Molecular basis of charge movement in voltage-gated sodium channels

Flecainide sensitivity of a Na channel long QT mutation shows an open-channel blocking mechanism for use-dependent block

A long QT mutation in the cardiac sodium channel, D1790G (DG), shows enhanced flecainide use-dependent block (UDB). The relative importance of open and inactivated states of the channel in flecainide UDB has been controversial. We used a modifiable, inactivation-deficient mutant channel that contains the F1486C mutation in the IFM motif to investigate the UDB difference between the wild-type (WT-ICM) and DG (DG-ICM) channels. UDB at 5 Hz was greater in DG-ICM than WT-ICM, and IC50 values for steady-state UDB were 7.19 and 18.06 microM, respectively. When [2-(trimethyammonium) ethyl]methanethiosulfonate bromide (MTSET) was included in the pipette and fast inactivation was disabled, IC50 was 5.04 microM for DG-ICM and 12.63 microM for WT-ICM. We measured open-channel block by flecainide directly in MTSET-treated, noninactivating ICM channels. Steady-state block was higher for DG-ICM than WT-ICM (IC50 was 2.34 microM for DG-ICM and 5.87 microM for WT-ICM), suggesting that open-channel block is an important determinant of flecainide UDB. We obtained association (kon) and dissociation (koff) rates for open-channel block by the Langmuir-isotherm model. They were koff = 31.37 s(-1), kon = 5.83 s(-1).microM(-1), and calculated Kd = 5.38 microM for WT-ICM (where Kd = koff/kon); and koff = 24.88 s(-1), kon = 9.54 s(-1).microM(-1), and calculated Kd = 2.61 microM for DG-ICM. These Kd values were similar to IC50 measured from steady-state open-channel block. Furthermore, we modeled UDB mathematically by using these kinetic rates and found that the model predicted experimental UDB accurately. The recovery from UDB had a minor contribution to UDB. Flecainide UDB is predominantly determined by an open-channel blocking mechanism, and DG-ICM channels appeared to have an altered open-channel state with higher flecainide affinity than WT-ICM.

Allosteric effects of external K+ ions mediated by the aspartate of the GYGD signature sequence in the Kv2.1 K+ channel

K+ channels achieve exquisite ion selectivity without jeopardizing efficient permeation by employing multiple, interacting K+-binding sites. Introduction ofa cadmium (Cd2+)-binding site in the external vestibule of Kv2.1 (drk1), allowed us to functionally characterize a binding site for external monovalent cations. Permeant ions displayed higher affinity for this site than non-permeant monovalent cations, although the selectivity profile was different from that of the channel. Point mutations identified the highly conserved aspartate residue immediately following the selectivity filter as a critical determinant of the antagonism between external K+ and Cd2+ ions. A conservative mutation at this position (D378E) significantly affected the open-state stability. Moreover, the mean open time was found to be modulated by external K+ concentration, suggesting a coupling between channel closing and the permeation process. Reducing the Rb+ conductance by mutating the selectivity filter to the sequence found inKv4.1, also significantly reduced the effectiveness ofRb+ ions to antagonize Cd2+ inhibition, thereby implicating the selectivity filter as the site at which K+ions exert their antagonistic effect on Cd2+ block. The equivalent of D378 in KcsA, D80, takes part in an inter-subunit hydrogen-bond network that allows D80to functionally interact with the selectivity filter. The results suggest that external K+ ions antagonize Cd2+inhibition (in I379C) and modulate the mean open time(in the wild-type Kv2.1) by altering the occupancy profile of the K+-binding sites in the selectivity filter.

Calibrated Measurement of Gating-Charge Arginine Displacement in the KvAP Voltage-Dependent K+ Channel

Voltage-dependent ion channels open and conduct ions in response to changes in cell-membrane voltage. The voltage sensitivity of these channels arises from the motion of charged arginine residues located on the S4 helices of the channel's voltage sensors. In KvAP, a prokaryotic voltage-dependent K+ channel, the S4 helix forms part of a helical hairpin structure, the voltage-sensor paddle. We have measured the membrane depth of residues throughout the KvAP channel using avidin accessibility to different-length tethered biotin reagents. From these measurements, we have calibrated the tether lengths and derived the thickness of the membrane that forms a barrier to avidin penetration, allowing us to determine the magnitude of displacement of the voltage-sensor paddles during channel gating. Here we show that the voltage-sensor paddles are highly mobile compared to other regions of the channel and transfer the gating-charge arginines 15-20 A through the membrane to open the pore.

Shedding light on membrane proteins

Membrane proteins are a cell's first line of communication with the world that exists just beyond the plasma membrane. These proteins afford the cell a peek at its external environment, signal the cell to adjust its internal chemistry in response to its surroundings, and ensure that the cell's metabolic state is faithfully coupled to the outside world. Because of their importance in cellular communication, membrane proteins have been the focus of intense study at the functional and structural levels. Here, we describe optical techniques that can either passively monitor or actively control the structural rearrangements that take place as these proteins peek at the outside world. Our focus is on ion channels, but the techniques described can be applied to a host of other proteins.

Inherited disorders of voltage-gated sodium channels

A variety of inherited human disorders affecting skeletal muscle contraction, heart rhythm, and nervous system function have been traced to mutations in genes encoding voltage-gated sodium channels. Clinical severity among these conditions ranges from mild or even latent disease to life-threatening or incapacitating conditions. The sodium channelopathies were among the first recognized ion channel diseases and continue to attract widespread clinical and scientific interest. An expanding knowledge base has substantially advanced our understanding of structure-function and genotype-phenotype relationships for voltage-gated sodium channels and provided new insights into the pathophysiological basis for common diseases such as cardiac arrhythmias and epilepsy.

Focused Electric Field across the Voltage Sensor of Potassium Channels

Voltage-gated ion channels respond to changes in membrane potential by movement of their voltage sensors across the electric field between cytoplasmic and extracellular solutions. The principal voltage sensors in these proteins are positively charged S4 segments. The absolute magnitude of S4 movement discriminates two competing classes of gating models. In one class, the movement is <10 Angstrom due to the fact that the electric field is focused by aqueous crevices in the channel protein. In an alternative model, based in part on the crystal structure of a potassium channel, the side chains of S4 arginines move their charges across the bilayer's electric field, a distance of >25 Angstrom. Here, using tethered charges attached to an S4 segment, we provide evidence that the electric field falls across a distance of <4 Angstrom, supporting a model in which the relative movement between S4 and the electric field is very small.

