Figure 3.

Biosynthesis of polysialic acids in E. coli using the ABC transporter pathway. The biosynthesis of homopolymeric O antigens (or capsules) is initiated at the cytosolic side of the inner membrane. Polysialic acids (PSA) of E. coli are proposed to be assembled in a processive fashion as shown, based on undecaprenol-phosphate. Membrane-associated polysialyltransferase (PST) transfers many units of sialic acid from CMP-sialic acid to the growing polysialic acid. The enzyme can act on a number of acceptor substrates to form repeated sialylα2–8 linkages. A termination reaction stops the growth of the long PSA chain. The PSA is then transported to the periplasmic space by the Wzm exporter, which is associated with the ATP-binding Wzt. Further processing occurs in the periplasm. The completed PSA is ligated to the core-lipid A and then translocated by export proteins to the outer membrane to serve as a highly charged and hydrophilic protective coat. Other homopolymeric O antigens such as poly-d-Mannose or poly-d-Rhamnose are processed in a similar fashion by the ABC transporter pathway.