a 3-O-[N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + CMP-N-acetyl-beta-neuraminate = a 3-O-[N-acetyl-alpha-neuraminosyl-(2->6)-N-acetyl-alpha-D-galactosaminyl]-L-threonyl-[protein] + CMP + H(+) — reaction