Polypeptide N-acetylgalactosaminyltransferases catalyze the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor. Displays the same enzyme activity toward MUC1, MUC4, and EA2. Not involved in glycosylation of erythropoietin (EPO). It can both act as a peptide transferase that transfers GalNAc onto unmodified peptide substrates, and as a glycopeptide transferase that requires the prior addition of a GalNAc on a peptide before adding additional GalNAc moieties. Protein modification by this enzyme might be important for cytokinesis and tube formation during embryogenesis. Together with Pgant3, regulates integrin levels and activity-dependent integrin signaling at the synapse in neurons and muscles