Nitrate, the major nitrogen source for most plants, is widely used as a fertilizer and as a result has become a predominant freshwater pollutant. Plants need nitrate for growth and store most of it in the central vacuole¹. Some members of the chloride channel (CLC) protein family, such as the torpedo-fish ClC-0 and mammalian ClC-1, are anion channels^{2, 3}, whereas the bacterial ClC-ec1 and mammalian ClC-4 and ClC-5 have recently been characterized as Cl^-/H⁺ exchangers with unknown cellular functions^{4, 5, 6}. Plant members of the CLC family are proposed to be anion channels^{7, 8} involved in nitrate homeostasis⁹; however, direct evidence for anion transport mediated by a plant CLC is still lacking. Here we show that Arabidopsis thaliana CLCa (AtCLCa) is localized to an intracellular membrane, the tonoplast of the plant vacuole, which is amenable to electrophysiological studies, and we provide direct evidence for its anion transport ability. We demonstrate that AtCLCa is able to accumulate specifically nitrate in the vacuole and behaves as a NO₃^-/H⁺ exchanger. For the first time, to our knowledge, the transport activity of a plant CLC is revealed, the antiporter mechanism of a CLC protein is investigated in a native membrane system, and this property is directly connected with its physiological role.

1. Institut des Sciences du Végétal, CNRS UPR 2355, 1 Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
2. Université Paris 7-Denis Diderot, UFR Biologie Sciences de la Nature, 2 place Jussieu, 75251 Paris Cedex 05, France
3. Istituto di Biofisica, CNR, Via De Marini 6, 16149 Genova, Italy

Correspondence to: H. Barbier-Brygoo¹ Correspondence and requests for materials should be addressed to H.B.-B. (Email: brygoo@isv.cnrs-gif.fr).

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