Widening the scope of constriction

JGP modeling study suggests that selectivity filter constriction is a plausible mechanism for C-type inactivation of the Shaker voltage-gated potassium channel.

In response to prolonged activation, many K⁺ channels spontaneously reduce the membrane conductance by undergoing C-type inactivation, a kinetic process crucial for the pacing of cardiac action potentials and the modulation of neuronal firing patterns. In the pH-activated bacterial channel KcsA, C-type inactivation appears to involve constriction of the channel’s selectivity filer that prohibits ion conduction, but whether voltage-gated channels like Drosophila Shaker use a similar mechanism is controversial (1). In this issue of JGP, a computational study by Li et al. suggests that filter constriction is indeed a plausible mechanism for the C-type inactivation of Shaker (2).(Left to right) Jing Li, Benoît Roux, and colleagues use computational modeling to show that selectivity filter constriction, allosterically promoted by opening of the intracellular activation gate, is a plausible mechanism for the C-type inactivation of voltage-gated K⁺ channels such as Drosophila Shaker. The selectivity filter is conductive (left) when the intracellular gate is partially open, but adopts a constricted conformation (right) when the gate is open wide.Various structural approaches have shown that C-type inactivation of KcsA channels is associated with the symmetrical constriction of all four channel subunits at the level of the central glycine residue in the selectivity filter. Benoît Roux and colleagues at The University of Chicago used MD simulations to show that the KcsA pore can transition from the conductive to the constricted conformation on an appropriate timescale, and that this transition is allosterically promoted by the wide opening of the pore’s intracellular gate (3). Modeling by Roux and colleagues suggests that C-type inactivation of cardiac hERG channels could also involve selectivity filter constriction, though in this case it appears to be an asymmetric process in which only two of the channel’s subunits move closer together (4).“In view of the high similarity between the pore domains of Shaker and KcsA (almost 40% sequence identity), we wanted to examine if it’s possible for the Shaker selectivity filter to constrict and, if so, how similar it is to KcsA,” Roux explains. Led by first author Jing Li—now an assistant professor at the University of Mississippi—Roux and colleagues developed several homology models of the Shaker pore domain with the intracellular gate open to various degrees (2).MD simulations and free energy calculations revealed that the Shaker selectivity filter can dynamically transition from a conductive to a constricted conformation, and that this transition is allosterically coupled to the intracellular gate; the constricted conformation is stable when the gate is wide open. “Our computations strongly suggest that constriction is a plausible mechanism for the C-type inactivation of Shaker,” Roux says. “There’s no reason based on the currently available information to reject the existence of a constricted state in Shaker channels.”As with KcsA, Shaker channels appear to constrict symmetrically at the level of the selectivity filter’s central glycine. But Li et al.’s simulations revealed some small variations between the two channels, including differences in the number of water molecules bound to each channel subunit and the arrangement of the hydrogen-bond network they form to stabilize the constricted state.Li et al. also modeled the pore domain of the Shaker W434F mutant, which is widely assumed to be trapped in a C-type inactivated state. The simulation suggests that the mutant channel’s filter adopts a stable constricted conformation even when the intracellular gate is only partially open, although the constriction is asymmetric and occurs at the level of a different filter residue (2).Constriction may therefore be a universal mechanism of C-type inactivation, even if the exact conformation varies from channel to channel. But, says Roux, confirming this will require more experimental work using the right conditions and mutations to capture the structure of inactivated channels.     

The Binding of κ-Conotoxin PVIIA and Fast C-Type Inactivation of Shaker K+ Channels are Mutually Exclusive

