Inverse coupling in leak and voltage-activated K+ channel gates underlies distinct roles in electrical signaling

K2P potassium channels, mysterious and paradoxically exciting

New evidence reveals that the common electrolyte disorder hypokalemia can induce K2P1 channels that are normally selective for K+ to break the rules and conduct Na+. This defiant behavior leads to paradoxical depolarization of many cells in the heart, increasing the risk for lethal arrhythmia. The new research resolves a mystery uncovered 50 years ago and bestows an array of new riddles. Here, I discuss how K2P1 might achieve this alchemy--through stable residence of the K+ selectivity filter in a Na+-conductive state between its open and C-inactive configurations--and predict that other K+ channels and environmental stimuli will be discovered to produce the same excitatory misconduct.

Multiple modalities converge on a common gate to control K2P channel function

K_2P potassium channels play important roles in the regulation of neuronal excitability. K_2P channels are gated chemical, thermal, and mechanical stimuli, and the present study identifies and characterizes a common molecular gate that responds to all different stimuli, both activating and inhibitory ones.

Members of the K_2P potassium channel family regulate neuronal excitability and are implicated in pain, anaesthetic responses, thermosensation, neuroprotection, and mood. Unlike other potassium channels, K_2Ps are gated by remarkably diverse stimuli that include chemical, thermal, and mechanical modalities. It has remained unclear whether the various gating inputs act through separate or common channel elements. Here, we show that protons, heat, and pressure affect activity of the prototypical, polymodal K_2P, K_2P2.1 (KCNK2/TREK-1), at a common molecular gate that comprises elements of the pore-forming segments and the N-terminal end of the M4 transmembrane segment. We further demonstrate that the M4 gating element is conserved among K_2Ps and is employed regardless of whether the gating stimuli are inhibitory or activating. Our results define a unique gating mechanism shared by K_2P family members and suggest that their diverse sensory properties are achieved by coupling different molecular sensors to a conserved core gating apparatus.

Gating of a pH-sensitive K(2P) potassium channel by an electrostatic effect of basic sensor residues on the selectivity filter

K⁺ channels share common selectivity characteristics but exhibit a wide diversity in how they are gated open. Leak K_2P K⁺ channels TASK-2, TALK-1 and TALK-2 are gated open by extracellular alkalinization. The mechanism for this alkalinization-dependent gating has been proposed to be the neutralization of the side chain of a single arginine (lysine in TALK-2) residue near the pore of TASK-2, which occurs with the unusual pK_a of 8.0. We now corroborate this hypothesis by transplanting the TASK-2 extracellular pH (pH_o) sensor in the background of a pH_o-insensitive TASK-3 channel, which leads to the restitution of pH_o-gating. Using a concatenated channel approach, we also demonstrate that for TASK-2 to open, pH_o sensors must be neutralized in each of the two subunits forming these dimeric channels with no apparent cross-talk between the sensors. These results are consistent with adaptive biasing force analysis of K⁺ permeation using a model selectivity filter in wild-type and mutated channels. The underlying free-energy profiles confirm that either a doubly or a singly charged pH_o sensor is sufficient to abolish ion flow. Atomic detail of the associated mechanism reveals that, rather than a collapse of the pore, as proposed for other K_2P channels gated at the selectivity filter, an increased height of the energetic barriers for ion translocation accounts for channel blockade at acid pH_o. Our data, therefore, strongly suggest that a cycle of protonation/deprotonation of pH_o-sensing arginine 224 side chain gates the TASK-2 channel by electrostatically tuning the conformational stability of its selectivity filter.

Detection of native-state nonadditivity in double mutant cycles via hydrogen exchange

Proteins have evolved to exploit long-range structural and dynamic effects as a means of regulating function. Understanding communication between sites in proteins is therefore vital to our comprehension of such phenomena as allostery, catalysis, and ligand binding/ejection. Double mutant cycle analysis has long been used to determine the existence of communication between pairs of sites, proximal or distal, in proteins. Typically, nonadditivity (or "thermodynamic coupling") is measured from global transitions in concert with a single probe. Here, we have applied the atomic resolution of NMR in tandem with native-state hydrogen exchange (HX) to probe the structure/energy landscape for information transduction between a large number of distal sites in a protein. Considering the event of amide proton exchange as an energetically quantifiable structural perturbation, m n-dimensional cycles can be constructed from mutation of n-1 residues, where m is the number of residues for which HX data is available. Thus, efficient mapping of a large number of couplings is made possible. We have applied this technique to one additive and two nonadditive double mutant cycles in a model system, eglin c. We find heterogeneity of HX-monitored couplings for each cycle, yet averaging results in strong agreement with traditionally measured values. Furthermore, long-range couplings observed at locally exchanging residues indicate that the basis for communication can occur within the native state ensemble, a conclusion not apparent from traditional measurements. We propose that higher-order couplings can be obtained and show that such couplings provide a mechanistic basis for understanding lower-order couplings via "spheres of perturbation". The method is presented as an additional tool for identifying a large number of couplings with greater coverage of the protein of interest.

Structural basis for the coupling between activation and inactivation gates in K(+) channels

The coupled interplay between activation and inactivation gating is a functional hallmark of K(+) channels. This coupling has been experimentally demonstrated through ion interaction effects and cysteine accessibility, and is associated with a well defined boundary of energetically coupled residues. The structure of the K(+) channel KcsA in its fully open conformation, in addition to four other partial channel openings, richly illustrates the structural basis of activation-inactivation gating. Here, we identify the mechanistic principles by which movements on the inner bundle gate trigger conformational changes at the selectivity filter, leading to the non-conductive C-type inactivated state. Analysis of a series of KcsA open structures suggests that, as a consequence of the hinge-bending and rotation of the TM2 helix, the aromatic ring of Phe 103 tilts towards residues Thr 74 and Thr 75 in the pore-helix and towards Ile 100 in the neighbouring subunit. This allows the network of hydrogen bonds among residues Trp 67, Glu 71 and Asp 80 to destabilize the selectivity filter, allowing entry to its non-conductive conformation. Mutations at position 103 have a size-dependent effect on gating kinetics: small side-chain substitutions F103A and F103C severely impair inactivation kinetics, whereas larger side chains such as F103W have more subtle effects. This suggests that the allosteric coupling between the inner helical bundle and the selectivity filter might rely on straightforward mechanical deformation propagated through a network of steric contacts. Average interactions calculated from molecular dynamics simulations show favourable open-state interaction-energies between Phe 103 and the surrounding residues. We probed similar interactions in the Shaker K(+) channel where inactivation was impaired in the mutant I470A. We propose that side-chain rearrangements at position 103 mechanically couple activation and inactivation in KcsA and a variety of other K(+) channels.

Gating of two pore domain potassium channels

Two-pore-domain potassium (K2P) channels are responsible for background leak currents which regulate the membrane potential and excitability of many cell types. Their activity is modulated by a variety of chemical and physical stimuli which act to increase or decrease the open probability of individual K2P channels. Crystallographic data and homology modelling suggest that all K(+) channels possess a highly conserved structure for ion selectivity and gating mechanisms. Like other K(+) channels, K2P channels are thought to have two primary conserved gating mechanisms: an inactivation (or C-type) gate at the selectivity filter close to the extracellular side of the channel and an activation gate at the intracellular entrance to the channel involving key, identified, hinge glycine residues. Zinc and hydrogen ions regulate Drosophila KCNK0 and mammalian TASK channels, respectively, by interacting with the inactivation gate of these channels. In contrast, the voltage dependence of TASK3 channels is mediated through its activation gate. For KCNK0 it has been shown that the gates display positive cooperativity. It is of much interest to determine whether other K2P regulatory compounds interact with either the activation gate or the inactivation gate to alter channel activity or, indeed, whether additional regulatory gating pathways exist.

Activation of Slo2.1 channels by niflumic acid

Slo2.1 channels conduct an outwardly rectifying K⁺ current when activated by high [Na⁺]_i. Here, we show that gating of these channels can also be activated by fenamates such as niflumic acid (NFA), even in the absence of intracellular Na⁺. In Xenopus oocytes injected with <10 ng cRNA, heterologously expressed human Slo2.1 current was negligible, but rapidly activated by extracellular application of NFA (EC₅₀ = 2.1 mM) or flufenamic acid (EC₅₀ = 1.4 mM). Slo2.1 channels activated by 1 mM NFA exhibited weak voltage dependence. In high [K⁺]_e, the conductance–voltage (G-V) relationship had a V_1/2 of +95 mV and an effective valence, z, of 0.48 e. Higher concentrations of NFA shifted V_1/2 to more negative potentials (EC₅₀ = 2.1 mM) and increased the minimum value of G/G_max (EC₅₀ = 2.4 mM); at 6 mM NFA, Slo2.1 channel activation was voltage independent. In contrast, V_1/2 of the G-V relationship was shifted to more positive potentials when [K⁺]_e was elevated from 1 to 300 mM (EC₅₀ = 21.2 mM). The slope conductance measured at the reversal potential exhibited the same [K⁺]_e dependency (EC₅₀ = 23.5 mM). Conductance was also [Na⁺]_e dependent. Outward currents were reduced when Na⁺ was replaced with choline or mannitol, but unaffected by substitution with Rb⁺ or Li⁺. Neutralization of charged residues in the S1–S4 domains did not appreciably alter the voltage dependence of Slo2.1 activation. Thus, the weak voltage dependence of Slo2.1 channel activation is independent of charged residues in the S1–S4 segments. In contrast, mutation of R190 located in the adjacent S4–S5 linker to a neutral (Ala or Gln) or acidic (Glu) residue induced constitutive channel activity that was reduced by high [K⁺]_e. Collectively, these findings indicate that Slo2.1 channel gating is modulated by [K⁺]_e and [Na⁺]_e, and that NFA uncouples channel activation from its modulation by transmembrane voltage and intracellular Na⁺.

Separate gating mechanisms mediate the regulation of K2P potassium channel TASK-2 by intra- and extracellular pH

TASK-2 (KCNK5 or K(2P)5.1) is a background K(+) channel that is opened by extracellular alkalinization and plays a role in renal bicarbonate reabsorption and central chemoreception. Here, we demonstrate that in addition to its regulation by extracellular protons (pH(o)) TASK-2 is gated open by intracellular alkalinization. The following pieces of evidence suggest that the gating process controlled by intracellular pH (pH(i)) is independent from that under the command of pH(o). It was not possible to overcome closure by extracellular acidification by means of intracellular alkalinization. The mutant TASK-2-R224A that lacks sensitivity to pH(o) had normal pH(i)-dependent gating. Increasing extracellular K(+) concentration acid shifts pH(o) activity curve of TASK-2 yet did not affect pH(i) gating of TASK-2. pH(o) modulation of TASK-2 is voltage-dependent, whereas pH(i) gating was not altered by membrane potential. These results suggest that pH(o), which controls a selectivity filter external gate, and pH(i) act at different gating processes to open and close TASK-2 channels. We speculate that pH(i) regulates an inner gate. We demonstrate that neutralization of a lysine residue (Lys(245)) located at the C-terminal end of transmembrane domain 4 by mutation to alanine abolishes gating by pH(i). We postulate that this lysine acts as an intracellular pH sensor as its mutation to histidine acid-shifts the pH(i)-dependence curve of TASK-2 as expected from its lower pK(a). We conclude that intracellular pH, together with pH(o), is a critical determinant of TASK-2 activity and therefore of its physiological function.

Principles of conduction and hydrophobic gating in K+ channels

We present the first atomic-resolution observations of permeation and gating in a K(+) channel, based on molecular dynamics simulations of the Kv1.2 pore domain. Analysis of hundreds of simulated permeation events revealed a detailed conduction mechanism, resembling the Hodgkin-Keynes "knock-on" model, in which translocation of two selectivity filter-bound ions is driven by a third ion; formation of this knock-on intermediate is rate determining. In addition, at reverse or zero voltages, we observed pore closure by a novel "hydrophobic gating" mechanism: A dewetting transition of the hydrophobic pore cavity-fastest when K(+) was not bound in selectivity filter sites nearest the cavity-caused the open, conducting pore to collapse into a closed, nonconducting conformation. Such pore closure corroborates the idea that voltage sensors can act to prevent pore collapse into the intrinsically more stable, closed conformation, and it further suggests that molecular-scale dewetting facilitates a specific biological function: K(+) channel gating. Existing experimental data support our hypothesis that hydrophobic gating may be a fundamental principle underlying the gating of voltage-sensitive K(+) channels. We suggest that hydrophobic gating explains, in part, why diverse ion channels conserve hydrophobic pore cavities, and we speculate that modulation of cavity hydration could enable structural determination of both open and closed channels.

