Biophysical mechanisms of myocardium sodium channelopathies.

Genetic variants of gene SCN5A encoding the alpha-subunit of cardiac voltage-gated sodium channel Nav1.5 are associated with various diseases, including long QT syndrome (LQT3), Brugada syndrome (BrS1), and progressive cardiac conduction disease (PCCD). In the last decades, the great progress in understanding molecular and biophysical mechanisms of these diseases has been achieved. The LQT3 syndrome is associated with gain-of-function of sodium channels Nav1.5 due to impaired inactivation, enhanced activation, accelerated recovery from inactivation or the late current appearance. In contrast, BrS1 and PCCD are associated with the Nav1.5 loss-of-function, which in electrophysiological experiments can be manifested as reduced current density, enhanced fast or slow inactivation, impaired activation, or decelerated recovery from inactivation. Genetic variants associated with congenital arrhythmias can also disturb interactions of the Nav1.5 channel with different proteins or drugs and cause unexpected reactions to drug administration. Furthermore, mutations can affect post-translational modifications of the channels and their sensitivity to pH and temperature. Here we briefly review the current knowledge on biophysical mechanisms of LQT3, BrS1 and PCCD. We focus on limitations of studies that use heterologous expression systems and induced pluripotent stem cells (iPSC) derived cardiac myocytes and summarize our understanding of genotype-phenotype relations of SCN5A mutations.

In silico analysis of TRPM4 variants of unknown clinical significance.

BACKGROUND: TRPM4 is a calcium-activated channel that selectively permeates monovalent cations. Genetic variants of the channel in cardiomyocytes are associated with various heart disorders, such as progressive familial heart block and Brugada syndrome. About97% of all known TRPM4 missense variants are classified as variants of unknown clinical significance (VUSs). The very large number of VUSs is a serious problem in diagnostics and treatment of inherited heart diseases. METHODS AND RESULTS: We collected 233 benign or pathogenic missense variants in the superfamily of TRP channels from databases ClinVar, Humsavar and Ensembl Variation to compare performance of 22 algorithms that predict damaging variants. We found that ClinPred is the best-performing tool for TRP channels. We also used the paralogue annotation method to identify disease variants across the TRP family. In the set of 565 VUSs of hTRPM4, ClinPred predicted pathogenicity of 299 variants. Among these, 12 variants are also categorized as LP/P variants in at least one paralogue of hTRPM4. We further used the cryo-EM structure of hTRPM4 to find scores of contact pairs between parental (wild type) residues of VUSs for which ClinPred predicts a high probability of pathogenicity of variants for both contact partners. We propose that 68 respective missense VUSs are also likely pathogenic variants. CONCLUSIONS: ClinPred outperformed other in-silico tools in predicting damaging variants of TRP channels. ClinPred, the paralogue annotation method, and analysis of residue contacts the hTRPM4 cryo-EM structure collectively suggest pathogenicity of 80 TRPM4 VUSs.

Mechanisms of dihydropyridine agonists and antagonists in view of cryo-EM structures of calcium and sodium channels.

Opposite effects of 1,4-dihydropyridine (DHP) agonists and antagonists on the L-type calcium channels are a challenging problem. Cryo-EM structures visualized DHPs between the pore-lining helices S6III and S6IV in agreement with published mutational data. However, the channel conformations in the presence of DHP agonists and antagonists are virtually the same, and the mechanisms of the ligands' action remain unclear. We docked the DHP agonist S-Bay k 8644 and antagonist R-Bay k 8644 in Cav1.1 channel models with or without π-bulges in helices S6III and S6IV. Cryo-EM structures of the DHP-bound Cav1.1 channel show a π-bulge in helix S6III but not in S6IV. The antagonist's hydrophobic group fits into the hydrophobic pocket formed by residues in S6IV. The agonists' polar NO2 group is too small to fill up the pocket. A water molecule could sterically fit into the void space, but its contacts with isoleucine in helix S6IV (motif INLF) would be unfavorable. In a model with π-bulged S6IV, this isoleucine turns away from the DHP molecule and its position is occupied by the asparagine from the same motif INLF. The asparagine provides favorable contacts for the water molecule at the agonist's NO2 group but unfavorable contacts for the antagonist's methoxy group. In our models, the DHP antagonist stabilizes entirely α-helical S6IV. In contrast, the DHP agonist stabilizes π-bulged helix S6IV whose C-terminal part turned and rearranged the activation-gate region. This would stabilize the open channel. Thus, agonists, but not antagonists, would promote channel opening by stabilizing π-bulged helix S6IV.

