The outcome of an antibiotic treatment on the growth capacity of bacteria is largely dependent on the initial population size (Inoculum Effect). We characterized and built a model of this effect in E. coli cultures using a large variety of antimicrobials, including conventional antibiotics, and for the first time, cationic antimicrobial peptides (CAMPs). Our results show that all classes of antimicrobial drugs induce an inoculum effect, which, as we explain, implies that the dynamic is bistable: For a range of anti-microbial densities, a very small inoculum decays whereas a larger inoculum grows, and the threshold inoculum depends on the drug concentration. We characterized three distinct classes of drug-induced bistable growth dynamics and demonstrate that in rich medium, CAMPs correspond to the simplest class, bacteriostatic antibiotics to the second class, and all other traditional antibiotics to the third, more complex class. These findings provide a unifying universal framework for describing the dynamics of the inoculum effect induced by antimicrobials with inherently different killing mechanisms.
Bacterial resistance to antibiotics is a major concern worldwide, leading to an extensive search for alternative drugs. Promising candidates are antimicrobial peptides (AMPs), innate immunity molecules, shown to be highly efficient against multidrug resistant bacteria. Therefore, it is essential to study bacterial resistance mechanisms against them. For that purpose, we used experimental evolution, and isolated a Salmonella enterica serovar typhimurium-resistant line to the AMP 4DK5L7. This AMP displayed promising features including widespread activity against Gram-negative bacteria and protection from proteolytic degradation. However, the resistance that evolved in the isolated strain was particularly high. Whole genome sequencing revealed that five spontaneous mutations had evolved. Of these, three are novel in the context of acquired AMP resistance. Two mutations are related to the AcrAB-TolC multidrug efflux pump. One occurred in AcrB, the substrate-binding domain of the system, and the second in RamR, a transcriptional regulator of the system. Together, the mutations increased the minimal inhibitory concentration (MIC) by twofold toward this AMP. Moreover, the mutation in AcrB induced hypersusceptibility toward ampicillin and colistin. The last mutation occurred in Skp, a periplasmic chaperone that participates in the biogenesis of outer membrane proteins (OMPs). This mutation increased the MIC by twofold to 4DK5L7 and by fourfold to another AMP, seg5D. Proteomic analysis revealed that the mutation abolished Skp expression, reduced OMP abundance, and increased DegP levels. DegP, a protease that was reported to have an additional chaperone activity, escorts OMPs through the periplasm along with Skp, but is also associated with AMP resistance. In conclusion, our data demonstrate that both loss of Skp and manipulation of the AcrAB-TolC system are alternative strategies of AMP acquired resistance in Salmonella typhimurium and might represent a common mechanism in other Gram-negative bacteria.
Recruited blood monocytes contribute to the establishment, perpetuation, and resolution of tissue inflammation. Specifically, in the inflamed intestine, monocyte ablation was shown to ameliorate colitis scores in preclinical animal models. However, the majority of intestinal macrophages that seed the healthy gut are also monocyte derived. Monocyte ablation aimed to curb inflammation would therefore likely interfere with intestinal homeostasis. In this study, we used a TLR2 trans-membrane peptide that blocks TLR2 dimerization that is critical for TLR2/1 and TLR2/6 heterodimer signaling to blunt inflammation in a murine colitis model. We show that although the TLR2 peptide treatment ameliorated colitis, it allowed recruited monocytes to give rise to macrophages that lack the detrimental proinflammatory gene signature and reduced potentially damaging neutrophil infiltrates. Finally, we demonstrate TLR blocking activity of the peptide on in vitro cultured human monocyte-derived macrophages. Collectively, we provide a significantly improved anti-inflammatory TLR2 peptide and critical insights in its mechanism of action toward future potential use in the clinic.
The human immunodeficiency virus enters its host cells by membrane fusion, initiated by the gp41 subunit of its envelope protein. gp41 has also been shown to bind T-cell receptor (TCR) complex components, interfering with TCR signaling leading to reduced T-cell activation. This immunoinhibitory activity is suggested to occur during the membrane fusion process and is attributed to various membranotropic regions of the gp41 ectodomain and to the transmembrane domain. Although extensively studied, the cytosolic region of gp41, termed the cytoplasmic tail (CT), has not been examined in the context of immune suppression. Here we investigated whether the CT inhibits T-cell activation in different T-cell models by utilizing gp41-derived peptides and expressed full gp41 proteins. We found that a conserved region of the CT, termed lentiviral lytic peptide 2 (LLP2), specifically inhibits the activation of mouse, Jurkat, and human primary T-cells. This inhibition resulted in reduced T-cell proliferation, gene expression, cytokine secretion, and cell surface expression of CD69. Differential activation of the TCR signaling cascade revealed that CT-based immune suppression occurs downstream of the TCR complex. Moreover, LLP2 peptide treatment of Jurkat and primary human T-cells impaired Akt but not NFκB and ERK1/2 activation, suggesting that immune suppression occurs through the Akt pathway. These findings identify a novel gp41 T-cell suppressive element with a unique inhibitory mechanism that can take place post-membrane fusion.
Hantavirus assembly and budding are governed by the surface glycoproteins Gn and Gc. In this study, we investigated the glycoproteins of Puumala, the most abundant Hantavirus species in Europe, using fluorescently labeled wild-type constructs and cytoplasmic tail (CT) mutants. We analyzed their intracellular distribution, co-localization and oligomerization, applying comprehensive live, single-cell fluorescence techniques, including confocal microscopy, imaging flow cytometry, anisotropy imaging and Number&Brightness analysis. We demonstrate that Gc is significantly enriched in the Golgi apparatus in absence of other viral components, while Gn is mainly restricted to the endoplasmic reticulum (ER). Importantly, upon co-expression both glycoproteins were found in the Golgi apparatus. Furthermore, we show that an intact CT of Gc is necessary for efficient Golgi localization, while the CT of Gn influences protein stability. Finally, we found that Gn assembles into higher-order homo-oligomers, mainly dimers and tetramers, in the ER while Gc was present as mixture of monomers and dimers within the Golgi apparatus. Our findings suggest that PUUV Gc is the driving factor of the targeting of Gc and Gn to the Golgi region, while Gn possesses a significantly stronger self-association potential.
The ability of the Lentivirus HIV-1 to inhibit T-cell activation by its gp41 fusion protein is well documented, yet limited data exists regarding other viral fusion proteins. HIV-1 utilizes membrane binding region of gp41 to inhibit T-cell receptor (TCR) complex activation. Here we examined whether this T-cell suppression strategy is unique to the HIV-1 gp41. We focused on T-cell modulation by the gp21 fusion peptide (FP) of the Human T-lymphotropic Virus 1 (HTLV-1), a Deltaretrovirus that like HIV infects CD4(+) T-cells. Using mouse and human in-vitro T-cell models together with in-vivo T-cell hyper activation mouse model, we reveal that HTLV-1's FP inhibits T-cell activation and unlike the HIV FP, bypasses the TCR complex. HTLV FP inhibition induces a decrease in Th1 and an elevation in Th2 responses observed in mRNA, cytokine and transcription factor profiles. Administration of the HTLV FP in a T-cell hyper activation mouse model of multiple sclerosis alleviated symptoms and delayed disease onset. We further pinpointed the modulatory region within HTLV-1's FP to the same region previously identified as the HIV-1 FP active region, suggesting that through convergent evolution both viruses have obtained the ability to modulate T-cells using the same region of their fusion protein. Overall, our findings suggest that fusion protein based T-cell modulation may be a common viral trait.
Multidrug resistant bacteria possess various mechanisms that can sense environmental stresses such as antibiotics and antimicrobial peptides and rapidly respond to defend themselves. Two known defense strategies are biofilm formation and lipopolysaccharide (LPS) modification. Though LPS modifications are observed in biofilm-embedded bacteria, their effect on biofilm formation is unknown. Using biochemical and biophysical methods coupled with confocal microscopy, atomic force microscopy, and transmission electron microscopy, we show that biofilm formation is promoted in a Pseudomonas aeruginosa PAO1 strain with a loss of function mutation in the arnB gene. This loss of function prevents the addition of the positively charged sugar 4-amino-4-deoxy-l-arabinose to lipid A of LPS under restrictive magnesium conditions. The data reveal that the arnB mutant, which is susceptible to antimicrobial peptides, forms a biofilm that is more robust than that of the wild type. This is in line with the observations that the arnB mutant exhibits outer surface properties such as hydrophobicity and net negative charge that promote the formation of biofilms. Moreover, when grown under Mg
2+ limitation, both the wild type and the arnB mutant exhibited a reduction in the level of membrane-bound polysaccharides. The data suggest that the loss of polysaccharides exposes the membrane and alters its biophysical properties, which in turn leads to more biofilm formation. In summary, we show for the first time that blocking a specific lipid A modification promotes biofilm formation, suggesting a trade-off between LPS remodeling and resistance mechanisms of biofilm formation.
Cell surfaces are densely populated with various proteins. Aggregation of these proteins to nanoscale clusters can be critical for various cellular functions such as signaling, motility and division. Quantitative characterization of corresponding structures and their changes might be useful to understand these basic cell processes and serve as an early marker of cellular stress or diseases. Atomic force microscopy (AFM) allows high-resolution imaging of cell surface structures, resolving fine details of these structures. Moreover, AFM enables simultaneous imaging of cell surface morphology and mapping of its’ mechanical characteristics. This review focuses on applying the fractal dimension measure as a sensitive method to quantify single cell surface structures and their changes from AFM images.
Recently, a single study revealed a new complex composed of Toll-like receptor 4 (TLR4), TLR6, and CD36 induced by fibrillary A beta peptides, the hallmark of Alzheimer's disease. Unlike TLRs located on the plasma membrane that dimerize on the membrane after ligand binding to their extracellular domain, the TLR4-TLR6-CD36 complex assembly has been suggested to be induced by intracellular signals from CD36, similar to integrin inside-out signaling. However, the assembly site of TLR4-TLR6-CD36 and the domains participating in A beta-induced signaling is still unknown. By interfering with TLR4-TLR6 dimerization using a TLR4-derived peptide, we show that receptor assembly is abrogated within the plasma membrane. Furthermore, we reveal that the transmembrane domains of TLR4 and TLR6 have an essential role in receptor dimerization and activation. Inhibition of TLR4-TLR6 assembly was associated with reduced secretion of proinflammatory mediators from microglia cells, ultimately rescuing neurons from death. Our findings support TLR4-TLR6 dimerization induced by A beta. Moreover, we shed new light on TLR4-TLR6 assembly and localization and show the potential of inhibiting TLR4-TLR6 dimerization as a treatment of Alzheimer's disease.
The HIV gp160 envelope fusion protein is situated in the viral membrane and mediates virus entry into its host cell. Increasing evidence suggests that virtually all parts of the HIV envelope are structurally and functionally dependent on membranes. Protein-lipid interactions and membrane properties influence the dynamics of a manifold of gp160 biological activities such as membrane fusion, immune suppression and gp160 incorporation into virions during HIV budding and assembly. In the following we will summarize our current understanding of this interdependence between membrane interaction, structural conformation and functionality of the different gp160 domains. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.
Pseudomonas aeruginosa is the major microorganism colonizing the respiratory epithelium in cystic fibrosis (CF) sufferers. The widespread use of available antibiotics has drastically reduced their efficacy, and antimicrobial peptides (AMPs) are a promising alternative. Among them, the frog skin-derived AMPs, i.e., Esc(1-21) and its diastereomer, Esc(1-21)-1c, have recently shown potent activity against free-living and sessile forms of P. aeruginosa. Importantly, this pathogen also escapes antibiotics treatment by invading airway epithelial cells. Here, we demonstrate that both AMPs kill Pseudomonas once internalized into bronchial cells which express either the functional or the ΔF508 mutant of the CF transmembrane conductance regulator. A higher efficacy is displayed by Esc(1-21)-1c (90% killing at 15 μM in 1 h). We also show the peptides' ability to stimulate migration of these cells and restore the induction of cell migration that is inhibited by Pseudomonas lipopolysaccharide when used at concentrations mimicking lung infection. This property of AMPs was not investigated before. Our findings suggest new therapeutics that not only eliminate bacteria but also can promote reepithelialization of the injured infected tissue. Confocal microscopy indicated that both peptides are intracellularly localized with a different distribution. Biochemical analyses highlighted that Esc(1-21)-1c is significantly more resistant than the all-l peptide to bacterial and human elastase, which is abundant in CF lungs. Besides proposing a plausible mechanism underlying the properties of the two AMPs, we discuss the data with regard to differences between them and suggest Esc(1-21)-1c as a candidate for the development of a new multifunctional drug against Pseudomonas respiratory infections.
The persistence of HIV in resting memory CD4+ T cells at a latent state is considered as the major barrier on the path to achieve a cure for HIV. Proteasome inhibitors (PIs) were previously reported as latency reversing agents (LRAs) but the mechanism underlying this function is yet unclear. Here we demonstrate that PIs reactivate latent HIV ex vivo without global T cell activation, and may facilitate host innate immune responses. Mechanistically, latent HIV reactivation induced by PIs is mediated by heat shock factor 1 (HSF1) via the recruitment of the heat shock protein (HSP) 90-positive transcriptional elongation factor b (p-TEFb) complex. Specifically, HSP90 downstream HSF1 gives positive feedback to the reactivation process through binding to cyclin-dependent kinase 9 (CDK9) and preventing it from undergoing degradation by the proteasome. Overall, these findings suggest proteasome inhibitors as potential latency reversing agents. In addition, HSF1/HSP90 involved in HIV transcription elongation, may serve as therapeutic targets in HIV eradication.
The development of antibacterial drugs to overcome various pathogenic species, which inhabit the oral cavity, faces several challenges, such as salivary flow and enzymatic activity that restrict dosage retention. Owing to their amphipathic nature, antimicrobial peptides (AMPs) serve as the first line of defense of the innate immune system. The ability to synthesize different types of AMPs enables exploitation of their advantages as alternatives to antibiotics. Sustained release of AMPs incorporated in biodegradable polymers can be advantageous in maintaining high levels of the peptides. In this study, four potent ultra-short lipopeptides, conjugated to an aliphatic acid chain (16C) were incorporated in two different biodegradable polymers: poly (lactic acid co castor oil) (PLACO) and ricinoleic acid-based poly (ester-anhydride) (P(SA-RA)) for sustained release. The lipopeptide and polymer formulations were tested for antibacterial activity during one week, by turbidometric measurements of bacterial outgrowth, anti-biofilm activity by live/dead staining, biocompatibility by hemolysis and XTT colorimetric assays, mode of action by fluorescence-activated cell sorting (FACS) and release profile by a fluorometric assay. The results show that an antibacterial and anti-biofilm effect, as well as membrane disruption, can be achieved by the use of a formulation of lipopeptide incorporated in biodegradable polymer.
For successful infection and propagation viruses must overcome many obstacles such as the immune system and entry into their host cells. HIV utilizes its trimeric envelope protein gp160, specifically the gp41 subunit, to enter its host cell. During this process, a gp41-central coiled coil is formed from three N- and three C-terminal heptad repeats, termed the six-helix bundle (SHB), which drives membrane fusion. Recently, T-cell suppression has been reported as an additional function for several regions of gp41 by interfering with the T-cell receptor (TCR) signalling cascade. One of these regions encompasses the conserved pocket binding domain (PBD) that is situated in the C-terminal heptad repeat (CHR) and stabilizes SHB formation. This could indicate that the PBD plays a role in T-cell suppression in addition to its role in membrane fusion. To investigate this dual function, we used two independent cell cultures coupled with biophysical techniques. The data reveal that the PBD mediates T-cell suppression by stabilizing a TCR-binding conformation in the membrane. Moreover, we show that the clinically used HIV fusion inhibitor T-20 did not show suppressive abilities, in contrast with the potent fusion inhibitor C34. In addition, by focusing on SHB conformation after its assembly, we shed light on a mechanism by which gp41's function alternates from membrane fusion facilitation to suppression of TCR activation.
The frog skin-derived antimicrobial peptide esculentin-1a(1-21)NH2 [Esc(1-21)], and its diastereomer Esc(1-21)-1c (containing two D-amino acids at positions 14 and 17), have been recently found to neutralize the toxic effect of Pseudomonas aeruginosa lipopolysaccharide (LPS), although to different extents. Here, we studied the three-dimensional structure of both peptides in complex with P. aeruginosa LPS, by transferred nuclear Overhauser effect spectroscopy. Lack of NOE peaks revealed that both the peptides adopted a random coil structure in aqueous solution. However, Esc(1-21) adopted an amphipathic helical conformation in LPS micelles with 5 basic Lys residues forming a hydrophilic cluster. In comparison, the diastereomer maintained an alpha helical conformation only at the N-terminal region, whereas the C-terminal portion was quite flexible. Isothermal titration calorimetry (ITC) revealed that the interaction of Esc(1-21) with LPS is an exothermic process associated with a dissociation constant of similar to 4 mu M. In contrast, Esc(1-21)-1c had almost 8 times weaker binding affinity to LPS micelles. Moreover, STD NMR data supported by docking analysis have identified those amino acid residues responsible for the peptide's binding to LPS micelles. Overall, the data provide important mechanistic insights on the interaction of esculentin-derived peptides with LPS and the reason for their different anti-endotoxin activity. These data might also assist to further design more potent antimicrobial peptides with antisepsis properties, which are highly needed to overcome the widespread concem of the available anti-infective agents. (C) 2015 Elsevier B.V. All rights reserved.
Monocytes have emerged as critical driving force of acute inflammation. Here, we show that inhibition of Toll-like receptor 2 (TLR2) dimerization by a TLR2 transmembrane peptide (TLR2-p) ameliorated DSS-induced colitis by interfering specifically with the activation of Ly6C(+) monocytes without affecting their recruitment to the colon. We report that TLR2-p directly interacts with TLR2 within the membrane, leading to inhibition of TLR2-TLR6/1 assembly induced by natural ligands. This was associated with decreased levels of extracellular signal-regulated kinases (ERK) signaling and reduced secretion of pro-inflammatory cytokines, such as interleukin (IL)-6, IL-23, IL-12, and IL-1 beta. Altogether, our study provides insights into the essential role of TLR2 dimerization in the activation of pathogenic pro-inflammatory Ly6C(hi) monocytes and suggests that inhibition of this aggregation by TLR2-p might have therapeutic potential in the treatment of acute gut inflammation.
To successfully infect and persist within its host, HIV-1 utilizes several immunosuppressive motifs within its gp41 envelope glycoprotein to manipulate and evade the immune system. The transmembrane domain (TMD) of gp41 downregulates T-cell receptor (TCR) signaling through a hitherto unknown mechanism. Interactions between TMDs within the membrane milieu have been shown to be typically mediated by particular amino acids, such as interactions between basic and acidic residues and dimerization motifs as GxxxG. The HIV-1 TMD exhibits both a polar arginine (Arg(696)) residue and a GxxxG motif, making them ideal candidates for mediators of TMD-TCR interaction. Using a primary T-cell activation assay and biochemical and biophysical methods, we demonstrate that the gp41 TMD directly interacts with TMDs of the TCR and the CD3 coreceptors (delta, gamma, and epsilon) within the membrane, presumably leading to impairment of complex assembly. Additionally, we reveal that although Arg(696) does not affect TMD immunosuppression, the GxxxG motif is crucial in mediating gp41's TMD interaction with the CD3 coreceptors of the TCR. These findings suggest that compared with other gp41 immunosuppressive motifs, the gp41 TMD has multiple targets within the TCR complex, suggesting less susceptibility to evolutionary pressure and consequently being advantageous for the virus over the host immune response. Furthermore, as the GxxxG motif mediates interactions of the gp41 TMD with multiple receptors, it emerges as an attractive drug target. This multitarget inhibitory mechanism might be a strategy utilized by HIV to interfere with the function of additional host receptors.
Naturally occurring antimicrobial peptides (AMPs) represent promising future antibiotics. We have previously isolated esculentin-1a(1-21)NH2, a short peptide derived from the frog skin AMP esculentin-1a, with a potent anti-Pseudomonal activity. Here, we investigated additional functions of the peptide and properties responsible for these activities. For that purpose, we synthesized the peptide, as well as its structurally altered analog containing two d-amino acids. The peptides were then biophysically and biologically investigated for their cytotoxicity and immunomodulating activities. The data revealed that compared to the wild-type, the diastereomer: (1) is significantly less toxic towards mammalian cells, in agreement with its lower alpha-helical structure, as determined by circular dichroism spectroscopy; (2) is more effective against the biofilm form of Pseudomonas aeruginosa (responsible for lung infections in cystic fibrosis sufferers), while maintaining a high activity against the free-living form of this important pathogen; (3) is more stable in serum; (4) has a higher activity in promoting migration of lung epithelial cells, and presumably in healing damaged lung tissue, and (5) disaggregates and detoxifies the bacterial lipopolysaccharide (LPS), albeit less than the wild-type. Light scattering studies revealed a correlation between anti-LPS activity and the ability to disaggregate the LPS. Besides shedding light on the multifunction properties of esculentin-1a(1-21)NH2, the d-amino acid containing isomer may serve as an attractive template for the development of new anti-Pseudomonal compounds with additional beneficial properties. Furthermore, together with other studies, incorporation of d-amino acids may serve as a general approach to optimize the future design of new AMPs.
Many bacteria live as biofilms to cope with unfavourable surroundings. Biofilms start from (i) a planktonic stage, (ii) initial adhesion to surfaces and (iii) formation of sessile micro-colonies that secrete extracellular polymeric substance (EPS), leading to bacterial resistance to antibiotics. Antimicrobial peptides (AMPs) are extensively studied with regard to planktonic bacteria but much less so with regard to biofilm formation. In the present study, we investigated how the above three steps are affected by the properties of the AMPs using a series of peptides composed of six lysines and nine leucines, which differ in their sequences and hence their biophysical properties. Treatment with bactericidal peptides at non-inhibitory concentrations resulted in reduced biofilm growth, for some starting from 25 nM which is 0.2 and 0.4% of their minimum inhibitory concentration (MIC 6.3 and 12.5 mu M, respectively), continuing in a dose-dependent manner. We suggest that reduced bacterial adhesion to surfaces and decreased biofilm growth are due to the peptide's ability to coat either the biomaterial surface or the bacterium itself. Degradation of established biofilms by bactericidal and non-bactericidal peptides, within 1 h of incubation, occurs by either killing of embedded bacteria or detachment of live ones. In addition to shedding light on the mechanism of biofilm inhibition and degradation, these data may assist in the design of anti-biofilm AMPs.
Cell-wall peptidoglycan (PG) of Gram-positive bacteria is a strong and elastic multi-layer designed to resist turgor pressure and determine the cell shape and growth. Despite its crucial role, its architecture remains largely unknown. Here using high-resolution multiparametric atomic force microscopy (AFM), we studied how the structure and elasticity of PG change when subjected to increasing turgor pressure in live Group B Streptococcus. We show a new net-like arrangement of PG, which stretches and stiffens following osmotic challenge. The same structure also exists in isogenic mutants lacking surface appendages. Cell aging does not alter the elasticity of the cell wall, yet destroys the net architecture and exposes single segmented strands with the same circumferential orientation as predicted for intact glycans. Together, we show a new functional PG architecture in live Gram-positive bacteria.
Antimicrobial peptides (AMPs) are conserved evolutionary components of the innate immune system that are being tested as alternatives to antibiotics. Slow release of AMPs using biodegradable polymers can be advantageous in maintaining high peptide levels for topical treatment, especially in the oral environment in which dosage retention is challenged by drug dilution with saliva flow and by drug inactivation by salivary enzymatic activity. Enterococcus faecalis is a multidrug resistant nosocomial pathogen and a persistent pathogen in root canal infections. In this study, four ultra-short lipopeptides (C16-KGGK, C16-KLLK, C16-KAAK and C16-KKK) and an amphipathic a-helical antimicrobial peptide (Amp-1D) were tested against E. faecalis. The antibacterial effect was determined against planktonic bacteria and bacteria grown in biofilm. Of the five tested AMPs, C16-KGGK was the most effective. Next C16-KGGK was formulated with one of two polymers poly (lactic acid co castor oil) (DLLA) or ricinoleic acid-based poly (ester-anhydride) P(SA-RA). Peptide-synthetic polymer conjugates, also referred to as biohybrid mediums were tested for antibacterial activity against E. faecalis grown in suspension and in biofilms. The new formulations exhibited strong and improved anti- E. faecalis activity.
Assembly of transmembrane domains (TMDs) is a critical step in the function of membrane proteins. In recent years, the role of specific amino acids in TMD-TMD interactions has been better characterized, with more emphasis on polar and aromatic residues. Despite the high abundance of proline residues in TMDs, contribution of proline to TMD-TMD association has not been intensively studied. Here, we evaluated statistically the frequency of appearance, and experimentally the contribution of proline, compared to other hydrophobic amino acids (Gly, Ala, Val, Leu, Ile, and Met), with regard to TMD-TMD self-assembly. Our model system is the assembly motif ((22)QxxS(25)) found previously in TMDs of the Escherichia coli aspartate receptor (Tar-1). Statistically, our data revealed that all different motifs, except PxxS (P/S), have frequencies similar to their theoretical random expectancy within a database of 41916 sequences of TMDs, while PxxS motif is underrepresented. Experimentally, using the ToxR assembly system, the SDS-gel running pattern of biotin-conjugated TMD peptides, and FRET experiments between fluorescence-labeled peptides, we found that only the P/S motif preserves the dimerization ability of wild-type Tar-1 TMD. Although proline is known as a helix breaker in solution, Circular Dichroism spectroscopy revealed that the secondary structure of the P/S and the wild-type peptides are similar. All together, these data suggest that proline can stabilize TM self-assembly when localized to the interaction interface of a transmembrane oligomer. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova. (C) 2014 Elsevier B.V. All rights reserved.
HIV-1 uses a number of means to manipulate the immune system, to avoid recognition and to highjack signaling pathways. HIV-1 infected cells show limited Toll-Like Receptor (TLR) responsiveness via as yet unknown mechanisms. Using biochemical and biophysical approaches, we demonstrate that the trans-membrane domain (TMD) of the HIV-1 envelope (ENV) directly interacts with TLR2 TMD within the membrane milieu. This interaction attenuates TNF alpha, IL-6 and MCP-1 secretion in macrophages, induced by natural ligands of TLR2 both in in vitro and in vivo models. This was associated with decreased levels of ERK phosphorylation. Furthermore, mutagenesis demonstrated the importance of a conserved GxxxG motif in driving this interaction within the membrane milieu. The administration of the ENV TMD in vivo to lipotechoic acid (LTA)/Galactosamine-mediated septic mice resulted in a significant decrease in mortality and in tissue damage, due to the weakening of systemic macrophage activation. Our findings suggest that the TMD of ENV is involved in modulation of the innate immune response during HIV infection. Furthermore, due to the high functional homology of viral ENV proteins this function may be a general character of viral-induced immune modulation.
Lipid-conjugated peptides have advanced. the understanding of membrane protein functions and the roles of lipids in the membrane milieu. These lipopeptides modulate various biological systems such as viral fusion. A single function has been suggested for the lipid, binding to the membrane and thus elevating the local concentration of the peptide at the target site. In the present paper, we challenged this argument by exploring in-depth the antiviral mechanism of lipopeptides, which comprise sphinganine, the lipid backbone of DHSM (dihydrosphingomyelin), and an HIV-1 envelope-derived peptide. Surprisingly, we discovered a partnership between the lipid and the peptide that impaired early membrane fusion events by reducing CD4 receptor lateral diffusion and HIV-1 fusion peptide-mediated lipid mixing. Moreover, only the joint function of sphinganine and its conjugate peptide disrupted HIV-1 fusion protein assembly and folding at the later fusion steps. Via imaging techniques we revealed for the first time the direct localization of these lipopeptides to the virus- cell and cell-cell contact sites. Overall, the findings of the present study may suggest lipid-protein interactions in various biological systems and may help uncover a role for elevated DHSM in HIV-1 and its target cell membranes.
The mammalian innate immune response is responsible for the early stages of defense against invading pathogens. One of the major receptor families facilitating innate immune activation is the Toll-like receptor (TLR) family. These receptors are type I membrane proteins spanning the membrane with a single transmembrane domain (TMD). All TLRs form homo- and hetero-dimers within membranes and new data suggest that the single transmembrane domain of some of these receptors is involved in their dimerization and function. Newly identified TLR dimers are continuously reported but only little is known about the importance of the TMDs for their dimer assembly and signaling regulation. Uncontrolled or untimely activation of TLRs is related to a large number of pathologies ranging from cystic fibrosis to sepsis and cancer. In this review we will focus on the contribution of the TMDs of innate immune receptors specifically TLR2 to their regulation and function. In addition, we will address the current issues remaining to be solved regarding the mechanistic insights of this regulation. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy. (C) 2014 Elsevier B.V. All rights reserved.
The growing number of microbial pathogens resistant to available antibiotics is a serious threat to human life. Among them is the bacterium Staphylococcus aureus, which colonizes keratinocytes, the most abundant cell type in the epidermis. Its intracellular accumulation complicates treatments against resulting infections, mainly due to the limited diffusion of conventional drugs into the cells. Temporins A (Ta) and B (Tb) are short frog skin antimicrobial peptides (AMPs). Despite extensive studies regarding their antimicrobial activity, very little is known about their activity on infected cells or involvement in various immunomodulatory functions. Here we show that Tb kills both ATCC-derived and multidrug- resistant clinical isolates of S. aureus within infected HaCaT keratinocytes ( 80% and 40% bacterial mortality, respectively) at a nontoxic concentration, i. e., 16 = M, whereas a weaker effect is displayed by Ta. Furthermore, the peptides prevent killing of keratinocytes by the invading bacteria. Further studies revealed that both temporins promote wound healing in a monolayer of HaCaT cells, with front speed migrations of 19 = mu m/h and 12 =mu m/h for Ta and Tb, respectively. Migration is inhibited by mitomycin C and involves the epidermal growth factor receptor (EGFR) signaling pathway. Finally, confocal fluorescence microscopy indicated that the peptides diffuse into the cells. By combining antibacterial and wound-healing activities, Ta and Tb may act as multifunctional mediators of innate immunity in humans. Particularly, their nonendogenous origin may reduce microbial resistance to them as well as the risk of autoimmune diseases in mammals.
Background: Recently, an immunosuppressive motif was identified in the HIV-1 envelope glycoprotein complex. Results: Both d- and l-stereoisomers of the motif inhibit T-cell receptor activation and preferentially bind T-cells over B-cells. Conclusion: The motif immunomodulates T-cells through interactions occurring within the membrane milieu. Significance: This study provides new insights into the immunosuppressive activity of the envelope glycoprotein complex and the molecular recognition within the membrane. An immunosuppressive motif was recently found within the HIV-1 gp41 fusion protein (termed immunosuppressive loop-associated determinant core motif (ISLAD CM)). Peptides containing the motif interact with the T-cell receptor (TCR) complex; however, the mechanism by which the motif exerts its immunosuppressive activity is yet to be determined. Recent studies showed that interactions between protein domains in the membrane milieu are not always sterically controlled. Therefore, we utilized the unique membrane leniency toward association between d- and l-stereoisomers to investigate the detailed mechanism by which ISLAD CM inhibits T-cell activation. We show that a d-enantiomer of ISLAD CM (termed ISLAD d-CM) inhibited the proliferation of murine myelin oligodendrocyte glycoprotein (MOG)-(35-55)-specific line T-cells to the same extent as the l-motif form. Moreover, the d- and l-forms preferentially bound spleen-derived T-cells over B-cells by 13-fold. Furthermore, both forms of ISLAD CM co-localized with the TCR on activated T-cells and interacted with the transmembrane domain of the TCR. FRET experiments revealed the importance of basic residues for the interaction between ISLAD CM forms and the TCR transmembrane domain. Ex vivo studies demonstrated that ISLAD d-CM administration inhibited the proliferation (72%) and proinflammatory cytokine secretion of pathogenic MOG(35-55)-specific T-cells. This study provides insights into the immunosuppressive mechanism
The gp41 disulfide loop region switches from a soluble state to a membrane-bound state during the human immunodeficiency virus type 1 (HIV-1) envelope-mediated membrane fusion process. The loop possesses a hydrophobic core at the center of the region with an unusual basic residue (Lys-601). Furthermore, two loop core mutations, K601A and L602A, are found to inhibit HIV-1 infectivity while keeping wild type-like levels of the envelope, implying that they exert an inhibitory effect on gp41 during the membrane fusion event. Here, we investigated the mode of action of these mutations on the loop region. We show that the K601A mutation, but not the L602A mutation, abolished the binding of a loop-specific monoclonal antibody to a loop domain peptide. Additionally, the K601A, but not the L602A, impaired disulfide bond formation in the peptides. This was correlated with changes in the circular dichroism spectrum imposed by the K601A mutation. In the membrane, however, the L602A, but not the K601A, reduced the lipid mixing ability of the loop peptides, which was correlated with decreased -helical content of the L602A mutant. The results suggest that the Lys-601 residue provides a moderate hydrophobicity level within the gp41 loop core that contributes to the proper structure and function of the loop inside and outside the membrane. Because basic residues are found between the loop Cys residues of several lentiviral fusion proteins, the findings may contribute to understanding the fusion mechanism of other viruses as well.
Fusion of the HIV membrane with that of a target T cell is an essential first step in the viral infection process. Here we describe single-partide tracking (SPT) studies of a 16-amino-acid peptide derived from the HIV fusion protein (FP16), as it interacts with a supported lipid bilayer. FP16 was found to spontaneously insert into and move within the bilayer with two different modes of diffusion, a fast mode with a diffusion coefficient typical of protein motion in membranes and a much slower one. We observed transitions between the two modes: slow peptides were found to speed up, and fast peptides could slow down. Hidden Markov model analysis was employed as a method for the identification of the two modes in single-molecule trajectories and analysis of their interconversion rates. Surprisingly, the diffusion coefficients of the two modes were found to depend differently on solution viscosity. Thus, whereas the fast diffusive mode behaved as predicted by the Saffman-Delbruck theory, the slow mode behaved according to the Stokes Einstein relation. To further characterize the two diffusive modes, FP16 molecules were studied in bilayers cooled through their liquid crystalline-to-gel phase transition. Our analysis suggested that the slow diffusive mode might originate from the formation of large objects, such as lipid domains or local protrusions, which are induced by the peptides and move together with them.
Infrared (IR) spectroscopy has been shown to be very reliable for the characterization, identification and quantification of structural data. Particularly, the Attenuated Total Reflectance (ATR) technique which became one of the best choices to study the structure and organization of membrane proteins and membrane-bound peptides in biologically relevant membranes. An important advantage of IR spectroscopy is its ability to analyze material under a very wide range of conditions including solids, liquids and gases. This method allows elucidation of component secondary structure elements of a peptide or protein in a global manner, and by using site specific isotope labeling allows determination of specific regions. A few advantages in using ATR-FTIR spectroscopy include; a relatively simple technique, allow the determination of peptide orientation in the membrane, allow the determination of secondary structures of very small peptides, and importantly, the method is sensitive to isotopic labeling on the scale of single amino acids. Many studies were reported on the use of ATR-FTIR spectroscopy in order to study the structure and orientation of membrane bound hydrophobic peptides and proteins. The list includes native and de-novo designed peptides, as well as those derived from trans-membrane domains of various receptors (TMDs). The present review will focus on several examples that demonstrate the potential and the simplicity in using the ATR-FTIR approach to determine secondary structures of proteins and peptides when bound, inserted, and oligomerized within membranes. The list indudes (i) a channel forming protein/peptide: the Ca2+ channel phospholamban, (ii) a cell penetrating peptide, (iii) changes in the structure of a transmembrane domain located within ordered and non-ordered domains, and (iv) isotope edited FUR to directly assign structure to the membrane associated fusion peptide in context of a Key gp41 Structural Motif. Importantly, a unique advantage of i
The T-cell receptor-CD3 complex (TCR-CD3) serves a critical role in protecting organisms from infectious agents. The TCR is a heterodimer composed of alpha- and beta-chains, which are responsible for antigen recognition. Within the transmembrane domain of the a-subunit, a region has been identified to be crucial for the assembly and function of the TCR. This region, termed core peptide (CP), consists of nine amino acids (GLRILLLKV), two of which are charged (lysine and arginine) and are crucial for the interaction with CD3. Earlier studies have shown that a synthetic peptide corresponding to the CP sequence can suppress the immune response in animal models of T-cell-mediated inflammation, by disrupting proper assembly of the TCR. As a step towards the understanding of the source of the CP activity, we focused on CP in egg phos-phatidylcholine/cholesterol (9: 1, mol/mol) model membranes and determined its secondary structure, oligomerization state, and orientation with respect to the membrane. To achieve this goal, 15-residue segments of TCR alpha, containing the CP, were synthesized and spin-labeled at different locations with a nitroxide derivative. Electron spin-echo envelope modulation spectroscopy was used to probe the position and orientation of the peptides within the membrane, and double electron-electron resonance measurements were used to probe its conformation and oligomerization state. We found that the peptide is predominantly helical in a membrane environment and tends to form oligomers (mostly dimers) that are parallel to the membrane plane.