Gating charge displacement in voltage-gated ion channels involves limited transmembrane movement

Voltage-gated ion channels are responsible for generating electrical impulses in nerves and other excitable cells. The fourth transmembrane helix (S4) in voltage-gated channels is the primary voltage-sensing unit that mediates the response to a changing membrane electric field. The molecular mechanism of voltage sensing, particularly with respect to the magnitude of the transmembrane movement of S4, remains controversial. To determine the extent of this transmembrane movement, we use fluorescent resonance energy transfer between the S4 domain and a reference point in the lipid bilayer. The lipophilic ion dipicrylamine distributes on either side of the lipid bilayer depending on the membrane potential, and is used here as a resonance-energy-transfer acceptor from donor molecules attached to several positions in the Shaker K+ channel. A voltage-driven transmembrane movement of the donor should produce a transient fluorescence change because the acceptor also translocates as a function of voltage. In Shaker K+ channels no such transient fluorescence is observed, indicating that the S4 segment does not translocate across the lipid bilayer. Based on these observations, we propose a molecular model of voltage gating that can account for the observed 13e gating charge with limited transmembrane S4 movement.

Voltage-sensor activation with a tarantula toxin as cargo

The opening and closing of voltage-activated Na+, Ca2+ and K+ (Kv) channels underlies electrical and chemical signalling throughout biology, yet the structural basis of voltage sensing is unknown. Hanatoxin is a tarantula toxin that inhibits Kv channels by binding to voltage-sensor paddles, crucial helix-turn-helix motifs within the voltage-sensing domains that are composed of S3b and S4 helices. The active surface of the toxin is amphipathic, and related toxins have been shown to partition into membranes, raising the possibility that the toxin is concentrated in the membrane and interacts only weakly and transiently with the voltage sensors. Here we examine the kinetics and state dependence of the toxin-channel interaction and the physical location of the toxin in the membrane. We find that hanatoxin forms a strong and stable complex with the voltage sensors, far outlasting fluctuations of the voltage sensors between resting (closed) conformations at negative voltages and activated (open) conformations at positive voltages. Toxin affinity is reduced by voltage-sensor activation, explaining why the toxin stabilizes the resting conformation. We also find that when hanatoxin partitions into membranes it is localized to an interfacial region, with Trp 30 positioned about 8.5 A from the centre of the bilayer. These results demonstrate that voltage-sensor paddles activate with a toxin as cargo, and suggest that the paddles traverse no more than the outer half of the bilayer during activation.

Role of Domain IV/S4 outermost arginines in gating of T-type calcium channels

The role of the outermost three charged residues of Domain IV/S4 in controlling gating of Ca(v)3.2 was investigated using single substitutions of each arginine with glutamine, cysteine, histidine, and lysine in a Flp-In-293 cell line, in which expression levels could be compared. Channel density, based on gating charge measurements, was ~125,000 channels/cell (10 fC/pF), except for R2Q and R3C, which expressed at lower levels. Channels substituted at Arg-1715 (R1C, R1Q, R1H) demonstrated such modest changes that a role in voltage sensing could not be determined. Arg-1718 (R2) made a contribution to activation voltage sensing, and the channel was sensitive to the geometry of side-chain substitutions at this position. Arg-1721 (R3) substitutions produced complex kinetic changes that together suggested that geometry made a larger contribution than charge. Current decay at positive potentials (O-->I) exponentially approached a constant value for all mutants except R2K channels, which were biphasically dependent on potential. R2K channels also displayed slowed deactivation with reduced voltage dependence despite near control values for conductance. Voltage-dependent accessibility of R to C mutants, evaluated with intracellularly and extracellularly applied methanthiosulfonate (MTS) reagents, showed that both R2 and R3 were exposed only when cells were depolarized, although it was not necessary for channels to open. Together, the data indicate that Domain IV/S4 is an activation domain and is not involved in inactivation from the open state.

Voltage Sensor of Kv1.2: Structural Basis of Electromechanical Coupling

Voltage-dependent ion channels contain voltage sensors that allow them to switch between nonconductive and conductive states over the narrow range of a few hundredths of a volt. We investigated the mechanism by which these channels sense cell membrane voltage by determining the x-ray crystal structure of a mammalian Shaker family potassium ion (K+) channel. The voltage-dependent K+ channel Kv1.2 grew three-dimensional crystals, with an internal arrangement that left the voltage sensors in an apparently native conformation, allowing us to reach three important conclusions. First, the voltage sensors are essentially independent domains inside the membrane. Second, they perform mechanical work on the pore through the S4-S5 linker helices, which are positioned to constrict or dilate the S6 inner helices of the pore. Third, in the open conformation, two of the four conserved Arg residues on S4 are on a lipid-facing surface and two are buried in the voltage sensor. The structure offers a simple picture of how membrane voltage influences the open probability of the channel.

Beta-scorpion toxin effects suggest electrostatic interactions in domain II of voltage-dependent sodium channels

Beta-scorpion toxins specifically modulate the voltage dependence of sodium channel activation by acting through a voltage-sensor trapping model. We used mutagenesis, functional analysis and the action of beta-toxin as tools to investigate the existence and role in channel activation of molecular interactions between the charged residues of the S2, S3 and S4 segments in domain II of sodium channels. Mutating to arginine the acidic residues of the S2 and S3 transmembrane segments in domain II, or making charge-reversal mutation of the two outermost gating charges of the IIS4 voltage sensor, shifts the voltage dependence of channel activation to more positive potentials and enhances the effect of beta-scorpion toxin. Thus, mutations of acidic residues in IIS2 and IIS3 segments are able to promote voltage-sensor trapping in a way that is similar to the mutations of the arginines in the IIS4 segment. In order to disclose the network of interactions among acidic and basic residues we performed functional analysis of charge-inversion double mutants: our data suggest that the first arginine of the voltage sensor S4 in domain II (R850) interacts specifically with E805, D814 and E821 in the S2 and S3 segments, whereas the second arginine (R853) only interacts with D827 in the S3 segment. Our results suggest that the S2, S3 and S4 segments in domain II form a voltage-sensing structure, and that molecular interactions between the charged residues of this structure modulate the availability of the IIS4 voltage sensor for trapping by beta-toxins. They also provide unique insights into the molecular events that occur during channel activation, as well as into the structure of the channel.

Reduced voltage dependence of inactivation in the SCN5A sodium channel mutation delF1617

Deletion of a phenylalanine at position 1617 (delF1617) in the extracellular linker between segments S3 and S4 in domain IV of the human heart Na+ channel (hH1a) has been tentatively associated with long QT syndrome type 3 (LQT3). In a mammalian cell expression system, we compared whole cell, gating, and single-channel currents of delF1617 with those of wild-type hH1a. The half points of the peak activation-voltage curve for the two channels were similar, as were the deactivation time constants at hyperpolarized test potentials. However, delF1617 demonstrated a significant negative shift of −7 mV in the half point of the voltage-dependent Na+ channel availability curve compared with wild type. In addition, both the time course of decay of Na+ current ( INa) and two-pulse development of inactivation of delF1617 were faster at negative test potentials, whereas they tended to be slower at positive potentials compared with wild type. Mean channel open times for delF1617 were shorter at potentials <0 mV, whereas they were longer at potentials >0 mV compared with wild type. Using anthopleurin-A, a site-3 toxin that inhibits movement of segment S4 in domain IV (S4-DIV), we found that gating charge contributed by the S4-DIV in delF1617 was reduced 37% compared with wild type. We conclude that deletion of a single amino acid in the S3-S4 linker of domain IV alters the voltage dependence of fast inactivation via a reduction in the gating charge contributed by S4-DIV and can cause either a gain or loss of INa, depending on membrane potential.

