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

Venomics Reveals the Venom Complexity of Sea Anemone Heteractis magnifica

The venoms of various sea anemones are rich in diverse toxins, which usually play a dual role in capturing prey and deterring predators. However, the complex components of such venoms have not been well known yet. Here, venomics of integrating transcriptomic and proteomic technologies was applied for the first time to identify putative protein and peptide toxins from different tissues of the representative sea anemone, Heteractis magnifica. The transcriptomic analysis of H. magnifica identified 728 putative toxin sequences, including 442 and 381 from the tentacles and the column, respectively, and they were assigned to 68 gene superfamilies. The proteomic analysis confirmed 101 protein and peptide toxins in the venom, including 91 in the tentacles and 39 in the column. The integrated venomics also confirmed that some toxins such as the ShK-like peptides and defensins are co-expressed in both the tentacles and the column. Meanwhile, a homology analysis was conducted to predict the three-dimensional structures and potential activity of seven representative toxins. Altogether, this venomics study revealed the venom complexity of H. magnifica, which will help deepen our understanding of cnidarian toxins, thereby supporting the in-depth development of valuable marine drugs.

Mechanisms of Action of the Peptide Toxins Targeting Human and Rodent Acid-Sensing Ion Channels and Relevance to Their In Vivo Analgesic Effects

Acid-sensing ion channels (ASICs) are voltage-independent H⁺-gated cation channels largely expressed in the nervous system of rodents and humans. At least six isoforms (ASIC1a, 1b, 2a, 2b, 3 and 4) associate into homotrimers or heterotrimers to form functional channels with highly pH-dependent gating properties. This review provides an update on the pharmacological profiles of animal peptide toxins targeting ASICs, including PcTx1 from tarantula and related spider toxins, APETx2 and APETx-like peptides from sea anemone, and mambalgin from snake, as well as the dimeric protein snake toxin MitTx that have all been instrumental to understanding the structure and the pH-dependent gating of rodent and human cloned ASICs and to study the physiological and pathological roles of native ASICs in vitro and in vivo. ASICs are expressed all along the pain pathways and the pharmacological data clearly support a role for these channels in pain. ASIC-targeting peptide toxins interfere with ASIC gating by complex and pH-dependent mechanisms sometimes leading to opposite effects. However, these dual pH-dependent effects of ASIC-inhibiting toxins (PcTx1, mambalgin and APETx2) are fully compatible with, and even support, their analgesic effects in vivo, both in the central and the peripheral nervous system, as well as potential effects in humans.

Newly Discovered Peptides from the Coral Heliofungia actiniformis Show Structural and Functional Diversity

Scleractinian corals are crucially important to the health of some of the world's most biodiverse, productive, and economically important marine habitats. Despite this importance, analysis of coral peptidomes is still in its infancy. Here we show that the tentacle extract from the stony coral Heliofungia actiniformis is rich in peptides with diverse and novel structures. We have characterized the sequences and three-dimensional structures of four new peptides, three of which have no known homologues. We show that a 2 kDa peptide, Hact-2, promotes significant cell proliferation on human cells and speculate this peptide may be involved in the remarkable regenerative capacity of corals. We found a 3 kDa peptide, Hact-3, encoded within a fascin-like domain, and homologues of Hact-3 are present in the genomes of other coral species. Two additional peptides, Hact-4 and Hact-SCRiP1, with limited sequence similarity, both contain a beta-defensin-like fold and highlight a structural link with the small cysteine-rich proteins (SCRiP) family of proteins found predominantly in corals. Our results provide a first glimpse into the remarkable and unexplored structural diversity of coral peptides, providing insight into their diversity and putative functions and, given the ancient lineage of corals, potential insight into the evolution of structural motifs.

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.

