Cryo-EM structure of the zinc-activated channel (ZAC) in the Cys-loop receptor superfamily.

Cys-loop receptors are a large superfamily of pentameric ligand-gated ion channels with various physiological roles, especially in neurotransmission in the central nervous system. Among them, zinc-activated channel (ZAC) is a Zn(2+)-activated ion channel that is widely expressed in the human body and is conserved among eukaryotes. Due to its gating by extracellular Zn(2+), ZAC has been considered a Zn(2+) sensor, but it has undergone minimal structural and functional characterization since its molecular cloning. Among the families in the Cys-loop receptor superfamily, only the structure of ZAC has yet to be determined. Here, we determined the cryo-EM structure of ZAC in the apo state and performed structure-based mutation analyses. We identified a few residues in the extracellular domain whose mutations had a mild impact on Zn(2+) sensitivity. The constriction site in the ion-conducting pore differs from the one in other Cys-loop receptor structures, and further mutational analysis identified a key residue that is important for ion selectivity. In summary, our work provides a structural framework for understanding the ion-conducting mechanism of ZAC.