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

Effect of a sensing charge mutation on the deactivation of KV7.2 channels

Potassium-selective, voltage-gated channels of the KV7 family are critical regulators of electrical excitability in many cell types. Removing the outermost putative sensing charge (R198) of the human KV7.2 shifts its activation voltage dependence toward more negative potentials. This suggests that removing a charge "at the top" of the fourth (S4) segment of the voltage-sensing domain facilitates activation. Here, we hypothesized that restoring that charge would bring back the activation to its normal voltage range. We introduced the mutation R198H in KV7.2 with the idea that titrating the introduced histidine with protons would reinstate the sensing charge. As predicted, the mutant's activation voltage dependence changed as a function of the external pH (pHEXT) while modest changes in the activation voltage dependence were observed with the wild-type (WT) channel. On the other hand, the deactivation kinetics of the R198H mutant was remarkably sensitive to pHEXT changes, readily deactivating at pHEXT 6, while becoming slower to deactivate at pHEXT 8. In contrast, the KV7.2 WT displayed modest changes in the deactivation kinetics as a function of pHEXT. This suggested that the charge of residue 198 was critical for deactivation. However, in a surprising turn, the mutant R198Q-a non-titratable mutation-also displayed a high pHEXT sensitivity activity. We thus concluded that rather than the charge at position 198, the protonation status of the channel's extracellular face modulates the open channel stabilization and that the charge of residue 198 is required for the voltage sensor to effectively deactivate the channel, overcoming the stabilizing effect of high pHEXT.

Exploring Flexibility and Folding Patterns Throughout Time in Voltage Sensors

The voltage-sensing domain (VSD) is a module capable of responding to changes in the membrane potential through conformational changes and facilitating electromechanical coupling to open a pore gate, activate proton permeation pathways, or promote enzymatic activity in some membrane-anchored phosphatases. To carry out these functions, this module acts cooperatively through conformational changes. The VSD is formed by four transmembrane segments (S1-S4) but the S4 segment is critical since it carries positively charged residues, mainly Arg or Lys, which require an aqueous environment for its proper function. The discovery of this module in voltage-gated ion channels (VGICs), proton channels (Hv1), and voltage sensor-containing phosphatases (VSPs) has expanded our understanding of the principle of modularity in the voltage-sensing mechanism of these proteins. Here, by sequence comparison and the evaluation of the relationship between sequence composition, intrinsic flexibility, and structural analysis in 14 selected representatives of these three major protein groups, we report five interesting differences in the folding patterns of the VSD both in prokaryotes and eukaryotes. Our main findings indicate that this module is highly conserved throughout the evolutionary scale, however: (1) segments S1 to S3 in eukaryotes are significantly more hydrophobic than those present in prokaryotes; (2) the S4 segment has retained its hydrophilic character; (3) in eukaryotes the extramembranous linkers are significantly larger and more flexible in comparison with those present in prokaryotes; (4) the sensors present in the kHv1 proton channel and the ciVSP phosphatase, both of eukaryotic origin, exhibit relationships of flexibility and folding patterns very close to the typical ones found in prokaryotic voltage sensors; and (5) archaeal channels KvAP and MVP have flexibility profiles which are clearly contrasting in the S3-S4 region, which could explain their divergent activation mechanisms. Finally, to elucidate the obscure origins of this module, we show further evidence for a possible connection between voltage sensors and TolQ proteins.

Sweetening K-channels: what sugar taught us about permeation and gating

Because they enable for the modification of both viscosity and osmolarity, sugars have been used as a biophysical probe of voltage-gated K-channels for a while. Viscosity variations made it possible to measure the pore sizes in large and small conductance K-channels using techniques similar to those used in the 1980s to study the gramicidin A channel. These analyses led to the finding that the size of the internal mouth appears to be the primary cause of the conductance differences between Shaker-like channels and large conductance BK-channels. As an osmotic agent, adding sugar unilaterally causes streaming potentials that indicate H₂O/K⁺ cotransport across the BK-channel pore. Osmotic experiments on Shaker K-channels suggest that the pore gate operation and the slow inactivation displace comparable amounts of water. Functionally isolated voltage sensors allow estimation of individual osmotic work for each voltage sensing charge during voltage-activation, reporting dramatic internal and external remodeling of the Voltage Sensing Domain´s solvent exposed surfaces. Remarkably, each charge of the VSD appears to take a unique trajectory. Thus, manipulation of viscosity and osmolarity, together with 3D structures, brings in solid grounds to harmonize function and structure in membrane proteins such as K-channels and, in a wider scope, other structurally dynamic proteins.

Tracking the movement of discrete gating charges in a voltage-gated potassium channel

Positively charged amino acids respond to membrane potential changes to drive voltage sensor movement in voltage-gated ion channels, but determining the displacements of voltage sensor gating charges has proven difficult. We optically tracked the movement of the two most extracellular charged residues (R1 and R2) in the Shaker potassium channel voltage sensor using a fluorescent positively charged bimane derivative (qBBr) that is strongly quenched by tryptophan. By individually mutating residues to tryptophan within the putative pathway of gating charges, we observed that the charge motion during activation is a rotation and a tilted translation that differs between R1 and R2. Tryptophan-induced quenching of qBBr also indicates that a crucial residue of the hydrophobic plug is linked to the Cole-Moore shift through its interaction with R1. Finally, we show that this approach extends to additional voltage-sensing membrane proteins using the Ciona intestinalis voltage-sensitive phosphatase (CiVSP).

Comparison of K+ Channel Families

K⁺ channels enable potassium to flow across the membrane with great selectivity. There are four K⁺ channel families: voltage-gated K (K_v), calcium-activated (K_Ca), inwardly rectifying K (K_ir), and two-pore domain potassium (K_2P) channels. All four K⁺ channels are formed by subunits assembling into a classic tetrameric (4x1P = 4P for the K_v, K_Ca, and K_ir channels) or tetramer-like (2x2P = 4P for the K_2P channels) architecture. These subunits can either be the same (homomers) or different (heteromers), conferring great diversity to these channels. They share a highly conserved selectivity filter within the pore but show different gating mechanisms adapted for their function. K⁺ channels play essential roles in controlling neuronal excitability by shaping action potentials, influencing the resting membrane potential, and responding to diverse physicochemical stimuli, such as a voltage change (K_v), intracellular calcium oscillations (K_Ca), cellular mediators (K_ir), or temperature (K_2P).

