Novel architecture of family-9 glycoside hydrolases identified in cellulosomal enzymes of Acetivibrio cellulolyticus and Clostridium thermocellum

We have sequenced a new gene, cel9B, encoding a family-9 cellulase from a cellulosome-producing bacterium, Acetivibrio cellulolyticus. The gene includes a signal peptide, a family-9 glycoside hydrolases (GH9) catalytic module, two family-3 carbohydrate-binding modules (CBM3c-CBM3b tandem dyad) and a C-terminal dockerin module. An identical modular arrangement exists in two putative GH9 genes from the draft sequence of the Clostridium thermocellum genome. The three homologous CBM3b modules from A. cellulolyticus and C. thermocellum were overexpressed, but, surprisingly, none bound cellulosic substrates. The results raise fundamental questions concerning the possible role(s) of the newly described CBMs. Phylogenetic analysis and preliminary site-directed mutagenesis studies suggest that the catalytic module and the CBM3 dyad are distinctive in their sequences and are proposed to constitute a new GH9 architectural theme.