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J. Biol. Chem., Vol. 279, Issue 13, 12438-12447, March 26, 2004

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A Novel Membrane Protein Capable of Binding the Na⁺/H⁺ Antiporter (Nha1p) Enhances the Salinity-resistant Cell Growth of Saccharomyces cerevisiae^*

Keiji Mitsui, Fumihiro Ochi, Norihiro Nakamura, Yoshihide Doi, Hiroki Inoue, and Hiroshi Kanazawa

From the Department of Biological Sciences, Graduate School of Science, Osaka University, Machikaneyama-cho, Toynaka City, Osaka 560-0043, Japan

The Na⁺/H⁺ antiporter Nha1p of Saccharomyces cerevisiae plays an important role in maintaining intracellular pH and Na⁺ homeostasis. Nha1p has a two-domain structure composed of integral membrane and hydrophilic tail regions. Overexpression of a peptide of 40 residues (C1+C2 domains) that is localized in the juxtamembrane area of its cytoplasmic tail caused cell growth retardation in highly saline conditions, possibly by decreasing Na⁺/H⁺ antiporter activity. A multicopy suppressor gene of this growth retardation was identified from a yeast genome library. The clone encodes a novel membrane protein denoted as COS3 in the genome data base. Overexpression or deletion of COS3 increases or decreases salinity-resistant cell growth, respectively. However, in nha1 cells, overexpression of COS3 alone did not suppress the growth retardation. Cos3p and a hydrophilic portion of Cos3p interact with the C1+C2 peptide in vitro, and Cos3p is co-precipitated with Nha1p from yeast cell extracts. Cos3p-GFP mainly resides at the vacuole, but overexpression of Nha1p caused a portion of the Cos3p-GFP proteins to shift to the cytoplasmic membrane. These observations suggest that Cos3p is a novel membrane protein that can enhance salinity-resistant cell growth by interacting with the C1+C2 domain of Nha1p and thereby possibly activating the antiporter activity of this protein.

Received for publication, October 1, 2003 , and in revised form, December 23, 2003.

^* This work was supported in part by a grant-in-aid from the Ministry of Science, Education, Sports, Culture and Technology of Japan. 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.

To whom correspondence should be addressed: Dept. of Biological Sciences, Graduate School of Science, Osaka University, Machikaneyama Cho, 1-16 Osaka, 560-0043 Japan. Tel.: 81-6-6850-5812; Fax: 81-6-6850-5817; E-mail: kanazawa{at}bio.sci.osaka-u.ac.jp.

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