Biosynthesis of the polysialic acid capsule in Escherichia coli K1. The endogenous acceptor of polysialic acid is a membrane protein of 20 kDa.

The nature of endogenous acceptor molecules implicated in the membrane-directed synthesis of the polysialic acid (polySia) capsule in Escherichia coli K1 serotypes is not known. The capsule contains at least 200 sialic acid (Sia) residues that are elongated by the addition of new Sia residues to the nonreducing termini of growing nascent chains (Rohr, T. E., and Troy, F. A. (1980) J. Biol. Chem. 255, 2332-2342). Presumably, chain growth starts when activated Sia residues are transferred to acceptors that are not already sialylated. In the present study, we used an acapsular mutant defective in synthesis of CMP-NeuAc to label acceptors with [14C]NeuAc and an anti-polySia-specific antibody (H.46) to identify the molecules to which the polySia was attached. [14C]Sia-labeled acceptors were solubilized with 2% Triton X-100, immunoprecipitated with H.46, and partially depolymerized with poly-alpha-2,8-endo-N-acetylneuraminidase. Approximately 5% of the [14C]Sia incorporated remained attached to endogenous acceptors. Double-labeling experiments were used to show that the non-Sia moiety of the acceptor was labeled in vivo with [14C]leucine and elongated in vitro with CMP-[3H]NeuAc. Concomitant with desialylation of the [3H]polySia-[14C]Leu acceptor was the appearance of a new [14C]Leu-labeled protein at 20 kDa. After strong acid hydrolysis, the 20-kDa labeled protein was shown to contain [14C]Leu. The acceptor molecules were not labeled metabolically with D-[3H]GlcN, 35SO4, or 32PO4, indicating that they do not appear to contain lipopolysaccharide, peptidoglycan, phosphatidic acid, or phospholipid. Based on these results, we conclude that the endogenous acceptor molecule is a membrane protein of about 20 kDa. The nature of attachment of polySia to acceptor is unknown. There are only 400-500 acceptor molecules/cell, which is about 100-fold fewer than the 50,000 polySia chains/cell. This suggests that each acceptor molecule may participate in the shuttling of about 100 polySia chains/cell. We hypothesize that the acceptor protein may function to translocate polySia chains from their site of synthesis on the cytoplasmic surface of the inner membrane to the periplasm.