KCNQ1 Channels Do Not Undergo Concerted but Sequential Gating Transitions in Both the Absence and the Presence of KCNE1 Protein

The co-assembly of KCNQ1 with KCNE1 produces I_KS, a K⁺ current, crucial for the repolarization of the cardiac action potential. Mutations in these channel subunits lead to life-threatening cardiac arrhythmias. However, very little is known about the gating mechanisms underlying KCNQ1 channel activation. Shaker channels have provided a powerful tool to establish the basic gating mechanisms of voltage-dependent K⁺ channels, implying prior independent movement of all four voltage sensor domains (VSDs) followed by channel opening via a last concerted cooperative transition. To determine the nature of KCNQ1 channel gating, we performed a thermodynamic mutant cycle analysis by constructing a concatenated tetrameric KCNQ1 channel and by introducing separately a gain and a loss of function mutation, R231W and R243W, respectively, into the S4 helix of the VSD of one, two, three, and four subunits. The R231W mutation destabilizes channel closure and produces constitutively open channels, whereas the R243W mutation disrupts channel opening solely in the presence of KCNE1 by right-shifting the voltage dependence of activation. The linearity of the relationship between the shift in the voltage dependence of activation and the number of mutated subunits points to an independence of VSD movements, with each subunit incrementally contributing to channel gating. Contrary to Shaker channels, our work indicates that KCNQ1 channels do not experience a late cooperative concerted opening transition. Our data suggest that KCNQ1 channels in both the absence and the presence of KCNE1 undergo sequential gating transitions leading to channel opening even before all VSDs have moved.     

Episodic ataxia type-1 mutations in the hKv1.1 cytoplasmic pore region alter the gating properties of the channel.

Episodic ataxia type-1 is a rare human neurological syndrome which occurs during childhood and persists through the whole life of affected patients. Several heterozygous point mutations have been found in the coding sequence of the voltage-gated potassium channel gene hKv1.1 of different affected families. V408A and E325D mutations are located in the cytoplasmic putative pore region of hKv1.1 channels and profoundly alter their gating properties. V408A channels showed increased kinetic rates of activation, deactivation and C-type inactivation. Expression of E325D channels in Xenopus oocytes led to an approximately 13-fold current amplitude reduction and to a 52.4 mV positive shift in the voltage dependence of activation. Moreover, the E325D mutation altered the kinetics of activation, deactivation, C-type inactivation and channel open probability. Heteromeric channels composed of two wild-type and two mutated subunits, linked as dimers, showed gating properties intermediate between channels formed from four normal or four mutated subunits. The results demonstrate that the highly conserved residues Val408 and Glu325 play a pivotal role in several gating processes of a human potassium channel, and suggest a pathogenetic mechanism by which the impairment of the delayed-rectifier function of affected neurons is related to the type and number of mutated subunits which make up the hKv1.1 channels.     

Allosteric linkage between voltage and Ca(2+)-dependent activation of BK-type mslo1 K(+) channels

The activation of BK type Ca(2+)-activated K(+) channels depends on both voltage and Ca(2+). We studied three point mutations in the putative voltage sensor S4 or S4-S5 linker regions in the mslo1 BK channels to explore the relationship between voltage and Ca(2+) in activating the channel. These mutations reduced the steepness of the open probability - voltage (P(o) - V) relation and increased the shift of the P(o) - V relations on the voltage axis in response to increases in the calcium concentration. It is striking that these two effects were reciprocally related for all three mutations, despite different effects of the mutations on other aspects of the voltage dependence of channel gating. This reciprocal relationship suggests strongly that the free energy contributions to channel activation provided by voltage and by calcium binding are simply additive. We conclude that the Ca(2+) binding sites and the voltage sensors do not directly interact. Rather they both affect the mslo1 channel opening through an allosteric mechanism, by influencing the conformational change between the closed and open conformations. The mutations changed the channel's voltage dependence with little effect on its Ca(2+) affinitiy.     

