A localized interaction surface for voltage-sensing domains on the pore domain of a K+ channel

The role of Phe150 in human voltage-gated proton channel

The voltage-gated proton channel H_v1 is a member of voltage-gated ion channels containing voltage-sensing domains (VSDs). The VSDs are made of four membrane-spanning segments (S1 through S4), and their function is to detect changes in membrane potential in the cells. A highly conserved phenylalanine 150 (F150) is located in the S2 segment of human voltage-gated proton channels. We previously discovered that the F150 is a binding site for the open channel blocker 2GBI. Here, we show that the H_v1 VSD voltage-dependent activation requires a hydrophobic group at position F150. We perform double-mutant cycle analysis to probe interactions between F150 and positively charged arginines in the S4 segment of the channel. Our results indicate that F150 interacts with two arginines (R2 and R3) in the S4 segment and catalyzes the transfer of the S4 arginines in the process of voltage-dependent activation.

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Hydrophobicity of F150 is crucial for human H_v1 channel voltage-dependent activation

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F150 interacts with R2 to stabilize the closed state of the H_v1 channel

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When depolarized, R3 moves upward to interact with F150 stabilizing the open state of H_v1

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F150 is essential for the transfer of the H_v1 arginines in the process of voltage sensing

Dimer interaction in the Hv1 proton channel

The voltage-gated proton channel Hv1 is a member of the voltage-gated ion channel superfamily, which stands out in design: It is a dimer of two voltage-sensing domains (VSDs), each containing a pore pathway, a voltage sensor (S4), and a gate (S1) and forming its own ion channel. Opening of the two channels in the dimer is cooperative. Part of the cooperativity is due to association between coiled-coil domains that extend intracellularly from the S4s. Interactions between the transmembrane portions of the subunits may also contribute, but the nature of transmembrane packing is unclear. Using functional analysis of a mutagenesis scan, biochemistry, and modeling, we find that the subunits form a dimer interface along the entire length of S1, and also have intersubunit contacts between S1 and S4. These interactions exert a strong effect on gating, in particular on the stability of the open state. Our results suggest that gating in Hv1 is tuned by extensive VSD-VSD interactions between the gates and voltage sensors of the dimeric channel.

Voltage-dependent gating in K channels: experimental results and quantitative models

Voltage-dependent K channels open and close in response to voltage changes across the cell membrane. This voltage dependence was postulated to depend on the presence of charged particles moving through the membrane in response to voltage changes. Recording of gating currents originating from the movement of these particles fully confirmed this hypothesis, and gave substantial experimental clues useful for the detailed understanding of the process. In the absence of structural information, the voltage-dependent gating was initially investigated using discrete Markov models, an approach only capable of providing a kinetic and thermodynamic comprehension of the process. The elucidation of the crystal structure of the first voltage-dependent channel brought in a dramatic change of pace in the understanding of channel gating, and in modeling the underlying processes. It was now possible to construct quantitative models using molecular dynamics, where all the interactions of each individual atom with the surroundings were taken into account, and its motion predicted by Newton's laws. Unfortunately, this modeling is computationally very demanding, and in spite of the advances in simulation procedures and computer technology, it is still limited in its predictive ability. To overcome these limitations, several groups have developed more macroscopic voltage gating models. Their approaches understandably require a number of approximations, which must however be physically well justified. One of these models, based on the description of the voltage sensor as a Brownian particle, that we have recently developed, is able to simultaneously describe the behavior of a single voltage sensor and to predict the macroscopic gating current originating from a population of sensors. The basics of this model are here described, and a typical application using the Kv1.2/2.1 chimera channel structure is also presented.

Noncanonical mechanism of voltage sensor coupling to pore revealed by tandem dimers of Shaker

In voltage-gated potassium channels (VGKC), voltage sensors (VSD) endow voltage-sensitivity to pore domains (PDs) through a not fully understood mechanism. Shaker-like VGKC show domain-swapped configuration: VSD of one subunit is covalently connected to its PD by the protein backbone (far connection) and non-covalently to the PD of the next subunit (near connection). VSD-to-PD coupling is not fully explained by far connection only, therefore an additional mechanistic component may be based on near connection. Using tandem dimers of Shaker channels we show functional data distinguishing VSD-to-PD far from near connections. Near connections influence both voltage-dependence of C-type inactivation at the selectivity filter and overall PD open probability. We speculate a conserved residue in S5 (S412 in Shaker), within van der Waals distance from next subunit S4 residues is key for the noncanonical VSD-to-PD coupling. Natural mutations of S412-homologous residues in brain and heart VGKC are related to neurological and cardiac diseases.

S4-S5 linker movement during activation and inactivation in voltage-gated K+ channels

The S4-S5 linker physically links voltage sensor and pore domain in voltage-gated ion channels and is essential for electromechanical coupling between both domains. Little dynamic information is available on the movement of the cytosolic S4-S5 linker due to lack of a direct electrical or optical readout. To understand the movements of the gating machinery during activation and inactivation, we incorporated fluorescent unnatural amino acids at four positions along the linker of the Shaker K_V channel. Using two-color voltage-clamp fluorometry, we compared S4-S5 linker movements with charge displacement, S4 movement, and pore opening. We found that the proximal S4-S5 linker moves with the S4 helix throughout the gating process, whereas the distal portion undergoes a separate motion related to late gating transitions. Both pore and S4-S5 linker undergo rearrangements during C-type inactivation. In presence of accelerated C-type inactivation, the energetic coupling between movement of the distal S4-S5 linker and pore opening disappears.

Gating interaction maps reveal a noncanonical electromechanical coupling mode in the Shaker K+ channel

Membrane potential regulates the activity of voltage-dependent ion channels via specialized voltage-sensing modules but the mechanisms involved in coupling voltage-sensor movement to pore opening remain unclear due to lack of resting state structures and robust methods to identify allosteric pathways. Here, using a newly developed interaction energy analysis, we probe the interfaces of the voltage-sensing and pore modules in the drosophila Shaker K⁺ channel. Our measurements reveal unexpectedly strong equilibrium gating interactions between contacts at the S4 and S5 helices in addition to those between S6 and S4–S5 linker. Network analysis of MD trajectories shows that the voltage-sensor and pore motions are linked by two distinct pathways- canonical one through the S4–S5 linker and a hitherto unknown pathway akin to rack and pinion coupling involving S4 and S5 helices. Our findings highlight the central role of the S5 helix in electromechanical transduction in the VGIC superfamily.

