Functionality of the voltage-gated proton channel truncated in S4

Trp207 regulation of voltage-dependent activation of human Hv1 proton channel

In voltage-gated Na+ and K+ channels, the hydrophobicity of noncharged residues in the S4 helix has been shown to regulate the S4 movement underlying the process of voltage-sensing domain (VSD) activation. In voltage-gated proton channel Hv1, there is a bulky noncharged tryptophan residue located at the S4 transmembrane segment. This tryptophan remains entirely conserved across all Hv1 members but is not seen in other voltage-gated ion channels, indicating that the tryptophan contributes different roles in VSD activation. The conserved tryptophan of human voltage-gated proton channel Hv1 is Trp207 (W207). Here, we showed that W207 modifies human Hv1 voltage-dependent activation, and small residues replacement at position 207 strongly perturbs Hv1 channel opening and closing, and the size of the side chain instead of the hydrophobic group of W207 regulates the transition between closed and open states of the channel. We conclude that the large side chain of tryptophan controls the energy barrier during the Hv1 VSD transition.

Trapping Charge Mechanism in Hv1 Channels (CiHv1)

The majority of voltage-gated ion channels contain a defined voltage-sensing domain and a pore domain composed of highly conserved amino acid residues that confer electrical excitability via electromechanical coupling. In this sense, the voltage-gated proton channel (Hv1) is a unique protein in that voltage-sensing, proton permeation and pH-dependent modulation involve the same structural region. In fact, these processes synergistically work in concert, and it is difficult to separate them. To investigate the process of Hv1 voltage sensor trapping, we follow voltage-sensor movements directly by leveraging mutations that enable the measurement of Hv1 channel gating currents. We uncover that the process of voltage sensor displacement is due to two driving forces. The first reveals that mutations in the selectivity filter (D160) located in the S1 transmembrane interact with the voltage sensor. More hydrophobic amino acids increase the energy barrier for voltage sensor activation. On the other hand, the effect of positive charges near position 264 promotes the formation of salt bridges between the arginines of the voltage sensor domain, achieving a stable conformation over time. Our results suggest that the activation of the Hv1 voltage sensor is governed by electrostatic-hydrophobic interactions, and S4 arginines, N264 and selectivity filter (D160) are essential in the Ciona-Hv1 to understand the trapping of the voltage sensor.

Proton Paths in Models of the Hv1 Proton Channel

The voltage-gated proton channel (Hv1) plays an essential role in numerous biological processes, but a detailed molecular understanding of its function is lacking. The lack of reliable structures for the open and resting states is a major handicap. Several models have been built based on homologous voltage sensors and the structure of a chimera between the mouse homologue and a phosphatase voltage sensor, but their validity is uncertain. In addition, differing views exist regarding the mode of proton translocation, the role of specific residues, and the mechanism of pH effects on voltage gating. Here we use classical proton hopping simulations under a voltage biasing force to evaluate some of the proposed structural models and explore the mechanism of proton conduction. Paradoxically, some models proposed for the closed state allow for proton permeation more easily than models for the open state. An open state model with a D112-R211 salt bridge (R3D) allows proton transport more easily than models with a D112-R208 salt bridge (R2D). However, its permeation rate seems too high, considering experimental conductances. In all cases, the proton permeates through a water wire, bypassing the salt-bridged D112 rather than being shuttled by D112. Attempts to protonate D112 are rejected due to its strong interaction with an arginine. Consistent with proton selectivity, no Na+ permeation was observed in the R2D models. As a negative control, simulations with the Kv1.2-Kv2.1 paddle-chimera voltage sensor, which is not expected to conduct protons, did not show proton permeation under the same conditions. Hydrogen bond connectivity graphs show a constriction at D112, but cannot discriminate between open and closed states.

Voltage-Gated Proton Channels in the Tree of Life

With a single gene encoding H_V1 channel, proton channel diversity is particularly low in mammals compared to other members of the superfamily of voltage-gated ion channels. Nonetheless, mammalian H_V1 channels are expressed in many different tissues and cell types where they exert various functions. In the first part of this review, we regard novel aspects of the functional expression of H_V1 channels in mammals by differentially comparing their involvement in (1) close conjunction with the NADPH oxidase complex responsible for the respiratory burst of phagocytes, and (2) in respiratory burst independent functions such as pH homeostasis or acid extrusion. In the second part, we dissect expression of H_V channels within the eukaryotic tree of life, revealing the immense diversity of the channel in other phylae, such as mollusks or dinoflagellates, where several genes encoding H_V channels can be found within a single species. In the last part, a comprehensive overview of the biophysical properties of a set of twenty different H_V channels characterized electrophysiologically, from Mammalia to unicellular protists, is given.

