title: GPI7 is the second partner of PIG-F and involved in modification of glycosylphosphatidylinositol.
abstract: Many eukaryotic cell surface proteins are anchored to the membrane via glycosylphosphatidylinositol (GPI). GPI is synthesized from phosphatidylinositol by stepwise reactions and attached en bloc to nascent proteins. In mammalian cells, the major GPI species transferred to proteins is termed H7. By attachment of an additional ethanolamine phosphate (EtNP) to the second mannose, H7 can be converted to H8, which acts as a minor type of protein-linked GPI and also exists as a free GPI on the cell surface. Yeast GPI7 is involved in the transfer of EtNP to the second mannose, but the corresponding mammalian enzyme has not yet been clarified. Here, we report that the human homolog of Gpi7p (hGPI7) forms a protein complex with PIG-F and is involved in the H7-to-H8 conversion. We knocked down hGPI7 by RNA interference and found that H7 accumulated with little production of H8. Immunoprecipitation experiments revealed that hGPI7 was associated with and stabilized by PIG-F, which is known to bind to and stabilize PIG-O, a protein homologous to hGPI7. PIG-O is a transferase that adds EtNP to the third mannose, rendering GPI capable of attaching to proteins. We further found that the overexpression of hGPI7 decreased the level of PIG-O and, therefore, decreased the level of EtNP transferred to the third mannose. Finally, we propose a mechanism for the regulation of GPI biosynthesis through competition between the two independent enzymes, PIG-O and hGPI7, for the common stabilizer, PIG-F.
authors.affiliation: Department of Immunoregulation, Research Institute for Microbial Diseases, Osaka University, Osaka 565-0871, Japan.
authors.forename: Nobue
authors.initials: N
authors.lastname: Shishioh
authors.forename: Yeongjin
authors.initials: Y
authors.lastname: Hong
authors.forename: Kazuhito
authors.initials: K
authors.lastname: Ohishi
authors.forename: Hisashi
authors.initials: H
authors.lastname: Ashida
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Maeda
authors.forename: Taroh
authors.initials: T
authors.lastname: Kinoshita
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D016466:CHO Cells; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D003001:Cloning, Molecular; D017124:Conserved Sequence; D006224:Cricetinae; D018076:DNA, Complementary / Q000378:metabolism; D006023:Glycoproteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D017261:Glycosylphosphatidylinositols / Q000378:metabolism*; D006801:Humans; D007624:KB Cells; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D008969:Molecular Sequence Data; D010957:Plasmids; D011506:Proteins / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D002352:Carrier Proteins; D018076:DNA, Complementary; D006023:Glycoproteins; D017261:Glycosylphosphatidylinositols; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; C098069:PIGF protein, human; C488892:PIP protein, human; D011506:Proteins; D011994:Recombinant Proteins