Probing the outer vestibule of a sodium channel voltage sensor

Role of domain 4 in sodium channel slow inactivation

Depolarization of sodium channels initiates at least three gating pathways: activation, fast inactivation, and slow inactivation. Little is known about the voltage sensors for slow inactivation, a process believed to be separate from fast inactivation. Covalent modification of a cysteine substituted for the third arginine (R1454) in the S4 segment of the fourth domain (R3C) with negatively charged methanethiosulfonate-ethylsulfonate (MTSES) or with positively charged methanethiosulfonate-ethyltrimethylammonium (MTSET) produces a marked slowing of the rate of fast inactivation. However, only MTSES modification produces substantial effects on the kinetics of slow inactivation. Rapid trains of depolarizations (2-20 Hz) cause a reduction of the peak current of mutant channels modified by MTSES, an effect not observed for wild-type or unmodified R3C channels, or for mutant channels modified by MTSET. The data suggest that MTSES modification of R3C enhances entry into a slow-inactivated state, and also that the effects on slow inactivation are independent of alterations of either activation or fast inactivation. This effect of MTSES is observed only for cysteine mutants within the middle of this S4 segment, and the data support a helical secondary structure of S4 in this region. Mutation of R1454 to the negatively charged residues aspartate or glutamate cannot reproduce the effects of MTSES modification, indicating that charge alone cannot account for these results. A long-chained derivative of MTSES has similar effects as MTSES, and can produce these effects on a residue that does not show use-dependent current reduction after modification by MTSES, suggesting that the sulfonate moiety can reach a critical site affecting slow inactivation. The effects of MTSES on R3C are partially counteracted by a point mutation (W408A) that inhibits slow inactivation. Our data suggest that a region near the midpoint of the S4 segment of domain 4 plays an important role in slow inactivation.

The voltage sensor in voltage-dependent ion channels

In voltage-dependent Na, K, or Ca channels, the probability of opening is modified by the membrane potential. This is achieved through a voltage sensor that detects the voltage and transfers its energy to the pore to control its gate. We present here the theoretical basis of the energy coupling between the electric field and the voltage, which allows the interpretation of the gating charge that moves in one channel. Movement of the gating charge constitutes the gating current. The properties are described, along with macroscopic data and gating current noise analysis, in relation to the operation of the voltage sensor and the opening of the channel. Structural details of the voltage sensor operation were resolved initially by locating the residues that make up the voltage sensor using mutagenesis experiments and determining the number of charges per channel. The changes in conformation are then analyzed based on the differential exposure of cysteine or histidine-substituted residues. Site-directed fluorescence labeling is then analyzed as another powerful indicator of conformational changes that allows time and voltage correlation of local changes seen by the fluorophores with the global change seen by the electrophysiology of gating currents and ionic currents. Finally, we describe the novel results on lanthanide-based resonance energy transfer that show small distance changes between residues in the channel molecule. All of the electrophysiological and the structural information are finally summarized in a physical model of a voltage-dependent channel in which a change in membrane potential causes rotation of the S4 segment that changes the exposure of the basic residues from an internally connected aqueous crevice at hyperpolarized potentials to an externally connected aqueous crevice at depolarized potentials.

Deletion of the S3-S4 linker in the Shaker potassium channel reveals two quenching groups near the outside of S4

