Functional Domains in the Chitin Oligosaccharide Synthase NodC and Related β-Polysaccharide Synthases

Abstract

Structure-comparisons of glycosyltransferases is hampered by the absence of extended sequence conservations. Only short regions of limited homology have been reported for groups of closely-related transferases such as the β-galactosyltransferase family, the sialyltransferases, and the β-polysaccharide synthases: a group of glycosyltransferases involved in the synthesis of linear polysaccharides that consist of β-linked saccharides. Examples of such enzymes are chitin synthase, cellulose synthase, hyaluronic acid synthase, and the bacterial NodC protein which synthesizes chitin oligosaccharides. In this paper we summarize the known functional aspects of this group of transferases, and possible links with structural aspects. We have found that all members contain six short sequences which are conserved throughout this family. Site-directed mutagenesis studies reported in literature have shown that the conserved residues in these conserved β-polysaccharide synthase regions are important, or even essential for enzyme activity. Since a detailed study of these mutants with regard to nucleotide-sugar binding or glycosyl acceptor binding has not been reported, the data generated by these studies do not provide information about the precise roles of the conserved β-polysaccharide synthase regions in substrate-binding and catalysis. However, we report that a novel motif, conserved in all members of this β-polysaccharide synthase family, is homologous to known nucleotide-binding motifs in nucleoside-triphosphate-binding proteins. In addition we present a sequence analysis that indicates putative functions for the conserved regions in the β-polysaccharide synthase family in substrate-specificity, catalysis, and product chainlength control.