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:30158631). 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 (PubMed:30158631). N-acetylgalactosaminyltransferase which preferentially O-glycosylates negatively charge substrates. O-glycosylates mucin-like protein Sgs3 in the salivary gland but to a lesser extent than isoform B. By regulating the O-glycosylation of secretory cargo proteins plays a role in the morphology and maturation of salivary gland secretory granules. N-acetylgalactosaminyltransferase which preferentially O-glycosylates positively charge substrates. O-glycosylates mucin-like protein Sgs3 in the salivary gland. By regulating the O-glycosylation of secretory cargo proteins, plays a role in the morphology and maturation of salivary gland secretory granules. 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. Isoform A forms homotetramer. Isoform B forms homodimer. Expressed both maternally and zygotically (PubMed:16251381). Expressed through embryonic and larval stages (PubMed:16251381). During embryonic stages 9-11, expressed in the developing anterior midgut and amnioserosa (PubMed:16251381). Expressed in the salivary glands from embryonic stage 12 onwards (PubMed:16251381). During embryonic stages 12-13, still expressed in the amnioserosa region (PubMed:16251381). In third instar larvae, expressed ubiquitously in wing, with increased expression in the notum and ventral wing pouch, leg and haltere imaginal disks (PubMed:16251381). In eye-antennal imaginal disk, expressed in the presumptive eye region only (PubMed:16251381). Isoform A: Expressed in trachea, midgut, salivary gland, hindgut, central nervous system and Malpighian tubules (PubMed:30158631). Isoform B: Specifically expressed in the salivary gland (PubMed:30158631). There are two conserved domains in the glycosyltransferase region: the N-terminal domain (domain A, also called GT1 motif), which is involved in manganese coordination and substrate binding and the C-terminal domain (domain B, also called Gal/GalNAc-T motif), which is involved in catalytic reaction and UDP-Gal binding. The alpha subunit loop in the ricin B-type lectin domain regulates substrate specificity and modulates the substrate access to the active site. In isoform A, the alpha subunit loop is composed of positively charged residues and acts on negatively charged substrates. In isoform B, the alpha subunit loop is composed of negatively charged residues and acts on positively charged substrates. RNAi-mediated knockdown in the whole body, in the mesoderm, the respiratory system or the amnioserosa results in lethality (PubMed:22157008). RNAi-mediated knockdown in the epidermis or the digestive system and reproductive tract results in a reduction in viability (PubMed:22157008). RNAi-mediated knockdown in salivary glands results in aberrant secretory granule morphology showing angular, shard-like morphology (PubMed:30158631). Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. Intron retention. Intron retention.