Molecular template for a voltage sensor in a novel K+ channel. III. Functional reconstitution of a sensorless pore module from a prokaryotic Kv channel

Subconductance states in a semimicroscopic model for a tetrameric pore

A physical model for a structured tetrameric pore is studied. The pore is modeled as a device composed of four subunits, each one exhibiting two possible states (open and closed). The pore is located within a membrane that separates two reservoirs with ionic solutions. All variables of the model follow physical dynamical equations accounting for the internal structure of the pore, derived from a single energy functional and supplemented with thermal noises. An extensive study of the resulting ionic intensity is performed for different values of the control parameters, mainly membrane potential and reservoir ion concentrations. Two possible physical devices are studied: voltage-gated (including a voltage sensor in each subunit) and non-voltage-gated pores. The ionic flux through the pore exhibits several distinct dynamical configurations, in particular subconductance states, which indicate very different dynamical internal states of the subunits. Such subconductance states become much easier to observe in sensorless pores. These results are compared with available experimental data on tetrameric K channels and analytical predictions.

Identification and functional characterization of the Piezo1 channel pore domain

Mechanotransduction is the process by which cells convert physical forces into electrochemical responses. On a molecular scale, these forces are detected by mechanically activated ion channels, which constitute the basis for hearing, touch, pain, cold, and heat sensation, among other physiological processes. Exciting high-resolution structural details of these channels are currently emerging that will eventually allow us to delineate the molecular determinants of gating and ion permeation. However, our structural-functional understanding across the family remains limited. Piezo1 is one of the largest and least understood of these channels, with various structurally identified features within its trimeric assembly. This study seeks to determine the modularity and function of Piezo1 channels by constructing deletion proteins guided by cryo EM structural knowledge. Our comprehensive functional study identified, for the first time, the minimal amino acid sequence of the full-length Piezo1 that can fold and function as the channel's pore domain between E2172 and the last residue E2547. While the addition of an anchor region has no effect on permeation properties. The Piezo1 pore domain is not pressure-sensitive and the appending of Piezo Repeat-A did not restore pressure-dependent gating, hence the sensing module must exist between residues 1 to 1952. Our efforts delineating the permeation and gating regions within this complex ion channel have implications in identifying small molecules that exclusively regulate the activity of the channel's pore module to influence mechanotransduction and downstream processes.

Atomistic Insights of Calmodulin Gating of Complete Ion Channels

Intracellular calcium is essential for many physiological processes, from neuronal signaling and exocytosis to muscle contraction and bone formation. Ca²⁺ signaling from the extracellular medium depends both on membrane potential, especially controlled by ion channels selective to K⁺, and direct permeation of this cation through specialized channels. Calmodulin (CaM), through direct binding to these proteins, participates in setting the membrane potential and the overall permeability to Ca²⁺. Over the past years many structures of complete channels in complex with CaM at near atomic resolution have been resolved. In combination with mutagenesis-function, structural information of individual domains and functional studies, different mechanisms employed by CaM to control channel gating are starting to be understood at atomic detail. Here, new insights regarding four types of tetrameric channels with six transmembrane (6TM) architecture, Eag1, SK2/SK4, TRPV5/TRPV6 and KCNQ1–5, and its regulation by CaM are described structurally. Different CaM regions, N-lobe, C-lobe and EF3/EF4-linker play prominent signaling roles in different complexes, emerging the realization of crucial non-canonical interactions between CaM and its target that are only evidenced in the full-channel structure. Different mechanisms to control gating are used, including direct and indirect mechanical actuation over the pore, allosteric control, indirect effect through lipid binding, as well as direct plugging of the pore. Although each CaM lobe engages through apparently similar alpha-helices, they do so using different docking strategies. We discuss how this allows selective action of drugs with great therapeutic potential.

Extended beta distributions open the access to fast gating in bilayer experiments-assigning the voltage-dependent gating to the selectivity filter

Lipid bilayers provide many benefits for ion channel recordings, such as control of membrane composition and transport molecules. However, they suffer from high membrane capacitance limiting the bandwidth and impeding analysis of fast gating. This can be overcome by fitting the deviations of the open-channel noise from the baseline noise by extended beta distributions. We demonstrate this analysis step-by-step by applying it to the example of viral K+  channels (Kcv), from the choice of the gating model through the fitting process, validation of the results, and what kinds of results can be obtained. These voltage sensor-less channels show profoundly voltage-dependent gating with dwell times in the closed state of about 50 μs. Mutations assign it to the selectivity filter.

A new mechanism of voltage-dependent gating exposed by KV10.1 channels interrupted between voltage sensor and pore

A linker that connects the voltage-sensing domain and pore domain in voltage-gated K⁺ channels is thought to provide coupling during gating, but this view has been challenged in KCNH channels. Tomczak et al. investigate gating in K_V10.1 channels with disrupted linkers and reveal multiple mechanisms.

Voltage-gated ion channels couple transmembrane potential changes to ion flow. Conformational changes in the voltage-sensing domain (VSD) of the channel are thought to be transmitted to the pore domain (PD) through an α-helical linker between them (S4–S5 linker). However, our recent work on channels disrupted in the S4–S5 linker has challenged this interpretation for the KCNH family. Furthermore, a recent single-particle cryo-electron microscopy structure of K_V10.1 revealed that the S4–S5 linker is a short loop in this KCNH family member, confirming the need for an alternative gating model. Here we use “split” channels made by expression of VSD and PD as separate fragments to investigate the mechanism of gating in K_V10.1. We find that disruption of the covalent connection within the S4 helix compromises the ability of channels to close at negative voltage, whereas disconnecting the S4–S5 linker from S5 slows down activation and deactivation kinetics. Surprisingly, voltage-clamp fluorometry and MTS accessibility assays show that the motion of the S4 voltage sensor is virtually unaffected when VSD and PD are not covalently bound. Finally, experiments using constitutively open PD mutants suggest that the presence of the VSD is structurally important for the conducting conformation of the pore. Collectively, our observations offer partial support to the gating model that assumes that an inward motion of the C-terminal S4 helix, rather than the S4–S5 linker, closes the channel gate, while also suggesting that control of the pore by the voltage sensor involves more than one mechanism.

hERG S4-S5 linker acts as a voltage-dependent ligand that binds to the activation gate and locks it in a closed state

Delayed-rectifier potassium channels (hERG and KCNQ1) play a major role in cardiac repolarization. These channels are formed by a tetrameric pore (S5–S6) surrounded by four voltage sensor domains (S1-S4). Coupling between voltage sensor domains and the pore activation gate is critical for channel voltage-dependence. However, molecular mechanisms remain elusive. Herein, we demonstrate that covalently binding, through a disulfide bridge, a peptide mimicking the S4-S5 linker (S4-S5_L) to the channel S6 C-terminus (S6_T) completely inhibits hERG. This shows that channel S4-S5_L is sufficient to stabilize the pore activation gate in its closed state. Conversely, covalently binding a peptide mimicking S6_T to the channel S4-S5_L prevents its inhibiting effect and renders the channel almost completely voltage-independent. This shows that the channel S4-S5_L is necessary to stabilize the activation gate in its closed state. Altogether, our results provide chemical evidence that S4-S5_L acts as a voltage-controlled ligand that binds S6_T to lock the channel in a closed state, elucidating the coupling between voltage sensors and the gate in delayed rectifier potassium channels and potentially other voltage-gated channels.

Isolated pores dissected from human two-pore channel 2 are functional

Multi-domain voltage-gated ion channels appear to have evolved through sequential rounds of intragenic duplication from a primordial one-domain precursor. Whereas modularity within one-domain symmetrical channels is established, little is known about the roles of individual regions within more complex asymmetrical channels where the domains have undergone substantial divergence. Here we isolated and characterised both of the divergent pore regions from human TPC2, a two-domain channel that holds a key intermediate position in the evolution of voltage-gated ion channels. In HeLa cells, each pore localised to the ER and caused Ca²⁺ depletion, whereas an ER-targeted pore mutated at a residue that inactivates full-length TPC2 did not. Additionally, one of the pores expressed at high levels in E. coli. When purified, it formed a stable, folded tetramer. Liposomes reconstituted with the pore supported Ca²⁺ and Na⁺ uptake that was inhibited by known blockers of full-length channels. Computational modelling of the pore corroborated cationic permeability and drug interaction. Therefore, despite divergence, both pores are constitutively active in the absence of their partners and retain several properties of the wild-type pore. Such symmetrical ‘pore-only’ proteins derived from divergent channel domains may therefore provide tractable tools for probing the functional architecture of complex ion channels.