Mechanism of hERG inhibition by gating-modifier toxin, APETx1, deduced by functional characterization

Background

Human ether-à-go-go-related gene potassium channel 1 (hERG) is a voltage-gated potassium channel, the voltage-sensing domain (VSD) of which is targeted by a gating-modifier toxin, APETx1. APETx1 is a 42-residue peptide toxin of sea anemone Anthopleura elegantissima and inhibits hERG by stabilizing the resting state. A previous study that conducted cysteine-scanning analysis of hERG identified two residues in the S3-S4 region of the VSD that play important roles in hERG inhibition by APETx1. However, mutational analysis of APETx1 could not be conducted as only natural resources have been available until now. Therefore, it remains unclear where and how APETx1 interacts with the VSD in the resting state.

Results

We established a method for preparing recombinant APETx1 and determined the NMR structure of the recombinant APETx1, which is structurally equivalent to the natural product. Electrophysiological analyses using wild type and mutants of APETx1 and hERG revealed that their hydrophobic residues, F15, Y32, F33, and L34, in APETx1, and F508 and I521 in hERG, in addition to a previously reported acidic hERG residue, E518, play key roles in the inhibition of hERG by APETx1. Our hypothetical docking models of the APETx1-VSD complex satisfied the results of mutational analysis.

Conclusions

The present study identified the key residues of APETx1 and hERG that are involved in hERG inhibition by APETx1. These results would help advance understanding of the inhibitory mechanism of APETx1, which could provide a structural basis for designing novel ligands targeting the VSDs of K_V channels.

New opportunities and challenges of venom-based and bacteria-derived molecules for anticancer targeted therapy

Due to advances in detection and treatment of cancer, especially the rise in the targeted therapy, the five-year relative survival rate of all cancers has increased significantly. However, according to the analysis of the survival rate of cancer patients in 2019, the survival rate of most cancers is still less than five years. Therefore, to combat complex cancer and further improve the 5-year survival rate of cancer patients, it is necessary to develop some new anticancer drugs. Because of the adaptive evolution of toxic species for millions of years, the venom sac is a "treasure bank", which has millions of biomolecules with high affinity and stability awaiting further development. Complete utilization of venom-based and bacteria-derived drugs in the market is still staggering because of incomplete understanding regarding their mode of action. In this review, we focused on the currently identified targets for anticancer effects based on venomous and bacterial biomolecules, such as ion channels, membrane non-receptor molecules, integrins, and other related target molecules. This review will serve as the key for exploring the molecular mechanisms behind the anticancer potential of venom-based and bacteria-derived drugs and will also lay the path for the development of anticancer targeted therapy.

APETx-Like Peptides from the Sea Anemone Heteractis crispa, Diverse in Their Effect on ASIC1a and ASIC3 Ion Channels

Currently, five peptide modulators of acid-sensing ion channels (ASICs) attributed to structural class 1b of sea anemone toxins have been described. The APETx2 toxin is the first and most potent ASIC3 inhibitor, so its homologs from sea anemones are known as the APETx-like peptides. We have discovered that two APETx-like peptides from the sea anemone Heteractis crispa, Hcr 1b-3 and Hcr 1b-4, demonstrate different effects on rASIC1a and rASIC3 currents. While Hcr 1b-3 inhibits both investigated ASIC subtypes with IC₅₀ 4.95 ± 0.19 μM for rASIC1a and 17 ± 5.8 μM for rASIC3, Hcr 1b-4 has been found to be the first potentiator of ASIC3, simultaneously inhibiting rASIC1a at similar concentrations: EC₅₀ 1.53 ± 0.07 μM and IC₅₀ 1.25 ± 0.04 μM. The closest homologs, APETx2, Hcr 1b-1, and Hcr 1b-2, previously demonstrated the ability to inhibit hASIC3 with IC₅₀ 63 nM, 5.5, and 15.9 μM, respectively, while Hcr 1b-2 also inhibited rASIC1a with IC₅₀ 4.8 ± 0.3 μM. Computer modeling allowed us to describe the peculiarities of Hcr 1b-2 and Hcr 1b-4 interfaces with the rASIC1a channel and the stabilization of the expanded acidic pocket resulting from peptides binding which traps the rASIC1a channel in the closed state.