κ-Conotoxin PVIIA (κ-PVIIA), a 27-amino acid peptide identified from the venom of Conus purpurascens, inhibits the Shaker K⁺ channel by blocking its outer pore. The toxin appears as a gating modifier because its binding affinity decreases with relatively fast kinetics upon channel opening, but there is no indication that it interferes with the gating transitions of the wild-type channels (WT), including the structural changes of the outer pore that underlie its slow C-type inactivation. In this report we demonstrate that in two outer pore mutants of Shaker-IR (M448K and T449S), that have high toxin sensitivity and fast C-type inactivation, the latter process is instead antagonized by and incompatible with κ-PVIIA binding. Inactivation is slowed by the necessary preliminary unbinding of κ-PVIIA, whereas toxin rebinding must await recovery from inactivation causing a double-exponential relaxation of the second response to double-pulse stimulations. Compared with the lack of similar effects in WT, these results demonstrate the ability of peptide toxins like κ-PVIIA to reveal possibly subtle differences in structural changes of the outer pore of K⁺ channels; however, they also warn against a naive use of fast inactivating mutants as models for C-type inactivation. Unfolded from the antagonistic effect of inactivation, toxin binding to mutant noninactivated channels shows state- and voltage-dependencies similar to WT: slow and high affinity for closed channels; relatively fast dissociation from open channels at rate increasing with voltage. This supports the idea that these properties depend mainly on interactions with pore-permeation processes that are not affected by the mutations. In mutant channels the state-dependence also greatly enhances the protection of toxin binding against steady-state inactivation at low depolarizations while still allowing large responses to depolarizing pulses that relieve toxin block. Although not obviously applicable to any known combination of natural channel and outer-pore blocker, our biophysical characterization of such highly efficient mechanism of protection from steady-state outer-pore inactivation may be of general interest.     

Ion Conduction through C-Type Inactivated Shaker Channels

C-type inactivation of Shaker potassium channels involves entry into a state (or states) in which the inactivated channels appear nonconducting in physiological solutions. However, when Shaker channels, from which fast N-type inactivation has been removed by NH₂-terminal deletions, are expressed in Xenopus oocytes and evaluated in inside-out patches, complete removal of K⁺ ions from the internal solution exposes conduction of Na⁺ and Li⁺ in C-type inactivated conformational states. The present paper uses this observation to investigate the properties of ion conduction through C-type inactivated channel states, and demonstrates that both activation and deactivation can occur in C-type states, although with slower than normal kinetics. Channels in the C-type states appear “inactivated” (i.e., nonconducting) in physiological solutions due to the summation of two separate effects: first, internal K⁺ ions prevent Na⁺ ions from permeating through the channel; second, C-type inactivation greatly reduces the permeability of K⁺ relative to the permeability of Na⁺, thus altering the ion selectivity of the channel.     

Functional Consequences of a Decreased Potassium Affinity in a Potassium Channel Pore : Ion Interactions and C-Type Inactivation

Ions bound near the external mouth of the potassium channel pore impede the C-type inactivation conformational change (Lopez-Barneo, J., T. Hoshi, S. Heinemann, and R. Aldrich. 1993. Receptors Channels. 1:61– 71; Baukrowitz, T., and G. Yellen. 1995. Neuron. 15:951–960). In this study, we present evidence that the occupancy of the C-type inactivation modulatory site by permeant ions is not solely dependent on its intrinsic affinity, but is also a function of the relative affinities of the neighboring sites in the potassium channel pore. The A463C mutation in the S6 region of Shaker decreases the affinity of an internal ion binding site in the pore (Ogielska, E.M., and R.W. Aldrich, 1998). However, we have found that this mutation also decreases the C-type inactivation rate of the channel. Our studies indicate that the C-type inactivation effects observed with substitutions at position A463 most likely result from changes in the pore occupancy of the channel, rather than a change in the C-type inactivation conformational change. We have found that a decrease in the potassium affinity of the internal ion binding site in the pore results in lowered (electrostatic) interactions among ions in the pore and as a result prolongs the time an ion remains bound at the external C-type inactivation site. We also present evidence that the C-type inactivation constriction is quite local and does not involve a general collapse of the selectivity filter. Our data indicate that in A463C potassium can bind within the selectivity filter without interfering with the process of C-type inactivation.     