Effect of charge status on the ion transport and antimicrobial activity of synthetic channels

The charge status of channels formed by pillararene–gramicidin hybrid molecules has a significant impact on their trans-membrane transport properties, membrane-association abilities and antimicrobial activities.

Cholesterol-Dependent Gating Effects on Ion Channels

Biomembranes separate a live cell from its environment and keep it in an off-equilibrium, steady state. They contain both phospholipids and nonphospholipids, depending on whether there are phosphate groups in the headgroup regions. Cholesterol (CHOL) is one type of nonphospholipids, and one of the most abundant lipid molecules in humans. Its content in plasma membranes and intracellular membranes varies and is tightly regulated. Voltage-gated ion channels are universally present in every cell and are fairly diversified in the eukaryotic domain of life. Our lipid-dependent gating hypothesis postulates that the controlled switch of the voltage-sensor domains (VSDs) in a voltage-gated potassium (Kv) channel between the "down" and the "up" state (gating) is sensitive to the ratio of phospholipids:nonphospholipids in the annular layer around the channel. High CHOL content is found to exert strong inhibitory effects on Kv channels. Such effects have been observed in in vitro membranes, cultured cells, and animal models for cholesterol metabolic defects. Thermodynamic analysis of the CHOL-dependent gating suggests that the inhibitory effects of CHOL result from collective interactions between annular CHOL molecules and the channel, which appear to be a more generic principle behind the CHOL effects on other ion channels and transporters. We will review the recent progress in the CHOL-dependent gating of voltage-gated ion channels, discuss the current technical limitations, and then expand briefly the learned principles to other ion channels that are known to be sensitive to the CHOL-channel interactions.

Gating-induced large aqueous volumetric remodeling and aspartate tolerance in the voltage sensor domain of Shaker K+ channels

Neurons encode electrical signals with critically tuned voltage-gated ion channels and enzymes. Dedicated voltage sensor domains (VSDs) in these membrane proteins activate coordinately with an unresolved structural change. Such change conveys the transmembrane translocation of four positively charged arginine side chains, the voltage-sensing residues (VSRs; R1-R4). Countercharges and lipid phosphohead groups likely stabilize these VSRs within the low-dielectric core of the protein. However, the role of hydration, a sign-independent charge stabilizer, remains unclear. We replaced all VSRs and their neighboring residues with negatively charged aspartates in a voltage-gated potassium channel. The ensuing mild functional effects indicate that hydration is also important in VSR stabilization. The voltage dependency of the VSR aspartate variants approached the expected arithmetic summation of charges at VSR positions, as if negative and positive side chains faced similar pathways. In contrast, aspartates introduced between R2 and R3 did not affect voltage dependence as if the side chains moved outside the electric field or together with it, undergoing a large displacement and volumetric remodeling. Accordingly, VSR performed osmotic work at both internal and external aqueous interfaces. Individual VSR contributions to volumetric works approached arithmetical additivity but were largely dissimilar. While R1 and R4 displaced small volumes, R2 and R3 volumetric works were massive and vectorially opposed, favoring large aqueous remodeling during VSD activation. These diverse volumetric works are, at least for R2 and R3, not compatible with VSR translocation across a unique stationary charge transfer center. Instead, VSRs may follow separated pathways across a fluctuating low-dielectric septum.

Pore size matters for potassium channel conductance

Ion channels are membrane proteins that mediate efficient ion transport across the hydrophobic core of cell membranes, an unlikely process in their absence. K⁺ channels discriminate K⁺ over cations with similar radii with extraordinary selectivity and display a wide diversity of ion transport rates, covering differences of two orders of magnitude in unitary conductance. The pore domains of large- and small-conductance K⁺ channels share a general architectural design comprising a conserved narrow selectivity filter, which forms intimate interactions with permeant ions, flanked by two wider vestibules toward the internal and external openings. In large-conductance K⁺ channels, the inner vestibule is wide, whereas in small-conductance channels it is narrow. Here we raise the idea that the physical dimensions of the hydrophobic internal vestibule limit ion transport in K⁺ channels, accounting for their diversity in unitary conductance.

Functional diversity of potassium channel voltage-sensing domains

Voltage-gated potassium channels or Kv's are membrane proteins with fundamental physiological roles. They are composed of 2 main functional protein domains, the pore domain, which regulates ion permeation, and the voltage-sensing domain, which is in charge of sensing voltage and undergoing a conformational change that is later transduced into pore opening. The voltage-sensing domain or VSD is a highly conserved structural motif found in all voltage-gated ion channels and can also exist as an independent feature, giving rise to voltage sensitive enzymes and also sustaining proton fluxes in proton-permeable channels. In spite of the structural conservation of VSDs in potassium channels, there are several differences in the details of VSD function found across variants of Kvs. These differences are mainly reflected in variations in the electrostatic energy needed to open different potassium channels. In turn, the differences in detailed VSD functioning among voltage-gated potassium channels might have physiological consequences that have not been explored and which might reflect evolutionary adaptations to the different roles played by Kv channels in cell physiology.

A self-consistent approach for determining pairwise interactions that underlie channel activation

Net free-energy measurements can be combined with mutant cycle analysis to determine interaction energies between specific amino acid pairs during channel activation.

Signaling proteins such as ion channels largely exist in two functional forms, corresponding to the active and resting states, connected by multiple intermediates. Multiparametric kinetic models based on sophisticated electrophysiological experiments have been devised to identify molecular interactions of these conformational transitions. However, this approach is arduous and is not suitable for large-scale perturbation analysis of interaction pathways. Recently, we described a model-free method to obtain the net free energy of activation in voltage- and ligand-activated ion channels. Here we extend this approach to estimate pairwise interaction energies of side chains that contribute to gating transitions. Our approach, which we call generalized interaction-energy analysis (GIA), combines median voltage estimates obtained from charge-voltage curves with mutant cycle analysis to ascertain the strengths of pairwise interactions. We show that, for a system with an arbitrary gating scheme, the nonadditive contributions of amino acid pairs to the net free energy of activation can be computed in a self-consistent manner. Numerical analyses of sequential and allosteric models of channel activation also show that this approach can measure energetic nonadditivities even when perturbations affect multiple transitions. To demonstrate the experimental application of this method, we reevaluated the interaction energies of six previously described long-range interactors in the Shaker potassium channel. Our approach offers the ability to generate detailed interaction energy maps in voltage- and ligand-activated ion channels and can be extended to any force-driven system as long as associated “displacement” can be measured.

