Catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor (PubMed:12829714, PubMed:18669915). It can both act as a peptide transferase that transfers GalNAc onto unmodified peptide substrates, and as a glycopeptide transferase that requires the prior addition of a GalNAc on a peptide before adding additional GalNAc moieties. Prefers the monoglycosylated Muc5AC-3 as substrate (PubMed:12829714). O-glycosylation modification of proteins by this enzyme might be important for cytokinesis (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. No expression observed during embryonic stages 9-11. During embryonic stages 12-13, specific expression is observed in the developing tracheal branches and brain. During embryonic stages 14-17, expression is restricted to the dorsal longitudinal trachea. In third instar larvae imaginal wing disk, expressed in clusters of cells in the presumptive pleura and notum. In eye-antennal imaginal disk, shows a very distinct band of expression at the morphogenetic furrow and weaker expression in the presumptive eye posterior to the furrow, no expression is detected in the presumptive antennal or head region anterior to the furrow. No expression observed in leg or haltere imaginal disks. Expressed both maternally and zygotically. Expressed during embryonic, larval, pupal and adult stages. Weakly expressed in the male and female body and correspondingly low levels during early embryonic stages. Significantly expressed from embryonic stage 8-12 hours and reaches maximal levels in the pupae and male head. 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 is viable. Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily.