Simplified bacterial "pore" channel provides insight into the assembly, stability, and structure of sodium channels

Quaternary structure independent folding of voltage-gated ion channel pore domain subunits

Every voltage-gated ion channel (VGIC) has a pore domain (PD) made from four subunits, each comprising an antiparallel transmembrane helix pair bridged by a loop. The extent to which PD subunit structure requires quaternary interactions is unclear. Here, we present crystal structures of a set of bacterial voltage-gated sodium channel (BacNa_V) 'pore only' proteins that reveal a surprising collection of non-canonical quaternary arrangements in which the PD tertiary structure is maintained. This context-independent structural robustness, supported by molecular dynamics simulations, indicates that VGIC-PD tertiary structure is independent of quaternary interactions. This fold occurs throughout the VGIC superfamily and in diverse transmembrane and soluble proteins. Strikingly, characterization of PD subunit-binding Fabs indicates that non-canonical quaternary PD conformations can occur in full-length VGICs. Together, our data demonstrate that the VGIC-PD is an autonomously folded unit. This property has implications for VGIC biogenesis, understanding functional states, de novo channel design, and VGIC structural origins.

The T1-tetramerisation domain of Kv1.2 rescues expression and preserves function of a truncated NaChBac sodium channel

Cytoplasmic domains frequently promote functional assembly of multimeric ion channels. To investigate structural determinants of this process, we generated the ‘T1‐chimera’ construct of the NaChBac sodium channel by truncating its C‐terminal domain and splicing the T1‐tetramerisation domain of the Kv1.2 channel to the N terminus. Purified T1‐chimera channels were tetrameric, conducted Na⁺ when reconstituted into proteoliposomes, and were functionally blocked by the drug mibefradil. Both the T1‐chimera and full‐length NaChBac had comparable expression levels in the membrane, whereas a NaChBac mutant lacking a cytoplasmic domain had greatly reduced membrane expression. Our findings support a model whereby bringing the transmembrane regions into close proximity enables their tetramerisation. This phenomenon is found with other channels, and thus, our findings substantiate this as a common assembly mechanism.

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.

Sodium Binding Sites and Permeation Mechanism in the NaChBac Channel: A Molecular Dynamics Study

NaChBac was the first discovered bacterial sodium voltage-dependent channel, yet computational studies are still limited due to the lack of a crystal structure. In this work, a pore-only construct built using the NavMs template was investigated using unbiased molecular dynamics and metadynamics. The potential of mean force (PMF) from the unbiased run features four minima, three of which correspond to sites IN, CEN, and HFS discovered in NavAb. During the run, the selectivity filter (SF) is spontaneously occupied by two ions, and frequent access of a third one is often observed. In the innermost sites IN and CEN, Na⁺ is fully hydrated by six water molecules and occupies an on-axis position. In site HFS sodium interacts with a glutamate and a serine from the same subunit and is forced to adopt an off-axis placement. Metadynamics simulations biasing one and two ions show an energy barrier in the SF that prevents single-ion permeation. An analysis of the permeation mechanism was performed both computing minimum energy paths in the axial-axial PMF and through a combination of Markov state modeling and transition path theory. Both approaches reveal a knock-on mechanism involving at least two but possibly three ions. The currents predicted from the unbiased simulation using linear response theory are in excellent agreement with single-channel patch-clamp recordings.

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.

Lipid Regulation of Sodium Channels

The lipid landscapes of cellular membranes are complex and dynamic, are tissue dependent, and can change with the age and the development of a variety of diseases. Researchers are now gaining new appreciation for the regulation of ion channel proteins by the membrane lipids in which they are embedded. Thus, as membrane lipids change, for example, during the development of disease, it is likely that the ionic currents that conduct through the ion channels embedded in these membranes will also be altered. This chapter provides an overview of the complex regulation of prokaryotic and eukaryotic voltage-dependent sodium (Nav) channels by fatty acids, sterols, glycerophospholipids, sphingolipids, and cannabinoids. The impact of lipid regulation on channel gating kinetics, voltage-dependence, trafficking, toxin binding, and structure are explored for Nav channels that have been examined in heterologous expression systems, native tissue, and reconstituted into artificial membranes. Putative mechanisms for Nav regulation by lipids are also discussed.

