Agitoxin footprinting the shaker potassium channel pore

Deciphering the shape selective conformational equilibrium of E- and Z-locked azobenzene-tetraethylammonium ion in regulating photo-switchable K+-ion channel blocking

The search for photo-switchable optopharmacological agents that can block ion channels has been a prevalent area owing to its prime advantages of reversibility and specificity over the traditional blockers. However, the quest for a higher blocking ability shown by a less stable photo-isomer to perfectly suit the requirement of the optopharmacological agents is still ongoing. To date, only a marginal improvement in terms of blocking ability is observed by the less stable E-isomer of para-substituted locked azobenzene with TEA (LAB-TEA) for the K+-ion channel. Thus, rationalization of the limitation for achieving high activity by the E-isomer is rather essential to aid the improvement of the efficiency of photoswitchable blocker drugs. Herein, we report a molecular-level analysis on the mechanism of blocking by E- and Z-LAB-TEA with the bacterial KcsA K+-ion channel using Molecular Dynamics (MD) simulation and Quantum Mechanical (QM) calculations. The positively charged TEA fragment engages in stronger electrostatic interactions, while the neutral LAB fragment engages in weaker dispersive interactions. The binding free energy calculated by Molecular Mechanics Poisson-Boltzmann Surface Area (MMPBSA) for E-LAB-TEA (-22.3 kcal mol-1) shows less thermodynamic preference for binding with K+-ion channels than Z-LAB-TEA (-21.6 kcal mol-1) corroborating the experimental observation. The correlation between the structure and the binding ability of E- and Z-isomers of LAB-TEA indicates that the channel gate is narrow and acts as a bottleneck for the entry of the binder molecule inside the large cavity. Upon irradiation, the Z-isomer converts into a less stable but long and planar E-isomer (ΔE of photoisomerism = 7.0 kcal mol-1, at SA2-CASPT2(6,4)/6-31+G(d)//CASSCF(6,4)/6-31+G(d)), which is structurally more suitable to fit into the narrow channel gate rather than the curved and non-planar Z-LAB-TEA. Thus, a reduction in the ionic current is observed owing to the preferential entry and subsequent blocking by E-LAB-TEA. Discontinuing the irradiation leads to conversion to the Z-isomer, the curved nature of which hinders its spontaneous release outside the cavity, thereby contributing only a small increase in the ionic current.

Inactivation of the Kv2.1 channel through electromechanical coupling

The Kv2.1 voltage-activated potassium (Kv) channel is a prominent delayed-rectifier Kv channel in the mammalian central nervous system, where its mechanisms of activation and inactivation are critical for regulating intrinsic neuronal excitability1,2. Here we present structures of the Kv2.1 channel in a lipid environment using cryo-electron microscopy to provide a framework for exploring its functional mechanisms and how mutations causing epileptic encephalopathies3-7 alter channel activity. By studying a series of disease-causing mutations, we identified one that illuminates a hydrophobic coupling nexus near the internal end of the pore that is critical for inactivation. Both functional and structural studies reveal that inactivation in Kv2.1 results from dynamic alterations in electromechanical coupling to reposition pore-lining S6 helices and close the internal pore. Consideration of these findings along with available structures for other Kv channels, as well as voltage-activated sodium and calcium channels, suggests that related mechanisms of inactivation are conserved in voltage-activated cation channels and likely to be engaged by widely used therapeutics to achieve state-dependent regulation of channel activity.

Molecular determinants of inhibition of the human proton channel hHv1 by the designer peptide C6 and a bivalent derivative

We designed C6 peptide to address the absence of specific inhibitors of human voltage-gated proton channels (hHv1). Two C6 bind to the two hHv1 voltage sensors at the resting state, inhibiting activation on depolarization. Here, we identify the C6–hHv1 binding interface using tethered-toxin variants and channel mutants, unveil an important role for negatively charged lipids, and present a model of the C6–hHv1 complex. Inspired by nature, we create a peptide with two C6 epitopes (C6₂) that binds to both channel subunits simultaneously, yielding picomolar affinity and significantly improved inhibition at high potentials. C6 and C6₂ are peptides designed to regulate hHv1, a channel involved in innate immune-system inflammatory pathophysiology, sperm capacitation, cancer-cell proliferation, and tissue damage in ischemic stroke.

The human voltage-gated proton channel (hHv1) is important for control of intracellular pH. We designed C6, a specific peptide inhibitor of hHv1, to evaluate the roles of the channel in sperm capacitation and in the inflammatory immune response of neutrophils [R. Zhao et al., Proc. Natl. Acad. Sci. U.S.A. 115, E11847–E11856 (2018)]. One C6 binds with nanomolar affinity to each of the two S3–S4 voltage-sensor loops in hHv1 in cooperative fashion so that C6-bound channels require greater depolarization to open and do so more slowly. As depolarization drives hHv1 sensors outwardly, C6 affinity decreases, and inhibition is partial. Here, we identified residues essential to C6–hHv1 binding by scanning mutagenesis, five in the hHv1 S3–S4 loops and seven on C6. A structural model of the C6–hHv1 complex was then generated by molecular dynamics simulations and validated by mutant-cycle analysis. Guided by this model, we created a bivalent C6 peptide (C6₂) that binds simultaneously to both hHv1 subunits and fully inhibits current with picomolar affinity. The results help delineate the structural basis for C6 state-dependent inhibition, support an anionic lipid-mediated binding mechanism, and offer molecular insight into the effectiveness of engineered C6 as a therapeutic agent or lead.

Production of recombinant venom peptides as tools for ion channel research

Animal venom is a rich source for peptide toxins that bind and modulate the function of ion channels. Owing to their ability to bind receptor sites on the channel protein with high affinity and specificity, peptide neurotoxins have become an indispensable tool for ion channel research. Recent breakthroughs in structural biology and advances in computer simulations of biomolecules have sparked a new interest in animal toxins as probes of channel protein structure and function. Here, we focus on methods used to produce animal toxins for research purposes using recombinant expression. The specific challenges associated with heterologous production of venom peptides are discussed, and several methods targeting these issues are presented with an emphasis on E. coli based systems. An efficient protocol for the bacterial expression, folding, and purification of recombinant venom peptides is described.

Dual-Site Binding of Quaternary Ammonium Ions as Internal K+-Ion Channel Blockers: Nonclassical (C-H···O) H Bonding vs Dispersive (C-H···H-C) Interaction

A molecular-level study of the influence of the alkyl chain length of quaternary ammonium ions (QAs) on the blocking action and the mode of binding with the bacterial KcsA K⁺-ion channel is carried out by molecular dynamics (MD) simulations as well as quantum mechanics/molecular mechanics (QM/MM) methods. The present work unveils distinct modes of binding for different QAs, due to differences in size and hydrophobicity. The QAs bind near the channel gate as well as at the central cavity, leading to a possible dual-site blocking action. Small-sized tetraethylammonium (TEA) and tetrabutylammonium (TBA) ions enter inside the channel cavity in the open state of KcsA but bind strongly in the closed state. TEA binds to the polar hydroxyl group of threonine residues situated at the channel gate via nonclassical H-bonding interaction (C-H···O), while TBA binds to a second binding site, the central cavity, with hydrophobic benzyl and sec-butyl side chains of phenylalanine and isoleucine residues via alkyl-π and hydrophobic interactions (C-H···H-C). On the contrary, large tetrahexylammonium (THA) and tetraoctylammonium (TOA) ions bind the hydrophobic side-chain methyl and isopropyl of alanine and valine at the channel gate both in the open and closed states, thereby restricting the free movement of large QAs toward the center of the cavity. However, the binding to the hydrophobic benzyl and sec-butyl side chains of phenylalanine and isoleucine residues in the closed state is thermodynamically preferable. Also, the binding energy is found to increase with an increase in the alkyl chain length from ethyl (-16.4 kcal/mol) to octyl (-65.5 kcal/mol), due to an almost linear increase in dispersive interaction.

