Cation levels within the cytosol are coordinated by a network of transporters. Here, we examine the functional roles of calcium exchanger 1 (CAX1), a vacuolar H 1 /Ca 21 transporter, and the closely related transporter CAX3. We demonstrate that like CAX1, CAX3 is also localized to the tonoplast. We show that CAX1 is predominately expressed in leaves, while CAX3 is highly expressed in roots. Previously, using a yeast assay, we demonstrated that an N-terminal truncation of CAX1 functions as an H At the whole-plant level, it has been well documented that there is a complex interplay among various ions (Marschner, 1995). For example, supplemental Ca 21 is known to mitigate the adverse effects of salinity on plant growth (Epstein, 1972). Recently, it has become possible to measure the sum total of the plant's mineral nutrient and trace element composition, termed the ionome (Lahner et al., 2003). The ionome phenotypes now allow investigators to assess how alterations in specific transporters affect these ionic relationships.Ca 21 and other cations can accumulate to millimolar levels in the vacuole, whereas the concentrations of these cations are maintained in the micromolar range in the cytosol (Taiz et al., 1990; Marty, 1999 (Sze et al., 2000). The driving force for cation antiport activity is the pH gradient generated by two electrogenic proton pumps located on the membrane, an ATPase and a pyrophosphatase (PPase; Sze et al., 1999). In principle, the proton pumps and the H 1 / cation exchangers can both dramatically alter the cation content of the vacuoles.Plant H Article, publication date, and citation information can be found at www.plantphysiol.org/cgi

The Arabidopsis Ca 2 ؉ /H ؉ transporter CAX1 (Cation Exchanger1) may be an important regulator of intracellular Ca 2 ؉ levels. Here, we describe the preliminary localization of CAX1 to the tonoplast and the molecular and biochemical characterization of cax1 mutants. We show that these mutants exhibit a 50% reduction in tonoplast Ca 2 ؉ /H ؉ antiport activity, a 40% reduction in tonoplast V-type H ؉ -translocating ATPase activity, a 36% increase in tonoplast Ca 2 ؉ -ATPase activity, and increased expression of the putative vacuolar Ca 2 ؉ /H ؉ antiporters CAX3 and CAX4 . Enhanced growth was displayed by the cax1 lines under Mn 2 ؉ and Mg 2 ؉ stress conditions. The mutants exhibited altered plant development, perturbed hormone sensitivities, and altered expression of an auxin-regulated promoter-reporter gene fusion. We propose that CAX1 regulates myriad plant processes and discuss the observed phenotypes with regard to the compensatory alterations in other transporters.

Ca(2+)/cation antiporter (CaCA) proteins are integral membrane proteins that transport Ca(2+) or other cations using the H(+) or Na(+) gradient generated by primary transporters. The CAX (for CAtion eXchanger) family is one of the five families that make up the CaCA superfamily. CAX genes have been found in bacteria, Dictyostelium, fungi, plants, and lower vertebrates, but only a small number of CAXs have been functionally characterized. In this study, we explored the diversity of CAXs and their phylogenetic relationships. The results demonstrate that there are three major types of CAXs: type I (CAXs similar to Arabidopsis thaliana CAX1, found in plants, fungi, and bacteria), type II (CAXs with a long N-terminus hydrophilic region, found in fungi, Dictyostelium, and lower vertebrates), and type III (CAXs similar to Escherichia coli ChaA, found in bacteria). Some CAXs were found to have secondary structures that are different from the canonical six transmembrane (TM) domains-acidic motif-five TM domain structure. Our phylogenetic tree indicated no evidence to support the cyanobacterial origin of plant CAXs or the classification of Arabidopsis exchangers CAX7 to CAX11. For the first time, these results clearly define the CAX exchanger family and its subtypes in phylogenetic terms. The surprising diversity of CAXs demonstrates their potential range of biochemical properties and physiologic relevance.