Molecular basis of charge movement in voltage-gated sodium channels

Activation of shaker potassium channels. I. Characterization of voltage-dependent transitions

The conformational changes associated with activation gating in Shaker potassium channels are functionally characterized in patch-clamp recordings made from Xenopus laevis oocytes expressing Shaker channels with fast inactivation removed. Estimates of the forward and backward rates for transitions are obtained by fitting exponentials to macroscopic ionic and gating current relaxations at voltage extremes, where we assume that transitions are unidirectional. The assignment of different rates is facilitated by using voltage protocols that incorporate prepulses to preload channels into different distributions of states, yielding test currents that reflect different subsets of transitions. These data yield direct estimates of the rate constants and partial charges associated with three forward and three backward transitions, as well as estimates of the partial charges associated with other transitions. The partial charges correspond to an average charge movement of 0.5 e0 during each transition in the activation process. This value implies that activation gating involves a large number of transitions to account for the total gating charge displacement of 13 e0. The characterization of the gating transitions here forms the basis for constraining a detailed gating model to be described in a subsequent paper of this series.

Co- and posttranslational translocation mechanisms direct cystic fibrosis transmembrane conductance regulator N terminus transmembrane assembly

Transmembrane topology of most eukaryotic polytopic proteins is established cotranslationally at the endoplasmic reticulum membrane through the action of alternating signal and stop transfer sequences. Here we demonstrate that the cystic fibrosis transmembrane conductance regulator (CFTR) achieves its N terminus topology through a variation of this mechanism that involves both co- and posttranslational translocation events. Using a series of defined chimeric and truncated proteins expressed in a reticulocyte lysate system, we have identified two topogenic determinants encoded within the first (TM1) and second (TM2) membrane-spanning segments of CFTR. Each sequence independently (i) directed endoplasmic reticulum targeting, (ii) translocated appropriate flanking residues, and (iii) achieved its proper membrane-spanning orientation. Signal sequence activity of TM1, however, was inefficient due to the presence of two charged residues, Glu92 and Lys95, located within its hydrophobic core. As a result, TM1 was able to direct correct topology for less than half of nascent CFTR chains. In contrast to TM1, TM2 signal sequence activity was both efficient and specific. Even in the absence of a functional TM1 signal sequence, TM2 was able to direct CFTR N terminus topology through a ribosome-dependent posttranslational mechanism. Mutating charged residues Glu92 and Lys95 to alanine improved TM1 signal sequence activity as well as the ability of TM1 to independently direct CFTR N terminus topology. Thus, a single functional signal sequence in either the first or second TM segment was sufficient for directing proper CFTR topology. These results identify two distinct and redundant translocation pathways for CFTR N terminus transmembrane assembly and support a model in which TM2 functions to ensure correct topology of CFTR chains that fail to translocate via TM1. This novel arrangement of topogenic information provides an alternative to conventional cotranslational pathways of polytopic protein biogenesis.

A specific interaction between the cardiac sodium channel and site-3 toxin anthopleurin B

The polypeptide neurotoxin anthopleurin B (ApB) isolated from the venom of the sea anemone Anthopleura xanthogrammica is one of a family of toxins that bind to the extracellular face of voltage-dependent sodium channels and retard channel inactivation. Because most regions of the sodium channel known to contribute to inactivation are located intracellularly or within the membrane bilayer, identification of the toxin/channel binding site is of obvious interest. Recently, mutation of a glutamic acid residue on the extracellular face of the fourth domain of the rat neuronal sodium channel (rBr2a) was shown to disrupt toxin/channel binding (Rogers, J. C., Qu, Y. S., Tanada, T. N., Scheuer, T., and Catterall, W. A. (1996) J. Biol. Chem. 271, 15950-15962). A negative charge at this position is highly conserved between mammalian sodium channel isoforms. We have constructed mutations of the corresponding residue (Asp-1612) in the rat cardiac channel isoform (rH1) and shown that the lowered affinity occurs primarily through an increase in the toxin/channel dissociation rate koff. Further, we have used thermodynamic mutant cycle analysis to demonstrate a specific interaction between this anionic amino acid and Lys-37 of ApB (DeltaDeltaG = 1.5 kcal/mol), a residue that is conserved among many sea anemone toxins. Reversal of the charge at Asp-1612, as in the mutant D1612R, also affects channel inactivation independent of toxin (-14 mV shift in channel availability). Binding of the toxin to Asp-1612 may therefore contribute both to toxin/channel affinity and to transduction of the effects of the toxin on channel kinetics.

Slow inactivation does not affect movement of the fast inactivation gate in voltage-gated Na+ channels

Voltage-gated Na+ channels exhibit two forms of inactivation, one form (fast inactivation) takes effect on the order of milliseconds and the other (slow inactivation) on the order of seconds to minutes. While previous studies have suggested that fast and slow inactivation are structurally independent gating processes, little is known about the relationship between the two. In this study, we probed this relationship by examining the effects of slow inactivation on a conformational marker for fast inactivation, the accessibility of a site on the Na+ channel III-IV linker that is believed to form a part of the fast inactivation particle. When cysteine was substituted for phenylalanine at position 1304 in the rat skeletal muscle sodium channel (microl), application of [2-(trimethylammonium)ethyl]methanethiosulfonate (MTS-ET) to the cytoplasmic face of inside-out patches from Xenopus oocytes injected with F1304C RNA dramatically disrupted fast inactivation and displayed voltage-dependent reaction kinetics that closely paralleled the steady state availability (hinfinity) curve. Based on this observation, the accessibility of cys1304 was used as a conformational marker to probe the position of the fast inactivation gate during the development of and the recovery from slow inactivation. We found that burial of cys1304 is not altered by the onset of slow inactivation, and that recovery of accessibility of cys1304 is not slowed after long (2-10 s) depolarizations. These results suggest that (a) fast and slow inactivation are structurally distinct processes that are not tightly coupled, (b) fast and slow inactivation are not mutually exclusive processes (i.e., sodium channels may be fast- and slow-inactivated simultaneously), and (c) after long depolarizations, recovery from fast inactivation precedes recovery from slow inactivation.

Voltage-dependent membrane displacements measured by atomic force microscopy

Cells use polar molecules in the membrane to sense changes in the transmembrane potential. The opening of voltage-gated ion channels and membrane bending due to the inverse flexoelectric effect are two examples of such electromechanical coupling. We have looked for membrane motions in an electric field using atomic (or scanning) force microscopy (AFM) with the intent of studying voltage-dependent conformational changes of ion channels. Voltage-clamped HEK293 cells were either untransfected controls or transfected with Shaker K+ channels. Using a +/- 10-mV peak-peak AC carrier stimulus, untransfected cells moved 0.5-15 nm normal to the plane of the membrane. These movements tracked the voltage at frequencies >1 kHz with a phase lead of 60-120 degrees, as expected of a displacement current. The movement was outward with depolarization, but the holding potential only weakly influenced the amplitude of the movement. In contrast, cells transfected with a noninactivating mutant of Shaker K+channels showed similar movements, but these were sensitive to the holding potential; decreasing with depolarization between -80 and 0 mV. Searching for artifactual origins of these movements, we used open or sealed pipettes and AFM cantilever placements just above the cells. These results were negative, suggesting that the observed movements were produced by the cell membrane rather than by movement of the patch pipette, or by acoustic or electrical interactions of the membrane with the AFM tip. In control cells, the electrical motor may arise from the flexoelectric effect, where changes in potential induce changes in curvature. In transfected cells, it appears that channel-specific movements also occurred. These experiments demonstrate that the AFM may be able to exploit voltage-dependent movements as a source of contrast for imaging membrane components. The electrically induced motility will cause twitching during action potentials, and may have physiological consequences.

