Gating Pore Research Articles

Sweetening K-channels: what sugar taught us about permeation and gating.

Because they enable for the modification of both viscosity and osmolarity, sugars have been used as a biophysical probe of voltage-gated K-channels for a while. Viscosity variations made it possible to measure the pore sizes in large and small conductance K-channels using techniques similar to those used in the 1980s to study the gramicidin A channel. These analyses led to the finding that the size of the internal mouth appears to be the primary cause of the conductance differences between Shaker-like channels and large conductance BK-channels. As an osmotic agent, adding sugar unilaterally causes streaming potentials that indicate H2O/K+ cotransport across the BK-channel pore. Osmotic experiments on Shaker K-channels suggest that the pore gate operation and the slow inactivation displace comparable amounts of water. Functionally isolated voltage sensors allow estimation of individual osmotic work for each voltage sensing charge during voltage-activation, reporting dramatic internal and external remodeling of the Voltage Sensing Domain´s solvent exposed surfaces. Remarkably, each charge of the VSD appears to take a unique trajectory. Thus, manipulation of viscosity and osmolarity, together with 3D structures, brings in solid grounds to harmonize function and structure in membrane proteins such as K-channels and, in a wider scope, other structurally dynamic proteins.

Optical measurement of gating pore currents in hypokalemic periodic paralysis model cells.

Hypokalemic periodic paralysis (HypoPP) is a rare genetic disease associated with mutations in CACNA1S or SCN4A, encoding Cav1.1 or Nav1.4, respectively. Most HypoPP-associated missense changes occur at the arginine residues within the voltage-sensing domain (VSD) in Cav1.1 and Nav1.4. Vigorous structural and functional studies suggest that mutations of the arginine residues of VSDs destroy the hydrophobic seal separating the external water and the internal cytosolic crevices, resulting in the generation of an aberrant ion permeation pathway, the gating pore. Presently, non-physiological leak currents through the gating pore, known as the gating pore currents, are thought to underlie HypoPP. In this study, we generated HEK293T-based HypoPP-model cell lines with the Sleeping Beauty transposon system that co-express mouse inward-rectifier potassium ion channel (mKir2.1) and HypoPP2-associated Nav1.4 missense variants, p.R669H, p.R672H, p.R1135H or p.R669H/p.R672H (double mutations, DMT). Whole-cell patch clamp measurements confirmed that mKir2.1 successfully hyperpolarized the membrane electrical potential of the cells to −90 mV or below (comparable to myofibers), and that p.R669H and DMT induced notable proton-based gating pore currents. Using a ratiometric pH indicator, SNARF-4F, we observed that the proton transports at higher rate in p.R669H and DMT compared to wild-type (WT) control. The expression of the HypoPP2-associated Nav1.4 variants was similar to the WT level based on our newly developed membrane protein targeting assay, indicating that the higher proton transport rates found in our fluorometric measurements for p.R669H and DMT are indeed ascribed to the gating pore currents. Our optical method has a potential to serve as an in vitro platform for rapidly characterizing variants associated with HypoPP and other channelopathies, and for subsequent high-throughput drug screen. Support: JP19ek0109230 and JP22bm0804005(AMED), JP18K07524 and JP22K07493(JSPS), Takeda Science Foundation, Mochida Memorial Foundation, and DC01782(NIH).

A de novo arrhythmogenic Nav1.5 variant, E171Q, causes multiple biophysical defects.

An infant presenting antenatally with cardiac conduction disease and junctional tachycardia reminiscent of the multi-focal ectopic Purkinje-related premature contractions (MEPPC) syndrome was found to harbour the de novo E171Q variant of the cardiac voltage-gated sodium channel, Nav1.5. Functional analysis was performed using whole cell patch clamp and current clamp to compare WT Nav1.5 with E171Q expressed in HEK293 cells and iPSC-CMs. We found E171Q significantly right-shifted the voltage-dependence of both activation and steady-state fast inactivation, and increased the amplitude of late sodium current. These changes were reversed upon perfusion with 100 μM ranolazine. Incorporating the shifts in voltage-dependence and late current into the Ohara-Rudy cardiac action potential model produced a delay in action potential repolarization. Further investigation revealed a significant outward gating pore (omega) current in E171Q that was mitigated upon perfusion of 3 μM flecainide or with K+-free internal solution. Current clamp experiments in iPSC-CMs revealed action potential variability with delayed afterdepolarizations in myocytes expressing E171Q. The increase in gating pore current is consistent with disrupting the interaction between E171 and a corresponding positive charge in the S4 voltage sensor. This disruption could result in the completion of an ion-permeable pore, consistent with that previously described for Nav1.5 R222Q - a variant also associated with MEPPC - and Nav1.4 R222W.

High-level expression of voltage-gated sodium channels in thymidine-arrested human embryonic kidney cells.

The benefit of using heterologous mammalian cell lines such as human embryonic kidney (HEK293) cells for expressing voltage-gated ion channels includes efficient translation and processing of such large membrane proteins. Still, the biophysical assessment of these channels is hindered by their low expression and reduced localization on the surface of a cell. Having a higher expression level of ion channels is crucial for investigating mutations that cause a decrease in the current amplitude as well as for measuring tiny currents such as gating currents and gating pore currents that are in the range of 0.1 to 1% of the peak current. We aimed at improving the efficiency of ion channel expression and enhancing their localization in cell membranes by modifying the protocol of cell cycle arrest at the G1/S boundary using thymidine to direct the cells into increasing the expression machinery during viral transduction of mammalian voltage-gated sodium channels. The combination of cell arrest at the G1/S boundary and mammalian baculovirus system (BacMam) resulted in a 5-fold increase in the current density of different mammalian voltage-gated sodium channels in HEK293 cells compared to the current literature. By applying this modified protocol for expression, we succeeded to measure a pathogenic gating pore current in HEK293 cells of ∼0.5% of the central pore current induced by an autism-related mutation in the R2 gating charge in Domain II (R853Q) of Nav1.2, which is blocked in a voltage-dependent manner when the voltage sensors activate. These results show that our protocol is a universal one that can be applied on different voltage-gated ion channels and possibly other membrane proteins. This ultimately will pave the way to decipher the structure and function of ion channels and their association with ion channelopathies and other diseases.

The human TRPA1 intrinsic cold and heat sensitivity involves separate channel structures beyond the N-ARD domain

TRP channels sense temperatures ranging from noxious cold to noxious heat. Whether specialized TRP thermosensor modules exist and how they control channel pore gating is unknown. We studied purified human TRPA1 (hTRPA1) truncated proteins to gain insight into the temperature gating of hTRPA1. In patch-clamp bilayer recordings, ∆1–688 hTRPA1, without the N-terminal ankyrin repeat domain (N-ARD), was more sensitive to cold and heat, whereas ∆1–854 hTRPA1, also lacking the S1–S4 voltage sensing-like domain (VSLD), gained sensitivity to cold but lost its heat sensitivity. In hTRPA1 intrinsic tryptophan fluorescence studies, cold and heat evoked rearrangement of VSLD and the C-terminus domain distal to the transmembrane pore domain S5–S6 (CTD). In whole-cell electrophysiology experiments, replacement of the CTD located cysteines 1021 and 1025 with alanine modulated hTRPA1 cold responses. It is proposed that hTRPA1 CTD harbors cold and heat sensitive domains allosterically coupled to the S5–S6 pore region and the VSLD, respectively.

Magneto-Fluorescent Mesoporous Nanocarriers for the Dual-Delivery of Ofloxacin and Doxorubicin to Tackle Opportunistic Bacterial Infections in Colorectal Cancer

Cancer-related opportunistic bacterial infections are one major barrier for successful clinical therapies, often correlated to the production of genotoxic factors and higher cancer incidence. Although dual anticancer and antimicrobial therapies are a growing therapeutic fashion, they still fall short when it comes to specific delivery and local action in in vivo systems. Nanoparticles are seen as potential therapeutic vectors, be it by means of their intrinsic antibacterial properties and effective delivery capacity, or by means of their repeatedly reported modulation and maneuverability. Herein we report on the production of a biocompatible, antimicrobial magneto-fluorescent nanosystem (NANO3) for the delivery of a dual doxorubicin–ofloxacin formulation against cancer-related bacterial infections. The drug delivery capacity, rendered by its mesoporous silica matrix, is confirmed by the high loading capacity and stimuli-driven release of both drugs, with preference for tumor-like acidic media. The pH-dependent emission of its surface fluorescent SiQDs, provides an insight into NANO3 surface behavior and pore availability, with the SiQDs working as pore gates. Hyperthermia induces heat generation to febrile temperatures, doubling drug release. NANO3-loaded systems demonstrate significant antimicrobial activity, specifically after the application of hyperthermia conditions. NANO3 structure and antimicrobial properties confirm their potential use in a future dual anticancer and antimicrobial therapeutical vector, due to their drug loading capacity and their surface availability for further modification with bioactive, targeting species.

Pore opening, not voltage sensor movement, underpins the voltage-dependence of facilitation by a hERG blocker.

A drug that blocks the cardiac myocyte voltage-gated K+ channels encoded by the human Ether-à-go-go-Related Gene (hERG) carries a potential risk of long QT syndrome and life-threatening cardiac arrhythmia, including Torsade de Points Interestingly, certain hERG blockers can also facilitate hERG activation to increase hERG currents, which may reduce proarrhythmic potential. However, the molecular mechanism involved in the facilitation effect of hERG blockers remains unclear. The hallmark feature of the facilitation effect by hERG blockers is that a depolarizing preconditioning pulse shifts voltage-dependence of hERG activation to more negative voltages. Here we utilize a D540K hERG mutant to study the mechanism of the facilitation effect. D540K hERG is activated by not only depolarization but also hyperpolarization. This unusual gating property enables tests of the mechanism by which voltage induces facilitation of hERG by blockers. With D540K hERG, we find that nifekalant, a hERG blocker and Class III antiarrhythmic agent, blocks and facilitates not only current activation by depolarization but also current activation by hyperpolarization, suggesting a shared gating process upon depolarization and hyperpolarization. Moreover, in response to hyperpolarizing conditioning pulses, nifekalant facilitates D540K hERG currents but not wild-type currents. Our results indicate that induction of facilitation is coupled to pore opening, not voltage per se We propose that gated access to the hERG central cavity underlies the voltage-dependence of induction of facilitation. This study identifies hERG channel pore gate opening as the conformational change facilitated by nifekalant, a clinically important antiarrhythmic agent. Significance Statement Nifekalant is a clinically important antiarrhythmic agent and a hERG blocker which can also facilitate voltage-dependent activation of hERG channels after a preconditioning pulse. Here we show that the mechanism of action of the preconditioning pulse is to open a conductance gate to enable drug access to a facilitation site. Moreover, we find that facilitation increases hERG currents by altering pore dynamics, rather than acting through voltage sensors.

Structure of the Human BK Ion Channel in Lipid Environment.

Voltage-gated and ligand-modulated ion channels play critical roles in excitable cells. To understand the interplay among voltage sensing, ligand binding, and channel opening, the structures of ion channels in various functional states and in lipid membrane environments need to be determined. Here, the random spherically constrained (RSC) single-particle cryo-EM method was employed to study human large conductance voltage- and calcium-activated potassium (hBK or hSlo1) channels reconstituted into liposomes. The hBK structure was determined at 3.5 Å resolution in the absence of Ca2+. Instead of the common fourfold symmetry observed in ligand-modulated ion channels, a twofold symmetry was observed in hBK in liposomes. Compared with the structure of isolated hSlo1 Ca2+ sensing gating rings, two opposing subunits in hBK unfurled, resulting in a wider opening towards the transmembrane region of hBK. In the pore gate domain, two opposing subunits also moved downwards relative to the two other subunits.

A gating ultramicroporous metal-organic framework showing high adsorption selectivity, capacity and rate for xylene separation

Adsorptive separation of p-xylene (pX) from xylene isomers is a key process in chemical industry, but known adsorbents cannot simultaneously achieve high adsorption selectivity, capacity, and rate. Here, we demonstrate gating ultramicropore as a solution for this challenge. Slight modification of the synthetic condition gives rise to isomeric metal-organic frameworks α-[Zn(pba)] (MAF-88, H2pba = 4-(1H-pyrazol-4-yl)benzoic acid) and β-[Zn(pba)] (MAF-89) possessing similar pillared-column structures, porosities, and high pX capacities of 2.0 mmol g−1, but very different framework/pore topologies, pore sizes, and pX selectivities. For binary and ternary mixtures of liquid xylene isomers, MAF-88 with narrow one-dimensional (1D) channels shows pX selectivity of 11 and 1.6, while MAF-89 with 3D-connected quasi-discrete pores shows pX selectivity up to 221 and 46, respectively. Thermogravimetry, differential scanning calorimetry, and time-dependent separation experiments reveal that the kinetic effects of the gating pores play more important roles than the thermodynamic effects, which is further confirmed by single-crystal X-ray diffraction and computational simulations.

