Gating modifier toxins reveal a conserved structural motif in voltage-gated Ca2+ and K+ channels

Tarantula toxin ProTx-I differentiates between human T-type voltage-gated Ca2+ Channels Cav3.1 and Cav3.2

ProTx-I peptide, a venom toxin of the tarantula Thrixopelma pruriens, has been reported to interact with voltage-gated ion channels. ProTx-I reduced Ba(2+) currents through recombinant human T-type voltage-gated Ca(2+) channels, Ca(v)3.1 (hCa(v)3.1), with roughly 160-fold more potency than through hCa(v)3.2 channels. Chimeric channel proteins (hCa(v)3.1/S3S4 and hCa(v)3.2/S3S4) were produced by exchanging fourteen amino acids in the hCa(v)3.1 domain IV S3-S4 linker region and the corresponding region of hCa(v)3.2 between each other. The ProTx-I sensitivity was markedly reduced in the hCa(v)3.1/S3S4 chimera as compared to the original hCa(v)3.1 channel, while the hCa(v)3.2/S3S4 chimera exhibited greater ProTx-I sensitivity than the original hCa(v)3.2 channel. These results suggest that the domain IV S3-S4 linker in the hCa(v)3.1 channel may contain residues involved in the interaction of ProTx-I with T-type Ca(2+) channels.

Targeting voltage sensors in sodium channels with spider toxins

Voltage-activated sodium (Nav) channels are essential in generating and propagating nerve impulses, placing them amongst the most widely targeted ion channels by toxins from venomous organisms. An increasing number of spider toxins have been shown to interfere with the voltage-driven activation process of mammalian Nav channels, possibly by interacting with one or more of their voltage sensors. This review focuses on our existing knowledge of the mechanism by which spider toxins affect Nav channel gating and the possible applications of these toxins in the drug discovery process.

Target promiscuity and heterogeneous effects of tarantula venom peptides affecting Na+ and K+ ion channels

Venom-derived peptide modulators of ion channel gating are regarded as essential tools for understanding the molecular motions that occur during the opening and closing of ion channels. In this study, we present the characterization of five spider toxins on 12 human voltage-gated ion channels, following observations about the target promiscuity of some spider toxins and the ongoing revision of their "canonical" gating-modifying mode of action. The peptides were purified de novo from the venom of Grammostola rosea tarantulas, and their sequences were confirmed by Edman degradation and mass spectrometry analysis. Their effects on seven tetrodotoxin-sensitive Na(+) channels, the three human ether-à-go-go (hERG)-related K(+) channels, and two human Shaker-related K(+) channels were extensively characterized by electrophysiological techniques. All the peptides inhibited ion conduction through all the Na(+) channels tested, although with distinctive patterns. The peptides also affected the three pharmaceutically relevant hERG isoforms differently. At higher concentrations, all peptides also modified the gating of the Na(+) channels by shifting the activation to more positive potentials, whereas more complex effects were recorded on hERG channels. No effects were evident on the two Shaker-related K(+) channels at concentrations well above the IC(50) value for the affected channels. Given the sequence diversity of the tested peptides, we propose that tarantula toxins should be considered both as multimode and target-promiscuous ion channel modulators; both features should not be ignored when extracting mechanistic interpretations about ion channel gating. Our observations could also aid in future structure-function studies and might help the development of novel ion channel-specific drugs.

Coupling of S4 helix translocation and S6 gating analyzed by molecular-dynamics simulations of mutated Kv channels

The recently determined crystal structure of a chimeric Kv1.2-Kv2.1 Kv channel at 2.4 A resolution motivated this molecular-dynamics simulation study of the chimeric channel and its mutants embedded in a DPPC membrane. For the channel protein, we used two types of C-terminus: E+ and Eo. E+ contains, and Eo lacks, the EGEE residue quartet located distal to the S6 helix. For both E+ and Eo, the following trend was observed: When S4 helices were restrained at the same position as in the x-ray structure (S4high), the S6 gate remained open for 12 ns. The results were similar when the S4 helices were pulled downward 7 A (S4low). However, S4middle (or S4low) facilitated the S6 gate-narrowing for the following mutated channels (shown in order of increasing effect): 1), E395W; 2), E395W-F401A-F402A; and 3), E395W-F401A-F402A-V478W. The amino acid numbering system is that used for the Shaker channel. Even though all four subunits were set at S4low, S6 gate-narrowing was often brought about by movements of only two opposing S6 helices toward the central axis of the pore, resulting in a twofold symmetry-like structure. A free-energy profile analysis over the ion conduction pathway shows that the two opposing S6 helices whose peptide backbones are approximately 10.4 A distant from each other lead to an energetic barrier of approximately 25 kJ/mol. S6 movement was coupled with translocation of the S4-S5 linker toward the central axis of the same subunit, and the coupling was mediated by salt bridges formed between the inner (intracellular side) end of S4 and that of S6. Simulations in which S4 of only one subunit was pulled down to S4low showed that a weak intersubunit coordination is present for S5 movement, whereas the coupling between the S4-S5 linker and S6 is largely an intrasubunit one. In general, whereas subunit-based behavior appears to be dominant and to permit heteromeric conformations of the pore domain, direct intersubunit coupling of S5 or S6 is weak. Therefore, the "concerted transition" of the pore domain that has been predicted based on electrophysiological analyses is likely to be mediated mainly by the dual effects of S4 and the S4-S5 linker; these segments of one subunit can interact with both S5 of the same subunit and that of the adjacent subunit.

Sea anemone toxins affecting potassium channels

The great diversity of K(+) channels and their wide distribution in many tissues are associated with important functions in cardiac and neuronal excitability that are now better understood thanks to the discovery of animal toxins. During the past few decades, sea anemones have provided a variety of toxins acting on voltage-sensitive sodium and, more recently, potassium channels. Currently there are three major structural groups of sea anemone K(+) channel (SAK) toxins that have been characterized. Radioligand binding and electrophysiological experiments revealed that each group contains peptides displaying selective activities for different subfamilies of K(+) channels. Short (35-37 amino acids) peptides in the group I display pore blocking effects on Kv1 channels. Molecular interactions of SAK-I toxins, important for activity and binding on Kv1 channels, implicate a spot of three conserved amino acid residues (Ser, Lys, Tyr) surrounded by other less conserved residues. Long (58-59 amino acids) SAK-II peptides display both enzymatic and K(+) channel inhibitory activities. Medium size (42-43 amino acid) SAK-III peptides are gating modifiers which interact either with cardiac HERG or Kv3 channels by altering their voltage-dependent properties. SAK-III toxins bind to the S3C region in the outer vestibule of Kv channels. Sea anemones have proven to be a rich source of pharmacological tools, and some of the SAK toxins are now useful drugs for the diagnosis and treatment of autoimmune diseases.

Sensing voltage across lipid membranes

The detection of electrical potentials across lipid bilayers by specialized membrane proteins is required for many fundamental cellular processes such as the generation and propagation of nerve impulses. These membrane proteins possess modular voltage-sensing domains, a notable example being the S1-S4 domains of voltage-activated ion channels. Ground-breaking structural studies on these domains explain how voltage sensors are designed and reveal important interactions with the surrounding lipid membrane. Although further structures are needed to understand the conformational changes that occur during voltage sensing, the available data help to frame several key concepts that are fundamental to the mechanism of voltage sensing.

Deconstructing voltage sensor function and pharmacology in sodium channels

Voltage-activated sodium (Nav) channels are crucial for the generation and propagation of nerve impulses, and as such are amongst the most widely targeted ion channels by toxins and drugs. The four voltage sensors in Nav channels have distinct amino acid sequences, raising fundamental questions about their relative contributions to the function and pharmacology of the channel. Here we use four-fold symmetric voltage-activated potassium (Kv) channels as reporters to examine the contributions of individual Nav channel S3b-S4 paddle motifs to the kinetics of voltage sensor activation and to forming toxin receptors. Our results uncover binding sites for toxins from tarantula and scorpion venom on each of the four paddle motifs in Nav channels and reveal how paddle-specific interactions can be used to reshape Nav channel activity. One paddle motif is unique in that it slows voltage sensor activation and toxins selectively targeting this motif impede Nav channel inactivation. This reporter approach and the principles that emerge will be useful in developing new drugs for treating pain and Nav channelopathies.

Structure, function, and modification of the voltage sensor in voltage-gated ion channels

Voltage-gated ion channels are crucial for both neuronal and cardiac excitability. Decades of research have begun to unravel the intriguing machinery behind voltage sensitivity. Although the details regarding the arrangement and movement in the voltage-sensor domain are still debated, consensus is slowly emerging. There are three competing conceptual models: the helical-screw, the transporter, and the paddle model. In this review we explore the structure of the activated voltage-sensor domain based on the recent X-ray structure of a chimera between Kv1.2 and Kv2.1. We also present a model for the closed state. From this we conclude that upon depolarization the voltage sensor S4 moves approximately 13 A outwards and rotates approximately 180 degrees, thus consistent with the helical-screw model. S4 also moves relative to S3b which is not consistent with the paddle model. One interesting feature of the voltage sensor is that it partially faces the lipid bilayer and therefore can interact both with the membrane itself and with physiological and pharmacological molecules reaching the channel from the membrane. This type of channel modulation is discussed together with other mechanisms for how voltage-sensitivity is modified. Small effects on voltage-sensitivity can have profound effects on excitability. Therefore, medical drugs designed to alter the voltage dependence offer an interesting way to regulate excitability.

Ni2+ block of CaV3.1 (alpha1G) T-type calcium channels

Ni²⁺ inhibits current through calcium channels, in part by blocking the pore, but Ni²⁺ may also allosterically affect channel activity via sites outside the permeation pathway. As a test for pore blockade, we examined whether the effect of Ni²⁺ on Ca_V3.1 is affected by permeant ions. We find two components to block by Ni²⁺, a rapid block with little voltage dependence, and a slow block most visible as accelerated tail currents. Rapid block is weaker for outward vs. inward currents (apparent K_d = 3 vs. 1 mM Ni²⁺, with 2 mM Ca²⁺ or Ba²⁺) and is reduced at high permeant ion concentration (110 vs. 2 mM Ca²⁺ or Ba²⁺). Slow block depends both on the concentration and on the identity of the permeant ion (Ca²⁺ vs. Ba²⁺ vs. Na⁺). Slow block is 2–3× faster in Ba²⁺ than in Ca²⁺ (2 or 110 mM), and is ∼10× faster with 2 vs. 110 mM Ca²⁺ or Ba²⁺. Slow block is orders of magnitude slower than the diffusion limit, except in the nominal absence of divalent cations (∼3 μM Ca²⁺). We conclude that both fast and slow block of Ca_V3.1 by Ni²⁺ are most consistent with occlusion of the pore. The exit rate of Ni²⁺ for slow block is reduced at high Ni²⁺ concentrations, suggesting that the site responsible for fast block can “lock in” slow block by Ni²⁺, at a site located deeper within the pore. In contrast to the complex pore block observed for Ca_V3.1, inhibition of Ca_V3.2 by Ni²⁺ was essentially independent of voltage, and was similar in 2 mM Ca²⁺ vs. Ba²⁺, consistent with inhibition by a different mechanism, at a site outside the pore.

Lipid bilayer deformation and the free energy of interaction of a Kv channel gating-modifier toxin

A number of membrane proteins act via binding at the water/lipid bilayer interface. An important example of such proteins is provided by the gating-modifier toxins that act on voltage-gated potassium (Kv) channels. They are thought to partition to the headgroup region of lipid bilayers, and so provide a good system for probing the nature of interactions of a protein with the water/bilayer interface. We used coarse-grained molecular dynamics simulations to compute the one-dimensional potential of mean force (i.e., free energy) profile that governs the interaction between a Kv channel gating-modifier toxin (VSTx1) and model phospholipid bilayers. The reaction coordinate sampled corresponds to the position of the toxin along the bilayer normal. The course-grained representation of the protein and lipids enabled us to explore extended time periods, revealing aspects of toxin/bilayer dynamics and energetics that would be difficult to observe on the timescales currently afforded by atomistic molecular dynamics simulations. In particular, we show for this model system that the bilayer deforms as it interacts with the toxin, and that such deformations perturb the free energy profile. Bilayer deformation therefore adds an additional layer of complexity to be addressed in investigations of membrane/protein systems. In particular, one should allow for local deformations that may arise due to the spatial array of charged and hydrophobic elements of an interfacially located membrane protein.

Molecular dynamics simulation of Kv channel voltage sensor helix in a lipid membrane with applied electric field

In this article, we present the results of the molecular dynamics simulations of amphiphilic helix peptides of 13 amino-acid residues, placed at the lipid-water interface of dipalmitoylphosphatidylcholine bilayers. The peptides are identical with, or are derivatives of, the N-terminal segment of the S4 helix of voltage-dependent K channel KvAP, containing four voltage-sensing arginine residues (R1-R4). Upon changing the direction of the externally applied electric field, the tilt angle of the wild-type peptide changes relative to the lipid-water interface, with the N-terminus heading up with an outward electric field. These movements were not observed using an octane membrane in place of the dipalmitoylphosphatidylcholine membrane, and were markedly suppressed by 1), substituting Phe located one residue before the first arginine (R1) with a hydrophilic residue (Ser, Thr); or 2), changing the periodicity rule of Rs from at-every-third to at-every-fourth position; or 3), replacing R1 with a lysine residue (K). These and other findings suggest that the voltage-dependent movement requires deep positioning of Rs when the resting (inward) electric field is present. Later, we performed simulations of the voltage sensor domain (S1-S4) of Kv1.2 channel. In simulations with a strong electric field (0.1 V/nm or above) and positional restraints on the S1 and S2 helices, S4 movement was observed consisting of displacement along the S4 helix axis and a screwlike axial rotation. Gating-charge-carrying Rs were observed to make serial interactions with E183 in S1 and E226 in S2, in the outer water crevice. A 30-ns-backward simulation started from the open-state model gave rise to a structure similar to the recent resting-state model, with S4 moving vertically approximately 6.7 A. The energy landscape around the movement of S4 appears very ragged due to salt bridges formed between gating-charge-carrying residues and negatively charged residues of S1, S2, and S3 helices. Overall, features of S3 and S4 movements are consistent with the recent helical-screw model. Both forward and backward simulations show the presence of at least two stable intermediate structures in which R2 and R3 form salt bridges with E183 or E226, respectively. These structures are the candidates for the states postulated in previous gating kinetic models, such as the Zagotta-Hoshi-Aldrich model, to account for more than one transition step per subunit for activation.