Brevetoxin versus Brevenal Modulation of Human Nav1 Channels.

Brevetoxins (PbTx) and brevenal are marine ladder-frame polyethers. PbTx binds to and activates voltage-gated sodium (Nav) channels in native tissues, whereas brevenal antagonizes these actions. However, the effects of PbTx and brevenal on recombinant Nav channel function have not been systematically analyzed. In this study, the PbTx-3 and brevenal modulation of tissue-representative Nav channel subtypes Nav1.2, Nav1.4, Nav1.5, and Nav1.7 were examined using automated patch-clamp. While PbTx-3 and brevenal elicit concentration-dependent and subtype-specific modulatory effects, PbTx-3 is >1000-fold more potent than brevenal. Consistent with effects observed in native tissues, Nav1.2 and Nav1.4 channels were PbTx-3- and brevenal-sensitive, whereas Nav1.5 and Nav1.7 appeared resistant. Interestingly, the incorporation of brevenal in the intracellular solution caused Nav channels to become less sensitive to PbTx-3 actions. Furthermore, we generated a computational model of PbTx-2 bound to the lipid-exposed side of the interface between domains I and IV of Nav1.2. Our results are consistent with competitive antagonism between brevetoxins and brevenal, setting a basis for future mutational analyses of Nav channels' interaction with brevetoxins and brevenal. Our findings provide valuable insights into the functional modulation of Nav channels by brevetoxins and brevenal, which may have implications for the development of new Nav channel modulators with potential therapeutic applications.

Molecular Modeling of Cardiac Sodium Channel with Mexiletine.

A sodium channel blocker mexiletine (MEX) is used to treat chronic pain, myotonia and some arrhythmias. Mutations in the pore domain (PD) of voltage-gated sodium channels differently affect tonic block (TB) and use-dependent block (UDB) by MEX. Previous studies identified several MEX-sensing residues in the hNav1.5 channel and demonstrated that the channel block by MEX increases with activation of the voltage-sensing domain III (VSDIII), whereas MEX stabilizes the activated state of VSDIII. Structural rationales for these observations are unclear. Here, Monte Carlo (MC) energy minimizations were used to dock MEX and its more potent analog, Thio-Me2, into the hNav1.5 cryo-EM structure with activated VSDs and presumably inactivated PD. Computations yielded two ensembles of ligand binding poses in close contacts with known MEX-sensing residues in helices S6III, S6IV and P1IV. In both ensembles, the ligand NH3 group approached the cation-attractive site between backbone carbonyls at the outer-pore bottom, while the aromatic ring protruded ether into the inner pore (putative UDB pose) or into the III/IV fenestration (putative TB pose). In silico deactivation of VSDIII shifted helices S4-S5III, S5III, S6III and S6IV and tightened the TB site. In a model with activated VSDIII and three resting VSDs, MC-minimized energy profile of MEX pulled from the TB site towards lipids shows a deep local minimum due to interactions with 11 residues in S5III, P1III, S6III and S6IV. The minimum may correspond to an interim binding site for MEX in the hydrophobic path to the TB site along the lipid-exposed sides of repeats III and IV where 15 polar and aromatic residues would attract cationic blockers. The study explains numerous experimental data and suggests the mechanism of allosteric modification of the MEX binding site by VSDIII.

A lysine residue from an extracellular turret switches the ion preference in a Cav3 T-Type channel from calcium to sodium ions.