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.

Large conductance voltage- and calcium-dependent K+ channel, a distinct member of voltage-dependent ion channels with seven N-terminal transmembrane segments (S0-S6), an extracellular N terminus, and an intracellular (S9-S10) C terminus

Large conductance voltage- and Ca2+-dependent K+ (MaxiK) channels show sequence similarities to voltage-gated ion channels. They have a homologous S1-S6 region, but are unique at the N and C termini. At the C terminus, MaxiK channels have four additional hydrophobic regions (S7-S10) of unknown topology. At the N terminus, we have recently proposed a new model where MaxiK channels have an additional transmembrane region (S0) that confers beta subunit regulation. Using transient expression of epitope tagged MaxiK channels, in vitro translation, functional, and "in vivo" reconstitution assays, we now show that MaxiK channels have seven transmembrane segments (S0-S6) at the N terminus and a S1-S6 region that folds in a similar way as in voltage-gated ion channels. Further, our results indicate that hydrophobic segments S9-S10 in the C terminus are cytoplasmic and unequivocally demonstrate that S0 forms an additional transmembrane segment leading to an exoplasmic N terminus.

Contribution of the NH2 terminus of Kv2.1 to channel activation

Opening and closing of voltage-operated channels requires the interaction of diverse structural elements. One approach to the identification of channel domains that participate in gating is to locate the sites of action of modifiers. Covalent reaction of Kv2.1 channels with the neutral, sulfhydryl-specific methylmethanethiosulfonate (MMTS) caused a slowing of channel gating with a predominant effect on the kinetics of activation. These effects were also obtained after intracellular, but not extracellular, application of a charged MMTS analog. Single channel analysis revealed that MMTS acted primarily by prolonging the latency to first opening without substantially affecting gating transitions after the channel first opens and until it inactivates. To localize the channel cysteine(s) with which MMTS reacts, we generated NH2- and COOH-terminal deletion mutants and a construct in which all three cysteines in transmembrane regions were substituted. Only the NH2-terminal deletion construct gave rise to currents that activated slowly and displayed MMTS-insensitive kinetics. These results show that the NH2-terminal tail of Kv2.1 participates in transitions leading to activation through interactions involving reduced cysteine(s) that can be modulated from the cytoplasmic phase.

Ion regulation of the kinetics of potential-dependent potassium channels

We apply a theoretical approach developed earlier. The interaction ofions that permeate a channel with slowly relaxing charged channel-forminggroups (ion-conformational interaction - ICI) is addressed by thisapproach. One can describe the ion concentration influence (ion regulation)on channel functioning in this manner. A patch-clamp method in a'whole-cell' configuration is used to study the ICI. For this purpose theinfluence of an external concentration of potassium ions on thepotential-dependent potassium current (I(A)) in the externalmembrane of GH(3) cells was studied. The increase of[K(+) (out)] from 5 mM to 100 mM causes anon-monotonous shift of current-voltage dependencies. The dependence of bothan activation time constant tgr(n) and a steady-state activation(n(∞)) on [K(+)](out) have a minimum andmaximum respectively. The analysis of the results suggests that the observedeffects are caused by ICI. A physical model is developed to describe thedependence of the potassium channel kinetics on the external concentrationof the ions and the membrane potential. The 'deformation' of the closedstate of the gate and the corresponding energy shifts cause the observednon-monotonous dependencies due to ICI. Thus, the general theoreticalapproach has an experimental confirmation and is applied to concreteexamples. Formulas for concentrational dependencies of the channel kineticsare given for practical uses.

Role in fast inactivation of the IV/S4-S5 loop of the human muscle Na+ channel probed by cysteine mutagenesis

1. In order to investigate the role in fast inactivation of the cytoplasmic S4-S5 loop of the fourth domain (IV/S4-S5) within the alpha-subunit of the adult human muscle Na+ channel, every single amino acid from R1469 to G1486 was substituted by a cysteine and the mutants were studied by functional expression in human embryonic kidney cells (tsA201) using whole-cell patch clamping. Effects following intracellular application of the sulfhydryl reagents MTSET and MTSES on the mutants were investigated. 2. Sixteen of eighteen mutants resulted in the formation of functional channels. For P1480C and N1484C, no Na+ currents could be detected in transfected cells. In the absence of sulfhydryl reagents, F1473C and A1481C slowed fast Na+ channel inactivation by 2- and 1.5-fold, respectively, and L1482C induced a steady-state Na+ current (Iss) of 3% of peak current (Ipeak) (1% for wild-type). 3. Upon application of MTSET and MTSES, changes in fast inactivation gating occurred for most of the mutants. The most dramatic destabilizing effects on fast inactivation were observed for M1476C (9-fold slowing of inactivation; Iss/Ipeak, 3.6%; +15 mV shift in steady-state inactivation; 2- to 3-fold acceleration of recovery from inactivation), A1481C (3-fold; 14%; +20 mV; no change) and F1473C (2.5-fold; 2.4%; +8 mV; 1.5-fold). Less pronounced destabilizing effects were observed for M1477C and L1479C. Strongly stabilizing effects on the inactivated state, that is a 20-30 mV hyperpolarizing shift of the inactivation curve associated with a 3- to 4-fold decrease in the rate of recovery from inactivation, occurred for T1470C, L1471C and A1474C. Almost all effects were independent of the membrane potential; however, A1474C only reacted when cells were depolarized. Significant effects on activation were not observed. 4. We conclude that the IV/S4-S5 loop plays an important role in fast inactivation of the muscle Na+ channel and may contribute to the formation of a receptor for the putative inactivation particle. The effects of sulfhydryl reagents on the various mutations suggest an alpha-helical structure of IV/S4-S5 (up to P1480) with destabilizing effects on inactivation for one cluster of amino acids (1473/76/77/79) and a stabilized inactivation at the opposite side of the helix (1470/71/74).

Voltage-dependent proton transport by the voltage sensor of the Shaker K+ channel

In voltage-dependent ion channels, pore opening is initiated by electrically driven movements of charged residues, and this movement generates a gating current. To examine structural rearrangements in the Shaker K+ channel, basic residues R365 and R368 in the S4 segment were replaced with histidine, and gating currents were recorded. Changes in gating charge displacement with solvent pH reveal voltage-dependent changes in exposure of the histidine to solvent protons. This technique directly monitors accessibility changes during gating, probes the environment even in confined locations, and introduces minimal interference of gating charge motion. The results indicate that charges 365 and 368 traverse the entire electric field during gating. The remarkable implication of the successive exposure of histidine to each side of the membrane is that in a pH gradient, the voltage sensor transports protons.

