Clostridium cellulolyticum is a model mesophilic anaerobic bacterium that efficiently degrades plant cell walls. The recent genome release offers the opportunity to analyse its complete degradation system. A total of 148 putative carbohydrate-active enzymes were identified, and their modular structures and activities were predicted. Among them, 62 dockerin-containing proteins bear catalytic modules from numerous carbohydrate-active enzymes' families and whose diversity reflects the chemical and structural complexity of the plant carbohydrate. The composition of the cellulosomes produced by C. cellulolyticum upon growth on different substrates (cellulose, xylan, and wheat straw) was investigated by LC MS/MS. The majority of the proteins encoded by the cip-cel operon, essential for cellulose degradation, were detected in all cellulosome preparations. In the presence of wheat straw, the natural and most complex of the substrates studied, additional proteins predicted to be involved in hemicellulose degradation were produced. A 32-kb gene cluster encodes the majority of these proteins, all harbouring carbohydrate-binding module 6 or carbohydrate-binding module 22 xylan-binding modules along dockerins. This newly identified xyl-doc gene cluster, specialised in hemicellulose degradation, comes in addition of the cip-cel operon for plant cell wall degradation. Hydrolysis efficiencies determined on the different substrates corroborates the finding that cellulosome composition is adapted to the growth substrate.
The cross-species specificity of the cohesin-dockerin interaction, which defines the incorporation of the enzymatic subunits into the cellulosome complex, has been investigated. Cohesin-containing segments from the cellulosomes of two different species, Clostridium thermocellum and Clostridium cellulolyticum, were allowed to interact with cellulosomal (dockerin-containing) enzymes from each species. In both cases, the cohesin domain of one bacterium interacted with enzymes from its own cellulosome in a calcium-dependent manner, but the same cohesin failed to recognize enzymes from the other species. Thus, in the case of these two bacteria, the cohesin-dockerin interaction seems to be species-specific. Based on intra- and cross-species sequence comparisons among the different dockerins together with their known specificities, we tender a prediction as to the amino-acid residues critical to recognition of the cohesins. The suspected residues were narrowed down to only four, which comprise a repeated pair located within the calcium-binding motif of two duplicated sequences, characteristic of the dockerin domain. According to the proposed model, these four residues do not participate in the binding of calcium per se; instead, they appear to serve as recognition codes in promoting interaction with the cohesin surface.
The recombinant form of the cellulase CelF of Clostridium cellulolyticum, tagged by a C-terminal histine tail, was overproduced in Escherichia coli. The fusion protein was purified by affinity chromatography on a Ninitrilotriacetic acid column. The intact form of CelF (M r , 79,000) was rapidly degraded at the C terminus, giving a shorter stable form, called truncated CelF (M r , 71,000). Both the entire and the truncated purified forms degraded amorphous cellulose (k cat ؍ 42 and 30 min ؊1 , respectively) and microcrystalline cellulose (k cat ؍ 13 and 10 min ؊1 , respectively). The high ratio of soluble reducing ends to insoluble reducing ends released by truncated CelF from amorphous cellulose showed that CelF is a processive enzyme. Nevertheless, the diversity of the cellodextrins released by truncated CelF from phosphoric acid-swollen cellulose at the beginning of the reaction indicated that the enzyme might randomly hydrolyze -1,4 bonds. This hypothesis was supported by viscosimetric measurements and by the finding that CelF and the endoglucanase CelA are able to degrade some of the same cellulose sites. CelF was therefore called a processive endocellulase. The results of immunoblotting analysis showed that CelF was associated with the cellulosome of C. cellulolyticum. It was identified as one of the three major components of cellulosomes. The ability of the entire form of CelF to interact with CipC, the cellulosome integrating protein, or mini-CipC 1 , a recombinant truncated form of CipC, was monitored by interaction Western blotting (immunoblotting) and by binding assays using a BIAcore biosensor-based analytical system.
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