Portability of paddle motif function and pharmacology in voltage sensors

Voltage-sensing domains enable membrane proteins to sense and react to changes in membrane voltage. Although identifiable S1-S4 voltage-sensing domains are found in an array of conventional ion channels and in other membrane proteins that lack pore domains, the extent to which their voltage-sensing mechanisms are conserved is unknown. Here we show that the voltage-sensor paddle, a motif composed of S3b and S4 helices, can drive channel opening with membrane depolarization when transplanted from an archaebacterial voltage-activated potassium channel (KvAP) or voltage-sensing domain proteins (Hv1 and Ci-VSP) into eukaryotic voltage-activated potassium channels. Tarantula toxins that partition into membranes can interact with these paddle motifs at the protein-lipid interface and similarly perturb voltage-sensor activation in both ion channels and proteins with a voltage-sensing domain. Our results show that paddle motifs are modular, that their functions are conserved in voltage sensors, and that they move in the relatively unconstrained environment of the lipid membrane. The widespread targeting of voltage-sensor paddles by toxins demonstrates that this modular structural motif is an important pharmacological target.

Peptides Selected for the Protein Nanocage Pores Change the Rate of Iron Recovery from the Ferritin Mineral

Pores regulate access between ferric-oxy biomineral inside and reductants/chelators outside the ferritin protein nanocage to control iron demineralization rates. The pore helix/loop/helix motifs that are contributed by three subunits unfold independently of the protein cage, as observed by crystallography, Fe removal rates, and CD spectroscopy. Pore unfolding is induced in wild type ferritin by increased temperature or urea (1-10 mM), a physiological urea range, 0.1 mM guanidine, or mutation of conserved pore amino acids. A peptide selected for ferritin pore binding from a combinatorial, heptapeptide library increased the rate of Fe demineralization 3-fold (p<0.001), similarly to a mutation that unfolded the pores. Conjugating the peptide to Desferal (desferrioxamine B mesylate), a chelator in therapeutic use, increased the rates to 8-fold (p<0.001). A second pore binding peptide had the opposite effect and decreased the rate of Fe demineralization 60% (p<0.001). The peptides could have pharmacological uses and may model regulators of ferritin demineralization rates in vivo or peptide regulators of gated pores in membranes. The results emphasize that small peptides can exploit the structural plasticity of protein pores to modulate function.

Tarantula toxins interact with voltage sensors within lipid membranes

Voltage-activated ion channels are essential for electrical signaling, yet the mechanism of voltage sensing remains under intense investigation. The voltage-sensor paddle is a crucial structural motif in voltage-activated potassium (K_v) channels that has been proposed to move at the protein–lipid interface in response to changes in membrane voltage. Here we explore whether tarantula toxins like hanatoxin and SGTx1 inhibit K_v channels by interacting with paddle motifs within the membrane. We find that these toxins can partition into membranes under physiologically relevant conditions, but that the toxin–membrane interaction is not sufficient to inhibit K_v channels. From mutagenesis studies we identify regions of the toxin involved in binding to the paddle motif, and those important for interacting with membranes. Modification of membranes with sphingomyelinase D dramatically alters the stability of the toxin–channel complex, suggesting that tarantula toxins interact with paddle motifs within the membrane and that they are sensitive detectors of lipid–channel interactions.

Jingzhaotoxin-XII, a gating modifier specific for Kv4.1 channels

Jingzhaotoxin-XII (JZTX-XII), a 29-residue polypeptide, was purified from the venom of the Chinese tarantula Chilobrachys jingzhao. Electrophysiological recordings carried out in Xenopus laevis oocytes showed that JZTX-XII is specific for Kv4.1 channels, with the IC50 value of 0.363 microM. It interacts with the channels by modifying the gating behavior. JZTX-XII shares 80% sequence identity with phrixotoxin1, a potent inhibitor for Kv4.2 and Kv4.3 channels. Structure analysis indicates that the difference of the charge distribution in the interactive surface perhaps influences the specific pharmacology of the toxins. JZTX-XII should be a valuable tool for the investigation of the Kv4.1 channels.

Solution structure of Jingzhaotoxin-III, a peptide toxin inhibiting both Nav1.5 and Kv2.1 channels

Jingzhaotoxin-III (JZTX-III) is a peptide toxin isolated from the venom of the Chinese spider Chilobrachys jingzhao that inhibits Nav channels of rat cardiac myocytes by modifying voltage-dependent gating and also binds to Kv2.1 channel (Kd = 0.43 microM) with an action model similar to that of hanatoxin1 and SGTx1. The solution structure of JZTX-III was determined by (1)H 2D NMR method. The toxin adopts an ICK motif composed of three beta-strands connected by four turns. Structural comparison of JZTX-III with those of other ICK motif peptides shows that they all adopt a conserved surface profile, a hydrophobic patch surrounded by charged residues, which might be the crucial site for voltage-gating ion channel inhibition. Furthermore, the similar action model of JZTX-III affecting both Kv and Nav channels implies that JZTX-III recognized a conserved receptor within the voltage sensing domains, which is similar to that of hanatoxin1 binding to both Kv and Cav channels.

Isolation and characterization of a novel toxin from the venom of the spider Grammostola rosea that blocks sodium channels

This communication reports the chemical and physiological characterization of a novel peptide (GrTx1) isolated from the venom of the "rosean-tarantula"Grammostola rosea. This component was one among more than 15 distinct components separated from the soluble venom by high-performance liquid chromatography (HPLC). GrTx1 has 29 amino-acid residues, compactly folded by three disulfide bridges with a molecular weight of 3697 Da. Here we show that this peptide blocks Na(+) currents of neuroblastoma F-11 cells with an IC(50) of 2.8+/-0.1 microM, up to a maximum of about 85% at 10 microM. Moreover, the right-shift (+20.1+/-0.4 mV) of the fractional voltage-dependent conductance could be also compatible with a putative "gating-modifier" mechanism. No effects were seen on common K(+) channels, such as K(v)1.1 and 1.4, using concentrations of toxin up to 10 microM. Sequence analysis reveals that GrTx1 is closely related to other spider toxins reported to affect various distinct ion channel functions. A critical analysis of this study suggests the necessity to search for other potential receptor sites in order to establish the preferred specificity of these kind of peptides.