Unexpected expansion of the voltage-gated proton channel family

Voltage-gated ion channels, whose first identified function was to generate action potentials, are divided into subfamilies with numerous members. The family of voltage-gated proton channels (HV ) is tiny. To date, all species found to express HV have exclusively one gene that codes for this unique ion channel. Here we report the discovery and characterization of three proton channel genes in the classical model system of neural plasticity, Aplysia californica. The three channels (AcHV 1, AcHV 2, and AcHV 3) are distributed throughout the whole animal. Patch-clamp analysis confirmed proton selectivity of these channels but they all differed markedly in gating. AcHV 1 gating resembled HV in mammalian cells where it is responsible for proton extrusion and charge compensation. AcHV 2 activates more negatively and conducts extensive inward proton current, properties likely to acidify the cytosol. AcHV 3, which differs from AcHV 1 and AcHV 2 in lacking the first arginine in the S4 helix, exhibits proton selective leak currents and weak voltage dependence. We report the expansion of the proton channel family, demonstrating for the first time the expression of three functionally distinct proton channels in a single species.

Role of voltage-gated proton channel (Hv1) in cancer biology

The acid-base characteristics of tumor cells and the other elements that compose the tumor microenvironment have been topics of scientific interest in oncological research. There is much evidence confirming that pH conditions are maintained by changes in the patterns of expression of certain proton transporters. In the past decade, the voltage-gated proton channel (Hv1) has been added to this list and is increasingly being recognized as a target with onco-therapeutic potential. The Hv1 channel is key to proton extrusion for maintaining a balanced cytosolic pH. This protein-channel is expressed in a myriad of tissues and cell lineages whose functions vary from producing bioluminescence in dinoflagellates to alkalizing spermatozoa cytoplasm for reproduction, and regulating the respiratory burst for immune system response. It is no wonder that in acidic environments such as the tumor microenvironment, an exacerbated expression and function of this channel has been reported. Indeed, multiple studies have revealed a strong relationship between pH balance, cancer development, and the overexpression of the Hv1 channel, being proposed as a marker for malignancy in cancer. In this review, we present data that supports the idea that the Hv1 channel plays a significant role in cancer by maintaining pH conditions that favor the development of malignancy features in solid tumor models. With the antecedents presented in this bibliographic report, we want to strengthen the idea that the Hv1 proton channel is an excellent therapeutic strategy to counter the development of solid tumors.

Role of the time dependence of Boltzmann open probability in voltage-gated proton channels

The modeling and simulation of experimental families of current-time (I-t) curves of dimeric voltage-gated proton channels and of proton-conducting voltage sensing domains (VSDs) with a minimum of free parameters requires the movement of protons to be controlled by the rate of increase of the Boltzmann open probability p over time in passing from the holding to the depolarizing potential. Families of I-t curves of protomers and proton-conducting VSDs can be satisfactorily fitted by the use of a single free parameter expressing the rate constant k_p for the increase of p over time. Families of I-t curves of dimeric H_v1 channels can be fitted by a model that assumes an initial proton current I₁ flowing along the two monomeric units, while they are still operating separately; I₁ is gradually replaced by a slower and more potential-dependent current I₂ flowing when the two monomers start operating jointly under the control of the coiled-coil domain. Here too, p is assumed to increase over time with a rate constant k_p that doubles in passing from I₁ to I₂, with fit requiring three free parameters. Chord conductance yields erroneously high gating charges when fitted by the Boltzmann function, differently from slope conductance.

Assessing Structural Determinants of Zn2+ Binding to Human HV1 via Multiple MD Simulations

Voltage-gated proton channels (H_V1) are essential for various physiological tasks but are strongly inhibited by Zn²⁺ cations. Some determinants of Zn²⁺ binding have been elucidated experimentally and in computational studies. However, the results have always been interpreted under the assumption that Zn²⁺ binds to monomeric H_V1 despite evidence that H_V1 expresses as a dimer and that the dimer has a higher affinity for zinc than the monomer and experimental data that suggest coordination in the dimer interface. The results of former studies are also controversial, e.g., supporting either one single or two binding sites. Some structural determinants of the binding are still elusive. We performed a series of molecular dynamics simulations to address different structures of the human proton channel, the monomer and two plausible dimer conformations, to compare their respective potential to interact with and bind Zn²⁺ via the essential histidines. The series consisted of several copies of the system to generate independent trajectories and increase the significance compared to a single simulation. The amount of time simulated totals 29.9 μs for 126 simulations of systems comprising ∼59,000 to ∼187,000 atoms. Our approach confirms the existence of two binding sites in monomeric and dimeric human H_V1. The dimer interface is more efficient for attracting and binding Zn²⁺ via the essential histidines than the monomer or a dimer with the histidines in the periphery. The higher affinity is due to the residues in the dimer interface that create an attractive electrostatic potential funneling the zinc cations toward the binding sites.

