Conversion of a delayed rectifier K+ channel to a voltage-gated inward rectifier K+ channel by three amino acid substitutions

Kv1.3 voltage-gated potassium channels link cellular respiration to proliferation through a non-conducting mechanism

Cellular energy metabolism is fundamental for all biological functions. Cellular proliferation requires extensive metabolic reprogramming and has a high energy demand. The Kv1.3 voltage-gated potassium channel drives cellular proliferation. Kv1.3 channels localise to mitochondria. Using high-resolution respirometry, we show Kv1.3 channels increase oxidative phosphorylation, independently of redox balance, mitochondrial membrane potential or calcium signalling. Kv1.3-induced respiration increased reactive oxygen species production. Reducing reactive oxygen concentrations inhibited Kv1.3-induced proliferation. Selective Kv1.3 mutation identified that channel-induced respiration required an intact voltage sensor and C-terminal ERK1/2 phosphorylation site, but is channel pore independent. We show Kv1.3 channels regulate respiration through a non-conducting mechanism to generate reactive oxygen species which drive proliferation. This study identifies a Kv1.3-mediated mechanism underlying the metabolic regulation of proliferation, which may provide a therapeutic target for diseases characterised by dysfunctional proliferation and cell growth.

Voltage-dependent conformational changes of Kv1.3 channels activate cell proliferation

The voltage-dependent potassium channel Kv1.3 has been implicated in proliferation in many cell types, based on the observation that Kv1.3 blockers inhibited proliferation. By modulating membrane potential, cell volume, and/or Ca²⁺ influx, K⁺  channels can influence cell cycle progression. Also, noncanonical channel functions could contribute to modulate cell proliferation independent of K⁺ efflux. The specificity of the requirement of Kv1.3 channels for proliferation suggests the involvement of molecule-specific interactions, but the underlying mechanisms are poorly identified. Heterologous expression of Kv1.3 channels in HEK cells has been shown to increase proliferation independently of K⁺ fluxes. Likewise, some of the molecular determinants of Kv1.3-induced proliferation have been located in the C-terminus region, where individual point mutations of putative phosphorylation sites (Y447A and S459A) abolished Kv1.3-induced proliferation. Here, we investigated the mechanisms linking Kv1.3 channels to proliferation exploring the correlation between Kv1.3 voltage-dependent molecular dynamics and cell cycle progression. Using transfected HEK cells, we analyzed both the effect of changes in resting membrane potential on Kv1.3-induced proliferation and the effect of mutated Kv1.3 channels with altered voltage dependence of gating. We conclude that voltage-dependent transitions of Kv1.3 channels enable the activation of proliferative pathways. We also found that Kv1.3 associated with IQGAP3, a scaffold protein involved in proliferation, and that membrane depolarization facilitates their interaction. The functional contribution of Kv1.3-IQGAP3 interplay to cell proliferation was demonstrated both in HEK cells and in vascular smooth muscle cells. Our data indicate that voltage-dependent conformational changes of Kv1.3 are an essential element in Kv1.3-induced proliferation.

Opposite Effects of the S4-S5 Linker and PIP(2) on Voltage-Gated Channel Function: KCNQ1/KCNE1 and Other Channels

Voltage-gated potassium (Kv) channels are tetramers, each subunit presenting six transmembrane segments (S1-S6), with each S1-S4 segments forming a voltage-sensing domain (VSD) and the four S5-S6 forming both the conduction pathway and its gate. S4 segments control the opening of the intracellular activation gate in response to changes in membrane potential. Crystal structures of several voltage-gated ion channels in combination with biophysical and mutagenesis studies highlighted the critical role of the S4-S5 linker (S4S5(L)) and of the S6 C-terminal part (S6(T)) in the coupling between the VSD and the activation gate. Several mechanisms have been proposed to describe the coupling at a molecular scale. This review summarizes the mechanisms suggested for various voltage-gated ion channels, including a mechanism that we described for KCNQ1, in which S4S5(L) is acting like a ligand binding to S6(T) to stabilize the channel in a closed state. As discussed in this review, this mechanism may explain the reverse response to depolarization in HCN-like channels. As opposed to S4S5(L), the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)), stabilizes KCNQ1 channel in an open state. Many other ion channels (not only voltage-gated) require PIP(2) to function properly, confirming its crucial importance as an ion channel cofactor. This is highlighted in cases in which an altered regulation of ion channels by PIP(2) leads to channelopathies, as observed for KCNQ1. This review summarizes the state of the art on the two regulatory mechanisms that are critical for KCNQ1 and other voltage-gated channels function (PIP(2) and S4S5(L)), and assesses their potential physiological and pathophysiological roles.

Kv1.3 channels can modulate cell proliferation during phenotypic switch by an ion-flux independent mechanism

OBJECTIVE

Phenotypic modulation of vascular smooth muscle cells has been associated with a decreased expression of all voltage-dependent potassium channel (Kv)1 channel encoding genes but Kcna3 (which encodes Kv1.3 channels). In fact, upregulation of Kv1.3 currents seems to be important to modulate proliferation of mice femoral vascular smooth muscle cells in culture. This study was designed to explore if these changes in Kv1 expression pattern constituted a landmark of phenotypic modulation across vascular beds and to investigate the mechanisms involved in the proproliferative function of Kv1.3 channels.