The cooperative voltage sensor motion that gates a potassium channel

The four arginine-rich S4 helices of a voltage-gated channel move outward through the membrane in response to depolarization, opening and closing gates to generate a transient ionic current. Coupling of voltage sensing to gating was originally thought to operate with the S4s moving independently from an inward/resting to an outward/activated conformation, so that when all four S4s are activated, the gates are driven to open or closed. However, S4 has also been found to influence the cooperative opening step (Smith-Maxwell et al., 1998a), suggesting a more complex mechanism of coupling. Using fluorescence to monitor structural rearrangements in a Shaker channel mutant, the ILT channel (Ledwell and Aldrich, 1999), that energetically isolates the steps of activation from the cooperative opening step, we find that opening is accompanied by a previously unknown and cooperative movement of S4. This gating motion of S4 appears to be coupled to the internal S6 gate and to two forms of slow inactivation. Our results suggest that S4 plays a direct role in gating. While large transmembrane rearrangements of S4 may be required to unlock the gating machinery, as proposed before, it appears to be the gating motion of S4 that drives the gates to open and close.

Structure and function of clc channels

The CLC family comprises a group of integral membrane proteins whose major action is to translocate chloride (Cl-) ions across the cell membranes. Recently, the structures of CLC orthologues from two bacterial species, Salmonella typhimurium and Escherichia coli, were solved, providing the first framework for understanding the operating mechanisms of these molecules. However, most of the previous mechanistic understanding of CLC channels came from electrophysiological studies of a branch of the channel family, the muscle-type CLC channels in vertebrate species. These vertebrate CLC channels were predicted to contain two identical but independent pores, and this hypothesis was confirmed by the solved bacterial CLC structures. The opening and closing of the vertebrate CLC channels are also known to couple to the permeant ions via their binding sites in the ion-permeation pathway. The bacterial CLC structures can probably serve as a structural model to explain the gating-permeation coupling mechanism. However, the CLC-ec1 protein in E. coli was most recently shown to be a Cl- -H+ antiporter, but not an ion channel. The molecular basis to explain the difference between vertebrate and bacterial CLCs, especially the distinction between an ion channel and a transporter, remains a challenge in the structure/function studies for the CLC family.

Voltage-sensing arginines in a potassium channel permeate and occlude cation-selective pores

Voltage-gated ion channels sense voltage by shuttling arginine residues located in the S4 segment across the membrane electric field. The molecular pathway for this arginine permeation is not understood, nor is the filtering mechanism that permits passage of charged arginines but excludes solution ions. We find that substituting the first S4 arginine with smaller amino acids opens a high-conductance pathway for solution cations in the Shaker K(+) channel at rest. The cationic current does not flow through the central K(+) pore and is influenced by mutation of a conserved residue in S2, suggesting that it flows through a protein pathway within the voltage-sensing domain. The current can be carried by guanidinium ions, suggesting that this is the pathway for transmembrane arginine permeation. We propose that when S4 moves it ratchets between conformations in which one arginine after another occupies and occludes to ions the narrowest part of this pathway.

Models of the structure and voltage-gating mechanism of the shaker K+ channel

In the preceding, accompanying article, we present models of the structure and voltage-dependent gating mechanism of the KvAP bacterial K+ channel that are based on three types of evidence: crystal structures of portions of the KvAP protein, theoretical modeling criteria for membrane proteins, and biophysical studies of the properties of native and mutated voltage-gated channels. Most of the latter experiments were performed on the Shaker K+ channel. Some of these data are difficult to relate directly to models of the KvAP channel's structure due to differences in the Shaker and KvAP sequences. We have dealt with this problem by developing new models of the structure and gating mechanism of the transmembrane and extracellular portions of the Shaker channel. These models are consistent with almost all of the biophysical data. In contrast, much of the experimental data are incompatible with the "paddle" model of gating that was proposed when the KvAP crystal structures were first published. The general folding pattern and gating mechanisms of our current models are similar to some of our earlier models of the Shaker channel.

A model of voltage gating developed using the KvAP channel crystal structure

Having inspected the crystal structure of the complete KvAP channel protein, we suspect that the voltage-sensing domain is too distorted to provide reliable information about its native tertiary structure or its interactions with the central pore-forming domain. On the other hand, a second crystal structure of the isolated voltage-sensing domain may well correspond to a native open conformation. We also observe that the paddle model of gating developed from these two structures is inconsistent with many experimental results, and suspect it to be energetically unrealistic. Here we show that the isolated voltage-sensing domain crystal structure can be docked onto the pore domain portion of the full-length KvAP crystal structure in an energetically favorable way to create a model of the open conformation. Using this as a starting point, we have developed rather conventional models of resting and transition conformations based on the helical screw mechanism for the transition from the open to the resting conformation. Our models are consistent with both theoretical considerations and experimental results.

S3b amino acid residues do not shuttle across the bilayer in voltage-dependent Shaker K+ channels

In voltage-dependent channels, positive charges contained within the S4 domain are the voltage-sensing elements. The "voltage-sensor paddle" gating mechanism proposed for the KvAP K+ channel has been the subject of intense discussion regarding its general applicability to the family of voltage-gated channels. In this model, the voltage sensor composed of the S3b and the S4 segment shuttles across the lipid bilayer during channel activation. Guided by this mechanism, we assessed here the accessibility of residues in the S3 segment of the Shaker K+ channel by using cysteine-scanning mutagenesis. Mutants expressed robust K+ currents in Xenopus oocytes and reacted with methanethiosulfonate ethyltrimethylammonium in both closed and open conformations of the channel. Because Shaker has a long S3-S4 linker segment, we generated a deletion mutant with only three residues to emulate the KvAP structure. In this short linker mutant, all of the tested residues in the S3b were accessible to methanethiosulfonate ethyltrimethylammonium in both closed and open conformations. Because the S3b moves together with the S4 domain in the paddle model, we tested the effects of deleting two negative charges or adding a positive charge to this region of the channel. We found that altering the S3b net charge does not modify the total gating charge involved in channel activation. We conclude that the S3b segment is always exposed to the external milieu of the Shaker K+ channel. Our results are incompatible with any model involving a large membrane displacement of segment S3b.

Voltage-Gated Ion Channels

Voltage-dependent ion channels are membrane proteins that conduct ions at high rates regulated by the voltage across the membrane. They play a fundamental role in the generation and propagation of the nerve impulse and in cell homeostasis. The voltage sensor is a region of the protein bearing charged amino acids that relocate upon changes in the membrane electric field. The movement of the sensor initiates a conformational change in the gate of the conducting pathway thus controlling the flow of ions. Major advances in molecular biology, spectroscopy, and structural techniques are delineating the main features and possible structural changes that account for the function of voltage-dependent channels.

