CELS - A MAJOR EXOGLUCANASE COMPONENT OF CLOSTRIDIUM-THERMOCELLUM CELLULOSOME

The CelS protein (M(r) = 82,000) is the most abundant protein species secreted by Clostridium thermocellum. It has been identified as a key catalytic component of the C. thermocellum cellulosome, an extremely complicated and large protein aggregate responsible for the degradation of crystalline cellulose. We have proposed that CelS acts synergistically with CelL (M(r) = 250,000), another major cellulosome component, to degrade crystalline cellulose through a novel enzyme-anchor mechanism distinguishable from that of the fungal cellulase system. However, the function and the properties of this key catalytic component remain to be elucidated. We have recently cloned the celS gene. Its DNA sequence cannot be classified into any of the known cellulase families. In fact, the celS sequence has led to the discovery of a new cellulase family. Two previously un-characterized gene fragments from other bacteria have been found to contain sequences highly homologous to the celS sequence. Furthermore, the N-terminal amino acid sequence of a novel exo-beta-glucanase (Avicelase II) purified from C. stercorarium matches that of CelS. The members of this new cellulase family are so far found only in strict anaerobic bacteria. CelS may therefore be one of the key components distinguishing the anaerobic cellulase system from its fungal counterpart. Expression of the celS gene in Escherichia coli resulted in the formation of inclusion bodies. The solubilized and refolded gene product displayed activity typical of an exo-beta-glucanase. Thermostability was enhanced by Ca++, as for the crude enzyme. The celS gene therefore encodes a novel exoglucanase.