Bacterial β-Kdo glycosyltransferases represent a new glycosyltransferase family (GT99)

Kdo (3-deoxy-D-manno-oct-2-ulosonic acid) is an eight-carbon sugar mostly confined to Gram-negative bacteria. It is often involved in attaching surface polysaccharides to their lipid anchors. alpha-Kdo provides a bridge between lipid A and the core oligosaccharide in all bacterial LPSs, whereas an oligosaccharide of beta-Kdo residues links " group 2" capsular polysaccharides to (lyso) phosphatidylglycerol. beta-Kdo is also found in a small number of other bacterial polysaccharides. The structure and function of the prototypical cytidine monophosphate-Kdo-dependent alpha-Kdo glycosyltransferase from LPS assembly is well characterized. In contrast, the beta-Kdo counterparts were not identified as glycosyltransferase enzymes by bioinformatics tools and were not represented among the 98 currently recognized glycosyltransferase families in the Carbohydrate-Active Enzymes database. We report the crystallographic structure and function of a prototype beta-Kdo GT from WbbB, a modular protein participating in LPS O-antigen synthesis in Raoultella terrigena. The beta-Kdo GT has dual Rossmann-fold motifs typical of GT-B enzymes, but extensive deletions, insertions, and rearrangements result in a unique architecture that makes it a prototype for a new GT family (GT99). The cytidine monophosphate-binding site in the C-terminal alpha/beta domain closely resembles the corresponding site in bacterial sialyltransferases, suggesting an evolutionary connection that is not immediately evident from the overall fold or sequence similarities.