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

Cholesterol modulates the structural dynamics of the paddle motif loop of KvAP voltage sensor

KvAP is a prokaryotic Kv channel, which has been widely used as a model system to understand voltage- and lipid-dependent gating mechanisms. In phospholipid membranes, the KvAP-VSD adopts the activated/'Up' conformation, whereas the presence of non-phospholipids in membranes favours the structural transition to resting/'Down' state. The S3b-S4 paddle motif loop of KvAP-VSD is functionally important as this participates in protein-protein interactions and is the target for animal toxins. In this study, we have monitored the modulatory role of cholesterol - the physiologically-relevant non-phospholipid - on the organization and dynamics of the S3b-S4 loop of the isolated KvAP-VSD in membranes by site-directed fluorescence approaches using the environmental sensitivity of 7-nitrobenz-2-oxa-1,3-diazol-4-yl-ethylenediamine (NBD) fluorescence. Our results show that cholesterol alters the dynamic nature (rotational and hydration dynamics) of S3b-S4 loop in a segmental fashion, i.e., the residues 110 to 114 and 115 to 117 behave differently in the presence of cholesterol, which is accompanied by considerable change in conformational heterogeneity. Further, quantitative depth measurements using the parallax quenching method reveal that the sensor loop is located at the shallow interfacial region of cholesterol-containing membranes, suggesting that the sensor loop organization is not directly correlated with S4 helix movement. Our results clearly show that cholesterol-induced changes in bilayer properties may not be the predominant factor for the sensor loop's altered structural dynamics, but can be attributed to the conformational change of the KvAP-VSD in cholesterol-containing membranes. Overall, these results are relevant for gating mechanisms, particularly the lipid-dependent gating, of Kv channels in membranes.

Not so optimal: The evolution of mutual information in potassium voltage-gated channels

Potassium voltage-gated (Kv) channels need to detect and respond to rapidly changing ionic concentrations in their environment. With an essential role in regulating electric signaling, they would be expected to be optimal sensors that evolved to predict the ionic concentrations. To explore these assumptions, we use statistical mechanics in conjunction with information theory to model how animal Kv channels respond to changes in potassium concentrations in their environment. By measuring mutual information in representative Kv channel types across a variety of environments, we find two things. First, under weak conditions, there is a gating charge that maximizes mutual information with the environment. Second, as Kv channels evolved, they have moved towards decreasing mutual information with the environment. This either suggests that Kv channels do not need to act as sensors of their environment or that Kv channels have other functionalities that interfere with their role as sensors of their environment.

Studying KcsA Channel Clustering Using Single Channel Voltage-Clamp Fluorescence Imaging*

Oligomerization and complex formation play a key role for many membrane proteins and has been described to influence ion channel function in both neurons and the heart. In this study, we observed clustering of single KcsA channels in planar lipid bilayer using single molecule fluorescence, while simultaneously measuring single channel currents. Clustering coincided with cooperative opening of KcsA. We demonstrate that clustering was not caused by direct protein-protein interactions or hydrophobic mismatch with the lipid environment, as suggested earlier, but was mediated via microdomains induced by the channel in the lipid matrix. We found that single channel activity of KcsA requires conically-shaped lipids in the lamellar liquid-crystalline (L_α) phase, and the need for a negative spontaneous curvature seem to lead to the deformations in the membrane that cause the clustering. The method introduced here will be applicable to follow oligomerization of a wide range of membrane proteins.

The pH-dependent gating of the human voltage-gated proton channel from computational simulations

Gating of the voltage-gated proton channel H_V1 is strongly controlled by pH. There is evidence that this involves the sidechains of titratable amino acids that change their protonation state with changes of the pH. Despite experimental investigations to identify the amino acids involved in pH sensing only few progress has been made, including one histidine at the cytoplasmic side of the channel that is involved in sensing cellular pH. We have used constant pH molecular dynamics simulations in symmetrical and asymmetrical pH conditions across the membrane to investigate the pH- and ΔpH-dependent gating of the human H_V1 channel. Therefore, the pK_a of every titratable amino acids has been assessed in single simulations. Our simulations captured initial conformational changes between a deactivated and an activated state of the channel induced solely by changes of the pH. The pH-dependent gating is accompanied by an outward displacement of the three S4 voltage sensing arginines that moves the second arginine past the hydrophobic gasket (HG) which separates the inner and outer pores of the channel. H_V1 activation, when outer pH increases, involves amino acids at the extracellular entrance of the channel that extend the network of interactions from the external solution down to the HG. Whereas, amino acids at the cytoplasmic entrance of the channel are involved in activation, when inner pH decreases, and in a network of interactions that extend from the cytoplasm up to the HG.

The Voltage-Dependent Deactivation of the KvAP Channel Involves the Breakage of Its S4 Helix

Voltage-gated potassium channels (Kv) allow ion permeation upon changes of the membrane electrostatic potential (Vm). Each subunit of these tetrameric channels is composed of six transmembrane helices, of which the anti-parallel helix bundle S1-S4 constitutes the voltage-sensor domain (VSD) and S5-S6 forms the pore domain. Here, using 82 molecular dynamics (MD) simulations involving 266 replicated VSDs, we report novel responses of the archaebacterial potassium channel KvAP to membrane polarization. We show that the S4 α-helix, which is straight in the experimental crystal structure solved under depolarized conditions (Vm ∼ 0), breaks into two segments when the cell membrane is hyperpolarized (Vm << 0), and reversibly forms a single straight helix following depolarization (Vm = 0). The outermost segment of S4 translates along the normal to the membrane, bringing new perspective to previously paradoxical accessibility experiments that were initially thought to imply the displacement of the whole VSD across the membrane. The novel model is applied through steered and unbiased MD simulations to the recently solved whole structure of KvAP. The simulations show that the resting state involves a re-orientation of the S5 α-helix by ∼ 5-6 degrees in respect to the normal of the bilayer, which could result in the constriction and closure of the selectivity filter. Our findings support the idea that the breakage of S4 under (hyper)polarization is a general feature of Kv channels with a non-swapped topology.