Insect-selective spider toxins targeting voltage-gated sodium channels

The voltage-gated sodium (Na(v)) channel is a target for a number of drugs, insecticides and neurotoxins. These bind to at least seven identified neurotoxin binding sites and either block conductance or modulate Na(v) channel gating. A number of peptide neurotoxins from the venoms of araneomorph and mygalomorph spiders have been isolated and characterized and determined to interact with several of these sites. These all conform to an 'inhibitor cystine-knot' motif with structural, but not sequence homology, to a variety of other spider and marine snail toxins. Of these, spider toxins several show phyla-specificity and are being considered as lead compounds for the development of biopesticides. Hainantoxin-I appears to target site-1 to block Na(v) channel conductance. Magi 2 and Tx4(6-1) slow Na(v) channel inactivation via an interaction with site-3. The delta-palutoxins, and most likely mu-agatoxins and curtatoxins, target site-4. However, their action is complex with the mu-agatoxins causing a hyperpolarizing shift in the voltage-dependence of activation, an action analogous to scorpion beta-toxins, but with both delta-palutoxins and mu-agatoxins slowing Na(v) channel inactivation, a site-3-like action. In addition, several other spider neurotoxins, such as delta-atracotoxins, are known to target both insect and vertebrate Na(v) channels most likely as a result of the conserved structures within domains of voltage-gated ion channels across phyla. These toxins may provide tools to establish the molecular determinants of invertebrate selectivity. These studies are being greatly assisted by the determination of the pharmacophore of these toxins, but without precise identification of their binding site and mode of action their potential in the above areas remains underdeveloped.

Functional interactions at the interface between voltage-sensing and pore domains in the Shaker K(v) channel

Voltage-activated potassium (K(v)) channels contain a central pore domain that is partially surrounded by four voltage-sensing domains. Recent X-ray structures suggest that the two domains lack extensive protein-protein contacts within presumed transmembrane regions, but whether this is the case for functional channels embedded in lipid membranes remains to be tested. We investigated domain interactions in the Shaker K(v) channel by systematically mutating the pore domain and assessing tolerance by examining channel maturation, S4 gating charge movement, and channel opening. When mapped onto the X-ray structure of the K(v)1.2 channel the large number of permissive mutations support the notion of relatively independent domains, consistent with crystallographic studies. Inspection of the maps also identifies portions of the interface where residues are sensitive to mutation, an external cluster where mutations hinder voltage sensor activation, and an internal cluster where domain interactions between S4 and S5 helices from adjacent subunits appear crucial for the concerted opening transition.

Voltage-gated ion channels and gating modifier toxins

Voltage-gated sodium, calcium, and potassium channels generate electrical signals required for action potential generation and conduction and are the molecular targets for a broad range of potent neurotoxins. These channels are built on a common structural motif containing six transmembrane segments and a pore loop. Their pores are formed by the S5/S6 segments and the pore loop between them, and they are gated by bending of the S6 segments at a hinge glycine or proline residue. The voltage sensor domain consists of the S1-S4 segments, with positively charged residues in the S4 segment serving as gating charges. The diversity of toxin action on these channels is illustrated by sodium channels, which are the molecular targets for toxins that act at six or more distinct receptor sites on the channel protein. Both hydrophilic low molecular weight toxins and larger polypeptide toxins physically block the pore and prevent sodium conductance. Hydrophobic alkaloid toxins and related lipid-soluble toxins act at intramembrane sites and alter voltage-dependent gating of sodium channels via an allosteric mechanism. In contrast, polypeptide toxins alter channel gating by voltage-sensor trapping through binding to extracellular receptor sites, and this toxin interaction has now been modeled at the atomic level for a beta-scorpion toxin. The voltage-sensor trapping mechanism may be a common mode of action for polypeptide gating modifier toxins acting on all of the voltage-gated ion channels.

Gating modifier peptides as probes of pancreatic β-cell physiology

Pancreatic beta-cells depolarize in response to glucose and fire calcium-dependent actions potentials that trigger insulin secretion. The major current responsible for action potential repolarization in these cells is a delayed rectifier and Kv2.1 subunits are thought be a major contributor of the delayed rectifier channels. Hence, blockers of Kv2.1 channels might prolong action potentials and enhance calcium influx and insulin secretion. However, the lack of specific small molecule Kv2.1 inhibitors has hindered the testing of this mechanism. Importantly, several gating modifier peptides inhibit Kv2.1 channels in a relatively specific fashion. Hanatoxin (HaTX) and guangxitoxin-1 (GxTX-1) are examples that have been used to probe the role of Kv2.1 channels in beta-cell physiology. Both HaTX and GxTX-1 strongly inhibit the Kv current of beta-cells from various species, arguing that Kv2.1 subunits contribute significantly to the beta-cell delayed rectifier. GxTX-1 prolongs glucose-triggered action potentials, enhances glucose-dependent intracellular calcium elevations and augments glucose-dependent insulin secretion. Taken together, these data suggest that blockers of Kv2.1 channels may be a useful approach to the design of novel therapeutic agents for the treatment of type 2 diabetes. These studies highlight the utility of gating modifier peptides in the study of physiological systems.

The interaction of spider gating modifier peptides with voltage-gated potassium channels

Gating modifier peptides bind to ion channels and alter the gating process of these molecules. One of the most extensively studied peptides, Hanatoxin (HaTx), isolated from a Chilean tarantula, has been used to characterize the blocking properties of the voltage-gated potassium channel Kv2.1. These studies have provided some insight into the gating mechanism in Kv channels. In this review we will discuss the interaction of HaTx and related spider peptides with Kv channels illustrating the properties of the binding surface of these peptides, their membrane partitioning characteristics, and will provide a working hypothesis for how the peptides inhibit gating of Kv channels. Advanced simulation results support the concept of mutual conformational changes upon peptide binding to the S3b region of the channel which will restrict movement of S4 and compromise coupling of the gating machinery to opening of the pore.

Metals in membranes

In this critical review we discuss recent advances in understanding the modes of interaction of metal ions with membrane proteins, including channels, pumps, transporters, ATP-binding cassette proteins, G-protein coupled receptors, kinases and respiratory enzymes. Such knowledge provides a basis for elucidating the mechanism of action of some classes of metallodrugs, and a stimulus for the further exploration of the coordination chemistry of metal ions in membranes. Such research offers promise for the discovery of new drugs with unusual modes of action. The article will be of interest to bioinorganic chemists, chemical biologists, biochemists, pharmacologists and medicinal chemists. (247 references).

