In this study, we have shown that, when expressed in Xenopus oocytes, trout anion exchanger 1 (tAE1) was able to act as a bifunctional protein, either an anion exchanger or a chloride conductance. Point mutations of tAE1 were carried out and their effect on Cl À conductance and Cl À unidirectional flux were studied. We have shown that mutations made in transmembrane domain 7 had dramatic effects on tAE1 function. Indeed, when these residues were mutated, either individually or together (mutants E632K, D633G, and ED/KG), Cl À conductance was reduced to 28-44% that of wild-type tAE1. Moreover, ion substitution experiments showed that anion selectivity was altered. However, the exchanger function was unchanged, as evidenced by the fact that Cl À influx and K m were identical for each of these mutants and similar to the wild-type protein parameters. By contrast, mutations made in the C-terminal domains of the protein (R819M, Q829K) affected both transport functions. Cl À conductance was increased by $200% with respect to tAE1 and anion selectivity was impaired. Likewise, Cl À influx was increased by $260% and was no longer saturable. These and other mutations carried out in transmembrane domains 7, 8, 12-14 of tAE1 allow us to demonstrate without doubt that, in addition to its anion exchanger activity, tAE1 can also function as a chloride channel. Above all, this work led us to identify amino acids involved in this double function organization.