Tuning Scorpion Toxin Selectivity: Switching From KV1.1 to KV1.3

Voltage-gated potassium channels (K_Vs) perform vital physiological functions and are targets in different disorders ranging from ataxia and arrhythmia to autoimmune diseases. An important issue is the search for and production of selective ligands of these channels. Peptide toxins found in scorpion venom named KTx excel in both potency and selectivity with respect to some potassium channel isoforms, which may present only minute differences in their structure. Despite several decades of research the molecular determinants of KTx selectivity are still poorly understood. Here we analyze MeKTx13-3 (Kalium ID: α-KTx 3.19) from the lesser Asian scorpion Mesobuthus eupeus, a high-affinity K_V1.1 blocker (IC₅₀ ~2 nM); it also affects K_V1.2 (IC₅₀ ~100 nM), 1.3 (~10 nM) and 1.6 (~60 nM). By constructing computer models of its complex with K_V1.1-1.3 channels we identify specific contacts between the toxin and the three isoforms. We then perform mutagenesis to disturb the identified contacts with K_V1.1 and 1.2 and produce recombinant MeKTx13-3_AAAR, which differs by four amino acid residues from the parent toxin. As predicted by the modeling, this derivative shows decreased activity on K_V1.1 (IC₅₀ ~550 nM) and 1.2 (~200 nM). It also has diminished activity on K_V1.6 (~1500 nM) but preserves K_V1.3 affinity as measured using the voltage-clamp technique on mammalian channels expressed in Xenopus oocytes. In effect, we convert a selective K_V1.1 ligand into a new specific K_V1.3 ligand. MeKTx13-3 and its derivatives are attractive tools to study the structure-function relationship in potassium channel blockers.

Distinct expression of potassium channels regulates visual response properties of lamina neurons in Drosophila melanogaster

The computational organization of sensory systems depends on the diversification of individual cell types with distinct signal-processing capabilities. The Drosophila visual system, for instance, splits information into channels with different temporal properties directly downstream of photoreceptors in the first-order interneurons of the OFF pathway, L2 and L3. However, the biophysical mechanisms that determine this specialization are largely unknown. Here, we show that the voltage-gated K_a channels Shaker and Shal contribute to the response properties of the major OFF pathway input L2. L3 calcium response kinetics postsynaptic to photoreceptors resemble the sustained calcium signals of photoreceptors, whereas L2 neurons decay transiently. Based on a cell-type-specific RNA-seq data set and endogenous protein tagging, we identified Shaker and Shal as the primary candidates to shape L2 responses. Using in vivo two-photon imaging of L2 calcium signals in combination with pharmacological and genetic perturbations of these K_a channels, we show that the wild-type Shaker and Shal function is to enhance L2 responses and cell-autonomously sharpen L2 kinetics. Our results reveal a role for K_a channels in determining the signal-processing characteristics of a specific cell type in the visual system.

Exploring structural dynamics of a membrane protein by combining bioorthogonal chemistry and cysteine mutagenesis

The functional mechanisms of membrane proteins are extensively investigated with cysteine mutagenesis. To complement cysteine-based approaches, we engineered a membrane protein with thiol-independent crosslinkable groups using azidohomoalanine (AHA), a non-canonical methionine analogue containing an azide group that can selectively react with cycloalkynes through a strain-promoted azide-alkyne cycloaddition (SPAAC) reaction. We demonstrate that AHA can be readily incorporated into the Shaker Kv channel in place of methionine residues and modified with azide-reactive alkyne probes in Xenopus oocytes. Using voltage-clamp fluorometry, we show that AHA incorporation permits site-specific fluorescent labeling to track voltage-dependent conformational changes similar to cysteine-based methods. By combining AHA incorporation and cysteine mutagenesis in an orthogonal manner, we were able to site-specifically label the Shaker Kv channel with two different fluorophores simultaneously. Our results identify a facile and straightforward approach for chemical modification of membrane proteins with bioorthogonal chemistry to explore their structure-function relationships in live cells.

Living cells can sense cues from their environment via molecules located at the interface between the inside and the outside of the cell. These molecules are mostly proteins and are made up of building blocks known as amino acids. To understand how these proteins work, fluorescent probes can be attached to amino acids within them – which can then tell when different parts of proteins move in response to a signal. Scientists often target fluorescent probes at the amino acid cysteine, because it has a chemically reactive side group and is rare enough so that unique positions can be labeled in the protein of interest. However, being able to target other amino acids would allow scientists to ask, and potentially solve, more precise questions about these proteins.

Methionine is another amino acid that has a low abundance in most proteins. Previous research has shown that the cell’s normal protein-building machinery can incorporate synthetic versions of methionine into proteins. This suggested that the introduction of chemically reactive alternatives to methionine could offer a way to label membrane proteins with fluorescent probes and free up the cysteines to be targeted with other approaches. Gupta et al. set out to develop a straightforward method to achieve this and started with a well-studied membrane protein, called Shaker, and cells from female African clawed frogs, which are widely used to study membrane proteins.

Gupta et al. found that the cells could readily take up a chemically reactive methionine alternative called azidohomoalanine (AHA) from their surrounding solution and incorporate it within the Shaker protein. The AHA took the place of the methionines that are normally found in Shaker, and just like in cysteine-based methods, fluorescent probes could be easily attached to the AHAs in this membrane protein. Shaker is a protein that allows potassium ions to flow across the cell membrane by changing shape in response to the membrane voltage. The fluorescence from those probes also changed with the membrane voltage in a way that was comparable to cysteine-mediated approaches. This indicated that the AHA modification could also be used to track structural changes in the Shaker protein. Finally, Gupta et al. showed that AHA- and cysteine-mediated labeling approaches could be combined to attach two different fluorescent probes onto the Shaker protein.

This method will expand the toolbox for researchers studying the relationship between the structure and function of membrane proteins in live cells. In future, it could be applied more widely once the properties of the fluorescent probes for AHA-mediated labeling can be optimized.

Trans-toxin ion-sensitivity of charybdotoxin-blocked potassium-channels reveals unbinding transitional states

In silico and in vitro studies have made progress in understanding protein–protein complex formation; however, the molecular mechanisms for their dissociation are unclear. Protein–protein complexes, lasting from microseconds to years, often involve induced-fit, challenging computational or kinetic analysis. Charybdotoxin (CTX), a peptide from the Leiurus scorpion venom, blocks voltage-gated K⁺-channels in a unique example of binding/unbinding simplicity. CTX plugs the external mouth of K⁺-channels pore, stopping K⁺-ion conduction, without inducing conformational changes. Conflicting with a tight binding, we show that external permeant ions enhance CTX-dissociation, implying a path connecting the pore, in the toxin-bound channel, with the external solution. This sensitivity is explained if CTX wobbles between several bound conformations, producing transient events that restore the electrical and ionic trans-pore gradients. Wobbling may originate from a network of contacts in the interaction interface that are in dynamic stochastic equilibria. These partially-bound intermediates could lead to distinct, and potentially manipulable, dissociation pathways.

Diverse Structural Features of Potassium Channels Characterized by Scorpion Toxins as Molecular Probes

Scorpion toxins are well-known as the largest potassium channel peptide blocker family. They have been successfully proven to be valuable molecular probes for structural research on diverse potassium channels. The potassium channel pore region, including the turret and filter regions, is the binding interface for scorpion toxins, and structural features from different potassium channels have been identified using different scorpion toxins. According to the spatial orientation of channel turrets with differential sequence lengths and identities, conformational changes and molecular surface properties, the potassium channel turrets can be divided into the following three states: open state with less hindering effects on toxin binding, half-open state or half-closed state with certain effects on toxin binding, and closed state with remarkable effects on toxin binding. In this review, we summarized the diverse structural features of potassium channels explored using scorpion toxin tools and discuss future work in the field of scorpion toxin-potassium channel interactions.

