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J. Biol. Chem., Vol. 279, Issue 35, 36390-36396, August 27, 2004

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HOR7, a Multicopy Suppressor of the Ca²⁺-induced Growth Defect in Sphingolipid Mannosyltransferase-deficient Yeast^*

Quirine Lisman¶, Dorothy Urli-Stam, and Joost C. M. Holthuis||

From the Department of Membrane Enzymology, Faculty of Chemistry, Utrecht University, 3584 CH Utrecht, The Netherlands and the Department of Cell Biology and Histology, Academic Medical Center, University of Amsterdam, 1105 AZ Amsterdam, The Netherlands

Yeast mutants defective in sphingolipid mannosylation accumulate inositol phosphorylceramide C (IPC-C), which renders cells Ca²⁺-sensitive. A screen for loss of function suppressors of the Ca²⁺-sensitive phenotype previously led to the identification of numerous genes involved in IPC-C synthesis. To better understand the molecular basis of the Ca²⁺-induced growth defect in IPC-C-overaccumulating cells, we searched for genes whose overexpression restored Ca²⁺ tolerance in a mutant lacking the IPC mannosyltransferases Csg1p and Csh1p. Here we report the isolation of HOR7 as a multicopy suppressor of the Ca²⁺-sensitive phenotype of csg1csh1 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 Ca²⁺ 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 Ca²⁺-induced influx of toxic cations in IPC-C-overaccumulating cells.

Received for publication, June 3, 2004

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ Supported by a grant from the Meelmeijer Foundation.

^|| Supported by a grant from the Royal Academy of Arts and Sciences. To whom correspondence should be addressed: Dept. of Membrane Enzymology, Center for Biomembranes and Lipid Enzymology, H. R. Kruytgebouw N605, Padualaan 8, 3584 CH Utrecht, The Netherlands. Tel.: 31-30-253-6630; Fax: 31-30-252-2478; E-mail: j.c.holthuis{at}chem.uu.nl.

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