Lignocellulose is the most abundant renewable carbon source in the biosphere. However, the main bottleneck in its conversion to produce second generation biofuels is the saccharification step: the hydrolysis of lignocellulosic material into soluble fermentable sugars. Some anaerobic bacteria have developed an extracellular multi-enzyme complex called the cellulosome that efficiently degrades cellulosic substrates. Cellulosome complexes rely on enzyme-integrating scaffoldins that are large non-catalytic scaffolding proteins comprising several cohesin modules and additional functional modules that mediate the anchoring of the complex to the cell surface and the specific binding to its cellulosic substrate. It was proposed that mechanical forces may affect the cohesins positioned between the cell- and cellulose-anchoring points in the so-called connecting region. Consequently, the mechanical resistance of cohesins within the scaffoldin is of great importance, both to understand cellulosome function and as a parameter of industrial interest, to better mimic natural complexes through the use of the established designer cellulosome technology. Here we study how the mechanical stability of cohesins in a scaffoldin affects the enzymatic activity of a cellulosome. We found that when a cohesin of low mechanical stability is positioned in the connecting region of a scaffoldin, the activity of the resulting cellulosome is reduced as opposed to a cohesin of higher mechanical stability. This observation directly relates mechanical stability of the scaffoldin-borne cohesins to cellulosome activity and provides a rationale for the design of artificial cellulosomes for industrial applications, by incorporating mechanical stability as a new industrial parameter in the biotechnology toolbox.
The multi-enzyme cellulosome complex can mediate the valorization of lignocellulosic biomass into soluble sugars that can serve in the production of biofuels and valuable products. A potent bacterial chassis for the production of active cellulosomes displayed on the cell surface is the bacteriumLactobacillus plantarum, a lactic acid bacterium used in many applications. Here, we developed a methodological pipeline to produce improved designer cellulosomes, using a cell-consortium approach, whereby the different components self-assemble on the surface ofL. plantarum.The pipeline served as a vehicle to select and optimize the secretion efficiency of potent designer cellulosome enzyme components, to screen for the most efficient enzymatic combinations and to assess attempts to grow the engineered bacterial cells on wheat straw as a sole carbon source. Using this strategy, we were able to improve the secretion efficiency of the selected enzymes and to secrete a fully functional high-molecular-weight scaffoldin component. The adaptive laboratory process served to increase significantly the enzymatic activity of the most efficient cell consortium. Internal plasmid re-arrangement towards a higher enzymatic performance attested for the suitability of the approach, which suggests that this strategy represents an efficient way for microbes to adapt to changing conditions.
Real-time atomic force microscopy imaging reveals dynamic structural changes of the multienzyme cellulosome complex and its cellulosic substrate.
Clostridium saccharoperbutylacetonicum is a mesophilic, anaerobic, butanol-producing bacterium, originally isolated from soil. It was recently reported that C. saccharoperbutylacetonicum possesses multiple cellulosomal elements and would potentially form the smallest cellulosome known in nature. Its genome contains only eight dockerin-bearing enzymes, and its unique scaffoldin bears two cohesins (Cohs), three X2 modules, and two carbohydrate-binding modules (CBMs). In this study, all of the cellulosome-related modules were cloned, expressed, and purified. The recombinant cohesins, dockerins, and CBMs were tested for binding activity using enzyme-linked immunosorbent assay (ELISA)-based techniques. All the enzymes were tested for their comparative enzymatic activity on seven different cellulosic and hemicellulosic substrates, thus revealing four cellulases, a xylanase, a mannanase, a xyloglucanase, and a lichenase. All dockerin-containing enzymes interacted similarly with the second cohesin (Coh2) module, whereas Coh1 was more restricted in its interaction pattern. In addition, the polysaccharide-binding properties of the CBMs within the scaffoldin were examined by two complementary assays, affinity electrophoresis and affinity pulldown. The scaffoldin of C. saccharoperbutylacetonicum exhibited high affinity for cellulosic and hemicellulosic substrates, specifically to microcrystalline cellulose and xyloglucan. Evidence that supports substrate-dependent in vivo secretion of cellulosomes is presented. The results of our analyses contribute to a better understanding of simple cellulosome systems by identifying the key players in this minimalistic system and the binding pattern of its cohesin-dockerin interaction. The knowledge gained by our study will assist further exploration of similar minimalistic cellulosomes and will contribute to the significance of specific sets of defined cellulosomal enzymes in the degradation of cellulosic biomass.IMPORTANCE Cellulosome-producing bacteria are considered among the most important bacteria in both mesophilic and thermophilic environments, owing to their capacity to deconstruct recalcitrant plant-derived polysaccharides (and notably cellulose) into soluble saccharides for subsequent processing. In many ecosystems, the cellulosome-producing bacteria are particularly effective "first responders." The massive amounts of sugars produced are potentially amenable in industrial settings to further fermentation by appropriate microbes to biofuels, notably ethanol and butanol. Among the solvent-producing bacteria, Clostridium saccharoperbutylacetonicum has the smallest cellulosome system known thus far. The importance of investigating the building blocks of such a small, multifunctional nanomachine is crucial to understanding the fundamental activities of this efficient enzymatic complex.
Biomass deconstruction remains integral for enabling second-generation biofuel production at scale. However, several steps necessary to achieve significant solubilization of biomass, notably harsh pretreatment conditions, impose economic barriers to commercialization. By employing hyperthermostable cellulase machinery, biomass deconstruction can be made more efficient, leading to milder pretreatment conditions and ultimately lower production costs. The hyperthermophilic bacteriumCaldicellulosiruptor besciiproduces extremely active hyperthermostable cellulases, including the hyperactive multifunctional cellulaseCbCel9A/Cel48A. RecombinantCbCel9A/Cel48A components have been previously produced inEscherichia coliand integrated into synthetic hyperthermophilic designer cellulosome complexes. Since then, glycosylation has been shown to be vital for the high activity and stability ofCbCel9A/Cel48A. Here, we studied the impact of glycosylation on a hyperthermostable designer cellulosome system in which two of the cellulosomal components, the scaffoldin and the GH9 domain ofCbCel9A/Cel48A, were glycosylated as a consequence of employingCa. besciias an expression host.Inclusion of the glycosylated components yielded an active cellulosome system that exhibited long-term stability at 75 degrees C. The resulting glycosylated designer cellulosomes showed significantly greater synergistic activity compared to the enzymatic components alone, as well as higher thermostability than the analogous nonglycosylated designer cellulosomes. These results indicate that glycosylation can be used as an essential engineering tool to improve the properties of designer cellulosomes. Additionally,Ca. besciiwas shown to be an attractive candidate for production of glycosylated designer cellulosome components, which may further promote the viability of this bacterium both as a cellulase expression host and as a potential consolidated bioprocessing platform organism.
Cellulolytic clostridia use a highly efficient cellulosome system to degrade polysaccharides. To regulate genes encoding enzymes of the multi-enzyme cellulosome complex, certain clostridia contain alternative sigma I (σ I) factors that have cognate membrane-associated anti-σ I factors (RsgIs) which act as polysaccharide sensors. In this work, we analyzed the structure-function relationship of the extracellular sensory elements of Clostridium (Ruminiclostridium) thermocellum and Clostridium clariflavum (RsgI3 and RsgI4, respectively). These elements were selected for comparison, as each comprised two tandem PA14-superfamily motifs. The X-ray structures of the PA14 modular dyads from the two bacterial species were determined, both of which showed a high degree of structural and sequence similarity, although their binding preferences differed. Bioinformatic approaches indicated that the DNA sequence of promoter of sigI/rsgI operons represents a strong signature, which helps to differentiate binding specificity of the structurally similar modules. The σ I4-dependent C. clariflavum promoter sequence correlates with binding of RsgI4_PA14 to xylan and was identified in genes encoding xylanases, whereas the σ I3-dependent C. thermocellum promoter sequence correlates with RsgI3_PA14 binding to pectin and regulates pectin degradation-related genes. Structural similarity between clostridial PA14 dyads to PA14-containing proteins in yeast helped identify another crucial signature element: the calcium-binding loop 2 (CBL2), which governs binding specificity. Variations in the five amino acids that constitute this loop distinguish the pectin vs xylan specificities. We propose that the first module (PA14 A) is dominant in directing the binding to the ligand in both bacteria. The two X-ray structures of the different PA14 dyads represent the first reported structures of tandem PA14 modules.
Rapid decomposition of plant biomass in soda lakes is associated with microbial activity of anaerobic cellulose-degrading communities. The alkaliphilic bacterium, Clostridium alkalicellulosi, is the single known isolate from a soda lake that demonstrates cellulolytic activity. This microorganism secretes cellulolytic enzymes that degrade cellulose under anaerobic and alkaliphilic conditions. A previous study indicated that the protein fraction of cellulose-grown cultures showed similarities in composition and size to known components of the archetypical cellulosome Clostridium thermocellum. Bioinformatic analysis of the C. alkalicellulosi draft genome sequence revealed 44 cohesins, organized into 22 different scaffoldins, and 142 dockerin-containing proteins. The modular organization of the scaffoldins shared similarities to those of C. thermocellum and Acetivibrio cellulolyticus, whereas some exhibited unconventional arrangements containing peptidases and oxidative enzymes. The binding interactions among cohesins and dockerins assessed by ELISA, revealed a complex network of cellulosome assemblies and suggested both cell-associated and cell-free systems. Based on these interactions, C. alkalicellulosi cellulosomal systems have the genetic potential to create elaborate complexes, which could integrate up to 105 enzymatic subunits. The alkalistable C. alkalicellulosi cellulosomal systems and their enzymes would be amenable to biotechnological processes, such as treatment of lignocellulosic biomass following prior alkaline pretreatment.
Can molecular dynamics simulations predict the mechanical behavior of protein complexes? Can simulations decipher the role of protein domains of unknown function in large macromolecular complexes? Here, we employ a wide-sampling computational approach to demonstrate that molecular dynamics simulations, when carefully performed and combined with single-molecule atomic force spectroscopy experiments, can predict and explain the behavior of highly mechanostable protein complexes. As a test case, we studied a previously unreported homologue from Ruminococcus flavefaciens called X-module-Dockerin (XDoc) bound to its partner Cohesin (Coh). By performing dozens of short simulation replicas near the rupture event, and analyzing dynamic network fluctuations, we were able to generate large simulation statistics and directly compare them with experiments to uncover the mechanisms involved in mechanical stabilization. Our single-molecule force spectroscopy experiments show that the XDoc-Coh homologue complex withstands forces up to 1 nN at loading rates of 10(5) pN/s. Our simulation results reveal that this remarkable mechanical stability is achieved by a protein architecture that directs molecular deformation along paths that run perpendicular to the pulling axis. The X-module was found to play a crucial role in shielding the adjacent protein complex from mechanical rupture. These mechanisms of protein mechanical stabilization have potential applications in biotechnology for the development of systems exhibiting shear enhanced adhesion or tunable mechanics.
β-Glucosidases are key enzymes in the process of cellulose utilization. It is the last enzyme in the cellulose hydrolysis chain, which converts cellobiose to glucose. Since cellobiose is known to have a feedback inhibitory effect on a variety of cellulases, β-glucosidase can prevent this inhibition by hydrolyzing cellobiose to non-inhibitory glucose. While the optimal temperature of the Clostridium thermocellum cellulosome is 70 °C, C. thermocellum β-glucosidase A is almost inactive at such high temperatures. Thus, in the current study, a random mutagenesis directed evolutionary approach was conducted to produce a thermostable mutant with Kcat and Km, similar to those of the wild-type enzyme. The resultant mutant contained two mutations, A17S and K268N, but only the former was found to affect thermostability, whereby the inflection temperature (Ti) was increased by 6.4 °C. A17 is located near the central cavity of the native enzyme. Interestingly, multiple alignments revealed that position 17 is relatively conserved, whereby alanine is replaced only by serine. Upon the addition of the thermostable mutant to the C. thermocellum secretome for subsequent hydrolysis of microcrystalline cellulose at 70 °C, a higher soluble glucose yield (243%) was obtained compared to the activity of the secretome supplemented with the wild-type enzyme.
70 family alternative σ
I factors and their cognate anti-σ
I factors are widespread in Clostridia and Bacilli and play a role in heat stress response, virulence, and polysaccharide sensing. Multiple σ
I factors exist in some lignocellulolytic clostridial species, specifically for regulation of components of a multienzyme complex, termed the cellulosome. The σ
I and anti-σ
I factors are unique, because the C-terminal domain of σ
I (SigIC) and the N-terminal inhibitory domain of anti-σ
I (RsgIN) lack homology to known proteins. Here, we report structure and interaction studies of a pair of σ
I and anti-σ
I factors, SigI1 and RsgI1, from the cellulosome-producing bacterium, Clostridium thermocellum. In contrast to other known anti-σ factors that have N-terminal helical structures, RsgIN has a β-barrel structure. Unlike other anti-σ factors that bind both σ2 and σ4 domains of the σ factors, RsgIN binds SigIC specifically. Structural analysis showed that SigIC contains a positively charged surface region that recognizes the promoter -35 region, and the synergistic interactions amongmultiple interfacial residues result in the specificity displayed by different σ
I pairs. We suggest that the σ
I factors represent a distinctive mode of σ/anti-σ complex formation, which provides the structural basis for understanding the molecular mechanism of the intricate σ
Background(Pseudo)Bacteroides cellulosolvens is a cellulolytic bacterium that produces the most extensive and intricate cellulosomal system known in nature. Recently, the elaborate architecture of the B. cellulosolvens cellulosomal system was revealed from analysis of its genome sequence, and the first evidence regarding the interactions between its structural and enzymatic components were detected in vitro. Yet, the understanding of the cellulolytic potential of the bacterium in carbohydrate deconstruction is inextricably linked to its high-molecular-weight protein complexes, which are secreted from the bacterium.ResultsThe current proteome-wide work reveals patterns of protein expression of the various cellulosomal components, and explores the signature of differential expression upon growth of the bacterium on two major carbon sourcescellobiose and microcrystalline cellulose. Mass spectrometry analysis of the bacterial secretome revealed the expression of 24 scaffoldin structural units and 166 dockerin-bearing components (mainly enzymes), in addition to free enzymatic subunits. The dockerin-bearing components comprise cell-free and cell-bound cellulosomes for more efficient carbohydrate degradation. Various glycoside hydrolase (GH) family members were represented among 102 carbohydrate-degrading enzymes, including the omnipresent, most abundant GH48 exoglucanase. Specific cellulosomal components were found in different molecular-weight fractions associated with cell growth on different carbon sources. Overall, microcrystalline cellulose-derived cellulosomes showed markedly higher expression levels of the structural and enzymatic components, and exhibited the highest degradation activity on five different cellulosic and/or hemicellulosic carbohydrates. The cellulosomal activity of B. cellulosolvens showed high degradation rates that are very promising in biotechnological terms and were compatible with the activity levels exhibited by Clostridium thermocellum purified cellulosomes.ConclusionsThe current research demonstrates the involvement of key cellulosomal factors that participate in the mechanism of carbohydrate degradation by B. cellulosolvens. The powerful ability of the bacterium to exhibit different degradation strategies on various carbon sources was revealed. The novel reservoir of cellulolytic components of the cellulosomal degradation machineries may serve as a pool for designing new cellulolytic cocktails for biotechnological purposes.
Background: Renewable energy has become a field of high interest over the past decade, and production of biofuels from cellulosic substrates has a particularly high potential as an alternative source of energy. Industrial deconstruction of biomass, however, is an onerous, exothermic process, the cost of which could be decreased significantly by use of hyperthermophilic enzymes. An efficient way of breaking down cellulosic substrates can also be achieved by highly efficient enzymatic complexes called cellulosomes. The modular architecture of these multi-enzyme complexes results in substrate targeting and proximity-based synergy among the resident enzymes. However, cellulosomes have not been observed in hyperthermophilic bacteria. Results: Here, we report the design and function of a novel hyperthermostable "designer cellulosome" system, which is stable and active at 75 °C. Enzymes from Caldicellulosiruptor bescii, a highly cellulolytic hyperthermophilic anaerobic bacterium, were selected and successfully converted to the cellulosomal mode by grafting onto them divergent dockerin modules that can be inserted in a precise manner into a thermostable chimaeric scaffoldin by virtue of their matching cohesins. Three pairs of cohesins and dockerins, selected from thermophilic microbes, were examined for their stability at extreme temperatures and were determined stable at 75 °C for at least 72 h. The resultant hyperthermostable cellulosome complex exhibited the highest levels of enzymatic activity on microcrystalline cellulose at 75 °C, compared to those of previously reported designer cellulosome systems and the native cellulosome from Clostridium thermocellum. Conclusion: The functional hyperthermophilic platform fulfills the appropriate physico-chemical properties required for exothermic processes. This system can thus be adapted for other types of thermostable enzyme systems and could serve as a basis for a variety of cellulolytic and non-cellulolytic industrial objectives at high temperatures.
Cell-surface display of designer cellulosomes complexes has attracted increased interest in recent years. These engineered microorganisms can efficiently degrade lignocellulosic biomass that represents an abundant resource for conversion into fermentable sugars, suitable for production of biofuels. The designer cellulosome is an artificial enzymatic complex that mimics the architecture of the natural cellulosome and allows the control of the positions, type, and copy number of the cellulosomal enzymes within the complex. Lactobacillus plantarum is an attractive candidate for metabolic engineering of lignocellulosic biomass to biofuels, as its natural characteristics include high ethanol and acid tolerance and the ability to metabolize hexose sugars. In recent years, successful expression of a variety of designer cellulosomes on the cell surface of this bacterium has been demonstrated using the cell-consortium approach. This strategy minimized genomic interference on each strain upon genetic engineering, thereby maximizing the ability of each strain to grow, express, and secrete each enzyme. In addition, this strategy allows stoichiometric control of the cellulosome elements and facile exchange of the secreted proteins. A detailed procedure for display of designer cellulosomes on the cell surface of L. plantarum is described in this chapter.
Efficient degradation of plant cell walls by selected anaerobic bacteria is performed by large extracellular multienzyme complexes termed cellulosomes. The spatial arrangement within the cellulosome is organized by a protein called scaffoldin, which recruits the cellulolytic subunits through interactions between cohesin modules on the scaffoldin and dockerin modules on the enzymes. Although many structural studies of the individual components of cellulosomal scaffoldins have been performed, the role of interactions between individual cohesin modules and the flexible linker regions between them are still not entirely understood. Here, we report single-molecule measurements using FRET to study the conformational dynamics of a bimodular cohesin segment of the scaffoldin protein CipA of Clostridium thermocellum. We observe compacted structures in solution that persist on the timescale of milliseconds. The compacted conformation is found to be in dynamic equilibrium with an extended state that shows distance fluctuations on the microsecond timescale. Shortening of the intercohesin linker does not destabilize the interactions but reduces the rate of contact formation. Upon addition of dockerin-containing enzymes, an extension of the flexible state is observed, but the cohesin-cohesin interactions persist. Using allatom molecular-dynamics simulations of the system, we further identify possible intercohesin binding modes. Beyond the view of scaffoldin as "beads on a string," we propose that cohesincohesin interactions are an important factor for the precise spatial arrangement of the enzymatic subunits in the cellulosome that leads to the high catalytic synergy in these assemblies and should be considered when designing cellulosomes for industrial applications.
The cellulosome provides a fully worked out example of evolved radical nanotechnology. Improved understanding, and first steps toward re-engineering this biological nanomachine, is providing design rules for the formulation of advanced synthetic materials that can harness molecular flexibility and sticking interactions for applications in clean energy, environmental monitoring, and miniaturized devices. Computer simulations provide atomic scale insights into the mechanical stability of the component protein units, flexibility of short peptides that tether the units into scaffolds, and thermodynamic stability of protein-protein and protein-carbohydrate complexes, complementing and in some cases directing experiments. In the present work, a systematic computational study of cohesin-dockerin pairs, the strongly-bound protein complexes that glue the cellulosome nano-architecture in place, reveals that a short alpha-helix in the middle of the smaller dockerin protein becomes disordered at elevated temperatures and weakens cohesin-dockerin binding in mesophilic species. In thermophilic species, a more extensive and more thermally resistant H-bond network ensures the structure remains ordered at elevated temperatures of up to 400 K. The simulations predict that simply grafting the most crucial eight-residue peptide sequence into the mesophilic complex can, for one species and one of two possible binding modes, potentially create a new thermally resistant complex, providing leads for future experiments to re-engineer designer cellulosomes that can withstand elevated temperatures and so provide clean, renewable biocatalysts.
Bacteria can adjust their genetic programs via alternative σ factors to face new environmental pressures. Here, we analyzed a unique set of paralogous alternative σ factors, termed σ
Is, which fine-tune the regulation of one of the most intricate cellulolytic systems in nature, the bacterial cellulosome, that is involved in degradation of environmental polysaccharides. We combined bioinformatics with experiments to decipher the regulatory networks of five σ
Is in Clostridium thermocellum, the epitome of cellulolytic microorganisms, and one σ
I in Pseudobacteroides cellulosolvens which produces the cellulosomal system with the greatest known complexity. Despite high homology between different σ
Is, our data suggest limited cross-talk among them. Remarkably, the major cross-talk occurs within the main cellulosomal genes which harbor the same σ
I-dependent promoter elements, suggesting a promoter-based mechanism to guarantee the expression of relevant genes. Our findings provide insights into the mechanisms used by σ
Is to differentiate among their corresponding regulons, representing a comprehensive overview of the regulation of the cellulosome to date. Finally, we show the advantage of using a heterologous host system for analysis of multiple σ
Is, since information generated by their analysis in their natural host can be misinterpreted owing to a cascade of interactions among the different σ
Background: During the process of bioethanol production, cellulose is hydrolyzed into its monomeric soluble units. For efficient hydrolysis, a chemical and/or mechanical pretreatment step is required. Such pretreatment is designed to increase enzymatic digestibility of the cellulose chains inter alia by de-crystallization of the cellulose chains and by removing barriers, such as lignin from the plant cell wall. Biological pretreatment, in which lignin is decomposed or modified by white-rot fungi, has also been considered. One disadvantage in biological pretreatment, however, is the consumption of the cellulose by the fungus. Thus, fungal species that attack lignin with only minimal cellulose loss are advantageous. The secretomes of white-rot fungi contain carbohydrate-active enzymes (CAZymes) including lignin-modifying enzymes. Thus, modification of secretome composition can alter the ratio of lignin/cellulose degradation.Results: Pleurotus ostreatus PC9 was genetically modified to either overexpress or eliminate (by gene replacement) the transcriptional regulator CRE1, known to act as a repressor in the process of carbon catabolite repression. The cre1-overexpressing transformant demonstrated lower secreted cellulolytic activity and slightly increased selectivity (based on the chemical composition of pretreated wheat straw), whereas the knockout transformant demonstrated increased cellulolytic activity and significantly reduced residual cellulose, thereby displaying lower selectivity. Pre-treatment of wheat straw using the wild-type PC9 resulted in 2.8-fold higher yields of soluble sugar compared to untreated wheat straw. The overexpression transformant showed similar yields (2.6-fold), but the knockout transformant exhibited lower yields (1.2-fold) of soluble sugar. Based on proteomic secretome analysis, production of numerous CAZymes was affected by modification of the expression level of cre1.Conclusions: The gene cre1 functions as a regulator for expression of fungal CAZymes active against plant cell wall lignocelluloses, hence altering the substrate preference of the fungi tested. While the cre1 knockout resulted in a less efficient biological pretreatment, i.e., less saccharification of the treated biomass, the converse manipulation of cre1 (overexpression) failed to improve efficiency. Despite the inverse nature of the two genetic alterations, the expected "mirror image" (i.e., opposite regulatory response) was not observed, indicating that the secretion level of CAZymes, was not exclusively dependent on CRE1 activity.
Cellulosomes are highly sophisticated molecular nanomachines that participate in the deconstruction of complex polysaccharides, notably cellulose and hemicellulose. Cellulosomal assembly is orchestrated by the interaction of enzyme-borne dockerin (Doc) modules to tandem cohesin (Coh) modules of a noncatalytic primary scaffoldin. In some cases, as exemplified by the cellulosome of the major cellulolytic ruminal bacterium Ruminococcus flavefaciens, primary scaffoldins bind to adaptor scaffoldins that further interact with the cell surface via anchoring scaffoldins, thereby increasing cellulosome complexity. Here we elucidate the structure of the unique Doc of R. flavefaciens FD-1 primary scaffoldin ScaA, bound to Coh 5 of the adaptor scaffoldin ScaB. The RfCohScaB5-DocScaA complex has an elliptical architecture similar to previously described complexes from a variety of ecological niches. ScaA Doc presents a single-binding mode, analogous to that described for the other two Coh-Doc specificities required for cellulosome assembly in R. flavefaciens. The exclusive reliance on a single-mode of Coh recognition contrasts with the majority of cellulosomes from other bacterial species described to date, where Docs contain two similar Coh-binding interfaces promoting a dual-binding mode. The discrete Coh-Doc interactions observed in ruminal cellulosomes suggest an adaptation to the exquisite properties of the rumen environment.
Cellulosomes are bacterial protein complexes that bind and efficiently degrade lignocellulosic substrates. These are formed by multimodular scaffolding proteins known as scaffoldins, which comprise cohesin modules capable of binding dockerin-bearing enzymes and usually a carbohydrate-binding module that anchors the system to a substrate. It has been suggested that cellulosomes bound to the bacterial cell surface might be exposed to significant mechanical forces. Accordingly, the mechanical properties of these anchored cellulosomes may be important to understand and improve cellulosome function. Here we used single-molecule force spectroscopy to study the mechanical properties of selected cohesin modules from scaffoldins of different cellulosomes. We found that cohesins located in the region connecting the cell and the substrate are more robust than those located outside these two anchoring points. This observation applies to cohesins from primary scaffoldins (i.e. those that directly bind dockerin-bearing enzymes) from different cellulosomes despite their sequence differences. Furthermore, we also found that cohesin nanomechanics (specifically, mechanostability and the position of the mechanical clamp of cohesin) are not significantly affected by other cellulosomal components, including linkers between cohesins, multiple cohesin repeats, and dockerin binding. Finally, we also found that cohesins (from both the connecting and external regions) have poor refolding efficiency but similar refolding rates, suggesting that the high mechanostability of connecting cohesins may be an evolutionarily conserved trait selected to minimize the occurrence of cohesin unfolding, which could irreversibly damage the cellulosome. We conclude that cohesin mechanostability is a major determinant of the overall mechanical stability of the cellulosome.
Processive hydrolysis of crystalline cellulose by cellulases is a critical step for lignocellulose deconstruction. The classic Trichoderma reesei exoglucanase TrCel7A, which has a closed active-site tunnel, starts each processive run by threading the tunnel with a cellulose chain. Loop regions are necessary for tunnel conformation, resulting in weak thermostability of fungal exoglucanases. However, endoglucanase CcCel9A, from the thermophilic bacterium Clostridium cellulosi, comprises a glycoside hydrolase (GH) family 9 module with an open cleft and five carbohydrate-binding modules (CBMs) and hydrolyzes crystalline cellulose processively. How CcCel9A and other similar GH9 enzymes bind to the smooth surface of crystalline cellulose to achieve processivity is still unknown. Our results demonstrate that the C-terminal CBM3b and three CBMX2s enhance productive adsorption to cellulose, while the CBM3c adjacent to the GH9 is tightly bound to 11 glucosyl units, thereby extending the catalytic cleft to 17 subsites, which facilitates decrystallization by forming a supramodular binding surface. In the open cleft, the strong interaction forces between substrate-binding subsites and glucosyl rings enable cleavage of the hydrogen bonds and extraction of a single cellulose chain. In addition, subsite 4 is capable of drawing the chain to its favored location. Cellotetraose is released from the open cleft as the initial product to achieve high processivity, which is further hydrolyzed to cellotriose, cellobiose and glucose by the catalytic cleft of the endoglucanase. On this basis, we propose a wirewalking mode for processive degradation of crystalline cellulose by an endoglucanase, which provides insights for rational design of industrial cellulases.
Heterologous display of enzymes on microbial cell surfaces is an extremely desirable approach, since it enables the engineered microbe to interact directly with the plant wall extracellular polysaccharide matrix. In recent years, attempts have been made to endow noncellulolytic microbes with genetically engineered cellulolytic capabilities for improved hydrolysis of lignocellulosic biomass and for advanced probiotics. Thus far, however, owing to the hurdles encountered in secreting and assembling large, intricate complexes on the bacterial cell wall, only free cellulases or relatively simple cellulosome assemblies have been introduced into live bacteria. Here, we employed the "adaptor scaffoldin" strategy to compensate for the low levels of protein displayed on the bacterial cell surface. That strategy mimics natural elaborated cellulosome architectures, thus exploiting the exponential features of their Lego-like combinatorics. Using this approach, we produced several bacterial consortia of Lactobacillus plantarum, a potent gut microbe which provides a very robust genetic framework for lignocellulosic degradation. We successfully engineered surface display of large, fully active self-assembling cellulosomal complexes containing an unprecedented number of catalytic subunits all produced in vivo by the cell consortia. Our results demonstrate that the enzyme stability and performance of the cellulosomal machinery, which are superior to those seen with the equivalent secreted free enzyme system, and the high cellulase-to-xylanase ratios proved beneficial for efficient degradation of wheat straw.
The assembly of the polysaccharide degradating cellulosome machinery is mediated by tight binding between cohesin and dockerin domains. We have used an empirical model known as FoldX as well as molecular mechanics methods to determine the free energy of binding between a cohesin and a dockerin from Clostridium thermocellum in two possible modes that differ by an approximately 180 degrees rotation. Our studies suggest that the full-length wild-type complex exhibits dual binding at room temperature, i.e., the two modes of binding have comparable probabilities at equilibrium. The ability to bind in the two modes persists at elevated temperatures. However, single-point mutations or truncations of terminal segments in the dockerin result in shifting the equilibrium towards one of the binding modes. Our molecular dynamics simulations of mechanical stretching of the full-length wildtype cohesin-dockerin complex indicate that each mode of binding leads to two kinds of stretching pathways, which may be mistakenly taken as evidence of dual binding.
The cellulosome is a remarkably intricate multienzyme nano-machine produced by anaerobic bacteria to degrade plant cell wall polysaccharides. Cellulosome assembly is mediated through binding of enzyme-borne dockerin modules to cohesin modules of the primary scaffoldin subunit. The anaerobic bacterium Acetivibrio cellulolyticus produces a highly intricate cellulosome comprising an adaptor scaffoldin, ScaB, whose cohesins interact with the dockerin of the primary scaffoldin (ScaA) that integrates the cellulosomal enzymes. The ScaB dockerin selectively binds to cohesin modules in ScaC that anchors the cellulosome onto the cell surface. Correct cellulosome assembly requires distinct specificities displayed by structurally related type-I cohesin-dockerin pairs that mediate ScaC-ScaB and ScaA-enzyme assemblies. To explore the mechanism by which these two critical protein interactions display their required specificities, we determined the crystal structure of the dockerin of a cellulosomal enzyme in complex with a ScaA cohesin. The data revealed that the enzyme-borne dockerin binds to the ScaA cohesin in two orientations, indicating two identical cohesin-binding sites. Combined mutagenesis experiments served to identify amino acid residues that modulate type-I cohesin-dockerin specificity in A. cellulolyticus. Rational design was used to test the hypothesis that the ligand-binding surfaces of ScaA-and ScaB-associated dockerins mediate cohesin recognition, independent of the structural scaffold. Novel specificities could thus be engineered into one, but not both, of the ligand-binding sites of ScaB, whereas attempts at manipulating the specificity of the enzyme-associated dockerin were unsuccessful. These data indicate that dockerin specificity requires critical interplay between the ligand-binding surface and the structural scaffold of these modules.
Transformation of cellulose into monosaccharides can be achieved by hydrolysis of the cellulose chains, carried out by a special group of enzymes known as cellulases. The enzymatic mechanism of cellulases is well described, but the role of non-enzymatic components of the cellulose-degradation machinery is still poorly understood, and difficult to measure using experiments alone. In this study, we use a comprehensive set of atomistic molecular dynamics simulations to probe the molecular details of binding of the family-3a carbohydrate-binding module (CBM3a) and the bacterial expansin protein (EXLX1) to a range of cellulose substrates. Our results suggest that CBM3a behaves in a similar way on both crystalline and amorphous cellulose, whereas binding of the dual-domain expansin protein depends on the substrate crystallinity, and we relate our computed binding modes to the experimentally measured features of CBM and expansin action on cellulose.
Cellulosomes are multienzyme complexes produced by anaerobic, cellulolytic bacteria for highly efficient breakdown of plant cell wall polysaccharides. Clostridium clariflavum is an anaerobic, thermophilic bacterium that produces the largest assembled cellulosome complex in nature to date, comprising three types of scaffoldins: a primary scaffoldin, ScaA; an adaptor scaffoldin, ScaB; and a cell surface anchoring scaffoldin, ScaC. This complex can contain 160 polysaccharide-degrading enzymes. In previous studies, we proposed potential types of cellulosome assemblies in C. clariflavum and demonstrated that these complexes are released into the extracellular medium. In the present study, we explored the disposition of the highly structured, four-tiered cell-anchored cellulosome complex of this bacterium. Four separate, integral cellulosome components were subjected to immunolabeling: ScaA, ScaB, ScaC, and the cellulosome's most prominent enzyme, GH48. Imaging of the cells by correlating scanning electron microscopy and three-dimensional (3D) super-resolution fluorescence microscopy revealed that some of the protuberance-like structures on the cell surface represent cellulosomes and that the components are highly colocalized and organized by a defined hierarchy on the cell surface. The display of the cellulosome on the cell surface was found to differ between cells grown on soluble or insoluble substrates. Cell growth on microcrystalline cellulose and wheat straw exhibited dramatic enhancement in the amount of cellulosomes displayed on the bacterial cell surface.IMPORTANCE Conversion of plant biomass into soluble sugars is of high interest for production of fermentable industrial materials, such as biofuels. Biofuels are a very attractive alternative to fossil fuels, both for recycling of agricultural wastes and as a source of sustainable energy. Cellulosomes are among the most efficient enzymatic degraders of biomass known to date, due to the incorporation of a multiplicity of enzymes into a potent, multifunctional nanomachine. The intimate association with the bacterial cell surface is inherent in its efficient action on lignocellulosic substrates, although this property has not been properly addressed experimentally. The dramatic increase in cellulosome performance on recalcitrant feedstocks is critical for the design of cost-effective processes for efficient biomass degradation.
Ruminococcus bromii is a dominant member of the human colonic microbiota that plays a 'keystone' role in degrading dietary resistant starch. Recent evidence from one strain has uncovered a unique cell surface 'amylosome' complex that organizes starch-degrading enzymes. New genome analysis presented here reveals further features of this complex and shows remarkable conservation of amylosome components between human colonic strains from three different continents and a R. bromii strain from the rumen of Australian cattle. These R. bromii strains encode a narrow spectrum of carbohydrate active enzymes (CAZymes) that reflect extreme specialization in starch utilization. Starch hydrolysis products are taken up mainly as oligosaccharides, with only one strain able to grow on glucose. The human strains, but not the rumen strain, also possess transporters that allow growth on galactose and fructose. R. bromii strains possess a full complement of sporulation and spore germination genes and we demonstrate the ability to form spores that survive exposure to air. Spore formation is likely to be a critical factor in the ecology of this nutritionally highly specialized bacterium, which was previously regarded as 'non-sporing', helping to explain its widespread occurrence in the gut microbiota through the ability to transmit between hosts.
The bacterial cellulosome is an extracellular, multi-enzyme machinery, which efficiently depolymerizes plant biomass by degrading plant cell wall polysaccharides. Several cellulolytic bacteria have evolved various elaborate modular architectures of active cellulosomes. We present here a genome-wide analysis of a dozen mesophilic clostridia species, including both well-studied and yet-undescribed cellulosome-producing bacteria. We first report here, the presence of cellulosomal elements, thus expanding our knowledge regarding the prevalence of the cellulosomal paradigm in nature. We explored the genomic organization of key cellulosome components by comparing the cellulosomal gene clusters in each bacterial species, and the conserved sequence features of the specific cellulosomal modules (cohesins and dockerins), on the background of their phylogenetic relationship. Additionally, we performed comparative analyses of the species-specific repertoire of carbohydrate-degrading enzymes for each of the clostridial species, and classified each cellulosomal enzyme into a specific CAZy family, thus indicating their putative enzymatic activity (e.g., cellulases, hemicellulases, and pectinases). Our work provides, for this large group of bacteria, a broad overview of the blueprints of their multi-component cellulosomal complexes. The high similarity of their scaffoldin clusters and dockerin-based recognition residues suggests a common ancestor, and/or extensive horizontal gene transfer, and potential cross-species recognition. In addition, the sporadic spatial organization of the numerous dockerin-containing genes in several of the genomes, suggests the importance of the cellulosome paradigm in the given bacterial species. The information gained in this work may be utilized directly or developed further by genetically engineering and optimizing designer cellulosome systems for enhanced biotechnological biomass deconstruction and biofuel production.
In the shadow of a burgeoning biomass-to-fuels industry, biological conversion of lignocellulose to fermentable sugars in a cost-effective manner is key to the success of second-generation and advanced biofuel production. For the effective comparison of one cellulase preparation to another, cellulase assays are typically carried out with one or more engineered cellulase formulations or natural exoproteomes of known performance serving as positive controls. When these formulations have unknown composition, as is the case with several widely used commercial products, it becomes impossible to compare or reproduce work done today to work done in the future, where, for example, such preparations may not be available. Therefore, being a critical tenet of science publishing, experimental reproducibility is endangered by the continued use of these undisclosed products. We propose the introduction of standard procedures and materials to produce specific and reproducible cellulase formulations. These formulations are to serve as yardsticks to measure improvements and performance of new cellulase formulations.
Background: (Pseudo) Bacteroides cellulosolvens is an anaerobic, mesophilic, cellulolytic, cellulosome-producing clostridial bacterium capable of utilizing cellulose and cellobiose as carbon sources. Recently, we sequenced the B. cellulosolvens genome, and subsequent comprehensive bioinformatic analysis, herein reported, revealed an unprecedented number of cellulosome-related components, including 78 cohesin modules scattered among 31 scaffoldins and more than 200 dockerin-bearing ORFs. In terms of numbers, the B. cellulosolvens cellulosome system represents the most intricate, compositionally diverse cellulosome system yet known in nature. Results: The organization of the B. cellulosolvens cellulosome is unique compared to previously described cellulosome systems. In contrast to all other known cellulosomes, the cohesin types are reversed for all scaffoldins i.e., the type II cohesins are located on the enzyme-integrating primary scaffoldin, whereas the type I cohesins are located on the anchoring scaffoldins. Many of the type II dockerin-bearing ORFs include X60 modules, which are known to stabilize type II cohesin-dockerin interactions. In the present work, we focused on revealing the architectural arrangement of cellulosome structure in this bacterium by examining numerous interactions between the various cohesin and dockerin modules. In total, we cloned and expressed 43 representative cohesins and 27 dockerins. The results revealed various possible architectures of cell-anchored and cell-free cellulosomes, which serve to assemble distinctive cellulosome types via three distinct cohesin-dockerin specificities: type I, type II, and a novel-type designated R (distinct from type III interactions, predominant in ruminococcal cellulosomes). Conclusions: The results of this study provide novel insight into the architecture and function of the most intricate and extensive cellulosomal system known today, thereby extending significantly our overall knowledge base of cellulosome systems and their components. The robust cellulosome system of B. cellulosolvens, with its unique binding specificities and reversal of cohesin–dockerin types, has served to amend our view of the cellulosome paradigm. Revealing new cellulosomal interactions and arrangements is critical for designing high-efficiency artificial cellulosomes for conversion of plant-derived cellulosic biomass towards improved production of biofuels.
