Coupling between an electrostatic network and the Zn2+ binding site modulates Hv1 activation

The voltage sensor (VS) domain in Hv1 proton channels mediates a voltage-dependent and H+-selective “aqueous” conductance (GAQ) that is potently modulated by extracellular Zn2+. Although two conserved His residues are required for Zn2+ effects on GAQ gating, the atomic structure of the Zn2+ coordination site and mechanism by which extracellular Zn2+ stabilizes a closed-state conformation remain unknown. Here we use His mutagenesis to identify residues that increase Zn2+ potency and are therefore likely to participate in first solvation shell interactions with Zn2+. Experimental Zn2+-mapping data were then used to constrain the structure of a new resting-state Hv1 model (Hv1 F). Molecular dynamics (MD) simulations show how protein and water atoms directly contribute to octahedral Zn2+ coordination spheres in Zn2+-bound and -unbound Hv1 F models. During MD simulations, we observed correlated movements of Zn2+-interacting side chains and residues in a highly conserved intracellular Coulombic network (ICN) that contains highly conserved Arg “gating charges” in S4 as well as acidic “counter-charges” in S2 and S3 and is known to control VS activation, suggesting that occupancy of the extracellular Zn2+ site is conformationally coupled to reorganization of the ICN. To test this hypothesis, we neutralized an ICN Glu residue (E153) and show that in addition to shifting GAQ activation to more negative voltages, E153A also decreases Zn2+ potency. We speculate that extracellular gating-modifier toxins and other ligands may use a generally similar long-range conformational coupling mechanism to modulate VS activation in related ion channel proteins.