Regulation of a mammalian Shaker-related potassium channel, hKv1.5, by extracellular potassium and pH

Ion channels in sarcoma: pathophysiology and treatment options

Sarcomas are characterized by aggressive growth and a high metastasis potentially leading in most cases to a lethal outcome. These malignant tumors of the connective tissue have a high heterogeneity with numerous genetic mutations resulting in more than 100 types of sarcoma that can be grouped into two main kinds: soft tissue sarcoma and bone sarcoma. Sarcomas are often diagnosed at late disease stage, whereas a guaranteed diagnosis of the sarcoma type is fundamental for successful therapy. However, there is no appropriate therapy available. Therefore, the need for new therapies, which prolong survival and improve quality of life, is high. In the last two decades, the role of ion channels in cancer has emerged. Ion channels seem to be an ideal target for anti-tumor therapies. However, different cancer types have their own altered ion channel pattern, and the knowledge about the tumor-associated ion channel expression is fundamental. Here, we focus on the role of different ion channels in sarcoma, their pathophysiology, and possible treatment options.

Regulation of human cardiac Kv1.5 channels by extracellular acidification

Human Kv1.5 channels (hKv1.5) conduct the ultra-rapid delayed rectifier potassium current (I _Kur), which plays an important role in action potential repolarization of atrial myocytes. The present study was undertaken to examine the effects of acidic pH on hKv1.5 wild-type (WT) and its pore mutant channels heterologously expressed in Chinese hamster ovary (CHO) cells using site-directed mutagenesis combined with whole-cell patch-clamp technique. Both extracellular and intracellular acidifications equally and reversely reduced the amplitude of hKv1.5 currents. The extracellular acidification significantly shifted the voltage dependence of current activation to more depolarized potentials and accelerated deactivation kinetics of the current. The ancillary β subunits Kvβ1.3 and Kvβ1.2, known to modify the pharmacological sensitivities of hKv1.5, enhanced the extracellular proton-induced inhibitory effect on hKv1.5 current. In addition, several mutants (T462C, T479A, T480A, and I508A) exhibited significantly higher sensitivity to acidic pH-induced inhibition compared with WT channel, whereas the inhibitory effect of acidic pH was markedly reduced in H463G mutant. These observations indicate that (1) extracellular acidification modifies hKv1.5 gating and activity, (2) β subunits and several residues (T462, T479, T480, and I508) play critical roles in determining the sensitivity of the channel to acidic exposure, and (3) H463 may be a critical sensor for the channel inhibition by extracellular protons.

Intrinsic Relative Stabilities of the Neutral Tautomers of Arginine Side-Chain Models

The specific protonation state of amino acids is crucial for the physicochemical properties of proteins and their biological functions. These protonation states influence, for instance, properties related to hydrogen bonding, solubility, and folding. pKa calculations for proteins are, therefore, important and require, in principle, a specification of the most stable protonated and deprotonated forms of each titratable group. This is complicated by the existence of multiple tautomers, like the five neutral tautomers of the guanidine moiety in arginine. In this study, the compounds N-methyl-guanidine and N-ethyl-guanidine were used to model the charged and all neutral protonation states of the arginine side chain. The relative stabilities of all five neutral tautomers were investigated systematically for the first time, using quantum-mechanical calculations. These relative stabilities were obtained in vacuo, water and chloroform, by combining the quantum-mechanical calculations with a continuum solvation model. The water model was used to represent arginines exposed to an aqueous solution, whereas the chloroform model has a polarity representative of a protein core or a membrane. This allowed determining the relative pKa's associated with each neutral tautomer in these environments. A key result is that significant differences in stability are found between the neutral tautomers, in both water and chloroform. Some tautomers are consistently found to be the most stable. These findings will be helpful to refine pKa calculations in proteins.

