Copper and protons directly activate the zinc-activated channel

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The zinc-activated channel (ZAC) is a cationic ion channel belonging to the superfamily of Cys-loop receptors, which consists of pentameric ligand-gated ion channels. ZAC is the least understood member of this family so in the present study we sought to characterize the properties of this channel further. We demonstrate that not only zinc (Zn2+) but also copper (Cu2+) and protons (H+) are agonists of ZAC, displaying potencies and efficacies in the rank orders of H+ > Cu2+ > Zn2+ and H+ > Zn2+ > Cu2+, respectively. The responses elicited by Zn2+, Cu2+ and H+ through ZAC are all characterized by low degrees of desensitization. In contrast, currents evoked by high concentrations of the three agonists comprise distinctly different activation and decay components, with transitions to and from an open state being significantly faster for H+ than for the two metal ions. The permeabilities of ZAC for Na+ and K+ relative to Cs+ are indistinguishable, whereas replacing all of extracellular Na+ and K+ with the divalent cations Ca2+ or Mg2+ results in complete elimination of Zn2+-activated currents at both negative and positive holding potentials. This indicates that ZAC is non-selectively permeable to monovalent cations, whereas Ca2+ and Mg2+ inhibit the channel. In conclusion, this is the first report of a Cys-loop receptor being gated by Zn2+, Cu2+ and H+. ZAC could be an important mediator of some of the wide range of physiological functions regulated by or involving Zn2+, Cu2+ and H+.

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