Cryo-EM structure of the KvAP channel reveals a non-domain-swapped voltage sensor topology

Conductance in voltage-gated ion channels is regulated by membrane voltage through structural domains known as voltage sensors. A single structural class of voltage sensor domain exists, but two different modes of voltage sensor attachment to the pore occur in nature: domain-swapped and non-domain-swapped. Since the more thoroughly studied Kv1-7, Nav and Cav channels have domain-swapped voltage sensors, much less is known about non-domain-swapped voltage-gated ion channels. In this paper, using cryo-EM, we show that KvAP from Aeropyrum pernix has non-domain-swapped voltage sensors as well as other unusual features. The new structure, together with previous functional data, suggests that KvAP and the Shaker channel, to which KvAP is most often compared, probably undergo rather different voltage-dependent conformational changes when they open.

Simulation of Gating Currents of the Shaker K Channel Using a Brownian Model of the Voltage Sensor

The physical mechanism underlying the voltage-dependent gating of K channels is usually addressed theoretically using molecular dynamics simulations. However, besides being computationally very expensive, this approach is presently unable to fully predict the behavior of fundamental variables of channel gating such as the macroscopic gating current, and hence, it is presently unable to validate the model. To fill this gap, here we propose a voltage-gating model that treats the S4 segment as a Brownian particle moving through a gating channel pore and adjacent internal and external vestibules. In our model, charges on the S4 segment are screened by charged residues localized on neighboring segments of the channel protein and by ions present in the vestibules, whose dynamics are assessed using a flux conservation equation. The electrostatic voltage spatial profile is consistently assessed by applying the Poisson equation to all the charges present in the system. The treatment of the S4 segment as a Brownian particle allows description of the dynamics of a single S4 segment using the Langevin stochastic differential equation or the behavior of a population of S4 segments-useful for assessing the macroscopic gating current-using the Fokker-Planck equation. The proposed model confirms the gating charge transfer hypothesis with the movement of the S4 segment among five different stable positions where the gating charges interact in succession with the negatively charged residues on the channel protein. This behavior produces macroscopic gating currents quite similar to those experimentally found.

Voltage-Dependent Profile Structures of a Kv-Channel via Time-Resolved Neutron Interferometry

Available experimental techniques cannot determine high-resolution three-dimensional structures of membrane proteins under a transmembrane voltage. Hence, the mechanism by which voltage-gated cation channels couple conformational changes within the four voltage sensor domains, in response to either depolarizing or polarizing transmembrane voltages, to opening or closing of the pore domain's ion channel remains unresolved. Single-membrane specimens, composed of a phospholipid bilayer containing a vectorially oriented voltage-gated K+ channel protein at high in-plane density tethered to the surface of an inorganic multilayer substrate, were developed to allow the application of transmembrane voltages in an electrochemical cell. Time-resolved neutron reflectivity experiments, enhanced by interferometry enabled by the multilayer substrate, were employed to provide directly the low-resolution profile structures of the membrane containing the vectorially oriented voltage-gated K+ channel for the activated, open and deactivated, closed states of the channel under depolarizing and hyperpolarizing transmembrane voltages applied cyclically. The profile structures of these single membranes were dominated by the voltage-gated K+ channel protein because of the high in-plane density. Importantly, the use of neutrons allowed the determination of the voltage-dependent changes in both the profile structure of the membrane and the distribution of water within the profile structure. These two key experimental results were then compared to those predicted by three computational modeling approaches for the activated, open and deactivated, closed states of three different voltage-gated K+ channels in hydrated phospholipid bilayer membrane environments. Of the three modeling approaches investigated, only one state-of-the-art molecular dynamics simulation that directly predicted the response of a voltage-gated K+ channel within a phospholipid bilayer membrane to applied transmembrane voltages by utilizing very long trajectories was found to be in agreement with the two key experimental results provided by the time-resolved neutron interferometry experiments.

Molecular mechanisms of coupling to voltage sensors in voltage-evoked cellular signals

The voltage sensor domain (VSD) has long been studied as a unique domain intrinsic to voltage-gated ion channels (VGICs). Within VGICs, the VSD is tightly coupled to the pore-gate domain (PGD) in diverse ways suitable for its specific function in each physiological context, including action potential generation, muscle contraction and relaxation, hormone and neurotransmitter secretion, and cardiac pacemaking. However, some VSD-containing proteins lack a PGD. Voltage-sensing phosphatase contains a cytoplasmic phosphoinositide phosphatase with similarity to phosphatase and tensin homolog (PTEN). H_v1, a voltage-gated proton channel, also lacks a PGD. Within H_v1, the VSD operates as a voltage sensor, gate, and pore for both proton sensing and permeation. H_v1 has a C-terminal coiled coil that mediates dimerization for cooperative gating. Recent progress in the structural biology of VGICs and VSD proteins provides insights into the principles of VSD coupling conserved among these proteins as well as the hierarchy of protein organization for voltage-evoked cell signaling.

Thermodynamics of voltage-gated ion channels

Ion channels are essential for cellular signaling. Voltage-gated ion channels (VGICs) are the largest and most extensively studied superfamily of ion channels. They possess modular structural features such as voltage-sensing domains that encircle and form mechanical connections with the pore-forming domains. Such features are intimately related to their function in sensing and responding to changes in the membrane potential. In the present work, we discuss the thermodynamic mechanisms of the VGIC superfamily, including the two-state gating mechanism, sliding-rocking mechanism of the voltage sensor, subunit cooperation, lipid-infiltration mechanism of inactivation, and the relationship with their structural features.

