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The halothane-inhibited K 2P leak potassium channel K 2P 13.1 (THIK-1) 1-3 is found in diverse cells 1,4 including neurons 1,5 and microglia 6-8 where it affects surveillance6, synaptic pruning7, phagocytosis7, and inflammasome-mediated interleukin-1β release 6,8,9 . As with many K 2P s 1,5,10-14 and other voltage-gated ion channel (VGIC) superfamily members 3,15,16 , polyunsaturated fatty acid (PUFA) lipids modulate K 2P 13.1 (THIK-1) 1,5,14,17 via a poorly understood mechanism. Here, we present cryo-electronmicroscopy (cryo-EM) structures of human K 2P 13.1 (THIK-1) and mutants in lipid nanodiscs and detergent. These reveal that, unlike other K 2P s 13,18-24 , K 2P 13.1 (THIK-1) has a two-chamber aqueous inner cavity obstructed by a M4 transmembrane helix tyrosine (Tyr273, the flow restrictor). This hydrophilic barrier can be opened by an activatory mutation, S136P 25 , at natural break in the M2 transmembrane helix and by intrinsic channel dynamics. The structures also reveal a buried lipid in the P1/M4 intersubunit interface at a location, the PUFA site, that coincides with the TREK subfamily K 2P modulator pocket for small molecule agonists 18,26,27 . This overlap, together with the effects of mutation on K 2P 13.1 (THIK-1) PUFA responses, indicates that the PUFA site lipids are K 2P 13.1 (THIK-1) cofactors. Comparison with the PUFA-responsive VGIC Kv7.1 (KCNQ1) 28-31 reveals a shared role for the equivalent pore domain intersubunit interface in lipid modulation, providing a framework for dissecting the effects of PUFAs on the VGIC superfamily. Our findings reveal the unique architecture underlying K 2P 13.1 (THIK-1) function, highlight the importance of the P1/M4 interface in control of K 2P s by both natural and synthetic agents, and should aid development of THIK subfamily modulators for diseases such as neuroinflammation 6,32 and autism 6 ., Competing Interests: Competing interests The authors declare no competing interests.

Voltage-gated ion channels (VGICs) comprise multiple structural units, the assembly of which is required for function 1,2 . Structural understanding of how VGIC subunits assemble and whether chaperone proteins are required is lacking. High-voltage-activated calcium channels (Ca V s) 3,4 are paradigmatic multisubunit VGICs whose function and trafficking are powerfully shaped by interactions between pore-forming Ca V 1 or Ca V 2 Ca V α 1 (ref. 3 ), and the auxiliary Ca V β 5 and Ca V α 2 δ subunits 6,7 . Here we present cryo-electron microscopy structures of human brain and cardiac Ca V 1.2 bound with Ca V β 3 to a chaperone-the endoplasmic reticulum membrane protein complex (EMC) 8,9 -and of the assembled Ca V 1.2-Ca V β 3 -Ca V α 2 δ-1 channel. These structures provide a view of an EMC-client complex and define EMC sites-the transmembrane (TM) and cytoplasmic (Cyto) docks; interaction between these sites and the client channel causes partial extraction of a pore subunit and splays open the Ca V α 2 δ-interaction site. The structures identify the Ca V α 2 δ-binding site for gabapentinoid anti-pain and anti-anxiety drugs 6 , show that EMC and Ca V α 2 δ interactions with the channel are mutually exclusive, and indicate that EMC-to-Ca V α 2 δ hand-off involves a divalent ion-dependent step and Ca V 1.2 element ordering. Disruption of the EMC-Ca V complex compromises Ca V function, suggesting that the EMC functions as a channel holdase that facilitates channel assembly. Together, the structures reveal a Ca V assembly intermediate and EMC client-binding sites that could have wide-ranging implications for the biogenesis of VGICs and other membrane proteins., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

American bullfrog ( Rana castesbeiana ) saxiphilin ( Rc Sxph) is a high-affinity "toxin sponge" protein thought to prevent intoxication by saxitoxin (STX), a lethal bis-guanidinium neurotoxin that causes paralytic shellfish poisoning (PSP) by blocking voltage-gated sodium channels (Na V s). How specific Rc Sxph interactions contribute to STX binding has not been defined and whether other organisms have similar proteins is unclear. Here, we use mutagenesis, ligand binding, and structural studies to define the energetic basis of Sxph:STX recognition. The resultant STX "recognition code" enabled engineering of Rc Sxph to improve its ability to rescue Na V s from STX and facilitated discovery of 10 new frog and toad Sxphs. Definition of the STX binding code and Sxph family expansion among diverse anurans separated by ∼140 My of evolution provides a molecular basis for understanding the roles of toxin sponge proteins in toxin resistance and for developing novel proteins to sense or neutralize STX and related PSP toxins.

Voltage-gated sodium channels (Na V s) are targets for a number of acute poisons. Many of these agents act as allosteric modulators of channel activity and serve as powerful chemical tools for understanding channel function. Herein, we detail studies with batrachotoxin (BTX), a potent steroidal amine, and three ester derivatives prepared through de novo synthesis against recombinant Na V subtypes (rNa V 1.4 and hNa V 1.5). Two of these compounds, BTX-B and BTX- c Hx, are functionally equivalent to BTX, hyperpolarizing channel activation and blocking both fast and slow inactivation. BTX-yne-a C20-n-heptynoate ester-is a conspicuous outlier, eliminating fast but not slow inactivation. This property differentiates BTX-yne among other Na V modulators as a unique reagent that separates inactivation processes. These findings are supported by functional studies with bacterial Na V s (BacNa V s) that lack a fast inactivation gate. The availability of BTX-yne should advance future efforts aimed at understanding Na V gating mechanisms and designing allosteric regulators of Na V activity., Competing Interests: Declaration of interests J.D. is a cofounder, executive board member, and holds equity shares in SiteOne Therapeutics, Inc., a start-up company interested in developing subtype-selective modulators of Na(V)., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Many poisonous organisms carry small-molecule toxins that alter voltage-gated sodium channel (NaV) function. Among these, batrachotoxin (BTX) from Pitohui poison birds and Phyllobates poison frogs stands out because of its lethality and unusual effects on NaV function. How these toxin-bearing organisms avoid autointoxication remains poorly understood. In poison frogs, a NaV DIVS6 pore-forming helix N-to-T mutation has been proposed as the BTX resistance mechanism. Here, we show that this variant is absent from Pitohui and poison frog NaVs, incurs a strong cost compromising channel function, and fails to produce BTX-resistant channels in poison frog NaVs. We also show that captivity-raised poison frogs are resistant to two NaV-directed toxins, BTX and saxitoxin (STX), even though they bear NaVs sensitive to both. Moreover, we demonstrate that the amphibian STX "toxin sponge" protein saxiphilin is able to protect and rescue NaVs from block by STX. Taken together, our data contradict the hypothesis that BTX autoresistance is rooted in the DIVS6 N→T mutation, challenge the idea that ion channel mutations are a primary driver of toxin resistance, and suggest the possibility that toxin sequestration mechanisms may be key for protecting poisonous species from the action of small-molecule toxins., (© 2021 Abderemane-Ali et al.)

K 2P potassium channels regulate cellular excitability using their selectivity filter (C-type) gate. C-type gating mechanisms, best characterized in homotetrameric potassium channels, remain controversial and are attributed to selectivity filter pinching, dilation, or subtle structural changes. The extent to which such mechanisms control C-type gating of innately heterodimeric K 2P s is unknown. Here, combining K 2P 2.1 (TREK-1) x-ray crystallography in different potassium concentrations, potassium anomalous scattering, molecular dynamics, and electrophysiology, we uncover unprecedented, asymmetric, potassium-dependent conformational changes that underlie K 2P C-type gating. These asymmetric order-disorder transitions, enabled by the K 2P heterodimeric architecture, encompass pinching and dilation, disrupt the S1 and S2 ion binding sites, require the uniquely long K 2P SF2-M4 loop and conserved "M3 glutamate network," and are suppressed by the K 2P C-type gate activator ML335. These findings demonstrate that two distinct C-type gating mechanisms can operate in one channel and underscore the SF2-M4 loop as a target for K 2P channel modulator development., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

Prokaryotic Na V channels are tetramers and eukaryotic Na V channels consist of a single subunit containing four domains. Each monomer/domain contains six transmembrane segments (S1-S6), S1-S4 being the voltage-sensor domain and S5-S6 the pore domain. A crystal structure of Na V Ms, a prokaryotic Na V channel, suggests that the S4-S5 linker (S4-S5 L ) interacts with the C-terminus of S6 (S6 T ) to stabilize the gate in the open state. However, in several voltage-gated potassium channels, using specific S4-S5 L -mimicking peptides, we previously demonstrated that S4-S5 L /S6 T interaction stabilizes the gate in the closed state. Here, we used the same strategy on another prokaryotic Na V channel, Na V Sp1, to test whether equivalent peptides stabilize the channel in the open or closed state. A Na V Sp1-specific S4-S5 L peptide, containing the residues supposed to interact with S6 T according to the Na V Ms structure, induced both an increase in Na V Sp1 current density and a negative shift in the activation curve, consistent with S4-S5 L stabilizing the open state. Using this approach on a human Na V channel, hNa V 1.4, and testing 12 hNa V 1.4 S4-S5 L peptides, we identified four activating S4-S5 L peptides. These results suggest that, in eukaryotic Na V channels, the S4-S5 L of DI, DII and DIII domains allosterically modulate the activation gate and stabilize its open state.

Calcium-dependent inactivation (CDI) is a fundamental autoregulatory mechanism in Ca V 1 and Ca V 2 voltage-gated calcium channels. Although CDI initiates with the cytoplasmic calcium sensor, how this event causes CDI has been elusive. Here, we show that a conserved selectivity filter (SF) domain II (DII) aspartate is essential for CDI. Mutation of this residue essentially eliminates CDI and leaves key channel biophysical characteristics untouched. DII mutants regain CDI by placing an aspartate at the analogous SF site in DIII or DIV, but not DI, indicating that Ca V SF asymmetry is key to CDI. Together, our data establish that the Ca V SF is the CDI endpoint. Discovery of this SF CDI gate recasts the Ca V inactivation paradigm, placing it squarely in the framework of voltage-gated ion channel (VGIC) superfamily members in which SF-based gating is important. This commonality suggests that SF inactivation is an ancient process arising from the shared VGIC pore architecture., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Kv7 (KCNQ) voltage-gated potassium channels control excitability in the brain, heart, and ear. Calmodulin (CaM) is crucial for Kv7 function, but how this calcium sensor affects activity has remained unclear. Here, we present X-ray crystallographic analysis of CaM:Kv7.4 and CaM:Kv7.5 AB domain complexes that reveal an Apo/CaM clamp conformation and calcium binding preferences. These structures, combined with small-angle X-ray scattering, biochemical, and functional studies, establish a regulatory mechanism for Kv7 CaM modulation based on a common architecture in which a CaM C-lobe calcium-dependent switch releases a shared Apo/CaM clamp conformation. This C-lobe switch inhibits voltage-dependent activation of Kv7.4 and Kv7.5 but facilitates Kv7.1, demonstrating that mechanism is shared by Kv7 isoforms despite the different directions of CaM modulation. Our findings provide a unified framework for understanding how CaM controls different Kv7 isoforms and highlight the role of membrane proximal domains for controlling voltage-gated channel function. VIDEO ABSTRACT., (Copyright © 2018 Elsevier Inc. All rights reserved.)

For many voltage-gated ion channels (VGICs), creation of a properly functioning ion channel requires the formation of specific protein-protein interactions between the transmembrane pore-forming subunits and cystoplasmic accessory subunits. Despite the importance of such protein-protein interactions in VGIC function and assembly, their potential as sites for VGIC modulator development has been largely overlooked. Here, we develop meta-xylyl (m-xylyl) stapled peptides that target a prototypic VGIC high affinity protein-protein interaction, the interaction between the voltage-gated calcium channel (Ca V ) pore-forming subunit α-interaction domain (AID) and cytoplasmic β-subunit (Ca V β). We show using circular dichroism spectroscopy, X-ray crystallography, and isothermal titration calorimetry that the m-xylyl staples enhance AID helix formation are structurally compatible with native-like AID:Ca V β interactions and reduce the entropic penalty associated with AID binding to Ca V β. Importantly, electrophysiological studies reveal that stapled AID peptides act as effective inhibitors of the Ca V α 1 :Ca V β interaction that modulate Ca V function in an Ca V β isoform-selective manner. Together, our studies provide a proof-of-concept demonstration of the use of protein-protein interaction inhibitors to control VGIC function and point to strategies for improved AID-based Ca V modulator design.

Introduction: Phosphatidylinositol-4,5-bisphosphate (PIP2) is a cofactor necessary for the activity of KCNQ1 channels. Some Long QT mutations of KCNQ1, including R243H, R539W and R555C have been shown to decrease KCNQ1 interaction with PIP2. A previous study suggested that R539W is paradoxically less sensitive to intracellular magnesium inhibition than the WT channel, despite a decreased interaction with PIP2. In the present study, we confirm this peculiar behavior of R539W and suggest a molecular mechanism underlying it., Methods and Results: COS-7 cells were transfected with WT or mutated KCNE1-KCNQ1 channel, and patch-clamp recordings were performed in giant-patch, permeabilized-patch or ruptured-patch configuration. Similar to other channels with a decreased PIP2 affinity, we observed that the R243H and R555C mutations lead to an accelerated current rundown when membrane PIP2 levels are decreasing. As opposed to R243H and R555C mutants, R539W is not more but rather less sensitive to PIP2 decrease than the WT channel. A molecular model of a fragment of the KCNQ1 C-terminus and the membrane bilayer suggested that a potential novel interaction of R539W with cholesterol stabilizes the channel opening and hence prevents rundown upon PIP2 depletion. We then carried out the same rundown experiments under cholesterol depletion and observed an accelerated R539W rundown that is consistent with this model., Conclusions: We show for the first time that a mutation may shift the channel interaction with PIP2 to a preference for cholesterol. This de novo interaction wanes the sensitivity to PIP2 variations, showing that a mutated channel with a decreased affinity to PIP2 could paradoxically present a slowed current rundown compared to the WT channel. This suggests that caution is required when using measurements of current rundown as an indicator to compare WT and mutant channel PIP2 sensitivity.

Voltage-gated sodium channels (NaVs) are central elements of cellular excitation. Notwithstanding advances from recent bacterial NaV (BacNaV) structures, key questions about gating and ion selectivity remain. Here, we present a closed conformation of NaVAe1p, a pore-only BacNaV derived from NaVAe1, a BacNaV from the arsenite oxidizer Alkalilimnicola ehrlichei found in Mono Lake, California, that provides insight into both fundamental properties. The structure reveals a pore domain in which the pore-lining S6 helix connects to a helical cytoplasmic tail. Electrophysiological studies of full-length BacNaVs show that two elements defined by the NaVAe1p structure, an S6 activation gate position and the cytoplasmic tail "neck", are central to BacNaV gating. The structure also reveals the selectivity filter ion entry site, termed the "outer ion" site. Comparison with mammalian voltage-gated calcium channel (CaV) selectivity filters, together with functional studies, shows that this site forms a previously unknown determinant of CaV high-affinity calcium binding. Our findings underscore commonalities between BacNaVs and eukaryotic voltage-gated channels and provide a framework for understanding gating and ion permeation in this superfamily., (© 2013. Published by Elsevier Ltd. All rights reserved.)

Phosphatidylinositol (4,5)-bisphosphate (PIP(2)) is a phospholipid of the plasma membrane that has been shown to be a key regulator of several ion channels. Functional studies and more recently structural studies of Kir channels have revealed the major impact of PIP(2) on the open state stabilization. A similar effect of PIP(2) on the delayed rectifiers Kv7.1 and Kv11.1, two voltage-gated K(+) channels, has been suggested, but the molecular mechanism remains elusive and nothing is known on PIP(2) effect on other Kv such as those of the Shaker family. By combining giant-patch ionic and gating current recordings in COS-7 cells, and voltage-clamp fluorimetry in Xenopus oocytes, both heterologously expressing the voltage-dependent Shaker channel, we show that PIP(2) exerts 1) a gain-of-function effect on the maximal current amplitude, consistent with a stabilization of the open state and 2) a loss-of-function effect by positive-shifting the activation voltage dependence, most likely through a direct effect on the voltage sensor movement, as illustrated by molecular dynamics simulations.

Voltage-gated potassium (Kv) channels are tetramers, each subunit presenting six transmembrane segments (S1-S6), with each S1-S4 segments forming a voltage-sensing domain (VSD) and the four S5-S6 forming both the conduction pathway and its gate. S4 segments control the opening of the intracellular activation gate in response to changes in membrane potential. Crystal structures of several voltage-gated ion channels in combination with biophysical and mutagenesis studies highlighted the critical role of the S4-S5 linker (S4S5(L)) and of the S6 C-terminal part (S6(T)) in the coupling between the VSD and the activation gate. Several mechanisms have been proposed to describe the coupling at a molecular scale. This review summarizes the mechanisms suggested for various voltage-gated ion channels, including a mechanism that we described for KCNQ1, in which S4S5(L) is acting like a ligand binding to S6(T) to stabilize the channel in a closed state. As discussed in this review, this mechanism may explain the reverse response to depolarization in HCN-like channels. As opposed to S4S5(L), the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)), stabilizes KCNQ1 channel in an open state. Many other ion channels (not only voltage-gated) require PIP(2) to function properly, confirming its crucial importance as an ion channel cofactor. This is highlighted in cases in which an altered regulation of ion channels by PIP(2) leads to channelopathies, as observed for KCNQ1. This review summarizes the state of the art on the two regulatory mechanisms that are critical for KCNQ1 and other voltage-gated channels function (PIP(2) and S4S5(L)), and assesses their potential physiological and pathophysiological roles.

Voltage-dependent potassium (Kv) channels are tetramers of six transmembrane domain (S1-S6) proteins. Crystallographic data demonstrate that the tetrameric pore (S5-S6) is surrounded by four voltage sensor domains (S1-S4). One key question remains: how do voltage sensors (S4) regulate pore gating? Previous mutagenesis data obtained on the Kv channel KCNQ1 highlighted the critical role of specific residues in both the S4-S5 linker (S4S5(L)) and S6 C terminus (S6(T)). From these data, we hypothesized that S4S5(L) behaves like a ligand specifically interacting with S6(T) and stabilizing the closed state. To test this hypothesis, we designed plasmid-encoded peptides corresponding to portions of S4S5(L) and S6(T) of the voltage-gated potassium channel KCNQ1 and evaluated their effects on the channel activity in the presence and absence of the ancillary subunit KCNE1. We showed that S4S5(L) peptides inhibit KCNQ1, in a reversible and state-dependent manner. S4S5(L) peptides also inhibited a voltage-independent KCNQ1 mutant. This inhibition was competitively prevented by a peptide mimicking S6(T), consistent with S4S5(L) binding to S6(T). Val(254) in S4S5(L) is known to contact Leu(353) in S6(T) when the channel is closed, and mutations of these residues alter the coupling between the two regions. The same mutations introduced in peptides altered their effects, further confirming S4S5(L) binding to S6(T). Our results suggest a mechanistic model in which S4S5(L) acts as a voltage-dependent ligand bound to its receptor on S6 at rest. This interaction locks the channel in a closed state. Upon plasma membrane depolarization, S4 pulls S4S5(L) away from S6(T), allowing channel opening.