When attached outside the voltage-sensing S4 segment of the Shaker potassium channel, the fluorescent probe tetramethylrhodamine (TMRM) undergoes voltage-dependent fluorescence changes (DeltaF) due to differential interaction with a pH-titratable external protein-lined vestibule (Cha, A., and F. Bezanilla. 1998. J. Gen. Physiol. 112:391-408.). We attached TMRM at the same sites [corresponding to M356C and A359C in the wild-type (wt) channel] in a deletion mutant of Shaker where all but the five amino acids closest to S4 had been removed from the S3-S4 linker. In the deletion mutant, the maximal DeltaF/F seen was diminished 10-fold, and the DeltaF at M356C became pH independent, suggesting that the protein-lined vestibule is made up in large part by the S3-S4 linker. The residual DeltaF showed that the probe still interacted with two putative quenching groups near the S4 segment. One group was detected by M356C-TMRM (located outside of S3 in the deletion mutant) and reported on deactivation gating charge movement when applying hyperpolarizing voltage steps from a holding potential of 0 mV. During activating voltage steps from a holding potential of -90 mV, the fluorescence lagged considerably behind the movement of gating charge over a range of potentials. Another putative quenching group was seen by probes attached closer to the S4 and caused a DeltaF at extreme hyperpolarizations (more negative than -90 mV) only. A signal from the interaction with this group in the wt S3-S4 linker channel (at L361C) correlated with gating charge moving in the hyperpolarized part of the Q-V curve. Probe attached at A359C in the deletion mutant and at L361C in wt channel showed a biphasic DeltaF as the probe oscillated between the two groups, revealing that there is a transient state of the voltage sensor in between, where the probe has maximal fluorescence. We conclude that the voltage sensor undergoes two distinct conformational changes as seen from probes attached outside the S4 segment.

Differential effects of homologous S4 mutations in human skeletal muscle sodium channels on deactivation gating from open and inactivated states

1. The outermost charged amino acid of S4 segments in the alpha subunit of human skeletal muscle sodium channels was mutated to cysteine in domains I (R219C), II (R669C), III (K1126C), and IV (R1448C). Double mutations in DIS4 and DIVS4 (R219C/R1448C), DIIS4 and DIVS4 (R669C/R1448C), and DIIIS4 and DIVS4 (K1126C/R1448C) were introduced in other constructs. Macropatch recordings of mutant and wild-type (hSkM1-wt) skeletal muscle sodium channels expressed in Xenopus oocytes were used to measure deactivation kinetics from open or fast inactivated states. 2. Conductance (voltage) curves (G (V)) derived from current (voltage) (I (V)) relations indicated a right-shifted G (V) relationship for R669C and for R669C/R1448C, but not for other mutations. The apparent valency was decreased for all mutations. Time-to-peak activation at -20 mV was increased for R1448C and for double mutations. 3. Deactivation kinetics from the open state were determined from the monoexponential decay of tail currents. Outermost charge-to-cysteine mutations in the S4 segments of domains III and IV slowed deactivation, with the greatest effect produced by R1448C. The deactivation rate constant was slowed to a greater extent for the DIII/DIV double mutation than that calculated from additive effects of single mutations in each of these two domains. Mutation in DIIS4 accelerated deactivation from the open state, whereas mutation in DIS4 had little effect. 4. Delays in the onset to recovery from fast inactivation were determined to assess deactivation kinetics from the inactivated state. Delay times for R219C and R669C were not significantly different from those for hSkM1-wt. Recovery delay was increased for K1126C, and was accelerated for R1448C. 5. Homologous charge mutations of S4 segments produced domain-specific effects on deactivation gating from the open and from the fast inactivated state. These results are consistent with the hypothesis that translocations of S4 segments in each domain during deactivation are not identical and independent processes. Non-identical effects of these mutations raise several possibilities regarding deactivation gating; translocation of DIVS4 may constitute the rate-limiting step in deactivation from the open state, DIVS4 may be part of the immobilizable charge, and S4 translocations underlying deactivation in human skeletal muscle sodium channel may exhibit co-operativity.

The screw-helical voltage gating of ion channels

In the voltage-gated ion channels of every animal, whether they are selective for K+, Na+ or Ca2+, the voltage sensors are the S4 transmembrane segments carrying four to eight positive charges always separated by two uncharged residues. It is proposed that they move across the membrane in a screw-helical fashion in a series of three or more steps that each transfer a single electronic charge. The unit steps are stabilized by ion pairing between the mobile positive charges and fixed negative charges, of which there are invariably two located near the inner ends of segments S2 and S3 and a third near the outer end of either S2 or S3. Opening of the channel involves three such steps in each domain.