The Role of Proton Transport in Gating Current in a Voltage Gated Ion Channel, as Shown by Quantum Calculations

Over two-thirds of a century ago, Hodgkin and Huxley proposed the existence of voltage gated ion channels (VGICs) to carry Na⁺ and K⁺ ions across the cell membrane to create the nerve impulse, in response to depolarization of the membrane. The channels have multiple physiological roles, and play a central role in a wide variety of diseases when they malfunction. The first channel structure was found by MacKinnon and coworkers in 1998. Subsequently, the structure of a number of VGICs was determined in the open (ion conducting) state. This type of channel consists of four voltage sensing domains (VSDs), each formed from four transmembrane (TM) segments, plus a pore domain through which ions move. Understanding the gating mechanism (how the channel opens and closes) requires structures. One TM segment (S4) has an arginine in every third position, with one such segment per domain. It is usually assumed that these arginines are all ionized, and in the resting state are held toward the intracellular side of the membrane by voltage across the membrane. They are assumed to move outward (extracellular direction) when released by depolarization of this voltage, producing a capacitive gating current and opening the channel. We suggest alternate interpretations of the evidence that led to these models. Measured gating current is the total charge displacement of all atoms in the VSD; we propose that the prime, but not sole, contributor is proton motion, not displacement of the charges on the arginines of S4. It is known that the VSD can conduct protons. Quantum calculations on the K_v1.2 potassium channel VSD show how; the key is the amphoteric nature of the arginine side chain, which allows it to transfer a proton. This appears to be the first time the arginine side chain has had its amphoteric character considered. We have calculated one such proton transfer in detail: this proton starts from a tyrosine that can ionize, transferring to the NE of the third arginine on S4; that arginine’s NH then transfers a proton to a glutamate. The backbone remains static. A mutation predicted to affect the proton transfer has been qualitatively confirmed experimentally, from the change in the gating current-voltage curve. The total charge displacement in going from a normal closed potential of −70 mV across the membrane to 0 mV (open), is calculated to be approximately consistent with measured values, although the error limits on the calculation require caution in interpretation.

Gating currents

A voltage change across a membrane protein moves charges or dipoles producing a gating current that is an electrical expression of a conformational change.

Many membrane proteins sense the voltage across the membrane where they are inserted, and their function is affected by voltage changes. The voltage sensor consists of charges or dipoles that move in response to changes in the electric field, and their movement produces an electric current that has been called gating current. In the case of voltage-gated ion channels, the kinetic and steady-state properties of the gating charges provide information of conformational changes between closed states that are not visible when observing ionic currents only. In this Journal of General Physiology Milestone, the basic principles of voltage sensing and gating currents are presented, followed by a historical description of the recording of gating currents. The results of gating current recordings are then discussed in the context of structural changes in voltage-dependent membrane proteins and how these studies have provided new insights on gating mechanisms.

Time-Resolved Neutron Interferometry and the Mechanism of Electromechanical Coupling in Voltage-Gated Ion Channels

The mechanism of electromechanical coupling for voltage-gated ion channels (VGICs) involved in neurological signal transmission, primarily Nav- and Kv-channels, remains unresolved. Anesthetics have been shown to directly impact this mechanism, at least for Kv-channels. Molecular dynamics computer simulations can now predict the structures of VGICs embedded within a hydrated phospholipid bilayer membrane as a function of the applied transmembrane voltage, but significant assumptions are still necessary. Nevertheless, these simulations are providing new insights into the mechanism of electromechanical coupling at the atomic level in 3-D. We show that time-resolved neutron interferometry can be used to investigate directly the profile structure of a VGIC, vectorially oriented within a single hydrated phospholipid bilayer membrane at the solid-liquid interface, as a function of the applied transmembrane voltage in the absence of any assumptions or potentially perturbing modifications of the VGIC protein and/or the host membrane. The profile structure is a projection of the membrane's 3-D structure onto the membrane normal and, in the absence of site-directed deuterium labeling, is provided at substantially lower spatial resolution than the atomic level. Nevertheless, this novel approach can be used to directly test the validity of the predictions from molecular dynamics simulations. We describe the key elements of our novel experimental approach, including why each is necessary and important to providing the essential information required for this critical comparison of "simulation" vs "experiment." In principle, the approach could be extended to higher spatial resolution and to include the effects of anesthetics on the electromechanical coupling mechanism in VGICs.

Voltage and pH sensing by the voltage-gated proton channel, HV1

Voltage-gated proton channels are unique ion channels, membrane proteins that allow protons but no other ions to cross cell membranes. They are found in diverse species, from unicellular marine life to humans. In all cells, their function requires that they open and conduct current only under certain conditions, typically when the electrochemical gradient for protons is outwards. Consequently, these proteins behave like rectifiers, conducting protons out of cells. Their activity has electrical consequences and also changes the pH on both sides of the membrane. Here we summarize what is known about the way these proteins sense the membrane potential and the pH inside and outside the cell. Currently, it is hypothesized that membrane potential is sensed by permanently charged arginines (with very high pKa) within the protein, which results in parts of the protein moving to produce a conduction pathway. The mechanism of pH sensing appears to involve titratable side chains of particular amino acids. For this purpose their pKa needs to be within the operational pH range. We propose a 'counter-charge' model for pH sensing in which electrostatic interactions within the protein are selectively disrupted by protonation of internally or externally accessible groups.

Illuminating Brain Activities with Fluorescent Protein-Based Biosensors

Fluorescent protein-based biosensors are indispensable molecular tools for life science research. The invention and development of high-fidelity biosensors for a particular molecule or molecular event often catalyze important scientific breakthroughs. Understanding the structural and functional organization of brain activities remain a subject for which optical sensors are in desperate need and of growing interest. Here, we review genetically encoded fluorescent sensors for imaging neuronal activities with a focus on the design principles and optimizations of various sensors. New bioluminescent sensors useful for deep-tissue imaging are also discussed. By highlighting the protein engineering efforts and experimental applications of these sensors, we can consequently analyze factors influencing their performance. Finally, we remark on how future developments can fill technological gaps and lead to new discoveries.