Interactions of local anesthetics with voltage-gated Na+ channels

Voltage-gated Na+ channels are dynamic transmembrane proteins responsible for the rising phase of the action potential in excitable membranes. Local anesthetics (LAs) and structurally related antiarrhythmic and anticonvulsant compounds target specific sites in voltage-gated Na+ channels to block Na+ currents, thus reducing excitability in neuronal, cardiac, or central nervous tissue. A high-affinity LA block is produced by binding to open and inactivated states of Na+ channels rather than to resting states and suggests a binding site that converts from a low- to a high-affinity conformation during gating. Recent findings using site-directed mutagenesis suggest that multiple S6 segments together form an LA binding site within the Na+ channel. While the selectivity filter may form the more extracellular-located part of this binding site, the role of the fast inactivation gate in LA binding has not yet been resolved. The receptor of the neurotoxin batrachotoxin (BTX) is adjacent to or even overlaps with the LA binding site. The close proximity of the LA and BTX binding sites to residues critical for inactivation, together with gating transitions through S6 segments, might explain the strong impact of LAs and BTX on inactivation of voltage-gated Na+ channels and might help elucidate the mechanisms underlying voltage- and frequency-dependent LA block.

MTSEA prevents ligand binding to the human melanocortin-4 receptor by modification of cysteine 130 in transmembrane helix 3

We have investigated the effect of the sulfhydryl-reactive reagent, methyl thiosulfonate ethylammonium (MTSEA), on ligand binding to the human melanocortin-4 (MC4) receptor stably expressed in HEK-293 cells. MTSEA inhibited binding of the agonist, 125I-NDPalpha-MSH, and the antagonist, 125I-SHU9119, in a concentration-dependent manner. Pre-incubation of cells with either the agonist or antagonist protected from subsequent MTSEA inhibition of radioligand binding. Mutation of Cys130 in transmembrane helix 3 to alanine, whilst not affecting ligand binding, led to a complete loss of the inhibitory effect of MTSEA. Since other types of sulfhydryl-reactive reagents had no effect on ligand binding, we conclude that covalent modification of Cys130 by MTSEA disrupts ligand binding by neutralising a close-by negative charge, most likely on Asp126.

Hysteresis in the voltage dependence of HCN channels: conversion between two modes affects pacemaker properties

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) ion channels are important for rhythmic activity in the brain and in the heart. In this study, using ionic and gating current measurements, we show that cloned spHCN channels undergo a hysteresis in their voltage dependence during normal gating. For example, both the gating charge versus voltage curve, Q(V), and the conductance versus voltage curve, G(V), are shifted by about +60 mV when measured from a hyperpolarized holding potential compared with a depolarized holding potential. In addition, the kinetics of the tail current and the activation current change in parallel to the voltage shifts of the Q(V) and G(V) curves. Mammalian HCN1 channels display similar effects in their ionic currents, suggesting that the mammalian HCN channels also undergo voltage hysteresis. We propose a model in which HCN channels transit between two modes. The voltage dependence in the two modes is shifted relative to each other, and the occupancy of the two modes depends on the previous activation of the channel. The shifts in the voltage dependence are fast (τ ≈ 100 ms) and are not accompanied by any apparent inactivation. In HCN1 channels, the shift in voltage dependence is slower in a 100 mM K extracellular solution compared with a 1 mM K solution. Based on these findings, we suggest that molecular conformations similar to slow (C-type) inactivation of K channels underlie voltage hysteresis in HCN channels. The voltage hysteresis results in HCN channels displaying different voltage dependences during different phases in the pacemaker cycle. Computer simulations suggest that voltage hysteresis in HCN channels decreases the risk of arrhythmia in pacemaker cells.

Towards a structural view of gating in potassium channels

Voltage-activated cation channels have pores that are selective for K(+), Na(+) or Ca(2+). Neurons use these channels to generate and propagate action potentials, release neurotransmitters at synaptic terminals and integrate incoming signals in dendrites. Recent X-ray and electron microscopy studies of an archaebacterial voltage-activated K(+) (Kv) channel have provided the first atomic resolution images of the voltage-sensing domains in Kv channels. Although these structures are consistent with previous biophysical analyses of eukaryotic channels, they also contain surprises, which have provoked new ideas about the structure and movements of these proteins during gating. This review summarizes our current understanding of these intriguing membrane proteins and highlights the open questions.

Structure acquisition of the T1 domain of Kv1.3 during biogenesis

The T1 recognition domains of voltage-gated K(+) (Kv) channel subunits form tetramers and acquire tertiary structure while still attached to their individual ribosomes. Here we ask when and in which compartment secondary and tertiary structures are acquired. We answer this question using biogenic intermediates and recently developed folding and accessibility assays to evaluate the status of the nascent Kv peptide both inside and outside of the ribosome. A compact structure (likely helical) that corresponds to a region of helicity in the mature structure is already manifest in the nascent protein within the ribosomal tunnel. The T1 domain acquires tertiary structure only after emerging from the ribosomal exit tunnel and complete synthesis of the T1-S1 linker. These measurements of ion channel folding within the ribosomal tunnel and its exit port bear on basic principles of protein folding and pave the way for understanding the molecular basis of protein misfolding, a fundamental cause of channelopathies.

Charge immobilization of the voltage sensor in domain IV is independent of sodium current inactivation

Recovery from fast inactivation in voltage-dependent Na+ channels is associated with a slow component in the time course of gating charge during repolarization (i.e. charge immobilization), which results from the slow movement of the S4 segments in domains III and IV (S4-DIII and S4-DIV). Previous studies have shown that the non-specific removal of fast inactivation by the proteolytic enzyme pronase eliminated charge immobilization, while the specific removal of fast inactivation (by intracellular MTSET modification of a cysteine substituted for the phenylalanine in the IFM motif, ICMMTSET, in the inactivation particle formed by the linker between domains III and IV) only reduced the amount of charge immobilization by nearly one-half. To investigate the molecular origin of the remaining slow component of charge immobilization we studied the human cardiac Na+ channel (hH1a) in which the outermost arginine in the S4-DIV, which contributes approximately 20% to total gating charge (Qmax), was mutated to a cysteine (R1C-DIV). Gating charge could be fully restored in R1C-DIV by exposure to extracellular MTSEA, a positively charged methanethiosulphonate reagent. The RIC-DIV mutation was combined with ICMMTSET to remove fast inactivation, and the gating currents of R1C-DIV-ICM(MTSET) were recorded before and after modification with MTSEAo. Prior to MTSEAo, the time course of the gating charge during repolarization (off-charge) was best described by a single fast time constant. After MTSEA, the off-charge had both fast and slow components, with the slow component accounting for nearly 35% of Qmax. These results demonstrate that the slow movement of the S4-DIV during repolarization is not dependent upon the normal binding of the inactivation particle.