Molecular analysis of PIP2 regulation of HERG and IKr

We previously reported that cloned human ether-a-go-go-related gene (HERG) K+ channels are regulated by changes in phosphatidylinositol 4,5-bisphosphate (PIP2) concentration. Here we investigated the molecular determinants of PIP2 interactions with HERG channel protein. To establish the molecular nature of the PIP2-HERG interaction, we examined a segment of the HERG COOH terminus with a high concentration of positively charged amino acids (nos. 883-894) as a possible site of interaction with negatively charged PIP2. When we excised deletion-HERG (D-HERG) or mutated methionine-substituted-HERG (M-HERG) this segment of HERG to neutralize the amino acid charge, the mutant channels produced current that was indistinguishable from wild-type HERG. Elevating internal PIP2, however, no longer accelerated the activation kinetics of the mutant HERG. Moreover, PIP2-dependent hyperpolarizing shifts in the voltage dependence of activation were abolished with both mutants. PIP2 effects on channel-inactivation kinetics remained intact, which suggests an uncoupling of inactivation and activation regulation by PIP2. The specific binding of radiolabeled PIP2 to both mutant channel proteins was nearly abolished. Stimulation of alpha1A-adrenergic receptors produced a reduction in current amplitude of the rapidly activating delayed rectifier K+ current (the current carried by ERG protein) from rabbit ventricular myocytes. The alpha-adrenergic-induced current reduction was accentuated by PKC blockers and also unmasked a depolarizing shift in the voltage dependence of activation, which supports the conclusion that receptor activation of PLC results in PIP2 consumption that alters channel activity. These results support a physiological role for PIP2 regulation of the rapidly activating delayed rectifier K+ current during autonomic stimulation and localize a site of interaction to the COOH-terminal tail of the HERG K+ channel.     

MinK, MiRP1, and MiRP2 diversify Kv3.1 and Kv3.2 potassium channel gating

High frequency firing in mammalian neurons requires ultra-rapid delayed rectifier potassium currents generated by homomeric or heteromeric assemblies of Kv3.1 and Kv3.2 potassium channel alpha subunits. Kv3.1 alpha subunits can also form slower activating channels by coassembling with MinK-related peptide 2 (MiRP2), a single transmembrane domain potassium channel ancillary subunit. Here, using channel subunits cloned from rat and expressed in Chinese hamster ovary cells, we show that modulation by MinK, MiRP1, and MiRP2 is a general mechanism for slowing of Kv3.1 and Kv3.2 channel activation and deactivation and acceleration of inactivation, creating a functionally diverse range of channel complexes. MiRP1 also negatively shifts the voltage dependence of Kv3.1 and Kv3.2 channel activation. Furthermore, MinK, MiRP1, and MiRP2 each form channels with Kv3.1-Kv3.2 heteromers that are kinetically distinct from one another and from MiRP/homomeric Kv3 channels. The findings illustrate a mechanism for dynamic expansion of the functional repertoire of Kv3.1 and Kv3.2 potassium currents and suggest roles for these alpha subunits outside the scope of sustained rapid neuronal firing.     

Mg(2+) modulates voltage-dependent activation in ether-à-go-go potassium channels by binding between transmembrane segments S2 and S3

Extracellular Mg(2+) directly modulates voltage-dependent activation in ether-à-go-go (eag) potassium channels, slowing the kinetics of ionic and gating currents (Tang, C.-Y., F. Bezanilla, and D.M. Papazian. 2000. J. Gen. Physiol. 115:319-337). To exert its effect, Mg(2+) presumably binds to a site in or near the eag voltage sensor. We have tested the hypothesis that acidic residues unique to eag family members, located in transmembrane segments S2 and S3, contribute to the Mg(2+)-binding site. Two eag-specific acidic residues and three acidic residues found in the S2 and S3 segments of all voltage-dependent K(+) channels were individually mutated in Drosophila eag, mutant channels were expressed in Xenopus oocytes, and the effect of Mg(2+) on ionic current kinetics was measured using a two electrode voltage clamp. Neutralization of eag-specific residues D278 in S2 and D327 in S3 eliminated Mg(2+)-sensitivity and mimicked the slowing of activation kinetics caused by Mg(2+) binding to the wild-type channel. These results suggest that Mg(2+) modulates activation kinetics in wild-type eag by screening the negatively charged side chains of D278 and D327. Therefore, these residues are likely to coordinate the bound ion. In contrast, neutralization of the widely conserved residues D284 in S2 and D319 in S3 preserved the fast kinetics seen in wild-type eag in the absence of Mg(2+), indicating that D284 and D319 do not mediate the slowing of activation caused by Mg(2+) binding. Mutations at D284 affected the eag gating pathway, shifting the voltage dependence of Mg(2+)-sensitive, rate limiting transitions in the hyperpolarized direction. Another widely conserved residue, D274 in S2, is not required for Mg(2+) sensitivity but is in the vicinity of the binding site. We conclude that Mg(2+) binds in a water-filled pocket between S2 and S3 and thereby modulates voltage-dependent gating. The identification of this site constrains the packing of transmembrane segments in the voltage sensor of K(+) channels, and suggests a molecular mechanism by which extracellular cations modulate eag activation kinetics.     