Selectivity Changes during Activation of Mutant Shaker Potassium Channels

Mutations of the pore-region residue T442 in Shaker channels result in large effects on channel kinetics. We studied mutations at this position in the backgrounds of NH₂-terminal–truncated Shaker H4 and a Shaker -NGK2 chimeric channel having high conductance (Lopez, G.A., Y.N. Jan, and L.Y. Jan. 1994. Nature (Lond.). 367: 179–182). While mutations of T442 to C, D, H, V, or Y resulted in undetectable expression in Xenopus oocytes, S and G mutants yielded functional channels having deactivation time constants and channel open times two to three orders of magnitude longer than those of the parental channel. Activation time courses at depolarized potentials were unaffected by the mutations, as were first-latency distributions in the T442S chimeric channel. The mutant channels show two subconductance levels, 37 and 70% of full conductance. From single-channel analysis, we concluded that channels always pass through the larger subconductance state on the way to and from the open state. The smaller subconductance state is traversed in ∼40% of activation time courses. These states apparently represent kinetic intermediates in channel gating having voltage-dependent transitions with apparent charge movements of ∼1.6 e₀. The fully open T442S chimeric channel has the conductance sequence Rb⁺ > NH₄ ⁺ > K⁺. The opposite conductance sequence, K⁺ > NH₄ ⁺ > Rb⁺, is observed in each of the subconductance states, with the smaller subconductance state discriminating most strongly against Rb⁺.     

How Does the W434F Mutation Block Current in Shaker Potassium Channels?

The mutation W434F produces an apparently complete block of potassium current in Shaker channels expressed in Xenopus oocytes. Tandem tetrameric constructs containing one or two subunits with this mutation showed rapid inactivation, although the NH₂-terminal inactivation domain was absent from these constructs. The inactivation showed a selective dependence on external cations and was slowed by external TEA; these properties are characteristic of C-type inactivation. Inactivation was, however, incompletely relieved by hyperpolarization, suggesting the presence of a voltage-independent component. The hybrid channels had near-normal conductance and ion selectivity. Single-channel recordings from patches containing many W434F channels showed occasional channel openings, consistent with open probabilities of 10⁻⁵ or less. We conclude that the W434F mutation produces a channel that is predominantly found in an inactivated state.     

Role of Outer-pore Residue Y380 in U-type Inactivation of KV2.1 Channels

The interpretation of slow inactivation in potassium channels has been strongly influenced by work on C-type inactivation in Shaker channels. Slow inactivation in Shaker and some other potassium channels can be dramatically modulated by the state of the pore, including mutations at outer pore residue T449, which altered inactivation kinetics up to 100-fold. K_V2.1, another voltage-dependent potassium channel, exhibits a biophysically distinct inactivation mechanism with a U-shaped voltage-dependence and preferential closed-state inactivation, termed U-type inactivation. However, it remains to be demonstrated whether U-type and C-type inactivation have different molecular mechanisms. This study examines mutations at Y380 (homologous to Shaker T449) to investigate whether C-type and U-type inactivation have distinct molecular mechanisms, and whether C-type inactivation can occur at all in K_V2.1. Y380 mutants do not introduce C-type inactivation into K_V2.1 and have little effect on U-type inactivation of K_V2.1. Interestingly, two of the mutants tested exhibit twofold faster recovery from inactivation compared to wild-type channels. The observation that mutations have little effect suggests K_V2.1 lacks C-type inactivation as it exists in Shaker and that C-type and U-type inactivation have different molecular mechanisms. Kinetic modeling predicts that all mutants inactivate preferentially, but not exclusively, from partially activated closed states. Therefore, K_V2.1 exhibits a single U-type inactivation process including some inactivation from open as well as closed states.     

Fast Inactivation in Shaker K+ Channels : Properties of Ionic and Gating Currents

Fast inactivating Shaker H4 potassium channels and nonconducting pore mutant Shaker H4 W434F channels have been used to correlate the installation and recovery of the fast inactivation of ionic current with changes in the kinetics of gating current known as “charge immobilization” (Armstrong, C.M., and F. Bezanilla. 1977. J. Gen. Physiol. 70:567–590.). Shaker H4 W434F gating currents are very similar to those of the conducting clone recorded in potassium-free solutions. This mutant channel allows the recording of the total gating charge return, even when returning from potentials that would largely inactivate conducting channels. As the depolarizing potential increased, the OFF gating currents decay phase at −90 mV return potential changed from a single fast component to at least two components, the slower requiring ∼200 ms for a full charge return. The charge immobilization onset and the ionic current decay have an identical time course. The recoveries of gating current (Shaker H4 W434F) and ionic current (Shaker H4) in 2 mM external potassium have at least two components. Both recoveries are similar at −120 and −90 mV. In contrast, at higher potentials (−70 and −50 mV), the gating charge recovers significantly more slowly than the ionic current. A model with a single inactivated state cannot account for all our data, which strongly support the existence of “parallel” inactivated states. In this model, a fraction of the charge can be recovered upon repolarization while the channel pore is occupied by the NH₂-terminus region.     

