Catalyzes the addition of sialic acid in alpha 2,8-linkage to the sialic acid moiety of the ganglioside GM3 to form ganglioside GD3; gangliosides are a subfamily of complex glycosphinglolipds that contain one or more residues of sialic acid (PubMed:7937974, PubMed:8058740, PubMed:8195250, PubMed:8631981, PubMed:8706663, PubMed:18348864, PubMed:22885356). Can catalyze the addition of a second alpha-2,8-sialic acid to GD3 to form GT3 (PubMed:8631981). Can use GM1b, GD1a and GT1b as acceptor substrates to synthesize GD1c, GT1a and GQ1b respectively (PubMed:8706663). Can synthesize unusual tetra- and pentasialylated lactosylceramide derivatives identified as GQ3 (II3Neu5Ac4-Gg2Cer) and GP3 (II3Neu5Ac5-Gg2Cer) in breast cancer cells (PubMed:22885356). an N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl derivative + CMP-N-acetyl-beta-neuraminate = an N-acetyl-alpha-neuraminyl-(2->8)-N-acetyl-alpha-neuraminyl-(2->3)-beta-D-galactosyl derivative + CMP + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GM3 (d18:1(4E)) = CMP + ganglioside GD3 (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GD3 (d18:1(4E)) = CMP + ganglioside GT3 (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 GT1b (d18:1(4E)) = CMP + ganglioside GQ1b (d18:1(4E)) + H(+) CMP-N-acetyl-beta-neuraminate + ganglioside GM1b (d18:1(4E)) = CMP + ganglioside GD1c (d18:1(4E)) + H(+) Protein modification; protein glycosylation. Lipid metabolism; sphingolipid metabolism. Strongly expressed in melanoma cell lines, adult and fetal brain and to a lesser extent in adult and fetal lung. Belongs to the glycosyltransferase 29 family. ST8Sia I