Defensins, a novel type of animal toxin-like potassium channel inhibitor

The classical potassium channel inhibitors are toxin peptides from venomous animals, and whether there are peptide inhibitors from other species is an open question. Due to both the independent and interdependent relationships between the spear (peptide inhibitors) and the shield (potassium channels), human defensins were first identified by our group as endogenous potassium channel inhibitors. Encouraged by the discovery of human defensins as potassium channel inhibitors, defensins from invertebrates and fungi were successively found by our group to be potassium channel inhibitors. In addition, a plant defensin was reported to be a potassium channel inhibitor. Since defensins are widely produced by vertebrate, invertebrate, plant and fungi species, the recent work established a new research field on defensin-potassium channel interactions. Here, we review the current work on defensins from vertebrate, invertebrate, plant and fungi species as inhibitors of potassium channels and discuss future work in this research field.

Deglycosylation of Shaker KV channels affects voltage sensing and the open-closed transition

Voltage-gated ion channels are subject to posttranslational modification, including glycosylation. Lopez-Rodriguez and Holmgren show that, in Shaker K_V channels, deglycosylation influences voltage sensing and open–closed transitions but not binding of ligands to the protein.

Most membrane proteins are subject to posttranslational glycosylation, which influences protein function, folding, solubility, stability, and trafficking. This modification has been proposed to protect proteins from proteolysis and modify protein–protein interactions. Voltage-activated ion channels are heavily glycosylated, which can result in up to 30% of the mature molecular mass being contributed by glycans. Normally, the functional consequences of glycosylation are assessed by comparing the function of fully glycosylated proteins with those in which glycosylation sites have been mutated or by expressing proteins in model cells lacking glycosylation enzymes. Here, we study the functional consequences of deglycosylation by PNGase F within the same population of voltage-activated potassium (K_V) channels. We find that removal of sugar moieties has a small, but direct, influence on the voltage-sensing properties and final opening–closing transition of Shaker K_V channels. Yet, we observe that the interactions of various ligands with different domains of the protein are not affected by deglycosylation. These results imply that the sugar mass attached to the voltage sensor neither represents a cargo for the dynamics of this domain nor imposes obstacles to the access of interacting molecules.

Heat activation is intrinsic to the pore domain of TRPV1

The TRPV1 channel is a sensitive detector of pain-producing stimuli, including noxious heat, acid, inflammatory mediators, and vanilloid compounds. Although binding sites for some activators have been identified, the location of the temperature sensor remains elusive. Using available structures of TRPV1 and voltage-activated potassium channels, we engineered chimeras wherein transmembrane regions of TRPV1 were transplanted into the Shaker Kv channel. Here we show that transplanting the pore domain of TRPV1 into Shaker gives rise to functional channels that can be activated by a TRPV1-selective tarantula toxin that binds to the outer pore of the channel. This pore-domain chimera is permeable to Na+, K+, and Ca2+ ions, and remarkably, is also robustly activated by noxious heat. Our results demonstrate that the pore of TRPV1 is a transportable domain that contains the structural elements sufficient for activation by noxious heat.

Molecular Dynamics Simulation Reveals Specific Interaction Sites between Scorpion Toxins and Kv1.2 Channel: Implications for Design of Highly Selective Drugs

The K_v1.2 channel plays an important role in the maintenance of resting membrane potential and the regulation of the cellular excitability of neurons, whose silencing or mutations can elicit neuropathic pain or neurological diseases (e.g., epilepsy and ataxia). Scorpion venom contains a variety of peptide toxins targeting the pore region of this channel. Despite a large amount of structural and functional data currently available, their detailed interaction modes are poorly understood. In this work, we choose four K_v1.2-targeted scorpion toxins (Margatoxin, Agitoxin-2, OsK-1, and Mesomartoxin) to construct their complexes with K_v1.2 based on the experimental structure of ChTx-K_v1.2. Molecular dynamics simulation of these complexes lead to the identification of hydrophobic patches, hydrogen-bonds, and salt bridges as three essential forces mediating the interactions between this channel and the toxins, in which four K_v1.2-specific interacting amino acids (D353, Q358, V381, and T383) are identified for the first time. This discovery might help design highly selective K_v1.2-channel inhibitors by altering amino acids of these toxins binding to the four channel residues. Finally, our results provide new evidence in favor of an induced fit model between scorpion toxins and K⁺ channel interactions.

The envenomation of general physiology throughout the last century

Sack discusses the evolution of toxin research in JGP over the last 100 years.

Toxins are the poisonous products of organisms. Toxins serve vital defensive and offensive functions for those that harbor them: stinging scorpions, pesticidal plants, sanguinary snakes, fearless frogs, sliming snails, noxious newts, and smarting spiders. For physiologists, toxins are integral chemical tools that hijack life’s fundamental processes with remarkable molecular specificity. Our understanding of electrophysiological phenomena has been transformed time and time again with the help of some terrifying toxins. For this reason, studies of toxin mechanism are an important and enduring facet of The Journal of General Physiology (JGP). This Milestone in Physiology reflects on toxins studied in JGP over its first 100 years, what they have taught us, and what they have yet to reveal.

Hanatoxin inserts into phospholipid membranes without pore formation

Hanatoxin (HaTx), a 35-residue polypeptide from spider venom, functions as an inhibitor of Kv2.1 channels by interacting with phospholipids prior to affecting the voltage-sensor. However, how this water-soluble peptide modifies the gating remains poorly understood, as the voltage-sensor is deeply embedded within the bilayer. To determine how HaTx interacts with phospholipid bilayers, in this study, we examined the toxin-induced partitioning of liposomal membranes. HPLC-results from high-speed spin-down vesicles with HaTx demonstrated direct binding. Dynamic light scattering (DLS) and leakage assay results further indicated that neither membrane pores nor membrane fragmentations were observed in the presence of HaTx. To clarify the binding details, Langmuir trough experiments were performed with phospholipid monolayers by mimicking the external leaflet of membrane bilayers, indicating the involvement of acyl chains in such interactions between HaTx and phospholipids. Our current study thus describes the interaction pattern of HaTx with vesicle membranes, defining a membrane-partitioning mechanism for peptide insertion involving the membrane hydrocarbon core without pore formation.

Complexes of Peptide Blockers with Kv1.6 Pore Domain: Molecular Modeling and Studies with KcsA-Kv1.6 Channel

Potassium voltage-gated Kv1.6 channel, which is distributed primarily in neurons of central and peripheral nervous systems, is of significant physiological importance. To date, several high-affinity Kv1.6-channel blockers are known, but the lack of selective ones among them hampers the studies of tissue localization and functioning of Kv1.6 channels. Here we present an approach to advanced understanding of interactions of peptide toxin blockers with a Kv1.6 pore. It combines molecular modeling studies and an application of a new bioengineering system based on a KcsA-Kv1.6 hybrid channel for the quantitative fluorescent analysis of blocker-channel interactions. Using this system we demonstrate that peptide toxins agitoxin 2, kaliotoxin1 and OSK1 have similar high affinity to the extracellular vestibule of the K⁺-conducting pore of Kv1.6, hetlaxin is a low-affinity ligand, whereas margatoxin and scyllatoxin do not bind to Kv1.6 pore. Binding of toxins to Kv1.6 pore has considerable inverse dependence on the ionic strength. Model structures of KcsA-Kv1.6 and Kv1.6 complexes with agitoxin 2, kaliotoxin 1 and OSK1 were obtained using homology modeling and molecular dynamics simulation. Interaction interfaces, which are formed by 15-19 toxin residues and 10 channel residues, are described and compared. Specific sites of Kv1.6 pore recognition are identified for targeting of peptide blockers. Analysis of interactions between agitoxin 2 derivatives with point mutations (S7K, S11G, L19S, R31G) and KcsA-Kv1.6 confirms reliability of the calculated complex structure.

Scorpion toxins prefer salt solutions

There is a wide variety of ion channel types with various types of blockers, making research in this field very complicated. To reduce this complexity, it is essential to study ion channels and their blockers independently. Scorpion toxins, a major class of blockers, are charged short peptides with high affinities for potassium channels. Their high selectivity and inhibitory properties make them an important pharmacological tool for treating autoimmune or nervous system disorders. Scorpion toxins typically have highly charged surfaces and-like other proteins-an intrinsic ability to bind ions (Friedman J Phys Chem B 115(29):9213-9223, 1996; Baldwin Biophys J 71(4):2056-2063, 1996; Vrbka et al. Proc Natl Acad Sci USA 103(42):15440-15444, 2006a; Vrbka et al. J Phys Chem B 110(13):7036-43, 2006b). Thus, their effects on potassium channels are usually investigated in various ionic solutions. In this work, computer simulations of protein structures were performed to analyze the structural properties of the key residues (i.e., those that are presumably involved in contact with the surfaces of the ion channels) of 12 scorpion toxins. The presence of the two most physiologically abundant cations, Na(+) and K(+), was considered. The results indicated that the ion-binding properties of the toxin residues vary. Overall, all of the investigated toxins had more stable structures in ionic solutions than in water. We found that both the number and length of elements in the secondary structure varied depending on the ionic solution used (i.e., in the presence of NaCl or KCl). This study revealed that the ionic solution should be chosen carefully before performing experiments on these toxins. Similarly, the influence of these ions should be taken into consideration in the design of toxin-based pharmaceuticals.

Paxilline inhibits BK channels by an almost exclusively closed-channel block mechanism

Paxilline, a tremorogenic fungal alkaloid, potently inhibits large conductance Ca(2+)- and voltage-activated K(+) (BK)-type channels, but little is known about the mechanism underlying this inhibition. Here we show that inhibition is inversely dependent on BK channel open probability (Po), and is fully relieved by conditions that increase Po, even in the constant presence of paxilline. Manipulations that shift BK gating to more negative potentials reduce inhibition by paxilline in accordance with the increase in channel Po. Measurements of Po times the number of channels at negative potentials support the idea that paxilline increases occupancy of closed states, effectively reducing the closed-open equilibrium constant, L(0). Gating current measurements exclude an effect of paxilline on voltage sensors. Steady-state inhibition by multiple paxilline concentrations was determined for four distinct equilibration conditions, each with a distinct Po. The IC50 for paxilline shifted from around 10 nM when channels were largely closed to near 10 µM as maximal Po was approached. Model-dependent analysis suggests a mechanism of inhibition in which binding of a single paxilline molecule allosterically alters the intrinsic L(0) favoring occupancy of closed states, with affinity for the closed conformation being >500-fold greater than affinity for the open conformation. The rate of inhibition of closed channels was linear up through 2 µM paxilline, with a slope of 2 × 10(6) M(-1)s(-1). Paxilline inhibition was hindered by either the bulky cytosolic blocker, bbTBA, or by concentrations of cytosolic sucrose that hinder ion permeation. However, paxilline does not hinder MTSET modification of the inner cavity residue, A313C. We conclude that paxilline binds more tightly to the closed conformation, favoring occupancy of closed-channel conformations, and propose that it binds to a superficial position near the entrance to the central cavity, but does not hinder access of smaller molecules to this cavity.

The genome of Mesobuthus martensii reveals a unique adaptation model of arthropods

Representing a basal branch of arachnids, scorpions are known as ‘living fossils’ that maintain an ancient anatomy and are adapted to have survived extreme climate changes. Here we report the genome sequence of Mesobuthus martensii, containing 32,016 protein-coding genes, the most among sequenced arthropods. Although M. martensii appears to evolve conservatively, it has a greater gene family turnover than the insects that have undergone diverse morphological and physiological changes, suggesting the decoupling of the molecular and morphological evolution in scorpions. Underlying the long-term adaptation of scorpions is the expansion of the gene families enriched in basic metabolic pathways, signalling pathways, neurotoxins and cytochrome P450, and the different dynamics of expansion between the shared and the scorpion lineage-specific gene families. Genomic and transcriptomic analyses further illustrate the important genetic features associated with prey, nocturnal behaviour, feeding and detoxification. The M. martensii genome reveals a unique adaptation model of arthropods, offering new insights into the genetic bases of the living fossils.

Scorpions have maintained the primary anatomical features of their Paleozoic arthropod ancestors. Here, the authors report the genome sequence of Mesobuthus martensii and highlight evidence of genetic and morphological evolution that represents a unique adaptation model of arthropods.

OcyKTx2, a new K⁺-channel toxin characterized from the venom of the scorpion Opisthacanthus cayaporum

Opisthacanthus cayaporum belongs to the Liochelidae family, and the scorpions from this genus occur in southern Africa, Central America and South America and, therefore, can be considered a true Gondwana heritage. In this communication, the isolation, primary structure characterization, and K⁺-channel blocking activity of new peptide from this scorpion venom are reported. OcyKTx2 is a 34 amino acid long peptide with four disulfide bridges and molecular mass of 3807 Da. Electrophysiological assays conducted with pure OcyKTx2 showed that this toxin reversibly blocks Shaker B K⁺-channels with a Kd of 82 nM, and presents an even better affinity toward hKv1.3, blocking it with a Kd of ∼18 nM. OcyKTx2 shares high sequence identity with peptides belonging to subfamily 6 of α-KTxs that clustered very closely in the phylogenetic tree included here. Sequence comparison, chain length and number of disulfide bridges analysis classify OcyKTx2 into subfamily 6 of the α-KTx scorpion toxins (systematic name, α-KTx6.17).

Structure of a pore-blocking toxin in complex with a eukaryotic voltage-dependent K(+) channel

Pore-blocking toxins inhibit voltage-dependent K⁺ channels (K_v channels) by plugging the ion-conduction pathway. We have solved the crystal structure of paddle chimera, a K_v channel in complex with charybdotoxin (CTX), a pore-blocking toxin. The toxin binds to the extracellular pore entryway without producing discernable alteration of the selectivity filter structure and is oriented to project its Lys27 into the pore. The most extracellular K⁺ binding site (S1) is devoid of K⁺ electron-density when wild-type CTX is bound, but K⁺ density is present to some extent in a Lys27Met mutant. In crystals with Cs⁺ replacing K⁺, S1 electron-density is present even in the presence of Lys27, a finding compatible with the differential effects of Cs⁺ vs K⁺ on CTX affinity for the channel. Together, these results show that CTX binds to a K⁺ channel in a lock and key manner and interacts directly with conducting ions inside the selectivity filter.

DOI:http://dx.doi.org/10.7554/eLife.00594.001

The deadly toxins produced by many creatures, including spiders, snakes, and scorpions, work by blocking the ion channels that are essential for the normal operation of many different types of cells. Ion channels are proteins and, as their name suggests, they allow ions—usually sodium, potassium, or calcium ions—to move in and out of cells. They are especially important for cells that generate or respond to electrical signals, such as neurons and the cells in heart muscle.

