Synthesizes the galactose-alpha(1,3)-galactose group on the glycosphingolipid isoglobotrihexosylceramide or isogloboside 3 (iGb3) by catalyzing the transfer of galactose from UDP-Galactose to its acceptor molecule Gal-beta-1,4-Glc-ceramide. Can also catalyze the addition of galactose to iGb3 itself to form polygalactose structures. Synthesis of iGb3 is the initial step in the formation of the isoglobo-series glycolipid pathway and is the precursor to isogloboside 4 (iGb4) and isoForssman glycolipids. Can glycosylate only lipids and not proteins and is solely responsible for initiating the synthesis of isoglobo-series glycosphingolipids. a beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + UDP-alpha-D-galactose = an alpha-D-galactosyl-(1->3)-beta-D-galactosyl-(1->4)-N-acetyl-beta-D-glucosaminyl derivative + H(+) + UDP a beta-D-Gal-(1->4)-beta-D-Glc-(1<->1)-Cer(d18:1(4E)) + UDP-alpha-D-galactose = H(+) + isogloboside iGb3Cer (d18:1(4E)) + UDP globoside Gb3Cer + UDP-alpha-D-galactose = GalGb3Cer + H(+) + UDP Binds 1 Mn(2+) ion per subunit. Also found in numerous large vesicles throughout the cytoplasm of the soma. Thymus and lung. The conserved DXD motif is involved in cofactor binding. The manganese ion interacts with the beta-phosphate group of UDP and may also have a role in catalysis (By similarity). Mice are fertile, develop normally and exhibit no overt behavioral abnormalities. However, compared to heterozygous mice they lack expression of the glycosphingolipid isoglobotrihexosylceramide (iGb3) in the dorsal root ganglion. Belongs to the glycosyltransferase 6 family.