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Published on September 5, 2008; 10.1104/pp.108.118810

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Received May 8, 2008
Accepted August 30, 2008

AtCCX3 is an Arabidopsis endomembrane H+-dependent K+ transporter

Jay Morris , Hui Tian , Sunghun Park , Coimbatore S. Sreevidya , John M. Ward , and Kendal D. Hirschi *

Vegetable and Fruit Improvement Center, Texas A&M University, College Station, Texas 77845; 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; Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108; Department of Immunology, M.D.Anderson Cancer Center, Houston, Texas, 77030

* Corresponding author; email: kendalh{at}bcm.tmc.edu.

The Arabidopsis Cation Calcium eXchangers (CCXs) were recently identified as a subfamily of cation transporters; though, no plant CCXs have been functionally characterized. Here we show Arabidopsis thaliana 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 PCR showed 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 CaMV 35S promoter. Overexpression of AtCCX3 in tobacco produced lesions in the leaves, stunted growth and resulted in the accumulation of higher levels of numerous cations. In summary, these findings suggest AtCCX3 is an endomembrane localized H+-dependent K+ transporter with apparent Na+ and Mn2+ transport properties distinct from previously characterized plant transporters.






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