Ion channels are located in the lipid membranes that surround all cells, and the ions enter or leave the cell via a pore that runs through the channel protein. They can be opened and closed (or ‘gated’) in different ways: some ion channels open and close in response to voltages, whereas others are gated by biomolecules, such as neurotransmitters, that bind to them.

Now, Banerjee et al. have used x-ray crystallography to study the structure of the complex that is formed when charybdotoxin (CTX), a toxin that is found in scorpion venom, blocks a voltage-gated potassium channel. Previous studies have shown that CTX binds to the channel on the extracellular side of the pore. Banerjee et al. show that the toxin fits into the entrance to the channel like a key into a lock, which means the toxin is preformed to fit the shape of the channel.

The potassium ion channel is made up of four subunits, and the pore contains four ion-binding sites that form a ‘selectivity filter’: it is this filter that ensures that only potassium ions can pass through the channel when it is open. When CTX binds to the channel, a lysine residue poised at a critical position on the toxin is so close to the outermost ion-binding site that it prevents potassium ions binding to the site. The structure determined by Banerjee et al. explains many previous findings, including the fact that ions entering the pore from inside the cell can disrupt the binding between the toxin and the ion channel protein. It remains to be seen if the toxins that target the pore of other types of ion channels work in the same way.

DOI:http://dx.doi.org/10.7554/eLife.00594.002

Two conserved arginine residues from the SK3 potassium channel outer vestibule control selectivity of recognition by scorpion toxins

Potassium channel functions are often deciphered by using selective and potent scorpion toxins. Among these toxins, only a limited subset is capable of selectively blocking small conductance Ca(2+)-activated K(+) (SK) channels. The structural bases of this selective SK channel recognition remain unclear. In this work, we demonstrate the key role of the electric charges of two conserved arginine residues (Arg-485 and Arg-489) from the SK3 channel outer vestibule in the selective recognition by the SK3-blocking BmP05 toxin. Indeed, individually substituting these residues with histidyl or lysyl (maintaining the positive electric charge partially or fully), although decreasing BmP05 affinity, still preserved the toxin sensitivity profile of the SK3 channel (as evidenced by the lack of recognition by many other types of potassium channel-sensitive charybdotoxin). In contrast, when Arg-485 or Arg-489 of the SK3 channel was mutated to an acidic (Glu) or alcoholic (Ser) amino acid residue, the channel lost its sensitivity to BmP05 and became susceptible to the "new" blocking activity by charybdotoxin. In addition to these SK3 channel basic residues important for sensitivity, two acidic residues, Asp-492 and Asp-518, also located in the SK3 channel outer vestibule, were identified as being critical for toxin affinity. Furthermore, molecular modeling data indicate the existence of a compact SK3 channel turret conformation (like a peptide screener), where the basic rings of Arg-485 and Arg-489 are stabilized by strong ionic interactions with Asp-492 and Asp-518. In conclusion, the unique properties of Arg-485 and Arg-489 (spatial orientations and molecular interactions) in the SK3 channel account for its toxin sensitivity profile.

Scorpion toxins specific for potassium (K+) channels: a historical overview of peptide bioengineering

Scorpion toxins have been central to the investigation and understanding of the physiological role of potassium (K⁺) channels and their expansive function in membrane biophysics. As highly specific probes, toxins have revealed a great deal about channel structure and the correlation between mutations, altered regulation and a number of human pathologies. Radio- and fluorescently-labeled toxin isoforms have contributed to localization studies of channel subtypes in expressing cells, and have been further used in competitive displacement assays for the identification of additional novel ligands for use in research and medicine. Chimeric toxins have been designed from multiple peptide scaffolds to probe channel isoform specificity, while advanced epitope chimerization has aided in the development of novel molecular therapeutics. Peptide backbone cyclization has been utilized to enhance therapeutic efficiency by augmenting serum stability and toxin half-life in vivo as a number of K⁺-channel isoforms have been identified with essential roles in disease states ranging from HIV, T-cell mediated autoimmune disease and hypertension to various cardiac arrhythmias and Malaria. Bioengineered scorpion toxins have been monumental to the evolution of channel science, and are now serving as templates for the development of invaluable experimental molecular therapeutics.

A diverse family of novel peptide toxins from an unusual cone snail, Conus californicus

Diversity among Conus toxins mirrors the high species diversity in the Indo-Pacific region, and evolution of both is thought to stem from feeding-niche specialization derived from intra-generic competition. This study focuses on Conus californicus, a phylogenetic outlier endemic to the temperate northeast Pacific. Essentially free of congeneric competitors, it preys on a wider variety of organisms than any other cone snail. Using molecular cloning of cDNAs and mass spectrometry, we examined peptides isolated from venom ducts to elucidate the sequences and post-translational modifications of two eight-cysteine toxins (cal12a and cal12b of type 12 framework) that block voltage-gated Na(+) channels. Based on homology of leader sequence and mode of action, these toxins are related to the O-superfamily, but differ significantly from other members of that group. Six of the eight cysteine residues constitute the canonical framework of O-members, but two additional cysteine residues in the N-terminal region define an O+2 classification within the O-superfamily. Fifteen putative variants of Cal12.1 toxins have been identified by mRNAs that differ primarily in two short hypervariable regions and have been grouped into three subtypes (Cal12.1.1-3). This unique modular variation has not been described for other Conus toxins and suggests recombination as a diversity-generating mechanism. We propose that these toxin isoforms show specificity for similar molecular targets (Na(+) channels) in the many species preyed on by C. californicus and that individualistic utilization of specific toxin isoforms may involve control of gene expression.

Differential molecular information of maurotoxin peptide recognizing IK(Ca) and Kv1.2 channels explored by computational simulation

BACKGROUND

Scorpion toxins are invaluable tools for ion channel research and are potential drugs for human channelopathies. However, it is still an open task to determine the molecular basis underlying the diverse interactions between toxin peptides and ion channels. The inhibitory peptide Maurotoxin (MTX) recognized the distantly related IK(Ca) and Kv1.2 channel with approximately the same potency and using the same functional residues, their differential binding mechanism remain elusive. In this study, we applied computational methods to explore the differential binding modes of MTX to Kv1.2 and IK(Ca) channels, which would help to understand the diversity of channel-toxin interactions and accelerate the toxin-based drug design.

RESULTS

A reasonably stable MTX-IK(Ca) complex was obtained by combining various computational methods and by in-depth comparison with the previous model of the MTX-Kv1.2 complex. Similarly, MTX adopted the β-sheet structure as the interacting surface for binding both channels, with Lys23 occluding the pore. In contrast, the other critical residues Lys27, Lys30, and Tyr32 of MTX adopted distinct interactions when associating with the IK(Ca) channel. In addition, the residues Gln229, Ala230, Ala233, and Thr234 on the IK(Ca) channel turret formed polar and non-polar interactions with MTX, whereas the turret of Kv1.2 was almost not involved in recognizing MTX. In all, the pairs of interacting residues on MTX and the IK(Ca) channel of the bound complex indicated that electrostatic and Van der Waal interactions contributed equally to the formation of a stable MTX-IK(Ca) complex, in contrast to the MTX-Kv1.2 binding that is dominantly mediated by electrostatic forces.

CONCLUSIONS

Despite sharing similar pharmacological profiles toward both IK(Ca) and Kv1.2 channels, MTX adopted totally diverging modes in the two association processes. All the molecular information unveiled here could not only offer a better understanding about the structural differences between the IK(Ca) and Kv1.2 channels, but also provide novel structural clues that will help in the designing of more selective molecular probes to discriminate between these two channels.

Scorpion venom and the inflammatory response

Scorpion venoms consist of a complex of several toxins that exhibit a wide range of biological properties and actions, as well as chemical compositions, toxicity, and pharmacokinetic and pharmacodynamic characteristics. These venoms are associated with high morbility and mortality, especially among children. Victims of envenoming by a scorpion suffer a variety of pathologies, involving mainly both sympathetic and parasympathetic stimulation as well as central manifestations such as irritability, hyperthermia, vomiting, profuse salivation, tremor, and convulsion. The clinical signs and symptoms observed in humans and experimental animals are related with an excessive systemic host inflammatory response to stings and stings, respectively. Although the pathophysiology of envenomation is complex and not yet fully understood, venom and immune responses are known to trigger the release of inflammatory mediators that are largely mediated by cytokines. In models of severe systemic inflammation produced by injection of high doses of venom or venoms products, the increase in production of proinflammatory cytokines significantly contributes to immunological imbalance, multiple organ dysfunction and death. The cytokines initiate a cascade of events that lead to illness behaviors such as fever, anorexia, and also physiological events in the host such as activation of vasodilatation, hypotension, and increased of vessel permeability.

Molecular recognition and self-assembly special feature: Calix[4]arene-based conical-shaped ligands for voltage-dependent potassium channels

Potassium channels are among the core functional elements of life because they underpin essential cellular functions including excitability, homeostasis, and secretion. We present here a series of multivalent calix[4]arene ligands that bind to the surface of voltage-dependent potassium channels (K(v)1.x) in a reversible manner. Molecular modeling correctly predicts the best candidates with a conical C(4) symmetry for optimal binding, and the effects on channel function are assessed electrophysiologically. Reversible inhibition was observed, without noticeable damage of the oocytes, for tetraacylguanidinium or tetraarginine members of the series with small lower rim O-substituents. Apparent binding constants were in the low micromolar range and had Hill coefficients of 1, consistent with a single site of binding. Suppression of current amplitude was accompanied by a positive shift in the voltage dependence of gating and slowing of both voltage sensor motion and channel opening. These effects are in keeping with expectations for docking in the central pore and interaction with the pore domain "turret."

Characterization of a new Kv1.3 channel-specific blocker, J123, from the scorpion Buthus martensii Karsch

The potassium channel Kv1.3 is an attractive pharmacological target for T-cell-mediated autoimmune diseases, and specific and selective peptidic blockers of Kv1.3 channels have served as valuable therapeutic leads for treating these diseases. Here, we found a new peptide toxin, J123, with 43 amino acids including six cysteine residues by screening the venomous cDNA library of scorpion Buthus martensii Karsch, which has been used as traditional medicine in China for more than 2000 years. The sequence analysis suggested that peptide J123 constituted a new member of the alpha-KTx toxins. The electrophysiological experiments further indicated that peptide J123 has a novel pharmacological profiles: it blocked Kv1.3 channel with high potency (IC50=0.79 nM), and exhibited good selectivity on Kv1.3 over Kv1.1 (>1000-fold) and Kv1.2 (about 30-fold), respectively. Furthermore, peptide J123 had no activity on SKCa2 and SKCa3 channels at micromolar concentration level. Based on the pharmacological activities, the possible channel-interacting surface of peptide J123 was also predicted by molecular modeling and docking. All these data not only enrich the knowledge of the structure-function relationship of the new Kv1.3-speicific peptide but also present a potential drug candidate for selectively targeting Kv1.3 channels.

Molecular basis of inhibitory peptide maurotoxin recognizing Kv1.2 channel explored by ZDOCK and molecular dynamic simulations

Inhibitory peptide-channel interactions have been utilized to characterize both channels and peptides; however, the fundamental basis for these interactions remains elusive. Here, combined computation methods were employed to study the specific binding of maurotoxin (MTX) peptide to Kv1.2 channel. In the first stage, numerous predicted complexes were generated by docking an ensemble of all 35 NMR conformations of MTX to Kv1.2 channel with ZDOCK program. Then the resulted complexes were clustered and classified into four main binding modes, based on experimental information and interaction energy analysis after the energy minimization and molecular dynamics (MD) simulations. By examining the stability of the plausible candidates through unrestrained MD simulations and calculation of the binding free energies, a final reasonable MTX-Kv1.2 complex was identified, with an overall high degree of correlation between the calculation and experiment on mutational effects. In the obtained complex structure model, MTX mainly used its beta-sheet domains to associate the channel mouth instead of the well-recognized functionally important S5P linkers of Kv1.2 channel. Structure analysis characterized that the most essential Tyr(32) residue of MTX was surrounded by a "pocket" formed by many nonpolar and polar residues of Kv1.2 channel, and revealed a pore-blocking Lys(23) and an important Lys(7) stabilized by strong electrostatic interactions with Asp(379) of Kv1.2. Furthermore, a stepwise structural arrangement for both ligand and receptor was found to accompany the tighter interaction of MTX into the target channel. The starting conformation of MTX, the side-chain conformation of the most important residue Tyr(32), and proper introduction of flexibility for candidate complexes were demonstrated to be considerably important factors for obtaining the final reasonable complex structure model. All these findings should not only be helpful for identifying more plausible K(+) channel-inhibitory peptide complex structures, but also provide intrinsically valuable structural biology information to interpret binding affinities, specificities, and diversity of K(+) channel-nature toxin interactions.

Sequential shrinkage and swelling underlie P2X7-stimulated lymphocyte phosphatidylserine exposure and death

Patterns of change in cell volume and plasma membrane phospholipid distribution during cell death are regarded as diagnostic means of distinguishing apoptosis from necrosis, the former being associated with cell shrinkage and early phosphatidylserine (PS) exposure, whereas necrosis is associated with cell swelling and consequent lysis. We demonstrate that cell volume regulation during lymphocyte death stimulated via the purinergic receptor P2X7 is distinct from both. Within seconds of stimulation, murine lymphocytes undergo rapid shrinkage concomitant with, but also required for, PS exposure. However, within 2 min shrinkage is reversed and swelling ensues ending in cell rupture. P2X7-induced shrinkage and PS translocation depend upon K+ efflux via KCa3.1, but use a pathway of Cl- efflux distinct from that previously implicated in apoptosis. Thus, P2X7 stimulation activates a novel pathway of cell death that does not conform to those conventionally associated with apoptosis and necrosis. The mixed apoptotic/necrotic phenotype of P2X7-stimulated cells is consistent with a potential role for this death pathway in lupus disease.

State-dependent inhibition of cystic fibrosis transmembrane conductance regulator chloride channels by a novel peptide toxin

Peptide toxins from animal venom have been used for many years for the identification and study of cation-permeable ion channels. However, no peptide toxins have been identified that interact with known anion-selective channels, including cystic fibrosis transmembrane conductance regulator (CFTR), the protein defective in cystic fibrosis and a member of the ABC transporter superfamily. Here, we describe the identification and initial characterization of a novel 3.7-kDa peptide toxin, GaTx1, which is a potent and reversible inhibitor of CFTR, acting from the cytoplasmic side of the membrane. Thus, GaTx1 is the first peptide toxin identified that inhibits a chloride channel of known molecular identity. GaTx1 exhibited high specificity, showing no effect on a panel of nine transport proteins, including Cl(-) and K(+) channels, and ABC transporters. GaTx1-mediated inhibition of CFTR channel activity is strongly state-dependent; both potency and efficacy are reduced under conditions of elevated [ATP], suggesting that GaTx1 may function as a non-competitive inhibitor of ATP-dependent channel gating. This tool will allow the application of new quantitative approaches to study CFTR structure and function, particularly with respect to the conformational changes that underlie transitions between open and closed states.

