Molecular determinant of ion selectivity of a (Na⁺ + K⁺)-coupled rat brain glutamate transporter

Glutamate transporters remove this neurotransmitter from the synaptic cleft by a two-stage electrogenic process, in which glutamate is first cotransported with three sodium ions and a proton. Subsequently, the cycle is completed by translocation of a potassium ion in the opposite direction. Recently, we have identified an amino acid residue of the glutamate transporter GLT-1 (Glu-404) that influences potassium coupling. We have now analyzed the effect of seven other amino acid residues in the highly conserved region surrounding this site. One of these residues, Tyr-403, also proved important for potassium coupling, because mutation to Phe (Y403F) resulted in an electroneutral obligate exchange mode of glutamate transport. This mutation in the transporter also caused an approximately 8-fold increase in the apparent sodium affinity, with no change in the apparent affinity for L-glutamate or D-aspartate. Strikingly, although exchange catalyzed by the wild-type transporter is strictly dependent on sodium, the selectivity of Y403F mutant transporters is altered so that sodium can be replaced by other alkaline metal cations including lithium and cesium. These results indicate the presence of interacting sites in or near the transporter pore that control selectivity for sodium and potassium.