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 EA2 peptide substrate, but has a weak activity toward Muc2, Muc1b, rMuc-2 or mG-Muc substrates. 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. Interacts with EXT2. Does not interact with EXT1, EXTL1 or EXTL3 (By similarity). Predominantly expressed in sublingual gland. Expressed at lower level in stomach and small intestine. Weakly or not expressed in submandibular gland, parotid gland, kidney, liver, heart, brain, spleen, lung, skeletal muscle, testis, ovary, cervix and uterus. 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.