Probing the outer vestibule of a sodium channel voltage sensor

The second and third basic residues of the S4 segment of domain 4 (D4:R2 and D4:R3) of the human skeletal muscle Na+ channel are known to be translocated from a cytoplasmic to an extracellular position during depolarization. Accessibilities of individual S4 residues were assayed by alteration of inactivation kinetics during modification of cysteine mutants by hydrophilic methanethiosulfonate reagents. The voltage dependences of the reaction rates are identical for extracellular application of cationic methanethiosulfonate-ethyltrimethylammonium (MTSET) and anionic methanethiosulfonate-ethylsulfonate (MTSES), suggesting that D4:R3C is situated outside the membrane electric field at depolarized voltages. The absolute rate of R3C modification is 281-fold greater for MTSET than for MTSES, however, suggesting that at depolarized voltages this S4 thiol resides in a negatively charged hydrophilic crevice. The two hydrophobic residues between D4:R2C and D4:R3C in the primary sequence (L1452 and A1453) are not externally exposed at any voltage. An alpha-helical representation of D4/S4 shows that the basic residues D4:R2 and D4:R3 are on the face opposite that of L1452 and A1453. We propose that in the depolarized conformation, the hydrophobic face of this portion of D4/S4 remains in contact with a hydrophobic region of the extracellular vestibule of the S4 channel.

Characterizing voltage-dependent conformational changes in the Shaker K+ channel with fluorescence

We examined voltage-dependent conformational changes in three specific regions of the Shaker potassium channel with site-directed fluorescent labeling: the fourth transmembrane segment (S4), the second transmembrane segment (S2), and the putative pore region. The fluorescence changes displayed distinctive properties that correlate with gating, activation, and slow inactivation of the channel. The fluorescence signals measured near the S2 and S4 segments suggest that the S2 segment may undergo voltage-sensitive conformational changes that precede those in the S4 segment. In contrast, fluorescence changes in the pore correlated with the voltage dependence and time course of ionic activation and slow inactivation. Spectroscopy indicated that the mechanism of fluorescence change involves voltage-dependent quenching of the probe in an aqueous environment by other parts of the protein.

Insertion of a SNS-specific tetrapeptide in S3-S4 linker of D4 accelerates recovery from inactivation of skeletal muscle voltage-gated Na channel mu1 in HEK293 cells

Na channel subunits alphaSNS (PN3) and alpha mu1(SkM1) produce slowly inactivating/TTX-resistant and rapidly inactivating/TTX-sensitive currents, respectively. AlphaSNS (PN3) current recovers from inactivation (reprimes) rapidly. Sequence alignment identified the tetrapeptide SLEN, in the S3-S4 linker of D4, as alphaSNS-specific. To determine whether SLEN endows Na channels with slow kinetics and/or rapid repriming, we analyzed the transient Na current produced by a chimera mu1SLEN in HEK293 cells. Neither kinetics nor voltage dependence of activation and inactivation was affected. However, repriming was twice as fast as in the wild type at -100 mV. This suggests that SLEN may contribute to the rapid repriming of TTX-resistant Na current.

Cytoskeleton modulates coupling between availability and activation of cardiac sodium channel

The aim of this study was to investigate modulation of voltage-dependent steady-state activation and availability from inactivation of the cardiac Na+ channel by the cytoskeleton. As an experimental approach, we used long-lasting monitoring [63 +/- 5 (SE) min] of the half-point potentials of the steady-state availability curve (V(1/2A)) and normalized conductance curve (V(1/2G)) in 116 rat ventricular cardiomyocytes by whole cell patch clamp at 22-24 degrees C. Both half-point potentials shifted in the negative direction with time as an exponentially saturating change, with the shift of V(1/2G) being smaller and faster. An F-actin disrupter, cytochalasin D (Cyto-D, 20 microM), accelerated the rate of the V(1/2A) shift but decreased the range of the V(1/2G) shift. An F-actin stabilizer, phalloidin (100 microM), temporarily (for 28.2 +/- 2.2 min, n = 15) prevented the V(1/2A) shift but did not influence the V(1/2G) shift. The best fit for the V(1/2G)-V(1/2A) relationship in untreated cells (1,021 data points measured in 51 cells) was a second-degree (2.06) power function. Cytoskeleton-directed agents modified the relationship. In Cyto-D-treated cells, the V(1/2G)-V(1/2A) relationship was shifted (by 2.5 mV) toward positive V(1/2G). On the contrary, a microtubule stabilizer, taxol (100 microM), shifted the relationship toward negative V(1/2G) (by 12.2 mV). We conclude that coupling between availability and activation is modulated by F-actin-based and microtubular cytoskeleton.

Sodium channel activation gating is affected by substitutions of voltage sensor positive charges in all four domains

The role of the voltage sensor positive charges in the activation and deactivation gating of the rat brain IIA sodium channel was investigated by mutating the second and fourth conserved positive charges in the S4 segments of all four homologous domains. Both charge-neutralizing (by glutamine substitution) and -conserving mutations were constructed in a cDNA encoding the sodium channel alpha subunit that had fast inactivation removed by the incorporation of the IFMQ3 mutation in the III-IV linker (West, J.W., D.E. Patton, T. Scheuer, Y. Wang, A.L. Goldin, and W.A. Catterall. 1992. 89:10910-10914.). A total of 16 single and 2 double mutants were constructed and analyzed with respect to voltage dependence and kinetics of activation and deactivation. The most significant effects were observed with substitutions of the fourth positive charge in each domain. Neutralization of the fourth positive charge in domain I or II produced the largest shifts in the voltage dependence of activation, both in the positive direction. This change was accompanied by positive shifts in the voltage dependence of activation and deactivation kinetics. Combining the two mutations resulted in an even larger positive shift in half-maximal activation and a significantly reduced gating valence, together with larger positive shifts in the voltage dependence of activation and deactivation kinetics. In contrast, neutralization of the fourth positive charge in domain III caused a negative shift in the voltage of half-maximal activation, while the charge-conserving mutation resulted in a positive shift. Neutralization of the fourth charge in domain IV did not shift the half-maximal voltage of activation, but the conservative substitution produced a positive shift. These data support the idea that both charge and structure are determinants of function in S4 voltage sensors. Overall, the data supports a working model in which all four S4 segments contribute to voltage-dependent activation of the sodium channel.

Allosteric gating of a large conductance Ca-activated K+ channel

Large-conductance Ca-activated potassium channels (BK channels) are uniquely sensitive to both membrane potential and intracellular Ca2+. Recent work has demonstrated that in the gating of these channels there are voltage-sensitive steps that are separate from Ca2+ binding steps. Based on this result and the macroscopic steady state and kinetic properties of the cloned BK channel mslo, we have recently proposed a general kinetic scheme to describe the interaction between voltage and Ca2+ in the gating of the mslo channel (Cui, J., D.H. Cox, and R.W. Aldrich. 1997. J. Gen. Physiol. In press.). This scheme supposes that the channel exists in two main conformations, closed and open. The conformational change between closed and open is voltage dependent. Ca2+ binds to both the closed and open conformations, but on average binds more tightly to the open conformation and thereby promotes channel opening. Here we describe the basic properties of models of this form and test their ability to mimic mslo macroscopic steady state and kinetic behavior. The simplest form of this scheme corresponds to a voltage-dependent version of the Monod-Wyman-Changeux (MWC) model of allosteric proteins. The success of voltage-dependent MWC models in describing many aspects of mslo gating suggests that these channels may share a common molecular mechanism with other allosteric proteins whose behaviors have been modeled using the MWC formalism. We also demonstrate how this scheme can arise as a simplification of a more complex scheme that is based on the premise that the channel is a homotetramer with a single Ca2+ binding site and a single voltage sensor in each subunit. Aspects of the mslo data not well fitted by the simplified scheme will likely be better accounted for by this more general scheme. The kinetic schemes discussed in this paper may be useful in interpreting the effects of BK channel modifications or mutations.