Molecular Architecture of the KvAP Voltage-Dependent K+ Channel in a Lipid Bilayer

We have analyzed the local structure and dynamics of the prokaryotic voltage-dependent K+ channel (KvAP) at 0 millivolts, using site-directed spin labeling and electron paramagnetic resonance spectroscopy. We show that the S4 segment is located at the protein/lipid interface, with most of its charges protected from the lipid environment. Structurally, S4 is highly dynamic and is separated into two short helices by a flexible linker. Accessibility and dynamics data indicate that the S1 segment is surrounded by other parts of the protein. We propose that S1 is at the contact interface between the voltage-sensing and pore domains. These results establish the general principles of voltage-dependent channel structure in a biological membrane.

Coupling interactions between voltage sensors of the sodium channel as revealed by site-specific measurements

The voltage-sensing S4 segments in the sodium channel undergo conformational rearrangements in response to changes in the electric field. However, it remains unclear whether these structures move independently or in a coordinated manner. Previously, site-directed fluorescence measurements were shown to track S4 transitions in each of the four domains (Chanda, B., and F. Bezanilla. 2002. J. Gen. Physiol. 120:629–645). Here, using a similar technique, we provide direct evidence of coupling interactions between voltage sensors in the sodium channel. Pairwise interactions between S4s were evaluated by comparing site-specific conformational changes in the presence and absence of a gating perturbation in a distal domain. Reciprocity of effect, a fundamental property of thermodynamically coupled systems, was measured by generating converse mutants. The magnitude of a local gating perturbation induced by a remote S4 mutation depends on the coupling strength and the relative equilibrium positions of the two voltage sensors. In general, our data indicates that the movement of all four voltage sensors in the sodium channel are coupled to a varying extent. Moreover, a gating perturbation in S4-DI has the largest effect on the activation of S4-DIV and vice versa, demonstrating an energetic linkage between S4-DI and S4-DIV. This result suggests a physical mechanism by which the activation and inactivation process may be coupled in voltage-gated sodium channels. In addition, we propose that cooperative interactions between voltage sensors may be the mechanistic basis for the fast activation kinetics of the sodium channel.

Coupled motions between pore and voltage-sensor domains: a model for Shaker B, a voltage-gated potassium channel

A high-resolution crystal structure of KvAP, an archeabacterial voltage-gated potassium (Kv) channel, complexed with a monoclonal Fab fragment has been recently determined. Based on this structure, a mechanism for the activation (opening) of Kv channels has been put forward. This mechanism has since been criticized, suggesting that the resolved structure is not representative of the family of voltage-gated potassium channels. Here, we propose a model of the transmembrane domain of Shaker B, a well-characterized Kv channel, built by homology modeling and docking calculations. In this model, the positively charged S4 helices are oriented perpendicular to the membrane and localized in the groove between segments S5 and S6 of adjacent subunits. The structure and the dynamics of the full atomistic model embedded in a hydrated lipid bilayer were investigated by means of two large-scale molecular dynamics simulations under transmembrane-voltage conditions known to induce, respectively, the resting state (closed) and the activation (opening) of voltage-gated channels. Upon activation, the model undergoes conformational changes that lead to an increase of the hydration of the charged S4 helices, correlated with an upward translation and a tilting of the latter, concurrently with movements of the S5 helices and the activation gate. Although small, these conformational changes ultimately result in an alteration of the ion-conduction pathway. Our findings support the transporter model devised by Bezanilla and collaborators, and further underline the crucial role played by internal hydration in the activation of the channel.

Gating of the bacterial sodium channel, NaChBac: voltage-dependent charge movement and gating currents

The bacterial sodium channel, NaChBac, from Bacillus halodurans provides an excellent model to study structure–function relationships of voltage-gated ion channels. It can be expressed in mammalian cells for functional studies as well as in bacterial cultures as starting material for protein purification for fine biochemical and biophysical studies. Macroscopic functional properties of NaChBac have been described previously (Ren, D., B. Navarro, H. Xu, L. Yue, Q. Shi, and D.E. Clapham. 2001. Science. 294:2372–2375). In this study, we report gating current properties of NaChBac expressed in COS-1 cells. Upon depolarization of the membrane, gating currents appeared as upward inflections preceding the ionic currents. Gating currents were detectable at −90 mV while holding at −150 mV. Charge–voltage (Q–V) curves showed sigmoidal dependence on voltage with gating charge saturating at −10 mV. Charge movement was shifted by −22 mV relative to the conductance–voltage curve, indicating the presence of more than one closed state. Consistent with this was the Cole-Moore shift of 533 μs observed for a change in preconditioning voltage from −160 to −80 mV. The total gating charge was estimated to be 16 elementary charges per channel. Charge immobilization caused by prolonged depolarization was also observed; Q–V curves were shifted by approximately −60 mV to hyperpolarized potentials when cells were held at 0 mV. The kinetic properties of NaChBac were simulated by simultaneous fit of sodium currents at various voltages to a sequential kinetic model. Gating current kinetics predicted from ionic current experiments resembled the experimental data, indicating that gating currents are coupled to activation of NaChBac and confirming the assertion that this channel undergoes several transitions between closed states before channel opening. The results indicate that NaChBac has several closed states with voltage-dependent transitions between them realized by translocation of gating charge that causes activation of the channel.

A possible molecular mechanism of hanatoxin binding-modified gating in voltage-gated K+-channels

While S4 is known as the voltage sensor in voltage-gated potassium channels, the carboxyl terminus of S3 (S3C) is of particular interest concerning the site for gating modifier toxins like hanatoxin. The thus derived helical secondary structural arrangement for S3C, as well as its surrounding environment, has since been intensively and vigorously debated. Our previous structural analysis based on molecular simulation has provided sufficient information to describe reasonable docking conformation and further experimental designs (Lou et al., 2002. J. Mol. Recognit. 15: 175-179). However, if one only relies on such information, more advanced structure-functional interpretations for the roles S3C may play in the modification of gating behavior upon toxin binding will remain unknown. In order to have better understanding of the molecular details regarding this issue, we have performed the docking simulation with the S3C sequence from the hanatoxin-insensitive K+-channel, shaker, and analyzed the conformational changes resulting from such docking. Compared with other functional data from previous studies with respect to the proximity of the S3-S4 linker region, we suggested a significant movement of drk1 S3C, but not shaker S3C, in the direction presumably towards S4, which was comprehended as a possible factor interfering with S4 translocation during drk1 gating in the presence of toxin. In combination with the discussions for structural roles of the length of the S3-S4 linker, a possible molecular mechanism to illustrate the hanatoxin binding-modified gating is proposed.

