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). 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 EA2 as substrate. Has weak activity toward Muc5AC-3, -13 and -3/13 substrates (PubMed:12829714). Plays a critical role in the regulation of integrin-mediated cell adhesion during wing development by influencing, via glycosylation, the secretion and localization of the integrin ligand Tig to the basal cell layer interface (PubMed:18835818, PubMed:20371600, PubMed:20807760, PubMed:22157008). 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). Together with Pgant35A, regulates integrin levels and activity-dependent integrin signaling at the synapse in neurons and muscles (PubMed:25253852). 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 primordiums of the foregut, midgut and hindgut. During embryonic stages 12-13, expression is found uniquely in the posterior spiracle. During embryonic stages 14-17, expressed in the pharynx, esophagus and posterior spiracles. Expression observed in the epidermis during embryonic stages 16-17. In third instar larvae, expressed ubiquitously in wing, with increased expression in pleura and notum, eye-antennal, leg and haltere imaginal disks. Expressed both maternally and zygotically. Expressed throughout embryonic, larval, pupal and adult stages. 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. Mutant larval wing disks show a decrease in thickness and in the content of O-glycoproteins specifically along the basal surface of the columnar epithelial cells (PubMed:18835818, PubMed:20371600). Adult mutants display blistered wings (PubMed:18835818, PubMed:20371600). Mutant larval shows down-regulation of synaptic O-linked glycosylation, integrin level and signaling via Ten-m and if. Synapses show smaller synaptic boutons, expanded activity-dependent postsynaptic pockets which affect synaptic plasticity and synaptic strength in both the pre-synaptic and post-synaptic assembly, no differences in neuromuscular junction morphology (PubMed:25253852). Simultaneous knockout of Pgant35A, restores normal synaptic strength (PubMed:25253852). RNAi-mediated knockdown in the developing wing results in a blistered phenotype (PubMed:22157008). Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily.