Inverted allosteric coupling between activation and inactivation gates in K+ channels.
ABSTRACT: The selectivity filter and the activation gate in potassium channels are functionally and structurally coupled. An allosteric coupling underlies C-type inactivation coupled to activation gating in this ion-channel family (i.e., opening of the activation gate triggers the collapse of the channel's selectivity filter). We have identified the second Threonine residue within the TTVGYGD signature sequence of K+ channels as a crucial residue for this allosteric communication. A Threonine to Alanine substitution at this position was studied in three representative members of the K+-channel family. Interestingly, all of the mutant channels exhibited lack of C-type inactivation gating and an inversion of their allosteric coupling (i.e., closing of the activation gate collapses the channel's selectivity filter). A state-dependent crystallographic study of KcsA-T75A proves that, on activation, the selectivity filter transitions from a nonconductive and deep C-type inactivated conformation to a conductive one. Finally, we provide a crystallographic demonstration that closed-state inactivation can be achieved by the structural collapse of the channel's selectivity filter.
Project description:Ion channels regulate ion flow by opening and closing their pore gates. K(+) channels commonly possess two pore gates, one at the intracellular end for fast channel activation/deactivation and the other at the selectivity filter for slow C-type inactivation/recovery. The large-conductance calcium-activated potassium (BK) channel lacks a classic intracellular bundle-crossing activation gate and normally show no C-type inactivation. We hypothesized that the BK channel's activation gate may spatially overlap or coexist with the C-type inactivation gate at or near the selectivity filter. We induced C-type inactivation in BK channels and studied the relationship between activation/deactivation and C-type inactivation/recovery. We observed prominent slow C-type inactivation/recovery in BK channels by an extreme low concentration of extracellular K(+) together with a Y294E/K/Q/S or Y279F mutation whose equivalent in Shaker channels (T449E/K/D/Q/S or W434F) caused a greatly accelerated rate of C-type inactivation or constitutive C-inactivation. C-type inactivation in most K(+) channels occurs upon sustained membrane depolarization or channel opening and then recovers during hyperpolarized membrane potentials or channel closure. However, we found that the BK channel C-type inactivation occurred during hyperpolarized membrane potentials or with decreased intracellular calcium ([Ca(2+)]i) and recovered with depolarized membrane potentials or elevated [Ca(2+)]i Constitutively open mutation prevented BK channels from C-type inactivation. We concluded that BK channel C-type inactivation is closed state-dependent and that its extents and rates inversely correlate with channel-open probability. Because C-type inactivation can involve multiple conformational changes at the selectivity filter, we propose that the BK channel's normal closing may represent an early conformational stage of C-type inactivation.
Project description:Flux-dependent inactivation that arises from functional coupling between the inner gate and the selectivity filter is widespread in ion channels. The structural basis of this coupling has only been well characterized in KcsA. Here we present NMR data demonstrating structural and dynamic coupling between the selectivity filter and intracellular constriction point in the bacterial nonselective cation channel, NaK. This transmembrane allosteric communication must be structurally different from KcsA because the NaK selectivity filter does not collapse under low-cation conditions. Comparison of NMR spectra of the nonselective NaK and potassium-selective NaK2K indicates that the number of ion binding sites in the selectivity filter shifts the equilibrium distribution of structural states throughout the channel. This finding was unexpected given the nearly identical crystal structure of NaK and NaK2K outside the immediate vicinity of the selectivity filter. Our results highlight the tight structural and dynamic coupling between the selectivity filter and the channel scaffold, which has significant implications for channel function. NaK offers a distinct model to study the physiologically essential connection between ion conduction and channel gating.
Project description:Voltage-gated potassium (Kv) channels display several types of inactivation processes, including N-, C-, and U-types. C-type inactivation is attributed to a nonconductive conformation of the selectivity filter (SF). It has been proposed that the activation gate and the channel's SF are allosterically coupled because the conformational changes of the former affect the structure of the latter and vice versa. The second threonine of the SF signature sequence (e.g., TTVGYG) has been proven to be essential for this allosteric coupling. To further study the role of the SF in U-type inactivation, we substituted the second threonine of the TTVGYG sequence by an alanine in the hKv2.1 and hKv3.1 channels, which are known to display U-type inactivation. Both hKv2.1-T377A and hKv3.1-T400A yielded channels that were resistant to inactivation, and as a result, they displayed noninactivating currents upon channel opening; i.e., hKv2.1-T377A and hKv3.1-T400A remained fully conductive upon prolonged moderate depolarizations, whereas in wild-type hKv2.1 and hKv3.1, the current amplitude typically reduces because of U-type inactivation. Interestingly, increasing the extracellular K+ concentration increased the macroscopic current amplitude of both hKv2.1-T377A and hKv3.1-T400A, which is similar to the response of the homologous T to A mutation in Shaker and hKv1.5 channels that display C-type inactivation. Our data support an important role for the second threonine of the SF signature sequence in the U-type inactivation gating of hKv2.1 and hKv3.1.
