A quantitative description of KcsA gating II: single-channel currents

Amphipathic antenna of an inward rectifier K+ channel responds to changes in the inner membrane leaflet

Membrane lipids modulate the function of membrane proteins. In the case of ion channels, they bias the gating equilibrium, although the underlying mechanism has remained elusive. Here we demonstrate that the N-terminal segment (M0) of the KcsA potassium channel mediates the effect of changes in the lipid milieu on channel gating. The M0 segment is a membrane-anchored amphipathic helix, bearing positively charged residues. In asymmetric membranes, the M0 helix senses the presence of negatively charged phospholipids on the inner leaflet. Upon gating, the M0 helix revolves around the axis of the helix on the membrane surface, inducing the positively charged residues to interact with the negative head groups of the lipids so as to stabilize the open conformation (i.e., the "roll-and-stabilize model"). The M0 helix is thus a charge-sensitive "antenna," capturing temporary changes in lipid composition in the fluidic membrane. This unique type of sensory device may be shared by various types of membrane proteins.

Functional equilibrium of the KcsA structure revealed by NMR

KcsA is a tetrameric K(+) channel that is activated by acidic pH. Under open conditions of the helix bundle crossing, the selectivity filter undergoes an equilibrium between permeable and impermeable conformations. Here we report that the population of the permeable conformation (p(perm)) positively correlates with the tetrameric stability and that the population in reconstituted high density lipoprotein, where KcsA is surrounded by the lipid bilayer, is lower than that in detergent micelles, indicating that dynamic properties of KcsA are different in these two media. Perturbation of the membrane environment by the addition of 1-3% 2,2,2-trifluoroethanol increases p(perm) and the open probability, revealed by NMR and single-channel recording analyses. These results demonstrate that KcsA inactivation is determined not only by the protein itself but also by the surrounding membrane environments.

A mechanistic description of gating of the human cardiac ryanodine receptor in a regulated minimal environment

Cardiac muscle contraction, triggered by the action potential, is mediated by the release of Ca²⁺ from the sarcoplasmic reticulum through ryanodine receptor (RyR)2 channels. In situ, RyR2 gating is modulated by numerous physiological and pharmacological agents, and altered RyR2 function underlies the occurrence of arrhythmias in both inherited and acquired diseases. To understand fully the mechanisms underpinning the regulation of RyR2 in the normal heart and how these systems are altered in pathological conditions, we must first gain a detailed knowledge of the fundamental processes of RyR2 gating. In this investigation, we provide key novel mechanistic insights into the physical reality of RyR2 gating revealed by new experimental and analytical approaches. We have examined in detail the single-channel gating kinetics of the purified human RyR2 when activated by cytosolic Ca²⁺ in a stringently regulated environment where the modulatory influence of factors external to the channel were minimized. The resulting gating schemes are based on an accurate description of single-channel kinetics using hidden Markov model analysis and reveal several novel aspects of RyR2 gating behavior: (a) constitutive gating is observed as unliganded opening events; (b) binding of Ca²⁺ to the channel stabilizes it in different open states; (c) RyR2 exists in two preopening closed conformations in equilibrium, one of which binds Ca²⁺ more readily than the other; (d) the gating of RyR2 when bound to Ca²⁺ can be described by a kinetic scheme incorporating bursts; and (e) analysis of flicker closing events within bursts reveals gating activity that is not influenced by ligand binding. The gating schemes generated in this investigation provide a framework for future studies in which the mechanisms of action of key physiological regulatory factors, disease-linked mutations, and potential therapeutic compounds can be described precisely.

Thermodynamic coupling between activation and inactivation gating in potassium channels revealed by free energy molecular dynamics simulations

The amount of ionic current flowing through K⁺ channels is determined by the interplay between two separate time-dependent processes: activation and inactivation gating. Activation is concerned with the stimulus-dependent opening of the main intracellular gate, whereas inactivation is a spontaneous conformational transition of the selectivity filter toward a nonconductive state occurring on a variety of timescales. A recent analysis of multiple x-ray structures of open and partially open KcsA channels revealed the mechanism by which movements of the inner activation gate, formed by the inner helices from the four subunits of the pore domain, bias the conformational changes at the selectivity filter toward a nonconductive inactivated state. This analysis highlighted the important role of Phe103, a residue located along the inner helix, near the hinge position associated with the opening of the intracellular gate. In the present study, we use free energy perturbation molecular dynamics simulations (FEP/MD) to quantitatively elucidate the thermodynamic basis for the coupling between the intracellular gate and the selectivity filter. The results of the FEP/MD calculations are in good agreement with experiments, and further analysis of the repulsive, van der Waals dispersive, and electrostatic free energy contributions reveals that the energetic basis underlying the absence of inactivation in the F103A mutation in KcsA is the absence of the unfavorable steric interaction occurring with the large Ile100 side chain in a neighboring subunit when the intracellular gate is open and the selectivity filter is in a conductive conformation. Macroscopic current analysis shows that the I100A mutant indeed relieves inactivation in KcsA, but to a lesser extent than the F103A mutant.