Molecular diversification in spider venoms: a web of combinatorial peptide libraries

Spider venoms are a rich source of novel pharmacologically and agrochemically interesting compounds that have received increased attention from pharmacologists and biochemists in recent years. The application of technologies derived from genomics and proteomics have led to the discovery of the enormous molecular diversity of those venoms, which consist mainly of peptides and proteins. The molecular diversity of spider peptides has been revealed by mass spectrometry and appears to be based on a limited set of structural scaffolds. Genetic analysis has led to a further understanding of the molecular evolution mechanisms presiding over the generation of these combinatorial peptide libraries. Gene duplication and focal hypermutation, which has been described in cone snails, appear to be common mechanisms to venomous mollusks and spiders. Post-translational modifications, fine structural variations and new molecular scaffolds are other potential mechanisms of toxin diversification, leading to the pharmacologically complex cocktails used for predation and defense.

Tarantula toxins interacting with voltage sensors in potassium channels

Voltage-activated ion channels open and close in response to changes in membrane voltage, a process that is crucial for electrical signaling in the nervous system. The venom from many poisonous creatures contains a diverse array of small protein toxins that bind to voltage-activated channels and modify the gating mechanism. Hanatoxin and a growing number of related tarantula toxins have been shown to inhibit activation of voltage-activated potassium (K(v)) channels by interacting with their voltage-sensing domains. This review summarizes our current understanding of the mechanism by which these toxins alter gating, the location of the toxin receptor within K(v) channels and the disposition of this receptor with respect to the lipid membrane. The conservation of tarantula toxin receptors among voltage-activated ion channels will also be discussed.

Voltage-gated sodium channel modulation by scorpion alpha-toxins

Voltage-gated Na(+) channels are integral membrane proteins that function as a gateway for a selective permeation of sodium ions across biological membranes. In this way, they are crucial players for the generation of action potentials in excitable cells. Voltage-gated Na(+) channels are encoded by at least nine genes in mammals. The different isoforms have remarkably similar functional properties, but small changes in function and pharmacology are biologically well-defined, as underscored by mutations that cause several diseases and by modulation of a myriad of compounds, respectively. This review will stress on the modulation of voltage-gated Na(+) channels by scorpion alpha-toxins. Nature has designed these two classes of molecules as if they were predestined to each other: an inevitable 'encounter' between a voltage-gated Na(+) channel isoform and an alpha-toxin from scorpion venom indeed results in a dramatically changed Na(+) current phenotype with clear-cut consequences on electrical excitability and sometimes life or death. This fascinating aspect justifies an overview on scorpion venoms, their alpha-toxins and the Na(+) channel targets they are built for, as well as on the molecular determinants that govern the selectivity and affinity of this 'inseparable duo'.

Gating modifier toxins of voltage-gated calcium channels

Some of the most potent and specific inhibitors of voltage-gated calcium channels are peptide toxins that inhibit channel function not by occlusion of the channel pore, but rather by interfering with the voltage dependence and kinetics of channel opening and closing. Many such gating modifier toxins conform to the inhibitor cystine knot structural family and have primary sequence or functional mechanism similar to toxins that target voltage-gated sodium or potassium channels. This review introduces known gating modifiers of calcium channels, discusses the selectivity, binding sites, and mechanism of the toxin-channel interaction, and reviews the usefulness of these toxins as research tools and as the basis for novel calcium channel pharmacology and therapeutics.

Kv2 channels oppose myogenic constriction of rat cerebral arteries

By hyperpolarizing arterial smooth muscle, voltage-gated, Ca2+-independent K+ (Kv) channels decrease calcium influx and thus oppose constriction. However, the molecular nature of the Kv channels function in arterial smooth muscle remains controversial. Recent investigations have emphasized a predominant role of Kv1 channels in regulating arterial tone. In this study, we tested the hypothesis Kv2 channels may also significantly regulate tone of rat cerebral arteries. We found that Kv2.1 transcript and protein are present in cerebral arterial smooth muscle. In addition, our analysis indicates that a substantial component (≈50%) of the voltage dependencies and kinetics of Kv currents in voltage-clamped cerebral arterial myocytes is consistent with Kv2 channels. Accordingly, we found that stromatoxin, a specific inhibitor of Kv2 channels, significantly decreased Kv currents in these cells. Furthermore, stromatoxin enhanced myogenic constriction of pressurized arterial segments. We also found that during angiotensin II-induced hypertension, Kv2 channel function was reduced in isolated myocytes and in intact arteries. This suggests that impaired Kv2 channel activity may contribute to arterial dysfunction during hypertension. On the basis of these novel observations, we propose a new model of Kv channel function in arterial smooth muscle in which Kv2 channels (in combination with Kv1 channels) contribute to membrane hyperpolarization and thus oppose constriction.

Structure and function of the voltage sensor of sodium channels probed by a beta-scorpion toxin

Voltage sensing by voltage-gated sodium channels determines the electrical excitability of cells, but the molecular mechanism is unknown. beta-Scorpion toxins bind specifically to neurotoxin receptor site 4 and induce a negative shift in the voltage dependence of activation through a voltage sensor-trapping mechanism. Kinetic analysis showed that beta-scorpion toxin binds to the resting state, and subsequently the bound toxin traps the voltage sensor in the activated state in a voltage-dependent but concentration-independent manner. The rate of voltage sensor trapping can be fit by a two-step model, in which the first step is voltage-dependent and correlates with the outward gating movement of the IIS4 segment, whereas the second step is voltage-independent and results in shifted voltage dependence of activation of the channel. Mutations of Glu(779) in extracellular loop IIS1-S2 and both Glu(837) and Leu(840) in extracellular loop IIS3-S4 reduce the binding affinity of beta-scorpion toxin. Mutations of positively charged and hydrophobic amino acid residues in the IIS4 segment do not affect beta-scorpion toxin binding but alter voltage dependence of activation and enhance beta-scorpion toxin action. Structural modeling with the Rosetta algorithm yielded a three-dimensional model of the toxin-receptor complex with the IIS4 voltage sensor at the extracellular surface. Our results provide mechanistic and structural insight into the voltage sensor-trapping mode of scorpion toxin action, define the position of the voltage sensor in the resting state of the sodium channel, and favor voltage-sensing models in which the S4 segment spans the membrane in both resting and activated states.

Potassium channel blockers in multiple sclerosis: Neuronal Kv channels and effects of symptomatic treatment

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS) characterized by demyelination, with a relative sparing of axons. In MS patients, many neurologic signs and symptoms have been attributed to the underlying conduction deficits. The idea that neurologic function might be improved if conduction could be restored in CNS demyelinated axons led to the testing of potassium (K(+)) channel blockers as a symptomatic treatment. To date, only 2 broad-spectrum K(+) channel blockers, 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP), have been tested in MS patients. Although both 4-AP and 3,4-DAP produce clear neurologic benefits, their use has been limited by toxicity. Here we review the current status of basic science and clinical research related to the therapeutic targeting of voltage-gated K(+) channels (K(v)) in MS. By bringing together 3 distinct but interrelated disciplines, we aim to provide perspective on a vast body of work highlighting the lengthy and ongoing process entailed in translating fundamental K(v) channel knowledge into new clinical treatments for patients with MS and other demyelinating diseases. Covered are (1) K(v) channel nomenclature, structure, function, and pharmacology; (2) classic and current experimental morphology and neurophysiology studies of demyelination and conduction deficits; and (3) a comprehensive overview of clinical trials utilizing 4-AP and 3,4-DAP in MS patients.