Role of the S2 and S3 Segment in Determining the Activation Kinetics in Kv2.1 Channels

We constructed chimeras between the rapidly activating Kv1.2 channel and the slowly activating Kv2.1 channel in order to study to what extent sequence differences within the S1–S4 region contribute to the difference in activation kinetics. The channels were expressed in Xenopus oocytes and the currents were measured with a two-microelectrode voltage-clamp technique. Substitution of the S1–S4 region of Kv2.1 subunits by the ones of Kv1.2 resulted in chimeric channels which activated more rapidly than Kv2.1. Furthermore, activation kinetics were nearly voltage-independent in contrast to the pronounced voltage-dependent activation kinetics of both parent channels. Systematic screening of the S1–S4 region by the replacement of smaller protein parts resolved that the main functional changes generated by the S1–S4 substitution were generated by the S2 and the S3 segment. However, the effects of these segments were different: The S3 substitution reduced the effective gating charge and accelerated both a voltage-dependent and a voltage-independent component of the activation time course. In contrast, the S2 substitution accelerated predominantly the voltage-dependent component of the activation time course thereby leaving the effective gating charge unchanged. It is concluded that the S2 and the S3 segment determine the activation kinetics in a specific manner. Received: 13 November 2000/Revised: 5 April 2001     

Binding site in eag voltage sensor accommodates a variety of ions and is accessible in closed channel

In ether-a-go-go K+ channels, voltage-dependent activation is modulated by ion binding to a site located in an extracellular-facing crevice between transmembrane segments S2 and S3 in the voltage sensor. We find that acidic residues D278 in S2 and D327 in S3 are able to coordinate a variety of divalent cations, including Mg2+, Mn2+, and Ni2+, which have qualitatively similar functional effects, but different half-maximal effective concentrations. Our data indicate that ions binding to individual voltage sensors in the tetrameric channel act without cooperativity to modulate activation gating. We have taken advantage of the unique phenotype of Ni2+ in the D274A channel, which contains a mutation of a nonbinding site residue, to demonstrate that ions can access the binding site from the extracellular solution when the voltage sensor is in the resting conformation. Our results are difficult to reconcile with the x-ray structure of the KvAP K+ channel, in which the binding site residues are widely separated, and with the hydrophobic paddle model for voltage-dependent activation, in which the voltage sensor domain, including the S3-S4 loop, is near the cytoplasmic side of the membrane in the closed channel.     

Properties of KvLQT1 K+ channel mutations in Romano-Ward and Jervell and Lange-Nielsen inherited cardiac arrhythmias.

Mutations in the delayed rectifier K+ channel subunit KvLQT1 have been identified as responsible for both Romano-Ward (RW) and Jervell and Lange-Nielsen (JLN) inherited long QT syndromes. We report the molecular cloning of a human KvLQT1 isoform that is expressed in several human tissues including heart. Expression studies revealed that the association of KvLQT1 with another subunit, IsK, reconstitutes a channel responsible for the IKs current involved in ventricular myocyte repolarization. Six RW and two JLN mutated KvLQT1 subunits were produced and co-expressed with IsK in COS cells. All the mutants, except R555C, fail to produce functional homomeric channels and reduce the K+ current when co-expressed with the wild-type subunit. Thus, in both syndromes, the main effect of the mutations is a dominant-negative suppression of KvLQT1 function. The JLN mutations have a smaller dominant-negative effect, in agreement with the fact that the disease is recessive. The R555C subunit forms a functional channel when expressed with IsK, but with altered gating properties. The voltage dependence of the activation is strongly shifted to more positive values, and deactivation kinetics are accelerated. This finding indicates the functional importance of a small positively charged cytoplasmic region of the KvLQT structure where two RW and one JLN mutations have been found to take place.     

Single-channel properties of ionic channels gated by cyclic nucleotides.

