Transfers sialic acid from CMP-sialic acid to galactose-containing acceptor substrates. a beta-D-galactoside + CMP-N-acetyl-beta-neuraminate = an N-acetyl-alpha-neuraminyl-(2->6)-beta-D-galactosyl derivative + CMP + H(+) Inhibited by CTP. Protein modification; protein glycosylation. Monomer and homodimer. Membrane-bound form in trans cisternae of Golgi. Secreted into the body fluid. The soluble form derives from the membrane form by proteolytic processing. The HB-6, CDW75, and CD76 differentiation antigens are cell-surface carbohydrate determinants generated by this enzyme. N-glycosylated. Belongs to the glycosyltransferase 29 family. ST6Gal I