Regulation of Deactivation by an Amino Terminal Domain in Human Ether-à-go-go?–related Gene Potassium Channels

Abnormalities in repolarization of the cardiac ventricular action potential can lead to life-threatening arrhythmias associated with long QT syndrome. The repolarization process depends upon the gating properties of potassium channels encoded by the human ether-à-go-go–related gene (HERG), especially those governing the rate of recovery from inactivation and the rate of deactivation. Previous studies have demonstrated that deletion of the NH₂ terminus increases the deactivation rate, but the mechanism by which the NH₂ terminus regulates deactivation in wild-type channels has not been elucidated. We tested the hypothesis that the HERG NH₂ terminus slows deactivation by a mechanism similar to N-type inactivation in Shaker channels, where it binds to the internal mouth of the pore and prevents channel closure. We found that the regulation of deactivation by the HERG NH₂ terminus bears similarity to Shaker N-type inactivation in three respects: (a) deletion of the NH₂ terminus slows C-type inactivation; (b) the action of the NH₂ terminus is sensitive to elevated concentrations of external K⁺, as if its binding along the permeation pathway is disrupted by K⁺ influx; and (c) N-ethylmaleimide, covalently linked to an aphenotypic cysteine introduced within the S4–S5 linker, mimics the N deletion phenotype, as if the binding of the NH₂ terminus to its receptor site were hindered. In contrast to N-type inactivation in Shaker, however, there was no indication that the NH₂ terminus blocks the HERG pore. In addition, we discovered that separate domains within the NH₂ terminus mediate the slowing of deactivation and the promotion of C-type inactivation. These results suggest that the NH₂ terminus stabilizes the open state and, by a separate mechanism, promotes C-type inactivation.     

Slow Inactivation in Shaker K Channels Is Delayed by Intracellular Tetraethylammonium

After removal of the fast N-type inactivation gate, voltage-sensitive Shaker (Shaker IR) K channels are still able to inactivate, albeit slowly, upon sustained depolarization. The classical mechanism proposed for the slow inactivation observed in cell-free membrane patches—the so called C inactivation—is a constriction of the external mouth of the channel pore that prevents K⁺ ion conduction. This constriction is antagonized by the external application of the pore blocker tetraethylammonium (TEA). In contrast to C inactivation, here we show that, when recorded in whole Xenopus oocytes, slow inactivation kinetics in Shaker IR K channels is poorly dependent on external TEA but severely delayed by internal TEA. Based on the antagonism with internally or externally added TEA, we used a two-pulse protocol to show that half of the channels inactivate by way of a gate sensitive to internal TEA. Such gate had a recovery time course in the tens of milliseconds range when the interpulse voltage was −90 mV, whereas C-inactivated channels took several seconds to recover. Internal TEA also reduced gating charge conversion associated to slow inactivation, suggesting that the closing of the internal TEA-sensitive inactivation gate could be associated with a significant amount of charge exchange of this type. We interpreted our data assuming that binding of internal TEA antagonized with U-type inactivation (Klemic, K.G., G.E. Kirsch, and S.W. Jones. 2001. Biophys. J. 81:814–826). Our results are consistent with a direct steric interference of internal TEA with an internally located slow inactivation gate as a “foot in the door” mechanism, implying a significant functional overlap between the gate of the internal TEA-sensitive slow inactivation and the primary activation gate. But, because U-type inactivation is reduced by channel opening, trapping the channel in the open conformation by TEA would also yield to an allosteric delay of slow inactivation. These results provide a framework to explain why constitutively C-inactivated channels exhibit gating charge conversion, and why mutations at the internal exit of the pore, such as those associated to episodic ataxia type I in hKv1.1, cause severe changes in inactivation kinetics.     

