Zn2+ inhibits the anion conductance of the glutamate transporter EAAT4

Glutamate transport by the excitatory amino acid transporters (EAATs) is co upled to the co-transport of 3 Na+ ions and 1 H+ and the counter-transport of 1 K+ ion, which ensures that extracellular glutamate concentrations are maintained in the submicromolar range. In addition to the coupled ion fluxe s, glutamate transport activates an uncoupled anion conductance that does n ot influence the rate or direction of transport but may have the capacity t o influence the excitability of the cell. Free Zn2+ ions are often co-local ized with glutamate in the central nervous system and have the capacity to modulate the dynamics of excitatory neurotransmission. In this study we dem onstrate that Zn2+ ions inhibit the uncoupled anion conductance and also re duce the affinity of L-aspartate for EAAT4. The molecular basis for this ef fect was investigated using site-directed mutagenesis. Two histidine residu es in the extracellular loop between transmembrane domains three and four o f EAAT4 appear to confer Zn2+ inhibition of the anion conductance.