Contribution of ion binding affinity to ion selectivity and permeation in KcsA, a model potassium channel

Ion permeation and selectivity, key features in ion channel function, are believed to arise from a complex ensemble of energetic and kinetic variables. Here we evaluate the contribution of pore cation binding to ion permeation and selectivity features of KcsA, a model potassium channel. For this, we used E71A and M96V KcsA mutants in which the equilibrium between conductive and nonconductive conformations of the channel is differently shifted. E71A KcsA is a noninactivating channel mutant. Binding of K(+) to this mutant reveals a single set of low-affinity K(+) binding sites, similar to that seen in the binding of K(+) to wild-type KcsA that produces a conductive, low-affinity complex. This seems consistent with the observed K(+) permeation in E71A. Nonetheless, the E71A mutant retains K(+) selectivity, which cannot be explained on the basis of just its low affinity for this ion. At variance, M96V KcsA is a rapidly inactivating mutant that has lost selectivity for K(+) and also conducts Na(+). Here, low-affinity binding and high-affinity binding of both cations are detected, seemingly in agreement with both being permeating species in this mutant channel. In conclusion, binding of the ion to the channel protein seemingly explains certain gating, ion selectivity, and permeation properties. Ion binding stabilizes greatly the channel and, depending upon ion type and concentration, leads to different conformations and ion binding affinities. High-affinity states guarantee binding of specific ions and mediate ion selectivity but are nonconductive. Conversely, low-affinity states would not discriminate well among different ions but allow permeation to occur.

Modes of glutamate receptor gating

The time course of excitatory synaptic currents, the major means of fast communication between neurons of the central nervous system, is encoded in the dynamic behaviour of post-synaptic glutamate-activated channels. First-pass attempts to explain the glutamate-elicited currents with mathematical models produced reaction mechanisms that included only the most basic functionally defined states: resting vs. liganded, closed vs. open, responsive vs. desensitized. In contrast, single-molecule observations afforded by the patch-clamp technique revealed an unanticipated kinetic multiplicity of transitions: from microseconds-lasting flickers to minutes-long modes. How these kinetically defined events impact the shape of the synaptic response, how they relate to rearrangements in receptor structure, and whether and how they are physiologically controlled represent currently active research directions. Modal gating, which refers to the slowest, least frequently observed ion-channel transitions, has been demonstrated for representatives of all ion channel families. However, reaction schemes have been largely confined to the short- and medium-range time scales. For glutamate receptors as well, modal gating has only recently come under rigorous scrutiny. This article reviews the evidence for modal gating of glutamate receptors and the still developing hypotheses about the mechanism(s) by which modal shifts occur and the ways in which they may impact the time course of synaptic transmission.

Potassium-dependent activation of Kir4.2 K⁺ channels

The inwardly rectifying potassium channel Kir4.2 is sensitive to changes in the extracellular potassium concentration ([K(+)](o)). This form of regulation is manifest as a slow (tens of minutes) increase in the whole-cell currents when [K(+)](o) is increased. Here we have investigated the mechanism of K(o)(+) sensitivity of Kir4.2 expressed in Xenopus oocytes. Using two-electrode voltage clamp we found that the sensitivity is specific for the homomeric form of the channel and is completely abolished when coexpressed with Kir5.1. Furthermore, unlike Kir1.1, there is no coupling between the intracellular pH sensitivity and K(o)(+) sensitivity, as is evident by introducing a mutation (K66M), which greatly decreases the pH(i) sensitivity while the K(o)(+) sensitivity remains unchanged. K(o)(+)-dependent activation of Kir4.2 does not involve an increase in the surface expression of the channel, nor is there a difference in the open probability between high and low [K(+)] as determined through patch-clamp measurements. We also found that there is an inverse relationship between the rates of both activation and deactivation and [K(+)](o). Using a kinetic model we argue that Kir4.2 exists in at least three states at the plasma membrane: a deactivated state, an intermediate unstable state and an active state, and that [K(+)](o) affects the rate of transition from the intermediate state to the active state.

