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:17333390, PubMed:18095347). Glycosphingolipids are required for convergence extension movements during early development (PubMed:18095347). Can catalyze the addition of a second alpha-2,8- sialic acid to GD3 to form GT3 (By similarity). Can use GM1b, GD1a and GT1b as acceptor substrates to synthesize GD1c, GT1a and GQ1b respectively (By similarity). 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. PubMed:18095347 report localization in the endoplasmic reticulum. Expressed in the dorsal blastopore lip and in the presumptive neuroectoderm in stage 11 embryos. During gastrulation, strongly expressed in the involuting mesoderm. At stages 13 and 16, expressed in the neural plate and neural fold, paraxial mesoderm and notochord. At stages 19 and 22 (neural tube and early tailbud), strongly expressed in the neural tube and notochord. At the tadpole stage, expressed in the head region, branchial arches and otic and optic primordia. Also localized in the notochord and weakly expressed in the somites. In adults, expressed in the brain and ovary. Isoform 2 (short) is expressed at a low level in the adult testis and muscle, and at a high level in the skin. Isoform 1 (long) is expressed at a high level in the adult lung and kidney. Both isoforms 1 and 2 are expressed in the gut and liver. Expressed zygotically from the beginning of gastrulation, with expression increasing at stage 12. Expression then persists until stage 35 and declines afterwards. Belongs to the glycosyltransferase 29 family. PubMed:18095347 report an endoplasmic reticulum localization and a lack of enzymatic activity, which could be a result of impaired N-glycosylation.