Identification of the Cellulose-Binding Domain of aBacillus subtilisEndoglucanase Distinct from Its Catalytic Domain

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.