Cav3 T-type calcium channels from great pond snail Lymnaea stagnalis have a selectivity-filter ring of five acidic residues, EE(D)DD. Splice variants with exons 12b or 12a spanning the extracellular loop between the outer helix IIS5 and membrane-descending pore helix IIP1 (IIS5-P1) in Domain II of the pore module possess calcium selectivity or dominant sodium permeability, respectively. Here, we use AlphaFold2 neural network software to predict that a lysine residue in exon 12a is salt-bridged to the aspartate residue immediately C terminal to the second-domain glutamate in the selectivity filter. Exon 12b has a similar folding but with an alanine residue in place of lysine in exon 12a. We express LCav3 channels with mutated exons Ala-12b-Lys and Lys-12a-Ala and demonstrate that they switch the ion preference to high sodium permeability and calcium selectivity, respectively. We propose that in the calcium-selective variants, a calcium ion chelated between Domain II selectivity-filter glutamate and aspartate is knocked-out by the incoming calcium ion in the process of calcium permeation, whereas sodium ions are repelled. The aspartate is neutralized by the lysine residue in the sodium-permeant variants, allowing for sodium permeation through the selectivity-filter ring of four negatively charged residues akin to the prokaryotic sodium channels with four glutamates in the selectivity filter. The evolutionary adaptation in invertebrate LCav3 channels highlight the involvement of a key, ubiquitous aspartate, "a calcium beacon" of sorts in the outer pore of Domain II, as determinative for the calcium ion preference over sodium ions through eukaryotic Cav1, Cav2, and Cav3 channels.

Pyrethroids in an AlphaFold2 Model of the Insect Sodium Channel.

Pyrethroid insecticides stabilize the open state of insect sodium channels. Previous mutational, electrophysiological, and computational analyses led to the development of homology models predicting two pyrethroid receptor sites, PyR1 and PyR2. Many of the naturally occurring sodium channel mutations, which confer knockdown resistance (kdr) to pyrethroids, are located within or close to these receptor sites, indicating that these mutations impair pyrethroid binding. However, the mechanism of the state-dependent action of pyrethroids and the mechanisms by which kdr mutations beyond the receptor sites confer resistance remain unclear. Recent advances in protein structure prediction using the AlphaFold2 (AF2) neural network allowed us to generate a new model of the mosquito sodium channel AaNav1-1, with the activated voltage-sensing domains (VSMs) and the presumably inactivated pore domain (PM). We further employed Monte Carlo energy minimizations to open PM and deactivate VSM-I and VSM-II to generate additional models. The docking of a Type II pyrethroid deltamethrin in the models predicted its interactions with many known pyrethroid-sensing residues in the PyR1 and PyR2 sites and revealed ligand-channel interactions that stabilized the open PM and activated VSMs. Our study confirms the predicted two pyrethroid receptor sites, explains the state-dependent action of pyrethroids, and proposes the mechanisms of the allosteric effects of various kdr mutations on pyrethroid action. The AF2-based models may assist in the structure-based design of new insecticides.

Characterization of two kdr mutations at predicted pyrethroid receptor site 2 in the sodium channels of Aedes aegypti and Nilaparvata lugens.

Pyrethroid insecticides prolong the opening of insect sodium channels by binding to two predicted pyrethroid receptor sites (PyR), PyR1 and PyR2. Many naturally-occurring sodium channel mutations that confer pyrethroid resistance (known as knockdown resistance, kdr) are located at PyR1. Recent studies identified two new mutations, V253F and T267A, at PyR2, which co-exist with two well-known mutations F1534C or M918T, at PyR1, in pyrethroid-resistant populations of Aedes aegypti and Nilaparvata lugens, respectively. However, the role of the V253F and T267A mutations in pyrethroid resistance has not been functionally examined. Here we report functional characterization of the V253F and T267A mutations in the Ae. aegypti sodium channel AaNav2-1 and the N. lugens sodium channel NlNav1 expressed in Xenopus oocytes. Both mutations alone reduced channel sensitivity to pyrethroids, including etofenprox. We docked etofenprox in a homology model of the pore module of the NlNav1 channel based on the crystal structure of an open prokaryotic sodium channel NavMs. In the low-energy binding pose etofenprox formed contacts with V253, T267 and a previously identified L1014 within PyR2. Combining of V253F or T267A with F1534C or M918T results in a higher level of pyrethroid insensitivity. Furthermore, both V253F and T267A mutations altered channel gating properties. However, V253F- and T267A-induced gating modifications was not observed in the double mutant channels. Our findings highlight the first example in which naturally-found combinational mutations in PyR1 and PyR2 not only confer higher level pyrethroid insensitivity, but also reduce potential fitness tradeoff in pyrethroid-resistant mosquitoes caused by kdr mutation-induced sodium channel gating modifications.