Role of Arginine Residues on the S4 Segment of the Bacillus halodurans Na+ Channel in Voltage-sensing

The one-domain voltage-gated sodium channel of Bacillus halodurans (NaChBac) is composed of six transmembrane segments (S1-S6) comprising a pore-forming region flanked by segments S5 and S6 and a voltage-sensing element composed of segment S4. To investigate the role of the S4 segment in NaChBac channel activation, we used the cysteine mutagenesis approach where the positive charges of single and multiple arginine (R) residues of the S4 segment were replaced by the neutrally charged amino acid cysteine (C). To determine whether it was the arginine residue itself or its positive charge that was involved in channel activation, arginine to lysine (R to K) mutations were constructed. Wild-type (WT) and mutant NaChBac channels were expressed in tsA201 cells and Na+ currents were recorded using the whole-cell configuration of the patch-clamp technique. The current/voltage (I-V) and conductance/voltage (G-V) relationships steady-state inactivation (h(infinity)) and recovery from inactivation were evaluated to determine the effects of the S4 mutations on the biophysical properties of the NaChBac channel. R to C on the S4 segment resulted in a slowing of both activation and inactivation kinetics. Charge neutralization of arginine residues mostly resulted in a shift toward more positive potentials of G-V and h(infinity) curves. The G-V curve shifts were associated with a decrease in slope, which may reflect a decrease in the gating charge involved in channel activation. Single neutralization of R114, R117, or R120 by C resulted in a very slow recovery from inactivation. Double neutralization of R111 and R129 confirmed the role of R111 in activation and suggested that R129 is most probably not part of the voltage sensor. Most of the R to K mutants retained WT-like current kinetics but exhibited an intermediate G-V curve, a steady-state inactivation shifted to more hyperpolarized potentials, and intermediate time constants of recovery from inactivation. This indicates that R, at several positions, plays an important role in channel activation. The data are consistent with the notion that the S4 is most probably the voltage sensor of the NaChBac channel and that both positive charges and the nature of the arginine residues are essential for channel activation.

Noninactivating voltage-gated sodium channels in severe myoclonic epilepsy of infancy

Mutations in SCN1A, the gene encoding the brain voltage-gated sodium channel alpha(1) subunit (Na(V)1.1), are associated with at least two forms of epilepsy, generalized epilepsy with febrile seizures plus and severe myoclonic epilepsy of infancy (SMEI). We examined the functional properties of five SMEI mutations by using whole-cell patch-clamp analysis of heterologously expressed recombinant human SCN1A. Two mutations (F902C and G1674R) rendered SCN1A channels nonfunctional, and a third allele (G1749E) exhibited minimal functional alterations. However, two mutations within or near the S4 segment of the fourth repeat domain (R1648C and F1661S) conferred significant impairments in fast inactivation, including persistent, noninactivating channel activity resembling the pattern of channel dysfunction observed for alleles associated with generalized epilepsy with febrile seizures plus. Our data provide evidence for a range of SCN1A functional abnormalities in SMEI, including gain-of-function defects that were not anticipated in this disorder. Our results further indicate that a complex relationship exists between phenotype and aberrant sodium channel function in these inherited epilepsies.

Electrostatic model of S4 motion in voltage-gated ion channels

The S4 transmembrane domain of the family of voltage-gated ion channels is generally thought to be the voltage sensor, whose translocation by an applied electric field produces the gating current. Experiments on hSkMI Na(+) channels and both Shaker and EAG K(+) channels indicate which S4 residues cross the membrane-solution interface during activation gating. Using this structural information, we derive the steady-state properties of gating-charge transfer for wild-type and mutant Shaker K(+) channels. Assuming that the energetics of gating is dominated by electrostatic forces between S4 charges and countercharges on neighboring transmembrane domains, we calculate the total energy as a function of transmembrane displacement and twist of the S4 domain. The resulting electrostatic energy surface exhibits a series of deep energy minima, corresponding to the transition states of the gating process. The steady-state gating-charge distribution is then given by a Boltzmann distribution among the transition states. The resulting gating-charge distributions are compared to experimental results on wild-type and charge-neutralized mutants of the Shaker K(+) channel.

Molecular coupling between voltage sensor and pore opening in the Arabidopsis inward rectifier K+ channel KAT1

Animal and plant voltage-gated ion channels share a common architecture. They are made up of four subunits and the positive charges on helical S4 segments of the protein in animal K⁺ channels are the main voltage-sensing elements. The KAT1 channel cloned from Arabidopsis thaliana, despite its structural similarity to animal outward rectifier K⁺ channels is, however, an inward rectifier. Here we detected KAT1-gating currents due to the existence of an intrinsic voltage sensor in this channel. The measured gating currents evoked in response to hyperpolarizing voltage steps consist of a very fast (τ = 318 ± 34 μs at −180 mV) and a slower component (4.5 ± 0.5 ms at −180 mV) representing charge moved when most channels are closed. The observed gating currents precede in time the ionic currents and they are measurable at voltages (less than or equal to −60) at which the channel open probability is negligible (≈10⁻⁴). These two observations, together with the fact that there is a delay in the onset of the ionic currents, indicate that gating charge transits between several closed states before the KAT1 channel opens. To gain insight into the molecular mechanisms that give rise to the gating currents and lead to channel opening, we probed external accessibility of S4 domain residues to methanethiosulfonate-ethyltrimethylammonium (MTSET) in both closed and open cysteine-substituted KAT1 channels. The results demonstrate that the putative voltage–sensing charges of S4 move inward when the KAT1 channels open.

Voltage sensing in ion channels: a 50-year-old mystery resolved?

CONTEXT

Membrane-protein ion channels control electrical activity in the nervous system. Voltage-gated channels have four-fold symmetry, with a central pore domain surrounded by voltage-sensor regions. Each voltage-sensor region has a positively charged transmembrane helix, S4, which carries gating charges through the membrane on opening or closing. How S4 moves at gating is debated: either S4 moves in a helical screw or in a helical twist pattern. In both cases S4 is assumed to move inside the densely packed channel protein. The pore region was visualised when Roderick MacKinnon's group crystallised a bacterial K+ channel in 1998. The voltage-sensor region and the S4 movement are more difficult to visualise, because voltage-gated channels are harder to crystallise.

STARTING POINT

Recently, Yuoxing Jiang and colleagues in MacKinnon's group reported the first successful crystallisation and X-ray analysis of a voltage-gated channel (Nature 2003; 423: 33-41, 42-48), with unexpected results. S4 forms a hairpin loop with another helix, stretching out from the channel perimeter rather than being located inside the densely-packed protein core. This finding suggests a novel type of S4 movement at gating, a paddle sweeping through the lipid bilayer in response to voltage changes. WHERE NEXT? Jiang and colleagues' unexpected results are controversial. Several predictions of the model are incompatible with other experimental data. However, if the paddle configuration is real, it opens up a new field for pharmacological rethinking. Such a configuration may help to understand certain aspects of the action of lipid-soluble gating modifiers, such as local and general anaesthetics and antiarrythmic and antiepileptic agents.

