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(+) Protein modification; protein glycosylation. Monomer and homodimer. Membrane-bound form in trans cisternae of Golgi. Secreted into the body fluid. Highly expressed in testis, brain and liver and to a lesser extent in lung and heart. The soluble form derives from the membrane form by proteolytic processing. N-glycosylated. Belongs to the glycosyltransferase 29 family.