Energetics of side-chain partitioning of β-signal residues in unassisted folding of a transmembrane β-barrel protein

The free energy of water-to-interface amino acid partitioning is a major contributing factor in membrane protein folding and stability. The interface residues at the C terminus of transmembrane β-barrels form the β-signal motif required for assisted β-barrel assembly in vivo but are believed to be less important for β-barrel assembly in vitro. Here, we experimentally measured the thermodynamic contribution of all 20 amino acids at the β-signal motif to the unassisted folding of the model β-barrel protein PagP. We obtained the partitioning free energy for all 20 amino acids at the lipid-facing interface (ΔΔG⁰_w,i(φ)) and the protein-facing interface (ΔΔG⁰_w,i(π)) residues and found that hydrophobic amino acids are most favorably transferred to the lipid-facing interface, whereas charged and polar groups display the highest partitioning energy. Furthermore, the change in non-polar surface area correlated directly with the partitioning free energy for the lipid-facing residue and inversely with the protein-facing residue. We also demonstrate that the interface residues of the β-signal motif are vital for in vitro barrel assembly, because they exhibit a side chain–specific energetic contribution determined by the change in nonpolar accessible surface. We further establish that folding cooperativity and hydrophobic collapse are balanced at the membrane interface for optimal stability of the PagP β-barrel scaffold. We conclude that the PagP C-terminal β-signal motif influences the folding cooperativity and stability of the folded β-barrel and that the thermodynamic contributions of the lipid- and protein-facing residues in the transmembrane protein β-signal motif depend on the nature of the amino acid side chain.

A new mechanism of voltage-dependent gating exposed by KV10.1 channels interrupted between voltage sensor and pore

A linker that connects the voltage-sensing domain and pore domain in voltage-gated K⁺ channels is thought to provide coupling during gating, but this view has been challenged in KCNH channels. Tomczak et al. investigate gating in K_V10.1 channels with disrupted linkers and reveal multiple mechanisms.

Voltage-gated ion channels couple transmembrane potential changes to ion flow. Conformational changes in the voltage-sensing domain (VSD) of the channel are thought to be transmitted to the pore domain (PD) through an α-helical linker between them (S4–S5 linker). However, our recent work on channels disrupted in the S4–S5 linker has challenged this interpretation for the KCNH family. Furthermore, a recent single-particle cryo-electron microscopy structure of K_V10.1 revealed that the S4–S5 linker is a short loop in this KCNH family member, confirming the need for an alternative gating model. Here we use “split” channels made by expression of VSD and PD as separate fragments to investigate the mechanism of gating in K_V10.1. We find that disruption of the covalent connection within the S4 helix compromises the ability of channels to close at negative voltage, whereas disconnecting the S4–S5 linker from S5 slows down activation and deactivation kinetics. Surprisingly, voltage-clamp fluorometry and MTS accessibility assays show that the motion of the S4 voltage sensor is virtually unaffected when VSD and PD are not covalently bound. Finally, experiments using constitutively open PD mutants suggest that the presence of the VSD is structurally important for the conducting conformation of the pore. Collectively, our observations offer partial support to the gating model that assumes that an inward motion of the C-terminal S4 helix, rather than the S4–S5 linker, closes the channel gate, while also suggesting that control of the pore by the voltage sensor involves more than one mechanism.

Gating energetics of a voltage-dependent K+ channel pore domain

We used targeted molecular dynamics, informed by experimentally determined inter-atomic distances defining the pore region of open and closed states of the KvAP voltage-gated potassium channel, to generate a gating pathway of the pore domain in the absence of the voltage-sensing domains. We then performed umbrella sampling simulations along this pathway to calculate a potential of mean force that describes the free energy landscape connecting the closed and open conformations of the pore domain. The resulting energetic landscape displays three minima, corresponding to stable open, closed, and intermediate conformations with roughly similar stabilities. We found that the extent of hydration of the interior of the pore domain could influence the free energy landscape for pore opening/closing. © 2017 Wiley Periodicals, Inc.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Cryo-EM structure of the open high-conductance Ca2+-activated K+ channel

The Ca²⁺-activated K⁺ channel, Slo1, has an unusually large conductance and contains a voltage sensor and multiple chemical sensors. Dual activation by membrane voltage and Ca²⁺ renders Slo1 central to processes that couple electrical signalling to Ca²⁺-mediated events such as muscle contraction and neuronal excitability. Here we present the cryo-electron microscopy structure of a full-length Slo1 channel from Aplysia californica in the presence of Ca²⁺ and Mg²⁺ at a resolution of 3.5 Å. The channel adopts an open conformation. Its voltage-sensor domain adopts a non-domain-swapped attachment to the pore and contacts the cytoplasmic Ca²⁺-binding domain from a neighbouring subunit. Unique structural features of the Slo1 voltage sensor suggest that it undergoes different conformational changes than other known voltage sensors. The structure reveals the molecular details of three distinct divalent cation-binding sites identified through electrophysiological studies of mutant Slo1 channels.

Disulfide mapping the voltage-sensing mechanism of a voltage-dependent potassium channel

Voltage-dependent potassium (Kv) channels allow for the selective permeability of potassium ions in a membrane potential dependent manner, playing crucial roles in neurotransmission and muscle contraction. Kv channel is a tetramer, in which each subunit possesses a voltage-sensing domain (VSD) and a pore domain (PD). Although several lines of evidence indicated that membrane depolarization is sensed as the movement of helix S4 of the VSD, the detailed voltage-sensing mechanism remained elusive, due to the difficulty of structural analyses at resting potential. In this study, we conducted a comprehensive disulfide locking analysis of the VSD using 36 double Cys mutants, in order to identify the proximal residue pairs of the VSD in the presence or absence of a membrane potential. An intramolecular SS-bond was formed between 6 Cys pairs under both polarized and depolarized environment, and one pair only under depolarized environment. The multiple conformations captured by the SS-bond can be divided by two states, up and down, where S4 lies on the extracellular and intracellular sides of the membrane, respectively, with axial rotation of 180°. The transition between these two states is caused by the S4 translocation of 12 Å, enabling allosteric regulation of the gating at the PD.