ShakerIR and Kv1.5 mutant channels with enhanced slow inactivation also exhibit K⁺ o-dependent resting inactivation

Previous studies have shown that in N-type inactivation-removed Shaker (ShakerIR) channels, the T449K and T449A mutations result in enhanced slow inactivation. These mutant channels also show a loss of conductance in 0 mM K⁺ o that was attributed to an inactivation process occurring from the closed, resting state and which we refer to as resting inactivation. Similar behavior has also been observed in the Kv1.5 H463G mutant channel. To date, the time courses for the onset of and recovery from resting inactivation have been unknown, but a comparison of the kinetics for resting inactivation induced at -80 mV and slow inactivation evoked at +50 mV may provide information on whether these two processes are mechanistically related. Here, we present an analysis of the time courses for the onset of and recovery from [K⁺]o-dependent resting inactivation and depolarization-induced inactivation of these mutant channels. Despite the enhancement of slow inactivation in the ShakerIR T449K, T449A, and Kv1.5 H463G mutants, the time constant for slow inactivation at +50 mV (τ inact) was relatively insensitive to the increases or decreases of [K(+)]o, confirming that accelerated inactivation from the open state does not underlie the loss of conductance in 0 mM K⁺. For all three mutants, the time constant for resting inactivation (τ RI), induced by exposure to 0 mM K⁺ o solution at -80 mV, was at least an order of magnitude larger than τ inact. On the other hand, the time course of recovery at -80 mV of each mutant from 0 mM K(+) o-induced resting inactivation was the same as that from depolarization-induced slow inactivation. This latter result suggests that the 0 mM K⁺ o-induced resting inactivation of these mutant ShakerIR and Kv1.5 channels is mechanistically related to slow inactivation.

Proton block of the pore underlies the inhibition of hERG cardiac K+ channels during acidosis

Human ether-a-go-go-related gene (hERG) potassium channels are critical determinants of cardiac repolarization. Loss of function of hERG channels is associated with Long QT Syndrome, arrhythmia, and sudden death. Acidosis occurring as a result of myocardial ischemia inhibits hERG channel function and may cause a predisposition to arrhythmias. Acidic pH inhibits hERG channel maximal conductance and accelerates deactivation, likely by different mechanisms. The mechanism underlying the loss of conductance has not been demonstrated and is the focus of the present study. The data presented demonstrate that, unlike in other voltage-gated potassium (Kv) channels, substitution of individual histidine residues did not abolish the pH dependence of hERG channel conductance. Abolition of inactivation, by the mutation S620T, also did not affect the proton sensitivity of channel conductance. Instead, voltage-dependent channel inhibition (δ = 0.18) indicative of pore block was observed. Consistent with a fast block of the pore, hERG S620T single channel data showed an apparent reduction of the single channel current amplitude at low pH. Furthermore, the effect of protons was relieved by elevating external K(+) or Na(+) and could be modified by charge introduction within the outer pore. Taken together, these data strongly suggest that extracellular protons inhibit hERG maximal conductance by blocking the external channel pore.

Kinetic analysis of the effects of H+ or Ni2+ on Kv1.5 current shows that both ions enhance slow inactivation and induce resting inactivation

External H+ and Ni2+ ions inhibit Kv1.5 channels by increasing current decay during a depolarizing pulse and reducing the maximal conductance. Although the former may be attributed to an enhancement of slow inactivation occurring from the open state, the latter cannot. Instead, we propose that the loss of conductance is due to the induction, by H+ or Ni2+, of a resting inactivation process. To assess whether the two inactivation processes are mechanistically related, we examined the time courses for the onset of and recovery from H+- or Ni2+-enhanced slow inactivation and resting inactivation. Compared to the time course of H+- or Ni2+-enhanced slow inactivation at +50 mV, the onset of resting inactivation induced at 80 mV with either ion involves a relatively slower process. Recovery from slow inactivation under control conditions was bi-exponential, indicative of at least two inactivated states. Recovery following H+- or Ni2+-enhanced slow inactivation or resting inactivation had time constants similar to those for recovery from control slow inactivation, although H+ and Ni2+ biased inactivation towards states from which recovery was fast and slow, respectively. The shared time constants suggest that the H+- and Ni2+-enhanced slow inactivated and induced resting inactivated states are similar to those visited during control slow inactivation at pH 7.4. We conclude that in Kv1.5 H+ and Ni2+ differentially enhance a slow inactivation process that involves at least two inactivated states and that resting inactivation is probably a close variant of slow inactivation.

