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: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:12829714). Some peptide transferase activity is however not excluded, considering that its appropriate peptide substrate may remain unidentified (PubMed:12829714). Prefers the diglycosylated Muc5AC-3/13 as substrate (PubMed:12829714). 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 during oogenesis, in the somatically derived follicle cells that surround the developing oocyte, which are involved in the maturation of the oocyte and construction of the egg shell, as well as playing a role in subsequent embryonic pattern formation. Expressed in the salivary glands from embryonic stage 12 onwards, becoming stronger at stage 13. During embryonic stages 12-13, also expressed in the posterior midgut and hindgut. During embryonic stages 14-15, expression continues in the hindgut. Expression is detected in the epidermis and antennomaxillary complex during embryonic stages 16-17. In third instar larvae, ubiquitously expressed in wing, eye-antennal, leg and haltere imaginal disks. Expressed throughout embryonic, larval, pupal and adult stages, with increasing levels during larval development. Transcripts first detected during embryonic stages 12-13. 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 reduces viability. Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily.