P2X receptor channels (P2XRs) are allosterically modulated by several compounds, mainly acting at the ectodomain of the receptor. Like copper, mercury, a metal that induces oxidative stress in cells, also stimulates the activity of P2X 2 R and inhibits the activity of P2X 4 R. However, the mercury modulation is not related to the extracellular residues critical for copper modulation. To identify the site(s) for mercury action, we generated two chimeras using the full size P2X 2 subunit, termed P2X 2a , and a splice variant lacking a 69 residue segment in the C terminal, termed P2X 2b , as the donors for intracellular and transmembrane segments and the P2X 4 subunit as the donor for ectodomain segment of chimeras. The potentiating effect of mercury on ATP-induced current was preserved in Xenopus oocytes expressing P2X 4/2a chimera but was absent in oocytes expressing P2X 4/2b chimera. Site-directed mutagenesis experiments revealed that the Cys 430 residue mediates effects of mercury on the P2X 2a R activity. Because mercury could act as an oxidative stress inducer, we also tested whether hydrogen peroxide (H 2 O 2 ) and mitochondrial stress inducers myxothiazol and rotenone mimicked mercury effects. These experiments, done in both oocytes and human embryonic kidney HEK293 cells, revealed that these compounds potentiated the ATPevoked P2X 2a R and P2X 4/2a R currents but not P2X 2b R and P2X 2a -C430A and P2X 2a -C430S mutant currents, whereas antioxidants dithiothreitrol and N-acetylcysteine prevented the H 2 O 2 potentiation. Alkylation of Cys 430 residue with methylmethane-thiosulfonate also abolished the mercury and H 2 O 2 potentiation. Altogether, these results are consistent with the hypothesis that the Cys 430 residue is an intracellular P2X 2a R redox sensor.

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