METHODS AND RESULTS

Changes in Kv1.3 and Kv1.5 channel expression were reproduced in mesenteric and aortic vascular smooth muscle cells, and their correlate with protein expression was electrophysiologicaly confirmed using selective blockers. Heterologous expression of Kv1.3 and Kv1.5 channels in HEK cells has opposite effects on the proliferation rate. The proproliferative effect of Kv1.3 channels was reproduced by "poreless" mutants but disappeared when voltage-dependence of gating was suppressed.

CONCLUSIONS

These findings suggest that the signaling cascade linking Kv1.3 functional expression to cell proliferation is activated by the voltage-dependent conformational change of the channels without needing ion conduction. Additionally, the conserved upregulation of Kv1.3 on phenotypic modulation in several vascular beds makes this channel a good target to control unwanted vascular remodeling.

What makes a gate? The ins and outs of Kv-like K+ channels in plants

Gating of K(+) and other ion channels is 'hard-wired' within the channel protein. So it remains a puzzle how closely related channels in plants can show an unusually diverse range of biophysical properties. Gating of these channels lies at the heart of K(+) mineral nutrition, signalling, abiotic and biotic stress responses in plants. Thus, our knowledge of the molecular mechanics underpinning K(+) channel gating will be important for rational engineering of related traits in agricultural crops. Several key studies have added significantly to our understanding of channel gating in plants and have challenged current thinking about analogous processes found in animal K(+) channels. Such studies highlight how much of K(+) channel gating remains to be explored in plants.

Potassium channel opening: a subtle two-step

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

Biological pacemakers as a therapy for cardiac arrhythmias

PURPOSE OF REVIEW

Cardiac rhythm disorders are caused by malfunctions of impulse generation or conduction. Malfunctions of impulse generation, that is, defects in pacemaking, are often life-threatening. Present therapies span a wide array of approaches, but remain largely palliative. Recent progress in understanding of the underlying biology of pacemaking opens up new prospects for better alternatives to the present routine. Specifically, development and use of biological pacemakers could prove to be advantageous to the conventional approaches.

RECENT FINDINGS

We review the current state of the art in gene and cell-based approaches to correct cardiac rhythm disturbances. These include genetic suppression of an ionic current, embryonic as well as adult stem cell therapies, novel synthetic pacemaker channels, and adult somatic cell-fusion approach.

SUMMARY

Biological pacemaking can be achieved by modulating ionic currents by gene transfer or by delivering engineered pacemaker cells into normally quiescent myocardium. The present state of development is proof-of-concept; we are now working on reducing to practice a stable, reliable biological product as an alternative to electronic pacemakers.

Creation of a biological pacemaker by gene- or cell-based approaches

Cardiac rhythm-associated disorders are caused by mal-functions of impulse generation and conduction. Present therapies for the impulse generation span a wide array of approaches but remain largely palliative. The progress in the understanding of the biology of the diseases with related biological tools beckons for new approaches to provide better alternatives to the present routine. Here, we review the current state of the art in gene- and cell-based approaches to correct cardiac rhythm disturbances. These include genetic suppression of an ionic current, stem cell therapies, adult somatic cell-fusion approach, novel synthetic pacemaker channel, and creating a self-contained pacemaker activity in non-excitable cells. We then conclude by discussing advantages and disadvantages of the new possibilities.

Gene transfer of a synthetic pacemaker channel into the heart: a novel strategy for biological pacing

BACKGROUND

One key element of natural pacemakers is the pacemaker current encoded by the hyperpolarization-activated nucleotide-gated channel (HCN) gene family. Although HCN gene transfer has been used to engineer biological pacemakers, this strategy may be confounded by unpredictable consequences of heteromultimerization with endogenous HCN family members and limited flexibility with regard to frequency tuning of the engineered pacemaker.

METHODS AND RESULTS

To circumvent these limitations, we converted a depolarization-activated potassium-selective channel, Kv1.4, into a hyperpolarization-activated nonselective channel by site-directed mutagenesis (R447N, L448A, and R453I in S4 and G528S in the pore). Gene transfer into ventricular myocardium demonstrated the ability of this construct to induce pacemaker activity with spontaneous action potential oscillations in adult ventricular myocytes and idioventricular rhythms by in vivo electrocardiography.

CONCLUSIONS

Given the sparse expression of Kv1 family channels in the human ventricle, gene transfer of a synthetic pacemaker channel based on the Kv1 family has novel therapeutic potential as a biological alternative to electronic pacemakers.

Reversal of HCN channel voltage dependence via bridging of the S4-S5 linker and Post-S6

Voltage-gated ion channels possess charged domains that move in response to changes in transmembrane voltage. How this movement is transduced into gating of the channel pore is largely unknown. Here we show directly that two functionally important regions of the spHCN1 pacemaker channel, the S4–S5 linker and the C-linker, come into close proximity during gating. Cross-linking these regions with high-affinity metal bridges or disulfide bridges dramatically alters channel gating in the absence of cAMP; after modification the polarity of voltage dependence is reversed. Instead of being closed at positive voltage and activating with hyperpolarization, modified channels are closed at negative voltage and activate with depolarization. Mechanistically, this reversal of voltage dependence occurs as a result of selectively eliminating channel deactivation, while retaining an existing inactivation process. Bridging also alters channel activation by cAMP, showing that interaction of these two regions can also affect the efficacy of physiological ligands.