Structure of the C-terminal domain of the prokaryotic sodium channel orthologue NsvBa

Crystallographic and electrophysiological studies have recently provided insight into the structure, function, and drug binding of prokaryotic sodium channels. These channels exhibit significant sequence identities, especially in their transmembrane regions, with human voltage-gated sodium channels. However, rather than being single polypeptides with four homologous domains, they are tetramers of single domain polypeptides, with a C-terminal domain (CTD) composed of an inter-subunit four helix coiled coil. The structures of the CTDs differ between orthologues. In NavBh and NavMs, the C-termini form a disordered region adjacent to the final transmembrane helix, followed by a coiled-coil region, as demonstrated by synchrotron radiation circular dichroism (SRCD) and double electron-electron resonance electron paramagnetic resonance spectroscopic measurements. In contrast, in the crystal structure of the NavAe orthologue, the entire C-terminus is comprised of a helical region followed by a coiled coil. In this study, we have examined the CTD of the NsvBa from Bacillus alcalophilus, which unlike other orthologues is predicted by different methods to have different types of structures: either a disordered region adjacent to the transmembrane region, followed by a helical coiled coil, or a fully helical CTD. To discriminate between the two possible structures, we have used SRCD spectroscopy to experimentally determine the secondary structure of the C-terminus of this orthologue and used the results as the basis for modeling the open and closed conformations of the channel.

Molecular basis of ion permeability in a voltage-gated sodium channel

Voltage‐gated sodium channels are essential for electrical signalling across cell membranes. They exhibit strong selectivities for sodium ions over other cations, enabling the finely tuned cascade of events associated with action potentials. This paper describes the ion permeability characteristics and the crystal structure of a prokaryotic sodium channel, showing for the first time the detailed locations of sodium ions in the selectivity filter of a sodium channel. Electrostatic calculations based on the structure are consistent with the relative cation permeability ratios (Na⁺ ≈ Li⁺ ≫ K⁺, Ca²⁺, Mg²⁺) measured for these channels. In an E178D selectivity filter mutant constructed to have altered ion selectivities, the sodium ion binding site nearest the extracellular side is missing. Unlike potassium ions in potassium channels, the sodium ions in these channels appear to be hydrated and are associated with side chains of the selectivity filter residues, rather than polypeptide backbones.

Characterization of the Prokaryotic Sodium Channel NavSp Pore with a Microfluidic Bilayer Platform

This paper describes the use of a newly-developed micro-chip bilayer platform to examine the electrophysiological properties of the prokaryotic voltage-gated sodium channel pore (Na(v)Sp) from Silicibacter pomeroyi. The platform allows up to 6 bilayers to be analysed simultaneously. Proteoliposomes were incorporated into suspended lipid bilayers formed within the microfluidic bilayer chips. The chips provide access to bilayers from either side, enabling the fast and controlled titration of compounds. Dose-dependent modulation of the opening probability by the channel blocking drug nifedipine was measured and its IC50 determined.

Cell-free expression of a functional pore-only sodium channel

Voltage-gated sodium channels participate in the propagation of action potentials in excitable cells. Eukaryotic Navs are pseudo homotetrameric polypeptides, comprising four repeats of six transmembrane segments (S1-S6). The first four segments form the voltage-sensing domain and S5 and S6 create the pore domain with the selectivity filter. Prokaryotic Navs resemble these characteristics, but are truly tetrameric. They can typically be efficiently synthesized in bacteria, but production in vitro with cell-free synthesis has not been demonstrated. Here we report the cell-free expression and purification of a prokaryotic tetrameric pore-only sodium channel. We produced milligram quantities of the functional channel protein as characterized by size-exclusion chromatography, infrared spectroscopy and electrophysiological recordings. Cell-free expression enables advanced site-directed labelling, post-translational modifications, and special solubilization schemes. This enables next-generation biophysical experiments to study the principle of sodium ion selectivity and transport in sodium channels.

