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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

AtCCX3 Is an Arabidopsis Endomembrane H⁺-Dependent K⁺ Transporter^1,[W],[OA]

Vegetable and Fruit Improvement Center, Texas A&M University, College Station, Texas 77845 (J.M., S.P., K.D.H.); Plant Physiology Group, United States Department of Agriculture/Agriculture Research Service, Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030 (J.M., K.D.H.); Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108 (H.T., J.M.W.); and Department of Immunology, M.D. Anderson Cancer Center, Houston, Texas 77030 (C.S.S.)

The Arabidopsis (Arabidopsis thaliana) cation calcium exchangers (CCXs) were recently identified as a subfamily of cation transporters; however, no plant CCXs have been functionally characterized. Here, we show that Arabidopsis AtCCX3 (At3g14070) and AtCCX4 (At1g54115) can suppress yeast mutants defective in Na⁺, K⁺, and Mn²⁺ transport. We also report high-capacity uptake of ⁸⁶Rb⁺ in tonoplast-enriched vesicles from yeast expressing AtCCX3. Cation competition studies showed inhibition of ⁸⁶Rb⁺ uptake in AtCCX3 cells by excess Na⁺, K⁺, and Mn²⁺. Functional epitope-tagged AtCCX3 fusion proteins were localized to endomembranes in plants and yeast. In Arabidopsis, AtCCX3 is primarily expressed in flowers, while AtCCX4 is expressed throughout the plant. Quantitative polymerase chain reaction showed that expression of AtCCX3 increased in plants treated with NaCl, KCl, and MnCl₂. Insertional mutant lines of AtCCX3 and AtCCX4 displayed no apparent growth defects; however, overexpression of AtCCX3 caused increased Na⁺ accumulation and increased ⁸⁶Rb⁺ transport. Uptake of ⁸⁶Rb⁺ increased in tonoplast-enriched membranes isolated from Arabidopsis lines expressing CCX3 driven by the cauliflower mosaic virus 35S promoter. Overexpression of AtCCX3 in tobacco (Nicotiana tabacum) produced lesions in the leaves, stunted growth, and resulted in the accumulation of higher levels of numerous cations. In summary, these findings suggest that AtCCX3 is an endomembrane-localized H⁺-dependent K⁺ transporter with apparent Na⁺ and Mn²⁺ transport properties distinct from those of previously characterized plant transporters.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Kendal D. Hirschi (kendalh{at}bcm.tmc.edu).

^[W] The online version of this article contains Web-only data.

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.118810

^* Corresponding author; e-mail kendalh{at}bcm.tmc.edu.

Received May 8, 2008; accepted August 30, 2008; published September 5, 2008.