This paper presents an extensive analysis of single-channel properties of cyclic nucleotide gated (CNG) channels, obtained by injecting into Xenopus laevis oocytes the mRNA encoding for the alpha and beta subunits from bovine rods. When the alpha and beta subunits of the CNG channel are coexpressed, at least three types of channels with different properties are observed. One type of channel has well-resolved, multiple conductive levels at negative voltages, but not at positive voltages. The other two types of channel are characterized by flickering openings, but are distinguished because they have a low and a high conductance. The alpha subunit of CNG channels has a well-defined conductance of about 28 pS, but multiple conductive levels are observed in mutant channels E363D and T364M. The conductance of these open states is modulated by protons and the membrane voltage, and has an activation energy around 44 kJ/mol. The relative probability of occupying any of these open states is independent of the cGMP concentration, but depends on extracellular protons. The open probability in the presence of saturating cGMP was 0.78, 0.47, 0.5, and 0.007 in the w.t. and mutants E363D, T364M, and E363G, and its dependence on temperature indicates that the thermodynamics of the transition between the closed and open state is also affected by mutations in the pore region. These results suggest that CNG channels have different conductive levels, leading to the existence of multiple open states in homomeric channels and to the flickering behavior in heteromeric channels, and that the pore is an essential part of the gating of CNG channels.     

The role of S4 charges in voltage-dependent and voltage-independent KCNQ1 potassium channel complexes

Voltage-gated potassium (Kv) channels extend their functional repertoire by coassembling with MinK-related peptides (MiRPs). MinK slows the activation of channels formed with KCNQ1 alpha subunits to generate the voltage-dependent I(Ks) channel in human heart; MiRP1 and MiRP2 remove the voltage dependence of KCNQ1 to generate potassium "leak" currents in gastrointestinal epithelia. Other Kv alpha subunits interact with MiRP1 and MiRP2 but without loss of voltage dependence; the mechanism for this disparity is unknown. Here, sequence alignments revealed that the voltage-sensing S4 domain of KCNQ1 bears lower net charge (+3) than that of any other eukaryotic voltage-gated ion channel. We therefore examined the role of KCNQ1 S4 charges in channel activation using alanine-scanning mutagenesis and two-electrode voltage clamp. Alanine replacement of R231, at the N-terminal side of S4, produced constitutive activation in homomeric KCNQ1 channels, a phenomenon not observed with previous single amino acid substitutions in S4 of other channels. Homomeric KCNQ4 channels were also made constitutively active by mutagenesis to mimic the S4 charge balance of R231A-KCNQ1. Loss of single S4 charges at positions R231 or R237 produced constitutively active MinK-KCNQ1 channels and increased the constitutively active component of MiRP2-KCNQ1 currents. Charge addition to the CO2H-terminal half of S4 eliminated constitutive activation in MiRP2-KCNQ1 channels, whereas removal of homologous charges from KCNQ4 S4 produced constitutively active MiRP2-KCNQ4 channels. The results demonstrate that the unique S4 charge paucity of KCNQ1 facilitates its unique conversion to a leak channel by ancillary subunits such as MiRP2.     

Long QT syndrome-associated mutations in the S4-S5 linker of KvLQT1 potassium channels modify gating and interaction with minK subunits.

Long QT syndrome is an inherited disorder of cardiac repolarization caused by mutations in cardiac ion channel genes, including KVLQT1. In this study, the functional consequences of three long QT-associated missense mutations in KvLQT1 (R243C, W248R, E261K) were characterized using the Xenopus oocyte heterologous expression system and two-microelectrode voltage clamp techniques. These mutations are located in or near the intracellular linker between the S4 and S5 transmembrane domains, a region implicated in activation gating of potassium channels. The E261K mutation caused loss of function and did not interact with wild-type KvLQT1 subunits. R243C or W248R KvLQT1 subunits formed functional channels, but compared with wild-type KvLQT1 current, the rate of activation was slower, and the voltage dependence of activation and inactivation was shifted to more positive potentials. Co expression of minK and KvLQT1 channel subunits induces a slow delayed rectifier K(+) current, I(Ks), characterized by slow activation and a markedly increased magnitude compared with current induced by KvLQT1 subunits alone. Coexpression of minK with R243C or W248R KvLQT1 subunits suppressed current, suggesting that coassembly of mutant subunits with minK prevented normal channel gating. The decrease in I(Ks) caused by loss of function or altered gating properties explains the prolonged QT interval and increased risk of arrhythmia and sudden death associated with these mutations in KVLQT1.     