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

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

Atypical Phenotypes From Flatworm Kv3 Channels

Divergence of the Shaker superfamily of voltage-gated (Kv) ion channels early in metazoan evolution created numerous electrical phenotypes that were presumably selected to produce a wide range of excitability characteristics in neurons, myocytes, and other cells. A comparative approach that emphasizes this early radiation provides a comprehensive sampling of sequence space that is necessary to develop generally applicable models of the structure–function relationship in the Kv potassium channel family. We have cloned and characterized two Shaw-type potassium channels from a flatworm ( Notoplana atomata) that is arguably a representative of early diverging bilaterians. When expressed in Xenopus oocytes, one of these cloned channels, N.at-Kv3.1, exhibits a noninactivating, outward current with slow opening kinetics that are dependent on both the holding potential and the activating potential. A second Shaw-type channel, N.at-Kv3.2, has very different properties, showing weak inward rectification. These results demonstrate that broad phylogenetic sampling of proteins of a single family will reveal unexpected properties that lead to new interpretations of structure–function relationships.

Domain analysis of Kv6.3, an electrically silent channel

The subunit Kv6.3 encodes a voltage-gated potassium channel belonging to the group of electrically silent Kv subunits, i.e. subunits that do not form functional homotetrameric channels. The lack of current, caused by retention in the endoplasmic reticulum (ER), was overcome by coexpression with Kv2.1. To investigate whether a specific section of Kv6.3 was responsible for ER retention, we constructed chimeric subunits between Kv6.3 and Kv2.1, and analysed their subcellular localization and functionality. The results demonstrate that the ER retention of Kv6.3 is not caused by the N-terminal A and B box (NAB) domain nor the intracellular N- or C-termini, but rather by the S1-S6 core protein. Introduction of individual transmembrane segments of Kv6.3 in Kv2.1 was tolerated, with the exception of S6. Indeed, introduction of the S6 domain of Kv6.3 in Kv2.1 was enough to cause ER retention, which was due to the C-terminal section of S6. The S4 segment of Kv6.3 could act as a voltage sensor in the Kv2.1 context, albeit with a major hyperpolarizing shift in the voltage dependence of activation and inactivation, apparently caused by the presence of a tyrosine in Kv6.3 instead of a conserved arginine. This study suggests that the silent behaviour of Kv6.3 is largely caused by the C-terminal part of its sixth transmembrane domain that causes ER retention of the subunit.

Coupling of voltage sensing to channel opening reflects intrasubunit interactions in kv channels

Voltage-gated K(+) channels play a central role in the modulation of excitability. In these channels, the voltage-dependent movement of the voltage sensor (primarily S4) is coupled to the (S6) gate that opens the permeation pathway. Because of the tetrameric structure, such coupling could occur within each subunit or between adjacent subunits. To discriminate between these possibilities, we analyzed various combinations of a S4 mutation (R401N) and a S6 mutation (P511G) in hKv1.5, incorporated into tandem constructs to constrain subunit stoichiometry. R401N shifted the voltage dependence of activation to negative potentials while P511G did the opposite. When both mutations were introduced in the same alpha-subunit of the tandem, the positive shift of P511G was compensated by the negative shift of R401N. With each mutation in a separate subunit of a tandem, this compensation did not occur. This suggests that for Kv channels, the coupling between voltage sensing and gating reflects primarily an intrasubunit interaction.

Arranging the elements of the potassium channel: the T1 domain occludes the cytoplasmic face of the channel

The voltage-gated potassium channel is currently one of the few membrane proteins where functional roles have been mapped onto specific segments of sequence. Although high-resolution structures of the transmembrane portions of three bacterial potassium channels, the tetramerization domain and the cytoplasmic "ball" are available, their relative spatial arrangement in mammalian channels remains a matter of ongoing debate. Cryo-electron microscopic images of the six transmembrane voltage-gated Kv channel have been reconstructed at up to 18 A resolution, revealing that the T1 domain tetramerizes and is suspended below the transmembrane segments. However, the resolution of these images is insufficient to reveal the location of the third piece of the puzzle, the inactivating ball domain. We have used the aberrant interactions observed in a series of chimaeric channels to establish that an assembled T1 domain restricts access to the cytoplasmic face of the channel, suggesting that the N-terminal "ball and chain" may be confined in the space between the T1 domain and the transmembrane portion of the channel.

Hyperpolarization-activated cation currents: from molecules to physiological function

Hyperpolarization-activated cation currents, termed If, Ih, or Iq, were initially discovered in heart and nerve cells over 20 years ago. These currents contribute to a wide range of physiological functions, including cardiac and neuronal pacemaker activity, the setting of resting potentials, input conductance and length constants, and dendritic integration. The hyperpolarization-activated, cation nonselective (HCN) gene family encodes the channels that underlie Ih. Here we review the relation between the biophysical properties of recombinant HCN channels and the pattern of HCN mRNA expression with the properties of native Ih in neurons and cardiac muscle. Moreover, we consider selected examples of the expanding physiological functions of Ih with a view toward understanding how the properties of HCN channels contribute to these diverse functional roles.

Inward and outward potassium currents through the same chimeric human Kv channel

Voltage-gated ion channels are among the most intensely studied membrane proteins today and a variety of techniques has led to a basic mapping of functional roles onto specific regions of their structure. The architecture of the proteins appears to be modular and segments associated with voltage sensing and the pore lining have been identified. However, the means by which movement of the sensor is transduced into channel opening is still unclear. In this communication, we report on a chimeric potassium channel construct which can function in two distinct operating voltage ranges, spanning both inward and outward currents with a non-conducting intervening regime. The observed changes in operating range could be brought about by perturbing either the direction of sensor movement or the process of transducing movements of the sensor into channel opening and closing. The construct could thus provide a means to identify the machinery underlying these processes.