Protein Rearrangements Underlying Slow Inactivation of the Shaker K+ Channel

Voltage-dependent ion channels transduce changes in the membrane electric field into protein rearrangements that gate their transmembrane ion permeation pathways. While certain molecular elements of the voltage sensor and gates have been identified, little is known about either the nature of their conformational rearrangements or about how the voltage sensor is coupled to the gates. We used voltage clamp fluorometry to examine the voltage sensor (S4) and pore region (P-region) protein motions that underlie the slow inactivation of the Shaker K⁺ channel. Fluorescent probes in both the P-region and S4 changed emission intensity in parallel with the onset and recovery of slow inactivation, indicative of local protein rearrangements in this gating process. Two sequential rearrangements were observed, with channels first entering the P-type, and then the C-type inactivated state. These forms of inactivation appear to be mediated by a single gate, with P-type inactivation closing the gate and C-type inactivation stabilizing the gate''s closed conformation. Such a stabilization was due, at least in part, to a slow rearrangement around S4 that stabilizes S4 in its activated transmembrane position. The fluorescence reports of S4 and P-region fluorophore are consistent with an increased interaction of the voltage sensor and inactivation gate upon gate closure, offering insight into how the voltage-sensing apparatus is coupled to a channel gate.     

Macroscopic Na+ Currents in the “Nonconducting” Shaker Potassium Channel Mutant W434F

C-type inactivation in Shaker potassium channels inhibits K⁺ permeation. The associated structural changes appear to involve the outer region of the pore. Recently, we have shown that C-type inactivation involves a change in the selectivity of the Shaker channel, such that C-type inactivated channels show maintained voltage-sensitive activation and deactivation of Na⁺ and Li⁺ currents in K⁺-free solutions, although they show no measurable ionic currents in physiological solutions. In addition, it appears that the effective block of ion conduction produced by the mutation W434F in the pore region may be associated with permanent C-type inactivation of W434F channels. These conclusions predict that permanently C-type inactivated W434F channels would also show Na⁺ and Li⁺ currents (in K⁺-free solutions) with kinetics similar to those seen in C-type-inactivated Shaker channels. This paper confirms that prediction and demonstrates that activation and deactivation parameters for this mutant can be obtained from macroscopic ionic current measurements. We also show that the prolonged Na⁺ tail currents typical of C-type inactivated channels involve an equivalent prolongation of the return of gating charge, thus demonstrating that the kinetics of gating charge return in W434F channels can be markedly altered by changes in ionic conditions.     

Dihydropyridine Action on Voltage-dependent Potassium Channels Expressed in Xenopus Oocytes

Dihydropyridines (DHPs) are well known for their effects on L-type voltage-dependent Ca²⁺ channels. However, these drugs also affect other voltage-dependent ion channels, including Shaker K⁺ channels. We examined the effects of DHPs on the Shaker K⁺ channels expressed in Xenopus oocytes. Intracellular applications of DHPs quickly and reversibly induced apparent inactivation in the Shaker K⁺ mutant channels with disrupted N- and C-type inactivation. We found that DHPs interact with the open state of the channel as evidenced by the decreased mean open time. The DHPs effects are voltage-dependent, becoming more effective with hyperpolarization. A model which involves binding of two DHP molecules to the channel is consistent with the results obtained in our experiments.     

Computational study of non-conductive selectivity filter conformations and C-type inactivation in a voltage-dependent potassium channel

C-type inactivation is a time-dependent process of great physiological significance that is observed in a large class of K⁺ channels. Experimental and computational studies of the pH-activated KcsA channel show that the functional C-type inactivated state, for this channel, is associated with a structural constriction of the selectivity filter at the level of the central glycine residue in the signature sequence, TTV(G)YGD. The structural constriction is allosterically promoted by the wide opening of the intracellular activation gate. However, whether this is a universal mechanism for C-type inactivation has not been established with certainty because similar constricted structures have not been observed for other K⁺ channels. Seeking to ascertain the general plausibility of the constricted filter conformation, molecular dynamics simulations of a homology model of the pore domain of the voltage-gated potassium channel Shaker were performed. Simulations performed with an open intracellular gate spontaneously resulted in a stable constricted-like filter conformation, providing a plausible nonconductive state responsible for C-type inactivation in the Shaker channel. While there are broad similarities with the constricted structure of KcsA, the hypothetical constricted-like conformation of Shaker also displays some subtle differences. Interestingly, those are recapitulated by the Shaker-like E71V KcsA mutant, suggesting that the residue at this position along the pore helix plays a pivotal role in determining the C-type inactivation behavior. Free energy landscape calculations show that the conductive-to-constricted transition in Shaker is allosterically controlled by the degree of opening of the intracellular activation gate, as observed with the KcsA channel. The behavior of the classic inactivating W434F Shaker mutant is also characterized from a 10-μs MD simulation, revealing that the selectivity filter spontaneously adopts a nonconductive conformation that is constricted at the level of the second glycine in the signature sequence, TTVGY(G)D.     