Project description:Ca(2+)/calmodulin- and voltage-dependent inactivation (CDI and VDI) comprise vital prototypes of Ca(2+) channel modulation, rich with biological consequences. Although the events initiating CDI and VDI are known, their downstream mechanisms have eluded consensus. Competing proposals include hinged-lid occlusion of channels, selectivity filter collapse, and allosteric inhibition of the activation gate. Here, novel theory predicts that perturbations of channel activation should alter inactivation in distinctive ways, depending on which hypothesis holds true. Thus, we systematically mutate the activation gate, formed by all S6 segments within Ca(V)1.3. These channels feature robust baseline CDI, and the resulting mutant library exhibits significant diversity of activation, CDI, and VDI. For CDI, a clear and previously unreported pattern emerges: activation-enhancing mutations proportionately weaken inactivation. This outcome substantiates an allosteric CDI mechanism. For VDI, the data implicate a "hinged lid-shield" mechanism, similar to a hinged-lid process, with a previously unrecognized feature. Namely, we detect a "shield" in Ca(V)1.3 channels that is specialized to repel lid closure. These findings reveal long-sought downstream mechanisms of inactivation and may furnish a framework for the understanding of Ca(2+) channelopathies involving S6 mutations.
Project description:Potassium channels are presumed to have two allosterically coupled gates, the activation gate and the selectivity filter gate, that control channel opening, closing, and inactivation. However, the molecular mechanism of how these gates regulate K+ ion flow through the channel remains poorly understood. An activation process, occurring at the selectivity filter, has been recently proposed for several potassium channels. Here, we use X-ray crystallography and extensive molecular dynamics simulations, to study ion permeation through a potassium channel MthK, for various opening levels of both gates. We find that the channel conductance is controlled at the selectivity filter, whose conformation depends on the activation gate. The crosstalk between the gates is mediated through a collective motion of channel helices, involving hydrophobic contacts between an isoleucine and a conserved threonine in the selectivity filter. We propose a gating model of selectivity filter-activated potassium channels, including pharmacologically relevant two-pore domain (K2P) and big potassium (BK) channels.
Project description:In the prokaryotic potassium channel KcsA activation gating at the inner bundle gate is followed by C-type inactivation at the selectivity filter. Entry into the C-type inactivated state has been directly linked to the strength of the H-bond interaction between residues Glu-71 and Asp-80 behind the filter, and is allosterically triggered by the rearrangement of the inner bundle gate. Here, we show that H-bond pairing between residues Trp-67 and Asp-80, conserved in most K? channels, constitutes another critical interaction that determines the rate and extent of KcsA C-type inactivation. Disruption of the equivalent interaction in Shaker (Trp-434-Asp-447) and Kv1.2 (Trp-366-Asp-379) leads also to modulation of the inactivation process, suggesting that these residues also play an analogous role in the inactivation gating of Kv channels. The present results show that in KcsA C-type inactivation gating is governed by a multipoint hydrogen-bond network formed by the triad Trp-67-Glu71-Asp-80. This triad exerts a critical role in the dynamics and conformational stability of the selectivity filter and might serve as a general modulator of selectivity filter gating in other members of the K? channel family.