The S1 helix critically regulates the finely tuned gating of Kv11.1 channels

Congenital mutations in the cardiac Kv11.1 channel can cause long QT syndrome type 2 (LQTS2), a heart rhythm disorder associated with sudden cardiac death. Mutations act either by reducing protein expression at the membrane and/or by perturbing the intricate gating properties of Kv11.1 channels. A number of clinical LQTS2-associated mutations have been reported in the first transmembrane segment (S1) of Kv11.1 channels, but the role of this region of the channel is largely unexplored. In part, this is due to problems defining the extent of the S1 helix, as a consequence of its low sequence homology with other Kv family members. Here, we used NMR spectroscopy and electrophysiological characterization to show that the S1 of Kv11.1 channels extends seven helical turns, from Pro-405 to Phe-431, and is flanked by unstructured loops. Functional analysis suggests that pre-S1 loop residues His-402 and Tyr-403 play an important role in regulating the kinetics and voltage dependence of channel activation and deactivation. Multiple residues within the S1 helix also play an important role in fine-tuning the voltage dependence of activation, regulating slow deactivation, and modulating C-type inactivation of Kv11.1 channels. Analyses of LQTS2-associated mutations in the pre-S1 loop or S1 helix of Kv11.1 channels demonstrate perturbations to both protein expression and most gating transitions. Thus, S1 region mutations would reduce both the action potential repolarizing current passed by Kv11.1 channels in cardiac myocytes, as well as the current passed in response to premature depolarizations that normally helps protect against the formation of ectopic beats.

The isolated voltage sensing domain of the Shaker potassium channel forms a voltage-gated cation channel

Domains in macromolecular complexes are often considered structurally and functionally conserved while energetically coupled to each other. In the modular voltage-gated ion channels the central ion-conducting pore is surrounded by four voltage sensing domains (VSDs). Here, the energetic coupling is mediated by interactions between the S4-S5 linker, covalently linking the domains, and the proximal C-terminus. In order to characterize the intrinsic gating of the voltage sensing domain in the absence of the pore domain, the Shaker Kv channel was truncated after the fourth transmembrane helix S4 (Shaker-iVSD). Shaker-iVSD showed significantly altered gating kinetics and formed a cation-selective ion channel with a strong preference for protons. Ion conduction in Shaker-iVSD developed despite identical primary sequence, indicating an allosteric influence of the pore domain. Shaker-iVSD also displays pronounced 'relaxation'. Closing of the pore correlates with entry into relaxation suggesting that the two processes are energetically related.

A Disease Mutation Causing Episodic Ataxia Type I in the S1 Links Directly to the Voltage Sensor and the Selectivity Filter in Kv Channels

The mutation F184C in Kv1.1 leads to development of episodic ataxia type I (EA1). Although the mutation has been said to alter activation kinetics and to lower expression, we show here that the underlying molecular mechanisms may be more complex. Although F184 is positioned in the "peripheral" S1 helix, it occupies a central position in the 3D fold. We show in cut-open oocyte voltage-clamp recordings of gating and ionic currents of the Shaker Kv channel expressed in Xenopus oocytes that F184 not only interacts directly with the gating charges of the S4, but also creates a functional link to the selectivity filter of the neighboring subunit. This link leads to impaired fast and slow inactivation. The effect on fast inactivation is of an allosteric nature considering that fast inactivation is caused by a linked cytosolic ball peptide. The extensive effects of F184C provide a new mechanism underlying EA.

SIGNIFICANCE STATEMENT

Episodic ataxia (EA) is an inherited disease that leads to occasional loss of motor control in combination with variable other symptoms such as vertigo or migraine. EA type I (EA1), studied here, is caused by mutations in a voltage-gated potassium channel that contributes to the generation of electrical signals in the brain. The mechanism by which mutations in voltage-gated potassium channels lead to EA is still unknown and there is no consistent pharmacological treatment. By studying in detail one disease-causing mutation in Kv1.1, we describe a novel molecular mechanism distinct from mechanisms described previously. This mechanism contributes to the understanding of potassium channel function in general and might lead to a better understanding of how EA develops.

Tryptophan Scanning Mutagenesis Identifies the Molecular Determinants of Distinct Barttin Functions

CLC-K chloride channels are expressed in the kidney and in the inner ear and require the accessory subunit barttin for proper function and membrane insertion. Barttin exerts multiple functions on CLC-proteins: it modifies protein stability and intracellular trafficking as well as channel activity, ion conduction, and gating. So far, the molecular determinants of these distinct barttin functions have remained elusive. Here we performed serial perturbation mutagenesis to identify the sequence determinants of barttin function. Barttin consists of two transmembrane helices followed by a long intracellular carboxyl terminus, and earlier work demonstrated that the transmembrane core of barttin suffices for most effects on the α-subunit. We individually substituted every amino acid of the predicted transmembrane core (amino acids 9-26 and 35-55) with tryptophan, co-expressed mutant barttin with hClC-Ka or V166E rClC-K1, and characterized CLC-K/barttin channels by patch clamp techniques, biochemistry, and confocal microscopy. The majority of mutations left the chaperone function of barttin, i.e. the effects on endoplasmic reticulum exit and surface membrane insertion, unaffected. In contrast, tryptophan insertion at multiple positions resulted in impaired activity of hClC-Ka/barttin and changes in gating of V166E rClC-K1/barttin. These results demonstrate that mutations in a cluster of hydrophobic residues within transmembrane domain 1 affect barttin-CLC-K interaction and impair gating modification by the accessory subunit. Whereas tight interaction is necessary for functional modification, even impaired association of barttin and CLC-K suffices for normal intracellular trafficking. Our findings allow definition of a likely interaction surface and clarify the mechanisms underlying CLC-K channel modification by barttin.

Moving gating charges through the gating pore in a Kv channel voltage sensor

Voltage sensor domains (VSDs) regulate ion channels and enzymes by transporting electrically charged residues across a hydrophobic VSD constriction called the gating pore or hydrophobic plug. How the gating pore controls the gating charge movement presently remains debated. Here, using saturation mutagenesis and detailed analysis of gating currents from gating pore mutations in the Shaker Kv channel, we identified statistically highly significant correlations between VSD function and physicochemical properties of gating pore residues. A necessary small residue at position S240 in S1 creates a "steric gap" that enables an intracellular access pathway for the transport of the S4 Arg residues. In addition, the stabilization of the depolarized VSD conformation, a hallmark for most Kv channels, requires large side chains at positions F290 in S2 and F244 in S1 acting as "molecular clamps," and a hydrophobic side chain at position I237 in S1 acting as a local intracellular hydrophobic barrier. Finally, both size and hydrophobicity of I287 are important to control the main VSD energy barrier underlying transitions between resting and active states. Taken together, our study emphasizes the contribution of several gating pore residues to catalyze the gating charge transfer. This work paves the way toward understanding physicochemical principles underlying conformational dynamics in voltage sensors.

A novel mechanism for fine-tuning open-state stability in a voltage-gated potassium channel

Voltage-gated potassium channels elicit membrane hyperpolarization through voltage-sensor domains that regulate the conductive status of the pore domain. To better understand the inherent basis for the open-closed equilibrium in these channels, we undertook an atomistic scan using synthetic fluorinated derivatives of aromatic residues previously implicated in the gating of Shaker potassium channels. Here we show that stepwise dispersion of the negative electrostatic surface potential of only one site, Phe481, stabilizes the channel open state. Furthermore, these data suggest that this apparent stabilization is the consequence of the amelioration of an inherently repulsive open-state interaction between the partial negative charge on the face of Phe481 and a highly co-evolved acidic side chain, Glu395, and this interaction is potentially modulated through the Tyr485 hydroxyl. We propose that the intrinsic open-state destabilization via aromatic repulsion represents a new mechanism by which ion channels, and likely other proteins, fine-tune conformational equilibria.

Voltage-gated potassium channels cycle between closed and open states through poorly-defined transitions. Pless and colleagues incorporate artificial amino acids into Shaker potassium channels and find that that the negative electrostatic surface potential of Phe481, destabilizes the channel open state.

Interactions between voltage sensor and pore domains in a hERG K+ channel model from molecular simulations and the effects of a voltage sensor mutation

The hERG K(+) channel is important for establishing normal electrical activity in the human heart. The channel's unique gating response to membrane potential changes indicates specific interactions between voltage sensor and pore domains that are poorly understood. In the absence of a crystal structure we constructed a homology model of the full hERG membrane domain and performed 0.5 μs molecular dynamics (MD) simulations in a hydrated membrane. The simulations identify potential interactions involving residues at the extracellular surface of S1 in the voltage sensor and at the N-terminal end of the pore helix in the hERG model. In addition, a diffuse interface involving hydrophobic residues on S4 (voltage sensor) and pore domain S5 of an adjacent subunit was stable during 0.5 μs of simulation. To assess the ability of the model to give insight into the effects of channel mutation we simulated a hERG mutant that contains a Leu to Pro substitution in the voltage sensor S4 helical segment (hERG L532P). Consistent with the retention of gated K(+) conductance, the L532P mutation was accommodated in the S4 helix with little disruption of helical structure. The mutation reduced the extent of interaction across the S4-S5 interface, suggesting a structural basis for the greatly enhanced deactivation rate in hERG L532P. The study indicates that pairwise comparison of wild-type and mutated channel models is a useful approach to interpreting functional data where uncertainty in model structures exist.

Probing the energy landscape of activation gating of the bacterial potassium channel KcsA

The bacterial potassium channel KcsA, which has been crystallized in several conformations, offers an ideal model to investigate activation gating of ion channels. In this study, essential dynamics simulations are applied to obtain insights into the transition pathways and the energy profile of KcsA pore gating. In agreement with previous hypotheses, our simulations reveal a two phasic activation gating process. In the first phase, local structural rearrangements in TM2 are observed leading to an intermediate channel conformation, followed by large structural rearrangements leading to full opening of KcsA. Conformational changes of a highly conserved phenylalanine, F114, at the bundle crossing region are crucial for the transition from a closed to an intermediate state. 3.9 µs umbrella sampling calculations reveal that there are two well-defined energy barriers dividing closed, intermediate, and open channel states. In agreement with mutational studies, the closed state was found to be energetically more favorable compared to the open state. Further, the simulations provide new insights into the dynamical coupling effects of F103 between the activation gate and the selectivity filter. Investigations on individual subunits support cooperativity of subunits during activation gating.

Energetic role of the paddle motif in voltage gating of Shaker K(+) channels

Voltage-gated ion channels underlie rapid electric signaling in excitable cells. Electrophysiological studies have established that the N-terminal half of the fourth transmembrane segment (^NTS4) of these channels functions as the primary voltage sensor, whereas crystallographic studies have shown that ^NTS4 is not located within a proteinaceous pore. Rather, ^NTS4 and the C-terminal half of S3 (^CTS3 or S3b) form a helix-turn-helix motif, termed the voltage-sensor paddle. This unexpected structural finding raises two fundamental questions: does the paddle motif also exist in voltage-gated channels in a biological membrane and, if so, what is its function in voltage gating. Here, we provide evidence that the paddle motif exists in the open state of Drosophila Shaker voltage-gated K⁺ channels expressed in Xenopus oocytes and that ^CTS3 acts as an extracellular hydrophobic "stabilizer" for ^NTS4, biasing the gating chemical equilibrium towards the open state.

The ladder-shaped polyether toxin gambierol anchors the gating machinery of Kv3.1 channels in the resting state

Voltage-gated potassium (Kv) and sodium (Nav) channels are key determinants of cellular excitability and serve as targets of neurotoxins. Most marine ciguatoxins potentiate Nav channels and cause ciguatera seafood poisoning. Several ciguatoxins have also been shown to affect Kv channels, and we showed previously that the ladder-shaped polyether toxin gambierol is a potent Kv channel inhibitor. Most likely, gambierol acts via a lipid-exposed binding site, located outside the K⁺ permeation pathway. However, the mechanism by which gambierol inhibits Kv channels remained unknown. Using gating and ionic current analysis to investigate how gambierol affected S6 gate opening and voltage-sensing domain (VSD) movements, we show that the resting (closed) channel conformation forms the high-affinity state for gambierol. The voltage dependence of activation was shifted by >120 mV in the depolarizing direction, precluding channel opening in the physiological voltage range. The (early) transitions between the resting and the open state were monitored with gating currents, and provided evidence that strong depolarizations allowed VSD movement up to the activated-not-open state. However, for transition to the fully open (ion-conducting) state, the toxin first needed to dissociate. These dissociation kinetics were markedly accelerated in the activated-not-open state, presumably because this state displayed a much lower affinity for gambierol. A tetrameric concatemer with only one high-affinity binding site still displayed high toxin sensitivity, suggesting that interaction with a single binding site prevented the concerted step required for channel opening. We propose a mechanism whereby gambierol anchors the channel’s gating machinery in the resting state, requiring more work from the VSD to open the channel. This mechanism is quite different from the action of classical gating modifier peptides (e.g., hanatoxin). Therefore, polyether toxins open new opportunities in structure–function relationship studies in Kv channels and in drug design to modulate channel function.

Coupling of voltage-sensors to the channel pore: a comparative view

The activation of voltage-dependent ion channels is initiated by potential-induced conformational rearrangements in the voltage-sensor domains that propagates to the pore domain (PD) and finally opens the ion conduction pathway. In potassium channels voltage-sensors are covalently linked to the pore via S4-S5 linkers at the cytoplasmic site of the PD. Transformation of membrane electric energy into the mechanical work required for the opening or closing of the channel pore is achieved through an electromechanical coupling mechanism, which involves local interaction between residues in S4-S5 linker and pore-forming alpha helices. In this review we discuss present knowledge and open questions related to the electromechanical coupling mechanism in most intensively studied voltage-gated Shaker potassium channel and compare structure-functional aspects of coupling with those observed in distantly related ion channels. We focus particularly on the role of electromechanical coupling in modulation of the constitutive conductance of ion channels.