Voltage vs. Ligand II: Structural insights of the intrinsic flexibility in cyclic nucleotide-gated channels

In the preceding article, we present a flexibility analysis of the voltage-gated ion channel (VGIC) superfamily. In this study, we describe in detail the flexibility profile of the voltage-sensor domain (VSD) and the pore domain (PD) concerning the evolution of 6TM ion channels. In particular, we highlight the role of flexibility in the emergence of CNG channels and describe a significant level of sequence similarity between the archetypical VSD and the TolQ proteins. A highly flexible S4-like segment exhibiting Lys instead Arg for these membrane proteins is reported. Sequence analysis indicates that, in addition to this S4-like segment, TolQ proteins also show similarity with specific motifs in S2 and S3 from typical V-sensors. Notably, S3 flexibility profiles from typical VSDs and S3-like in TolQ proteins are also similar. Interestingly, TolQ from early divergent prokaryotes are comparatively more flexible than those in modern counterparts or true V-sensors. Regarding the PD, we also found that 2TM K+-channels in early prokaryotes are considerably more flexible than the ones in modern microbes, and such flexibility is comparable to the one present in CNG channels. Voltage dependence is mainly exhibited in prokaryotic CNG channels whose VSD is rigid whereas the eukaryotic CNG channels are considerably more flexible and poorly V-dependent. The implication of the flexibility present in CNG channels, their sensitivity to cyclic nucleotides and the cation selectivity are discussed. Finally, we generated a structural model of the putative cyclic nucleotide-modulated ion channel, which we coined here as AqK, from the thermophilic bacteria Aquifex aeolicus, one of the earliest diverging prokaryotes known. Overall, our analysis suggests that V-sensors in CNG-like channels were essentially rigid in early prokaryotes but raises the possibility that this module was probably part of a very flexible stator protein of the bacterial flagellum motor complex.

Gating Currents in the Hv1 Proton Channel

The Hv1 proton channel shares striking structural homology with fourth transmembrane helical segment-type voltage-sensor (VS) domains but manifests distinctive functional properties, including a proton-selective "aqueous" conductance and allosteric control of voltage-dependent gating by changes in the transmembrane pH gradient. The mechanisms responsible for Hv1's functional properties remain poorly understood, in part because methods for measuring gating currents that directly report VS activation have not yet been described. Here, we describe an approach that allows robust and reproducible measurement of gating-associated charge movements in Hv1. Gating currents reveal that VS activation and proton-selective aqueous conductance opening are thermodynamically distinct steps in the Hv1 activation pathway and show that pH changes directly alter VS activation. The availability of an assay for gating currents in Hv1 may aid future efforts to elucidate the molecular mechanisms of gating cooperativity, pH-dependent modulation, and H⁺ selectivity in a model VS domain protein.

Gating charge displacement in a monomeric voltage-gated proton (Hv1) channel

The voltage-gated proton (Hv1) channel, a voltage sensor and a conductive pore contained in one structural module, plays important roles in many physiological processes. Voltage sensor movements can be directly detected by measuring gating currents, and a detailed characterization of Hv1 charge displacements during channel activation can help to understand the function of this channel. We succeeded in detecting gating currents in the monomeric form of the Ciona-Hv1 channel. To decrease proton currents and better separate gating currents from ion currents, we used the low-conducting Hv1 mutant N264R. Isolated ON-gating currents decayed at increasing rates with increasing membrane depolarization, and the amount of gating charges displaced saturates at high voltages. These are two hallmarks of currents arising from the movement of charged elements within the boundaries of the cell membrane. The kinetic analysis of gating currents revealed a complex time course of the ON-gating current characterized by two peaks and a marked Cole-Moore effect. Both features argue that the voltage sensor undergoes several voltage-dependent conformational changes during activation. However, most of the charge is displaced in a single central transition. Upon voltage sensor activation, the charge is trapped, and only a fast component that carries a small percentage of the total charge is observed in the OFF. We hypothesize that trapping is due to the presence of the arginine side chain in position 264, which acts as a blocking ion. We conclude that the movement of the voltage sensor must proceed through at least five states to account for our experimental data satisfactorily.

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.

Insights into the structure and function of HV1 from a meta-analysis of mutation studies

The voltage-gated proton channel (HV1) is a widely distributed, proton-specific ion channel with unique properties. Since 2006, when genes for HV1 were identified, a vast array of mutations have been generated and characterized. Accessing this potentially useful resource is hindered, however, by the sheer number of mutations and interspecies differences in amino acid numbering. This review organizes all existing information in a logical manner to allow swift identification of studies that have characterized any particular mutation. Although much can be gained from this meta-analysis, important questions about the inner workings of HV1 await future revelation.

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.