Characterization of the novel heterozygous SCN5A genetic variant Y739D associated with Brugada syndrome.

Genetic variants in SCN5A gene were identified in patients with various arrhythmogenic conditions including Brugada syndrome. Despite significant progress of last decades in studying the molecular mechanism of arrhythmia-associated SCN5A mutations, the understanding of relationship between genetics, electrophysiological consequences and clinical phenotype is lacking. We have found a novel genetic variant Y739D in the SCN5A-encoded sodium channel Nav1.5 of a male patient with Brugada syndrome (BrS). The objective of the study was to characterize the biophysical properties of Nav1.5-Y739D and provide possible explanation of the phenotype observed in the patient. The WT and Y739D channels were heterologously expressed in the HEK-293T cells and the whole-cell sodium currents were recorded. Substitution Y739D reduced the sodium current density by 47 ± 2% at -20 mV, positively shifted voltage-dependent activation, accelerated both fast and slow inactivation, and decelerated recovery from the slow inactivation. The Y739D loss-of-function phenotype likely causes the BrS manifestation. In the hNav1.5 homology models, which are based on the cryo-EM structure of rat Nav1.5 channel, Y739 in the extracellular loop IIS1-S2 forms H-bonds with K1381 and E1435 and pi-cation contacts with K1397 (all in loop IIIS5-P1). In contrast, Y739D accepts H-bonds from K1397 and Y1434. Substantially different contacts of Y739 and Y739D with loop IIIS5-P1 would differently transmit allosteric signals from VSD-II to the fast-inactivation gate at the N-end of helix IIIS5 and slow-inactivation gate at the C-end of helix IIIP1. This may underlie the atomic mechanism of the Y739D channel dysfunction.

P-Loop Channels: Experimental Structures, and Physics-Based and Neural Networks-Based Models.

The superfamily of P-loop channels includes potassium, sodium, and calcium channels, as well as TRP channels and ionotropic glutamate receptors. A rapidly increasing number of crystal and cryo-EM structures have revealed conserved and variable elements of the channel structures. Intriguing differences are seen in transmembrane helices of channels, which may include π-helical bulges. The bulges reorient residues in the helices and thus strongly affect their intersegment contacts and patterns of ligand-sensing residues. Comparison of the experimental structures suggests that some π-bulges are dynamic: they may appear and disappear upon channel gating and ligand binding. The AlphaFold2 models represent a recent breakthrough in the computational prediction of protein structures. We compared some crystal and cryo-EM structures of P-loop channels with respective AlphaFold2 models. Folding of the regions, which are resolved experimentally, is generally similar to that predicted in the AlphaFold2 models. The models also reproduce some subtle but significant differences between various P-loop channels. However, patterns of π-bulges do not necessarily coincide in the experimental and AlphaFold2 structures. Given the importance of dynamic π-bulges, further studies involving experimental and theoretical approaches are necessary to understand the cause of the discrepancy.

Functional Analysis of SCN5A Genetic Variants Associated with Brugada Syndrome.

BACKGROUND: Brugada syndrome (BrS) is a rare inherited cardiac arrhythmia with increased risk of sudden cardiac death. Mutations in gene SCN5A, which encodes the α-subunit of cardiac voltage-gated sodium channel NaV1.5, have been identified in over 20% of patients with BrS. However, only a small fraction of NaV1.5 variants, which are associated with BrS, are characterized in electrophysiological experiments. RESULTS: Here we explored variants V281A and L1582P, which were found in our patients with BrS, and variants F543L and K1419E, which are reportedly associated with BrS. Heterologous expression of the variants in CHO-K1 cells and the Western blot analysis demonstrated that each variant appeared at the cell surface. We further measured sodium current in the whole-cell voltage clamp configuration. Variant F543L produced robust sodium current with a hyperpolarizing shift in the voltage dependence of steady-state fast inactivation. Other variants did not produce detectable sodium currents, indicating a complete loss of function. In a recent cryoEM structure of the hNaV1.5 channel, residues V281, K1419, and L1582 are in close contacts with residues whose mutations are reportedly associated with BrS, indicating functional importance of respective contacts. CONCLUSIONS: Our results support the notion that loss of function of NaV1.5 or decrease of the channel activity is involved in the pathogenesis of BrS.

