Polypeptide N-acetylgalactosaminyltransferases catalyze the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor (PubMed:11925450, PubMed:12829714). Displays the same enzyme activity toward MUC1, MUC4, and EA2 (PubMed:11925450, PubMed:12829714). Not involved in glycosylation of erythropoietin (EPO) (PubMed:11925450). 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:11925450, PubMed:12829714). Protein modification by this enzyme might be important for cytokinesis and tube formation during embryogenesis (PubMed:16251381, PubMed:20807760). Together with Pgant3, 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 at high level in ovaries. Expressed at low level in testis. Expressed at higher level in adult females than males. During oogenesis, it is detected in germ cells and follicle epithelia of all developmental stages. Initially expressed during early stages of oogenesis in region I and reaches high levels in regions IIa and IIb of the germarium. Highly expressed in stage 2 egg chambers. Remains highly expressed during later stages of oogenesis. During embryonic stages 9-11, expressed in the primordium of the foregut, midgut and hindgut. Expressed in salivary glands from embryonic stage 12 onwards. During embryonic stages 12-13, expressed in the posterior midgut and hindgut. During embryonic stages 14-15, expression continues in the hindgut. During embryonic stages 16-17, expressed in the dorsal longitudinal trachea and posterior spiracles. In third instar larvae, ubiquitously expressed in wing, eye-antennal, leg and haltere imaginal disks. Expressed both maternally and zygotically. Expressed throughout embryonic, larval, pupal and adult stages, with increasing levels during larval development. 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 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 Pgant3, restores normal synaptic strength (PubMed:25253852). RNAi-mediated knockdown is lethal (PubMed:22157008). RNAi-mediated knockdown in the mesoderm, respiratory system, digestive system or reproductive tract results in a reduction in viability (PubMed:22157008). The human ortholog GALNT11 (AC Q8NCW6) is not able to rescue lethality caused by the SF32 mutation. Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. Truncated N-terminus.