Differences in protein structural regions that impact functional specificity in GT2 family β-glucan synthases

Most cell wall and secreted beta-glucans are synthesised by the CAZy Glycosyltransferase 2 family (www.cazyorg), with different members catalysing the formation of (1,4)-beta-, (1,3)-beta-, or both (1,4)-and (1,3)-beta-glucosidic linkages. Given the distinct physicochemical properties of each of the resultant beta-glucans (cellulose, curdlan, and mixed linkage glucan, respectively) are crucial to their biological and biotechnological functions, there is a desire to understand the molecular evolution of synthesis and how linkage specificity is determined. With structural studies hamstrung by the instability of these proteins to solubilisation, we have utilised in silico techniques and the crystal structure for a bacterial cellulose synthase to further understand how these enzymes have evolved distinct functions. Sequence and phylogenetic analyses were performed to determine amino acid conservation, both family-wide and within each sub-family. Further structural analysis centred on comparison of a bacterial curdlan synthase homology model with the bacterial cellulose synthase crystal structure, with molecular dynamics simulations performed with their respective beta-glucan products bound in the trans-membrane channel. Key residues that differentially interact with the different beta-glucan chains and have sub-family-specific conservation were found to reside at the entrance of the trans-membrane channel. The linkage-specific catalytic activity of these enzymes and hence the type of beta-glucan chain built is thus likely determined by the different interactions between the proteins and the first few glucose residues in the channel, which in turn dictates the position of the acceptor glucose. The sequence-function relationships for the bacterial beta-glucan synthases pave the way for extending this understanding to other kingdoms, such as plants.