Natural and synthetic membrane active peptides as well as fragments from membrane proteins interact with membranes. In several cases, such interactions cause the insertion of the peptides to the membrane and their assembly within the lipid bilayer. Here we present spectroscopic approaches utilizing NBD and rhodamine fluorescently labeled peptides to measure peptide-membrane interaction and peptide-peptide interaction within the membrane. The usage of the physical properties of NBD and rhodamine in solution and in membranes provides useful information on the interplay between peptides and lipids.
One of the routes by which HIV-1 is able to escape the immune response is by immunosuppression. The gp41 fusion protein of the HIV-1 envelope mediates virus entry by membrane fusion and also functions as an inhibitor of T cell activation. Here, we review the recent studies suggesting that some of the gp41 immunosuppressive processes are initiated by novel motifs, located within the hydrophobic regions of the protein. This indicates that the immunosuppressive process mediated by gp41 is much more complex than initially thought. Additionally, we propose a model illustrating the interactions and interferences of these regions with the T cell receptor complex.
TLR2, together with TLR1 and TLR6, is essential for detecting lipopeptides and bacterial cell wall components such as lipoteichoic acid from Gram-positive bacteria. In this study, we report that transmembrane domain (TMD)-derived peptides from TLR2 and TLR6 specifically inhibit TLR2 activation. Secretion of the cytokines TNF-alpha and IL-6 by cultured macrophages (RAW264.7 cell line) was inhibited by these peptides in response to TLR2 activation by lipoteichoic acid (TLR2/6 activator) or palmitoyl (3)-CysSer- Lys(4)-OH (TLR2/1 activator) but not by LPS (TLR4 activator). Extensive biophysical and biochemical assays, combined with GALLEX experiments, show that these peptides heterodimerize with their complementary TMDs on their reciprocal protein. These results suggest that TLR2/6/1 TMD assembly is essential for activating this complex. Importantly, when administered to mice inflicted by TLR2, but not TLR4-driven lethal inflammation, a selected peptide rescued 60% of these septic mice, showing potent in vivo inhibition of TNF-alpha and IL-6 secretion. Furthermore, this peptide also showed high protection in a whole bacteria model. Owing to the importance of TLR2 regulation under a variety of pathological conditions, compounds that can fine-tune this activity are of great importance.
Modulation of T-cell responses by HIV occurs via distinct mechanisms, 1 of which involves inactivation of T cells already at the stage of virus-cell fusion. Hydrophobic portions of the gp41 protein of the viral envelope that contributes to membrane fusion may modulate T-cell responsiveness. Here we found a highly conserved sequence (termed "ISLAD") that is associated with the membranotropic gp41 loop region. We showed that ISLAD has the ability to bind the T-cell membrane and to interact with the T-cell receptor (TCR) complex. Furthermore, ISLAD inhibited T-cell proliferation and interferon-gamma secretion that resulted from TCR engagement through antigen-presenting cells. Moreover, administering ISLAD (10 mu g per mouse) to an experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis reduced the severity of the disease. This was related to the inhibition of pathogenic T-cell proliferation and to reduced pro-inflammatory cytokine secretion in the lymph nodes of ISLAD-treated EAE mice. The data suggest that T-cell inactivation by HIV during membrane fusion may lie in part in this conserved sequence associated with the gp41 loop region.
The organization and orientation of membrane-inserted helices is important for better understanding the mode of action of membrane-active peptides and of protein-membrane interactions. Here we report on the application of ESEEM (electron spin-echo envelope modulation) and DEER (double electron-electron resonance) techniques to probe the orientation and oligomeric state of an a-helical trans-membrane model peptide, WALP23, under conditions of negative mismatch between the hydrophobic cores of the model membrane and the peptide. Using ESEEM, we measured weak dipolar interactions between spin-labeled WALP23 and H-2 nuclei of either the solvent (D2O) or of lipids specifically deuterated at the choline group. The ESEEM data obtained from the deuterated lipids were fitted using a model that provided the spin label average distance from a layer of H-2 nuclei in the hydrophilic region of the membrane and the density of the H-2 nuclei in the layer. DEER was used to probe oligomerization through the dipolar interaction between two spin-labels on different peptides. We observed that the center of WALP23 does not coincide with the bilayer midplane and its N-terminus is more buried than the C-terminus. In addition, the ESEEM data fitting yielded a H-2 layer density that was much lower than expected. The DEER experiments revealed the presence of oligomers, the presence of which was attributable to the negative mismatch and the electrostatic dipole of the peptide. A discussion of a possible arrangement of the individual helices in the oligomers that is consistent with the ESEEM and DEER data is presented.
Complexity of cell membrane poses difficulties to quantify corresponding morphology changes during cell proliferation and damage. We suggest using fractal dimension of the cell membrane to quantify its complexity and track changes produced by various treatments. Glutaraldehyde fixed mouse RAW 264.7 macrophage membranes were chosen as model system and imaged in PeakForce QNM (quantitative nanomechanics) mode of AFM (atomic force microscope). The morphology of the membranes was characterized by fractal dimension. The parameter was calculated for set of AFM images by three different methods. The same calculations were done for the AFM images of macrophages treated with colchicine, an inhibitor of the microtubule polymerization, and microtubule stabilizing agent taxol. We conclude that fractal dimension can be additional and useful parameter to characterize the cell membrane complexity and track the morphology changes produced by different treatments. (C) 2012 Elsevier Ltd. All rights reserved.
Understanding the structural organization of lipids in the cell and viral membranes is essential for elucidating mechanisms of viral fusion that lead to entry of enveloped viruses into their host cells. The HIV lipidome shows a remarkable enrichment in dihydrosphingomyelin, an unusual sphingolipid formed by a dihydrosphingosine backbone. Here we investigated the ability of dihydrosphingosine to incorporate into the site of membrane fusion mediated by the HIV envelope (Env) protein. Dihydrosphingosine as well as cholesterol, fatty acid, and tocopherol was conjugated to highly conserved, short HIV-1 Env-derived peptides with no antiviral activity otherwise. We showed that dihydrosphingosine exclusively endowed nanomolar antiviral activity to the peptides (IC50 as low as 120 nM) in HIV-1 infection on TZM-bl cells and on Jurkat T cells, as well as in the cell-cell fusion assay. These sphingopeptides were active against enfuvirtide-resistant and wild-type CXCR4 and CCR5 tropic HIV strains. The anti-HIV activity was determined by both the peptides and their dihydrosphingosine conjugate. Moreover, their mode of action involved accumulation in the cells and viruses and binding to membranes enriched in sphingomyelin and cholesterol. The data suggest that sphingopeptides are recruited to the HIV membrane fusion site and provide a general concept in developing inhibitors of sphingolipid-mediated biological systems.-Ashkenazi, A., Viard, M., Unger, L., Blumenthal, R., Shai, Y. Sphingopeptides: dihydrosphingosine-based fusion inhibitors against wild-type and enfuvirtide-resistant HIV-1. FASEB J. 26, 4628-4636 (2012). www.fasebj.org
The human immunodeficiency virus utilizes its gp41 fusion protein to mediate virus-cell membrane fusion. The conserved disulfide loop region in the gp41 hairpin conformation reverses the protein chain, such that the N-terminal heptad repeat and the C-terminal heptad repeat regions interact to form the six-helix bundle. Hence, it is conceivable that the sequential folded N- and C-terminal parts of the loop region also interact. We show that the N- and C-terminal parts of the loop preferably form disulfide-bonded heterodimers with slow oxidation kinetics. Furthermore, when the two parts were linked to a single polypeptide to form the full-length loop, only an intramolecular disulfide-bonded loop was formed. Fluorescence studies revealed that an interaction takes place between the N- and C-terminal parts of the loop in solution, which was sustained in membranes. Functionally, only a combination of the N- and C-loop parts induced lipid mixing of model liposomes, the level of which increased 8-fold when they were connected to a single polypeptide chain. In both cases, the activity was independent of the oxidation state of the cysteines. Overall, the data (i) provide evidence of a specific interaction between the N- and C-terminal parts of the loop, which can further stabilize gp41 hairpin conformation, and (ii) suggest that the interaction between the N- and C-terminal parts of the loop is sufficient to induce lipid merging without forming a disulfide bond.
To thrive in the human body, HIV fuses to its target cell and evades the immune response via several mechanisms. The fusion cascade is initiated by the fusion peptide (FP), which is located at the N-terminal of gp41, the transmembrane protein of HIV. Recently, it has been shown that the HIV-1 FP, particularly its 5-13 amino acid region (FP5-13), suppresses T-cell activation and interacts with the transmembrane domain (TMD) of the T-cell receptor (TCR) complex. Specific amino acid motifs often contribute to such interactions in TMDs of membrane proteins. Using bioinformatics and experimental studies, we report on a GxxxG-like motif (AxxxG), which is conserved in the FP throughout different clades and strains of HIV-1. Biological activity studies and FTIR spectroscopy revealed that HIV FP5-13-derived peptides, in which the motif was altered either by randomization or by a single amino acid shift, lost their immunosuppressive activity concomitant with a loss of the beta-sheet structure in a membranous environment. Furthermore, fluorescence studies revealed that the inactive mutants lost their ability to interact with their target site, namely, the TMD of TCR alpha, designated CP. Importantly, lipotechoic acid activated macrophages (lacking TCR) were not affected by FP, further demonstrating the specificity of the immunosuppressant activity of CP. Finally, although the AxxxG WT and the GxxxG analog both associated with the CP and immunosuppressed T-cells, the AxxxG WT but not the GxxxG analog induced lipid mixing. Overall, the data support an important role for the AxxxG motif in the function of FP and might explain the natural selection of the AxxxG motif rather than the classical GxxxG motif in FP.
Cationic antimicrobial peptides (CAMPs) serve as the first line of defense of the innate immune system against invading microbial pathogens. Gram-positive bacteria can resist CAMPs by modifying their anionic teichoic acids (TAs) with D-alanine, but the exact mechanism of resistance is not fully understood. Here, we utilized various functional and biophysical approaches to investigate the interactions of the human pathogen Group B Streptococcus (GBS) with a series of CAMPs having different properties. The data reveal that: (i) D-alanylation of lipoteichoic acids (LTAs) enhance GBS resistance only to a subset of CAMPs and there is a direct correlation between resistance and CAMPs length and charge density; (ii) resistance due to reduced anionic charge of LTAs is not attributed to decreased amounts of bound peptides to the bacteria; and (iii) Dalanylation most probably alters the conformation of LTAs which results in increasing the cell wall density, as seen by Transmission Electron Microscopy, and reduces the penetration of CAMPs through the cell wall. Furthermore, Atomic Force Microscopy reveals increased surface rigidity of the cell wall of the wild-type GBS strain to more than 20-fold that of the dltA mutant. We propose that D-alanylation of LTAs confers protection against linear CAMPs mainly by decreasing the flexibility and permeability of the cell wall, rather than by reducing the electrostatic interactions of the peptide with the cell surface. Overall, our findings uncover an important protective role of the cell wall against CAMPs and extend our understanding of mechanisms of bacterial resistance.
Protein-protein interactions within the membrane are involved in many vital cellular processes. Consequently, deficient oligomerization is associated with known diseases. The interactions can be partially or fully mediated by transmembrane domains (TMD). However, in contrast to soluble regions, our knowledge of the factors that control oligomerization and recognition between the membrane-embedded domains is very limited. Due to the unique chemical and physical properties of the membrane environment, rules that apply to interactions between soluble segments are not necessarily valid within the membrane. This review summarizes our knowledge on the sequences mediating TMD-TMD interactions which include conserved motifs such as the GxxxG, QxxS, glycine and leucine zippers, and others. The review discusses the specific role of polar, charged and aromatic amino acids in the interface of the interacting TMD helices. Strategies to determine the strength, dynamics and specificities of these interactions by experimental (ToxR, TOXCAT, GALLEX and FRET) or various computational approaches (molecular dynamic simulation and bioinformatics) are summarized. Importantly, the contribution of the membrane environment to the TMD-TMD interaction is also presented. Studies utilizing exogenously added TMD peptides have been shown to influence in vivo the dimerizafion of intact membrane proteins involved in various diseases. The chirality independent TMD-TMD interactions allows for the design of novel short D- and L-amino acids containing TMD peptides with advanced properties. Overall these studies shed light on the role of specific amino acids in mediating the assembly of the TMDs within the membrane environment and their contribution to protein function. This article is part of a Special Issue entitled: Protein Folding in Membranes. (C) 2011 Elsevier B.V. All rights reserved.
Fusion of the human immunodeficiency virus (HIV) with target cells is mediated by the gp41 subunit of the envelope protein. Mutation and deletion studies within the transmembrane domain (TMD) of intact gp41 influenced its fusion activity. In addition, current models suggest that the TMD is in proximity with the fusion peptide (FP) at the late fusion stages, but there are no direct experimental data to support this hypothesis. Here, we investigated the TMD focusing on two regions: the N-terminal containing the GxxxG motif and the C-terminal containing the GLRI motif; which is conserved among the TMDs of HIV and the T-cell receptor. Studies utilizing the ToxR expression system combined with synthetic peptides and their fluorescent analogues derived from TMD revealed that the GxxxG motif is important for TMD self-association, whereas the C-terminal region is for its heteroassociation with FP. Functionally, all three TMD peptides induced lipid mixing that was enhanced significantly upon mixing with FP. Furthermore, the TMD peptides inhibited virus-cell fusion apparently through their interaction with their endogenous counterparts. Notably, the R2E mutant (in the GLRI) was significantly less potent than the two others. Overall, our findings provide experimental evidence that HIV-1 TMD contributes to membrane assembly and function of the HIV-1 envelope. Owing to similarities between functional domains within viruses, these findings suggest that the TMDs and FPs may contribute similarly in other viruses as well.
The MOM (mitochondrial outer membrane) contains SA (signal-anchored) proteins that bear at their N-terminus a single hydrophobic segment that serves as both a mitochondria] targeting signal and an anchor at the membrane. These proteins, like the vast majority of mitochondrial proteins, are encoded in the nucleus and have to be imported into the organelle. Currently, the mechanisms by which they are targeted to and inserted into the OM (outer membrane) are unclear. To shed light on these issues, we employed a recombinant version of the SA protein OM45 and a synthetic peptide corresponding to its signal-anchor segment. Both forms are associated with isolated mitochondria independently of cytosolic factors. Interaction with mitochondria was diminished when a mutated form of the signal-anchor was employed. We demonstrate that the signal-anchor peptide acquires an a-helical structure in a lipid environment and adopted a TM (transmembrane) topology within artificial lipid bilayers. Moreover, the peptide's affinity to artificial membranes with OM-like lipid composition was much higher than that of membranes with ER (endoplasmic reticulum)-like lipid composition. Collectively, our results suggest that SA proteins are specifically inserted into the MOM by a process that is not dependent on additional proteins, but is rather facilitated by the distinct lipid composition of this membrane.
Pathogenic bacteria can resist their microenvironment by changing the expression of virulence genes. In Salmonella typhimurium, some of these genes are controlled by the two-component system PhoP-PhoQ. Studies have shown that activation of the system by cationic antimicrobial peptides (AMPs) results, among other changes, in outer membrane remodeling. However, it is not fully clear what characteristics of AMPs are required to activate the PhoP-PhoQ system and whether activation can induce resistance to the various AMPs. For that purpose, we investigated the ability of a broad repertoire of AMPs to traverse the inner membrane, to activate the PhoP-PhoQ system, and to induce bacterial resistance. The AMPs differ in length, composition, and net positive charge, and the tested bacteria include two wild-type (WT) Salmonella strains and their corresponding PhoP-PhoQ knock-out mutants. A lacZ-reporting system was adapted to follow PhoP-PhoQ activation. The data revealed that: (i) a good correlation exists among the extent of the positive charge, hydrophobicity, and amphipathicity of an AMP and its potency to activate PhoP-PhoQ; (ii) a + 1 charged peptide containing histidines was highly potent, suggesting the existence of an additional mechanism independent of the peptide charge; (iii) the WT bacteria are more resistant to AMPs that are potent activators of PhoP-PhoQ; (iv) only a subset of AMPs, independent of their potency to activate the system, is more toxic to the mutated bacteria compared with the WT strains; and (v) short term exposure of WT bacteria to these AMPs does not enhance resistance. Overall, this study advances our understanding of the molecular mechanism by which AMPs activate PhoP-PhoQ and induce bacterial resistance. It also reveals that some AMPs can overcome such a resistance mechanism.
The activity of antimicrobial peptides (AMPs) that contain a large proportion of histidine residues (pK(a) similar to 6) depends on the physiological pH environment. Advantages of these AMPs include high activity in slightly acidic areas of the human body and relatively low toxicity in other areas. Also, many AMPs are highly active in a multivalent form, but this often increases toxicity. Here we designed pH dependent amphiphilic compounds consisting of multiple ultrashort histidine lipopeptides on a triazacyclophane scaffold, which showed high activity toward Aspergillus fumigatus and Cryptococcus neoformans at acidic pH, yet remained nontoxic. In vivo, treatment with a myristic acid conjugated trivalent histidine-histidine dipeptide resulted in 55% survival of mice (n = 9) in an otherwise lethal murine lung Aspergillus infection model. Fungal burden was assessed and showed completely sterile lungs in 80% of the mice (n = 5). At pH 5.5 and 7.5, differing peptide-membrane interactions and peptide nanostructures were observed. This study underscores the potential of unique AMPs to become the next generation of clinical antimicrobial therapy.
Many natural broad-spectrum cationic antimicrobial peptides (AMPs) possess a general mode of action that is dependent on lipophilicity and charge. Modulating the lipophilicity of AMPs by the addition of a fatty acid has been an effective strategy to increase the lytic activity and can further broaden the spectrum of AMPs. However, lipophilic modifications that narrow the spectrum of activity and exclusively direct peptides to fungi are less common. Here, we show that short peptide sequences can be targeted to fungi with structured lipophilic biomolecules, such as vitamin E and cholesterol. The conjugates were active against Aspergillus fumigatus, Cryptococcus neoformans, and Candida albicans but not against bacteria and were observed to cause membrane perturbation by transmission electron microscopy and in membrane permeability studies. However, for C. albicans, selected compounds were effective without the perturbation of the cell membrane, and synergism was seen with a vitamin E conjugate and amphotericin B. Moreover, in combination with beta-cyclodextrin, antibacterial activity emerged in selected compounds. Biocompatibility for selected active compounds was tested in vitro and in vivo using toxicity assays on erythrocytes, macrophages, and mice. In vitro cytotoxicity experiments led to selective toxicity ratios (50% lethal concentration/MIC) of up to 64 for highly active antifungal compounds, and no in vivo murine toxicity was seen. Taken together, these results highlight the importance of the conjugated lipophilic structure and suggest that the modulation of other biologically relevant peptides with hydrophobic moieties, such as cholesterol and vitamin E, generate compounds with unique bioactivity.
Studies of membrane peptide interactions at the molecular level are important for understanding essential processes such as membrane disruption or fusion by membrane active peptides. In a previous study, we combined several electron paramagnetic resonance (EPR) techniques, particularly continuous wave (CW) EPR, electron spin echo envelope modulation (ESEEM), and double electron-electron resonance (DEER) with Monte Carlo (MC) simulations to probe the conformation, insertion depth, and orientation with respect to the membrane of the membrane active peptide melittin. Here, we combined these EPR techniques with cryogenic transmission electron microscopy (cryo-TEM) to examine the effect of the peptide/phospholipid (P/PL) molar ratio, in the range of 1:400 to 1:25, on the membrane shape, lipids packing, and peptide orientation and penetration. Large unilamellar vesicles (LUVs) of DPPC/PG (7:3 dipalmitoylphosphatidylcholine/egg phosphatidylglycerol) were used as model membranes. Spin-labeled peptides were used to probe the peptide behavior whereas spin-labeled phspholipids were used to examine the membrane properties. The cryo-TEM results showed that melittin causes vesicle rupture and fusion into new vesicles with ill-defined structures. This new state was investigated by the EPR methods. In terms of the peptide, CW EPR showed decreased mobility, and ESEEM revealed increased insertion depth as the P/PL ratio was raised. DEER measurements did not reveal specific aggregates of melittin, thus excluding the presence of stable, well-defined pore structures. In terms of membrane properties, the CW EPR reported reduced mobility in both polar head and alkyl chain regions with increasing P/PL. ESEEM measurements showed that, as the P/PL ratio increased, a small increase in water content in the PL headgroup region took place and no change was observed in the alkyl chains part close to the hydrophilic region. In terms of lipid local density, opposite behavior was observed for the
Cancer continues to be a leading source of morbidity and mortality worldwide in spite of progress in oncolytic therapies. In addition, the incidence of cancers affecting the breast, kidney, prostate and skin among others continue to rise. Chemotherapeutic drugs are widely used in cancer treatment but have the serious drawback of nonspecific toxicity because these agents target any rapidly dividing cell without discriminating between healthy and malignant cells. In addition, many neoplasms eventually become resistant to conventional chemotherapy due to selection for multidrug-resistant variants. The limitations associated with existing chemotherapeutic drugs have stimulated the search for new oncolytic therapies. Host defense peptides (HDPs) may represent a novel family of oncolytic agents that can avoid the shortcomings of conventional chemotherapy because they exhibit selective cytotoxicity against a broad spectrum of malignant human cells, including multi-drug-resistant neoplastic cells. Oncolytic activity by HDPs is usually via necrosis due to cell membrane lysis, but some HDPs can trigger apoptosis in cancer cells via mitochondrial membrane disruption. In addition, certain HDPs are anti-angiogenic which may inhibit cancer progression. This paper reviews oncolytic HDP studies in order to address the suitability of selected HDPs as oncolytic therapies.
The viral peptide fusion inhibitor Fuzeon (T-20/DP178/enfuvirtide) is an essential part of the drug combination that has significantly increased the quality of life and life span of many acquired immunodeficiency syndrome (AIDS) patients. Its development as a drug preceded the elucidation of its precise inhibitory mechanism, as well as its molecular targets. The initial model was that Fuzeon inhibits human immunodeficiency virus (HIV) entry by targeting one site within the viral transmembrane envelope protein. Herein, we describe the emerging discoveries that extend this model towards a multifaceted mechanism for the drug in targeting HIV. This significantly advances the understanding of how viruses enter host cells and opens a new window of opportunity for designing future viral fusion inhibitors. (C) 2011 Elsevier B.V. All rights reserved.
Fusion of human immunodeficiency virus (HIV-1) with target cells is mediated by the gp41 transmembrane envelope protein. The loop region within gp41 contains 2 crucial cysteines that play an unknown role in HIV-cell fusion. On the basis of cell-cell fusion assay, using human T-cell lines [Jurkat E6-1 and Jurkat HXBc2(4)], and virus-cell fusion assay, using fully infectious HIV-1 HXBc2 virus and TZM-bl human cell line, we provide evidence that the oxidation state of the disulfide bond within a loop domain peptide determines its activity. The oxidized (closed) form inhibits fusion, while the reduced (opened) form enhances hemifusion. These opposite activities reach 60% difference in viral fusion. Both forms of the loop domain interact with gp41: the opened form enhances gp41 folding into a bundle, whereas the closed form inhibits this folding. Therefore, the transformation of the cysteines from a reduced to an oxidized state enables the loop to convert from opened to closed conformations, which assists gp41 to fold and induces hemifusion. The significant conservation of the loop region within many envelope proteins suggests a general mechanism, which is exploited by viruses to enhance entry into their host cells.-Ashkenazi, A., Viard, M., Wexler-Cohen, Y., Blumenthal, R., Shai, Y. Viral envelope protein folding and membrane hemifusion are enhanced by the conserved loop region of HIV-1 gp41. FASEB J. 25, 2156-2166 (2011). www.fasebj.org
Due to the rapid emergence of resistant microbes to the currently available antibiotics, cationic antimicrobial peptides have attracted considerable interest as a possible new generation of anti-infective compounds. However, low cost development for therapeutic or industrial purposes requires, among other properties, that the peptides will be small and with simple structure. Therefore, considerable research has been devoted to optimizing peptide length combined with a simple design. This review focuses on the similarities and differences in the mode of action and target cell specificity of two families of small peptides: the naturally occurring temporins from the skin of amphibia and the engineered ultrashort lipopeptides. We will also discuss the finding that acylation of cationic peptides results in molecules with a more potent spectrum of activity and a higher resistance to proteolytic degradation. Conjugation of fatty acids to linear native peptide sequences is a powerful strategy to engineer novel successful anti-infective drugs.
Sarcomas display a rare and heterogeneous group of tumors. Treatment options are limited. Host defense peptides (HDPs), effector molecules of the innate immune system, might provide a more effective treatment option. The aim of our study was to analyze the oncolytic activity and mode of action of a designer HDP. In vitro, the human liposarcoma cell line SW-872 and primary human fibroblasts as a control were exposed to [D]-K(3)H(3)L(9), a 15-mer D,L-amino acid designer peptide. Cell growth (MTT assay), proliferation (BrdU assay) and genotoxicity (TUNEL assay) were analyzed. The mode of action was examined via fluorescence-activated cell sorter (FACS) analysis and confocal laser scanning microscopy. In vivo, [D]-K(3)H(3)L(9) (n = 7) was administered intratumorally in a SW-872 xenograft mouse model (Foxn1nu/nu). Phosphate buffered saline served as a control (n 5 5). After 4 weeks, tumor sections were histologically analyzed with respect to proliferation, cytotoxicity, vessel density and signs of apoptosis and necrosis, respectively. In vitro, [D]-K(3)H(3)L(9) highly significantly (p <0.01) inhibited cell metabolism and proliferation. TUNEL assay revealed corresponding genotoxicity. FACS analysis suggested induction of necrosis as a cause of cell death. The mean tumor volume of the control group exponentially increased sevenfold, whereas the mean tumor growth was negligible in the treatment group. Macroscopically, [D]-K(3)H(3)L(9) induced full tumor remission in 43% of treated animals and partial remission in 43%. Vessel density was significantly reduced by 52%. Morphological analyses supported the hypothesis of cancer cell killing by necrosis. In summary, [D]-K(3)H(3)L(9) exerts very promising oncolytic activity on liposarcoma cells. Our study demonstrates the potential of HDPs as a novel therapeutic option in future soft tissue sarcoma therapy.
HIV-1 fusion with its target cells is mediated by the glycoprotein 41 (gp41) transmembrane subunit of the viral envelope glycoprotein (ENV). The current models propose that gp41 undergoes several conformational changes between the apposing viral and cell membranes to facilitate fusion. In this review we focus on the progress that has been made in revealing the dynamic role of the N-terminal heptad repeat (NHR) and the C-terminal heptad repeat (CHR) regions within gp41 to the fusion process. The involvement of these regions in the formation of the gp41 pre-hairpin and hairpin conformations during an ongoing fusion event was mainly discovered by their derived inhibitory peptides. For example, the core structure within the hairpin conformation in a dynamic fusion event is suggested to be larger than its high resolution structure and its minimal boundaries were determined in situ. Also, inhibitory peptides helped reveal the dual contribution of the NHR to the fusion process. Finally, we will also discuss several developments in peptide design that has led to a deeper understanding of the mechanism of viral membrane fusion.
Background: Streptococcus agalactiae (Group B Streptococcus) is a leading cause of sepsis and meningitis in newborns. Most bacterial pathogens, including gram-positive bacteria, have long filamentous structures known as pili extending from their surface. Although pili are described as adhesive organelles, they have been also implicated in many other functions including thwarting the host immune responses. We previously characterized the pilus-encoding operon PI-2a (gbs14791474) in strain NEM316. This pilus is composed of three structural subunit proteins: PilA (Gbs1478), PilB (Gbs1477), and PilC (Gbs1474), and its assembly involves two class C sortases (SrtC3 and SrtC4). PilB, the bona fide pilin, is the major component whereas PilA, the pilus associated adhesin, and PilC the pilus anchor are both accessory proteins incorporated into the pilus backbone. Methodology/Principal Findings: In this study, the role of the major pilin subunit PilB was tested in systemic virulence using 6-weeks old and newborn mice. Notably, the non-piliated Delta pilB mutant was less virulent than its wild-type counterpart in the newborn mice model. Next, we investigated the possible role(s) of PilB in resistance to innate immune host defenses, i.e. resistance to macrophage killing and to antimicrobial peptides. Phagocytosis and survival of wild-type NEM316 and its isogenic Delta pilB mutant in immortalized RAW 264.7 murine macrophages were not significantly different whereas the isogenic Delta sodA mutant was more susceptible to killing. These results were confirmed using primary peritoneal macrophages. We also tested the activities of five cationic antimicrobial peptides (AMP-1D, LL-37, colistin, polymyxin B, and mCRAMP) and found no significant difference between WT and Delta pilB strains whereas the isogenic dltA mutant showed increased sensitivity. Conclusions/Significance: These results question the previously described role of PilB pilus in resistance to the host immune defenses.
Background: Soft tissue sarcoma (STS) is an anatomically and histologically heterogeneous neoplasia that shares a putative mesenchymal cell origin. The treatment with common chemotherapeutics is still unsatisfying because of association with poor response rates. Although evidence is accumulating for potent oncolytic activity of host defense peptides (HDPs), their potential therapeutic use is often limited by poor bioavailability and inactivation in serum. Therefore, we tested the designer host defense-like lytic D, L-amino acid peptide [D]-K(3)H(3)L(9) on two STS cell lines in vitro and also in an athymic and syngeneic mouse model. In recent studies the peptide could show selectivity against prostate carcinoma cells and also an active state in serum. Methods: In vitro the human synovial sarcoma cell line SW982, the murine fibrosarcoma cell line BFS-1 and primary human fibroblasts as a control were exposed to [D]-K(3)H(3)L(9), a 15mer D, L-amino acid designer HDP. Cell vitality in physiological and acidic conditions (MTT-assay), cell growth (BrdU) and DNA-fragmentation (TUNEL) were investigated. Membrane damage at different time points could be analyzed with LDH assay. An antibody against the tested peptide and recordings using scanning electron microscopy could give an inside in the mode of action. In vivo [D]-K(3)H(3)L(9) was administered intratumorally in an athymic and syngeneic (immunocompetent) mouse model with SW982 and BFS-1 cells, respectively. After three weeks tumor sections were histologically analyzed. Results: The peptide exerts rapid and high significant cytotoxicity and antiproliferating activity against the malignant cell lines, apparently via a membrane disrupting mode of action. The local intratumoral administration of [D]-K(3)H(3)L(9) in the athymic and syngeneic mice models significantly inhibited tumor progression. The histological analyses of the tumor sections revealed a significant antiproliferative, antiangiogenic activity of the treatmen
We present high field DEER (double electron-electron resonance) distance measurements using Gd3+ (S = 7/2) spin labels for probing peptides' conformations in solution. The motivation for using Gd3+ spin labels as an alternative for the standard nitroxide spin labels is the sensitivity improvement they offer because of their very intense EPR signal at high magnetic fields. Gd3+ was coordinated by dipicolinic acid derivative (4MMDPA) tags that were covalently attached to two cysteine thiol groups. Cysteines were introduced in positions 15 and 27 of the peptide melittin and then two types of spin labeled melittins were prepared, one labeled with two nitroxide spin labels and the other with two 4MMDPA-Gd3+ labels. Both types were subjected to W-band (95 GHz, 3.5 T) DEER measurements. For the Gd3+ labeled peptide we explored the effect of the solution molar ratio of Gd3+ and the labeled peptide, the temperature, and the maximum dipolar evolution time T on the DEER modulation depth. We found that the optimization of the [Gd3+]/[Tag] ratio is crucial because excess Gd3+ masked the DEER effect and too little Gd3+ resulted in the formation of Gd3+-tag2 complexes, generating peptide dimers. In addition, we observed that the DEER modulation depth is sensitive to spectral diffusion processes even at Gd3+ concentrations as low as 0.2 mM and therefore experimental conditions should be chosen to minimize it as it decreases the DEER effect. Finally, the distance between the two Gd3+ ions, 3.4 nm, was found to be longer by 1.2 nm than the distance between the two nitroxides. The origin and implications of this difference are discussed.
The interactions between the N- and C-terminal heptad repeat (NHR and CHR) regions of the human immunodeficiency virus (HIV-1) glycoprotein gp41 create a structure comprising a 6-helix bundle (SHB). A sequence in the SHB named the "pocket" is crucial for the SHB's stability and for the fusion inhibitory activity of 36-residue NHR peptide N36. We report that a short 27-residue peptide, N27, which lacks the pocket sequence, exhibits potent inhibitory activity in both cell-cell and virus-cell fusion assays when fatty acids were conjugated to its N but not C terminus. Furthermore, mutations in the positions that prevent interaction with the CHR but not with the NHR resulted in a dramatic reduction in N27 activity. These data support a mechanism in which N27 mainly targets the CHR rather than the internal NHR coiled-coil, reveal the N-terminal edge of the endogenous core structure in situ and hence complement our recent findings of the C-terminal edge of the core, and provide a new approach for designing short inhibitors from the NHR region of other lentiviruses due to similarities in their envelope proteins.-Wexler-Cohen, Y., Ashkenazi, A., Viard, M., Blumenthal, R., Shai, Y. Virus-cell and cell-cell fusion mediated by the HIV-1 envelope glycoprotein is inhibited by short gp41 N-terminal membrane-anchored peptides lacking the critical pocket domain. FASEB J. 24, 4196-4202 (2010). www.fasebj.org
Viruses have evolved several strategies to modify cellular processes and evade the immune response in order to successfully infect, replicate, and persist in the host. By utilizing in-silico testing of a transmembrane sequence library derived from virus protein sequences, we have pin-pointed a nine amino-acid motif shared by a group of different viruses; this motif resembles the transmembrane domain of the alpha-subunit of the T-cell receptor (TCR alpha). The most striking similarity was found within the immunodeficiency virus (SIV and HIV) glycoprotein 41 TMD (gp41 TMD). Previous studies have shown that stable interactions between TCR alpha and CD3 are localized to this nine amino acid motif within TCR alpha, and a peptide derived from it (TCRa TMD, GLRILLLKV) interfered and intervened in the TCR function when added exogenously. We now report that the gp41 TMD peptide co-localizes with CD3 within the TCR complex and inhibits T cell proliferation in vitro. However, the inhibitory mechanism of gp41 TMD differs from that of the TCR alpha TMD and also from the other two known immunosuppressive regions within gp41.
A new method is presented for patterning surfaces with gradient properties. The method is based on magnetolithography in which the surface patterning is performed by applying a gradient of a magnetic field on the substrate, using paramagnetic metal masks in the presence of a constant magnetic field. Superparamagnetic nanoparticles (NPs) are deposited on the substrate, and they assemble according to the field and its gradients induced by the mask. Once they pattern the substrate, they protect their sites on the substrate from interacting with any other species. The areas not protected by the NPs can be covered by molecules that chemically bind to the substrate. After these molecules are bound, the NPs are removed, and other molecules may be adsorbed on the newly exposed area. The new technique is based on a parallel process that can be carried out on a full wafer. It provides high resolution, it creates gradient continuously from submicrometers to milimeters, and it can be performed on surfaces that are not flat and that are even on the inside of a tube. The gradient that is formed is not limited to a specific property or type of substrate.
