Effect of cysteine substitutions on the topology of the S4 segment of the Shaker potassium channel: implications for molecular models of gating

Voltage Sensing in Membranes: From Macroscopic Currents to Molecular Motions

Voltage-sensing domains (VSDs) are integral membrane protein units that sense changes in membrane electric potential, and through the resulting conformational changes, regulate a specific function. VSDs confer voltage-sensitivity to a large superfamily of membrane proteins that includes voltage-gated Na[Formula: see text], K[Formula: see text], Ca[Formula: see text] ,and H[Formula: see text] selective channels, hyperpolarization-activated cyclic nucleotide-gated channels, and voltage-sensing phosphatases. VSDs consist of four transmembrane segments (termed S1 through S4). Their most salient structural feature is the highly conserved positions for charged residues in their sequences. S4 exhibits at least three conserved triplet repeats composed of one basic residue (mostly arginine) followed by two hydrophobic residues. These S4 basic side chains participate in a state-dependent internal salt-bridge network with at least four acidic residues in S1-S3. The signature of voltage-dependent activation in electrophysiology experiments is a transient current (termed gating or sensing current) upon a change in applied membrane potential as the basic side chains in S4 move across the membrane electric field. Thus, the unique structural features of the VSD architecture allow for competing requirements: maintaining a series of stable transmembrane conformations, while allowing charge motion, as briefly reviewed here.

Voltage-dependent gating of HERG potassium channels

The mechanisms by which voltage-gated channels sense changes in membrane voltage and energetically couple this with opening of the ion conducting pore has been the source of significant interest. In voltage-gated potassium (Kv) channels, much of our knowledge in this area comes from Shaker-type channels, for which voltage-dependent gating is quite rapid. In these channels, activation and deactivation are associated with rapid reconfiguration of the voltage-sensing domain unit that is electromechanically coupled, via the S4-S5 linker helix, to the rate-limiting opening of an intracellular pore gate. However, fast voltage-dependent gating kinetics are not typical of all Kv channels, such as Kv11.1 (human ether-à-go-go related gene, hERG), which activates and deactivates very slowly. Compared to Shaker channels, our understanding of the mechanisms underlying slow hERG gating is much poorer. Here, we present a comparative review of the structure-function relationships underlying activation and deactivation gating in Shaker and hERG channels, with a focus on the roles of the voltage-sensing domain and the S4-S5 linker that couples voltage sensor movements to the pore. Measurements of gating current kinetics and fluorimetric analysis of voltage sensor movement are consistent with models suggesting that the hERG activation pathway contains a voltage independent step, which limits voltage sensor transitions. Constraints upon hERG voltage sensor movement may result from loose packing of the S4 helices and additional intra-voltage sensor counter-charge interactions. More recent data suggest that key amino acid differences in the hERG voltage-sensing unit and S4-S5 linker, relative to fast activating Shaker-type Kv channels, may also contribute to the increased stability of the resting state of the voltage sensor.

Fast and slow voltage sensor rearrangements during activation gating in Kv1.2 channels detected using tetramethylrhodamine fluorescence

The Kv1.2 channel, with its high resolution crystal structure, provides an ideal model for investigating conformational changes associated with channel gating, and fluorescent probes attached at the extracellular end of S4 are a powerful way to gain a more complete understanding of the voltage-dependent activity of these dynamic proteins. Tetramethylrhodamine-5-maleimide (TMRM) attached at A291C reports two distinct rearrangements of the voltage sensor domains, and a comparative fluorescence scan of the S4 and S3-S4 linker residues in Shaker and Kv1.2 shows important differences in their emission at other homologous residues. Kv1.2 shows a rapid decrease in A291C emission with a time constant of 1.5 +/- 0.1 ms at 60 mV (n = 11) that correlates with gating currents and reports on translocation of the S4 and S3-S4 linker. However, unlike any Kv channel studied to date, this fast component is dwarfed by a larger, slower quenching of TMRM emission during depolarizations between -120 and -50 mV (tau = 21.4 +/- 2.1 ms at 60 mV, V(1/2) of -73.9 +/- 1.4 mV) that is not seen in either Shaker or Kv1.5 and that comprises >60% of the total signal at all activating potentials. The slow fluorescence relaxes after repolarization in a voltage-dependent manner that matches the time course of Kv1.2 ionic current deactivation. Fluorophores placed directly in S1 and S2 at I187 and T219 recapitulate the time course and voltage dependence of slow quenching. The slow component is lost when the extracellular S1-S2 linker of Kv1.2 is replaced with that of Kv1.5 or Shaker, suggesting that it arises from a continuous internal rearrangement within the voltage sensor, initiated at negative potentials but prevalent throughout the activation process, and which must be reversed for the channel to close.