The core domain as the force sensor of the yeast mechanosensitive TRP channel

Stretch-activated conductances are commonly encountered in careful electric recordings. Those of known proteins (TRP, MscL, MscS, K_2p, Kv, etc.) all share a core, which houses the ion pathway and the gate, but no recognizable force-sensing domain. Like animal TRPs, the yeast TRPY1 is polymodal, activated by stretch force, Ca²⁺, etc. To test whether its S5–S6 core senses the stretch force, we tried to uncouple it from the peripheral domains by strategic peptide insertions to block the covalent core–periphery interactions. Insertion of long unstructured peptides should distort, if not disrupt, protein structures that transmit force. Such insertions between S6 and the C-terminal tail largely removed Ca²⁺ activation, showing their effectiveness. However, such insertions as well as those between S5 and the N-terminal region, which includes S1–S4, did not significantly alter mechanosensitivity. Even insertions at both locations flanking the S5–S6 core did not much alter mechanosensitivity. Tryptophan scanning mutations in S5 were also constructed to perturb possible noncovalent core–periphery contacts. The testable tryptophan mutations also have little or no effects on mechanosensitivity. Boltzmann fits of the wild-type force–response curves agree with a structural homology model for a stretch-induced core expansion of ∼2 nm² upon opening. We hypothesize that membrane tension pulls on S5–S6, expanding the core and opening the TRPY1 gate. The core being the major force sensor offers the simplest, though not the only, explanation of why so many channels of disparate designs are mechanically sensitive. Compared with the bacterial MscL, TRPY1 is much less sensitive to force, befitting a polymodal channel that relies on multiple stimuli.

Tryptophan scanning mutagenesis reveals distortions in the helical structure of the δM4 transmembrane domain of the Torpedo californica nicotinic acetylcholine receptor

The lipid-protein interface is an important domain of the nicotinic acetylcholine receptor (nAChR) that has recently garnered increased relevance. Several studies have made significant advances toward determining the structure and dynamics of the lipid-exposed domains of the nAChR. However, there is still a need to gain insight into the mechanism by which lipid-protein interactions regulate the function and conformational transitions of the nAChR. In this study, we extended the tryptophan scanning mutagenesis (TrpScanM) approach to dissect secondary structure and monitor the conformational changes experienced by the δM4 transmembrane domain (TMD) of the Torpedo californica nAChR, and to identify which positions on this domain are potentially linked to the regulation of ion channel kinetics. The difference in oscillation patterns between the closed- and open-channel states suggests a substantial conformational change along this domain as a consequence of channel activation. Furthermore, TrpScanM revealed distortions along the helical structure of this TMD that are not present on current models of the nAChR. Our results show that a Thr-Pro motif at positions 462-463 markedly bends the helical structure of the TMD, consistent with the recent crystallographic structure of the GluCl Cys-loop receptor which reveals a highly bent TMD4 in each subunit. This Thr-Pro motif acts as a molecular hinge that delineates two gating blocks in the δM4 TMD. These results suggest a model in which a hinge-bending motion that tilts the helical structure is combined with a spring-like motion during transition between the closed- and open-channel states of the δM4 TMD.

Estimating the voltage-dependent free energy change of ion channels using the median voltage for activation

Voltage-gated ion channels are crucial for electrical activity and chemical signaling in a variety of cell types. Structure-activity studies involving electrophysiological characterization of mutants are widely used and allow us to quickly realize the energetic effects of a mutation by measuring macroscopic currents and fitting the observed voltage dependence of conductance to a Boltzmann equation. However, such an approach is somewhat limiting, principally because of the inherent assumption that the channel activation is a two-state process. In this analysis, we show that the area delineated by the gating charge displacement curve and its ordinate axis is related to the free energy of activation of a voltage-gated ion channel. We derive a parameter, the median voltage of charge transfer (V_m), which is proportional to this area, and prove that the chemical component of free energy change of a system can be obtained from the knowledge of V_m and the maximum number of charges transferred. Our method is not constrained by the number or connectivity of intermediate states and is applicable to instances in which the observed responses show a multiphasic behavior. We consider various models of ion channel gating with voltage-dependent steps, latent charge movement, inactivation, etc. and discuss the applicability of this approach in each case. Notably, our method estimates a net free energy change of approximately −14 kcal/mol associated with the full-scale activation of the Shaker potassium channel, in contrast to −2 to −3 kcal/mol estimated from a single Boltzmann fit. Our estimate of the net free energy change in the system is consistent with those derived from detailed kinetic models (Zagotta et al. 1994. J. Gen. Physiol. doi:10.1085/jgp.103.2.321). The median voltage method can reliably quantify the magnitude of free energy change associated with activation of a voltage-dependent system from macroscopic equilibrium measurements. This will be particularly useful in scanning mutagenesis experiments.

Molecular determinants of U-type inactivation in Kv2.1 channels

Kv2.1 channels exhibit a U-shaped voltage-dependence of inactivation that is thought to represent preferential inactivation from preopen closed states. However, the molecular mechanisms underlying so-called U-type inactivation are unknown. We have performed a cysteine scan of the S3-S4 and S5-P-loop linkers and found sites that are important for U-type inactivation. In the S5-P-loop linker, U-type inactivation was preserved in all mutant channels except E352C. This mutation, but not E352Q, abolished closed-state inactivation while preserving open-state inactivation, resulting in a loss of the U-shaped voltage profile. The reducing agent DTT, as well as the C232V mutation in S2, restored U-type inactivation to the E352C mutant, which suggests that residues 352C and C232 may interact to prevent U-type inactivation. The R289C mutation, in the S3-S4 linker, also reduced U-type inactivation. In this case, DTT had little effect but application of MTSET restored wild-type-like U-type inactivation behavior, suggestive of the importance of charge at this site. Kinetic modeling suggests that the E352C and R289C inactivation phenotypes largely resulted from reductions in the rate constants for transitions from closed to inactivated states. The data indicate that specific residues within the S3-S4 and S5-P-loop linkers may play important roles in Kv2.1 U-type inactivation.

Fourier transform coupled tryptophan scanning mutagenesis identifies a bending point on the lipid-exposed δM3 transmembrane domain of the Torpedo californica nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (nAChR) is a member of a family of ligand-gated ion channels that mediate diverse physiological functions, including fast synaptic transmission along the peripheral and central nervous systems. Several studies have made significant advances toward determining the structure and dynamics of the lipid-exposed domains of the nAChR. However, a high-resolution atomic structure of the nAChR still remains elusive. In this study, we extended the Fourier transform coupled tryptophan scanning mutagenesis (FT-TrpScanM) approach to gain insight into the secondary structure of the δM3 transmembrane domain of the Torpedo californica nAChR, to monitor conformational changes experienced by this domain during channel gating, and to identify which lipid-exposed positions are linked to the regulation of ion channel kinetics. The perturbations produced by periodic tryptophan substitutions along the δM3 transmembrane domain were characterized by two-electrode voltage clamp and (125)I-labeled α-bungarotoxin binding assays. The periodicity profiles and Fourier transform spectra of this domain revealed similar helical structures for the closed- and open-channel states. However, changes in the oscillation patterns observed between positions Val-299 and Val-304 during transition between the closed- and open-channel states can be explained by the structural effects caused by the presence of a bending point introduced by a Thr-Gly motif at positions 300-301. The changes in periodicity and localization of residues between the closed-and open-channel states could indicate a structural transition between helix types in this segment of the domain. Overall, the data further demonstrate a functional link between the lipid-exposed transmembrane domain and the nAChR gating machinery.

