Characterization of CAX4, an Arabidopsis H+/Cation Antiporter

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Abstract
Ion compartmentalization is essential for plant growth and development. The Arabidopsis open reading frames for CAX1, CAX2, and CAX3 (cation exchangers 1, 2, and 3) were previously identified as transporters that may modulate ion fluxes across the vacuolar membrane. To understand the diversity and role of H+/cation transporters in controlling plant ion levels, another homolog of theCAX genes, CAX4, was cloned from an Arabidopsis cDNA library. CAX4 is 53% identical to CAX1 at the amino acid level, 42% identical to CAX2, and 54% identical to CAX3.CAX4 transcripts appeared to be expressed at low levels in all tissues and levels of CAX4 RNA increased after Mn2+, Na+, and Ni2+ treatment. An N-terminal CAX4-hemagglutinin fusion appeared to localize to both yeast and plant vacuolar membranes. When expressed in yeast, CAX4, like CAX3, failed to suppress the Ca2+ sensitivity of yeast strains deficient in vacuolar Ca2+ transport. Several modifications to CAX4 allowed the protein to transport Ca2+. Addition of amino acids to the N terminus of CAX4 and CAX3 caused both transporters to suppress the sensitivity of yeast strains deficient in vacuolar Ca2+ transport. These findings suggest that CAX transporters may modulate their ion transport properties through alterations at the N terminus.