Inhibition of K(Ca)2.2 and K(Ca)2.3 channel currents by protonation of outer pore histidine residues

Ion channels are often modulated by changes in extracellular pH, with most examples resulting from shifts in the ionization state of histidine residue(s) in the channel pore. The application of acidic extracellular solution inhibited expressed K(Ca)2.2 (SK2) and K(Ca)2.3 (SK3) channel currents, with K(Ca)2.3 (pIC(50) of approximately 6.8) being approximately fourfold more sensitive than K(Ca)2.2 (pIC(50) of approximately 6.2). Inhibition was found to be voltage dependent, resulting from a shift in the affinity for the rectifying intracellular divalent cation(s) at the inner mouth of the selectivity filter. The inhibition by extracellular protons resulted from a reduction in the single-channel conductance, without significant changes in open-state kinetics or open probability. K(Ca)2.2 and K(Ca)2.3 subunits both possess a histidine residue in their outer pore region between the transmembrane S5 segment and the pore helix, with K(Ca)2.3 also exhibiting an additional histidine residue between the selectivity filter and S6. Mutagenesis revealed that the outer pore histidine common to both channels was critical for inhibition. The greater sensitivity of K(Ca)2.3 currents to protons arose from the additional histidine residue in the pore, which was more proximal to the conduction pathway and in the electrostatic vicinity of the ion conduction pathway. The decrease of channel conductance by extracellular protons was mimicked by mutation of the outer pore histidine in K(Ca)2.2 to an asparagine residue. These data suggest that local interactions involving the outer turret histidine residues are crucial to enable high conductance openings, with protonation inhibiting current by changing pore shape.

External Ba2+ block of human Kv1.5 at neutral and acidic pH: evidence for Ho+-induced constriction of the outer pore mouth at rest

Previous studies have shown that low pHo accelerates depolarization-induced inactivation and decreases the macroscopic conductance by reducing channel availability. To test the hypothesis that outer pore constriction underlies the decreased conductance at low pHo, external Ba2+ was used to examine the accessibility of the channel pore at rest under neutral and acidic conditions. At pHo 7.4, Ba2+ block of closed channels follows a monoexponential time course and involves a low-affinity superficial site (KD congruent with 1 mM, -80 mV, 0 mM Ko(+)) and a high-affinity site (KD congruent with 4 microM) deeper in the pore. Depolarization promotes Ba2+ dissociation and an analytical model incorporating state-dependent changes of Ba2+ affinity is presented that replicates the frequency dependence of the time course and the extent of block. Open-channel block by Ba2+ is weak. At pHo 5.5, both the access to and exit from the deep site is inhibited. These results are consistent with a low-pHo-induced conformational change in the outer pore that prevents Ba2+ binding at rest or unbinding during depolarization. If a pore constriction is involved, similar to that proposed to occur during P/C-type inactivation, this would imply that closed-state inactivation in Kv1.5 occurs under acidic conditions.

Voltage clamp fluorimetry reveals a novel outer pore instability in a mammalian voltage-gated potassium channel