Voltage-insensitive Gating after Charge-neutralizing Mutations in the S4 Segment of Shaker Channels

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

A single charged voltage sensor is capable of gating the Shaker K+ channel

We sought to determine the contribution of an individual voltage sensor to Shaker''s function. Concatenated heterotetramers of Shaker zH4 Δ(6–46) wild type (wt) in combination with a neutral S4 segment Shaker mutant (mut) with stoichiometries 2wt/2mut and 1wt/3mut were studied and compared with the 4wt concatenated homotetramer. A single charged voltage sensor is sufficient to open Shaker conductance with reduced delay (<1 ms) and at more hyperpolarized voltages compared with 4wt. In addition, the wt-like slow inactivation of 1wt/3mut was almost completely eliminated by mutations T449V-I470C in its single wt subunit, indicating that the subunits bearing a neutral S4 were unable to trigger slow inactivation. Our results strongly suggest that a neutral S4 segment of Shaker''s subunit is voltage insensitive and its voltage sensor is in the activated position (i.e., ready for pore opening), and provide experimental support to the proposed model of independent voltage sensors with a final, almost voltage-independent concerted step.     

N-type Inactivation Features of Kv4.2 Channel Gating

We examined whether the N-terminus of Kv4.2 A-type channels (4.2NT) possesses an autoinhibitory N-terminal peptide domain, which, similar to the one of Shaker, mediates inactivation of the open state. We found that chimeric Kv2.1(4.2NT) channels, where the cytoplasmic Kv2.1 N-terminus had been replaced by corresponding Kv4.2 domains, inactivated relatively fast, with a mean time constant of 120 ms as compared to 3.4 s in Kv2.1 wild-type. Notably, Kv2.1(4.2NT) showed features typically observed for Shaker N-type inactivation: fast inactivation of Kv2.1(4.2NT) channels was slowed by intracellular tetraethylammonium and removed by N-terminal truncation (Δ40). Kv2.1(4.2NT) channels reopened during recovery from inactivation, and recovery was accelerated in high external K⁺. Moreover, the application of synthetic N-terminal Kv4.2 and ShB peptides to inside-out patches containing slowly inactivating Kv2.1 channels mimicked N-type inactivation. Kv4.2 channels, after fractional inactivation, mediated tail currents with biphasic decay, indicative of passage through the open state during recovery from inactivation. Biphasic tail current kinetics were less prominent in Kv4.2/KChIP2.1 channel complexes and virtually absent in Kv4.2Δ40 channels. N-type inactivation features of Kv4.2 open-state inactivation, which may be suppressed by KChIP association, were also revealed by the finding that application of Kv4.2 N-terminal peptide accelerated the decay kinetics of both Kv4.2Δ40 and Kv4.2/KChIP2.1 patch currents. However, double mutant cycle analysis of N-terminal inactivating and pore domains indicated differences in the energetics and structural determinants between Kv4.2 and Shaker N-type inactivation.     

Intermediate Conductances during Deactivation of Heteromultimeric Shaker Potassium Channels