The muscle chloride channel ClC-1 has a double-barreled appearance that is differentially affected in dominant and recessive myotonia

Single-channel recordings of the currents mediated by the muscle Cl- channel, ClC-1, expressed in Xenopus oocytes, provide the first direct evidence that this channel has two equidistant open conductance levels like the Torpedo ClC-0 prototype. As for the case of ClC-0, the probabilities and dwell times of the closed and conducting states are consistent with the presence of two independently gated pathways with approximately 1.2 pS conductance enabled in parallel via a common gate. However, the voltage dependence of the common gate is different and the kinetics are much faster than for ClC-0. Estimates of single-channel parameters from the analysis of macroscopic current fluctuations agree with those from single-channel recordings. Fluctuation analysis was used to characterize changes in the apparent double-gate behavior of the ClC-1 mutations I290M and I556N causing, respectively, a dominant and a recessive form of myotonia. We find that both mutations reduce about equally the open probability of single protopores and that mutation I290M yields a stronger reduction of the common gate open probability than mutation I556N. Our results suggest that the mammalian ClC-homologues have the same structure and mechanism proposed for the Torpedo channel ClC-0. Differential effects on the two gates that appear to modulate the activation of ClC-1 channels may be important determinants for the different patterns of inheritance of dominant and recessive ClC-1 mutations.

Interaction between the pore and a fast gate of the cardiac sodium channel

Permeant ions affect a fast gating process observed in human cardiac sodium channels (Townsend, C., H.A. Hartmann, and R. Horn. 1997. J. Gen. Physiol. 110:11-21). Removal of extracellular permeant ions causes a reduction of open probability at positive membrane potentials. These results suggest an intimate relationship between the ion-conducting pore and the gates of the channel. We tested this hypothesis by three sets of manipulations designed to affect the binding of cations within the pore: application of intracellular pore blockers, mutagenesis of residues known to contribute to permeation, and chemical modification of a native cysteine residue (C373) near the extracellular mouth of the pore. The coupling between extracellular permeant ions and this fast gating process is abolished both by pore blockers and by a mutation that severely affects selectivity. A more superficial pore mutation or chemical modification of C373 reduces single channel conductance while preserving both selectivity of the pore and the modulatory effects of extracellular cations. Our results demonstrate a modulatory gating role for a region deep within the pore and suggest that the structure of the permeation pathway is largely preserved when a channel is closed.

Three transmembrane conformations and sequence-dependent displacement of the S4 domain in shaker K+ channel gating

We have acquired structural evidence that two components evident previously in the depolarization-evoked gating currents from voltage-gated Shaker K+ channels have their origin in sequential, two-step outward movements of the S4 protein segments. A point mutation greatly destabilizes the "fully retracted" state of S4 transmembrane translocation, causing instead an intermediate state to predominate at resting potentials. This state is distinguishable topologically and fluorometrically. That a point mutation effectively excludes half the range of S4 motion from physiological voltages suggests that the diverse sensitivities among voltage-gated channels might reflect not only differences in S4 valence, but also displacement. Existence of an intermediate subunit state helps explain why modeling channel activation has required positing greater than four closed states.

State-dependent accessibility and electrostatic potential in the channel of the acetylcholine receptor. Inferences from rates of reaction of thiosulfonates with substituted cysteines in the M2 segment of the alpha subunit

Ion channel function depends on the chemical and physical properties and spatial arrangement of the residues that line the channel lumen and on the electrostatic potential within the lumen. We have used small, sulfhydryl-specific thiosulfonate reagents, both positively charged and neutral, to probe the environment within the acetylcholine (ACh) receptor channel. Rate constants were determined for their reactions with cysteines substituted for nine exposed residues in the second membrane-spanning segment (M2) of the alpha subunit. The largest rate constants, both in the presence and absence of ACh, were for the reactions with the cysteine substituted for alpha Thr244, near the intracellular end of the channel. In the open state of the channel, but not in the closed state, the rate constants for the reactions of the charged reagents with several substituted cysteines depended on the transmembrane electrostatic potential, and the electrical distance of these cysteines increased from the extracellular to the intracellular end of M2. Even at zero transmembrane potential, the ratios of the rate constants for the reactions of three positively charged reagents with alpha T244C, alpha L251C, and alpha L258C to the rate constant for the reaction of an uncharged reagent were much greater in the open than in the closed state. This dependence of the rate constants on reagent charge is consistent with an intrinsic electrostatic potential in the channel that is considerably more negative in the open state than in the closed state. The effects of ACh on the rate constants for the reactions of substituted Cys along the length of alpha M2, on the dependence of the rate constants on the transmembrane potential, and on the intrinsic potential support a location of a gate more intracellular than alpha Thr244.