Voltage-gated potassium (Kv) channel gating involves complex structural rearrangements that regulate the ability of channels to conduct K(+) ions. Fluorescence-based approaches provide a powerful technique to directly report structural dynamics underlying these gating processes in Shaker Kv channels. Here, we apply voltage clamp fluorimetry, for the first time, to study voltage sensor motions in mammalian Kv1.5 channels. Despite the homology between Kv1.5 and the Shaker channel, attaching TMRM or PyMPO fluorescent probes to substituted cysteine residues in the S3-S4 linker of Kv1.5 (M394C-V401C) revealed unique and unusual fluorescence signals. Whereas the fluorescence during voltage sensor movement in Shaker channels was monoexponential and occurred with a similar time course to ionic current activation, the fluorescence report of Kv1.5 voltage sensor motions was transient with a prominent rapidly dequenching component that, with TMRM at A397C (equivalent to Shaker A359C), represented 36 +/- 3% of the total signal and occurred with a tau of 3.4 +/- 0.6 ms at +60 mV (n = 4). Using a number of approaches, including 4-AP drug block and the ILT triple mutation, which dissociate channel opening from voltage sensor movement, we demonstrate that the unique dequenching component of fluorescence is associated with channel opening. By regulating the outer pore structure using raised (99 mM) external K(+) to stabilize the conducting configuration of the selectivity filter, or the mutations W472F (equivalent to Shaker W434F) and H463G to stabilize the nonconducting (P-type inactivated) configuration of the selectivity filter, we show that the dequenching of fluorescence reflects rapid structural events at the selectivity filter gate rather than the intracellular pore gate.

Effects of changes in extracellular pH and potassium concentration on Kv1.3 inactivation

The Kv1.3 channel inactivates via the P/C-type mechanism, which is influenced by a histidine residue in the pore region (H399, equivalent of Shaker 449). Previously we showed that the electric field of the protonated histidines at low extracellular pH (pHe) creates a potential barrier for K+ ions just outside the pore that hinders their exit from the binding site controlling inactivation (control site) thereby slowing inactivation kinetics. Here we examined the effects of extracellular potassium [K+]e and pHe on the rate of inactivation of Kv1.3 using whole-cell patch-clamp. We found that in 150 mM [K+]e inactivation was accelerated upon switching to pHe 5.5 as opposed to the slowing at 5 mM [K+]e. The transition from slowing to acceleration occurred at 40 mM [K+]e, whereas this "turning point" was at 20 mM [K+]e for inward currents. The rate of entry of Ba(2+) ions from the extracellular space to the control site was significantly slowed by low pHe in wild-type hKv1.3, but it was insensitive to pH(e) in H399K and H399L mutants. Based on these observations we expanded our model and propose that the potential barrier created by the protonated histidines impedes the passage of K+ ions between the extracellular medium and the control site in both directions and the effect on inactivation rate (acceleration or slowing) depends on the relative contribution of filling from the extracellular and intracellular sides.

A direct demonstration of closed-state inactivation of K+ channels at low pH

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.

SCAM analysis reveals a discrete region of the pore turret that modulates slow inactivation in Kv1.5

In Kv1.5, protonation of histidine 463 in the S5-P linker (turret) increases the rate of depolarization-induced inactivation and decreases the peak current amplitude. In this study, we examined how amino acid substitutions that altered the physico-chemical properties of the side chain at position 463 affected slow inactivation and then used the substituted cysteine accessibility method (SCAM) to probe the turret region (E456-P468) to determine whether residue 463 was unique in its ability to modulate the macroscopic current. Substitutions at position 463 of small, neutral (H463G and H463A) or large, charged (H463R, H463K, and H463E) side groups accelerated inactivation and induced a dependency of the current amplitude on the external potassium concentration. When cysteine substitutions were made in the distal turret (T462C-P468C), modification with either the positively charged [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET) or negatively charged sodium (2-sulfonatoethyl) methanethiosulfonate reagent irreversibly inhibited current. This inhibition could be antagonized either by the R487V mutation (homologous to T449V in Shaker) or by raising the external potassium concentration, suggesting that current inhibition by MTS reagents resulted from an enhancement of inactivation. These results imply that protonation of residue 463 does not modulate inactivation solely by an electrostatic interaction with residues near the pore mouth, as proposed by others, and that residue 463 is part of a group of residues within the Kv1.5 turret that can modulate P/C-type inactivation.

Modelling the pH-dependent properties of Kv1 potassium channels

It is known that the pH dependence of conductance for the rat potassium channel Kv1.4 is susbstantially reduced upon mutation of either H508 or K532. These residues lie in the extracellular mouth of the channel pore. We have used continuum electrostatics to investigate their interactions with K(+) sites in the pore. The predicted scale of interactions between H508/K532 and potassium sites is sufficient to significantly alter potassium occupancy and thus channel function. We interpret the effect of K532 mutation as indicating that the pH-dependent effect requires not only an ionisable group with a suitable pK(a) value (i.e. histidine), but also that other charged groups set the potential profile at a threshold level. This hypothesis is examined in the context of pH dependence for other members of the Kv1 family, and may represent a general tool with which to study potassium channels.

The external K+ concentration and mutations in the outer pore mouth affect the inhibition of kv1.5 current by Ni2+

By examining the consequences both of changes of [K+]o and of point mutations in the outer pore mouth, our goal was to determine if the mechanism of the block of Kv1.5 ionic currents by external Ni2+ is similar to that for proton block. Ni2+ block is inhibited by increasing [K+]o, by mutating a histidine residue in the pore turret (H463Q) or by mutating a residue near the pore mouth (R487V) that is the homolog of Shaker T449. Aside from a slight rightward shift of the Q-V curve, Ni2+ had no effect on gating currents. We propose that, as with Ho+, Ni2+ binding to H463 facilitates an outer pore inactivation process that is antagonized by Ko+ and that requires R487. However, whereas Ho+ substantially accelerates inactivation of residual currents, Ni2+ is much less potent, indicating incomplete overlap of the profiles of these two metal ions. Analyses with Co2+ and Mn2+, together with previous results, indicate that for the first-row transition metals the rank order for the inhibition of Kv1.5 in 0 mM Ko+ is Zn2+ (KD approximately 0.07 mM) > or = Ni2+) (KD approximately 0.15 mM) > Co2+ (KD approximately 1.4 mM) > Mn2+ (KD > 10 mM).

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Mechanisms of the inhibition of Shaker potassium channels by protons

Potassium channels are regulated by protons in various ways and, in most cases, acidification results in potassium current reduction. To elucidate the mechanisms of proton-channel interactions we investigated N-terminally truncated Shaker potassium channels (Kv1 channels) expressed in Xenopus oocytes, varying pH at the intracellular and the extracellular face of the membrane. Intracellular acidification resulted in rapid and reversible channel block. The block was half-maximal at pH 6.48, thus even physiological excursions of intracellular pH will have an impact on K+ current. The block displayed only very weak voltage dependence and C-type inactivation and activation were not affected. Extracellular acidification (up to pH 4) did not block the channel, indicating that protons are effectively excluded from the selectivity filter. Channel current, however, was reduced greatly due to marked acceleration of C-type inactivation at low pH. In contrast, inactivation was not affected in the T449V mutant channel, in which C-type inactivation is impaired. The pH effect on inactivation of the wild-type channel had an apparent pK of 4.7, suggesting that protonation of extracellular acidic residues in Kv channels makes them subject to pH regulation.

Rapid induction of P/C-type inactivation is the mechanism for acid-induced K+ current inhibition

Extracellular acidification is known to decrease the conductance of many voltage-gated potassium channels. In the present study, we investigated the mechanism of H(+)(o)-induced current inhibition by taking advantage of Na(+) permeation through inactivated channels. In hKv1.5, H(+)(o) inhibited open-state Na(+) current with a similar potency to K(+) current, but had little effect on the amplitude of inactivated-state Na(+) current. In support of inactivation as the mechanism for the current reduction, Na(+) current through noninactivating hKv1.5-R487V channels was not affected by [H(+)(o)]. At pH 6.4, channels were maximally inactivated as soon as sufficient time was given to allow activation, which suggested two possibilities for the mechanism of action of H(+)(o). These were that inactivation of channels in early closed states occurred while hyperpolarized during exposure to acid pH (closed-state inactivation) and/or inactivation from the open state was greatly accelerated at low pH. The absence of outward Na(+) currents but the maintained presence of slow Na(+) tail currents, combined with changes in the Na(+) tail current time course at pH 6.4, led us to favor the hypothesis that a reduction in the activation energy for the inactivation transition from the open state underlies the inhibition of hKv1.5 Na(+) current at low pH.