Allosteric features of KCNQ1 gating revealed by alanine scanning mutagenesis

Controlled opening and closing of an ion-selective pathway in response to changes of membrane potential is a fundamental feature of voltage-gated ion channels. In recent decades, various details of this process have been revealed with unprecedented precision based on studies of prototypic potassium channels. Though current scientific efforts are focused more on a thorough description of voltage-sensor movement, much less is known about the similarities and differences of the gating mechanisms among potassium channels. Here, we describe the peculiarities of the KCNQ1 gating process in parallel comparison to Shaker. We applied alanine scanning mutagenesis to the S4-S5 linker and pore region and followed the regularities of gating perturbations in KCNQ1. We found a fractional constitutive conductance for wild-type KCNQ1. This component increased significantly in mutants with considerably leftward-shifted steady-state activation curves. In contrast to Shaker, no correlation between V(1/2) and Z parameters was observed for the voltage-dependent fraction of KCNQ1. Our experimental findings are explained by a simple allosteric gating scheme with voltage-driven and voltage-independent transitions. Allosteric features are discussed in the context of extreme gating adaptability of KCNQ1 upon interaction with KCNE β-subunits.

Uncoupling charge movement from channel opening in voltage-gated potassium channels by ruthenium complexes

The Kv2.1 channel generates a delayed-rectifier current in neurons and is responsible for modulation of neuronal spike frequency and membrane repolarization in pancreatic β-cells and cardiomyocytes. As with other tetrameric voltage-activated K(+)-channels, it has been proposed that each of the four Kv2.1 voltage-sensing domains activates independently upon depolarization, leading to a final concerted transition that causes channel opening. The mechanism by which voltage-sensor activation is coupled to the gating of the pore is still not understood. Here we show that the carbon-monoxide releasing molecule 2 (CORM-2) is an allosteric inhibitor of the Kv2.1 channel and that its inhibitory properties derive from the CORM-2 ability to largely reduce the voltage dependence of the opening transition, uncoupling voltage-sensor activation from the concerted opening transition. We additionally demonstrate that CORM-2 modulates Shaker K(+)-channels in a similar manner. Our data suggest that the mechanism of inhibition by CORM-2 may be common to voltage-activated channels and that this compound should be a useful tool for understanding the mechanisms of electromechanical coupling.

Renewable Energy Driven by Le Chatelier's Principle, Enzyme Function, and Non-Additive Contributions to Ion Fluctuations: A Hypothesis in Biomechanical and Nanotechnology Energy Theory

The search for green energy sources has populated the research arena with significant emphasis on green electronics, green fuels, and green batteries that reduce waste, emissions, and environmental toxicity. Simultaneously, nanotechnology has developed substantially in the recent years and the emerging area of nanoenergetics has shown impressive discoveries that can aid in the search for alternative and green energies. The use of exotic materials in these fields and even enzymes has led scientists to be able to cross-link biomolecules and nanotechnology circuits, which can be important points in the search of novel energy searches. This paper discusses a biochemical energy-generating unit driven by ion fluctuations and spontaneous enzyme conformational changes. The paper lays also the theoretical thermodynamical foundation of the nanoenergy unit and to exploit the principle of nonadditivity and equilibrium as main forces in driving an energy-generating reaction.

Arrangement and mobility of the voltage sensor domain in prokaryotic voltage-gated sodium channels

Prokaryotic voltage-gated sodium channels (Na(V)s) form homotetramers with each subunit contributing six transmembrane α-helices (S1-S6). Helices S5 and S6 form the ion-conducting pore, and helices S1-S4 function as the voltage sensor with helix S4 thought to be the essential element for voltage-dependent activation. Although the crystal structures have provided insight into voltage-gated K channels (K(V)s), revealing a characteristic domain arrangement in which the voltage sensor domain of one subunit is close to the pore domain of an adjacent subunit in the tetramer, the structural and functional information on Na(V)s remains limited. Here, we show that the domain arrangement in NaChBac, a firstly cloned prokaryotic Na(V), is similar to that in K(V)s. Cysteine substitutions of three residues in helix S4, Q107C, T110C, and R113C, effectively induced intersubunit disulfide bond formation with a cysteine introduced in helix S5, M164C, of the adjacent subunit. In addition, substituting two acidic residues with lysine, E43K and D60K, shifted the activation of the channel to more positive membrane potentials and consistently shifted the preferentially formed disulfide bond from T110C/M164C to Q107C/M164C. Because Gln-107 is located closer to the extracellular side of helix S4 than Thr-110, this finding suggests that the functional shift in the voltage dependence of activation is related to a restriction of the position of helix S4 in the lipid bilayer. The domain arrangement and vertical mobility of helix S4 in NaChBac indicate that the structure and the mechanism of voltage-dependent activation in prokaryotic Na(V)s are similar to those in canonical K(V)s.

The S4-S5 linker of KCNQ1 channels forms a structural scaffold with the S6 segment controlling gate closure

In vivo, KCNQ1 α-subunits associate with the β-subunit KCNE1 to generate the slowly activating cardiac potassium current (I(Ks)). Structurally, they share their topology with other Kv channels and consist out of six transmembrane helices (S1-S6) with the S1-S4 segments forming the voltage-sensing domain (VSD). The opening or closure of the intracellular channel gate, which localizes at the bottom of the S6 segment, is directly controlled by the movement of the VSD via an electromechanical coupling. In other Kv channels, this electromechanical coupling is realized by an interaction between the S4-S5 linker (S4S5(L)) and the C-terminal end of S6 (S6(T)). Previously we reported that substitutions for Leu(353) in S6(T) resulted in channels that failed to close completely. Closure could be incomplete because Leu(353) itself is the pore-occluding residue of the channel gate or because of a distorted electromechanical coupling. To resolve this and to address the role of S4S5(L) in KCNQ1 channel gating, we performed an alanine/tryptophan substitution scan of S4S5(L). The residues with a "high impact" on channel gating (when mutated) clustered on one side of the S4S5(L) α-helix. Hence, this side of S4S5(L) most likely contributes to the electromechanical coupling and finds its residue counterparts in S6(T). Accordingly, substitutions for Val(254) resulted in channels that were partially constitutively open and the ability to close completely was rescued by combination with substitutions for Leu(353) in S6(T). Double mutant cycle analysis supported this cross-talk indicating that both residues come in close contact and stabilize the closed state of the channel.

Ion channel voltage sensors: structure, function, and pathophysiology

Voltage-gated ion channels generate electrical signals in species from bacteria to man. Their voltage-sensing modules are responsible for initiation of action potentials and graded membrane potential changes in response to synaptic input and other physiological stimuli. Extensive structure-function studies, structure determination, and molecular modeling are now converging on a sliding-helix mechanism for electromechanical coupling in which outward movement of gating charges in the S4 transmembrane segments catalyzed by sequential formation of ion pairs pulls the S4-S5 linker, bends the S6 segment, and opens the pore. Impairment of voltage-sensor function by mutations in Na+ channels contributes to several ion channelopathies, and gating pore current conducted by mutant voltage sensors in Na(V)1.4 channels is the primary pathophysiological mechanism in hypokalemic periodic paralysis. The emerging structural model for voltage sensor function opens the way to development of a new generation of ion-channel drugs that act on voltage sensors rather than blocking the pore.

