A coiled-coil domain acts as a molecular ruler to regulate O-antigen chain length in lipopolysaccharide

Abstract

Long-chain bacterial polysaccharides have important roles in pathogenicity. In Escherichia coli O9a, a model for ABC transporter–dependent polysaccharide assembly, a large extracellular carbohydrate with a narrow size distribution is polymerized from monosaccharides by a complex of two proteins, ​WbdA (polymerase) and ​WbdD (terminating protein). Combining crystallography and small-angle X-ray scattering, we found that the C-terminal domain of ​WbdD contains an extended coiled-coil that physically separates ​WbdA from the catalytic domain of ​WbdD. The effects of insertions and deletions in the coiled-coil region were analyzed in vivo, revealing that polymer size is controlled by varying the length of the coiled-coil domain. Thus, the coiled-coil domain of ​WbdD functions as a molecular ruler that, along with ​WbdA:​WbdD stoichiometry, controls the chain length of a model bacterial polysaccharide