Spreading acidification and depression in the cerebellar cortex

Optical imaging of activity-dependent pH changes using neutral red has revealed a novel form of propagated activity in the cerebellar cortex: spreading acidification and depression (SAD). Evoked by surface stimulation, SAD is characterized by a propagation geometry that reflects the parasagittal architecture of the cerebellum, high speed of propagation across several folia, and a transient depression of the molecular layer circuitry. The properties of SAD differentiate it from other forms of propagating activity in the nervous system including spreading depression and Ca++ waves. Involving several factors, SAD is hypothesized to be a regenerative process that requires a functioning parallel fibers-Purkinje cell circuit, glutamatergic neurotransmission, and is initiated by increased neuronal excitability. Three possible neuronal and glia substrates in the cerebellar cortex could account for the propagation geometry of SAD. Recently, the authors demonstrated that blocking voltage-gated Kv1.1 potassium channels plays a major role in the generation of SAD. This observation has lead to the hypothesis that the episodic and transient disruption in cerebellar function that characterizes episodic ataxia type 1, a Kv1.1 channelopathy, is due to SAD occurring in the cerebellar cortex.

Effect of external pH on activation of the Kv1.5 potassium channel

We studied the mechanism by which external acidification from pH 7.3 to 6.8 reduced current magnitude in the Kv1.5 potassium channel. At physiological external [K(+)], a shift in the voltage-dependence of activation was entirely responsible for the acidification-induced decrease in Kv1.5 current magnitude (pK = 7.15). Elevation of external [Ca(2+)] or [Mg(2+)] identically shifted activation curves to the right and identically shifted the pH-sensitivity of the activation curves to more acidic values. Similar observations were made with the Kv2.1 K(+) channel, except that the pK for the activation shift was out of the physiological range. These data are consistent with a mechanism by which acidification shifted activation via modification of a local surface potential. Elimination of eight positive charges within the outer vestibule of the conduction pathway had no effect on the voltage-dependence of activation at pH 7.3 or higher, which suggested that sites exposed to the conduction pathway within the outer vestibule did not directly contribute to the relevant local surface potential. However, mutations at position 487 (within the conduction pathway) displaced the pK of the pH-sensitive shift in activation, such that the sensitivity of Kv1.5 current to physiologically relevant changes in pH was reduced or eliminated. These results suggest that, among voltage-gated K(+) channels, activation in Kv1.5 is uniquely sensitive to physiologically relevant changes in pH because the pK for the sites that contribute to the local surface potential effect is near pH 7. Moreover, the pK for the activation shift depends not only on the nature of the sites involved but also on structural orientation conferred, in part, by at least one residue within the conduction pathway.

Molecular determinants of the inhibition of human Kv1.5 potassium currents by external protons and Zn(2+)

Using human Kv1.5 channels expressed in HEK293 cells we assessed the ability of H+o to mimic the previously reported action of Zn(2+) to inhibit macroscopic hKv1.5 currents, and using site-directed mutagenesis, we addressed the mechanistic basis for the inhibitory effects of H(+)(o) and Zn(2+). As with Zn(2+), H(+)(o) caused a concentration-dependent, K(+)(o)-sensitive and reversible reduction of the maximum conductance (g(max)). With zero, 5 and 140 mM K(+)(o) the pK(H) for this decrease of g(max) 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 K(D) 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 Q(max) 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 Zn(2+) 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 Zn(2+) binding to histidine residues in the pore turret stabilizes a channel conformation that is most likely an inactivated state.