Transmembrane communication: general principles and lessons from the structure and function of the M2 proton channel, K⁺ channels, and integrin receptors

Signal transduction across biological membranes is central to life. This process generally happens through communication between different domains and hierarchical coupling of information. Here, we review structural and thermodynamic principles behind transmembrane (TM) signal transduction and discuss common themes. Communication between signaling domains can be understood in terms of thermodynamic and kinetic principles, and complex signaling patterns can arise from simple wiring of thermodynamically coupled domains. We relate this to functions of several signal transduction systems: the M2 proton channel from influenza A virus, potassium channels, integrin receptors, and bacterial kinases. We also discuss key features in the structural rearrangements responsible for signal transduction in these systems.

In vitro reconstitution of eukaryotic ion channels using droplet interface bilayers

The ability to routinely study eukaryotic ion channels in a synthetic lipid environment would have a major impact on our understanding of how different lipids influence ion channel function. Here, we describe a straightforward, detergent-free method for the in vitro reconstitution of eukaryotic ion channels and ionotropic receptors into droplet interface bilayers and measure their electrical activity at both the macroscopic and single-channel level. We explore the general applicability of this method by reconstitution of channels from a wide range of sources including recombinant cell lines and native tissues, as well as preparations that are difficult to study by conventional methods including erythrocytes and mitochondria.

Mechanism for selectivity-inactivation coupling in KcsA potassium channels

Structures of the prokaryotic K(+) channel, KcsA, highlight the role of the selectivity filter carbonyls from the GYG signature sequence in determining a highly selective pore, but channels displaying this sequence vary widely in their cation selectivity. Furthermore, variable selectivity can be found within the same channel during a process called C-type inactivation. We investigated the mechanism for changes in selectivity associated with inactivation in a model K(+) channel, KcsA. We found that E71A, a noninactivating KcsA mutant in which a hydrogen-bond behind the selectivity filter is disrupted, also displays decreased K(+) selectivity. In E71A channels, Na(+) permeates at higher rates as seen with and flux measurements and analysis of intracellular Na(+) block. Crystal structures of E71A reveal that the selectivity filter no longer assumes the "collapsed," presumed inactivated, conformation in low K(+), but a "flipped" conformation, that is also observed in high K(+), high Na(+), and even Na(+) only conditions. The data reveal the importance of the E71-D80 interaction in both favoring inactivation and maintaining high K(+) selectivity. We propose a molecular mechanism by which inactivation and K(+) selectivity are linked, a mechanism that may also be at work in other channels containing the canonical GYG signature sequence.

Changes in single K(+) channel behavior induced by a lipid phase transition

We show that the activity of an ion channel is correlated with the phase state of the lipid bilayer hosting the channel. By measuring unitary conductance, dwell times, and open probability of the K(+) channel KcsA as a function of temperature in lipid bilayers composed of POPE and POPG in different relative proportions, we obtain that all those properties show a trend inversion when the bilayer is in the transition region between the liquid-disordered and the solid-ordered phase. These data suggest that the physical properties of the lipid bilayer influence ion channel activity likely via a fine-tuning of its conformations. In a more general interpretative framework, we suggest that other parameters such as pH, ionic strength, and the action of amphiphilic drugs can affect the physical behavior of the lipid bilayer in a fashion similar to temperature changes resulting in functional changes of transmembrane proteins.

Structural and dynamic mechanisms for the function and inhibition of the M2 proton channel from influenza A virus

The M2 proton channel from influenza A virus, a prototype for a class of viral ion channels known as viroporins, conducts protons along a chain of water molecules and ionizable sidechains, including His37. Recent studies highlight a delicate interplay between protein folding, proton binding, and proton conduction through the channel. Drugs inhibit proton conduction by binding to an aqueous cavity adjacent to M2's proton-selective filter, thereby blocking access of proton to the filter, and altering the energetic landscape of the channel and the energetics of proton-binding to His37.