Influence of permeant ions on voltage sensor function in the Kv2.1 potassium channel

We previously demonstrated that the outer vestibule of activated Kv2.1 potassium channels can be in one of two conformations, and that K⁺ occupancy of a specific selectivity filter site determines which conformation the outer vestibule is in. These different outer vestibule conformations result in different sensitivities to internal and external TEA, different inactivation rates, and different macroscopic conductances. The [K⁺]-dependent switch in outer vestibule conformation is also associated with a change in rate of channel activation. In this paper, we examined the mechanism by which changes in [K⁺] modulate the rate of channel activation. Elevation of symmetrical [K⁺] or [Rb⁺] from 0 to 3 mM doubled the rate of on-gating charge movement (Q_on), measured at 0 mV. Cs⁺ produced an identical effect, but required 40-fold higher concentrations. All three permeant ions occupied the selectivity filter over the 0.03–3 mM range, so simple occupancy of the selectivity filter was not sufficient to produce the change in Q_on. However, for each of these permeant ions, the speeding of Q_on occurred with the same concentration dependence as the switch between outer vestibule conformations. Neutralization of an amino acid (K356) in the outer vestibule, which abolishes the modulation of channel pharmacology and ionic currents by the K⁺-dependent reorientation of the outer vestibule, also abolished the K⁺-dependence of Q_on. Together, the data indicate that the K⁺-dependent reorientation in the outer vestibule was responsible for the change in Q_on. Moreover, similar [K⁺]-dependence and effects of mutagenesis indicate that the K⁺-dependent change in rate of Q_on can account for the modulation of ionic current activation rate. Simple kinetic analysis suggested that K⁺ reduced an energy barrier for voltage sensor movement. These results provide strong evidence for a direct functional interaction, which is modulated by permeant ions acting at the selectivity filter, between the outer vestibule of the Kv2.1 potassium channel and the voltage sensor.

Specificity of charge-carrying residues in the voltage sensor of potassium channels

Positively charged voltage sensors of sodium and potassium channels are driven outward through the membrane's electric field upon depolarization. This movement is coupled to channel opening. A recent model based on studies of the KvAP channel proposes that the positively charged voltage sensor, christened the “voltage-sensor paddle”, is a peripheral domain that shuttles its charged cargo through membrane lipid like a hydrophobic cation. We tested this idea by attaching charged adducts to cysteines introduced into the putative voltage-sensor paddle of Shaker potassium channels and measuring fractional changes in the total gating charge from gating currents. The only residues capable of translocating attached charges through the membrane-electric field are those that serve this function in the native channel. This remarkable specificity indicates that charge movement involves highly specialized interactions between the voltage sensor and other regions of the protein, a mechanism inconsistent with the paddle model.

Interaction between extracellular Hanatoxin and the resting conformation of the voltage-sensor paddle in Kv channels

In voltage-activated potassium (Kv) channels, basic residues in S4 enable the voltage-sensing domain to move in response to membrane depolarization and thereby trigger the activation gate to open. In the X-ray structure of the KvAP channel, the S4 helix is located near the intracellular boundary of the membrane where it forms a "voltage-sensor paddle" motif with the S3b helix. It has been proposed that the paddle is lipid-exposed and that it translocates through the membrane as it activates. We studied the interaction of externally applied Hanatoxin with the voltage-sensor paddle in Kv channels and show that the toxin binds tightly even at negative voltages where the paddle is resting and the channel is closed. Moreover, measurements of gating charge movement suggest that Hanatoxin interacts with and stabilizes the resting paddle. These findings point to an extracellular location for the resting conformation of the voltage-sensor paddle and constrain its transmembrane movements during activation.

Changes in local S4 environment provide a voltage-sensing mechanism for mammalian hyperpolarization-activated HCN channels

The positively charged S4 transmembrane segment of voltage-gated channels is thought to function as the voltage sensor by moving charge through the membrane electric field in response to depolarization. Here we studied S4 movements in the mammalian HCN pacemaker channels. Unlike most voltage-gated channel family members that are activated by depolarization, HCN channels are activated by hyperpolarization. We determined the reactivity of the charged sulfhydryl-modifying reagent, MTSET, with substituted cysteine (Cys) residues along the HCN1 S4 segment. Using an HCN1 channel engineered to be MTS resistant except for the chosen S4 Cys substitution, we determined the reactivity of 12 S4 residues to external or internal MTSET application in either the closed or open state of the channel. Cys substitutions in the NH₂-terminal half of S4 only reacted with external MTSET; the rates of reactivity were rapid, regardless of whether the channel was open or closed. In contrast, Cys substitutions in the COOH-terminal half of S4 selectively reacted with internal MTSET when the channel was open. In the open state, the boundary between externally and internally accessible residues was remarkably narrow (∼3 residues). This suggests that S4 lies in a water-filled gating canal with a very narrow barrier between the external and internal solutions, similar to depolarization-gated channels. However, the pattern of reactivity is incompatible with either classical gating models, which postulate a large translational or rotational movement of S4 within a gating canal, or with a recent model in which S4 forms a peripheral voltage-sensing paddle (with S3b) that moves within the lipid bilayer (the KvAP model). Rather, we suggest that voltage sensing is due to a rearrangement in transmembrane segments surrounding S4, leading to a collapse of an internal gating canal upon channel closure that alters the shape of the membrane field around a relatively static S4 segment.

The Orientation and Molecular Movement of a K+ Channel Voltage-Sensing Domain

Voltage-gated channels operate through the action of a voltage-sensing domain (membrane segments S1-S4) that controls the conformation of gates located in the pore domain (membrane segments S5-S6). Recent structural studies on the bacterial K(v)AP potassium channel have led to a new model of voltage sensing in which S4 lies in the lipid at the channel periphery and moves through the membrane as a unit with a portion of S3. Here we describe accessibility probing and disulfide scanning experiments aimed at determining how well the K(v)AP model describes the Drosophila Shaker potassium channel. We find that the S1-S3 helices have one end that is externally exposed, S3 does not undergo a transmembrane motion, and S4 lies in close apposition to the pore domain in the resting and activated state.

Altered gating and local anesthetic block mediated by residues in the I-S6 and II-S6 transmembrane segments of voltage-dependent Na+ channels

The cytoplasmic side of the voltage-dependent Na+ channel pore is putatively formed by the S6 segments of domains I to IV. The role of amino acid residues of I-S6 and II-S6 in channel gating and local anesthetic (LA) block was investigated using the cysteine scanning mutagenesis of the rat skeletal muscle Na+ channel (Nav1.4). G428C uniquely reduced sensitivity to rested state or first-pulse block by lidocaine without alterations in the voltage dependence or kinetics of gating that would otherwise account for the increase in the IC50 for block. Mutations in I-S6 (N434C and I436C) and in II-S6 (L785C and V787C) increased sensitivity to first-pulse block by lidocaine. Enhanced inactivation accounted for the increased sensitivity of N434C to lidocaine and hastening of inactivation of I436C in the absence of drug could account for higher affinity first-pulse block. Mutations in I-S6 (I424C, I425C, and F430C) and in II-S6 (I782C and V786C) reduced the use-dependent lidocaine block. The reduction in use-dependent block of F430C was consistent with alterations in inactivation gating; the other mutants did not exhibit gating changes that could explain the reduced sensitivity to lidocaine. Therefore, several amino acids (I424, I425, G428, I782, and V786), in addition to those previously identified (Yarov-Yarovoy et al., 2002), alter the sensitivity of Nav1.4 to lidocaine, independent of mutation-induced changes in gating. The magnitude of the change in the IC50 values, the isoform, and LA dependence of the changes in affinity suggest that the determinants of binding in I-S6 and II-S6 are subsidiary to those in IV-S6.