The G213D variant in Nav1.5 alters sodium current and causes an arrhythmogenic phenotype resulting in a multifocal ectopic Purkinje-related premature contraction phenotype in human-induced pluripotent stem cell-derived cardiomyocytes.

Variants in SCN5A encoding Nav1.5 are associated with cardiac arrhythmias. We aimed to determine the mechanism by which c.638G>A in SCNA5 resulting in p.Gly213Asp (G213D) in Nav1.5 altered Na+ channel function and how flecainide corrected the defect in a family with multifocal ectopic Purkinje-related premature contractions (MEPPC)-like syndrome. Five patients carrying the G213D variant were treated with flecainide. Gating pore currents were evaluated in Xenopus laevis oocytes. The 638G>A SCN5A variant was introduced to human-induced pluripotent stem cell (hiPSC) by CRISPR-Cas9 gene editing and subsequently differentiated to cardiomyocytes (hiPSC-CM). Action potentials and sodium currents were measured in the absence and presence of flecainide. Ca2+ transients were measured by confocal microscopy. The five patients exhibited premature atrial and ventricular contractions which were suppressed by flecainide treatment. G213D induced gating pore current at potentials negative to -50 mV. Voltage-clamp analysis in hiPSC-CM revealed the activation threshold of INa was shifted in the hyperpolarizing direction resulting in a larger INa window current. The G213D hiPSC-CMs had faster beating rates compared with wild-type and frequently showed Ca2+ waves and alternans. Flecainide applied to G213D hiPSC-CMs decreased window current by shifting the steady-state inactivation curve and slowed the beating rate. The G213D variant in Nav1.5 induced gating pore currents and increased window current. The changes in INa resulted in a faster beating rate and Ca2+ transient dysfunction. Flecainide decreased window current and inhibited INa, which is likely responsible for the therapeutic effectiveness of flecainide in MEPPC patients carrying the G213D variant.

Gating choreography and mechanism of the human proton-activated chloride channel ASOR.

The proton-activated chloride channel ASOR (TMEM206/PAC) permeates anions across cellular membranes in response to acidification, thereby enhancing acid-induced cell death and regulating endocytosis. The molecular mechanisms of pH-dependent control are not understood, in part because structural information for an activated conformation of ASOR is lacking. Here, we reconstitute function from purified protein and present a 3.1-Å-resolution cryo–electron microscopy structure of human ASOR at acidic pH in an activated conformation. The work contextualizes a previous acidic pH structure as a desensitized conformation. Combined with electrophysiological studies and high-resolution structures of resting and desensitized states, the work reveals mechanisms of proton sensing and ion pore gating. Clusters of extracellular acidic residues function as pH sensors and coalesce when protonated. Ensuing conformational changes induce metamorphosis of transmembrane helices to fashion an ion conduction pathway unique to the activated conformation. The studies identify a new paradigm of channel gating in this ubiquitous ion channel.

Differential roles for the M2-M3 linkers of specific GABAAR subunits in pore stability and benzodiazepine modulation

GABAA receptors are neurotransmitter-gated ion channel critical for inhibitory synaptic transmission as well as the molecular target for many psychotropic drugs. Of these, benzodiazepines (BZDs) are some of the most widely prescribed. However, the physical basis of their modulation is poorly understood. We previously exploited a gain of function mutant (α1L9'Tβ2γ2L) that can be directly activated by BZDs alone in the absence of GABA to identify a critical residue in the α1 M2—M3 linker (V279) mediating coupling between the BZD site and the pore gate (PMC7899671).

Functional rescue of periodic paralysis by in vivo gene editing of CaV1.1

Missense mutations of CaV1.1 cause hypokalemic periodic paralysis (HypoPP) by creating an anomalous leak—the gating pore current—that renders the fiber susceptible to paradoxical depolarization and loss of excitability in low extracellular K+. We previously generated CaV1.1-R528H knock-in mutant mice and validated a robust HypoPP phenotype when challenged by 2 mM K+. In this study, we tested the feasibility of gene editing technologies to rescue the HypoPP phenotype in our R528H mouse model.

Open-state structure and pore gating mechanism of the cardiac sodium channel

Voltage-gated sodium (Nav) channels open to initiate the action potential in excitable cells, then close within a few milliseconds via fast inactivation. Rapid opening and fast inactivation focus the depolarizing stimulus of the cardiac action potential and support the rapid and efficient conduction needed for a regular heartbeat. Fast inactivation is disrupted by mutations in the DIII/DIV linker of Nav channels, including mutations in the canonical IFM motif (isoleucine/phenylalanine/ methionine) of the “fast inactivation particle” .