Gating mechanisms of voltage-gated proton channels

Hv1 is a voltage-gated proton-selective channel that plays critical parts in host defense, sperm motility, and cancer progression. Hv1 contains a conserved voltage-sensor domain (VSD) that is shared by a large family of voltage-gated ion channels, but it lacks a pore domain. Voltage sensitivity and proton conductivity are conferred by a unitary VSD that consists of four transmembrane helices. The architecture of Hv1 differs from that of cation channels that form a pore in the center among multiple subunits (as in most cation channels) or homologous repeats (as in voltage-gated sodium and calcium channels). Hv1 forms a dimer in which a cytoplasmic coiled coil underpins the two protomers and forms a single, long helix that is contiguous with S4, the transmembrane voltage-sensing segment. The closed-state structure of Hv1 was recently solved using X-ray crystallography. In this article, we discuss the gating mechanism of Hv1 and focus on cooperativity within dimers and their sensitivity to metal ions.

A specialized molecular motion opens the Hv1 voltage-gated proton channel

The Hv1 proton channel is unique among voltage-gated channels for containing the pore and gate within its voltage-sensing domain. Pore opening has been proposed to include assembly of the selectivity filter between an arginine (R3) of segment S4 and an aspartate (D1) of segment S1. We determined whether gating involves motion of S1, using Ciona intestinalis Hv1. We found that channel opening is concomitant with solution access to the pore-lining face of S1, from the cytoplasm to deep inside the pore. Voltage- and patch-clamp fluorometry showed that this involves a motion of S1 relative to its surroundings. S1 motion and the S4 motion that precedes it are each influenced by residues on the other helix, thus suggesting a dynamic interaction between S1 and S4. Our findings suggest that the S1 of Hv1 has specialized to function as part of the channel's gate.

Long α helices projecting from the membrane as the dimer interface in the voltage-gated H(+) channel

Continuous helices extending from the transmembrane region to the cytoplasmic region form a dimeric interface to regulate activation of the voltage-gated H⁺ channel.

The voltage-gated H⁺ channel (Hv) is a H⁺-permeable voltage-sensor domain (VSD) protein that consists of four transmembrane segments (S1–S4). Hv assembles as a dimeric channel and two transmembrane channel domains function cooperatively, which is mediated by the coiled-coil assembly domain in the cytoplasmic C terminus. However, the structural basis of the interdomain interactions remains unknown. Here, we provide a picture of the dimer configuration based on the analyses of interactions among two VSDs and a coiled-coil domain. Systematic mutations of the linker region between S4 of VSD and the coiled-coil showed that the channel gating was altered in the helical periodicity with the linker length, suggesting that two domains are linked by helices. Cross-linking analyses revealed that the two S4 helices were situated closely in the dimeric channel. The interaction interface between the two S4 and the assembly interface of the coiled-coil domain were aligned in the same direction based on the phase angle calculation along α helices. Collectively, we propose that continuous helices stretching from the transmembrane to the cytoplasmic region in the dimeric interface regulate the channel activation in the Hv dimer.

Detergent screening of the human voltage-gated proton channel using fluorescence-detection size-exclusion chromatography

The human voltage-gated proton channel (Hv1) is a membrane protein consisting of four transmembrane domains and intracellular amino- and carboxy-termini. The protein is activated by membrane depolarization, similar to other voltage-sensitive proteins. However, the Hv1 proton channel lacks a traditional ion pore. The human Hv1 proton channel has been implicated in mediating sperm capacitance, stroke, and most recently as a biomarker/mediator of cancer metastasis. Recently, the three-dimensional structures for homologues of this voltage-gated proton channel were reported. However, it is not clear what artificial environment is needed to facilitate the isolation and purification of the human Hv1 proton channel for structural study. In the present study, we generated a chimeric protein that placed an enhanced green fluorescent protein (EGFP) to the amino-terminus of the human Hv1 proton channel (termed EGFP-Hv1). The chimeric protein was expressed in a baculovirus expression system using Sf9 cells and subjected to detergent screening using fluorescence-detection size-exclusion chromatography. The EGFP-Hv1 proton channel can be solubilized in the zwitterionic detergent Anzergent 3-12 and the nonionic n-dodecyl-β-d-maltoside (DDM) with little protein aggregation and a prominent monomeric protein peak at 48 h postinfection. Furthermore, we demonstrate that the chimeric protein exhibits a monomeric protein peak, which is distinguishable from protein aggregates, at the final size-exclusion chromatography purification step. Taken together, we can conclude that solubilization in DDM will provide a useable final product for further structural characterization of the full-length human Hv1 proton channel.

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.

