Zn2+ inhibits the anion conductance of the glutamate transporter EAAT4

Glutamate transport by the excitatory amino acid transporters (EAATs) is coupled 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 fluxes, glutamate transport activates an uncoupled anion conductance that does not influence the rate or direction of transport but may have the capacity to influence the excitability of the cell. Free Zn2+ ions are often co-localized with glutamate in the central nervous system and have the capacity to modulate the dynamics of excitatory neurotransmission. In this study we demonstrate that Zn2+ ions inhibit the uncoupled anion conductance and also reduce the affinity of L-aspartate for EAAT4. The molecular basis for this effect was investigated using site-directed mutagenesis. Two histidine residues in the extracellular loop between transmembrane domains three and four of EAAT4 appear to confer Zn2+ inhibition of the anion conductance.