Direct evidence that receptor site-4 of sodium channel gating modifiers is not dipped in the phospholipid bilayer of neuronal membranes

In a recent note to Nature, R. MacKinnon has raised the possibility that potassium channel gating modifiers are able to partition in the phospholipid bilayer of neuronal membranes and that by increasing their partial concentration adjacent to their receptor, they affect channel function with apparent high affinity (Lee and MacKinnon (2004) Nature 430, 232-235). This suggestion was adopted by Smith et al. (Smith, J. J., Alphy, S., Seibert, A. L., and Blumenthal, K. M. (2005) J. Biol. Chem. 280, 11127-11133), who analyzed the partitioning of sodium channel modifiers in liposomes. They found that certain modifiers were able to partition in these artificial membranes, and on this basis, they have extrapolated that scorpion beta-toxins interact with their channel receptor in a similar mechanism as that proposed by MacKinnon. Since this hypothesis has actually raised a new conception, we examined it in binding assays using a number of pharmacologically distinct scorpion beta-toxins and insect and mammalian neuronal membrane preparations, as well as by analyzing the rate by which the toxin effect on gating of Drosophila DmNa(v)1 and rat brain rNa(v)1.2a develops. We show that in general, scorpion beta-toxins do not partition in neuronal membranes and that in the case in which a depressant beta-toxin partitions in insect neuronal membranes, this partitioning is unrelated to its interaction with the receptor site and the effect on the gating properties of the sodium channel. These results negate the hypothesis that the high affinity of beta-toxins for sodium channels is gained by their ability to partition in the phospholipid bilayer and clearly indicate that the receptor site for scorpion beta-toxins is accessible to the extracellular solvent.

Modulation of Kv3 subfamily potassium currents by the sea anemone toxin BDS: significance for CNS and biophysical studies

Kv3 potassium channels, with their ultra-rapid gating and high activation threshold, are essential for high-frequency firing in many CNS neurons. Significantly, the Kv3.4 subunit has been implicated in the major CNS disorders Parkinson's and Alzheimer's diseases, and it is claimed that selectively targeting this subunit will have therapeutic utility. Previous work suggested that BDS toxins ("blood depressing substance," from the sea anemone Anemonia sulcata) were specific blockers for rapidly inactivating Kv3.4 channels, and consequently these toxins are increasingly used as diagnostic agents for Kv3.4 subunits in central neurons. However, precisely how selective are these toxins for this important CNS protein? We show that BDS is not selective for Kv3.4 but markedly inhibits current through Kv3.1 and Kv3.2 channels. Inhibition comes about not by "pore block" but by striking modification of Kv3 gating kinetics and voltage dependence. Activation and inactivation kinetics are slowed by BDS-I and BDS-II, and V(1/2) for activation is shifted to more positive voltages. Alanine substitution mutagenesis around the S3b and S4 segments of Kv3.2 reveals that BDS acts via voltage-sensing domains, and, consistent with this, ON gating currents from nonconducting Kv3.2 are markedly inhibited. The altered kinetics and gating properties, combined with lack of subunit selectivity with Kv3 subunits, seriously affects the usefulness of BDS toxins in CNS studies. Furthermore, our results do not easily fit with the voltage sensor "paddle" structure proposed recently for Kv channels. Our data will be informative for experiments designed to dissect out the roles of Kv3 subunits in CNS function and dysfunction.

Interaction between K+ channel gate modifier hanatoxin and lipid bilayer membranes analyzed by molecular dynamics simulation

Hanatoxin (HaTx) is an ellipsoidal-shaped peptide that binds to the voltage sensor of voltage-dependent channels. Of physicochemical interest, HaTx has a "ring" of charged residues around its periphery and a hydrophobic protrusion. It has previously been postulated that HaTx binds to and functions on the surface of membranes, but a recent fluorescent-quenching study has implied a fairly deep positioning of HaTx in the lipid bilayer membrane. We carried out numerous molecular dynamic simulations of HaTx1, a well-studied variant of HaTx, in fully hydrated phospholipid bilayers. The system reproduced the surface-binding mode of HaTx1, in which HaTx1 resided in the extracellular side (outer) of the water/membrane interface with the hydrophobic patch of HaTx1 facing the membrane interior. On the other hand, analyses with various parameter settings suggested that the surface-binding mode was unstable because of the substantial attractive electrostatic force between HaTx1 and the lipid head groups of the inner (opposite) leaflet. Compared with this electrostatic force, the energetic cost for membrane deformation involving meniscus formation appeared to be small. In an attempt to interpret the quenching data, we consider the possibility of dimpling (meniscus formation) that brings HaTx1 inward (only ~0.7-0.8 nm above the bilayer center), while accounting for the flexibility of both leaflets of the membrane and the long-range interaction between positively charged residues of the membrane-bound peptide and the polar head groups of the opposite leaflet of the membrane. It is suggested that molecular dynamics simulations taking into account the flexibility of the membrane surface is potentially useful in interpreting the fluorescence-quenching data.

Lipid membrane interaction and antimicrobial activity of GsMTx-4, an inhibitor of mechanosensitive channel

GsMTx-4, a polypeptide from the spider Grammostola spatulata, is an inhibitor of mechanosensitive channels. It is known to interact with lipid membranes, suggesting it partitions into the membrane to alter the channel gating, but the effect of the membrane charge on GsMTx-4 activity remains unknown. In this study, we found that GsMTx-4 more effectively interacts with anionic lipids than zwitterionic ones. The effect of GsMTx-4 on negatively charged membranes was similar to that of the antimicrobial peptide melittin, which led us to assess GsMTx-4's antimicrobial activity. Interestingly, we found that, in contrast to other neurotoxins, GsMTx-4 exhibited antimicrobial properties and was more active against Gram-positive than Gram-negative bacteria. These results suggest that GsMTx-4 exerts its antimicrobial effect by altering the packing of the membrane and/or inhibiting mechanosensitive channels. These findings could point the way towards a new class of antimicrobial peptides.

