Predominantly catalyzes the biosynthesis of ganglioside GD1alpha from GM1b in the brain, by transferring 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 GM1b (PubMed:10601645, PubMed:10521438). GD1alpha is a critical molecule in the communication and interaction between neuronal cells and their supportive cells, particularly in brain tissues, and functions as an adhesion molecule in the process of metastasis (PubMed:9157980). Also shows activity towards sialyl Lc4Cer (N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl-(1->3)-N-acetyl-beta-D-glucosaminyl-(1->3)-beta-D-galactosyl-(1->4)-beta-D-glucosyl-(1<->1')-N-acyl-sphing-4-enine) generating disialyl Lc4Cer, which can lead to the synthesis of disialyl Lewis a (Le(a)), suggested to be a cancer-associated antigen (By similarity). CMP-N-acetyl-beta-neuraminate + ganglioside GM1b (d18:1(4E)) = a ganglioside GD1alpha (d18:1(4E)) + CMP + H(+) CMP-N-acetyl-beta-neuraminate + N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl-(1->3)-N-acetyl-beta-D-glucosaminyl-(1->3)-beta-D-galactosyl-(1->4)-beta-D-glucosyl-(1<->1')-N-acyl-sphing-4-enine = CMP + H(+) + N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl-(1->3)-[N-acetyl-alpha-neuraminosyl-(2->6)]-N-acetyl-beta-D-glucosaminyl-(1->3)-beta-D-galactosyl-(1->4)-beta-D-glucosyl-(1<->1')-N-acyl-sphing-4-enine Glycolipid biosynthesis. High expression in forebrain and to a lesser extent in cerebellum. No expression in salivary gland, intestine, liver, kidney, heart, lung, thymus and spleen. Belongs to the glycosyltransferase 29 family. ST6GalNAc V