Transfers the sialyl group (N-acetyl-alpha-neuraminyl or NeuAc) from CMP-NeuAc onto glycoproteins and glycolipids, forming an alpha-2,6-linkage. Produces branched type disialyl structures by transfer of a sialyl group onto the GalNAc or GlcNAc residue inside backbone core chains having a terminal sialic acid with an alpha-2,3-linkage on Gal. ST6GalNAcVI prefers glycolipids to glycoproteins, predominantly catalyzing the biosynthesis of ganglioside GD1alpha from GM1b (PubMed:12668675, PubMed:17123352). Besides GMb1, MSGG and other glycolipids, it shows activity towards sialyl Lc4Cer generating disialyl Lc4Cer, which can lead to the synthesis of disialyl Lewis a (Le(a)), suggested to be a cancer-associated antigen (PubMed:12668675). Also has activity toward GD1a and GT1b, and can generate DSGG (disialylgalactosylgloboside) from MSGG (monosialylgalactosylgloboside) (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 CMP-N-acetyl-beta-neuraminate + globoside MSGG = CMP + globoside DSGG + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GD1a (d18:1(4E)) = CMP + ganglioside GT1aalpha (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GT1b (d18:1(4E)) = CMP + ganglioside GQ1b alpha (d18:1(4E)) + 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(+) Expressed in kidney, in proximal tubule epithelial cells. Expressed in colon cell lines. Down-regulated in renal cancers. The carbohydrate antigen disialyl Lewis a, which is at least partly synthesized by ST6GALNAC6, is a normal counterpart of sialyl Lewis a, better known as CA19-9, an antigen widely used as a serum marker for diagnosis of cancers in the digestive track. Disialyl Lewis a is predominantly expressed in non-malignant epithelial cells of the digestive organs, while sialyl Lewis a is preferentially expressed in cancers. Disialyl Lewis a in normal epithelial cells serves as a ligand for immunosuppressive receptors, such as SIGLEC7 and SIGLEC9, expressed on resident monocytes/macrophages and maintains immunological homeostasis of mucosal membranes in digestive organs. Sialyl Lewis a, as well as its positional isomer sialyl Lewis x, serves as a ligand for vascular cell adhesion molecule E-selectin and facilitates hematogenous metastasis through mediating adhesion of circulating cancer cells to vascular endothelium (PubMed:17760270). Belongs to the glycosyltransferase 29 family. ST6GalNAc VI