Launching deep subwavelength bulk plasmon polaritons through hyperbolic metamaterials for surface imaging with a tuneable ultra-short illumination depth

Hyperbolic metamaterials (HMMs) composed of multiple nanometal-dielectric films are proposed for launching deep subwavelength bulk plasmon polaritons (BPPs) as uniform, large area surface imaging illumination sources with a skin depth even beyond 10 nm. Benefiting from the coupled plasmon modes over a wide wavevector range in HMMs, the illumination depth could be continually tuned, simply by adjusting the incidence angle of light impinged on a grating structure for BPP excitation. As an example, the illumination depths of 19-63 nm at a light wavelength of 532 nm are demonstrated with SiO₂-Ag multifilms. Moreover, the structure holds its deep subwavelength illumination depth for a broad light wavelength range, resembling that of light total internal reflection in a prism with an ultra high refractive index. Furthermore, a fluorescent nanoparticle based micro-zone system was employed for estimating the illumination depth of the HMM structure. The method is believed to provide access for surface imaging features in ultra thin layers especially for bio-samples.

Proton currents constrain structural models of voltage sensor activation

The Hv1 proton channel is evidently unique among voltage sensor domain proteins in mediating an intrinsic 'aqueous' H⁺ conductance (G_AQ). Mutation of a highly conserved 'gating charge' residue in the S4 helix (R1H) confers a resting-state H⁺ 'shuttle' conductance (G_SH) in VGCs and Ci VSP, and we now report that R1H is sufficient to reconstitute G_SH in Hv1 without abrogating G_AQ. Second-site mutations in S3 (D185A/H) and S4 (N4R) experimentally separate G_SH and G_AQ gating, which report thermodynamically distinct initial and final steps, respectively, in the Hv1 activation pathway. The effects of Hv1 mutations on G_SH and G_AQ are used to constrain the positions of key side chains in resting- and activated-state VS model structures, providing new insights into the structural basis of VS activation and H⁺ transfer mechanisms in Hv1.

Voltage-Gated Potassium Channels: A Structural Examination of Selectivity and Gating

Voltage-gated potassium channels play a fundamental role in the generation and propagation of the action potential. The discovery of these channels began with predictions made by early pioneers, and has culminated in their extensive functional and structural characterization by electrophysiological, spectroscopic, and crystallographic studies. With the aid of a variety of crystal structures of these channels, a highly detailed picture emerges of how the voltage-sensing domain reports changes in the membrane electric field and couples this to conformational changes in the activation gate. In addition, high-resolution structural and functional studies of K(+) channel pores, such as KcsA and MthK, offer a comprehensive picture on how selectivity is achieved in K(+) channels. Here, we illustrate the remarkable features of voltage-gated potassium channels and explain the mechanisms used by these machines with experimental data.

Molecular Interactions between Tarantula Toxins and Low-Voltage-Activated Calcium Channels

Few gating-modifier toxins have been reported to target low-voltage-activated (LVA) calcium channels, and the structural basis of toxin sensitivity remains incompletely understood. Studies of voltage-gated potassium (Kv) channels have identified the S3b–S4 “paddle motif,” which moves at the protein-lipid interface to drive channel opening, as the target for these amphipathic neurotoxins. Voltage-gated calcium (Cav) channels contain four homologous voltage sensor domains, suggesting multiple toxin binding sites. We show here that the S3–S4 segments within Cav3.1 can be transplanted into Kv2.1 to examine their individual contributions to voltage sensing and pharmacology. With these results, we now have a more complete picture of the conserved nature of the paddle motif in all three major voltage-gated ion channel types (Kv, Nav, and Cav). When screened with tarantula toxins, the four paddle sequences display distinct toxin binding properties, demonstrating that gating-modifier toxins can bind to Cav channels in a domain specific fashion. Domain III was the most commonly and strongly targeted, and mutagenesis revealed an acidic residue that is important for toxin binding. We also measured the lipid partitioning strength of all toxins tested and observed a positive correlation with their inhibition of Cav3.1, suggesting a key role for membrane partitioning.

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.

Conformational changes of an ion-channel during gating and emerging electrophysiologic properties: Application of a computational approach to cardiac Kv7.1

Ion channels are the "building blocks" of the excitation process in excitable tissues. Despite advances in determining their molecular structure, understanding the relationship between channel protein structure and electrical excitation remains a challenge. The Kv7.1 potassium channel is an important determinant of the cardiac action potential and its adaptation to rate changes. It is subject to beta adrenergic regulation, and many mutations in the channel protein are associated with the arrhythmic long QT syndrome. In this theoretical study, we use a novel computational approach to simulate the conformational changes that Kv7.1 undergoes during activation gating and compute the resulting electrophysiologic function in terms of single-channel and macroscopic currents. We generated all possible conformations of the S4-S5 linker that couples the S3-S4 complex (voltage sensor domain, VSD) to the pore, and all associated conformations of VSD and the pore (S6). Analysis of these conformations revealed that VSD-to-pore mechanical coupling during activation gating involves outward translation of the voltage sensor, accompanied by a translation away from the pore and clockwise twist. These motions cause pore opening by moving the S4-S5 linker upward and away from the pore, providing space for the S6 tails to move away from each other. Single channel records, computed from the simulated motion trajectories during gating, have stochastic properties similar to experimentally recorded traces. Macroscopic current through an ensemble of channels displays two key properties of Kv7.1: an initial delay of activation and fast inactivation. The simulations suggest a molecular mechanism for fast inactivation; a large twist of the VSD following its outward translation results in movement of the base of the S4-S5 linker toward the pore, eliminating open pore conformations to cause inactivation.

Structure of the voltage-gated two-pore channel TPC1 from Arabidopsis thaliana

Two-pore channels (TPCs) contain two copies of a Shaker-like six-transmembrane (6-TM) domain in each subunit and are ubiquitously expressed in both animals and plants as organellar cation channels. Here, we present the first crystal structure of a vacuolar two-pore channel from Arabidopsis thaliana, AtTPC1, which functions as a homodimer. AtTPC1 activation requires both voltage and cytosolic Ca²⁺. Ca²⁺ binding to the cytosolic EF-hand domain triggers conformational changes coupled to the pair of pore-lining inner helices (IS6 helices) from the first 6-TM domains, whereas membrane potential only activates the second voltage-sensing domain (VSD2) whose conformational changes are coupled to the pair of inner helices (IIS6 helices) from the second 6-TM domains. Luminal Ca²⁺ or Ba²⁺ can modulate voltage activation by stabilizing VSD2 in the resting state and shifts voltage activation towards more positive potentials. Our Ba²⁺ bound AtTPC1 structure reveals a voltage sensor in the resting state, providing hitherto unseen structural insight into the general voltage-gating mechanism among voltage-gated channels.