Structure-affinity relationships and stereospecificity of several homologous series of local anesthetics for the beta2-adrenergic receptor

Local anesthetics inhibit binding of ligands to beta2-adrenergic receptors (beta2ARs), and, as a consequence, inhibit intracellular cAMP production. We hypothesized that among homologous local anesthetics, their avidity at inhibiting binding of tritiated dihydroalprenolol (3H-DHA) to beta2ARs would increase with increasing length of alkyl substituents and would demonstrate stereospecificity. Specific binding of 3H-DHA to human beta2ARs was assayed in the presence of six different members of the 1-alkyl-2,6-pipecoloxylidide class of local anesthetics (including mepivacaine, ropivacaine, and bupivacaine), the R(+) and S(-) bupivacaine enantiomers, lidocaine, prilocaine, etidocaine, procaine, and tetracaine. Avidity of binding to beta2ARs increased with increasing length of the alkyl chain (pKi values = 2.4, 3.6, 4.3, 4.1, 4.1, 5.9 for the methyl [mepivacaine], ethyl, S(-)propyl [ropivacaine], butyl [bupivacaine], pentyl, and octyl derivatives, respectively). We found no evidence for bupivacaine stereospecificity (pKi values = 4.3 and 4.9 for the S(-) and R(+) isomers, respectively). Other amide and ester local anesthetics also showed increasing potency with increasing length of alkyl substituents (pKi values = 3.6, 3.8, and 4.3 for lidocaine, prilocaine, and etidocaine; 4.2 and 5.6 for procaine and tetracaine, respectively). The correlation between increased inhibition of beta2AR binding and alkyl chain length resembles the correlation between local anesthetic potency at nerve block and increased alkyl chain length. The lack of clear stereospecificity is consistent with the relatively low potency these agents demonstrate at inhibition of beta2AR binding. Finally, the relatively potent inhibition of beta2ARs by etidocaine, tetracaine, and bupivacaine suggests that their propensity for cardiovascular depression after accidental intravenous overdose could result from beta2AR or beta1AR blockade and inhibition of cAMP production.

IMPLICATIONS

Local anesthetics demonstrate a rank order of avidity for displacing ligands from beta2-adrenergic receptors such that larger molecules displace ligands at lower concentrations than smaller local anesthetic molecules. This relationship between molecular size and receptor avidity could explain the greater propensity for cardiovascular toxicity of relatively large local anesthetics such as bupivacaine.

Allosteric effects of permeating cations on gating currents during K+ channel deactivation

K+ channel gating currents are usually measured in the absence of permeating ions, when a common feature of channel closing is a rising phase of off-gating current and slow subsequent decay. Current models of gating invoke a concerted rearrangement of subunits just before the open state to explain this very slow charge return from opening potentials. We have measured gating currents from the voltage-gated K+ channel, Kv1.5, highly overexpressed in human embryonic kidney cells. In the presence of permeating K+ or Cs+, we show, by comparison with data obtained in the absence of permeant ions, that there is a rapid return of charge after depolarizations. Measurement of off-gating currents on repolarization before and after K+ dialysis from cells allowed a comparison of off-gating current amplitudes and time course in the same cells. Parallel experiments utilizing the low permeability of Cs+ through Kv1.5 revealed similar rapid charge return during measurements of off-gating currents at ECs. Such effects could not be reproduced in a nonconducting mutant (W472F) of Kv1.5, in which, by definition, ion permeation was macroscopically absent. This preservation of a fast kinetic structure of off-gating currents on return from potentials at which channels open suggests an allosteric modulation by permeant cations. This may arise from a direct action on a slow step late in the activation pathway, or via a retardation in the rate of C-type inactivation. The activation energy barrier for K+ channel closing is reduced, which may be important during repetitive action potential spiking where ion channels characteristically undergo continuous cyclical activation and deactivation.

Molecular motions within the pore of voltage-dependent sodium channels

The pores of ion channel proteins are often modeled as static structures. In this view, selectivity reflects rigidly constrained backbone orientations. Such a picture is at variance with the generalization that biological proteins are flexible, capable of major internal motions on biologically relevant time scales. We tested for motions in the sodium channel pore by systematically introducing pairs of cysteine residues throughout the pore-lining segments. Two distinct pairs of residues spontaneously formed disulfide bonds bridging domains I and II. Nine other permutations, involving all four domains, were capable of disulfide bonding in the presence of a redox catalyst. The results are inconsistent with a single fixed backbone structure for the pore; instead, the segments that line the permeation pathway appear capable of sizable motions.

Voltage-dependent binding and calcium channel current inhibition by an anti-alpha 1D subunit antibody in rat dorsal root ganglion neurones and guinea-pig myocytes

1. The presence of calcium channel alpha 1D subunit mRNA in cultured rat dorsal root ganglion (DRG) neurones and guinea-pig cardiac myocytes was demonstrated using the reverse transcriptase-polymerase chain reaction. 2. An antipeptide antibody targeted at a region of the voltage-dependent calcium channel alpha 1D subunit C-terminal to the pore-forming SS1-SS2 loop in domain IV (amino acids 1417-1434) only bound to this exofacial epitope if the DRG neurones and cardiac myocytes were depolarized with 30 mM K+. 3. Incubation of cells under depolarizing conditions for 2-4 h with the antibody resulted in a maximal inhibition of inward current density of 49% (P < 0.005) for DRGs and 30% (P < 0.05) for cardiac myocytes when compared with controls. 4. S-(-)-Bay K 8644 (1 microM) enhanced calcium channel currents in DRGs by 75 +/- 19% (n = 5) in neurones incubated under depolarizing conditions with antibody that had been preabsorbed with its immunizing peptide (100 micrograms ml-1). This was significantly (P < 0.05) larger than the enhancement by S-(-)-Bay K 8644 that was seen with cells incubated under identical conditions but with antibody alone, which was 15 +/- 4% (n = 5). 5. These results demonstrate the presence of calcium channel alpha 1D subunits in rat DRG neurones and guinea-pig cardiac myocytes. They also show that amino acids 1417-1434 of the alpha 1D subunit are only exposed to the extracellular face of the membrane following depolarization and that the binding of an antibody to these amino acids attenuates calcium channel current and reduces the ability of S-(-)-Bay K 8644 to enhance this current, indicating that it is an L-type current that is attenuated.

