Alpha-2,8-sialyltransferase that prefers O-glycans to N-glycans or glycolipids as acceptor substrates. The minimal acceptor substrate is the NeuAc-alpha-2,3(6)-Gal sequence at the non-reducing end of their carbohydrate groups. CMP-N-acetyl-beta-neuraminate + ganglioside GM3 = CMP + ganglioside GD3 + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GM3 (d18:1(4E)) = CMP + ganglioside GD3 (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GD1a (d18:1(4E)) = CMP + ganglioside GT1a (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GD1a = CMP + ganglioside GT1a + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GM1b (d18:1(4E)) = CMP + ganglioside GD1c (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GM1b = CMP + ganglioside GD1c + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GM4 (d18:1(4E)) = CMP + H(+) + N-acetyl-alpha-neuraminosyl-(2->8)-N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl-(1<->1')-N-acylsphing-4-enine CMP-N-acetyl-beta-neuraminate + N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl-(1<->1')-ceramide = CMP + H(+) + N-acetyl-alpha-neuraminosyl-(2->8)-N-acetyl-alpha-neuraminosyl-(2->3)-beta-D-galactosyl-(1<->1')-ceramide CMP-N-acetyl-beta-neuraminate + ganglioside GT1b (d18:1(4E)) = CMP + ganglioside GQ1b (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GT1b = CMP + ganglioside GQ1b + H(+) Protein modification; protein glycosylation. Belongs to the glycosyltransferase 29 family. ST8Sia VI