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 changes, 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.     

Independent and joint modulation of rat Nav1.6 voltage-gated sodium channels by coexpression with the auxiliary β1 and β2 subunits

The Na_v1.6 voltage-gated sodium channel α subunit isoform is the most abundant isoform in the brain and is implicated in the transmission of high frequency action potentials. Purification and immunocytochemical studies imply that Na_v1.6 exist predominantly as Na_v1.6 + β1 + β2 heterotrimeric complexes. We assessed the independent and joint effects of the rat β1 and β2 subunits on the gating and kinetic properties of rat Na_v1.6 channels by recording whole-cell currents in the two-electrode voltage clamp configuration following transient expression in Xenopus oocytes. The β1 subunit accelerated fast inactivation of sodium currents but had no effect on the voltage dependence of their activation and steady-state inactivation and also prevented the decline of currents following trains of high-frequency depolarizing prepulses. The β2 subunit selectively retarded the fast phase of fast inactivation and shifted the voltage dependence of activation towards depolarization without affecting other gating properties and had no effect on the decline of currents following repeated depolarization. The β1 and β2 subunits expressed together accelerated both kinetic phases of fast inactivation, shifted the voltage dependence of activation towards hyperpolarization, and gave currents with a persistent component typical of those recorded from neurons expressing Na_v1.6 sodium channels. These results identify unique effects of the β1 and β2 subunits and demonstrate that joint modulation by both auxiliary subunits gives channel properties that are not predicted by the effects of individual subunits.     

An extracellular domain of the accessory β1 subunit is required for modulating BK channel voltage sensor and gate

A family of tissue-specific auxiliary β subunits modulates large conductance voltage- and calcium-activated potassium (BK) channel gating properties to suit their diverse functions. Paradoxically, β subunits both promote BK channel activation through a stabilization of voltage sensor activation and reduce BK channel openings through an increased energetic barrier of the closed-to-open transition. The molecular determinants underlying β subunit function, including the dual gating effects, remain unknown. In this study, we report the first identification of a β1 functional domain consisting of Y74, S104, Y105, and I106 residues located in the extracellular loop of β1. These amino acids reside within two regions of highest conservation among related β1, β2, and β4 subunits. Analysis in the context of the Horrigan-Aldrich gating model revealed that this domain functions to both promote voltage sensor activation and also reduce intrinsic gating. Free energy calculations suggest that the dual effects of the β1 Y74 and S104-I106 domains can be largely accounted for by a relative destabilization of channels in open states that have few voltage sensors activated. These results suggest a unique and novel mechanism for β subunit modulation of voltage-gated potassium channels wherein interactions between extracellular β subunit residues with the external portions of the gate and voltage sensor regulate channel opening.     

Role of charged residues in the S1-S4 voltage sensor of BK channels

The activation of large conductance Ca(2+)-activated (BK) potassium channels is weakly voltage dependent compared to Shaker and other voltage-gated K(+) (K(V)) channels. Yet BK and K(V) channels share many conserved charged residues in transmembrane segments S1-S4. We mutated these residues individually in mSlo1 BK channels to determine their role in voltage gating, and characterized the voltage dependence of steady-state activation (P(o)) and I(K) kinetics (tau(I(K))) over an extended voltage range in 0-50 microM [Ca(2+)](i). mSlo1 contains several positively charged arginines in S4, but only one (R213) together with residues in S2 (D153, R167) and S3 (D186) are potentially voltage sensing based on the ability of charge-altering mutations to reduce the maximal voltage dependence of P(O). The voltage dependence of P(O) and tau(I(K)) at extreme negative potentials was also reduced, implying that the closed-open conformational change and voltage sensor activation share a common source of gating charge. Although the position of charged residues in the BK and K(V) channel sequence appears conserved, the distribution of voltage-sensing residues is not. Thus the weak voltage dependence of BK channel activation does not merely reflect a lack of charge but likely differences with respect to K(V) channels in the position and movement of charged residues within the electric field. Although mutation of most sites in S1-S4 did not reduce gating charge, they often altered the equilibrium constant for voltage sensor activation. In particular, neutralization of R207 or R210 in S4 stabilizes the activated state by 3-7 kcal mol(-1), indicating a strong contribution of non-voltage-sensing residues to channel function, consistent with their participation in state-dependent salt bridge interactions. Mutations in S4 and S3 (R210E, D186A, and E180A) also unexpectedly weakened the allosteric coupling of voltage sensor activation to channel opening. The implications of our findings for BK channel voltage gating and general mechanisms of voltage sensor activation are discussed.     

