Molecular function

UDP-GalNAc:polypeptide N-acetylgalactosaminyl transferase activity, UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase activity, UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyl-transferase activity, UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase activity, UDP-N-acetylgalactosamine-glycoprotein N-acetylgalactosaminyltransferase activity, UDP-N-acetylgalactosamine-protein N-acetylgalactosaminyltransferase activity, UDP-N-acetylgalactosamine:kappa-casein polypeptide N-acetylgalactosaminyltransferase activity, UDP-N-acetylgalactosamine:polypeptide N-acetylgalactosaminyltransferase activity, UDP-N-acetylgalactosamine:protein N-acetylgalactosaminyl transferase activity, UDP-acetylgalactosamine-glycoprotein acetylgalactosaminyltransferase activity, UDP-acetylgalactosamine:peptide-N-galactosaminyltransferase activity, glycoprotein acetylgalactosaminyltransferase activity, polypeptide-N-acetylgalactosamine transferase activity, protein-UDP acetylgalactosaminyltransferase activity, uridine diphosphoacetylgalactosamine-glycoprotein acetylgalactosaminyltransferase activity

Catalysis of the reaction: UDP-N-acetyl-D-galactosamine + polypeptide = UDP + N-acetyl-D-galactosaminyl-polypeptide. This reaction is the modification of serine or threonine residues in polypeptide chains by the transfer of a N-acetylgalactose from UDP-N-acetylgalactose to the hydroxyl group of the amino acid; it is the first step in O-glycan biosynthesis.