Cellulosomes can be described as one of nature's most elaborate and highly efficient nanomachines. These cell bound multienzyme complexes orchestrate the deconstruction of cellulose and hemicellulose, two of the most abundant polymers on Earth, and thus play a major role in carbon turnover. Integration of cellulosomal components occurs via highly ordered protein:protein interactions between cohesins and dockerins, whose specificity allows the incorporation of cellulases and hemicellulases onto a molecular scaffold. Cellulosome assembly promotes the exploitation of enzyme synergism because of spatial proximity and enzyme-substrate targeting. Recent structural and functional studies have revealed how cohesin-dockerin interactions mediate both cellulosome assembly and cell-surface attachment, while retaining the spatial flexibility required to optimize the catalytic synergy within the enzyme complex. These emerging advances in our knowledge of cellulosome function are reviewed here.

The utilization of organized supramolecular assemblies to exploit the synergistic interactions afforded by close proximity, both for enzymatic synthesis and for the degradation of recalcitrant substrates, is an emerging theme in cellular biology. Anaerobic bacteria harness a multiprotein complex, termed the “cellulosome,” for efficient degradation of the plant cell wall. This megadalton catalytic machine organizes an enzymatic consortium on a multifaceted molecular scaffold whose “cohesin” domains interact with corresponding “dockerin” domains of the enzymes. Here we report the structure of the cohesin–dockerin complex from Clostridium thermocellum at 2.2-Å resolution. The data show that the β-sheet cohesin domain interacts predominantly with one of the helices of the dockerin. Whereas the structure of the cohesin remains essentially unchanged, the loop–helix–helix–loop–helix motif of the dockerin undergoes conformational change and ordering compared with its solution structure, although the classical 12-residue EF-hand coordination to two calcium ions is maintained. Significantly, internal sequence duplication within the dockerin is manifested in near-perfect internal twofold symmetry, suggesting that both “halves” of the dockerin may interact with cohesins in a similar manner, thus providing a higher level of structure to the cellulosome and possibly explaining the presence of “polycellulosomes.” The structure provides an explanation for the lack of cross-species recognition between cohesin–dockerin pairs and thus provides a blueprint for the rational design, construction, and exploitation of these catalytic assemblies.

Endo-beta-1,4-xylanases (xylanases), which cleave beta-1,4 glycosidic bonds in the xylan backbone, are important components of the repertoire of enzymes that catalyze plant cell wall degradation. The mechanism by which these enzymes are able to hydrolyze a range of decorated xylans remains unclear. Here we reveal the three-dimensional structure, determined by x-ray crystallography, and the catalytic properties of the Cellvibrio mixtus enzyme Xyn10B (CmXyn10B), the most active GH10 xylanase described to date. The crystal structure of the enzyme in complex with xylopentaose reveals that at the +1 subsite the xylose moiety is sandwiched between hydrophobic residues, which is likely to mediate tighter binding than in other GH10 xylanases. The crystal structure of the xylanase in complex with a range of decorated xylooligosaccharides reveals how this enzyme is able to hydrolyze substituted xylan. Solvent exposure of the O-2 groups of xylose at the +4, +3, +1, and -3 subsites may allow accommodation of the alpha-1,2-linked 4-O-methyl-d-glucuronic acid side chain in glucuronoxylan at these locations. Furthermore, the uronic acid makes hydrogen bonds and hydrophobic interactions with the enzyme at the +1 subsite, indicating that the sugar decorations in glucuronoxylan are targeted to this proximal aglycone binding site. Accommodation of 3'-linked l-arabinofuranoside decorations is observed in the -2 subsite and could, most likely, be tolerated when bound to xylosides in -3 and +4. A notable feature of the binding mode of decorated substrates is the way in which the subsite specificities are tailored both to prevent the formation of "dead-end" reaction products and to facilitate synergy with the xylan degradation-accessory enzymes such as alpha-glucuronidase. The data described in this report and in the accompanying paper indicate that the complementarity in the binding of decorated substrates between the glycone and aglycone regions appears to be a conserved feature of GH10 xylanases.

The assembly of proteins that display complementary activities into macromolecular complexes is critical to cellular function. One such enzyme complex, of environmental significance, is the plant cell wall degrading apparatus of anaerobic bacteria, termed the cellulosome. The complex assembles through the interaction of enzyme-derived “type I dockerin” modules with the multiple “cohesin” modules of the scaffolding protein. Clostridium thermocellum type I dockerin modules contain a duplicated 22-residue sequence that comprises helix-1 and helix-3, respectively. The crystal structure of a C. thermocellum type I cohesin-dockerin complex showed that cohesin recognition was predominantly through helix-3 of the dockerin. The sequence duplication is reflected in near-perfect 2-fold structural symmetry, suggesting that both repeats could interact with cohesins by a common mechanism in wild-type (WT) proteins. Here, a helix-3 disrupted mutant dockerin is used to visualize the reverse binding in which the dockerin mutant is indeed rotated 180° relative to the WT dockerin such that helix-1 now dominates recognition of its protein partner. The dual binding mode is predicted to impart significant plasticity into the orientation of the catalytic subunits within this supramolecular assembly, which reflects the challenges presented by the degradation of a heterogeneous, recalcitrant, insoluble substrate by a tethered macromolecular complex.