Characterization of a multi-modular bacterial enzyme with binding and hydrolysis activities on fungal cell wall components: Insights into bacterial-fungal interactions in the soil

Abstract

Bacteria from the phylum Bacteroidetes is known as good degraders because of their abilities in secreting glycoside hydrolases (GHs). As a member in Bacteroidetes, Chitinophaga pinensis is capable to hydrolyze several glycans (McKee et al., 2019). We found that a multi-modular CAZyme produced by C. pinensis contains not only two GHs but also two uncharacterized domains. Although it is common for CAZmes to have non-catalytic modules like carbohydrate-binding domains (CBMs), the domains in this protein are uncommon, and their functions unknown. Our enzyme contains five domains – a GH5_46 enzyme, a GH18 enzyme, two non-identical uncharacterized domains, and a Por C-terminal secretion domain. I have shown that the GH5 can specifically degrade pustulan (β-1,6-glucan), and is the only β-1,6-glucanase found in its sub-family. The GH18 is a chitinase that reacts with β-chitin. Interestingly, the two uncharacterized domains both bind polysaccharides, and improve enzyme stability. However, they do not necessarily bind the same glycan as their GH partners. Our result demonstrates how two distinct polysaccharide-degrading modules within one protein can cooperate. We also discuss the impact of the binding domains on the GH activity. We anticipate our research can stimulate the exploration of β-1,6-glucanase activity in family GH5, and expand the database of carbohydrate specificities in bacteria. The two unusual binding domains we have characterized suggest an unexplored aspect of bacterial physiology in the soil.