Gated access to the pore of a voltage-dependent K+ channel

Voltage-activated K+ channels are integral membrane proteins that open or close a K(+)-selective pore in response to changes in transmembrane voltage. Although the S4 region of these channels has been implicated as the voltage sensor, little is known about how opening and closing of the pore is accomplished. We explored the gating process by introducing cysteines at various positions thought to lie in or near the pore of the Shaker K+ channel, and by testing their ability to be chemically modified. We found a series of positions in the S6 transmembrane region that react rapidly with water-soluble thiol reagents in the open state but not the closed state. An open-channel blocker can protect several of these cysteines, showing that they lie in the ion-conducting pore. At two of these sites, Cd2+ ions bind to the cysteines without affecting the energetics of gating; at a third site, Cd2+ binding holds the channel open. The results suggest that these channels open and close by the movement of an intracellular gate, distinct from the selectivity filter, that regulates access to the pore.

Anomalous effect of permeant ion concentration on peak open probability of cardiac Na+ channels

Human heart Na+ channels were expressed transiently in both mammalian cells and Xenopus oocytes, and Na+ currents measured using 150 mM intracellular Na+. Decreasing extracellular permeant ion concentration decreases outward Na+ current at positive voltages while increasing the driving force for the current. This anomalous effect of permeant ion concentration, especially obvious in a mutant (F1485Q) in which fast inactivation is partially abolished, is due to an alteration of open probability. The effect is only observed when a highly permeant cation (Na+, Li+, or hydrazinium) is substituted for a relatively impermeant cation (K+, Rb+, Cs+, N-methylglucamine, Tris, choline, or tetramethylammonium). With high concentrations of extracellular permeant cations, the peak open probability of Na+ channels increases with depolarization and then saturates at positive voltages. By contrast, with low concentrations of permeant ions, the open probability reaches a maximum at approximately 0 mV and then decreases with further depolarization. There is little effect of permeant ion concentration on activation kinetics at depolarized voltages. Furthermore, the lowered open probability caused by a brief depolarization to +60 mV recovers within 5 ms upon repolarization to -140 mV, indicative of a gating process with rapid kinetics. Tail currents at reduced temperatures reveal the rapid onset of this gating process during a large depolarization. A large depolarization may drive a permeant cation out of a site within the extracellular mouth of the pore, reducing the efficiency with which the channel opens.

Role of S4 segments and the leucine heptad motif in the activation of an L-type calcium channel

Basic residues in the S4 segments of voltage-dependent channels and leucines within the heptad repeat motif in the S4-S5 region of Shaker potassium channels have been shown to have important influences on activation. Here we have compared the relative importance for activation of S4 arginines (mutated to neutral or negative residues) in each of the four repeats of a chimeric L-type calcium channel. Significant effects on midpoint potential and time constant of activation were produced by mutations in repeats I and III but not in repeats II and IV. Leucine or isoleucine mutations in repeats I and III had the same effect on the voltage dependence of calcium channel activation as the mutations at equivalent positions in the Shaker channel, indicating that the heptad motif plays a fundamental role in channel activation.

Intrinsic voltage dependence and Ca2+ regulation of mslo large conductance Ca-activated K+ channels

The kinetic and steady-state properties of macroscopic mslo Ca-activated K+ currents were studied in excised patches from Xenopus oocytes. In response to voltage steps, the timecourse of both activation and deactivation, but for a brief delay in activation, could be approximated by a single exponential function over a wide range of voltages and internal Ca2+ concentrations ([Ca]i). Activation rates increased with voltage and with [Ca]i, and approached saturation at high [Ca]i. Deactivation rates generally decreased with [Ca]i and voltage, and approached saturation at high [Ca]i. Plots of the macroscopic conductance as a function of voltage (G-V) and the time constant of activation and deactivation shifted leftward along the voltage axis with increasing [Ca]i. G-V relations could be approximated by a Boltzmann function with an equivalent gating charge which ranged between 1.1 and 1.8 e as [Ca]i varied between 0.84 and 1,000 microM. Hill analysis indicates that at least three Ca2+ binding sites can contribute to channel activation. Three lines of evidence indicate that there is at least one voltage-dependent unimolecular conformational change associated with mslo gating that is separate from Ca2+ binding. (a) The position of the mslo G-V relation does not vary logarithmically with [Ca]i. (b) The macroscopic rate constant of activation approaches saturation at high [Ca]i but remains voltage dependent. (c) With strong depolarizations mslo currents can be nearly maximally activated without binding Ca2+. These results can be understood in terms of a channel which must undergo a central voltage-dependent rate limiting conformational change in order to move from closed to open, with rapid Ca2+ binding to both open and closed states modulating this central step.

Fast and slow activation kinetics of voltage-gated sodium channels in molluscan neurons

Whole cell patch-clamp recordings of Na current (I(Na)) were made under identical experimental conditions from isolated neurons from cephalopod (Loligo, Octopus) and gastropod (Aplysia, Pleurobranchaea, Doriopsilla) species to compare properties of activation gating. Voltage dependence of peak Na conductance (gNa) is very similar in all cases, but activation kinetics in the gastropod neurons studied are markedly slower. Kinetic differences are very pronounced only over the voltage range spanned by the gNa-voltage relation. At positive and negative extremes of voltage, activation and deactivation kinetics of I(Na) are practically indistinguishable in all species studied. Voltage-dependent rate constants underlying activation of the slow type of Na channel found in gastropods thus appear to be much more voltage dependent than are the equivalent rates in the universally fast type of channel that predominates in cephalopods. Voltage dependence of inactivation kinetics shows a similar pattern and is representative of activation kinetics for the two types of Na channels. Neurons with fast Na channels can thus make much more rapid adjustments in the number of open Na channels at physiologically relevant voltages than would be possible with only slow Na channels. This capability appears to be an adaptation that is highly evolved in cephalopods, which are well known for their high-speed swimming behaviors. Similarities in slow and fast Na channel subtypes in molluscan and mammalian neurons are discussed.

Cloning and characterization of a mouse sensory neuron tetrodotoxin-resistant voltage-gated sodium channel gene, Scn10a

Small-diameter sensory neurons associated with unmyelinated axons express a tetrodotoxin-insensitive (TTXi) voltage-gated sodium channel (VGSC) that may play an important role in the transmission of nociceptive information to the spinal cord. A TTXi VGSC, named SNS, that accounts for the tetrodotoxin-resistant sodium current described in sensory neurons has been cloned from rat dorsal root ganglia. Using recombinant lambda phage clones encoding a mouse 129/SV genomic library, we have determined the detailed structure of the mouse SNS gene (Scn10a), including the location of exon-intron boundaries and the nucleotide sequence of the exons. The gene consists of 27 exons spanning approximately 90 kb on chromosome 9. Mouse SNS shows 95.3% overall amino acid identity to rat SNS and 98.5% identity throughout the putative transmembrane segments and the intracellular loop linking domains 3 and 4. The sizes of the exons and the exon-intron junction positions of the mouse SNS and the human skeletal muscle VGSC genes are remarkably conserved. These results provide the basis for an evolutionary comparison of sodium channels, the construction and analysis of a mouse SNS null mutant as a direct approach to understanding the biological function of SNS, and the identification of regulatory elements that are responsible for the tissue- and cell-specific expression of SNS.

