title: HOR7, a multicopy suppressor of the Ca2+-induced growth defect in sphingolipid mannosyltransferase-deficient yeast.
abstract: Yeast mutants defective in sphingolipid mannosylation accumulate inositol phosphorylceramide C (IPC-C), which renders cells Ca(2+)-sensitive. A screen for loss of function suppressors of the Ca(2+)-sensitive phenotype previously led to the identification of numerous genes involved in IPC-C synthesis. To better understand the molecular basis of the Ca(2+)-induced growth defect in IPC-C-overaccumulating cells, we searched for genes whose overexpression restored Ca(2+) tolerance in a mutant lacking the IPC mannosyltransferases Csg1p and Csh1p. Here we report the isolation of HOR7 as a multicopy suppressor of the Ca(2+)-sensitive phenotype of Deltacsg1Deltacsh1 cells. HOR7 belongs to a group of hyperosmolarity-responsive genes and encodes a small (59-residue) type I membrane protein that localizes at the plasma membrane. Hor7p is not required for high Ca(2+) or Na(+) tolerance. Instead, we find that Hor7p-overproducing cells display an increased resistance to high salt, sensitivity to low pH, and a reduced uptake of methylammonium, an indicator of the plasma membrane potential. These phenotypes are induced through a mechanism independent of the plasma membrane H(+)-ATPase, Pma1p. Our findings suggest that induction of Hor7p causes a depolarization of the plasma membrane that may counteract a Ca(2+)-induced influx of toxic cations in IPC-C-overaccumulating cells.
authors.affiliation: Department of Membrane Enzymology, Faculty of Chemistry, Utrecht University, H.R. Kruytgebouw N605, Padualaan 8, 3584 CH Utrecht, The Netherlands.
authors.forename: Quirine
authors.initials: Q
authors.lastname: Lisman
authors.forename: Dorothy
authors.initials: D
authors.lastname: Urli-Stam
authors.forename: Joost C M
authors.initials: JC
authors.lastname: Holthuis
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000595:Amino Acid Sequence; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism*; D002455:Cell Division; D002462:Cell Membrane / Q000201:enzymology / Q000378:metabolism; D005656:Fungal Proteins / Q000378:metabolism; D016695:Glycosyltransferases; D006863:Hydrogen-Ion Concentration; D008364:Mannosyltransferases / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology*; D008744:Methylamines / Q000737:chemistry / Q000493:pharmacokinetics; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D008957:Models, Genetic; D008969:Molecular Sequence Data; D010641:Phenotype; D010957:Plasmids / Q000378:metabolism; D017434:Protein Structure, Tertiary; D006180:Proton-Translocating ATPases / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction; D012964:Sodium / Q000737:chemistry; D013107:Sphingolipids / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism; D013997:Time Factors
chemical_list: C430064:CSH1 protein, Candida albicans; D005656:Fungal Proteins; C491425:HOR7 protein, S cerevisiae; D008565:Membrane Proteins; D008744:Methylamines; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins; D013107:Sphingolipids; D012964:Sodium; C027451:methylamine; D016695:Glycosyltransferases; C092031:SUR1 protein, S cerevisiae; D008364:Mannosyltransferases; D000251:Adenosine Triphosphatases; C092477:PMA1 protein, S cerevisiae; D006180:Proton-Translocating ATPases; D002118:Calcium