Mapping of sites facing aqueous environment of voltage-gated proton channel at resting state: a study with PEGylation protection

Hv1 (also named, voltage-sensor only protein, VSOP) lacks an authentic pore domain, and its voltage sensor domain plays both roles in voltage sensing and proton permeation. The activities of a proton channel are intrinsic to protomers of Hv1, while Hv1 is dimeric in biological membranes; cooperative gating is exerted by interaction between two protomers. As the signature pattern conserved among voltage-gated channels and voltage-sensing phosphatase, Hv1 has multiple arginines intervened by two hydrophobic residues on the fourth transmembrane segment, S4. S4 moves upward relative to other helices upon depolarization, causing conformational change possibly leading to the formation of a proton-selective conduction pathway. However, detailed mechanisms of proton-selectivity and gating of Hv1 are unknown. Here we took an approach of PEGylation protection assay to define residues facing the aqueous environment of mouse Hv1 (mHv1). Accessibilities of two maleimide molecules, N-ethylmaleimide (NEM) and 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid (AMS), were examined on cysteine introduced into individual sites. Only the first arginine on S4 (R1: R201) was inaccessible by NEM and AMS in mHv1. This is consistent with previous results of electrophysiology on the resting state channel, suggesting that the accessibility profile represents the resting state of mHv1. D108, critical for proton selectivity, was accessible by AMS and NEM, suggesting that D108 faces the vestibule. F146, a site critical for blocking by a guanidinium-reagent, was accessible by NEM, suggesting that F146 also faces the inner vestibule. These findings suggest an inner vestibule lined by several residues on S2 including F146, D108 on S1, and the C-terminal half of S4.

Construction and validation of a homology model of the human voltage-gated proton channel hHV1

The topological similarity of voltage-gated proton channels (H_V1s) to the voltage-sensing domain (VSD) of other voltage-gated ion channels raises the central question of whether H_V1s have a similar structure. We present the construction and validation of a homology model of the human H_V1 (hH_V1). Multiple structural alignment was used to construct structural models of the open (proton-conducting) state of hH_V1 by exploiting the homology of hH_V1 with VSDs of K⁺ and Na⁺ channels of known three-dimensional structure. The comparative assessment of structural stability of the homology models and their VSD templates was performed using massively repeated molecular dynamics simulations in which the proteins were allowed to relax from their initial conformation in an explicit membrane mimetic. The analysis of structural deviations from the initial conformation based on up to 125 repeats of 100-ns simulations for each system reveals structural features consistently retained in the homology models and leads to a consensus structural model for hH_V1 in which well-defined external and internal salt-bridge networks stabilize the open state. The structural and electrostatic properties of this open-state model are compatible with proton translocation and offer an explanation for the reversal of charge selectivity in neutral mutants of Asp¹¹². Furthermore, these structural properties are consistent with experimental accessibility data, providing a valuable basis for further structural and functional studies of hH_V1. Each Arg residue in the S4 helix of hH_V1 was replaced by His to test accessibility using Zn²⁺ as a probe. The two outermost Arg residues in S4 were accessible to external solution, whereas the innermost one was accessible only to the internal solution. Both modeling and experimental data indicate that in the open state, Arg²¹¹, the third Arg residue in the S4 helix in hH_V1, remains accessible to the internal solution and is located near the charge transfer center, Phe¹⁵⁰.

Structural characteristics of the redox-sensing coiled coil in the voltage-gated H+ channel

Oxidation is an important biochemical defense mechanism, but it also elicits toxicity; therefore, oxidation must be under strict control. In phagocytotic events in neutrophils, the voltage-gated H(+) (Hv) channel is a key regulator of the production of reactive oxygen species against invading bacteria. The cytoplasmic domain of the Hv channel forms a dimeric coiled coil underpinning a dimerized functional unit. Importantly, in the alignment of the coiled-coil core, a conserved cysteine residue forms a potential intersubunit disulfide bond. In this study, we solved the crystal structures of the coiled-coil domain in reduced, oxidized, and mutated (Cys → Ser) states. The crystal structures indicate that a pair of Cys residues forms an intersubunit disulfide bond dependent on the redox conditions. CD spectroscopy revealed that the disulfide bond increases the thermal stability of the coiled-coil protein. We also reveal that two thiol modifier molecules are able to bind to Cys in a redox-dependent manner without disruption of the dimeric coiled-coil assembly. Thus, the biochemical properties of the cytoplasmic coiled-coil domain in the Hv channel depend on the redox condition, which may play a role in redox sensing in the phagosome.

Voltage-gated proton channels: molecular biology, physiology, and pathophysiology of the H(V) family

Voltage-gated proton channels (H(V)) are unique, in part because the ion they conduct is unique. H(V) channels are perfectly selective for protons and have a very small unitary conductance, both arguably manifestations of the extremely low H(+) concentration in physiological solutions. They open with membrane depolarization, but their voltage dependence is strongly regulated by the pH gradient across the membrane (ΔpH), with the result that in most species they normally conduct only outward current. The H(V) channel protein is strikingly similar to the voltage-sensing domain (VSD, the first four membrane-spanning segments) of voltage-gated K(+) and Na(+) channels. In higher species, H(V) channels exist as dimers in which each protomer has its own conduction pathway, yet gating is cooperative. H(V) channels are phylogenetically diverse, distributed from humans to unicellular marine life, and perhaps even plants. Correspondingly, H(V) functions vary widely as well, from promoting calcification in coccolithophores and triggering bioluminescent flashes in dinoflagellates to facilitating killing bacteria, airway pH regulation, basophil histamine release, sperm maturation, and B lymphocyte responses in humans. Recent evidence that hH(V)1 may exacerbate breast cancer metastasis and cerebral damage from ischemic stroke highlights the rapidly expanding recognition of the clinical importance of hH(V)1.