A mutation in segment I-S6 alters slow inactivation of sodium channels

Slow inactivation occurs in voltage-gated Na+ channels when the membrane is depolarized for several seconds, whereas fast inactivation takes place rapidly within a few milliseconds. Unlike fast inactivation, the molecular entity that governs the slow inactivation of Na+ channels has not been as well defined. Some regions of Na+ channels, such as mu1-W402C and mu1-T698M, have been reported to affect slow inactivation. A mutation in segment I-S6 of mu1 Na+ channels, N434A, shifts the voltage dependence of activation and fast inactivation toward the depolarizing direction. The mutant Na+ current at +50 mV is diminished by 60-80% during repetitive stimulation at 5 Hz, resulting in a profound use-dependent phenomenon. This mutant phenotype is due to the enhancement of slow inactivation, which develops faster than that of wild-type channels (tau = 0.46 +/- 0.01 s versus 2.11 +/- 0.10 s at +30 mV, n = 9). An oxidant, chloramine-T, abolishes fast inactivation and yet greatly accelerates slow inactivation in both mutant and wild-type channels (tau = 0.21 +/- 0.02 s and 0.67 +/- 0.05 s, respectively, n = 6). These findings together demonstrate that N434 of mu1 Na+ channels is also critical for slow inactivation. We propose that this slow form of Na+ channel inactivation is analogous to the "C-type" inactivation in Shaker K+ channels.

Electrostatic interactions between transmembrane segments mediate folding of Shaker K+ channel subunits

In voltage-dependent Shaker K+ channels, charged residues E293 in transmembrane segment S2 and R365, R368, and R371 in S4 contribute significantly to the gating charge movement that accompanies activation. Using an intragenic suppression strategy, we have now probed for structural interaction between transmembrane segments S2, S3, and S4 in Shaker channels. Charge reversal mutations of E283 in S2 and K374 in S4 disrupt maturation of the protein. Maturation was specifically and efficiently rescued by second-site charge reversal mutations, indicating that electrostatic interactions exist between E283 in S2 and R368 and R371 in S4, and between K374 in S4 and E293 in S2 and D316 in S3. Rescued subunits were incorporated into functional channels, demonstrating that a native structure was restored. Our data indicate that K374 interacts with E293 and D316 within the same subunit. These electrostatic interactions mediate the proper folding of the protein and are likely to persist in the native structure. Our results raise the possibility that the S4 segment is tilted relative to S2 and S3 in the voltage-sensing domain of Shaker channels. Such an arrangement might provide solvent access to voltage-sensing residues, which we find to be highly tolerant of mutations.

Mapping the receptor site for hanatoxin, a gating modifier of voltage-dependent K+ channels

Hanatoxin (HaTx) binds to multiple sites on the surface of the drk1 voltage-gated K+ channel and modifies channel gating. We set out to identify channel residues that contribute to form these HaTx binding sites. Chimeras constructed using the drk1 and shaker K+ channels suggest that the S3-S4 linker may contain influential residues. Alanine scanning mutagenesis of the region extending from the C terminal end of S3 through S4 identified a number of residues that likely contribute to form the HaTx binding sites. The pore blocker Agitoxin2 and the gating modifier HaTx can simultaneously bind to individual K+ channels. These results suggest that residues near the outer edges of S3 and S4 form the HaTx binding sites and are eccentrically located at least 15 A from the central pore axis on the surface of voltage-gated K+ channels.

Effects of Tityus serrulatus scorpion toxin gamma on voltage-gated Na+ channels

The effects of Brazilian scorpion Tityus serrulatus toxin gamma (TiTx gamma) were studied on voltage-gated Na+ channels from human heart (hHl) and rat skeletal muscle (rSkM1). The Na+ channels were expressed in Xenopus laevis oocytes, and Na+ currents were recorded using two-microelectrode voltage-clamp techniques. In control experiments, the threshold of activation of hH1 is more negative than that of rSkM1 by approximately 20 mV. The toxin induces a shift of the voltage dependence of activation toward more negative potential values and reduces the amplitude of the current when administered to rSkM1. In contrast, TiTx gamma has little discernible effect on the current-voltage curve for hH1 at 100 nmol/L. Chimeric channels formed from these two isoforms were constructed to localize the binding site of TiTx gamma on rSkM1. TiTx gamma shifts the activation of a chimera (SSHH) in which domains 1 (D1) and 2 (D2) derive from rSkM1 and domain 3(D3) and 4 (D4) derive from hH1. This finding suggests that the toxin acts on the activation of rSkM1 by binding either to D1 and/or D2. TiTx gamma shifted the activation of another chimera with D2-D3-D4 from rSkM1 (HSSS) toward more hyperpolarizing potentials and had no effect on the activation of other chimeras with only D1-D3-D4 from rSkM1 (SHSS) or only D3 from rSkM1 (HHSH). Finally, a chimera in which D2 is from rSkM1 and all others domains are from hH1 (HSHH) provides further compelling support for our hypothesis. TiTx gamma shifts the activation of this chimera toward more negative potential values. Thus, TiTx gamma action on chimeras segregates with the source of D2: when D2 is from rSkM1, the toxin affects activation. We infer that D2 plays an important role in the activation process of voltage-gated Na+ channels.

Cloning and tissue distribution of the Aplysia Na+ channel alpha-subunit cDNA

Voltage-gated Na+ channels generate the depolarizing inward current that is critical for the initiation and conduction of action potentials. To study the roles of Na+ channels in neuronal signaling, we have begun the molecular analysis of Na+ channels in Aplysia californica. We have isolated cDNAs that encode a neuronal Na+ channel alpha-subunit, which we have named SCAP1. DNA sequence analysis of the SCAP1 cDNA revealed an open reading frame that predicts a protein of 1,993 amino acids, which is highly similar to other members of the Na+ channel alpha-subunit gene family. RNase protection assays carried out on various Aplysia tissues indicated that SCAP1 is expressed predominantly in the nervous system. All of the nonneuronal tissues tested were negative with the exceptions that low levels of expression were observed in ovotestis and parapodium, probably due to the presence of small numbers of neurons within these tissue preparations. Southern blot hybridization at reduced stringency indicated that the genome of Aplysia contains more than one Na+ channel alpha-subunit gene.

■ REVIEW : Ionic Channels with Weak Voltage-dependence

In light of the recent identification of a class of voltage-dependent Na+ channels with a comparatively weak peak conductance/voltage relationship, this article reviews the structural elements by which proteins are rendered voltage-dependent and the manner in which these proteins are studied. The physicochemical principles used to analyze voltage-dependent ionic channels are reviewed, together with examples drawn from Na+ currents recorded from the somatic membranes of dorsal root ganglion neurons. The structure of the voltage dependent Na + channel and known structural/functional correlates are described. A functional definition of ionic channels with weak voltage-dependence and a discussion of inherent pitfalls of classical analytic methods when applied to the study of such channels are offered. Ultimately, determining how these channels function as they do, i.e., elucidating their gating properties, will be accomplished by cloning and reexpressing the channels in otherwise non-excitable cell systems, and recording their gating currents. Na+ and K+ channels with weak voltage-dependence may serve an important functional role by modulating threshold in excitable cells. NEUROSCIENTIST 3:102-111, 1997

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.

