Exploring structural features of the interaction between the scorpion toxinCnErg1 and ERG K+ channels

In Silico Analysis of a Drosophila Parasitoid Venom Peptide Reveals Prevalence of the Cation-Polar-Cation Clip Motif in Knottin Proteins

As generalist parasitoid wasps, Leptopilina heterotoma are highly successful on many species of fruit flies of the genus Drosophila. The parasitoids produce specialized multi-strategy extracellular vesicle (EV)-like structures in their venom. Proteomic analysis identified several immunity-associated proteins, including the knottin peptide, LhKNOT, containing the structurally conserved inhibitor cysteine knot (ICK) fold, which is present in proteins from diverse taxa. Our structural and docking analysis of LhKNOT's 36-residue core knottin fold revealed that in addition to the knottin motif itself, it also possesses a Cation-Polar-Cation (CPC) clip. The CPC clip motif is thought to facilitate antimicrobial activity in heparin-binding proteins. Surprisingly, a majority of ICKs tested also possess the CPC clip motif, including 75 bona fide plant and arthropod knottin proteins that share high sequence and/or structural similarity with LhKNOT. Like LhKNOT and these other 75 knottin proteins, even the Drosophila Drosomycin antifungal peptide, a canonical target gene of the fly's Toll-NF-kappa B immune pathway, contains this CPC clip motif. Together, our results suggest a possible defensive function for the parasitoid LhKNOT. The prevalence of the CPC clip motif, intrinsic to the cysteine knot within the knottin proteins examined here, suggests that the resultant 3D topology is important for their biochemical functions. The CPC clip is likely a highly conserved structural motif found in many diverse proteins with reported heparin binding capacity, including amyloid proteins. Knottins are targets for therapeutic drug development, and insights into their structure-function relationships will advance novel drug design.

Peptide Toxins Targeting KV Channels

A number of peptide toxins isolated from animals target potassium ion (K⁺) channels. Many of them are particularly known to inhibit voltage-gated K⁺ (K_V) channels and are mainly classified into pore-blocking toxins or gating-modifier toxins. Pore-blocking toxins directly bind to the ion permeation pores of K_V channels, thereby physically occluding them. In contrast, gating-modifier toxins bind to the voltage-sensor domains of K_V channels, modulating their voltage-dependent conformational changes. These peptide toxins are useful molecular tools in revealing the structure-function relationship of K_V channels and have potential for novel treatments for diseases related to K_V channels. This review focuses on the inhibition mechanism of pore-blocking and gating-modifier toxins that target K_V channels.

Colombian Scorpion Centruroides margaritatus: Purification and Characterization of a Gamma Potassium Toxin with Full-Block Activity on the hERG1 Channel

The Colombian scorpion Centruroides margaritatus produces a venom considered of low toxicity. Nevertheless, there are known cases of envenomation resulting in cardiovascular disorders, probably due to venom components that target ion channels. Among them, the humanether-à-go-go-Related gene (hERG1) potassium channels are critical for cardiac action potential repolarization and alteration in its functionality are associated with cardiac disorders. This work describes the purification and electrophysiological characterization of a Centruroides margaritatus venom component acting on hERG1 channels, the CmERG1 toxin. This novel peptide is composed of 42 amino acids with a MW of 4792.88 Da, folded by four disulfide bonds and it is classified as member number 10 of the γ-KTx1 toxin family. CmERG1 inhibits hERG1 currents with an IC₅₀ of 3.4 ± 0.2 nM. Despite its 90.5% identity with toxin ɣ-KTx1.1, isolated from Centruroides noxius, CmERG1 completely blocks hERG1 current, suggesting a more stable plug of the hERG channel, compared to that formed by other ɣ-KTx.