Two separate interfaces between the voltage sensor and pore are required for the function of voltage-dependent K(+) channels

Voltage-dependent K⁺ (Kv) channels gate open in response to the membrane voltage. To further our understanding of how cell membrane voltage regulates the opening of a Kv channel, we have studied the protein interfaces that attach the voltage-sensor domains to the pore. In the crystal structure, three physical interfaces exist. Only two of these consist of amino acids that are co-evolved across the interface between voltage sensor and pore according to statistical coupling analysis of 360 Kv channel sequences. A first co-evolved interface is formed by the S4-S5 linkers (one from each of four voltage sensors), which form a cuff surrounding the S6-lined pore opening at the intracellular surface. The crystal structure and published mutational studies support the hypothesis that the S4-S5 linkers convert voltage-sensor motions directly into gate opening and closing. A second co-evolved interface forms a small contact surface between S1 of the voltage sensor and the pore helix near the extracellular surface. We demonstrate through mutagenesis that this interface is necessary for the function and/or structure of two different Kv channels. This second interface is well positioned to act as a second anchor point between the voltage sensor and the pore, thus allowing efficient transmission of conformational changes to the pore's gate.

Voltage-dependent ion channels open with a voltage dependence that is remarkably steep. This steep voltage dependence, which is essential to the propagation of nerve impulses, originates in the interaction between voltage-sensor domains of the ion channel and its pore. The voltage-sensor domains transmit voltage-driven conformational changes to the pore. To understand how this “electromechanical coupling” mechanism works, we have studied the protein–protein interfaces that connect the voltage sensors to the pore using bioinformatics, electrophysiological recordings, site-directed mutagenesis, and chemical cross-linking. We identify two functionally important interfaces: one links the mobile “voltage-sensor paddle” to the pore's gate near the intracellular membrane surface, while the other links an immobile region of the voltage sensor to the pore near the extracellular membrane surface. The two interfaces encompass only a small fraction of the voltage-sensor surface area, but appear to operate in unison to enable voltage-driven conformational changes within the voltage sensor so as to efficiently regulate the pore's gate.

Voltage-sensor domains contact the pore in potassium channels through two surfaces: one attaches the voltage sensor to the pore, while the other imparts force on the gate, in a process that is central to the generation of action potentials in nerve cells.

Dynamic coupling of voltage sensor and gate involved in closed-state inactivation of kv4.2 channels

Voltage-gated potassium channels related to the Shal gene of Drosophila (Kv4 channels) mediate a subthreshold-activating current (I_SA) that controls dendritic excitation and the backpropagation of action potentials in neurons. Kv4 channels also exhibit a prominent low voltage–induced closed-state inactivation, but the underlying molecular mechanism is poorly understood. Here, we examined a structural model in which dynamic coupling between the voltage sensors and the cytoplasmic gate underlies inactivation in Kv4.2 channels. We performed an alanine-scanning mutagenesis in the S4-S5 linker, the initial part of S5, and the distal part of S6 and functionally characterized the mutants under two-electrode voltage clamp in Xenopus oocytes. In a large fraction of the mutants (>80%) normal channel function was preserved, but the mutations influenced the likelihood of the channel to enter the closed-inactivated state. Depending on the site of mutation, low-voltage inactivation kinetics were slowed or accelerated, and the voltage dependence of steady-state inactivation was shifted positive or negative. Still, in some mutants these inactivation parameters remained unaffected. Double mutant cycle analysis based on kinetic and steady-state parameters of low-voltage inactivation revealed that residues known to be critical for voltage-dependent gate opening, including Glu 323 and Val 404, are also critical for Kv4.2 closed-state inactivation. Selective redox modulation of corresponding double-cysteine mutants supported the idea that these residues are involved in a dynamic coupling, which mediates both transient activation and closed-state inactivation in Kv4.2 channels.

The pore domain outer helix contributes to both activation and inactivation of the HERG K+ channel

Ion flow in many voltage-gated K(+) channels (VGK), including the (human ether-a-go-go-related gene) hERG channel, is regulated by reversible collapse of the selectivity filter. hERG channels, however, exhibit low sequence homology to other VGKs, particularly in the outer pore helix (S5) domain, and we hypothesize that this contributes to the unique activation and inactivation kinetics in hERG K(+) channels that are so important for cardiac electrical activity. The S5 domain in hERG identified by NMR spectroscopy closely corresponded to the segment predicted by bioinformatics analysis of 676 members of the VGK superfamily. Mutations to approximately every third residue, from Phe(551) to Trp(563), affected steady state activation, whereas mutations to approximately every third residue on an adjacent face and spanning the entire S5 segment perturbed inactivation, suggesting that the whole span of S5 experiences a rearrangement associated with inactivation. We refined a homology model of the hERG pore domain using constraints from the mutagenesis data with residues affecting inactivation pointing in toward S6. In this model the three residues with maximum impact on activation (W563A, F559A, and F551A) face out toward the voltage sensor. In addition, the residues that when mutated to alanine, or from alanine to valine, that did not express (Ala(561), His(562), Ala(565), Trp(568), and Ile(571)), all point toward the pore helix and contribute to close hydrophobic packing in this region of the channel.

Tryptophan scanning of the acetylcholine receptor's betaM4 transmembrane domain: decoding allosteric linkage at the lipid-protein interface with ion-channel gating

The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel protein that mediates fast excitatory synaptic transmission in the peripheral and central nervous systems. Changes in the structure and function of the AChR can lead to serious impairment of physiological processes. In this study, we combined site-directed mutagenesis, radioligand binding assays, electrophysiological recordings and Fourier analyses to characterize the functional role and structural aspects of the betaM4 transmembrane domain of the Torpedo AChR. We performed tryptophan replacements, from residues L438 through F455, along the betaM4 transmembrane domain. Expression levels of mutants F439W-G450W and F452W-I454W produced peak currents similar to or lower than those in wild-type (WT). Tryptophan substitutions at positions L438 and T451 led to a deficiency in either subunit expression or receptor assembly. Mutations L440W, V442W, C447W and S453W produced a gain-of-function response. Mutation F455W produced a loss of ion channel function. The periodicity profile of the normalized expression level (closed state) and EC(50) (open state) revealed a minor conformational change of 0.4 residues/turn of the betaM4 domain. These findings suggest that a minor movement of the betaM4 domain occurs during channel activation.