Cardiac ion channels

The normal electrophysiologic behavior of the heart is determined by ordered propagation of excitatory stimuli that result in rapid depolarization and slow repolarization, thereby generating action potentials in individual myocytes. Abnormalities of impulse generation, propagation, or the duration and configuration of individual cardiac action potentials form the basis of disorders of cardiac rhythm, a continuing major public health problem for which available drugs are incompletetly effective and often dangerous. The integrated activity of specific ionic currents generates action potentials, and the genes whose expression results in the molecular components underlying individual ion currents in heart have been cloned. This review discusses these new tools and how their application to the problem of arrhythmias is generating new mechanistic insights to identify patients at risk for this condition and developing improved antiarrhythmic therapies.

KCNKØ: opening and closing the 2-P-domain potassium leak channel entails "C-type" gating of the outer pore

Essential to nerve and muscle function, little is known about how potassium leak channels operate. KCNKØ opens and closes in a kinase-dependent fashion. Here, the transition is shown to correspond to changes in the outer aspect of the ion conduction pore. Voltage-gated potassium (VGK) channels open and close via an internal gate; however, they also have an outer pore gate that produces "C-type" inactivation. While KCNKØ does not inactivate, KCNKØ and VGK channels respond in like manner to outer pore blockers, potassium, mutations, and chemical modifiers. Structural relatedness is confirmed: VGK residues that come close during C-type gating predict KCNKØ sites that crosslink (after mutation to cysteine) to yield channels controlled by reduction and oxidization. We conclude that similar outer pore gates mediate KCNKØ opening and closing and VGK channel C-type inactivation despite their divergent structures and physiological roles.

The S4-S5 linker couples voltage sensing and activation of pacemaker channels

Voltage-gated channels are normally opened by depolarization and closed by repolarization of the membrane. Despite sharing significant sequence homology with voltage-gated K(+) channels, the gating of hyperpolarization-activated, cyclic-nucleotide-gated (HCN) pacemaker channels has the opposite dependence on membrane potential: hyperpolarization opens, whereas depolarization closes, these channels. The mechanism and structural basis of the process that couples voltage sensor movement to HCN channel opening and closing is not understood. On the basis of our previous studies of a mutant HERG (human ether-a-go-go-related gene) channel, we hypothesized that the intracellular linker that connects the fourth and fifth transmembrane domains (S4-S5 linker) of HCN channels might be important for channel gating. Here, we used alanine-scanning mutagenesis of the HCN2 S4-S5 linker to identify three residues, E324, Y331, and R339, that when mutated disrupted normal channel closing. Mutation of a basic residue in the S4 domain (R318Q) prevented channel opening, presumably by disrupting S4 movement. However, channels with R318Q and Y331S mutations were constitutively open, suggesting that these channels can open without a functioning S4 domain. We conclude that the S4-S5 linker mediates coupling between voltage sensing and HCN channel activation. Our findings also suggest that opening of HCN and related channels corresponds to activation of a gate located near the inner pore, rather than recovery of channels from a C-type inactivated state.

Ammonium ions induce inactivation of Kir2.1 potassium channels expressed in Xenopus oocytes

1. The decay of inward currents was studied using the giant patch-clamp technique and a cloned inward rectifier K(+) channel, Kir2.1, expressed in Xenopus oocytes. 2. In inside-out patches, inward currents carried by NH4(+) or Tl(+) decayed over time. When the voltage was more negative, the degree and rate of decay were greater. The rate of NH4(+)-induced decay saturated at a symmetrical [NH4(+)] of approximately 100 mM. The decay rate was slow (2.6 x 10(3) M(-1) s(-1)) at -140 mV with 10 mM [NH4(+)]. 3. Upon a 10 degrees C increase in temperature, the single-channel NH4(+) current amplitude increased by a factor of 1.57, whereas the NH4(+)-induced decay rate increased by a factor of 2.76. In the R148Y Kir2.1 mutant (tyrosine 148 is at the external pore mouth), NH4(+)-induced inactivation was no longer observed. 4. NH4(+) single-channel currents revealed one open and one closed state. The entry rate into the closed state was voltage dependent whereas the exit rate from the closed state was not. An increase of internal [NH4(+)] not only decreased the entry rate into but also elevated the exit rate from the closed state, consistent with the occupancy model modified from the foot-in-the-door model of gating. 5. These results suggest that the decay of NH4(+) current is unlikely to be due to a simple bimolecular reaction leading to channel block. We propose that NH4(+) binding to Kir2.1 channels induces a conformational change followed by channel closure. 6. The decay induced by permeant ions other than K(+) may serve as a secondary selectivity filter, such that K(+) is the preferred permeant ion for Kir2.1 channels.

Functional characterization of the C-terminus of the human ether-à-go-go-related gene K(+) channel (HERG)

1. In the present study the functional role of the C-terminus of the human ether-à-go-go-related gene K(+) channel HERG was investigated using a series of C-terminal deletion constructs expressed in Xenopus oocytes. 2. Constructs with deletions of 311 or more amino acid residues failed to form functional channels. Truncation by 215 amino acid residues or fewer had no discernable effects on channel activity. Truncation by 236 or 278 amino acid residues accelerated deactivation, and caused a faster recovery from inactivation. 3. In high extracellular K(+), channel deactivation of HERG results from the binding of the N-terminus to a site within the pore. This slows channel deactivation by a knock-off mechanism. Here, it was shown that C-terminal deletions also abolished this effect of high extracellular K(+). Mutants containing deletions in both the N- and C-termini deactivated with rates similar to those observed in individual deletion mutants. 4. In contrast, experiments with double-deletion constructs showed additive effects of the N- and C-termini on the voltage dependence of activation, and on the kinetics of inactivation and recovery from inactivation. The reduction of inactivation in these mutants contributed to an increase in peak current amplitude. 5. These results indicate that residues within the C-terminus of HERG play a role in channel expression as well as in most aspects of channel gating. The regulation of channel deactivation is likely to be mediated by an interaction with the N-terminus, but the regulation of the voltage dependence of activation, and of rate processes associated with inactivation, does not require the N-terminus.