Three Arg-rich nonapeptides, containing the same amino acid composition but different sequences, PFWRIRIRR-amide (PR-9), RRPFWIIRR-amide (RR-9) and PRFRWRIRI-amide (PI-9), are able to induce segregation of anionic lipids from zwitterionic lipids, as shown by changes in the phase transition properties of lipid mixtures detected by differential scanning calorimetry and freeze fracture electron microscopy. The relative Minimal Inhibitory Concentration (MIC) of these three peptides against several strains of Gram positive bacteria correlated well with the extent to which the lipid composition of the bacterial membrane facilitated peptide-induced clustering of anionic lipids. The lower activity of these three peptides against Gram negative bacteria could be explained by the retention of these peptides in the LPS layer. The membrane morphologies produced by PR-9 as well as by a cathelicidin fragment. KR-12 that had previously been shown to induce anionic lipid clustering, was directly visualized using freeze fracture electron microscopy. This work shows the insensitivity of phase segregation to the specific arrangement of the cationic charges in the peptide sequence as well as to their tendency to form different secondary structures. It also establishes the role of anionic lipid clustering in the presence of zwitterionic lipids in determining antimicrobial selectivity. (C) 2010 Elsevier B.V. All rights reserved.
One of the most important steps in the process of viral infection is a fusion between cell membrane and virus, which is mediated by the viral envelope glycoprotein. The study of activity of the glycoprotein in the post-fusion state is important for understanding the progression of infection. Here we present a first real-time kinetic study of the activity of gp41 (the viral envelope glycoprotein of human immunodeficiency virus-HIV) and its two mutants in the post-fusion state with nanometer resolution by atomic force microscopy (AFM). Tracking the changes in the phosphatidylcholine (PC) and phosphatidylcholine-phosphatidylserine (PC:PS) membrane integrity over one hour by a set of AFM images revealed differences in the interaction of the three types of protein with zwitterionic and negatively charged membranes. A quantitative analysis of the slow kinetics of hole formation in the negatively charged lipid bilayer is presented. Specifically, analysis of the rate of roughness change for the three types of proteins suggests that they exhibit different types of kinetic behavior. (C) 2010 Elsevier B.V. All rights reserved.
Fusion between viral and host cell membranes is the initial step of human immunodeficiency virus infection and is mediated by the gp41 protein, which is embedded in the viral membrane. The similar to 20-residue N-terminal fusion peptide (FP) region of gp41 binds to the host cell membrane and plays a critical role in fusion catalysis. Key gp41 fusion conformations include an early pre-hairpin intermediate (PHI) characterized by extended coiled-coil structure in the region C-terminal of the FP and a final hairpin state with compact six-helix bundle structure. The large "N70" (gp41 1-70) and "FP-Hairpin" constructs of the present study contained the FP and respectively modeled the PHI and hairpin conformations. Comparison was also made to the shorter "FP34" (gp41 1-34) fragment. Studies were done in membranes with physiologically relevant cholesterol content and in membranes without cholesterol. In either membrane type, there were large differences in fusion function among the constructs with little fusion Induced by FP-Hairpin, moderate fusion for FP34, and very rapid fusion for N70 Overall, our findings support acceleration of gp41-induced membrane fusion by early PHI conformation and fusion arrest after folding to the final six-helix bundle structure. FP secondary structure at Leu7 of the membrane-associated constructs was probed by solid-state nuclear magnetic resonance and showed populations of molecules with either beta-sheet or helical structure with greater beta-sheet population observed for FP34 than for N70 or FP-Hairpin The large differences in fusion function among the constructs were not obviously correlated with FP secondary structure. Observation of cholesterol-dependent FP structure for fusogenic FP34 and N70 and cholesterol-independent structure for non-fusogenic FP-Hairpin was consistent with membrane insertion of the FP for FP34 and N70 and with lack of insertion for FP-Hairpin. Membrane insertion of the FP may therefore be associated with the ear
The interaction between host-defense antimicrobial peptides (AMPS) and the bacterial lipopolysaccharide (LPS) governs both the susceptibility of the bacteria to the peptide and the ability of the peptide to inhibit LPS activation of immune cells. Both functions depend on the biophysical properties of the peptides. However, the sequence and structural diversity of AMPS makes it difficult to determine common denominators required for antimicrobial and LPS neutralizing activities. Toward this end, we synthesized and investigated a series of nine 12-amino acid peptides and their fatty acid-conjugated analogues composed of both D- and L-isomers of Leu and Lys at various ratios. The positions of the D-amino acids were preserved. These peptides differ in their net positive charge and hydrophobicity. However, their overall structure in the membrane is similar, as determined by Fourier transform infrared spectroscopy. The peptides and their analogues were functionally tested for their antibacterlal and hemolytic activity, their ability to permeate LPS vesicles, their ability to neutralize LPS activation of macrophages, and their effect on LPS morphology, determined by negative staining electron microscopy. The data revealed that increasing the ratio between hydrophobicity and the net positive charge increases both antimicrobial and LPS neutralization activities, but with different modes of contributions, Whereas antimicrobial activity increases linearly with the increase in the peptides' hydrophobicity, peptides with different hydrophobicities are endowed with similar LPS neutrallzing activities. Besides adding important information regarding AMP parameters involved in antimicrobial and anti-LPS activities, this study suggests the use or such diastereomers as potential templates for the development of simple molecules that conduct both types of functions.
We present a new approach to obtain details on the distribution and average structure and locations of membrane-associated peptides. The approach combines (i) pulse double electron-electron resonance (DEER) to determine intramolecular distances between residues in spin labeled peptides, (ii) electron spin echo envelope modulation (ESEEM) experiments to measure water exposure and the direct interaction of spin labeled peptides with deuterium nuclei on the phospholipid molecules, and (iii) Monte Carlo (MC) simulations to derive the peptide-membrane populations, energetics, and average conformation of the native peptide and mutants mimicking the spin labeling. To demonstrate the approach, we investigated the membrane-bound and solution state of the well-known antimicrobial peptide melittin, used as a model system. A good agreement was obtained between the experimental results and the MC simulations regarding the distribution of distances between the labeled amino acids, the side chain mobility, and the peptide's orientation. A good agreement in the extent of membrane penetration of amino acids in the peptide core was obtained as well, but the EPR data reported a somewhat deeper membrane penetration of the termini compared to the simulations. Overall, melittin adsorbed on the membrane Surface, in a monomeric state. as an amphipatic helix with its hydrophobic residues in the hydrocarbon region of the membrane and its charged and polar residues in the lipid headgroup region.
Ribosomally synthesized antimicrobial peptides (AMPs) represent an essential component of the ancient and non-specific innate immune system in all forms of life, with the primary role of killing infectious microorganisms. Amphibian skin is one of the richest storehouses for them. Each frog species produces its own set of peptides with up to 10 isoforms, as in the case of the species Rana temporaria. Nowadays, human health is facing two major threats: (i) the increasing emergence of resistant pathogens to one or more available drugs, and (ii) the onset of septic shock, which is associated with the release of lipopolysaccharide (LPS) from the cell walls of Gram-negative bacteria, particularly upon antibiotic treatment. AMPs are considered as potential new and-infective compounds with a novel mode of action, because many of them can kill bacteria and, at the same time, neutralize the toxic effects of LIPS. Recent studies have suggested that the production of large number of structurally similar AMPs within the same animal is a strategy used by nature to increase the spectrum of antimicrobial activities, by using combinations of the peptide's isoforms. The biological rationale for their coexistence within the same organism is discussed. In addition, the distinctive and attractive synergistic effects of temporins in both antimicrobial and anti-endotoxin activities are reviewed, along with their plausible underlying molecular mechanism. (C) 2009 Elsevier B.V. All rights reserved.
We have designed and chemically synthesized all artificial beta-defensin based on a minimal template derived from the comparative analysis of over 80 naturally occurring Sequences. This molecule has the disulfide-bridged beta-sheet core structure of natural beta-defensins and shows a robust salt-sensitive antimicrobial activity against bacteria and yeast. as well as a chemotactic activity against immature dendritic cells. Ail SAR (structure-activity relationship) Study using two truncated fragments on a CYS -> Ser point-mutated analogue, from which one or two of the three disulfide bridges were absent, indicated that altering the structure resulted ill a different type of membrane interaction and a switch to different modes of action towards both microbial and host cells, and that covalent dimerization Could favour antimicrobial activity. Comparison of the structural, aggregational and biological activities of the artificial defensin with those of three human beta-defensins and their primate orthologues provided useful information on how their mode of action may relate to specific structural features.
A new family of synthetic, membrane-active, ultrashort lipopeptides composed of only four amino acids linked to fatty acids was tested for the ability to induce systemic resistance and defense responses in plants. We found that two peptides wherein the third residue is a D-enantiomer (italic), C(16)-KKKK and C(16)-KLLK, can induce medium alkalinization of tobacco suspension-cultured cells and expression of defense-related genes in cucumber and Arabidopsis seedlings. Moreover, these compounds can prime systemic induction of antimicrobial compounds in cucumber leaves similarly to the plant-beneficial fungus Trichoderma asperellum T203 and provide systemic protection against the phytopathogens Botrytis cinerea B05, Pseudomonas syringae pv. lachrimans, and P. syringae pv. tomato DC3000. Thus, short cationic lipopeptides are a new category of compounds with potentially high utility in the induction of systemic resistance in plants.
Peptide inhibitors derived from HIV-gp41 envelope protein play a pivotal role in deciphering the molecular mechanism of HIV-cell fusion. According to accepted models, N-heptad repeat (NHR) peptides can bind two targets in an intermediate fusion conformation, thereby inhibiting progression of the fusion process. In both cases the orientation towards the endogenous intermediate conformation should be important. To test this, we anchored NHR to the cell membrane by conjugating fatty acids with increasing lengths to the N- or C-terminus of N36, as well as to two known N36 mutants; one that cannot bind C-heptad repeat (CHR) but can bind NHR (N36 MUTe,g), and the second cannot bind to either NHR or CHR (N36 MUTa,d). Importantly, the IC(50) increased up to 100-fold in a lipopeptide-dependent manner. However, no preferred directionality was observed for the wild type derived lipopeptides, suggesting a planar orientation of the peptides as well as the endogenous NHR region on the cell membrane. Furthermore, based on: (i) specialized analysis of the inhibition curves, (ii) the finding that N36 conjugates reside more on the target cells that occupy the receptors, and (iii) the finding that N36 MUTe, g acts as a monomer both in its soluble form and when anchored to the cell membrane, we suggest that anchoring N36 to the cell changes the inhibitory mode from a trimer which can target both the endogenous NHR and CHR regions, to mainly monomeric lipopetides that target primarily the internal NHR. Besides shedding light on the mode of action of HIV-cell fusion, the similarity between functional regions in the envelopes of other viruses suggests a new approach for developing potent HIV-1 inhibitors.
One of the most extensively studied receptor tyrosine kinases is EGFR/ErbB1. Although our knowledge of the role of the extracellular domains and ligands in ErbB1 activation has increased dramatically based on solved domain structures, the exact mechanism of signal transduction across the membrane remains unknown. The transmembrane domains are expected to play an important role in the dimerization process, but the contribution of ErbB1 TM domain to dimer stability is not known, with published results contradicting one another. We address this controversy by showing that ErbB1 TM domain dimerizes in lipid bilayers and by calculating its contribution to stability as -2.5 kcal/mol. The stability calculations use two different methods based on Forster resonance energy transfer, which give the same result. The ErbB1 TM domain contribution to stability exceeds the change in receptor tyrosine kinases dimerization propensities that can convert normal signaling processes into pathogenic processes, and is thus likely important for biological function.
Previously, we reported that intratumor or systemic inoculation of a cationic 15-mer, innate immunity-like lytic peptide composed of D- and L-amino acids ([D]-K(6)L(9)) caused growth arrest of 22RV1 prostate carcinoma xenografts in a mouse model. However, despite its therapeutic potential, this peptide has significant systemic toxicity at concentrations slightly higher than the therapeutic one. Here, we used the acidic environment created by solid tumors as a trigger to activate anticancer lytic peptides by making them cationic only at low pH levels. We achieved this selectivity by substituting lysines (pKa, similar to 10.5) for histidines (pKa, similar to 6.1) in the parental peptide [D]-K6L9. Histidine is protonated below pH 7. For that purpose, we replaced either three or all six lysines in the parental peptide with histidines to obtain the peptides [D]-K(3)H(3)L(9) and [D]-H(6)L(9). Interestingly, in vitro experiments showed pH-dependent activity only with [D]-H6L9 mainly toward cancer cell lines. However, both peptides showed reduced systemic toxicity compared with the parental peptide. Intratumor and systemic inoculation of these peptides resulted in a significant decrease in the 22RV1 prostate cancer tumor volume and systemic secretion of prostate-specific antigen in a xenograft mice model. Moreover, histologic modifications revealed a significant reduction in new blood vessels selectively in tumor tissues after treatment with the peptides compared with the untreated tumors. The lytic mode of action of these new peptides, which makes it difficult for the cancer cells to develop resistance, and their selective and pH-dependent activity make them potential candidates for treatment of solid cancer tumors. [Cancer Res 2009;69(8):3458-63]
Membrane fusion between the human immunodeficiency virus (HIV) and the target cell plasma membrane is correlated with conformational changes in the HIV gp41 glycoprotein, which include an early exposed conformation (prehairpin) and a late low energy six helix bundle (SHB) conformation also termed hairpin. Peptides resembling regions from the exposed prehairpin have been previously studied for their interaction with membranes. Here we report on the expression, purification, SHB stability, and membrane interaction of the full-length ectodomain of the HIV gp41 and its two deletion mutants, all in their SHB-folded state. The interaction of the proteins with zwitterionic and negatively charged membranes was examined by using various biophysical methods including circular dichroism spectroscopy, differential scanning calorimetry, lipid mixing of large unilamellar vesicles, and atomic force microscopy (AFM). All experiments were done in an acidic environment in which the protein remains in its soluble trimeric state. The data reveal that all three proteins fold into a stable coiled-coil core in aqueous solution and retain a stable helical fold with reduced coiled-coil characteristics in a zwitterionic and negatively charged membrane mimetic environment. Furthermore, in contrast with the extended exposed N-terminal domain, the folded gp41 ectodomain does not induce lipid mixing of zwitterionic membranes. However, it disrupts and induces lipid mixing of negatively charged phospholipid membranes (similar to 100-fold more effective than fusion peptide alone), which are known to be expressed more in HIV-1-infected T cells or macrophages. The results support the emerging model in which one of the roles of gp41 folding into the SHB conformation is to slow down membrane disruption effects induced by early exposed gp41. However, it can further affect membrane morphology once exposed to negatively charged membranes during late stages.
Conformational changes in the HIV gp41 protein are directly correlated with fusion between the HIV and target cell plasma membranes, which is the initial step of infection. Ke gp41 fusion conformations include an early extended conformation termed prehairpin which contains exposed regions and a final low-energy conformation termed hairpin which has a compact six-helix bundle structure. Current fusion models debate the roles of hairpin and prehairpin conformations in the process of membrane merger. In the present work, gp41 constructs have been engineered which correspond to fusion relevant parts of both prehairpin and hairpin conformations and have been analyzed for their ability to induce lipid mixing between membrane vesicles. The data correlate membrane fusion function with the prehairpin conformation and suggest that one of the roles of the final hairpin conformation is sequestration of membrane-perturbing gp41 regions with consequent loss of the membrane disruption induced earlier by the prehairpin structure. To our knowledge, this is the first biophysical study to delineate the membrane fusion potential of gp41 constructs modeling key fusion conformations.
Lipid membranes act as catalysts for protein folding. Both alpha-helical and beta-sheet structures can be induced by the interaction of peptides or proteins with lipid surfaces. Melittin, the main component of bee venom, is a particularly well-studied example for the membrane-induced random coil-to-alpha-helix transition. Melittin in water adopts essentially a random coil conformation. The cationic amphipathic molecule has a high affinity for neutral and anionic lipid membranes and exhibits similar to 50-65% alpha-helix conformation in the membrane-bound state. At higher melittin concentrations, the peptide forms aggregates or pores in the membrane. In spite of the long-standing interest in melittin-lipid interactions, no systematic thermodynamic study is available. This is probably caused by the complexity of the binding process. Melittin binding to lipid vesicles is fast and occurs within milliseconds, but the binding process involves at least four steps, namely, (i) the electrostatic attraction of the cationic peptide to an anionic membrane surface, (ii) the hydrophobic insertion into the lipid membrane, (iii) the conformational change from random coil to alpha-helix, and (iv) peptide aggregation in the lipid phase. We have combined microelectrophoresis (measurement of the zeta potential), isothermal titration calorimetry, and circular dichroism spectroscopy to provide a thermodynamic analysis of the individual binding steps. We have compared melittin with a synthetic analogue, [D]-V(5,8),I(17),K(21)-melittin, for which alpha-helix formation is suppressed and replaced by beta-structure formation. The comparison reveals that the thermodynamic parameters for the membrane-induced alpha-helix formation of melittin are identical to those observed earlier for other peptides with an enthalpy h(helix) of -0.7 kcal/mol and a free energy g(helix) of -0.2 kcal/mol per peptide residue. These thermodynamic parameters hence appear to be of general validity for lipid-induced
Antimicrobial lipopeptides are produced nonribosomally in bacteria and fungi during cultivation. They are composed of a cationic or an anionic peptide covalently bound to a specifically modified aliphatic chain. Most of the peptidic moieties have complex cyclic structures. Here we report that conjugation of a palmitic acid to the N-terminus of very short cationic di- and tripeptides composed of all L- and D,L-amino acids endowed them with potent antimicrobial activities. Interestingly, cell specificity was determined by the sequence of the short peptidic chain. Palmitoyllysine served as a control and was inactive toward all microorganisms tested. Replacing an L-amino acid with its D-enantiomer did not affect the activity of the corresponding lipopeptides. Importantly, selected lipopeptides were also potent in vivo in a mouse model of Candida albicans infection. Bacterial leakage experiments and negative staining electron microscopy suggest that their mode of action involves permeation and disintegration of the microorganism's membrane, similar to many long antimicrobial peptides and lipopeptides. Interestingly, each lipopeptide assembled in solution into a nanostructure with a unique morphology which could partially explain differences in their biological activity. Besides adding important information on the parameters necessary for antimicrobial lipopeptides to kill microorganisms, the simple composition of these minilipopeptides and their diverse cell specificities make them attractive candidates for various applications.
Aspergillus fumigatus is an opportunistic fungal pathogen responsible for invasive aspergillosis in immuno-compromised individuals. The inefficiency of antifungal agents and high mortality rate resulting from invasive aspergillosis remain major clinical concerns. Recently, we reported on a new family of ultrashort cationic lipopeptides active in vitro against fungi. Mode of action studies supported a membranolytic or a detergent-like effect. Here, we screened several lipopeptides in vitro for their anti-A. fumigatus activity. To investigate the therapeutic properties of the selected peptides in vivo, we challenged immunosuppressed C57BL/6 wild-type mice intranasally with DsRed-labeled A. fumigatus conidia and subsequently treated the animals locally with the lipopeptides. Confocal microscopic analysis revealed the degradation of DsRed-labeled hyphal forms and residual conidia in the lungs of the mice. The most efficient peptide was tested further using a survival assay and was found to significantly prolong the life of the treated animals, whereas no mice survived with the current standard antifungal treatment with amphotericin B. Moreover, as opposed to the drug-treated lungs, the peptide-treated lungs did not display any toxicity of the peptide. Our results highlight the potential of this family of lipopeptides for the treatment of pulmonary invasive aspergillosis.
Lipopolysaccharide (LPS) is the major structural component of the outer membrane of Gram-negative bacteria and shields them from a variety of host defense factors, including antimicrobial peptides (AMPs). LPS is also recognized by immune cells as a pathogen-associated molecular pattern and stimulates them to secrete pro-inflammatory cytokines that, in extreme cases, lead to a harmful host response known as septic shock. Previous studies have revealed that a few isoforms of the AMP temporin, produced within the same frog specimen, can synergize to overcome bacterial resistance imposed by the physical barrier of LPS. Here we found that temporins can synergize in neutralizing the LPS-induced macrophage activation. Furthermore, the synergism between temporins, to overcome the protective function of LPS as well as its endotoxic effect, depends on the length of the polysaccharide chain of LPS. Importantly, mode of action studies, using spectroscopic and thermodynamic methods, have pointed out different mechanisms underlying the synergism of temporins in antimicrobial and anti-endotoxin activities. To the best of our knowledge, such a dual synergism between isoforms of AMPs from the same species has not been observed before, and it might explain the ability of such amphibians to resist a large repertoire of microorganisms.
Endotoxin [lipopolysaccharide (LPS)] covers more than 90% of the outer monolayer of the outer membrane of Gram-negative bacteria, and it plays a dual role in its pathogenesis: as a protective barrier against antibiotics and as an effector molecule, which is recognized by and activates the innate immune system. The ability of host-defense antimicrobial peptides to bind LPS on intact bacteria and in suspension has been implicated in their antimicrobial and LPS detoxification activities. However, the mechanisms involved and the properties of the peptides that enable them to traverse the LPS barrier or to neutralize LPS endotoxic activity are not yet fully understood. Here we investigated a series of antimicrobial peptides and their analogues with drastically altered sequences and structures, all of which share the same amino acid composition (K6L9). The list includes both all-L-amino acid peptides and their diastereomers (composed of both L- and D-amino acids). The peptides were investigated functionally for their antibacterial. activity and their ability to block LPS-dependent TNF-alpha secretion by macrophages. Fluorescence spectroscopy and transmission electron microscopy were used to detect their ability to bind LPS and to affect its oligomeric state. Their secondary structure was characterized in solution, in LPS suspension, and in LPS multibilayers by using CD and FTIR spectroscopy. Our data reveal specific biophysical properties of the peptides that are required to kill bacteria and/or to detoxify LPS. Besides shedding light on the mechanisms of these two important functions, the information gathered should assist in the development of AMPs with potent antimicrobial and LPS detoxification activities.
HIV infection is initiated by the fusion of the viral membrane with the target T-cell membrane. The HIV envelope glycoprotein, gp41, contains a fusion peptide (FP) in the N terminus that functions together with other gp41 domains to fuse the virion with the host cell membrane. We recently reported that FP co-localizes with CD4 and T-cell receptor (TCR) molecules, co-precipitates with TCR, and inhibits antigen-specific T-cell proliferation and pro-inflammatory cytokine secretion. Molecular dynamic simulation implicated an interaction between an alpha-helical transmembrane domain (TM) of the TCR alpha chain (designated CP) and the beta-sheet 5-13 region of the 16 N-terminal amino acids of FP (FP1-16). To correlate between the theoretical prediction and experimental data, we synthesized a series of mutants derived from the interacting motif GALFLGFLG stretch (FP5-13) and investigated them structurally and functionally. The data reveal a direct correlation between the beta-sheet structure of FP5-13 and its mutants and their ability to interact with CP and induce immunosuppressive activity; the pherylalanines play an important role. Furthermore, studies with fluorescently labeled peptides revealed that this interaction leads to penetration of the N terminus of FP and its active analogues into the hydrophobic core of the membrane. A detailed understanding of the molecular interactions mediating the immunosuppressive activity of the FP5-13 Motif should facilitate evaluating its contribution to HIV. pathology and its exploitation as an immunotherapeutic tool.
Objectives: Cationic antimicrobial peptides (AMPs) are indispensable components of innate immune systems and promising candidates for novel anti-infective strategies. We rationally designed a series of peptides based on a template derived from known alpha-helical AMPs, which were then analysed regarding efficacy against clinical isolates and antibiotic mechanisms. Methods: Efficacy tests included standard MIC and synergy assays. Whole cell assays with staphylococcal strains included killing kinetics, efflux experiments and determination of membrane depolarization. The transcriptional response of AMP-treated Staphylococcus aureus SG511 was analysed using a Scienion genomic microarray covering (similar to 90% of) the S. aureus N315 genome and AMP P16(6 vertical bar E). Results: The AMPs showed remarkable broad-spectrum activity against bacteria and fungi regardless of any pre-existing antibiotic resistance mechanism. Whole cell assays indicated that the AMPs target the cytoplasmic membrane; however, significant membrane leakage and depolarization was only observed with a standard laboratory test strain. Transcriptional profiling identified up-regulation of putative efflux pumps and of aerobic energy generation mechanisms as major counter activities. Important components of the staphylococcal cell wall stress stimulon were up-regulated and the lipid metabolism was also affected. Conclusions: The broad spectrum activity of amphiphilic helical AMPs is based on multiple stresses resulting from interactions with microbial membranes; however, rather than killing through formation of pores, the AMPs appear to interfere with the coordinated and highly dynamic functioning of membrane bound multienzyme complexes such as electron transport chains and cell wall or lipid biosynthesis machineries.
The core complex is a structure involved in the fusion mechanism of many viruses, as well as in intracellular vesicle fusion. A powerful approach for studying the dynamic stages of HIV-1-cell fusion utilizes DP178, a core complex inhibitory peptide derived from the known sequence of the virus. Strikingly, we show that fatty acids can replace the entire C-terminal region of DP178, known to play a crucial role in the activity of the peptide. The inhibitory activity correlated with the length of the fatty acid, with the direction of fatty acid attachment (N-or C-terminus) and, as envisioned by a new triple staining assay, with the concentration of the peptides on cells. Our findings indicate, for the first time, the C-terminal boundary of the endogenous core structure in situ and establish that the C-terminal region of DP178 functions mainly as an anchor to the cell membrane. Apart from the mechanistic implications, such short lipopeptides provide new, promising fusion inhibitors. Because the fusion mechanism of HIV-1 is shared by other pathogen-enveloped viruses and by intracellular vesicle fusion, our results might influence the research and therapeutic efforts in these systems as well.
Various fusion proteins from eukaryotes and viruses share structural similarities such as a coiled coil motif. However, compared with eukaryotic proteins, a viral fusion protein contains a fusion pepticle (FP), which is an N-terminal hydrophobic fragment that is primarily involved in directing fusion via anchoring the protein to the target cell membrane. In various eukaryotic fusion proteins the membrane targeting domain is cysteine-rich and must undergo palmitoylation prior to the fusion process. Here we examined whether fatty acids can replace the FP of human immunodeficiency virus type 1 (HIV-1), thereby discerning between the contributions of the sequence versus hydrophobicity of the FP in the lipid-merging process. For that purpose, we structurally and functionally characterized peptides derived from the N terminus of HIV fusion protein - gp41 in which the FP is lacking or replaced by fatty acids. We found that fatty acid conjugation dramatically enhanced the capability of the peptides to induce lipid mixing and aggregation of zwitterionic phospholipids composing the outer leaflet of eukaryotic cell membranes. The enhanced effect of the acylated peptides on membranes was further supported by real-time atomic force microscopy (AFM) showing nanoscale holes in zwitterionic membranes. Membrane-binding experiments revealed that fatty acid conjugation did not increase the affinity of the peptides to the membrane significantly. Furthermore, all free and acylated peptides exhibited similar alpha-helical structures in solution and in zwitterionic membranes. Interestingly, the fusogenic active conformation of N36 in negatively charged membranes composing the inner leaflet of eukaryotic cells is beta-sheet. Apparently, N-terminal heptad repeat (NHR) can change its conformation as a response to a change in the charge of the membrane head group. Overall, the data suggest an analogy between the eukaryotic cysteine-rich domains and the viral fusion peptide, and mark the hy
Conus venoms are estimated to comprise over 100,000 distinct pharmacologically active peptides, the majority probably targeting ion channels. Through the characterization of a cytolytic peptide from the venom of Conus mustelinus, conolysin-Mt, we expand the known conopeptide mechanisms to include association with and destruction of cellular membranes. A new 23AA conopeptide, conolysin-Mt has potent hemolytic activity when tested on human erythrocytes. At a concentration of 0.25 mu M, the peptide permeabilized both negatively charged prokaryotic (PE:PG) and zwitterionic eukaryotic (PC: cholesterol) model membranes. The affinity constants (K-A) of conolysin-Mt for PE:PG and PC:cholesterol model membranes were 0.9 +/- 0.3 x 10(7) and 3 +/- 1 1 x 10(7) M-1, respectively. In contrast, conolysin-Mt exhibited low antimicrobial activity (MIC > 50 mu M) against two Escherichia coli strains, with an MIC for the Gram-positive S. aureus of 25-50 mu M. The specificity of conolysin-Mt for native eukaryotic membranes is a novel feature of the peptide compared to other well-characterized cytolytic peptides such as melittin.
Plant diseases constitute an emerging threat to global food security. Many of the currently available antimicrobial agents for agriculture are highly toxic and nonbiodegradable and cause extended environmental pollution. Moreover, an increasing number of phytopathogens develop resistance to them. Recently, we have reported on a new family of ultrashort antimicrobial lipopeptides which are composed of only four amino acids linked to fatty acids (A. Makovitzki, D. Avrahami, and Y. Shai, Proc. Natl. Acad. Sci. USA 103:15997-16002, 2006). Here, we investigated the activities in vitro and in planta and the modes of action of these short lipopeptides against plant-pathogenic bacteria and fungi. They act rapidly, at low micromolar concentrations, on the membranes of the microorganisms via a lytic mechanism. In vitro microscopic analysis revealed wide-scale damage to the microorganism's membrane, in addition to inhibition of pathogen growth. In planta potent antifungal activity was demonstrated on cucumber fruits and leaves infected with the pathogen Botrytis cinerea as well as on corn leaves infected with Cochliobolus heterostrophus. Similarly, treatment with the lipopeptides of Arabidopsis leaves infected with the bacterial leaf pathogen Pseudomonas syringae efficiently and rapidly reduced the number of bacteria. Importantly, in contrast to what occurred with many native lipopeptides, no toxicity was observed on the plant tissues. These data suggest that the ultrashort lipopeptides could serve as native-like antimicrobial agents economically feasible for use in plant protection.
Aromatic residues have been previously shown to mediate the self-assembly of different soluble proteins through pi-pi interactions ( McGaughey, G. B., Gagne, M., and Rappe, A. K. ( 1998) J. Biol. Chem. 273, 15458-15463). However, their role in transmembrane (TM) assembly is not yet clear. In this study, we performed statistical analysis of the frequency of occurrence of aromatic pairs in a bacterial TM data base that provided an initial indication that the appearance of a specific aromatic pattern, Aromatic-XX-Aromatic, is not coincidental, similar to the well characterized QXXS motif. The QXXS motif was previously shown to be both critical and sufficient for stabilizing TM self-assembly. Using the ToxR system, we monitored the dimerization propensities of TM domains that contain mutations of interacting residues to aromatic amino acids and demonstrated that aromatic residues can adequately stabilize self-association. Importantly, we have provided an example of a natural TM domain, the cholera toxin secretion protein EpsM, whose TM self-assembly is mediated by an aromatic motif (WXXW). This is, in fact, the first evidence that aromatic residues are involved in the dimerization of a wild type TM domain. The association mediated by aromatic residues was found to be sensitive to the TM sequence, suggesting that aromatic residue motifs can provide a general means for specificity in TM assembly. Molecular dynamics provided a structural explanation for this backbone sequence sensitivity.
Low molecular weight compounds were isolated by high-performance liquid chromatography from the maggot or haemolymph extracts of Lucilia sericata (Meigen) (Diptera: Calliphoridae). Using gas chromatography-mass spectrometry analysis, three compounds were obtained: p-hydroxybenzoic acid (molecular weight 138 Da), p-hydroxyphenylacetic acid (molecular weight 152 Da) and octahydro-dipyrrolo[1,2-a;1',2'-d] pyrazine-5,10-dione (molecular weight 194 Da), also known as the cyclic dimer of proline (or proline diketopiperazine or cyclo[Pro,Pro]). All three molecules revealed antibacterial activity when tested against Micrococcus luteus and/or Pseudomonas aeruginosa, and the effect was even more pronounced when these molecules were tested in combination and caused lysis of these bacteria.
Model peptides composed of alanine and leucine residues are often used to mimic single helical transmembrane domains. Many studies have been carried out to determine how they interact with membranes. However, few studies have investigated their lipid-destabilizing effect. We designed three peptides designated KALRs containing a hydrophobic stretch of 14, 18, or 22 alanines/leucines surrounded by charged amino acids. Molecular modeling simulations in an implicit membrane model as well as attenuated total reflection-Fourier transform infrared analyses show that KALR is a good model of a transmembrane helix. However, tryptophan fluorescence and attenuated total reflection-Fourier transform infrared spectroscopy indicate that the extent of binding and insertion into lipids increases with the length of the peptide hydrophobic core. Although binding can be directly correlated to peptide hydrophobicity, we show that insertion of peptides into a membrane is determined by the length of the peptide hydrophobic core. Functional studies were performed by measuring the ability of peptides to induce lipid mixing and leakage of liposomes. The data reveal that whereas KALR(14) does not destabilize liposomal membranes, KALR(18) and KALR(22) induce 40 and 50% of lipid-mixing, and 65 and 80% of leakage, respectively. These results indicate that a transmembrane model peptide can induce liposome fusion in vitro if it is long enough. The reasons for the link between length and fusogenicity are discussed in relation to studies of transmembrane domains of viral fusion proteins. We propose that fusogenicity depends not only on peptide insertion but also on the ability of peptides to destabilize the two leaflets of the liposome membrane.
Antibiotics are designed to support host defense in controlling infection. Here we describe a paradoxical inhibitory effect of bacteriostatic antibiotics on key mediators of mammalian innate immunity. When growth of species including Escherichia coli and Staphylococcus aureus is suppressed by chloramphenicol or erythromycin, the susceptibility of the bacteria to cathelicidin antimicrobial peptides or serum complement was markedly diminished. Survival of the bacteria in human whole blood, human wound fluid, or a mouse wound infection model was in turn increased after antibiotic-induced bacteriostasis. These findings provide a further rationale against the indiscriminate use of antibiotics.
Protein-protein interactions in the membrane are pivotal for the cellular response to receptor-sensed stimuli. Recently, it has been demonstrated that an all-D-amino acids analogue of the TCR alpha transmembrane peptide (CP) is recruited to the TCR complex and inhibits T-cell activation in vitro and in vivo, similarly to the wild-type CP peptide. Here we investigated the relative contributions of the secondary structure of CP compared to its side chains in the association of CP with the TCR. We disrupted the secondary structure of CP by replacing two positive residues, needed for the interaction of CP with the TCR complex, by their D-enantiomers (2D-CP). Structure disruption was demonstrated by CD and FTIR spectroscopy, and molecular dynamics simulation in a bilayer environment. In vitro, 2D-CP colocalized with the TCR (visualized with confocal microscopy), immunoprecipitated with TCR but not MHC I, and inhibited T-cell activation. The peptide was effective also in vivo: it inhibited adjuvant arthritis in rats and delayed type hypersensitivity in BALB/c mice. Moreover, 2D-CP manifested greater immunosuppressive activity than wild-type CP, both in vivo and in vitro, which can be attributed to the greater solubility and resistance to degradation of 2D-CP. In molecular terms, these findings suggest that, under certain conditions, protein-protein interactions within the membrane might be more dependent on side chain interactions than on a specific secondary structure. The new altered secondary structure probably determines how the Lys and the Arg are positioned with respect to each other, so they can interact with the TM domain of the receptor. In clinical terms, the increased solubility and resistance to degradation of D-stereoisomers might be exploited in the targeted inactivation of pathogenic signaling pathways such as those arising from TCR-triggered activation of T-cells in immune-mediated disorders.
Fusion peptide (FP) of the HIV gp41 molecule inserts into the T cell membrane during virus-cell fusion. FP also blocks the TCR/CD3 interaction needed for antigen-triggered T cell activation. Here we used in vitro (fluorescence and immunoprecipitation), in vivo (T cell mediated autoinumme disease adjuvant arthritis), and in silico methods to identify the FP-TCR novel interaction motif: the alpha-helical transmembrane domain (TMD) of the TCR alpha chain, and the beta-sheet 5-13 region of the 16 N-terminal aa of FP (FP1-16). Deciphering the molecular mechanism of the immunosuppressive activity of FP provides a new potential target to overcome the immunosuppressant activity of HIV, and in addition a tool for down-regulating immune mediated inflammation.