Four Novel Tarantula Toxins as Selective Modulators of Voltage-Gated Sodium Channel Subtypes

Four novel peptide toxins that act on voltage-gated sodium channels have been isolated from tarantula venoms: ceratotoxins 1, 2, and 3 (CcoTx1, CcoTx2, and CcoTx3) from Ceratogyrus cornuatus and phrixotoxin 3 (PaurTx3) from Phrixotrichus auratus. The pharmacological profiles of these new toxins were characterized by electrophysiological measurements on six cloned voltage-gated sodium channel subtypes expressed in Xenopus laevis oocytes (Na(v)1.1/beta(1), Na(v)1.2/beta(1), Na(v)1.3/beta(1), Na(v)1.4/beta(1), Na(v)1.5/beta(1), and Na(v)1.8/beta(1)). These novel toxins modulate voltage-gated sodium channels with properties similar to those of typical gating-modifier toxins, both by causing a depolarizing shift in gating kinetics and by blocking the inward component of the sodium current. PaurTx3 is one of the most potent peptide modulators of voltage-gated sodium channels described thus far from spider venom, modulating Na(v)1.2 with an IC(50) value of 0.6 +/- 0.1 nM. CcoTx1 and CcoTx2, differing by only one amino acid, are potent modulators of different voltage-gated sodium channel subtypes from the central nervous system, except for Na(v)1.3, which is only affected by CcoTx2. The potency of CcoTx3 is lower, although this toxin seems to be more selective for the tetrodotoxin-resistant channel subtype Na(v)1.5/beta(1) (IC(50) = 447 +/- 32 nM). In addition to these results, molecular modeling indicates that subtle differences in toxin surfaces may relate to their different pharmacological profiles. Furthermore, an evolutionary trace analysis of these toxins and other structurally related three-disulfide spider toxins provides clues for the exploration of toxin-channel interaction and future structure-function research.

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.

Delayed-rectifier (KV2.1) regulation of pancreatic beta-cell calcium responses to glucose: inhibitor specificity and modeling

The delayed-rectifier (voltage-activated) K(+) conductance (K(V)) in pancreatic islet beta-cells has been proposed to regulate plasma membrane repolarization during responses to glucose, thereby determining bursting and Ca(2+) oscillations. Here, we verified the expression of K(V)2.1 channel protein in mouse and human islets of Langerhans. We then probed the function of K(V)2.1 channels in islet glucose responses by comparing the effect of hanatoxin (HaTx), a specific blocker of K(V)2.1 channels, with a nonspecific K(+) channel blocker, tetraethylammonium (TEA). Application of HaTx (1 microM) blocked delayed-rectifier currents in mouse beta-cells, resulting in a 40-mV rightward shift in threshold of activation of the voltage-dependent outward current. In the presence of HaTx, there was negligible voltage-activated outward current below 0 mV, suggesting that K(V)2.1 channels form the predominant part of this current in the physiologically relevant range. We then employed HaTx to study the role of K(V)2.1 in the beta-cell Ca(2+) responses to elevated glucose in comparison with TEA. Only HaTx was able to induce slow intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations in cells stimulated with 20 mM glucose, whereas TEA induced an immediate rise in [Ca(2+)](i) followed by rapid oscillations. In human islets, HaTx acted in a similar fashion. The data were analyzed using a detailed mathematical model of ionic flux and Ca(2+) regulation in beta-cells. The results can be explained by a specific HaTx effect on the K(V) current, whereas TEA affects multiple K(+) conductances. The results underscore the importance of K(V)2.1 channel in repolarization of the pancreatic beta-cell plasma membrane and its role in regulating insulin secretion.

Biophysical and pharmacological properties of the voltage-gated potassium current of human pancreatic beta-cells

Voltage-gated potassium (Kv) currents of human pancreatic islet cells were studied by whole-cell patch clamp recording. On average, 75% of the cells tested were identified as beta-cells by single cell, post-recording RT-PCR for insulin mRNA. In most cells, the dominant Kv current was a delayed rectifier. The delayed rectifier activated at potentials above -20 mV and had a V(1/2) for activation of -5.3 mV. Onset of inactivation was slow for a major component (tau = 3.2 s at +20 mV) observed in all cells; a smaller component (tau = 0.30 s) with an amplitude of approximately 25% was seen in some cells. Recovery from inactivation had a tau of 2.5 s at -80 mV and steady-state inactivation had a V(1/2) of -39 mV. In 12% of cells (21/182) a low-threshold, transient Kv current (A-current) was present. The A-current activated at membrane potentials above -40 mV, inactivated with a time constant of 18.5 ms at -20 mV, and had a V(1/2) for steady-state inactivation of -52 mV. TEA inhibited total Kv current with an IC50 = 0.54 mm and PAC, a disubstituted cyclohexyl Kv channel inhibitor, inhibited with an IC50 = 0.57 microm. The total Kv current was insensitive to margatoxin (100 nm), agitoxin-2 (50 nm), kaliotoxin (50 nm) and ShK (50 nm). Hanatoxin (100 nm) inhibited total Kv current by 65% at +20 mV. Taken together, these data provide evidence of at least two distinct types of Kv channels in human pancreatic beta-cells and suggest that more than one type of Kv channel may be involved in the regulation of glucose-dependent insulin secretion.

Differential Phospholipid Binding by Site 3 and Site 4 Toxins

It has been shown recently that polypeptide toxins that modulate the gating properties of voltage-sensitive cation channels are able to bind to phospholipid membranes, leading to the suggestion that these toxins are able to access a channel-binding site that remains membrane-restricted (Lee, S.-Y., and MacKinnon, R. (2004) Nature 430, 232-235). We therefore examined the ability of anthopleurin B (ApB), a sea anemone toxin that selectively modifies inactivation kinetics of Na(V)1.x channels, and ProTx-II, a spider toxin that modifies activation kinetics of the same channels, to bind to liposomes. Whereas ProTx-II can be quantitatively depleted from solution upon incubation with phosphatidylcholine/phosphatidylserine liposomes, ApB displays no discernible phospholipid binding activity. We therefore examined the activities of structurally unrelated site 3 and site 4 toxins derived from Leiurus and Centruroides venoms, respectively, in the same assay. Like ApB, the site 3 toxin LqqV shows no lipid binding activity, whereas the site 4 toxin Centruroides toxin II, like ProTx-II, is completely bound. We conclude that toxins that modify inactivation kinetics via binding to Na(V)1.x site 3 lack the ability to bind phospholipids, whereas site 4 toxins, which modify activation, have this activity. This inherent difference suggests that the conformation of domain II more closely resembles that of the K(V)AP channel than does the conformation of domain IV.

Molecular surface of tarantula toxins interacting with voltage sensors in K(v) channels

The venom from spiders, scorpions, and sea anemone contain a rich diversity of protein toxins that interact with ion channel voltage sensors. Although atomic structures have been solved for many of these toxins, the surfaces that are critical for interacting with voltage sensors are poorly defined. Hanatoxin and SGTx are tarantula toxins that inhibit activation of K(v) channels by interacting with each of the four voltage sensors. In this study we set out to identify the active surface of these toxins by alanine-scanning SGTx and characterizing the interaction of each mutant with the K(v)2.1 channel. Examination of the concentration dependence for inhibition identified 15 mutants with little effect on the concentration dependence for toxin inhibition of the K(v)2.1 channel, and 11 mutants that display moderate to dramatic perturbations. Mapping of these results onto the structure of SGTx identifies one face of the toxin where mutations with pronounced perturbations cluster together, and a backside of the toxin where mutations are well tolerated. The active surface of SGTx contains a ring-like assembly of highly polar residues, with two basic residues that are particularly critical, concentrically arranged around a hydrophobic protrusion containing critical aliphatic and aromatic residues. These results identify the active surface of the toxin and reveal the types of side chains that are important for interacting with voltage sensors.

