Using human Kv1.5 channels expressed in HEK293 cells we assessed the ability of H+o to mimic the previously reported action of Zn2+ to inhibit macroscopic hKv1.5 currents, and using site‐directed mutagenesis, we addressed the mechanistic basis for the inhibitory effects of H+o and Zn2+. As with Zn2+, H+o caused a concentration‐dependent, K+o‐sensitive and reversible reduction of the maximum conductance (gmax). With zero, 5 and 140 mm K+o the pKH for this decrease of gmax was 6.8, 6.2 and 6.0, respectively. The concentration dependence of the block relief caused by increasing [K+]o was well fitted by a non‐competitive interaction between H+o and K+o, for which the KD for the K+ binding site was 0.5‐1.0 mm. Additionally, gating current analysis in the non‐conducting mutant hKv1.5 W472F showed that changing from pH 7.4 to pH 5.4 did not affect Qmax and that charge immobilization, presumed to be due to C‐type inactivation, was preserved at pH 5.4. Inhibition of hKv1.5 currents by H+o or Zn2+ was substantially reduced by a mutation either in the channel turret (H463Q) or near the pore mouth (R487V). In light of the requirement for R487, the homologue of Shaker T449, as well as the block‐relieving action of K+o, we propose that H+ or Zn2+ binding to histidine residues in the pore turret stabilizes a channel conformation that is most likely an inactivated state.

Lowering external pH reduces peak current and enhances current decay in Kv and Shaker-IR channels. Using voltage-clamp fluorimetry we directly determined the fate of Shaker-IR channels at low pH by measuring fluorescence emission from tetramethylrhodamine-5-maleimide attached to substituted cysteine residues in the voltage sensor domain (M356C to R362C) or S5-P linker (S424C). One aspect of the distal S3-S4 linker α-helix (A359C and R362C) reported a pH-induced acceleration of the slow phase of fluorescence quenching that represents P/C-type inactivation, but neither site reported a change in the total charge movement at low pH. Shaker S424C fluorescence demonstrated slow unquenching that also reflects channel inactivation and this too was accelerated at low pH. In addition, however, acidic pH caused a reversible loss of the fluorescence signal (pKa = 5.1) that paralleled the reduction of peak current amplitude (pKa = 5.2). Protons decreased single channel open probability, suggesting that the loss of fluorescence at low pH reflects a decreased channel availability that is responsible for the reduced macroscopic conductance. Inhibition of inactivation in Shaker S424C (by raising external K+ or the mutation T449V) prevented fluorescence loss at low pH, and the fluorescence report from closed Shaker ILT S424C channels implied that protons stabilized a W434F-like inactivated state. Furthermore, acidic pH changed the fluorescence amplitude (pKa = 5.9) in channels held continuously at −80 mV. This suggests that low pH stabilizes closed-inactivated states. Thus, fluorescence experiments suggest the major mechanism of pH-induced peak current reduction is inactivation of channels from closed states from which they can activate, but not open; this occurs in addition to acceleration of P/C-type inactivation from the open state.

Hyperpolarization-activated cyclic nucleotide–sensitive nonselective cation (HCN) channels are activated by membrane hyperpolarization, in contrast to the vast majority of other voltage-gated channels that are activated by depolarization. The structural basis for this unique characteristic of HCN channels is unknown. Interactions between the S4–S5 linker and post-S6/C-linker region have been implicated previously in the gating mechanism of HCN channels. We therefore introduced pairs of cysteines into these regions within the sea urchin HCN channel and performed a Cd2+-bridging scan to resolve their spatial relationship. We show that high affinity metal bridges between the S4–S5 linker and post-S6/C-linker region can induce either a lock-open or lock-closed phenotype, depending on the position of the bridged cysteine pair. This suggests that interactions between these regions can occur in both the open and closed states, and that these regions move relative to each other during gating. Concatenated constructs reveal that interactions of the S4–S5 linker and post-S6/C-linker can occur between neighboring subunits. A structural model based on these interactions suggests a mechanism for HCN channel gating. We propose that during voltage-dependent activation the voltage sensors, together with the S4–S5 linkers, drive movement of the lower ends of the S5 helices around the central axis of the channel. This facilitates a movement of the pore-lining S6 helices, which results in opening of the channel. This mechanism may underlie the unique voltage dependence of HCN channel gating.

Changing the extracellular concentration of divalent cations can have dramatic effects on the behaviour of electrically excitable membranes as illustrated by the lowering or raising of the threshold for cell firing caused by hypo-or hypercalcaemia, respectively (Frankenhaeuser & Hodgkin, 1957). Voltage-clamp analyses have shown that the divalent cation-induced change of excitability can be linked to a shift of the voltage dependence of channel gating. For example, an elevation of the external concentration of Ca 2+ ([Ca 2+ ] o ) produces a depolarizing shift of the midpoint (V 1/2 ) of the activation curves of both voltage-gated Na + and K + channels of the squid giant axon (Hille et al. 1975;Hahin & Campbell, 1983 1. We used the whole-cell recording technique to examine the effect of extracellular Zn 2+ on macroscopic currents due to Kv1.5 channels expressed in the human embryonic kidney cell line HEK293.2. Fits of a Boltzmann function to tail current amplitudes showed that 1 mM Zn 2+ shifted the half-activation voltage from _10.2 ± 0.4 to 21.1 ± 0.7 mV and the slope factor increased from 6.8 ± 0.4 to 9.4 ± 0.7 mV. The maximum conductance in 1 mM Zn 2+ and with 3.5 mM K + o was 33 ± 7 % of the control value.3. In physiological saline the apparent K D for the Zn 2+ block was 650 ± 24 µM and was voltage independent. A Hill coefficient of 1.0 ± 0.03 implied that block is mediated by the occupation of a single binding site. 6. We propose that the gating shift and the block caused by Zn 2+ are mediated by two distinct sites and that the blocking site is located in the external mouth of the pore.

The increasing prevalence of influenza viruses with resistance to approved antivirals highlights the need for new anti-influenza therapeutics. Here we describe the functional properties of hexamethylene amiloride (HMA)-derived compounds that inhibit the wildtype and adamantane-resistant forms of the influenza A M2 ion channel. For example, 6-(azepan-1-yl)-N-carbamimidoylnicotinamide (9) inhibits amantadine-sensitive M2 currents with 3-to 6-fold greater potency than amantadine or HMA (IC 50 5 0.2 vs. 0.6 and 1.3 mM, respectively). Compound 9 competes with amantadine for M2 inhibition, and molecular docking simulations suggest that 9 binds at site(s) that overlap with amantadine binding. In addition, tert-butyl 49-(carbamimidoylcarbamoyl)-29,3-dinitro-