Interfacial gating triad is crucial for electromechanical transduction in voltage-activated potassium channels

Gating interaction analysis reveals a cluster of three conserved amino acids that couple structural transitions in the potassium channel voltage sensor to those in the pore.

Voltage-dependent potassium channels play a crucial role in electrical excitability and cellular signaling by regulating potassium ion flux across membranes. Movement of charged residues in the voltage-sensing domain leads to a series of conformational changes that culminate in channel opening in response to changes in membrane potential. However, the molecular machinery that relays these conformational changes from voltage sensor to the pore is not well understood. Here we use generalized interaction-energy analysis (GIA) to estimate the strength of site-specific interactions between amino acid residues putatively involved in the electromechanical coupling of the voltage sensor and pore in the outwardly rectifying K_V channel. We identified candidate interactors at the interface between the S4–S5 linker and the pore domain using a structure-guided graph theoretical approach that revealed clusters of conserved and closely packed residues. One such cluster, located at the intracellular intersubunit interface, comprises three residues (arginine 394, glutamate 395, and tyrosine 485) that interact with each other. The calculated interaction energies were 3–5 kcal, which is especially notable given that the net free-energy change during activation of the Shaker K_V channel is ∼14 kcal. We find that this triad is delicately maintained by balance of interactions that are responsible for structural integrity of the intersubunit interface while maintaining sufficient flexibility at a critical gating hinge for optimal transmission of force to the pore gate.

Mechanisms of activation of voltage-gated potassium channels

Voltage-gated potassium ion channels (Kv) play an important role in a variety of cellular processes, including the functioning of excitable cells, regulation of apoptosis, cell growth and differentiation, the release of neurotransmitters and hormones, maintenance of cardiac activity, etc. Failure in the functioning of Kv channels leads to severe genetic disorders and the development of tumors, including malignant ones. Understanding the mechanisms underlying Kv channels functioning is a key factor in determining the cause of the diseases associated with mutations in the channels, and in the search for new drugs. The mechanism of activation of the channels is a topic of ongoing debate, and a consensus on the issue has not yet been reached. This review discusses the key stages in studying the mechanisms of functioning of Kv channels and describes the basic models of their activation known to date.

K(+) channels: function-structural overview

Potassium channels are particularly important in determining the shape and duration of the action potential, controlling the membrane potential, modulating hormone secretion, epithelial function and, in the case of those K(+) channels activated by Ca(2+), damping excitatory signals. The multiplicity of roles played by K(+) channels is only possible to their mammoth diversity that includes at present 70 K(+) channels encoding genes in mammals. Today, thanks to the use of cloning, mutagenesis, and the more recent structural studies using x-ray crystallography, we are in a unique position to understand the origins of the enormous diversity of this superfamily of ion channels, the roles they play in different cell types, and the relations that exist between structure and function. With the exception of two-pore K(+) channels that are dimers, voltage-dependent K(+) channels are tetrameric assemblies and share an extremely well conserved pore region, in which the ion-selectivity filter resides. In the present overview, we discuss in the function, localization, and the relations between function and structure of the five different subfamilies of K(+) channels: (a) inward rectifiers, Kir; (b) four transmembrane segments-2 pores, K2P; (c) voltage-gated, Kv; (d) the Slo family; and (e) Ca(2+)-activated SK family, SKCa.

Exploring structure-function relationships between TRP and Kv channels

The molecular mechanisms underlying the activation of Transient Receptor Potential (TRP) ion channels are poorly understood when compared to those of the voltage-activated potassium (Kv) channels. The architectural and pharmacological similarities between the members of these two families of channels suggest that their structure-function relationships may have common features. We explored this hypothesis by replacing previously identified domains and critical structural motifs of the membrane-spanning portions of Kv2.1 with corresponding regions of two TRP channels, TRPM8 and TRPV1. Our results show that the S3b-S4 paddle motif of Kv2.1, but not other domains, can be replaced by the analogous regions of both TRP channels without abolishing voltage-activation. In contrast, replacement of portions of TRP channels with those of Kv2.1 consistently yielded non-functional channels. Taken together, these results suggest that most structural elements within TRP channels and Kv channels are not sufficiently related to allow for the creation of hybrid channels.

The gating charge pathway of an epilepsy-associated potassium channel accommodates chemical ligands

Voltage-gated potassium (Kv) channels derive their voltage sensitivity from movement of gating charges in voltage-sensor domains (VSDs). The gating charges translocate through a physical pathway in the VSD to open or close the channel. Previous studies showed that the gating charge pathways of Shaker and Kv1.2-2.1 chimeric channels are occluded, forming the structural basis for the focused electric field and gating charge transfer center. Here, we show that the gating charge pathway of the voltage-gated KCNQ2 potassium channel, activity reduction of which causes epilepsy, can accommodate various small molecule ligands. Combining mutagenesis, molecular simulation and electrophysiological recording, a binding model for the probe activator, ztz240, in the gating charge pathway was defined. This information was used to establish a docking-based virtual screening assay targeting the defined ligand-binding pocket. Nine activators with five new chemotypes were identified, and in vivo experiments showed that three ligands binding to the gating charge pathway exhibit significant anti-epilepsy activity. Identification of various novel activators by virtual screening targeting the pocket supports the presence of a ligand-binding site in the gating charge pathway. The capability of the gating charge pathway to accommodate small molecule ligands offers new insights into the gating charge pathway of the therapeutically relevant KCNQ2 channel.

Leaky channels make weak muscles

Mutations in the skeletal muscle voltage-gated calcium channel (CaV1.1) have been associated with hypokalemic periodic paralysis, but how the pathogenesis of this disorder relates to the functional consequences of mutations was unclear. In this issue of the JCI, Wu and colleagues recapitulate the disease by generating a novel knock-in CaV1.1 mutant mouse and use this model to investigate the cellular and molecular features of pathogenesis. They demonstrated an aberrant muscle cell current conducted through the CaV1.1 voltage-sensor domain (gating pore current) that explains an abnormally depolarized muscle membrane and the failure of muscle action potential firing during challenge with agents known to provoke periodic paralysis. Their work advances understanding of molecular and cellular mechanisms underlying an inherited channelopathy.

