The Lectin Domain of UDP-N-acetyl-d-galactosamine:PolypeptideN-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 -acetylgalactosaminyltransferases (GalNAc-transferases). Differences in kinetic properties, substrate specificities, and expression patterns of these isoenzymes provide for differential regulation of O -glycan attachment sites and density. Recently, it has emerged that some GalNAc-transferase isoforms in vitro selectively function 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 dependent 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 functions as a GalNAc lectin and confers its glycopeptide specificity. A single amino acid substitution in the lectin domain of a secreted form of GalNAc-T4 selectively blocked GalNAc-glycopeptide activity, while the general activity to peptides exerted by this enzyme was unaffected. Furthermore, the GalNAc-glycopeptide activity of wild-type secreted GalNAc-T4 was selectively inhibited by free GalNAc, while the activity with peptides was unaffected.