Functional roles of charged residues in the putative voltage sensor of the HCN2 pacemaker channel

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels contribute to pacemaking activity in specialized neurons and cardiac myocytes. HCN channels have a structure similar to voltage-gated K(+) channels but have a much larger putative S4 transmembrane domain and open in response to membrane hyperpolarization instead of depolarization. As an initial attempt to define the structural basis of HCN channel gating, we have characterized the functional roles of the charged residues in the S2, S3, and S4 transmembrane domains. The nine basic residues and a single Ser in S4 were mutated individually to Gln, and the function of mutant channels was analyzed in Xenopus oocytes using two-microelectrode voltage clamp techniques. Surface membrane expression of hemagglutinin-epitope-tagged channel proteins was examined by chemiluminescence. Our results suggest that 1) Lys-291, Arg-294, Arg-297, and Arg-300 contribute to the voltage dependence of gating but not to channel folding or trafficking to the surface membrane; 2) Lys-303 and Ser-306 are essential for gating, but not for channel folding/trafficking; 3) Arg-312 is important for folding but not gating; and 4) Arg-309, Arg-315, and Arg-318 are crucial for normal protein folding/trafficking and may charge-pair with Asp residues located in the S2 and S3 domains.

Mutations in the S4 domain of a pacemaker channel alter its voltage dependence

In an attempt to study the functional role of the positively charged amino acids present in the S4 segment of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels, we have introduced single and sequential amino acid replacements throughout this domain in the mouse type 2 HCN channel (mHCN2). Sequential neutralization of the first three positively charged amino acids resulted in cumulative shifts of the midpoint voltage activation constant towards more hyperpolarizing potentials. The contribution of each amino acid substitution was approximately -20 mV. Amino acid replacements to neutralize either the first (K291Q) or fourth (R300Q) positively charged amino acid resulted in the same shift (about 20 mV) towards more hyperpolarized potentials. Replacing the first positively charged amino acid with the negatively charged glutamic acid (K291E) produced a shift of approximately -50 mV in the same direction. None of the above amino acid substitutions had any measurable effect on the time course of channel activation. This suggests that the S4 domain of HCN channels critically controls the voltage dependence of channel opening but is not involved in regulating activation kinetics. No channel activity was detected in mutants with neutralization of the last six positively charged amino acids from the S4 domain, suggesting that these amino acids cannot be altered without impairing channel function.

Molecular insights into the structure and function of plant K(+) transport mechanisms

Our understanding of plant potassium transport has increased in the past decade through the application of molecular biological techniques. In this review, recent work on inward and outward rectifying K(+) channels as well as high affinity K(+) transporters is described. Through the work on inward rectifying K(+) channels, we now have precise details on how the structure of these proteins determines functional characteristics such as ion conduction, pH sensitivity, selectivity and voltage sensing. The physiological function of inward rectifying K(+) channels in plants has been clarified through the analysis of expression patterns and mutational analysis. Two classes of outward rectifying K(+) channels have now been cloned from plants and their initial characterisation is reviewed. The physiological role of one class of outward rectifying K(+) channel has been demonstrated to be involved in long distance transport of K(+) from roots to shoots. The molecular structure and function of two classes of energised K(+) transporters are also reviewed. The first class is energised by Na(+) and shares structural similarities with K(+) transport mechanisms in bacteria and fungi. Structure-function studies suggest that it should be possible to increase the K(+) and Na(+) selectivity of these transporters, which will enhance the salt tolerance of higher plants. The second class of K(+) transporter is comprised of a large gene family and appears to have a dual affinity for K(+). A suite of molecular techniques, including gene cloning, oocyte expression, RNA localisation and gene inactivation, is now being used to fully characterise the biophysical and physiological function of plants K(+) transport mechanisms.

Ultrafast inactivation causes inward rectification in a voltage-gated K(+) channel from Caenorhabditis elegans

The exp-2 gene in the nematode Caenorhabditis elegans influences the shape and duration of the action potential of pharyngeal muscle cells. Several loss-of-function mutations in exp-2 lead to broadening of the action potential and to a concomitant slowing of the pumping action of the pharynx. In contrast, a gain-of-function mutation leads to narrow action potentials and shallow pumping. We cloned and functionally characterized the exp-2 gene. The exp-2 gene is homologous to genes of the family of voltage-gated K(+) channels (Kv type). The Xenopus oocyte-expressed EXP-2 channel, although structurally closely related to Kv-type channels, is functionally distinct and very similar to the human ether-à-gogo-related gene (HERG) K(+) channel. In response to depolarization, EXP-2 activates slowly and inactivates very rapidly. On repolarization, recovery from inactivation is also rapid and strongly voltage-dependent. These kinetic properties make the Kv-type EXP-2 channel an inward rectifier that resembles the structurally unrelated HERG channel. Apart from many similarities to HERG, however, the molecular mechanism of fast inactivation appears to be different. Moreover, the single-channel conductance is 5- to 10-fold larger than that of HERG and most Kv-type K(+) channels. It appears that the inward rectification mechanism by rapid inactivation has evolved independently in two distinct classes of structurally unrelated, voltage-gated K(+) channels.

