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bioRxiv · 10.1101/2023.09.07.556657

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

Zuehlke, M.-K.·Ficko-Blean, E.·Bartosik, D.·Terrapon, N.·Jeudy, A.·Jam, M.·Wang, F.·Welsch, N.·Larocque, R.·Jouanneau, D.·Eisenack, T.·Thomas, F.·Trautwein-Schult, A.·Teeling, H.·Becher, D.·Schweder, T.·Czjzek, M.
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Abstract

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.

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Zuehlke, M.-K., Ficko-Blean, E., Bartosik, D., Terrapon, N., Jeudy, A., Jam, M., Wang, F., Welsch, N., Larocque, R., Jouanneau, D., Eisenack, T., Thomas, F., Trautwein-Schult, A., Teeling, H., Becher, D., Schweder, T., Czjzek, M.. 2023-09-08. Novel laminarin-binding CBMs in multimodular proteins of marine Bacteroidota feature prominently in phytoplankton blooms. https://doi.org/10.1101/2023.09.07.556657

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