HIV-1 entry into its host cell involves a sequential interaction whereby gp41 is in direct contact with the plasma membrane. Understanding the effect of membrane composition on the fusion mechanism can shed light on the unsolved phases of this complex mechanism. Here, we studied N36, a peptide derived from the N-heptad-repeat (NHR) of the gp41 ectodomain, its six helix bundle (SHB) forming counterpart C34, together with the N-terminal 70-mer wild-type peptide (N70), and additional gp41 ectodomain-derived peptides in the presence of two membranes, modeling inner and outer leaflets of the plasma membrane. Information on the structure of these peptides, their affinity towards phospholipids and their ability to induce vesicle fusion was gathered by a variety of fluorescence, spectroscopic and microscopy methods. We found that N36, having strong affinity towards phospholipids, prominently shifts conformation from alpha-hehix in an outer leaflet-like zwitterionic membrane to beta-sheet in a membrane mimicking the negatively charged inner leaflet environment, leading to pronounced fusion-activity. Real-time atomic force microscopy (AFM) was used to study the peptides' effect on the membrane morphology, revealing severe bilayer perturbation and extensive pore formation. We also found, that the N36/C34 core is destabilized by electronegative, but not zwitterionic phospholipids. Taken together, our data suggest that the fusion-active pore forming conformation of gp41 is extended, upstream of the SHB. In this manner, folding of the ectodomain into a SHB might also serve as a negative regulator of fusion by impeding gp41 fusion-active surfaces, thus preventing irreversible damage to the cell membrane. This assumption is supported by the finding that pre-incubation of large unilamellar vesicles (LUV) with C-heptad repeat (CHR)-derived fusion inhibitors reduces the fusogenic activity of N-terminal peptides in a dose-dependant manner, and suggests that CHR-derived fusion inhibi
Endogenous peptide antibiotics (termed also host-defense or antimicrobial peptides) are known as evolutionarily old components of innate immunity. They were found initially in invertebrates, but later on also in vertebrates, including humans. This secondary, chemical immune system provides organisms with a repertoire of small peptides that act against invasion (for both offensive and defensive purposes) by occasional and obligate pathogens. Each antimicrobial peptide has a broad but not identical spectrum of antimicrobial activity, predominantly against bacteria, providing the host maximum coverage against a rather broad spectrum of microbial organisms. Many of these peptides interact with the target cell membranes and increase their permeability, which results in cell lysis. A second important family includes lipopeptides. They are produced in bacteria and fungi during cultivation on various carbon sources, and possess a strong antifungal activity. Unfortunately, native lipopeptides are non-cell selective and therefore extremely toxic to mammalian cells. Whereas extensive studies have emerged on the requirements for a peptide to be antibacterial, very little is known concerning the parameters that contribute to antifungal activity. This review summarizes recent studies aimed to understand how antimicrobial peptides and lipopeptides select their target cell. This includes a new group of lipopeptides highly potent against pathogenic fungi and yeast. They are composed of inert cationic peptides conjugated to aliphatic acids with different lengths. Deep understanding of the molecular mechanisms underlying the differential cells specificity of these families of host defense molecule is required to meet the challenges imposed by the life-threatening infections.
Host-defense cationic antimicrobial peptides (approximate to 12-50 aa long) play an essential protective role in the innate immune system of all organisms. Lipopeptides, however, are produced only in bacteria and fungi during cultivation, and they are composed of specific lipophilic moieties attached to anionic peptides (six to seven amino acids). Here we report the following. (i) The attachment of an aliphatic chain to otherwise inert, cationic D,L tetrapeptide's endows them with potent activity against various microorganisms including antibiotic resistance strains. (h) Cell specificity is determined by the sequence of the short peptidic chain and the length of the aliphatic moiety. (iii) Despite the fact that the peptidic chains are very short, their mode of action involves permeation and disintegration of membranes, similar to that of many long antimicrobial peptides. Besides adding important information on the parameters necessary for host-defense lipopeptides to kill microorganisms, the simple composition of these lipopeptides and their diverse specificities should make them economically available, innate immunity-mimicking antimicrobial and antifungal compounds for various applications.
Temporins are short and homologous antimicrobial peptides (AMPs) isolated from the frog skin of Rana genus. To date, very little is known about the biological significance of the presence of closely related AMPs in single living organisms. Here we addressed this question using temporins A, B, and L isolated from Rana temporaria. We found that temporins A and B are only weakly active toward Gram-negative bacteria. However, a marked synergism occurs when each is mixed with temporin L. To shed light on the underlying mechanisms involved in these activities, we used various experimental strategies to investigate: (i) the effect of the peptides' interaction on both the viability and membrane permeability of intact bacteria and spheroplasts; (ii) their interaction with lipopolysaccharides (LPS) and the effect of LPS on the oligomeric state of temporins, alone or combining one with another; (iii) their structure in solution and when bound to LPS, by using circular dichroism and ATR-FTIR spectroscopies. Our data reveal that temporin L synergizes with A and B by preventing their oligomerization in LPS. This should promote their translocation across the outer membrane into the cytoplasmic membrane. To the best of our knowledge, this is the first study that explains how a combination of native AMPs from the same species can overcome bacterial resistance imposed by the LPS leaflet.
Lipopolysaccharide (LPS) is the major molecular component of the outer membrane of Gram-negative bacteria and serves as a physical barrier providing the bacteria protection from its surroundings. LPS is also recognized by the immune system as a marker for the detection of bacterial pathogen invasion, responsible for the development of inflammatory response, and in extreme cases to endotoxic shock. Because of these functions, the interaction of LPS with LPS binding molecules attracts great attention. One example of such molecules are antimicrobial peptides (AMPs). These are large repertoire of gene-encoded peptides produced by living organisms of all types, which serve as part of the innate immunity protecting them from pathogen invasion. AMPs are known to interact with LPS with high affinities. The biophysical properties of AMPs and their mode of interaction with LPS determine their biological function, susceptibility of bacteria to them, as well as the ability of LPS to activate the immune system. This review will discuss recent studies on the molecular mechanisms underlying these interactions, their effects on the resistance of the bacteria to AMPs, as well as their potential to neutralize LPS-induced endotoxic shock. (c) 2006 Elsevier B.V. All rights reserved.
Central to our understanding of human immunodeficiency virus-induced fusion is the high resolution structure of fragments of the gp41 fusion protein folded in a low energy core conformation. However, regions fundamental to fusion, like the fusion peptide ( FP), have yet to be characterized in the context of the cognate protein regardless of its conformation. Based on conformation-specific monoclonal antibody recognition, we identified the polar region consecutive to the N36 fragment as a stabilizer of trimeric coiled-coil assembly, thereby enhancing inhibitory potency. This tertiary organization is retained in the context of the hydrophobic FP ( N70 fragment). Our data indicate that the N70 fragment recapitulates the expected organization of this region in the viral fusion intermediate ( N-terminal half of the pre- hairpin intermediate ( N-PHI)), which happens to be the prime target for fusion inhibitors. Regarding the low energy conformation, we show for the first time core formation in the context of the FP ( N70 core). The alpha-helical and coiled-coil stabilizing polar region confers substantial thermal stability to the core, whereas the hydrophobic FP does not add further stability. For the two key fusion conformations, N- PHI and N70 core, we find that the FP adopts a nonhelical structure and directs higher order assembly ( assembly of coiled coils in N- PHI and assembly of bundles in the N70 core). This supra-molecular organization of coiled coils or folded cores is seen only in the context of the FP. This study is the first to characterize the FP region in the context of the folded core and provides a basic understanding of the role of the elusive FP for key gp41 fusion conformations.
We report on a short host defense-like peptide that targets and arrests the growth of aggressive and hormone-resistant primary human prostate and breast tumors and prevents their experimental and spontaneous metastases, respectively, when systemically inoculated to immuodeficient mice. These effects are correlated with increased necrosis of the tumor cells and a significant decrease in the overall tumor microvessel density, as well as newly formed capillary tubes and prostate-specific antigen secretion (in prostate tumors). Growth inhibition of orthotopic tumors derived from stably transfected highly fluorescent human breast cancer cells and prevention of their naturally occurring metastases were visualized in real time by using noninvasive whole-body optical imaging. The exclusive selectivity of the peptide towards cancer derives from its specific binding to surface phosphatidylserine and the killing of the cancer cells via cytoplasmic membrane depolarization. These data indicate that membrane disruption can provide a therapeutic means of inhibiting tumor growth and preventing metastases of various cancers.
Human immunodeficiency virus 1 gp41 folds into a six-helix bundle whereby three C-terminal heptad repeat regions pack in an anti-parallel manner against the coiled-coil formed by three N-terminal heptad repeats (NHR). Peptides that inhibit bundle formation contributed significantly to the understanding of the entry mechanism of the virus. DP178, which partially overlaps C-terminal heptad repeats, prevents bundle formation through an undefined mechanism; additionally it has been suggested to bind other ENV regions and arrest fusion in an unknown manner. We used two structurally altered DP178 peptides; in each, two sequential amino acids were substituted into their D configuration, D-SQ in the hydrophilic N-terminal region and D-LW in the hydrophobic C-terminal. Importantly, we generated an elongated NHR peptide, N54, obtaining the full N-helix docking site for DP178. Interestingly, D-LW retained wild type fusion inhibitory activity, whereas D-SQ exhibited significantly reduced activity. In correlation with the inhibitory data, CD spectroscopy and fluorescence studies revealed that all the DP178 peptides interact with N54, albeit with different stabilities of the bundles. We conclude that strong binding of DP178 N-terminal region to the endogenous NHR, without significant contribution of the C-terminal sequence of DP178 to core formation, is vital for DP178 inhibition. The finding that D-amino acid incorporation in the C terminus did not affect activity or membrane binding as revealed by surface plasmon resonance correlates with an additional membrane binding site, or membrane anchoring role, for the C terminus, which works synergistically with the N terminus to inhibit fusion.
Antimicrobial peptides (AMPS) are evolutionarily old components of innate immunity found in all living pluricellular organisms. Interestingly, some organisms express families of AMPS with only a slight variation among their members, possibly to increase their spectrum of activity. Despite the growing body of knowledge about their biological activity and mode of action on bacteria, only a few of them have been tested on Leishmania, a worldwide spread protozoan pathogen, and the parameters contributing to this activity are yet to be determined. We report on the anti-Leishmania activity and mode of action of bombinins H2 and H4 isolated from the skin secretion of the frog Bombina variegata. H4, the most active, is the first natural AMP of animal origin with a single L- to D-amino acid isomerization. Membrane depolarization and membrane permeation assays, as well as electron microscopy, suggest that the lethal mechanism involves plasma membrane permeation and/or disruption. To better understand the enhanced activity of H4, we determined the peptide's structure in membranes mimicking those of mammals, bacteria, and Leishmania by using ATR-FTIR and CD spectroscopies and assessed their membrane binding by using surface plasmon resonance. The data reveal that (1) H2 but not H4 partially aggregates in membranes mimicking those of Leishmania, (ii) H2 is slightly more helical than H4 in all membranes, and (iii) H4 binds the Leishmania model membrane similar to 5-fold better than H2. This study highlights the importance of a single alpha-amino acid epimerization as a tool used by nature to modulate the activity of AMPS. In addition, our findings suggest bombinins H as potential templates for the development of new drugs with a new mode of action against Leishmania.
Membrane-active peptides participate in many cellular processes, and therefore knowledge of their mode of interaction with phospholipids is essential for understanding their biological function. Here we present a new methodology based on electron spin-echo envelope modulation to probe, at a relatively high resolution, the location of membrane-bound lytic peptides and to study their effect on the water concentration pro. le of the membrane. As a first example, we determined the location of the N-terminus of two membrane-active amphipathic peptides, the 26- mer bee venom melittin and a de novo designed 15-mer D, L-amino acid amphipathic peptide (5D-L9K6C), both of which are antimicrobial and bind and act similarly on negatively charged membranes. A nitroxide spin label was introduced to the N-terminus of the peptides and measurements were performed either in H2O solutions with deuterated model membranes or in D2O solutions with nondeuterated model membranes. The lipids used were dipalmitoyl phosphatidylcholine ( DPPC) and phosphatidylglycerol ( PG), (DPPC/PG (7:3w/w)), egg phosphatidylcholine ( PC) and PG (PC/PG (7:3w/w)), and phosphatidylethanolamine ( PE) and PG (PE/PG, 7:3w/w). The modulation induced by the 2 H nuclei was determined and compared with a series of controls that produced a reference ''ruler''. Actual estimated distances were obtained from a quantitative analysis of the modulation depth based on a simple model of an electron spin situated at a certain distance from the bottom of a layer with homogeneously distributed deuterium nuclei. The N-terminus of both peptides was found to be in the solvent layer in both the DPPC/PG and PC/PG membranes. For PE/PG, a further displacement into the solvent was observed. The addition of the peptides was found to change the water distribution in the membrane, making it ''flatter'' and increasing the penetration depth into the hydrophobic region.
Mammalian antimicrobial peptides provide rapid defense against infection by inactivating pathogens and by influencing the functions of cells involved in defense responses. Although the direct antibacterial properties of these peptides have been widely characterized, their multiple effects on host cells are only beginning to surface. Here we investigated the mechanistic and functional aspects of the interaction of the proline- rich antimicrobial peptide Bac7( 1 - 35) with mammalian cells, as compared with a truncated analog, Bac7( 5 - 35), lacking four critical N- terminal residues ( RRIR) of the Bac7( 1 - 35) sequence. By using confocal microscopy and flow cytometry, we showed that although the truncated analog Bac7( 5 - 35) remains on the cell surface, Bac7( 1 - 35) is rapidly taken up into 3T3 and U937 cells through a nontoxic energy- and temperature dependent process. Cell biology-based assays using selective endocytosis inhibitors and spectroscopic and surface plasmon resonance studies of the interaction of Bac7( 1 - 35) with phosphatidyl-choline/ cholesterol model membranes collectively suggest the concurrent contribution of macropinocytosis and direct membrane translocation. Structural studies with model membranes indicated that membrane-bound Bac7( 5 - 35) is significantly more aggregated than Bac7( 1 - 35) due to the absence of the N- terminal cationic cluster, thus providing an explanation for hampered cellular internalization of the truncated form. Further investigations aimed to reveal functional implications of intracellular uptake of Bac7( 1 - 35) demonstrated that it correlates with enhanced S phase entry of 3T3 cells, indicating a novel function for this proline- rich peptide.
Binding of lipopolysaccharide (LPS) to macrophages results in proinflammatory cytokine secretion. In extreme cases it leads to endotoxic shock. A few innate immunity antimicrobial peptides (AMPs) neutralize LPS activity. However, the underlying mechanism and properties of the peptides are not yet clear. Toward meeting this goal we investigated four AMPs and their fluorescently labeled analogs. These AMPs varied in composition, length, structure, and selectivity toward cells. The list included human LL-37 (37-mer), magainin (24- mer), a 15-mer amphipathic alpha-helix, and its D, L-amino acid structurally altered analog. The peptides were investigated for their ability to inhibit LPS-mediated cytokine release from RAW264.7 and bone marrow-derived primary macrophages, to bind LPS in solution, and when LPS is already bound to macrophages (fluorescence spectroscopy and confocal microscopy), to compete with LPS for its binding site on the CD14 receptor (flow cytometry) and affect LPS oligomerization. We conclude that a strong binding of a peptide to LPS aggregates accompanied by aggregate dissociation prevents LPS from binding to the carrier protein lipopolysaccharide-binding protein, or alternatively to its receptor, and hence inhibits cytokine secretion.
Many studies have shown that an amphipathic structure and a threshold of hydrophobicity of the peptidic chain are crucial for the biological function of AMPs (antimicrobial peptides). However, the factors that dictate their cell selectivity are not yet clear. In the present study, we show that the attachment of aliphatic acids with different lengths (10, 12, 14 or 16 carbon atoms) to the N-terminus of a biologically inactive cationic peptide is sufficient to endow the resulting lipopeptides with lytic activity against different cells. Mode-of-action studies were performed with model phospholipid membranes mimicking those of bacterial, mammalian and fungal cells. These include determination of the structure in solution and membranes by using CD and ATR-FTIR (attenuated total reflectance Fourier-transform infrared) spectroscopy, membrane leakage experiments and by visualizing bacterial and fungal damage via transmission electron microscopy. The results obtained reveal that: (i) the short lipopeptides (10 and 12 carbons atoms) are non-haemolytic, active towards both bacteria and fungi and monomeric in solution. (ii) The long lipopeptides (14 and 16 carbons atoms) are highly antifungal, haemolytic only at concentrations above their MIC (minimal inhibitory concentration) values and aggregate in solution. (iii) All the lipopeptides adopt a partial alpha-helical structure in 1% lysophosphatidylcholine and bacterial and mammalian model membranes. However, the two short lipopeptides contain a significant fraction of random coil in fungal membranes, in agreement with their reduced antifungal activity. (iv) All the lipopeptides have a membranolytic effect on all types of cells assayed. Overall, the results reveal that the length of the aliphatic chain is sufficient to control the pathogen specificity of the lipopeptides, most probably by controlling both the overall hydrophobicity and the oligomeric state of the lipopeptides in solution. Besides providing us with basic impo
A novel method, based on the rational and systematic modulation of macroscopic structural characteristics on a template originating from a large number of natural, cell-lytic, amphipathic a-helical peptides, was used to probe how the depths and shapes of hydrophobic and polar faces and the conformational stability affect antimicrobial activity and selectivity with respect to eukaryotic cells. A plausible mode of action explaining the peptides' behaviour in model membranes, bacteria and host cells is proposed. Cytotoxic activity, in general, correlated strongly with the hydrophobic sector depth, and required a majority of aliphatic residue side chains having more than two carbon atoms. It also correlated significantly with the size of polar sector residues, which determines the penetration depth of the peptide via the so-called snorkel effect. Both an oblique gradient of long to short aliphatic residues along the hydrophobic face and a stabilized helical structure increased activity against host cells but not against bacteria, as revealed by haemolysis, flow cytofluorimetric studies on lymphocytes and surface plasmon resonance studies with model phosphatidylcholine/cholesterol membranes. The mode of interaction changes radically for a peptide with a stable, preformed helical conformation compared with others that form a structure only on membrane binding. The close correlation between effects observed in biological and model systems suggests that the carpet model' correctly represents the type of peptides that are bacteria-selective, whereas the behaviour of those that lyse host cells is more complex.
The fusion peptide (FP) in the N terminus of the HIV envelope glycoprotein, gp41, functions together with other gp41 domains to fuse the virion with the host cell membrane. We now report that FP colocalizes with CD4 and TCR molecules, coprecipitates with the TCR, and inhibits antigen-specific T cell proliferation and proinflammatory cytokine secretion in vitro. These effects are specific: T cell activation by PMA/ionomycin or mitogenic antibodies is not affected by FPs, and FPs do not interfere with antigen-presenting cell function. In vivo, FPs inhibit the activation of arthritogenic T cells in the autoimmune disease model of adjuvant arthritis and reduce the disease-associated IFN-gamma response. Hence, FPs might play 2 roles in HIV infection: mediating membrane fusion while downregulating T cell responses to itself that could block infection. Disassociated from HIV, however, the FP molecule provides a novel reagent for downregulating undesirable immune responses, exemplified here by adjuvant arthritis.
The HIV gp41 protein mediates fusion with target host cells. The region primarily involved in directing fusion, the fusion peptide (FP), is poorly understood at the level of structure and function due to its toxic effect in expression systems. To overcome this, we used a synthetic approach to generate the N70 construct, whereby the FP is stabilized in context of the adjacent auto oligomerization domain. The amide I profile of unlabeled N70 in membranes reveals prominent a-helical contribution, along with significant beta-structure. By truncating the N terminus (FP region) of N70, beta-structure is eliminated, suggesting that the FP adopts a beta-structure in membranes. To assess this directly, C-13 Fourier-transformed infra-red analysis was carried out to map secondary structure of the 16 N-terminal hydrophobic residues of the fusion peptide (FP16). The C-13 isotope shifted absorbance of the FP was filtered from the global secondary structure of the 70 residue construct (N70). On the basis of the peak shift induced by the C-13-labeled residues of FP16, we directly assign beta-sheet structure in ordered membranes. A differential labeling scheme in FP16 allows us to distinguish the type of beta-sheet structure as parallel. Dilution of each FP16 labeled N70 peptide, by mixing with unlabeled N70, shows directly that the FP16-straud region self-assembles. We discuss our structural findings in the context of the prevailing gp41 fusion paradigm. Specifically, we address the role of the FP region in organizing supramolecular gp41 assembly, and we also discuss the mechanism by which exogenous, free FP constructs inhibit gp41-induced fusion. (c) 2005 Elsevier Ltd. All rights reserved.
Assembly of transmembrane (TM) domains is a critical step in the function of membrane proteins, and therefore, determining the amino acid motifs that mediate this process is important. Studies along this line have shown that the GXXXG motif is involved in TM assembly. In this study we characterized the minimal dimerization motif in the bacterial Tar-1 homodimer TM domain, which does not contain a GXXXG sequence. We found that a short polar motif QXXS is sufficient to induce stable TM-TM interactions. Statistical analysis revealed that this motif appears to be significantly over-represented in a bacterial TM data base compared with its theoretical expectancy, suggesting a general role for this motif in TM assembly. A truncated short TM peptide ( 9 residues) that contains the QXXS motif interacted slightly with the wild-type Tar-1. However, the same short TM peptide regained wild-type-like activity when conjugated to an octanoyl aliphatic moiety. Biophysical studies indicated that this modification compensated for the missing hydrophobicity, stabilized alpha-helical structure, and enabled insertion of the peptide into the membrane core. These findings serve as direct evidence that even a short peptide containing a minimal recognition motif is sufficient to inhibit the proper assembly of TM domains. Interestingly, electron microscopy revealed that above the critical micellar concentration, the TM lipopeptide forms a network of nanofibers, which can serve for the slow release of the active lipopeptide.
Antimicrobial peptides and lipopeptides play an essential protective role in the innate immune system of all organisms. Despite many studies, the factors that dictate their cell-selectivity and pH-dependent activity are yet not clear. This is important because various organs of the human body have an acidic pH environment, for example, the vagina, gastric lumen, cryogenic dental foci, and lung-lining fluids in cystic fibrosis and asthma. In this study we synthesized a new group of lipopeptides by conjugating dodecanoic acid (DDA) to the N-termini of 12-mer peptides LXXLLXXLLXXL (L6X6, X = Lys, His, Arg, and all the leucines are D-amino acid enantiomers) and investigated their pH-dependent biological activity and a plausible mode of action by using model phospholipids mimicking bacterial, mammalian, and fungal membranes. The data revealed that, depending on the basic amino acid incorporated, the lipopeptides are active against both bacteria and fungi or solely toward fungi. Furthermore, their activity is expressed at an acidic pH alone, neutral pH alone, or at both environments. Determination of secondary structure, membrane leakage experiments, surface plasmon resonance (SPR) binding experiments, and transmission electron microscopy suggest the involvement of a membranolytic effect. This mode of action, which should make it hard for the microorganism to develop resistance, their selective and pH-dependent activity, as well as pharmacological advantages due to the presence Of D-amino acids, make them potential candidates for the treatment of mycoses in organs, under various pH environments, especially in cases where the bacterial flora should not be harmed.
The gp41 glycoprotein of HIV-1 is considered to be responsible for the actual fusion process between the virus and the host membranes. According to a prevailing model, gp41 trimer organization, directed by the N-terminal coiled-coil region (NHR), is essential for steps involved in the actual merging of viral and cellular membranes. This study addresses a major question: Is the specific sequence of the NHR obligatory for the fusion process, or can it be replaced by distant coiled coils that form different oligomeric states in solution? For this purpose we synthesized three known GCN4 coiled-coil mutants that oligomerize in solution into either dimers, trimers, or tetramers. These peptides were chemically ligated to the fusion peptide thereby creating three chimera peptides with different oligomeric tendencies in solution. These peptides were investigated, together with the 70-mer wild-type peptide (N70), regarding their structure in solution and membrane by using circular dichroism (CD) and FTIR spectroscopies, their ability to induce vesicle fusion, and their ability to bind phospholipid membranes by using surface plasmon resonance (SPR). Our results suggest that local assembly of fusion peptides, facilitated by coiled-coil oligomers, increases lipid mixing ability, probably by facilitating stronger binding of the fusion peptides to the opposing membrane as revealed by SPR. However, N70 is significantly more active than the other chimeras. Overall, the data indicate a correlation between the distinct conformation of N70 in solution and in membranes and its enhanced lipid mixing relative to the GCN4 chimeras.
T cell activation requires the cross-talk between the CD3-signaling complex and the T cell receptor (TCR). A synthetic peptide coding for the TCR alpha. transmembrane domain (CP) binds CD3 molecules, interferes with the CD3/TCR cross-talk, and inhibits T cell activation. Intermolecular interactions are sterically constrained; accordingly no sequence-specific interactions are thought to occur between D- and L-stereoisomers. This argument was recently challenged when applied to intra-membrane protein assembly. In this paper we studied the ability of a D-stereoisomer of CP (D-CP) to inhibit T cell activation. L-CP and D-CP co-localized with the TCR in the membrane and inhibited T cell activation in a sequence-specific manner. In vivo, both L-CP and D-CP inhibited adjuvant arthritis. In molecular terms, these results suggest the occurrence of structural reorientation that facilitates native-like interactions between D-CP and CD3 within the membrane. In clinical terms, our results demonstrate that D-stereoisomers retain the therapeutic properties of their L-stereoisomers, while they benefit from an increased resistance to degradation.
In the last decade intensive research has been conducted to determine the role of innate immunity host defense peptides (also termed antimicrobial peptides) in the killing of prokaryotic and eukaryotic cells. Many antimicrobial peptides damage the cellular membrane as part of their killing mechanism. However, it is not clear what makes cancer cells more susceptible to some of these peptides, and what the molecular mechanisms underlying these activities are. Two general mechanisms were suggested: (i) plasma membrane disruption via micellization or pore formation, and (ii) induction of apoptosis via mitochondrial membrane disruption. To be clinically used, these peptides need to combine high and specific anticancer activity with stability in serum. Although so far very limited, new studies have paved the way for promising anticancer host defense peptides with a new mode of action and with a broad spectrum of anticancer activity
Cationic antimicrobial peptides serve as the first chemical barrier between all organisms and microbes. One of their main targets is the cytoplasmic membrane of the microorganisms. However, it is not yet clear why some peptides are active against one particular bacterial strain but not against others. Recent studies have suggested that the lipopolysaccharide (LPS) outer membrane is the first protective layer that actually controls peptide binding and insertion into Gram-negative bacteria. In order to shed light on these interactions, we synthesized and investigated a 12-mer amphipathic alpha-helical antimicrobial peptide (K5L7) and its diastereomer (4D-K5L7) (containing four D-amino acids). Interestingly, although both peptides strongly bind LPS bilayers and depolarize bacterial cytoplasmic membranes, only the diastereomer kills Gram-negative bacteria. Attenuated total reflectance Fourier transform infrared, CD, and surface plasmon resonance spectroscopies revealed that only the diastereomer penetrates the LPS layer. In contrast, K5L7 binds cooperatively to the polysaccharide chain and the outer phosphate groups. As a result, the self-associated K5L7 is unable to traverse through the tightly packed LPS molecules, revealed by epifluorescence studies with LPS giant unilamellar vesicles. The difference in the peptides' modes of binding is further demonstrated by the ability of the diastereomer to induce LPS miscellization, as shown by transmission electron microscopy. In addition to increasing our understanding of the molecular basis of the protection of bacteria by LPS, this study presents a potential strategy to overcome resistance by LPS, and it should help in the design of antimicrobial peptides for future therapeutic purposes.
The interactions between the TM (transmembrane) domains of many membrane proteins are important for their proper functioning. Mutations of residues into positively charged ones within TM domains were reported to be involved in many genetic diseases, possibly because these mutations affect the self- and/or hetero-assembly of the corresponding proteins. To our knowledge, despite significant progress in understanding the role of various amino acids in TM-TM interactions in vivo, the direct effect of positively charged residues on these interactions has not been studied. To address this issue, we employed the N-terminal TM domain of the aspartate receptor (Tar-1) as a dimerization model system. We expressed within the ToxR TM assembly system several Tar-1 constructs that dimerize via polar- or non-polar amino acid motifs, and mutated these by replacement with a single arginine residue. Our results have revealed that a mutation in each of the motifs significantly reduced the ability of the TMs to dimerize. Furthermore, a Tar-1 construct that contained two arginine residues was unable to correctly integrate itself into the membrane. Nevertheless, an exogenous synthetic Tar-1 peptide containing these two arginine residues was able to inhibit in vivo the marked dimerization of a mutant Tar-1 construct that contained two glutamate residues at similar positions. This indicates that hetero-assembly of TM domains can be mediated by the interaction of two oppositely charged residues, probably by formation of ion pairs. This study broadens our knowledge regarding the effect of positively charged residues on TM-TM interactions in vivo, and provides a potential therapeutic approach to inhibit uncontrolled dimerization of TM domains caused by mutations of polar amino acids.
The N-terminal fusion peptide ( FP) of human immunodeficiency virus-1 (HIV-1) is a potent inhibitor of cell-cell fusion, possibly because of its ability to recognize the corresponding segments inside the fusion complex within the membrane. Here we show that a fusion peptide in which the highly conserved Ile(4), Phe(8), Phe(11), and Ala(14) were replaced by their D-enantiomers (IFFA) is a potent inhibitor of cell-cell fusion. Fourier transform infrared spectroscopy confirmed that despite these drastic modifications, the peptide preserved most of its structure within the membrane. Fluorescence energy transfer studies demonstrated that the diastereomeric peptide interacted with the wild type FP, suggesting this segment as the target site for inhibition of membrane fusion. This is further supported by the similar localization of the wild type and IFFA FPs to microdomains in T cells and the preferred partitioning into ordered regions within sphingomyelin/phosphatidyl-choline/cholesterol giant vesicles. These studies provide insight into the mechanism of molecular recognition within the membrane milieu and may serve in designing novel HIV entry inhibitors.
Protein-protein interactions within the membrane, partially or fully mediated by transmembrane (TM) domains, are involved in many vital cellular processes. Since the unique feature of the membrane environment enables protein-protein assembly that otherwise is not energetically favored in solution, the structural restrictions involved in the assembly of soluble proteins are not necessarily valid for the assembly of TM domains. Here we used the N-terminal TM domain (Tar-1) of the Escherichia coli aspartate receptor as a model system for examining the stereospecificity of TM-TM interactions in vitro and in vivo in isolated systems, and in the context of the full receptor. For this propose, we synthesized Tar-1 all-L and all-D amino acid TM peptides, a mutant TM peptide and an unrelated TM peptide. The data revealed: (i) Tar-1 all-D specifically associated with Tar-1 all-L within a model lipid membrane, as determined by using fluorescence energy transfer experiments; (ii) Tar-1 all-L and all-D, but not the control peptides, demonstrated a dose-dependant dominant negative effect on the Tar-1 TM homodimerization in the bacterial ToxR assembly system, suggesting a wild-type-like interaction; and most interestingly, (iii) both Tar-1 all-L and all-D showed a remarkable ability to inhibit the chemotaxis response of the full-length receptor, in vivo. Peptide binding to the bacteria was confirmed through confocal imaging, and Western blotting confirmed that ToxR Tar-1 chimera protein levels are not affected by the presence of the exogenous peptides. These findings present the first evidence that an all-D TM domain peptide acts in vivo similarly to its parental all-L peptide and suggest that the dimerization of the TM domains is mainly mediated by side-chain interactions, rather than geometrically fitted conformations. In addition, the study provides a new approach for modifying the function of membrane proteins by proteolysis-free peptides. (C) 2004 Elsevier Ltd. All rights
Gene-encoded host defense peptides are used as part of the innate immunity, and many of them act by directly lysing the cell membrane of the pathogen. A few of these peptides showed anticancer activity it? vitro but could not be used in vivo because of their inactivation by serum. We designed a 15-amino acid peptide, composed of D- and L-amino acids (diastereomer), which targets both androgen-independent and androgen-dependent human prostate carcinoma cell lines (CL1, 22RV1, and LN-CaP). Most importantly, we observed a complete arrest of growth in CL1 and 22RV1 xenografts; treated intratumorally with the diastercomer. This was also accompanied by a lowering of prostate-specific antigen serum levels secreted by the 22RV1 xenograft. Furthermore, the diastereomer synergized with conventional chemotherapeutics. In contrast, the parental all L-amino acids peptide was highly active only in vitro and could not discriminate between tumor and nontumor cells. Fluorescent confocal microscopy, histopathologic examination, and cell permeability studies (depolarization of transmembrane potential and release of an encapsulated dye) suggest a necrotic mechanism of killing, after a threshold concentration of peptide has been reached. Its destructive killing effect and the simple sequence of the diastereomer make it an attractive chemotherapeutic candidate possessing a new mode of action, with potential to be developed additionally for the treatment of prostate carcinoma.
We report that intravenous injection (3 mg/kg of body weight twice daily) of a diastereomer (containing 33% D amino acids) of an antimicrobial peptide, K6L9 (LKLLKKLLKKLLKLL-NH2), but not the all-L-amino-acid parental peptide, cures neutropenic mice infected with gentamicin-sensitive Pseudomonas aeruginosa and gentamicin-resistant Acinetobacter baumannii bacteria. Various biophysical experiments suggest a membranolytic-like effect.
The amphipathic alpha-helix is a common motif found in many cell lytic peptides including antimicrobial peptides. We have recently shown that significantly altering the amphipathic structure of a lytic peptide by reshuffling its sequence and/or replacing a few L-amino acids with their D-enantiomers did not significantly affect the antimicrobial activity of the peptides nor their ability to bind and permeate negatively charged (PE/PG) membranes. However, a pronounced effect was observed regarding their hemolytic activity and their ability to bind and permeate zwitterionic (PC/Cho) membranes. To shed light on these findings, here we used surface plasmon resonance (SPR) with mono- and bilayer membranes. We found that the L-amino acid (aa) peptides bound 10-25-fold stronger to PC/Cho bilayers compared with monolayers, whereas the diastereomers bound similarly to both membranes. A two-state reaction model analysis of the data indicated that this difference is due to the insertion of the L-aa peptides into the PC/Cho bilayers, whereas the diastereomers are surface-localized. In contrast, only an similar to2-fold difference was found with negatively charged membranes. Changes in the amphipathicity markedly affected only the insertion of the L-aa peptides into PC/Cho bilayers. Furthermore, whereas the all-L-aa peptides bound similarly to the PC/Cho and PE/PG membranes, the diastereomers bound similar to100-fold better to PE/PG compared with PC/Cho membranes, and selectivity was determined only in the first binding step. The effect of the peptides on the lipid order determined by using ATR-FTIR studies supported these findings. Besides shedding light on the mode of action of these peptides, the present study demonstrates SPR as a powerful tool to differentiate between non-cell-selective compared with bacteria-selective peptides, based on differences in their membrane binding behavior.