The S631A mutation causes a mechanistic switch in the block of hERG channels by CnErg1

We have studied the interaction of CnErg1, a member of the gamma-KTX subfamily of scorpion toxins with the inactivation-deficient S631A hERG channel. In the background of this mutation, we observed a mechanistic switch from turret block, characteristic of the action of gamma-KTXs on Kv11-type channels, to pore plugging, characteristic of alpha-KTX block of Kv1-type channels. We suggest this reflects destabilization of the outer pore (turret region) of hERG allowing access of the toxin molecule to directly plug the conduction pathway.

Mechanism of block of the hERG K+ channel by the scorpion toxin CnErg1

The scorpion toxin CnErg1 binds to human ether-a-go-go related gene (hERG) K(+) channels with a 1:1 stoichiometry and high affinity. However, in contrast to other scorpion toxin-ion channel interactions, the inhibition of macroscopic hERG currents by high concentrations of CnErg1 is incomplete. In this study, we have probed the molecular basis for this incomplete inhibition. High concentrations of CnErg1 had only modest effects on hERG gating that could not account for the incomplete block. Furthermore, the residual current in the presence of 1 microM CnErg1 had normal single channel conductance. Analysis of the kinetics of CnErg1 interaction with hERG indicated that CnErg1 binding is not diffusion-limited. A bimolecular binding scheme that incorporates an initial encounter complex and permits normal ion conduction was able to completely reproduce both the kinetics and steady-state level of CnErg1-hERG binding. This scheme provides a simple kinetic explanation for incomplete block; that is, relatively fast backward compared to forward rate constants for the interconversion of the toxin-channel encounter complex and the blocked toxin-channel complex. We have also examined the temperature-dependence of CnErg1 binding to hERG. The dissociation constant, K(d), for CnErg1 increases from 7.3 nM at 22 degrees C to 64 nM at 37 degrees C (i.e., the affinity decreases as temperature increases) and the proportion of binding events that lead to channel blockade decreases from 70% to 40% over the same temperature range. These temperature-dependent effects on CnErg1 binding correlate with a temperature-dependent decrease in the stability of the putative CnErg1 binding site, the amphipathic alpha-helix in the outer pore domain of hERG, assayed using circular dichroism spectropolarimetry. Collectively, our data provides a plausible kinetic explanation for incomplete blockade of hERG by CnErg1 that is consistent with the proposed highly dynamic conformation of the outer pore domain of hERG.

Probing the outer mouth structure of the HERG channel with peptide toxin footprinting and molecular modeling

Previous studies have shown that the unusually long S5-P linker lining human ether a-go-go related gene's (hERG's) outer vestibule is critical for its channel function: point mutations at high-impact positions here can interfere with the inactivation process and, in many cases, also reduce the pore's K+ selectivity. Because no data are available on the equivalent region in the available K channel crystal structures to allow for homology modeling, we used alternative approaches to model its three-dimensional structure. The first part of this article describes mutant cycle analysis used to identify residues on hERG's outer vestibule that interact with specific residues on the interaction surface of BeKm-1, a peptide toxin with known NMR structure and a high binding affinity to hERG. The second part describes molecular modeling of hERG's pore domain. The transmembrane region was modeled after the crystal structure of KvAP pore domain. The S5-P linker was docked to the transmembrane region based on data from previous NMR and mutagenesis experiments, as well as a set of modeling criteria. The models were further restrained by contact points between hERG's outer vestibule and the bound BeKm-1 toxin molecule deduced from the mutant cycle analysis. Based on these analyses, we propose a working model for the open conformation of the outer vestibule of the hERG channel, in which the S5-P linkers interact with the pore loops to influence ion flux through the pore.

Interaction Simulation of hERG K+ Channel with Its Specific BeKm-1 Peptide: Insights into the Selectivity of Molecular Recognition

Potassium channels show a huge variability in the affinity when recognizing enormous bioactive peptides, and the elucidation of their recognition mechanism remains a great challenge due to an undetermined peptide-channel complex structure. Here, we employed combined computation methods to study the specific binding of BeKm-1 peptide to the hERG potassium channel, which is an essential determinant of the long-QT syndrome. By the use of a segment-assembly homology modeling method, the closed-state hERG structure containing unusual longer S5P linker was successfully constructed. It has a "petunia" shape, while four "petals" of symmetrically distributed S5P segments always decentralize. Starting from the hERG and BeKm-1 structures, a considerably reasonable BeKm-1-hERG complex structure was then screened out and identified by protein-protein docking, molecular dynamics (MD) simulations, and calculation of relative binding free energies. The validity of this predicted complex was further assessed by computational alanine-scanning, with the results correlating reasonably well with experimental data. In the novel complex structure, four considerably flexible S5P linkers are far from the BeKm-1 peptide. The BeKm-1 mainly uses its helical region to associate the channel outer vestibule, except for the S5P linker region; however, structural analysis further implies this neutral pore region with wiggling S5P linker is highly beneficial to the binding of BeKm-1 with lower positive charges. The most critical Lys18 of BeKm-1 plugs its side chain into the channel selectivity filter, while the secondarily important Arg20 forms three hydrogen bonds with spatially neighboring residues in the hERG channel. Different from the classical peptide-K+ channel interaction mainly induced by electrostatic interaction, a synergetic effect of the electrostatic and van der Waals interactions was found to mediate the molecular recognition between BeKm-1 and the hERG channel. And this specific binding process is revealed to be a dynamic change of reduction of binding free energy and conformational rearrangement mainly in the interface of both BeKm-1 and the hERG channel. All these structural and energy features yield deep insights on the high selective binding mechanism of hERG-specific peptides, present a diversity of peptide-K+ channel interactions, and also provide important clues to further study structure-function relationships of the hERG channel.

HgeTx1, the first K+-channel specific toxin characterized from the venom of the scorpion Hadrurus gertschi Soleglad

A novel toxin was identified, purified and characterized from the venom of the Mexican scorpion Hadrurus gertschi (abbreviated HgeTx1). It has a molecular mass of 3950 atomic mass units (a.m.u.) and contains 36 amino acids with four disulfide bridges established between Cys1-Cys5, Cys2-Cys6, Cys3-Cys7 and Cys4-Cys8. It blocks reversibly the Shaker B K(+)-channels with a Kd of 52nM. HgeTx1 shares 60%, 45% and 40% sequence identity, respectively, with Heterometrus spinnifer toxin1 (HsTX1), Scorpio maurus K(+)-toxin (maurotoxin) and Pandinus imperator toxin1 (Pi1), all four-disulfide bridged toxins. It is 57-58% identical with the other scorpion K(+)-channel toxins that contain only three disulfide bridges. Sequence comparison, chain length and number of disulfide bridges analysis classify HgeTx1 into subfamily 6 of the alpha-KTx scorpion toxins (systematic name: alpha-KTx 6.14).

Membrane cholesterol content modulates ClC-2 gating and sensitivity to oxidative stress

ClC-2 is a broadly expressed member of the voltage-gated ClC chloride channel family. In this study, we aimed to evaluate the role of the membrane lipid environment in ClC-2 function, and in particular the effect of cholesterol and ClC-2 distribution in membrane microdomains. Detergent-resistant and detergent-soluble microdomains (DSM) were isolated from stably transfected HEK293 cells by a discontinuous OptiPrep gradient. ClC-2 was found concentrated in detergent-insoluble membranes in basal conditions and relocalized to DSM upon cholesterol depletion by methyl-beta-cyclodextrin. As assessed by patch clamp recordings, relocalization was accompanied by acceleration of the activation kinetics of the channel. A similar distribution and activation pattern were obtained when cells were treated with the oxidant tert-butyl hydroperoxide and after ATP depletion. In both cases activation was prevented by cholesterol enrichment of cells. We conclude that the cholesterol environment regulates ClC-2 activity, and we provide evidence that the increase in ClC-2 activity in response to acute oxidative or metabolic stress involves relocalization of this channel to DSM.