Investigating the size and dynamics of voltage-gated sodium channel fenestrations

Eukaryotic voltage-gated sodium channels (VGSCs) are essential for the initiation and propagation of action potentials in electrically excitable cells, and are important pharmaceutical targets for the treatment of neurological disorders such as epilepsy, cardiac arrhythmias, and chronic pain. Evidence suggests that small, hydrophobic, VGSC-blocking drugs can gain access to binding residues within the central cavity of these channels by passing through lateral, lipid-filled “fenestrations” which run between the exterior of the protein and its central pore. Here, we use molecular dynamics simulations to investigate how the size and shape of fenestrations change over time in several bacterial VGSC models and a homology model of Nav1.4. We show that over the course of the simulations, the size of the fenestrations is primarily influenced by rapid protein motions, such as amino acid side-chain rotation, and highlight that differences between fenestration bottleneck-contributing residues are the primary cause of variations in fenestration size between the 6 bacterial models. In the eukaryotic channel model, 2 fenestrations are wide, but 2 are narrow due to differences in the amino acid sequence in the 4 domains. Lipid molecules are found to influence the size of the fenestrations by protruding acylchains into the fenestrations and displacing amino acid side-chains. Together, the results suggest that fenestrations provide viable pathways for small, flexible, hydrophobic drugs.

Selectivity filters and cysteine-rich extracellular loops in voltage-gated sodium, calcium, and NALCN channels

How nature discriminates sodium from calcium ions in eukaryotic channels has been difficult to resolve because they contain four homologous, but markedly different repeat domains. We glean clues from analyzing the changing pore region in sodium, calcium and NALCN channels, from single-cell eukaryotes to mammals. Alternative splicing in invertebrate homologs provides insights into different structural features underlying calcium and sodium selectivity. NALCN generates alternative ion selectivity with splicing that changes the high field strength (HFS) site at the narrowest level of the hourglass shaped pore where the selectivity filter is located. Alternative splicing creates NALCN isoforms, in which the HFS site has a ring of glutamates contributed by all four repeat domains (EEEE), or three glutamates and a lysine residue in the third (EEKE) or second (EKEE) position. Alternative splicing provides sodium and/or calcium selectivity in T-type channels with extracellular loops between S5 and P-helices (S5P) of different lengths that contain three or five cysteines. All eukaryotic channels have a set of eight core cysteines in extracellular regions, but the T-type channels have an infusion of 4–12 extra cysteines in extracellular regions. The pattern of conservation suggests a possible pairing of long loops in Domains I and III, which are bridged with core cysteines in NALCN, Cav, and Nav channels, and pairing of shorter loops in Domains II and IV in T-type channel through disulfide bonds involving T-type specific cysteines. Extracellular turrets of increasing lengths in potassium channels (Kir2.2, hERG, and K2P1) contribute to a changing landscape above the pore selectivity filter that can limit drug access and serve as an ion pre-filter before ions reach the pore selectivity filter below. Pairing of extended loops likely contributes to the large extracellular appendage as seen in single particle electron cryo-microscopy images of the eel Na_v1 channel.

Structural model of the open-closed-inactivated cycle of prokaryotic voltage-gated sodium channels

In excitable cells, the initiation of the action potential results from the opening of voltage-gated sodium channels. These channels undergo a series of conformational changes between open, closed, and inactivated states. Many models have been proposed for the structural transitions that result in these different functional states. Here, we compare the crystal structures of prokaryotic sodium channels captured in the different conformational forms and use them as the basis for examining molecular models for the activation, slow inactivation, and recovery processes. We compare structural similarities and differences in the pore domains, specifically in the transmembrane helices, the constrictions within the pore cavity, the activation gate at the cytoplasmic end of the last transmembrane helix, the C-terminal domain, and the selectivity filter. We discuss the observed differences in the context of previous models for opening, closing, and inactivation, and present a new structure-based model for the functional transitions. Our proposed prokaryotic channel activation mechanism is then compared with the activation transition in eukaryotic sodium channels.