C-peptides derived from the HIV envelope glycoprotein transmembrane subunit gp41 C-terminal heptad repeat (C-HR) region are potent HIV fusion inhibitors. These peptides interact with the gp41 N-terminal heptad repeat (N-HR) region and block the gp41 six-helix bundle formation that is required for fusion. However, the parameters that govern this inhibition have yet to be elucidated. We address this issue by comparing the ability of C34, derived from HIV-1, HIV-2 and SIV gp41, to inhibit HIVA, HIV-2 and SIV envelope-mediated fusion and the ability of these peptides to form stable six-helix bundles with N36 peptides derived from gp41 of these three viruses. The ability to form six-helix bundles was examined by circular dichroism spectroscopy, and HIV/SIV Env-mediated membrane fusion was monitored by a dye transfer assay. HIV-1 N36 formed stable helix bundles with HIV-1, HIV-2 and SIV C34, which all inhibited HIV-1 Env-mediated fusion at IC50 <10 nM. The three C34 peptides were poor inhibitors of HIV-2 and SIV fusion (IC50 > 100 nM), although HIV-2 and SIV N36 formed stable helix bundles with SIV C34. Priming experiments with sCD4 indicate that, in contrast to HIV-1, HIV-2 and SIV Env do not expose their N-HR region to SIV C34 following CD4 binding, but rapidly proceed to co-receptor engagement and six-helix bundle formation resulting in fusion. Our results suggest that several factors, including six-helix bundle stability and the ability of CD4 to destabilize the envelope glycoprotein, serve as determinants of sensitivity to entry inhibitors. (C) 2004 Elsevier Ltd. All rights reserved.
Protein assembly is a critical process involved in a wide range of cellular events and occurs through extra-cellular and/or transmembrane domains (TMs). Previous studies demonstrated that a GXXXG motif is crucial for homodimer formation. Here we selected the TMs of ErbB1 and ErbB2 as a model since these receptors function both as homodimers and as heterodimers. Both TMs contain two GXXXG-like motifs located at the C and N termini. The C-terminal motifs were implicated previously in homodimer formation, but the role of the N-terminal motifs was not clear. We used the ToxR system and expressed the TMs of both ErbB1 and ErbB2 containing only the N-terminal GXXXG motifs. The data revealed that the ErbB2 but not the ErbB1 construct formed homodimers. Importantly, a synthetic ErbB1 TM peptide was able to form a heterodimer with ErbB2, by displacing the ErbB2 TM homodimer. The specificity of the interaction was demonstrated by using three controls: (i) Two single mutations within the GXXXG-like motif of the ErbB1 peptide reduced or preserved its activity, in agreement with similar mutations in glycophorin A. (ii) A TM peptide of the bacterial Tar receptor did not assemble with the ErbB2 construct. (iii) The ErbB1 peptide had no effect on the dimerization of a construct containing the TM-1 domain of the Tar receptor. Fluorescence microscopy demonstrated that all the peptides localized on the membrane. Furthermore, incubation with the peptides had no effect on bacterial growth and protein expression levels. Our results suggest that the N-terminal GXXXG-like motif of the ErbB1 TM plays a role in heterodimerization with the ErbB2 transmembrane domain. To our knowledge, this is the first demonstration of a transmembrane domain with two distinct recognition motifs, one for homodimerization and the other for heterodimerization.
Protein-protein recognition is an essential process in life. The chemistry of these kind of interactions is predominately stereospecific (i.e. receptor-ligand, antibody-hapten binding). Here, we investigated whether the hydrophobic nature of the membrane affects this stereospecificity. To this end, we synthesized a diastereomer analogue (2D-GPA) of the glycophorin A transmembrane helix, with two L-valine residues replaced by their D-enantiomer. This ensures a disruption of the secondary structure. We investigated the ability of the diastereomer peptide to recognize the GPA chimera in the ToxR homodimer reporting system, in vivo. The peptide demonstrated a dose-dependent dominant negative effect on the GPA transmembrane in the bacterial ToxR system, suggesting a wildtype like interaction. This result was corroborated in vitro by fluorescence energy transfer between 2D-GPA and all-L GPA. Peptide binding to the bacteria was confirmed through confocal imaging, and Western blot confirmed that ToxR GPA receptor levels are not affected by the presence of the exogenous peptide. In order to understand the structural basis for heterodimer formation, homodimer and heterodimer structures, based on the NMR 3D structure of GPA, were subjected to a molecular dynamics simulation. The resulting heterodimer structure maintained most of the original inter-helical interactions, and its structure is similar to that of the homodimer. We postulate that the need to satisfy all H-bonds can compensate for the structural strain induced by the presence of the D-amino acid residues. (C) 2004 Elsevier Ltd. All rights reserved.
To infect target cells, HIV-1 employs a virally encoded transmembrane protein (gp41) to fuse its viral envelope with the target cell plasma membrane. We describe the gp41 ectodomain as comprised of N- and C-terminal subdomains, each containing a heptad repeat as well as a fusogenic region, whose organization is mirrored by the intervening loop region. Recent evidence indicates that the gp41 directed fusion reaction proceeds to initial pore formation prior to gp41 folding into its low energy hairpin conformation. This implies that exposed regions of the gp41 ectodomain are responsible for the bulk of the fusion work, probably through direct protein-membrane interactions. Prevalent fusion models contend that the gp41 ectodomain initially interacts with the target cell surface through its highly hydrophobic N terminus, which is believed to insert into the target membrane, thereby linking the virus to the target cell. This arrangement allows the N- terminal subdomain to interact with the target cell surface, whereas the C-terminal subdomain remains proximal to the virion, allowing interaction with the viral envelope. The composition of the viral envelope and the target cell surface differ due to the virus budding from raft microdomains. We show here that constructs corresponding to the C-terminal subdomain specifically destabilize ordered and cholesterol rich membranes ( 33 molar %), whereas the N- terminal subdomain is more effective in fusing both unordered cholesterol-free membranes and those containing lower amounts of cholesterol ( 10 molar %). Moreover we show that, in the context of the C-terminal subdomain, the heptad repeat contributes helical structure, which may describe the enhanced inhibitory effect of the C-terminal subdomain relative to the C-terminal heptad repeat (C34) alone. Our results are discussed in light of recent findings that showcase the role of exposed gp41 regions in effecting membrane fusion.
Transmembrane (TM) helix association is an important process affecting the function of many integral membrane proteins. Consequently, aberrations in this process are associated with diseases. Unfortunately, our knowledge of the factors that control this oligomerization process in the membrane milieu is limited at best. Previous studies have shown a role for polar residues in the assembly of synthetic peptides in vitro and the association of de novo-designed TM helices in vivo. Here we examined, for the first time, the involvement of polar residues in the dimerization of a biological TM domain in its natural environment. We analyzed both the involvement of polar residues in the dimerization process and whether their influence is position-dependent. For this purpose, we used the TM domain of the Escherichia coli aspartate receptor (Tar) and 10 single and double mutants. Polar to nonpolar mutations in the sequence demonstrated the role of the QxxS motif in the dimerization of the Tar TM domain. Moreover, creating a GxxxG motif, instead of the polar motif, almost completely abolished dimerization. Swapping positions between two wild-type polar residues did not affect dimerization, implying a similar contribution from both positions. Interestingly, mutants that contain two identical strong polar residues, EE and QQ, demonstrated a substantially higher level of dimerization than a QE mutant, although all three TM domains contain two strong polar residues. This result suggests that, in addition to the polarity of the residues, the formation of symmetric bonds also plays a role in dimer stability. The results of this study may facilitate a rational modulation of membrane protein function for therapeutic purposes.
We report on the synthesis, biological function, and a plausible mode of action of a new group of lipopeptides with potent antifungal and antibacterial activities. These lipopeptides are derived from positively charged peptides containing D- and L-amino acids (diastereomers) that are palmitoylated (PA) at their N terminus. The peptides investigated have the sequence K4X7W, where X designates Gly, Ala, Val, or Leu (designated D-X peptides). The data revealed that PA-D-G and PA-D-A gained potent antibacterial and antifungal activity despite the fact that both parental peptides were completely devoid of any activity toward microorganisms and model phospholipid membranes. In contrast, PA-D-L lost the potent antibacterial activity of the parental peptide but gained and preserved partial antifungal activity. Interestingly, both D- V and its palmitoylated analog were inactive toward bacteria, and only the palmitoylated peptide was highly potent toward yeast. Both PA-D-L and PA-D-V lipopeptides were also endowed with hemolytic activity. Mode of action studies were performed by using tryptophan fluorescence and attenuated total reflectance Fourier transform infrared and circular dichroism spectroscopy as well as transmembrane depolarization assays with bacteria and fungi. The data suggest that the lipopeptides act by increasing the permeability of the cell membrane and that differences in their potency and target specificity are the result of differences in their oligomeric state and ability to dissociate and insert into the cytoplasmic membrane. These results provide insight regarding a new approach of modulating hydrophobicity and the self-assembly of non-membrane interacting peptides in order to endow them with both antibacterial and antifungal activities urgently needed to combat bacterial and fungal infections.
Objectives: Increasing resistance of pathogenic bacteria to antibiotics is a severe problem in health care and has intensified the search for novel drugs. Cationic antibacterial peptides are the most abundant antibiotics in nature and have been frequently proposed as new anti-infective agents. Here, a group of diastereomeric (containing d- and l-amino acids) peptides is studied regarding their potency against multiply resistant clinical isolates and their modes of action against Gram-positive cocci. Methods: MIC determinations and chequerboard titrations followed established procedures. Mode of action studies included killing kinetics and a series of experiments designed to characterize the impact of the diastereomeric peptides on bacterial membranes. Results: The tested diastereomers displayed high antimicrobial and broad spectrum activity with amphipathic-2D being the most active peptide. Synergic activities were observed with individual strains. Mode of action studies clearly demonstrated that the cytoplasmic membrane is a primary target for the peptides and that membrane disruption constitutes a significant bactericidal activity for the major fraction of a bacterial population. However, depending on the indicator strain, the results also suggest that additional molecular events contribute to the overall activity.
Keywords: PHOSPHOLIPID-MEMBRANES; CHANNEL FORMATION; LYTIC PEPTIDES; CANCER-CELLS; PARDAXIN; ANALOGS; MODE; CYCLIZATION; DERMASEPTIN; MECHANISM
We report on two new cyclic 17-residue peptides that we named ranacyclins E and T, the first isolated from Rana esculenta frog skin secretions and the second discovered by screening a cDNA library from Rana temporaria. Ranacyclins have a loop region that is homologous with that of an 18-mer peptide, pLR, isolated from the skin of the Northern Leopard frog, Rana pipiens, with no reported antimicrobial activity. Here we show that ranacyclins and pLR have antimicrobial and antifungal activity. However, despite the high structural similarity, they differ in their spectrum of activity. The data reveal that ranacyclins and pLR have several properties that differentiate them from most known antimicrobial peptides. These include the following: (i) they adopt a significant portion of random coil structure in the membrane as revealed by ATR-FTIR and CD spectroscopy (50% for ranacyclin T and 70% for both ranacyclin E and pLR); (ii) they bind similarly to both zwitterionic and negatively charged membranes as revealed by using tryptophan fluorescence and surface plasmon resonance (SPR; BIAcore biosensor); (iii) they insert into the hydrophobic core of the membrane and presumably form transmembrane pores without damage to the bacterial wall, as revealed by SPR, ATR-FTIR, and transmission electron microscopy (TEM); and (iv) despite being highly and equally active in permeating bacterial spheroplasts and negatively charged membranes, they differ significantly in their potencies against target cells. Furthermore, a significant fraction of a given secondary structure is not prerequisite for membrane permeation and antimicrobial activity. However, increasing the fraction of a secondary structure and reducing peptide assembly in the membrane make it easier for the peptide to diffuse through the cell wall, which is different for each microorganism, into the cytoplasmic membrane.
The dramatically increased frequency of opportunistic fungal infections has prompted research to diversify the arsenal of antifungal agents. Antimicrobial peptides constitute a promising family for future antibiotics with a new mode of action. However, only a few are effective against fungal pathogens because of their ability to self-assemble. Recently, we showed that the conjugation of fatty acids to the potent antibacterial peptide magainin endowed it with antifungal activity concomitant with an increase in its oligomeric state in solution. To investigate whether a high potency of the parental peptide is prerequisite for antifungal activity, we conjugated undecanoic acid (UA) and palmitic acid (PA) to inactive diastereomers of magainin containing four D-amino acids ([D]-4-magainin), as well as to a weakly active diastereomeric lytic peptide containing Lys and Leu ([D]-K5L7). All lipopeptides gained potent activity toward Cryptococcus neoformans. Most importantly, [D]-K5L7-UA was highly potent against all microorganisms tested, including bacteria, yeast, and opportunistic fungi. All lipopeptides increased the permeability of Escherichia coli spheroplasts and intact C neoformans, as well as their corresponding membranes, phosphatidylethanol (PE)/phosphatidylglycerol (PG) and phosphatidylcholine (PC)/PE/phosphatidylinositol (PI)/ergosterol, respectively. The extent of membrane-permeating activity correlated with their biological function, suggesting that the plasma membrane was one of their major targets. Circular dichroism (CD) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy revealed that their mode of oligomerization in solution, structure, and organization in membranes have important roles regarding their antibacterial and antifungal activities. Together with the advantage of using diastereomers versus all L-amino acid peptides, this study paves the way to the design of a new group of potent antifungal peptides urgently neede
Cationic antibacterial peptides are produced in all living organisms and possess either selective activity toward a certain type of cell or microorganism, or a broad spectrum of activity toward several types of cells including prokaryotic and mammalian cells. In order to exert their activity, peptides first interact with and traverse an outer barrier, e.g., mainly LPS and peptidoglycan in bacteria or a glycocalix layer and matrix proteins in mammalian cells. Only then, can the peptides bind and insert into the cytoplasmic membrane. The mode of action of many antibacterial peptides is believed to be the disruption of the lipidic plasma membrane. Therefore, model phospholipid membranes have been used to study the mode of action of antimicrobial peptides. These studies have demonstrated that peptides that act preferentially on bacteria are also able to interact with and permeate efficiently anionic phospholipids, whereas peptides that lyse mammalian cells bind and permeate efficiently both acidic and zwitterionic phospholipids membranes, mimicking the plasma membranes of these cells. It is now becoming increasingly clear that selective activity of these peptides against different cells depends also on other parameters that characterize both the peptide and the target cell. With respect to the peptide's properties, these include the volume of the molecule, its structure, and its oligomeric state in solution and in membranes. Regarding the target membrane, these include the structure, length, and complexity of the hydrophilic polysaccharide found in its outer layer. These parameters affect the ability of the peptides to diffuse through the cell's outer barrier and to reach its cytoplasmic plasma membrane. (C) 2003 Elsevier Inc. All rights reserved.
To address the structure-function relationship of discrete regions within the gp41 ectodomain, 70-residue peptide constructs corresponding to the N-terminal subdomain of the HIV-1 gp41 ectodomain were examined in a membrane-associated context. These fragments encompass both fusion peptide (FP) and N-terminal heptad repeat (NHR) regions, and model the N-terminal half of the pre-hairpin intermediate (PHI), which is believed to be the target of the potent entry inhibitor DP-178, recently approved by the FDA. Using mutants, we attempted to map the structural organization of the N-terminal subdomain. Our results suggest that the N-terminal subdomain contains two discrete structural regions: the FP adopts a beta-sheet conformation and the NHR is alpha-helical. This structural make-up is essential for fusogenic function, since loss of function mutants exhibit both a significant reduction in region-specific secondary structure as well as significant impairment in lipid mixing of large unilamellar vesicles. Our results, delineating membrane-associated structure of the FP region differ from previous ones by inclusion of the autonomous oligomerization domain (NHR), which likely contributes to stabilization of the FP structure. Correspondingly, the a-helical structure for the NHR, in context of the FP, correlates with structural predictions for this region in both the hairpin and PHI conformations during fusion. Based on our results, we postulate how oligomerization of regions in this sub-domain is essential for fusion pore formation. (C) 2003 Elsevier Ltd. All rights reserved.
Despite significant advances in cancer therapy, there is an urgent need for drugs with a new mode of action that will preferentially kill cancer cells. Several cationic antimicrobial peptides, which bind strongly to negatively charged membranes, were shown to kill cancer cells slightly better than normal cells. This was explained by a slight increase (3-9%) in the level of the negatively charged membrane phosphatidylserine (PS) in many cancer cells compared to their normal counterparts. Unfortunately, however, these peptides are inactivated by serum components. Here we synthesized and investigated the anticancer activity and the role of peptide charge, peptide structure, and phospholipid headgroup charge on the activity of a new group of diastereomeric lytic peptides (containing D- and L-forms of leucine and lysine; 15-17 amino acids long). The peptides are highly toxic to cancer cells, to a degree similar to or larger than that of mitomycin C. However, compared with mitomycin C and many native antimicrobial peptides, they are more selective for cancer cells. The peptides were investigated for (i) their binding to mono- and bilayer membranes by using the surface plasmon resonance (SPR) technique, (ii) their ability to permeate membranes by using fluorescence spectroscopy, (iii) their structure and their effect on the lipid order by using ATR-FTIR spectroscopy, and (iv) their ability to bind to cancer versus normal cells by using confocal microscopy. The data suggest that the peptides disintegrate the cell membrane in a detergent-like manner. However, in contrast to native antimicrobial peptides, the diastereomers bind and permeate similarly zwitterionic and PS-containing model membranes. Therefore, cell selectivity is probably determined mainly by improved electrostatic attraction of the peptides to acidic components on the surface of cancer cells (e.g., O-glycosylation of mucines). The simple composition of the diastercomeric peptides and their stability regardi
In recent years, the simple picture of a viral fusion protein interacting with the cell and/or viral membranes by means of only two localized segments (i.e. the fusion peptide and the transmembrane domain) has given way to a more complex picture in which multiple regions from the viral proteins interact with membranes. Indeed, possible roles in membrane binding and/or destabilization have been postulated for the N-terminal heptad repeats, pre-transmembrane segments, and other internal regions of fusion proteins from distant viruses (such as orthomyxo-, retro-, paramyxo-, or flaviviruses). This review focuses on the experimental evidence and functional models postulated so far about the role of these regions in the process of virus-induced membrane fusion. (C) 2003 Elsevier B.V. All rights reserved.
The high toxicity of most chemotherapeutic drugs and their inactivation by multidrug resistance phenotypes motivated extensive search for drugs with new modes of action. We designed a short cationic diastereomeric peptide composed of D- and L-leucines, lysines, and arginines that has selective toxicity toward cancer cells and significantly inhibits lung metastasis formation in mice (86%) with no detectable side effects. Its ability to depolarize the transmembrane potential of cancer cells at the same rate (within minutes) and concentration (3 muM), at which it shows biological activity, suggests a killing mechanism that involves plasma membrane perturbation. Confocal microscopy experiments verified that the cells died as a result of acute injury, swelling, and bursting, suggesting necrosis. Biosensor binding experiments and attenuated total reflectance-Fourier transform infrared spectroscopy using model membranes have substantiated its high selectivity toward cancer cells. Although this is an initial study that looked at tumor formation rather than the ability of the peptides to reduce established tumors, the simple sequence of the peptide, its high solubility, substantial resistance to degradation, and inactivation by serum components might make it a good candidate for future anticancer treatment.
T20, a synthetic peptide corresponding to a C-terminal segment of the envelope glycoprotein (gp41) of human and simian immunodeficiency viruses, is a potent inhibitor of viral infection. We report here that C-terminal octylation of simian immunodeficiency virus gp41-derived T20 induces a significant increase in its inhibitory potency. Furthermore, when C-terminally octylated, an otherwise inactive mutant in which the C-terminal residues GNWF were replaced by ANAA has potency similar to that of the wild type T20. This effect cannot be explained by a trivial inhibitory effect of the octyl group added to the peptides, because the N-terminally octylated peptides have the same activity as the non-octylated parent peptides. The effects caused by octylation on the oligomerization, secondary structure, and membrane-interaction properties of the peptides were investigated. Our results shed light on the mechanism of inhibition by T20 and provide experimental support for the existence of a pre-hairpin intermediate.
gp41 is the protein responsible for the process of membrane fusion that allows primate lentiviruses (HIV and SIV) to enter into their host cells. gp41 ectodomain contains an N-terminal and a C-terminal heptad repeat region (NHR and CHR) connected by an immunodominant loop. In the absence of membranes, the NHR and CHR segments fold into a protease-resistant core with a trimeric helical hairpin structure. However, when the immunodominant loop is not present (either in a complex formed by HIV-1 gp41-derived NHR and CHR peptides or by mild treatment with protease of recombinant constructs of HIV-1 gp41 ectodomain, which also lack the N-terminal fusion peptide and the C-terminal Trp-rich region) membrane binding induces a conformational change in the gp41 core structure. Here, we further investigated whether covalently linking the NHR and CHR segments by the immunodominant loop affects this conformational change. Specifically, we analyzed a construct corresponding to a fragment of SIVmac239 gp41ectodomain (residues 27-149, named e-gp41) by means of surface plasmon resonance, Trp and rhodamine fluorescence, ATR-FTIR spectroscopy, and differential scanning calorimetry. Our results suggest that the presence of the loop stabilizes the trimeric helical hairpin both when e-gp41 is in aqueous solution and when it is bound to the membrane surface. Bearing in mind possible differences between HIV-1 and SIV gp41, and considering that the gp41 ectodomain constructs analyzed to date lack the N-terminal fusion peptide and the C-terminal Trp-rich region, we discuss our observations in relation to the mechanism of virus-induced membrane fusion. (C) 2003 Elsevier Science Ltd. All rights reserved.
Lytic peptides comprise a large group of membrane-active peptides used in the defensive and offensive systems of all organisms. Differentiating between their modes of interaction with membranes is crucial for understanding how these peptides select their target cells. Here we utilized SPR to study the interaction between lytic peptides and lipid bilayers (L1 sensor chip). Using studies also on hybrid monolayers (HPA sensor chip) revealed that SPR is a powerful tool for obtaining a real-time monitoring of the steps involved in the mode of action of membrane-active peptides, some of which previously could not be detected directly by other techniques and reported here for the first time. We investigated the mode of action of peptides that represent two major families: (i) the bee venom, melittin, as a model of a non-cell-selective peptide that forms transmembrane pores and (ii) magainin and a diastereomer of melittin (four amino acids were replaced by their D enantiomers), as models of bacteria-selective non-pore-forming peptides. Fitting the SPR data to different interaction models allows differentiating between two major steps: membrane binding and membrane insertion. Melittin binds to PC/cholesterol similar to450-fold better than its diastereomer and magainin, mainly because it is inserted into the inner leaflet (2/3 of the binding energy), whereas the other two are not. In contrast, there is only a slight difference in the binding of all the peptides to negatively charged PE/PG mono- and bilayer membranes (in the first and second steps), indicating that the inner leaflet contributes only slightly to their binding to PE/PG bilayers. Furthermore, the 100-fold stronger binding of the cell-selective peptides to PE/PG as compared with PC/cholesterol resulted only from electrostatic attraction to the negatively charged headgroups of the outer leaflet. These results clearly differentiate between the two general mechanisms: pore formation by melittin only in zwitterioni
Membrane-active peptides comprise a large group of toxins used in the defense and offense systems of all organisms including plants and humans. They act on diverse targets including microorganisms and mammalian cells, but the factors that determine their target cell selectivity are not yet clear. Here, we tested the role of peptide length and preassembly on the ability of diastereomeric cationic antimicrobial peptides to discriminate among bacteria, erythrocytes, and fungal cells, by using peptides with variable lengths (13, 16, and 19 amino acids long) and their covalently linked pentameric bundles. All the bundles expressed similar potent antifLingal activity (minimal inhibitory concentration of 0.2-0.3 muM) and high antimicrobial activity. Hemolytic activity was also observed at concentrations higher than those required for antifungal activity. In contrast, all the monomers showed length-dependent antimicrobial activity, were less active toward bacteria and fungi, and were devoid of hemolytic activity. BIAcore biosensor experiments revealed a similar to300-fold increase in peptide-membrane binding affinity between the 13- and 19-residue monomers toward zwitterionic (egg phosphatidylcholine (PC)/egg spingomyelin (SM)/cholesterol) vesicles. All the monomeric peptides display a similar high affinity to negatively charged (E. coli phosphatidylethanolamine (PE)/egg phosphatidylglycerol (PG)) vesicles regardless of their length. In contrast, irrespective of the size of the monomeric subunit, all the bundles bind irreversibly and strongly disrupt both PC/SM/cholesterol and PE/PG membranes. Attenuated total reflectance Fourier-trans form infrared spectroscopy revealed that peptide assembly also affects structure as observed by an increased alpha-helical and beta-sheet content in membranes and enhances acyl chain disruption of PC/cholesterol. The correlation between the antibacterial activity and ability to depolarize the transmembrane potential of E. coli spheroplasts
Stereospecificity in protein-protein recognition and docking is an unchallenged dogma. Soluble proteins provide the main source of evidence for stereospecificity. In contrast, within the membrane little is known about the role of stereospecificity in the recognition process. Here, we have reassessed the stereospecificity of protein-protein recognition by testing whether it holds true for the well-defined glycophorin A (GPA) transmembrane domain in vivo. We found that the all-D amino acid GPA transmembrane domain and two all-D mutants specifically associated with an all-L GIPA transmembrane domain, within the membrane milieu of Escherichia coli. Molecular dynamics techniques reveal a possible structural explanation to the observed interaction between all-D and all-L transmembrane domains. A very strong correlation was found between amino acid residues at the interface of both the all-L homodimer structure and the mixed L/D heterodimer structure, suggesting that the original interactions are conserved. The results suggest that GPA helix-helix recognition within the membrane is chirality-independent. (C) 2002 Elsevier Science Ltd. All rights reserved.
The N-terminal fusion peptide of Sendai virus F-1 envelope glycoprotein is a stretch of 14 amino acids, most of which are hydrophobic. Following this region, we detected a segment of 11 residues that are strikingly similar to the N-terminal fusion peptide. We found that, when anchored to the membrane by palmitoylation of its N-terminus, this segment (WT-palm-19-33) induces membrane fusion of large unilamellar liposomes to almost the same extent as a segment that includes the N-terminal fusion peptide. The activity of WT-palm-19-33 was dependent on its specific sequence, as a palmitoylated peptide with the same amino-acid composition but a scrambled sequence was inactive. Interestingly, two mutations (G7A and G12A) known to increase F-1-induced cell-cell fusion, also increased the homology between the N-terminal fusion peptide and WT-palm-19-33. The role of the amino-acid sequence on the fusogenicity, secondary structure, and mechanism of membrane fusion was analyzed by comparing a peptide comprising both homologous segments (WT 1-33), a G12A mutant (G12A 1-33), a G7A-G12A double mutant (G7A-G12A 1-33), and a peptide with a scrambled sequence (SC 1-33). Based on these experiments, we postulate that replacement of Gly 7 and Gly12 by Ala increases the alpha helical content of the N-terminal region, with a concomitant increase in its fusogenic activity. Furthermore, the dissimilar abilities of the different peptides to induce membrane negative curvature as well as to promote isotropic P-31 NMR signals, suggest that these mutations might also alter the extent of membrane penetration of the 33-residue peptide. Interestingly, our results serve to explain the effect of the G7A and G12A mutations on the fusogenic activity of the parent F-1 protein in vivo.
The search for antibiotics with a new mode of action led to numerous studies on antibacterial peptides. Most of the studies were carried out with L-amino acid peptides possessing amphipathic alpha-helix or beta-sheet structures, which are known to be important for biological activities. Here we compared the effect of significantly altering the sequence of an amphipathic a-helical peptide (15 amino acids long) and its diastereomer (composed of both L- and D-amino acids) regarding their structure, function, and interaction with model membranes and intact bacteria. Interestingly, the effect of sequence alteration on biological function was similar for the L-amino acid peptides and the diastereomers, despite some differences in their structure in the membrane as revealed by attenuated total reflectance Fourier-transform infrared spectroscopy. However, whereas the all L-amino acid peptides were highly hemolytic, had low solubility, lost their activity in serum, and were fully cleaved by trypsin and proteinase K, the diastereomers were nonhemolytic and maintained full activity in serum. Furthermore, sequence alteration allowed making the diastereomers either fully, partially, or totally protected from degradation by the enzymes. Transmembrane potential depolarization experiments in model membranes and intact bacteria indicate that although the killing mechanism of the diastereomers is via membrane perturbation, it is also dependent on their ability to diffuse into the inner bacterial membrane. These data demonstrate the advantage of the diastereomers over their all L-amino acid counterparts as candidates for developing a repertoire of new target antibiotics with a potential for systemic use.
The interaction of many lytic cationic antimicrobial peptides with their target cells involves electrostatic interactions, hydrophobic effects, and the formation of amphipathic secondary structures, such as a helices or beta sheets. We have shown in previous studies that incorporating 30% D-amino acids into a short alpha helical lytic peptide composed of leucine and lysine preserved the antimicrobial activity of the parent peptide, while the hemolytic activity was abolished. However, the mechanisms underlying the unique structural features induced by incorporating D-amino acids that enable short diastereomeric antimicrobial peptides to preserve membrane binding and lytic capabilities remain unknown. In this study, we analyze in detail the structures of a model amphipathic alpha helical cytolytic peptide KLLLKWLL KLLK-NH2 and its diastereomeric analog and their interactions with zwitterionic and negatively charged membranes. Calculations based on high-resolution NMR experiments in dodecylphosphocholine (DPCho) and sodium dodecyl sulfate (SDS) micelles yield three-dimensional structures of both peptides. Structural analysis reveals that the peptides have an amphipathic organization within both membranes. Specifically, the alpha helical structure of the L-type peptide causes orientation of the hydrophobic and polar amino acids onto separate surfaces, allowing interactions with both the hydrophobic core of the membrane and the polar head group region. Significantly, despite the absence of helical structures, the diastereomer peptide analog exhibits similar segregation between the polar and hydrophobic surfaces. Further insight into the membrane-binding properties of the peptides and their depth of penetration into the lipid bilayer has been obtained through tryptophan quenching experiments using brominated phospholipids and the recently developed lipid/polydiacetylene (PDA) colorimetric assay. The combined NMR, FTIR, fluorescence, and colorimetric studies shed light
For many different enveloped viruses the crystal structure of the fusion protein core has been established. A striking conservation in the tertiary and quaternary arrangement of these core structures is repeatedly revealed among members of diverse families. It has been proposed that the primary role of the core involves structural rearrangements which facilitate apposition between viral and target cell membranes. Forming the internal trimeric coiled coil of the core, the N-terminal heptad repeat (NHR) of HIV-1 gp41 was suggested to have additional roles, due to its ability to bind biological membranes. The NHR is adjacent to the N-terminal hydrophobic fusion peptide (FP), which alone can fuse biological membranes. To investigate the role of the NHR in membrane fusion, we synthesized and functionally characterized HIV-1 gp41 peptides corresponding to the FP and NHR alone, as well as continuous peptides made of both FP and NHR (wild type and mutant). We show here that a consecutive, 70-residue peptide consisting of both the FP and NHR (gp41/1-70) has dramatic fusogenic properties. The effect of including the complete NHR, as compared to shorter 23-, 33-, or 52-residue N-terminal peptides, is illustrated by a leap in lipid mixing of phosphatidylcholine (PC) large unilamellar vesicles (LUV) and clearly delineates the synergistic role of the NHR in the fusion event. Furthermore, a mutation in the NHR that renders the virus noninfectious is reflected by a significant reduction in in vitro lipid mixing induced by the mutant, gp41/1-70 (I62D). Additional spectroscopic studies, characterizing membrane binding and apposition induced by the peptides, help to clarify the role of the NHR in membrane fusion.
Infection by enveloped viruses requires fusion between the viral and cellular membranes, a process mediated by specific viral envelope glycoproteins. Information from studies with whole viruses, as well as protein dissection, has suggested that the fusion glycoprotein (F) from Paramyxoviridae, a family that includes major human pathogens, has two hydrophobic segments, termed fusion peptides. These peptides are directly responsible for the membrane fusion event. The recently determined three-dimensional structure of the pre-fusion conformation of the F protein supported these predictions and enabled the formulation of: (1) a detailed model for the initial interaction between F and the target membrane, (2) a new model for Paramyxovirus-induced membrane fusion that can be extended to other viral families, and (3) a novel strategy for developing better inhibitors of paramyxovirus infection.
Our basic understanding of how to combat fungal infections has not kept pace with the recent sharp rise in life-threatening cases found particularly among immuno-compromised individuals. Current investigations for new potential antifungal agents have focused on anti microbial peptides. which are used as a cell-free defense mechanism in all organisms. Unfortunately, despite their high antibacterial activity, most of them are not active toward fungi, the reason of which is not clear. Here, we present a new approach to modify an antibacterial peptide, a magainin analogue, to display antifungal activity by its conjugation with lipophilic acids. This approach has the advantage of producing well-defined changes in hydrophobicity, secondary structure, and self-association. These modifications were characterized in solution at physiological concentrations using CD spectroscopy, tryptophan fluorescence, and analytical ultracentrifugation. In order of increasing hydrophobicity, the attachment to the magainin-2 analogue of (i) heptanoic acid results in a monomeric, unordered structure, (ii) undecanoic acid yields concentration-dependent oligomers of alpha helices, and (iii) palmitic acid yields concentration-independent alpha-helical monomers, a novel lipopeptide structure, which is resistant to proteolytic digestion. Membrane-lipopeptide interactions and the membrane-bound structures were studied using fluorescence and ATR-FTIR in PC/PE/PI/ergosterol (5/2.5/2.5/1, w/w) SUV, which constitute the major components of Candida albicans bilayers. A direct correlation was found between oligomerization of the lipopeptides in solution and potent antifungal activity. These results provide insight to a new approach of modulating hydrophobicity and self-assembly of antimicrobial peptides in solution, without altering the sequence of the peptidic chain, These studies also provide a general means of developing a new group of lipopeptide candidates as therapeutic agents against fungal in
Antimicrobial peptides are a large group of gene-encoded, net positively A charged polypeptides, produced by living organisms of all types including human and plants, They are mobilized shortly after infection as part of the innate immunity of these species and act rapidly to neutralize a broad range of microbes. Nowadays, thousands of native and de-novo designed antimicrobial peptides are available. They vary considerably in length, composition, charge and secondary structure. Despite these variations most antimicrobial peptides use a similar target, which is the bacterial phospholipid membrane. Many of them use a common general mechanism, the "carpet" mechanism, in which they accumulate on the bacterial membrane up to a threshold concentration, and then effect membrane permeation/disintegration. However, the structure of the permeation pathway may vary for different peptides and may include channel aggregates, toroidal pores or channels. Target specificity is determined by the negatively charged bacterial membrane, the net positive charge of the peptide, its hydrophobicity, oligomeric state in solution and in the membrane, and the stability of its secondary structure. A novel group of non hemolytic antimicrobial peptides were derived from diastereomers (containing D- and L-amino acids) of lytic peptides based on parameters required by the carpet mechanism. Because these disastereomers exhibit several advantages over their all-L amino acid counterparts, they have a potential to be developed for therapeutic use both in vitro and in vivo.
Water membrane soluble protein and peptide toxins are used m the defense and offense systems of all organisms, including plants and humans. A major group includes antimicrobial peptides, which serve as a nonspecific defense system that complements the highly specific cell-mediated immune response. The increasing resistance of bacteria to conventional antibiotics stimulated the isolation and characterization of many antimicrobial peptides for potential use as new target antibiotics. The finding of thousands of antimicrobial peptides with variable lengths and sequences, all of which are active at similar concentrations, suggests a general mechanism for killing bacteria rather than a specific mechanism that requires preferred active structures. Such a mechanism is in agreement with the "carpet model" that does not require any specific structure or sequence. It seems that when there is an appropriate balance between hydrophobicity and a net positive charge the peptides are active an bacteria. However, selective activity depends also on other parameters, such as the volume of the molecule, its structure, and its oligomeric state in solution and membranes. Further, although many studies support that bacterial membrane damage is a lethal event for bacteria, other studies point to a multihit mechanism in which the peptide binds to several targets in the cytoplasmic region of the bacteria. (C) 2002 Wiley Periodicals, Inc.