Structure and function of voltage-gated sodium channels

1. Sodium channels mediate fast depolarization and conduct electrical impulses throughout nerve, muscle and heart. This paper reviews the links between sodium channel structure and function. 2. Sodium channels have a modular architecture, with distinct regions for the pore and the gates. The separation is far from absolute, however, with extensive interaction among the various parts of the channel. 3. At a molecular level, sodium channels are not static: they move extensively in the course of gating and ion translocation. 4. Sodium channels bind local anaesthetics and various toxins. In some cases, the relevant sites have been partially identified. 5. Sodium channels are subject to regulation at the levels of transcription, subunit interaction and post-translational modification (notably glycosylation and phosphorylation).

Independent versus coupled inactivation in sodium channels. Role of the domain 2 S4 segment

The voltage sensor of the sodium channel is mainly comprised of four positively charged S4 segments. Depolarization causes an outward movement of S4 segments, and this movement is coupled with opening of the channel. A mutation that substitutes a cysteine for the outermost arginine in the S4 segment of the second domain (D2:R1C) results in a channel with biophysical properties similar to those of wild-type channels. Chemical modification of this cysteine with methanethiosulfonate-ethyltrimethylammonium (MTSET) causes a hyperpolarizing shift of both the peak current-voltage relationship and the kinetics of activation, whereas the time constant of inactivation is not changed substantially. A conventional steady state inactivation protocol surprisingly produces an increase of the peak current at -20 mV when the 300-ms prepulse is depolarized from -190 to -110 mV. Further depolarization reduces the current, as expected for steady state inactivation. Recovery from inactivation in modified channels is also nonmonotonic at voltages more hyperpolarized than -100 mV. At -180 mV, for example, the amplitude of the recovering current is briefly almost twice as large as it was before the channels inactivated. These data can be explained readily if MTSET modification not only shifts the movement of D2/S4 to more hyperpolarized potentials, but also makes the movement sluggish. This behavior allows inactivation to have faster kinetics than activation, as in the HERG potassium channel. Because of the unique properties of the modified mutant, we were able to estimate the voltage dependence and kinetics of the movement of this single S4 segment. The data suggest that movement of modified D2/S4 is a first-order process and that rate constants for outward and inward movement are each exponential functions of membrane potential. Our results show that D2/S4 is intimately involved with activation but plays little role in either inactivation or recovery from inactivation.

Nonequilibrium response spectroscopy of voltage-sensitive ion channel gating

We describe a new electrophysiological technique called nonequilibrium response spectroscopy, which involves application of rapidly fluctuating (as high as 14 kHz) large-amplitude voltage clamp waveforms to ion channels. As a consequence of the irreversible (in the sense of Carnot) exchange of energy between the fluctuating field and the channel protein, the gating response is exquisitely sensitive to features of the kinetics that are difficult or impossible to adequately resolve by means of traditional stepped potential protocols. Here we focus on the application of dichotomous (telegraph) noise voltage fluctuations, a broadband Markovian colored noise that fluctuates between two values. Because Markov kinetic models of channel gating can be embedded within higher-dimensional Markov models that take into account the effects of the voltage fluctuations, many features of the response of the channels can be calculated algebraically. This makes dichotomous noise and its generalizations uniquely suitable for model selection and kinetic analysis. Although we describe its application to macroscopic ionic current measurements, the nonequilibrium response method can also be applied to gating and single channel current recording techniques. We show how data from the human cardiac isoform (hH1a) of the Na+ channel expressed in mammalian cells can be acquired and analyzed, and how these data reveal hidden aspects of the molecular kinetics that are not revealed by conventional methods.