Cellulose deconstruction can be achieved by three distinct enzymatic paradigms: free enzymes, multifunctional enzymes, and self-assembled, multi-enzyme complexes (cellulosomes). To study their comparative efficiency, the simple and efficient cellulolytic system of the aerobic bacterium, Thermobifida fusca, is developed as an enzymatic model. In previous studies, most of its cellulases are successfully converted to the cellulosomal mode and exhibited high cellulolytic activities, except for Cel6B, a key exoglucanase of the T. fusca enzymatic system. Here, the impact of the modular organization of Cel6B on enzymatic activity is investigated. The position of the cellulose-binding module (CBM), its family and linker segment are shown to affect activity. Surprisingly, exchange of the native family-2 CBM to family-3 generates an increase in Cel6B activity on cellulosic substrates. Conversion of Cel6B to the cellulosomal mode by fusing a cohesin to the catalytic module enables formation of divalent enzyme complexes with dockerin-bearing enzymes. The resultant pseudo-cellulosomes, containing Cel6B combined with endoglucanase Cel5A, exhibits enhanced enzymatic activity, compared to mixtures of wild-type enzymes or bifunctional enzymes, unlike similar pseudo-cellulosomes containing endoglucanase Cel6A or proccessive endoglucanase Cel9A. Insight into the different enzymatic paradigms benefits ongoing development of efficient cellulolytic systems for conversion of plant-derived biomass into valuable sugars.
Background: Bioethanol production processes involve enzymatic hydrolysis of pretreated lignocellulosic biomass into fermentable sugars. Due to the relatively high cost of enzyme production, the development of potent and cost-effective cellulolytic cocktails is critical for increasing the cost-effectiveness of bioethanol production. In this context, the multi-protein cellulolytic complex of Clostridium (Ruminiclostridium) thermocellum, the cellulosome, was studied here. C. thermocellum is known to assemble cellulosomes of various subunit (enzyme) compositions, in response to the available carbon source. In the current study, different carbon sources were used, and their influence on both cellulosomal composition and the resultant activity was investigated. Results: Glucose, cellobiose, microcrystalline cellulose, alkaline-pretreated switchgrass, alkaline-pretreated corn stover, and dilute acid-pretreated corn stover were used as sole carbon sources in the growth media of C. thermocellum strain DSM 1313. The purified cellulosomes were compared for their activity on selected cellulosic substrates. Interestingly, cellulosomes derived from cells grown on lignocellulosic biomass showed no advantage in hydrolyzing the original carbon source used for their production. Instead, microcrystalline cellulose- and glucose-derived cellulosomes were equal or superior in their capacity to deconstruct lignocellulosic biomass. Mass spectrometry analysis revealed differential composition of catalytic and structural subunits (scaffoldins) in the different cellulosome samples. The most abundant catalytic subunits in all cellulosome types include Cel48S, Cel9K, Cel9Q, Cel9R, and Cel5G. Microcrystalline cellulose- and glucose-derived cellulosome samples showed higher endoglucanase-to-exoglucanase ratios and higher catalytic subunit-per-scaffoldin ratios compared to lignocellulose-derived cellulosome types. Conclusion: The results reported here highlight the finding that cellulosomes derived from cells grown on glucose and microcrystalline cellulose are more efficient in their action on cellulosic substrates than other cellulosome preparations. These results should be considered in the future development of C. thermocellum-based cellulolytic cocktails, designer cellulosomes, or engineering of improved strains for deconstruction of lignocellulosic biomass.
The limitation of surface-display systems in biofuel cells to a single redox enzyme is a major drawback of hybrid biofuel cells, resulting in a low copy-number of enzymes per yeast cell and a limitation in displaying enzymatic cascades. Here we present the electrosome, a novel surface-display system based on the specific interaction between the cellulosomal scaffoldin protein and a cascade of redox enzymes that allows multiple electron-release by fuel oxidation. The electrosome is composed of two compartments: (i) a hybrid anode, which consists of dockerin-containing enzymes attached specifically to cohesin sites in the scaffoldin to assemble an ethanol oxidation cascade, and (ii) a hybrid cathode, which consists of a dockerin-containing oxygen-reducing enzyme attached in multiple copies to the cohesin-bearing scaffoldin. Each of the two compartments was designed, displayed, and tested separately. The new hybrid cell compartments displayed enhanced performance over traditional biofuel cells; in the anode, the cascade of ethanol oxidation demonstrated higher performance than a cell with just a single enzyme. In the cathode, a higher copy number per yeast cell of the oxygen-reducing enzyme copper oxidase has reduced the effect of competitive inhibition resulting from yeast oxygen consumption. This work paves the way for the assembly of more complex cascades using different enzymes and larger scaffoldins to further improve the performance of hybrid cells.
The robust plant cell wall polysaccharide-degrading properties of anaerobic bacteria are harnessed within elegant, marcomolecular assemblages called cellulosomes, in which proteins of complementary activities amass on scaffold protein networks. Research efforts have focused and continue to focus on providing detailed mechanistic insights into cellulosomal complex assembly, topology, and function. The accumulated information is expanding our fundamental understanding of the lignocellulosic biomass decomposition process and enhancing the potential of engineered cellulosomal systems for biotechnological purposes. Ongoing biochemical studies continue to reveal unexpected functional diversity within traditional cellulase families. Genomic, proteomic, and functional analyses have uncovered unanticipated cellulosomal proteins that augment the function of the native and designer cellulosomes. In addition, complementary structural and computational methods are continuing to provide much needed insights on the influence of cellulosomal interdomain linker regions on cellulosomal assembly and activity.
The opportunistic pathogen Clostridium perfringens assembles its toxins and carbohydrate-active enzymes by the high-affinity cohesin-dockerin (Coh-Doc) interaction. Coh-Doc interactions characterized previously have shown considerable resilience toward mechanical stress. Here; we aimed to determine the mechanics of this interaction from C. perfringens in the context of a pathogen. Using atomic force microscopy based single-molecule forte spectroscopy (AFM-SMFS) we probed the mechanical properties of the interaction of a dockerin from the mu-toxin with the GH84C X82 cohesin domain of C. perfringens. Most probable complex rupture forces were found to be approximately 60 pN and an estimate of the binding potential width was performed. The dockerin was expressed with its adjacent FIVAR (found in various architectures) domain, whose mechanostability we determined to be very similar to the complex. Additionally, fast refolding of this domain was observed. The Coh-Doc interaction from C. perfringens is the mechanically weakest observed to date. Our results establish the relevant force range of toxin assembly mechanics in pathogenic Clostridia.
Cellulosomes are sophisticated multi-enzymatic nanomachines produced by anaerobes to effectively deconstruct plant structural carbohydrates. Cellulosome assembly involves the binding of enzymeborne dockerins (Doc) to repeated cohesin (Coh) modules located in a non-catalytic scaffoldin. Docs appended to cellulosomal enzymes generally present two similar Coh-binding interfaces supporting a dual-binding mode, which may confer increased positional adjustment of the different complex components. Ruminococcus flavefaciens' cellulosome is assembled from a repertoire of 223 Doccontaining proteins classified into 6 groups. Recent studies revealed that Docs of groups 3 and 6 are recruited to the cellulosome via a single-binding mode mechanism with an adaptor scaffoldin. To investigate the extent to which the single-binding mode contributes to the assembly of R. flavefaciens cellulosome, the structures of two group 1 Docs bound to Cohs of primary (ScaA) and adaptor (ScaB) scaffoldins were solved. The data revealed that group 1 Docs display a conserved mechanism of Coh recognition involving a single-binding mode. Therefore, in contrast to all cellulosomes described to date, the assembly of R. flavefaciens cellulosome involves single but not dual-binding mode Docs. Thus, this work reveals a novel mechanism of cellulosome assembly and challenges the ubiquitous implication of the dual-binding mode in the acquisition of cellulosome flexibility.
Cellulosomes are considered to be one of the most efficient systems for the degradation of plant cell wall polysaccharides. The central cellulosome component comprises a large, noncatalytic protein subunit called scaffoldin. Multiple saccharolytic enzymes are incorporated into the scaffoldins via specific high-affinity cohesin-dockerin interactions. Recently, the regulation of genes encoding certain cellulosomal components by multiple RNA polymerase alternative sigma(I) factors has been demonstrated in Clostridium (Ruminiclostridium) thermocellum. In the present report, we provide experimental evidence demonstrating that the C. thermocellum cipA gene, which encodes the primary cellulosomal scaffoldin, is regulated by several alternative sigma(I) factors and by the vegetative sigma(A) factor. Furthermore, we show that previously suggested transcriptional start sites (TSSs) of C. thermocellum cipA are actually posttranscriptional processed sites. By using comparative bioinformatic analysis, we have also identified highly conserved sigma I and sigma(A)-dependent promoters upstream of the primary scaffoldin-encoding genes of other clostridia, namely, Clostridium straminisolvens, Clostridium clariflavum, Acetivibrio cellulolyticus, and Clostridium sp. strain Bc-iso-3. Interestingly, a previously identified TSS of the primary scaffoldin CbpA gene of Clostridium cellulovorans matches the predicted sigma(I)-dependent promoter identified in the present work rather than the previously proposed sigma(A) promoter. With the exception of C. cellulovorans, both sigma(I) and sigma(A) promoters of primary scaffoldin genes are located more than 600 nucleotides upstream of the start codon, yielding long 5=untranslated regions (5'-TRs).Furthermore, these 5'-Rs have highly conserved stem-loop structures located near the start codon. We propose that these large 5'-UTRs may be involved in the regulation of both the primary scaffoldin and other cellulosomal components.IMPORTANCE Cellulosome-producing bacteria are among the most effective cellulolytic microorganisms known. This group of bacteria has biotechnological potential for the production of second-generation biofuels and other biocommodities from cellulosic wastes. The efficiency of cellulose hydrolysis is due to their cellulosomes, which arrange enzymes in close proximity on the cellulosic substrate, thereby increasing synergism among the catalytic domains. The backbone of these multienzyme nanomachines is the scaffoldin subunit, which has been the subject of study for many years. However, its genetic regulation is poorly understood. Hence, from basic and applied points of view, it is imperative to unravel the regulatory mechanisms of the scaffoldin genes. The understanding of these regulatory mechanisms can help to improve the performance of the industrially relevant strains of C. thermocellum and related cellulosome-producing bacteria en route to the consolidated bioprocessing of biomass.
Cellulosomes are multienzyme complexes that are produced by anaerobic cellulolytic bacteria for the degradation of lignocellulosic biomass. They comprise a complex of scaffoldin, which is the structural subunit, and various enzymatic subunits. The intersubunit interactions in these multienzyme complexes are mediated by cohesin and dockerin modules. Cellulosome-producing bacteria have been isolated from a large variety of environments, which reflects their prevalence and the importance of this microbial enzymatic strategy. In a given species, cellulosomes exhibit intrinsic heterogeneity, and between species there is a broad diversity in the composition and configuration of cellulosomes. With the development of modern technologies, such as genomics and proteomics, the full protein content of cellulosomes and their expression levels can now be assessed and the regulatory mechanisms identified. Owing to their highly efficient organization and hydrolytic activity, cellulosomes hold immense potential for application in the degradation of biomass and are the focus of much effort to engineer an ideal microorganism for the conversion of lignocellulose to valuable products, such as biofuels.
Protein-protein interactions play a vital role in cellular processes as exemplified by assembly of the intricate multi-enzyme cellulosome complex. Cellulosomes are assembled by selective high-affinity binding of enzyme-borne dockerin modules to repeated cohesin modules of structural proteins termed scaffoldins. Recent sequencing of the fiber-degrading Ruminococcus flavefaciens FD-1 genome revealed a particularly elaborate cellulosome system. In total, 223 dockerin-bearing ORFs potentially involved in cellulosome assembly and a variety of multi-modular scaffoldins were identified, and the dockerins were classified into six major groups. Here, extensive screening employing three complementary medium- to high-throughput platforms was used to characterize the different cohesin-dockerin specificities. The platforms included (i) cellulose-coated microarray assay, (ii) enzyme-linked immunosorbent assay (ELISA) and (iii) in-vivo co-expression and screening in Escherichia coli. The data revealed a collection of unique cohesin-dockerin interactions and support the functional relevance of dockerin classification into groups. In contrast to observations reported previously, a dual-binding mode is involved in cellulosome cell-surface attachment, whereas single-binding interactions operate for cellulosome integration of enzymes. This sui generis cellulosome model enhances our understanding of the mechanisms governing the remarkable ability of R. flavefaciens to degrade carbohydrates in the bovine rumen and provides a basis for constructing efficient nano-machines applied to biological processes.
The cellulosome is an extracellular multi-enzyme complex that is considered one of the most efficient plant cell wall-degrading strategies devised by nature. Its unique modular architecture, achieved by high affinity and specific interaction between protein modules (cohesins and dockerins) enables formation of various enzyme combinations. Extensive research has been dedicated to the mechanistic nature of the cellulosome complex. Nevertheless, little is known regarding its distribution and abundance among microbes in natural plant fibre-rich environments. Here, we explored these questions in bovine rumen microbial communities, specialized in efficient degradation of lignocellulosic plant material. We bioinformatically screened for cellulosomal modules in this complex environment using a previously published ultra-deep fibre-adherent rumen metagenome. Intriguingly, a large portion of the functions of the dockerin-containing proteins were related to alternative biological processes, and not necessarily to the classic fibre degradation function. Our analysis was experimentally validated by characterizing specific interactions between selected cohesins and dockerins and revealed that cellulosome is a more generalized strategy used by diverse bacteria, some of which were not previously associated with cellulosome production. Remarkably, our results provide additional proof of similarity among rumen microbial communities worldwide. This study suggests a broader and widespread role for the cellulosomal machinery in nature.
The assembly of one of Nature's most elaborate multienzyme complexes, the cellulosome, results from the binding of enzyme-borne dockerins to reiterated cohesin domains located in a noncatalytic primary scaffoldin. Generally, dockerins present two similar cohesin-binding interfaces that support a dual binding mode. The dynamic integration of enzymes in cellulosomes, afforded by the dual binding mode, is believed to incorporate additional flexibility in highly populated multienzyme complexes. Ruminococcus flavefaciens, the primary degrader of plant structural carbohydrates in the rumen of mammals, uses a portfolio of more than 220 different dockerins to assemble the most intricate cellulosome known to date. A sequence-based analysis organized R. flavefaciens dockerins into six groups. Strikingly, a subset of R. flavefaciens cellulosomal enzymes, comprising dockerins of groups 3 and 6, were shown to be indirectly incorporated into primary scaffoldins via an adaptor scaffoldin termed ScaC. Here, we report the crystal structure of a group 3 R. flavefaciens dockerin, Doc3, in complex with ScaC cohesin. Doc3 is unusual as it presents a large cohesin-interacting surface that lacks the structural symmetry required to support a dual binding mode. In addition, dockerins of groups 3 and 6, which bind exclusively to ScaC cohesin, display a conserved mechanism of protein recognition that is similar to Doc3. Groups 3 and 6 dockerins are predominantly appended to hemicellulose-degrading enzymes. Thus, single binding mode dockerins interacting with adaptor scaffoldins exemplify an evolutionary pathway developed by R. flavefaciens to recruit hemicellulases to the sophisticated cellulosomes acting in the gastrointestinal tract of mammals.
During the course of evolution, the cellulosome, one of Nature's most intricate multi-enzyme complexes, has been continuously fine-tuned to efficiently deconstruct recalcitrant carbohydrates. To facilitate the uptake of released sugars, anaerobic bacteria use highly ordered protein-protein interactions to recruit these nanomachines to the cell surface. Dockerin modules located within a non-catalytic macromolecular scaffold, whose primary role is to assemble cellulosomal enzymatic subunits, bind cohesin modules of cell envelope proteins, thereby anchoring the cellulosome onto the bacterial cell. Here we have elucidated the unique molecular mechanisms used by anaerobic bacteria for cellulosome cellular attachment. The structure and biochemical analysis of five cohesin-dockerin complexes revealed that cell surface dockerins contain two cohesin-binding interfaces, which can present different or identical specificities. In contrast to the current static model, we propose that dockerins utilize multivalent modes of cohesin recognition to recruit cellulosomes to the cell surface, a mechanism that maximises substrate access while facilitating complex assembly.
Ruminococcus champanellensis is a keystone species in the human gut that produces an intricate cellulosome system of various architectures. A variety of cellulosomal enzymes have been identified, which exhibit a range of hydrolytic activities on lignocellulosic substrates. We describe herein a unique R. champanellensis scaffoldin, ScaK, which is expressed during growth on cellobiose and comprises a cohesin module and a family 25 glycoside hydrolase (GH25). The GH25 is non-autolytic and exhibits lysozyme-mediated lytic activity against several bacterial species. Despite the narrow acidic pH curve, the enzyme is active along a temperature range from 2 to 85 degrees C and is stable at very high temperatures for extended incubation periods. The ScaK cohesin was shown to bind selectively to the dockerin of a monovalent scaffoldin (ScaG), thus enabling formation of a cell-free cellulosome, whereby ScaG interacts with a divalent scaffodin (ScaA) that bears the enzymes either directly or through additional monovalent scaffoldins (ScaC and ScaD). The ScaK cohesin also interacts with the dockerin of a protein comprising multiple Fn3 domains that can potentially promote adhesion to carbohydrates and the bacterial cell surface. A cell-free cellulosomal GH25 lysozyme may provide a bacterial strategy to both hydrolyze lignocellulose and repel eventual food competitors and/or cheaters.
Cohesins and dockerins are complementary interacting protein modules that form stable and highly specific receptor ligand complexes. They play a crucial role in the assembly of cellulose-degrading multi-enzyme complexes called cellulosomes and have potential applicability in several technology areas, including biomass conversion processes. Here, we describe several exceptional properties of cohesin dockerin complexes, including their tenacious biochemical affinity, remarkably high mechanostability and a dual-binding mode of recognition that is contrary to the conventional lock and -key model of receptor-ligand interactions. We focus on structural aspects of the dual mode of cohesin dockerin binding, highlighting recent single-molecule analysis techniques for its explicit characterization.
Efficient breakdown of lignocellulose polymers into simple molecules is a key technological bottleneck limiting the production of plant-derived biofuels and chemicals. In nature, plant biomass degradation is achieved by the action of a wide range of microbial enzymes. In aerobic microorganisms, these enzymes are secreted as discrete elements in contrast to certain anaerobic bacteria, where they are assembled into large multienzyme complexes termed cellulosomes. These complexes allow for very efficient hydrolysis of cellulose and hemicellulose due to the spatial proximity of synergistically acting enzymes and to the limited diffusion of the enzymes and their products. Recently, designer cellulosomes have been developed to incorporate foreign enzymatic activities in cellulosomes so as to enhance lignocellulose hydrolysis further. In this study, we complemented a cellulosome active on cellulose and hemicellulose by addition of an enzyme active on lignin. To do so, we designed a dockerin-fused variant of a recently characterized laccase from the aerobic bacterium Thermobifida fusca. The resultant chimera exhibited activity levels similar to the wild-type enzyme and properly integrated into the designer cellulosome. The resulting complex yielded a twofold increase in the amount of reducing sugars released from wheat straw compared with the same system lacking the laccase. The unorthodox use of aerobic enzymes in designer cellulosome machinery effects simultaneous degradation of the three major components of the plant cell wall (cellulose, hemicellulose, and lignin), paving the way for more efficient lignocellulose conversion into soluble sugars en route to alternative fuels production.
Background: The concerted action of three complementary cellulases from Clostridium thermocellum, engineered to be stable at elevated temperatures, was examined on a cellulosic substrate and compared to that of the wild-type enzymes. Exoglucanase Cel48S and endoglucanase Cel8A, both key elements of the natural cellulosome from this bacterium, were engineered previously for increased thermostability, either by SCHEMA, a structure-guided, site-directed protein recombination method, or by consensus-guided mutagenesis combined with random mutagenesis using error-prone PCR, respectively. A thermostable beta-glucosidase BglA mutant was also selected from a library generated by error-prone PCR that will assist the two cellulases in their methodic deconstruction of crystalline cellulose. The effects of a thermostable scaffoldin versus those of a largely mesophilic scaffoldin were also examined. By improving the stability of the enzyme subunits and the structural component, we aimed to improve cellulosome-mediated deconstruction of cellulosic substrates. Results: The results demonstrate that the combination of thermostable enzymes as free enzymes and a thermostable scaffoldin was more active on the cellulosic substrate than the wild-type enzymes. Significantly, "thermostable" designer cellulosomes exhibited a 1.7-fold enhancement in cellulose degradation compared to the action of conventional designer cellulosomes that contain the respective wild-type enzymes. For designer cellulosome formats, the use of the thermostabilized scaffoldin proved critical for enhanced enzymatic performance under conditions of high temperatures. Conclusions: Simple improvement in the activity of a given enzyme does not guarantee its suitability for use in an enzyme cocktail or as a designer cellulosome component. The true merit of improvement resides in its ultimate contribution to synergistic action, which can only be determined experimentally. The relevance of the mutated thermostable enzymes employed in this study as components in multienzyme systems has thus been confirmed using designer cellulosome technology. Enzyme integration via a thermostable scaffoldin is critical to the ultimate stability of the complex at higher temperatures. Engineering of thermostable cellulases and additional lignocellulosic enzymes may prove a determinant parameter for development of state-of-the-art designer cellulosomes for their employment in the conversion of cellulosic biomass to soluble sugars.
Enzymatic breakdown of lignocellulose is a major limiting step in second generation biorefineries. Assembly of the necessary activities into designer cellulosomes increases the productivity of this step by enhancing enzyme synergy through the proximity effect. However, most cellulosomal components are obtained from mesophilic microorganisms, limiting the applications to temperatures up to 50 degrees C. We hypothesized that a scaffoldin, comprising modular components of mainly mesophilic origin, can function at higher temperatures when combined with thermophilic enzymes, and the resulting designer cellulosomes could be employed in higher temperature reactions. For this purpose, we used a tetravalent scaffoldin constituted of three cohesins of mesophilic origin as well as a cohesin and cellulose-binding module derived from the thermophilic bacterium Clostridium thermocellum. The scaffoldin was combined with four thermophilic enzymes from Geobacillus and Caldicellulosiruptor species, each fused with a dockerin whose specificity matched one of the cohesins. We initially verified that the biochemical properties and thermal stability of the resulting chimeric enzymes were not affected by the presence of the mesophilic dockerins. Then we examined the stability of the individual single-enzyme-scaffoldin complexes and the full tetravalent cellulosome showing that all complexes are stable and functional for at least 6 h at 60 degrees C. Finally, within this time frame and conditions, the full complex appeared over 50 % more efficient in the hydrolysis of corn stover compared to the free enzymes. Overall, the results support the utilization of scaffoldin components of mesophilic origin at relatively high temperatures and provide a framework for the production of designer cellulosomes suitable for high temperature biorefinery applications.
Designer cellulosomes consist of chimeric cohesin-bearing scaffoldins for the controlled incorporation of recombinant dockerin-containing enzymes. The largest designer cellulosome reported to date is a chimeric scaffoldin that contains 6 cohesins. This scaffoldin represented a technical limit of sorts, since adding another cohesin proved problematic, owing to resultant low expression levels, instability (cleavage) of the scaffoldin polypeptide, and limited numbers of available cohesin-dockerin specificities-the hallmark of designer cellulosomes. Nevertheless, increasing the number of enzymes integrated into designer cellulosomes is critical, in order to further enhance degradation of plant cell wall material. Adaptor scaffoldins comprise an intermediate type of scaffoldin that can both incorporate various enzymes and attach to an additional scaffoldin. Using this strategy, we constructed an efficient form of adaptor scaffoldin that possesses three type I cohesins for enzyme integration, a single type II dockerin for interaction with an additional scaffoldin, and a carbohydrate-binding module for targeting to the cellulosic substrate. In parallel, we designed a hexavalent scaffoldin capable of connecting to the adaptor scaffoldin by the incorporation of an appropriate type II cohesin. The resultant extended designer cellulosome comprised 8 recombinant enzymes-4 xylanases and 4 cellulases-thereby representing a potent enzymatic cocktail for solubilization of natural lignocellulosic substrates. The contribution of the adaptor scaffoldin clearly demonstrated that proximity between the two scaffoldins and their composite set of enzymes is crucial for optimized degradation. After 72 h of incubation, the performance of the extended designer cellulosome was determined to be approximately 70% compared to that of native cellulosomes.IMPORTANCE: Plant cell wall residues represent a major source of renewable biomass for the production of biofuels such as ethanol via breakdown to soluble sugars. The natural microbial degradation process, however, is inefficient for achieving cost-effective processes in the conversion of plant-derived biomass to biofuels, either from dedicated crops or human-generated cellulosic wastes. The accumulation of the latter is considered a major environmental pollutant. The development of designer cellulosome nanodevices for enhanced plant cell wall degradation thus has major impacts in the fields of environmental pollution, bioenergy production, and biotechnology in general. The findings reported in this article comprise a true breakthrough in our capacity to produce extended designer cellulosomes via synthetic biology means, thus enabling the assembly of higher-order complexes that can supersede the number of enzymes included in a single multienzyme complex.
Background: Expansins are relatively small proteins that lack enzymatic activity and are found in plants and microorganisms. The function of these proteins is to disrupt the plant cell walls by interfering with the non-covalent interchain bonding of the polysaccharides. Expansins were found to be important for plant growth, but they are also expressed by various bacteria known to have interactions with plants. Clostridium clariflavum is a plant cell wall-degrading bacterium with a highly elaborate cellulosomal system. Among its numerous dockerin-containing genes, two expansin-like proteins, Clocl_1862 and Clocl_1298 (termed herein CclEXL1 and CclEXL2) were identified, and CclEXL1 was found to be expressed as part of the cellulosome system. This is the first time that an expansin-like protein is identified in a cellulosome complex, which implicates its possible role in biomass deconstruction. Results: In the present article, we analyzed the functionality of CclEXL1. Its dockerin was characterized and shown to bind selectively to type-I cohesins of C. clariflavum, with preferential binding to the cohesin of ScaG, and additionally to a type-I cohesin of C. cellulolyticum. We demonstrated experimentally that the expansin-like protein binds preferentially to microcrystalline cellulose, but it also binds to acid-swollen cellulose, xylan, and wheat straw. CclEXL1 exhibited a pronounced loosening effect on filter paper, which resulted in substantial decrease in tensile stress. The C. clariflavum expansin-like protein thus enhances significantly enzymatic hydrolysis of cellulose, both by C. clariflavum cellulosomes and two major cellulosomal cellulases from this bacterium: GH48 (exoglucanase) and GH9 (endoglucanase). Finally, we demonstrated CclEXL1-mediated enhancement of microcrystalline cellulose degradation by different cellulosome fractions and the two enzymes. Conclusions: The results of this study confirm that the C. clariflavum expansin-like protein is part of the elaborate cellulosome system of this bacterium with capabilities of cellulose creeping. The data suggest that pretreatment of cellulosic materials with CclEXL1 can bring about substantial improvement of hydrolysis by cellulases.
Ruminococcus champanellensis is considered a keystone species in the human gut that degrades microcrystalline cellulose efficiently and contains the genetic elements necessary for cellulosome production. The basic elements of its cellulosome architecture, mainly cohesin and dockerin modules from scaffoldins and enzyme-borne dockerins, have been characterized recently. In this study, we cloned, expressed and characterized all of the glycoside hydrolases that contain a dockerin module. Among the 25 enzymes, 10 cellulases, 4 xylanases, 3 mannanases, 2 xyloglucanases, 2 arabinofuranosidases, 2 arabinanases and one -glucanase were assessed for their comparative enzymatic activity on their respective substrates. The dockerin specificities of the enzymes were examined by ELISA, and 80 positives out of 525 possible interactions were detected. Our analysis reveals a fine-tuned system for cohesin-dockerin specificity and the importance of diversity among the cohesin-dockerin sequences. Our results imply that cohesin-dockerin pairs are not necessarily assembled at random among the same specificity types, as generally believed for other cellulosome-producing bacteria, but reveal a more organized cellulosome architecture. Moreover, our results highlight the importance of the cellulosome paradigm for cellulose and hemicellulose degradation by R.champanellensis in the human gut.
The Gram-positive, anaerobic, cellulolytic, thermophile Clostridium (Ruminiclostridium) thermocellum secretes a multi-enzyme system called the cellulosome to solubilize plant cell wall polysaccharides. During the saccharolytic process, the enzymatic composition of the cellulosome is modulated according to the type of polysaccharide(s) present in the environment. C. thermocellum has a set of eight alternative RNA polymerase sigma (s) factors that are activated in response to extracellular polysaccharides and share sequence similarity to the Bacillus subtilis sigma(I) factor. The aim of the present work was to demonstrate whether individual C. thermocellum sI-like factors regulate specific cellulosomal genes, focusing on C. thermocellum sigma(I6) and sigma(I3) factors. To search for putative sigma(I6)-and sigma(I3)-dependent promoters, bioinformatic analysis of the upstream regions of the cellulosomal genes was performed. Because of the limited genetic tools available for C. thermocellum, the functionality of the predicted sigma(I6)-and sigma(I3)-dependent promoters was studied in B. subtilis as a heterologous host. This system enabled observation of the activation of 10 predicted sigma(I6)-dependent promoters associated with the C. thermocellum genes: sigI6 (itself, Clo1313_2778), xyn11B (Clo1313_0522), xyn10D (Clo1313_0177), xyn10Z (Clo1313_2635), xyn10Y (Clo1313_1305), cel9V (Clo1313_0349), cseP (Clo1313_2188), sigI1 (Clo1313_2174), cipA (Clo1313_0627), and rsgI5 (Clo1313_0985). Additionally, we observed the activation of 4 predicted sigma(I3)-dependent promoters associated with the C. thermocellum genes: sigI3 (itself, Clo1313_1911), pl11 (Clo1313_1983), ce12 (Clo1313_0693) and cipA. Our results suggest possible regulons of sigma(I6) and sigma(I3) in C. thermocellum, as well as the sigma(I6) and sigma(I3) promoter consensus sequences. The proposed -35 and -10 promoter consensus elements of sigma(I6) are CNNAAA and CGAA, respectively. Additionally, a less conserved CGA sequence next to the C in the -35 element and a highly conserved AT sequence three bases downstream of the -10 element were also identified as important nucleotides for promoter recognition. Regarding σI3, the proposed -35 and -10 promoter consensus elements are CCCYYAAA and CGWA, respectively. The present study provides new clues for understanding these recently discovered alternative σI factors.
A cellulolytic fiber-degrading bacterium, Ruminococcus champanellensis, was isolated from human faecal samples, and its genome was recently sequenced. Bioinformatic analysis of the R.champanellensis genome revealed numerous cohesin and dockerin modules, the basic elements of the cellulosome, and manual sequencing of partially sequenced genomic segments revealed two large tandem scaffoldin-coding genes that form part of a gene cluster. Representative R.champanellensis dockerins were tested against putative cohesins, and the results revealed three different cohesin-dockerin binding profiles which implied two major types of cellulosome architectures: (i) an intricate cell-bound system and (ii) a simplistic cell-free system composed of a single cohesin-containing scaffoldin. The cell-bound system can adopt various enzymatic architectures, ranging from a single enzyme to a large enzymatic complex comprising up to 11 enzymes. The variety of cellulosomal components together with adaptor proteins may infer a very tight regulation of its components. The cellulosome system of the human gut bacterium R.champanellensis closely resembles that of the bovine rumen bacterium Ruminococcus flavefaciens. The two species contain orthologous gene clusters comprising fundamental components of cellulosome architecture. Since R.champanellensis is the only human colonic bacterium known to degrade crystalline cellulose, it may thus represent a keystone species in the human gut.
Non-cellulosomal processive endoglucanase 9I (Cel9I) from Clostridium thermocellum is a modular protein, consisting of a family-9 glycoside hydrolase (GH9) catalytic module and two family-3 carbohydrate-binding modules (CBM3c and CBM3b), separated by linker regions. GH9 does not show cellulase activity when expressed without CBM3c and CBM3b and the presence of the CBM3c was previously shown to be essential for endoglucanase activity. Physical reassociation of independently expressed GH9 and CBM3c modules (containing linker sequences) restored 60-70% of the intact Cel9I endocellulase activity. However, the mechanism responsible for recovery of activity remained unclear. In this work we independently expressed recombinant GH9 and CBM3c with and without their interconnecting linker in Escherichia coli. We crystallized and determined the molecular structure of the GH9/linker-CBM3c heterodimer at a resolution of 1.68 angstrom to understand the functional and structural importance of the mutual spatial orientation of the modules and the role of the interconnecting linker during their re-association. Enzyme activity assays and isothermal titration calorimetry were performed to study and compare the effect of the linker on the re-association. The results indicated that reassembly of the modules could also occur without the linker, albeit with only very low recovery of endoglucanase activity. We propose that the linker regions in the GH9/CBM3c endoglucanases are important for spatial organization and fixation of the modules into functional enzymes.
Ruminococcus bromii is a dominant member of the human gut microbiota that plays a key role in releasing energy from dietary starches that escape digestion by host enzymes via its exceptional activity against particulate "resistant" starches. Genomic analysis of R. bromii shows that it is highly specialized, with 15 of its 21 glycoside hydrolases belonging to one family (GH13). We found that amylase activity in R. bromii is expressed constitutively, with the activity seen during growth with fructose as an energy source being similar to that seen with starch as an energy source. Six GH13 amylases that carry signal peptides were detected by proteomic analysis in R. bromii cultures. Four of these enzymes are among 26 R. bromii proteins predicted to carry dockerin modules, with one, Amy4, also carrying a cohesin module. Since cohesin-dockerin interactions are known to mediate the formation of protein complexes in cellulolytic ruminococci, the binding interactions of four cohesins and 11 dockerins from R. bromii were investigated after overexpressing them as recombinant fusion proteins. Dockerins possessed by the enzymes Amy4 and Amy9 are predicted to bind a cohesin present in protein scaffoldin 2 (Sca2), which resembles the Sea E cell wall-anchoring protein of a cellulolytic relative, R. flavefaciens. Further complexes are predicted between the dockerin-carrying amylases Amy4, Amy9, Amyl 0, and AmyI2 and two other cohesin-carrying proteins, while Amy4 has the ability to autoaggregate, as its dockerin can recognize its own cohesin. This organization of starch-degrading enzymes is unprecedented and provides the first example of cohesin-dockerin interactions being involved in an amylolytic system, which we refer to as an "amylosome." IMPORTANCE Fermentation of dietary nondigestible carbohydrates by the human colonic microbiota supplies much of the energy that supports microbial growth in the intestine. This activity has important consequences for health via modulation of microbiota composition and the physiological and nutritional effects of microbial metabolites, including the supply of energy to the host from short-chain fatty acids. Recent evidence indicates that certain human colonic bacteria play keystone roles in degrading nondigestible substrates, with the dominant but little-studied species Ruminococcus bromii displaying an exceptional ability to degrade dietary resistant starches (i.e., dietary starches that escape digestion by host enzymes in the upper gastrointestinal tract because of protection provided by other polymers, particle structure, retrogradation, or chemical cross-linking). In this report, we reveal the unique organization of the amylolytic enzyme system of R. bromii that involves cohesin-dockerin interactions between component proteins. While dockerins and cohesins are fundamental to the organization of cellulosomal enzyme systems of cellulolytic ruminococci, their contribution to organization of amylases has not previously been recognized and may help to explain the starch-degrading abilities of R. bromii.
Dimeric avidins are a newly discovered subgroup of the avidin family that bind biotin with high affinity. Their dimeric configuration is a quaternary substructure of the classical tetrameric avidins which lacks the requirement of the critical Trp that defines the tetramer and dictates the tenacious interaction with biotin. Hoefavidin, derived from the bacterium Hoeflea phototrophica DFL-43(T), is the third characterized member of the dimeric avidin subfamily. Like the other members of this group, hoefavidin is a thermostable protein that contains a disulfide bridge between Cys57 and Cys88, thereby connecting and stabilizing the L3,4 and L5,6 loops. This represents a distinctive characteristic of dimeric avidins that compensates for the lack of Trp and enables their dimeric configuration. The X-ray structure of the intact hoefavidin revealed unique crystal packing generated by an octameric cylindrical structure wherein the C-termini segments of each monomer is introduced into the entrance of the biotin-binding site of an adjacent non-canonical monomer. This anomaly in the protein structure served as a lead toward the design of specific binding peptides. We screened for specific hoefavidin binding peptides derived from the C-terminal region and two peptides were obtained that bind a truncated form of hoefavidin (lacking the last 10 amino acids) with dissociation constants of 10(-5) M. The crystal structure of short hoefavidin complexed with a C-terminal derived peptide revealed the mode of binding. These peptides may form the basis of novel and reversible binders for dimeric avidins.
Background: Cellulosomal cohesin-dockerin types are reversed in Bacteroides cellulosolvens. Results: Combined crystallographic and computational approaches of a lone cohesin yielded a structural model of the cohesin-dockerin complex that was verified experimentally. Conclusion: The dockerin dual-binding mode is not exclusive to enzyme integration into cellulosomes; it also characterizes cell-surface attachment. Significance: This combined approach provides a platform for generating testable hypotheses of the high affinity cohesin-dockerin interaction. Cohesin-dockerin interactions orchestrate the assembly of one of nature's most elaborate multienzyme complexes, the cellulosome. Cellulosomes are produced exclusively by anaerobic microbes and mediate highly efficient hydrolysis of plant structural polysaccharides, such as cellulose and hemicellulose. In the canonical model of cellulosome assembly, type I dockerin modules of the enzymes bind to reiterated type I cohesin modules of a primary scaffoldin. Each type I dockerin contains two highly conserved cohesin-binding sites, which confer quaternary flexibility to the multienzyme complex. The scaffoldin also bears a type II dockerin that anchors the entire complex to the cell surface by binding type II cohesins of anchoring scaffoldins. In Bacteroides cellulosolvens, however, the organization of the cohesin-dockerin types is reversed, whereby type II cohesin-dockerin pairs integrate the enzymes into the primary scaffoldin, and type I modules mediate cellulosome attachment to an anchoring scaffoldin. Here, we report the crystal structure of a type I cohesin from B. cellulosolvens anchoring scaffoldin ScaB to 1.84-angstrom resolution. The structure resembles other type I cohesins, and the putative dockerin-binding site, centered at -strands 3, 5, and 6, is likely to be conserved in other B. cellulosolvens type I cohesins. Combined computational modeling, mutagenesis, and affinity-based binding studies revealed similar hydrogen-bonding networks between putative Ser/Asp recognition residues in the dockerin at positions 11/12 and 45/46, suggesting that a dual-binding mode is not exclusive to the integration of enzymes into primary cellulosomes but can also characterize polycellulosome assembly and cell-surface attachment. This general approach may provide valuable structural information of the cohesin-dockerin interface, in lieu of a definitive crystal structure.
The cellulolytic bacterium Ruminococcus flavefaciens of the herbivore rumen produces an elaborate cellulosome system, anchored to the bacterial cell wall via the covalently bound scaffoldin ScaE. Dockerin-bearing scaffoldins also bind to an autonomous cohesin of unknown function, called cohesin G (CohG). Here, we demonstrate that CohG binds to the scaffoldin-borne dockerin in opposite orientation on a distinct site, relative to that of ScaE. Based on these structural data, we propose that the complexed dockerin is still available to bind ScaE on the cell surface. CohG may thus serve as a molecular shuttle for delivery of scaffoldins to the bacterial cell surface.
