Glycosylphosphatidylinositols (GPIs) are found in all eukaryotes and are synthesized in a pathway that starts with the transfer of N-acetylglucosamine (GIcNAc) from UDP-GlcNAc to phosphatidylinositol (PI). This reaction is carried out by a protein complex, three of whose subunits in humans, hGpi1p, Pig-Cp and Pig-Ap, have sequence and functional homologues in the Saccharomyces cerevisiae Gpi1, Gpi2 and Gpi3 proteins, respectively. Human GlcNAc-PI synthase contains two further subunits, Pig-Hp and PigPp. We report that the essential YNL038w gene encodes the S. cerevisiae homologue of Pig-Hp. Haploid YNL038w-deletion strains were created, in which Yn1038wp could be depleted by repressing YNL038w, expression using the GAL10 promoter. Depletion of Yn1038wp from membranes virtually abolished in vitro GlcNAc-PI synthetic activity, indicating that Yn1038wp is necessary for GlcNAc-PI synthesis in vitro. Further, depletion of Yn1038wp in an smp3 mutant background prevented the formation of the trimannosylated GPI intermediates that normally accumulate in this late-stage GPI assembly mutant. Yn1038wp is therefore required for GPI synthesis in vivo. Because YNL038w encodes a protein involved in GPI biosynthesis, we designate the gene GP115. Potential Pig-Hp/Gpil5p counterparts are also encoded in the genomes of Schizosacchomyces pombe and Candida albicans. Copyright (C) 2001 John Wiley & Sons, Ltd.