Molecular structures involved in L-type calcium channel inactivation. Role of the carboxyl-terminal region encoded by exons 40-42 in alpha1C subunit in the kinetics and Ca2+ dependence of inactivation

The pore-forming alpha1C subunit is the principal component of the voltage-sensitive L-type Ca2+ channel. It has a long cytoplasmic carboxyl-terminal tail playing a critical role in channel gating. The expression of alpha1C subunits is characterized by alternative splicing, which generates its multiple isoforms. cDNA cloning points to a diversity of human hippocampus alpha1C transcripts in the region of exons 40-43 that encode a part of the 662-amino acid carboxyl terminus. We compared electrophysiological properties of the well defined 2138-amino acid alpha1C,77 channel isoform with two splice variants, alpha1C,72 and alpha1C,86. They contain alterations in the carboxyl terminus due to alternative splicing of exons 40-42. The 2157-amino acid alpha1C,72 isoform contains an insertion of 19 amino acids at position 1575. The 2139-amino acid alpha1C,86 has 80 amino acids replaced in positions 1572-1651 of alpha1C,77 by a non-identical sequence of 81 amino acids. When expressed in Xenopus oocytes, all three splice variants retained high sensitivity toward dihydropyridine blockers but showed large differences in gating properties. Unlike alpha1C,77 and alpha1C,72, Ba2+ currents (IBa) through alpha1C,86 inactivated 8-10 times faster at +20 mV, and its inactivation rate was strongly voltage-dependent. Compared to alpha1C,77, the inactivation curves of IBa through alpha1C,86 and alpha1C,72 channels were shifted toward more negative voltages by 11 and 6 mV, respectively. Unlike alpha1C,77 and alpha1C,72, the alpha1C,86 channel lacks a Ca2+-dependent component of inactivation. Thus the segment 1572-1651 of the cytoplasmic tail of alpha1C is critical for the kinetics as well as for the Ca2+ and voltage dependence of L-type Ca2+ channel gating.

Activation-dependent subconductance levels in the drk1 K channel suggest a subunit basis for ion permeation and gating

Ion permeation and channel opening are two fundamental properties of ion channels, the molecular bases of which are poorly understood. Channels can exist in two permeability states, open and closed. The relative amount of time a channel spends in the open conformation depends on the state of activation. In voltage-gated ion channels, activation involves movement of a charged voltage sensor, which is required for channel opening. Single-channel recordings of drk1 K channels expressed in Xenopus oocytes suggested that intermediate current levels (sublevels) may be associated with transitions between the closed and open states. Because K channels are formed by four identical subunits, each contributing to the lining of the pore, it was hypothesized that these sublevels resulted from heteromeric pore conformations. A formal model based on this hypothesis predicted that sublevels should be more frequently observed in partially activated channels, in which some but not all subunits have undergone voltage-dependent conformational changes required for channel opening. Experiments using the drk1 K channel, as well as drk1 channels with mutations in the pore and in the voltage sensor, showed that the probability of visiting a sublevel correlated with voltage- and time-dependent changes in activation. A subunit basis is proposed for channel opening and permeation in which these processes are coupled.

Role of the S3-S4 linker in Shaker potassium channel activation

Structural models of voltage-gated channels suggest that flexibility of the S3-S4 linker region may be important in allowing the S4 region to undergo large conformational changes in its putative voltage-sensing function. We report here the initial characterization of 18 mutations in the S3-S4 linker of the Shaker channel, including deletions, insertions, charge change, substitution of prolines, and chimeras replacing the 25-residue Shaker linker with 7- or 9-residue sequences from Shab, Shaw, or Shal. As measured in Xenopus oocytes with a two-microelectrode voltage clamp, each mutant construct yielded robust currents. Changes in the voltage dependence of activation were small, with activation voltage shifts of 13 mV or less. Substitution of linkers from the slowly activating Shab and Shaw channels resulted in a three-to fourfold slowing of activation and deactivation. It is concluded that the S3-S4 linker is unlikely to participate in a large conformational change during channel activation. The linker, which in some channel subfamilies has highly conserved sequences, may however be a determinant of activation kinetics in potassium channels, as previously has been suggested in the case of calcium channels.

Identification of functionally distinct isoforms of the N-type Ca2+ channel in rat sympathetic ganglia and brain

The N channel is critical for regulating release of neurotransmitter at many synapses, and even subtle differences in its activity would be expected to influence the efficacy of synaptic transmission. Although several splice variants of the N channel are expressed in the mammalian nervous system, their biological importance is presently unclear. Here, we show that variants of the alpha1B subunit of the N channel are expressed in sympathetic ganglia and that alternative splicing within IIIS3-S4 and IVS3-S4 generate kinetically distinct channels. We further show a striking difference between the expression pattern of the S3-S4 variants in brain and peripheral ganglia and conclude that the brain-dominant form of the N channel gates 2-to-4-fold more rapidly than that predominant in ganglia.

Solvent-dependent rate-limiting steps in the conformational change of sodium channel gating in squid giant axon

1. The time course of sodium currents (INa) in squid giant axon was analysed using viscous non-electrolyte solutions on both sides of the axolemma. It slowed reversibly as the non-electrolyte concentration increased. The activation, deactivation (closing) and inactivation processes were slowed in a similar manner. The gating current of the sodium channel was also slowed to the same extent as the activation time constant. 2. The voltage dependence observed in a time constant vs. voltage relationship and a chord conductance vs. voltage relationship (activation curve), did not change significantly. 3. The gating kinetics have a similar temperature dependence in non-electrolyte solutions, showing that the basic gating mechanism did not change in these solutions and only a slight increase in the activation free energy was one of the main causes of slowing. 4. Eight non-electrolytes, formamide, ethylene glycol, glycerol, erythritol, glucose, sorbitol, sucrose and polyethylene glycol (mean molecular weight 600) were used. The amount of slowing was correlated with the gram concentration (g l-1) of non-electrolytes, but not with molar concentration (M) and solution osmolarity (osmol l-1). 5. The percentage changes of the time constant were expressed as a function of the relative change in solution viscosity, eta/eta0. The proportionality constants alpha in the relationship alpha (eta/eta0), and gamma in the relationship 100 (eta/eta0)gamma, obtained using different non-electrolytes, were close to 100% and 1, respectively. The simplest model to explain the results assumes that a slowing of a global conformational change is a consequence of sequential viscosity-dependent movements of local structures (viscosity model). 6. Values of alpha and gamma deviated frequently from those in an ideal case, i.e. 100% for alpha and 1 for gamma, and they scattered, having a tendency to decrease as a function of molecular weight. 7. The slowing was also expressed as an exponential function of the solution osmolarity. A predicted solute-inaccessible volume Va ranged (in nm3 per molecule) between 0.09 and 1.45. The value of Va increased as a logarithmic function of the molecular weight of the non-electrolyte. 8. This solute-inaccessible volume should be distributed in all hydrophilic parts of the sodium channel protein, but is not located in the channel conducting pore itself. The slowing of gating could be explained by a model in which a rate-limiting step is a hydration process that occurs after local small structural changes have exposed new, unhydrated faces (transient hydrated-states model). 9. Considering the opposite dependencies of parameters alpha (or gamma) and beta on the molecular weight, sodium channel gating is likely to reflect a combination of these two models, which are coupled in microscopic segment movements. We emphasize with this combination of models that fluctuating hydrophilic structures play an important role in determining time constants in the gating process.