Bacterial voltage-gated sodium channels (BacNa(V)s) from the soil, sea, and salt lakes enlighten molecular mechanisms of electrical signaling and pharmacology in the brain and heart

Voltage-gated sodium channels (Na(V)s) provide the initial electrical signal that drives action potential generation in many excitable cells of the brain, heart, and nervous system. For more than 60years, functional studies of Na(V)s have occupied a central place in physiological and biophysical investigation of the molecular basis of excitability. Recently, structural studies of members of a large family of bacterial voltage-gated sodium channels (BacNa(V)s) prevalent in soil, marine, and salt lake environments that bear many of the core features of eukaryotic Na(V)s have reframed ideas for voltage-gated channel function, ion selectivity, and pharmacology. Here, we analyze the recent advances, unanswered questions, and potential of BacNa(V)s as templates for drug development efforts.

Locating the route of entry and binding sites of benzocaine and phenytoin in a bacterial voltage gated sodium channel

Sodium channel blockers are used to control electrical excitability in cells as a treatment for epileptic seizures and cardiac arrhythmia, and to provide short term control of pain. Development of the next generation of drugs that can selectively target one of the nine types of voltage-gated sodium channel expressed in the body requires a much better understanding of how current channel blockers work. Here we make use of the recently determined crystal structure of the bacterial voltage gated sodium channel NavAb in molecular dynamics simulations to elucidate the position at which the sodium channel blocking drugs benzocaine and phenytoin bind to the protein as well as to understand how these drugs find their way into resting channels. We show that both drugs have two likely binding sites in the pore characterised by nonspecific, hydrophobic interactions: one just above the activation gate, and one at the entrance to the the lateral lipid filled fenestrations. Three independent methods find the same sites and all suggest that binding to the activation gate is slightly more favourable than at the fenestration. Both drugs are found to be able to pass through the fenestrations into the lipid with only small energy barriers, suggesting that this can represent the long posited hydrophobic entrance route for neutral drugs. Our simulations highlight the importance of a number of residues in directing drugs into and through the fenestration, and in forming the drug binding sites.

The treatment of cardiac arrhythmia, epilepsy and pain usually involves blocking the protein channels responsible for initiating electrical activity in nerves and muscles. Current drugs block all such channels, but improved medication requires compounds that can differentiate between the channels present in different parts of the body. Achieving this goal calls for a better understanding of the interactions of current drugs with the proteins. Here we use computer simulation to understand where a local anesthetic and an anti-epileptic bind to a bacterial sodium channel and how they find their way to this position, helping to uncover ways to selectively target different human channels.

Role of the C-terminal domain in the structure and function of tetrameric sodium channels

Voltage-gated sodium channels have essential roles in electrical signalling. Prokaryotic sodium channels are tetramers consisting of transmembrane (TM) voltage-sensing and pore domains, and a cytoplasmic carboxy-terminal domain. Previous crystal structures of bacterial sodium channels revealed the nature of their TM domains but not their C-terminal domains (CTDs). Here, using electron paramagnetic resonance (EPR) spectroscopy combined with molecular dynamics, we show that the CTD of the NavMs channel from Magnetococcus marinus includes a flexible region linking the TM domains to a four-helix coiled-coil bundle. A 2.9 Å resolution crystal structure of the NavMs pore indicates the position of the CTD, which is consistent with the EPR-derived structure. Functional analyses demonstrate that the coiled-coil domain couples inactivation with channel opening, and is enabled by negatively charged residues in the linker region. A mechanism for gating is proposed based on the structure, whereby splaying of the bottom of the pore is possible without requiring unravelling of the coiled-coil.