Subunit interactions during cooperative opening of voltage-gated proton channels

Voltage-gated proton (Hv1) channels are dimers, where each subunit has a separate permeation pathway. However, opening of the two pathways is highly cooperative. It is unclear how Hv1 channels open their permeation pathways, because Hv1 channels lack a classic pore domain. Using voltage-clamp fluorometry, we here detect two conformational changes reported by a fluorophore attached to the voltage sensor S4 in Hv1 channels. The first is voltage dependent and precedes channel opening, with properties consistent with reporting on independent S4 charge movements in the two subunits. The second is less voltage dependent and closely correlates with channel opening. Mutations that reduce dimerization or alter the intersubunit interface affect both the second conformational change and channel opening. These observations suggest that, following an initial S4 charge movement in the two subunits, there is a second, cooperative conformational change, involving interactions between subunits, that opens both pathways in Hv1 channels.

Molecular mechanism of voltage sensing in voltage-gated proton channels

Voltage-gated proton (Hv) channels play an essential role in phagocytic cells by generating a hyperpolarizing proton current that electrically compensates for the depolarizing current generated by the NADPH oxidase during the respiratory burst, thereby ensuring a sustained production of reactive oxygen species by the NADPH oxidase in phagocytes to neutralize engulfed bacteria. Despite the importance of the voltage-dependent Hv current, it is at present unclear which residues in Hv channels are responsible for the voltage activation. Here we show that individual neutralizations of three charged residues in the fourth transmembrane domain, S4, all reduce the voltage dependence of activation. In addition, we show that the middle S4 charged residue moves from a position accessible from the cytosolic solution to a position accessible from the extracellular solution, suggesting that this residue moves across most of the membrane electric field during voltage activation of Hv channels. Our results show for the first time that the charge movement of these three S4 charges accounts for almost all of the measured gating charge in Hv channels.

Gating of the designed trimeric/tetrameric voltage-gated H+ channel

The voltage-gated H(+) channel functions as a dimer, a configuration that is different from standard tetrameric voltage-gated channels. Each channel protomer has its own permeation pathway. The C-terminal coiled-coil domain has been shown to be necessary for both dimerization and cooperative gating in the two channel protomers. Here we report the gating cooperativity in trimeric and tetrameric Hv channels engineered by altering the hydrophobic core sequence of the coiled-coil assembly domain. Trimeric and tetrameric channels exhibited more rapid and less sigmoidal kinetics of activation of H(+) permeation than dimeric channels, suggesting that some channel protomers in trimers and tetramers failed to produce gating cooperativity observed in wild-type dimers. Multimerization of trimer and tetramer channels were confirmed by the biochemical analysis of proteins, including crystallography. These findings indicate that the voltage-gated H(+) channel is optimally designed as a dimeric channel on a solid foundation of the sequence pattern of the coiled-coil core, with efficient cooperative gating that ensures sustained and steep voltage-dependent H(+) conductance in blood cells.

Biophysical properties of the voltage gated proton channel H(V)1

The biophysical properties of the voltage gated proton channel (H(V)1) are the key elements of its physiological function. The voltage gated proton channel is a unique molecule that in contrast to all other ion channels is exclusively selective for protons. Alone among proton channels, it has voltage and time dependent gating like other "classical" ion channels. H(V)1 is furthermore a sensor for the pH in the cell and the surrounding media. Its voltage dependence is strictly coupled to the pH gradient across the membrane. This regulation restricts opening of the channel to specific voltages at any given pH gradient, therefore allowing H(V)1 to perform its physiological task in the tissue it is expressed in. For H(V)1 there is no known blocker. The most potent channel inhibitor is zinc (Zn(2+)) which prevents channel opening. An additional characteristic of H(V)1 is its strong temperature dependence of both gating and conductance. In contrast to single-file water filled pores like the gramicidin channel, H(V)1 exhibits pronounced deuterium effects and temperature effects on conduction, consistent with a different conduction mechanism than other ion channels. These properties may be explained by the recent identification of an aspartate in the pore of H(V)1 that is essential to its proton selectivity.