Microscopic origin of gating current fluctuations in a potassium channel voltage sensor

Voltage-dependent ion channels open and close in response to changes in membrane electrical potential due to the motion of their voltage-sensing domains (VSDs). VSD charge displacements within the membrane electric field are observed in electrophysiology experiments as gating currents preceding ionic conduction. The elementary charge motions that give rise to the gating current cannot be observed directly, but appear as discrete current pulses that generate fluctuations in gating current measurements. Here we report direct observation of gating-charge displacements in an atomistic molecular dynamics simulation of the isolated VSD from the KvAP channel in a hydrated lipid bilayer on the timescale (10-μs) expected for elementary gating charge transitions. The results reveal that gating-charge displacements are associated with the water-catalyzed rearrangement of salt bridges between the S4 arginines and a set of conserved acidic side chains on the S1-S3 transmembrane segments in the hydrated interior of the VSD.

In search of a consensus model of the resting state of a voltage-sensing domain

Voltage-sensing domains (VSDs) undergo conformational changes in response to the membrane potential and are the critical structural modules responsible for the activation of voltage-gated channels. Structural information about the key conformational states underlying voltage activation is currently incomplete. Through the use of experimentally determined residue-residue interactions as structural constraints, we determine and refine a model of the Kv channel VSD in the resting conformation. The resulting structural model is in broad agreement with results that originate from various labs using different techniques, indicating the emergence of a consensus for the structural basis of voltage sensing.

Lipid-dependent gating of a voltage-gated potassium channel

Recent studies hypothesized that phospholipids stabilize two voltage-sensing arginine residues of certain voltage-gated potassium channels in activated conformations. It remains unclear how lipids directly affect these channels. Here, by examining the conformations of the KvAP in different lipids, we showed that without voltage change, the voltage-sensor domains switched from the activated to the resting state when their surrounding lipids were changed from phospholipids to nonphospholipids. Such lipid-determined conformational change was coupled to the ion-conducting pore, suggesting that parallel to voltage gating, the channel is gated by its annular lipids. Our measurements recognized that the energetic cost of lipid-dependent gating approaches that of voltage gating, but kinetically it appears much slower. Our data support that a channel and its surrounding lipids together constitute a functional unit, and natural nonphospholipids such as cholesterol should exert strong effects on voltage-gated channels. Our first observation of lipid-dependent gating may have general implications to other membrane proteins.

Lipid phosphodiesters affect the conformation of certain potassium channels, but the details of the lipid-channel interactions are unclear. Here, the KvAP channel is found to switch from an active to a resting state when the channels are transferred from a phospholipid membrane to a bilayer lacking phosphodiesters.

Contributions of counter-charge in a potassium channel voltage-sensor domain

Voltage-sensor domains couple membrane potential to conformational changes in voltage-gated ion channels and phosphatases. Highly co-evolved acidic and aromatic side-chains assist the transfer of cationic side-chains across the transmembrane electric field during voltage-sensing. We investigated the functional contribution of negative electrostatic potentials from these residues to channel gating and voltage-sensing with unnatural amino acid mutagenesis, electrophysiology, voltage-clamp fluorometry and ab initio calculations. The data show that neutralization of two conserved acidic side-chains in transmembrane segments S2 and S3, Glu293 and Asp316 in Shaker potassium channels, have little functional effect on conductance-voltage relationships, although Glu293 appears to catalyze S4 movement. Our results suggest that neither Glu293 nor Asp316 engages in electrostatic state-dependent charge-charge interactions with S4, likely because they occupy, and possibly help create, a water-filled vestibule.

Independent and cooperative motions of the Kv1.2 channel: voltage sensing and gating

Voltage-gated potassium (Kv) channels, such as Kv1.2, are involved in the generation and propagation of action potentials. The Kv channel is a homotetramer, and each monomer is composed of a voltage-sensing domain (VSD) and a pore domain (PD). We analyzed the fluctuations of a model structure of Kv1.2 using elastic network models. The analysis suggested a network of coupled fluctuations of eight rigid structural units and seven hinges that may control the transition between the active and inactive states of the channel. For the most part, the network is composed of amino acids that are known to affect channel activity. The results suggested allosteric interactions and cooperativity between the subunits in the coupling between the motion of the VSD and the selectivity filter of the PD, in accordance with recent empirical data. There are no direct contacts between the VSDs of the four subunits, and the contacts between these and the PDs are loose, suggesting that the VSDs are capable of functioning independently. Indeed, they manifest many inherent fluctuations that are decoupled from the rest of the structure. In general, the analysis suggests that the two domains contribute to the channel function both individually and cooperatively.

A shaker K+ channel with a miniature engineered voltage sensor

Voltage-gated ion channels sense transmembrane voltage changes via a paddle-shaped motif that includes the C-terminal part of the third transmembrane segment (S3b) and the N-terminal part of the fourth segment ((NT)S4) that harbors voltage-sensing arginines. Here, we find that residue triplets in S3b and (NT)S4 can be deleted individually, or even in some combinations, without compromising the channels' basic voltage-gating capability. Thus, a high degree of complementarity between these S3b and (NT)S4 regions is not required for basic voltage gating per se. Remarkably, the voltage-gated Shaker K(+) channel remains voltage gated after a 43 residue paddle sequence is replaced by a glycine triplet. Therefore, the paddle motif comprises a minimal core that suffices to confer voltage gating in the physiological voltage range, and a larger, modulatory part. Our study also shows that the hydrophobic residues between the voltage-sensing arginines help set the sensor's characteristic chemical equilibrium between activated and deactivated states.

Structure of the full-length Shaker potassium channel Kv1.2 by normal-mode-based X-ray crystallographic refinement

Voltage-dependent potassium channels (Kv) are homotetramers composed of four voltage sensors and one pore domain. Because of high-level structural flexibility, the first mammalian Kv structure, Kv1.2 at 2.9 A, has about 37% molecular mass of the transmembrane portion not resolved. In this study, by applying a novel normal-mode-based X-ray crystallographic refinement method to the original diffraction data and structural model, we established the structure of full-length Kv1.2 in its native form. This structure offers mechanistic insights into voltage sensing. Particularly, it shows a hydrophobic layer of about 10 A at the midpoint of the membrane bilayer, which is likely the molecular basis for the observed "focused electric field" of Kv1.2 between the internal and external solutions. This work also demonstrated the potential of the refinement method in bringing up large chunks of missing densities, thus beneficial to structural refinement of many difficult systems.