Prokaryotic voltage-gated sodium channels possess dynamically disordered cytoplasmic C-terminal domains. Bagnéris et al. present the structure of the Magnetococcus marinus NavMs pore and C-terminal domain and reveal its role in coupling channel inactivation and opening.

Bacterial sodium channels: models for eukaryotic sodium and calcium channels

Eukaryotic sodium and calcium channels are made up of four linked homologous but different transmembrane domains. Bacteria express sodium channels comprised of four identical subunits, each being analogous to a single homologous domain of their eukaryotic counterparts. Key elements of primary structure are conserved between bacterial and eukaryotic sodium and calcium channels. The simple protein structure of the bacterial channels has allowed extensive structure-function probes of key regions as well as allowing determination of several X-ray crystallographic structures of these channels. The structures have revealed novel features of sodium and calcium channel pores and elucidated the structural importance of many of the conserved features of primary sequence. The structural information has also formed the basis for computational studies probing the basis for sodium and calcium selectivity and gating.

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?

Structure of a prokaryotic sodium channel pore reveals essential gating elements and an outer ion binding site common to eukaryotic channels

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.

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.

Differential lipid dependence of the function of bacterial sodium channels

The lipid bilayer is important for maintaining the integrity of cellular compartments and plays a vital role in providing the hydrophobic and charged interactions necessary for membrane protein structure, conformational flexibility and function. To directly assess the lipid dependence of activity for voltage-gated sodium channels, we compared the activity of three bacterial sodium channel homologues (NaChBac, NavMs, and NavSp) by cumulative (22)Na(+) uptake into proteoliposomes containing a 3∶1 ratio of 1-palmitoyl 2-oleoyl phosphatidylethanolamine and different "guest" glycerophospholipids. We observed a unique lipid profile for each channel tested. NavMs and NavSp showed strong preference for different negatively-charged lipids (phosphatidylinositol and phosphatidylglycerol, respectively), whilst NaChBac exhibited a more modest variation with lipid type. To investigate the molecular bases of these differences we used synchrotron radiation circular dichroism spectroscopy to compare structures in liposomes of different composition, and molecular modeling and electrostatics calculations to rationalize the functional differences seen. We then examined pore-only constructs (with voltage sensor subdomains removed) and found that in these channels the lipid specificity was drastically reduced, suggesting that the specific lipid influences on voltage-gated sodium channels arise primarily from their abilities to interact with the voltage-sensing subdomains.

Structure of a bacterial voltage-gated sodium channel pore reveals mechanisms of opening and closing

Sodium-gated ion channels open and close in response to the flow of ions. Here, McCusker et al. report the open structure of a sodium-gated ion channel pore from a bacterial homologue, and show, by comparison with the closed structure, that the movement of a C-terminal helix is sufficient to open the channel.

Voltage-gated sodium channels are vital membrane proteins essential for electrical signalling; in humans, they are key targets for the development of pharmaceutical drugs. Here we report the crystal structure of an open-channel conformation of NavMs, the bacterial channel pore from the marine bacterium Magnetococcus sp. (strain MC-1). It differs from the recently published crystal structure of a closed form of a related bacterial sodium channel (NavAb) by having its internal cavity accessible to the cytoplasmic surface as a result of a bend/rotation about a central residue in the carboxy-terminal transmembrane segment. This produces an open activation gate of sufficient diameter to allow hydrated sodium ions to pass through. Comparison of the open and closed structures provides new insight into the features of the functional states present in the activation cycles of sodium channels and the mechanism of channel opening and closing.

