Characterization and structural identification of a novel alginate-specific carbohydrate-binding module (CBM): The founding member of a new CBM family

Alginate is a commercially important polysaccharide widely distributed in brown algae. Carbohydrate-binding modules (CBMs), a class of commonly used polysaccharide-binding proteins, have greatly facilitated the investigations of polysaccharides. Few alginate-binding CBMs have been hitherto reported and structurally characterized. Herein, an unknown domain from a potential PL6 family alginate lyase in the marine bacterium Vibrio breoganii was discovered and recombinantly expressed. The obtained protein, designated VbCBMxx, displayed the favorable specificity to alginate. The unique sequence and well-defined function of VbCBMxx reveal a new CBM family. Moreover, the structure of VbCBMxx was determined at a 1.5 Å resolution by the X-ray crystallography, which shows a typical β-sandwich fold comprised of two antiparallel β-sheets. Site-directed mutagenesis assays confirmed that positively charged polar residues are crucial for the ligand binding of VbCBMxx. The discovery of VbCBMxx enriches the toolbox of alginate-binding proteins, and the elucidation of critical residues would guide the future practical applications of VbCBMxx.