Cloned potassium channels from eukaryotes and prokaryotes

Potassium channels contribute to the excitability of neurons and signaling in the nervous system. They arise from multiple gene families including one for voltage-gated potassium channels and one for inwardly rectifying potassium channels. Features of potassium permeation, channel gating and regulation, and subunit interaction have been analyzed. Potassium channels of similar design have been found in animals ranging from jellyfish to humans, as well as in plants, yeast, and bacteria. Structural similarities are evident for the pore-forming alpha subunits and for the beta subunits, which could potentially regulate channel activity according to the level of energy and/or reducing power of the cell.

Determinant for beta-subunit regulation in high-conductance voltage-activated and Ca(2+)-sensitive K+ channels: an additional transmembrane region at the N terminus

The pore-forming alpha subunit of large conductance voltage- and Ca(2+)-sensitive K (MaxiK) channels is regulated by a beta subunit that has two membrane-spanning regions separated by an extracellular loop. To investigate the structural determinants in the pore-forming alpha subunit necessary for beta-subunit modulation, we made chimeric constructs between a human MaxiK channel and the Drosophila homologue, which we show is insensitive to beta-subunit modulation, and analyzed the topology of the alpha subunit. A comparison of multiple sequence alignments with hydrophobicity plots revealed that MaxiK channel alpha subunits have a unique hydrophobic segment (S0) at the N terminus. This segment is in addition to the six putative transmembrane segments (S1-S6) usually found in voltage-dependent ion channels. The transmembrane nature of this unique S0 region was demonstrated by in vitro translation experiments. Moreover, normal functional expression of signal sequence fusions and in vitro N-linked glycosylation experiments indicate that S0 leads to an exoplasmic N terminus. Therefore, we propose a new model where MaxiK channels have a seventh transmembrane segment at the N terminus (S0). Chimeric exchange of 41 N-terminal amino acids, including S0, from the human MaxiK channel to the Drosophila homologue transfers beta-subunit regulation to the otherwise unresponsive Drosophila channel. Both the unique S0 region and the exoplasmic N terminus are necessary for this gain of function.

Ion channels: structural basis for function and disease

Ion channels are ubiquitous proteins that mediate nervous and muscular function, rapid transmembrane signaling events, and ionic and fluid balance. The cloning of genes encoding ion channels has led to major strides in understanding the mechanistic basis for their function. These advances have shed light on the role of ion channels in normal physiology, clarified the molecular basis for an expanding number of diseases, and offered new direction to the development of rational therapeutic interventions.

Interaction between fast and slow inactivation in Skm1 sodium channels

Rat skeletal muscle (Skm1) sodium channel alpha and beta 1 subunits were coexpressed in Xenopus oocytes, and resulting sodium currents were recorded from on-cell macropatches. First, the kinetics and steady-state probability of both fast and slow inactivation in Skm1 wild type (WT) sodium channels were characterized. Next, we confirmed that mutation of IFM to QQQ (IFM1303QQQ) in the DIII-IV 'inactivation loop' completely removed fast inactivation at all voltages. This mutation was then used to characterize Skm1 slow inactivation without the presence of fast inactivation. The major findings of this paper are as follows: 1) Even with complete removal of fast inactivation by the IFM1303QQQ mutation, slow inactivation remains intact. 2) In WT channels, approximately 20% of channels fail to slow-inactivate after fast-inactivating, even at very positive potentials. 3) Selective removal of fast inactivation by IFM1303QQQ allows slow inactivation to occur more quickly and completely than in WT. We conclude that fast inactivation reduces the probability of subsequent slow inactivation.

Characterization of human cardiac Na+ channel mutations in the congenital long QT syndrome

The congenital long QT syndrome (LQTS) is an inherited disorder characterized by a prolonged cardiac action potential. This delay in cellular repolarization can lead to potentially fatal arrhythmias. One form of LQTS (LQT3) has been linked to the human cardiac voltage-gated sodium channel gene (SCN5A). Three distinct mutations have been identified in the sodium channel gene. The biophysical and functional characteristics of each of these mutant channels were determined by heterologous expression of a recombinant human heart sodium channel in a mammalian cell line. Each mutation caused a sustained, non-inactivating sodium current amounting to a few percent of the peak inward sodium current, observable during long (> 50 msec) depolarizations. The voltage dependence and rate of inactivation were altered, and the rate of recovery from inactivation was changed compared with wild-type channels. These mutations in diverse regions of the ion channel protein, all produced a common defect in channel gating that can cause the long QT phenotype. The sustained inward current caused by these mutations will prolong the action potential. Furthermore, they may create conditions that promote arrhythmias due to prolonged depolarization and the altered recovery from inactivation. These results provide insights for successful intervention in the disease.

Faster voltage-dependent activation of Na+ channels in growth cones versus somata of neuroblastoma N1E-115 cells

Kinetics of voltage-gated ionic channels fundamentally reflect the response of the channels to local electric fields. In this report cell-attached patch-clamp studies reveal that the voltage-dependent activation rate of sodium channels residing in the growth cone membrane differs from that of soma sodium channels in differentiating N1E-115 neuroblastoma cells. Because other electrophysiological properties of these channels do not differ, this finding may be a reflection of the difference in intramembrane electric field in these two regions of the cell. This represents a new mechanism for channels to attain a range of activities both within and between cells.

Nonequilibrium gating and voltage dependence of the ClC-0 Cl- channel

The gating of ClC-0, the voltage-dependent Cl- channel from Torpedo electric organ, is strongly influenced by Cl- ions in the external solution. Raising external Cl- over the range 1-600 mM favors the fast-gating open state and disfavors the slow-gating inactivated state. Analysis of purified single ClC-0 channels reconstituted into planar lipid bilayers was used to identify the role of Cl- ions in the channel's fast voltage-dependent gating process. External, but not internal, Cl- had a major effect on the channel's opening rate constant. The closing rate was more sensitive to internal Cl- than to external Cl-. Both opening and closing rates varied with voltage. A model was derived that postulates (a) that in the channel's closed state, Cl- is accessible to a site located at the outer end of the conduction pore, where it binds in a voltage-independent fashion, (b) that this closed conformation can open, whether liganded by Cl- or not, in a weakly voltage-dependent fashion, (c) that the Cl(-)-liganded closed channel undergoes a conformational change to a different closed state, such that concomitant with this change, Cl- ion moves inward, conferring voltage-dependence to this step, and (d) that this new Cl(-)-liganded closed state opens with a very high rate. According to this picture, Cl- movement within the pre-open channel is the major source of voltage dependence, and charge movement intrinsic to the channel protein contributes very little to voltage-dependent gating of ClC-0. Moreover, since the Cl- activation site is probably located in the ion conduction pathway, the fast gating of ClC-0 is necessarily coupled to ion conduction, a nonequilibrium process.