Cellulosome Systems

Bacterial cellulosomal systems can be categorized into two major types: simple cellulosome systems contain a single scaffoldin and complex cellulosome systems exhibit multiple types of interacting scaffoldins. The genes encoding for many important cellulosome subunits are organized in “enzyme-linked gene clusters” on the chromosome. In the simple cellulosome systems, the scaffoldin gene is followed downstream by a series of genes that code for dockerin-bearing enzymes. In the complex cellulosome systems, the scaffoldin genes are organized into “multiple scaffoldin gene clusters” on the chromosome as shown in the following scheme:

The Scaffoldin Subunits

The scaffoldin subunit contains one or more cohesin modules connected to other types of functional modules. In a given scaffoldin, the latter types of modules may include a cellulose-specific carbohydrate-binding module (CBM), a dockerin, X modules of unknown function, an S-layer homology (SLH) module or a sortase anchoring motif. The arrangement of the modules on the scaffoldin subunit and the specificity of the cohesin(s) and/or dockerin for their modular counterpart dictate the overall architecture of the cellulosome. Several different types of scaffoldins have been described: the primary scaffoldins incorporate the various dockerin-bearing subunits directly into the cellulosome complex, adaptor scaffoldins increase the repertoire or number of components into the complex, and the anchoring scaffoldins attach the complex to the bacterial cell surface.

Simple Cellulosome Systems

In the simple cellulosome systems, the scaffoldins contain a single CBM, one or more X2 modules and numerous (5 to 9) cohesins. These scaffoldins are primary scaffoldins, which incorporate the dockerin-bearing enzymes into the complex. In several cases, the simple cellulosomes have been shown to be associated with the cell surface, but the molecular mechanism responsible for this is still unclear. The X2 module may play a role in attachment to the cell wall. The scaffoldins of simple cellulosome systems are given in the following scheme:

Complex Cellulosome Systems

To date, complex cellulosome systems have been described in four different bacterial species. In these systems, more than one scaffoldin interlocks with each other in various ways to produce a complex cellulosome architecture. At least one type of scaffoldin serves as a primary scaffoldin that incorporates the enzymes directly into the cellulosome complex. In each species, another type of scaffoldin attaches the cellulosome complex to the cell surface via a specialized module or sequence, designed for this purpose. Schematic representations of complex cellulosome systems of the following bacteria are shown:

The Clostridium thermocellum cellulosome paradigm consists of individual dockerin-containing enzymes integrated into the 9 cohesins of the primary scaffoldin. A set of anchoring scaffoldins possess 1, 2 and 7 cohesins, respectively, for attachment of the corresponding numbers of primary CipA scaffoldins and their enzyme complement to the cell surface.

The unexpected intricacy of the Acetivibrio cellulolyticus cellulosome system reveals 2 very different cellulosome systems (A and B) — unique from that of C. thermocellum. Both cellulosomes are attached to the cell surface by anchoring scaffoldins. System A contains a unique anchoring scaffoldin (ScaB) and System B contains a unique bifunctional scaffoldin (ScaD) that plays a dual primary/anchoring role.

The simplistic but massive cellulosome system of Bacteroides cellulosolvens contains only 2 scaffoldins, but they represent the largest cellulosomal components recorded to date – a 10-cohesin anchoring scaffoldin and an 11-cohesin primary scaffoldin, which would theoretically hold 110 different cellulosomal subunits in a single complex. Surprisingly, the roles of the cohesin types are reversed: the anchoring scaffodlin comprises type-I cohesins and the primary scaffoldin type-II cohesins.