The Arabidopsis Na+/H+Exchanger AtNHX1 Catalyzes Low Affinity Na+ and K+ Transport in Reconstituted Liposomes

Abstract

6 pages, 7 figures, 1 table, 44 references.In saline environments, plants accumulate Na+ in vacuoles through the activity of tonoplast Na+/H+ antiporters. The first gene for a putative plant vacuolar Na+/H+ antiporter,AtNHX1, was isolated from Arabidopsis and shown to increase plant tolerance to NaCl. However, AtNHX1mRNA was up-regulated by Na+ or K+ salts in plants and substituted for the homologous protein of yeast to restore tolerance to several toxic cations. To study the ion selectivity of the AtNHX1 protein, we have purified a histidine-tagged version of the protein from yeast microsomes by Ni2+ affinity chromatography, reconstituted the protein into lipid vesicles, and measured cation-dependent H+ exchange with the fluorescent pH indicator pyranine. The protein catalyzed Na+ and K+ transport with similar affinity in the presence of a pH gradient. Li+ and Cs+ ions were also transported with lower affinity. Ion exchange by AtNHX1 was inhibited 70% by the amiloride analog ethylisopropyl-amiloride. Our data indicate a role for intracellular antiporters in organelle pH control and osmoregulation.This work was supported by Grants PB97-1266 from the Spanish Dirección General de Enseñanza Superior e Investigación Cientifica (to J. P. D.) and FD97-0496-C03 from the Fondo Europeo de Desarrollo Regional program (to J. P. D. and J. M. P.) and by a grant Ramón y Cajal from the Spanish Ministerio de Ciencia y Tecnologia (to K. V.).Peer reviewe