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 (PubMed:16638743, PubMed:31932717, PubMed:8663203, PubMed:9295285). Has activity toward HIV envelope glycoprotein gp120, EA2, MUC2, MUC1A and MUC5AC (PubMed:8663203, PubMed:9295285). Probably glycosylates fibronectin in vivo (PubMed:9295285). Glycosylates FGF23 (PubMed:16638743, PubMed:31932717).