Reduced voltage sensitivity in a K+-channel voltage sensor by electric field remodeling

Propagation of the nerve impulse relies on the extreme voltage sensitivity of Na(+) and K(+) channels. The transmembrane movement of four arginine residues, located at the fourth transmembrane segment (S4), in each of their four voltage-sensing domains is mostly responsible for the translocation of 12 to 13 e(o) across the transmembrane electric field. Inserting additional positively charged residues between the voltage-sensing arginines in S4 would, in principle, increase voltage sensitivity. Here we show that either positively or negatively charged residues added between the two most external sensing arginines of S4 decreased voltage sensitivity of a Shaker voltage-gated K(+)-channel by up to approximately 50%. The replacement of Val363 with a charged residue displaced inwardly the external boundaries of the electric field by at least 6 A, leaving the most external arginine of S4 constitutively exposed to the extracellular space and permanently excluded from the electric field. Both the physical trajectory of S4 and its electromechanical coupling to open the pore gate seemed unchanged. We propose that the separation between the first two sensing charges at resting is comparable to the thickness of the low dielectric transmembrane barrier they must cross. Thus, at most a single sensing arginine side chain could be found within the field. The conserved hydrophobic nature of the residues located between the voltage-sensing arginines in S4 may shape the electric field geometry for optimal voltage sensitivity in voltage-gated ion channels.

Targeting voltage sensors in sodium channels with spider toxins

Voltage-activated sodium (Nav) channels are essential in generating and propagating nerve impulses, placing them amongst the most widely targeted ion channels by toxins from venomous organisms. An increasing number of spider toxins have been shown to interfere with the voltage-driven activation process of mammalian Nav channels, possibly by interacting with one or more of their voltage sensors. This review focuses on our existing knowledge of the mechanism by which spider toxins affect Nav channel gating and the possible applications of these toxins in the drug discovery process.

Potassium channel opening: a subtle two-step

Voltage-gated K(+) channels undergo a voltage-dependent conductance change that plays a key role in modulating cellular excitability. While the Open state is captured in crystal structures of Kv1.2 and a chimeric Kv1.2/Kv2.1 channel, the Close state and the mechanism of this transition are still a subject of debate. Here, we propose a model based on mutagenesis combined with measurements of both ionic and gating currents which is consistent with the idea that the Open state is the default state, the energy of the electric field being used to keep the channel closed. Our model incorporates an 'Activated state' where the bulk of sensor movement is completed without channel opening. The model accounts for the well characterized electrophysiology of the 'V2' and 'ILT' mutations in Shaker, where sensor movement and channel opening occur over distinct voltage ranges. Moreover, the model proposes relatively small protein rearrangements in going from the Activated to the Open state, consistent with the rapid transitions observed in single channel records of Shaker type channels at zero millivolts.

Probing S4 and S5 segment proximity in mammalian hyperpolarization-activated HCN channels by disulfide bridging and Cd2+ coordination

We explored the structural basis of voltage sensing in the HCN1 hyperpolarization-activated cyclic nucleotide-gated cation channel by examining the relative orientation of the voltage sensor and pore domains. The opening of channels engineered to contain single cysteine residues at the extracellular ends of the voltage-sensing S4 (V246C) and pore-forming S5 (C303) domains is inhibited by formation of disulfide or cysteine:Cd(2+) bonds. As Cd(2+) coordination is promoted by depolarization, the S4-S5 interaction occurs preferentially in the closed state. The failure of oxidation to catalyze dimer formation, as assayed by Western blotting, indicates the V246C:C303 interaction occurs within a subunit. Intriguingly, a similar interaction has been observed in depolarization-activated Shaker voltage-dependent potassium (Kv) channels at depolarized potentials but such an intrasubunit interaction is inconsistent with the X-ray crystal structure of Kv1.2, wherein S4 approaches S5 of an adjacent subunit. These findings suggest channels of opposite voltage-sensing polarity adopt a conserved S4-S5 orientation in the depolarized state that is distinct from that trapped upon crystallization.

Effect of voltage sensitive fluorescent proteins on neuronal excitability

Fluorescent protein voltage sensors are recombinant proteins that are designed as genetically encoded cellular probes of membrane potential using mechanisms of voltage-dependent modulation of fluorescence. Several such proteins, including VSFP2.3 and VSFP3.1, were recently reported with reliable function in mammalian cells. They were designed as molecular fusions of the voltage sensor of Ciona intestinalis voltage sensor containing phosphatase with a fluorescence reporter domain. Expression of these proteins in cell membranes is accompanied by additional dynamic membrane capacitance, or "sensing capacitance", with feedback effect on the native electro-responsiveness of targeted cells. We used recordings of sensing currents and fluorescence responses of VSFP2.3 and of VSFP3.1 to derive kinetic models of the voltage-dependent signaling of these proteins. Using computational neuron simulations, we quantitatively investigated the perturbing effects of sensing capacitance on the input/output relationship in two central neuron models, a cerebellar Purkinje and a layer 5 pyramidal neuron. Probe-induced sensing capacitance manifested as time shifts of action potentials and increased synaptic input thresholds for somatic action potential initiation with linear dependence on the membrane density of the probe. Whereas the fluorescence signal/noise grows with the square root of the surface density of the probe, the growth of sensing capacitance is linear. We analyzed the trade-off between minimization of sensing capacitance and signal/noise of the optical read-out depending on kinetic properties and cellular distribution of the probe. The simulation results suggest ways to reduce capacitive effects at a given level of signal/noise. Yet, the simulations indicate that significant improvement of existing probes will still be required to report action potentials in individual neurons in mammalian brain tissue in single trials.

Electromechanical coupling in the membranes of Shaker-transfected HEK cells

Membranes flex with changes in transmembrane potential as a result of changes in interfacial tension, the Lippman effect. We studied the membrane electromotility of Shaker K(+)-transfected HEK-293 cells in real time by using combined patch-clamp atomic force microscopy. In the voltage range where the channels were closed, Shaker expression had little effect on electromotility relative to wild-type cells. Depolarization between -120 and -40 mV resulted in a linear upward cantilever deflection equivalent to an increase in membrane tension. However, when depolarized sufficiently for channel opening, the electromotility saturated and only recovered over 10 s of milliseconds. This remarkable loss of motility was associated with channel opening, not ionic flux or movement of the voltage sensors. The IL mutant of Shaker, in which the voltage dependence of channel opening but not sensor movement is shifted to more positive potentials, caused the loss of electromotility saturation also to shift to more positive potentials. The temporary loss of electromotility associated with channel opening is probably caused by local buckling of the bilayer as the inner half of the channel expands as expected from X-ray structural data.

