Na(+):aspartate coupling stoichiometry in the glutamate transporter homologue Glt(Ph)

The Na(+) aspartate symporter Glt(Ph) from Pyrococcus horikoshii is the only member of the glutamate transporter family for which crystal structures have been determined. The cation:aspartate coupling stoichiometry is unknown, thus hampering the elucidation of the ion coupling mechanism. Here we measure transport of (22)Na(+) and [(14)C]aspartate in proteoliposomes containing purified Glt(Ph) and demonstrate that three Na(+) ions are symported with aspartate.