Structure and functional role of the SLHs-CBM54 tandem in bacterial multimodular glycoside hydrolases

Carbohydrate-binding modules (CBMs) enhance enzymatic degradation of polysaccharides by improving substrate binding. CBM54, a family of carbohydrate-binding modules, is found in surface-displayed multimodular glycoside hydrolases (MGHs) of gram-positive bacteria. These modules bind insoluble polysaccharides, including chitosan, chitin, xylan, cellulose, and fungal cell wall β-glucans. In MGHs, the CBM54 module is invariably positioned downstream of three S-layer homology modules (SLHs), which anchor the enzyme to the bacterial cell surface. The SLHs-CBM54 tandem promotes bacterial adherence to substrates, concentrating hydrolytic enzymes at the interface and facilitating efficient uptake of soluble degradation products. CBM54 contains a cleavage site that divides it into two structurally distinct parts, which remain associated via hydrogen bonding. The processing of CBM54 promotes bacterial detachment from the substrate and their migration toward new nutrient sources. Beyond MGHs, the SLHs-CBM54 tandem occurs in many other bacterial proteins with uncharacterized functions. SLH modules may self-assemble into 2D arrays on synthetic supports, enabling their use as matrices for protein nucleation and crystal growth. The SLHs-CBM54 tandem fusion with functional modules offers an innovative approach to surface modification, with broad biomedical and biotechnological applications. Due to its versatile architecture, this system holds promise for antigen display, vaccine development, and enhanced polysaccharide degradation processes.