Allosteric interactions among voltage-sensor modules of sodium channels probed by scorpion toxin modifiers.

Gating of voltage-dependent sodium channels involves coordinated movements of the voltage sensors in the voltage-sensing modules (VSMs) of the four domains (DI-DIV) in response to membrane depolarization. Zhu et al. have recently examined the effects of charge reversal substitutions at the VSM of domain III on the action of scorpion alpha- and beta-toxins that intercept the voltage sensors in domains IV and II, respectively. The increased activity of both toxin types on the mutant channels has suggested that the VSM module at domain III interacts allosterically with the VSM modules in domains IV and II during channel gating thus affecting indirectly the action of both scorpion toxin classes.

Intersegment Contacts of Potentially Damaging Variants of Cardiac Sodium Channel.

Over 1,500 missense variants of sodium channel hNav1.5, which are reported in the ClinVar database, are associated with cardiac diseases. For most of the variants, the clinical significance is uncertain (VUS), not provided (NP), or has conflicting interpretations of pathogenicity (CIP). Reclassifying these variants as pathogenic/likely pathogenic (P/LP) variants is important for diagnosing genotyped patients. In our earlier work, several bioinformatics tools and paralogue annotation method consensually predicted that 74 VUS/NP/CIP variants of 54 wild type residues (set w54) are potentially damaging variants (PDVs). Atomic mechanisms underlying dysfunction of the PDVs are unknown. Here we employed a recent cryo-EM structure of the hNav1.5 channel with likely inactivated pore domain (PD) and activated voltage-sensing domains (VSDs), and ad hoc models of the closed and open PD and resting VSDs to explore intersegment contacts of w54 residues. We found that 44 residues from set w54 contact 84 residues with 118 disease missense variants. These include 104 VUS/NP/CIP variants, most of which are associated with the loss-of-function Brugada syndrome (BrS1) or gain-of-function long QT syndrome (LQT3). Matrix representation of the PDVs and their contact variants facilitated recognition of coupled mutations associated with the same disease. In particular, BrS1-associated coupled mutations, which disturb the P-loops region with the selectivity filter slow inactivation gate, would cause the channel dysfunction. Other likely causes of the channel dysfunction include coupled BrS1-associated variants within VSDs that would destabilize their activated states and coupled LQT3-associated variants, which would stabilize the open PD or activated VSDs. Our study proposes mechanisms of channel dysfunction for scores of BrS1- and LQT3-associated variants, confirms status for 82% of PDVs, and suggests damaging status for their contact variants, which are currently categorized as VUS/NP/CIP variants.

Possible Interactions of Extracellular Loop IVP2-S6 With Voltage-Sensing Domain III in Cardiac Sodium Channel.

Motion transmission from voltage sensors to inactivation gates is an important problem in the general physiology of ion channels. In a cryo-EM structure of channel hNav1.5, residues N1736 and R1739 in the extracellular loop IVP2-S6 approach glutamates E1225 and E1295, respectively, in the voltage-sensing domain III (VSD-III). ClinVar-reported variants E1230K, E1295K, and R1739W/Q and other variants in loops IVP2-S6, IIIS1-S2, and IIIS3-S4 are associated with cardiac arrhythmias, highlighting the interface between IVP2-S6 and VSD-III as a hot spot of disease mutations. Atomic mechanisms of the channel dysfunction caused by these mutations are unknown. Here, we generated mutants E1295R, R1739E, E1295R/R1739E, and N1736R, expressed them in HEK-293T cells, and explored biophysical properties. Mutation E1295R reduced steady-state fast inactivation and enhanced steady-state slow inactivation. In contrast, mutation R1739E slightly enhanced fast inactivation and attenuated slow inactivation. Characteristics of the double mutant E1295R/R1739E were rather similar to those of the wild-type channel. Mutation N1736R attenuated slow inactivation. Molecular modeling predicted salt bridging of R1739E with the outermost lysine in the activated voltage-sensing helix IIIS4. In contrast, the loss-of-function substitution E1295R repelled R1739, thus destabilizing the activated VSD-III in agreement with our data that E1295R caused a depolarizing shift of the G-V curve. In silico deactivation of VSD-III with constraint-maintained salt bridge E1295-R1739 resulted in the following changes: 1) contacts between IIIS4 and IVS5 were switched; 2) contacts of the linker-helix IIIS4-S5 with IVS5, IVS6, and fast inactivation tripeptide IFM were modified; 3) contacts of the IFM tripeptide with helices IVS5 and IVS6 were altered; 4) mobile loop IVP2-S6 shifted helix IVP2 that contributes to the slow inactivation gate and helix IVS6 that contributes to the fast inactivation gate. The likelihood of salt bridge E1295-R1739 in deactivated VSD-III is supported by Poisson-Boltzmann calculations and state-dependent energetics of loop IVP2-S6. Taken together, our results suggest that loop IVP2-S6 is involved in motion transmission from VSD-III to the inactivation gates.