Voltage-dependent structural interactions in the Shaker K(+) channel

Using a strategy related to intragenic suppression, we previously obtained evidence for structural interactions in the voltage sensor of Shaker K(+) channels between residues E283 in S2 and R368 and R371 in S4 (Tiwari-Woodruff, S.K., C.T. Schulteis, A.F. Mock, and D. M. Papazian. 1997. Biophys. J. 72:1489-1500). Because R368 and R371 are involved in the conformational changes that accompany voltage-dependent activation, we tested the hypothesis that these S4 residues interact with E283 in S2 in a subset of the conformational states that make up the activation pathway in Shaker channels. First, the location of residue 283 at hyperpolarized and depolarized potentials was inferred by substituting a cysteine at that position and determining its reactivity with hydrophilic, sulfhydryl-specific probes. The results indicate that position 283 reacts with extracellularly applied sulfhydryl reagents with similar rates at both hyperpolarized and depolarized potentials. We conclude that E283 is located near the extracellular surface of the protein in both resting and activated conformations. Second, we studied the functional phenotypes of double charge reversal mutations between positions 283 and 368 and between 283 and 371 to gain insight into the conformations in which these positions approach each other most closely. We found that combining charge reversal mutations at positions 283 and 371 stabilized an activated conformation of the channel, and dramatically slowed transitions into and out of this state. In contrast, charge reversal mutations at positions 283 and 368 stabilized a closed conformation, which by virtue of the inferred position of 368 corresponds to a partially activated (intermediate) closed conformation. From these results, we propose a preliminary model for the rearrangement of structural interactions of the voltage sensor during activation of Shaker K(+) channels.

Plant ion channels: from molecular structures to physiological functions

Progress in identification of plant ion channels and development of electrophysiological analyses in heterologous expression systems and in planta, in combination with reverse genetic approaches, are providing the possibility of associating molecular entities with physiological functions. Recently, the first attempts to determine in vivo functions using knockout mutants demonstrated the roles of root ion channels. The search for proteins interacting with such channels leads to an even more complex view of the concerted action in protein networks.

Action of internal pronase on the f-channel kinetics in the rabbit SA node

1. The hyperpolarization-activated If current was recorded in inside-out macropatches from sino-atrial (SA) node myocytes during exposure of their intracellular side to pronase, in an attempt to verify if cytoplasmic f-channel domains are involved in both voltage- and cAMP-dependent gating. 2. Superfusion with pronase caused a quick, dramatic acceleration of channel opening upon hyperpolarization and slowing, rapidly progressing into full blockade, of channel closing upon depolarization; these changes persisted after wash off of pronase and were irreversible, indicating proteolytic cleavage of channel regions which contribute to gating. 3. If recorded from patches normally responding to cAMP became totally insensitive to cAMP following pronase treatment, indicating partial or total removal of channel regions involved in the cAMP-dependent activation. 4. The fully activated I-V relationship was not modified by pronase, indicating that internal proteolysis did not affect the f-channel conductance. 5. The changes in If kinetics induced by pronase were due to a large depolarizing shift of the f-channel open probability curve (56.5 +/- 1.1 mV, n = 7). 6. These results are consistent with the hypothesis that cytoplasmic f-channel regions are implicated in dual voltage- and cAMP-dependent gating; also, since pronase does not abolish hyperpolarization-activated opening, an intrinsic voltage-dependent gating mechanism must exist which is inaccessible to proteolytic cleavage. A model scheme able to account for these data thus includes an intrinsic gating mechanism operating at depolarized voltages, and a blocking mechanism coupled to cAMP binding to the channel.

The HCN gene family: molecular basis of the hyperpolarization-activated pacemaker channels

The molecular basis of the hyperpolarization-activated cation channels that underlie the anomalous rectifying current variously termed Ih, Iq, or I(f) is discussed. On the basis of the expression patterns and biophysical properties of the newly cloned HCN ion channels, an initial attempt at defining the identity and subunit composition of channels underlying native Ih is undertaken. By comparing the sequences of HCN channels to other members of the K channel superfamily, we discuss how channel opening may be coupled to membrane hyperpolarization and to direct binding of cyclic nucleotide. Finally, we consider some of the questions in cardiovascular physiology and neurobiology that can be addressed as a result of the demonstration that Ih is encoded by the HCN gene family.

Transplanting the N-terminus from Kv1.4 to Kv1.1 generates an inwardly rectifying K+ channel

A chimeric channel, 4N/1, was generated from two outwardly rectifying K+ channels by linking the N-terminal cytoplasmic domain of hKv1.4 (N terminus ball and chain of hKv1.4) with the transmembrane body of hKv1.1 (delta78N1 construct of hKv1.1). The recombinant channel has properties similar to the six transmembrane inward rectifiers and opens on hyperpolarization with a threshold of activation at -90 mV. Outward currents are seen on depolarization provided the channel is first exposed to a hyperpolarizing pulse of -100 mV or more. Hyperpolarization at and beyond -130 mV provides evidence of channel deactivation. Delta78N1 does not show inward currents on hyperpolarization but does open on depolarizing from -80 mV with characteristics similar to native hKv1.1. The outward currents seen in both delta78N1 and 4N/1 inactivate slowly at rates consistent with C-type inactivation. The inward rectification of the 4N/1 chimera is consistent with the inactivation gating mechanism. This implies that the addition of the N-terminus from hKv1.4 to hKv1.1 shifts channel activation to hyperpolarizing potentials. These results suggest a mechanism involving the N-terminal cytoplasmic domain for conversion of outward rectifiers to inward rectifiers.