Clostridium clariflavum is an anaerobic, cellulosome-forming thermophile, containing in its genome genes for a large number of cellulosomal enzyme and a complex scaffoldin system. Previously, we described the major cohesin-dockerin interactions of the cellulosome components, and on this basis a model of diverse cellulosome assemblies was derived. In this work, we cultivated C. clariflavum on cellobiose-, microcrystalline cellulose-, and switchgrass-containing media and isolated cell-free cellulosome complexes from each culture. Gel filtration separation of the cellulosome samples revealed two major fractions, which were analyzed by label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) in order to identify the key players of the cellulosome assemblies therein. From the 13 scaffoldins present in the C. clariflavum genome, 11 were identified, and a variety of enzymes from different glycoside hydrolase and carbohydrate esterase families were identified, including the glycoside hydrolase families GH48, GH9, GH5, GH30, GH11, and GH10. The expression level of the cellulosomal proteins varied as a function of the carbon source used for cultivation of the bacterium. In addition, the catalytic activity of each cellulosome was examined on different cellulosic substrates, xylan and switchgrass. The cellulosome isolated from the microcrystalline cellulose-containing medium was the most active of all the cellulosomes that were tested. The results suggest that the expression of the cellulosome proteins is regulated by the type of substrate in the growth medium. Moreover, both cell-free and cell-bound cellulosome complexes were produced which together may degrade the substrate in a synergistic manner. These observations are compatible with our previously published model of cellulosome assemblies in this bacterium. IMPORTANCE Because the reservoir of unsustainable fossil fuels, such as coal, petroleum, and natural gas, is overutilized and continues to contribute to environmental pollution and CO2 emission, the need for appropriate alternative energy sources becomes more crucial. Bioethanol produced from dedicated crops and cellulosic waste can provide a partial answer, yet a cost-effective production method must be developed. The cellulosome system of the anaerobic thermophile C. clariflavum comprises a large number of cellulolytic and hemicellulolytic enzymes, which self-assemble in a number of different cellulosome architectures for enhanced cellulosic biomass degradation. Identification of the major cellulosomal components expressed during growth of the bacterium and their influence on its catalytic capabilities provide insight into the performance of the remarkable cellulosome of this intriguing bacterium. The findings, together with the thermophilic characteristics of the proteins, render C. clariflavum of great interest for future use in industrial cellulose conversion processes.
Protein-protein interactions play a pivotal role in the assembly of the cellulosome, one of nature's most intricate nanomachines dedicated to the depolymerization of complex carbohydrates. The integration of cellulosomal components usually occurs through the binding of type I dockerin modules located at the C terminus of the enzymes to cohesin modules located in the primary scaffoldin subunit. Cellulosomes are typically recruited to the cell surface via type II cohesin-dockerin interactions established between primary and cell-surface anchoring scaffoldin subunits. In contrast with type II interactions, type I dockerins usually display a dual binding mode that may allow increased conformational flexibility during cellulosome assembly. Acetivibrio cellulolyticus produces a highly complex cellulosome comprising an unusual adaptor scaffoldin, ScaB, which mediates the interaction between the primary scaffoldin, ScaA, through type II cohesin-dockerin interactions and the anchoring scaffoldin, ScaC, via type I cohesin-dockerin interactions. Here, we report the crystal structure of the type I ScaB dockerin in complex with a type I ScaC cohesin in two distinct orientations. The data show that the ScaB dockerin displays structural symmetry, reflected by the presence of two essentially identical binding surfaces. The complex interface is more extensive than those observed in other type I complexes, which results in an ultra-high affinity interaction (K-a similar to 10(12) M). A subset of ScaB dockerin residues was also identified as modulating the specificity of type I cohesin-dockerin interactions in A. cellulolyticus. This report reveals that recruitment of cellulosomes onto the cell surface may involve dockerins presenting a dual binding mode to incorporate additional flexibility into the quaternary structure of highly populated multienzyme complexes.
Degradation of cellulose is of major interest in the quest for alternative sources of renewable energy, for its positive effects on environment and ecology, and for use in advanced biotechnological applications. Due to its microcrystalline organization, celluose is extremely difficult to degrade, although numerous microbes have evolved that produce the appropriate enzymes. The most efficient known natural cellulolytic system is produced by anaerobic bacteria, such as C. thermocellum, that possess a multi-enzymatic complex termed the cellulosome. Our laboratory has devised and developed the designer cellulosome concept, which consists of chimaeric scaffoldins for controlled incorporation of recombinant polysaccharide-degrading enzymes. Recently, we reported the creation of a combinatorial library of four cellulosomal modules comprising a basic chimaeric scaffoldin, i.e., a CBM and 3 divergent cohesin modules. Here, we employed selected members of this library to determine whether the position of defined cellulolytic enzymes is important for optimized degradation of a microcrystalline cellulosic substrate. For this purpose, 10 chimaeric scaffoldins were used for incorporation of three recombinant Thermobifida fusca enzymes: the processive endoglucanase Cel9A, endoglucanase Cel5A and exoglucanase Cel48A. In addition, we examined whether the characteristic properties of the T. fusca enzymes as designer cellulosome components are unique to this bacterium by replacing them with parallel enzymes from Clostridium thermocellum. The results support the contention that for a given set of cellulosomal enzymes, their relative position within a scaffoldin can be critical for optimal degradation of microcrystaline cellulosic substrates.
Interactions between cohesin and dockerin modules play a crucial role in the assembly of multienzyme cellulosome complexes. Although intraspecies cohesin and dockerin modules bind in general with high affinity but indiscriminately, cross-species binding is rare. Here, we combined ELISA-based experiments with Rosetta-based computational design to evaluate the contribution of distinct residues at the Clostridium thermocellum cohesin-dockerin interface to binding affinity, specificity, and promiscuity. We found that single mutations can show distinct and significant effects on binding affinity and specificity. In particular, mutations at cohesin position Asn(37) show dramatic variability in their effect on dockerin binding affinity and specificity: the N37A mutant binds promiscuously both to cognate (C. thermocellum) as well as to non-cognate Clostridium cellulolyticum dockerin. N37L in turn switches binding specificity: compared with the wild-type C. thermocellum cohesin, this mutant shows significantly increased preference for C. cellulolyticum dockerin combined with strongly reduced binding to its cognate C. thermocellum dockerin. The observation that a single mutation can overcome the naturally observed specificity barrier provides insights into the evolutionary dynamics of this system that allows rapid modulation of binding specificity within a high affinity background.
Cellulosomes are large multicomponent cellulose-degrading assemblies found on the surfaces of cellulolytic microorganisms. Often containing hundreds of components, the self-assembly of cellulosomes is mediated by the ultra-high-affinity cohesin-dockerin interaction, which allows them to adopt the complex architectures necessary for degrading recalcitrant cellulose. Better understanding of how the cellulosome assembles and functions and what kinds of structures it adopts will further effort to develop industrial applications of cellulosome components, including their use in bioenergy production. Ruminococcus flavefaciens is a well-studied anaerobic cellulolytic bacteria found in the intestinal tracts of ruminants and other herbivores. Key to cellulosomal self-assembly in this bacterium is the dockerin ScaADoc, found on the non-catalytic structural subunit scaffoldin ScaA, which is responsible for assembling arrays of cellulose-degrading enzymes. This work expands on previous efforts by conducting a series of binding studies on ScaADoc constructs that contain mutations in their cohesin recognition interface, in order to identify which residues play important roles in binding. Molecular dynamics simulations were employed to gain insight into the structural basis for our findings. A specific residue pair in the first helix of ScaADoc, as well as a glutamate near the C-terminus, was identified to be essential for cohesin binding. By advancing our understanding of the cohesin binding of ScaADoc, this study serves as a foundation for future work to more fully understand the structural basis of cellulosome assembly in R. flavefaciens.
Ruminococcus albus, a cellulolytic bacterium, is a critical member of the rumen community. Ruminococcus albus lacks a classical cellulosome complex, but it possesses a unique family 37 carbohydrate-binding module (CBM37), which is integrated into a variety of carbohydrate-active enzymes. We developed a potential molecular tool for functional phylotyping of the R. albus population in the rumen, based on a variable region in the cel48A gene. cel48A encodes a single copy of the CBM37-associated family 48 glycoside hydrolase in all known strains of this bacterium. A segment of the cel48A gene was amplified from rumen metagenomic samples of four bovines, and its abundance and diversity were evaluated. Analysis of the obtained sequences revealed the co-existence of multiple functional phylotypes of cel48A in all four animals. These included sequences identical or similar to those of R. albus isolates (reference strains), as well as several novel sequences. The dominant cel48A type varied among animals. This method can be used for detection of intraspecific diversity of R. albus in metagenomic samples. Together with scaC, a previously reported gene marker for R. flavefaciens, we present a set of two species-specific markers for phylotyping of Ruminococci in the herbivore rumen.
Challenging environments have guided nature in the development of ultrastable protein complexes. Specialized bacteria produce discrete multi-component protein networks called cellulosomes to effectively digest lignocellulosic biomass. While network assembly is enabled by protein interactions with commonplace affinities, we show that certain cellulosomal ligand-receptor interactions exhibit extreme resistance to applied force. Here, we characterize the ligand-receptor complex responsible for substrate anchoring in the Ruminococcus flavefaciens cellulosome using single-molecule force spectroscopy and steered molecular dynamics simulations. The complex withstands forces of 600-750 pN, making it one of the strongest bimolecular interactions reported, equivalent to half the mechanical strength of a covalent bond. Our findings demonstrate force activation and inter-domain stabilization of the complex, and suggest that certain network components serve as mechanical effectors for maintaining network integrity. This detailed understanding of cellulosomal network components may help in the development of biocatalysts for production of fuels and chemicals from renewable plant-derived biomass.
Lactic acid bacteria (LAB) have long been used in industrial applications mainly as starters for food fermentation or as biocontrol agents or as probiotics. However, LAB possess several characteristics that render them among the most promising candidates for use in future biorefineries in converting plant-derived biomass either from dedicated crops or from municipal/industrial solid wastes into biofuels and high value-added products. Lactic acid, their main fermentation product, is an attractive building block extensively used by the chemical industry, owing to the potential for production of polylactides as biodegradable and biocompatible plastic alternative to polymers derived from petrochemicals. LA is but one of many high-value compounds which can be produced by LAB fermentation, which also include biofuels such as ethanol and butanol, biodegradable plastic polymers, exopolysaccharides, antimicrobial agents, health-promoting substances and nutraceuticals. Furthermore, several LAB strains have ascertained probiotic properties, and their biomass can be considered a high-value product. The present contribution aims to provide an extensive overview of the main industrial applications of LAB and future perspectives concerning their utilization in biorefineries. Strategies will be described in detail for developing LAB strains with broader substrate metabolic capacity for fermentation of cheaper biomass. (C) 2014 Elsevier Inc. All rights reserved.
Dockerin modules of the cellulosomal enzyme subunits play an important role in the assembly of the cellulosome by binding tenaciously to cohesin modules of the scaffoldin subunit. A previously reported NMR-derived solution structure of the type-I dockerin module from Ce148S of Clostridium thermocellum, which utilized two-dimensional homonuclear H-1-H-1 NOESY and three-dimensional N-15-edited NOESY distance restraints, displayed substantial conformational differences from subsequent structures of dockerin modules in complex with their cognate cohesin modules, raising the question whether the source of the observed differences resulted from cohesin-induced structural rearrangements. Here, we determined the solution structure of the Ce148S type-I dockerin based on N-15- and C-13-edited NOESY-derived distance restraints. The structure adopted a fold similar to X-ray crystal structures of dockerin modules in complex with their cohesin partners. A unique cis-peptide bond between Leu-65 and Pro-66 in the Ce148S type-I dockerin module was also identified in the present structure. Our structural analysis of the Ce148S type-I dockerin module indicates that it does not undergo appreciable cohesin-induced structural alterations but rather assumes an inherent calcium-dependent cohesin-primed conformation. (C) 2014 Elsevier Inc. All rights reserved.
Cellulosic waste represents a significant and underutilized carbon source for the biofuel industry. Owing to the recalcitrance of crystalline cellulose to enzymatic degradation, it is necessary to design economical methods of liberating the fermentable sugars required for bioethanol production. One route towards unlocking the potential of cellulosic waste lies in a highly complex class of molecular machines, the cellulosomes. Secreted mainly by anaerobic bacteria, cellulosomes are structurally diverse, cell surface-bound protein assemblies that can contain dozens of catalytic components. The key feature of the cellulosome is its modularity, facilitated by the ultra-high affinity cohesin-dockerin interaction. Due to the enormous number of cohesin and dockerin modules found in a typical cellulolytic organism, a major bottleneck in understanding the biology of cellulosomics is the purification of each cohesin-and dockerin-containing component, prior to analyses of their interaction. As opposed to previous approaches, the present study utilized proteins contained in unpurified whole-cell extracts. This strategy was made possible due to an experimental design that allowed for the relevant proteins to be "purified" via targeted affinity interactions as a function of the binding assay. The approach thus represents a new strategy, appropriate for future medium-to high-throughput screening of whole genomes, to determine the interactions between cohesins and dockerins. We have selected the cellulosome of Acetivibrio cellulolyticus for this work due to its exceptionally complex cellulosome systems and intriguing diversity of its cellulosomal modular components. Containing 41 cohesins and 143 dockerins, A. cellulolyticus has one of the largest number of potential cohesin-dockerin interactions of any organism, and contains unusual and novel cellulosomal features. We have surveyed a representative library of cohesin and dockerin modules spanning the cellulosome's total cohesin
Cellulosomes are massive cell-bound multienzyme complexes tethered by macromolecular scaffolds that coordinate the efforts of many anaerobic bacteria to hydrolyze plant cell-wall polysaccharides, which are a major untapped source of carbon and energy. Integration of cellulosomal components occurs via highly ordered protein protein interactions between cohesin modules, located in the scaffold, and dockerin modules, found in the enzymes and other cellulosomal proteins. The proposed cellulosomal architecture for Ruminococcus flavefaciens strain FD-1 consists of a major scaffoldin (ScaB) that acts as the backbone to which other components attach. It has nine cohesins and a dockerin with a fused X-module that binds to the cohesin on ScaE, which in turn is covalently attached to the cell wall. The ScaA dockerin binds to ScaB cohesins allowing more carbohydrate-active modules to be assembled. ScaC acts as an adaptor that binds to both ScaA and selected ScaB cohesins, thereby increasing the repertoire of dockerin-bearing proteins that integrate into the complex. In previous studies, a screen for novel cohesin-dockerin complexes was performed which led to the identification of a total of 58 probable cohesin-dockerin pairs. Four were selected for subsequent structural and biochemical characterization based on the quality of their expression and the diversity in their specificities. One of these is C12D22, which comprises the cohesin from the adaptor ScaC protein bound to the dockerin of a CBM-containing protein. This complex has been purified and crystallized, and data were collected to resolutions of 2.5 angstrom (hexagonal, P6(5)), 2.16 angstrom (orthorhombic, P2(1)2(1)2(1)) and 2.4 angstrom (orthorhombic, P2(1)2(1)2) from three different crystalline forms.
Background: Lactobacillus plantarum is an attractive candidate for metabolic engineering towards bioprocessing of lignocellulosic biomass to ethanol or polylactic acid, as its natural characteristics include high ethanol and acid tolerance and the ability to metabolize the two major polysaccharide constituents of lignocellulolytic biomass (pentoses and hexoses). We recently engineered L. plantarum via separate introduction of a potent cellulase and xylanase, thereby creating two different L. plantarum strains. We used these strains as a combined cell-consortium for synergistic degradation of cellulosic biomass. Results: To optimize enzymatic degradation, we applied the cell-consortium approach to assess the significance of enzyme localization by comparing three enzymatic paradigms prevalent in nature: (i) a secreted enzymes system, (ii) enzymes anchored to the bacterial cell surface and (iii) enzymes integrated into cellulosome complexes. The construction of the three paradigmatic systems involved the division of the production and organization of the enzymes and scaffold proteins into different strains of L. plantarum. The spatial differentiation of the components of the enzymatic systems alleviated the load on the cell machinery of the different bacterial strains. Active designer cellulosomes containing a xylanase and a cellulase were thus assembled on L. plantarum cells by co-culturing three distinct engineered strains of the bacterium: two helper strains for enzyme secretion and one producing only the anchored scaffoldin. Alternatively, the two enzymes were anchored separately to the cell wall. The secreted enzyme consortium appeared to have a slight advantage over the designer cellulosome system in degrading the hypochlorite pretreated wheat straw substrate, and both exhibited significantly higher levels of activity compared to the anchored enzyme consortium. However, the secreted enzymes appeared to be less stable than the enzymes integrated into designer c
Background: A complex community of microorganisms is responsible for efficient plant cell wall digestion by many herbivores, notably the ruminants. Understanding the different fibrolytic mechanisms utilized by these bacteria has been of great interest in agricultural and technological fields, reinforced more recently by current efforts to convert cellulosic biomass to biofuels. Methodology/Principal Findings: Here, we have used a bioinformatics-based approach to explore the cellulosome-related components of six genomes from two of the primary fiber-degrading bacteria in the rumen: Ruminococcus flavefaciens (strains FD-1, 007c and 17) and Ruminococcus albus (strains 7, 8 and SY3). The genomes of two of these strains are reported for the first time herein. The data reveal that the three R. flavefaciens strains encode for an elaborate reservoir of cohesin-and dockerin-containing proteins, whereas the three R. albus strains are cohesin-deficient and encode mainly dockerins and a unique family of cell-anchoring carbohydrate-binding modules (family 37). Conclusions/Significance: Our comparative genome-wide analysis pinpoints rare and novel strain-specific protein architectures and provides an exhaustive profile of their numerous lignocellulose-degrading enzymes. This work provides blueprints of the divergent cellulolytic systems in these two prominent fibrolytic rumen bacterial species, each of which reflects a distinct mechanistic model for efficient degradation of cellulosic biomass.
Efficient conversion of cellulose into soluble sugars is a key technological bottleneck limiting efficient production of plant-derived biofuels and chemicals. In nature, the process is achieved by the action of a wide range of cellulases and associated enzymes. In aerobic microrganisms, cellulases are secreted as free enzymes. Alternatively, in certain anaerobic microbes, cellulases are assembled into large multienzymes complexes, termed "cellulosomes," which allow for efficient hydrolysis of cellulose. Recently, it has been shown that enzymes classified as lytic polysaccharide monooxygenases (LPMOs) were able to strongly enhance the activity of cellulases. However, LPMOs are exclusively found in aerobic organisms and, thus, cannot benefit from the advantages offered by the cellulosomal system. In this study, we designed several dockerin-fused LPMOs based on enzymes from the bacterium Thermobifida fusca. The resulting chimeras exhibited activity levels on microcrystalline cellulose similar to that of the wild-type enzymes. The dockerin moieties of the chimeras were demonstrated to be functional and to specifically bind to their corresponding cohesin partner. The chimeric LPMOs were able to self-assemble in designer cellulosomes alongside an endo-and an exo-cellulase also converted to the cellulosomal mode. The resulting complexes showed a 1.7-fold increase in the release of soluble sugars from cellulose, compared with the free enzymes, and a 2.6-fold enhancement compared with free cellulases without LPMO enhancement. These results highlight the feasibility of the conversion of LPMOs to the cellulosomal mode, and that these enzymes can benefit from the proximity effects generated by the cellulosome architecture.
Thermophilic microorganisms are attractive candidates for conversion of lignocellulose to biofuels because they produce robust, effective, carbohydrate-degrading enzymes and survive under harsh bioprocessing conditions that reflect their natural biotopes. However, no naturally occurring thermophile is known that can convert plant biomass into a liquid biofuel at rates, yields and titers that meet current bioprocessing and economic targets. Meeting those targets requires either metabolically engineering solventogenic thermophiles with additional biomass-deconstruction enzymes or engineering plant biomass degraders to produce a liquid biofuel. Thermostable enzymes from microorganisms isolated from diverse environments can serve as genetic reservoirs for both efforts. Because of the sheer number of enzymes that are required to hydrolyze plant biomass to fermentable oligosaccharides, the latter strategy appears to be the preferred route and thus has received the most attention to date. Thermophilic plant biomass degraders fall into one of two categories: cellulosomal (i.e. multienzyme complexes) and noncellulosomal (i.e. free' enzyme systems). Plant-biomass-deconstructing thermophilic bacteria from the genera Clostridium (cellulosomal) and Caldicellulosiruptor (noncellulosomal), which have potential as metabolic engineering platforms for producing biofuels, are compared and contrasted from a systems biology perspective.
Improved stability of cellulosomal enzymes is of great significance in order to provide efficient degradation of cellulosic derivatives for production of biofuels. In previous reports, we created a quadruple mutant of the endoglucanase Cel8A from Clostridium thermocellum resulting from a combination of both random error-prone PCR and a bioinformatics-based consensus mutagenesis approach. The quadruple mutant exhibited an increased half-life of activity by 14-fold at 85 degrees C with no apparent loss of catalytic activity compared to the wild-type form. Connection of the wild-type enzyme to its respective cohesin partner conferred increased thermostability, but no increase was observed for the cohesin-complexed mutant enzyme. The mutant and the wild-type enzymes were integrated into divalent chimeric scaffoldins with a family 48 exoglucanase partner, and the cellulose-degradation activities of resultant designer cellulosomes were examined. Despite the heightened thermostability of the mutant as a free enzyme, its substitution for the wild-type endoglucanase within the cellulosome context failed to exhibit an improvement in overall degradation of cellulose. (C) 2014 Elsevier Ltd. All rights reserved.
Ruminococcus flavefaciens is a cellulolytic bacterium found in the rumen of herbivores and produces one of the most elaborate and variable cellulosome systems. The structure of an R. flavefaciens protein (RfCohG, ZP_06142108), representing a freestanding (non-cellulosomal) type III cohesin module, has been determined. A selenomethionine derivative with a C-terminal histidine tag was crystallized and diffraction data were measured to 2.44 angstrom resolution. Its structure was determined by single-wavelength anomalous dispersion, revealing eight molecules in the asymmetric unit. RfCohG exhibits the most complex among all known cohesin structures, possessing four alpha-helical elements and a topographical protuberance on the putative dockerin-binding surface.
The anaerobic, thermophilic, cellulosome-producing bacterium Clostridium thermocellum relies on a variety of carbohydrate-active enzymes in order to efficiently break down complex carbohydrates into utilizable simple sugars. The regulation mechanism of the cellulosomal genes was unknown until recently, when genomic analysis revealed a set of putative operons in C. thermocellum that encode sigma(I) factors (i.e. alternative sigma factors that control specialized regulon activation) and their cognate anti-sigma(I) factor (RsgI). These putative anti-sigma(I)-factor proteins have modules that are believed to be carbohydrate sensors. Three of these modules were crystallized and their three-dimensional structures were solved. The structures show a high overall degree of sequence and structural similarity to the cellulosomal family 3 carbohydrate-binding modules (CBM3s). The structures of the three carbohydrate sensors (RsgI-CBM3s) and a reference CBM3 are compared in the context of the structural determinants for the specificity of cellulose and complex carbohydrate binding. Fine structural variations among the RsgI-CBM3s appear to result in alternative substrate preferences for each of the sensors.
Mammals rely entirely on symbiotic microorganisms within their digestive tract to gain energy from plant biomass that is resistant to mammalian digestive enzymes. Especially in herbivorous animals, specialized organs (the rumen, cecum, and colon) have evolved that allow highly efficient fermentation of ingested plant biomass by complex anaerobic microbial communities. We consider here the two most intensively studied, representative gut microbial communities involved in degradation of plant fiber: those of the rumen and the human large intestine. These communities are dominated by bacteria belonging to the Firmicutes and Bacteroidetes phyla. In Firmicutes, degradative capacity is largely restricted to the cell surface and involves elaborate cellulosome complexes in specialized cellulolytic species. By contrast, in the Bacteroidetes, utilization of soluble polysaccharides, encoded by gene clusters (PULs), entails outer membrane binding proteins, and degradation is largely periplasmic or intracellular. Biomass degradation involves complex interplay between these distinct groups of bacteria as well as (in the rumen) eukaryotic microorganisms.
Background: Select cellulolytic bacteria produce multi-enzymatic cellulosome complexes that bind to the plant cell wall and catalyze its efficient degradation. The multi-modular interconnecting cellulosomal subunits comprise dockerin-containing enzymes that bind cohesively to cohesin-containing scaffoldins. The organization of the modules into functional polypeptides is achieved by intermodular linkers of different lengths and composition, which provide flexibility to the complex and determine its overall architecture. Results: Using a synthetic biology approach, we systematically investigated the spatial organization of the scaffoldin subunit and its effect on cellulose hydrolysis by designing a combinatorial library of recombinant trivalent designer scaffoldins, which contain a carbohydrate-binding module (CBM) and 3 divergent cohesin modules. The positions of the individual modules were shuffled into 24 different arrangements of chimaeric scaffoldins. This basic set was further extended into three sub-sets for each arrangement with intermodular linkers ranging from zero (no linkers), 5 (short linkers) and native linkers of 27-35 amino acids (long linkers). Of the 72 possible scaffoldins, 56 were successfully cloned and 45 of them expressed, representing 14 full sets of chimaeric scaffoldins. The resultant 42-component scaffoldin library was used to assemble designer cellulosomes, comprising three model C. thermocellum cellulases. Activities were examined using Avicel as a pure microcrystalline cellulose substrate and pretreated cellulose-enriched wheat straw as a model substrate derived from a native source. All scaffoldin combinations yielded active trivalent designer cellulosome assemblies on both substrates that exceeded the levels of the free enzyme systems. A preferred modular arrangement for the trivalent designer scaffoldin was not observed for the three enzymes used in this study, indicating that they could be integrated at any position in the designer cellulosome without significant effect on cellulose-degrading activity. Designer cellulosomes assembled with the long-linker scaffoldins achieved higher levels of activity, compared to those assembled with short-and no-linker scaffoldins. Conclusions The results demonstrate the robustness of the cellulosome system. Long intermodular scaffoldin linkers are preferable, thus leading to enhanced degradation of cellulosic substrates, presumably due to the increased flexibility and spatial positioning of the attached enzymes in the complex. These findings provide a general basis for improved designer cellulosome systems as a platform for bioethanol production.
Bacterial cellulosomes are generally believed to assemble at random, like those produced by Clostridiumcellulolyticum. They are composed of one scaffolding protein bearing eight homologous typeI cohesins that bind to any of the typeI dockerins borne by the 62 cellulosomal subunits, thus generating highly heterogeneous complexes. In the present study, the heterogeneity and random assembly of the cellulosomes were evaluated with a simpler model: a miniscaffoldin containing three C.cellulolyticum cohesins and three cellulases of the same bacterium bearing the cognate dockerin (Cel5A, Cel48F, and Cel9G). Surprisingly, rather than the expected randomized integration of enzymes, the assembly of the minicellulosome generated only three distinct types of complex out of the 10 possible combinations, thus indicating preferential integration of enzymes upon binding to the scaffoldin. A hybrid scaffoldin that displays one cohesin from C.cellulolyticum and one from C.thermocellum, thus allowing sequential integration of enzymes, was exploited to further characterize this phenomenon. The initial binding of a given enzyme to the C.thermocellum cohesin was found to influence the type of enzyme that subsequently bound to the C.cellulolyticum cohesin. The preferential integration appears to be related to the length of the inter-cohesin linker. The data indicate that the binding of a cellulosomal enzyme to a cohesin has a direct influence on the dockerin-bearing proteins that will subsequently interact with adjacent cohesins. Thus, despite the general lack of specificity of the cohesin-dockerin interaction within a given species and type, bacterial cellulosomes are not necessarily assembled at random.
Cellulosomes are multi-enzyme complexes produced by anaerobic bacteria for the efficient deconstruction of plant cell wall polysaccharides. The assembly of enzymatic subunits onto a central non-catalytic scaffoldin subunit is mediated by a highly specific interaction between the enzyme-bearing dockerin modules and the resident cohesin modules of the scaffoldin, which affords their catalytic activities to work synergistically. The scaffoldin also imparts substrate-binding and bacterial-anchoring properties, the latter of which involves a second cohesin-dockerin interaction. Recent structure-function studies reveal an ever-growing array of unique and increasingly complex cohesin-dockerin complexes and cellulosomal enzymes with novel activities. A 'build' approach involving multimodular cellulosomal segments has provided a structural model of an organized yet conformationally dynamic supramolecular assembly with the potential to form higher order structures.
Lactobacillus plantarum is an attractive candidate for bioprocessing of lignocellulosic biomass due to its high metabolic variability, including its ability to ferment both pentoses and hexoses, as well as its high acid tolerance, a quality often utilized in industrial processes. This bacterium grows naturally on biomass; however, it lacks the inherent ability to deconstruct lignocellulosic substrates. As a first step toward engineering lignocellulose-converting lactobacilli, we have introduced genes coding for a GH6 cellulase and a GH11 xylanase from a highly active cellulolytic bacterium into L. plantarum. For this purpose, we employed the recently developed pSIP vectors for efficient secretion of heterologous proteins. Both enzymes were secreted by L. plantarum at levels estimated at 0.33 nM and 3.3 nM, for the cellulase and xylanase, respectively, in culture at an optical density at 600 nm (OD600) of 1. Transformed cells demonstrated the ability to degrade individually either cellulose or xylan and wheat straw. When mixed together to form a two-strain cell-based consortium secreting both cellulase and xylanase, they exhibited synergistic activity in the overall release of soluble sugar from wheat straw. This result paves the way toward metabolic harnessing of L. plantarum for novel biorefining applications, such as production of ethanol and polylactic acid directly from plant biomass.
The cellulosome of the cellulolytic bacterium Clostridium thermocellum has a structural multi-modular protein called CipA (cellulosome-integrating protein A) that includes nine enzyme-binding cohesin modules and a family 3 cellulose-binding module (CBM3a). In the CipA protein, the CBM3a module is located between the second and third cohesin modules and is connected to them via proline/threonine-rich linkers. The structure of CBM3a with portions of the C-and N-terminal flanking linker regions, CBM3a-L, has been determined to a resolution of 1.98 angstrom. The structure is a beta-sandwich with a structural Ca2+ ion. The structure is consistent with the previously determined CipA CBM structure; however, the structured linker regions provide a deeper insight into the overall cellulosome structure and assembly.
The rumen bacterium Ruminococcus flavefaciens produces a highly organized multienzyme cellulosome complex that plays a key role in the degradation of plant cell wall polysaccharides, notably cellulose. The R. flavefaciens cellulosomal system is anchored to the bacterial cell wall through a relatively small ScaE scaffoldin subunit, which bears a single type IIIe cohesin responsible for the attachment of two major dockerin-containing scaffoldin proteins, ScaB and the cellulose-binding protein CttA. Although ScaB recruits the catalytic machinery onto the complex, CttA mediates attachment of the bacterial substrate via its two putative carbohydrate-binding modules. In an effort to understand the structural basis for assembly and cell surface attachment of the cellulosome in R. flavefaciens, we determined the crystal structure of the high affinity complex (Kd = 20.83 nM) between the cohesin module of ScaE (CohE) and its cognate X-dockerin (XDoc) modular dyad from CttA at 1.97-angstrom resolution. The structure reveals an atypical calcium-binding loop containing a 13-residue insert. The results further pinpoint two charged specificity-related residues on the surface of the cohesin module that are responsible for specific versus promiscuous cross-strain binding of the dockerin module. In addition, a combined functional role for the three enigmatic dockerin inserts was established whereby these extraneous segments serve as structural buttresses that reinforce the stalklike conformation of the X-module, thus segregating its tethered complement of cellulosomal components from the cell surface. The novel structure of the RfCohE-XDoc complex sheds light on divergent dockerin structure and function and provides insight into the specificity features of the type IIIe cohesin-dockerin interaction.
Background: Ruminococcus flavefaciens is an important fibre-degrading bacterium found in the mammalian gut. Cellulolytic strains from the bovine rumen have been shown to produce complex cellulosome structures that are associated with the cell surface. R. flavefaciens 007 is a highly cellulolytic strain whose ability to degrade dewaxed cotton, but not Avicel cellulose, was lost following initial isolation in the variant 007S. The ability was recovered after serial subculture to give the cotton-degrading strain 007C. This has allowed us to investigate the factors required for degradation of this particularly recalcitrant form of cellulose. Methodology/Principal Findings: The major proteins associated with the bacterial cell surface and with the culture supernatant were analyzed for R. flavefaciens 007S and 007C grown with cellobiose, xylan or Avicel cellulose as energy sources. Identification of the proteins was enabled by a draft genome sequence obtained for 007C. Among supernatant proteins a cellulosomal GH48 hydrolase, a rubrerthyrin-like protein and a protein with type IV pili N-terminal domain were the most strongly up-regulated in 007C cultures grown on Avicel compared with cellobiose. Strain 007S also showed substrate-related changes, but supernatant expression of the Pil protein and rubrerythrin in particular were markedly lower in 007S than in 007C during growth on Avicel. Conclusions/Significance: This study provides new information on the extracellular proteome of R. flavefaciens and its regulation in response to different growth substrates. Furthermore it suggests that the cotton cellulose non-degrading strain (007S) has altered regulation of multiple proteins that may be required for breakdown of cotton cellulose. One of these, the type IV pilus was previously shown to play a role in adhesion to cellulose in R. albus, and a related pilin protein was identified here for the first time as a major extracellular protein in R. flavefaciens.
Nature has evolved multiple enzymatic strategies for the degradation of plant cell wall polysaccharides, which are central to carbon flux in the biosphere and an integral part of renewable biofuels production. Many biomass-degrading organisms secrete synergistic cocktails of individual enzymes with one or several catalytic domains per enzyme, whereas a few bacteria synthesize large multi-enzyme complexes, termed cellulosomes, which contain multiple catalytic units per complex. Both enzyme systems employ similar catalytic chemistries; however, the physical mechanisms by which these enzyme systems degrade polysaccharides are still unclear. Here we examine a prominent example of each type, namely a free-enzyme cocktail expressed by the fungus Hypocrea jecorina and a cellulosome preparation secreted from the anaerobic bacterium Clostridium thermocellum. We observe striking differences in cellulose saccharification exhibited by these systems at the same protein loading. Free enzymes are more active on pretreated biomass and in contrast cellulosomes are much more active on purified cellulose. When combined, these systems display dramatic synergistic enzyme activity on cellulose. To gain further insights, we imaged free enzyme- and cellulosome-digested cellulose and biomass by transmission electron microscopy, which revealed evidence for different mechanisms of cellulose deconstruction by free enzymes and cellulosomes. Specifically, the free enzymes employ an ablative, fibril-sharpening mechanism, whereas cellulosomes physically separate individual cellulose microfibrils from larger particles resulting in enhanced access to cellulose surfaces. Interestingly, when the two enzyme systems are combined, we observe changes to the substrate that suggests mechanisms of synergistic deconstruction. Insight into the different mechanisms underlying these two polysaccharide deconstruction paradigms will eventually enable new strategies for enzyme engineering to overcome biomass rec
Background: Many bacteria efficiently degrade lignocellulose yet the underpinning genome-wide metabolic and regulatory networks remain elusive. Here we revealed the "cellulose degradome" for the model mesophilic cellulolytic bacterium Clostridium cellulolyticum ATCC 35319, via an integrated analysis of its complete genome, its transcriptomes under glucose, xylose, cellobiose, cellulose, xylan or corn stover and its extracellular proteomes under glucose, cellobiose or cellulose. Results: Proteins for core metabolic functions, environment sensing, gene regulation and polysaccharide metabolism were enriched in the cellulose degradome. Analysis of differentially expressed genes revealed a "core" set of 48 CAZymes required for degrading cellulose-containing substrates as well as an "accessory" set of 76 CAZymes required for specific non-cellulose substrates. Gene co-expression analysis suggested that Carbon Catabolite Repression (CCR) related regulators sense intracellular glycolytic intermediates and control the core CAZymes that mainly include cellulosomal components, whereas 11 sets of Two-Component Systems (TCSs) respond to availability of extracellular soluble sugars and respectively regulate most of the accessory CAZymes and associated transporters. Surprisingly, under glucose alone, the core cellulases were highly expressed at both transcript and protein levels. Furthermore, glucose enhanced cellulolysis in a dose-dependent manner, via inducing cellulase transcription at low concentrations. Conclusion: A molecular model of cellulose degradome in C. cellulolyticum (Ccel) was proposed, which revealed the substrate-specificity of CAZymes and the transcriptional regulation of core cellulases by CCR where the glucose acts as a CCR inhibitor instead of a trigger. These features represent a distinct environment-sensing strategy for competing while collaborating for cellulose utilization, which can be exploited for process and genetic engineering of microbial celluloly
Clostridium thermocellum produces the prototypical cellulosome, a large multienzyme complex that efficiently hydrolyzes plant cell wall polysaccharides into fermentable sugars. This ability has garnered great interest in its potential application in biofuel production. The core non-catalytic scaffoldin subunit, CipA, bears nine type I cohesin modules that interact with the type I dockerin modules of secreted hydrolytic enzymes and promotes catalytic synergy. Because the large size and flexibility of the cellulosome preclude structural determination by traditional means, the structural basis of this synergy remains unclear. Small angle x-ray scattering has been successfully applied to the study of flexible proteins. Here, we used small angle x-ray scattering to determine the solution structure and to analyze the conformational flexibility of two overlapping N-terminal cellulosomal scaffoldin fragments comprising two type I cohesin modules and the cellulose-specific carbohydrate-binding module from CipA in complex with Cel8A cellulases. The pair distribution functions, ab initio envelopes, and rigid body models generated for these two complexes reveal extended structures. These two N-terminal cellulosomal fragments are highly dynamic and display no preference for extended or compact conformations. Overall, our work reveals structural and dynamic features of the N terminus of the CipA scaffoldin that may aid in cellulosome substrate recognition and binding.
Background: Ruminococcus flavefaciens is one of the predominant fiber-degrading bacteria found in the rumen of herbivores. Bioinformatic analysis of the recently sequenced genome indicated that this bacterium produces one of the most intricate cellulosome systems known to date. A distinct ORF, encoding for a multi-modular protein, RflaF_05439, was discovered during mining of the genome sequence. It is composed of two tandem modules of currently undefined function that share 45% identity and a C-terminal X-dockerin modular dyad. Gaining insight into the diversity, architecture and organization of different types of proteins in the cellulosome system is essential for broadening our understanding of a multi-enzyme complex, considered to be one of the most efficient systems for plant cell wall polysaccharide degradation in nature. Methodology/Principal Findings: Following bioinformatic analysis, the second tandem module of RflaF_05439 was cloned and its selenium-labeled derivative was expressed and crystallized. The crystals belong to space group P21 with unit-cell parameters of alpha = 65.81, b = 60.61, c = 66.13 angstrom, beta = 107.66 degrees and contain two protein molecules in the asymmetric unit. The crystal structure was determined at 1.38-angstrom resolution by X-ray diffraction using the single-wavelength anomalous dispersion (SAD) method and was refined to R-factor and R-free of 0.127 and 0.152 respectively. The protein molecule mainly comprises a beta-sheet flanked by short alpha-helixes, and a globular a-helical domain. The structure was found to be structurally similar to members of the NlpC/P60 superfamily of cysteine peptidases. Conclusions/Significance: The 3D structure of the second repeat of the RflaF_05439 enabled us to propose a role for the currently undefined function of this protein. Its putative function as a cysteine peptidase is inferred from in silico structural homology studies. It is therefore apparent that cellulosomes integrate proteins
Cellulosomes are modular, super-molecular enzyme systems secreted by anaerobic bacteria to degrade recalcitrant plant cell wall polysaccharides to simple sugars. The components of this molecular machine include a manifold of enzymatic units and carbohydrate-binding modules, as well as the cohesin and dockerin modules responsible for the system's unique self-assembly and connectivity Known to be one of the highest affinity protein protein interactions discovered to date, the cohesin dockerin interaction is also the key to engineering the cellulosome. By mixing and matching the spare parts of the cellulosome and connecting them via cohesins and dockerins, biotechnologists have begun pursuing the concept of 'designer cellulosomes', which one day may be an important contributor to production of sustainable biomass-derived fuels. By incorporating molecules from other systems into the cellulosome paradigm, nanotechnologists have begun to harness the potential of this molecular construction kit to create diverse, self-assembling nanostructures for a broad variety of biotechnological applications.