Project description:The coupled interplay between activation and inactivation gating is a functional hallmark of K(+) channels. This coupling has been experimentally demonstrated through ion interaction effects and cysteine accessibility, and is associated with a well defined boundary of energetically coupled residues. The structure of the K(+) channel KcsA in its fully open conformation, in addition to four other partial channel openings, richly illustrates the structural basis of activation-inactivation gating. Here, we identify the mechanistic principles by which movements on the inner bundle gate trigger conformational changes at the selectivity filter, leading to the non-conductive C-type inactivated state. Analysis of a series of KcsA open structures suggests that, as a consequence of the hinge-bending and rotation of the TM2 helix, the aromatic ring of Phe 103 tilts towards residues Thr 74 and Thr 75 in the pore-helix and towards Ile 100 in the neighbouring subunit. This allows the network of hydrogen bonds among residues Trp 67, Glu 71 and Asp 80 to destabilize the selectivity filter, allowing entry to its non-conductive conformation. Mutations at position 103 have a size-dependent effect on gating kinetics: small side-chain substitutions F103A and F103C severely impair inactivation kinetics, whereas larger side chains such as F103W have more subtle effects. This suggests that the allosteric coupling between the inner helical bundle and the selectivity filter might rely on straightforward mechanical deformation propagated through a network of steric contacts. Average interactions calculated from molecular dynamics simulations show favourable open-state interaction-energies between Phe 103 and the surrounding residues. We probed similar interactions in the Shaker K(+) channel where inactivation was impaired in the mutant I470A. We propose that side-chain rearrangements at position 103 mechanically couple activation and inactivation in KcsA and a variety of other K(+) channels.
Project description:Bestrophin (BEST1-4) ligand-gated chloride (Cl-) channels are activated by calcium (Ca2+). Mutation of BEST1 causes retinal disease. Partly because bestrophin channels have no sequence or structural similarity to other ion channels, the molecular mechanisms underlying gating are unknown. Here, we present a series of cryo-electron microscopy structures of chicken BEST1, determined at 3.1 Å resolution or better, that represent the channel's principal gating states. Unlike other channels, opening of the pore is due to the repositioning of tethered pore-lining helices within a surrounding protein shell that dramatically widens a neck of the pore through a concertina of amino acid rearrangements. The neck serves as both the activation and the inactivation gate. Ca2+ binding instigates opening of the neck through allosteric means whereas inactivation peptide binding induces closing. An aperture within the otherwise wide pore controls anion permeability. The studies define a new molecular paradigm for gating among ligand-gated ion channels.
Project description:C-type inactivation is a time-dependent process observed in many K<sup>+</sup> channels whereby prolonged activation by an external stimulus leads to a reduction in ionic conduction. While C-type inactivation is thought to be a result of a constriction of the selectivity filter, the local dynamics of the process remain elusive. Here, we use molecular dynamics (MD) simulations of the KcsA channel to elucidate the nature of kinetically delayed activation/inactivation gating coupling. Microsecond-scale MD simulations based on the truncated form of the KcsA channel (C-terminal domain deleted) provide a first glimpse of the onset of C-type inactivation. We observe over multiple trajectories that the selectivity filter consistently undergoes a spontaneous and rapid (within 1-2 µs) transition to a constricted conformation when the intracellular activation gate is fully open, but remains in the conductive conformation when the activation gate is closed or partially open. Multidimensional umbrella sampling potential of mean force calculations and nonequilibrium voltage-driven simulations further confirm these observations. Electrophysiological measurements show that the truncated form of the KcsA channel inactivates faster and greater than full-length KcsA, which is consistent with truncated KcsA opening to a greater degree because of the absence of the C-terminal domain restraint. Together, these results imply that the observed kinetics underlying activation/inactivation gating reflect a rapid conductive-to-constricted transition of the selectivity filter that is allosterically controlled by the slow opening of the intracellular gate.
Project description:The allosteric coupling between activation and inactivation processes is a common feature observed in K<sup>+</sup> channels. Particularly, in the prokaryotic KcsA channel the K<sup>+</sup> conduction process is controlled by the inner gate, which is activated by acidic pH, and by the selectivity filter (SF) or outer gate, which can adopt non-conductive or conductive states. In a previous study, a single tryptophan mutant channel (W67 KcsA) enabled us to investigate the SF dynamics using time-resolved homo-Förster Resonance Energy Transfer (homo-FRET) measurements. Here, the conformational changes of both gates were simultaneously monitored after labelling the G116C position with tetramethylrhodamine (TMR) within a W67 KcsA background. At a high degree of protein labeling, fluorescence anisotropy measurements showed that the pH-induced KcsA gating elicited a variation in the homo-FRET efficiency among the conjugated TMR dyes (TMR homo-FRET), while the conformation of the SF was simultaneously tracked (W67 homo-FRET). The dependence of the activation p<i>K</i><sub>a</sub> of the inner gate with the ion occupancy of the SF unequivocally confirmed the allosteric communication between the two gates of KcsA. This simple TMR homo-FRET based ratiometric assay can be easily extended to study the conformational dynamics associated with the gating of other ion channels and their modulation.