Molecular pharmacology of high voltage-activated calcium channels

Voltage-gated calcium channels are key sources of calcium entry into the cytosol of many excitable tissues. A number of different types of calcium channels have been identified and shown to mediate specialized cellular functions. Because of their fundamental nature, they are important targets for therapeutic intervention in disorders such as hypertension, pain, stroke, and epilepsy. Calcium channel antagonists fall into one of the following three groups: small inorganic ions, large peptide blockers, and small organic molecules. Inorganic ions nonselectively inhibit calcium entry by physical pore occlusion and are of little therapeutic value. Calcium-channel-blocking peptides isolated from various predatory animals such as spiders and cone snails are often highly selective blockers of individual types of calcium channels, either by preventing calcium flux through the pore or by antagonizing channel activation. There are many structure-activity-relation classes of small organic molecules that interact with various sites on the calcium channel protein, with actions ranging from selective high affinity block to relatively nondiscriminatory action on multiple calcium channel isoforms. Detailed interactions with the calcium channel protein are well understood for the dihydropyridine and phenylalkylamine drug classes, whereas we are only beginning to understand the molecular actions of some of the more recently discovered calcium channel blockers. Here, we provide a comprehensive review of pharmacology of high voltage-activated calcium channels.

A possible molecular mechanism of hanatoxin binding-modified gating in voltage-gated K+-channels

While S4 is known as the voltage sensor in voltage-gated potassium channels, the carboxyl terminus of S3 (S3C) is of particular interest concerning the site for gating modifier toxins like hanatoxin. The thus derived helical secondary structural arrangement for S3C, as well as its surrounding environment, has since been intensively and vigorously debated. Our previous structural analysis based on molecular simulation has provided sufficient information to describe reasonable docking conformation and further experimental designs (Lou et al., 2002. J. Mol. Recognit. 15: 175-179). However, if one only relies on such information, more advanced structure-functional interpretations for the roles S3C may play in the modification of gating behavior upon toxin binding will remain unknown. In order to have better understanding of the molecular details regarding this issue, we have performed the docking simulation with the S3C sequence from the hanatoxin-insensitive K+-channel, shaker, and analyzed the conformational changes resulting from such docking. Compared with other functional data from previous studies with respect to the proximity of the S3-S4 linker region, we suggested a significant movement of drk1 S3C, but not shaker S3C, in the direction presumably towards S4, which was comprehended as a possible factor interfering with S4 translocation during drk1 gating in the presence of toxin. In combination with the discussions for structural roles of the length of the S3-S4 linker, a possible molecular mechanism to illustrate the hanatoxin binding-modified gating is proposed.

A membrane-access mechanism of ion channel inhibition by voltage sensor toxins from spider venom

Venomous animals produce small protein toxins that inhibit ion channels with high affinity. In several well-studied cases the inhibitory proteins are water-soluble and bind at a channel's aqueous-exposed extracellular surface. Here we show that a voltage-sensor toxin (VSTX1) from the Chilean Rose Tarantula (Grammostola spatulata) reaches its target by partitioning into the lipid membrane. Lipid membrane partitioning serves two purposes: to localize the toxin in the membrane where the voltage sensor resides and to exploit the free energy of partitioning to achieve apparent high-affinity inhibition. VSTX1, small hydrophobic poisons and anaesthetic molecules reveal a common theme of voltage sensor inhibition through lipid membrane access. The apparent requirement for such access is consistent with the recent proposal that the sensor in voltage-dependent K+ channels is located at the membrane-protein interface.

Tarantulas: eight-legged pharmacists and combinatorial chemists

Tarantula venoms represent a cornucopia of novel ligands for a variety of cell receptors and ion channels. The diversity of peptide toxin pharmacology has been barely explored as indicated by pharmacological, toxicological and mass spectrometry investigations on more than 55 tarantula venoms. MALDI-TOF MS analysis reveals that the pharmacological diversity is based on relatively small size peptides, which seem to fall into a limited number of structural patterns. Properties and biological activities of the 33 known peptide toxins from tarantula venoms are described. Most known toxins conform to the Inhibitory Cystine Knot (ICK) motif, with differences in the length of intercysteine loops. Recently described peptides show that tarantula toxins can fold according to an elaboration of the Disulfide-Directed beta-Hairpin (DDH) motif which is also the canonical motif for the ICK fold. The ICK fold itself offers many variations leading to differing toxin properties. Examination of pharmacological data gives insights on the possible conserved site of action of toxins acting on voltage-gated ion channels and other toxins acting by a pore-blocking mechanism. Structure-activity data shows the versatility of the toxin scaffolds and the importance of surface features in the selectivity and specificity of these toxins. Tarantulas appear to be a good model for the discovery of novel compounds with important therapeutic potential, and for the study of the molecular evolution of peptide toxins.

Mechanism of Rectification in Inward-Rectifier K+ Channels

Inward rectifiers are a class of K+ channels that can conduct much larger inward currents at membrane voltages negative to the K+ equilibrium potential than outward currents at voltages positive to it, even when K+ concentrations on both sides of the membrane are made equal. This conduction property, called inward rectification, enables inward rectifiers to perform many important physiological tasks. Rectification is not an inherent property of the channel protein itself, but reflects strong voltage dependence of channel block by intracellular cations such as Mg2+ and polyamines. This voltage dependence results primarily from the movement of K+ ions across the transmembrane electric field along the pore, which is energetically coupled to the blocker binding and unbinding. This mutual displacement mechanism between several K+ ions and a blocker explains the signature feature of inward rectifier K+ channels, namely, that at a given concentration of intracellular K+, their macroscopic conductance depends on the difference between membrane voltage and the K+ equilibrium potential rather than on membrane voltage itself.

Interaction between extracellular Hanatoxin and the resting conformation of the voltage-sensor paddle in Kv channels

In voltage-activated potassium (Kv) channels, basic residues in S4 enable the voltage-sensing domain to move in response to membrane depolarization and thereby trigger the activation gate to open. In the X-ray structure of the KvAP channel, the S4 helix is located near the intracellular boundary of the membrane where it forms a "voltage-sensor paddle" motif with the S3b helix. It has been proposed that the paddle is lipid-exposed and that it translocates through the membrane as it activates. We studied the interaction of externally applied Hanatoxin with the voltage-sensor paddle in Kv channels and show that the toxin binds tightly even at negative voltages where the paddle is resting and the channel is closed. Moreover, measurements of gating charge movement suggest that Hanatoxin interacts with and stabilizes the resting paddle. These findings point to an extracellular location for the resting conformation of the voltage-sensor paddle and constrain its transmembrane movements during activation.