Examining cooperative gating phenomena in voltage-dependent potassium channels: taking the energetic approach

Allosteric regulation of protein function is often achieved by changes in protein conformation induced by changes in chemical or electrical potential. In multisubunit proteins, such conformational changes may give rise to cooperativity in ligand binding. Conformational changes between open and closed states are central to the function of voltage-activated potassium (Kv) channel proteins, homotetrameric pore-forming membrane proteins involved in generating and shaping action potentials in excitable cells. Accessible to extremely high signal-to-noise ratio in functional measurements, combined with the availability of high-resolution structural data for different conformations of the protein, the Kv channel represents an excellent allosteric model system to further understand the aspects of synergism and cooperative effects in protein function. In this chapter, we demonstrate how the use of the simple law of mass action combined with thermodynamic mutant cycle energetic coupling analysis of Kv channel gating can be used to provide valuable information regarding (1) how cooperativity in Kv channel pore opening can be assessed; (2) how one can directly discriminate whether conformational transitions during Kv channel pore opening occur in a concerted or sequential manner; and (3) how mechanistically, the coupling between distant activation gate and selectivity filter functional elements of the prototypical Shaker Kv channel protein might be achieved. In addition to providing valuable insight into the function of this important protein, the conclusions reached at using high-order thermodynamic energetic coupling analysis applied to the Kv channel allosteric model system reveal much about the function of allosteric proteins, in general.

The response of the tandem pore potassium channel TASK-3 (K(2P)9.1) to voltage: gating at the cytoplasmic mouth

Although the tandem pore potassium channel TASK-3 is thought to open and shut at its selectivity filter in response to changes of extracellular pH, it is currently unknown whether the channel also shows gating at its inner, cytoplasmic mouth through movements of membrane helices M2 and M4. We used two electrode voltage clamp and single channel recording to show that TASK-3 responds to voltage in a way that reveals such gating. In wild-type channels, P(open) was very low at negative voltages, but increased with depolarisation. The effect of voltage was relatively weak and the gating charge small, 0.17. Mutants A237T (in M4) and N133A (in M2) increased P(open) at a given voltage, increasing mean open time and the number of openings per burst. In addition, the relationship between P(open) and voltage was shifted to less positive voltages. Mutation of putative hinge glycines (G117A, G231A), residues that are conserved throughout the tandem pore channel family, reduced P(open) at a given voltage, shifting the relationship with voltage to a more positive potential range. None of these mutants substantially affected the response of the channel to extracellular acidification. We have used the results from single channel recording to develop a simple kinetic model to show how gating occurs through two classes of conformation change, with two routes out of the open state, as expected if gating occurs both at the selectivity filter and at its cytoplasmic mouth.

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.

A structural model for K2P potassium channels based on 23 pairs of interacting sites and continuum electrostatics

K(2P)Ø, the two-pore domain potassium background channel that determines cardiac rhythm in Drosophila melanogaster, and its homologues that establish excitable membrane activity in mammals are of unknown structure. K(2P) subunits have two pore domains flanked by transmembrane (TM) spans: TM1-P1-TM2-TM3-P2-TM4. To establish spatial relationships in K(2P)Ø, we identified pairs of sites that display electrostatic compensation. Channels silenced by the addition of a charge in pore loop 1 (P1) or P2 were restored to function by countercharges at specific second sites. A three-dimensional homology model was determined using the crystal structure of K(V)1.2, effects of K(2P)Ø mutations to establish alignment, and compensatory charge-charge pairs. The model was refined and validated by continuum electrostatic free energy calculations and covalent linkage of introduced cysteines. K(2P) channels use two subunits arranged so that the P1 and P2 loops contribute to one pore, identical P loops face each other diagonally across the pore, and the channel complex has bilateral symmetry with a fourfold symmetric selectivity filter.

Gating the pore of potassium leak channels

A key feature of potassium channel function is the ability to switch between conducting and non-conducting states by undergoing conformational changes in response to cellular or extracellular signals. Such switching is facilitated by the mechanical coupling of gating domain movements to pore opening and closing. Two-pore domain potassium channels (K(2P)) conduct leak or background potassium-selective currents that are mostly time- and voltage-independent. These channels play a significant role in setting the cell resting membrane potential and, therefore modulate cell responsiveness and excitability. Thus, K(2P) channels are key players in numerous physiological processes and were recently shown to also be involved in human pathologies. It is well established that K(2P) channel conductance, open probability and cell surface expression are significantly modulated by various physical and chemical stimuli. However, in understanding how such signals are translated into conformational changes that open or close the channels gate, there remain more open questions than answers. A growing line of evidence suggests that the outer pore area assumes a critical role in gating K(2P) channels, in a manner reminiscent of C-type inactivation of voltage-gated potassium channels. In some K(2P) channels, this gating mechanism is facilitated in response to external pH levels. Recently, it was suggested that K(2P) channels also possess a lower activation gate that is positively coupled to the outer pore gate. The purpose of this review is to present an up-to-date summary of research describing the conformational changes and gating events that take place at the K(2P) channel ion-conducting pathway during the channel regulation.