Voltage-insensitive gating after charge-neutralizing mutations in the S4 segment of Shaker channels

Shaker channel mutants, in which the first (R362), second (R365), and fourth (R371) basic residues in the S4 segment have been neutralized, are found to pass potassium currents with voltage-insensitive kinetics when expressed in Xenopus oocytes. Single channel recordings clarify that these channels continue to open and close from -160 to +80 mV with a constant opening probability (Po). Although Po is low ( approximately 0.15) in these mutants, mean open time is voltage independent and similar to that of control Shaker channels. Additionally, these mutant channels retain characteristic Shaker channel selectivity, sensitivity to block by 4-aminopyridine, and are partially blocked by external Ca2+ ions at very negative potentials. Furthermore, mean open time is approximately doubled, in both mutant channels and control Shaker channels, when Rb+ is substituted for K+ as the permeant ion species. Such strong similarities between mutant channels and control Shaker channels suggests that the pore region has not been substantially altered by the S4 charge neutralizations. We conclude that single channel kinetics in these mutants may indicate how Shaker channels would behave in the absence of voltage sensor input. Thus, mean open times appear primarily determined by voltage-insensitive transitions close to the open state rather than by voltage sensor movement, even in control, voltage-sensitive Shaker channels. By contrast, the low and voltage-insensitive Po seen in these mutant channels suggests that important determinants of normal channel opening derive from electrostatic coupling between S4 charges and the pore domain.

Voltage-dependent gating of single wild-type and S4 mutant KAT1 inward rectifier potassium channels

The voltage-dependent gating mechanism of KAT1 inward rectifier potassium channels was studied using single channel current recordings from Xenopus oocytes injected with KAT1 mRNA. The inward rectification properties of KAT1 result from an intrinsic gating mechanism in the KAT1 channel protein, not from pore block by an extrinsic cation species. KAT1 channels activate with hyperpolarizing potentials from -110 through -190 mV with a slow voltage-dependent time course. Transitions before first opening are voltage dependent and account for much of the voltage dependence of activation, while transitions after first opening are only slightly voltage dependent. Using burst analysis, transitions near the open state were analyzed in detail. A kinetic model with multiple closed states before first opening, a single open state, a single closed state after first opening, and a closed-state inactivation pathway accurately describes the single channel and macroscopic data. Two mutations neutralizing charged residues in the S4 region (R177Q and R176L) were introduced, and their effects on single channel gating properties were examined. Both mutations resulted in depolarizing shifts in the steady state conductance-voltage relationship, shortened first latencies to opening, decreased probability of terminating bursts, and increased burst durations. These effects on gating were well described by changes in the rate constants in the kinetic model describing KAT1 channel gating. All transitions before the open state were affected by the mutations, while the transitions after the open state were unaffected, implying that the S4 region contributes to the early steps in gating for KAT1 channels.

Inwardly rectifying potassium (IRK) currents are correlated with IRK subunit expression in rat nucleus accumbens medium spiny neurons

Inwardly rectifying K+ (IRK) channels are critical for shaping cell excitability. Whole-cell patch-clamp and single-cell RT-PCR techniques were used to characterize the inwardly rectifying K+ currents found in projection neurons of the rat nucleus accumbens. Inwardly rectifying currents were highly selective for K+ and blocked by low millimolar concentrations of Cs+ or Ba2+. In a subset of neurons, the inwardly rectifying current appeared to inactivate at hyperpolarized membrane potentials. In an attempt to identify this subset, neurons were profiled using single-cell RT-PCR. Neurons expressing substance P mRNA exhibited noninactivating inward rectifier currents, whereas neurons expressing enkephalin mRNA exhibited inactivating inward rectifier currents. The inactivation of the inward rectifier was correlated with the expression of IRK1 mRNA. These results demonstrate a clear physiological difference in the properties of medium spiny neurons and suggest that this difference could influence active state transitions driven by cortical and hippocampal excitatory input.

The N-terminus of the K channel KAT1 controls its voltage-dependent gating by altering the membrane electric field

Functional roles of different domains (pore region, S4 segment, N-terminus) of the KAT1 potassium channel in its voltage-dependent gating were electrophysiologically studied in Xenopus oocytes. The KAT1 properties did not depend on the extracellular K+ concentration or on residue H267, equivalent to one of the residues known to be important in C-type inactivation in Shaker channels, indicating that the hyperpolarization-induced KAT1 inward currents are related to the channel activation rather than to recovery from inactivation. Neutralization of a positively charged amino acid in the S4 domain (R176S) reduced the gating charge movement, suggesting that it acts as a voltage-sensing residue in KAT1. N-terminal deletions alone (e.g., delta20-34) did not affect the gating charge movement. However, the deletions paradoxically increased the voltage sensitivity of the R176S mutant channel, but not that of the wild-type channel. We propose a simple model in which the N-terminus determines the KAT1 voltage sensitivity by contributing to the electric field sensed by the voltage sensor.

