Expression cloning of a new member of the ABO blood group glycosyltransferases, iGb₃ synthase, that directs the synthesis of isoglobo-glycosphingolipids

The large array of different glycolipids described in mammalian tissues is a reflection, in part, of diverse glycosyltransferase expression. Herein, we describe the cloning of a UDP-galactose: β-D-galactosyl-1,4-glucosyl-ceramide α-1,3-galactosyltransferase (iGb₃ synthase) from a rat placental cDNA expression library, iGb₃ synthase acts on lactosylceramide, LacCer (Galβ1,4Glc-β1Cer) to form iGb₃ (Galα1,3Galβ1,4Glcβ1Cer) initiating the synthesis of the isoglobo-series of glycosphingolipids. The isolated cDNA encoded a predicted protein of 339 amino acids, which shows extensive homology (40-50% identity) to members of the ABO gene family that includes: murine α1,3-galactosyltransferase, Forssman (Gb₅)synthase, and the ABO glycosyltransferases. In contrast to the murine α1,3-galactosyltransferase, iGb₃ synthase preferentially modifies glycolipids over glyco-protein substrates. Reverse transcriptase-polymerase chain reaction revealed a widespread tissue distribution of iGb₃ synthase RNA expression, with high levels observed in spleen, thymus, and skeletal muscle. As an indirect consequence of the expression cloning strategy used, we have been able to identify several potential glycolipid biosynthetic pathways where iGb₃ functions, including the globo- and isoglobo-series of glycolipids.