Novel laminarin-binding CBMs in multimodular proteins of marine Bacteroidota feature prominently in phytoplankton blooms

The {beta}-(1,3)-glucan laminarin functions as storage polysaccharide in marine stramenophiles such as diatoms. Laminarin is abundant, water-soluble and structured simply, making it an attractive substrate for marine bacteria. As a consequence, many marine bacteria have developed competitive strategies to scavenge and decompose laminarin, which involves carbohydrate-binding modules (CBMs) as key players. We therefore functionally and structurally characterized two yet unassigned domains as laminarin-binding CBMs in multimodular proteins from our model bacterium Christiangramia forsetii KT0803T, hereby unveiling the novel laminarin-binding CBM families CBMxx and CBMyy (official CAZy numbering will be provided upon acceptance of the manuscript in a peer-reviewed journal). We discovered four CBMxx repeats in a surface glycan-binding protein (SGBP) and a single CBMyy combined with a glycoside hydrolase module from family 16 (GH16_3). Our analyses revealed that both modular proteins have an elongated shape, and that the GH16_3 displayed a higher flexibility than the SGBP. While motility of both polypeptide chains may facilitate recognition and/or degradation of laminarin, constraints in the SGBP may support docking of laminarin onto the bacterial surface. The exploration of bacterial metagenome-assembled genomes (MAGs) from phytoplankton blooms in the North Sea revealed that both laminarin-binding CBM families are widely distributed among marine Bacteroidota, illustrating the high adaptability of modularity in sugar-binding and -degrading proteins. High expression of CBMxx- and CBMyy-containing proteins during phytoplankton blooms further underpins their importance in marine laminarin usage.