On the structural basis of modal gating behavior in K(+) channels

Modal-gating shifts represent an effective regulatory mechanism by which ion channels control the extent and time course of ionic fluxes. Under steady-state conditions, the K(+) channel KcsA shows three distinct gating modes, high-P(o), low-P(o) and a high-frequency flicker mode, each with about an order of magnitude difference in their mean open times. Here we show that in the absence of C-type inactivation, mutations at the pore-helix position Glu71 unmask a series of kinetically distinct modes of gating in a side chain-specific way. These gating modes mirror those seen in wild-type channels and suggest that specific interactions in the side chain network surrounding the selectivity filter, in concert with ion occupancy, alter the relative stability of pre-existing conformational states of the pore. The present results highlight the key role of the selectivity filter in regulating modal gating behavior in K(+) channels.

Conformational dynamics in the selectivity filter of KcsA in response to potassium ion concentration

Conformational change in the selectivity filter of KcsA as a function of ambient potassium concentration is studied with solid-state NMR. This highly conserved region of the protein is known to chelate potassium ions selectively. We report solid-state NMR chemical shift fingerprints of two distinct conformations of the selectivity filter; significant changes are observed in the chemical shifts of key residues in the filter as the potassium ion concentration is changed from 50 mM to 1 muM. Potassium ion titration studies reveal that the site-specific K(d) for K(+) binding at the key pore residue Val76 is on the order of approximately 7 muM and that a relatively high sample hydration is necessary to observe the low-K(+) conformer. Simultaneous detection of both conformers at low ambient potassium concentration suggests that the high-K(+) and low-K(+) states are in slow exchange on the NMR timescale (k(ex)<500 s(-)(1)). The slow rate and tight binding for evacuating both inner sites simultaneously differ from prior observations in detergent in solution, but agree well with measurements by electrophysiology and appear to result from our use of a hydrated bilayer environment. These observations strongly support a common assumption that the low-K(+) state is not involved in ion transmission, and that during transmission one of the two inner sites is always occupied. On the other hand, these kinetic and thermodynamic characteristics of the evacuation of the inner sites certainly could be compatible with participation in a control mechanism at low ion concentration such as C-type inactivation, a process that is coupled to activation and involves closing of the outer mouth of the channel.

Inactivation of the KcsA potassium channel explored with heterotetramers

The tetrameric prokaryotic potassium channel KcsA is activated by protons acting on the intracellular aspect of the protein and inactivated through conformational changes in the selectivity filter. Inactivation is modulated by a network of interactions within each protomer between the pore helix and residues at the external entrance of the channel. Inactivation is suppressed by the E71A mutation, which perturbs the stability of this network. Here, cell-free protein synthesis followed by protein purification by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used to produce heterotetramers of KcsA that contain different combinations of wild-type and E71A subunits. Single-channel recordings from these heterotetramers reveal how the network of interactions in individual protomers affects ionic conduction and channel inactivation, suggesting that the latter is a cooperative process.

Design and characterization of a constitutively open KcsA

The molecular nature of the structure responsible for proton sensitivity in KcsA has been identified as a charge cluster that surrounds the inner helical bundle gate. Here, we show that this proton sensor can be modified to engineer a constitutively open form of KcsA, amenable to functional, spectroscopic and structural analyses. By combining charge neutralizations for all acidic and basic residues in the cluster at positions 25, 117-122 and 124 (but not E118), a mutant KcsA is generated that displays constitutively open channel activity up to pH 9. The structure of this mutant revealed that full opening appears to be inhibited by lattice forces since the activation gate seems to be only on the early stages of opening.

Mechanism of potassium-channel selectivity revealed by Na(+) and Li(+) binding sites within the KcsA pore

Potassium channels allow K(+) ions to diffuse through their pores while preventing smaller Na(+) ions from permeating. Discrimination between these similar, abundant ions enables these proteins to control electrical and chemical activity in all organisms. Selection occurs at the narrow selectivity filter containing structurally identified K(+) binding sites. Selectivity is thought to arise because smaller ions such as Na(+) do not bind to these K(+) sites in a thermodynamically favorable way. Using the model K(+) channel KcsA, we examined how intracellular Na(+) and Li(+) interact with the pore and the permeant ions using electrophysiology, molecular dynamics simulations and X-ray crystallography. Our results suggest that these small cations have a separate binding site within the K(+) selectivity filter. We propose that selective permeation from the intracellular side primarily results from a large energy barrier blocking filter entry for Na(+) and Li(+) in the presence of K(+), not from a difference of binding affinity between ions.