The cytoplasmic coiled-coil mediates cooperative gating temperature sensitivity in the voltage-gated H(+) channel Hv1

Hv1/VSOP is a dimeric voltage-gated H(+) channel in which the gating of one subunit is reportedly coupled to that of the other subunit within the dimer. The molecular basis for dimer formation and intersubunit coupling, however, remains unknown. Here we show that the carboxy terminus ends downstream of the S4 voltage-sensor helix twist in a dimer coiled-coil architecture, which mediates cooperative gating. We also show that the temperature-dependent activation of H(+) current through Hv1/VSOP is regulated by thermostability of the coiled-coil domain, and that this regulation is altered by mutation of the linker between S4 and the coiled-coil. Cooperative gating within the dimer is also dependent on the linker structure, which circular dichroism spectrum analysis suggests is α-helical. Our results indicate that the cytoplasmic coiled-coil strands form continuous α-helices with S4 and mediate cooperative gating to adjust the range of temperatures over which Hv1/VSOP operates.

Voltage-gated proton channels

Voltage-gated proton channels, HV1, have vaulted from the realm of the esoteric into the forefront of a central question facing ion channel biophysicists, namely, the mechanism by which voltage-dependent gating occurs. This transformation is the result of several factors. Identification of the gene in 2006 revealed that proton channels are homologues of the voltage-sensing domain of most other voltage-gated ion channels. Unique, or at least eccentric, properties of proton channels include dimeric architecture with dual conduction pathways, perfect proton selectivity, a single-channel conductance approximately 10(3) times smaller than most ion channels, voltage-dependent gating that is strongly modulated by the pH gradient, ΔpH, and potent inhibition by Zn(2+) (in many species) but an absence of other potent inhibitors. The recent identification of HV1 in three unicellular marine plankton species has dramatically expanded the phylogenetic family tree. Interest in proton channels in their own right has increased as important physiological roles have been identified in many cells. Proton channels trigger the bioluminescent flash of dinoflagellates, facilitate calcification by coccolithophores, regulate pH-dependent processes in eggs and sperm during fertilization, secrete acid to control the pH of airway fluids, facilitate histamine secretion by basophils, and play a signaling role in facilitating B-cell receptor mediated responses in B-lymphocytes. The most elaborate and best-established functions occur in phagocytes, where proton channels optimize the activity of NADPH oxidase, an important producer of reactive oxygen species. Proton efflux mediated by HV1 balances the charge translocated across the membrane by electrons through NADPH oxidase, minimizes changes in cytoplasmic and phagosomal pH, limits osmotic swelling of the phagosome, and provides substrate H(+) for the production of H2O2 and HOCl, reactive oxygen species crucial to killing pathogens.

The pore of the voltage-gated proton channel

In classical tetrameric voltage-gated ion channels four voltage-sensing domains (VSDs), one from each subunit, control one ion permeation pathway formed by four pore domains. The human Hv1 proton channel has a different architecture, containing a VSD, but lacking a pore domain. Since its location is not known, we searched for the Hv permeation pathway. We find that mutation of the S4 segment's third arginine R211 (R3) compromises proton selectivity, enabling conduction of a metal cation and even of the large organic cation guanidinium, reminiscent of Shaker's omega pore. In the open state, R3 appears to interact with an aspartate (D112) that is situated in the middle of S1 and is unique to Hv channels. The double mutation of both residues further compromises cation selectivity. We propose that membrane depolarization reversibly positions R3 next to D112 in the transmembrane VSD to form the ion selectivity filter in the channel's open conformation.

Voltage-gated proton channel in a dinoflagellate

Fogel and Hastings first hypothesized the existence of voltage-gated proton channels in 1972 in bioluminescent dinoflagellates, where they were thought to trigger the flash by activating luciferase. Proton channel genes were subsequently identified in human, mouse, and Ciona intestinalis, but their existence in dinoflagellates remained unconfirmed. We identified a candidate proton channel gene from a Karlodinium veneficum cDNA library based on homology with known proton channel genes. K. veneficum is a predatory, nonbioluminescent dinoflagellate that produces toxins responsible for fish kills worldwide. Patch clamp studies on the heterologously expressed gene confirm that it codes for a genuine voltage-gated proton channel, kH(V)1: it is proton-specific and activated by depolarization, its g(H)-V relationship shifts with changes in external or internal pH, and mutation of the selectivity filter (which we identify as Asp(51)) results in loss of proton-specific conduction. Indirect evidence suggests that kH(V)1 is monomeric, unlike other proton channels. Furthermore, kH(V)1 differs from all known proton channels in activating well negative to the Nernst potential for protons, E(H). This unique voltage dependence makes the dinoflagellate proton channel ideally suited to mediate the proton influx postulated to trigger bioluminescence. In contrast to vertebrate proton channels, whose main function is acid extrusion, we propose that proton channels in dinoflagellates have fundamentally different functions of signaling and excitability.

