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. Has activity toward HIV envelope glycoprotein gp120, EA2, Muc2 and Muc5. Glycosylates FGF23 (By similarity). Probably glycosylates fibronectin in vivo. May be involved in phosphate homeostasis. 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. Resides preferentially in the trans and medial parts of the Golgi stack. Highly expressed in the reproductive tract, principally in the testis and uterus, and to a lesser degree in the cervix with only trace levels in the ovary. Also expressed at high level in sublingual gland, stomach and colon, with more moderate amounts present in the submandibular and parotid gland as well as the kidney. 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. Polypeptide N-acetylgalactosaminyltransferase 3