Structural dynamics of an isolated voltage-sensor domain in a lipid bilayer

A strong interplay between the voltage-sensor domain (VSD) and the pore domain (PD) underlies voltage-gated channel functions. In a few voltage-sensitive proteins, the VSD has been shown to function without a canonical PD, although its structure and oligomeric state remain unknown. Here, using EPR spectroscopy, we show that the isolated VSD of KvAP can remain monomeric in a reconstituted bilayer and retain a transmembrane conformation. We find that water-filled crevices extending deep into the membrane around S3, a scaffold conducive to transport of protons/cations, are intrinsic to the VSD. Differences in solvent accessibility in comparison to the full-length KvAP allowed us to define an interacting footprint of the PD on the VSD. This interaction is centered around S1 and S2 and suggests a rotation of 70 degrees -100 degrees relative to Kv1.2-Kv2.1 chimera. Sequence-conservation patterns in Kv channels, Hv channels, and voltage-sensitive phosphatases reveal several near-universal features suggesting a common molecular architecture for all VSDs.

Fourier transform coupled to tryptophan-scanning mutagenesis: lessons from its application to the prediction of secondary structure in the acetylcholine receptor lipid-exposed transmembrane domains

Although Fourier transform (FT) and tryptophan-scanning mutagenesis (TrpScanM) have been extremely useful for predicting secondary structures of membrane proteins, they are deemed to be low-resolution techniques. Herein, we describe the combined use of FT and TrpScanM (FT-TrpScanM) as a more reliable approach for the prediction of secondary structure. Five TrpScanM studies of the acetylcholine receptor lipid-exposed transmembrane domains (LETMDs) were revisited and analyzed by FT-TrpScanM. FT analysis of the raw data from the aforementioned TrpScanM studies supports and validates the conclusions derived from their tryptophan-periodicity profiles. Furthermore, by FT-TrpScanM, we were able to determine the minimum number of consecutive tryptophan substitutions necessary for more robust prediction of alpha-helical secondary structures and evaluate the quality of structure predictions by alpha-helical character curves. Finally, this study encourages future utilization of FT-TrpScanM to more reliably predict secondary structures of the membrane protein LETMDs.

Role of the S6 C-terminus in KCNQ1 channel gating

Co-assembly of KCNQ1 alpha-subunits with KCNE1 beta-subunits results in the channel complex underlying the cardiac IKs current in vivo. Like other voltage-gated K+ channels, KCNQ1 has a tetrameric configuration. The S6 segment of each subunit lines the ion channel pore with the lower part forming the activation gate. To determine residues involved in protein-protein interactions in the C-terminal part of S6 (S6T), alanine and tryptophan perturbation scans were performed from residue 348-362 in the KCNQ1 channel. Several residues were identified to be relevant in channel gating, as substitutions affected the activation and/or deactivation process. Some mutations (F351A and V355W) drastically altered the gating characteristics of the resultant KCNQ1 channel, to the point of mimicking the IKs current. Furthermore, mutagenesis of residue L353 to an alanine or a charged residue impaired normal channel closure upon hyperpolarization, generating a constitutively open phenotype. This indicates that the L353 residue is essential for stabilizing the closed conformation of the channel gate. These findings together with the identification of several LQT1 mutations in the S6 C-terminus of KCNQ1 underscore the relevance of this region in KCNQ1 and IKs channel gating.

Functional interactions at the interface between voltage-sensing and pore domains in the Shaker K(v) channel

Voltage-activated potassium (K(v)) channels contain a central pore domain that is partially surrounded by four voltage-sensing domains. Recent X-ray structures suggest that the two domains lack extensive protein-protein contacts within presumed transmembrane regions, but whether this is the case for functional channels embedded in lipid membranes remains to be tested. We investigated domain interactions in the Shaker K(v) channel by systematically mutating the pore domain and assessing tolerance by examining channel maturation, S4 gating charge movement, and channel opening. When mapped onto the X-ray structure of the K(v)1.2 channel the large number of permissive mutations support the notion of relatively independent domains, consistent with crystallographic studies. Inspection of the maps also identifies portions of the interface where residues are sensitive to mutation, an external cluster where mutations hinder voltage sensor activation, and an internal cluster where domain interactions between S4 and S5 helices from adjacent subunits appear crucial for the concerted opening transition.

Secondary structure of a KCNE cytoplasmic domain

Type I transmembrane KCNE peptides contain a conserved C-terminal cytoplasmic domain that abuts the transmembrane segment. In KCNE1, this region is required for modulation of KCNQ1 K(+) channels to afford the slowly activating cardiac I(Ks) current. We utilized alanine/leucine scanning to determine whether this region possesses any secondary structure and to identify the KCNE1 residues that face the KCNQ1 channel complex. Helical periodicity analysis of the mutation-induced perturbations in voltage activation and deactivation kinetics of KCNQ1-KCNE1 complexes defined that the KCNE1 C terminus is alpha-helical when split in half at a conserved proline residue. This helical rendering assigns all known long QT mutations in the KCNE1 C-terminal domain as protein facing. The identification of a secondary structure within the KCNE1 C-terminal domain provides a structural scaffold to map protein-protein interactions with the pore-forming KCNQ1 subunit as well as the cytoplasmic regulatory proteins anchored to KCNQ1-KCNE complexes.

The interaction of spider gating modifier peptides with voltage-gated potassium channels

Gating modifier peptides bind to ion channels and alter the gating process of these molecules. One of the most extensively studied peptides, Hanatoxin (HaTx), isolated from a Chilean tarantula, has been used to characterize the blocking properties of the voltage-gated potassium channel Kv2.1. These studies have provided some insight into the gating mechanism in Kv channels. In this review we will discuss the interaction of HaTx and related spider peptides with Kv channels illustrating the properties of the binding surface of these peptides, their membrane partitioning characteristics, and will provide a working hypothesis for how the peptides inhibit gating of Kv channels. Advanced simulation results support the concept of mutual conformational changes upon peptide binding to the S3b region of the channel which will restrict movement of S4 and compromise coupling of the gating machinery to opening of the pore.

Tryptophan-scanning mutagenesis in the alphaM3 transmembrane domain of the muscle-type acetylcholine receptor. A spring model revealed

Membrane proteins constitute a large fraction of all proteins, yet very little is known about their structure and conformational transitions. A fundamental question that remains obscure is how protein domains that are in direct contact with the membrane lipids move during the conformational change of the membrane protein. Important structural and functional information of several lipid-exposed transmembrane domains of the acetylcholine receptor (AChR) and other ion channel membrane proteins have been provided by the tryptophan-scanning mutagenesis. Here, we use the tryptophan-scanning mutagenesis to monitor the conformational change of the alphaM3 domain of the muscle-type AChR. The perturbation produced by the systematic tryptophan substitution along the alphaM3 domain were characterized through two-electrode voltage clamp and 125I-labeled alpha-bungarotoxin binding. The periodicity profiles of the changes in AChR expression (closed state) and ACh EC50 (open-channel state) disclose two different helical structures; a thinner-elongated helix for the closed state and a thicker-shrunken helix for the open-channel state. The existence of two different helical structures suggest that the conformational transition of the alphaM3 domain between both states resembles a spring motion and reveals that the lipid-AChR interface plays a key role in the propagation of the conformational wave evoked by agonist binding. In addition, the present study also provides evidence about functional and structural differences between the alphaM3 domains of the Torpedo and muscle-type receptors AChR.

