Contribution of extracellular negatively charged residues to ATP action and zinc modulation of rat P2X2 receptors

Two histidines are known to be essential for zinc potentiation of rat P2X₂ receptors, but the chemistry of zinc coordination would suggest that other residues also participate in this zinc-binding site. There is also a second lower affinity zinc-binding site in P2X₂ receptors whose constituents are unknown. To assess whether the extracellular acidic residues of the P2X₂ receptor contribute to zinc potentiation or inhibition, site-directed mutagenesis was used to produce alanine substitutions at each extracellular glutamate or aspartate. Two electrode voltage clamp recordings from Xenopus oocytes indicated that 7 of the 34 mutants (D82A, E85A, E91A, E115A, D136A, D209A, and D281A) were deficient in zinc potentiation and one mutant (E84A) was deficient in zinc inhibition. Additional tests on cysteine mutants at these eight positions indicated that D136 is the only residue that is a strong candidate to be at the potentiating zinc-binding site, and that E84 is unlikely to be at the inhibitory zinc-binding site.