Arrhythmic Phenotypes Are a Defining Feature of Dilated Cardiomyopathy-Associated SCN5A Variants: A Systematic Review.

Variants in the SCN5A gene, that encodes the cardiac sodium channel, Nav1.5, are associated with a highly arrhythmogenic form of dilated cardiomyopathy (DCM). Our aim was to review the phenotypes, natural history, functional effects, and treatment outcomes of DCM-associated rare SCN5A variants. A systematic review of reported DCM-associated rare SCN5A variants was undertaken using PubMed and Embase. Eighteen SCN5A rare variants in 29 families with DCM (173 affected individuals) were identified. Eleven variants had undergone experimental evaluation, with 7 of these resulting in increased sustained current flow during the action potential (eg, increased window current) and at resting membrane potentials (eg, creation of a new gating pore current). These variants were located in transmembrane voltage-sensing domains and had a consistent phenotype characterized by frequent multifocal narrow and broad complex ventricular premature beats (VPB; 72% of affected relatives), ventricular arrhythmias (33%), atrial arrhythmias (32%), sudden cardiac death (13%), and DCM (56%). This VPB-predominant phenotype was not seen with 1 variant that increased late sodium current, or with variants that reduced peak current density or had mixed effects. In the latter groups, affected individuals mainly showed sinus node dysfunction, conduction defects, and atrial arrhythmias, with infrequent VPB and ventricular arrhythmias. DCM did not occur in the absence of arrhythmias for any variant. Twelve studies (23 total patients) reported treatment success in the VPB-predominant cardiomyopathy using sodium channel-blocking drug therapy. SCN5A variants can present with a diverse spectrum of primary arrhythmic features. A majority of DCM-associated variants cause a multifocal VPB-predominant cardiomyopathy that is reversible with sodium channel blocking drug therapy. Early recognition of the distinctive phenotype and prompt genetic testing to identify variant carriers are needed. Our findings have implications for interpretation and management of SCN5A variants found in DCM patients with and without arrhythmias.

Phosphoinositide regulates dynamic movement of the S4 voltage sensor in the second repeat in two-pore channel 3

The two-pore channels (TPCs) are voltage-gated cation channels consisting of single polypeptides with two repeats of a canonical 6-transmembrane unit. TPCs are known to be regulated by various physiological signals such as membrane voltage and phosphoinositide (PI). The fourth helix in the second repeat (second S4) plays a major role in detecting membrane voltage, whereas the first repeat contains a PI binding site. Therefore, each of these stimuli is detected by a unique repeat to regulate the gating of the TPC central pore. How these various stimuli regulate the dynamic structural rearrangement of the TPC molecule remain unknown. Here, we found that PI binding to the first repeat in TPC3 regulates the movement of the distally located second S4 helix, showing that the PI-binding signal is not confined to the pore gate but also transmitted to the voltage sensor. Using voltage clamp fluorometry, measurement of gating charges, and Cys-accessibility analysis, we observed that PI binding significantly potentiates the voltage dependence of the movement of the second S4 helix. Notably, voltage clamp fluorometry analysis revealed that the voltage-dependent movement of the second S4 helix occurred in two phases, of which the second phase corresponds to the transfer of the gating charges. This movement was observed in the voltage range where gate-opening occurs and was potentiated by PI. In conclusion, this regulation of the second S4 helix by PI indicates a tight inter-repeat coupling within TPC3, a feature which might be conserved among TPC family members to integrate various physiological signals.

CaV1.1 defects in hypokalemic periodic paralysis and harnessing multiple approaches for therapeutic intervention

