Voltage-gated proton (Hv1) channels play important roles in the respiratory burst, in pH regulation, in spermatozoa, in apoptosis, and in cancer metastasis. Unlike other voltage-gated cation channels, the Hv1 channel lacks a centrally located pore formed by the assembly of subunits. Instead, the proton permeation pathway in the Hv1 channel is within the voltage-sensing domain of each subunit. The gating mechanism of this pathway is still unclear. Mutagenic and fluorescence studies suggest that the fourth transmembrane (TM) segment (S4) functions as a voltage sensor and that there is an outward movement of S4 during channel activation. Using thermodynamic mutant cycle analysis, we find that the conserved positively charged residues in S4 are stabilized by countercharges in the other TM segments both in the closed and open states. We constructed models of both the closed and open states of Hv1 channels that are consistent with the mutant cycle analysis. These structural models suggest that electrostatic interactions between TM segments in the closed state pull hydrophobic residues together to form a hydrophobic plug in the center of the voltage-sensing domain. Outward S4 movement during channel activation induces conformational changes that remove this hydrophobic plug and instead insert protonatable residues in the center of the channel that, together with water molecules, can form a hydrogen bond chain across the channel for proton permeation. This suggests that salt bridge networks and the hydrophobic plug function as the gate in Hv1 channels and that outward movement of S4 leads to the opening of this gate.voltage gating | mutagenesis cycle | molecular dynamics | VSOP | blocker T he best-studied function of voltage-gated proton (Hv1) channels is in the immune system, where the activity of Hv1 channels has been shown to play a key role in charge compensation for the electron extrusion by NADPH oxidase during the respiratory burst in phagocytes (1, 2). In addition, this channel is also found in many other cell types including neurons, sperm, and lung airway epithelia cells, where it has been implicated in acid extrusion, male fertility, and the pathology of asthma, respectively (3, 4). During stroke, the activity of Hv1 channels exacerbates neuronal death (5). In 2006, two independent groups identified the genes coding for the Hv1 channel in humans (hHv1) (6) and mice and Ciona intestinalis (voltage-sensing only protein; we call it "Ci-Hv1" in this paper) (7). The sequences of Hv1/(VSOP) are homologous to the sequences of the voltagesensing domain (VSD) of other voltage-gated ion channels, such as voltage-gated sodium, potassium, and calcium channels (Nav, Kv, and Cav channels), and the voltage sensor-containing phosphatase VSP (6, 7). Hydropathy analysis of the Hv1 sequence suggests the existence of four transmembrane (TM) segments (Fig. 1A), designated S1 to S4, similar to the four TM segments of the VSD of Kv channels (Fig. S1). However, Hv1 channels lack the last two TM segments of Kv channels that make up t...

The molecular strategy for alkali cation selectivity by a bacterial sodium channel resembles those of eukaryotic calcium and potassium channels, rather than those of eukaryotic sodium channels.

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The differential modulation of agonist and antagonist binding to opioid receptors (ORs) by sodium (Na+) has been known for decades. To shed light on the molecular determinants, thermodynamics, and kinetics of Na+ translocation through the μ-OR (MOR), we used a multi-ensemble Markov model framework combining equilibrium and non-equilibrium atomistic molecular dynamics simulations of Na+ binding to MOR active or inactive crystal structures embedded in an explicit lipid bilayer. We identify an energetically favorable, continuous ion pathway through the MOR active conformation only, and provide, for the first time: i) estimates of the energy differences and required timescales of Na+ translocation in inactive and active MORs, ii) estimates of Na+-induced changes to agonist binding validated by radioligand measurements, and iii) testable hypotheses of molecular determinants and correlated motions involved in this translocation, which are likely to play a key role in MOR signaling.