Insights into the Molecular Inhibition of the Oncogenic Channel KV10.1 by Globular Toxins

Inhibition of the expression of the human ether-à-go-go (hEAG1 or hK_V10.1) channel is associated with a dramatic reduction in the growth of several cancerous tumors. The modulation of this channel's activity is a promising target for the development of new anticancer drugs. Although some small molecules have shown inhibitory activity against K_V10.1, their lack of specificity has prevented their use in humans. In vitro studies have recently identified a limited number of peptide toxins with proven specificity in their hK_V10.1 channel inhibitory effect. These peptide toxins have become desirable candidates to use as lead compounds to design more potent and specific hK_V10.1 inhibitors. However, the currently available studies lack the atomic resolution needed to characterize the molecular features that favor their binding to hK_V10.1. In this work, we present the first attempt to locate the possible hK_V10.1 binding sites of the animal peptide toxins APETx4, Aa1a, Ap1a, and k-hefutoxin 1, all of which described as hK_V10.1 inhibitors. Our studies incorporated homology modeling to construct a robust three-dimensional (3D) model of hK_V10.1, applied protein docking, and multiscale molecular dynamics techniques to reveal in atomic resolution the toxin-channel interactions. Our approach suggests that some peptide toxins bind in the outer vestibule surrounding the pore of hK_V10.1; it also identified the channel residues Met397 and Asp398 as possible anchors that stabilize the binding of the evaluated toxins. Finally, a description of the possible mechanism for inhibition and gating is presented.

Beyond hemostasis: a snake venom serine protease with potassium channel blocking and potential antitumor activities

Snake venom serine proteases (SVSPs) are complex and multifunctional enzymes, acting primarily on hemostasis. In this work, we report the hitherto unknown inhibitory effect of a SVSP, named collinein-1, isolated from the venom of Crotalus durissus collilineatus, on a cancer-relevant voltage-gated potassium channel (hEAG1). Among 12 voltage-gated ion channels tested, collinein-1 selectively inhibited hEAG1 currents, with a mechanism independent of its enzymatic activity. Corroboratively, we demonstrated that collinein-1 reduced the viability of human breast cancer cell line MCF7 (high expression of hEAG1), but does not affect the liver carcinoma and the non-tumorigenic epithelial breast cell lines (HepG2 and MCF10A, respectively), which present low expression of hEAG1. In order to obtain both functional and structural validation of this unexpected discovery, where an unusually large ligand acts as an inhibitor of an ion channel, a recombinant and catalytically inactive mutant of collinein-1 (His43Arg) was produced and found to preserve its capability to inhibit hEAG1. A molecular docking model was proposed in which Arg79 of the SVSP 99-loop interacts directly with the potassium selectivity filter of the hEAG1 channel.

Characterization of Kbot21 Reveals Novel Side Chain Interactions of Scorpion Toxins Inhibiting Voltage-Gated Potassium Channels

Scorpion toxins are important pharmacological tools for probing the physiological roles of ion channels which are involved in many physiological processes and as such have significant therapeutic potential. The discovery of new scorpion toxins with different specificities and affinities is needed to further characterize the physiology of ion channels. In this regard, a new short polypeptide called Kbot21 has been purified to homogeneity from the venom of Buthus occitanus tunetanus scorpion. Kbot21 is structurally related to BmBKTx1 from the venom of the Asian scorpion Buthus martensii Karsch. These two toxins differ by only two residues at position 13 (R /V) and 24 (D/N).Despite their very similar sequences, Kbot21 and BmBKTx1 differ in their electrophysiological activities. Kbot21 targets K_V channel subtypes whereas BmBKTx1 is active on both big conductance (BK) and small conductance (SK) Ca²⁺-activated K⁺ channel subtypes, but has no effects on Kv channel subtypes. The docking model of Kbot21 with the Kv1.2 channel shows that the D24 and R13 side-chain of Kbot21 are critical for its interaction with K_V channels.