Molecular motions of the outer ring of charge of the sodium channel: do they couple to slow inactivation?

In contrast to fast inactivation, the molecular basis of sodium (Na) channel slow inactivation is poorly understood. It has been suggested that structural rearrangements in the outer pore mediate slow inactivation of Na channels similar to C-type inactivation in potassium (K) channels. We probed the role of the outer ring of charge in inactivation gating by paired cysteine mutagenesis in the rat skeletal muscle Na channel (rNav1.4). The outer charged ring residues were substituted with cysteine, paired with cysteine mutants at other positions in the external pore, and coexpressed with rat brain beta1 in Xenopus oocytes. Dithiolthreitol (DTT) markedly increased the current in E403C+E758C double mutant, indicating the spontaneous formation of a disulfide bond and proximity of the alpha carbons of these residues of no more than 7 A. The redox catalyst Cu(II) (1,10-phenanthroline)3 (Cu(phe)3) reduced the peak current of double mutants (E403C+E758C, E403C+D1241C, E403C+D1532C, and D1241C+D1532C) at a rate proportional to the stimulation frequency. Voltage protocols that favored occupancy of slow inactivation states completely prevented Cu(phe)3 modification of outer charged ring paired mutants E403C+E758C, E403C+D1241C, and E403C+D1532C. In contrast, voltage protocols that favored slow inactivation did not prevent Cu(phe)3 modification of other double mutants such as E403C+W756C, E403C+W1239C, and E403C+W1531C. Our data suggest that slow inactivation of the Na channel is associated with a structural rearrangement of the outer ring of charge.

The position of the fourth segment of domain 4 determines status of the inactivation gate in Na+ channels

The fourth segment of domain 4 (S4/D4) in Na+ channels is a voltage sensor especially implicated in channel inactivation. Although there has been evidence that S4/D4 moves externally during membrane depolarization, whether (and if so, how) the movement leads to conformational changes of the inactivation gate remains unknown. We added a positive charge just external to the outermost charged residue in S4/D4 by point mutations of residue F1625 (i.e., F1625R and F1625K). The inactivation curves as well as the kinetics of recovery from inactivation in these mutant channels are split into two components, one happening with and the other without channel activation/deactivation. This is as if the "extra" positive charge induces new intermediate positions of S4/D4 and consequently new intermediate inactivation states uncoupled from channel activation/deactivation. The qualitatively similar but quantitatively very different findings between the F1625R and F1625K mutants further suggest a significantly different effect on the inactivation gate by a slight difference in the localization of the positive charge. On the other hand, neutral mutations of residue F1625 do not induce new inactivation states but shift the voltage dependence of different inactivation parameters in the voltage axis, as if only the relative tendency of S4/D4 to stay in its original outermost and innermost positions is altered. We conclude that S4/D4 movement not only decisively but also delicately controls the inactivation gate. Electrostatic interaction between the top charges of S4/D4 and the corresponding countercharges may play an essential role in the determination of S4/D4 position and therefore the inactivation status of the Na+ channel.

Voltage-gated K channels

Ion channels and the electrical properties they confer on cells are involved in every human characteristic that distinguishes us from the stones in a field. Every perception, thought, movement, and heartbeat depends on electrical signals generated by the activity of ion channels. Early views of the relationship between channel structure and function have undergone substantial modification following the cloning of various ion channels and the determination of the structure of a simple bacterial K channel, the KcsA channel. This review focuses on the relationship between the structure and function of voltage-dependent K channels, covering the molecular bases of channel selectivity, conduction, and gating. The evolution of ion channels in bacteria is discussed, as well as the basis of channel selectivity and conduction in the KcsA channel. More complex channels have evolved molecular "gatekeepers," allowing them to respond to appropriate stimuli by opening, closing, and inactivating.

Point mutations at L1280 in Nav1.4 channel D3-S6 modulate binding affinity and stereoselectivity of bupivacaine enantiomers

Local anesthetics (LAs) block voltage-gated sodium channels. Parts of the LA binding site are located in the pore-lining transmembrane segments 6 of domains 1, 3, and 4 (D1-S6, D3-S6, D4-S6). We suggested previously that residue N434 in D1-S6 interacts directly with bupivacaine enantiomers in inactivated channels because side-chain properties of different residues substituted at N434 correlated with changes in blocking potencies of bupivacaine enantiomers. Furthermore, mutation N434R exhibited significant stereoselectivity for block of inactivated channels that resulted from a selective decrease in block by S(-)-bupivacaine. In the present study, we analyzed the role of residue L1280 in D3-S6 of the rat skeletal muscle Nav1.4 channel in interactions with the enantiomers of bupivacaine. We substituted native leucine at L1280 with amino acids of different physicochemical properties. Wild-type and mutant channels were expressed transiently in human embryonic kidney 293t cells and were investigated under whole-cell voltage clamp. Block of resting mutant channels by bupivacaine enantiomers revealed little difference compared with wild-type channels. Block of inactivated channels was increased in a mutation containing an aromatic group (L1280W) and decreased in mutations containing a positive charge (L1280K, L1280R). Surprisingly, mutants L1280E, L1280N, L1280Q, and L1280R exhibited significant stereoselectivity for block of inactivated channels. More surprisingly, stereoselectivity resulted from a selective decrease in block by R(+)-bupivacaine, in contrast to mutation N434R in D1-S6. We propose that in inactivated channels, residues L1280 in D3-S6 and N434 in D1-S6 interact directly with LAs and thereby face each other in the ion-conducting pore.

A physical model of potassium channel activation: from energy landscape to gating kinetics

We have developed a method for rapidly computing gating currents from a multiparticle ion channel model. Our approach is appropriate for energy landscapes that can be characterized by a network of well-defined activation pathways with barriers. To illustrate, we represented the gating apparatus of a channel subunit by an interacting pair of charged gating particles. Each particle underwent spatial diffusion along a bistable potential of mean force, with electrostatic forces coupling the two trajectories. After a step in membrane potential, relaxation of the smaller barrier charge led to a time-dependent reduction in the activation barrier of the principal gate charge. The resulting gating current exhibited a rising phase similar to that measured in voltage-dependent ion channels. Reduction of the two-dimensional diffusion landscape to a circular Markov model with four states accurately preserved the time course of gating currents on the slow timescale. A composite system containing four subunits leading to a concerted opening transition was used to fit a series of gating currents from the Shaker potassium channel. We end with a critique of the model with regard to current views on potassium channel structure.