Water wires in atomistic models of the Hv1 proton channel

The voltage-gated proton channel (Hv1) is homologous to the voltage-sensing domain (VSD) of voltage-gated potassium (Kv) channels but lacks a separate pore domain. The Hv1 monomer has dual functions: it gates the proton current and also serves as the proton conduction pathway. To gain insight into the structure and dynamics of the yet unresolved proton permeation pathway, we performed all-atom molecular dynamics simulations of two different Hv1 homology models in a lipid bilayer in excess water. The structure of the Kv1.2-Kv2.1 paddle-chimera VSD was used as template to generate both models, but they differ in the sequence alignment of the S4 segment. In both models, we observe a water wire that extends through the membrane, whereas the corresponding region is dry in simulations of the Kv1.2-Kv2.1 paddle-chimera. We find that the kinetic stability of the water wire is dependent upon the identity and location of the residues lining the permeation pathway, in particular, the S4 arginines. A measurement of water transport kinetics indicates that the water wire is a relatively static feature of the permeation pathway. Taken together, our results suggest that proton conduction in Hv1 may occur via Grotthuss hopping along a robust water wire, with exchange of water molecules between inner and outer ends of the permeation pathway minimized by specific water-protein interactions. This article is part of a Special Issue entitled: Membrane protein structure and function.

Oligomerization of the voltage-gated proton channel

The voltage-gated proton channel exists as a dimer, although each protomer has a separate conduction pathway, and when forced to exist as a monomer, most major functions are retained. However, the proton channel protomers appear to interact during gating. Proton channel dimerization is thought to result mainly from coiled-coil interaction of the intracellular C-termini. Several types of evidence are discussed that suggest that the dimer conformation may not be static, but is dynamic and can sample different orientations. Zn(2+) appears to link the protomers in an orientation from which the channel(s) cannot open. A tandem WT-WT dimer exhibits signs of cooperative gating, indicating that despite the abnormal linkage, the correct orientation for opening can occur. We propose that C-terminal interaction functions mainly to tether the protomers together. Comparison of the properties of monomeric and dimeric proton channels speaks against the hypothesis that enhanced gating reflects monomer-dimer interconversion.

An aqueous H+ permeation pathway in the voltage-gated proton channel Hv1

Hv1 voltage-gated proton channels mediate rapid and selective transmembrane H(+) flux and are gated by both voltage and pH gradients. Selective H(+) transfer in membrane proteins is commonly achieved by Grotthuss proton 'hopping' in chains of ionizable amino acid side chains and intraprotein water molecules. To identify whether ionizable residues are required for proton permeation in Hv1, we neutralized candidate residues and measured expressed voltage-gated H(+) currents. Unexpectedly, charge neutralization was insufficient to abrogate either the Hv1 conductance or coupling of pH gradient and voltage-dependent activation. Molecular dynamics simulations revealed water molecules in the central crevice of Hv1 model structures but not in homologous voltage-sensor domain (VSD) structures. Our results indicate that Hv1 most likely forms an internal water wire for selective proton transfer and that interactions between water molecules and S4 arginines may underlie coupling between voltage- and pH-gradient sensing.

Zinc inhibition of monomeric and dimeric proton channels suggests cooperative gating

Voltage-gated proton channels are strongly inhibited by Zn(2+), which binds to His residues. However, in a molecular model, the two externally accessible His are too far apart to coordinate Zn(2+). We hypothesize that high-affinity Zn(2+) binding occurs at the dimer interface between pairs of His residues from both monomers. Consistent with this idea, Zn(2+) effects were weaker in monomeric channels. Mutation of His(193) and His(140) in various combinations and in tandem dimers revealed that channel opening was slowed by Zn(2+) only when at least one His was present in each monomer, suggesting that in wild-type (WT) H(V)1, Zn(2+) binding between His of both monomers inhibits channel opening. In addition, monomeric channels opened exponentially, and dimeric channels opened sigmoidally. Monomeric channel gating had weaker temperature dependence than dimeric channels. Finally, monomeric channels opened 6.6 times faster than dimeric channels. Together, these observations suggest that in the proton channel dimer, the two monomers are closely apposed and interact during a cooperative gating process. Zn(2+) appears to slow opening by preventing movement of the monomers relative to each other that is prerequisite to opening. These data also suggest that the association of the monomers is tenuous and allows substantial freedom of movement. The data support the idea that native proton channels are dimeric. Finally, the idea that monomer-dimer interconversion occurs during activation of phagocytes appears to be ruled out.

Voltage-gated proton channels find their dream job managing the respiratory burst in phagocytes

The voltage-gated proton channel bears surprising resemblance to the voltage-sensing domain (S1-S4) of other voltage-gated ion channels but is a dimer with two conduction pathways. The proton channel seems designed for efficient proton extrusion from cells. In phagocytes, it facilitates the production of reactive oxygen species by NADPH oxidase.