Voltage-Dependent Gating: Novel Insights from KCNQ1 Channels

Gating of voltage-dependent cation channels involves three general molecular processes: voltage sensor activation, sensor-pore coupling, and pore opening. KCNQ1 is a voltage-gated potassium (Kv) channel whose distinctive properties have provided novel insights on fundamental principles of voltage-dependent gating. 1) Similar to other Kv channels, KCNQ1 voltage sensor activation undergoes two resolvable steps; but, unique to KCNQ1, the pore opens at both the intermediate and activated state of voltage sensor activation. The voltage sensor-pore coupling differs in the intermediate-open and the activated-open states, resulting in changes of open pore properties during voltage sensor activation. 2) The voltage sensor-pore coupling and pore opening require the membrane lipid PIP2 and intracellular ATP, respectively, as cofactors, thus voltage-dependent gating is dependent on multiple stimuli, including the binding of intracellular signaling molecules. These mechanisms underlie the extraordinary KCNE1 subunit modification of the KCNQ1 channel and have significant physiological implications.

The Sensorless Pore Module of Voltage-gated K+ Channel Family 7 Embodies the Target Site for the Anticonvulsant Retigabine

KCNQ (voltage-gated K(+) channel family 7 (Kv7)) channels control cellular excitability and underlie the K(+) current sensitive to muscarinic receptor signaling (the M current) in sympathetic neurons. Here we show that the novel anti-epileptic drug retigabine (RTG) modulates channel function of pore-only modules (PMs) of the human Kv7.2 and Kv7.3 homomeric channels and of Kv7.2/3 heteromeric channels by prolonging the residence time in the open state. In addition, the Kv7 channel PMs are shown to recapitulate the single-channel permeation and pharmacological specificity characteristics of the corresponding full-length proteins in their native cellular context. A mutation (W265L) in the reconstituted Kv7.3 PM renders the channel insensitive to RTG and favors the conductive conformation of the PM, in agreement to what is observed when the Kv7.3 mutant is heterologously expressed. On the basis of the new findings and homology models of the closed and open conformations of the Kv7.3 PM, we propose a structural mechanism for the gating of the Kv7.3 PM and for the site of action of RTG as a Kv7.2/Kv7.3 K(+) current activator. The results validate the modular design of human Kv channels and highlight the PM as a high-fidelity target for drug screening of Kv channels.

PIP2 regulation of KCNQ channels: biophysical and molecular mechanisms for lipid modulation of voltage-dependent gating

Voltage-gated potassium (Kv) channels contain voltage-sensing (VSD) and pore-gate (PGD) structural domains. During voltage-dependent gating, conformational changes in the two domains are coupled giving rise to voltage-dependent opening of the channel. In addition to membrane voltage, KCNQ (Kv7) channel opening requires the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂). Recent studies suggest that PIP₂ serves as a cofactor to mediate VSD-PGD coupling in KCNQ1 channels. In this review, we put these findings in the context of the current understanding of voltage-dependent gating, lipid modulation of Kv channel activation, and PIP₂-regulation of KCNQ channels. We suggest that lipid-mediated coupling of functional domains is a common mechanism among KCNQ channels that may be applicable to other Kv channels and membrane proteins.

Conduits of life's spark: a perspective on ion channel research since the birth of neuron

Heartbeats, muscle twitches, and lightning-fast thoughts are all manifestations of bioelectricity and rely on the activity of a class of membrane proteins known as ion channels. The basic function of an ion channel can be distilled into, "The hole opens. Ions go through. The hole closes." Studies of the fundamental mechanisms by which this process happens and the consequences of such activity in the setting of excitable cells remains the central focus of much of the field. One might wonder after so many years of detailed poking at such a seemingly simple process, is there anything left to learn?

Lipid bilayer modules as determinants of K+ channel gating

The crystal structure of the sensorless pore module of a voltage-gated K(+) (Kv) channel showed that lipids occupy a crevice between subunits. We asked if individual lipid monolayers of the bilayer embody independent modules linked to channel gating modulation. Functional studies using single channel current recordings of the sensorless pore module reconstituted in symmetric and asymmetric lipid bilayers allowed us to establish the deterministic role of lipid headgroup on gating. We discovered that individual monolayers with headgroups that coat the bilayer-aqueous interface with hydroxyls stabilize the channel open conformation. The hydroxyl need not be at a terminal position and the effect is not dependent on the presence of phosphate or net charge on the lipid headgroup. Asymmetric lipid bilayers allowed us to determine that phosphoglycerides with glycerol or inositol on the extracellular facing monolayer stabilize the open conformation of the channel. This indirect effect is attributed to a change in water structure at the membrane interface. By contrast, inclusion of the positively charged lysyl-dioleoyl-phosphatidylglycerol exclusively on the cytoplasmic facing monolayer of the bilayer increases drastically the probability of finding the channel open. Such modulation is mediated by a π-cation interaction between Phe-19 of the pore module and the lysyl moiety anchored to the phosphatidylglycerol headgroup. The new findings imply that the specific chemistry of the lipid headgroup and its selective location in either monolayer of the bilayer dictate the stability of the open conformation of a Kv pore module in the absence of voltage-sensing modules.

