Chick neural retina N-acetylgalactosaminyltransferase/acceptor complex: catalysis involves transfer of N-acetylgalactosamine phosphate to endogenous acceptors

Abstract

Homogenates of embryonic chick neural retina prepared in 1% Triton X-100 have the ability to transfer N-acetyl[32P]galactosamine [( 32P]GalNAc) from beta-32P-labeled uridine diphosphate N-acetylgalactosamine [( beta-32P]UDP-GalNAc) to endogenous macromolecular acceptors. The phosphotransferase activity sediments as three distinct peaks upon centrifugation on sucrose gradients. These peaks are coincident with the transferase/acceptor complexes previously described [Balsamo, J., & Lilien, J. (1982) J. Biol. Chem. 257, 345-354]. The parameters of the 32P transfer reaction closely parallel those observed with UDP-[3H]GalNAc as substrate when the densest particles, H, are used as a source of transferase/acceptors. Treatment of 3H- and 32P-labeled products with alpha-N-acetylgalactosaminidase removes [3H]GalNAc residues and exposes 32P-labeled groups. These data suggest that the sugar-phosphate is transferred intact, resulting in a terminal phosphodiester linkage. The resistance of the macromolecular products to digestion by endoglycosidase F and its sensitivity to hydrolysis under mild alkaline conditions suggest that the alpha-linked sugar is transferred to an oligosaccharide chain attached to the protein core via an O-serine or threonine residue. Characterization of the 32P- and 3H-labeled H particle products by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals a series of coincident high molecular weight polypeptides.