Glycopeptide transferase involved in O-linked oligosaccharide biosynthesis, which catalyzes the transfer of an N-acetyl-D-galactosamine residue to an already glycosylated peptide. 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. Some peptide transferase activity is however not excluded, considering that its appropriate peptide substrate may remain unidentified. Prefers the diglycosylated Muc5AC-3/13 as substrate. 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. During embryonic stages 12-13, shows specific expression in the proventriculus that continues until the end of embryogenesis. In third instar larvae, ubiquitously expressed in wing, eye-antennal, leg and haltere imaginal disks. Expressed both maternally and zygotically. Expressed during embryonic, larval, pupal and adult stages. Weakly expressed during early embryonic stages but displays a dramatic increase at 12-24 hours of embryonic development. Continues to be in adult but displays much lower levels in the female adult as compared with the male. 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 the digestive system and reproductive tract is lethal (PubMed:22157008). RNAi-mediated knockdowin in hemocytes, epiderm, endoderm, mesoderm, respiratory system or nervous system causes no defect (PubMed:22157008). Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. Extended N-terminus.