A previous study of the T442S mutant Shaker channel revealed activation-coupled subconductance levels that apparently represent kinetic intermediates in channel activation (Zheng, J., and F.J. Sigworth. 1997. J. Gen. Physiol. 110:101–117). We have now extended the study to heteromultimeric channels consisting of various numbers of mutant subunits as well as channels without mutant subunits, all in the background of a chimeric Shaker channel having increased conductance. It has been found that activation-coupled sublevels exist in all these channel types, and are traversed in at least 80% of all deactivation time courses. In symmetric K⁺ solutions, the currents in the two sublevels have a linear voltage dependence, being 23–44% and 54–70% of the fully open conductance. Sublevels in different channel types share similar voltage dependence of the mean lifetime and similar ion selectivity properties. However, the mean lifetime of each current level depends approximately geometrically on the number of mutant subunits in the channel, becoming shorter in channels having fewer mutant subunits. Each mutant subunit appears to stabilize all of the conducting states by ∼0.5 kcal/mol. Consistent with previous results in the mutant channel, sublevels in channels with two or no mutant subunits also showed ion selectivities that differ from that of the fully open level, having relatively higher K⁺ than Rb⁺ conductances. A model is presented in which Shaker channels have two coupled activation gates, one associated with the selectivity filter and a second associated with the S6 helix bundle.     

Temperature and pressure dependence of Shaker K+ channel N- and C-type inactivation

Shaker B potassium channels undergo rapid N-type and slow C-type inactivation. While N-type inactivation is supposed to be mediated by occlusion of the pore by the N-terminal protein structure, the molecular mechanisms leading to C-type inactivation are less well understood. Considering N-type inactivation as a model for a protein conformational transition, we investigated inactivation of heterologously expressed Shaker B potassium channels and mutants thereof, showing various degrees of C-type inactivation, under high hydrostatic (oil) pressure. In addition to the derived apparent activation and reaction volumes (ΔV), experiments at various temperatures yielded estimates for enthalpic (ΔH) and entropic (TΔS) contributions. N-type inactivation was accelerated by increasing temperature and slowed by high hydrostatic pressure yielding at equilibrium ΔH = 76 kJ/mole, TΔS = 82 kJ/mole, and ΔV = 0.18 nm³ indicating that the transition to the N-type inactivated state is accompanied by an increase in volume and a decrease in order. N-terminally deleted ShΔ6–46 constructs with additional mutations at either position 449 or 463 were used to investigate C-type inactivation. In particular at high temperatures, inactivation occurred in two phases indicating more than one process. At equilibrium the following values were estimated for the major inactivation component of mutant ShΔ6–46 T449A: ΔH = –64 kJ/mole, TΔS = –60 kJ/mole, and ΔV = –0.25 nm³, indicating that the C-type inactivated state occupies a smaller volume and is more ordered than the noninactivated state. Thus, hydrostatic pressure affects N- and C-type inactivation in opposite ways. Received: 17 May 1997 / Accepted: 18 July 1997     

Correlation between Charge Movement and Ionic Current during Slow Inactivation in Shaker K+ Channels

Prolonged depolarization induces a slow inactivation process in some K⁺ channels. We have studied ionic and gating currents during long depolarizations in the mutant Shaker H4-Δ(6–46) K⁺ channel and in the nonconducting mutant (Shaker H4-Δ(6–46)-W434F). These channels lack the amino terminus that confers the fast (N-type) inactivation (Hoshi, T., W.N. Zagotta, and R.W. Aldrich. 1991. Neuron. 7:547–556). Channels were expressed in oocytes and currents were measured with the cut-open-oocyte and patch-clamp techniques. In both clones, the curves describing the voltage dependence of the charge movement were shifted toward more negative potentials when the holding potential was maintained at depolarized potentials. The evidences that this new voltage dependence of the charge movement in the depolarized condition is associated with the process of slow inactivation are the following: (a) the installation of both the slow inactivation of the ionic current and the inactivation of the charge in response to a sustained 1-min depolarization to 0 mV followed the same time course; and (b) the recovery from inactivation of both ionic and gating currents (induced by repolarizations to −90 mV after a 1-min inactivating pulse at 0 mV) also followed a similar time course. Although prolonged depolarizations induce inactivation of the majority of the channels, a small fraction remains non–slow inactivated. The voltage dependence of this fraction of channels remained unaltered, suggesting that their activation pathway was unmodified by prolonged depolarization. The data could be fitted to a sequential model for Shaker K⁺ channels (Bezanilla, F., E. Perozo, and E. Stefani. 1994. Biophys. J. 66:1011–1021), with the addition of a series of parallel nonconducting (inactivated) states that become populated during prolonged depolarization. The data suggest that prolonged depolarization modifies the conformation of the voltage sensor and that this change can be associated with the process of slow inactivation.