An electrostatic interaction between TEA and an introduced pore aromatic drives spring-in-the-door inactivation in Shaker potassium channels

Slow inactivation of Kv1 channels involves conformational changes near the selectivity filter. We examine such changes in Shaker channels lacking fast inactivation by considering the consequences of mutating two residues, T449 just external to the selectivity filter and V438 in the pore helix near the bottom of the selectivity filter. Single mutant T449F channels with the native V438 inactivate very slowly, and the canonical foot-in-the-door effect of extracellular tetraethylammonium (TEA) is not only absent, but the time course of slow inactivation is accelerated by TEA. The V438A mutation dramatically speeds inactivation in T449F channels, and TEA slows inactivation exactly as predicted by the foot-in-the-door model. We propose that TEA has this effect on V438A/T449F channels because the V438A mutation produces allosteric consequences within the selectivity filter and may reorient the aromatic ring at position 449. We investigated the possibility that the blocker promotes the collapse of the outer vestibule (spring-in-the-door) in single mutant T449F channels by an electrostatic attraction between a cationic TEA and the quadrupole moments of the four aromatic rings. To test this idea, we used in vivo nonsense suppression to serially fluorinate the introduced aromatic ring at the 449 position, a manipulation that withdraws electrons from the aromatic face with little effect on the shape, net charge, or hydrophobicity of the aromatic ring. Progressive fluorination causes monotonically enhanced rates of inactivation. In further agreement with our working hypothesis, increasing fluorination of the aromatic gradually transforms the TEA effect from spring-in-the-door to foot-in-the-door. We further substantiate our electrostatic hypothesis by quantum mechanical calculations.

Coupling of activation and inactivation gate in a K+-channel: potassium and ligand sensitivity

Potassium (K(+))-channel gating is choreographed by a complex interplay between external stimuli, K(+) concentration and lipidic environment. We combined solid-state NMR and electrophysiological experiments on a chimeric KcsA-Kv1.3 channel to delineate K(+), pH and blocker effects on channel structure and function in a membrane setting. Our data show that pH-induced activation is correlated with protonation of glutamate residues at or near the activation gate. Moreover, K(+) and channel blockers distinctly affect the open probability of both the inactivation gate comprising the selectivity filter of the channel and the activation gate. The results indicate that the two gates are coupled and that effects of the permeant K(+) ion on the inactivation gate modulate activation-gate opening. Our data suggest a mechanism for controlling coordinated and sequential opening and closing of activation and inactivation gates in the K(+)-channel pore.

Protein dynamics detected in a membrane-embedded potassium channel using two-dimensional solid-state NMR spectroscopy

We report longitudinal (15)N relaxation rates derived from two-dimensional ((15)N, (13)C) chemical shift correlation experiments obtained under magic angle spinning for the potassium channel KcsA-Kv1.3 reconstituted in multilamellar vesicles. Thus, we demonstrate that solid-state NMR can be used to probe residue-specific backbone dynamics in a membrane-embedded protein. Enhanced backbone mobility was detected for two glycine residues within the selectivity filter that are highly conserved in potassium channels and that are of core relevance to the filter structure and ion selectivity.

KirBac1.1: it's an inward rectifying potassium channel

KirBac1.1 is a prokaryotic homologue of eukaryotic inward rectifier potassium (Kir) channels. The crystal structure of KirBac1.1 and related KirBac3.1 have now been used extensively to generate in silico models of eukaryotic Kir channels, but functional analysis has been limited to (86)Rb(+) flux experiments and bacteria or yeast complementation screens, and no voltage clamp analysis has been available. We have expressed pure full-length His-tagged KirBac1.1 protein in Escherichia coli and obtained voltage clamp recordings of recombinant channel activity in excised membrane patches from giant liposomes. Macroscopic currents of wild-type KirBac1.1 are K(+) selective and spermine insensitive, but blocked by Ba(2+), similar to "weakly rectifying" eukaryotic Kir1.1 and Kir6.2 channels. The introduction of a negative charge at a pore-lining residue, I138D, generates high spermine sensitivity, similar to that resulting from the introduction of a negative charge at the equivalent position in Kir1.1 or Kir6.2. KirBac1.1 currents are also inhibited by PIP(2), consistent with (86)Rb(+) flux experiments, and reversibly inhibited by short-chain di-c8-PIP(2). At the single-channel level, KirBac1.1 channels show numerous conductance states with two predominant conductances (15 pS and 32 pS at -100 mV) and marked variability in gating kinetics, similar to the behavior of KcsA in recombinant liposomes. The successful patch clamping of KirBac1.1 confirms that this prokaryotic channel behaves as a bona fide Kir channel and opens the way for combined biochemical, structural, and electrophysiological analysis of a tractable model Kir channel, as has been successfully achieved for the archetypal K(+) channel KcsA.