Sensing voltage across lipid membranes

The detection of electrical potentials across lipid bilayers by specialized membrane proteins is required for many fundamental cellular processes such as the generation and propagation of nerve impulses. These membrane proteins possess modular voltage-sensing domains, a notable example being the S1-S4 domains of voltage-activated ion channels. Ground-breaking structural studies on these domains explain how voltage sensors are designed and reveal important interactions with the surrounding lipid membrane. Although further structures are needed to understand the conformational changes that occur during voltage sensing, the available data help to frame several key concepts that are fundamental to the mechanism of voltage sensing.

Gating pore currents in DIIS4 mutations of NaV1.4 associated with periodic paralysis: saturation of ion flux and implications for disease pathogenesis

S4 voltage–sensor mutations in CaV1.1 and NaV1.4 channels cause the human muscle disorder hypokalemic periodic paralysis (HypoPP). The mechanism whereby these mutations predispose affected sarcolemma to attacks of sustained depolarization and loss of excitability is poorly understood. Recently, three HypoPP mutations in the domain II S4 segment of NaV1.4 were shown to create accessory ionic permeation pathways, presumably extending through the aqueous gating pore in which the S4 segment resides. However, there are several disparities between reported gating pore currents from different investigators, including differences in ionic selectivity and estimates of current amplitude, which in turn have important implications for the pathological relevance of these aberrant currents. To clarify the features of gating pore currents arising from different DIIS4 mutants, we recorded gating pore currents created by HypoPP missense mutations at position R666 in the rat isoform of Nav1.4 (the second arginine from the outside, at R672 in human NaV1.4). Extensive measurements were made for the index mutation, R666G, which created a gating pore that was permeable to K⁺ and Na⁺. This current had a markedly shallow slope conductance at hyperpolarized voltages and robust inward rectification, even when the ionic gradient strongly favored outward ionic flow. These characteristics were accounted for by a barrier model incorporating a voltage-gated permeation pathway with a single cation binding site oriented near the external surface of the electrical field. The amplitude of the R666G gating pore current was similar to the amplitude of a previously described proton-selective current flowing through the gating pore in rNaV1.4-R663H mutant channels. Currents with similar amplitude and cation selectivity were also observed in R666S and R666C mutant channels, while a proton-selective current was observed in R666H mutant channels. These results add support to the notion that HypoPP mutations share a common biophysical profile comprised of a low-amplitude inward current at the resting potential that may contribute to the pathological depolarization during attacks of weakness.

Structure, function, and modification of the voltage sensor in voltage-gated ion channels

Voltage-gated ion channels are crucial for both neuronal and cardiac excitability. Decades of research have begun to unravel the intriguing machinery behind voltage sensitivity. Although the details regarding the arrangement and movement in the voltage-sensor domain are still debated, consensus is slowly emerging. There are three competing conceptual models: the helical-screw, the transporter, and the paddle model. In this review we explore the structure of the activated voltage-sensor domain based on the recent X-ray structure of a chimera between Kv1.2 and Kv2.1. We also present a model for the closed state. From this we conclude that upon depolarization the voltage sensor S4 moves approximately 13 A outwards and rotates approximately 180 degrees, thus consistent with the helical-screw model. S4 also moves relative to S3b which is not consistent with the paddle model. One interesting feature of the voltage sensor is that it partially faces the lipid bilayer and therefore can interact both with the membrane itself and with physiological and pharmacological molecules reaching the channel from the membrane. This type of channel modulation is discussed together with other mechanisms for how voltage-sensitivity is modified. Small effects on voltage-sensitivity can have profound effects on excitability. Therefore, medical drugs designed to alter the voltage dependence offer an interesting way to regulate excitability.

Double bilayers and transmembrane gradients: a molecular dynamics study of a highly charged peptide

The position and extent of movement of a charged peptide within a membrane bilayer provides much controversy. In our study, we have examined the nature of the highly charged helix-turn-helix motif (S3b and S4) to address how a highly charged peptide is stabilized within a bilayer in the presence of various transmembrane electrical potentials. Our double-bilayer simulation results show how the variation of the salt concentrations between the inner and outer bath establishes a transmembrane potential. Our results also show that important features of the peptide affected by changes in electrical potential are the center of mass depth, the swivel/kink degrees of conformation, and the hydrogen-bonding patterns. As the voltage gradient across the bilayer increased, the center of mass of the peptide shifted in a direction toward the outer bath. The peptide also has a higher percent helical content and the swivel/kink conformation is more rigid for nonpolarized systems where no voltage drop occurred between salt baths. Our results also provide some suggestions for how this domain may be affected by environmental changes as part of the voltage sensor in a K-channel.

Portability of paddle motif function and pharmacology in voltage sensors

Voltage-sensing domains enable membrane proteins to sense and react to changes in membrane voltage. Although identifiable S1-S4 voltage-sensing domains are found in an array of conventional ion channels and in other membrane proteins that lack pore domains, the extent to which their voltage-sensing mechanisms are conserved is unknown. Here we show that the voltage-sensor paddle, a motif composed of S3b and S4 helices, can drive channel opening with membrane depolarization when transplanted from an archaebacterial voltage-activated potassium channel (KvAP) or voltage-sensing domain proteins (Hv1 and Ci-VSP) into eukaryotic voltage-activated potassium channels. Tarantula toxins that partition into membranes can interact with these paddle motifs at the protein-lipid interface and similarly perturb voltage-sensor activation in both ion channels and proteins with a voltage-sensing domain. Our results show that paddle motifs are modular, that their functions are conserved in voltage sensors, and that they move in the relatively unconstrained environment of the lipid membrane. The widespread targeting of voltage-sensor paddles by toxins demonstrates that this modular structural motif is an important pharmacological target.

