Synthesis and Secretion of Microbial Extracellular Polysaccharides

Abstract

Complex carbohydrates are frequently deposited on the cell surface to form cell walls, capsules, or 3-dimensional matrices called biofilms. Often, the high molecular weight polymers are synthesized intracellularly and transported across at least one biological membrane by dedicated translocation machineries. We are interested in how polysaccharides are assembled and deposited on the cell surface. In particular, bacterial cellulose is a major biofilm component of many enterobacteria and secreted during its synthesis by the membrane-embedded cellulose synthase, a processive glycosyltransferase. Using the tools of structural and molecular biology, we delineated the mechanism by which the enzyme elongates a cellulose polymer and translocates it across the membrane through a pore formed by its own transmembrane region. We have also examined O antigens, which constitute the variable region of lipopolysaccharides in the outer membrane of Gram-negative bacteria. The polymers are synthesized inside the cell on a lipid anchor and transported to the periplasm by the WzmWzt ABC transporter. Crystal structures of the transporter in nucleotide-free and ATP-bound states suggest a processive translocation mechanism through a channel formed by the transporter. Molecular dynamics simulations suggest that membrane lipids are a fundamental part of the transporter's function by sealing its channel in a resting state.