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J. Biol. Chem., Vol. 279, Issue 4, 2922-2926, January 23, 2004

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The Protein Kinase SOS2 Activates the Arabidopsis H⁺/Ca²⁺ Antiporter CAX1 to Integrate Calcium Transport and Salt Tolerance^*

Ning-Hui Cheng, Jon K. Pittman, Jian-Kang Zhu, and Kendal D. Hirschi¶||**

From the United States Department of Agriculture/Agricultural Research Service Children's Nutrition Research Center, Baylor College of Medicine, Houston, Texas 77030, the Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721, the ^¶Department of Human and Molecular Genetics, Baylor College of Medicine, Houston, Texas 77030, and the ^||Vegetable and Fruit Improvement Center, Texas A&M University, College Station, Texas 77845

The regulation of ions within cells is an indispensable component of growth and adaptation. The plant SOS2 protein kinase and its associated Ca²⁺ sensor, SOS3, have been demonstrated to modulate the plasma membrane H⁺/Na⁺ antiporter SOS1; however, how these regulators modulate Ca²⁺ levels within cells is poorly understood. Here we demonstrate that SOS2 regulates the vacuolar H⁺/Ca²⁺ antiporter CAX1. Using a yeast growth assay, co-expression of SOS2 specifically activated CAX1, whereas SOS3 did not. CAX1-like chimeric transporters were activated by SOS2 if the chimeric proteins contained the N terminus of CAX1. Vacuolar membranes from CAX1-expressing cells were made to be H⁺/Ca²⁺-competent by the addition of SOS2 protein in a dose-dependent manner. Using a yeast two-hybrid assay, SOS2 interacted with the N terminus of CAX1. In each of these yeast assays, the activation of CAX1 by SOS2 was SOS3-independent. In planta, the high level of expression of a deregulated version of CAX1 caused salt sensitivity. These findings suggest multiple functions for SOS2 and provide a mechanistic link between Ca²⁺ and Na⁺ homeostasis in plants.

Received for publication, August 15, 2003 , and in revised form, October 27, 2003.

^* This work was supported by the United States Department of Agriculture/Agricultural Research Service under Cooperative Agreement 58-6250-6001 and by National Science Foundation Grant 0209777. 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: Baylor College of Medicine, Plant Physiology Group, USDA/ARS Children's Nutrition Research Center, 1100 Bates St., Houston, TX 77030. Tel.: 713-798-7011; Fax: 713-798-7078; E-mail: kendalh{at}bcm.tmc.edu.

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