The voltage-sensing domain of a phosphatase gates the pore of a potassium channel

The modular architecture of voltage-gated K(+) (Kv) channels suggests that they resulted from the fusion of a voltage-sensing domain (VSD) to a pore module. Here, we show that the VSD of Ciona intestinalis phosphatase (Ci-VSP) fused to the viral channel Kcv creates Kv(Synth1), a functional voltage-gated, outwardly rectifying K(+) channel. Kv(Synth1) displays the summed features of its individual components: pore properties of Kcv (selectivity and filter gating) and voltage dependence of Ci-VSP (V(1/2) = +56 mV; z of ~1), including the depolarization-induced mode shift. The degree of outward rectification of the channel is critically dependent on the length of the linker more than on its amino acid composition. This highlights a mechanistic role of the linker in transmitting the movement of the sensor to the pore and shows that electromechanical coupling can occur without coevolution of the two domains.

Potassium ion channels: could they have evolved from viruses?

Phylogenetic analyses of small viral K+ channels suggests that they did not originate from their hosts, but instead could be the source of the postulated pore precursor in the evolution of K+ channels.

Functional characterization of the cardiac ryanodine receptor pore-forming region

Ryanodine receptors are homotetrameric intracellular calcium release channels. The efficiency of these channels is underpinned by exceptional rates of cation translocation through the open channel and this is achieved at the expense of the high degree of selectivity characteristic of many other types of channel. Crystallization of prokaryotic potassium channels has provided insights into the structures and mechanisms responsible for ion selection and movement in these channels, however no equivalent structural detail is currently available for ryanodine receptors. Nevertheless both molecular modeling and cryo-electron microscopy have identified the probable pore-forming region (PFR) of the ryanodine receptor (RyR) and suggest that this region contains structural elements equivalent to those of the PFRs of potassium-selective channels. The aim of the current study was to establish if the isolated putative cardiac RyR (RyR2) PFR could form a functional ion channel. We have expressed and purified the RyR2 PFR and shown that function is retained following reconstitution into planar phospholipid bilayers. Our data provide the first direct experimental evidence to support the proposal that the conduction pathway of RyR2 is formed by structural elements equivalent to those of the potassium channel PFR.

Stabilization of the conductive conformation of a voltage-gated K+ (Kv) channel: the lid mechanism

Voltage-gated K(+) (Kv) channels are molecular switches that sense membrane potential and in response open to allow K(+) ions to diffuse out of the cell. In these proteins, sensor and pore belong to two distinct structural modules. We previously showed that the pore module alone is a robust yet dynamic structural unit in lipid membranes and that it senses potential and gates open to conduct K(+) with unchanged fidelity. The implication is that the voltage sensitivity of K(+) channels is not solely encoded in the sensor. Given that the coupling between sensor and pore remains elusive, we asked whether it is then possible to convert a pore module characterized by brief openings into a conductor with a prolonged lifetime in the open state. The strategy involves selected probes targeted to the filter gate of the channel aiming to modulate the probability of the channel being open assayed by single channel recordings from the sensorless pore module reconstituted in lipid bilayers. Here we show that the premature closing of the pore is bypassed by association of the filter gate with two novel open conformation stabilizers: an antidepressant and a peptide toxin known to act selectively on Kv channels. Such stabilization of the conductive conformation of the channel is faithfully mimicked by the covalent attachment of fluorescein at a cysteine residue selectively introduced near the filter gate. This modulation prolongs the occupancy of permeant ions at the gate. It is this longer embrace between ion and gate that we conjecture underlies the observed stabilization of the conductive conformation. This study provides a new way of thinking about gating.

Crystal structure of a voltage-gated K+ channel pore module in a closed state in lipid membranes

Voltage-gated K(+) channels underlie the electrical excitability of cells. Each subunit of the functional tetramer consists of the tandem fusion of two modules, an N-terminal voltage-sensor and a C-terminal pore. To investigate how sensor coupling to the pore generates voltage-dependent channel opening, we solved the crystal structure and characterized the function of a voltage-gated K(+) channel pore in a lipid membrane. The structure of a functional channel in a membrane environment at 3.1 Å resolution establishes an unprecedented connection between channel structure and function. The structure is unique in delineating an ion-occupied ready to conduct selectivity filter, a confined aqueous cavity, and a closed activation gate, embodying a dynamic entity trapped in an unstable closed state.