MOLECULAR BIOLOGY OF CATION TRANSPORT IN PLANTS

This review summarizes current knowledge about genes whose products function in the transport of various cationic macronutrients (K, Ca) and micronutrients (Cu, Fe, Mn, and Zn) in plants. Such genes have been identified on the basis of function, via complementation of yeast mutants, or on the basis of sequence similarity, via database analysis, degenerate PCR, or low stringency hybridization. Not surprisingly, many of these genes belong to previously described transporter families, including those encoding Shaker-type K+ channels, P-type ATPases, and Nramp proteins. ZIP, a novel cation transporter family first identified in plants, also seems to be ubiquitous; members of this family are found in protozoa, yeast, nematodes, and humans. Emerging information on where in the plant each transporter functions and how each is controlled in response to nutrient availability may allow creation of food crops with enhanced mineral content as well as crops that bioaccumulate or exclude toxic metals.

Identification of a gene encoding a hyperpolarization-activated pacemaker channel of brain

The generation of pacemaker activity in heart and brain is mediated by hyperpolarization-activated cation channels that are directly regulated by cyclic nucleotides. We previously cloned a novel member of the voltage-gated K channel family from mouse brain (mBCNG-1) that contained a carboxy-terminal cyclic nucleotide-binding domain (Santoro et al., 1997) and hence proposed it to be a candidate gene for pacemaker channels. Heterologous expression of mBCNG-1 demonstrates that it does indeed code for a channel with properties indistinguishable from pacemaker channels in brain and similar to those in heart. Three additional mouse genes and two human genes closely related to mBCNG-1 display unique patterns of mRNA expression in different tissues, including brain and heart, demonstrating that these channels constitute a widely expressed gene family.

Voltage-dependent gating characteristics of the K+ channel KAT1 depend on the N and C termini

We studied how the C and N termini of the plant K+ channel KAT1 influence the voltage-dependent gating behavior by generating C- and N-terminal deletion mutants. Functional expression was observed only when C-terminal deletions were downstream of the putative cyclic nucleotide binding site. Treatments of oocytes expressing KAT1 channels with anticytoskeletal agents indicated that intact microtubules are important for functional expression. C-terminal deletions altered the voltage sensitivity of the KAT1 channel with greater deletions resulting in smaller equivalent charge movements. In contrast, a deletion in the N terminus (delta20-34) shifted the half-activation voltage by approximately -65 mV without markedly affecting the number of equivalent charges. The results reveal novel roles of the N and C termini in regulation of the voltage-dependent gating of KAT1.

Transfer of voltage independence from a rat olfactory channel to the Drosophila ether-à-go-go K+ channel

The S4 segment is an important part of the voltage sensor in voltage-gated ion channels. Cyclic nucleotide-gated channels, which are members of the superfamily of voltage-gated channels, have little inherent sensitivity to voltage despite the presence of an S4 segment. We made chimeras between a voltage-independent rat olfactory channel (rolf) and the voltage-dependent ether-à-go-go K+ channel (eag) to determine the basis of their divergent gating properties. We found that the rolf S4 segment can support a voltage-dependent mechanism of activation in eag, suggesting that rolf has a potentially functional voltage sensor that is silent during gating. In addition, we found that the S3-S4 loop of rolf increases the relative stability of the open conformation of eag, effectively converting eag into a voltage-independent channel. A single charged residue in the loop makes a significant contribution to the relative stabilization of the open stage in eag. Our data suggest that cyclic nucleotide-gated channels such as rolf contain a voltage sensor which, in the physiological voltage range, is stabilized in an activated conformation that is permissive for pore opening.

Inward rectifier potassium channels

The past three years have seen remarkable progress in research on the molecular basis of inward rectification, with significant implications for basic understanding and pharmacological manipulation of cellular excitability. Expression cloning of the first inward rectifier K channel (Kir) genes provided the necessary break-through that has led to isolation of a family of related clones encoding channels with the essential functional properties of classical inward rectifiers, ATP-sensitive K channels, and muscarinic receptor-activated K channels. High-level expression of cloned channels led to the discovery that classical inward so-called anomalous rectification is caused by voltage-dependent block of the channel by polyamines and Mg2+ ions, and it is now clear that a similar mechanism results in inward rectification of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-kainate receptor channels. Knowledge of the primary structures of Kir channels and the ability to mutate them also has led to the determination of many of the structural requirements of inward rectification.

A HERG-like K+ channel in rat F-11 DRG cell line: pharmacological identification and biophysical characterization

1. The relationships between the K+ inward rectifier current present in neuroblastoma cells (IIR) and the current encoded by the human ether-á-go-go-related gene (HERG), IHERG, and the rapidly activating repolarizing cardiac current IK(r), were investigated in a rat dorsal root ganglion (DRG) x mouse neuroblastoma hybrid cell line (F-11) using pharmacological and biophysical treatments. 2. IIR shared the pharmacological features described for IK(r), including the sensitivity to the antiarrhythmic drugs E4301 and WAY-123,398, whilst responding to Cs+, Ba2+ and La3+ in a similar way to IHERG. 3. The voltage-dependent gating properties of IIR were similar to those of IK(r) and IHERG, although IIR outward currents were negligible in comparison. 4. In high K+ extracellular solutions devoid of divalent cations, IIR deactivation kinetics were removed resulting in long-lasting currents apparently activated in hyperpolarization, with a marked (2.7-fold) increase in conductance, as recorded from the instantaneous linear current-voltage relationship at -120 mV. Re-addition of Ca2+ restored the original closure of the channel whereas re-addition of Mg2+ reduced the peak current. 5. The IIR described here, the heart IK(r) and the IHERG could be successfully predicted by a unique kinetic model where the voltage dependencies of the activation/inactivation gates were properly voltage shifted. On the whole, IIR seems to be the first example of a HERG-type current constitutively expressed and operating in mammalian cells of the neuronal lineage.