Electrophysiological characterization of Ts6 and Ts7, K⁺ channel toxins isolated through an improved Tityus serrulatus venom purification procedure

In Brazil, Tityus serrulatus (Ts) is the species responsible for most of the scorpion related accidents. Among the Ts toxins, the neurotoxins with action on potassium channels (α-KTx) present high interest, due to their effect in the envenoming process and the ion channel specificity they display. The α-KTx toxins family is the most relevant because its toxins can be used as therapeutic tools for specific target cells. The improved isolation method provided toxins with high resolution, obtaining pure Ts6 and Ts7 in two chromatographic steps. The effects of Ts6 and Ts7 toxins were evaluated in 14 different types of potassium channels using the voltage-clamp technique with two-microelectrodes. Ts6 toxin shows high affinity for Kv1.2, Kv1.3 and Shaker IR, blocking these channels in low concentrations. Moreover, Ts6 blocks the Kv1.3 channel in picomolar concentrations with an IC₅₀ of 0.55 nM and therefore could be of valuable assistance to further designing immunosuppressive therapeutics. Ts7 toxin blocks multiple subtypes channels, showing low selectivity among the channels analyzed. This work also stands out in its attempt to elucidate the residues important for interacting with each channel and, in the near future, to model a desired drug.

Positive selection-guided mutational analysis revealing two key functional sites of scorpion ERG K(+) channel toxins

Scorpion γ-KTx toxins are important molecular tools for studying physiological and pharmacological functions of human ether-á-go-go related gene (hERG) K(+) channels. To pinpoint functional residues of this class of toxins involved in channel binding, we employed a combined approach that integrates evolutionary information and site-directed mutagenesis. Among three positively selected sites (PSSs) identified here, two (Gln18 and Met35) were found to be associated with the toxin's function because their changes significantly decreased the potency of ErgTx1 (also called CnErg1) on hERG1 channel. On the contrary, no potency alteration was observed at the third PSS (Ala42) when the mutation was introduced, which could be due to its location far from the functional surface of the toxin. Our strategy will accelerate the research of structure-function relationship of scorpion K(+) channel toxins.

Toxin modulators and blockers of hERG K(+) channels

The K(+) channel encoded by the Ether-á-go-go-Related Gene (ERG) is expressed in different tissues of different animal species. There are at least three subtypes of this channel, being the sub-type 1 (ERG1) crucial in the repolarization phase of the cardiac action potential. Mutations in this gene can affect the properties of the channel producing the type II long QT syndrome (LQTS2) and many drugs are also known to affect this channel with a similar side effect. Various scorpion, spider and sea anemone toxins affect the ERG currents by blocking the ion-conducting pore from the external side or by modulating channel gating through binding to the voltage-sensor domain. By doing so, these toxins become very useful tools for better understanding the structural and functional characteristics of these ion channels. This review discusses the interaction between the ERG channels and the peptides isolated from venoms of these animals. Special emphasis is placed on scorpion toxins, although the effects of several spider venom toxins and anemone toxins will be also revised.

Interacting sites of scorpion toxin ErgTx1 with hERG1 K+ channels

Peptides purified from scorpion venoms were shown to interact with specific amino acid residues present in the outer vestibule of various sub-types of potassium channels, occluding the pore and causing a decrement of K(+) permeability through the membrane of excitable and non excitable cells. This communication describes the identification of several interacting sites of toxin ErgTx1, a toxin purified from the venom of the scorpion Centruroides noxius, with the human ERG1 K(+) channels, by means of site-directed mutagenesis of specific residues of the toxin. Recombinant mutants of the gene coding for ErgTx1 were expressed heterologously in Escherichia coli, properly folded and their affinities and interactions with hERG1 channels were determined by patch-clamp techniques. Residues in position Y14, Y17 and F37 of the solvent exposed hydrophobic surface, and charged residues at the position K13 and K38 of ErgTx1 were shown to cause a decrement of the affinity from 20 folds to 3 orders of magnitude, thus suggesting that they are certainly participating on the binding surface of this toxin towards the hERG1 channels. Double mutants at positions K13 and F37, Y14 and F37, Y17 and F37 and K13 and K38 were also prepared and assayed, but the results obtained are not much different from the single point mutants of ErgTx1. The results of the present work indicate the most probable surface area of ErgTx1 that makes contact with the hERG channels.

