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. Displays activity toward mucin-derived peptide substrates such as Muc2, Muc5AC, Muc7, and Muc13 (-58). May be involved in O-glycosylation in kidney. 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. Detected in renal tubules (at protein level). Highly expressed in fetal and adult kidney. Widely expressed at low level. Weakly expressed in whole brain, cerebellum, thymus, lung, mammary gland, liver, stomach, small intestine, colon, pancreas, spleen, bladder, uterus, placenta, testis, ovary, skeletal muscle, leukocyte, B-cell, bone marrow, fetal brain, fetal thymus, fetal lung, fetal liver, fetal small intestine, fetal spleen, fetal skeletal and fetus. Detected in renal tubules (at protein level). 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. Major isoform. Minor isoform. Belongs to the glycosyltransferase 2 family. GalNAc-T subfamily. Chimeric at the C-terminus. Polypeptide N-acetylgalactosaminyltransferase 14