Biophysical properties of zebrafish ether-à-go-go related gene potassium channels

The zebrafish is increasingly recognized as an animal model for the analysis of hERG-related diseases. However, functional properties of the zebrafish orthologue of hERG have not been analyzed yet. We heterologously expressed cloned ERG channels in Xenopus oocytes and analyzed biophysical properties using the voltage clamp technique. zERG channels conduct rapidly activating and inactivating potassium currents. However, compared to hERG, the half-maximal activation voltage of zERG current is shifted towards more positive potentials and the half maximal steady-state inactivation voltage is shifted towards more negative potentials. zERG channel activation is delayed and channel deactivation is accelerated significantly. However, time course of zERG conducted current under action potential clamp is highly similar to the human orthologue. In summary, we show that ERG channels in zebrafish exhibit biophysical properties similar to the human orthologue. Considering the conserved channel function, the zebrafish represents a valuable model to investigate human ERG channel related diseases.     

High-affinity Zn block in recombinant N-methyl-D-aspartate receptors with cysteine substitutions at the Q/R/N site.

In ionotropic glutamate receptors, many channel properties (e.g., selectivity, ion permeation, and ion block) depend on the residue (glutamine, arginine, or asparagine) located at the tip of the pore loop (the Q/R/N site). We substituted a cysteine for the asparagine present at that position in both NR1 and NR2 N-methyl-D-aspartate (NMDA) receptor subunits. Under control conditions, receptors containing mutated NR1 and NR2 subunits show much smaller glutamate responses than wild-type receptors. However, this difference disappears upon addition of heavy metal chelators in the extracellular bath. The presence of cysteines at the Q/R/N site in both subunits of NR1/NR2C receptors results in a 220,000-fold increase in sensitivity of the inhibition by extracellular Zn. In contrast with the high-affinity Zn inhibition of wild-type NR1/NR2A receptors, the high-affinity Zn inhibition of mutated NR1/NR2C receptors shows a voltage dependence, which resembles very much that of the block by extracellular Mg. This indicates that the Zn inhibition of the mutated receptors results from a channel block involving Zn binding to the thiol groups introduced into the selectivity filter. Taking advantage of the slow kinetics of the Zn block, we show that both blocking and unblocking reactions require prior opening of the channel.     

Extracellular Mg(2+) modulates slow gating transitions and the opening of Drosophila ether-à-Go-Go potassium channels

We have characterized the effects of prepulse hyperpolarization and extracellular Mg(2+) on the ionic and gating currents of the Drosophila ether-à-go-go K(+) channel (eag). Hyperpolarizing prepulses significantly slowed channel opening elicited by a subsequent depolarization, revealing rate-limiting transitions for activation of the ionic currents. Extracellular Mg(2+) dramatically slowed activation of eag ionic currents evoked with or without prepulse hyperpolarization and regulated the kinetics of channel opening from a nearby closed state(s). These results suggest that Mg(2+) modulates voltage-dependent gating and pore opening in eag channels. To investigate the mechanism of this modulation, eag gating currents were recorded using the cut-open oocyte voltage clamp. Prepulse hyperpolarization and extracellular Mg(2+) slowed the time course of ON gating currents. These kinetic changes resembled the results at the ionic current level, but were much smaller in magnitude, suggesting that prepulse hyperpolarization and Mg(2+) modulate gating transitions that occur slowly and/or move relatively little gating charge. To determine whether quantitatively different effects on ionic and gating currents could be obtained from a sequential activation pathway, computer simulations were performed. Simulations using a sequential model for activation reproduced the key features of eag ionic and gating currents and their modulation by prepulse hyperpolarization and extracellular Mg(2+). We have also identified mutations in the S3-S4 loop that modify or eliminate the regulation of eag gating by prepulse hyperpolarization and Mg(2+), indicating an important role for this region in the voltage-dependent activation of eag.     

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.