Isolation and biochemical characterization of an endo-1,3-β-glucanase from Streptomyces sioyaensis containing a C-terminal family 6 carbohydrate-binding module that binds to 1,3-β-glucan

A gene encoding 1,3-beta-glucanase was isolated from Streptomyces sioyaensis based on an activity plate assay. Analysis of the deduced amino acid sequence of the gene revealed that the matured 1,3-beta-glucanase has two functional domains separated by a stretch of nine glycine residues. The N-terminal domain shares sequence similarity with bacterial endo-1,3-beta-glucanases classified in glycosyl hydrolase family 16 (GHF 16), while the C-terminal domain is a putative carbohydrate-binding module (CBM) grouped into CBM family 6. To characterize the function of each domain, both the full-length and the CBM-truncated versions of the protein were expressed in Escherichia coli and purified to homogeneity. Biochemical data suggest that the glycosyl hydrolase domain preferentially catalyses the hydrolysis of glucans with 1,3-beta linkage, and has an endolytic mode of action. Binding assay indicated that the C-terminal CBM binds to various insoluble beta-glucans (1,3-, 1,3-1,4- and 1,4-linkages) but not to xylan, a primary binding target for most members of CBM family 6. The full-length and the CBM-truncated proteins had similar specific activity (units per mol of hydrolase domain) on soluble 1,3-beta-glucan, whereas the former had much stronger specific activity on insoluble 1,3-beta-glucans, suggesting that the C-terminal CBM enhances the activity of the S. sioyaensis 1,3-beta-glucanase against insoluble substrates, presumably by increasing the frequency of encounter events between the hydrolase domain and the substrate.