Glycosyl transferases of baby hamster kidney cells and ricin-resistant mutants. O-glycan biosynthesis.

Abstract

Extracts of baby hamster kidney (BHK) cells catalyzed the incorporation of N-acetylgalactosamine from UDP-N-acetyl[14C]galactosamine into myelin basic protein and an acylated tetrapeptide, N-acetylthreonyl-triproline, based on the threonine residue 98, glycosylated in myelin basic protein. The incorporated N-acetylgalactosamine residues were shown to be in alpha linkage to the peptide moieties. Several ricin-resistant BHK cell lines contained enhanced (approximately twofold) levels of the transferase activity. Apomucins obtained from bovine submaxillary gland mucin by chemical or enzymic degradation were relatively poor acceptors. Using asialomucin as acceptor, galactosyl, transferase activities and a weak sialyl transferase activity were detected in BHK cell extracts. Galactose transfer occurred at two sites: to peptide-linked N-acetylgalactosamine residues to form the linkage, galactosyl-(beta 1 leads to 3)-N-acetylgalactosamine and to terminally linked N-acetylglucosamine residues that exist as a minor constituent in bovine submaxillary mucin O-glycans, to form a galactosyl N-acetylglucosamine linkage. This reaction was not inhibited by ovalbumin, an efficient acceptor of the beta 1 leads to 4 galactosyl transferase involved in N-glycan assembly. Incorporation of galactose and N-acetylgalactosamine into endogenous proteins of BHK cell extracts was also detected. Sialic acid, fucose and N-acetylglucosamine residues were not incorporated. The incorporated N-acetylgalactosamine residues were shown to be in alpha linkage to polypeptide, and galactose incorporation represented synthesis of the galactosyl-(beta 1 leads to 3)-N-acetylgalactosamine sequence linked to polypeptide. The major endogenous protein labelled by either sugar had a molecular weight of approximately 80 000. A BHK-cell-associated glycoprotein, analogous to the urinary Tamm-Horsfall glycoprotein of molecular weight similar to the major endogenous acceptor of glycosylation, was not glycosylated in the experiments in vitro.