Expression cloning of a new member of the ABO blood group glycosyltransferases, iGb(3) 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, w e describe the cloning of a UDP-galactose: beta-D-galactosyl-1,4-glucosyl-c eramide alpha-1,3-galactosyltransferase (iGb(3) synthase) from a rat placen tal cDNA expression library. iGb(3) synthase acts on lactosylceramide, LacC er (Ga1 beta 1,4Glc-beta 1Cer) to form iGb(3) (Ga1 alpha 1,3Gal beta 1,4Glc beta 1Cer) initiating the synthesis of the isoglobo-series of glycosphingo lipids. The isolated cDNA encoded a predicted protein of 339 amino acids, w hich shows extensive homology (40-50% identity) to members of the ABO gene family that includes: murine alpha 1,3-galactosyltransferase, Forssman (Gb, ) synthase, and the ABO glycosyltransferases. In contrast to the murine alp ha 1,3-galactosyltransferase, iGb(3) synthase preferentially modifies glyco lipids over glycoprotein substrates, Reverse transcriptase-polymerase chain reaction revealed a widespread tissue distribution of iGb(3) synthase RNA expression, with high levels observed in spleen, thymus, and skeletal muscl e. As an indirect consequence of the expression cloning strategy used, we h ave been able to identify several potential glycolipid biosynthetic pathway s where iGb(3) functions, including the globo- and isoglobo-series of glyco lipids.