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J. Biol. Chem., Vol. 280, Issue 8, 6621-6626, February 25, 2005

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High Affinity Transport of Taurine by the Drosophila Aspartate Transporter dEAAT2^*

Marie Thérèse Besson, Diane B. Ré, Matthieu Moulin, and Serge Birman¶

From the Laboratoire de Génétique et Physiologie du Développement, UMR 6545 CNRS-Université de la Méditerranée, Developmental Biology Institute of Marseille, Campus de Luminy, Case 907, 13288 Marseille Cedex 9, France and the Laboratoire Interactions Cellulaires, Neurodégénérescence et Neuroplasticité, UMR 6186 CNRS-Université de la Méditerranée, Campus du GLM, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 9, France

Excitatory amino acid transporters (EAATs) are structurally related plasma membrane proteins known to mediate the Na⁺/K⁺-dependent uptake of the amino acids L-glutamate and DL-aspartate. In the nervous system, these proteins contribute to the clearance of glutamate from the synaptic cleft and maintain excitatory amino acid concentrations below excitotoxic levels. Two homologues exist in Drosophila melanogaster, dEAAT1 and dEAAT2, which are specifically expressed in the nervous tissue. We previously reported that dEAAT2 shows unique substrate discrimination as it mediates high affinity transport of aspartate but not glutamate. We now show that dEAAT2 can also transport the amino acid taurine with high affinity, a property that is not shared by two other transporters of the same family, Drosophila dEAAT1 and human hEAAT2. Taurine transport by dEAAT2 was efficiently blocked by an EAAT antagonist but not by inhibitors of the structurally unrelated mammalian taurine transporters. Taurine and aspartate are transported with similar K_m and relative efficacy and behave as mutually competitive inhibitors. dEAAT2 can mediate either net uptake or the heteroexchange of its two substrates, both being dependent on the presence of Na⁺ ions in the external medium. Interestingly, heteroexchange only occurs in one preferred substrate orientation, i.e. with taurine transported inwards and aspartate outwards, suggesting a mechanism of transinhibition of aspartate uptake by intracellular taurine. Therefore, dEAAT2 is actually an aspartate/taurine transporter. Further studies of this protein are expected to shed light on the role of taurine as a candidate neuromodulator and cell survival factor in the Drosophila nervous system.

Received for publication, November 3, 2004

^* This work was supported by an Action Concertée Incitative grant from the Ministère de la Recherche and funding from the Association Française contre les Myopathies and Fondation pour la Recherche Médicale (to S. B.). 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. Tel.: 33-491-26-96-06; Fax: 33-491-82-06-82; E-mail: birman{at}ibdm.univ-mrs.fr.

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