The lectin domain of UDP-N-acetyl-D-galactosamine : polypeptide N-acetylgalactosaminyltransferase-T4 directs its glycopeptide specificities

The initiation step of mucin-type O-glycosylation is controlled by a large family of homologous UDP-GalNAc: polypeptide N-acetylgalactosaminyltransfer ases (GalNAc-transferases), Differences in kinetic properties, substrate sp ecificities, and expression patterns of these isoenzymes provide for differ ential regulation of O-glycan attachment sites and density, Recently, it ha s emerged that some GalNAc-transferase isoforms in, vitro selectively funct ion with partially GalNAc O-glycosylated acceptor peptides rather than with the corresponding unglycosylated peptides, O-Glycan attachment to selected sites, most notably two sites in the MUC1 tandem repeat, is entirely depen dent on the glycosylation-dependent function of GalNAc-T4. Here we present data that a putative lectin domain found in the C terminus of GalNAc-T4 fun ctions as a GalNAc lectin and confers its glycopeptide specificity. A singl e amino acid substitution in the lectin domain of a secreted form of GalNAc -T4 selectively blocked GalNAc-glycopeptide activity, while the general act ivity to peptides exerted by this enzyme was unaffected. Furthermore, the G alNAc-glycopeptide activity of wild-type secreted GalNAc-T4 was selectively inhibited by free GalNAc, while the activity with peptides was unaffected.