Cloning of Gb3 synthase, the key enzyme in globo-series glycosphingolipid synthesis, predicts a family of α1,4-glycosyltransferases conserved in plants, insects, and mammals

We have cloned Gb, synthase, the key alpha 1,4-galactosyl-transferase in globe-series glycosphingolipid (GSL) synthesis, via a phenotypic screen, which previously yielded iGb(3) synthase, the alpha 1,3-galactosyltransferase required in iso-globe-series GSL (Keusch, J,J., Manzella, S. M., Nyame, K. A., Cummings, R. D., and Baenziger, J. U. (2000) J. Biol. Chem. 33), Both transferases act on lactosylceramide, Gal beta 1,4Glc beta 1Cer (LacCer), to produce Gb(3) (Gal alpha 1,4LacCer) or iGb(3) (Ga1 alpha 1,3LacCer), respectively. GalNAc can be added sequentially to either Gb(3) or iGb(3) yielding globoside and Forssman from Gb,, and isogloboside and isoForssman hem iGb(3). Gb, synthase is not homologous to iGb(3) synthase but shows 43% identity to a human alpha 1,4GlcNAc transferase that transfers a UDP-sugar in an alpha 1,4-linkage to a beta-linked Gal found in mucin. Extensive homology (35% identity) is also present between Gb, synthase and genes in Drosophila melanogaster and Arabidopsis thaliana, supporting conserved expression of an alpha 1,4-glycosyltransferase, possibly Gb, synthase, throughout evolution. The isolated Gb, synthase cDNA encodes a type II transmembrane glycosyltransferase of 360 amino acids. The highest tissue expression of Gb, synthase RNA is found in the kidney, mesenteric lymph node, spleen, and brain. Gb, glycolipid, also called pk antigen or CD77, is a known receptor for verotoxins. CHO cells that do not express Gb, and are resistant to verotoxin become susceptible to the toxin following transfection with Gb, synthase cDNA.