Tetrameric assembly of KvLm K+ channels with defined numbers of voltage sensors

Voltage-gated K(+) (Kv) channels are tetrameric assemblies in which each modular subunit consists of a voltage sensor and a pore domain. KvLm, the voltage-gated K(+) channel from Listeria monocytogenes, differs from other Kv channels in that its voltage sensor contains only three out of the eight charged residues previously implicated in voltage gating. Here, we ask how many sensors are required to produce a functional Kv channel by investigating heterotetramers comprising combinations of full-length KvLm (FL) and its sensorless pore module. KvLm heterotetramers were produced by cell-free expression, purified by electrophoresis, and shown to yield functional channels after reconstitution in droplet interface bilayers. We studied the properties of KvLm channels with zero, one, two, three, and four voltage sensors. Three sensors suffice to promote channel opening with FL(4)-like voltage dependence at depolarizing potentials, but all four sensors are required to keep the channel closed during membrane hyperpolarization.

Piezo proteins are pore-forming subunits of mechanically activated channels

Mechanotransduction has an important role in physiology. Biological processes including sensing touch and sound waves require as-yet-unidentified cation channels that detect pressure. Mouse Piezo1 (MmPiezo1) and MmPiezo2 (also called Fam38a and Fam38b, respectively) induce mechanically activated cationic currents in cells; however, it is unknown whether Piezo proteins are pore-forming ion channels or modulate ion channels. Here we show that Drosophila melanogaster Piezo (DmPiezo, also called CG8486) also induces mechanically activated currents in cells, but through channels with remarkably distinct pore properties including sensitivity to the pore blocker ruthenium red and single channel conductances. MmPiezo1 assembles as a ∼1.2-million-dalton homo-oligomer, with no evidence of other proteins in this complex. Purified MmPiezo1 reconstituted into asymmetric lipid bilayers and liposomes forms ruthenium-red-sensitive ion channels. These data demonstrate that Piezo proteins are an evolutionarily conserved ion channel family involved in mechanotransduction.

Voltage-gated sodium channel (NaV) protein dissection creates a set of functional pore-only proteins

Many voltage-gated ion channel (VGIC) superfamily members contain six-transmembrane segments in which the first four form a voltage-sensing domain (VSD) and the last two form the pore domain (PD). Studies of potassium channels from the VGIC superfamily together with identification of voltage-sensor only proteins have suggested that the VSD and the PD can fold independently. Whether such transmembrane modularity is common to other VGIC superfamily members has remained untested. Here we show, using protein dissection, that the Silicibacter pomeroyi voltage-gated sodium channel (Na(V)Sp1) PD forms a stand-alone, ion selective pore (Na(V)Sp1p) that is tetrameric, α-helical, and that forms functional, sodium-selective channels when reconstituted into lipid bilayers. Mutation of the Na(V)Sp1p selectivity filter from LESWSM to LDDWSD, a change similar to that previously shown to alter ion selectivity of the bacterial sodium channel Na(V)Bh1 (NaChBac), creates a calcium-selective pore-only channel, Ca(V)Sp1p. We further show that production of PDs can be generalized by making pore-only proteins from two other extremophile Na(V)s: one from the hydrocarbon degrader Alcanivorax borkumensis (Na(V)Ab1p), and one from the arsenite oxidizer Alkalilimnicola ehrlichei (Na(V)Ae1p). Together, our data establish a family of active pore-only ion channels that should be excellent model systems for study of the factors that govern both sodium and calcium selectivity and permeability. Further, our findings suggest that similar dissection approaches may be applicable to a wide range of VGICs and, thus, serve as a means to simplify and accelerate biophysical, structural, and drug development efforts.

An engineered right-handed coiled coil domain imparts extreme thermostability to the KcsA channel

KcsA, a potassium channel from Streptomyces lividans, was the first ion channel to have its transmembrane domain structure determined by crystallography. Previously we have shown that its C-terminal cytoplasmic domain is crucial for the thermostability and the expression of the channel. Expression was almost abolished in its absence, but could be rescued by the presence of an artificial left-handed coiled coil tetramerization domain GCN4. In this study, we noticed that the handedness of GCN4 is not the same as the bundle crossing of KcsA. Therefore, a compatible right-handed coiled coil structure was identified from the Protein Data Bank and used to replace the C-terminal domain of KcsA. The hybrid channel exhibited a higher expression level than the wild-type and is extremely thermostable. Surprisingly, this stable hybrid channel is equally active as the wild-type channel in conducting potassium ions through a lipid bilayer at an acidic pH. We suggest that a similar engineering strategy could be applied to other ion channels for both functional and structural studies.