N-acetyl-D-galactosamine + L-threonyl-[protein] => a 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + H2O
n-acetyl-d-galactosamine + l-threonyl-[protein] => a 3-o-[n-acetyl-alpha-d-galactosaminyl]-l-threonyl-[protein] + h2o