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

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

Characterization of the PCMBS-dependent modification of KCa3.1 channel gating

Intermediate conductance, calcium-activated potassium channels are gated by the binding of intracellular Ca(2+) to calmodulin, a Ca(2+)-binding protein that is constitutively associated with the C terminus of the channel. Although previous studies indicated that the pore-lining residues along the C-terminal portion of S6 contribute to the activation mechanism, little is known about whether the nonluminal face of S6 contributes to this process. Here we demonstrate that the sulfhydral reagent, parachloromercuribenze sulfonate (PCMBS), modifies an endogenous cysteine residue predicted to have a nonluminal orientation (Cys(276)) along the sixth transmembrane segment (S6). Modification of Cys(276) manipulates the steady-state and kinetic behavior of the channel by shifting the gating equilibrium toward the open state, resulting in a left shift in apparent Ca(2+) affinity and a slowing in the deactivation process. Using a six-state gating scheme, our analysis shows that PCMBS slows the transition between the open state back to the third closed state. Interpreting this result in the context of the steady-state and kinetic data suggests that PCMBS functions to shift the gating equilibrium toward the open state by disrupting channel closing. In an attempt to understand whether the nonluminal face of S6 participates in the activation mechanism, we conducted a partial tryptophan scan of this region. Substituting a tryptophan for Leu(281) recapitulated the effect on the steady-state and kinetic behavior observed with PCMBS. Considering the predicted nonluminal orientation of Cys(276) and Leu(281), a simple physical interpretation of these results is that the nonluminal face of S6 forms a critical interaction surface mediating the transition into the closed conformation, suggesting the nonluminal C-terminal portion of S6 is allosterically coupled to the activation gate.

Constraints on voltage sensor movement in the shaker K+ channel

In nerve and muscle cells, the voltage-gated opening and closing of cation-selective ion channels is accompanied by the translocation of 12-14 elementary charges across the membrane's electric field. Although most of these charges are carried by residues in the S4 helix of the gating module of these channels, the precise nature of their physical movement is currently the topic of spirited debate. Broadly speaking, two classes of models have emerged: those that suggest that small-scale motions can account for the extensive charge displacement, and those that invoke a much larger physical movement. In the most recent incarnation of the latter type of model, which is based on structural and functional data from the archaebacterial K(+) channel KvAP, a "voltage-sensor paddle" comprising a helix-turn-helix of S3-S4 translocates approximately 20 A through the bilayer during the gating cycle (Jiang, Y., A. Lee, J. Chen, V. Ruta, M. Cadene, B.T. Chait, and R. MacKinnon. 2003. Nature. 423:33-41; Jiang, Y., V. Ruta, J. Chen, A. Lee, and R. MacKinnon. 2003. Nature. 423:42-48.; Ruta, V., J. Chen, and R. MacKinnon. 2005. Cell. 123:463-475). We used two methods to test for analogous motions in the Shaker K(+) channel, each examining the aqueous exposure of residues near S3. In the first, we employed a pore-blocking maleimide reagent (Blaustein, R.O., P.A. Cole, C. Williams, and C. Miller. 2000. Nat. Struct. Biol. 7:309-311) to probe for state-dependent changes in the chemical reactivity of substituted cysteines; in the second, we tested the state-dependent accessibility of a tethered biotin to external streptavidin (Qiu, X.Q., K.S. Jakes, A. Finkelstein, and S.L. Slatin. 1994. J. Biol. Chem. 269:7483-7488; Slatin, S.L., X.Q. Qiu, K.S. Jakes, and A. Finkelstein. 1994. Nature. 371:158-161). In both types of experiments, residues predicted to lie near the top of S3 did not exhibit any change in aqueous exposure during the gating cycle. This lack of state dependence argues against large-scale movements, either axially or radially, of Shaker's S3-S4 voltage-sensor paddle.

How Does Voltage Open an Ion Channel?

