Transfers the sialyl group (N-acetyl-alpha-neuraminyl or NeuAc) from CMP-NeuAc to the GalNAc residue on the NeuAc-alpha-2,3-Gal-beta-1,3-GalNAc sequence of glycoproteins and glycolipids forming an alpha-2,6-linkage. Produces branched type disialyl structures by transfer of a sialyl group onto a GalNAc residue inside the backbone core chains (PubMed:10207017, PubMed:10601645). Prefers O-glycans to glycoproteins or glycolipids (PubMed:10207017). an alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-D-GlcNAc derivative + CMP-N-acetyl-beta-neuraminate = an alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-[alpha-Neu5Ac-(2->6)]-D-GlcNAc-R + CMP + H(+) CMP-N-acetyl-beta-neuraminate + N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl-(1->3)-N-acetyl-D-galactosamine = CMP + H(+) + N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl-(1->3)-[N-acetyl-alpha-neuraminosyl-(2->6)]-N-acetyl-D-galactosamine CMP-N-acetyl-beta-neuraminate + ganglioside GM1b (d18:1(4E)) = a ganglioside GD1alpha (d18:1(4E)) + CMP + H(+) 3-O-[alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-alpha-D-GalNAc]-L-Ser-[protein] + CMP-N-acetyl-beta-neuraminate = 3-O-{alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-[alpha-Neu5Ac-(2->6)]-alpha-D-GalNAc}-L-Ser-[protein] + CMP + H(+) 3-O-[alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-alpha-D-GalNAc]-L-Thr-[protein] + CMP-N-acetyl-beta-neuraminate = 3-O-{alpha-Neu5Ac-(2->3)-beta-D-Gal-(1->3)-[alpha-Neu5Ac-(2->6)]-alpha-D-GalNAc}-L-Thr-[protein] + CMP + H(+) Protein modification; protein glycosylation. Glycolipid biosynthesis. Experimental confirmation may be lacking for some isoforms. High expression in brain and colon and to a lesser extent in lung, heart, kidney, spleen and thymus. Developmentally regulated. Belongs to the glycosyltransferase 29 family. ST6GalNAc IV