Structural basis for the inhibition of voltage-dependent K+ channel by gating modifier toxin

Voltage-dependent K⁺ (K_v) channels play crucial roles in nerve and muscle action potentials. Voltage-sensing domains (VSDs) of K_v channels sense changes in the transmembrane potential, regulating the K⁺-permeability across the membrane. Gating modifier toxins, which have been used for the functional analyses of K_v channels, inhibit K_v channels by binding to VSD. However, the structural basis for the inhibition remains elusive. Here, fluorescence and NMR analyses of the interaction between VSD derived from K_vAP channel and its gating modifier toxin, VSTx1, indicate that VSTx1 recognizes VSD under depolarized condition. We identified the VSD-binding residues of VSTx1 and their proximal residues of VSD by the cross-saturation (CS) and amino acid selective CS experiments, which enabled to build a docking model of the complex. These results provide structural basis for the specific binding and inhibition of K_v channels by gating modifier toxins.

Voltage-gated proton (H(v)1) channels, a singular voltage sensing domain

The main role of voltage-gated proton channels (Hv1) is to extrude protons from the intracellular milieu when, mediated by different cellular processes, the H(+) concentration increases. Hv1 are exquisitely selective for protons and their structure is homologous to the voltage sensing domain (VSD) of other voltage-gated ion channels like sodium, potassium, and calcium channels. In clear contrast to the classical voltage-dependent channels, Hv1 lacks a pore domain and thus permeation necessarily occurs through the voltage sensing domain. Hv1 channels are activated by depolarizing voltages, and increases in internal proton concentration. It has been proposed that local conformational changes of the transmembrane segment S4, driven by depolarization, trigger the molecular rearrangements that open Hv1. However, it is still unclear how the electromechanical coupling is achieved between the VSD and the potential pore, allowing the proton flux from the intracellular to the extracellular side. Here we provide a revised view of voltage activation in Hv1 channels, offering a comparative scenario with other voltage sensing channels domains.

Proton channel models filling the gap between experimental data and the structural rationale

Voltage-gated proton channels are integral membrane proteins with the capacity to permeate elementary particles in a voltage and pH dependent manner. These proteins have been found in several species and are involved in various physiological processes. Although their primary topology is known, lack of details regarding their structures in the open conformation has limited analyses toward a deeper understanding of the molecular determinants of their function and regulation. Consequently, the function-structure relationships have been inferred based on homology models. In the present work, we review the existing proton channel models, their assumptions, predictions and the experimental facts that support them. Modeling proton channels is not a trivial task due to the lack of a close homolog template. Hence, there are important differences between published models. This work attempts to critically review existing proton channel models toward the aim of contributing to a better understanding of the structural features of these proteins.

Designs and sensing mechanisms of genetically encoded fluorescent voltage indicators

Neurons tightly regulate the electrical potential difference across the plasma membrane with millivolt accuracy and millisecond resolution. Membrane voltage dynamics underlie the generation of an impulse, the transduction of impulses from one end of the neuron to the other, and the release of neurotransmitters. Imaging these voltage dynamics in multiple neurons simultaneously is therefore crucial for understanding how neurons function together within circuits in intact brains. Genetically encoded fluorescent voltage sensors have long been desired to report voltage in defined subsets of neurons with optical readout. In this review, we discuss the diverse strategies used to design and optimize protein-based voltage sensors, and highlight the chemical mechanisms by which different classes of reporters sense voltage. To guide neuroscientists in choosing an appropriate sensor for their applications, we also describe operating trade-offs of each class of voltage indicators.

Caution is required in interpretation of mutations in the voltage sensing domain of voltage gated channels as evidence for gating mechanisms

The gating mechanism of voltage sensitive ion channels is generally considered to be the motion of the S4 transmembrane segment of the voltage sensing domains (VSD). The primary supporting evidence came from R→C mutations on the S4 transmembrane segment of the VSD, followed by reaction with a methanethiosulfonate (MTS) reagent. The cys side chain is –SH (reactive form –S⁻); the arginine side chain is much larger, leaving space big enough to accommodate the MTS sulfonate head group. The cavity created by the mutation has space for up to seven more water molecules than were present in wild type, which could be displaced irreversibly by the MTS reagent. Our quantum calculations show there is major reorientation of three aromatic residues that face into the cavity in response to proton displacement within the VSD. Two phenylalanines reorient sufficiently to shield/unshield the cysteine from the intracellular and extracellular ends, depending on the proton positions, and a tyrosine forms a hydrogen bond to the cysteine sulfur with its side chain –OH. These could produce the results of the experiments that have been interpreted as evidence for physical motion of the S4 segment, without physical motion of the S4 backbone. The computations strongly suggest that the interpretation of cysteine substitution reaction experiments be re-examined in the light of these considerations.

Conformational dynamics of shaker-type Kv1.1 ion channel in open, closed, and two mutated states

The dynamic properties of shaker-type Kv1.1 ion channel in its open, closed, & two mutated (E325D & V408A) states embedded in DPPC membrane have been investigated using all-atom force field-based MD simulation. Here, we represent the detailed channel stability, gating environment of charge-carrying residues, salt bridge interaction among the voltage-sensing domains (VSDs), movement of S4 helix, and ion conduction of pore. At positive potential, the S4 helix undergoes lateral fluctuations in accordance with their gating motions found in every model. During transition from closed to active state conformation, charged residues of S4 move "up" across the membrane with an average tilt angle difference of 24°, which is more consistent with the paddle model of channel gating. The E325D mutation at C-terminal end of S4-S5 helical linker leads the channel to a rapid activated state by pushing the gating charge residues upward beside the VSDs resulting in more prominent tilt of S4. Similarly in V408A mutant model, disruption of hydrophobic gate at S6 C-terminal end takes place, which causes the violation of channel-active conformation by bringing the C-terminal end of S4 to its corresponding resting state. The ion permeation is observed only in open-state conformation.