The recurrent attacks of weakness in hypokalemic periodic paralysis (HypoPP) are caused by failure to maintain the resting potential, with paradoxical depolarization in low K+. Remarkably, 24 out of 25 HypoPP mutations are R/X substitutions in S4 segments of voltage-sensing domains of CaV1.1 (70% of cases) or NaV1.4 (10% of cases). Expression studies in oocytes and murine muscle show anomalous gating pore leakage currents (ω-pore) for six of eight CaV1.1-HypoPP mutations, with one exception being the charge-conserving R897K. The proposed consensus pathomechanism, whereby a gating pore leak predisposes to paradoxical depolarization in low K+, is now verified by continuous recording of Vm. Selective measurement of voltage-dependent Ca2+ release, in "healthy appearing" HypoPP fibers, shows only a modest decrease in the Ca2+-dependent peak fluorescence (Oregon green 488/EGTA), and supports the notion that stabilizing Vrest will be sufficient to prevent low-K+-induced loss of force. In our knockin mouse models of HypoPP (CaV1.1-R528H and NaV1.4-R669H), pretreatment with K+-channel openers protects against the loss of force with a 2 mM K+ challenge. Alternatively, gene editing offers the possibility of sustained protection from attacks of weakness, and may prevent the late-onset permanent myopathy. In a proof-of-principle study of cultured myoblasts and in vivo electroporation, we show selective editing of the mutant HypoPP allele, without compromise of the WT allele, using CRISPR/Cas-mediated indel formation to destroy the HypoPP allele or a CRISPR/Cas base editor to correct the missense mutation.

Autism-associated mutations in KV7 channels induce gating pore current

Autism spectrum disorder (ASD) adversely impacts >1% of children in the United States, causing social interaction deficits, repetitive behaviors, and communication disorders. Genetic analysis of ASD has advanced dramatically through genome sequencing, which has identified >500 genes with mutations in ASD. Mutations that alter arginine gating charges in the voltage sensor of the voltage-gated potassium (KV) channel KV7 (KCNQ) are among those frequently associated with ASD. We hypothesized that these gating charge mutations would induce gating pore current (also termed ω-current) by causing an ionic leak through the mutant voltage sensor. Unexpectedly, we found that wild-type KV7 conducts outward gating pore current through its native voltage sensor at positive membrane potentials, owing to a glutamine in the third gating charge position. In bacterial and human KV7 channels, gating charge mutations at the R1 and R2 positions cause inward gating pore current through the resting voltage sensor at negative membrane potentials, whereas mutation at R4 causes outward gating pore current through the activated voltage sensor at positive potentials. Remarkably, expression of the KV7.3/R2C ASD-associated mutation in vivo in midbrain dopamine neurons of mice disrupts action potential generation and repetitive firing. Overall, our results reveal native and mutant gating pore current in KV7 channels and implicate altered control of action potential generation by gating pore current through mutant KV7 channels as a potential pathogenic mechanism in autism.

Open-state structure and pore gating mechanism of the cardiac sodium channel

The heartbeat is initiated by voltage-gated sodium channel NaV1.5, which opens rapidly and triggers the cardiac action potential; however, the structural basis for pore opening remains unknown. Here, we blocked fast inactivation with a mutation and captured the elusive open-state structure. The fast inactivation gate moves away from its receptor, allowing asymmetric opening of pore-lining S6 segments, which bend and rotate at their intracellular ends to dilate the activation gate to ∼10Å diameter. Molecular dynamics analyses predict physiological rates of Na+ conductance. The open-state pore blocker propafenone binds in a high-affinity pose, and drug-access pathways are revealed through the open activation gate and fenestrations. Comparison with mutagenesis results provides a structural map of arrhythmia mutations that target the activation and fast inactivation gates. These results give atomic-level insights into molecular events that underlie generation of the action potential, open-state drug block, and fast inactivation of cardiac sodium channels, which initiate the heartbeat.

Gating pore currents occur in CaV1.1 domain III mutants associated with HypoPP.

Mutations in the voltage sensor domain (VSD) of CaV1.1, the α1S subunit of the L-type calcium channel in skeletal muscle, are an established cause of hypokalemic periodic paralysis (HypoPP). Of the 10 reported mutations, 9 are missense substitutions of outer arginine residues (R1 or R2) in the S4 transmembrane segments of the homologous domain II, III (DIII), or IV. The prevailing view is that R/X mutations create an anomalous ion conduction pathway in the VSD, and this so-called gating pore current is the basis for paradoxical depolarization of the resting potential and weakness in low potassium for HypoPP fibers. Gating pore currents have been observed for four of the five CaV1.1 HypoPP mutant channels studied to date, the one exception being the charge-conserving R897K in R1 of DIII. We tested whether gating pore currents are detectable for the other three HypoPP CaV1.1 mutations in DIII. For the less conserved R1 mutation, R897S, gating pore currents with exceptionally large amplitude were observed, correlating with the severe clinical phenotype of these patients. At the R2 residue, gating pore currents were detected for R900G but not R900S. These findings show that gating pore currents may occur with missense mutations at R1 or R2 in S4 of DIII and that the magnitude of this anomalous inward current is mutation specific.