Recombinant expression of the toxic peptide ErgTx1 and role of Met35 on its stability and function

Ergtoxin 1 (ErgTx1) is a 42 amino acid peptide purified from the venom of the Mexican scorpion Centruroides noxius Hoffmann, capable of blocking specifically human potassium channels of the ether-á-go-go-related gene family (hERG). This peptide binds to a partially overlapping site on the channel outer mouth, in which residues of the S5-P linker are critically involved. Here we describe results of site directed mutagenesis of the ErgTx1 gene and its heterologous expression in Escherichia coli. The recombinant products show the fundamental role played by methionine in position 35 (Met35) of the primary structure. Naturally oxidized Met35 decreases by three orders of magnitude the affinity of the peptide for the hERG1 channels. This result is quite relevant, because it shows two possible situations: either Met35 is involved in the proper folding of the molecule or it plays a direct role in the interaction with the channel, i.e., constitutes part of the interacting surfaces. These two situations were evaluated by preparing heterologously expressed ErgTx1 gene and a mutant containing alanine in position 35. Additionally circular dichroism measurements of both native and recombinant peptides were performed. The electrophysiological recordings and the structural values obtained by optical measurements, strongly support the idea that Met35 is indeed a key residue on the interacting surfaces of the toxin with the channels.

A common "hot spot" confers hERG blockade activity to alpha-scorpion toxins affecting K+ channels

While alpha-KTx peptides are generally known for their modulation of the Shaker-type and the Ca(2+)-activated potassium channels, gamma-KTxs are associated with hERG channels modulation. An exception to the rule is BmTx3 which belongs to subfamily alpha-KTx15 and can block hERG channels. To explain the peculiar behavior of BmTx3, a tentative "hot spot" formed of 2 basic residues (R18 and K19) was suggested but never further studied [Huys I, et al. BmTx3, a scorpion toxin with two putative functional faces separately active on A-type K(+) and HERG currents. Biochem J 2004;378:745-52]. In this work, we investigated if the "hot spot" is a commonality in subfamily alpha-KTx15 by testing the effect of (AmmTx3, Aa1, discrepin). Furthermore, single mutations altering the "hot spot" in discrepin, have introduced for the very first time a hERG blocking activity to a previously non-active alpha-KTx. Additionally, we could extend our results to other alpha-KTx subfamily members belonging to alpha-KTx1, 4 and 6, therefore, the "hot spot" represents a common pharmacophore serving as a predictive tool for yet to be discovered alpha-KTxs.

APETx1 from sea anemone Anthopleura elegantissima is a gating modifier peptide toxin of the human ether-a-go-go- related potassium channel

We studied the mechanism of action and the binding site of APETx1, a peptide toxin purified from sea anemone, on the human ether-a-go-go-related gene (hERG) channel. Similar to the effects of gating modifier toxins (hanatoxin and SGTx) on the voltage-gated potassium (Kv) 2.1 channel, APETx1 shifts the voltage-dependence of hERG activation in the positive direction and suppresses its current amplitudes elicited by strong depolarizing pulses that maximally activate the channels. The APETx1 binding site is distinctly different from that of a pore-blocking peptide toxin, BeKm-1. Mutations in the S3b region of hERG have dramatic impact on the responsiveness to APETx1: G514C potentiates whereas E518C abolishes the APETx1 effect. Restoring the negative charge at position 518 (methanethiosulfonate ethylsulfonate modification of 518C) partially restores APETx1 responsiveness, supporting an electrostatic interaction between E518 and APETx1. Among the three hERG isoforms, hERG1 and hERG3 are equally responsive to APETx1, whereas hERG2 is insensitive. The key feature seems to be an arginine residue uniquely present at the 514-equivalent position in hERG2, where the other two isoforms possess a glycine. Our data show that APETx1 is a gating modifier toxin of the hERG channel, and its binding site shares characteristics with those of gating modifier toxin binding sites on other Kv channels.