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

The cooperative voltage sensor motion that gates a potassium channel

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

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

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

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

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

Gating charges in the activation and inactivation processes of the HERG channel

The hERG channel has a relatively slow activation process but an extremely fast and voltage-sensitive inactivation process. Direct measurement of hERG's gating current (Piper, D.R., A. Varghese, M.C. Sanguinetti, and M. Tristani-Firouzi. 2003. PNAS. 100:10534-10539) reveals two kinetic components of gating charge transfer that may originate from two channel domains. This study is designed to address three questions: (1) which of the six positive charges in hERG's major voltage sensor, S4, are responsible for gating charge transfer during activation, (2) whether a negative charge in the cytoplasmic half of S2 (D466) also contributes to gating charge transfer, and (3) whether S4 serves as the sole voltage sensor for hERG inactivation. We individually mutate S4's positive charges and D466 to cysteine, and examine (a) effects of mutations on the number of equivalent gating charges transferred during activation (z(a)) and inactivation (z(i)), and (b) sidedness and state dependence of accessibility of introduced cysteine side chains to a membrane-impermeable thiol-modifying reagent (MTSET). Neutralizing the outer three positive charges in S4 and D466 in S2 reduces z(a), and cysteine side chains introduced into these positions experience state-dependent changes in MTSET accessibility. On the other hand, neutralizing the inner three positive charges in S4 does not affect z(a). None of the charge mutations affect z(i). We propose that the scheme of gating charge transfer during hERG's activation process is similar to that described for the Shaker channel, although hERG has less gating charge in its S4 than in Shaker. Furthermore, channel domain other than S4 contributes to gating charge involved in hERG's inactivation process.

Molecular mechanism of voltage sensor movements in a potassium channel

Voltage-gated potassium channels are six-transmembrane (S1-S6) proteins that form a central pore domain (4 x S5-S6) surrounded by four voltage sensor domains (S1-S4), which detect changes in membrane voltage and control pore opening. Upon depolarization, the S4 segments move outward carrying charged residues across the membrane field, thereby leading to the opening of the pore. The mechanism of S4 motion is controversial. We have investigated how S4 moves relative to the pore domain in the prototypical Shaker potassium channel. We introduced pairs of cysteines, one in S4 and the other in S5, and examined proximity changes between each pair of cysteines during activation, using Cd2+ and copper-phenanthroline, which crosslink the cysteines with metal and disulphide bridges, respectively. Modelling of the results suggests a novel mechanism: in the resting state, the top of the S3b-S4 voltage sensor paddle lies close to the top of S5 of the adjacent subunit, but moves towards the top of S5 of its own subunit during depolarization--this motion is accompanied by a reorientation of S4 charges to the extracellular phase.

A technical consideration concerning the removal of oocyte vitelline membranes for patch clamp recording

We have developed an efficient method for removing the vitelline membrane of Xenopus oocytes for patch clamp recording. Functional studies using oocytes as models provide insights into the biological profiles and physiological properties of ion channels. A methodological modification is described in this paper. The important feature of this modification is that protease treatment is used to remove the oocyte's vitelline membrane. This method is simple and the oocytes produced remain in a healthy state during the recording process.

Electrostatic model of S4 motion in voltage-gated ion channels

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

Cellular processing of cone photoreceptor cyclic GMP-gated ion channels: a role for the S4 structural motif

We examined cellular protein processing and functional expression of photoreceptor cyclic nucleotide-gated (CNG) ion channels. In a mammalian cell line, wild type bovine cone photoreceptor channel alpha subunits (bCNGA3) convert from an unglycosylated state, at 90 kDa, to two glycosylated states at 93 and 102 kDa as they transit within the cell to their final location at the plasma membrane. Glycosylation per se is not required to yield functional channels, yet it is a milestone that distinguishes sequential steps in channel protein maturation. CNG ion channels are not gated by membrane voltage although their structure includes the transmembrane S4 motif known to function as the membrane voltage sensor in all voltage-gated ion channels. S4 must be functionally important because its natural mutation in cone photoreceptor CNG channels is associated with achromatopsia, a human autosomal inherited loss of cone function. Point mutation of specific, not all, charged and neutral residues within S4 cause failure of functional channel expression. Cellular channel protein processing fails in every one of the non-functional S4 mutations we studied. Mutant proteins do not reach the 102-kDa glycosylated state and do not arrive at the plasma membrane. They remain trapped within the endoplasmic reticulum and fail to transit out to the Golgi apparatus. Coexpression of cone CNG beta subunit (CNGB3) does not rescue the consequence of S4 mutations in CNGA3. It is likely that an intact S4 is required for proper protein folding and/or assembly in the endoplasmic reticulum membrane.