The initial stages leading to the binding and functioning of membrane-active polypeptides including hormones, signal sequences, and lytic peptides are mainly governed by electrostatic attraction and hydrophobic partitioning between water and lipid bilayers. Antimicrobial peptides serve as an important model for studying the details of these initial steps. However, a systematic analysis of the contribution of multiple hydrophobic amino acids to these steps have been hindered by the propensity of many peptides to aggregate and become inactivated in solution. To this end, we synthesized a series of model amphipathic all L-amino acid peptides and their diastereomers with the sequence KX3KWX2KX2K, where X = Gly, Ala, Val, Ile, or Leu. The effect of the aliphatic amino acids on the biological activity, binding, structure, membrane localization, and mode of action of these peptides was investigated. Most of the L-amino acid peptides oligomerized and adopted distinct structures in solution and in a membrane mimetic environment. Among this group only the Leu containing peptide was hemolytic and highly active on most bacteria tested. The Val- and Leu-containing peptides were hemolytic but inactive toward most bacteria tested. In contrast, the diastercomeric peptides were monomeric and unstructured in solution, but they adopted distinct structures upon membrane binding. While hemolytic activity was drastically reduced, the spectrum of antibacterial activity was preserved or increased. Importantly, we found a direct correlation with the diastercomers between hydrophobicity and propensity to form a helical/distorted-helix and activity (induced membrane leakage and antibacterial activity), despite the fact that they contained 30% D-amino acids. Furthermore, efficient increase in membrane permeability can proceed through different mechanisms. Specifically, the Leu-containing diastereomeric peptide micellized vesicles and possibly bacterial membranes while the Ile-containing dia
Living organisms of all types produce a large repertoire of gene-encoded, net positively charged, antimicrobial peptides as part of their innate immunity to microbial invasion. Despite significant variations in composition, length and secondary structure most antimicrobial peptides are active in micromolar concentrations, suggesting a common general mechanism for their mode of action. Man antimicrobial peptides bind bacterial phospholipid membranes up to a threshold concentration, followed by membrane permeation/disintegration (the "carpet" mechanism). Recent data suggest that the details of the permeation pathways may vary for different peptides and are assigned to different modes of action. Accumulating, data reveal that the molecular basis for cell selectivity is the ability of peptides to specifically bind the negatively charged bacterial membrane, as well as their oligomeric state in solution and in the membrane. Based on the "carpet" mechanism and the role of the peptide oligomeric state, a novel group of diastereomeric (containing D- and L-amino acids) antimicrobial peptides were developed. These peptides may serve as promising templates for the future designs of antimicrobial peptides. (C) 2001 Elsevier Science Inc. All rights reserved.
Protein recognition within the membrane is a crucial process for numerous biological activities. Detection of such interaction is limited because of difficulties that arise from the hydrophobic environment of the membrane. We detected direct hetero-oligomerization of the glycophorin-A (GPA) transmembrane segments in vivo through inhibition of ToxR transcription activator dimer formation. We investigated the amino acids important for hetero-oligomerization within the membrane, using peptide analog segments of the transmembrane domain of glycophorin A. The wild type ([WT]GPA) and alanine mutant ([A]GPA) were able to interfere with and inhibit the proper dimerization of the ToxR-GPA transcription factor. Conversely, a second alanine mutant ([A(2)]GPA), a glycine mutant ([G]GPA), and a scrambled analog ([SC]GPA) were virtually inactive. Binding studies reveal similar membrane partitions for [WT]GPA, [G]GPA, and [SC]GPA, whereas membrane partition of [A]GPA and [A]GPA are lower. Spectral analysis of fluorescent-labeled analogs revealed a significant blue shift, indicating membrane insertion. Our results suggest that the GXXXG motif, found in homo-oligomerization, is not sufficient for hetero-oligomerization in a biological membrane, whereas an extended motif, LIXXGXXXGXXXT, is sufficient. Interfering with hetero-oligomerization within the membrane can be a useful strategy for characterizing such interactions and possibly modulating membrane protein activity.
The viral envelope glycoprotein gp41 mediates membrane fusion in HIV/SIV infection. gp41 ectodomain (e-gop41, residues 27-149), which was shown to interact with phospholipid membranes, exists in an equilibrium between the monomeric and trimeric states. Here, we analyzed, by intrinsic Trp fluorescence and resonance energy transfer, whether SIV e-gp41-membrane interaction depends on the gp41 oligomeric state. We found that both gp41 monomers and trimers bind membranes, with the monomers' full binding being reached at substantially lower lipid to protein ratios. Furthermore, the different characteristics of the Trp fluorescence of monomers and trimers enabled us to detect binding of each form at concentrations at which both species were present. CD spectroscopy revealed that the secondary structure of gp41 monomers does not change upon membrane binding, suggesting that membrane-bound monomeric-gp41 is a possible target for DP-178, a potent peptide inhibitor of HIV infection. The consequences of the interaction between monomeric and trimeric gp41 with membranes in HIV/SIV infection, its inhibition, and its associated neuropathologies are discussed. (C) 2001 Academic Press.
The amphipathic alpha -helical structure is a common motif found in membrane binding polypeptides including cell lytic peptides, antimicrobial peptides, hormones, and signal sequences. Numerous studies have been undertaken to understand the driving forces for partitioning of amphipathic alpha -helical peptides into membranes, many of them based on the antimicrobial peptide magainin 2 and the non-cell-selective cytolytic peptide melittin, as paradigms. These studies emphasized the role of linearity in their mode of action. Here we synthesized and compared the structure, biological function, and interaction with model membranes of linear and cyclic analogues of these peptides. Cyclization altered the binding of melittin and magainin analogues to phospholipid membranes. However, at similar bound peptide:lipid molar ratios, both linear and cyclic analogues preserved their high potency to permeate membranes. Furthermore, the cyclic analogues preserved similar to 75% of the helical structure of the linear peptides when bound to membranes. Biological activity studies revealed that the cyclic melittin analogue had increased antibacterial activity but decreased hemolytic activity, whereas the cyclic magainin 2 analogue had a marked decrease in both antibacterial and hemolytic activities. The results indicate that the linearity of the peptides is not essential for the disruption of the target phospholipid membrane, but rather provides the means to reach it. In addition, interfering with the coil-helix transition by cyclization, while maintaining the same sequence of hydrophobic and positively charged amino acids, allows a separated evaluation of the hydrophobic and electrostatic contributions to binding of peptides to membranes.
Paramyxoviruses penetrate into their host cells by fusing their membranes with the plasma membrane. The hydrophobic N terminus of their F1 protein, termed the 'fusion peptide', is thought to be responsible for this process. Recently, an additional internal fusion peptide, homologous in sequence to the N-terminal fusion peptide of HIV-1, was identified in the Sendai virus Fl protein. Here, we investigated whether the presence of an additional internal fusion peptide is a general feature of paramyxoviridae. To this end, we synthesized and structurally and functionally characterized three peptides: (i) MV-197, which corresponds to an internal segment of the Fl protein of the measles virus (amino acids 197-225), homologous in location but not in sequence to the internal fusion peptide of the Sendai virus, (ii) MuMV-197, a randomized version of MV-197, and (iii) the 33 amino acid N-terminal fusion peptide of the measles virus. Remarkably, only MV-197 was highly fusogenic toward large unilamellar vesicles composed of either zwitterionic (phosphatidylcholine or phosphatidylcholine/sphingomyelin/cholesterol a composition similar to that of human cell membranes) or negatively charged phospholipids. Binding experiments, circular dichroism spectroscopy in phospholipid membranes, and home energy-transfer studies with fluorescently labeled peptides revealed that MV-197 adopts a predominant alpha -helical structure and shares properties similar to those reported for known fusion peptides. These results suggest that the presence of two fusion peptides in the F1 protein is a general feature of paramyxoviruses.
The spore-forming bacterium Bacillus thuringiensis produces intracellular inclusions comprised of protoxins active on several orders of insects. These highly effective and specific toxins have great potential in agriculture and for the control of disease-related insect vectors. Inclusions ingested by larvae are solubilized and converted to active toxins, in the midgut. There are two major classes, the cytolytic toxins and the delta -endotoxins. The former are produced by B. thuringiensis subspecies active on Diptera. The latter, which will be the focus of this review, are more prevalent and active on at least three orders of insects. They have a three-domain structure with extensive functional interactions among the domains. The initial reversible binding to receptors on larval midgut cells is largely dependent upon domains II and III. Subsequent steps involve toxin insertion into the membrane and aggregation, leading to the formation of gated, cation-selective channels. The channels are comprised of certain amphipathic helices in domain I, but the three processes of insertion. aggregation and the formation of functional channels are probably dependent upon all three domains. Lethality is believed to be due to destruction of the transmembrane potential, with the subsequent osmotic lysis of cells lining the midgut. In this review, the mode of action of these delta -endotoxins will be discussed with emphasis on unique features. (C) 2001 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.
DP178, a synthetic peptide corresponding to a segment of the transmembrane envelope glycoprotein (gp41) of human immunodeficiency virus, type 1 (HIV-l), is a potent inhibitor of viral infection and virus-mediated cell-cell fusion, Nevertheless, DP178 does not contain gp41 coiled-coil cavity binding residues postulated to be essential for inhibiting HIV-1 entry. We find that DP178 inhibits phospholipid redistribution mediated by the HIV-1 envelope glycoprotein at a concentration 8 times greater than that of solute redistribution (the IC,, values are 43 and 335 nM, respectively), In contrast, C34, a synthetic peptide which overlaps with DP178 but contains the cavity binding residues, did not show this phenomenon (11 and 25 nM, respectively). The ability of DP178 to inhibit membrane fusion at a post-lipid mixing stage correlates with its ability to bind and oligomerize on the surface of membranes. Furthermore, our results are consistent with a model in which DP178 inhibits the formation of gp41 viral hairpin structure at low affinity, whereas C34 inhibits its formation at high affinity: the failure to form the viral hairpin prevents both lipid and solute from redistributing between cells, However, our data also suggest an additional membrane-bound inhibitory site for DP178 in the ectodomain of gp41 within a region immediately adjacent to the membrane-spanning domain. By binding to this higher affinity site, DP178 inhibits the recruitment of several. gp41-membrane complexes, thus inhibiting fusion pore formation.
Keywords: THURINGIENSIS DELTA-ENDOTOXIN; GLYCOPHORIN-A DIMERIZATION; MAGNETIC-RESONANCE DATA; PEPTIDE CECROPIN P1; PORE-FORMING DOMAIN; MODEL ION CHANNELS; SHAKER K+ CHANNEL; ISK MINK PROTEIN; SEA MOSES SOLE; BACILLUS-THURINGIENSIS
The entry of enveloped viruses into host cells is accomplished by fusion of the viral envelope and target plasma membrane and is mediated by fusion proteins. Recently, several functional domains within fusion proteins from different viral families were identified. Some are directly involved in conformational changes after receptor binding, as suggested by the recent release of crystallographically determined structures of a highly stable core structure of the fusion proteins in the absence of membranes. However, in the presence of membranes, this core binds strongly to the membrane's surface and dissociates therein, Other regions, besides the N-terminal fusion peptide, which include the core region and an internal fusion peptide in paramyxoviruses, are directly involved in the actual membrane fusion event, suggesting an "umbrella" like model for the membrane induced conformational change of fusion proteins. Peptides resembling these regions have been shown to have specific antiviral activity, presumably because they interfere with the corresponding domains within the viruses. Overall, these studies shed light into the molecular mechanism of membrane fusion induced by envelope glycoproteins and suggest that fusion proteins from different viral families share common structural and functional motifs.
Viral glycoproteins catalyze the fusion between viral and cellular membranes. The fusion protein (F-1) of Sendai virus has two fusion peptides. One is located at its N-terminus and the other, highly homologous to the HIV-I and RSV fusion peptides, in the interior of the F-1 protein. A synthetic peptide corresponding to the internal fusogenic domain, namely, SV-201, was found to inhibit virus-cell fusion without interfering with the binding of the virus to the target cells, thus highlighting the importance of this region in Sendai virus-induced membrane fusion. However, its detailed mechanism of inhibition remains unknown. Here, we synthesized a shorter version of SV-201, namely, SV-208, an elongated one, SV-197, and two mutants of SV-201, and compared them functionally and structurally with SV-201. In contrast to SV-201, SV-208 and the two mutants do not inhibit virus-cell fusion. The differences in the oligomerization state of these peptides in aqueous solution and within the membrane, and in their ability to bind to Sendai virions, enabled us to postulate a possible mechanism of viral entry inhibition: SV-201 binds to its target in Sendai virions before the Fl internal fusion peptide binds to the membrane, therefore blocking the Fl conformational change required for fusion. In addition, we further characterized the fusogenic activity of the internal fusion peptide, compared to the N-terminal one, and determined its structure in the membrane-bound state by means of fluorescence, CD, and ATR-FTIR spectroscopy. Remarkably, we found that SV-201 and its elongated form, SV-197, are highly potent in inducing fusion of the highly stable large unilamellar vesicles composed of egg phosphatidylcholine, a property found only in an extended version of the HIV-I fusion peptide. The inhibitory activity of SV-201 and its fusogenic ability are discussed in terms of the "umbrella" model of Sendai virus-induced membrane fusion.
The human immunodeficiency virus type 1 gp41 ectodomain forms a three-hairpin protease-resistant core in the absence of membranes, namely, the putative gp41 fusion-active state. Here, we show that recombinant proteins corresponding to the ectodomain of gp41, but lacking the fusion peptide, bind membranes and consequently undergo a major conformational change. As a result, the protease-resistant core becomes susceptible to proteolytic digestion. Accordingly, synthetic peptides corresponding to the segments that construct this core bind the membrane. It is remarkable that the hetero-oligomer formed by these peptides dissociates upon binding to the membrane. These results are consistent with a model in which, after the three-hairpin conformation is formed, membrane binding induces opening of the gp41 core complex. We speculate that binding of the segments that constructed the core to the viral and cellular membranes could bring the membranes closer together and facilitate their merging. (C) 2000 Academic Press.
The pore-forming domain of Bacillus thuringiensis Cry1Ac insecticidal protein comprises of a seven alpha-helix bundle (alpha 1-alpha 7). According to the "umbrella model," alpha 4 and alpha 5 helices form a hairpin structure thought to be inserted into the membrane upon binding. Here, we have synthesized and characterized the hairpin domain, alpha 4-loop-alpha 5, its alpha 4 and alpha 5 helices, as well as mutant alpha 4 peptides based on mutations that increased or decreased toxin toxicity. Membrane permeation studies revealed that the alpha 4-loop-alpha 5 hairpin is extremely active compared with the isolated helices or their mixtures, indicating the complementary role of the two helices and the need for the loop for efficient insertion into membranes. Together with spectrofluorometric studies, we provide direct evidence for the role of alpha 4-loop-alpha 5 as the membrane-inserted pore-forming hairpin in which alpha 4 and alpha 5 line the lumen of the channel and alpha 5 also participates in the oligomerization of the toxin. Strikingly, the addition of the active alpha 4 mutant peptide completely inhibits alpha 4-loop-alpha 5 pore formation, thus providing, to our knowledge, the first example that a mutated helix within a pore can function as an "immunity protein" by directly interacting with the segments that form the pore. This presents a potential means of interfering with the assembly and function of other membrane proteins as well.
The amphipathic alpha-helical structure is considered to be a prerequisite for the lytic activity of a large group of cytolytic peptides. However, despite numerous studies on the contribution of various parameters to their structure and activity, the importance of linearity has not been examined. In the present study we functionally and structurally characterized a linear amphipathic alpha-helical peptide (wt peptide), its diastereomer, and cyclic analogues of both. Using analogues with the same sequence of hydrophobic and positively charged amino acids, but with different propensities to form a helical structure, we were able to examine the contribution of linearity to helix formation, bilogical function, and membrane binding and permeation. Importantly, we found that cyclization increases the selectivity between bacteria and human erythrocytes by substantially reducing the hemolytic activity of the cyclic peptides compared with the linear peptides. Moreover, whereas the wt peptide was highly active toward Gram(+) bacteria, its cyclic counterpart is active toward both Gram(+) and Gram(-) bacteria. These findings are correlated with an impaired ability of the cyclic analogues to bind and permeate zwitterionic phospholipid membranes compared with their linear counterparts and an increase in the binding and permeating activity of the cyclic wt peptide toward negatively charged membranes. Furthermore, cyclization abolished the oligomerization of the linear wt peptide in solution and in SDS, suggesting an additional factor that may account for the difference in the spectrum of antibacterial activity between the linear and the cyclic wt peptides. Interestingly, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy revealed that, despite cyclization and incorporation of 33% D-amino acids along the peptide backbone, the membrane environment can impose a predominantly helical structure on the peptides, which is required for their bilogical funct
Viral fusion proteins contain a highly hydrophobic segment, named the fusion peptide, which is thought to be responsible for the merging of the cellular and viral membranes. Paramyxoviruses are believed to contain a single fusion peptide at the N terminus of the F1 protein. However, here we identified an additional internal segment in the Sendai virus F1 protein (amino acids 214-226) highly homologous to the fusion peptides of HIV-1 and RSV. A synthetic peptide, which includes this region, was found to induce membrane fusion of large unilamellar vesicles, at concentrations where the known N-terminal fusion peptide is not effective. A scrambled peptide as well as several peptides from other regions of the F1 protein, which strongly bind to membranes, are not fusogenic. The functional and structural characterization of this active segment suggest that the F1 protein has an additional internal fusion peptide that could participate in the actual fusion event. The presence of homologous regions in other members of the same family suggests that the concerted action of two fusion peptides, one N-terminal and the other internal, is a general feature of paramyxoviruses. (C) 2000 Academic Press.
The fusion peptide of HIV-1 gp41 is formed by the 16 N-terminal residues of the protein. This 16-amino acid peptide, in common with several other viral fusion peptides, caused a reduction in the bilayer to hexagonal phase transition temperature of dipalmitoleoylphosphatidylethanolamine (Tw), suggesting its ability to promote negative curvature in membranes. Surprisingly, an elongated peptide corresponding to the 33 N-terminal amino acids raised T-H, although it was more potent than the 16-amino acid fusion peptide in inducing lipid mixing with large unilamellar liposomes of 1:1:1 dioleoylphosphatidylethanolamine/dioleoylphosphatidylchol. The 17-amino acid C-terminal fragment of the peptide can induce membrane fusion by itself, if it is anchored to a membrane by palmitoylation of the amino terminus, indicating that the additional 17 hydrophilic amino acids contribute to the fusogenic potency of the peptide. This is not solely a consequence of the palmitoylation, as a random peptide with the same amino acid composition with a palmitoyl anchor was less potent in promoting membrane fusion and palmitic acid itself had no fusogenic activity. The 16-amino acid N-terminal fusion peptide and the longer 33-amino acid peptide were labeled with NBD. Fluorescence binding studies indicate that both peptides bind to the membrane with similar affinities, indicating that the increased fusogenic activity of the longer peptide was not a consequence of a greater extent of membrane partitioning. We also determined the secondary structure of the peptides using FTIR spectroscopy. We find that the amino-terminal fusion peptide is inserted into the membrane as a P-sheet and the 17 C-terminal amino acids lie on the surface of the membrane, adopting an cr-helical conformation. It was further demonstrated with the use of rhodamine-labeled peptides that the 33-amino acid peptide self-associated in the membrane while the 16-amino acid N-terminal peptide did not. Thus, the 16-amino acid N-term
Androctonin is a 25-residue non-haemolytic anti-microbial peptide isolated from the scorpion Androctonus australis and contains two disulphide bridges. Androctonin is different from known native anti-microbial peptides, being a relatively hydrophilic and non-amphipathic molecule. This raises the possibility that the target of androctonin might not be the bacterial membrane, shown to be a target for most amphipathic lytic peptides. To shed light on its mode of action on bacteria and its nonhaemolytic activity, we synthesized androctonin, its fluorescent derivatives and its all-D-amino acid enantiomer. The enantiomer preserved high activity, suggesting a lipid-peptide interaction between androctonin and bacterial membranes. In Gram-positive and (at higher concentrations) Gram-negative bacteria, androctonin induced an immediate perturbation of the permeability properties of the cytoplasmic membrane of the bacterial energetic state, concomitant with perturbation of the morphology of the cell envelope as revealed by electron microscopy. Androctonin binds only to negatively charged lipid vesicles and induces the leakage of markers at high concentrations and with a slow kinetics, in contrast with amphipathic alpha-helical anti-microbial peptides that bind and permeate negatively charged vesicles, and to a smaller extent also zwitterionic ones. This might explain the selective lytic activity of androctonin towards bacteria but not red blood cells. Polarized attenuated total reflection-Fourier transform infrared spectroscopy revealed that androctonin adopts a beta-sheet structure in membranes and did not affect the lipid acyl chain order, which supports a detergent-like effect. The small size of androctonin, its hydrophilic character and its physicochemical properties are favourable features for its potential application as a replacement for commercially available antibiotics to which bacteria have developed resistance.
HIV-1 entry into its host cell is modulated by its transmembrane envelope glycoprotein (gp41). The core of the activated conformation of gp41 consists of a trimer of heterodimers comprising a leucine/isoleucine zipper sequence (represented here by the synthetic peptide N36 or by the longer N51 peptide) and a C-terminal highly conserved region (represented here by C34). A correlation was found between the action of DP178, which is a potent inhibitor of HIV-1 entry into its host cell, and its ability to interact with the leucine/isoleucine zipper sequence. This correlation was further tested and confirmed by circular dichroism spectroscopy. We found that whereas DP178 perturbs the partial alpha-helix nature of peptides corresponding to the leucine/isoleucine zipper sequence (N36 or N51), it cannot perturb the trimer of heterodimers conformation, modeled by the complex of N36 or N51 with C34. Therefore, we suggest that the already formed trimer of heterodimers is not the target of inhibition by DP178. Our results are consistent with a model in which DP178 acquires its inhibitory activity by binding to an earlier intermediate of gp41, in which the N and C peptide regions are not yet associated, thus allowing DP178 to bind to the leucine/isoleucine zipper sequence and consequently to inhibit transition to the fusion-active conformation. (C) 2000 Academic Press.
Some amphipathic bitter tastants and non-sugar sweeteners are direct activators of G proteins and stimulate transduction pathways in cells not related to taste. We demonstrate that the amphipathic bitter tastants quinine and cyclo(Leu-Trp) and the non-sugar sweetener saccharin translocate rapidly through multilamellar liposomes. Furthermore, when rat circumvallate (CV) taste buds were incubated with the above tastants for 30 s, their intracellular concentrations increased by 3.5- to 7-fold relative to their extracellular concentrations. The time course of this dramatic accumulation was also monitored in situ in rat single CV taste buds under a confocal laser-scanning microscope. Tastants were clearly localized to the taste cell cytosol. It is proposed that, due to their rapid permeation into taste cells, these amphipathic tastants may be available for activation of signal transduction components (e.g., G proteins) directly within the time course of taste sensation. Such activation may occur in addition to the action of these tastants on putative G protein-coupled receptors. This phenomenon may be related to the slow taste onset and lingering aftertaste typically produced by many bitter tastants and non-sugar sweeteners.
Recently, we reported on a new group of diastereomers of short-model peptides (12 amino acids long) composed of leucine and lysine with varying ratios, possessing several properties that make them potentially better than native or de novo-designed all L-amino acid antimicrobial peptides. Preliminary studies have revealed that modulating the hydrophobicity and positive charges of these diastereomers is sufficient to confer antibacterial activity and cell selectivity. However, the relationship between their biological function, structure, and mode of action was not investigated. Here we synthesized and investigated three types of linear model diastereomers (12 amino acids long) with varying lysine:leucine (or tryptophan) ratios (i.e., K3L8W, K5L6W, and K7L4W), which confer different levels of lytic activities. For each K:L ratio, tryptophan was introduced in the middle or the N- or C-terminus of the peptides, as an intrinsic fluorescent probe. Only the hemolytic peptide K3L8W binds to both negatively charged and zwitterionic phospholipid membranes. K5L6W and K7L4W bind similarly, but only to negatively charged membranes, despite the fact that K5L6W is substantially more lytic to bacteria than K7L4W. Interestingly, although K3L8W contains 33% D-amino acids, ATR-FTIR spectroscopy revealed a structure of similar to 90% alpha-helix in both types of membranes. In addition, K5L6W contains similar to 40% 3(10)-helix and K7L4W is predominantly a random coil in membranes. Polarized ATR-FTIR and tryptophan-quenching experiments, using brominated phospholipids, revealed a similar depth of penetration and an orientation that was parallel to the membrane surface for all the peptides, but with K3L8W affecting the lipid order more than the others. The results provide insight into the mode of action of this group of diastereomeric peptides, and the effect of hydrophobicity and positive charges on their membrane structure, function, and cell selectivity. Moreover, this research sho
Permeation of the cell membrane leading to cell death is a mechanism used by a large number of membrane-lytic peptides. Some are linear, mostly helical, and others contain one or more disulfide bonds forming beta-sheet or both beta-sheet and alpha-helix structures. They are all soluble in solution but when they reach the target membrane, conformational changes occur which let them associate with and lyse the membrane. Some lytic peptides are not cell-selective and lyse different microorganisms and normal mammalian cells, while others are specific to either type of cells. Despite extensive studies, the mode of action of membrane-lytic peptides is not fully understood and the basis for their selectivity towards specific target cells is not known. Many studies have shown that peptide-lipid interactions leading to membrane permeation play a major role in their activity. Membrane permeation by amphipathic alpha-helical peptides has been proposed to occur via one of two general mechanisms: (i) transmembrane pore formation via a 'barrel-stave' mechanism; and (ii) membrane destruction/solubilization via a 'carpet' mechanism. This review, which is focused on the different stages of membrane permeation induced by representatives of amphipathic alpha-helical antimicrobial and cell non-selective lytic peptides distinguishes between the 'carpet' mechanism, which holds for antimicrobial peptides versus the 'barrel-stave' mechanism, which holds for cell non-selective lytic peptides. (C) 1999 Elsevier Science B.V. All rights reserved.
'Melittin, a 26 residue, non-cell-selective cycolytic peptide, is the major component of the venom of the honey bee Apis mellifera. In a previous study, a diastereomer ([D]-V-5,V-8,I-17,K-21-melittin, D-amino acids at positions V-5,V-8,I-17,K-21) of melittin was synthesized and its function was investigated [Oren, Z., and Shai, Y. (1997) Biochemistry 36, 1826-1835]. [D]-V-5 8,I-17,K-21-melittin lost its cytotoxic effects on mammalian cells; however, it retained antibacterial activity. Furthermore, [D]-V-5,V-8,I-17,K-21-melittin binds strongly and destabilizes only negatively charged phospholipid vesicles, in contrast to native melittin, which binds strongly also zwitterionic phospholipids. To understand the differences in the properties of melittin and its diastereomer, 2D-NMR experiments were carried out with [D]-V-5,V-8,I-17,K-21-melittin, and polarized attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy experiments were-done with both melittin and [D]-V-5,V-8,I-17,K-21-melittin. The structure of the diastereomer was characterized by NMR in water, as well as in three different membrane-mimicking environment, 40% 2,2,2-trifluoroethanol (TFE)/water, methanol, and dodecylphosphocholine/phosphatidylglycerol (DPC/DMPG) micelles. The NMR data revealed an amphipathic alpha-helix only in the C-terminal region of the diastereomer in TFE/water and methanol solutions and in DPC/DMPG micelles. ATR-FTIR experiments revealed that melittin and [D]-V-5,V-8,I-17,K-21-melittin are oriented parallel to the membrane surface. This study indicates the role of secondary structure formation in selective cytolytic activity of [D]-V-5,V-8 ,I-17,K-21- melittin. While the N-terminal helical structure is not required for the cytolytic activity toward negatively charged membranes and bacterial cells, it appears to be a crucial structural element for binding and insertion into zwitterionic membranes and for hemolytic activity.
Recent studies have demonstrated the importance of heptad repeat regions within envelope proteins of viruses in mediating conformational changes at various stages of viral infection. However, it is not clear if heptad repeats have a direct role in the actual fusion event. Here we have synthesized, fluorescently labeled and functionally and structurally characterized a wild-type 70 residue peptide (SV-117) composed of both the fusion peptide and the N-terminal heptad repeat of Sendai virus fusion protein, two of its mutants, as well as the fusion peptide and heptad repeat separately. One mutation was introduced in the fusion peptide (G119K) and another in the heptad repeat region (I154K). Similar mutations have been shown to drastically reduce the fusogenic ability of the homologous fusion protein of Newcastle disease virus. We found that only SV-117 was active in inducing lipid mixing of egg phosphatidyl-choline/phosphatidyiglycerol (PC/PG) large unilamellar vesicles (LUV), and not the mutants nor the mixture of the fusion peptide and the heptad repeat. Functional characterization revealed that SV-117, and to a lesser extent its two mutants, were potent inhibitors of Sendai virus-mediated hemolysis of red blood cells, while the fusion peptide and SV-150 were negligibly active alone or in a mixture. Hemagglutinin assays revealed that none of the peptides disturb the binding of virions to red blood cells. Further studies revealed that SV-117 and its mutants oligomerize similarly in solution and in membrane, and have similar potency in inducing vesicle aggregation. Circular dichroism and FTIR spectroscopy revealed a higher helical content for SV-117 compared to its mutants in 40% tifluorethanol and in PC/PG multibilayer membranes, respectively, ATR-FTIR studies indicated that SV-117 lies more parallel with the surface of the membrane than its mutants. These observations suggest a direct role for the N-terminal heptad repeat in assisting the fusion peptide in mediati
The antimicrobial peptide LL-37 belongs to the cathelicidin family and is the first amphipathic ct-helical peptide isolated from human. LL-37 is considered to play an important role in the first line of defence against local infection and systemic invasion of pathogens at sites of inflammation and wounds. Understanding its mode of action may assist in the development of antimicrobial agents mimicking those of the human immune system. In vitro studies revealed that LL-37 is cytotoxic to both bacterial and normal eukaryotic cells. To gain insight into the mechanism of its non-cell-selective cytotoxicity, we synthesized and structurally and functionally characterized LL-37, its N-terminal truncated form FF-33, and their fluorescent derivatives (which retained structure and activity). The results showed several differences, between LL-37 and other native antimicrobial peptides, that may shed light on its in vivo activities. Most interestingly, LL-37 exists in equilibrium between monomers and oligomers in solution at very low concentrations. Also, it is significantly resistant to proteolytic degradation in solution, and when bound to both zwitterionic (mimicking mammalian membranes) and negatively charged membranes (mimicking bacterial membranes). The results also showed a role for the N-terminus in proteolytic resistance and haemolytic activity, but not in antimicrobial activity. The LL-37 mode of action with negatively charged membranes suggests a detergent-like effect via a 'carpet-like' mechanism. However, the ability of LL-37 to oligomerize in zwitterionic membranes might suggest the formation of a transmembrane pore in normal eukaryotic cells. To examine this possibility we used polarized attenuated total reflectance Fourier-transform infrared spectroscopy and found that the peptide is predominantly alpha-helical and oriented nearly parallel with the surface of zwitterionic-lipid membranes. This result does not support the channel-forming hypothesis, but rather
The fusion domain of the HIV-1 envelope glycoprotein (gp120-gp41) is a conserved hydrophobic region located at the N-terminus of the transmembrane subunit (gp41). A prominent feature of this domain is a conserved five-residue "FLGFL" sequence at positions 8-12. Mutation of the highly conserved Phe(11) to Val (F11V), presumed not to significantly affect the hydrophobicity and the structure of this region, has been shown to decrease the level of syncytium formation and virus infectivity. Here we show that the substitution of Gly for Phe(11) (F11G) reduces cell-cell fusion activity by 80-90%, To determine the effect of these mutations on the properties of the fusion peptide, a 33-residue peptide (WT) identical to the extended fusion domain and its F11V and F11G mutants were synthesized, fluorescently labeled, and studied with respect to their function, structure, and organization in phospholipid membranes. The WT peptide alone induced fusion of both zwitterionic (PC/Chol) and negatively charged (PS/PC/Chol and POPG) vesicles, in contrast to a 23-mer fusion peptide lacking the C-terminal domain which has been shown to be inactive with PC vesicles but able to induce fusion of POPG vesicles which had been preaggragated with Ca2+ or Mg2+, The F11V peptide preserved 50% activity, and the F11G peptide was virtually inactive, ATR-FTIR spectroscopy indicated similar secondary structure of the peptides in multibilayers that was independent of membrane composition. Furthermore, all the peptides increased the extent of lipid disorder to a similar extent, but the kinetics of amide II H to D exchange was in the following order: F11G > F11V > WT. Fluorescence studies in the presence of membranes, as well as SDS -PAGE, revealed that the WT and Fl IV peptides self-associate to similar levels while F11G exhibited a decreased level of self-association. The data suggest that the FLGFL motif contributes to the functional organization of the HIV-1 fusion peptide and that the C-terminal
Dermaseptins, a family of antimicrobial peptides, are believed to act by forming amphipathic alpha-helices which associate with the cell membrane, leading to its permeabilisation and disruption. A simple mean field method is described for simulation of the interactions of peptides with lipid bilayers which includes an approximate representation of the electrostatic effects of the head-group region of the bilayer. Starting from an atomistic model of a PC phospholipid bilayer we calculate an average electrostatic potential along the bilayer normal. By combining the interaction of the peptide with this electrostatic potential and with the hydrophobic core of the membrane we arrive at a more complete description of peptide-bilayer energetics than would be obtained using sidechain hydrophobicities alone. Using this interaction potential in MD simulations of the frog skin peptide dermaseptin B reveals that the lipid bilayer stabilises the alpha-helical conformation of the peptide. This is in agreement with FTIR data. A surface associated orientation thus appears to be the most stable arrangement of the peptide, at least at zero ionic strength and without taking account of possible peptide-peptide interactions. (C) 1999 Elsevier Science B.V. All rights reserved.
Peptides derived from conserved heptad-repeat regions of several viruses have been shown recently to inhibit virus-cell fusion. To find out their possible role in the fusion process, two biologically active heptad-repeat segments of the fusion protein (F) of Sendai virus, SV-150 (residues 150-186), and SV-473 (residues 473-495) were synthesized, fluorescently labeled and spectroscopically characterized for their structure and organization in solution and within the membrane. SV-150 was found to be 50-fold less active than SV-473 in inhibiting Sendai virus-cell fusion. Circular dichroism (CD) spectroscopy revealed that in aqueous solution, the peptides are self-associated and adopt low alpha-helical structure. However, when the two peptides are mixed together, their alpha-helical content significantly increases. Fluorescence studies, CD, and polarized attenuated total reflection infrared (ATR-FTIR) spectroscopy showed that both peptides, alone or as a complex, bind strongly to negatively charged and zwitterionic phospholipid membranes, dissociate therein into alpha-helical monomers, but do not perturb the lipid packing of the membrane. The ability of the peptides to interact with each other in solution may be correlated with antiviral activity, whereas their ability to interact with the membrane, together with their location near the fusion peptide and the transmembrane domain, suggests a revision to the currently accepted model for viral-induced membrane fusion. in the revised model, in the sequence of events associated with viral entry, the two heptad-repeat sequences may assist in bringing the viral and cellular membranes closer, thus facilitating membrane fusion. (C) 1999 Academic Press.
