A bifunctional O-antigen polymerase structure reveals a new glycosyltransferase family

Abstract

Lipopolysaccharide O-antigen is an attractive candidate for immunotherapeutic strategies targeting antibiotic-resistant Klebsiella pneumoniae. Several K. pneumoniae O-serotypes are based on a shared O2a-antigen backbone repeating unit; [→3)-α-Galp-(1→3)-β-Galf-(1→]. O2a antigen is synthesized on undecaprenol diphosphate in a pathway involving the O2a polymerase, WbbM, prior to its export by an ABC transporter. This dual domain polymerase possesses a C-terminal galactopyranosyltransferase resembling known GT8 family enzymes, and an N-terminal DUF4422 domain identified here as a galactofuranosyltransferase defining a previously unrecognized family (GT111). Functional assignment of DUF4422 explains how galactofuranose is incorporated into various polysaccharides of importance in vaccine production and the food industry. In the 2.1 Å-resolution structure, three WbbM protomers associate to form a flattened triangular prism connected to a central stalk that orients the active sites towards the membrane. The biochemical, structural and topological properties of WbbM offer broader insight into the mechanisms of assembly of bacterial cell-surface glycans.