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

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

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

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

Depolarization induces intersubunit cross-linking in a S4 cysteine mutant of the Shaker potassium channel

Voltage-gated potassium (K(v)) channels are integral membrane proteins, composed of four subunits, each comprising six (S1-S6) transmembrane segments. S1-S4 comprise the voltage-sensing domain, and S5-S6 with the linker P-loop forms the ion conducting pore domain. During activation, S4 undergoes structural rearrangements that lead to the opening of the channel pore and ion conduction. To obtain details of these structural changes we have used the engineered disulfide bridge approach. For this we have introduced the L361C mutation at the extracellular end of S4 of the Shaker K channel and expressed the mutant channel in Xenopus oocytes. When exposed to mild oxidizing conditions (ambient oxygen or copper phenanthroline), Cys-361 formed an intersubunit disulfide bridge as revealed by the appearance of a dimeric band on Western blotting. As a consequence, the mutant channel suffered a significant loss in conductance (measured by two-electrode voltage clamp). Removal of native cysteines failed to prevent the disulfide formation, indicating that Cys-361 forms a disulfide with its counterpart in the neighboring subunit. The effect was voltage-dependent and occurred during channel activation after Cys-361 has been exposed to the extracellular phase. Although the disulfide bridge reduced the maximal conductance, it caused a hyperpolarizing shift in the conductance-voltage relationship and reduced the deactivation kinetics of the channel. The latter two effects suggest stabilization of the open state of the channel. In conclusion, we report that during activation the intersubunit distance between the N-terminal ends of the S4 segments of the L361C mutant Shaker K channel is reduced.

Conformational switch between slow and fast gating modes: allosteric regulation of voltage sensor mobility in the EAG K+ channel

Voltage-gated EAG K+ channels switch between fast and slow gating modes in a Mg2+-dependent manner by an unknown mechanism. We analyzed molecular motions in and around the voltage-sensing S4 in bEAG1. Using accessibility and perturbation analyses, we found that activation increases both the charge occupancy and volume of S4 side chains in the gating canal. Fluorescence measurements suggest that mode switching is due to a motion of the S2/S3 side of the gating canal. We propose that when S4 is in the resting state and its thin end is in the gating canal, a conformational rearrangement of S2/S3 narrows the canal around S4, forming the Mg2+ binding site. Binding of Mg2+ is proposed to stabilize this conformation and to slow opening of the gate by impeding S4's voltage-sensing outward motion.

Amino acid substitution within the S2 and S4 transmembrane segments in Shaker potassium channel modulates channel gating

To investigate of the gating properties in the voltage-activated potassium channel, we have mutated a variety of S2 and S4 residues in the Shaker potassium protein. Results showed that the R365C and R368C, but not the E283C, R362C, R365S, R368S or the ShB-IR, were sensitive to micromolar concentrations of Cd(2+) ions. This indicates that R365 and R368 play a crucial role in the channel gating due to a conformational modulation of the channel structure. Doubly mutated channels of the E283C/R365E and E283C/R368E caused a transient increase in current amplitude, which reached a peak within a few seconds and then decreased toward initial levels, despite the continual presence of Cd(2+). Taken together, our results suggest that E283, R365, and R368 form a network of strong, local, and electrostatic interactions that relate closely to the mechanism of the channel gating.

Reconstructing voltage sensor-pore interaction from a fluorescence scan of a voltage-gated K+ channel

X-ray crystallography has made considerable recent progress in providing static structures of ion channels. Here we describe a complementary method-systematic fluorescence scanning-that reveals the structural dynamics of a channel. Local protein motion was measured from changes in the fluorescent intensity of a fluorophore attached at one of 37 positions in the pore domain and in the S4 voltage sensor of the Shaker K+ channel. The local rearrangements that accompany activation and slow inactivation were mapped onto the homologous structure of the KcsA channel and onto models of S4. The results place clear constraints on S4 location, voltage-dependent movement, and the mechanism of coupling of S4 motion to the operation of the slow inactivation gate in the pore domain.