Phylogenetic analysis of known dockerins in Ruminococcus flavefaciens revealed a novel subtype, type-Ill, in the scaffoldin proteins, ScaA, ScaB, ScaC and ScaE. In this study, we explored the Ca2+-binding properties of the type-Ill dockerin from the ScaA scaffoldin (ScaADoc) using a battery of structural and biophysical approaches including circular dichroism spectroscopy, isothermal titration calorimetry, differential scanning calorimetry, and nuclear magnetic resonance spectroscopy. Despite the lack of a second canonical Ca2+-binding loop, the behaviour of ScaADoc is similar with respect to other dockerin protein modules in terms of its responsiveness to Ca2+ and affinity for the cohesin from the ScaB scaffoldin. Our results highlight the robustness of dockerin modules and how their Ca2+-binding properties can be exploited in the construction of designer cellulosomes. Structured summary of protein interactions: ScaB cohesin and ScaADoc bind by isothermal titration calorimetry (View interaction) ScaB cohesin and ScaADoc bind by molecular sieving (View interaction) ScaADoc and ScaB cohesin bind by biophysical (View interaction) ScaB cohesin binds to ScaADoc by enzyme linked immunosorbent assay (View interaction) (C) 2012 Federation of European Biochemical Societies. Published by Elsevier B. V. All rights reserved.
Cellulose-degrading enzyme systems are of significant interest from both a scientific and technological perspective due to the diversity of cellulase families, their unique assembly and substrate binding mechanisms, and their potential applications in several key industrial sectors, notably cellulose hydrolysis for second-generation biofuel production. Particularly fascinating are cellulosomes, the multimodular extracellular complexes produced by numerous anaerobic bacteria. Using single-molecule force spectroscopy, we analyzed the mechanical stability of the intermolecular interfaces between the cohesin and the dockerin modules responsible for self-assembly of the cellulosomal components into the multienzyme complex. The observed cohesin-dockerin rupture forces (>120 pN) are among the highest reported for a receptor-ligand system to date. Using an atomic force microscope protocol that quantified single-molecule binding activity, we observed force-induced dissociation of calcium ions from the duplicated loop-helix F-hand motif located within the dockerin module, which in the presence of EDTA resulted in loss of affinity to the cohesin partner. A cohesin amino acid mutation (D39A) that eliminated hydrogen bonding with the dockerin's critically conserved serine residues reduced the observed rupture forces. Consequently, no calcium loss occurred and dockerin activity was maintained throughout multiple forced dissociation events. These results offer insights at the single-molecule level into the stability and folding of an exquisite class of high-affinity protein-protein interactions that dictate fabrication and architecture of cellulose-degrading molecular machines.
Greater understanding of the mechanisms contributing to chemical and enzymatic solubilization of plant cell walls is critical for enabling cost-effective industrial conversion of cellulosic biomass to biofuels. Here, we report the use of correlative imaging in real time to assess the impact of pretreatment, as well as the resulting nanometer-scale changes in cell wall structure, upon subsequent digestion by two commercially relevant cellulase systems. We demonstrate that the small, noncomplexed fungal cellulases deconstruct cell walls using mechanisms that differ considerably from those of the larger, multienzyme complexes (cellulosomes). Furthermore, high-resolution measurement of the microfibrillar architecture of cell walls suggests that digestion is primarily facilitated by enabling enzyme access to the hydrophobic cellulose face. The data support the conclusion that ideal pretreatments should maximize lignin removal and minimize polysaccharide modification, thereby retaining the essentially native microfibrillar structure.
Lignocellulosic biomass, the most abundant polymer on Earth, is typically composed of three major constituents: cellulose, hemicellulose, and lignin. The crystallinity of cellulose, hydrophobicity of lignin, and encapsulation of cellulose by the lignin-hemicellulose matrix are three major factors that contribute to the observed recalcitrance of lignocellulose. By means of designer cellulosome technology, we can overcome the recalcitrant properties of lignocellulosic substrates and thus increase the level of native enzymatic degradation. In this context, we have integrated six dockerin-bearing cellulases and xylanases from the highly cellulolytic bacterium, Thermobifida fusca, into a chimeric scaffoldin engineered to bear a cellulose-binding module and the appropriate matching cohesin modules. The resultant hexavalent designer cellulosome represents the most elaborate artificial enzyme composite yet constructed, and the fully functional complex achieved enhanced levels (up to 1.6-fold) of degradation of untreated wheat straw compared to those of the wild-type free enzymes. The action of these designer cellulosomes on wheat straw was 33 to 42% as efficient as the natural cellulosomes of Clostridium thermocellum. In contrast, the reduction of substrate complexity by chemical or biological pretreatment of the substrate removed the advantage of the designer cellulosomes, as the free enzymes displayed higher levels of activity, indicating that enzyme proximity between these selected enzymes was less significant on pretreated substrates. Pretreatment of the substrate caused an increase in activity for all the systems, and the native cellulosome completely converted the substrate into soluble saccharides. IMPORTANCE Cellulosic biomass is a potential alternative resource which could satisfy future demands of transportation fuel. However, overcoming the natural lignocellulose recalcitrance remains challenging. Current research and development efforts have concentrated on t
The interaction between the cohesin and dockerin modules serves to attach cellulolytic enzymes (carrying dockerins) to non-catalytic scaffoldin units (carrying multiple cohesins) in cellulosome, a multienzyme plant cell-wall degrading complex. This interaction is species-specific, for example, the enzyme-borne dockerin from Clostridium thermocellum bacteria binds to scaffoldin cohesins from the same bacteria but not to cohesins from Clostridium cellulolyticum and vice versa. We studied the role of interface residues, contributing either to affinity or specificity, by mutating these residues on the cohesin counterpart from C. thermocellum. The high affinity of the cognate interactions makes it difficult to evaluate the effect of these mutations by common methods used for measuring proteinprotein interactions, especially when subtle discrimination between the mutants is needed. We described in this article an approach based on indirect enzyme-linked immunosorbent assay (ELISA) that is able to detect differences in binding between the various cohesin mutants, whereas surface plasmon resonance and standard ELISA failed to distinguish between high-affinity interactions. To be able to calculate changes in energy of binding (Delta Delta G) and dissociation constants (K-d) of mutants relative to wild type, a pre-equilibrium step was added to the standard indirect ELISA procedure. Thus, the cohesindockerin interaction under investigation occurs in solution rather than between soluble and immobilized proteins. Unbound dockerins are then detected through their interaction with immobilized cohesins. Because our method allows us to assess the effect of mutations on particularly tenacious proteinprotein interactions much more accurately than do other prevalent methods used to measure binding affinity, we therefore suggest this approach as a method of choice for comparing relative binding in high-affinity interactions. Copyright (c) 2012 John Wiley & Sons, Ltd.
Background: Microorganisms employ a multiplicity of enzymes to efficiently degrade the composite structure of plant cell wall cellulosic polysaccharides. These remarkable enzyme systems include glycoside hydrolases (cellulases, hemicellulases), polysaccharide lyases, and the carbohydrate esterases. To accomplish this challenging task, several strategies are commonly observed either separately or in combination. These include free enzyme systems, multifunctional enzymes, and multi-enzyme self-assembled designer cellulosome complexes. Results: In order to compare these different paradigms, we employed a synthetic biology approach to convert two different cellulases from the free enzymatic system of the well-studied bacterium, Thermobifida fusca, into bifunctional enzymes with different modular architectures. We then examined their performance compared to those of the combined parental free-enzyme and equivalent designer-cellulosome systems. The results showed that the cellulolytic activity displayed by the different architectures of the bifunctional enzymes was somewhat inferior to that of the wild-type free enzyme system. Conclusions: The activity exhibited by the designer cellulosome system was equal or superior to that of the free system, presumably reflecting the combined proximity of the enzymes and high flexibility of the designer cellulosome components, thus enabling efficient enzymatic activity of the catalytic modules.
In Ruminococcus flavefaciens, a predominant fibre-degrading bacterium found in ruminants, cellulosomal proteins are anchored to the bacterial cell wall through a relatively small ScaE scaffoldin which includes a single type III cohesin. The cotton-binding protein CttA consists of two cellulose-binding modules and a C-terminal modular pair (XDoc) comprising an X-module and a contiguous dockerin, which exhibits high affinity towards the ScaE cohesin. Seleno-l-methionine-labelled derivatives of the ScaE cohesin module and the XDoc from CttA have been expressed, copurified and cocrystallized. The crystals belonged to the tetragonal space group P43212, with unit-cell parameters a = b = 78.7, c = 203.4 angstrom, and the unit cell contains a single cohesinXDoc complex in the asymmetric unit. The diffraction data were phased to 2.0 angstrom resolution using the anomalous signal of the Se atoms.
Cellulosomes are multienzyme complexes responsible for efficient degradation of plant cell wall polysaccharides. The nonenzymatic scaffoldin subunit provides a platform for cellulolytic enzyme binding that enhances the overall activity of the bound enzymes. Understanding the unique quaternary structural elements responsible for the enzymatic synergy of the cellulosome is hindered by the large size and inherent flexibility of these multiprotein complexes. Herein, we have used x-ray crystallography and small angle x-ray scattering to structurally characterize a ternary protein complex from the Clostridium thermocellum cellulosome that comprises a C-terminal trimodular fragment of the CipA scaffoldin bound to the SdbA type II cohesin module and the type I dockerin module from the Cel9D glycoside hydrolase. This complex represents the largest fragment of the cellulosome solved by x-ray crystallography to date and reveals two rigid domains formed by the type I cohesin.dockerin complex and by the X module-type II cohesin.dockerin complex, which are separated by a 13-residue linker in an extended conformation. The type I dockerin modules of the four structural models found in the asymmetric unit are in an alternate orientation to that previously observed that provides further direct support for the dual mode of binding. Conserved intermolecular contacts between symmetry-related complexes were also observed and may play a role in higher order cellulosome structure. SAXS analysis of the ternary complex revealed that the 13-residue intermodular linker of the scaffoldin subunit is highly dynamic in solution. These studies provide fundamental insights into modular positioning, linker flexibility, and higher order organization of the cellulosome.
The crystal structure of the family 3b carbohydrate-binding module (CBM3b) of the cellulosomal multimodular hydrolytic enzyme cellobiohydrolase 9A (Cbh9A) from Clostridium thermocellum has been determined. Cbh9A CBM3b crystallized in space group P4(1) with four molecules in the asymmetric unit and diffracted to a resolution of 2.20 angstrom using synchrotron radiation. The structure was determined by molecular replacement using C. thermocellum Cel9V CBM3b' (PDB entry 2wnx) as a model. The C. thermocellum Cbh9A CBM3b molecule forms a nine-stranded antiparallel beta-sandwich similar to other family 3 carbohydrate-binding modules (CBMs). It has a short planar array of two aromatic residues that are assumed to bind cellulose, yet it lacks the ability to bind cellulose. The molecule contains a shallow groove of unknown function that characterizes other family 3 CBMs with high sequence homology. In addition, it contains a calcium-binding site formed by a group of amino-acid residues that are highly conserved in similar structures. After determination of the three-dimensional structure of Cbh9A CBM3b, the site-specific N126W mutant was produced with the intention of enhancing the cellulose-binding ability of the CBM. Cbh9A CBM3b(N126W) crystallized in space group P4(1)2(1)2, with one molecule in the asymmetric unit. The crystals diffracted to 1.04 angstrom resolution using synchrotron radiation. The structure of Cbh9A CBM3b(N126W) revealed incorporation of the mutated Trp126 into the putative cellulose-binding strip of residues. Cellulose-binding experiments demonstrated the ability of Cbh9A CBM3b(N126W) to bind cellulose owing to the mutation. This is the first report of the engineered conversion of a non-cellulose-binding CBM3 to a binding CBM3 by site-directed mutagenesis. The three-dimensional structure of Cbh9A CBM3b(N126W) provided a structural correlation with cellulose-binding ability, revealing a longer planar array of definitive cellulose-binding residues.
Clostridium thermocellum wild-type strain YS is an anaerobic, thermophilic, cellulolytic bacterium capable of directly converting cellulosic substrates into ethanol. Strain YS and a derived cellulose adhesion-defective mutant strain, AD2, played pivotal roles in describing the original cellulosome concept. We present their draft genome sequences.
The conversion of recalcitrant plant-derived cellulosic biomass into biofuels is dependent on highly efficient cellulase systems that produce near-quantitative levels of soluble saccharides. Similar to other fungal and bacterial cellulase systems, the multienzyme cellulosome system of the anaerobic, cellulolytic bacterium Clostridium thermocellum is strongly inhibited by the major end product cellobiose. Cellobiose-induced inhibition can be relieved via its cleavage to noninhibitory glucose by the addition of exogenous noncellulosomal enzyme beta-glucosidase; however, because the cellulosome is adsorbed to the insoluble substrate only a fraction of beta-glucosidase would be available to the cellulosome. Towards this end, we designed a chimeric cohesin-fused beta-glucosidase (BglA-CohII) that binds directly to the cellulosome through an unoccupied dockerin module of its major scaffoldin subunit. The beta-glucosidase activity is thus focused at the immediate site of cellobiose production by the cellulosomal enzymes. BglA-CohII was shown to retain cellobiase activity and was readily incorporated into the native cellulosome complex. Surprisingly, it was found that the native C. thermocellum cellulosome exists as a homooligomer and the high-affinity interaction of BglA-CohII with the scaffoldin moiety appears to dissociate the oligomeric state of the cellulosome. Complexation of the cellulosome and BglA-CohII resulted in higher overall degradation of microcrystalline cellulose and pretreated switchgrass compared to the native cellulosome alone or in combination with wild-type BglA in solution. These results demonstrate the effect of enzyme targeting and its potential for enhanced degradation of cellulosic biomass.
The use of thermostable cellulases is advantageous for the breakdown of lignocellulosic biomass toward the commercial production of biofuels. Previously, we have demonstrated the engineering of an enhanced thermostable family 8 cellulosomal endoglucanase (EC 188.8.131.52), Cel8A, from Clostridium thermocellum, using random error-prone PCR and a combination of three beneficial mutations, dominated by an intriguing serine-to-glycine substitution (M. Anbar, R. Lamed, E. A. Bayer, ChemCatChem 2: 997-1003, 2010). In the present study, we used a bioinformatics-based approach involving sequence alignment of homologous family 8 glycoside hydrolases to create a library of consensus mutations in which residues of the catalytic module are replaced at specific positions with the most prevalent amino acids in the family. One of the mutants (G283P) displayed a higher thermal stability than the wild-type enzyme. Introducing this mutation into the previously engineered Cel8A triple mutant resulted in an optimized enzyme, increasing the half-life of activity by 14-fold at 85 degrees C. Remarkably, no loss of catalytic activity was observed compared to that of the wild-type endoglucanase. The structural changes were simulated by molecular dynamics analysis, and specific regions were identified that contributed to the observed thermostability. Intriguingly, most of the proteins used for sequence alignment in determining the consensus residues were derived from mesophilic bacteria, with optimal temperatures well below that of C. thermocellum Cel8A.
Background: Microbial degradation of plant cell walls and its conversion to sugars and other byproducts is a key step in the carbon cycle on Earth. In order to process heterogeneous plant-derived biomass, specialized anaerobic bacteria use an elaborate multi-enzyme cellulosome complex to synergistically deconstruct cellulosic substrates. The cellulosome was first discovered in the cellulolytic thermophile, Clostridium thermocellum, and much of our knowledge of this intriguing type of protein composite is based on the cellulosome of this environmentally and biotechnologically important bacterium. The recently sequenced genome of the cellulolytic mesophile, Acetivibrio cellulolyticus, allows detailed comparison of the cellulosomes of these two select cellulosome-producing bacteria. Results: Comprehensive analysis of the A. cellulolyticus draft genome sequence revealed a very sophisticated cellulosome system. Compared to C. thermocellum, the cellulosomal architecture of A. cellulolyticus is much more extensive, whereby the genome encodes for twice the number of cohesin- and dockerin-containing proteins. The A. cellulolyticus genome has thus evolved an inflated number of 143 dockerin-containing genes, coding for multimodular proteins with distinctive catalytic and carbohydrate-binding modules that play critical roles in biomass degradation. Additionally, 41 putative cohesin modules distributed in 16 different scaffoldin proteins were identified in the genome, representing a broader diversity and modularity than those of Clostridium thermocellum. Although many of the A. cellulolyticus scaffoldins appear in unconventional modular combinations, elements of the basic structural scaffoldins are maintained in both species. In addition, both species exhibit similarly elaborate cella anchoring and cellulosome-related gene-regulatory elements. Conclusions: This work portrays a particularly intricate, cell-surface cellulosome system in A. cellulolyticus and provides a blueprin
Shwanavidin is an avidin-like protein from the marine proteo-bactrium Shewanella denitrificans, which exhibits an innate dimeric structure while maintaining high affinity toward biotin. A unique residue (Phe-43) from the L3,4 loop and a distinctive disulfide bridge were shown to account for the high affinity toward biotin. Phe-43 emulates the function and position of the critical intermonomeric Trp that characterizes the tetrameric avidins but is lacking in shwanavidin. The 18 copies of the apo-monomer revealed distinctive snapshots of L3,4 and Phe-43, providing rare insight into loop flexibility, binding site accessibility, and psychrophilic adaptation. Nevertheless, as in all avidins, shwanavidin also displays high thermostability properties. The unique features of shwanavidin may provide a platform for the design of a long sought after monovalent form of avidin, which would be ideal for novel types of biotechnological application.
beta-Xylosidases are hemicellulases that hydrolyze short xylo-oligosaccharides into xylose units, thus complementing endoxylanase degradation of the hemicellulose component of lignocellulosic substrates. Here, we describe the cloning, characterization, and kinetic analysis of a glycoside hydrolase family 43 beta-xylosidase (Xyl43A) from the aerobic cellulolytic bacterium, Thermobifida fusca. Temperature and pH optima of 55-60 degrees C and 5.5-6, respectively, were determined. The apparent K-m value was 0.55 mM, using p-nitrophenyl xylopyranoside as substrate, and the catalytic constant (k(cat)) was 6.72 s(-1). T. fusca Xyl43A contains a catalytic module at the N terminus and an ancillary module (termed herein as Module-A) of undefined function at the C terminus. We expressed the two recombinant modules independently in Escherichia coli and examined their remaining catalytic activity and binding properties. The separation of the two Xyl43A modules caused the complete loss of enzymatic activity, whereas potent binding to xylan was fully maintained in the catalytic module and partially in the ancillary Module-A. Nondenaturing gel electrophoresis revealed a specific noncovalent coupling of the two modules, thereby restoring enzymatic activity to 66.7% (relative to the wild-type enzyme). Module-A contributes a phenylalanine residue that functions as an essential part of the active site, and the two juxtaposed modules function as a single functional entity.
Viruses are the most abundant biological entities on the planet and play an important role in balancing microbes within an ecosystem and facilitating horizontal gene transfer. Although bacteriophages are abundant in rumen environments, little is known about the types of viruses present or their interaction with the rumen microbiome. We undertook random pyrosequencing of virus-enriched metagenomes (viromes) isolated from bovine rumen fluid and analysed the resulting data using comparative metagenomics. A high level of diversity was observed with up to 28 000 different viral genotypes obtained from each environment. The majority (similar to 78%) of sequences did not match any previously described virus. Prophages outnumbered lytic phages approximately 2: 1 with the most abundant bacteriophage and prophage types being associated with members of the dominant rumen phyla (Firmicutes and Proteobacteria). Metabolic profiling based on SEED subsystems revealed an enrichment of sequences with putative functional roles in DNA and protein metabolism, but a surprisingly low proportion of sequences assigned to carbohydrate and amino acid metabolism. We expanded our analysis to include previously described metagenomic data and 14 reference genomes. Clustered regularly interspaced short palindromic repeats (CRISPR) were detected in most of the microbial genomes, suggesting previous interactions between viral and microbial communities.
The carbohydrate-binding module (CBM) of the major scaffoldin subunit ScaA of the cellulosome of Acetivibrio cellulolyticus is classified as a family 3b CBM and binds strongly to cellulose. The CBM3b was overexpressed, purified and crystallized, and its three-dimensional structure was determined. The structure contained a nickel-binding site located at the N-terminal region in addition to a 'classical' CBM3b calcium-binding site. The structure was also determined independently by the SAD method using data collected at the Ni-absorption wavelength of 1.48395 angstrom and even at a wavelength of 0.97625 angstrom in a favourable case. The new scaffoldin-borne CBM3 structure reported here provides clear evidence for the proposition that a family 3b CBM may be accommodated in scaffoldin subunits and functions as the major substrate-binding entity of the cellulosome assembly.
Many efforts have been invested to reduce the cost of biofuel production to substitute renewable sources of energy for fossil-based fuels. At the forefront of these efforts are the initiatives to convert plant-derived cellulosic material to biofuels. Although significant improvements have been achieved recently in cellulase engineering in both efficiency and cost reduction, complete degradation of lignocellulosic material still requires very long periods of time and high enzyme loads. Thermostable cellulases offer many advantages in the bioconversion process, which include increase in specific activity, higher levels of stability, inhibition of microbial growth, increase in mass transfer rate due to lower fluid viscosity, and greater flexibility in the bioprocess. Besides rational design methods, which require deep understanding of protein structure function relationship, two of the major methods for improvement in specific cellulase properties are directed evolution and knowledge-based library design based on multiple sequence alignments. In this chapter, we provide protocols for constructing and screening of improved thermostable cellulases. Modifications of these protocols may also be used for screening for other improved properties of cellulases such as pH tolerance, high salt, and more.
The cellulosome is a large bacterial extracellular multienzyme complex able to degrade crystalline cellulosic substrates. The complex contains catalytic and noncatalytic subunits, interconnected by high-affinity cohesin dockerin interactions. In this chapter, we introduce an optimized method for comparative binding among different cohesins or cohesin mutants to the dockerin partner. This assay offers advantages over other methods (such as ELISA, cELIA, SPR, and ITC) for particularly high-affinity binding interactions. In this approach, the high-affinity interaction of interest occurs in the liquid phase during the equilibrated binding step, whereas the interaction with the immobilized phase is used only for detection of the unbound dockerins that remain in the solution phase. Once equilibrium conditions are reached, the change in free energy of binding (Delta Delta G(binding)) as well as the affinity constant of mutants, can be estimated against the known affinity constant of the wild-type interaction. In light of the above, we propose this method as a preferred alternative for the relative quantification of high-affinity protein interactions.
The specificity of cohesin dockerin interactions is critically important for the assembly of cellulosomal enzymes into the multienzyme cellulolytic complex (cellulosome). In order to investigate the origins of the observed specificity, a variety of selected amino acid positions at the cohesin dockerin interface can be subjected to mutagenesis, and a library of mutants can be constructed. In this chapter, we describe a protein protein microarray technique based on the high affinity of a carbohydrate-binding module (CBM), attached to mutant cohesins. Using cellulose-coated glass slides, libraries of mutants can be screened for binding to complementary partners. The advantages of this tool are that crude cell lysate can be used without additional purification, and the microarray can be used for screening both large libraries as initial scanning for "positive" plates, and for small libraries, wherein individual colonies are printed on the slide. Since the time-consuming step of purifying proteins can be circumvented, the approach is also appropriate for providing molecular insight into the multicomponent organization of complex cellulosomes.
During the past several years, major progress has been accomplished in the production of "designer cellulosomes," artificial enzymatic complexes that were demonstrated to efficiently degrade crystalline cellulose. This progress is part of a global attempt to promote biomass waste solutions and biofuel production. In designer cellulosomes, each enzyme is equipped with a dockerin module that interacts specifically with one of the cohesin modules of the chimeric scaffoldin. Artificial scaffoldins serve as docking backbones and contain a cellulose-specific carbohydrate-binding module that directs the enzymatic complex to the cellulosic substrate, and one or more cohesin modules from different natural cellulosomal species, each exhibiting a different specificity, that allows the specific incorporation of the desired matching dockerin-bearing enzymes. With natural cellulosomal components, the insertion of the enzymes in the scaffold would presumably be random, and we would not be able to control the contents of the resulting artificial cellulosome. There are an increasing number of papers describing the production of designer cellulosomes either in vitro, ex vivo, or in vivo. These types of studies are particularly intricate, and a number of such publications are less meaningful in the final analysis, as important controls are frequently excluded. In this chapter, we hope to give a complete overview of the methodologies essential for designing and examining cellulosome complexes.
In nature, the complex composition and structure of the plant cell wall pose a barrier to enzymatic degradation. Nevertheless, some anaerobic bacteria have evolved for this purpose an intriguing, highly efficient multienzyme complex, the cellulosome, which contains numerous cellulases and hemicellulases. The rod-like cellulose component of the plant cell wall is embedded in a colloidal blend of hemicelluloses, a major component of which is xylan. In order to enhance enzymatic degradation of the xylan component of a natural complex substrate (wheat straw) and to study the synergistic action among different xylanases, we have employed a variation of the designer cellulosome approach by fabricating a tetravalent complex that includes the three endoxylanases of Thermobifida fusca (Xyn10A, Xyn10B, and Xyn11A) and an Xy143A beta-xylosidase from the same bacterium. Here, we describe the conversion of Xyn10A and Xy143A to the cellulosomal mode. The incorporation of the Xy143A enzyme together with the three endoxylanases into a common designer cellulosome served to enhance the level of reducing sugars produced during wheat straw degradation. The enhanced synergistic action of the four xylanases reflected their immediate juxtaposition in the complex, and these tetravalent xylanolytic designer cellulosomes succeeded in degrading significant (similar to 25%) levels of the total xylan component of the wheat straw substrate. The results suggest that the incorporation of xylanases into cellulosome complexes is advantageous for efficient decomposition of recalcitrant cellulosic substrates-a distinction previously reserved for cellulose-degrading enzymes. IMPORTANCE Xylanases are important enzymes for our society, due to their variety of industrial applications. Together with cellulases and other glycoside hydrolases, xylanases may also provide cost-effective conversion of plant-derived cellulosic biomass into soluble sugars en route to biofuels as an alternative to fossil fuels.
Background: The bovine rumen maintains a diverse microbial community that serves to break down indigestible plant substrates. However, those bacteria specifically adapted to degrade cellulose, the major structural component of plant biomass, represent a fraction of the rumen microbiome. Previously, we proposed scaC as a candidate for phylotyping Ruminococcus flavefaciens, one of three major cellulolytic bacterial species isolated from the rumen. In the present report we examine the dynamics and diversity of scaC-types both within and between cattle temporally, following a dietary switch from corn-silage to grass-legume hay. These results were placed in the context of the overall bacterial population dynamics measured using the 16S rRNA. Principal Findings: As many as 117 scaC-types were estimated, although just nineteen were detected in each of three rumens tested, and these collectively accounted for the majority of all types present. Variation in scaC populations was observed between cattle, between planktonic and fiber-associated fractions and temporally over the six-week survey, and appeared related to scaC phylogeny. However, by the sixth week no significant separation of scaC populations was seen between animals, suggesting enrichment of a constrained set of scaC-types. Comparing the amino-acid translation of each scaC-type revealed sequence variation within part of the predicted dockerin module but strong conservation in the N-terminus, where the cohesin module is located. Conclusions: The R. flavefaciens species comprises a multiplicity of scaC-types in-vivo. Enrichment of particular scaC-types temporally, following a dietary switch, and between fractions along with the phylogenetic congruence suggests that functional differences exist between types. Observed differences in dockerin modules suggest at least part of the functional heterogeneity may be conferred by scaC. The polymorphic nature of scaC enables the relative distribution of R. flavefaciens str
The composition of the cellulase system in the cellulosome-producing bacterium, Clostridium thermocellum, has been reported to change in response to growth on different carbon sources. Recently, an extensive carbohydrate-sensing mechanism, purported to regulate the activation of genes coding for polysaccharide-degrading enzymes, was suggested. In this system, CBM modules, comprising extracellular components of RsgI-like anti-sigma factors, were proposed to function as carbohydrate sensors, through which a set of cellulose utilization genes are activated by the associated sigma(I)-like factors. An extracellular module of one of these RsgI-like proteins (Cthe_2119) was annotated as a family 10 glycoside hydrolase, RsgI6-GH10, and a second putative anti-sigma factor (Cthe_1471), related in sequence to Rsi24, was found to contain a module that resembles a family 5 glycoside hydrolase (termed herein Rsi24C-GH5). The present study examines the relevance of these two glycoside hydrolases as sensors in this signal-transmission system. The RsgI6-GH10 was found to bind xylan matrices but exhibited low enzymatic activity on this substrate. In addition, this glycoside hydrolase module was shown to interact with crystalline cellulose although no hydrolytic activity was detected on cellulosic substrates. Bioinformatic analysis of the Rsi24C-GH5 showed a glutamate-to-glutamine substitution that would presumably preclude catalytic activity. Indeed, the recombinant module was shown to bind to cellulose, but showed no hydrolytic activity. These observations suggest that these two glycoside hydrolases underwent an evolutionary adaptation to function as polysaccharide binding agents rather than enzymatic components and thus serve in the capacity of extracellular carbohydrate sensors.
The potent cellulose-binding modules of cellulosomal scaffoldin subunits belong to the greater family of carbohydrate-binding modules (CBMs). They have generally been classified as belonging to family 3a on the basis of sequence similarity. They form nine-stranded beta-sandwich structures with jelly-roll topology. The members of this family possess on their surface a planar array of aromatic amino-acid residues (known as the linear strip) that form stacking interactions with the glucose rings of cellulose chains and have a conserved Ca2+-binding site. Intriguingly, the CBM3 from scaffoldin A (ScaA) of Bacteroides cellulosolvens exhibits alterations in sequence that make it more similar to the CBMs of free cellulolytic enzymes, which are classified into CBM family 3b. X-ray structural analysis was undertaken in order to examine the structural consequences of the sequence changes and the consequent family affiliation. The CBM3 crystallized in space group I4(1)22 with one molecule in the asymmetric unit, yielding diffraction to a resolution of 1.83 A using X-ray synchrotron radiation. Compared with the known structures of other scaffoldin-borne CBMs, a sequence insertion and deletion appear to compensate for each other as both contained an aromatic residue that is capable of contributing to cellulose binding; hence, even though there are alterations in the composition and localization of the aromatic residues in the linear strip its binding ability was not compromised. Interestingly, no Ca2+ ions were detected in the conserved calcium-binding site, although the module was properly folded; this suggests that the structural role of Ca2+ is less important than originally supposed. These observations indicate that despite their conserved function the scaffoldin-borne CBMs are more diverse in their sequences and structures than previously assumed.
Understanding the molecular-level mechanisms that enzymes employ to deconstruct plant cell walls is a fundamental scientific challenge with significant ramifications for renewable fuel production from biomass. In nature, bacteria and fungi use enzyme cocktails that include processive and non-processive cellulases and hemicellulases to convert cellulose and hemicellulose to soluble sugars. Catalyzed by an accelerated biofuels R&D portfolio, there is now a wealth of new structural and experimental insights related to cellulases and the structure of plant cell walls. From this background, computational approaches commonly used in other fields are now poised to offer insights complementary to experiments designed to probe mechanisms of plant cell wall deconstruction. Here we outline the current status of computational approaches for a collection of critical problems in cellulose deconstruction. We discuss path sampling methods to measure rates of elementary steps of enzyme action, coarse-grained modeling for understanding macromolecular, cellulosomal complexes, methods to screen for enzyme improvements, and studies of cellulose at the molecular level. Overall, simulation is a complementary tool to understand carbohydrate-active enzymes and plant cell walls, which will enable industrial processes for the production of advanced, renewable fuels.
The complex polysaccharide-degrading marine bacterium Saccharophagus degradans strain 2-40 produces putative proteins that contain numerous cadherin and cadherin-like domains involved in intercellular contact interactions. The current study reveals that both domain types exhibit reversible calcium-dependent binding to different complex polysaccharides which serve as growth substrates for the bacterium.
The increasing numbers of published genomes has enabled extensive survey of protein sequences in nature. During the course of our studies on cellulolytic bacteria that produce multienzyme cellulosome complexes designed for efficient degradation of cellulosic substrates, we have investigated the intermodular cohesin-dockerin interaction, which provides the molecular basis for cellulosome assembly. An early search of the genome databases yielded the surprising existence of a dockerin-like sequence and two cohesin-like sequences in the hyperthermophilic noncellulolytic archaeon, Archaeoglobus fulgidus, which clearly contradicts the cellulosome paradigm. Here, we report a biochemical and biophysical analysis, which revealed particularly strong-and specific-binding interactions between these two cohesins and the single dockerin. The crystal structure of one of the recombinant cohesin modules was determined and found to resemble closely the type-I cohesin structure from the cellulosome of Clostridium thermocellum, with certain distinctive features: two of the loops in the archaeal cohesin structure are shorter than those of the C. thermocellum structure, and a large insertion of 27-amino acid residues, unique to the archaeal cohesin, appears to be largely disordered. Interestingly, the cohesin module undergoes reversible dimer and tetramer formation in solution, a property, which has not been observed previously for other cohesins. This is the first description of cohesin and dockerin interactions in a noncellulolytic archaeon and the first structure of an archaeal cohesin. This finding supports the notion that interactions based on the cohesin-dockerin paradigm are of more general occurrence and are not unique to the cellulosome system. Proteins 2011; 79: 50-60. (C) 2010 Wiley-Liss, Inc.
Cellulose, a major component of plant matter, is degraded by a cell surface multiprotein complex called the cellulosome produced by several anaerobic bacteria. This complex coordinates the assembly of different glycoside hydrolases, via a high-affinity Ca(2+)-dependent interaction between the enzyme-borne dockerin and the scaffoldin-borne cohesin modules. In this study, we characterized a new protein affinity tag, Delta Doc, a truncated version (48 residues) of the Clostridium thermocellum CeI48S dockerin. The truncated dockerin tag has a binding affinity (K(A)) of 7.7 x 10(8) M(-1), calculated by a competitive enzyme-linked assay system. In order to examine whether the tag can be used for general application in affinity chromatography, it was fused to a range of target proteins, including Aequorea victoria green fluorescent protein (GFP), C. thermocellum beta-glucosidase, Escherichia toll thioesterase/protease I (TEP1), and the antibody-binding ZZ-domain from Staphylococcus aureus protein A. The results of this study significantly extend initial studies performed using the Geobacillus stearothermophilus xylanase T-6 as a model system. In addition, the enzymatic activity of a C. thermocellum beta-glucosidase, purified using this approach, was tested and found to be similar to that of a beta-glucosidase preparation (without the Delta Doc tag) purified using the standard His-tag. The truncated dockerin derivative functioned as an effective affinity tag through specific interaction with a cognate cohesin, and highly purified target proteins were obtained in a single step directly from crude cell extracts. The relatively inexpensive beaded cellulose-based affinity column was reusable and maintained high capacity after each cycle. This study demonstrates that deletion into the first Ca(2+)-binding loop of the dockerin module results in an efficient and robust affinity tag that can be generally applied for protein purification. Copyright (C) 2010 John Wiley & Sons, Ltd
Designer cellulosomes are precision-engineered multienzyme complexes in which the molecular architecture and enzyme content are exquisitely controlled. This system was used to examine enzyme cooperation for improved synergy among Thermobifida fusca glycoside hydrolases. Two T. fusca cellulases, Cel48A exoglucanase and Cel5A endoglucanase, and two T. fusca xylanases, endoxylanases Xyn10B and Xyn11A, were selected as enzymatic components of a mixed cellulase/xylanase-containing designer cellulosome. The resultant mixed multienzyme complex was fabricated on a single scaffoldin subunit bearing all four enzymes. Conversion of T. fusca enzymes to the cellulosomal mode followed by their subsequent incorporation into a tetravalent cellulosome led to assemblies with enhanced activity (similar to 2.4-fold) on wheat straw as a complex cellulosic substrate. The enhanced synergy was caused by the proximity of the enzymes on the complex compared to the free-enzyme systems. The hydrolytic properties of the tetravalent designer cellulosome were compared with the combined action of two separate divalent cellulase-and xylanase-containing cellulosomes. Significantly, the tetravalent designer cellulosome system exhibited an similar to 2-fold enhancement in enzymatic activity compared to the activity of the mixture of two distinct divalent scaffoldin-borne enzymes. These results provide additional evidence that close proximity between cellulases and xylanases is key to the observed concerted degradation of the complex cellulosic substrate in which the integrated enzymes complement each other by promoting access to the relevant polysaccharide components of the substrate. The data demonstrate that cooperation among xylanases and cellulases can be augmented by their integration into a single designer cellulosome. IMPORTANCE Global efforts towards alternative energy programs are highlighted by processes for converting plant-derived carbohydrates to biofuels. The major barrier in such pro
Clostridium thermocellum produces a highly efficient cellulolytic extracellular complex, termed the cellulosome, for hydrolyzing plant cell wall biomass. The composition of the cellulosome is affected by the presence of extracellular polysaccharides; however, the regulatory mechanism is unknown. Recently, we have identified in C. thermocellum a set of putative sigma and anti-sigma s factors that include extracellular polysaccharide-sensing components [Kahel-Raifer et al. (2010) FEMS Microbiol Lett 308:84-93]. These factor-encoding genes are homologous to the Bacillus subtilis bicistronic operon sigI-rsgI, which encodes for an alternative s I factor and its cognate anti-sigma(1) regulator RsgI that is functionally regulated by an extracytoplasmic signal. In this study, the binding of C. thermocellum putative anti-s I factors to their corresponding s factors was measured, demonstrating binding specificity and dissociation constants in the range of 0.02 to 1 mu M. Quantitative real-time RT-PCR measurements revealed three-to 30-fold up-expression of the alternative s factor genes in the presence of cellulose and xylan, thus connecting their expression to direct detection of their extracellular polysaccharide substrates. Cellulosomal genes that are putatively regulated by two of these s factors, sigma(I1) or sigma(I6), were identified based on the sequence similarity of their promoters. The ability of sigma(I1) to direct transcription from the sigI1 promoter and from the promoter of celS (encodes the family 48 cellulase) was demonstrated in vitro by runoff transcription assays. Taken together, the results reveal a regulatory mechanism in which alternative s factors are involved in regulating the cellulosomal genes via an external carbohydrate-sensing mechanism.
The thermostability of endoglucanase (E.C. 184.108.40.206) Cel8A, a major component of the cellulosome complex from Clostridium thermocellum, was significantly enhanced using a directed evolution strategy. To ensure that thermostability would not compromise enzyme activity, a two-step screening strategy was employed that involved consecutive activity and thermostability assays. We have combined three of the mutations from the thermostability screen to obtain a Cel8A variant with a significant increase in thermal resistance without substantial alteration of kinetic parameters. One of the three mutations (S329G) provided the highest contribution to enzyme stability. This single mutation served to increase the T(m) by 7.0 degrees C and the half-life of activity by eight fold at 85 degrees C. Site-saturation mutagenesis at position 329 revealed that only the glycine residue could confer thermostability. The structural changes responsible for the properties of the mutant enzymes are discussed.