Antimicrobial peptides which adopt mainly or only beta-sheet structures have two or more disulfide bonds stabilizing their structure. The disruption of the disulfide bonds results in most cases in a large decrease in their antimicrobial activity. In the present study we examined the effect of D-amino acids incorporation on the structure and function of a cytolytic alpha-helical peptide which acts on erythrocytes and bacteria. The influence of a single or double D-amino acid replacement in alpha-helical peptides on their structure was reported previously in 50% 2,2,2,trifluoroethanol/water [Krause et al. (1995) Anal. Chem. 67, 252-258]. Here we used Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) spectroscopy and found that the predominant structure of the wild-type peptide is alpha-helix in phospholipid membranes, whereas the structure of the diastereomer is beta-sheet. However, the linear, beta-sheet diastereomer preserved its cytolytic activity on bacteria but not on erythrocytes. Previous studies have shown that the ability of antimicrobial peptides to lyse bacteria but not normal mammalian cells correlated with their ability to disintegrate preferentially negatively charged, but not zwitterionic phospholipid membranes. In contrast, the diastereomer described here disrupts zwitterionic and negatively charged vesicles with similar potencies to those of the hemolytic wild-type peptide. Interestingly, whereas addition of a positive charge to the N-terminus of the wildtype peptide (which caused a minor effect on its structure) increased activity only towards some of the bacteria tested, similar modification in the diastereomer increased activity towards all of them. Furthermore, the modified wild-type peptide preserved its potency to destabilize zwitterionic and negatively charged vesicles, whereas the modified diastereomer had a reduced potency on zwitterionic vesicles but increased potency on negatively charged vesicles. Overall our results su
The increasing resistance of bacteria to conventional antibiotics resulted in a strong effort to develop antimicrobial compounds with new mechanisms of action. Antimicrobial peptides seem to be a promising solution to this problem. Many studies aimed at understanding their mode of action were described in the past few years. The most studied group includes the linear, mostly ct-helical peptides. Although the exact mechanism by which they kill bacteria is not clearly understood, it has been shown that peptide-lipid interactions leading to membrane permeation play a role in their activity. Membrane permeation by amphipathic ct-helical peptides can proceed via either one of the Two mechanisms: (a) transmembrane pore formation via a "barrel-stave" mechanism; and bi membrane destruction/solubilization via a "carpet-like" mechanism. The purpose of this review is to summarize recent studies aimed at understanding the mode of action of linear a-helical antimicrobial peptides. This review which is focused on magainins, cecropins, and dermaseptins as representatives of the amphipathic a-helical antimicrobial peptides, supports the carpet-like rather the barrel-stave mechanism. That these peptides vary with regard to their length, amino acid composition, and net positive charge, but act via a common mechanism, may imply that other linear antimicrobial peptides that share the same properties also share the same mechanism (C) 1999 John Wiley & Sons, Inc.
A synthetic heptad repeat, SV-473, derived from Sendal virus fusion protein is a potent inhibitor of virus-cell fusion. In order to understand the mechanism of the inhibitory effect, we synthesized and fluorescently labeled SV-465, an extended version of SV-473 by one more heptad, its mutant peptide A(17,24)-SV-465, in which two heptadic leucines were substituted with two alanines, and its enatiomer D-SV-465, composed entirely of D-amino acids. Similar mutations in the homologous fusion protein of the Newcastle disease virus drastically reduced its activity. The data revealed that SV-465, but not A(17,24)-SV-465 or. its enantiomer, is highly active in inhibiting Sendai virus-induced hemolysis of red blood cells. None of the peptides interfere with the binding of virions to the target red blood cells as demonstrated by hemagglutinin assay. Fluorescence and circular dichroism (CD) spectroscopy indicated that: (i) only SV-465 could self-assemble in aqueous environment; (ii) only SV-465 could co-assemble with two other biologically active heptad repeats derived from Sendai virus fusion protein; (iii) SV-465 has a higher helical content than A(17,24)-SV-465 in solution, and (iv) all the peptides bind strongly to zwitterionic and negatively charged phospholipids. Polarized attenuated total reflection infrared spectroscopy revealed that they bound as monomers onto the surface of zwitterionic membranes with predominantly alpha-helical structures. The functional role of the amino acid 465-497 domain in Sendai virus-mediated membrane fusion is discussed in light of these findings.
The aim of this study was to elucidate the mechanism of membrane insertion and the structural organization of pores formed by Bacillus thuringiensis delta-endotoxin, We determined the relative affinities for membranes of peptides corresponding to the seven helices that compose the toxin pore-forming domain, their modes of membrane interaction, their structures within membranes, and their orientations relative to the membrane normal, In addition, we used resonance energy transfer measurements of all possible combinatorial pairs of membrane-bound helices to map the network of interactions between helices in their membrane-bound state. The interaction of the helices with the bilayer membrane was also probed by a Monte Carlo simulation protocol to determine lowest-energy orientations. Our results are consistent with a situation in which helices alpha 4 and alpha 5 insert into the membrane as a helical hairpin in an antiparallel manner, while the other helices lie on the membrane surface like the ribs of an umbrella (the "umbrella model"). Our results also support the suggestion that (alpha 7 may serve as a binding sensor to initiate the structural rearrangement of the pore-forming domain.
Recent studies demonstrated that a synthetic fusion peptide of HIV-1 self-associates in phospholipid membranes and inhibits HIV-1 envelope glycoprotein-mediated cell fusion, presumably by interacting with the N-terminal domain of gp41 and forming inactive heteroaggregates [Kliger, Y., Aharoni, A., Rapaport, D,, Jones, P., Blumenthal, R, & Shai, Y. (1997) J. Biol. Chem, 272, 13496-13505]. Here, we show that a synthetic all D-amino acid peptide corresponding to the N-terminal sequence of HIV-1 gp41 (D-WT) of HIV-1 associates with its enantiomeric wild-type fusion (WT) peptide in the membrane and inhibits cell fusion mediated by the HIV-1 envelope glycoprotein, D-WT does not inhibit cell fusion mediated by the HIV-2 envelope glycoprotein. WT and D-WT are equally potent in inducing membrane fusion, D-WT peptide but not WT peptide is resistant to proteolytic digestion. Structural analysis showed that the CD spectra of D-WT in trifluoroethanol/water is a mirror image of that of WT, and attenuated total reflectance-fourier transform infrared spectroscopy revealed similar structures and orientation for the two enantiomers in the membrane. The results reveal that the chirality of the synthetic peptide corresponding to the HIV-1 gp41 N-terminal sequence does not play a role in liposome fusion and that the peptides' chirality is not necessarily required for peptide-peptide interaction within the membrane environment. Furthermore, studies along these lines may provide criteria to design protease-resistant therapeutic agents against HIV and other viruses.
SV-201, a peptide derived from a conserved and potentially amphipathic region (amino acids 201-229) in the Sendai virus ectodomain, specifically inhibited virus-mediated hemolysis only when added to virions prior to their attachment to red blood cells. Sendai virus-mediated hemagglutinin assay in the presence of SV-201 demonstrated that the peptide does not disturb the binding of virions to the target red blood cells. A mutated peptide with 2 amino acids substitution, rendering the peptide neutral, was biologically inactive. A second mutant with 7 amino acids randomized at the N terminus keeping the hydrophobicity of the peptide unaltered was only slightly active. A hydrophobic peptide corresponding to the fusion peptide domain was also inactive. SV-201, the two mutants, and the fusion peptide bind similarly with high affinity to both negatively charged phosphatidylserine/phosphatidylcholine and zwitterionic phosphatidylcholine lipid vesicles, suggesting that the inhibitory effect is not due merely to membrane modulation. Fluorescence studies with rhodamine-labeled peptides and SV-201-induced inhibition assays, demonstrated that the SV-201 binding site is most probably located in the region corresponding to amino acids 201-229 of the Sendai virus fusion protein. The data presented here suggest that SV-201 disturbs a functional domain in the Sendai virus fusion protein, which is most probably associated with the assembly of the fusion protein and/or membrane apposition. The existence of homologous SV-201 regions in other viruses suggests that these regions may have a similar role, and their synthetic counterparts may act as inhibitors for the corresponding viruses.
We have detected a leucine zipper-like motif in the ectodomain of the Sendai virus fusion protein (aa 269-307) which is extremely conserved in the family of Sendai viruses, To find a possible role for this motif, we synthesized SV-269, a 39 amino acid peptide corresponding to this domain, and a mutant peptide, MuSV-269, with an amino acid pair interchanged their positions. The peptides were labeled with fluorescent probes at their N-terminal amino acid and functionally and structurally characterized, The data show that SV-269, but not MuSV-269, specifically binds Sendai virus. Expectedly, SV-269 is more active than the mutant MuSV-269 in inhibiting Sendai virus-mediated hemolysis. Fluorescence studies reveal that SV-269 assembles in aqueous solution, binds to zwitterionic PC and negatively-charged PS/PC vesicles, and assembles therein, Although MuSV-269 similarly binds to both types of vesicles, it only slightly assembles in solution and nor at all in membranes. Moreover, SV-269, but not MuSV-269, coassembles with the biologically-active heptad repeats SV-150 and SV-473 (Rapaport et al., 1995) in solution as revealed by fluorescence and circular dichroism (CD) spectroscopy, and with SV-150 within negatively-charged PS/PC and zwitterionic PC vesicles, Despite these differences, both SV-269 and MuSV-269 adopt similar secondary structures in 40% TFE and 1% SDS as revealed by CD spectroscopy, and disrupt the packing of the lipid bilayers to the same extent, as shown by the dissipation of diffusion potential, The role of this leucine zipper motif is discussed in terms of the assembly of the Sendai virus fusion protein in solution and within membranes, Since most of the heptadic leucines are also conserved in the corresponding domains of other paramyxoviruses such as rinderpest, measles, SV5, and parainfluenza, it may indicate a similar role of this domain in these viruses as well.
The antimicrobial activity of various naturally occurring microbicidal peptides was reported to result from their interaction with microbial membrane. In this study, we investigated the cytotoxicity of the hemolytic peptide dermaseptin S4 (DS4) and the nonhemolytic peptide dermaseptin S3 (DS3) toward human erythrocytes infected by the malaria parasite Plasmodium falciparum. Both DS4 and DS3 inhibited the parasite's ability to incorporate [H-3]hypoxanthine. However, while DS4 was toxic toward both the parasite and the host erythrocyte, DS3 was toxic only toward the intraerythrocytic parasite. To gain insight into the mechanism of this selective cytotoxicity, we labeled the peptides with fluorescent probes and investigated their organization in solution and in membranes. In Plasmodium-infected cells, rhodamine-labeled peptides interacted directly with the intracellular parasite, in contrast to noninfected cells, where the peptides remained bound to the erythrocyte plasma membrane. Binding experiments to phospholipid membranes revealed that DS3 and DS4 had similar binding characteristics. Membrane permeation studies indicated that the peptides were equally potent in permeating phosphatidylserine/phosphatidylcholine vesicles, whereas DS4 was more permeative with phosphatidylcholine vesicles. In aqueous solutions, DS4 was found to be in a higher aggregation state. Nevertheless, both DS3 and DS4 spontaneously dissociated to monomers upon interaction with vesicles, albeit with different kinetics. In light of these results, we propose a mechanism by which dermaseptins permeate cells and affect intraerythrocytic parasites.
The CytA toxin exerts its activity by the formation of pores within target cell membranes. However, the exact mechanism of pore formation and the structural elements that are involved in the toxic activity are yet to be determined, Recently, the structure of the highly similar CytB toxin was solved (Ei et al., 1996), and a beta-barrel was suggested as a possible structure of the pores. Due to the similarity between the toxins, the existence and positioning of alpha-helices and beta-sheets in CytA were predicted from the alignment of the sequences. Here peptides corresponding to beta 5, beta 6, and beta 7 strands, to a conserved nonhelical region of the CytA toxin (p(149-170)), to helices B and D, and to an analogue of helix A were synthesized, fluorescently labeled. and characterized, We found that, unlike helices A and C (Gazit and Shai, 1993), neither the beta-strand peptides nor helix B could interact with lipid membranes. whereas P149-170 and helix D bind the membrane weakly. Membrane permeation experiments suggested that CytA toxin exerts its activity by aggregation or several monomers, To learn about the structural elements that may mediate CytA oligomerization, the ability of the synthetic peptides to interact with membrane-bound CytA was studied, Helices A and C, but not the beta-strands, helix D, or a control peptide, caused a large increase in the fluorescence of membrane-bound fluorescein-labeled CytA, whereas helix B had only a slight erect, Moreover, the addition of rearranged helix A, a peptide with the same composition as helix A, but with only two pairs of amino acids rearranged, did not affect the fluorescence. The addition of unlabeled CytA also caused an increase in the fluorescence intensity, further demonstrating, the interaction between CytA monomers within the membrane, Taken together, our results provide further support for the suggestion that the CytA toxin self-assembles within membrane and that helices A and C are major structural eleme
This review describes the numerous and innovative methods used to study the structure and function of viral fusion peptides. The systems studied include both intact fusion proteins and synthetic peptides interacting with model membranes. The strategies and methods include dissecting the fusion process into intermediate stages, comparing the effects of sequence mutations, electrophysiological patch clamp methods, hydrophobic photolabelling, video microscopy of the redistribution of both aqueous and lipophilic fluorescent probes between cells, standard optical spectroscopy of peptides in solution (circular dichroism and fluorescence) and attenuated total reflection-Fourier transform infrared spectroscopy of peptides bound to planar bilayers. Although the goal of a detailed picture of the fusion pore has not been achieved for any of the intermediate stages, important properties useful for constraining the development of models are emerging. For example, the presence of alpha-helical structure in at least part of the fusion peptide is strongly correlated with activity; whereas, beta-structure tends to be less prevalent, associated with non-native experimental conditions, and more related to vesicle aggregation than fusion. The specific angle of insertion of the peptides into the membrane plane is also found to be an important characteristic for the fusion process. A shallow penetration, extending only to the central aliphatic core region, is likely responsible for the destabilization of the lipids required for coalescence of the apposing membranes and fusion. The functional role of the fusion peptides (which tend to be either nonpolar or aliphatic) is then to bind to and dehydrate the outer bilayers at a localized site; and thus reduce the energy barrier for the formation of highly curved, lipidic 'stalk' intermediates. In addition, the importance of the formation of specific, 'higher-order' fusion peptide complexes has also been shown. Recent crystallographic struct
The increase in infectious diseases and bacterial resistance to antibiotics has resulted in intensive studies focusing on the use of linear, alpha-helical, cytolytic peptides from insects and mammals as potential drugs for new target sites in bacteria. Recent studies with diastereomers of the highly potent cytolytic peptides, pardaxin and melittin, indicate that alpha-helical structure is required for mammalian cells lysis but is not necessary for antibacterial activity, Thus, hydrophobicity and net positive charge of the polypeptide might confer selective antibacterial lytic activity. To test this hypothesis, a series of diastereomeric model peptides (12 amino acids long) composed of varying ratios of leucine and lysine were synthesized, and their structure and biological function were investigated, Peptide length and the position of D-amino acids were such that short peptides with stretches of only 1-3 consecutive L-amino acids that cannot form an alpha-helical structure were constructed, Circular dichroism spectroscopy showed that the peptides do not retain any detectable secondary structure in a hydrophobic environment, This enabled examination of the sole effect of hydrophobicity and positive charge on activity, The data reveal that modulating hydrophobicity and positive charge is sufficient to confer antibacterial activity and cell selectivity, A highly hydrophobic diastereomer that permeated both zwitterionic and negatively charged phospholipid vesicles, lysed eukaryotic and prokaryotic cells, In contrast, a highly positively charged diastereomer that only permeated slightly negatively charged phospholipid vesicles had low antibacterial activity and could not lyse eukaryotic cells, In the boundary between high hydrophobicity and high positive charge, the diastereomers acquired selective and potent antibacterial activity, Furthermore, they were completely resistant to human serum inactivation, which dramatically reduces the activity of native antibacterial
The fusion domain of human immunodeficiency virus (HIV-1) envelope glycoprotein (gp120-gp41) is a conserved hydrophobic region located at the N terminus of the transmembrane glycoprotein (gp41). A V2E mutant has been shown to dominantly interfere with wild type envelope-mediated syncytium formation and virus infectivity. To understand this phenomenon, a 33-residue peptide (wild type, WT) identical to the N-terminal segment of gp41 and its V2E mutant were synthesized, fluorescently labeled, and characterized, Both peptides inhibited HIV-1 envelope-mediated cell cell fusion and had similar alpha-helical content in membrane mimetic environments, Studies with fluorescently labeled peptide analogues revealed that both peptides have high affinity for phospholipid membranes, are susceptible to digestion by proteinase-K in their membrane-bound state, and tend to self- and coassemble in the membranes. In SDS-polyacrylamide gel electrophoresis the WT peptide formed dimers as well as higher order oligomers, whereas the V2E mutant only formed dimers. The WT, but not the V2E mutant, induced liposome aggregation, destabilization, and fusion. Moreover, the V2E mutant inhibited vesicle fusion induced by the WT peptide, probably by forming inactive heteroaggregates. These data form the basis for an explanation of the mechanism by which the gp41 V2E mutant inhibits HIV-1 infectivity in cells when co expressed with WT gp41.
HIV-1 transmembrane envelope glycoprotein (gp41) has an unusually long cytoplasmic domain that has secondary associations with the inner leaflet of the membrane. Two highly amphiphatic alpha-helices in the cytoplasmic domain of gp41 have previously been shown to interact with lipid bilayers, We have detected a highly conserved leucine zipper-like sequence between the two alpha-helices. A peptide corresponding to this segment (residues 789-815, LLP-3) aggregates in aqueous solution, but spontaneously inserts into phospholipid membranes and dissociates into alpha-helical monomers. The peptide perturbs the bilayer structure resulting in the formation of micelles and other non-bilayer structures. Tryptophan fluorescence quenching experiments using brominated phospholipids revealed that the peptide penetrates deeply into the hydrophobic milieu of the membrane bilayer. The peptide interacts equally with zwitterionic and negatively-charged phospholipid membranes and is protected from proteolytic digestion in its membrane-bound state. Polarized attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy showed that the LLP-3 alpha-helix axis is about 70 degrees from the normal to the membrane plane, The ATR-FTIR CH2-stretching dichroic ratio increases when the peptide is incorporated into pure phospholipid membranes, further indicating that the peptide can deeply penetrate and perturb the bilayer structure. Integrating these data with what is already known about the membrane-associating features of adjacent segments, we propose a revised structural model in which a large portion of the cytoplasmic tail of the HIV-1 envelope glycoprotein is associated with the membrane.
Studies on lipid-peptide interactions of cytolytic polypeptides tend to emphasize the importance of the amphipathic alpha-helical structure for their cytolytic activity. In this study, diasetereomers of the bee venom melittin (26 a.a.), a non-cell-selective cytolysin, were synthesized and investigated for their structure and cytolytic activity toward bacteria and mammalian cells. Similarly to the findings with the diastereomers of the less cytolytic peptide pardaxin (33 a.a.) (Shai & Oren, 1996), the melittin diastereomers lost their alpha-helical structure, which abrogated their hemolytic activity toward human erythrocytes. However, they retained their antibacterial activity and completely lysed both Gram-positive and Gram-negative bacteria, as revealed by transmission electron microscopy. To understand the molecular mechanism underlying this selectivity, binding experiments utilizing the intrinsic tryptophan of melittin, tryptophan quenching experiments using brominated phospholipids, and membrane destabilization studies were done. The data revealed that the melittin diastereomers bound to and destabilized only negatively-charged phospholipid vesicles, in contrast to native melittin, which binds strongly to both negatively-charged and zwitterionic phospholipids. However, the partition coefficient, the depth of penetration into the membrane, and the membrane-permeating activity of the diastereomers with negatively-charged phospholipids were similar to those obtained with melittin. The results obtained do not support the formation of transmembrane pores as the mode of action of the diastereomers, but rather suggest that these peptides bind to the surface of the bacterial membrane, cover it in a ''carpet-like'' manner, and dissolve it like a detergent. The results presented here together with those obtained with the cytolytic peptide pardaxin suggest that the combination of hydrophobicity and net positive charge may be sufficient in the design of potent diastereom
The hydropathy plot of ROMK1, an inwardly rectifying K+ channel, suggests that the channel contains two transmembrane domains (M1 and M2) and a linker between them with significant homology to the H5 pore region of voltage-gated K+ channels. To gain structural information on the pore region of the ROMK1 channel, we used a spectrofluorimetric approach and characterized the structure, the organization state, and the ability of the putative membranous domains of the ROMK1 channel to self-assemble and coassemble within lipid membranes. Circular dichroism (CD) spectroscopy revealed that M1 and M2 adopt high alpha-helical structures in egg phosphatidylcholine small unilamellar vesicles and 40% trifluoroethanol (TFE)/water, whereas H5 is not alpha-helical in either egg phosphatidylcholine small unilamellar vesicles or 40% TFE/water. Binding experiments with 4-fluoro-7-nitrobenz-2-oxa-1,3-diazole (NBD)-labeled peptide demonstrated that all of the peptides bind to zwitterionic phospholipid membranes with partition coefficients on the order of 10(5) M(-1). Tryptophan quenching experiments using brominated phospholipids revealed that M1 is dipped into the hydrophobic core of the membrane. Resonance energy transfer (RET) measurements between fluorescently labeled pairs of donor (NBD)/acceptor (rhodamine) peptides revealed that H5 and M2 can self-associate in their membrane-bound state, but M1 cannot. Moreover, the membrane-associated nonhelical H5 serving as a donor can coassemble with the alpha-helical M2 but not with M1, and M1 can coassemble with M2. No coassembly was observed between any of the segments and a membrane-embedded alpha-helical control peptide, pardaxin. The results are discussed in terms of their relevance to the proposed topology of the ROMK1 channel, and to general aspects of molecular recognition between membrane-bound polypeptides.
The hydropathy plot of the inwardly rectifying ROMK1 K+ channel, which reveals two transmembrane and a pore region domains, also reveals areas of intermediate hydrophobicity in the N terminus (M0) and in the C terminus (post-M2). Peptides that correspond to M0, post-M2, and a control peptide, pre-M0, were synthesized and characterized for their structure, affinity to phospholipid membranes, organizational state in membranes, and ability to self-assemble and coassemble in the membrane-bound state. CD spectroscopy revealed thar both M0 and post-M2 adopt highly cu-helical structures in 1% SDS and 40% TFE/water, whereas pre-M0 is not cu-helical in either 1% SDS or 40% TFE/water. Binding experiments with NBD-labeled peptides demonstrated that both M0 and post-M2, but not pre-M0, bind to zwitterionic phospholipid membranes with partition coefficients of 10(3)-10(5) M(-1). A surface localization for both post-M2 and M0 was indicated by NBD shift, tryptophan quenching experiments with brominated phospholipids, and enzymatic cleavage. Resonance energy transfer measurements between fluorescently labeled pairs of donor (NBD)/ acceptor (rhodamine) peptides revealed that M0 and post-M2 can coassemble in their membrane-bound state, but cannot self-associate when membrane-bound. The results are in agreement with recent data indicating that amino acids in the carboxy terminus of inwardly rectifying K+ channels have a major role in specifying the pore properties of the channels (Taglialatela M, Wible BA, Caporaso R, Brown AM, 1994, Science 264:844-847; Pessia M, Bond CT, Kavanaugh MP, Adelman JP, 1995, Neuron 14:1039-1045). The relevance of the results presented herein to the suggested model for the structure of the ROMK1 channel and to general aspects of molecular recognition between membrane-bound polypeptides are also discussed.
The mechanism of leakage induced by surface active peptides is not yet fully understood. To gain insight into the molecular events underlying this process, the leakage induced by the peptide pardaxin from phosphatidylcholine/phosphatidylserine/cholesterol large unilamellar vesicles was studied by monitoring the rate and extent of dye release and by theoretical modeling. The leakage occurred by an all-or-none mechanism: vesicles either leaked or retained all of their contents. We further developed a mathematical model that includes the assumption that certain peptides become incorporated into the vesicle bilayer and aggregate to form a pore. The current experimental results can be explained by the model only if the surface aggregation of the peptide is reversible. Considering this reversibility, the model can explain the final extents of calcein leakage for lipid/peptide ratios of >2000:1 to 25:1 by assuming that only a fraction of the bound peptide forms pores consisting of M = 6 +/- 3 peptides. Interestingly, less leakage occurred at 43 degrees C than at 30 degrees C, although peptide partitioning into the bilayer was enhanced upon elevation of the temperature. We deduced that the increased leakage at 30 degrees C was due to an increase in the extent of reversible surface aggregation at the lower temperature. Experiments employing fluorescein-labeled pardaxin demonstrated reversible aggregation of the peptide in suspension and within the membrane, and exchange of the peptide between liposomes. In summary, our experimental and theoretical results support reversible surface aggregation as the mechanism of pore formation by pardaxin.
Cecropins are positively charged antibacterial peptides that act by permeating the membrane of susceptible bacteria. To gain insight into the mechanism of membrane permeation, the secondary structure and the orientation within phospholipid membranes of the mammalian cecropin P1 (CecP) was studied using attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy and molecular dynamics simulations. The shape and frequency of the amide I and II absorption peaks of CecP within acidic PE/PG multibilayers (phosphatidylethanolamine/phosphatidylglycerol) in a 7:3 (w/w) ratio (a phospholipid composition similar to that of many bacterial membranes), indicated that the peptide is predominantly alpha-helical. Polarized ATR-FTIR spectroscopy was used to determine the orientation of the peptide relative to the bilayer normal of phospholipid multibilayers. The ATR dichroic ratio of the amide I band of CecP peptide reconstituted into oriented PE/PG phospholipid membranes indicated that the peptide is preferentially oriented nearly parallel to the surface of the lipid membranes. A similar secondary structure and orientation were found when zwitterionic phosphatidylcholine phospholipids were used. The incorporation of CecP did not significantly change the order parameters of the acyl chains of the multibilayer, further suggesting that CecP does not penetrate the hydrocarbon core of the membranes. Molecular dynamics simulations were used to gain insight into possible effects of transmembrane potential on the orientation of CecP relative to the membrane. The simulations arrear to confirm that CecP adopts an orientation parallel to the membrane surface and does not insert into the bilayer in response to a cis positive transmembrane voltage difference. Taken together, the results further support a ''carpet-like'' mechanism, rather than the formation of transmembrane pores, as the mode of action of CecP According to this model, formation of a layer of peptide monome
Increasing evidence suggests that membrane-embedded hydrophobic segments can interact within the phospholipid milieu of the membrane with varying degrees of specificity and thus contribute to the folding and oligomerization of proteins. We have used synthetic peptides corresponding to segments from the hydrophobic core of the Shaker potassium channel as a model system to study interactions between membrane-embedded segments. Three synthetic segments of the Shaker K+ channel, comprising the hydrophobic S2, S3, and S4 sequences, were used, and their secondary structure, their interactions with, and orientation within phospholipid membranes were examined. Secondary structure studies revealed that though S3 and S4 both adopt certain fractions of alpha-helical structures in membrane mimetic environments, the alpha-helical content of S3 is lower. Both S3 and S4 bind strongly to zwitterionic phospholipids, with partition coefficients in the order of 10(4) and 10(5) M(-1). ATR-FTIR studies showed. that while the S4 peptide is oriented parallel to the membrane surface, S3 tends to a more transmembranal orientation. Enzymatic cleavage experiments demonstrated that the presence of S3 induces some change in the proteolytic accessibility of the S4 segment. Resonance energy transfer measurements, done in high lipid/peptide molar ratios, revealed that S3 and S4 cannot self-associate in zwitterionic phospholipid vesicles but can associate with each other and with the 52 segment of the channel. Furthermore, S3 does not interact with the homologous S4 region from the first repeat of the eel sodium channel, demonstrating specificity in the interactions. These results are in line with data indicating that functionally important interactions indeed exist between the negatively charged S2 and S3 regions and the positively charged S4 region [Papazian, D. M., ct al. (1995) Neuron 14, 1293-1301; Planells-Cases, R., et al. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 9422-9426]. From a broade
Pardaxin, a 33-amino-acid pore-forming polypetide toxin isolated from the Red Sea Moses sole Pardachirus marmoratus, has a helix-hinge-helix structure. This is a common structural motif found both in antibacterial peptides that can act selectively on bacterial membranes (e.g., cecropin), and in cytotoxic peptides that can lyse both mammalian and bacterial cells (e.g., melittin). Herein we show that pardaxin possesses a high antibacterial activity with a significantly reduced hemolytic activity towards human red blood cells (hRBC), compared with melittin. Its potency is comparable to that of other known native antibacterial peptides such as magainin, cecropins and dermaseptins. To determine the structural features responsible for the selective hemolytic and antibacterial activities, and the structural requirements for a high antibacterial activity, 8 truncated and modified pardaxin analogues were synthesized and structurally and functionally characterized. Each peptide was synthesized with a free carboxylate or amino group (i.e., aminated form) at its C-terminus. The aminated form of pardaxin has both high hemolytic and antibacterial activity. A truncated analogue, with 11 amino acids removed from the C-terminal domain, had dramatically reduced hemolytic activity. However, the aminated form of this analogue was significantly more potent than pardaxin against most bacteria tested, suggesting that the C-terminal tail of pardaxin is responsible for non-selective activity against erythrocytes and bacteria. Furthermore, a positive charge added to its N-terminus significantly increased its antibacterial activity and abolished its low hemolytic activity. The 22-amino-acid C-terminal domain and the short 11-amino-acid N-terminal domain were, in their aminated forms, active only against gram-positive bacteria. Secondary-structure determination using circular dichroism spectroscopy revealed that all the aminated analogues had 25-80% more alpha-helical content in 40% CF3CH2O
IsK is a 14.5-kDa type III membrane glycoprotein which induces slowly activating K+ and Cl- currents when expressed in Xenopus oocytes and HEK 293 cells. Recently, mutagenesis experiments identified amino- and carboxyl-terminal domains of IsK: as critical for induction of Cl- and K+ currents, respectively, This hypothesis was tested by examining effects of synthetic IsK hydrophilic peptides on untreated Xenopus oocytes. In agreement with IsK membrane topology, we show here that peptides derived from carboxyl and amino termini are sufficient to activate slow K+ and Cl- channels whose biophysical and pharmacological characteristics are similar to those exhibited by the native IsK protein. That data provide further evidence that IsK is a regulatory subunit of pre-existing silent channel complexes rather than a channel per se.
An amphipathic alpha-helical structure is considered to be a prerequisite for the lytic activity of most short linear cytolytic polypeptides that act on both mammalian cells and bacteria, This structure allows them also to exert diverse pathological and pharmacological effects, presumably by mimicking protein components that are involved in membrane-related events, In this study D-amino acid-incorporated analogues (diastereomers) of the cytolysin pardaxin, which is active against mammalian cells and bacteria, were synthesized and structurally and functionally characterized, We demonstrate that the diastereomers do not retain the alpha-helical structure, which in turn abolishes their cytotoxic effects on mammalian cells. However, they retain a high antibacterial activity, which is expressed in a complete lysis of the bacteria, as revealed by negative staining electron microscopy. The disruption of the alpha-helical structure should prevent the diastereomer analogues from permeating the bacterial wall by forming transmembrane pores but rather by dissolving the membrane as a detergent, These findings open the way for a new strategy in developing a novel class of highly potent antibacterial polypeptides for the treatment of infectious diseases, due to the increasing resistance of bacteria to the available antibacterial drugs.
Neuropeptide Y (NPY) and polypeptide YY (PYY) are two ubiquitous neuropeptides, found in brain and intestines, respectively, where they exert important regulatory functions, In this study, a new member of the YY family recently isolated from amphibian skin, skin-PW (SPYY), is reported to inhibit irreversibly the proliferation of a broad spectrum of pathogenic microorganisms, NPY and PW are shown to be endowed with the same activity, Their potency is similar to that of other antibacterial peptides which have been shown to exert their function by disintegrating the bacterial membrane, These findings and the fact that the C-terminal alpha-helical domain SPYY14-36, which is highly conserved among the family members, was responsible for killing microorganisms and for permeation of phospholipid vesicles, suggested that the antibiotic activity may emerge via a membrane permeation mechanism, These findings also raise the question whether NPY and PYY exert in vivo a similar function in mammals.
Integral membrane proteins have recently been shown to recognize and interact with other proteins within the membrane, either mimicking or altering their function, and with the lipid bilayer itself, resulting in a reorganization of native membrane protein. Membrane proteins are difficult to study using conventional methods such as X-ray crystallography, and so both synthesized and naturally occurring segments of membrane proteins have been used in the assessment of the mechanisms involved in their structure and organization.
A series of peptides derived from three domains within the fusion protein of Sendai virus was synthesized and examined for their potential to inhibit the fusion of the virus with human red blood cells. These domains include the 'fusion peptide) and two heptad repeats, one adjacent to the fusion peptide (SV-163) and the other to the transmembrane domain (SV-473). Of all the peptides tested, only SV-473 was highly inhibitive. Using fluorescently-labelled peptides, the mechanism through which the SV-473 peptide inhibits the haemolytic activity of the virus was investigated, The results suggest that interactions of the active peptide with virion elements and lipid membranes are involved, Since it has recently been found that synthetic peptides corresponding to putative coiled-coil domains of the human immunodeficiency virus (HIV) type 1 transmembrane protein gp41 are potent inhibitors of HIV, we discuss the general property of virus-derived coiled-coil peptides as inhibitors of viral infection.
Cecropins are positively charged antibacterial polypeptides that were originally isolated from insects. Later on a mammalian homologue, cecropin P1 (CecP), was isolated from pig intestines. While insect cecropins are highly potent against both Gram-negative and Gram-positive bacteria, CecP is as active as insect cecropins against Gram-negative but has reduced activity against Gram-positive bacteria. To gain insight into the mechanism of action of CecP and the molecular basis of its antibacterial specificity, the peptide and its proline incorporated analogue (at the conserved position found in insect cecropins), P-22-CecP, were synthesized and labeled on their N-terminal amino acids with fluorescent probes, without significantly affecting their antibacterial activities. Fluorescence studies indicated that the N-terminal of CecP is located on the surface of phospholipid membranes. Binding experiments revealed that CecP binds acidic phosphatidylserine/phosphatidylcholine (PS/PC) vesicles better than zwitterionic PC vesicles, which correlates with its ability to permeate the former better than the latter. The shape of the binding isotherms suggest that CecP, like insect cecropin, binds phospholipids in a simple, noncooperative manner. However, resonance energy transfer (RET) measurements revealed that, unlike insect cecropins, CecP does not aggregate in the membrane even at relatively high peptide to lipid ratios. The stoichiometry of CecP binding to vesicles suggests that amount of CecP sufficient to form a monolayer causes vesicle permeation. In spite of the incorporation of the conserved proline in P-22-CecP, the analogue has reduced antibacterial activity, which correlates with its reduced alpha-helical structure and its lower partitioning and membrane permeating activity with phospholipid vesicles. Taken together, our results support a mechanism in which CecP disrupts the structure of the bacterial membrane by (i) binding of peptide monomers to the acidic surfac
The hydropathy plot of the alpha subunit of the voltage-gated Naf channel reveals four homologous repeats, each of which is homologous to Shaker type K+ channel monomer and contains six putative transmembrane segments and a hydrophobic segment within the loop connecting transmembrane segments S5 and S6. Current models predict that the four homologous segments [designated H5 or P regions (PR)I from the S5-S6 loop of each repeat lie in the aqueous pore. Peptides corresponding to the P regions of the four domains of the Electrophorus electricus (eel) Na+ channel (25-36 aa long, designated as PR-I, PR-II, PR-III, and PR-TV) and a 23-mer preceding PR-II (designated pre-PR-II) were synthesized and flourescently labeled. The segments were then structurally and functionally characterized for their interaction with phospholipid membranes. Although the sequences of the four P regions are significantly different, they all bind to zwitterionic phospholipid membranes with similar partition coefficients (similar to 10(4) M(-1)). The pre-PR-II does not bind membranes at all. Resonance energy transfer measurements, between donor/acceptor-labeled pairs of peptides, revealed that besides the PR-I/PR-III pair, all other pairs form heteroaggregates but do not coassemble with unrelated membrane-bound peptide. Circular dichroism (CD) spectroscopy revealed that PR-I, PR-II, and PR-III adopt similar partial a-helical structures (similar to 30%) in 40% trifluoroethanol and in solutions of 1% sodium dodecylsulfate (SDS). The PR-IV (36 aa) adopts similar to 18% alpha-helical structure, and pre-PR-II gives a low CD signal. These findings are in line with proposed models in which the P regions are packed in close proximity in the lumen of the hydrophobic core of the channel. Furthermore, the finding that the PRs adopt similar partial alpha-helical structures in two different hydrophobic environments might suggest that partial alpha-helical structures also exist in the native channel as propo
The pore-forming domain of Bacillus thuringiensis insecticidal CryIIIA delta-endotoxin contains two helices, alpha 5 and alpha 7, that are highly conserved within all different Cry S-endotoxins, To gain information on the mode of action of delta-endotoxins, we have used a spectrofluorimetric approach and characterized the structure, the organization state, and the ability to self-assemble and to co-assemble within lipid membranes of alpha 5 and alpha 7. Circular dichroism (CD) spectroscopy revealed that alpha 7 adopts a predominantly alpha-helical structure in methanol, similar to what has been found for alpha 5, and consistent with its structure in the intact molecule, The hydrophobic moment of alpha 7 is higher than that calculated for alpha 5; however, alpha 7 has a lesser ability to permeate phospholipids as compared to alpha 5. Binding experiments with 7-nitrobenz-2-oxa-1,3-diazole-4-yl (NBD)-labeled peptide demonstrated that alpha 7 binds to phospholipid vesicles with a partition coefficient in the order of 10(4) M(-1) similar to alpha 5, but with reduced kinetics and in a noncooperative manner, as opposed to the fast kinetics and cooperativity found with (alpha 5. Resonance energy transfer measurements between fluorescently labeled pairs of donor (NBD)/acceptor (rhodamine) peptides revealed that, in their membrane bound state, alpha 5 self-associates but alpha 7 does not, and that (alpha 5 coassembles with alpha 7 but not with an unrelated membrane bound alpha-helical peptide, Furthermore, resonance energy transfer experiments, using alpha 5 segments, specifically labeled in either the Nor C-terminal sides, suggest a parallel organization of alpha 5 monomers within the membranes, Taken together the results are consistent with an umbrella model suggested for the pore forming activity of delta-endotoxin (Li, J., Caroll, J., and Ellar, D. J. (1991) Nature 353, 815-821), where alpha 5 has transmembrane localization and may be part of the pore lining segment(s)
The amphipathic helix, in which hydrophobic and hydrophilic residues are grouped on opposing faces, is a structural motif found in many peptides and proteins that bind to membranes. One of the physical properties of membranes that can be altered by the binding of amphipathic helices is membrane monolayer curvature strain. Class A amphipathic helices, which are present in exchangeable plasma lipoproteins, can stabilize membranes by reducing negative monolayer curvature strain, proline-punctuated class A amphipathic helical segments are particularly effective in this regard. This property is suggested to be associated with some of the beneficial biological effects of this protein. On the other hand, lytic amphipathic helical peptides can act by increasing negative curvature strain or by forming pores composed of helical clusters. Thus, different amphipathic helical peptides can be membrane stabilizing or be lytic to membranes, depending on the structural motif of the helix, which in rum determines the nature of its association with membranes. Features of these peptides that are responsible for their specific properties are discussed. (C) 1995 John Wiley & Sons, Inc.