L-Type Calcium Channel: Predicting Pathogenic/Likely Pathogenic Status for Variants of Uncertain Clinical Significance.

(1) Background: Defects in gene CACNA1C, which encodes the pore-forming subunit of the human Cav1.2 channel (hCav1.2), are associated with cardiac disorders such as atrial fibrillation, long QT syndrome, conduction disorders, cardiomyopathies, and congenital heart defects. Clinical manifestations are known only for 12% of CACNA1C missense variants, which are listed in public databases. Bioinformatics approaches can be used to predict the pathogenic/likely pathogenic status for variants of uncertain clinical significance. Choosing a bioinformatics tool and pathogenicity threshold that are optimal for specific protein families increases the reliability of such predictions. (2) Methods and Results: We used databases ClinVar, Humsavar, gnomAD, and Ensembl to compose a dataset of pathogenic/likely pathogenic and benign variants of hCav1.2 and its 20 paralogues: voltage-gated sodium and calcium channels. We further tested the performance of sixteen in silico tools in predicting pathogenic variants. ClinPred demonstrated the best performance, followed by REVEL and MCap. In the subset of 309 uncharacterized variants of hCav1.2, ClinPred predicted the pathogenicity for 188 variants. Among these, 36 variants were also categorized as pathogenic/likely pathogenic in at least one paralogue of hCav1.2. (3) Conclusions: The bioinformatics tool ClinPred and the paralogue annotation method consensually predicted the pathogenic/likely pathogenic status for 36 uncharacterized variants of hCav1.2. An analogous approach can be used to classify missense variants of other calcium channels and novel variants of hCav1.2.

Computational Analysis of the Crystal and Cryo-EM Structures of P-Loop Channels with Drugs.

The superfamily of P-loop channels includes various potassium channels, voltage-gated sodium and calcium channels, transient receptor potential channels, and ionotropic glutamate receptors. Despite huge structural and functional diversity of the channels, their pore-forming domain has a conserved folding. In the past two decades, scores of atomic-scale structures of P-loop channels with medically important drugs in the inner pore have been published. High structural diversity of these complexes complicates the comparative analysis of these structures. Here we 3D-aligned structures of drug-bound P-loop channels, compared their geometric characteristics, and analyzed the energetics of ligand-channel interactions. In the superimposed structures drugs occupy most of the sterically available space in the inner pore and subunit/repeat interfaces. Cationic groups of some drugs occupy vacant binding sites of permeant ions in the inner pore and selectivity-filter region. Various electroneutral drugs, lipids, and detergent molecules are seen in the interfaces between subunits/repeats. In many structures the drugs strongly interact with lipid and detergent molecules, but physiological relevance of such interactions is unclear. Some eukaryotic sodium and calcium channels have state-dependent or drug-induced π-bulges in the inner helices, which would be difficult to predict. The drug-induced π-bulges may represent a novel mechanism of gating modulation.

Mapping the interaction surface of scorpion ß-toxins with an insect sodium channel.

