CMP-N-acetylneuraminate-poly-alpha-2,8-sialyltransferase
Golgi apparatus
In vivo autopolysialylation and localization of the polysialyltransferases PST and STX.
A select group of mammalian proteins have been shown to possess alpha2,8-polysialylated oligosaccharide chains. The best studied of these proteins is the neural cell adhesion molecule (NCAM). Polysialylation of NCAM has been shown to decrease NCAM-dependent and independent cell adhesion. PST (ST8Sia IV) and STX (ST8Sia II) are the two polysialyltransferases responsible for NCAM polysialylation. Recent studies revealed that PST itself is autopolysialylated in vitro (Muhlenhoff, M., Eckhardt, M., Bethe, A., Frosch, M., and Gerardy-Schahn, R. (1996) EMBO J. 15, 6943-6950). Here we report studies on the biosynthesis and localization of the PST and STX polysialyltransferases. Both PST and STX are expressed as high molecular mass, polydisperse forms that are associated with the cell and found soluble in the medium. Analysis of these high molecular mass forms by glycosidase digestion and serial immunoprecipitation/immunoblot experiments demonstrated that PST and STX are autopolysialylated in vivo. Indirect immunofluorescence microscopy and immunoprecipitation analyses demonstrated that autopolysialylated PST and STX are localized in the Golgi, on the cell surface, and in the extracellular space. The cell surface and extracellular localization of these polysialylated polysialyltransferases suggest that their polysialic acid chains, like those of NCAM, may modulate cell interactions.