Fluorescence detection of the movement of single KcsA subunits reveals cooperativity

The prokaryotic KcsA channel is gated at the helical bundle crossing by intracellular protons and inactivates at the extracellular selectivity filter. The C-terminal transmembrane helix has to undergo a conformational change for potassium ions to access the central cavity. Whereas a partial opening of the tetrameric channel is suggested to be responsible for subconductance levels of ion channels, including KcsA, a cooperative opening of the 4 subunits is postulated as the final opening step. In this study, we used single-channel fluorescence spectroscopy of KcsA to directly observe the movement of each subunit and the temporal correlation between subunits. Purified KcsA channels labeled at the C terminus near the bundle crossing have been inserted into supported lipid bilayer, and the fluorescence traces analyzed by means of a cooperative or independent Markov model. The analysis revealed that the 4 subunits do not move fully independently but instead showed a certain degree of cooperativity. However, the 4 subunits do not simply open in 1 concerted step.

Ion channels in microbes

Studies of ion channels have for long been dominated by the animalcentric, if not anthropocentric, view of physiology. The structures and activities of ion channels had, however, evolved long before the appearance of complex multicellular organisms on earth. The diversity of ion channels existing in cellular membranes of prokaryotes is a good example. Although at first it may appear as a paradox that most of what we know about the structure of eukaryotic ion channels is based on the structure of bacterial channels, this should not be surprising given the evolutionary relatedness of all living organisms and suitability of microbial cells for structural studies of biological macromolecules in a laboratory environment. Genome sequences of the human as well as various microbial, plant, and animal organisms unambiguously established the evolutionary links, whereas crystallographic studies of the structures of major types of ion channels published over the last decade clearly demonstrated the advantage of using microbes as experimental organisms. The purpose of this review is not only to provide an account of acquired knowledge on microbial ion channels but also to show that the study of microbes and their ion channels may also hold a key to solving unresolved molecular mysteries in the future.

Molecular mechanism of pH sensing in KcsA potassium channels

The bacterial potassium channel KcsA is gated by high concentrations of intracellular protons, allowing the channel to open at pH < 5.5. Despite prior attempts to determine the mechanism responsible for pH gating, the proton sensor has remained elusive. We have constructed a KcsA channel mutant that remains open up to pH 9.0 by replacing key ionizable residues from the N and C termini of KcsA with residues mimicking their protonated counterparts with respect to charge. A series of individual and combined mutations were investigated by using single-channel recordings in lipid bilayers. We propose that these residues are the proton-binding sites and at neutral pH they form a complex network of inter- and intrasubunit salt bridges and hydrogen bonds near the bundle crossing that greatly stabilize the closed state. In our model, these residues change their ionization state at acidic pH, thereby disrupting this network, modifying the electrostatic landscape near the channel gate, and favoring channel opening.

A quantitative description of KcsA gating I: macroscopic currents

The prokaryotic K⁺ channel KcsA is activated by intracellular protons and its gating is modulated by transmembrane voltage. Typically, KcsA functions have been studied under steady-state conditions, using macroscopic Rb⁺-flux experiments and single-channel current measurements. These studies have provided limited insights into the gating kinetics of KcsA due to its low open probability, uncertainties in the number of channels in the patch, and a very strong intrinsic kinetic variability. In this work, we have carried out a detailed analysis of KcsA gating under nonstationary conditions by examining the influence of pH and voltage on the activation, deactivation, and slow-inactivation gating events. We find that activation and deactivation gating of KcsA are predominantly modulated by pH without a significant effect of voltage. Activation gating showed sigmoidal pH dependence with a pKa of ∼4.2 and a Hill coefficient of ∼2. In the sustained presence of proton, KcsA undergoes a time-dependent decay of conductance. This inactivation process is pH independent but is modulated by voltage and the nature of permeant ion. Recovery from inactivation occurs via deactivation and also appears to be voltage dependent. We further find that inactivation in KcsA is not entirely a property of the open-conducting channel but can also occur from partially “activated” closed states. The time course of onset and recovery of the inactivation process from these pre-open closed states appears to be different from the open-state inactivation, suggesting the presence of multiple inactivated states with diverse kinetic pathways. This information has been analyzed together with a detailed study of KcsA single-channel behavior (in the accompanying paper) in the framework of a kinetic model. Taken together our data constitutes the first quantitative description of KcsA gating.