Structural interactions of a voltage sensor toxin with lipid membranes

Protein toxins from tarantula venom alter the activity of diverse ion channel proteins, including voltage, stretch, and ligand-activated cation channels. Although tarantula toxins have been shown to partition into membranes, and the membrane is thought to play an important role in their activity, the structural interactions between these toxins and lipid membranes are poorly understood. Here, we use solid-state NMR and neutron diffraction to investigate the interactions between a voltage sensor toxin (VSTx1) and lipid membranes, with the goal of localizing the toxin in the membrane and determining its influence on membrane structure. Our results demonstrate that VSTx1 localizes to the headgroup region of lipid membranes and produces a thinning of the bilayer. The toxin orients such that many basic residues are in the aqueous phase, all three Trp residues adopt interfacial positions, and several hydrophobic residues are within the membrane interior. One remarkable feature of this preferred orientation is that the surface of the toxin that mediates binding to voltage sensors is ideally positioned within the lipid bilayer to favor complex formation between the toxin and the voltage sensor.

Normal mode dynamics of voltage-gated K(+) channels: gating principle, opening mechanism, and inhibition

Voltage-dependent potassium channels open in response to changes in membrane potential and become partially inactivated upon binding of inhibitors. Here we calculate normal mode motion of two voltage-dependent K(+) channels, KvAP and Shaker, and their complexes with inhibitors and address the gating principle, opening mechanism, and inhibition. The normal modes indicate that pore expansion and channel opening is correlated with a displacement of the arginine gating charges and a tilting of the voltage-sensor paddles. Normal modes of Shaker in complex with agitoxin, which blocks the central pore, do not display significantly altered paddle tilting and pore expansion. In contrast, normal modes of Shaker in complex with hanatoxin, which binds to the voltage sensor paddle, display decreased paddle tilting and pore expansion. This study presents a unified motion for the gating principle and channel opening, and offers insight into the voltage sensor paddle motion and its inhibition.

Tethered spectroscopic probes estimate dynamic distances with subnanometer resolution in voltage-dependent potassium channels

Measurements of inter- and intramolecular distances are important for monitoring structural changes and understanding protein interaction networks. Fluorescence resonance energy transfer and functionalized chemical spacers are the two predominantly used strategies to map short-range distances in living cells. Here, we describe the development of a hybrid approach that combines the key advantages of spectroscopic and chemical methods to estimate dynamic distance information from labeled proteins. Bifunctional spectroscopic probes were designed to make use of adaptable-anchor and length-varied spacers to estimate molecular distances by exploiting short-range collisional electron transfer. The spacers were calibrated using labeled polyproline peptides of defined lengths and validated by molecular simulations. This approach was extended to estimate distance restraints that enable us to evaluate the resting-state model of the Shaker potassium channel.

Direct evidence of conformational changes associated with voltage gating in a voltage sensor protein by time-resolved X-ray/neutron interferometry

The voltage sensor domain (VSD) of voltage-gated cation (e.g., Na(+), K(+)) channels central to neurological signal transmission can function as a distinct module. When linked to an otherwise voltage-insensitive, ion-selective membrane pore, the VSD imparts voltage sensitivity to the channel. Proteins homologous with the VSD have recently been found to function themselves as voltage-gated proton channels or to impart voltage sensitivity to enzymes. Determining the conformational changes associated with voltage gating in the VSD itself in the absence of a pore domain thereby gains importance. We report the direct measurement of changes in the scattering-length density (SLD) profile of the VSD protein, vectorially oriented within a reconstituted phospholipid bilayer membrane, as a function of the transmembrane electric potential by time-resolved X-ray and neutron interferometry. The changes in the experimental SLD profiles for both polarizing and depolarizing potentials with respect to zero potential were found to extend over the entire length of the isolated VSD's profile structure. The characteristics of the changes observed were in qualitative agreement with molecular dynamics simulations of a related membrane system, suggesting an initial interpretation of these changes in terms of the VSD's atomic-level 3-D structure.

Structural mechanism of voltage-dependent gating in an isolated voltage-sensing domain

The transduction of transmembrane electric fields into protein motion has an essential role in the generation and propagation of cellular signals. Voltage-sensing domains (VSDs) carry out these functions through reorientations of positive charges in the S4 helix. Here, we determined crystal structures of the Ciona intestinalis VSD (Ci-VSD) in putatively active and resting conformations. S4 undergoes an ~5-Å displacement along its main axis, accompanied by an ~60° rotation. This movement is stabilized by an exchange in countercharge partners in helices S1 and S3 that generates an estimated net charge transfer of ~1 eo. Gating charges move relative to a ''hydrophobic gasket' that electrically divides intra- and extracellular compartments. EPR spectroscopy confirms the limited nature of S4 movement in a membrane environment. These results provide an explicit mechanism for voltage sensing and set the basis for electromechanical coupling in voltage-dependent enzymes and ion channels.

Structural basis of lipid-driven conformational transitions in the KvAP voltage-sensing domain

Voltage-gated ion channels respond to transmembrane electric fields through reorientations of the positively charged S4 helix within the voltage-sensing domain (VSD). Despite a wealth of structural and functional data, the details of this conformational change remain controversial. Recent electrophysiological evidence showed that equilibrium between the resting ('down') and activated ('up') conformations of the KvAP VSD from Aeropyrum pernix can be biased through reconstitution in lipids with or without phosphate groups. We investigated the structural transition between these functional states, using site-directed spin-labeling and EPR spectroscopic methods. Solvent accessibility and interhelical distance determinations suggest that KvAP gates through S4 movements involving an ∼3-Å upward tilt and simultaneous ∼2-Å axial shift. This motion leads to large accessibly changes in the intracellular water-filled crevice and supports a new model of gating that combines structural rearrangements and electric-field remodeling.

