Glycopeptide transferase involved in O-linked oligosaccharide biosynthesis, which catalyzes the transfer of an N-acetyl-D-galactosamine residue to an already glycosylated peptide (PubMed:11925450, PubMed:12829714). In contrast to other proteins of the family, it does not act as a peptide transferase that transfers GalNAc onto serine or threonine residue on the protein receptor, but instead requires the prior addition of a GalNAc on a peptide before adding additional GalNAc moieties (PubMed:11925450). Some peptide transferase activity is however not excluded, considering that its appropriate peptide substrate may remain unidentified. Prefers the monoglycosylated Muc5AC-3 as substrate (PubMed:11925450). Might have a role in protein O-glycosylation in the Golgi and thereby in establishing and/or maintaining a proper secretory apparatus structure (PubMed:20807760). L-seryl-[protein] + UDP-N-acetyl-alpha-D-galactosamine = 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-seryl-[protein] + H(+) + UDP L-threonyl-[protein] + UDP-N-acetyl-alpha-D-galactosamine = 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + H(+) + UDP Protein modification; protein glycosylation. Expressed in developing oocytes and egg chambers. During embryonic stages 9-11, expressed in the primordium of the foregut, midgut and hindgut. Expressed in the salivary glands from embryonic stage 12 onwards. During embryonic stages 12-13, expressed in the posterior midgut and hindgut. During embryonic stages 14-15, expression continues in the hindgut. During embryonic stages 16-17, expressed in the antennomaxillary complex. In third instar larvae, ubiquitously expressed in wing, with increased expression in the notum and ventral wing pouch, eye-antennal, leg and haltere imaginal disks. Expressed both maternally and zygotically. Expressed throughout embryonic, larval, pupal and adult stages, with increasing levels during larval development. There are two conserved domains in the glycosyltransferase region: the N-terminal domain (domain A, also called GT1 motif), which is probably involved in manganese coordination and substrate binding and the C-terminal domain (domain B, also called Gal/GalNAc-T motif), which is probably involved in catalytic reaction and UDP-Gal binding. The ricin B-type lectin domain binds to GalNAc and contributes to the glycopeptide specificity. RNAi-mediated knockdown in the whole body or respiratory system during development is lethal (PubMed:22157008). RNAi-mediated knockdown in hemocytes, amnioserosa, mesoderm or digestive system and reproductive tract causes no defect (PubMed:22157008). Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. Truncated N-terminus. Truncated N-terminus.