CBM3d, a novel subfamily of family 3 carbohydrate-binding modules identified in Cel48A exoglucanase of Cellulosilyticum ruminicola.

Previously, we found that exoglucanase Cel48A from Cellulosilyticum ruminicola H1 bound intensively to Avicel; however, no known carbohydrate-binding module (CBM) was observed in the protein. Bioinformatics suggested that a C-terminal fragment of 127 amino acids, named the Cellulosilyticum-specific paralogous module (CPM), could function in binding. CPM-appended proteins are all putative (hemi)cellulases from Cellulosilyticum spp. In the present work, we demonstrated that Cel48A without the CPM retained only exoglucanase activity and lost the Avicel-binding ability, while the isolated CPM exhibited a high affinity for Avicel. In addition, the CPM bound to chitin, but not to soluble polysaccharides, making it a type A CBM, which binds only insoluble polysaccharides. Phylogenetic analysis clustered the CPM and its homologs as a separate branch that was distantly related to CBM subfamilies 3a (28% identity), 3b (24% identity), and 3c (21% identity). Sequence alignment revealed distinct secondary structures of the new CBM 3 group, in particular, a conserved Pro66-Trp67 insert preceding strand β4', a deletion preceding strand β6, and incomplete strands β8 and β9. An alanine scan for six aromatic and three nonaromatic amino acid residues (D66, P66, and R111) by site-directed mutagenesis determined that Phe62, Pro66, Trp67, Tyr68, Arg111, and Trp117 were the functional residues for binding. Among them, Phe62, Pro66, and Trp67 were the newly determined key sites in the CPM for binding. Three-dimensional homolog modeling revealed two types of substrate-binding sites, planar and groove, in the CPM. Thus, a new subfamily, CBM family 3d, is proposed.