Structure of a full-length bacterial polysaccharide co-polymerase.

Lipopolysaccharides are important components of the bacterial cell envelope that among other things act as a protective barrier against the environment and toxic molecules such as antibiotics. One of the most widely disseminated pathways of polysaccharide biosynthesis is the inner membrane bound Wzy-dependent pathway. Here we present the 3.0 A structure of the co-polymerase component of this pathway, WzzB from E. coli solved by single-particle cryo-electron microscopy. The overall architecture is octameric and resembles a box jellyfish containing a large bell-shaped periplasmic domain with the 2-helix transmembrane domain from each protomer, positioned 32 A apart, encircling a large empty transmembrane chamber. This structure also reveals the architecture of the transmembrane domain, including the location of key residues for the Wzz-family of proteins and the Wzy-dependent pathway present in many Gram-negative bacteria, explaining several of the previous biochemical and mutational studies and lays the foundation for future investigations. Lipopolysaccharides, important components of the bacterial cell envelope, are synthesized at the inner membrane by the Wzx/Wzy-dependent assembly pathway. A cryo-EM structure of an intact E. coli WzzB, the polysaccharide co-polymerase component of this pathway, reveals details of the transmembrane, cytoplasmic domains and a conserved a proline-rich segment proximal to the C-terminal transmembrane helix.