Binding of a gating modifier toxin induces intersubunit cooperativity early in the Shaker K channel's activation pathway

Potassium currents from voltage-gated Shaker K channels activate with a sigmoid rise. The degree of sigmoidicity in channel opening kinetics confirms that each subunit of the homotetrameric Shaker channel undergoes more than one conformational change before the channel opens. We have examined effects of two externally applied gating modifiers that reduce the sigmoidicity of channel opening. A toxin from gastropod mucus, 6-bromo-2-mercaptotryptamine (BrMT), and divalent zinc are both found to slow the same conformational changes early in Shaker's activation pathway. Sigmoidicity measurements suggest that zinc slows a conformational change independently in each channel subunit. Analysis of activation in BrMT reveals cooperativity among subunits during these same early steps. A lack of competition with either agitoxin or tetraethylammonium indicates that BrMT binds channel subunits outside of the external pore region in an allosterically cooperative fashion. Simulations including negatively cooperative BrMT binding account for its ability to induce gating cooperativity during activation. We conclude that cooperativity among K channel subunits can be greatly altered by experimental conditions.

Inhibition of ClC-2 Chloride Channels by a Peptide Component or Components of Scorpion Venom

ClC chloride channels play essential roles in membrane excitability and maintenance of osmotic balance. Despite the recent crystallization of two bacterial ClC-like proteins, the gating mechanism for these channels remains unclear. In this study we tested scorpion venom for the presence of novel peptide inhibitors of ClC channels, which might be useful tools for dissecting the mechanisms underlying ClC channel gating. Recently, it has been shown that a peptide component of venom from the scorpion L. quinquestriatus hebraeus inhibits the CFTR chloride channel from the intracellular side. Using two-electrode voltage clamp we studied the effect of scorpion venom on ClC-0, -1, and -2, and found both dose- and voltage-dependent inhibition only of ClC-2. Comparison of voltage-dependence of inhibition by venom to that of known pore blockers revealed opposite voltage dependencies, suggesting different mechanisms of inhibition. Kinetic data show that venom induced slower activation kinetics compared to pre-venom records, suggesting that the active component(s) of venom may function as a gating modifier at ClC-2. Trypsinization abolished the inhibitory activity of venom, suggesting that the component(s) of scorpion venom that inhibits ClC-2 is a peptide.

Slow inactivation in voltage gated potassium channels is insensitive to the binding of pore occluding peptide toxins

Voltage gated potassium channels open and inactivate in response to changes of the voltage across the membrane. After removal of the fast N-type inactivation, voltage gated Shaker K-channels (Shaker-IR) are still able to inactivate through a poorly understood closure of the ion conduction pore. This, usually slower, inactivation shares with binding of pore occluding peptide toxin two important features: i), both are sensitive to the occupancy of the pore by permeant ions or tetraethylammonium, and ii), both are critically affected by point mutations in the external vestibule. Thus, mutual interference between these two processes is expected. To explore the extent of the conformational change involved in Shaker slow inactivation, we estimated the energetic impact of such interference. We used kappa-conotoxin-PVIIA (kappa-PVIIA) and charybdotoxin (CTX) peptides that occlude the pore of Shaker K-channels with a simple 1:1 stoichiometry and with kinetics 100-fold faster than that of slow inactivation. Because inactivation appears functionally different between outside-out patches and whole oocytes, we also compared the toxin effect on inactivation with these two techniques. Surprisingly, the rate of macroscopic inactivation and the rate of recovery, regardless of the technique used, were toxin insensitive. We also found that the fraction of inactivated channels at equilibrium remained unchanged at saturating kappa-PVIIA. This lack of interference with toxin suggests that during slow inactivation the toxin receptor site remains unaffected, placing a strong geometry-conservative constraint on the possible structural configurations of a slow inactivated K-channel. Such a constraint could be fulfilled by a concerted rotation of the external vestibule.

Electrostatic Recognition and Induced Fit in the κ-PVIIA Toxin Binding to Shaker Potassium Channel

Brownian dynamics (BD) and molecular dynamics (MD) simulations and electrostatic calculations were performed to study the binding process of kappa-PVIIA to the Shaker potassium channel and the structure of the resulting complex. BD simulations, guided by electrostatic interactions, led to an initial alignment between the toxin and the channel protein. MD simulations were then carried out to allow for rearrangements from this initial structure. After approximately 4 ns, a critical "induced fit" process was observed to last for approximately 2 ns. In this process, the interface was reorganized, and side chains were moved so that favorable atomic contacts were formed or strengthened, while unfavorable contacts were eliminated. The final complex structure was stabilized through electrostatic interactions with the positively charged side chain of Lys7 of kappa-PVIIA deeply inserted into the channel pore and other hydrogen bonds and by hydrophobic interactions involving Phe9 and Phe23 of the toxin. The validity of the predicted structure for the complex was assessed by calculating the effects of mutating charged and polar residues of both the toxin and the channel protein, with the calculated effects correlating reasonably well with experimental data. The present study suggests a general binding mechanism, whereby proteins are pre-aligned in their diffusional encounter by long-range electrostatic attraction, and nanosecond-scale rearrangements within the initial complex then lead to a specifically bound complex.

Voltage-sensing arginines in a potassium channel permeate and occlude cation-selective pores

Voltage-gated ion channels sense voltage by shuttling arginine residues located in the S4 segment across the membrane electric field. The molecular pathway for this arginine permeation is not understood, nor is the filtering mechanism that permits passage of charged arginines but excludes solution ions. We find that substituting the first S4 arginine with smaller amino acids opens a high-conductance pathway for solution cations in the Shaker K(+) channel at rest. The cationic current does not flow through the central K(+) pore and is influenced by mutation of a conserved residue in S2, suggesting that it flows through a protein pathway within the voltage-sensing domain. The current can be carried by guanidinium ions, suggesting that this is the pathway for transmembrane arginine permeation. We propose that when S4 moves it ratchets between conformations in which one arginine after another occupies and occludes to ions the narrowest part of this pathway.

Oxidation and reduction control of the inactivation gating of Torpedo ClC-0 chloride channels

Oxidation and reduction (redox) are known to modulate the function of a variety of ion channels. Here, we report a redox regulation of the function of ClC-0, a chloride (Cl(-)) channel from the Torpedo electric organ. The study was motivated by the occasional observation of oocytes with hyperpolarization-activated Cl(-) current when these oocytes expressed ClC-0. We find that these atypical recording traces can be turned into typical ClC-0 current by incubating the oocyte in millimolar concentrations of reducing agents, suggesting that the channel function is regulated by oxidation and reduction. The redox control apparently results from an effect of oxidation on the slow (inactivation) gating: oxidation renders it more difficult for the channel to recover from the inactivated states. Introducing the point mutation C212S in ClC-0 suppresses the inactivation state, and this inactivation-suppressed mutant is no longer sensitive to the inhibition by oxidizing reagents. However, C212 is probably not the target for the redox reaction because the regulation of the inactivation gating by oxidation is still present in a pore mutant (K165C/K165 heterodimer) in which the C212S mutation is present. Taking advantage of the K165C/K165 heterodimer, we further explore the oxidation effect in ClC-0 by methane thiosulfonate (MTS) modifications. We found that trimethylethylammonium MTS modification of the introduced cysteine can induce current in the K165C/K165 heterodimer, an effect attributed to the recovery of the channel from the inactivation state. The current induction by MTS reagents is subjected to redox controls, and thus the extent of this current induction can serve as an indicator to report the oxidation state of the channel. These results together suggest that the inactivation gating of ClC-0 is affected by redox regulation. The finding also provides a convenient method to "cure" those atypical recording traces of ClC-0 expressed in Xenopus oocytes.