Background: The cellulosome is a multi-enzyme machine, which plays a key role in the breakdown of plant cell walls in many anaerobic cellulose-degrading microorganisms. Ruminococcus flavefaciens FD-1, a major fiber-degrading bacterium present in the gut of herbivores, has the most intricate cellulosomal organization thus far described. Cellulosome complexes are assembled through high-affinity cohesin-dockerin interactions. More than two-hundred dockerin-containing proteins have been identified in the R. flavefaciens genome, yet the reason for the expansion of these crucial cellulosomal components is yet unknown. Methodology/Principal Findings: We have explored the full spectrum of 222 dockerin-containing proteins potentially involved in the assembly of cellulosome-like complexes of R. flavefaciens. Bioinformatic analysis of the various dockerin modules showed distinctive conservation patterns within their two Ca2+-binding repeats and their flanking regions. Thus, we established the conceptual framework for six major groups of dockerin types, according to their unique sequence features. Within this framework, the modular architecture of the parent proteins, some of which are multi-functional proteins, was evaluated together with their gene expression levels. Specific dockerin types were found to be associated with selected groups of functional components, such as carbohydrate-binding modules, numerous peptidases, and/or carbohydrate-active enzymes. In addition, members of other dockerin groups were linked to structural proteins, e. g., cohesin-containing proteins, belonging to the scaffoldins. Conclusions/Significance: This report profiles the abundance and sequence diversity of the R. flavefaciens FD-1 dockerins, and provides the molecular basis for future understanding of the potential for a wide array of cohesin-dockerin specificities. Conserved differences between dockerins may be reflected in their stability, function or expression within the context of the p
Genome analysis of the Gram-positive cellulolytic bacterium Clostridium thermocellum revealed the presence of multiple negative regulators of alternative Sigma factors. Nine of the deduced proteins share a strong similarity in their N-terminal sequences to the Bacillus subtilis membrane-associated anti-Sigma I factor RsgI and have an unusual domain organization. In six RsgI-like proteins, the C-terminal sequences contain predicted carbohydrate-binding modules. Three of these modules were overexpressed and shown to bind specifically to cellulose and/or pectin. Bioinformatic analysis of > 1200 bacterial genomes revealed that the C. thermocellum RsgI-like proteins are unique to this species and are not present in other cellulolytic clostridial species (e.g. Clostridium cellulolyticum and Clostridium papyrosolvens). Eight of the nine genes encoding putative C. thermocellum RsgI-like anti-Sigma factors form predicted bicistronic operons, in which the first gene encodes a putative alternative Sigma factor, similar to B. subtilis Sigma I, but lacking in one of its domains. These observations suggest a novel carbohydrate-sensing mechanism in C. thermocellum, whereby the presence of polysaccharide biomass components is detected extracellularly and the signal is transmitted intracellularly, resulting in the disruption of the interaction between RsgI-like proteins and Sigma I-like factors, the latter of which serve to activate appropriate genes encoding proteins involved in cellulose utilization.
Protein molecular scaffolds are attracting interest as natural candidates for the presentation of enzymes and acceleration of catalytic reactions. We have previously reported evidence that the stable protein 1 (SP1) from Populus tremula can be employed as a molecular scaffold for the presentation of either catalytic or structural binding (cellulosomal cohesin) modules. In the present work, we have displayed a potent exoglucanase (Cel6B) from the aerobic cellulolytic bacterium, Thermobifida jusca, on a cohesin-bearing SP1 scaffold. For this purpose, a chimaeric form of the enzyme, fused to a cellulosomal dockerin module, was prepared. Full incorporation of 12 dockerin-bearing exoglucanase molecules onto the cohesinbearing scaffold was achieved. Cellulase activity was tested on two cellulosic substrates with different levels of crystallinity, and the activity of the scaffold-linked exoglucanase was significantly reduced, compared to the free dockerin-containing enzyme. However, addition of relatively low concentrations of a free wild-type endoglucanase (T. jusca Cel5A) that bears a cellulose-binding module, in combination with the complexed exoglucanase resulted in a marked rise in activity on both cellulosic substrates. The endoglucanase cleaves internal sites of the cellulose chains, and the new chain ends of the substrate were now readily accessible to the scaffold-borne exoglucanase, thereby resulting in highly effective, synergistic degradation of cellulosic substrates. (C) 2010 Elsevier B.V. All rights reserved.
The cellulosome complex is composed of a conglomerate of subunits, each of which comprises a set of interacting functional modules. Scaffoldin (Sca), a major cellulosomal subunit, is responsible for organizing the cellulolytic subunits into the complex. This is accomplished by the interaction of two complementary classes of modules-a cohesin (Coh) module on the Sca subunit and a dockerin module on each of the enzymatic subunits. Although individual Coh modules from different cellulosomal scaffoldins have been subjected to intensive structural investigation, the Sca subunit in its entirety has not, and there remains a paucity of information on the arrangement and interactions of Cohs within the Sca subunit. In the present work, we describe the crystal structure of a type II Coh dyad from the ScaB "adaptor" Sca of Acetivibrio cellulolyticus. The ScaB Cohs are oriented in an "antiparallel" manner relative to one another, with their dockerin-interacting surfaces (beta-strands 8-3-6-5) facing the same direction-aligned on the same plane. A set of extensive hydrophobic and hydrogen-bond contacts between the Cohs and the short interconnecting linker segment between them stabilizes the modular orientation. This Coh dyad structure provides novel information about Coh-Coh association and arrangement in the Sca and further insight into intermodular linker interactions. Putative structural arrangements of a hexamodular complex, composed of the Coh dyad bound to two X-dockerin modules, were suggested. (C) 2010 Published by Elsevier Ltd.
Conversion of components of the Thermobifida fusca free-enzyme system to the cellulosomal mode using the designer cellulosome approach can be employed to discover the properties and inherent advantages of the cellulosome system. In this article, we describe the conversion of the T. fusca xylanases Xyn11A and Xyn10B and their synergistic interaction in the free state or within designer cellulosome complexes in order to enhance specific degradation of hatched wheat straw as a model for a complex cellulosic substrate. Endoglucanase Cel5A from the same bacterium and its recombinant dockerin-containing chimera were also studied for their combined effect, together with the xylanases, on straw degradation. Synergism was demonstrated when Xyn11A was combined with Xyn10B and/or Cel5A, and similar to 1.5-fold activity enhancements were achieved by the designer cellulosome complexes compared to the free wild-type enzymes. These improvements in activity were due to both substrate-targeting and proximity effects among the enzymes contained in the designer cellulosome complexes. The intrinsic cellulose/xylan-binding module (XBM) of Xyn11A appeared to be essential for efficient substrate degradation. Indeed, only designer cellulosomes in which the XBM was maintained as a component of Xyn11A achieved marked enhancement in activity compared to the combination of wild-type enzymes. Moreover, integration of the XBM in designer cellulosomes via a dockerin module (separate from the Xyn11A catalytic module) failed to enhance activity, suggesting a role in orienting the parent xylanase toward its preferred polysaccharide component of the complex wheat straw substrate. The results provide novel mechanistic insight into the synergistic activity of designer cellulosome components on natural plant cell wall substrates.
The anaerobic, thermophilic cellulolytic bacterium Clostridium thermocellum is known for its elaborate cellulosome complex, but it also produces a separate free cellulase system. Among the free enzymes, the noncellulosomal enzyme Cel9I is a processive endoglucanase whose sequence and architecture are very similar to those of the cellulosomal enzyme Cel9R; likewise, the noncellulosomal exoglucanase Cel48Y is analogous to the principal cellulosomal enzyme Cel48S. In this study we used the designer cellulosome approach to examine the interplay of prominent cellulosomal and noncellulosomal cellulases from C. thermocellum. Toward this end, we converted the cellulosomal enzymes to noncellulosomal chimeras by swapping the dockerin module of the cellulosomal enzymes with a carbohydrate-binding module from the free enzyme analogues and vice versa. This enabled us to study the importance of the targeting effect of the free enzymes due to their carbohydrate-binding module and the proximity effect for cellulases on the designer cellulosome. C. thermocellum is the only cellulosome-producing bacterium known to express two different glycoside hydrolase family 48 enzymes and thus the only bacterial system that can currently be used for such studies. The different activities with crystalline cellulose were examined, and the results demonstrated that the individual chimeric cellulases were essentially equivalent to the corresponding wild-type analogues. The wild-type cellulases displayed a synergism of about 1.5-fold; the cellulosomal pair acted synergistically when they were converted into free enzymes, whereas the free enzymes acted synergistically mainly in the wild-type state. The targeting effect was found to be the major factor responsible for the elevated activity observed for these specific enzyme combinations, whereas the proximity effect appeared to play a negligible role.
Cellulosomes are large, multienzyme, plant cell wall-degrading protein complexes found affixed to the surface of a variety of anaerobic microbes. The core of the cellulosome is a noncatalytic scaffoldin protein, which contains several type-I cohesin modules that bind type-I dockerin-containing enzymatic subunits, a cellulose-binding module, an X module, and a type-II dockerin that interacts with type-II cohesin-containing cell surface proteins. The unique arrangement of the enzymatic subunits in the cellulosome complex, made possible by the scaffoldin subunit, promotes enhanced substrate degradation relative to the enzymes free in solution. Despite representative high-resolution structures of all of the individual modules of the cellulosome, this mechanism of enzymatic synergy remains poorly understood. Consequently, a model of the entire cellulosome and a detailed picture of intermodular contacts will provide more detailed insight into cellulosome activity. Toward this goal, we have solved the structure of a multimodular heterodimeric complex from Clostridium thermocellum composed of the type-II cohesin module of the cell surface protein SdbA bound to a trimodular C-terminal fragment of the scaffoldin subunit CipA to a resolution of 1.95 angstrom. The linker that connects the ninth type-I cohesin module and the X module has elevated temperature factors, reflecting an inherent flexibility within this region. Interestingly, a novel dimer interface was observed between CipA and a second, symmetry-related CipA molecule within the crystal structure, mediated by contacts between a type-I cohesin and an X module of a symmetry mate, resulting in two intertwined scaffoldins. Sedimentation velocity experiments confirmed that dimerization also occurs in solution. These observations support the intriguing possibility that individual cellulosomes can associate with one another via inter-scaffoldin interactions, which may play a role in the mechanism of action of the complex.
Family 3 carbohydrate-binding modules (CBM3s) are associated with both cellulosomal scaffoldins and family 9 glycoside hydrolases (GH9s), which are multi-modular enzymes that act on cellulosic substrates. CBM3s bind cellulose. X-ray crystal structures of these modules have established an accepted cellulose-binding mechanism based on stacking interactions between the sugar rings of cellulose and a planar array of aromatic residues located on the CBM3 surface. These planar-strip residues are generally highly conserved, although some CBM3 sequences lack one or more of these residues. In particular, CBM3b' from Clostridium thermocellum Cel9V exhibits such sequence changes and fails to bind cellulosic substrates. A crystallographic investigation of CBM3b' has been initiated in order to understand the structural reason(s) for this inability. CBM3b' crystallized in space group C222(1) (diffraction was obtained to 2.0 angstrom resolution in-house) with three independent molecules in the asymmetric unit and in space group P4(1)2(1)2 (diffraction was obtained to 1.79 angstrom resolution in-house and to 1.30 angstrom resolution at a synchrotron) with one molecule in the asymmetric unit. The molecular structure of Cel9V CBM3b' revealed that in addition to the loss of several cellulose-binding residues in the planar strip, changes in the backbone create a surface 'hump' which could interfere with the formation of cellulose-protein surface interactions and thus prevent binding to crystalline cellulose.
The high-affinity cohesin-dockerin interaction dictates the suprastructural assembly of the multienzyme cellulosome Complex. The interaction between these two complementary families 4 protein modules was found to be species-specific and type-dependent. The structure of the type II cohesin module possesses additional intrinsic secondary structural elements absent in the type I cohesin, i.e., an alpha-helix and two singular "beta-flaps". The role of these extra secondary structures in dockerin recognition was Studied in this work using gene swapping, in which corresponding homologous stretches of types I and II cohesins were interchanged. The specificity of binding of the resultant chimaeric cohesins was determined by enzyme-linked affinity assay. Several chimaeric cohesins retained dockerin recognition properties. Hence, these cohesins may undergo manipulations (insertion/deletion of peptide segments) without altering their affinity toward their counterpart dockerin, although type-dependent binding specificity cannot be converted by swapping of the additional secondary structural elements between the two cohesin types. The results further emphasize the strong affinity and plasticity between the cohesin and dockerin pair and are consistent with the known findings on specificity of the types I and II interactions. These Studies provide insight into the structural and functional resilience of the cohesins and thus have direct bearing on their potential use in biotechnological applications.
We have been developing the cellulases of Thermobifida fusca as a model to explore the conversion from a free cellulase system to the cellulosomal mode. Three of the six T. fusca cellulases (endoglucanase Cel6A and exoglucanases Cel6B and Cel48A) have been converted in previous work by replacing their cellulose-binding modules (CBMs) with a dockerin, and the resultant recombinant "cellulosomized" enzymes were incorporated into chimeric scaffolding proteins that contained cohesin(s) together with a CBM. The activities of the resultant designer cellulosomes were compared with an equivalent mixture of wild-type enzymes. In the present work, a fourth T. fusca cellulase, Cel5A, was equipped with a dockerin and intervening linker segments of different lengths to assess their contribution to the overall activity of simple one-and two-enzyme designer cellulosome complexes. The results demonstrated that cellulose binding played a major role in the degradation of crystalline cellulosic substrates. The combination of the converted Cel5A endoglucanase with the converted Cel48A exoglucanase also exhibited a measurable proximity effect for the most recalcitrant cellulosic substrate (Avicel). The length of the linker between the catalytic module and the dockerin had little, if any, effect on the activity. However, positioning of the dockerin on the opposite (C-terminal) side of the enzyme, consistent with the usual position of dockerins on most cellulosomal enzymes, resulted in an enhanced synergistic response. These results promote the development of more complex multienzyme designer cellulosomes, which may eventually be applied for improved degradation of plant cell wall biomass.
The cellulosome is an intriguing multienzyme complex found in cellulolytic bacteria that plays a key role in the degradation of plant cell-wall polysaccharides. In Ruminococcus flavefaciens, a predominant fiber-degrading bacterium found in ruminants, the cellulosome is anchored to the bacterial cell wall through a relatively short ScaE scaffoldin. Determination of the crystal structure of the lone type-III ScaE cohesin from R. flavefaciens (Rf-CohE) was initiated as a part of a structural effort to define cellulosome assembly. The structure was determined at 1.95 angstrom resolution by single-wavelength anomalous diffraction. This is the first detailed description of a crystal structure for a type-III cohesin, and its features were compared with those of the known type-I and type-II cohesin structures. The Rf-CohE module folds into a nine-stranded beta-sandwich with jellyroll topology, typically observed for cohesins, and includes two beta-flaps in the midst of beta-strands 4 and 8, similar to the type-II cohesin structures. However, the presence in Rf-CohE of an additional 13-residue alpha-helix located between beta-strands 8 and 9 represents a dramatic divergence from other known cohesin structures. The prominent alpha-helix is enveloped by an extensive N-terminal loop, not observed in any other known cohesin, which embraces the helix presumably enhancing its stability. A planar surface at the upper portion of the front face of the molecule, bordered by beta-flap 8, exhibits plausible dimensions and exposed amino acid residues to accommodate the dockerin-binding site. Proteins 2009; 77:699-709. (C) 2009 Wiley-Liss, Inc.
Editorial introduction to the thematic series on the life and lifework of Mary Mandels.
Background: Ruminococcus flavefaciens is a predominant cellulolytic rumen bacterium, which forms a multi-enzyme cellulosome complex that could play an integral role in the ability of this bacterium to degrade plant cell wall polysaccharides. Identifying the major enzyme types involved in plant cell wall degradation is essential for gaining a better understanding of the cellulolytic capabilities of this organism as well as highlighting potential enzymes for application in improvement of livestock nutrition and for conversion of cellulosic biomass to liquid fuels. Methodology/Principal Findings: The R. flavefaciens FD-1 genome was sequenced to 29x-coverage, based on pulsed-field gel electrophoresis estimates (4.4 Mb), and assembled into 119 contigs providing 4,576,399 bp of unique sequence. As much as 87.1% of the genome encodes ORFs, tRNA, rRNAs, or repeats. The GC content was calculated at 45%. A total of 4,339 ORFs was detected with an average gene length of 918 bp. The cellulosome model for R. flavefaciens was further refined by sequence analysis, with at least 225 dockerin-containing ORFs, including previously characterized cohesin-containing scaffoldin molecules. These dockerin-containing ORFs encode a variety of catalytic modules including glycoside hydrolases (GHs), polysaccharide lyases, and carbohydrate esterases. Additionally, 56 ORFs encode proteins that contain carbohydrate-binding modules (CBMs). Functional microarray analysis of the genome revealed that 56 of the cellulosome-associated ORFs were up-regulated, 14 were down-regulated, 135 were unaffected, when R. flavefaciens FD-1 was grown on cellulose versus cellobiose. Three multi-modular xylanases (ORF01222, ORF03896, and ORF01315) exhibited the highest levels of up-regulation. Conclusions/Significance: The genomic evidence indicates that R. flavefaciens FD-1 has the largest known number of fiber-degrading enzymes likely to be arranged in a cellulosome architecture. Functional analysis of the genom
Cellulosome complexes comprise an intercalated set of multimodular dockerin-containing enzymatic subunits connected to cohesin-containing nonenzymatic subunits called scaffoldins. The adjoining modules in each cellulosomal subunit are interconnected by a variety of linker segments of different lengths and composition. The exact role of the cellulosomal linkers has yet to be described, although it is assumed that they contribute to the architecture and action of the cellulosome by providing the protein subunits with flexibility and by providing spacers between the enzymatic modules that could enhance interactions with the cellulose substrate. Here we present four crystal structures of Acetivibrio cellulolyticus cellulosomal type II cohesins with linker extensions. Two of the structures represent two different crystal forms (trigonal and orthorhombic) of the same N-terminal cohesin module (CohB1) together with its full (6-residue) native C-terminal linker, derived from scaffoldin B. The other two structures belong to the adjacent (second) cohesin module (CohB2), each of which was crystallized with the same 6-residue linker segment, but now positioned at the N-terminus and with either a truncated (5-residue) or a full-length (45-residue) C-terminal linker, respectively. Comparison between the two CohB1 structures revealed significant differences in the conformation of their equivalent C-terminal linker segment. In one crystal form a helical conformation was observed, as opposed to an extended conformation in the other. The CohB2 structures also displayed diverse conformations in their linker segments. In these structures, different linker conformations were observed in the individual molecules within the asymmetric unit of each structure. This conformational diversity implies that the linkers may adopt alternative conformations in their natural environment, consistent with varying environmental conditions. The findings suggest that linkers can play an important role
Clostridium thermocellum cellulase 9I (Cel9I) is a non-cellulosomal tri-modular enzyme, consisting of a family-9 glycoside hydrolase (GH9) catalytic module and two family-3 carbohydrate-binding modules (CBM3c and CBM3b). The presence of CBM3c was previously shown to be essential for activity, however the mechanism by which it functions is unclear. We expressed the three recombinant modules independently in Escherichia coli and examined their interactions. Non-denaturing gel electrophoresis, isothermal titration calorimetry, and affinity purification of the GH9-CBM3c complex revealed a specific non-covalent binding interaction between the GH9 module and CBM3c. Their physical association was shown to recover 60-70% of the intact Cel9I endoglucanase activity. (C) 2009 Federation of European Biochemical Societies. Published by Elsevier B. V. All rights reserved.
Efficient degradation of cellulose by the anaerobic thermophilic bacterium, Clostridium thermocellum, is carried out by the mufti-enzyme cellulosome complex. The enzymes on the complex are attached in a calcium-dependent manner via their dockerin (Doc) module to a cohesin (Coh) module of the cellulosomal scaffoldin subunit. In this study, we have optimized the Coh-Doc interaction for the purpose of protein affinity purification. A C. thermocellum Coh module was thus fused to a carbohydrate-binding module, and the resultant fusion protein was applied directly onto beaded cellulose, thereby serving as a non-covalent "activation" procedure. A complementary Doc module was then fused to a model protein target: xylanase T-6 from Geobacillus stearothermophilus. However, the binding to the immobilized Coh was only partially reversible upon treatment with EDTA, and only negligible amounts of the target protein were eluted from the affinity column. In order to improve protein elution, a series of truncated Docs were designed in which the calcium-coordinating function was impaired without appreciably affecting high-affinity binding to Coh. A shortened Doc of only 48 residues was sufficient to function as an effective affinity tag, and highly purified target protein was achieved directly from crude cell extracts in a single step with near-quantitative recovery of the target protein. Effective EDTA-mediated elution of the sequestered protein from the column was the key step of the procedure. The affinity column was reusable and maintained very high levels of capacity upon repeated rounds of loading and elution. Reusable Coh-Doc affinity columns thus provide an efficient and attractive approach for purifying proteins in high yield by modifying the calcium-binding loop of the Doc module. Copyright (c) 2008 John Wiley & Sons, Ltd.
A cohesin-like module of 160 amino-acid residues from the hypothetical protein AF2375 of the noncellulolytic, hyperthermophilic, sulfate-reducing archaeon Archaeoglobus fulgidus was cloned, expressed, purified, crystallized and subjected to X-ray structural study in order to compare its structure with those of cellulolytic cohesins. The crystals had cubic symmetry, with unit-cell parameters a = b = c = 101.75 angstrom in space group P4(3)32, and diffracted to 1.82 angstrom resolution. The asymmetric unit contained a single cohesin molecule. A model assembled from six cohesin structures (PDB entries 1anu, 1aoh, 1g1k, 1qzn, 1zv9 and 1tyj) of very low sequence identity to the cohesin-like module was used in molecular-replacement attempts, producing a marginal solution.
Rhizavidin, from the proteobacterium Rhizobium etli, exhibits high affinity towards biotin but maintains an inherent dimeric quaternary structure and thus, differs from all other known tetrameric avidins. Rhizavidin also differs from the other avidins, since it lacks the characteristic tryptophan residue positioned in the L7,8 loop that plays a crucial role in high-affinity binding and oligomeric stability of the tetrameric avidins. The question is, therefore, how does the dimer exist and how is the high biotin-binding affinity retained? For this purpose, the crystal structures of apo- and biotin-complexed rhizavidin were determined. The structures reveal that the rhizavidin monomer exhibits a topology similar to those of other members of the avidin family, that is, eight antiparallel beta-strands that form the conventional avidin beta-barrel. The quaternary structure comprises the sandwich-like dinner, in which the extensive 1-4 intermonomer interface is intact, but the 1-2 and 1-3 interfaces are nonexistent. Consequently, the biotin-binding site is partially accessible, due to the lack of the tryptophan "lid" that distinguishes the tetrameric structures. In rhizavidin, a disulfide bridge connecting the L3,4 and L5,6 loops restrains the L3,4 loop conformation, leaving the binding-site residues essentially unchanged upon biotin binding. Our study suggests that in addition to the characteristic hydrogen bonding and hydrophobic interactions, the preformed architecture of the binding site and consequent shape complementarity play a decisive role in the high-affinity biotin binding of rhizavidin. The structural description of a novel dimeric avidin-like molecule will greatly contribute to the design of improved and unique avidin derivatives for diversifying the capabilities of avidin-biotin technology. (C) 2008 Elsevier Ltd. All rights reserved.
The high-affinity cohesin-dockerin interaction was originally discovered as modular components, which mediate the assembly of the various subunits of the multienzyme cellulosome complex that characterizes some cellulolytic bacteria. Until recently, the presence of cohesins and dockerins within a bacterial proteome was considered a definitive signature of a cellulosome-producing bacterium. Widespread genome sequencing has since revealed a wealth of putative cohesin- and dockerin-containing proteins in Bacteria, Archaea, and in primitive eukaryotes. The newly identified modules appear to serve diverse functions that are clearly distinct from the classical cellulosome archetype, and the vast majority of parent proteins are not predicted glycoside hydrolases. In most cases, only a few such genes have been identified in a given microorganism, which encode proteins containing but a single cohesin and/or dockerin. In some cases, one or the other module appears to be missing from a given species, and in other cases both modules occur within the same protein. This review provides a bioinformatics-based survey of the current status of cohesin- and dockerin-like sequences in species from the Bacteria, Archaea, and Eukarya. Surprisingly, many identified modules and their parent proteins are clearly unrelated to cellulosomes. The cellulosome paradigm may thus be the exception rather than the rule for bacterial, archaeal, and eukaryotic employment of cohesin and dockerin modules.
Clostridium thermocellum is an anaerobic thermophilic bacterium that grows efficiently on cellulosic biomass. This bacterium produces and secretes a highly active multienzyme complex, the cellulosome, that mediates the cell attachment to and hydrolysis of the crystalline cellulosic substrate. C. thermocellum can efficiently utilize only beta-1,3 and beta-1,4 glucans and prefers long cellodextrins. Since the bacterium can also produce ethanol, it is considered an attractive candidate for a consolidated fermentation process in which cellulose hydrolysis and ethanol fermentation occur in a single process. In this study, we have identified and characterized five sugar ABC transporter systems in C. thermocellum. The putative transporters were identified by sequence homology of the putative solute-binding lipoprotein to known sugar-binding proteins. Each of these systems is transcribed from a gene cluster, which includes an extracellular solute-binding protein, one or two integral membrane proteins, and, in most cases, an ATP-binding protein. The genes of the five solute-binding proteins were cloned, fused to His tags, overexpressed, and purified, and their abilities to interact with different sugars was examined by isothermal titration calorimetry. Three of the sugar-binding lipoproteins (CbpB to -D) interacted with different lengths of cellodextrins (G(2) to G(5)), with disassociation constants in the micromolar range. One protein, CbpA, binds only cellotriose (G(3)), while another protein, Lbp (laminaribiose-binding protein) interacts with laminaribiose. The sugar specificity of the different binding lipoproteins is consistent with the observed substrate preference of C. thermocellum, in which cellodextrins (G(3) to G(5)) are assimilated faster than cellobiose.
The rumen bacterium Ruminococcus albus binds to and degrades crystalline cellulosic substrates via a unique cellulose degradation system. A unique family of carbohydrate-binding modules (CBM37), located at the C terminus of different glycoside hydrolases, appears to be responsible both for anchoring these enzymes to the bacterial cell surface and for substrate binding.
The genome of the opportunistic pathogen Clostridium perfringens encodes a large number of secreted glycoside hydrolases. Their predicted activities indicate that they are involved in the breakdown of complex carbohydrates and other glycans found in the mucosal layer of the human gastrointestinal tract, within the extracellular matrix, and on the surface of host cells. One such group of these enzymes is the family 84 glycoside hydrolases, which has predicted hyaluronidase activity and comprises five members [C. perfringens glycoside hydrolase family 84 (CpGH84) A-E]. The first identified member, CpGH84A, corresponds to the mu-toxin whose modular architecture includes an N-terminal catalytic domain, four family 32 carbohydrate-binding modules, three FIVAR modules of unknown function, and a C-terminal putative calcium-binding module. Here, we report the solution NMR structure of the C-terminal modular pair from the mu-toxin. The three-helix bundle FIVAR module displays structural homology to a heparin-binding module within the N-terminal of the a C protein from group B Streptoccocus. The C-terminal module has a typical calcium-binding dockerin fold comprising two anti-parallel helices that form a planar face with EF-hand calcium-binding loops at opposite ends of the module. The size of the helical face of the mu-toxin dockerin module is approximately equal to the planar region recently identified on the surface of a cohesin-like X82 module of CpGH84C. Size-exclusion chromatography and heteronuclear NMR-based chemical shift mapping studies indicate that the helical face of the dockerin module recognizes the CpGH84C X82 module. These studies represent the structural characterization of a noncellulolytic dockerin module and its interaction with a cohesin-like X82 module. Dockerin/X82-mediated enzyme complexes may have important implications in the pathogenic properties of C. perfringens. (c) 2008 Elsevier Ltd. All rights reserved.
Ruminococcus flavefaciens is a vital cellulosome-producing fibrolytic rumen bacterium. The arrangement of the cellulosomal scaffoldin gene cluster (scaC-scaA-scaB-cttA-scaE) is conserved in two R. flavefaciens strains (17 and FD-1). Sequence analysis revealed a high mosaic conservation of the intergenic regions in the two strains that contrasted sharply with the divergence of the structural sca gene sequences. Based on the conserved intergenic regions, we designed PCR primers in order to examine the sca gene cluster in additional R. flavefaciens strains (C94, B34b, C1a and JM1). Using these conserved and/or degenerate primers, the scaC, scaA and scaB genes were amplified in all six strains, while the entire sca gene cluster and the proximal genes cttA and scaE were successfully amplified in four of the strains (17, FD-1, C94 and JM1). The sequencing of scaA and scaC genes in all the strains yielded additional insight into the variability of the structural genes with regard to the number and type of cohesin modules contained in a conserved molecular skeleton. Moreover, the scaC gene, being short and variable, appears to be a promising functional phylotyping target for metagenomic population studies of R. flavefaciens in the rumen as a function of the individual host animal.
The virulent properties of the common human and livestock pathogen Clostridium perfringens are attributable to a formidable battery of toxins. Among these are a number of large and highly modular carbohydrate-active enzymes, including the mu-toxin and sialidases, whose catalytic properties are consistent with degradation of the mucosal layer of the human gut, glycosaminoglycans, and other cellular glycans found throughout the body. The conservation of noncatalytic ancillary modules among these enzymes suggests they make significant contributions to the overall functionality of the toxins. Here, we describe the structural basis of an ultra-tight interaction (K-a = 1.44 x 10(11) M-1) between the X82 and dockerin modules, which are found throughout numerous C perfringens carbohydrate-active enzymes. Extensive hydrogen-bonding and van der Waals contacts between the X82 and dockerin modules give rise to the observed high affinity. The mu-toxin dockerin module in this complex is positioned approximate to 180 degrees relative to the orientation of the dockerin modules on the cohesin module surface within cellulolytic complexes. These observations represent a unique property of these clostridial toxins whereby they can associate into large, noncovalent multitoxin complexes that allow potentiation of the activities of the individual toxins by combining complementary toxin specificities.
Cellulosomes are multi-enzyme complexes produced by certain anaerobic bacteria that exhibit efficient degradation of plant cell wall polysaccharides. To understand their enhanced levels of hydrolysis, we are investigating the effects of converting a free-cellulase system into a cellulosomal one. To achieve this end, we are replacing the cell ulose-binding module of the native cellulases, produced by the aerobic bacterium Thermobifida fusca, with a cellulosome-derived dockerin module of established specificity, to allow their incorporation into defined "designer cellulosomes". In this communication, we have attached divergent dockerins to the two exoglucanases produced by T. fusca exoglucanase, Cel6B and Cel48A. The resultant fusion proteins were shown to bind efficiently and specifically to their matching cohesins, and their activities on several different cellulose substrates were compared. The lack of a cellulose-binding module in Cel6B had a deleterious effect on its activity on crystalline substrates. In contrast, the dockerin-hearing family-48 exoglucanase showed increased levels of hydrolytic activity on carboxymethyl cellulose and on both Crystalline Substrates tested, compared to the wild-type enzyme. The marked difference in the response of the two exoglucanases to incorporation into a cellulosome, Suggests that the family-48 cellulase is more appropriate than the family-6 enzyme as a designer cellulosome component. (C) 2008 Elsevier B.V. All rights reserved.
The cellulosome is a multiprotein complex, produced primarily by anaerobic microorganisms, which functions to degrade lignocellulosic materials. An important topic of current debate is whether cellulosomal systems display greater ability to deconstruct complex biomass materials (e.g. plant cell walls) than nonaggregated enzymes, and in so doing would be appropriate for improved, commercial bioconversion processes. To sufficiently understand the complex macromolecular processes between plant cell wall polymers, cellulolytic microbes, and their secreted enzymes, a highly concerted research approach is required. Adaptation of existing biophysical techniques and development of new science tools must be applied to this system. This review focuses on strategies likely to permit improved understanding of the bacterial cellulosome using biophysical approaches, with emphasis on advanced imaging and computational techniques.
The plant cell wall degrading apparatus of anaerobic bacteria includes a large multienzyme complex termed the "cellulosome." The complex assembles through the interaction of enzyme-derived dockerin modules with the multiple cohesin modules of the noncatalytic scaffolding protein. Here we report the crystal structure of the Clostridium cellulolyticum cohesin-dockerin complex in two distinct orientations. The data show that the dockerin displays structural symmetry reflected by the presence of two essentially identical cohesin binding surfaces. In one binding mode, visualized through the A16S/L17T dockerin mutant, the C-terminal helix makes extensive interactions with its cohesin partner. In the other binding mode observed through the A47S/F48T dockerin variant, the dockerin is reoriented by 180 and interacts with the cohesin primarily through the N-terminal helix. Apolar interactions dominate cohesin-dockerin recognition that is centered around a hydrophobic pocket on the surface of the cohesin, formed by Leu-87 and Leu-89, which is occupied, in the two binding modes, by the dockerin residues Phe-19 and Leu-50, respectively. Despite the structural similarity between the C. cellulolyticum and Clostridium thermocellum cohesins and dockerins, there is no cross-specificity between the protein partners from the two organisms. The crystal structure of the C. cellulolyticum complex shows that organism-specific recognition between the protomers is dictated by apolar interactions primarily between only two residues, Leu-17 in the dockerin and the cohesin amino acid Ala-129. The biological significance of the plasticity in dockerin-cohesin recognition, observed here in C. cellulolyticum and reported previously in C. thermocellum, is discussed.
The marine bacterium Saccharophagus degradans strain 2-40 (Sde 2-40) is emerging as a vanguard of a recently discovered group of marine and estuarine bacteria that recycles complex polysaccharides. We report its complete genome sequence, analysis of which identifies an unusually large number of enzymes that degrade >10 complex polysaccharides. Not only is this an extraordinary range of catabolic capability, many of the enzymes exhibit unusual architecture including novel combinations of catalytic and substrate-binding modules. We hypothesize that many of these features are adaptations that facilitate depolymerization of complex polysaccharides in the marine environment. This is the first sequenced genome of a marine bacterium that can degrade plant cell walls, an important component of the carbon cycle that is not well-characterized in the marine environment.
The cellulosome is an intricate multienzyme complex, designed for efficient degradation of plant cell wall polysaccharides, notably cellulose. The supramolecular cellulosome architecture in different bacteria is the consequence of the types and specificities of the interacting cohesin and dockerin modules, borne by the different cellulosomal subunits. In this study, we describe a microarray system for determining cohesin-dockerin specificity, which allows global comparison among the interactions between various members of these two complementary families of interacting protein modules. Matching recombinant fusion proteins were prepared that contained one of the interacting modules: cohesins were joined to an appropriate cellulose-binding module (CBM) and the dockerins were fused to a thermostable xylanase that served to enhance expression and proper folding. The CBM-fused cohesins were immobilized on cellulose-coated glass slides, to which xylanase-fused dockerin samples were applied. Knowledge of the specificity characteristics of native and mutated members of the cohesin and dockerin families provides insight into the architecture of the parent cellulosome and allows selection of suitable cohesin-dockein pairs for biotechnological. and nanotechnological. application. Using this approach, extensive cross-species interaction among type-II cohesins and dockerins is shown for the first time. Selective intraspecies binding of an archaeal dockerin to two complementary cohesins is also demonstrated.
The cellulosome is an intricate multi-enzyme complex, known for its efficient degradation of recalcitrant cellulosic substrates. Its supramolecular architecture is determined by the high-affinity intermodular cohesin-dockerin interaction. The dockerin module comprises a calcium-binding, duplicated 'F-hand' loop-helix motif that bears striking similarity to the EF-hand loop-helix-loop motif of eukaryotic calcium-binding proteins. In the present study, we demonstrate by progressive truncation and alanine scanning of a representative type-I dockerin module from Clostridium thermocellum, that only one of the repeated motifs is critical for high-affinity cohesin binding. The results suggest that the near-symmetry in sequence and structure of the repeated elements of the dockerin is not essential to cohesin binding. The first calcium-binding loop can be deleted entirely, with almost full retention of binding. Likewise, significant deletion of the second repeated segment can be achieved, provided that its calcium-binding loop remains intact. Essentially the same conclusion was verified by systematically mutating the highly conserved residues in the calcium-binding loop. Mutations in one of the calcium-binding loops failed to disrupt cohesin recognition and binding, whereas a single mutation in both loops served to reduce the affinity significantly. The results are mutually compatible with recent crystal structures of the type-I cohesin-dockerin heterodimer, which demonstrate that the dockerin can bind in an equivalent manner to its cohesin counterpart through either its first or second repeated motif. The observed plasticity in cohesin-dockerin binding may facilitate cellulosome assembly in vivo or, alternatively, provide a conformational switch that promotes access of the tethered cellulosomal enzymes to their polysaccharide substrates.
The microbiota of the mammalian intestine depend largely on dietary polysaccharides as energy sources. Most of these polymers are not degradable by the host, but herbivores can derive 70% of their energy intake from microbial breakdown - a classic example of mutualism. Moreover, dietary polysaccharides that reach the human large intestine have a major impact on gut microbial ecology and health. Insight into the molecular mechanisms by which different gut bacteria use polysaccharides is, therefore, of fundamental importance. Genomic analyses of the gut microbiota could revolutionize our understanding of these mechanisms and provide new biotechnological tools for the conversion of polysaccharides, including lignocellulosic biomass, into monosaccharides.
Ruminococcus flavefaciens is an anaerobic bacterium that resides in the gastrointestinal tract of ruminants. It produces a highly organized multi-enzyme cellulosome complex that plays a key role in the degradation of plant cell walls. ScaE is one of the critical structural components of its cellulosome that serves to anchor the complex to the cell wall. The seleno-L-methionine-labelled derivative of the ScaE cohesin module has been cloned, expressed, purified and crystallized. The crystals belong to space group C2, with unit-cell parameters a = 155.6, b = 69.3, c = 93.0 angstrom, beta = 123.4 degrees, and contain four molecules in the asymmetric unit. Diffraction data were phased to 1.95 angstrom using the anomalous signal from the Se atoms.
Cellulosomes are intricate multienzyme systems produced by several cellulolytic bacteria, the first example of which was discovered in the anaerobic thermophilic bacterium, Clostridium thermocellum. Cellulosomes are designed for efficient degradation of plant cell wall polysaccharides, notably cellulose-the most abundant renewable polymer on earth. The component parts of the multicomponent complex are integrated by virtue of a unique family of integrating modules, the cohesins and the dockerins, whose distribution and specificity dictate the overall cellulosome architecture. A full generation of research has elapsed since the original publications that documented the cellulosome concept. In this review, we provide a personal account on the discovery process, while describing how divergent cellulosome systems were identified and investigated, culminating in the collaboration of several labs worldwide to tackle together the challenging field of cellulosome genomics and metagenomics. (c) 2008 The Japan Chemical journal Forum and Wiley Periodicals, Inc. Chem Rec 8: 364-377; 2008: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20160
The genomes of myonecrotic strains of Clostridium perftingens encode a large number of secreted glycoside hydrolases. The activities of these enzymes are consistent with degradation of the mucosal layer of the human gastrointestinal tract, glycosaminoglycans and other cellular glycans found throughout the body. In many cases this is thought to aid in the propagation of the major toxins produced by C. perftingens. One such example is the family 84 glycoside hydrolases, which contains five C. perfringens members (CpGH84A-E), each displaying a unique modular architecture. The smallest and most extensively studied member, CpGH84C, comprises an N-terminal catalytic domain with beta-N-acetylglucosaminidase activity, a family 32 carbohydrate-binding module, a family 82 X-module (X82) of unknown function, and a fibronectin type-III-like module. Here we present the structure of the X82 module from CpGH84C, determined by both NMR spectroscopy and X-ray crystallography. CpGH84C X82 adopts a jell-roll fold comprising two beta-sheets formed by nine beta-strands. CpGH84C X82 displays distant amino acid sequence identity yet close structural similarity to the cohesin modules of cellulolytic anaerobic bacteria. Cohesin modules are responsible for the assembly of numerous hydrolytic enzymes m a cellulose-degrading multi-enzyme complex, termed the cellulosome, through a high-affinity interaction with the calcium-binding dockerin module. A planar surface is located on the face of the CpGH84 X82 structure that corresponds to the dockerin-binding region of cellulolytic cohesin modules and has the approximate dimensions to accommodate a dockerin module. The presence of cohesin-like X82 modules in glycoside hydrolases of C. perfringens is an indication that the formation of novel X82-dockerin mediated multi-enzyme complexes, with potential roles in pathogenesis, is possible. (c) 2007 Elsevier Ltd. All rights reserved.
The second type II cohesin module of the cellulosomal scaffoldin polypeptide ScaB from Acetivibrio cellulolyticus (CohB2) was cloned into two constructs: one containing a short (five-residue) C-terminal linker (CohB2_S) and the second incorporating the full native 45-residue linker (CohB2_L). Both constructs encode proteins that also include the full native six-residue N-terminal linker. The CohB2_S and CohB2_L proteins were expressed, purified and crystallized in the orthorhombic crystal system, but with different unit cells and symmetries: space group P2(1)2(1)2(1) with unit-cell parameters a = 90.36, b = 68.65, c = 111.29 angstrom for CohB2_S and space group P2(1)2(1)2 with unit-cell parameters a = 68.76, b = 159.22, c = 44.21 angstrom for CohB2_L. The crystals diffracted to 2.0 and 2.9 angstrom resolution, respectively. The asymmetric unit of CohB2_S contains three cohesin molecules, while that of CohB2_L contains two molecules.