BcIV, a new paralyzing peptide obtained from the venom of the sea anemone Bunodosoma caissarum. A comparison with the Na+ channel toxin BcIII

Sea anemones produce a wide variety of biologically active compounds, such as the proteinaceous neurotoxins and cytolysins. Herein we report a new peptide, purified to homogeneity from the neurotoxic fraction of B. caissarum venom, by using gel filtration followed by rp-HPLC, naming it as BcIV. BcIV is a 41 amino acid peptide (molecular mass of 4669 amu) possessing 6 cysteines covalently linked by three disulfide bonds. This toxin has 45 and 48% of identity when compared to APETx1 and APETx2 from Anthopleura elegantissima, respectively, and 42% of identity with Am-II and BDS-I and-II obtained from Antheopsis maculata and Anemonia sulcata, respectively. This neurotoxin presents only a weak-paralyzing action (minimal Lethal Dose close to 2000 microg/kg) in swimming crabs Callinectes danae. This appears to be a different effect to that caused by the type 1 sea anemone toxin BcIII that is lethal to the same animals at lower doses (LD50=219 microg/kg). Circular dichroism spectra of BcIII and BcIV show a high content of beta-strand secondary structure in both peptides, very similar to type 1 sodium channel toxins from various sea anemones, and to APETx1 and APETx2 from A. elegantissima, a HERG channel modulator and an ASIC3 inhibitor, respectively. Interestingly, BcIII and BcIV have similar effects on the action potential of the crab leg nerves, suggesting the same target in this tissue. As BcIII was previously reported as a Na+ channel effector and BcIV is inactive over Na+ currents of mammalian GH3 cells, we propose a species-specific action for this new molecule. A molecular model of BcIV was constructed using the structure of the APETx1 as template and putative key residues are discussed.

The receptor site of the spider toxin PcTx1 on the proton-gated cation channel ASIC1a

Acid-sensing ion channels (ASICs) are excitatory neuronal cation channels, involved in physiopathological processes related to extracellular pH fluctuation such as nociception, ischaemia, perception of sour taste and synaptic transmission. The spider peptide toxin psalmotoxin 1 (PcTx1) has previously been shown to inhibit specifically the proton-gated cation channel ASIC1a. To identify the binding site of PcTx1, we produced an iodinated form of the toxin ((125)I-PcTx1Y(N)) and developed a set of binding and electrophysiological experiments on several chimeras of ASIC1a and the PcTx1-insensitive channels ASIC1b and ASIC2a. We show that (125)I-PcTx1Y(N) binds specifically to ASIC1a at a single site, with an IC(50) of 128 pM, distinct from the amiloride blocking site. Results obtained from chimeras indicate that PcTx1 does not bind to ASIC1a transmembrane domains (M1 and M2), involved in formation of the ion pore, but binds principally on both cysteine-rich domains I and II (CRDI and CRDII) of the extracellular loop. The post-M1 and pre-M2 regions, although not involved in the binding site, are crucial for the ability of PcTx1 to inhibit ASIC1a current. The linker domain between CRDI and CRDII is important for their correct spatial positioning to form the PcTx1 binding site. These results will be useful for the future identification or design of new molecules acting on ASICs.

