Biosynthesis of a conserved glycolipid anchor for Gram-negative bacterial capsules.

Several important Gram-negative bacterial pathogens possess surface capsular layers composed of hypervariable long-chain polysaccharides linked via a conserved 3-deoxy-ß-D-manno-oct-2-ulosonic acid (ß-Kdo) oligosaccharide to a phosphatidylglycerol residue. The pathway for synthesis of the terminal glycolipid was elucidated by determining the structures of reaction intermediates. In Escherichia coli, KpsS transfers a single Kdo residue to phosphatidylglycerol; this primer is extended using a single enzyme (KpsC), possessing two cytidine 5'-monophosphate (CMP)-Kdo-dependent glycosyltransferase catalytic centers with different linkage specificities. The structure of the N-terminal ß-(2→4) Kdo transferase from KpsC reveals two α/ß domains, supplemented by several helices. The N-terminal Rossmann-like domain, typically responsible for acceptor binding, is severely reduced in size compared with canonical GT-B folds in glycosyltransferases. The similar structure of the C-terminal ß-(2→7) Kdo transferase indicates a past gene duplication event. Both Kdo transferases have a narrow active site tunnel, lined with key residues shared with GT99 ß-Kdo transferases. This enzyme provides the prototype for the GT107 family.