Family 3 carbohydrate-binding modules (CBM3s) are associated with the scaffoldin subunit of the multi-enzyme cellulosome complex and with the family 9 glycoside hydrolases, which are multimodular enzymes that act on plant cell-wall polysaccharides, notably cellulose. Here, the crystallization of CBM3b from cellobiohydrolase 9A is reported. The crystals are tetragonal and belong to space group P4(1) or P4(3). X-ray diffraction data for CBM3b have been collected to 2.68 angstrom resolution on beamline ID14-4 at the ESRF.
In this study, novel cellulosome chimeras exhibiting atypical geometries and binding modes, wherein the targeting and proximity functions were directly incorporated as integral parts of the enzyme components, were designed. Two pivotal cellulosomal enzymes (family 48 and 9 cellulases) were thus appended with an efficient cellulose-binding module (CBM) and an optional cohesin and/or dockerin. Compared to the parental enzymes, the chimeric cellulases exhibited improved activity on crystalline cellulose as opposed to their reduced activity on amorphous cellulose. Nevertheless, the various complexes assembled using these engineered enzymes were somewhat less active on crystalline cellulose than the conventional designer cellulosomes containing the parental enzymes. The diminished activity appeared to reflect the number of protein-protein interactions within a given complex, which presumably impeded the mobility of their catalytic modules. The presence of numerous CBMs in a given complex, however, also reduced their performance. Furthermore, a "covalent cellulosome" that combines in a single polypeptide chain a CBM, together with family 48 and family 9 catalytic modules, also exhibited reduced activity. This study also revealed that the cohesin-dockerin interaction may be reversible under specific conditions. Taken together, the data demonstrate that cellulosome components can be used to generate higher-order functional composites and suggest that enzyme mobility is a critical parameter for cellulosome efficiency.
Self assembly is a prerequisite for fabricating nanoscale structures. Here we present a new fusion protein based on the stress-responsive homooligomeric protein, SP1. This ring-shaped protein is a highly stable homododecamer, which can be potentially utilized to self-assemble different modules and enzymes in a predicted and oriented manner. For that purpose, a cohesin module (a component of the bacterial cellulosome) was selected, its gene fused in-frame to SP1, and the fusion protein was expressed in Escherichia coli. The cohesin module, specialized to incorporate different enzymes through specific recognition of a dockerin modular counterpart, is used to display new moieties on the SP1 scaffold. The SP1 scaffold displayed 12 active cohesin modules and specific binding to a dockerin-fused cellulase enzyme from Thermobifida fusca. Moreover, we found a significant increase in specific activity of the scaffold-displayed enzymes. (c) 2007 Elsevier B.V. All rights reserved.
Ruminococcus flavefaciens produces a cellulosomal enzyme complex, based on the structural proteins ScaA, -B, and -C, that was recently shown to attach to the bacterial cell surface via the wall-anchored protein ScaE. ScaA, -B, -C, and -E are all cohesin-bearing proteins encoded by linked genes in the sca cluster. The product of an unknown open reading frame within the sea cluster, herein designated CttA, is similar in sequence at its C terminus to the corresponding region of ScaB, which contains an X module together with a dockerin sequence. The ScaB-XDoc dyad was shown previously to interact tenaciously with the cohesin of ScaE. Likewise, avid binding was confirmed between purified recombinant fragments of the CttA-XDoc dyad and the ScaE cohesin. In addition, the N-terminal regions of CttA were shown to bind to cellulose, thus suggesting that CttA is a cell wall-anchored, cellulose-binding protein. Proteomic analysis showed that the native CttA protein (similar to 130 kDa) corresponds to one of the three most abundant polypeptides binding tightly to insoluble cellulose in cellulose-grown R. flavefaciens 17 cultures. Interestingly, this protein was also detected among cellulose-bound proteins in the related strain R. flavefaciens 007C but not in a mutant derivative, 007S, that was previously shown to have lost the ability to grow on dewaxed cotton fibers. In R. flavefaciens, the presence of CttA on the cell surface is likely to provide an important mechanism for substrate binding, perhaps compensating for the absence of an identified cellulose-binding module in the major cellulosomal scaffolding proteins of this species.
Artificial designer minicellulosomes comprise a chimeric scaffoldin that displays an optional cellulosebinding module (CBM) and bacterial cohesins from divergent species which bind strongly to enzymes engineered to bear complementary dockerins. Incorporation of cellulosomal cellulases from Clostridium cellulolyticum into minicellulosomes leads to artificial complexes with enhanced activity on crystalline cellulose, due to enzyme proximity and substrate targeting induced by the scaffoldin-borne CBM. In the present study, a bacterial dockerin was appended to the family 6 fungal cellulase CeBA, produced by NeocallimastLr patriciarum, for subsequent incorporation into minicellulosomes in combination with various cellulosomal cellulases from C. cellulolyticum. The binding of the fungal Cel6A with a bacterial family 5 endoglucanase onto chimeric miniscaffoldins had no impact on their activity toward crystalline cellulose. Replacement of the bacterial family 5 enzyme with homologous endoglucanase Cel5D from N. patriciarum bearing a clostridial dockerin gave similar results. In contrast, enzyme pairs comprising the fungal Cel6A and bacterial family 9 endoglucanases were substantially stimulated (up to 2.6-fold) by complexation on chimeric scaffoldins, compared to the free-enzyme system. Incorporation of enzyme pairs including Cel6A and a processive bacterial cellulase generally induced lower stimulation levels. Enhanced activity on crystalline cellulose appeared to result from either proximity or CBM effects alone but never from both simultaneously, unlike minicellulosomes composed exclusively of bacterial cellulases. The present study is the first demonstration that viable designer minicellulosomes can be produced that include (i) free (noncellulosomal) enzymes, (ii) fungal enzymes combined with bacterial enzymes, and (iii) a type (family 6) of cellulase never known to occur in natural cellulosomes.
Lignocellulose is the most abundant plant cell wall component of the biosphere and the most voluminous waste produced by our society. Fortunately, it is not toxic or directly harmful, but our major waste disposal facilities - the landfills - are rapidly filling up with few realistic alternatives. Because cellulose is pure glucose, its conversion to fine products or fuels has remained a romantic and popular notion; however, the heterogeneous and recalcitrant nature of cellulosic waste presents a major obstacle for conventional conversion processes. One paradigm for the conversion of biomass to products in nature relies on a multienzyme complex, the cellulosome. Microbes that produce cellulosomes convert lignocelluose to microbial cell mass and products (e.g. ethanol) simultaneously. The combination of designer cellulosomes with novel production concepts could in the future provide the breakthroughs necessary for economical conversion of cellulosic biomass to biofuels.
The assembly of proteins that display complementary activities into macromolecular complexes is critical to cellular function. One such enzyme complex, of environmental significance, is the plant cell wall degrading apparatus of anaerobic bacteria, termed the cellulosome. The complex assembles through the interaction of enzyme-derived "type I dockerin" modules with the multiple "cohesin" modules of the scaffolding protein. Clostridium thermocellum type I dockerin modules contain a duplicated 22-residue sequence that comprises helix-1 and helix-3, respectively. The crystal structure of a C thermocellum type I cohesin-dockerin complex showed that cohesin recognition was predominantly through helix-3 of the dockerin. The sequence duplication is reflected in near-perfect 2-fold structural symmetry, suggesting that both repeats could interact with cohesins by a common mechanism in wild-type (WT) proteins. Here, a helix-3 disrupted mutant dockerin is used to visualize the reverse binding in which the dockerin mutant is indeed rotated 180 degrees relative to the WT dockerin such that helix-1 now dominates recognition of its protein partner. The dual binding mode is predicted to impart significant plasticity into the orientation of the catalytic subunits within this supramolecular assembly, which reflects the challenges presented by the degradation of a heterogeneous, recalcitrant, insoluble substrate by a tethered macromolecular complex.
A 17-kb scaffoldin gene cluster in Ruminococcus flavefaciens strain FD-1 was compared with the homologous segment published for strain 17. Although the general design of the cluster is identical in the two strains, significant differences in the modular architecture of the scaffoldin proteins were discovered, implying strain-specific divergence in cellulosome organization.
The innate binding specificity of different carbohydrate-binding modules (CBMs) offers a versatile approach for mapping the chemistry and structure of surfaces that contain complex carbohydrates. We have employed the distinct recognition properties of a double His-tagged recombinant CBM tagged with semiconductor quantum dots for direct imaging of crystalline cellulose at the molecular level of resolution, using transmission and scanning transmission electron microscopy. In addition, three different types of CBMs from families 3, 6, and 20 that exhibit different carbohydrate specificities were each fused with either green fluorescent protein (GFP) or red fluorescent protein (RFP) and employed for double-labeling fluorescence inicroscopy studies of primary cell walls and various mixtures of complex carbohydrate target molecules. CBM probes can be used for characterizing both native complex carbohydrates and engineered biomaterials.
Many of the Firmicutes bacteria responsible for plant polysaccharide degradation in Nature produce a multiprotein complex called a cellulosome, which co-ordinates glycoside hydrolase assembly, bacterial adhesion to substrate and polysaccharide hydrolysis. Cellulosomal proteins possess a dockerin module, which mediates their attachment to the scaffoldin protein via its interaction with cohesin modules, and only glycoside hydrolases and other carbohydrate active enzymes were known to reside within the cellulosome. We show here with Clostridium thermocellum ATCC 27405 that members of the serpin superfamily of serine proteinase inhibitors, which are best recognized for their conformational flexibility and co-ordination of key regulatory functions in multicellular eukaryotes, also reside within the cellulosome. These studies are the first to expand the cellulosome paradigm of protein complex assembly beyond glycoside hydrolase and carbohydrate active enzymes, and to include a newly identified functionality in the Firmicutes.
The hydrolysis of biotinyl p-nitrophenyl ester (BNP) by a series of avidin derivatives was examined. Surprisingly, a hyperthermostable avidin-related protein (AVR4) was shown to display extraordinary yet puzzling hydrolytic activity. In order to evaluate the molecular determinants that contribute to the reaction, the crystal structure of AVR4 was compared with those of avidin, streptavidin and key mutants of the two proteins in complex with biotinyl p-nitroanilide (BNA), the inert amide analogue of BNP. The structures revealed that a critical lysine residue contributes to the hydrolysis of BNP by avidin but has only a minor contribution to the AVR4-mediated reaction. Indeed, the respective rates of hydrolysis among the different avidins reflect several molecular parameters, including binding-site architecture, the availability of the ligand to solvent and the conformation of the ligand and consequent susceptibility to efficient nucleophilic attack. In avidin, the interaction of BNP with Lys111 and disorder of the L3,4 loop (and consequent solvent availability) together comprise the major driving force behind the hydrolysis, whereas in AVR4 the status of the ligand (the pseudo-substrate) is a major distinguishing feature. In the latter protein, a unique conformation of the L3,4 loop restrains the pseudo-substrate, thereby exposing the carbonyl carbon atom to nucleophilic attack. In addition, due to its conformation, the pseudosubstrate in the AVR4 complex cannot interact with the conserved lysine analogue (Lys109); instead, this function is superseded by polar interactions with Arg112. The results demonstrate that, in highly similar proteins, different residues can perform the same function and that subtle differences in the active-site architecture of such proteins can result in alternative modes of reaction. (c) 2006 Elsevier Ltd. All rights reserved.
Molecular recognition or biorecognition is as the heart of all biological interactions. These interactions are characterized by a collection of noncovalent bonds, namely ionic, hydrogen-bonding and hydrophobic interactions. In addition, shape complementarity appears to play a pivotal role in the process of biorecognition. In this review, we examine the versatile avidin-biotin complex as a model system for study of the biorecognition phenomenon with respect to protein-protein, protein-peptide, protein-ligand and protein-DNA interactions. (c) 2005 Elsevier B.V. All rights reserved.
From an anthropocentric point of view, for millennia, human culture has been intricately involved with cellulose, the major component of the plant cell wall. The development of the wood, paper and textile industries has served to incorporate cellulosic materials into the fabric of our society. Within the past century, however, cellulosic wastes, derived mainly from the same industries, have also become a major source of environmental pollution. This chapter will concentrate mainly on cellulose and the cellulolytic bacteria, in view of their importance to mankind and world ecology. Nevertheless, the true substrate of these bacteria—i.e., the complement of plant cell wall polysaccharides in general—is much more complex than cellulose alone. Likewise, the complement of enzymes—both the cellulolytic and the non-cellulolytic glycosyl hydrolases—are produced concurrently in these bacteria for the purpose of efficient synergistic degradation of the complete substrate composite as it appears in nature. Consequently, when we discuss the cellulose-decomposing bacteria and their enzyme systems, we cannot ignore the related noncellulolytic enzymes, and these will also be treated, albeit secondarily, in the present chapter.
We have sequenced a new gene, cel9B, encoding a family-9 cellulase from a cellulosome-producing bacterium, Acetivibrio cellulolyticus. The gene includes a signal peptide, a family-9 glycoside hydrolases (GH9) catalytic module, two family-3 carbohydrate-binding modules (CBM3c-CBM3b tandem dyad) and a C-terminal dockerin module. An identical modular arrangement exists in two putative GH9 genes from the draft sequence of the Clostridium thermocellum genome. The three homologous CBM3b modules from A. cellulolyticus and C. thermocellum were overexpressed, but, surprisingly, none bound cellulosic substrates. The results raise fundamental questions concerning the possible role(s) of the newly described CBMs. Phylogenetic analysis and preliminary site-directed mutagenesis studies suggest that the catalytic module and the CBM3 dyad are distinctive in their sequences and are proposed to constitute a new GH9 architectural theme.
During the course of our studies on the structure-function relationship of cellulosomes, we were interested in converting the free cellulase system of the aerobic bacterium, Thermobifida fusca, to a cellulosomal system. For this purpose, the cellulose-binding modules (CBM) of two T. fusca family-6 cellulases, endoglucanase Ce16A and exoglucanase Ce16B, were replaced by divergent dockerin modules. Thus far, family-6 cellulases have not been shown to be members of natural cellulosome systems. The resultant chimaeric proteins, 6A-c and t-6B, respectively, were purified and found to interact specifically and stoichiometrically with their corresponding cohesin modules, indicating their suitability for use as components in 'designer cellulosomes'. Both chimaeric enzymes showed somewhat decreased but measurable levels of activity on carboxymethyl cellulose, consistent with the known endo- and exo-glucanase character of the parent enzymes. The activity of 6A-c on phosphoric acid swollen cellulose was also consistent with that of the wild-type endoglucanase Ce16A. The startling finding of the present research was the extent of degradation of this substrate by the chimaeric enzyme t-6B. Wild-type exoglucanase Ce16B exhibited very low activity on this substrate, while the specific activity of t-6B was 14-fold higher than the parent enzyme.
Sequence extension of the scaffoldin gene cluster from Ruminococcus flavefaciens revealed a new gene (scaE) that encodes a protein with an N-terminal cohesin domain and a C terminus with a typical gram-positive anchoring signal for sortase-mediated attachment to the bacterial cell wall. The recombinant cohesin of ScaE was recovered after expression in Escherichia coli and was shown to bind to the C-terminal domain of the cellulosomal structural protein ScaB, as well as to three unknown polypeptides derived from native cellulose-bound Ruminococcus flavefaciens protein extracts. The ScaB C terminus includes a cryptic dockerin domain that is unusual in its sequence, and considerably larger than conventional dockerins. The ScaB dockerin binds to ScaE, suggesting that this interaction occurs through a novel cohesin-dockerin pairing. The novel ScaB dockerin was expressed as a xylanase fusion protein, which was shown to bind tenaciously and selectively to a recombinant form of the ScaE cohesin. Thus, ScaE appears to play a role in anchoring the cellulosomal complex to the bacterial cell envelope via its interaction with ScaB. This sortase-mediated mechanism for covalent cell-wall anchoring of the cellulosome in R. flavefaciens differs from those reported thus far for any other cellulosome system.
Cellulosomes are multi-enzyme complexes that orchestrate the efficient degradation of cellulose and related plant cell wall polysaccharides. The complex is maintained by the high-affinity protein-protein interaction between two complementary modules: the cohesin and the dockerin. In order to characterize the interaction between different cohesins and dockerins, we have developed matching fusion-protein systems, which harbor either the cohesin or the dockerin component. For this purpose, corresponding plasmid cassettes were designed, which encoded for the following carrier proteins: (i) a thermostable xylanase with an appended His-tag; and (ii) a highly stable cellulose-binding module (CBM). The resultant xylanase-dockerin and CBM-cohesin fusion products exhibited high expression levels of soluble protein. The expressed, affinity-purified proteins were extremely stable, and the functionality of the cohesin or dockerin component was retained. The fusion protein system was used to establish a sensitive and reliable, semiquantitative enzyme-linked affinity assay for determining multiple samples of cohesin-dockerin interactions in microtiter plates. A variety of cohesin-dockerin systems, which had been examined previously using other methodologies, were revisited applying the affinity-based enzyme assay, the results of which served to verify the validity of the approach. Copyright (c) 2005 John Wiley & Sons, Ltd.
Cellulose, the main structural component of plant cell walls, is the most abundant carbohydrate polymer in nature. To break down plant cell walls, anaerobic microorganisms have evolved a large extracellular enzyme complex termed cellulosome. This megadalton catalytic machinery organizes an enzymatic assembly, tenaciously bound to a scaffolding protein via specialized intermodular "cohesin-dockerin" interactions that serve to enhance synergistic activity among the different catalytic subunits. Here, we report the solution structure properties of cellulosome-like assemblies analyzed by small angle x-ray scattering and molecular dynamics. The atomic models, generated by our strategy for the free chimeric scaffoldin and for binary and ternary complexes, reveal the existence of various conformations due to intrinsic structural flexibility with no, or only coincidental, inter-cohesin interactions. These results provide primary evidence concerning the mechanisms by which these protein assemblies attain their remarkable synergy. The data suggest that the motional freedom of the scaffoldin allows precise positioning of the complexed enzymes according to the topography of the substrate, whereas short-scale motions permitted by residual flexibility of the enzyme linkers allow "fine-tuning" of individual catalytic domains.
Non-DNA microarrays, such as protein, peptide and small molecule microarrays, can potentially revolutionize the high-throughput screening tools currently used in basic and pharmaceutical research. However, fundamental obstacles remain that limit their rapid and widespread implementation as an alternative bioanalytical approach. These include the prerequisite for numerous proteins in active and purified form, ineffectual immobilization strategies and inadequate means for quality control of the considerable numbers of multiple reagents. This study describes a simple yet efficient strategy for the production of non-DNA microarrays, based on the tenacious affinity of a carbohydrate-binding module (CBM) for its three-dimensional substrate, i.e., cellulose. Various microarray formats are described, e.g., conventional and single-chain antibody microarrays and peptide microarrays for serodiagnosis of human immunodeficiency virus patients. CBM-based microarray technology overcomes many of the previous obstacles that have hindered fabrication of non-DNA microarrays and provides a technically simple but effective alternative to conventional microarray technology.
The chicken avidin gene belongs to an extended gene family encoding seven avidin-related genes (AVRs), of which only avidin is expressed in the chicken. The sequences of AVR4 and AVR5 are identical and the common protein ( AVR4) has been expressed both in insect and bacterial systems. The recombinant proteins are similarly hyperthermostable and bind biotin with similarly high affinities. AVR4 was crystallized in the apo and biotin-complexed forms and their structures were determined at high resolution. Its tertiary and quaternary structures are very similar to those of avidin and streptavidin. Its biotin-binding site shows only a few alterations compared with those of avidin and streptavidin, which account for the observed differences in binding affinities. The increased hyperthermostability can be attributed to the conformation of the critical L3,4 loop and the extensive network of 1 - 3 inter-monomeric interactions. The loop contains a tandem Pro-Gly sequence and an Asp-Arg ion pair that collectively induce rigidity, thus maintaining its closed and ordered conformation in both the apo and biotin-complexed forms. In addition, Tyr115 is present on the AVR4 1 - 3 monomer - monomer interface, which is absent in avidin and streptavidin. The interface tyrosine generates intermonomeric interactions, i.e. a tyrosine - tyrosine pi-pi interaction and a hydrogen bond with Lys92. The resultant network of interactions confers a larger 1 - 3 dimer - dimer contact surface on AVR4, which correlates nicely with its higher thermostability compared with avidin and streptavidin. Several of the proposed thermostability-determining factors were found to play a role in strengthening the tertiary and quaternary integrity of AVR4.
The incorporation of enzymes into the multi-enzyme cellulosome complex and its anchoring to the bacterial cell surface are dictated by a set of binding interactions between two complementary protein modules: the cohesin and the dockerin. In this work, the X-ray crystal structure of a type-II cohesin from scaffoldin A of Bacteroides cellulosolvens has been determined to a resolution of 1.6 angstrom using molecular replacement. The type-II cellulosolvens cohesin (Bc-cohesin-II) is the first detailed description of a B. crystal structure for a type-II cohesin, and its features were compared with the known type-I cohesins from Clostridium thermocellum and Clostridium cellulolyticum (Ct-cohesin-I and Cc-cohesin-I, respectively). The overall jelly-roll topology of the type-II Bc-cohesin is very similar to that observed for the type-I cohesins with three additional secondary structures: an alpha-helix and two "beta-flaps" that disrupt the normal course of a beta-strand. In addition, beta-strand 5 is elevated by approximately 4 angstrom on the surface of the molecule, relative to the type-I Ct and Cc-cohesins. Like its type-I analogue, the hydrophobic/aromatic core of Bc-cohesin-II comprises an upper and but an additional aromatic patch and conserved tryptophan at lower core, the crown of the molecule serves to stabilize the alpha-helix of the type-II cohesin. Comparison of Bc-cohesin-II with the known type-I cohesin-dockerin heterodimer suggests that each of the additional secondary structural elements assumes a flanking position relative to the putative dockerin-binding surface. The raised ridge formed by beta-strand 5 confers additional distinctive topographic features to the proposed binding interface that collectively distinguish between the type-II and type-I cohesins. (c) 2005 Elsevier Ltd. All rights reserved.
The expression of scaffoldin-anchoring genes and one of the major processive endoglucanases (CelS) from the cellulosome of Clostridium thermocellum has been shown to be dependent on the growth rate. For the present work, we studied the gene regulation of selected cellulosomal endoglucanases and a major xylanase in order to examine the previously observed subs trate-linked alterations in cellulosome composition. For this purpose, the transcript levels of genes encoding endoglucanases CelB, CelG, and COD and the family 10 xylanase XynC were determined in batch cultures, grown on either cellobiose or cellulose, and in carbon-limited continuous cultures at different dilution rates. Under all conditions tested, the transcript levels of celB and celG were at least 10-fold higher than that of celD. Like the major processive endoglucanase CelS, the transcript levels of these endoglucanase genes were also dependent on the growth rate. Thus, at a rate of 0.04 h(-1), the levels of celB, celG, and celD were threefold higher than those obtained in cultures grown at maximal rates (0.35 h(-1)) on cellobiose. In contrast, no clear correlation was observed between the transcript level of xynC and the growth rate-the levels remained relatively high, fluctuating between 30 and 50 transcripts per cell. The results suggest that the regulation of C. thermocellum endoglucanases is similar to that of the processive endoglucanase celS but differs from that of a major cellulosomal xylanase in that expression of the latter enzyme is independent of the growth rate.
In recent work (Fierobe, H.- P., Bayer, E. A., Tardif, C., Czjzek, M., Mechaly, A., Belai " ch, A., Lamed, R., Shoham, Y., and Belaich, J.- P. ( 2002) J. Biol. Chem. 277, 49621 49630), we reported the self-assembly of a comprehensive set of defined " bifunctional" chimeric cellulosomes. Each complex contained the following: ( i) a chimeric scaffoldin possessing a cellulose-binding module and two cohesins of divergent specificity and (ii) two cellulases, each bearing a dockerin complementary to one of the divergent cohesins. This approach allowed the controlled integration of desired enzymes into a multiprotein complex of predetermined stoichiometry and topology. The observed enhanced synergy on recalcitrant substrates by the bifunctional designer cellulosomes was ascribed to two major factors: substrate targeting and proximity of the two catalytic components. In the present work, the capacity of the previously described chimeric cellulosomes was amplified by developing a third divergent cohesin-dockerin device. The resultant trifunctional designer cellulosomes were assayed on homogeneous and complex substrates ( microcrystalline cellulose and straw, respectively) and found to be considerably more active than the corresponding free enzyme or bifunctional systems. The results indicate that the synergy between two prominent cellulosomal enzymes ( from the family-48 and -9 glycoside hydrolases) plays a crucial role during the degradation of cellulose by cellulosomes and that one dominant family-48 processive endoglucanase per complex is sufficient to achieve optimal levels of synergistic activity. Furthermore cooperation within a cellulosome chimera between cellulases and a hemicellulase from different microorganisms was achieved, leading to a trifunctional complex with enhanced activity on a complex substrate.
Avidin, the basic biotin-binding glycoprotein from chicken egg white, is known to interact with DNA, whereas streptavidin, its neutral non-glycosylated bacterial analog, does not. In the present study we investigated the DNA-binding properties of avidin. Its affinity for DNA in the presence and absence of biotin was compared with that of other positively charged molecules, namely the protein lysozyme, the cationic polymers polylysine and polyarginine and an avidin derivative with higher isoelectric point (pI approximate to 11) in which most of the lysine residues were converted to homoarginines. Gel-shift assays, transmission electron microscopy and dynamic light scattering experiments demonstrated an unexpectedly strong interaction between avidin and DNA. The most pronounced gel-shift retardation occurred with the avidin-biotin complex, followed by avidin alone and then guanidylated avidin. Furthermore, ultrastructural and light-scattering studies showed that avidin assembles on the DNA molecule in an organized manner. The assembly leads to the formation of nanoparticles that are about 50-100 nm in size (DNA approximate to 5 kb) and have a rod-like or toroidal shape. In these particles the DNA is highly condensed and one avidin is bound to each 18 4 DNA base pairs. The complexes are very stable even at high dilution ([DNA] = 10 pm) and are not disrupted in the presence of buffers or salt (up to 200 mm NaCl). The other positively charged molecules also condense DNA and form particles with a globular shape. However, in this case, these particles disassemble by dilution or in the presence of low salt concentration. The results indicate that the interaction of avidin with DNA may also occur under physiological conditions, further enhanced by the presence of biotin. This DNA-binding property of avidin may thus shed light on a potentially new physiological role for the protein in its natural environment. Copyright (C) 2004 John Wiley Sons, Ltd.
The high affinity cohesin-dockerin interaction dictates the suprastructural assembly of the multienzyme cellulosome complex. The connection between affinity and species specificity was studied by exploring the recognition properties of two structurally related cohesin species of divergent specificity. The cohesins were examined by progressive rounds of swapping, in which corresponding homologous stretches were interchanged. The specificity of binding of the resultant chimeric cohesins was determined by enzyme-linked affinity assay and complementary protein microarray. In succeeding rounds, swapped segments were systematically contracted, according to the binding behavior of previously generated chimeras. In the fourth and final round we discerned three residues, reputedly involved in interspecies binding specificity. By replacing only these three residues, we were able to convert the specificity of the resultant mutated cohesin, which bound preferentially to the rival dockerin with similar to 20% capacity of the wild-type interaction. These residues represent but 3 of the 16 contact residues that participate in the cohesin-dockerin interaction. This approach allowed us to differentiate, in a structure-independent fashion, between residues critical for interspecies recognition and binding residues per se.
Sequencing of a cellulosome-integrating gene cluster in Acetivibrio cellulolyticus was completed. The cluster contains four tandem scaffoldin genes (scaA, scaB, scaC, and scaD) bounded upstream and downstream, respectively, by a presumed cellobiose phosphorylase and a nucleotide methylase. The sequences and properties of scaA, scaB, and scaC were reported previously, and those of scaD are reported here. The scaD gene encodes an 852-residue polypeptide that includes a signal peptide, three cohesins, and a C-terminal S-layer homology (SLH) module. The calculated molecular weight of the mature ScaD is 88,960; a 67-residue linker segment separates cohesins 1 and 2, and two similar to30-residue linkers separate cohesin 2 from 3 and cohesin 3 from the SLH module. The presence of an SLH module in ScaD indicates its role as an anchoring protein. The first two ScaD cohesins can be classified as type II, similar to the four cohesins of ScaB. Surprisingly, the third ScaD cohesin belongs to the type I cohesins, like the seven ScaA cohesins. ScaD is the first scaffoldin to be described that contains divergent types of cohesins as integral parts of the polypeptide chain. The recognition properties among selected recombinant cohesins and dockerins from the different scaffoldins of the gene cluster were investigated by affinity blotting. The results indicated that the divergent types of ScaD cohesins also differ in their preference of dockerins. ScaD thus plays a dual role, both as a primary scaffoldin, capable of direct incorporation of a single dockerin-borne enzyme, and as a secondary scaffoldin that anchors the major primary scaffoldin, ScaA and its complement of enzymes to the cell surface.
The cohesive cellulosome complex is sustained by the high-affinity cohesin-dockerin interaction. In previous work [J. Biol. Chem. 276 (2001) 9883], we demonstrated that a single Thr-to-Leu replacement in the Clostridium thermocellum dockerin component differentiates between non-recognition and high-affinity recognition by the interspecies rival cohesin from C. cellulolyticum. In this report, we show that a single Asp-to-Asn substitution on the cohesin counterpart also disrupts normal recognition of the dockerin. The Asp34 carboxyl group of the cohesin appears to play a central role in the resultant hydrogen-bonding network as an acceptor of two crucial hydrogen bonds from Ser45 of the dockerin domain. The results underscore the fragile nature of the intermolecular contact interactions that maintain this very high-affinity protein-protein interaction. (C) 2004 Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies.
We recently showed that some of the enzymes underpinning cellulose solubilization by Rundnacoccus albus 8 lack the conventional type of dockerin module characteristic of cellulosomal proteins and instead, bear an "X" domain of unknown function at their C-termini. We have now subcloned and expressed six X domains and showed that five of them bind to xylan, chitin, microcrystalline and phosphoric-acid swollen cellulose, as well as more heterogenous substrates such as alfalfa cell walls, banana stem and wheat straw. The X domain that did not bind to these substrates was derived from a family-5 glycoside hydrolase (Cel5G), which possesses two X domains in tandem. Whereas the internal X domain failed to bind to the substrates, the recombinant dyad exhibited markedly enhanced binding relative to that observed for the C-terminal X domain alone. The evidence supports a distinctive carbohydrate-binding role of broad specificity for this type of domain, and we propose a novel family (designated family 37) of carbohydrate-binding modules that appear to be peculiar to R. albus. (C) 2004 Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies.
A new gene, designated scaC and encoding a protein carrying a single cohesin, was identified in the cellulolytic rumen anaerobe Ruminococcus flavefaciens 17 as part of a gene cluster that also codes for the cellulosome structural components ScaA and ScaB. Phylogenetic analysis showed that the sequence of the ScaC cohesin is distinct from the sequences of other cohesins, including the sequences of R. flavefaciens ScaA and ScaB. The scaC gene product also includes at its C terminus a dockerin module that closely resembles those found in R. flavefaciens enzymes that bind to the cohesins of the primary ScaA scaffoldin. The putative cohesin domain and the C-terminal dockerin module were cloned and overexpressed in Escherichia coli as His(6)-tagged products (ScaC-Coh and ScaC-Doc, respectively). Affinity probing of protein extracts of R. flavefaciens 17 separated in one-dimensional and two-dimensional gels with recombinant cohesins from ScaC and ScaA revealed that two distinct subsets of native proteins interact with ScaC-Coh and ScaA-Coh. Furthermore, ScaC-Coh failed to interact with the recombinant dockerin module from the enzyme EndB that is recognized by ScaA cohesins. On the other hand, ScaC-Doc was shown to interact specifically with the recombinant cohesin domain from ScaA, and the ScaA-Coh probe was shown to interact with a native 29-kDa protein spot identified as ScaC by matrix-assisted laser desorption ionization-time of flight mass spectrometry. These results suggest that ScaC plays the role of an adaptor scaffoldin that is bound to ScaA via the ScaC dockerin module, which, via the distinctive ScaC cohesin, expands the range of proteins that can bind to the ScaA-based enzyme complex.
Avidin enhances the hydrolysis of biotinyl p-nitrophenyl ester (BNP) under mild alkaline conditions, whereas streptavidin prevents hydrolysis of BNP up to pH 12. Recently, we imposed hydrolytic activity on streptavidin by rational mutagenesis, based on the molecular elements responsible for the hydrolysis by avidin. Three mutants were designed, whereby the desired features, the distinctive L124R point mutation (M1), the L3,4 loop replacement (M2), and the combined mutation (M3), were transferred from avidin to streptavidin. The crystal structures of the mutants, in complex with biotinyl p-nitroanilide (BNA), the stable amide analogue of BNP, were determined. The results demonstrate that the point mutation alone has little effect on hydrolysis, and BNA exhibits a conformation similar to that of streptavidin. Substitution of a lengthier L3,4 loop (from avidin to streptavidin), resulted in an open conformation, thus exposing the ligand to solvent. Moreover, the amide bond of BNA was flipped relative to that of the streptavidin and M1 complexes, thus deflecting the nitro group toward Lys-121. Consequently, the leaving group potential of the nitrophenyl group of BNP is increased, and M2 hydrolyzes BNP at pH values >8.5. To better emulate the hydrolytic potential of avidin, M3 was required. The combination of loop replacement and point mutation served to further increase the leaving group potential by interaction of the nitro group with Arg-124 and Lys-121. The information derived from this study may provide insight into the design of enzymes and transfer of desired properties among homologous proteins.
A large gene downstream of the primary Bacteroides cellulosolvens cellulosomal scaffoldin (cipBc, now renamed scaA) was sequenced. The gene, termed scaB, contained an N-terminal leader peptide followed by 10 type I cohesins, an "X" domain of unknown structure and function, and a C-terminal S-layer homology (SLH) surface-anchoring module. In addition, a previously identified gene in a different part of the genome, encoding for a dockerin-borne family 48 cellulosomal glycoside hydrolase (Ce148), was sequenced completely, and a putative cellulosome-related family 9 glycosyl hydrolase was detected. Recombinant fusion proteins, comprising dockerins derived from either the ScaA scaffoldin or Ce148, were overexpressed. Their interaction with ScaA and ScaB cohesins was examined by immunoassay. The results indicated that the ScaB type I cohesin of the new anchoring protein binds selectively to the ScaA dockerin, whereas the Ce148 dockerin binds specifically to the type II ScaA cohesin 5. Thus, by virtue of the 11 type II ScaA cohesins and the 10 type I ScaB cohesins, the relatively simple two-component cellulosome-integrating complex would potentially incorporate 110 enzyme molecules onto the cell surface via the ScaB SLH module. Compared to previously described cellulosome systems, the apparent roles of the B. cellulosolvens cohesins are reversed, in that the type II cohesins are located on the enzyme-binding primary scaffoldin, whereas the type I cohesins are located on the anchoring scaffoldin. The results underscore the extensive diversity in the supramolecular architecture of cellulosome systems in nature.
The discrete multicomponent, multienzyme cellulosome complex of anaerobic cellulolytic bacteria provides enhanced synergistic activity among the different resident enzymes to efficiently hydrolyze intractable cellulosic and hemicellulosic substrates of the plant cell wall. A pivotal noncatalytic subunit called scaffoldin secures the various enzymatic subunits into the complex via the cohesin-dockerin interaction. The specificity characteristics and tenacious binding between the scaffoldin-based cohesin modules and the enzyme-borne dockerin domains dictate the supramolecular architecture of the cellulosome. The diversity in cellulosome architecture among the known cellulosome-producing bacteria is manifest in the arrangement of their genes in either multiple-scaffoldin or enzyme-linked clusters on the genome. The recently described three-dimensional crystal structure of the cohesin-dockerin heterodimer sheds light on the critical amino acids that contribute to this high-affinity protein-protein interaction. In addition, new information regarding the regulation of cellulosome-related 66 genes, budding genetic tools, and emerging genomics of cellulosome-producing bacteria promises new insight into the assembly and consequences of the multienzyme complex.
The cellulolytic bacterium Ruminococcus albus 8 adheres tightly to cellulose, but the molecular biology underpinning this process is not well characterized. Subtractive enrichment procedures were used to isolate mutants of R. albus 8 that are defective in adhesion to cellulose. Adhesion of the mutant strains was reduced 50% compared to that observed with the wild-type strain, and cellulose solubilization was also shown to be slower in these mutant strains, suggesting that bacterial adhesion and cellulose solubilization are inextricably linked. Two-dimensional polyacrylamide gel electrophoresis showed that all three mutants studied were impaired in the production of two high-molecular-mass, cell-bound polypeptides when they were cultured with either cellobiose or cellulose. The identities of these proteins were determined by a combination of mass spectrometry methods and genome sequence data for R. albus 8. One of the polypeptides is a family 9 glycoside hydrolase (Cel9B), and the other is a family 48 glycoside hydrolase (Cel48A). Both Cel9B and Cel48A possess a modular architecture, Cel9B possesses features characteristic of the B-2 (or theme D) group of family 9 glycoside hydrolases, and Cel48A is structurally similar to the processive endocellulases CelF and CelS from Clostridium cellulolyticum and Clostridium thermocellum, respectively. Both Cel9B and Cel48A could be recovered by cellulose affinity procedures, but neither Cel9B nor Cel48A contains a dockerin, suggesting that these polypeptides are retained on the bacterial cell surface, and recovery by cellulose affinity procedures did not involve a clostridium-like cellulosome complex. Instead, both proteins possess a single copy of a novel X module with an unknown function at the C terminus. Such X modules are also present in several other R. albus glycoside hydrolases and are phylogentically distinct from the fibronectin III-like and X modules identified so far in other cellulolytic bacteria.
The N-terminal type II cohesin from the cellulosomal ScaB subunit of Acetivibrio cellulolyticus was crystallized in two different crystal systems: orthorhombic (space group P2(1)2(1)2(1)), with unit-cell parameters a = 37.455, b = 55.780, c = 87.912 Angstrom, and trigonal (space group P3(1)21), with unit-cell parameters a = 55.088, b = 55.088, c = 112.553 Angstrom. The two crystals diffracted to 1.2 and 1.9 Angstrom, respectively. A selenomethionine derivative was also crystallized and exhibited trigonal symmetry (space group P3(1)21), with unit-cell parameters a = 55.281, b = 55.281, c = 112.449 Angstrom and a diffraction limit of 1.97 Angstrom. Initial phasing of the trigonal crystals was successfully performed by the SIRAS method using Cu Kalpha radiation with the selenomethionine derivative as a heavy-atom derivative. The structure of the orthorhombic crystal form was solved by molecular replacement using the coordinates of the trigonal form.
Clostridium thermocellum produces an extracellular multienzyme complex, termed the cellulosome, that allows efficient solubilization of crystalline cellulose. The complex is organized around a large noncatalytic protein subunit, termed CipA or scaffoldin, and is found either free in the supernatant or cell bound. The binding of the complex to the cell is mediated by three cell surface anchoring proteins, OlpB, Orf2p, and SdbA, that interact with the CipA scaffoldin. The transcriptional level of the olpB, orf2, sdbA, and cipA genes was determined quantitatively by RNase protection assays in batch and continuous cultures, under carbon and nitrogen limitation. The mRNA level of olpB, orf2, and cipA varied with growth rate, reaching 40 to 60 transcripts per cell under carbon limitation at a low growth rate of 0.04 h(-1) and 2 to 10 transcripts per cell at a growth rate of 0.35 h(-1) in batch culture. The mRNA level of sdbA was about three transcripts per cell and was not influenced by growth rate. Primer extension analysis revealed two major transcriptional start sites, at -81 and -50 bp, upstream of the translational start site of the cipA gene. The potential promoters exhibited homology to the known sigma factors sigma(A) and sigma(L) (sigma(54)) of Bacillus subtilis. Transcription from the sigma(L)-Iike promoter was found under all growth conditions, whereas transcription from the sigma(A)-like promoter was significant only under carbon limitation. The overall expression level obtained in the primer extension analysis was in good agreement with the results of the RNase-protection assays.