A peptide with a sequence corresponding to the highly conserved alpha-5 segment of the Cry delta-endotoxin family (amino acids 193-215 of Bacillus thuringiensis CryIIIA [Gazit and Shai (1993) Biochemistry 32, 3429-3436]), was investigated with respect to its interaction with insect membranes, cytotoxicity in vitro towards Spodoptera frugiperda (Sf-9) cells, and its propensity to form ion channels in planar lipid membranes (PLMs). Selectively labelled analogues of alpha-5 at either the N-terminal animo acid or the epsilon-amine of its lysine, were used to monitor the interaction of the peptides with insect membranes. The fluorescent emission spectra of the 7-nitrobenz-2-oxa-1,3-diazole-4-yl (NBD)-labelled alpha-5 peptides displayed a blue shift upon binding to insect (Spodoptera littoralis) mid-gut membranes, reflecting the relocation of the fluorescent probes to an environment of increased apolarity, i.e. within the lipidic constituent of the membrane. Moreover, midgut membrane-bound NBD-labelled alpha-5 peptides were protected from enzymic proteolysis. Functional characterization of alpha-5 has revealed that it is cytotoxic to Sf-9 insect cells, and that it forms ion channels in PLMs with conductances ranging from 30 to 1000 pS. A proline-substituted analogue of alpha-5 is less cytolytic and slightly more exposed to enzymic digestion. Molecular modelling utilizing simulated annealing via molecular dynamics suggests that a transbilayer pore may be formed by alpha-5 monomers that assemble to form a left-handed coiled coil of approximately parallel helices. These findings further support a role for alpha-5 in the toxic mechanism, of delta-endotoxins, and assign alpha-5 as one of the transmembrane helices which form the toxic pore. The suggested role is consistent with the recent finding that cleavage of CryIVB delta-endotoxin in a loop between alpha-5 and alpha-6 is highly important for its larvicidal activity [Angsuthanasombat, Crickmore and Ellar (1993) FEMS Micr
Peptide-induced vesicle fusion is frequently accompanied by leakage of vesicle contents. To determine the correlation between these two processes, we studied the interaction of the amphiphilic peptide pardaxin and two of its analogues with large unilamellar vesicles composed of phosphatidylserine. A pardaxin analogue with a positive charge at both its C- and N-termini induced significantly more fusion but less leakage than the parent peptide. Fusion and leakage were studied with large unilamellar vesicles of two sizes. Aggregation of vesicles was found to be the rate-limiting step in the overall fusion process induced by the peptides. The rates and extents of fusion, determined by membrane mixing, increase in vesicle size, and mixing of aqueous contents, were significantly enhanced in the presence of 2.5-5 mM Mg2+ which promoted vesicle aggregation. Model calculations showed that increasing the peptide to lipid ratio resulted in a parallel increase in the fusion rate constants. As the average vesicle diameter was increased, the extent of leakage was enhanced, as more peptide molecules were bound to each vesicle. The mode of leakage induced by the peptides was also investigated. Our results suggest that the potency of a peptide to induce vesicle fusion is not necessarily associated with its capacity to induce leakage, and we further elucidate how these capacities depend on the structures of the peptides.
Dermaseptins are 27-34 amino acid antimicrobial peptides that irreversibly inhibit growth of pathogenic filamentous fungi, in addition to their ability to inhibit the growth of bacteria, yeasts, and protozoa. Synthetic peptides, with sequences corresponding to dermaseptin-b (DS-b) and its N-terminal extended precursor form dermaseptin-B (DS-B), were synthesized and investigated with respect to their spectrum of antimicrobial activity and their mode of interaction with model membranes composed of PS or PC/PS phospholipids. We found that DS-B is much more potent than DS-b against all microorganisms tested. Furthermore, despite significant structural identity between DS-B and DS-S (Pouny et al., 1992), only the former is highly effective at inhibiting the growth of filamentous fungi. The peptides were labeled selectively at their N-terminal amino acid with either 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or rhodamine fluorescent probes, which facilitated the determination of their partition coefficients with phospholipid membranes and their organization in their membrane-bound state. The partition coefficients of DS-B are 10-fold higher than those of DS-b and DS-S, with both acidic and zwitterionic phospholipid vesicles. This may explain the ability of DS-B to permeate both types of vesicles efficiently. Furthermore, while both DS-b and DS-B interact with phospholipid membranes in a noncooperative manner, they are self-associated in their membrane-bound state. This noncooperative binding probably prevents aggregation of the peptides on the surface of outer bacterial membranes, and assists them in efficiently diffusing into the inner target membranes. The exceptional property of DS-B to bind strongly to phospholipid membranes and to form small bundles correlates with its high potential to kill yeast and filamentous fungi. As a molecular model, dermaseptins may be of potential interest in drug design, particularly in antifungal warfare.
Cecropin B2 (CecB) is a 35 amino acid residue, antibacterial peptide that was isolated from the hemolymph and cuticular matrix of the silkworm, Bombyx mori. Synthetic peptides with sequences corresponding to CecB and its truncated analogue, [3-->35] CecB, were synthesized and selectively labeled at their N-terminal amino acids with either 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or rhodamine fluorescent probes. Utilization of these probes facilitated study of the interaction of cecropin with model phospholipid membranes at a high lipid/peptide molar ratio (similar to 3000:1), permitting investigation of the initial steps involved in this process. The surface partition coefficient of CecB, derived from binding isotherms of the NBD-labeled peptide, was 10-fold higher with acidic phospholipids than with zwitterionic ones, which correlates with the high efficiency of CecB and its analogues in permeating acidic phospholipid vesicles. Furthermore, a direct correlation was found between the antibacterial activity of CecB or its truncated analogues and the ability of their Rho-labeled analogues to interact with bacteria and human red blood cells. We propose that CecB binds phospholipid membranes preferentially as monomers lying on the surface, rather than cooperatively as bundles that form transmembranal pores via a ''barrel stave'' mechanism. This is based on the following: (i) the linearity of CecB's binding isotherms; (ii) the low energy transfer between membrane-embedded donor and acceptor-labeled CecB, even in the presence of a transmembrane potential; (iii) the surface localization of CecB's N-terminus; (iv) the need for more than 100 peptide molecules per phospholipid vesicle to induce initial ion leakage; and (v) the fact that CecB is a highly positively charged amphipathic alpha-helix, and therefore it is not expected to transverse the membrane as a monomer. We speculate that the non-cooperative binding of the peptides on the outer surface of the bacteria (i.e.,
Current models of voltage-activated K+ channels predict that the channels are formed by the coassembly of four polypeptide monomers, each of which consists of six transmembrane segments (S1-S6) and long terminal domains. The aqueous pores are thought to be composed of the conserved H-5 regions contributed by four monomers. In this study, two putative membrane-embedded segments of the Shaker K+ channel were synthesized. One segment corresponds to the putative, transmembrane helix S-2 (amino acids 275-300), and the other corresponds to the highly conserved 12 amino acid residues within the H-5 region [amino acids 432-443, designated (12)H-5)]. Structural and functional characterization at elevated lipid/peptide molar ratios (> 3000:1) was performed on the two segments, as well as on a previously synthesized 21 amino acid long peptide with a sequence resembling the entire H-5 region (designated (21)H-5) (Peled and Shai, 1993). Circular dichroism spectroscopy revealed that S-2 adopts predominantly alpha-helical structure in both trifluoroethanol and 35 mM SDS (78% or 99%, respectively), while (12)H-5 and (21)H-5 adopt low alpha-helical structure only in the presence of 35 mM SDS. Functional characterization demonstrated that S-2 and (12)H-5 segments bind to zwitterionic phospholipids, with partition coefficients on the order of 10(4) M(-1). Resonance energy transfer measurements, between donor/acceptor-labeled pairs of peptides, revealed that the peptides self-associate in their membrane-bound state, which may correlate with the existence of functional interactions between the conserved (12)H-5 regions of different subunits of K+ channels (Kirsch et al., 1993). Furthermore, membrane-bound (12)H-5 or (21)H-5 associates with membrane-bound S-2, but does not associate with unrelated, membrane-bound alpha-helical peptides. These results demonstrate molecular recognition between transmembrane segments of the Shaker K+ channel that might contribute to the oligomerization a
IsK, also referred to as minK, is a membrane protein consisting of 130 amino acids and localized mainly in epithelial cells but also in human T lymphocytes. Depending on the cRNA concentration that was injected into Xenopus oocytes, IsK and its truncated forms can induce either a K+ current alone or both K+ and Cl- currents [Attali et al. (1993) Nature 365, 850-852]. To obtain information on the secondary structure and the topology of IsK in a membrane-bound state, the synthesis, fluorescent-labeling, and structural and functional characterization of five polypeptides of 20-63 amino acids within the rat IsK protein were examined. The alpha-helical content of the segments, assessed in methanol using circular dichroism, suggests that both the N-terminal and transmembrane segments of IsK adopt alpha-helical structures. Binding experiments and the blue shift of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled peptides suggest that while both the alpha-helical transmembrane segment and the N-terminal of IsK are located within the lipid bilayer, the linking segment between the two segments lies on the surface of the membrane. The fluorescence energy transfer, between donor and acceptor-labeled truncated IsK, suggests that it aggregates within phospholipid membranes. Although a protein whose sequence is similar to that of truncated IsK can induce K+ channel activity when expressed in Xenopus oocytes, the inability of a truncated IsK to form functional K+ channels in planar lipid membranes supports increasing evidence that the protein alone cannot form a K+ channel.
A peptide representing the NH2-terminal (33 amino acid residues) of the fusion protein (F) of Sendai virus, as well as its Gly(12) --> Ala(12) mutant, were synthesized, fluorescently labeled, and spectroscopically and functionally characterized. Peptide-induced vesicle fusion was demonstrated by a combination of increased visible absorbance, lipid mixing assay, and electron microscopy. Both peptides, with the mutant peptide being significantly more potent, were shown to induce membrane fusion and bilayer perturbation of negatively charged phospholipid vesicles. These results are consistent with a previous study that showed that a similar mutation in the homologous NH2-terminal segment of simian virus 5 greatly enhanced syncytium formation (Horvath, C. M., and Lamb, R. A. (1992) J. Virol. 66, 2443-2455). Circular dichroism spectroscopy revealed similar high alpha-helical contents of both peptides in methanol and in trifluoroethanol. Using fluorescently labeled peptide analogues we found that (i) the peptides' membrane partition coefficients are in the range of 10(5) M(-1); (ii) the NH2 terminus of the wild-type peptide is located within the lipid bilayer, whereas that of the variant peptide lies on the surface; and (iii) both peptides tend to self-associate in their membrane-bound state. The results support a model in which an alpha-helical secondary structure and self-aggregation of peptides are necessary conditions for membrane fusion. The observed differences in the peptides' fusogenic abilities are hypothesized to result from differences in the peptides' degree of penetration into the membrane, induction of membrane destabilization, and ability to cause vesicles to aggregate. The data support Sendai virus-cell fusion models in which the fusion peptide plays a crucial role in fusion induction by destabilizing the bilayer and by triggering the association of viral fusion protein molecules.
The results of the various studies describing the mechanism involved in pore formation by pardaxin and some of its analogues, support a 'barrel-stave' model (Ehrenstein amd Lecar, 1977). In this model pardaxin exerts its activity via three successive steps: (i) a fast binding step (as reflected by the rapid increase of NBD fluorescence in the presence of vesicles); (ii) insertion of peptides into the lipid bilayer; and (iii) the monomers aggregate into a barrel-like formation in which a central aqueous pore surrounded by proteins is formed. This pore increases in diameter through the progressive recruitment of additional monomers. Both the fluorescence energy transfer (FET) studies and the observation of a significant difference in the increase of NBD fluorescence, depending on which terminal was labelled by the fluorophore, support a model by which aggregates are formed in an ordered parallel manner, where the C-terminus is more exposed to the aqueous phase.
The Bacillus thuringiensis var. israelensis (Bti) cytolytic toxin is hypothesized to exert its toxic activity via pore formation in the cell membrane as a result of the aggregation of several monomers. To gain insight into the toxin's mode of action, 2 putative hydrophobic 22 amino acid peptides were synthesized and characterized spectroscopically and functionally. One peptide corresponded to the putative amphiphilic alpha-helical region (amino acids 110-131, termed helix-2), and the other to amino acids 50-71 (termed helix-1) [Ward, E. S., Ellar, D. J., & Chilcott, C. N. (1988) J. Mol. Biol. 202, 527-5351 of the toxin. Circular dichroism spectroscopy revealed that both segments adopt high alpha-helical content in a hydrophobic environment,in agreement with previous models. To monitor peptide-lipid and peptide-peptide interactions, the peptides were labeled selectively with either 7-nitro-2,1,3-benzoxadiazol-4-yl (NBD) (to serve as donor) or tetramethylrhodamine (to serve as an acceptor), at their N-terminal amino acids. Both segments bind strongly to small unilamellar vesicles, composed of zwitterionic phospholipids, with surface partition coefficients on the order of 10(4) M-1. The shape of the binding isotherms indicates that helix-2 forms large aggregates within phospholipid membranes. Resonance energy transfer experiments demonstrated that the segments self-associate and interact with each other, but do not associate with unrelated membrane-bound peptides. Functional characterization demonstrated that helix-2 permeates phospholipid SUV with a potency similar to that of naturally occurring pore-forming peptides. Thus, the results support a role for helices-1 and -2 in the assembly and in the pore formation by Bti toxin.
The voltage-activated K+ channels are assumed to be formed by the coassembly of four polypeptide monomers. Each of these monomers is postulated to consist of six transmembrane segments (S1 to S6), and long N- and C-terminal domains. The highly conserved linker, H-5, between the fifth and the sixth transmembrane segments, is hypothesized to line the lumen of the K+ channel formed by the bundle of the transmembrane segments of the monomers. Here in we utilize the spectrofluorometric approach and investigate the interaction with phospholipid membranes of fluorescently-labeled synthetic peptides, whose sequences are derived from the H-5 region. Binding experiments reveal that the peptides can strongly bind to phospholipid membranes with partition coefficients on the order of 10(4) M-1. However, a truncated peptide without four amino acids within the most conserved region (amino acids 432-435) did not bind to the membranes at all. Moreover, the single substitution of a conserved tryptophan at position 435 to serine reduced the partition coefficient of the peptide approximately 5-fold, which may account for a mutated K+ channel with this substitution not producing functional channels (Yool & Schwarz, 1991). Structural characterization using circular dichroism spectroscopy (CD) reveals that H-5 can partially adopt an alpha-helix structure in hydrophobic environments. Resonance energy transfer (RET) experiments reveal that the H-5-derived segments can self-assemble within the membrane but cannot coassemble with other unrelated membrane-bound peptides. The results herein support the hypothesis that H-5 segments are packed in close proximity and might participate in mediating the appropriate assembly of the core region of K+ channel monomers.
Synthetic calmodulin-binding (CaM-binding) peptides (CBPs) representing CaM-binding domains of Ca2+/CaM-dependent enzymes have been reported to interfere with the activity of the melanocyte-stimulating hormone (MSH) receptor function in melanoma cells [Gerst, J. E., & Salomon, Y. (1988) J. Biol. Chem. 263, 7073-7078]. We postulated that membrane lipids may play an important role in the mode of action of CBPs on cells. We therefore tested the ability of CBPs to interact with membrane bilayers. Using artificial phospholipid vesicles, or M2R melanoma cells and cell membranes derived therefrom, as models, we report here that synthetic peptides representing the CaM-binding domains of skeletal muscle myosin light chain kinase (M5) and the human erythrocyte calcium pump (C28W), as well as other CBPs, interact with lipid bilayers and cell membranes. Significant interactions of CBPs with the lipid bilayer were detected in both model systems. M5 and C28W were found to partition into the lipid bilayer of melanoma cell membranes and soybean lecithin vesicles, and surface partition constants obtained (for the liposome model) were in the range 10(3)-10(4) M-1. In addition, C28W and its N-modified NBD derivative were found to inhibit [I-125]iodo-[Nle4,D-Phe7]alphaMSH binding to cultured M2R melanoma cells. These and other CBPs were also found to induce the release of cations and calcein from liposomes, suggesting that the interaction of CBPs with the lipid bilayer increases membrane permeability. Nonrelevant peptides used as controls were found ineffective. Melittin, a bee venom derived CBP, and pardaxin, a shark-repellent neurotoxin, both membrane-permeating peptides, were in comparison more potent than the enzyme-derived CBPs that were not lytic when applied to cells. It is proposed that the tested CBPs act as permeators that partition into the lipid bilayer of the cell membrane, thereby also promoting their interaction with hydrophobic domains of membrane proteins such as th
The fusogenic properties of, the neurotoxin pardaxin and eight of its analogues with small unilamellar vesicles (SUV), composed of egg phosphatidylcholine and phosphatidylserine (PC/PS), were investigated. Fusion was demonstrated by a lipid-mixing assay and by an increase in vesicle size as revealed by electron microscopy. The lipid-mixing assay was performed at either neutral (pH 6.8) or acidic (pH 4.5) conditions, in solutions containing either high or low salt concentrations. A low level of fusion could be induced at neutral pH only by pardaxin derivatives with amino groups at both the peptide's backbone and N-terminus. However, a marked enhancement in the.fusogenic activity occurred when amino groups were present also in the C-terminus. Pardaxin analogues in which amino groups were substituted by carboxylic groups induced elevated levels of fusion only at high salt concentrations where enhancement of aggregation occurs, and acidic pH, which increased alpha-helicity. The influence of mutual interactions between pardaxin's analogues possessing complementary charges on the lipid-mixing process was also studied. At neutral pH and high salt, an inactive acidic analogue increased the fusogenic activity of a complementary-charged basic peptide. However, such mutual interactions at low salt concentrations reduced the fusogenic activity of the pardaxin analogues. Analogues containing D-amino acids were not fusogenic, thus demonstrating the structural specificity of these observations. The results indicate that the charge, alpha-helical structure, and aggregation of peptide monomers play an important role in the fusogenic ability of polypeptides.
One of the most conserved sequences in various delta-endotoxins is the 30 amino acid long block 1. Block I of cryIIIA delta-endotoxin contains a 23 amino acid amphiphilic alpha-helix termed alpha5. The potential involvement of this alpha5 helix in the toxic mechanism of delta-endotoxin was examined. For this purpose, a peptide corresponding to the alpha5 segment and its proline incorporated analogue (P-alpha5) were synthesized and characterized. The alpha-helical content of the peptides, assessed in methanol by circular dichroism (CD), was 58% and 24% for alpha5 and P-alpha5, respectively. To monitor the interaction of alpha5 peptides with phospholipid membranes, they were selectively labeled at their N-terminal amino acids with the fluorescent probes 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or carboxyfluorescein. Fluorometric studies allowed the calculation of membrane surface partition constants, which were about 10(4) M-1 for both alpha5 and P-alpha5, and revealed that their N-terminals are located within the lipid bilayers. The shape of the binding isotherms indicated that alpha5 aggregated in both zwitterionic and acidic vesicles. Functional characterization of the alpha5 peptides was determined by assessing their ability to dissipate a diffusion potential from sonicated small unilamellar vesicles (SUV) composed of zwitterionic or acidic phospholipids and to lyse human erythrocytes. alpha5 was much more active than P-alpha5 in both assays. Moreover, membrane-bound alpha5 was more protected from enzymatic proteolysis than P-alpha5. The results reveal a good correlation between the structure and function of alpha5 and experimentally support the hypothesis that alpha5 is a structural component of the pores formed by delta-endotoxins via aggregation of amphiphilic alpha-helices.
MinK (I(sk)) is a voltage-dependent K+ channel whose gene has been recently cloned and which consists of 130 amino acids [Takumi, T., Ohkubo, H., & Nakanishi, S. (1988) Science 242, 1042-1045]. The protein contains one putative transmembrane segment by hydropathy analysis. Whether this putative transmembrane segment is involved in the function of the protein was studied. A 32 amino acid peptide (residues 41-72) with the sequence SKLEALYILMVLGFFGFFTLGIMLSYIRSKKL, containing the hypothesized transmembrane domain, designated TM-minK, was synthesized and fluorescently labeled. The alpha-helical content of TM-minK, assessed in methanol using circular dichroism (CD), was 57%. The fluorescent emission spectrum of 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled TM-minK displayed a blue shift upon binding to small unilamellar vesicles (SUV), reflecting a relocation of the fluorescent probe to an environment of increased apolarity, i.e., within the lipid bilayer. The increase in NBD's fluorescence upon mixing NBD-labeled TM-minK with small unilamellar vesicles (SUV) was used to generate a binding isotherm, from which was derived a surface partition coefficient of 5.5 x 10(4) M-1. Fluorescence energy transfer measurements between carboxyfluoresceine-labeled and rhodamine-labeled analogues suggest that TM-minK aggregates within membranes. In addition, single-channel experiments revealed that TM-minK can form single channels in planar lipid membranes only when a trans negative potential is applied. The findings herein experimentally support a role of the transmembrane segment of minK both in the assembly and as a constituent of the pore formed by the protein.
The mechanism of pore formation employed by pardaxin, a shark-repellent neurotoxin, and three of its charge-modified analogues, (N1,Lys8,Lys16-trisuccinylamido)pardaxin, (N1,Lys8,Lys16-triacetyl) pardaxin and (C33-di-ethanolamido)pardaxin, was investigated. The kinetics of Tl+ and IO3- release from large unilamellar phospholipid vesicles was measured under acidic, neutral and basic conditions. Both the kinetics of pore formation and the ability of the analogues to release ions increase when the N-terminal amino acid residue and the peptide backbone are either neutrally or positively charged. Moreover, five- or sixfold higher concentrations of the polypeptides are required for IO3- release than for Tl+ release. Our results are consistent with the ''barrel stave'' model by which pardaxin and its analogues exert their pore-forming activities. The smallest pore permeable to Tl+ is impermeable to IO3-, for which larger pores obtained at higher polypeptide concentrations are required.
Dermaseptin, a 34 amino-acid residue antimicrobial polypeptide [Mor, A., Nguyen, V. H., Delfour, A., Migliore-Samour, D., & Nicolas, P. (1991) Biochemistry 30, 8824-88301 was synthesized and selectively labeled at its N-terminal amino acid with either 7-nitrobenz-2-oxa-1,3-diazole-4-yl (NBD), rhodamine, or fluorescein. The fluorescent emission spectra of the NBD-labeled dermaseptin displayed a blue-shift upon binding to small unilamellar vesicles (SUV), reflecting the relocation of the fluorescent probe to an environment of increased a polarity. Titrations of solutions containing NBD-labeled dermaseptin with SUV composed of zwitterionic or acidic phospholipids were used to generate binding isotherms, from which were derived surface partition constants of (0.66 +/- 0.06) X 10(4) M-1 and (2.8 +/- 0.3) x 10(4) M-1, respectively. The shape of the binding isotherms, as well as fluorescence energy transfer measurements, suggests that some aggregation of membrane-bound peptide monomers occurs in acidic but not in zwitterionic vesicles. The preferential susceptibility of the peptide to proteolysis when bound to zwitterionic but not to acidic SUV suggests that these aggregates might then penetrate a relatively short distance into the hydrophobic region of the acidic membrane. Furthermore, the results provide good correlation between the peptide's strong binding and its ability to permeate membranes composed of acidic phospholipids, as revealed by a dissipation of diffusion potential and a release of entrapped calcein from SUV.
The influence of specific L- to D-amino acid substitutions on the interaction of pardaxin, a shark repellent neurotoxin polypeptide, with phospholipid vesicles and human erythrocytes is described. Twelve modified, truncated, or fluorescently labeled [with the fluorophore 7-nitrobenz-2-oxa- 1,3-diazole-4-yl (NBD) at their N-terminal amino acid] analogues of pardaxin were synthesized by a solid-phase method. Fluorescence measurements were used to monitor the interaction of the analogues with membranes [Rapaport, D., & Shai, Y. (1991) J. Biol. Chem. 266, 23769-23775]. Upon titration of solutions containing the NBD-labeled peptides with small unilamellar vesicles, the fluorescent emission spectra of all NBD-labeled peptides displayed similar blue-shifts, in addition to enhanced intensities, upon relocation of the probe to the more apolar environment. Binding isotherms were constructed from which surface partition constants, in the range of 10(4) M-1, were derived. The existence of an aggregation process, suggested by the shape of the binding isotherms, could be associated only with those analogues in which the N-helix (residues 1-9) was not perturbed. The alpha-helical content of the analogues was estimated by circular dichroism (CD) spectroscopy, both before and after binding to vesicles at neutral pH. The ability of the peptides to dissipate a diffusion potential and to cause calcein release, as well as to lyse human erythrocytes, served to functionally characterize the peptides. The results support a two alpha-helix model, with a bend at position 13, as best describing pardaxin in its membrane-bound state. The study also demonstrates that local configurational changes of amino acids, including prolines, do not affect the abilities of the analogues to bind to phospholipid membranes, but do have some effect on their membrane-permeating activities.
A 24-amino acid peptide corresponding to the S4 segment of the sodium channel was synthesized. In order to perform fluorescence energy transfer measurements and to monitor the interaction of the peptide with lipid vesicles, the peptide was selectively labeled with fluorescence probes at either its N- or C-terminal amino acids. The fluorescent emission spectra of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-(NBD-)labeled analogues displayed blue shifts upon binding to small unilamellar vesicles (SUV), reflecting the relocation of the fluorescent probe to an environment of increased apolarity. The results revealed that both the N- and C-terminus of the S4 segment are located within the lipid bilayer. Titration of solutions containing NBD-labeled peptides with SUV was used to generate binding isotherms, from which surface partition constants, in the range of 10(4) M-1, Were derived. The shape of the binding isotherms as well as fluorescence energy transfer measurements suggest that aggregation of peptide monomers within the membrane readily occurs in acidic but not in zwitterionic vesicles. Furthermore, the results provide good correlation between the incidence of aggregation in PC/PS vesicles and the ability of the peptides to permeate the vesicle's membrane. However, a transmembrane diffusion potential had no detectable effect on the location of the peptide within the lipid bilayer or on its aggregation state. Taken together, these results provide experimental support for a transmembranal localization for the sodium channel S4 segment as well as for its potential in forming part of the channel's lining, both properties in agreement with the "propagating helix" model, suggested by Guy and Conti [Guy, H. R., & Conti, F. (1990) Trends Neurosci. 13, 201-206].
We previously described a prosthetic group methodology for incorporating F-18 into peptides and showed that F-18-labeled insulin (F-18-insulin) binds to insulin receptors on human cells (IM-9 lymphoblastoid cells) with affinity equal to that of native insulin (1). We now report studies using F-18-insulin with positron emission tomography to study binding to insulin receptors in vivo. Positron emission tomography scans were performed in six rhesus monkeys injected with 0.3-1.4 mCi of F-18-insulin (approximately 0.1 nmol, SA 4-11 Ci/mu-mol). Integrity of the tracer in blood, determined by immunoprecipitation, was 94% of control for the first 5 min and decreased to 31% by 30 min. Specific, saturable uptake of F-18 was observed in the liver and kidney. Coinjection of unlabeled insulin (200 U, approximately 1 nmol) with the F-18-insulin reduced liver and increased kidney uptake of the labeled insulin. Liver radioactivity was decreased by administration of unlabeled insulin at 3 min, but not 5 min, after administration of the tracer, while some kidney radioactivity could be displaced 5 min after injection. Clearance of F-18 was predominantly in bile and urine. F-18-insulin is a suitable analogue for studying insulin receptor-ligand interactions in vivo, especially in the liver and kidney.
Fluorescence resonance energy transfer has been used to study the aggregation and organization of pardaxin and its analogues within lipid membranes. Peptide molecules labeled with 5- (and 6-) carboxyfluorescein at their N-terminal amino acid served as donors in these energy transfer measurements, whereas peptides labeled with 5- (and 6-) carboxytetramethylrhodamine at either their N- or C-terminal amino acid, served as acceptors. The membrane-permeating activity of the native molecule was maintained in the labeled peptides. Upon aggregation of the labeled peptides, fluorescence energy transfer was detected as a quenching of the donor fluorescence (520 nm), as well as an enhancement of the acceptor fluorescence (575 nm). Correlation exists between self-aggregation of the different analogues within membranes and their pore-forming abilities. A comparison of the degrees of fluorescence energy transfer from N1-donor-labeled pardaxin to N1-acceptor-labeled pardaxin with the transfer efficiency observed in the interaction between the same donor and C1-acceptor-labeled pardaxin suggests that aggregates are formed in an ordered manner, with a preferentially parallel orientation of monomers within the aggregate. The extent of hetero-oligomer formation, i.e. complexes composed of two different analogue species, revealed that complementary charges contribute to peptide-peptide recognition within the lipid bilayer. Taken together, these results provide further support for the barrel stave model, involving parallel organization of monomers within the aggregate, as a description of the pore formation mechanism of pardaxin and its analogues.
Fluorescence measurements were used to monitor the interaction of the neurotoxin pardaxin and its analogues with membranes. Eight peptides were selectively labeled with the fluorophore 7-nitrobenz-2-oxa-1,3-diazole-4-yl, either at their N-terminal or at their C-terminal. No detectable changes in membrane permeability or hemolytic activity were observed upon modification. Upon the titration of solutions containing the different peptides with small unilamellar vesicles, the fluorescent emission spectra of 7-nitrobenz-2-oxa-1,3-diazole-4-yl-labeled pardaxin and its analogues, but not those of control peptides, displayed blue shifts in addition to enhanced intensities upon relocation of the probe to a more apolar environment. The results revealed that the N terminus of pardaxin is buried within the lipid bilayer while the C terminus is located at the bilayer's surface. Binding isotherms were obtained from the observed increases in the fluorescence emission yields, from which surface partition constants, in the range of 10(4) M-1, were in turn derived. The existence of an aggregation process was suggested by the shape of the binding isotherms. Furthermore, the results show good correlation between the incidence of aggregation and the ability of the different analogues to induce the release of relatively large molecules from vesicles. As such, our results suggest that the mechanism of pore formation employed by pardaxin and its analogues could be described by the "barrel stave" model.
The interaction of pardaxin, a shark-repellent neurotoxin, and its charge-modified analogues with vesicles and human erythrocytes is described. The following six analogues and derivatives were synthesized by a solid phase method: [Glu8,Glu16]pardaxin, [N1-succinamido, Glu8,Glu16]pardaxin, [N1,Lys8,Lys16-trisuccinamido] pardaxin, [N1,Lys8,Lys16-triacetyl]pardaxin, des-[l --> 9]pardaxin (Shai, Y., Bach, D., and Yanovsky, A. (1990) J. Biol. Chem. 265, 2020220209), and des-[l --> 9][Glu16]pardaxin. The relative hydrophobic characteristics of the analogues were examined using reverse-phase high performance liquid chromatography. The pH-dependent spectroscopic and functional characteristics of the analogues were also investigated at either neutral or acidic pH. Spectroscopic characterization was achieved by measuring circular dichroism both before and after binding to vesicles, at either neutral or acidic pH. The ability of the peptides to dissipate a diffusion potential, to cause calcein release or the pH-dependent release of 8-aminonaphthalene, 1,3,6-trisulfonic acid disodium salt/p-xylene-bis [pyridinium bromide] from sonicated unilamellar liposomes, as well as measurements of cytolytic activity on human erythrocytes, served to functionally characterize the peptides. We show a direct correlation between alpha-helical content, the analogues' hydrophobicity, and their pore-forming properties at the different pH values tested. We also demonstrate that the charge of the N terminus and of the peptide backbone, but not of the C terminus, affects the secondary structure as well as the activities of the analogues. Finally, we show that the cytolytic activity of pardaxin at neutral pH is not retained by any of the analogues.
Six analogues of teh 33-residue shark repellent neurotoxin pardaxin were synthesized by the solid phase method: [Ala13]pardaxin, [Gly14,Gly15]pardaxin, des[1----9]pardaxin, [N1-succinamido]pardaxin, C33-dihydroxyethylamido]pardaxin, and C33-[diaminoethylamido]pardaxin. The spectroscopic and functional characterizations of the analogues are described. The peptides were characterized spectroscopically by circular dichroism (CD) before and after binding to soybean vesicles. They were characterized functionally by measuring their potential to evoke the dissipation of diffusion potential and calcein release from sonicated unilamellar soybean liposomes, by determining their ability to create single channels in planar bilayers, and by measuring their cytolytic activity on human erythrocytes. The behavior of the analogues modified at the C terminus is similar to that of pardaxin. [N'-succinamido]Pardaxin, however, reveals an increase in alpha-helicity both alone and in the presence of liposomes. It has the same potency as pardaxin to dissipate diffusion potential, to evoke calcein release and to produce single channels in lipid bilayers, but at a slower rate than that of pardaxin. It has more than 70-fold less cytolytic activity than pardaxin. [Ala13] Pardaxin has twice the alpha-helical content than pardaxin, both alone and in the presence of vesicles, yet it has less effect on the diffusion potential and calcein release, and it does not have cytolytic activity on human erythrocytes. Both [Gly14,Gly15]pardaxin and des[1----9]pardaxin are much less potent than pardaxin in all effects. However des[1----9]pardaxin exhibits a slight change in alpha-helicity upon binding to vesicles, whereas [Gly14,Gly15]pardaxin does not. The results support a model in which pardaxin is composed of two putative alpha-helices separated by proline. The N-terminal alpha-helix is important for the insertion of the peptide to the lipid bilayer, and the C-terminal amphiphilic alpha-helix is the ion channel lining segment of pardaxin.
Keywords: Pharmacology & Pharmacy
The use of transfer polymeric reagents (PRs) as excellent acylating agents for high yield and high purity peptide synthesis is described. Three methodologies are compared: the classical solution method, the Merrifield approach and an automated (the "mediator"-shadchan) method with continuous monitoring. The utilization of PRs as general acyl transfer reagents is also elaborated. The described approaches are not limited to peptide synthesis, but may be applicable to a wide range of organic reaction types.