MUTATION OF AN AMINO-ACID RESIDUE INFLUENCING POTASSIUM COUPLING IN THE GLUTAMATE TRANSPORTER GET-1 INDUCES OBLIGATE EXCHANGE

Glutamate transporters maintain low synaptic concentrations of neurotr ansmitter by coupling uptake to flux of other ions. After cotransport of glutamic acid with Na+, the cycle is completed by countertransport of K+. We have identified an amino acid residue (glutamate 404) influe ncing ion coupling in a domain of the transporter implicated previousl y in kainate binding. Mutation of this residue to aspartate (E404D) pr events both forward and reverse transport induced by K+. Sodium depend ent transmitter exchange and a transporter-mediated chloride conductan ce are unaffected by the mutation, indicating that this residue select ively influences potassium flux coupling. The results support a kineti c model in which sodium and potassium are translocated in distinct ste ps and suggest that this highly conserved region of the transporter is intimately associated with the ion permeation pathway.