APETx1 from Sea Anemone Anthopleura elegantissima Is a Gating Modifier Peptide Toxin of the Human Ether-a-go-go- Related Potassium Channel

We studied the mechanism of action and the binding site of APETx1, a peptide toxin purified from sea anemone, on the human ether-a-go-go-related gene (hERG) channel. Similar to the effects of gating modifier toxins (hanatoxin and SGTx) on the voltage-gated potassium (Kv) 2.1 channel, APETx1 shifts the voltage-dependence of hERG activation in the positive direction and suppresses its current amplitudes elicited by strong depolarizing pulses that maximally activate the channels. The APETx1 binding site is distinctly different from that of a pore-blocking peptide toxin, BeKm-1. Mutations in the S3b region of hERG have dramatic impact on the responsiveness to APETx1: G514C potentiates whereas E518C abolishes the APETx1 effect. Restoring the negative charge at position 518 (methanethiosulfonate ethylsulfonate modification of 518C) partially restores APETx1 responsiveness, supporting an electrostatic interaction between E518 and APETx1. Among the three hERG isoforms, hERG1 and hERG3 are equally responsive to APETx1, whereas hERG2 is insensitive. The key feature seems to be an arginine residue uniquely present at the 514-equivalent position in hERG2, where the other two isoforms possess a glycine. Our data show that APETx1 is a gating modifier toxin of the hERG channel, and its binding site shares characteristics with those of gating modifier toxin binding sites on other Kv channels.

Life Among the Axons

A blink in history's eye has brought us an understanding of electricity, and with it a revolution in human life. From the frog leg twitch experiments of Galvani and the batteries of Volta, we have progressed to telegraphs, motors, telephones, computers, and the Internet. In the same period, the ubiquitous role of electricity in animal and plant life has become clear. A great milestone in this journey was the elucidation of electrical signaling by Hodgkin & Huxley in 1952. This chapter gives a personal account of a small part of this story, the transformation of the rather abstract electrical conductances of Hodgkin & Huxley into the more tangible gated ion channel.

How Does Voltage Open an Ion Channel?

Neurons transmit information through electrical signals generated by voltage-gated ion channels. These channels consist of a large superfamily of proteins that form channels selective for potassium, sodium, or calcium ions. In this review we focus on the molecular mechanisms by which these channels convert changes in membrane voltage into the opening and closing of "gates" that turn ion conductance on and off. An explosion of new studies in the last year, including the first X-ray crystal structure of a mammalian voltage-gated potassium channel, has led to radically different interpretations of the structure and molecular motion of the voltage sensor. The interpretations are as distinct as the techniques employed for the studies: crystallography, fluorescence, accessibility analysis, and electrophysiology. We discuss the likely causes of the discrepant results in an attempt to identify the missing information that will help resolve the controversy and reveal the mechanism by which a voltage sensor controls the channel's gates.

Molecular template for a voltage sensor in a novel K+ channel. I. Identification and functional characterization of KvLm, a voltage-gated K+ channel from Listeria monocytogenes

The fundamental principles underlying voltage sensing, a hallmark feature of electrically excitable cells, are still enigmatic and the subject of intense scrutiny and controversy. Here we show that a novel prokaryotic voltage-gated K⁺ (Kv) channel from Listeria monocytogenes (KvLm) embodies a rudimentary, yet robust, sensor sufficient to endow it with voltage-dependent features comparable to those of eukaryotic Kv channels. The most conspicuous feature of the KvLm sequence is the nature of the sensor components: the motif is recognizable; it appears, however, to contain only three out of eight charged residues known to be conserved in eukaryotic Kv channels and accepted to be deterministic for folding and sensing. Despite the atypical sensor sequence, flux assays of KvLm reconstituted in liposomes disclosed a channel pore that is highly selective for K⁺ and is blocked by conventional Kv channel blockers. Single-channel currents recorded in symmetric K⁺ solutions from patches of enlarged Escherichia coli (spheroplasts) expressing KvLm showed that channel open probability sharply increases with depolarization, a hallmark feature of Kv channels. The identification of a voltage sensor module in KvLm with a voltage dependence comparable to that of other eukaryotic Kv channels yet encoded by a sequence that departs significantly from the consensus sequence of a eukaryotic voltage sensor establishes a molecular blueprint of a minimal sequence for a voltage sensor.

Molecular template for a voltage sensor in a novel K+ channel. II. Conservation of a eukaryotic sensor fold in a prokaryotic K+ channel

KvLm, a novel bacterial depolarization-activated K⁺ (Kv) channel isolated from the genome of Listeria monocytogenes, contains a voltage sensor module whose sequence deviates considerably from the consensus sequence of a Kv channel sensor in that only three out of eight conserved charged positions are present. Surprisingly, KvLm exhibits the steep dependence of the open channel probability on membrane potential that is characteristic of eukaryotic Kv channels whose sensor sequence approximates the consensus. Here we asked if the KvLm sensor shared a similar fold to that of Shaker, the archetypal eukaryotic Kv channel, by examining if interactions between conserved residues in Shaker known to mediate sensor biogenesis and function were conserved in KvLm. To this end, each of the five non-conserved residues in the KvLm sensor were mutated to their Shaker-like charged residues, and the impact of these mutations on the voltage dependence of activation was assayed by current recordings from excised membrane patches of Escherichia coli spheroplasts expressing the KvLm mutants. Conservation of pairwise interactions was investigated by comparison of the effect of single mutations to the impact of double mutations presumed to restore wild-type fold and voltage sensitivity. We observed significant functional coupling between sites known to interact in Shaker Kv channels, supporting the notion that the KvLm sensor largely retains the fold of its eukaryotic homologue.

Structure and function of the voltage sensor of sodium channels probed by a beta-scorpion toxin

Voltage sensing by voltage-gated sodium channels determines the electrical excitability of cells, but the molecular mechanism is unknown. beta-Scorpion toxins bind specifically to neurotoxin receptor site 4 and induce a negative shift in the voltage dependence of activation through a voltage sensor-trapping mechanism. Kinetic analysis showed that beta-scorpion toxin binds to the resting state, and subsequently the bound toxin traps the voltage sensor in the activated state in a voltage-dependent but concentration-independent manner. The rate of voltage sensor trapping can be fit by a two-step model, in which the first step is voltage-dependent and correlates with the outward gating movement of the IIS4 segment, whereas the second step is voltage-independent and results in shifted voltage dependence of activation of the channel. Mutations of Glu(779) in extracellular loop IIS1-S2 and both Glu(837) and Leu(840) in extracellular loop IIS3-S4 reduce the binding affinity of beta-scorpion toxin. Mutations of positively charged and hydrophobic amino acid residues in the IIS4 segment do not affect beta-scorpion toxin binding but alter voltage dependence of activation and enhance beta-scorpion toxin action. Structural modeling with the Rosetta algorithm yielded a three-dimensional model of the toxin-receptor complex with the IIS4 voltage sensor at the extracellular surface. Our results provide mechanistic and structural insight into the voltage sensor-trapping mode of scorpion toxin action, define the position of the voltage sensor in the resting state of the sodium channel, and favor voltage-sensing models in which the S4 segment spans the membrane in both resting and activated states.