Mortality and antibody responses of mice to three successive episodes of experimental scorpion (Centruroides limpidus limpidus) envenomation and immunological rescue

Mortality rates of mice and their levels of anti-venom and anti-F(ab')2 antibodies were assessed after three episodes of subcutaneous envenomations with or without treatment with horse F(ab')2. Soluble venom from the Mexican scorpion Centruroides limpidus limpidus was used for these experiments. Repetition of episodes did not induce different mortality rates in untreated mice. F(ab')2 rescued about 85% of the mice in the first two episodes and 66% in the third, without distinction of gender or ostensible side-effects: a suggestion of selection of the most resistant mice. Surviving mice produced in vitro neutralizing antibodies to the scorpion venom and also antibodies to F(ab')2, when injected alone but more so if combined: a possible immunological adjuvant or alarm effect of the venom or of the cascading physiopathology of envenomation. In the few surviving mice, both anti-venom and anti-F(ab')2 antibodies increased significantly after the first envenomation but not thereafter, showing no correlation with mortality rates: a suggestion of their clinical irrelevance, the few hard-to kill mice appeared to resist envenomation by mechanisms other than antibody response. Injection of F(ab')2 alone induced production of detectable anti-venom antibodies in a few mice and injection of venom alone induced that of anti-F(ab')2 antibodies, perhaps due to trace amounts of venom in the high affinity fraction of F(ab')2 and to anti-idiotypic antibodies or polyclonal activity in the envenomation episode, respectively.

Detection of subunit interfacial modifications by tracing the evolution of clamp–loader complex

The archaeal and eukaryal clamp-loader and clamp proteins were investigated with the evolutionary trace method. The molecular phylogeny of the proteins suggested that the hetero-pentameric complex of the archaeal clamp-loader with two subunits (RFCL and RFCS) was not a preserved ancestral type, but a degenerated version of the eukaryal complex of five subunits (RFC1-5). The evolutionary trace of amino acid replacements during the course of subunit differentiation revealed that the replacements had accumulated preferentially at the subunit interface regions. Some of the interfacial modifications that might be responsible for the specific interaction between the subunits were conserved in the current complex.

Solution structure of APETx1 from the sea anemone Anthopleura elegantissima: A new fold for an HERG toxin

APETx1 is a 42-amino acid toxin purified from the venom of the sea anemone Anthopleura elegantissima. This cysteine-rich peptide possesses three disulfide bridges (C4-C37, C6-C30, and C20-C38). Its pharmacological target is the Ether-a-gogo potassium channel. We herein determine the solution structure of APETx1 by use of conventional two-dimensional 1H-NMR techniques followed by torsion angle dynamics and refinement protocols. The calculated structure of APETx1 belongs to the disulfide-rich all-beta structural family, in which a three-stranded anti-parallel beta-sheet is the only secondary structure. APETx1 is the first Ether-a-gogo effector discovered to fold in this way. We therefore compare the structure of APETx1 to those of the two other known effectors of the Ether-a-gogo potassium channel, CnErg1 and BeKm-1, and analyze the topological disposition of key functional residues proposed by analysis of the electrostatic anisotropy. The interacting surface is made of a patch of aromatic residues (Y5, Y32, and F33) together with two basic residues (K8 and K18) at the periphery of the surface. We pinpoint the absence of the central lysine present in the functional surface of the two other Ether-a-gogo effectors.

Current views on scorpion toxins specific for K+-channels

Much of our knowledge on K+-channels was elucidated using specific peptide ligands isolated from a number of venomous organisms. Recently, this field received a strong support and increased interest due to the solution of the three-dimensional structure of a couple of K+-channels. At the same time, several new subfamilies of specific toxins for K+-channels were isolated from scorpion venoms, enhancing the availability and diversity of such useful molecular tools. It opened new lines of research for the better understanding of K+-channel biophysics and pharmacology. In this review, we listed 120 amino acid sequences of peptides isolated from scorpion venoms. They were demonstrated or assumed to be specific for K+-channels. These sequences were aligned and used to generate a rooted phylogenetic tree. The evolutionary tree indicates that several clusters of divergent peptides show preference for specific subtypes of channels. The three-dimensional structures of representative examples of these peptides were drawn and analysed concerning the molecular fitness of their interactions with the channel targets. Four different interacting modes were identified to exist between scorpion toxins and the various subtypes of K+-channels.