Domain organization of the ATP-sensitive potassium channel complex examined by fluorescence resonance energy transfer

K(ATP) channels link cell metabolism to excitability in many cells. They are formed as tetramers of Kir6.2 subunits, each associated with a SUR1 subunit. We used mutant GFP-based FRET to assess domain organization in channel complexes. Full-length Kir6.2 subunits were linked to YFP or cyan fluorescent protein (CFP) at N or C termini, and all such constructs, including double-tagged YFP-Kir6.2-CFP (Y6.2C), formed functional K(ATP) channels. In intact COSm6 cells, background emission of YFP excited by 430-nm light was ∼6%, but the Y6.2C construct expressed alone exhibited an apparent FRET efficiency of ∼25%, confirmed by trypsin digestion, with or without SUR1 co-expression. Similar FRET efficiency was detected in mixtures of CFP- and YFP-tagged full-length Kir6.2 subunits and transmembrane domain only constructs, when tagged at the C termini but not at the N termini. The FRET-reported Kir6.2 tetramer domain organization was qualitatively consistent with Kir channel crystal structures: C termini and M2 domains are centrally located relative to N termini and M1 domains, respectively. Additional FRET analyses were performed on cells in which tagged full-length Kir6.2 and tagged SUR1 constructs were co-expressed. These analyses further revealed that 1) NBD1 of SUR1 is closer to the C terminus of Kir6.2 than to the N terminus; 2) the Kir6.2 cytoplasmic domain is not essential for complexation with SUR1; and 3) the N-terminal half of SUR1 can complex with itself in the absence of either the C-terminal half or Kir6.2.

Opposite Effects of the S4-S5 Linker and PIP(2) on Voltage-Gated Channel Function: KCNQ1/KCNE1 and Other Channels

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.

Transmembrane and extramembrane contributions to membrane protein thermal stability: studies with the NaChBac sodium channel

The thermal stabilities of the extramembranous and transmembranous regions of the bacterial voltage-gated sodium channel NaChBac have been characterised using thermal-melt synchrotron radiation circular dichroism (SRCD) spectroscopy. A series of constructs, ranging from the full-length protein containing both the C-terminal cytoplasmic and the transmembranous domains, to proteins with decreasing amounts of the cytoplasmic domain, were examined in order to separately define the roles of these two types of domains in the stability and processes of unfolding of a membrane protein. The sensitivity of the SRCD measurements over a wide range of wavelengths and temperatures has meant that subtle but reproducible conformational changes could be detected with accuracy. The residues in the C-terminal extramembranous domain were highly susceptible to thermal denaturation, but for the most part the transmembrane residues were not thermally-labile and retained their helical character even at very elevated temperatures. The process of thermal unfolding involved an initial irreversible unfolding of the highly labile distal extramembranous C-terminal helical region, which was accompanied by a reversible unfolding of a small number of helical residues in the transmembrane domain. This was then followed by the irreversible unfolding of a limited number of additional transmembrane helical residues at greatly elevated temperatures. Hence this study has been able to determine the different contributions and roles of the transmembrane and extramembrane residues in the processes of thermal denaturation of this multipass integral membrane protein.

NaChBac: the long lost sodium channel ancestor

In excitable cells, the main mediators of sodium conductance across membranes are voltage-gated sodium channels (Na_Vs). Eukaryotic Na_Vs are essential elements in neuronal signaling and muscular contraction and in humans have been causally related to a variety of neurological and cardiovascular channelopathies. They are complex heavily glycosylated intrinsic membrane proteins present in only trace quantities that have proven to be challenging objects of study. However, in recent years, a number of simpler prokaryotic sodium channels have been identified, with NaChBac from Bacillus halodurans being the most well-characterized to date. The availability of a bacterial Na_V that is amenable to heterologous expression and functional characterization in both bacterial and mammalian systems has provided new opportunities for structure–function studies. This review describes features of NaChBac as an exemplar of this class of bacterial channels, compares prokaryotic and eukaryotic Na_Vs with respect to their structural organization, pharmacological profiling, and functional kinetics, and discusses how voltage-gated ion channels may have evolved to deal with the complex functional demands of higher organisms.

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.