Molecular recognition of the beta-glucans laminarin and pustulan by a SusD-like glycan-binding protein of a marine Bacteroidetes.

Marine bacteria catabolize carbohydrate polymers of algae, which synthesize these structurally diverse molecules in ocean surface waters. Although algal glycans are an abundant carbon and energy source in the ocean, the molecular details that enable specific recognition between algal glycans and bacterial degraders remain largely unknown. Here we characterized a surface protein, GMSusD from the planktonic Bacteroidetes-Gramella sp. MAR_2010_102 that thrives during algal blooms. Our biochemical and structural analyses show that GMSusD binds glucose polysaccharides such as branched laminarin and linear pustulan. The 1.8 Å crystal structure of GMSusD indicates that three tryptophan residues form the putative glycan-binding site. Mutagenesis studies confirmed that these residues are crucial for laminarin recognition. We queried metagenomes of global surface water datasets for the occurrence of SusD-like proteins and found sequences with the three structurally conserved residues in different locations in the ocean. The molecular selectivity of GMSusD underscores that specific interactions are required for laminarin recognition. In conclusion, our findings provide insight into the molecular details of ß-glucan binding by GMSusD and our bioinformatic analysis reveals that this molecular interaction may contribute to glucan cycling in the surface ocean.

Crystal structure of a marine glycoside hydrolase family 99-related protein lacking catalytic machinery.

Algal polysaccharides of diverse structures are one of the most abundant carbon resources for heterotrophic, marine bacteria with coevolved digestive enzymes. A putative sulfo-mannan polysaccharide utilization locus, which is conserved in marine flavobacteria, contains an unusual GH99-like protein that lacks the conserved catalytic residues of glycoside hydrolase family 99. Using X-ray crystallography, we structurally characterized this protein from the marine flavobacterium Ochrovirga pacifica to help elucidate its molecular function. The structure reveals the absence of potential catalytic residues for polysaccharide hydrolysis, which-together with additional structural features-suggests this protein may be noncatalytic and involved in carbohydrate binding.