A scaffoldin gene cluster was identified in the mesophilic cellulolytic anaerobe Acetivibrio cellulolyticus. The previously described scaffoldin gene, cipV, encodes an N-terminal family 9 glycoside hydrolase, a family 3b cellulose-binding domain, seven cohesin domains, and a C-terminal dockerin. The gene immediately downstream of cipV was sequenced and designated scaB. The protein encoded by this gene has 942 amino acid residues and a calculated molecular weight of 100,358 and includes an N-terminal signal peptide, four type II cohesions, and a C-terminal dockerin. ScaB cohesins 1 and 2 are very closely linked. Similar, but not identical, 39-residue Thr-rich linker segments separate cohesin 2 from cohesin 3 and cohesin 3 from cohesin 4, and an 84-residue Thr-rich linker connects the fourth cohesin to a C-terminal dockerin. The scaC gene downstream of scaB codes for a 1,237-residue polypeptide that includes a signal peptide, three cohesins, and a C-terminal S-layer homology (SLH) module. A long, ca. 550-residue linker separates the third cohesin and the SLH module of ScaC and is characterized by an 18-residue Pro-Thr-Ala-Ser-rich segment that is repeated 27 times. The calculated molecular weight of the mature ScaC polypeptide (excluding the signal peptide) is 124,162. The presence of the cohesins and the conserved SLH module implies that ScaC acts as an anchoring protein. The ScaC cohesins are on a separate branch of the phylogenetic tree that is close to, but distinct from, the type I cohesins. Affinity blotting with representative recombinant probes revealed the following specific intermodular interactions: (i) an expressed CipV cohesin binds selectively to an enzyme-borne dockerin, (ii) a representative ScaB cohesin binds to the CipV band of the cell-free supernatant fraction, and (iii) a ScaC cohesin binds to the ScaB dockerin. The experimental evidence thus indicates that CipV acts as a primary (enzyme-recognizing) scaffoldin, and the protein was also designa
The biotin-binding tetrameric proteins, streptavidin from Streptomyces avidinii and chicken egg white avidin, are excellent models for the study of subunit-subunit interactions of a multimeric protein. Efforts are thus being made to prepare mutated forms of streptavidin and avidin, which would form monomers or dimers, in order to examine their effect on quaternary structure and assembly. In the present communication, we compared the crystal structures of binding site W-K mutations in streptavidin and avidin. In solution, both mutant proteins are known to form dimers, but upon crystallization, both formed tetramers with the same parameters as the native proteins. All of the intersubunit bonds were conserved, except for the hydrophobic interaction between biotin and the tryptophan that was replaced by lysine. In the crystal structure, the binding site of the mutated apo-avidin contains 3 molecules of structured water instead of the 5 contained in the native protein. The lysine side chain extends in a direction opposite that of the native tryptophan, the void being partially filled by an adjacent lysine residue. Nevertheless, the binding-site conformation observed for the mutant tetramer is an artificial consequence of crystal packing that would not be maintained in the solution-phase dimer. It appears that the dimer-tetramer transition may be concentration dependent, and the interaction among subunits obeys the law of mass action.
Clostridium thermocellum produces an extracellular multienzyme complex, termed cellulosome, that allows efficient solubilization of crystalline cellulose. One of the major enzymes in this complex is the CelS (Ce148A) exoglucanase. The regulation of CelS at the protein and transcriptional levels was studied using batch and continuous cultures. The results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analyses indicated that the amount of CelS in the supernatant fluids of cellobiose-grown cultures is lower than that of cellulose-grown cultures. The transcriptional level of celS mRNA was determined quantitatively by RNase protection assays with batch and continuous cultures under carbon and nitrogen limitation. The amount of celS mRNA transcripts per cell was about 180 for cells grown under carbon limitation at growth rates of 0.04 to 0.21 h(-1) and 80 and 30 transcripts per cell for batch cultures at growth rates of 0.23 and 0.35 h(-1), respectively. Under nitrogen limitation, the corresponding levels were 110, 40, and 30 transcripts/cell for growth rates of 0.07, 0.11, and 0.14 h(-1), respectively. Two major transcriptional start sites were detected at positions - 140 and - 145 bp, upstream of the translational start site of the celS gene. The potential promoters exhibited homology to known sigma factors (i.e., sigma(A) and sigma(B)) of Bacillus subtilis. The relative activity of the two promoters remained constant under the conditions studied and was in agreement with the results of the RNase protection assay, in which the observed transcriptional activity was inversely proportional to the growth rate.
The DNA sequence coding for putative cellulosomal scaffolding protein ScaA from the rumen cellulolytic anaerobe Ruminococcus flavefaciens 17 was completed. The mature protein exhibits a calculated molecular mass of 90,198 Da and comprises three cohesin domains, a C-terminal dockerin, and a unique N-terminal X domain of unknown function. A novel feature of ScaA is the absence of an identifiable cellulose-binding module. Nevertheless, native ScaA was detected among proteins that attach to cellulose and appeared as a glycosylated band migrating at around 130 kDa. The ScaA dockerin was previously shown to interact with the cohesin-containing putative surface-anchoring protein ScaB. Here, six of the seven cohesins from ScaB were overexpressed as histidine-tagged products in E. coli; despite their considerable sequence differences, each ScaB cohesin specifically recognized the native 130-kDa ScaA protein. The binding specificities of clockerins found in R.flavefaciens plant cell wall-degrading enzymes were examined next. The dockerin sequences of the enzymes EndA, EndB, XynB, and XynD are all closely related but differ from those of XynE and CesA. A recombinant ScaA cohesin bound selectively to dockerin-containing fragments of EndB, but not to those of XynE or CesA. Furthermore, dockerin-containing EndB and XynB, but not XynE or CesA, constructs bound specifically to native ScaA. XynE- and CesA-derived probes did however bind a number of alternative R. flavefaciens bands, including an similar to110-kDa supernatant protein expressed selectively in cultures grown on xylan. Our findings indicate that in addition to the ScaA dockerin-ScaB cohesin interaction, at least two distinct dockerin-binding specificities are involved in the novel organization of plant cell wall-degrading enzymes in this species and suggest that different scaffoldins and perhaps multiple enzyme complexes may exist in R. flavefaciens.
Homotetrameric chicken avidin that binds four molecules of biotin was converted to a monomeric form (monoavidin) by mutations of two interface residues: tryptophan 110 in the 1-->2 interface was mutated to lysine and asparagine 54 in the 1-->4 interface was converted to alanine. The affinity for biotin binding of the mutant decreased from K-d similar to10(-15) M of the wild-type tetramer to K-d similar to10(-7) M, which was studied by an optical biosensor IAsys and by a fluorescence spectroscopical method in solution. The binding was completely reversible. Conversion of the tetramer to a monomer results in increased sensitivity to proteinase K digestion. The antigenic properties of the mutated protein were changed, such that monoavidin was only partially recognized by a polyclonal antibody whereas two different monoclonal antibodies entirely failed to recognize the avidin monomer. This new monomeric avidin, which binds biotin reversibly, may be useful for applications both in vitro and in vivo. It may also shed light on the effect of inter-subunit interactions on the binding of ligands.
Introduction of enzymatic activity into proteins or other types of polymers by rational design is a major objective in the life sciences. To date, relatively low levels of enzymatic activity could be introduced into antibodies by using transition-state analogues of haptens. In the present study, we identify the structural elements that contribute to the observed hydrolytic activity in egg white avidin, which promote the cleavage of active biotin esters (notably biotinyl p-nitrophenyl ester). The latter elements were then incorporated into bacterial streptavidin via genetic engineering. The streptavidin molecule was thus converted from a protector to an enhancer of hydrolysis of biotin esters. The conversion was accomplished by the combined replacement of a "lid-like loop" (L3,4) and a leucine-to-arginine point mutation in streptavidin. Interestingly, neither of these elements play a direct role in the hydrolytic reaction. The latter features were thus shown to be responsible for enhanced substrate hydrolysis. This work indicates that structural and non-catalytic elements of a protein can be modified to promote the induced fit of a substrate for subsequent interaction with either a catalytic residue or water molecules. This approach complements the conventional design of active sites that involves direct modifications of catalytic residues.
The family 9 cellulase gene cell of Clostridium thermocellum, was previously cloned, expressed, and characterized (G. P. Hazlewood, K. Davidson, J. 1. Laurie, N. S. Huskisson, and H. J. Gilbert, J. Gen. Microbiol. 139:307-316, 1993). We have recloned and sequenced the entire cell gene and found that the published sequence contained a 53-bp deletion that generated a frameshift mutation, resulting in a truncated and modified C-terminal segment of the protein. The enzymatic properties of the wild-type protein were characterized and found to conform to those of other family 9 glycoside hydrolases with a so-called theme B architecture, where the catalytic module is fused to a family 3c carbohydrate-binding module (CBM3c); Cell also contains a C-terminal CBM3b. The intact recombinant Cell exhibited high levels of activity on all cellulosic substrates tested, with pH and temperature optima of 5.5 and 70degreesC, respectively, using carboxymethylcellulose as a substrate. Native Cell was capable of solubilizing filter paper, and the distribution of reducing sugar between the soluble and insoluble fractions suggests that the enzyme acts as a processive cellulase. A truncated form of the enzyme, lacking the C terminal CBM3b, failed to bind to crystalline cellulose and displayed reduced activity toward insoluble substrates. A truncated form of the enzyme, in which both the cellulose-binding CBM3b and the fused CBM3c were removed, failed to exhibit significant levels of activity on any of the substrates examined. This study underscores the general nature of this type of enzymatic theme, whereby the fused CBM3c plays a critical accessory role for the family 9 catalytic domain and changes its character to facilitate processive cleavage of recalcitrant cellulose substrates.
The strong interaction between avidin and biotin is so tight (dissociation constant 10(-15) M) that conditions usually sufficient for protein denaturing fail to dislodge biotin from the avidin-biotin complex. This kind of irreversible binding hinders the use of avidin in applications such as affinity purification or protein immobilization. To address this concern, we have constructed a series of mutants of the strategically positioned Tyr-33 in order to study the role of this residue in biotin binding, and to create avidin variants with more reversible ligand-binding properties. Unexpectedly, an avidin mutant in which Tyr-33 was replaced with phenylalanine (Avm-Y33F) displayed similar biotin-binding characteristics to the native avidin, indicating that the hydrogen bond formed between the hydroxy group of Tyr-33 and the carbonyl oxygen of biotin is not as important for the tight binding of biotin as previously suggested. In terms of the reversibility of biotin binding, Avm-Y33H was the most successful substitution constructed in this study. Interestingly, the binding of this mutant exhibited clear pH-dependence, since at neutral pH it bound to the biotin surface in an irreversible fashion, whereas, at pH 9, 50% of the bound protein could be released with free biotin. Furthermore, although Tyr-33 is located relatively distant from the monomer-monomer interfaces, the mutagenesis of this residue also weakened the quaternary structure of avidin, indicating that the high ligand binding and the high stability of avidin have evolved together and it is difficult to modify one without affecting the other.
A library of 75 different chimeric cellulosomes was constructed as an extension of our previously described approach for the production of model functional complexes (Fierobe, H.-P., Mechaly, A., Tardif, C., Belaich, A., Lamed, R., Shoham, Y., Belaich, J.-P., and Bayer, E. A (2001) J. BioL Chem. 276,21257-21261), based on the high affinity species-specific cohesin-dockerin interaction. Each complex contained three protein components: (i) a chimeric scaffoldin possessing an optional cellulose-binding module and two cohesins of divergent specificity, and (ii) two cellulases, each bearing a dockerin complementary to one of the divergent cohesins. The activities of the resultant ternary complexes were assayed using different types of cellulose substrates. Organization of cellulolytic enzymes into cellulosome chimeras resulted in characteristically high activities on recalcitrant substrates, whereas the cellulosome chimeras showed little or no advantage over free enzyme systems on tractable substrates. On recalcitrant cellulose, the presence of a cellulose-binding domain on the scaffoldin and enzyme proximity on the resultant complex contributed almost equally to their elevated action on the substrate. For certain enzyme pairs, however, one effect appeared to predominate over the other. The results also indicate that substrate recalcitrance is not necessarily a function of its crystallinity but reflects the overall accessibility of reactive sites.
We have studied the structural elements that affect ligand exchange between the two high affinity biotin-binding proteins, egg white avidin and its bacterial analogue, streptavidin. For this purpose, we have developed a simple assay based on the antipodal behavior of the two proteins toward hydrolysis of biotinyl p-nitrophenyl ester (BNP). The assay provided the experimental basis for these studies. It was found that biotin migrates unidirectionally from streptavidin to avidin. Conversely, the biotin derivative, BNP, is transferred in the opposite direction, from avidin to streptavidin. A previous crystallographic study (Huberman, T., Eisenberg-Domovich, Y., Gitlin, G., Kulik, T., Bayer, E. A., Wilchek, M., and Livnah, O. (2001) J. Bio. Chem. 276, 32031-32039) provided insight into a plausible explanation for these results. These data revealed that the non-hydrolyzable BNP analogue, biotinylp-nitroanilide, was almost completely sheltered in streptavidin as opposed to avidin in which the disordered conformation of a critical loop resulted in the loss of several hydrogen bonds and concomitant exposure of the analogue to the solvent. In order to determine the minimal modification of the biotin molecule required to cause the disordered loop conformation, the structures of avidin and streptavidin were determined with norbiotin, homobiotin, and a common long-chain biotin derivative, biotinyl epsilon-aminocaproic acid. Six new crystal structures of the avidin and streptavidin complexes with the latter biotin analogues and derivatives were thus elucidated. It was found that extending the biotin side chain by a single CH2 group (i.e. homobiotin) is sufficient to result in this remarkable conformational change in the loop of avidin. These results bear significant biotechnological importance, suggesting that complexes containing biotinylated probes with streptavidin would be more stable than those with avidin. These findings should be heeded when developing new drugs based
Bacterial streptavidin and chicken avidin are homotetrameric proteins that share an exceptionally high affinity towards the vitamin biotin. The biotin-binding sites in both proteins contain a crucial tryptophan residue contributed from an adjacent subunit. This particular tryptophan (W110 in avidin and W120 in streptavidin) plays an important role in both biotin binding and in the quaternary stabilities of the proteins. An intriguing naturally occurring alteration of tryptophan to lysine was previously described in the C-terminal domain of sea-urchin fibropellins, which share a relatively high sequence similarity with avidin and streptavidin. Avidin (Avm-W110K) and streptavidin (Savm-W120K) mutations show substantially reduced affinities towards biotin as well as the dissociation of their tetrameric structure into stable avidin and streptavidin dimers. Savm-W120K was crystallized at 293 K using the hanging-drop vapour-diffusion method. The crystals diffract to 1.7 Angstrom resolution using synchrotron radiation and belong to the monoclinic space group P2(1), with unit-cell parameters a=50.43, b=100.41, c=52.51 Angstrom, beta =112.12 degrees. The asymmetric unit contains four molecules of Savm-W120K, with a corresponding V-M of 2.3 Angstrom (3) Da(-1) and a solvent content of 46%.
Aims: To compare the subcellular distribution of glycanase-related components between wildtype Ruminococcus albus SY3 and an adhesion-defective mutant, to identify their possible contribution to the adhesion process, and to determine their association with cellulosome-like complexes. Methods and Results: Cell fractionation revealed that most of the cellulases and xylanases were associated with capsular and cell-wall fractions. SDS-PAGE and gel filtration indicated that most of the bacterial enzyme activity was not integrated into cellulosome-like complexes. The adhesion-defective mutant produced significantly less (5- to 10-fold) overall glycanase activity, and the 'true cellulase activity' appeared to be entirely confined to the cell membrane fractions. Antibodies specific for the cellulosomal scaffoldin of Clostridium thermocellum recognized a single 240 kDa band in R. albus SY3. Conclusions: The adhesion-defective mutant appeared to be blocked in exocellular transport of enzymes involved in true cellulase activity. A potential cellulosomal scaffoldin candidate was identified in R. albus SY3. Significance and Impact of the Study: Several glycanase-related proteins and more than one mechanism appear to be involved in the adhesion of R. albus SY3 to cellulose.
Avidin and its bacterial analogue streptavidin exhibit similarly high affinities toward the vitamin biotin. The extremely high affinity of these two proteins has been utilized as a powerful tool in many biotechnological applications. Although avidin and streptavidin have similar tertiary and quaternary structures, they differ in many of their properties. Here we show that avidin enhances the alkaline hydrolysis of biotinyl p-nitrophenyl ester, whereas streptavidin protects this reaction even under extreme alkaline conditions (pH > 12). Unlike normal enzymatic catalysis, the hydrolysis reaction proceeds as a single cycle with no turnover because of the extremely high affinity of the protein for one of the reaction products (i.e. free biotin). The three-dimensional crystal structures of avidin (2 Angstrom) and streptavidin (2.4 Angstrom) complexed with the amide analogue, biotinyl p-nitroanilide, as a model for the p-nitrophenyl ester, revealed structural insights into the factors that enhance or protect the hydrolysis reaction. The data demonstrate that several molecular features of avidin are responsible for the enhanced hydrolysis of biotinyl p-nitrophenyl ester. These include the nature of a decisive flexible loop, the presence of an obtrusive arginine 114, and a newly formed critical interaction between lysine III and the nitro group of the substrate. The open conformation of the loop serves to expose the substrate to the solvent, and the arginine shifts the p-nitroanilide moiety toward the interacting lysine, which increases the electron withdrawing characteristics and consequent electrophilicity of the carbonyl group of the substrate. Streptavidin lacked such molecular properties, and analogous interactions with the substrate were consequently absent. The information derived from these structures may provide insight into the action of artificial protein catalysts and the evolution of catalytic sites in general.
Defined chimeric cellulosomes were produced in which selected enzymes were incorporated in specific locations within a multicomponent complex, The molecular building blocks of this approach are based on complementary protein modules from the cellulosomes of two clostridia, Clostridium thermocellum and Clostridium cellulolyticum, wherein cellulolytic enzymes are incorporated into the complexes by means of high-affinity species-specific cohesin-dockerin interactions. To construct the desired complexes, a series of chimeric scaffoldins was prepared by recombinant means. The scaffoldin chimeras were designed to include two cohesin modules from the different species, optionally connected to a cellulose-binding domain. The two divergent cohesins exhibited distinct specificities such that leach recognized selectively and bound strongly to its dockerin counterpart. Using this strategy, appropriate dockerin-containing enzymes could be assembled precisely and by design into a desired complex. Compared with the mixture of free cellulases, the resultant cellulosome chimeras exhibited enhanced synergistic action on crystalline cellulose.
Two tandem cellulosome-associated genes were identified in the cellulolytic rumen bacterium, Ruminococcus flavefaciens. The deduced gene products represent multimodular scaffoldin-related proteins (termed ScaA and ScaB), both of which include several copies of explicit cellulosome signature sequences. The scaB gene was completely sequenced, and its upstream neighbor scaA was partially sequenced. The sequenced portion of scaA contains repeating cohesin modules and a C-terminal dockerin domain. ScaB contains seven relatively divergent cohesin modules, two extremely long T-rich linkers, and a C-terminal domain of unknown function. Collectively, the cohesins of ScaA and ScaB are phylogenetically distinct from the previously described type I and type II cohesins, and we propose that they define a new group, which we designated here type III cohesins, Selected modules from both genes were overexpressed in Escherichia coli, and the recombinant proteins were used as probes in affinity-blotting experiments. The results strongly indicate that ScaA serves as a cellulosomal scaffoldin-like protein for several R. flavefaciens enzymes. The data are supported by the direct interaction of a recombinant ScaA cohesin with an expressed dockerin-containing enzyme construct from the same bacterium. The evidence also demonstrates that the ScaA dockerin binds to a specialized cohesin(s) on ScaB, suggesting that ScaB may act as an anchoring protein, linked either directly or indirectly to the bacterial cell surface. This study is the first direct demonstration in a cellulolytic rumen bacterium of a cellulosome system, mediated by distinctive cohesin-dockerin interactions.
Chicken avidin, a homotetramer that binds four molecules of biotin was converted to a monomeric form by successive mutations of interface residues to alanine, The major contribution to monomer formation was the mutation of two aspartic acid residues, which together account for ten hydrogen bonding interactions at the 1-4 interface, Mutation of these residues, together with the three hydrophobic residues at the 1-3 interface, led to stable monomer formation in the absence of biotin. Upon addition of biotin, the monomeric avidin reassociated to the tetramer, which exhibited properties similar to those of native avidin, with respect to biotin binding, thermostability, and protease resistance. To our knowledge, these unexpected results represent the first example of a small monovalent ligand that induces oligomerization of a monomeric protein. This study may suggest a biological role for low molecular weight ligands in inducing oligomerization and in maintaining the stability of multimeric protein assemblies.
The assembly of enzyme components into the cellulosome complex is dictated by the cohesin-dockerin interaction. In a recent article (Mechaly, A, Yaron, S,, Lamed, R., Fierobe, H.-P., Belaich, A., Belaich, J.-P., Shoham, Y., and Bayer, E. A. (2000) Proteins 39, 170-177), we provided experimental evidence that four previously predicted dockerin residues play a decisive role in the specificity of this high affinity interaction, although additional residues were also implicated. In the present communication, we examine further the contributing factors for the recognition of a dockerin by a cohesin domain between the respective cellulosomal systems of Clostridium thermocellum and Clostridium cellulolyticum, In this context, the four confirmed residues were analyzed for their individual effect on selectivity, In addition, other dockerin residues were discerned that could conceivably contribute to the interaction, and the suspected residues were similarly modified by site-directed mutagenesis, The results indicate that mutation of a single residue from threonine to leucine at a given position of the C. thermocellum dockerin differentiates between its nonrecognition and high affinity recognition (K-a similar to 10(9) M-1) by a cohesin from C. cellulolyticum, This suggests that the presence or absence of a single decisive hydroxyl group is critical to the observed biorecognition, This study further implicates additional residues as secondary determinants in the specificity of interaction, because interconversion of selected residues reduced intraspecies self-recognition by at least three orders of magnitude. Nevertheless, as the latter mutageneses served to reduce but not annul the cohesin-dockerin interaction within this species, it follows that other subtle alterations play a comparatively minor role in the recognition between these two modules.
Previous work from our group [Morag (Morgenstern), E., Bayer, E. A., and La med, R. (1991), Appl. Biochem. Biotechnol. 30, 129-136] has demonstrated an anomalous electrophoretic mobility pattern for scaffoldin, the 210-kDa cellulosome-integrating subunit of Clostridium thermocellum. Subsequent evidence [Morag, E., Bayer, E. A., and Lamed, R. (1992), Appl. Biochem. Biotechnol. 33, 205-217] indicated that the effect could be attributed to a nonproteolytic fragmentation of the subunit into a defined series of lower-molecular-weight bands. In the present work, a recombinant segment of the scaffoldin subunit was employed to determine the site(s) of bond breakage. An Asp-Pro sequence within the cohesin domain was identified to be the sensitive peptide bond. This sequence appears quite frequently in the large cellulosomal proteins, and the labile bond may be related to an as yet undescribed physiological role in the hydrolysis of cellulose by cellulosomes.
The crystal structure of the family IIIa cellulose-binding domain (CBD) from the cellulosomal scaffoldin subunit (CipC) of Clostridium cellulolyticum has been determined. The structure reveals a nine-stranded jelly-roll topology which exhibits distinctive structural elements consistent with family III CBDs that bind crystalline cellulose. These include a well conserved calcium-binding site, a putative cellulose-binding surface and a conserved shallow groove of unknown function. The CipC CBD structure is very similar to the previously elucidated family IIIa CBD from the CipA scaffoldin of C. thermocellum, with some minor differences. The CipC CBD structure was also compared with other previously described CBD structures from families IIIc and IV derived from the endoglucanases of Thermomonospora fusca and Cellulomonas fimi, respectively. The possible functional consequences of structural similarities and differences in the shallow groove and cellulose-binding faces among various CBD families and subfamilies are discussed.
We introduce a new nonradioactive, chromogenic label based on 4-hydroxyazobenzene-2-carboxylic acid (HABA), which is suitable for bioanalytical application, e.g., detection, localization, isolation, and purification. The HABA label is superior to other systems where it is difficult to separate labeled from unlabeled molecules or to determine the amount of label. HABA is readily detected spectroscopically by its absorption at 350 nm or by its interaction with avidin that results in a red shift to 500 nm. The HABA reagents described can be conjugated to a variety of functional groups on biomolecules and purified thereafter by affinity chromatography on an avidin column. The interaction of the HABAylated biomolecules with their corresponding targets is detected with high-affinity anti-HABA antibodies or with avidin. The nonradioactive, chromogenic HABA-based reagents form a homogeneous system that can complement or replace systems where facile quantification of the label is desired. (C) 2000 Academic Press.
A cellulosomal scaffoldin gene, termed cipBc, was identified and sequenced from the mesophilic cellulolytic anaerobe Bacteroides cellulosolvens, The gene encodes a 2,292-residue polypeptide (excluding the signal sequence) with a calculated molecular weight of 242,437, CipBc contains an N-terminal signal peptide, 11 type II cohesin domains, an internal family III cellulose-binding domain (CBD), and a C-terminal dockerin domain. Its CBD belongs to family IIIb, like that of CipV from Acetivibrio cellulolyticus but unlike the family IIIa CBDs of other clostridial scaffoldins, In contrast to all other scaffoldins thus far described, CipBc lacks a hydrophilic domain or domain X of unknown function. The singularity of CipBc, however, lies in its numerous type II cohesin domains, all of which are very similar in sequence. One of the latter cohesin domains was expressed, and the expressed protein interacted selectively with cellulosomal enzymes, one of which was identified as a family 48 glycosyl hydrolase on the basis of partial sequence alignment. By definition, the dockerins, carried by the cellulosomal enzymes of this species, would be considered to be type II, This is the first example of authentic type II cohesins that are confirmed components of a cellulosomal scaffoldin subunit rather than a cell surface anchoring component, The results attest to the emerging diversity of cellulosomes and their component sequences in nature.
The cohesin-dockerin interaction provides the basis for incorporation of the individual enzymatic subunits into the cellulosome complex, In a previous article (Pages et al,, Proteins 1997;29:517-527) we predicted that four amino acid residues of the similar to 70-residue dockerin domain would serve as recognition codes for binding to the cohesin domain. The validity of the prediction was examined by site-directed mutagenesis of the suspected residues, whereby the species-specificity of the cohesin-dockerin interaction was altered. The results support the premise that the four residues indeed play a role in biorecognition, while additional residues may also contribute to the specificity of the interaction, Proteins 2000;39:170-177. (C) 2000 Wiley-Liss, Inc.
s-Triazines including atrazine are heavily used agricultural herbicides, and their extensive removal from industrial wastewater is required before these effluents can be disposed. The use of enzymes for this purpose is an important potential alternative to conventional methods of detoxification. The purpose of the present study was to develop an enzyme-based technology for the treatment of atrazine contaminated water. In this paper we describe the construction and expression of two fusion proteins which dechlorinate atrazine while being firmly bound to an insoluble cellulose matrix. Dechlorination of atrazine produces in one enzymatic step the nonregulated compound hydroxyatrazine from the regulated mother compound, atrazine.
A recombinant non-glycosylated and acidic form of avidin was designed and expressed in soluble form in baculovirus-infected insect cells, The mutations were based on the same principles that guided the design of the chemically and enzymatically modified avidin derivative, known as NeutraLite Avidin. In this novel recombinant avidin derivative, five out of the eight arginine residues were replaced with neutral amino acids, and two of the lysine residues were replaced by glutamic acid. In addition, the carbohydrate-bearing asparagine-17 residue was altered to an isoleucine, according to the known sequences of avidin-related, genes. The resultant mutant protein, termed recombinant NeutraLite Avidin, exhibited superior properties compared to those of avidin, streptavidin and the conventional NeutraLite Avidin, prepared by chemo-enzymatic means, In this contest, the recombinant mutant is a single molecular species, which possesses strong biotin-binding characteristics. Due to its acidic pi, it is relatively free from non-specific binding to DNA and cells. The recombinant NeutraLite Avidin retains seven lysines per subunit, which are available for further conjugation and derivatization. (C) 2000 Federation of European Biochemical Societies.
The distribution of cellulosomal cohesin domains among the sequences currently compiled in various sequence databases was investigated, Two cohesin domains were detected in two consecutive open reading frames (ORFs) of the recently sequenced genome of the archaeon Archaeoglobus fulgidus. Otherwise, no cohesin-like sequence could be detected in organisms other than those of the Eubacteria, One of the A. fulgidus cohesin-containing ORFs also harbored a dockerin domain, but the additional modular portions of both genes are undefined, both with respect to sequence homology and function. It is currently unclear what function(s) the putative cohesin and dockerin-containing proteins play in the life cycle of this organism, In particular, since A. fulgidus contains no known glycosyl hydrolase gene, the presence of a cellulosome can be excluded. The results suggest that cohesin and dockerin signature sequences cannot be used alone for the definitive identification of cellulosomes in genomes. (C) 1999 Federation of European Biochemical Societies.
Sea urchin fibropellins are epidermal growth factor homologues that harbor a C-terminal domain, similar in sequence to hen egg-white avidin and bacterial streptavidin, The fibropellin sequence was used as a conceptual template for mutation of designated conserved tryptophan residues in the biotin-binding sites of the tetrameric proteins, avidin and streptavidin. Three different mutations of avidin, Trp-110-Lys, Trp-70-Arg and the double mutant, were expressed in a baculovirus-infected insect cell system, A mutant of streptavidin, Trp-120-Lys, was similarly expressed, The homologous tryptophan to lysine (W-->K) mutations of avidin and streptavidin were both capable of binding biotin and biotinylated material. Their affinity for the vitamin was, however, significantly reduced: from K-d similar to 10(-15) M of the wild-type tetramer down to K-d similar to 10(-8) M for both W --> K mutants. In fact, their binding to immobilized biotin matrices could be reversed by the presence of free biotin, The Trp-70-Arg mutant of avidin bound biotin very poorly and the double mutant (which emulates the fibropellin domain) failed to bind biotin at all. Using a gel filtration fast-protein liquid chromatography assay, both W --> K mutants were found to form stable dimers in solution, These findings may indicate that mimicry in the nature of the avidin sequence and fold by the fibropellins is not designed to generate biotin-binding, but may serve to secure an appropriate structure for facilitating dimerization, (C) 1999 Federation of European Biochemical Societies.
A novel cellulosomal scaffoldin gene, termed cipV, was identified and sequenced from the mesophilic cellulolytic anaerobe Acetivibrio cellulolyticus. Initial identification of the protein was based on a combination of properties, including its high molecular weight, cellulose-binding activity, glycoprotein nature, and immuno-cross-reactivity with the cellulosomal scaffoldin of Clostridium thermocellum. The cipV gene is 5,748 bp in length and encodes a 1,915-residue polypeptide with a calculated molecular weight of 199,496. CipV contains an N-terminal signal peptide, seven type I cohesin domains, an internal family III cellulose-binding domain (CBD), and an X2 module of unknown function in tandem with a type II dockerin domain at the C terminus. Surprisingly, CipV also possesses at its N terminus a catalytic module that belongs to the family 9 glycosyl hydrolases. Sequence analysis indicated the following. (i) The repeating cohesin domains are very similar to each other, ranging between 70 and 90% identity, and they also have about 30 to 40% homology with each of the other known type I scaffoldin cohesins. (ii) The internal CBD belongs to family III but differs from other known scaffoldin CBDs by the omission of a 9-residue stretch that constitutes a characteristic loop previously associated with the scaffoldins. (iii) The C-terminal type II dockerin domain is only the second such domain to have been discovered; its predicted "recognition codes" differ from those proposed for the other known dockerins. The putative calcium-binding loop includes an unusual insert, lacking in all the known type I and type II dockerins. (iv) The X2 module has about 60% sequence homology with that of C. thermocellum and appears at the same position in the scaffoldin. (v) Unlike the other known family 9 catalytic modules of bacterial origin, the CipV catalytic module is not accompanied by a flanking helper module, e.g., an adjacent family IIIc CBD or an immunoglobulin-like domain. Comparative sequence analysis of the CipV functional modules with those of the previously sequenced scaffoldins provides new insight into the structural arrangement and phylogeny of this intriguing family of microbial proteins. The modular organization of CipV is reminiscent of that of the CipA scaffoldin from C. thermocellum as opposed to the known scaffoldins from the mesophilic clostridia. The phylogenetic relationship of the different functional modules appears to indicate that the evolution of the scaffoldins reflects a collection of independent events and mechanisms whereby individual modules and other constituents are incorporated into the scaffoldin gene from different microbial sources.
We describe a method for the isolation of recombinant single-chain antibodies in a biologically active form, The single-chain antibodies are fused to a cellulose binding domain as a single-chain protein that accumulates as insoluble inclusion bodies upon expression in Escherichia coli. The inclusion bodies are then solubilized and denatured by an appropriate chaotropic solvent, then reversibly immobilized onto a cellulose matrix via specific interaction of the matrix with the cellulose binding domain (CBD) moiety. The efficient immobilization that minimizes the contact between folding protein molecules, thus preventing their aggregation, is facilitated by the robustness of the Clostridium thermocellum CBD we use. This CBD is unique in retaining its specific cellulose binding capability when solubilized in up to 6 M urea, while the proteins fused to it are fully denatured. Refolding of the fusion proteins is induced by reducing with time the concentration of the denaturing solvent while in contact with the cellulose matrix. The refolded single-chain antibodies in their native state are then recovered by releasing them from the cellulose matrix in high yield of 60% or better, which is threefold or higher than the yield obtained by using published refolding protocols to recover the same scFvs. The described method should have general applicability for the production of many protein-CBD fusions in which the fusion partner is insoluble upon expression. (C) 1999 Academic Press.
The cellulosome is an extracellular supramolecular machine that can efficiently degrade crystalline cellulosic substrates and associated plant cell wall polysaccharides. The cellulosome arrangement can also promote adhesion to the insoluble substrate, thus providing individual microbial cells with a direct competitive advantage in the utilization of the soluble hydrolysis products.
The action of cellulosomes from Clostridium thermocellum on model cellulose microfibrils from Acetobacter xylinum and cellulose microcrystals from Vonia ventricosa was investigated. The biodegradation of these substrates was followed by transmission electron microscopy, Fourier-transform IR spectroscopy and Xray diffraction analysis, as a function of the extent of degradation. The cellulosomes were very effective in catalysing the complete digestion of bacterial cellulose, but the total degradation of Valonia microcrystals was achieved more slowly. Ultrastructural observations during the digestion process suggested that the rapid degradation of bacterial cellulose was the result of a very efficient synergistic action of the various enzymic components that are attached to the scaffolding protein of the cellulosomes. The degraded Valonia sample assumed various shapes, ranging from thinned-down microcrystals to crystals where one end was pointed and the other intact. This complexity may be correlated with the multi-enzyme content of the cellulosomes and possibly to a diversity of the cellulosome composition within a given batch. Another aspect of the digestion of model celluloses by cellulosomes is the relative invariability of their crystallinity, together with their I-alpha/I-beta composition throughout the degradation process. Comparison of the action of cellulosomes with that of fungal enzymes indicated that the degradation of cellulose crystals by cellulosomes occurred with only limited levels of processivity, in contrast with the observations reported for fungal enzymes. The findings were consistent with a mechanism whereby initial attack by a cellulosome of an individual cellulose crystal results in its 'commitment' towards complete degradation.
The cellulosome concept is one of the major paradigms by which microorganisms bring about the efficient enzymatic degradation of crystalline cellulosic substrates. The principal structural element of a cellulosome is a scaffoldin subunit which integrates the other enzymatic subunits into the cellulosome complex. Ln recent years, the three-dimensional structures of various functional modules, which characterize the typical cellulosome, and in some cases free cellulases as well, have been determined. Currently, phylogenetic analysis of cellulosome-related sequences is contributing novel evidence concerning the intra- and interspecies relationship of the functional modules that comprise a typical cellulosome.
Ligands containing amino or hydroxyl groups were converted to their corresponding activated N-hydroxysuccinimidyl carbamate and carbonate by reaction with disuccinimidyl carbonate (DSC). The latter reagents can be used for the group-specific modification of primary amines as an alternative to the widespread usage of N-hydroxysuccinimide esters. Biotin and 2,4-dinitrophenyl (DNP) derivatives were used as examples to demonstrate the approach. Biotin and DNP were each extended by attaching two different spacer arms, carrying either a hydroxyl group or a primary amine as terminal functions. The latter were then activated via their conversion to N-hydroxysuccinimide carbonates and carbamates, respectively. The usefulness of these reagents for protein modification was investigated. The modified proteins obtained exhibited similar stability and activity characteristics compared to those modified with active N-hydroxysuccinimdyl esters. The activation of hydroxy- or amino-terminating compounds with DSC represents a general method that can be applied to any ligand which contains these functional groups for its covalent coupling to amines. (C) 1999 Elsevier Science B.V. All rights reserved.
The cellulosome is a macromolecular machine, whose components interact in a synergistic manner to catalyze the efficient degradation of cellulose. The cellulosome complex is composed of numerous kinds of cellulases and related enzyme subunits, which are assembled into the complex by virtue of a unique type of scaffolding subunit (scaffoldin). Each of the cellulosomal subunits consists of a multiple set of modules, two classes of which (dockerin domains on the enzymes and cohesin domains on scaffoldin) govern the incorporation of the enzymatic subunits into the cellulosome complex. Another scaffoldin module-the cellulose-binding domain-is responsible for binding to the substrate. Some cellulosomes appear to be tethered to the cell envelope via similarly intricate, multiple-domain anchoring proteins. The assemblage is organized into dynamic polycellulosomal organelles, which adorn the cell surface. The cellulosome dictates both the binding of the cell to the substrate and its extracellular decomposition to soluble sugars, which are then taken up and assimilated by normal cellular processes. (C) 1998 Academic Press.
Avidin, a positively charged egg-white glycoprotein, is a widely used tool in biotechnological applications because of its ability to bind biotin strongly, The high pI of avidin (similar to 10.5), however, is a hindrance in certain applications due to non-specific (charge-related) binding. Here we report a construction of a series of avidin charge mutants with pIs ranging from 9.4 to 4.7. Rational design of the avidin mutants was based on known crystallographic data together with comparative sequence alignment of avidin, streptavidin and a set of avidin-related genes which occur in the chicken genome. All charge mutants retained the ability to bind biotin tightly according to optical biosensor interaction analysis. In most cases, their thermal stability characteristics were indistinguishable from those of the wild-type avidin, Our results demonstrate that the charge properties of avidin can be modified without disturbing the crucial biotin-binding activity. (C) 1998 Federation of European Biochemical Societies.
A new approach to illustrate the principle of signal transduction and to assemble protein multilayers is described. It is based on competing affinities of two different ligands for the same binding site of a protein. A low-affinity ligand can be attached covalently to the protein, where it will be buried in the binding site and thus be prevented to interact with other proteins that recognize it. However, if a high-affinity ligand (or a molecule containing this ligand) is added, it will displace the low-affinity ligand (which still remains covalently bound) from the binding site to the periphery. The low-affinity ligand is now available for interaction with other molecules, thus providing the means through which to assemble multilayers of proteins by a "recognition cascade". This principle was demonstrated using the protein avidin which binds two ligands, biotin and 4-hydroxyazobenzene-2-carboxylic acid (HABA), with markedly different affinities. Avidin was affinity labeled with HABA, the low-affinity ligand, to produce a red, covalently conjugated avidin-HABA derivative (red avidin). Anti-HABA antibodies failed to recognize HABA buried in the binding site of avidin. However, upon addition of the high-affinity ligand biotin, HABA was expelled from the binding site and immediately bound by the antibodies. Multilayer assemblies of HABAylated avidin and biotinylated anti-HABA antibodies could thus be constructed. This concept may find application in numerous fields, such as medicine, diagnostics, nanotechnology, and artificial intelligence.
