Na+ transport by the neural glutamine transporter ATA1

Transfer of glutamine between astrocytes and neurons is an essential part of the glutamate-glutamine cycle in the brain. Here we have investigated how the neural glutamine transporter (rATA1/GlnT) works. Rat ATA1 was expressed in Xenopus laevis oocytes and examined using two-electrode voltage-clamp recordings, ion-sensitive microelectrodes and tracer flux experiments. Glutamine transport via rATA1 was electrogenic and caused inward currents that did not reverse at positive holding potentials. Currents were induced by a variety of neutral amino acids in the following relative order Ala > Ser/Gln/Asn/His/Cys/Met > MeAIB/Gly > Thr/Pro/Tyr/Val, where MeAIB is the amino acid analogue N-methylaminoisobutyric acid. The uptake of glutamine and the corresponding currents depended on Na+ and PH. Hill-coefficient and flux studies with (NaCl)-Na-22 indicated a cotransport stoichiometry 1 Na+ per transport cycle. The transporter also showed uncoupled Na+ transport, particularly when alanine was used as the substrate. Although substrate uptake increased strongly with increasing PH, no change of intracellular PH was observed during transport. A decrease of the intracellular PH similarly inhibited glutamine transport via ATA1, suggesting that the PH dependence was an allosteric effect on the transporter.