Structure prediction for the down state of a potassium channel voltage sensor

Voltage-gated potassium (Kv) channels, essential for regulating potassium uptake and cell volume in plants and electrical excitability in animals, switch between conducting and non-conducting states as a result of conformational changes in the four voltage-sensing domains (VSDs) that surround the channel pore. This process, known as gating, is initiated by a cluster of positively charged residues on the fourth transmembrane segment (S4) of each VSD, which drives the VSD into a 'down state' at negative voltages and an 'up state' at more positive voltages. The crystal structure of Kv1.2 probably corresponds to the up state, but the local environment of S4 in the down state and its motion in voltage gating remains unresolved. Here we employed several conditional lethal/second-site suppressor yeast screens to determine the transmembrane packing of the VSD in the down state. This screen relies on the ability of KAT1, a eukaryotic Kv channel, to conduct potassium when its VSDs are in the down state, thereby rescuing potassium-transport-deficient yeast. Starting with KAT1 channels bearing conditional lethal mutations, we identified second-site suppressor mutations throughout the VSD that recover yeast growth. We then constructed a down state model of the channel using six pairs of interacting residues as structural constraints and verified this model by engineering suppressor mutations on the basis of spatial considerations. A comparison of this down state model with the up state Kv1.2 structure suggests that the VSDs undergo large rearrangements during gating, whereas the S4 segment remains positioned between the central pore and the remainder of the VSD in both states.

Modulation of HERG gating by a charge cluster in the N-terminal proximal domain

Human ether-a-go-go-related gene (HERG) potassium channels contribute to the repolarization of the cardiac action potential and display unique gating properties with slow activation and fast inactivation kinetics. Deletions in the N-terminal 'proximal' domain (residues 135-366) have been shown to induce hyperpolarizing shifts in the voltage dependence of activation, suggesting that it modulates activation. However, we did not observe a hyperpolarizing shift with a subtotal deletion designed to preserve the local charge distribution, and other deletions narrowed the region to the KIKER containing sequence 362-372. Replacing the positively charged residues of this sequence by negative ones (EIEEE) resulted in a -45 mV shift of the voltage dependence of activation. The shifts were intermediate for individual charge reversals, whereas E365R resulted in a positive shift. Furthermore, the shifts in the voltage dependence were strongly correlated with the net charge of the KIKER region. The apparent speeding of the activation was attributable to the shifted voltage dependence of activation. Additionally, the introduction of negative charges accelerated the intermediate voltage-independent forward rate constant. We propose that the modulatory effects of the proximal domain on HERG gating are largely electrostatic, localized to the charged KIKER sequence.

Molecular regions responsible for differences in activation between heag channels

The ether-a-go-go potassium channels heag1 and heag2 are highly homologous; however, the activation properties between the two channels are different. We have studied the molecular regions that determine differences in activation properties by making chimeras between the two channels, expressing them in oocytes, and recording currents with two-electrode voltage-clamp. The activation time course has an initial sigmoidal component dependent on the Cole-Moore shift, followed by a faster component. We show that not only is the extreme N terminus involved in differences between heag1 and heag2 channels, but also the PAS domain itself. Also multiple regions of the membrane-spanning part of the channel appear to be involved, with different regions involved for the early and late time courses, reflecting their different mechanisms. The later time course involved S1 and P-S6 regions. Taken together, our data show that activation involves multiple regions of the N terminal region and membrane-spanning regions of the channel.

How far will you go to sense voltage?

Despite tremendous progress in the study of voltage-gated channels, the molecular mechanism underlying voltage sensing has remained a matter of debate. We review five new studies that make major progress in the field. The studies employ a battery of distinct approaches that have the common aim of measuring the motion of the voltage sensor. We interpret the results in light of the recent crystal structure of the mammalian potassium channel Kv1.2. We focus on the transmembrane movement of the voltage sensor as a key element to the detection of membrane potential and to the control of channel gating.

Electrifying phosphatases

Ci-VSP, a recently described protein with sequence similarity to both the voltage-sensing domain of a voltage-gated potassium channel and the phosphatase PTEN, functions as a transmembrane phosphoinositide phosphatase that is regulated by changes in voltage across the plasma membrane. Ci-VSP dephosphorylated phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3] in vitro, exhibited capacitative currents that resembled ion channel gating currents, and, when coexpressed with potassium channels that are regulated by PtdIns(4,5)P2, conferred sensitivity of potassium current amplitude to prolonged changes in membrane potential. How the voltage-sensing (VS) domain of Ci-VSP communicates with the phosphatase domain, and how the VS domain moves its charges across the membrane electric field in the absence of a transmembrane pore domain, remain to be determined.

The S4 voltage sensor packs against the pore domain in the KAT1 voltage-gated potassium channel

In voltage-gated ion channels, the S4 transmembrane segment responds to changes in membrane potential and controls channel opening. The local environment of S4 is still unknown, even regarding the basic question as to whether S4 is close to the pore domain. Relying on the ability of functional KAT1 channels to rescue potassium (K+) transport-deficient yeast, we have performed an unbiased mutagenesis screen aimed at determining whether S4 packs against S5 of the pore domain. Starting with semilethal mutations of surface-exposed S5 residues of the KAT1 pore domain, we have screened randomly mutagenized libraries of S4 or S1-S3 for second-site suppressors. Our study identifies two S4 residues that interact in a highly specific manner with two S5 residues in the middle of the membrane-spanning regions, supporting a model in which the S4 voltage sensor packs against the pore domain in the hyperpolarized, or "down," state of S4.

Tryptophan scanning mutagenesis of the HERG K+ channel: the S4 domain is loosely packed and likely to be lipid exposed

Inherited mutations or drug-induced block of voltage-gated ion channels, including the human ether-à-go-go-related gene (HERG) K+ channel, are significant causes of malignant arrhythmias and sudden death. The fourth transmembrane domain (S4) of these channels contains multiple positive charges that move across the membrane electric field in response to changes in transmembrane voltage. In HERG K+ channels, the movement of the S4 domain across the transmembrane electric field is particularly slow. To examine the basis of the slow movement of the HERG S4 domain and specifically to probe the relationship between the S4 domain with the lipid bilayer and rest of the channel protein, we individually mutated each of the S4 amino acids in HERG (L524-L539) to tryptophan, and characterized the activation and deactivation properties of the mutant channels in Xenopus oocytes, using two-electrode voltage-clamp methods. Tryptophan has a large bulky hydrophobic sidechain and so should be tolerated at positions that interact with lipid, but not at positions involved in close protein-protein interactions. Significantly, we found that all S4 tryptophan mutants were functional. These data indicate that the S4 domain is loosely packed within the rest of the voltage sensor domain and is likely to be lipid exposed. Further, we identified residues K525, R528 and K538 as being the most important for slow activation of the channels.