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 a highest activity toward EA2 peptide substrate and a much lower activity with EPO-T, Muc2, Muc1a, Muc1b. 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. Highly expressed in sublingual gland, stomach, colon, small intestine and cervix. Expressed at intermediate levels in kidney, ovary, lung and uterus. Weakly expressed in spleen, liver, heart and brain. Not expressed in submandibular and parotid glands, skeletal muscle and testis. 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 directs the glycopeptide specificity. It is required in the glycopeptide specificity of enzyme activity but not for activity with naked peptide substrates, suggesting that it triggers the catalytic domain to act on GalNAc-glycopeptide substrates. Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. Polypeptide N-acetylgalactosaminyltransferase 4