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J. Biol. Chem., Vol. 283, Issue 7, 4145-4154, February 15, 2008

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Dimeric Structure of Human Na⁺/H⁺ Exchanger Isoform 1 Overproduced in Saccharomyces cerevisiae^*

From the Department of Biochemistry, University of Alberta, Edmonton, Alberta T6G 2H7, Canada

The Na⁺/H⁺ exchanger isoform 1 (NHE1) is an integral membrane protein that regulates intracellular pH by extruding an intracellular H⁺ in exchange for one extracellular Na⁺. The human NHE1 isoform is involved in heart disease and cell growth and proliferation. Although details of NHE1 regulation and transport are being revealed, there is little information available on the structure of the intact protein. In this report, we demonstrate overexpression, purification, and characterization of the human NHE1 (hNHE1) protein in Saccharomyces cerevisiae. Overproduction of the His-tagged protein followed by purification via nickel-nitrilotriacetic acid-agarose chromatography yielded 0.2 mg of pure protein/liter of cell culture. Reconstitution of hNHE1 in proteoliposomes demonstrated that the protein was active and responsive to an NHE1-specific inhibitor. Circular dichroism spectroscopy of purified hNHE1 revealed that the protein contains 41% -helix, 23% β-sheet, and 36% random coil. Size exclusion chromatography indicated that the protein-detergent micelle was in excess of 200 kDa, consistent with an hNHE1 dimer. Electron microscopy and single particle reconstruction of negatively stained hNHE1 confirmed that the protein was a dimer, with a compact globular domain assigned to the transmembrane region and an apical ridge assigned to the cytoplasmic domain. The transmembrane domain of the hNHE1 reconstruction was clearly dimeric, where each monomer had a size and shape consistent with the predicted 12 membrane-spanning segments for hNHE1.

Received for publication, June 12, 2007 , and in revised form, November 8, 2007.

^* This work was supported in part by grants from the Canadian Institutes of Health Research (to H. S. Y. and L. F.). 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.

¹ Supported by the Canadian Institutes of Health Research Strategic Training Initiative in Membrane Proteins and Cardiovascular Disease. Both authors have contributed equally to this work.

² Supported by an Alberta Heritage Foundation for Medical Research Scientist Award.

³ To whom correspondence should be addressed. E-mail: hyoung{at}ualberta.ca.

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