Neutral amino acid transporter ASCT2 displays substrate-induced Na+ exchange and a substrate-gated anion conductance.
The neutral amino acid transporter ASCT2 mediates electroneutral obligatory antiport but at the same time requires Na(+) for its function. To elucidate the mechanism, ASCT2 was expressed in Xenopus laevis oocytes and transport was analysed by flux studies and two-electrode voltage clamp recordings. Flux studies with (22)NaCl indicated that the uptake of one molecule of glutamine or alanine is accompanied by the uptake of four to seven Na(+) ions. Similarly to the transport of amino acids, the Na(+) uptake was mediated by an obligatory Na(+) exchange mechanism that depended on the presence of amino acids but was not stoichiometrically coupled to the amino acid transport. Other cations could not replace Na(+) in this transport mechanism. When NaCl was replaced by NaSCN in the transport buffer, the superfusion of oocytes with amino acid substrates resulted in large inward currents, indicating the presence of a substrate-gated anion channel in the ASCT2 transporter. The K(m) for glutamine derived from these experiments is in good agreement with the K(m) derived from flux studies; it varied between 40 and 90 microM at holding potentials of -60 and -20 mV respectively. The permeability of the substrate-gated anion conductance decreased in the order SCN(-)>>NO(3)(-)>I(-)>Cl(-) and also required the presence of Na(+).
Bröer
A
A
Universität Tübingen, Physiologisches Institut, Gmelinstrasse 5, 72076 Tübingen, Germany. stefan.broeer@uni-tuebingen.de
Wagner
C
C
Lang
F
F
Bröer
S
S
D026905:Amino Acid Transport Systems; D000818:Animals; D000838:Anions; D002352:Carrier Proteins / Q000378:metabolism*; D005260:Female; D015640:Ion Channel Gating; D017136:Ion Transport; D009865:Oocytes / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D014982:Xenopus laevis
D026905:Amino Acid Transport Systems; D000838:Anions; D002352:Carrier Proteins; D012964:Sodium