Voltage sensor conformations in the open and closed states in ROSETTA structural models of K(+) channels

Voltage-gated ion channels control generation and propagation of action potentials in excitable cells. Significant progress has been made in understanding structure and function of the voltage-gated ion channels, highlighted by the high-resolution open-state structure of the voltage-gated potassium channel, K(v)1.2. However, because the structure of the closed state is unknown, the gating mechanism remains controversial. We adapted the rosetta membrane method to model the structures of the K(v)1.2 and KvAP channels using homology, de novo, and domain assembly methods and selected the most plausible models using a limited number of experimental constraints. Our model of K(v)1.2 in the open state is very similar in overall topology to the x-ray structure of this channel. Modeling of KvAP in the open state suggests that orientation of the voltage-sensing domain relative to the pore-forming domain is considerably different from the orientation in the K(v)1.2 open state and that the magnitude of the vertical movement of S4 is significantly greater. Structural modeling of closed state of K(v)1.2 suggests gating movement that can be viewed as a sum of two previously suggested mechanisms: translation (2-4 A) plus rotation ( approximately 180 degrees ) of the S4 segment as proposed in the original "sliding helix" or "helical screw" models coupled with a rolling motion of the S1-S3 segments around S4, similar to recent "transporter" models of gating. We propose a unified mechanism of voltage-dependent gating for K(v)1.2 and KvAP in which this major conformational change moves the gating charge across the electric field in an analogous way for both channels.

Environment of the gating charges in the Kv1.2 Shaker potassium channel

Recently, the structure of the Shaker channel Kv1.2 has been determined at a 2.9-angstroms resolution. This opens new possibilities in deciphering the mechanism underlying the function of voltage-gated potassium (Kv) channels. Molecular dynamics simulations of the channel, embedded in a membrane environment show that the channel is in its open state and that the gating charges carried by S4 are exposed to the solvent. The hydrated environment of S4 favors a local collapse of the electrostatic potential, which generates high electric-field gradients around the arginine gating charges. Comparison to experiments suggests furthermore that activation of the channel requires mainly a lateral displacement of S4. Overall, the results agree with the transporter model devised for Kv channels from electrophysiology experiments, and provide a possible pathway for the mechanistic response to membrane depolarization.

Interactions between charged residues in the transmembrane segments of the voltage-sensing domain in the hERG channel

Studies on voltage-gated K channels such as Shaker have shown that positive charges in the voltage-sensor (S4) can form salt bridges with negative charges in the surrounding transmembrane segments in a state-dependent manner, and different charge pairings can stabilize the channels in closed or open states. The goal of this study is to identify such charge interactions in the hERG channel. This knowledge can provide constraints on the spatial relationship among transmembrane segments in the channel's voltage-sensing domain, which are necessary for modeling its structure. We first study the effects of reversing S4's positive charges on channel activation. Reversing positive charges at the outer (K525D) and inner (K538D) ends of S4 markedly accelerates hERG activation, whereas reversing the 4 positive charges in between either has no effect or slows activation. We then use the 'mutant cycle analysis' to test whether D456 (outer end of S2) and D411 (inner end of S1) can pair with K525 and K538, respectively. Other positive charges predicted to be able, or unable, to interact with D456 or D411 are also included in the analysis. The results are consistent with predictions based on the distribution of these charged residues, and confirm that there is functional coupling between D456 and K525 and between D411 and K538.

Differential roles of S6 domain hinges in the gating of KCNQ potassium channels

Voltage-gated K+ channel activation is proposed to result from simultaneous bending of all S6 segments away from the central axis, enlarging the aperture of the pore sufficiently to permit diffusion of K+ into the water-filled central cavity. The hinge position for the bending motion of each S6 segment is proposed to be a Gly residue and/or a Pro-Val-Pro motif in Kv1-Kv4 channels. The KCNQ1 (Kv7.1) channel has Ala-336 in the Gly-hinge position and Pro-Ala-Gly. Here we show that mutation of Ala-336 to Gly in KCNQ1 increased current amplitude and shifted the voltage dependence of activation to more negative potentials, consistent with facilitation of hinge activity that favors the open state. In contrast, mutation of Ala-336 to Cys or Thr shifted the voltage dependence of activation to more positive potentials and reduced current amplitude. Mutation of the putative Gly hinge to Ala in KCNQ2 (Kv7.2) abolished channel function. Mutation-dependent changes in current amplitude, but not kinetics, were found in heteromeric KCNQ1/KCNE1 channels. Mutation of the Pro or Gly of the Pro-Ala-Gly motif to Ala abolished KCNQ1 function and introduction of Gly in front of the Ala-mutations partially recovered channel function, suggesting that flexibility at the PAG is important for channel activation.

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.

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.

Ion permeation through a voltage- sensitive gating pore in brain sodium channels having voltage sensor mutations

Voltage-gated sodium channels activate in response to depolarization, but it is unknown whether the voltage-sensing arginines in their S4 segments pivot across the lipid bilayer as voltage sensor paddles or move through the protein in a gating pore. Here we report that mutation of pairs of arginine gating charges to glutamine induces cation permeation through a gating pore in domain II of the Na(V)1.2a channel. Mutation of R850 and R853 induces a K(+)-selective inward cationic current in the resting state that is blocked by activation. Remarkably, mutation of R853 and R856 causes an outward cationic current with the opposite gating polarity. These results support a model in which the IIS4 gating charges move through a narrow constriction in a gating pore in the sodium channel protein during gating. Paired substitutions of glutamine allow cation movement through the constriction when appropriately positioned by the gating movements of the S4 segment.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.