Synthesizes the galactose-alpha(1,3)-galactose group by catalyzing the transfer of a galactose residue, with an alpha-1,3 linkage, on terminal lactosaminide (Gal-beta-1,4-GlcNAc-R) disaccharide borne by a glycoprotein or a glycolipid. Preferentially glycosylates proteins, can synthesize galactose-alpha(1,3)-galactose on glycoproteins but cannot synthesize the glycolipid called isogloboside 3 (iGb3) (By similarity).
Subcellular locations: Golgi apparatus, Golgi stack membrane
Membrane-bound form in trans cisternae of Golgi.