The structural complexity and rigidity of cellulosic substrates have given rise to a phenomenal diversity of degradative enzymes - the cellulases. Cellulolytic microorganisms produce a wide variety of different catalytic and noncatalytic enzyme modules, which form the cellulases and act synergistically on their substrate. In some microbes, several types of cellulases are organized into an elaborate multifunctional supramolecular complex, known as the cellulosome. A combination of molecular genetic, biochemical, chemical, crystallographic and microscopic techniques are paving the way for new insights into both the structure of cellulose and the mechanisms of its hydrolysis.
An adhesion defective mutant of Ruminococcus albus SY3 was isolated by a subtractive enrichment procedure, which involved repetitive adsorption of cellobiose-grown cells to cellulose. The growth characteristics of the mutant were compared With those of the wild type. Like the wild-type cells, the mutant was capable of growing on soluble substrates, i.e. cellobiose and xylan. However, in contrast to the wild type strain, the mutant was impaired in its capacity to utilize insoluble substrates, e.g. crystalline cellulose, acid-swollen cellulose or alfalfa cell walls. Scanning electron microscopy revealed protuberance-like surface structures on the wild-type strain which were absent on the mutant. The levels of endoglucanase and xylanase enzymatic activities released into the extracellular culture fluid were higher in the wild type compared to the mutant. However, Avicelase activity was not detected in the extracellular culture fluid of either strains when grown on cellobiose.
The cross-species specificity of the cohesin-dockerin interaction, which defines the incorporation of the enzymatic subunits into the cellulosome complex, has been investigated, Cohesin-containing segments from the cellulosomes of two different species, Clostridium thermocellum and Clostridium cellulolyticum, were allowed to interact with cellulosomal (dockerin-containing) enzymes from each species, In both cases, the cohesin domain of one bacterium interacted with enzymes from its own cellulosome in a calcium-dependent manner, but the same cohesin failed to recognize enzymes from the other species, Thus, in the case of these two bacteria, the cohesin-dockerin interaction seems to be species-specific, Based on intra-and cross-species sequence comparisons among the different dockerins together with their known specificities, we tender a prediction as to the aminoacid residues critical to recognition of the cohesins. The suspected residues were narrowed down to only four, which comprise a repeated pair located within the calcium-binding motif of two duplicated sequences, characteristic of the dockerin domain, According to the proposed model, these four residues do not participate in the binding of calcium per se; instead, they appear to serve as recognition codes in promoting interaction with the cohesin surface. (C) 1997 Wiley-Liss, Inc.
Background: The scaffoldin component of the cellulolytic bacterium Clostridium thermocellum is a non-hydrolytic protein which organizes the hydrolytic enzymes in a large complex, called the cellulosome. Scaffoldin comprises a series of functional domains, amongst which is a single cellulose-binding domain and nine cohesin domains which are responsible for integrating the individual enzymatic subunits into the complex, The cohesin domains are highly conserved in their primary amino acid sequences. These domains interact with a complementary domain, termed the dockerin domain, one of which is located on each enzymatic subunit. The cohesin-dockerin interaction is the crucial interaction for complex formation in the cellulosome, The determination of structural information about the cohesin domain will provide insights into cellulosome assembly and activity. Results: We have determined the three-dimensional crystal structure of one of the cohesin domains from C. thermocellum (cohesin 2) at 2.15 Angstrom resolution, The domain forms a nine-stranded beta sandwich with a jelly-roll topology, somewhat similar to the fold displayed by its neighboring cellulose-binding domain. Conclusions: The compact nature of the cohesin structure and its lack of a defined binding pocket suggests that binding between the cohesin and dockerin domains is characterized by interactions between exposed surface residues. As the cohesin-dockerin interaction appears to be rather nonselective, the binding face would presumably be characterized by surface residues which exhibit both intraspecies conservation and interspecies dissimilarity, Within the same species, unconserved surface residues may reflect the position of a given cohesin domain within the scaffoldin subunit, its orientation and interactions with neighboring domains.
An efficient lepidopteran insect cell system was established for the expression of a recombinant form of chicken egg-white avidin. The gene product was obtained in both secreted and intracellular forms, and biologically active recombinant avidin was isolated using affinity chromatography on an iminobiotin-agarose column. Similar to the known quaternary structure of the native egg-white protein, the purified recombinant protein was glycosylated and assembled mainly into tetramers. Like native avidin, the recombinant tetramer also exhibited a high level of thermostability, and was further stabilized upon binding biotin. The biotin-binding and structural properties of the recombinant avidin are thus similar to those of the natural egg-white protein, and the insect system is appropriate both for future site-directed mutagenesis studies and for the production of avidin fusion proteins. (C) 1997 Academic Press.
Recombinant cohesin-2, a unique type of protein-recognition domain from the cellulosome of Clostridium thermocellum, has been crystallized by tile hanging-drop vapor-diffusion method. The crystals are monoclinic, space group C2 with unit-cell dimensions a = 79.91, b = 47.86, c = 51.13 Angstrom, beta = 126.77 degrees. There is most likely to be one molecule per asymmetric unit, corresponding to a packing density of 2.16 Angstrom(3) Da(-1). The crystals diffract to beyond 2.3 Angstrom on a conventional laboratory rotating-anode source.
A novel approach for the selection of high-affinity phage from phage-peptide Libraries is described. The methodology employs a chemically modified form of streptavidin, termed nitrostreptavidin, which exhibits a reversible attraction for biotin. The new approach emulates conventional procedures in that a biotinylated probe, in this case biotinylated alpha-bungarotoxin, is attached to an immobilized streptavidin matrix The phage library is introduced, and interacting phage particles are released under conventional acidic conditions (pH 2.2). At this stage, the primary peptide sequences characterizing the released phage are found to be identical with those previously known to interact with the toxin. However, other phage particles, which presumably interact more strongly than those released by acid, remain attached to the immobilized toxin. These can be released by virtue of the reversible biotin-binding properties of nitrostreptavidin. For this purpose, alkaline solutions (pH 10) or free biotin can be used. Using this approach, phage particles that recognize alpha-bungarotoxin were isolated; their peptide sequences were found to be similar to, but clearly distinct from, those collected by conventional acid elution. The affinity of the isolated phage was dramatically higher than that of phage obtained by the conventional methodology. In contrast, their synthetically prepared 15-mer peptides actually exhibited a lower affinity for the toxin than that shown by peptides prepared on the basis of the sequence obtained from conventional acid-eluted phage, This apparent discrepancy can be explained by an altered conformational state of the peptides in solution, compared to the epitopes expressed in situ on the phage surface. (C) 1996 Academic Press, Inc
Chemically modified forms of egg-white avidin and bacterial streptavidin (termed nitro-avidin and nitro-streptavidin, respectively), in which the binding-site tyrosine was nitrated, were used for several biotechnological applications. The fundamental difference between nitro-avidin and the native protein is that interaction of the modified protein with biotin can be reversed under relatively mild conditions. Consequently, nitro-avidin affinity columns or immobilizing matrices can be reused. Three examples are given to demonstrate the possible uses of such columns: (a) biotinylated protein A was attached to a nitro-avidin affinity column, and immunoglobulin was purified directly from whole rabbit serum; (b) biotinylated transferrin was attached to a nitro-streptavidin column, and anti-transferrin was isolated directly from rabbit anti-serum; and (c) biotinylated beta-glucosidase was immobilized onto a nitro-avidin column and used as an enzyme reactor. In each example, the immobilized biotinylated probe could be released selectively from the column and recovered following its utilization. Reusable nitro-avidin thus provides an easy and attractive reversible form of avidin and thereby serves to expand the versatility of avidin-biotin technology. (C) 1996 Academic Press, Inc.
The cellulosome from the anaerobic, thermophilic, cellulolytic bacterium, Clostridium thermocellum, was dissociated under nondenaturing conditions by incubation at 60 degrees C in the combined presence of EDTA and cellulose. This approach differs from previous strategies in that detergents were not necessary for dissociation of the subunits. Selected enzymatic subunits could be isolated with relative ease. One of these is the major cellulosomal cellobiohydrolase, CelS (previously designated subunit S8 or S-s). Both the intact cellulosome complex and the combined dissociated subunits exhibited similar levels of activities on soluble and acid-swollen cellulosic substrates. In contrast, the mixture of the free enzymatic subunits was markedly deficient in its degradation of crystalline cellulose compared with that of the intact cellulosome. The results support the critical role of divalent cations. e.g. calcium, in the stability of the cellulosome complex. In addition, the results reinforce previous indications that the cellulosome undergoes conformational changes upon binding to cellulose. Copyright (C) 1996 Elsevier Science B.V.
The cellulosome of the cellulolytic bacterium, Clostridium thermocellum, is a multi-enzyme complex in which the enzymatic (cellulolytic) subunits are attached to a unique nonhydrolytic subunit called scaffoldin. The attachment is mediated by two mutually interacting subunits. Knowledge of the three-dimensional structure of each of the interacting components would be critical to a better understanding of the cohesin-dockerin interaction at the molecular level. In this report, we describe the purification of one of the nine cohesin domains of the scaffoldin subunit from C. thermocellum. A DNA segment containing the cohesin 2 sequence was fused to a hexa-histidine tag, and the resultant construct was expressed in Escherichia coli. The expressed peptide was efficiently isolated by metal-chelate affinity chromatography. The purified recombinant form of the cohesin was crystallized pending determination of its structure. Copyright (C) 1996 Elsevier Science B.V.
The crystal structure of a family-III cellulose-binding domain (CBD) from the cellulosomal scaffoldin subunit of Clostridium thermocellum has been determined at 1.75 Angstrom resolution, The protein forms a nine-stranded beta sandwich with a jelly roll topology and binds a calcium ion, Conserved, surface-exposed residues map into two defined surfaces located on opposite sides of the molecule, One of these faces is dominated by a planar linear strip of aromatic and polar residues which are proposed to interact with crystalline cellulose, The other conserved residues are contained in a shallow groove, the function of which is currently unknown, and which has not been observed previously in other families of CBDs. On the basis of modeling studies combined with comparisons of recently determined NMR structures for other CBDs, a general model for the binding of CBDs to cellulose is presented. Although the proposed binding of the CBD to cellulose is essentially a surface interaction, specific types and combinations of amino acids appear to interact selectively with glucose moieties positioned on three adjacent chains of the cellulose surface, The major interaction is characterized by the planar strip of aromatic residues, which align along one of the chains, In addition, polar amino acid residues are proposed to anchor the CBD molecule to two other adjacent chains of crystalline cellulose.
Avidin, a positively charged egg-white protein, aggregates extensively when mixed at ambient temperatures with anionic detergents, such as sodium dodecyl sulfate (SDS). The resultant aggregates fail to penetrate the stacking gel during polyacrylamide gel electrophoresis (PAGE). To prevent the formation of such aggregates, avidin was acetylated and the pI was thus reduced. Acetylated avidin was found to behave in a manner similar to that of streptavidin; under nondenaturing conditions (i.e., incubation of samples at room temperature), both proteins normally migrated mainly as tetramers with a tendency to form oligomers of the tetramer. When samples were boiled, both proteins migrated mainly as the monomer. The comparative stability properties of avidin and streptavidin were also examined using SDS-PAGE by heating samples and determining the extent of dissociation of tetramers to monomers as a function of temperature. A distinctive transition temperature could be defined for individual samples. Using this assay, it was determined that, in the absence of biotin, the quaternary structure of streptavidin is more stable than that of avidin. Biotin appears to stabilize structures of both avidin and streptavidin to a similar degree. Acetylation of avidin thus provides a simple means to analyze the quaternary structure of the molecule using SDS-PAGE.
The tight interaction between the vitamin biotin and the protein avidin is so strong (K-a similar to 10(-15) M(-1)) that conditions which are usually sufficient for protein denaturation fail to dissociate the avidin-biotin complex. In order to form a reversible interaction between the two biomolecules, we have modified the binding-site tyrosine by nitration, thus reducing the pK(a) of the phenol group which forms a crucial hydrogen bond with the ureido group of biotin. At relatively low pH values (4-5), the resultant modified forms of avidin bind biotin with a very high association constant (> 10(9) M(-1)). The modified avidins are thus capable of supporting stable, long-term binding of biotin or biotinylated macromolecules. The latter molecules can be detached by increasing the pH of the medium or by introduction of excess levels of biotin at neutral pH. These findings demonstrate the importance of a single hydrogen bond for strong biotin binding. The new derivatives of avidin should be useful for applications whereby a reversible interaction between the four biotin-binding sites and biotin is desired, thus increasing the versatility of the avidin-biotin system for biotechnological application.
Two strains of Streptomyces venezuelae were found to produce high-affinity, biotin-binding proteins, termed streptavidin vl and v2, respectively. Both proteins were isolated to purity, and their corresponding genes were cloned and sequenced. Compared to streptavidin from S. avidinii, streptavidin v1 had only a single amino acid substitution and streptavidin v2 showed 9 such differences. The substitutions were remarkably conservative, none of which affected the amino acid residues known to be important to the biotin-binding properties or to the structure of the tetrameric protein. The results also indicate that the biosynthesis of such biotin-binding proteins is not simply a curious anomaly in a single species of Streptomyces. It is suggested that the classification of S, avidinii as a unique species should be reconsidered. The occurrence of these proteins appears to be linked to the production of an unusual synergistic antibiotic complex.
A simple procedure for the preparation of deglycosylated avidin is described. Commercially obtained avidin was treated with a mixed microbial culture. The cells were capable of growing on the oligosaccharide residues, but generally ignored the polypeptide portion of the egg white glycoprotein. The resultant deglycosylated avidin retained its biotin-binding characteristics. The major bacterial strain (strain BECH080), responsible for the deglycosylation, was isolated. On the basis of elementary biochemical tests, fatty acid, and phenotypic analyses, the isolate was identified as a strain of Flavobacterium meningosepticum. The primary enzymatic activity that caused the removal of the oligosaccharide residues of avidin appeared to be similar to endoglycosidase F.
A heat-stable enzyme was isolated from the cellulase complex of a thermophilic strain of the micromycete Thielavia terrestris. The purified enzyme exhibited both endoglucanase and xylanase activities and had a mol mass of 69,000 Daltons and an isoelectric point of 6.4. When the cells were grown at 48 degrees C, the initial activity of the purified enzyme using carboxymethylcellulose as a substrate was 150 nkat/mg apd the Michaelis constant was 6.6 g/L. The heat stability of the enzyme was high, losing only 20% of the initial activity after a 6-h incubation at 65 degrees C. When cultures were grown on microcrystalline cellulose and xylose was added after 48 h of growth, endoglucanase and xylanase activities were more than doubled. Similar increases in these activities were observed by growing the cultures on straw.
The enzymatic subunits of the cellulosome of Clostridium thermocellum are integrated into the complex by a major non-catalytic polypeptide, called scaffoldin. Its numerous functional domains include a single cellulose-binding domain (CBD) and nine subunit-binding domains, or cohesin domains, Two of the cohesin domains, together with the adjacent CBD, have been cloned and expressed in Escherichia coli, and the recombinant constructs were purified by affinity chromatography on a cellulosic matrix. Both cohesin domains, which differ by about 30% in their primary structure, showed a similar binding profile to the cellulosomal subunits, Calcium ions enhanced dramatically this binding. Under the conditions of the assay, only one major catalytic subunit of the cellulosome failed to bind to either cohesin domain. The results indicate a lack of selectivity in the binding of cohesin domains to the catalytic subunits and also suggest that additional mechanisms may be involved in cellulosome assembly,
The cellulose-binding domain from the scaffoldin subunit of the cellulosome from Clostridium thermocellum strain YS has been expressed in Escherichia coli, purified to homogeneity and crystallized. Crystals were grown by vapor diffusion using polyethylene glycol as precipitant. They belong to the monoclinic space group C2 with unit cell dimensions of a 64.68 Angstrom, b = 50.38 Angstrom C = 96.27 Angstrom; beta = 99.43 degrees, and density packing considerations suggest that the asymmetric unit contains two molecules. The crystals diffract beyond 2.0 Angstrom resolution using a laboratory rotating anode source.
The cellulases of many cellulolytic bacteria are organized into discrete multienzyme complexes, called cellulosomes. The multiple subunits of cellulosomes are composed of numerous functional domains, which interact with each other and with the cellulosic substrate. One of these subunits comprises a distinctive new class of noncatalytic scaffolding polypeptide, which selectively integrates the various cellulase and xylanase subunits into the cohesive complex. Intelligent application of cellulosome hybrids and chimeric constructs of cellulosomal domains should enable better use of cellulosic biomass and may offer a wide range of novel applications in research, medicine and industry.
The cellulase complexes of two cellulolytic bacteria, Clostridium thermocellum and Bacteroides cellulosolvens, were subjected to extensive Pronase digestion. Glycopeptide fractions were isolated by gel permeation and fast protein liquid chromatography and analyzed by monosaccharide analysis, amino acid analysis, methylation analysis, and H-1 NMR spectroscopy. Alkaline boro-hydride-induced deglycosylation/amino acid conversion and periodate oxidation studies on the glycopeptide fraction of the C. thermocellum cellulosome demonstrated that the earlier established collection of carbohydrate moieties with 3-O-Me-D-GlcpNAc-alpha(1-->2)-[D-Galp-alpha(1-->3)]-D-Galf-alpha(1-->2)-D-Gal (where 3-O-Me-D-GlcpNAc is 3-O-methyl-N-acetylglucopyranosamine, Galp is galactopyranose, and Galf is galactofuranose) as the major component, is O-linked to threonine via galactopyranose. Using the same approach for the glycopeptide fraction of the cellulase complex of B. cellulosolvens, it was found that the reported collection of carbohydrate moieties with D-Galf-alpha(1-->3)-D-GlcpNAc-alpha(1-->2)-D-Galf-alpha(1-->2)-[D-Galf-beta(1-->3)]-D-Gal as the major component, is O-linked mainly to threonine and partly to serine via galactopyranose. In both species, the hydroxyamino-acid-bound galactopyranose residue has probably an alpha-configuration. The carbohydrate chains appear as clusters located in highly Thr/Pro-rich peptide regions of the glycoproteins. The results are consistent with the notion that the glycosylation sites are localized in linker sequences which connect the various binding domains of the noncatalytic S1 subunit of the cellulosome.
The controversy regarding the identity of a major cellulosomal component type from two different strains of Clostridium thermocellum has been resolved. The principal cellobiohydrolase, subunit S8, from the cellulosome of strain YS has been demonstrated to be synonymous with cellulase component S(s) (CelS) from the cellulosome of ATCC strain 27405. This component is not related to any other cellulosomal subunit or cloned endoglucanase in this organism.
In the present study, we have demonstrated the feasibility of targeting a proteolytic enzyme, via the high-affinity avidin-biotin system, to act in a highly selective manner upon a cell surface-associated antibody. As an example of this approach, a cell-bound biotinylated monoclonal antibody could be removed efficiently by means of biotinylated proteinase K, bridged to streptavidin. Only low levels of cell death were observed using this procedure. The approach may prove useful for a variety of applications including the recovery of antibody-free positively selected cell populations.
The crystal structure of the complex formed between the egg-white biotin-binding protein, avidin, and the dye, 2-(4'-hydroxyazobenzene) benzoic acid (HABA), was determined to a resolution of 2.5 angstrom. The interaction of avidin with the benzoate ring of HABA is essentially identical to that of the complex formed between HABA and streptavidin (the bacterial analogue of the egg-white protein). This interaction emulates the definitive high-affinity interaction of both proteins with the ureido moiety of biotin. The major difference between the avidin- and streptavidin-HABA complexes lies in their interaction with the hydroxypheny) ring of the dye molecule; in avidin, two adjacent amino acid residues (Phe72 and Ser73), which are not present in streptavidin, form additional interactions with this ring. These are suggested to account for the higher affinity of avidin for HABA. The characteristic red shift, which accompanies the interaction of both proteins with the dye, was traced to a proposed charge-transfer complex formed between the hydroxyphenyl ring of HABA and the indole ring of Trp79 in avidin (Trp79 in streptavidin). Comparison of binding site residues of two such similar proteins versus their markedly different affinities for two such different substrates should eventually contribute to a better design of biomimetic reagents and drugs.
The crystal structures of a deglycosylated form of the egg-white glycoprotein avidin and of its complex with biotin have been determined to 2.6 and 3.0 angstrom, respectively. The structures reveal the amino acid residues critical for stabilization of the tetrameric assembly and for the exceptionally tight binding of biotin. Each monomer is an eight-stranded antiparallel beta-barrel, remarkably similar to that of the genetically distinct bacterial analog streptavidin. As in streptavidin, binding of biotin involves a highly stabilized network of polar and hydrophobic interactions. There are, however, some differences. The presence of additional hydrophobic and hydrophilic groups in the binding site of avidin (which are missing in streptavidin) may account for its higher affinity constant. Two amino acid substitutions are proposed to be responsible for its susceptibility to denaturation relative to streptavidin. Unexpectedly, a residual N-acetylglucosamine moiety was detected in the deglycosylated avidin monomer by difference Fourier synthesis.
The sequence of the V(H) gene of a monoclonal anti-biotin antibody was determined. Biotin-binding motifs, similar to those in avidin and streptavidin, were identified in complementarity determining regions 2 and 3, suggesting that natural selection of functional motifs may occur in unrelated protein types.
Streptavidin is a biotin-binding analogue of egg-white avidin which is secreted by the bacterium Streptomyces avidinii. We have recently reported that streptavidin contains an Arg-Tyr-Asp-Ser (RYDS) sequence which exhibits structural homology to the Arg-Gly-Asp-Ser (RGDS) cell adhesion domain of fibronectin and other matrix-associated glycoproteins. Competition studies with RGD peptides indicated that streptavidin binds to cells via this site and that the binding is independent of biotin recognition. Since the RGD-containing peptide has been shown to play a key role in integrin-mediated cell adhesion., we assumed that streptavidin may utilize the RYDS site to bind to immune cells and thereby abrogate their adhesion-dependent functions. We now report that streptavidin modulates several matrix-dependent interactions of immune cells. In this context, immobilized streptavidin was found to support activated human CD4+ T cell adhesion in an RGD-specific, alpha5beta1-dependent manner. In addition, soluble streptavidin (the commercially available or biotin-blocked forms) inhibited T cell adhesion to fibronectin and interfered with its co-stimulatory effect on tumor necrosis factor-alpha secretion by co-cultures of CD4+ T cells and macrophages. These results suggest that streptavidin is a novel example of a bacterial protein which utilizes RGD mimicry to interfere with integrin-mediated immune responses.
Human serum was found to contain natural antibodies to the egg-white glycoprotein avidin. Of 270 samples tested, all contained antibodies to different extents, mainly of the IgG and IgM classes. Anti-avidin antibodies could be isolated by affinity chromatography.
Biotin-blocked streptavidin binds specifically (K(d) approximately 3 x 10(-8) M) to cell surfaces, presumably via an RYD-containing sequence. This site is distinct from the biotin-binding cleft of the protein and bears high homology to the RGD-containing cell-binding domain of fibronectin. We show here that various cell types adhere to immobilized streptavidin and that the soluble protein interferes specifically with cell adhesion to fibronectin substrata (with an IC50 of about 1 X 10(-7) M) but less so to other adhesive glycoproteins (e.g., collagen type I, vitronectin). Immunochemical evidence combined with peptide competition studies demonstrated that cells bind to streptavidin primarily via the major fibronectin receptor (the alpha5beta1 integrin). The results suggest that streptavidin acts as a relatively strict fibronectin mimetic, thus reflecting the great similarity in their respective RYD/RGD sequence and the immediate flanking regions. The bacterial protein emulates and competes with fibronectin and other extracellular matrix adhesive proteins in the initial recognition and binding to cell surfaces, but appears not to induce subsequent processes (e.g., anchorage and spreading). Streptavidin may thus represent a novel example of bacterial protein mimicry of a key adhesion motif.
The 3' region of a gene designated cipB, which shows strong homology with cipA that encodes the cellulosome S(L) subunit of Clostridium thermocellum ATCC 27405, was isolated from a gene library of C. thermocellum strain YS. The truncated S1 protein encoded by the cipB derivative bound tightly to cellulose. The cellulose-binding domain in this polypeptide consisted of a C-terminal proximal 167 residue sequence which showed complete identity with residues 337-503 of mature S(L) from C. thermocellum strain ATCC 27405. The cellulose-binding domain interacted with both crystalline and amorphous cellulose, but not with xylan.
The interaction of streptavidin with various cell systems was studied using fluorescent derivatives of the protein. The native unprocessed form of streptavidin bound to cells at low levels and in a nonspecific manner. In contrast, both the truncated "core" streptavidin (the commercially available form) and the biotin-blocked unprocessed protein bound to cells in enhanced levels and in a specific, saturable manner. This suggests that the binding of biotin or cleavage of the terminal portion(s) of the native protein molecule causes conformational changes which lead to the exposure of sites which presumably interact with cell surface receptors. Peptide inhibition studies demonstrated that the majority of binding to cells appears to be dependent on RGD-like specificity, suggesting that the GRYDS sequence of the streptavidin molecule may exhibit such specificity. Indirect immunofluorescence assays revealed that the protein is associated mainly with the cell surface. Moreover, streptavidin was demonstrated to compete with specific monoclonal antibodies to the RGD-binding site on the GpIIbIIIa integrin of activated platelets, thus suggesting that streptavidin may facilitate binding to ubiquitous cell-surface adhesion receptors via RGD mimicry.
Clostridium thermocellum, an anaerobic thermophilic cellulolytic bacterium, produces an extremely cohesive, very high-molecular-mass, multicellulase-containing complex termed the cellulosome. One of its components, the S1 subunit, is a nonenzymatic, 210 kDa glycopolypeptide. Upon preconditioning of the intact cellulosome with low-ionic-strength or low-pH solutions, the S1 subunit separates in hot sodium dodecyl sulfate (SDS) solutions into a series of defined lower-molecular-mass subcomponents. Under the same conditions, the purified S1 subunit demonstrated the same behavior. Higher levels of glycosylation associated with the larger S1 subcomponents. The data support alterations in the conformational state of the S1 structure that lead to its disintegration induced by combined treatments with SDS and heating. Evidence is provided that this phenomenon may reflect a physiological response of the cellulosome, since similar alterations in S1 appear to accompany its binding to cellulose.
The affinity properties displayed by the cellulosome, the multienzyme complex from Clostridium thermocellum, for its insoluble polymeric substrate, cellulose, have previously been employed for its purification by affinity chromatography. In the present communication, a new purification procedure is described which provides five-fold higher yields (> 90%) of the cellulosome with enhanced solubilizing activity (approximately 1.5-fold). The new method is based on the efficient adsorption of the cellulosome onto phosphoric acid-treated substrate (amorphous cellulose) and the subsequent digestion of the carrier/substrate by the adsorbed enzyme complex. The term "affinity digestion" is proposed for such systems in which the affinity matrix is degraded totally by the adsorbed enzyme(s), thus facilitating its recovery.
The multiple cellulase-containing protein complex, isolated from the cellulolytic bacterium Bacteroides cellulosolvens, contains oligosaccharides which are O-linked mainly to a 230-kDa subunit. The oligosaccharide chains were liberated by alkaline-borohydride treatment and fractionated as oligosaccharide alditols via gel-permeation chromatography and HPLC. The fractions were investigated by one- and two-dimensional (correlation, homonuclear Hartmann-Hahn, rotating-frame nuclear Overhauser enhancement) 500-MHz H-1-NMR spectroscopy in combination with monosaccharide and methylation analyses and with fast-atom-bombardment mass spectrometry. The following carbohydrate structures could be established: [GRAPHICS]. The results indicate an interesting similarity between the oligosaccharide moieties of the cellulase complex of B. cellulosolvens and of Clostridium thermocellum [Gerwig, G. J., Kamerling, J. P., Vliegenthart, J. F. G., Morag (Morgenstern), E., Lamed, R. & Bayer, E. A. (1991) Eur. J. Biochem. 196, 115-122], having 3, 5 and 6 as common elements. The furanose form of a terminal alpha-D-galactose residue demonstrated an inhibitory effect on the interaction of Griffonia simplicifolia I isolectin B4 with the cellulosome-like entity of B. cellulosolvens.
The object of this study was to define minimized biotin-binding fragments, or 'prorecognition sites', of either the egg-white glycoprotein avidin or its bacterial analogue streptavidin. Because of the extreme stability to enzymic hydrolysis, fragments of avidin were prepared by chemical means and examined for their individual biotin-binding capacity. Treatment of avidin with hydroxylamine was shown to result in new cleavage sites in addition to the known Asn-Gly cleavage site (position 88-89 in avidin). Notably, the Asn-Glu and Asp-Lys peptide bonds (positions 42-43 and 57-58 respectively) were readily cleaved; in addition, lesser levels of hydrolysis of the Gln-Pro (61-62) and Asn-Asp (12-13 and 104-105) bonds could be detected. The smallest biotin-binding peptide fragment, derived from hydroxylamine cleavage of either native or non-glycosylated avidin, was identified to comprise residues 1-42. CNBr cleavage resulted in a 78-amino acid-residue fragment (residues 19-96) that still retained activity. The data ascribe an important biotin-binding function to the overlapping region (residues 19-42) of avidin, which bears the single tyrosine moiety. This contention was corroborated by synthesizing a tridecapeptide corresponding to residues 26-38 of avidin; this peptide was shown to recognize biotin. Streptavidin was not susceptible to either enzymic or chemical cleavage methods used in this work. The approach taken in this study enabled the experimental distinction between the chemical and structural elements of the binding site. The capacity to assign biotin-binding activity to the tyrosine-containing domain of avidin underscores its primary chemical contribution to the binding of biotin by avidin.
The cellulosome of Clostridium thermocellum is a highly cohesive multienzyme complex that is capable of completely solubilizing insoluble cellulose. One of the major cellulosomal components, the glycosylated S1 subunit, is believed to play an important structural role and normal migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an M(r) of 210,000. It is shown here that by simply altering the conditions (pH or ionic strength) of the environment prior to electrophoresis, a different migratory profile for S1 emerges, yielding a collection of bands, all of which migrate faster than the parent band. The original electrophoretic behavior of S1 can be reproduced on restoration of the original pH and ionic strength. These results may bear important significance for the physiological role of the S1 subunit in facilitating the observed synergistic action of the other (cellulolytic) components of the cellulosome.
We report on the interactions of urea and guanidinium salts with streptavidin. Gel filtration chromatography in 0, 4, 6, and 7 m urea indicates that the streptavidin tetramer remains intact in urea. Biotin alters the electrophoretic mobility of streptavidin whether or not 6 M urea is present. The intrinsic fluorescence of streptavidin is increased and blue-shifted in 6 m urea. The fluorescence changes indicate the absence of unfolding. A conformational response to urea is possible, but much of the fluorescence change is due to urea binding as a weak biotin analog (K(a) almost-equal-to 1.3 m-1). The resistance to structural perturbation by urea reflects the structural stability of streptavidin's antiparallel beta-barrel motif. Unfolding is sluggish in 6 M guanidinium hydrochloride (half-time, almost-equal-to 50 days). After guanidinium thiocyanate unfolding, streptavidin can be refolded, but the unfolding and refolding transitions are centered at different concentrations of perturbant. Slow unfolding, with a 15th power dependence on guanidinium thiocyanate concentration, may be partially responsible for the noncoincidence of the unfolding and refolding processes. Nonequilibrium behavior is also seen in 6 M urea, as native streptavidin does not unfold and guanidinium thiocyanate unfolded streptavidin does not refold. Refolding does occur at lower concentrations of urea. Guanidinium thiocyanate only slowly unfolds the biotin-streptavidin complex. In the presence of biotin, unfolded streptavidin does not refold in 6 M guanidinium thiocyanate or in 6 M urea.
The extensive use of antibody-containing affinity columns in the purification of biologically active compounds (e.g., genetically engineered proteins) is severely hampered by the leaching of antibody (or portions thereof) from the immunoaffinity resin during elution of the target antigen. One of the major problems in this context is the combined use of reducing (i.e., thiols) and chaotropic (e.g., detergents and denaturants) agents in the elution step, which causes the disassociation of heavy and/or light chains from the immobilized antibody, thereby contaminating the resultant product. In order to overcome this problem, we have cross-linked the four antibody chains at their sites of disulfide interlinkage, thus producing a single antibody chain. To accomplish this, interchain disulfide bonds were reduced, and the resultant thiol groups were cross-linked by using bifunctional SH-specific reagents (particularly bismaleimides). Cross-linking of up to 95% of the available SH groups produced was achieved with concomitant retention of antigen-binding activity. The cross-linked antibody was immobilized onto CNBr-activated Sepharose, and the resultant column was found to be substantially more stable to harsh elution conditions than similar columns which contain the un-cross-linked antibody.
In the anaerobic, thermophilic, cellulolytic bacterium Clostridium thermocellum, efficient solubilization of the insoluble cellulose substrate is accomplished largely through the action of a cellulose-binding multienzyme complex, the cellulosome. A major cellobiohydrolase activty from the cellulosome has been traced to its M(r) 75,000 S8 subunit, and an active fragment of this subunit was prepared by a novel procedure involving limited proteolytic cleavage. The truncated M(r) 68,000 fragment, termed S8-tr, was purified by gel filtration and high-performance ion-exchange chromatography. The purified protein adsorbed weakly to amorphous cellulose, and its enzymatic action yielded cellobiose as the major end product from both amorphous and crystalline cellulose preparations. The high ratio of exo- to endo-beta-glucanase activities was supported by viscosimetric measurements. The use of model substrates showed that the smallest cellodextrin to be degraded was cellotetraose, but cellopentaose was degraded at a much greater rate. Cellobiose dramatically inhibited the cellulolytic activities. In the absence of calcium or other bivalent metal ions, both the truncated cellobiohydrolase activity of S8-tr and the true cellulase activity of the parent cellulosome were relatively unstable at temperatures above 50-degrees-C. Cysteine further enhanced the stabilizing effect of calcium. This is the first report of a defined cellobiohydrolase in C. thermocellum. Its association with the cellulosome and the correspondence of several of their major distinctive properties suggest that this cellobiohydrolase plays a key role in the solubilization of cellulose by the intact cellulosomal complex.
The cell-free forms of the multiple cellulase-containing protein complex (cellulosome), isolated from the cellulolytic bacterium Clostridium thermocellum strains YS, ATCC 27405 and LQRI, have a total carbohydrate content of 5-7% (by mass), consisting of O-linked oligosaccharide chains. The carbohydrate chains were liberated by alkaline-borohydride treatment and fractionated as oligosaccharide alditols via gel-permeation chromatography and HPLC. The fractions were investigated by 500-MHz H-1-NMR spectroscopy in combination with monosaccharide and methylation analysis and with fast-atom-bombardment mass spectrometry (FAB-MS). In addition to the previously described major oligosaccharide, [GRAPHICS] [Gerwig, G. J., de Waard, P., Kamerling, J. P., Vliegenthart, J. F. G., Morgenstern, E., Lamed, R. & Bayer, E. A. (1989) J. Biol. Chem. 264, 1027-1035], the following partial structures of this compound could be established: [GRAPHICS] Cell-free and cell-associated forms of the cellulosome of C. thermocellum, as determined for strain YS, have the same oligosaccharide pattern. Based on the oligosaccharide structures, a biosynthetic pathway is suggested.
The cellulosome, the multienzyme complex of the cellulase system of Clostridium thermocellum, that mediates the solubilization of insoluble cellulose, is strongly inhibited by the major end product, cellobiose. By combining a purified beta-glucosidase from Aspergillus niger with the cellulosome, accumulated cellobiose was hydrolyzed thereby resulting in a dramatic enhancement (up to 10-fold) of cellulose degradation. The observed enhancement was expressed both in the rate and degree of solubilization of microcrystalline cellulose, compared with that observed for the unsupplemented cellulosome. Near-complete conversion of cellulose to glucose could be obtained from dense substrate suspensions (up to at least 200 g/L).
A multi-coupled enzyme assay system for determining sialidase activity is described. Enzymes, substrates and chromogens are reacted in situ and determined spectrophotometrically in ELISA microtiter plates. Sialidase is assayed by the extent of desialylated galactose on an appropriate sialoglycoconjugate (fetuin), which is otherwise unavailable for oxidation by galactose oxidase. The oxidation is monitored by the coupling of H2O2 released to a third enzyme, peroxidase. The rate of change of absorbance at 405 nm, resulting from the oxidized chromogen is a measure of the reaction rate of the coupled enzyme system. A similar system can be used for determining galactose oxidase in solution, or on blots using galactose as substrate. Due to the small-scale single-step measurement, the described assay is a sensitive, convenient, and inexpensive alternative to the classic colorimetric determination.
Biotin binding reduces the tryptophan fluorescence emissions of streptavidin by 39%, blue shifts the emission peak from 333 to 329 nm, and reduces the bandwidth at half height from 53 to 46 nm. The biotin-induced emission difference spectrum resembles that of a moderately polar tryptophan. Streptavidin fluorescence can be described by two lifetime classes: 2.6 nsec (34%) and 1.3 nsec (66%). With biotin bound, lifetimes are 1.3 nsec (26%) and 0.8 nsec (74%). Biotin binding reduces the average fluorescence lifetime form 1.54 to 0.88 nsec. Biotin does not quench the fluorescence of indoles. The fluorescence changes are consistent with biotin binding causing a conformational change which moves tryptophans into proximity to portions of streptavidin which reduce the quantum yield and lifetimes. Fluorescence quenching by acrylamide revealed two classes of fluorophores. Analysis indicated a shielded component comprising 20-28% of the initial fluorescence with (K(SV) + V) less-than-or-equal-to 0.55 M-1. The more accessible component has a predominance of static quenching. Measurements of fluorescence lifetimes at different acrylamide concentrations confirmed the strong static quenching. Since static quenching could be due to acrylamide binding to streptavidin, a dye displacement assay for acrylamide binding was constructed. Acrylamide does bind to streptavidin (Ka = 5 M-1), and probably binds within the biotin-binding site. In the absence of biotin, none of streptavidin's fluorescence is particularly accessible to iodide. In the presence of biotin, iodide neither quenches fluorescence nor alters emission spectra, and acrylamide access is dramatically reduced. We propose that the three tryptophans which always line the biotin site are sufficiently close to the surface of the binding site to be quenched by bound acrylamide. These tryptophans are shielded from iodide, most probably due to steric or ionic hindrances against diffusion into the binding site. Most of the shieldi
The avidin-biotin system was used in order to target enzymes to their substrates in complex mixtures of proteins in solution. The approach described here thus mimics natural systems in which enzymes usually act in selective fashion, due, perhaps, to proximity effects. For affinity cleavage studies, biotinyl transferrin was used as a model target substrate. Avidin or streptavidin was then employed to bridge between the biotinylated target protein and a biotinyl protease. Bovine serum albumin was included in the reaction mixtures to assess the level of nonspecific cleavage. In the case of an unbiotinylated target protein, avidin could be used to inhibit the hydrolytic action of the biotinyl protease. In some systems, a biotinyl antibody could be used to direct the avidin-bridge biotinyl protease to an unbiotinylated target antigen. The data support the contention that preferential cleavage reflects two separate phenomena: (i) avidin confers a conformational alteration of the biotinylated target protein, and (ii) the biotinyl protease is targeted (via the avidin bridge) to the proximity of the biotinylated target protein, thereby promoting cleavage of the conformationally altered molecule. This is the first report in which a proteolytic enzyme could be selectively targeted to specifically hydrolyze a defined protein substrate in solutions containing a complex mixture of other proteins. The approach appears to be a general phenomenon for "targeted catalysis", appropriate for other application, particularly for affinity cleavage and targeted catalysis of cell-based macromolecules.