Identification of the cellulose-binding domain of a Bacillus subtilis endoglucanase distinct from its catalytic...

The endoglucanase (BSC) from Bacillus subtilis IFO 3034, which shows no ability to hydrolyze microcrystalline cellulose, was found to bind to Avicel. Ninety-eight amino acids-truncation at the COOH-terminus of BSC did not abolish the carboxymethyl cellulose (CMC)-hydrolyzing ability, but removed the Avicel-binding ability. These data suggested the presence of an Avicel-binding domain at the COOH-terminus of BSC, despite its inability to hydrolyze crystalline cellulose. A mutant enzyme with Phe at the 131st His, generated by site-directed mutagenesis, had no enzymatic activity with CMC as the substrate, as predicted from hydrophobic cluster analysis, while the cellulose-binding ability of the mutant enzyme still remained. Similarly, the mutation at the 169th Glu severely affected the enzyme activity, but not the cellulose-binding ability. All these data clearly show that BSC is composed of the catalytic domain at its NH2-terminal portion and the cellulose-binding domain at its COOH-terminal portion, and that the two domains are independently functional in the absence of the other.