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. Although it displays a much weaker activity toward all substrates tested compared to GALNT2, it is able to transfer up to seven GalNAc residues to the Muc5AC peptide, suggesting that it can fill vicinal Thr/Ser residues in cooperation with other GALNT proteins. Prefers Muc1a 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. Widely expressed. Highly expressed in small intestine, placenta, spleen, cerebral cortex and ovary. Expressed at intermediate level in uterus, mammary gland, stomach, cerebellum and whole brain. Weakly expressed in fetal brain, bone marrow, thyroid gland, thymus, heart, skeletal muscle, lung, liver, colon, pancreas, kidney and testis. Not expressed in leukocyte. Expressed in both normal and osteoarthritic cartilage. Expressed at low level in chondrocytes in all zones of both normal and osteoarthritic cartilage. 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. Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. Was originally termed Galnt15/pp-GaNTase 15. Polypeptide N-acetylgalactosaminyltransferase-like protein 2