Optically detected structural change in the N-terminal region of the voltage-sensor domain

The voltage-sensor domain (VSD) is a functional module that undergoes structural transitions in response to membrane potential changes and regulates its effectors, thereby playing a crucial role in amplifying and decoding membrane electrical signals. Ion-conductive pore and phosphoinositide phosphatase are the downstream effectors of voltage-gated channels and the voltage-sensing phosphatase, respectively. It is known that upon transition, the VSD generally acts on the region C-terminal to S4. However, whether the VSD also induces any structural changes in the N-terminal region of S1 has not been addressed directly. Here, we report the existence of such an N-terminal effect. We used two distinct optical reporters-one based on the Förster resonance energy transfer between a pair of fluorescent proteins, and the other based on fluorophore-labeled HaloTag-and studied the behavior of these reporters placed at the N-terminal end of the monomeric VSD derived from voltage-sensing phosphatase. We found that both of these reporters were affected by the VSD transition, generating voltage-dependent fluorescence readouts. We also observed that whereas the voltage dependencies of the N- and C-terminal effects appear to be tightly coupled, the local structural rearrangements reflect the way in which the VSD is loaded, demonstrating the flexible nature of the VSD.

Modulation of TRP ion channels by venomous toxins

Venoms are evolutionarily fine-tuned mixtures of small molecules, peptides, and proteins-referred to as toxins-that have evolved to specifically modulate and interfere with the function of diverse molecular targets within the envenomated animal. Many of the identified toxin targets are membrane receptors and ion channels. Due to their high specificity, toxins have emerged as an invaluable tool set for the molecular characterization of ion channels, and a selected group of toxins even have been developed into therapeutics. More recently, TRP ion channels have been included as targets for venomous toxins. In particular, a number of apparently unrelated peptide toxins target the capsaicin receptor TRPV1 to produce inflammatory pain. These toxins have turned out to be invaluable for structural and functional characterizations of the capsaicin receptor. If toxins will serve similar roles for other TRP ion channels, only future will tell.

Hydrophobic plug functions as a gate in voltage-gated proton channels

Voltage-gated proton (Hv1) channels play important roles in the respiratory burst, in pH regulation, in spermatozoa, in apoptosis, and in cancer metastasis. Unlike other voltage-gated cation channels, the Hv1 channel lacks a centrally located pore formed by the assembly of subunits. Instead, the proton permeation pathway in the Hv1 channel is within the voltage-sensing domain of each subunit. The gating mechanism of this pathway is still unclear. Mutagenic and fluorescence studies suggest that the fourth transmembrane (TM) segment (S4) functions as a voltage sensor and that there is an outward movement of S4 during channel activation. Using thermodynamic mutant cycle analysis, we find that the conserved positively charged residues in S4 are stabilized by countercharges in the other TM segments both in the closed and open states. We constructed models of both the closed and open states of Hv1 channels that are consistent with the mutant cycle analysis. These structural models suggest that electrostatic interactions between TM segments in the closed state pull hydrophobic residues together to form a hydrophobic plug in the center of the voltage-sensing domain. Outward S4 movement during channel activation induces conformational changes that remove this hydrophobic plug and instead insert protonatable residues in the center of the channel that, together with water molecules, can form a hydrogen bond chain across the channel for proton permeation. This suggests that salt bridge networks and the hydrophobic plug function as the gate in Hv1 channels and that outward movement of S4 leads to the opening of this gate.

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.

Removal of human ether-à-go-go related gene (hERG) K+ channel affinity through rigidity: a case of clofilium analogues

Cardiotoxicity is a side effect that plagues modern drug design and is very often due to the off-target blockade of the human ether-à-go-go related gene (hERG) potassium channel. To better understand the structural determinants of this blockade, we designed and synthesized a series of 40 derivatives of clofilium, a class III antiarrhythmic agent. These were evaluated in radioligand binding and patch-clamp assays to establish structure-affinity relationships (SAR) for this potassium channel. Efforts were especially focused on studying the influence of the structural rigidity and the nature of the linkers composing the clofilium scaffold. It was shown that introducing triple bonds and oxygen atoms in the n-butyl linker of the molecule greatly reduced affinity without significantly modifying the pKa of the essential basic nitrogen. These findings could prove useful in the first stages of drug discovery as a systematic way of reducing the risk of hERG K(+) channel blockade-induced cardiotoxicity.

Intermediate state trapping of a voltage sensor

Voltage sensor domains (VSDs) regulate ion channels and enzymes by undergoing conformational changes depending on membrane electrical signals. The molecular mechanisms underlying the VSD transitions are not fully understood. Here, we show that some mutations of I241 in the S1 segment of the Shaker Kv channel positively shift the voltage dependence of the VSD movement and alter the functional coupling between VSD and pore domains. Among the I241 mutants, I241W immobilized the VSD movement during activation and deactivation, approximately halfway between the resting and active states, and drastically shifted the voltage activation of the ionic conductance. This phenotype, which is consistent with a stabilization of an intermediate VSD conformation by the I241W mutation, was diminished by the charge-conserving R2K mutation but not by the charge-neutralizing R2Q mutation. Interestingly, most of these effects were reproduced by the F244W mutation located one helical turn above I241. Electrophysiology recordings using nonnatural indole derivatives ruled out the involvement of cation-Π interactions for the effects of the Trp inserted at positions I241 and F244 on the channel’s conductance, but showed that the indole nitrogen was important for the I241W phenotype. Insight into the molecular mechanisms responsible for the stabilization of the intermediate state were investigated by creating in silico the mutations I241W, I241W/R2K, and F244W in intermediate conformations obtained from a computational VSD transition pathway determined using the string method. The experimental results and computational analysis suggest that the phenotype of I241W may originate in the formation of a hydrogen bond between the indole nitrogen atom and the backbone carbonyl of R2. This work provides new information on intermediate states in voltage-gated ion channels with an approach that produces minimum chemical perturbation.

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.