Biosynthesis and assembly of the polysialic acid capsule in Escherichia coli K1. Activation of sialyl polymer synthesis in inactivate sialyltransferase complexes requires protein synthesis.

The polysialosyl capsule in Escherichia coli K1 is not synthesized when cells are grown at 15 degrees C. Membranous sialyltransferase complexes isolated from cells grown at 15 degrees C spontaneously gain the ability to synthesize sialyl polymers when incubated at 33 degrees C for 2-4 h. Activation of the endogenous synthesis of polysialic acid is localized in a low-density vesicle fraction (Whitfield, C., Adams, D.A., and Troy, F.A. (1984) J. Biol. Chem. 259, 12769-12775). The low density vesicles catalyzed protein synthesis, and spontaneous activation required protein synthesis. Immune blotting confirmed the presence of protein synthesis initiation factor, IF2, and ribosomal protein S6, in the low-density vesicle fraction. Temperature-upshift experiments identified four membrane proteins whose synthesis was temporally correlated with in vitro activation. These proteins have apparent molecular masses of 40, 21, 17.5, and 16 kDA. Although the function of these proteins is not known, they may relate to a sialyltransferase activity required to initiate sialyl polymer assembly, to an endogenous acceptor, or to a porin that may facilitate channeling of the polysialosyl chains through the outer membrane.