The interaction of insect-selective scorpion depressant ß-toxins (LqhIT2 and Lqh-dprIT3 from Leiurus quinquestriatus hebraeus) with the Blattella germanica sodium channel, BgNav1-1a, was investigated using site-directed mutagenesis, electrophysiological analyses, and structural modeling. Focusing on the pharmacologically defined binding site-4 of scorpion ß-toxins at the voltage-sensing domain II (VSD-II), we found that charge neutralization of D802 in VSD-II greatly enhanced the channel sensitivity to Lqh-dprIT3. This was consistent with the high sensitivity of the splice variant BgNav2-1, bearing G802, to Lqh-dprIT3, and low sensitivity of BgNav2-1 mutant, G802D, to the toxin. Further mutational and electrophysiological analyses revealed that the sensitivity of the WT = D802E < D802G < D802A < D802K channel mutants to Lqh-dprIT3 correlated with the depolarizing shifts of activation in toxin-free channels. However, the sensitivity of single mutants involving IIS4 basic residues (K4E = WT << R1E < R2E < R3E) or double mutants (D802K = K4E/D802K = R3E/D802K > R2E/D802K > R1E/D802K > WT) did not correlate with the activation shifts. Using the cryo-EM structure of the Periplaneta americana channel, NavPaS, as a template and the crystal structure of LqhIT2, we constructed structural models of LqhIT2 and Lqh-dprIT3-c in complex with BgNav1-1a. These models along with the mutational analysis suggest that depressant toxins approach the salt-bridge between R1 and D802 at VSD-II to form contacts with linkers IIS1-S2, IIS3-S4, IIIP5-P1 and IIIP2-S6. Elimination of this salt-bridge enables deeper penetration of the toxin into a VSD-II gorge to form new contacts with the channel, leading to increased channel sensitivity to Lqh-dprIT3.

Possible Mechanism of Ion Selectivity in Eukaryotic Voltage-Gated Sodium Channels.

The outer pore of Nav1.x channels is lined by the selectivity-filter ring Asp-Glu-Lys-Ala (DEKA), an outer ring of carboxylates, and two inner rings of backbone carbonyls. A key role of Lys in the Na+/K+ selectivity is known, but the mechanism is unclear. Here, contacts involving DEKA residues in 15 cryo-EM structures of Nav1.x channels were analyzed and Monte Carlo (MC) energy minimizations of models with the DEKA residues in different protonation states, with or without Na+ or K+, were performed. In MC-minimized structures, protonated Lys+ was salt-bridged with Glu, whereas deprotonated Lys•• "dunked" to the inner rings. When Na+ was pulled through the outer pore, it was inevitably chelated by Glu and Lys•• at the narrow pore levels. Lys•• further escorted Na+ to the inner rings and in several steps mutual dispositions of the DEKA residues are similar to those seen in cryo-EM structures. Analogous results were obtained in models with DEKA mutants, which have high, but not low Na+/K+ selectivity. When K+ was pulled through the pore, it was also chelated between Glu and Lys••, but respective distances were larger and K+ energy was higher than in models with Na+. The computations suggest that salt-bridged Lys+ and Glu block the pore. Approaching Na+ would knock out H+, squeeze between Glu and Lys••, and move down escorted by Lys••, whereas the displaced H+ would stay nearby in a H-bond involving Glu or/and Asp. When Na+ leaves the outer pore, reprotonated Lys•• would rejoin Glu to complete the permeation cycle.

Mutations in transmembrane proteins: diseases, evolutionary insights, prediction and comparison with globular proteins.

Membrane proteins are unique in that they interact with lipid bilayers, making them indispensable for transporting molecules and relaying signals between and across cells. Due to the significance of the protein's functions, mutations often have profound effects on the fitness of the host. This is apparent both from experimental studies, which implicated numerous missense variants in diseases, as well as from evolutionary signals that allow elucidating the physicochemical constraints that intermembrane and aqueous environments bring. In this review, we report on the current state of knowledge acquired on missense variants (referred to as to single amino acid variants) affecting membrane proteins as well as the insights that can be extrapolated from data already available. This includes an overview of the annotations for membrane protein variants that have been collated within databases dedicated to the topic, bioinformatics approaches that leverage evolutionary information in order to shed light on previously uncharacterized membrane protein structures or interaction interfaces, tools for predicting the effects of mutations tailored specifically towards the characteristics of membrane proteins as well as two clinically relevant case studies explaining the implications of mutated membrane proteins in cancer and cardiomyopathy.