Sea Anemone Toxins: A Structural Overview

Sea anemones produce venoms of exceptional molecular diversity, with at least 17 different molecular scaffolds reported to date. These venom components have traditionally been classified according to pharmacological activity and amino acid sequence. However, this classification system suffers from vulnerabilities due to functional convergence and functional promiscuity. Furthermore, for most known sea anemone toxins, the exact receptors they target are either unknown, or at best incomplete. In this review, we first provide an overview of the sea anemone venom system and then focus on the venom components. We have organised the venom components by distinguishing firstly between proteins and non-proteinaceous compounds, secondly between enzymes and other proteins without enzymatic activity, then according to the structural scaffold, and finally according to molecular target.

The Anemonia viridis Venom: Coupling Biochemical Purification and RNA-Seq for Translational Research

Blue biotechnologies implement marine bio-resources for addressing practical concerns. The isolation of biologically active molecules from marine animals is one of the main ways this field develops. Strikingly, cnidaria are considered as sustainable resources for this purpose, as they possess unique cells for attack and protection, producing an articulated cocktail of bioactive substances. The Mediterranean sea anemone Anemonia viridis has been studied extensively for years. In this short review, we summarize advances in bioprospecting of the A. viridis toxin arsenal. A. viridis RNA datasets and toxin data mining approaches are briefly described. Analysis reveals the major pool of neurotoxins of A. viridis, which are particularly active on sodium and potassium channels. This review therefore integrates progress in both RNA-Seq based and biochemical-based bioprospecting of A. viridis toxins for biotechnological exploitation.

PhcrTx2, a New Crab-Paralyzing Peptide Toxin from the Sea Anemone Phymanthus crucifer

Sea anemones produce proteinaceous toxins for predation and defense, including peptide toxins that act on a large variety of ion channels of pharmacological and biomedical interest. Phymanthus crucifer is commonly found in the Caribbean Sea; however, the chemical structure and biological activity of its toxins remain unknown, with the exception of PhcrTx1, an acid-sensing ion channel (ASIC) inhibitor. Therefore, in the present work, we focused on the isolation and characterization of new P. crucifer toxins by chromatographic fractionation, followed by a toxicity screening on crabs, an evaluation of ion channels, and sequence analysis. Five groups of toxic chromatographic fractions were found, and a new paralyzing toxin was purified and named PhcrTx2. The toxin inhibited glutamate-gated currents in snail neurons (maximum inhibition of 35%, IC₅₀ 4.7 µM), and displayed little or no influence on voltage-sensitive sodium/potassium channels in snail and rat dorsal root ganglion (DRG) neurons, nor on a variety of cloned voltage-gated ion channels. The toxin sequence was fully elucidated by Edman degradation. PhcrTx2 is a new β-defensin-fold peptide that shares a sequence similarity to type 3 potassium channels toxins. However, its low activity on the evaluated ion channels suggests that its molecular target remains unknown. PhcrTx2 is the first known paralyzing toxin in the family Phymanthidae.

Revisiting venom of the sea anemone Stichodactyla haddoni: Omics techniques reveal the complete toxin arsenal of a well-studied sea anemone genus

More than a century of research on sea anemone venoms has shown that they contain a diversity of biologically active proteins and peptides. However, recent omics studies have revealed that much of the venom proteome remains unexplored. We used, for the first time, a combination of proteomic and transcriptomic techniques to obtain a holistic overview of the venom arsenal of the well-studied sea anemone Stichodactyla haddoni. A purely search-based approach to identify putative toxins in a transcriptome from tentacles regenerating after venom extraction identified 508 unique toxin-like transcripts grouped into 63 families. However, proteomic analysis of venom revealed that 52 of these toxin families are likely false positives. In contrast, the combination of transcriptomic and proteomic data enabled positive identification of 23 families of putative toxins, 12 of which have no homology known proteins or peptides. Our data highlight the importance of using proteomics of milked venom to correctly identify venom proteins/peptides, both known and novel, while minimizing false positive identifications from non-toxin homologues identified in transcriptomes of venom-producing tissues. This work lays the foundation for uncovering the role of individual toxins in sea anemone venom and how they contribute to the envenomation of prey, predators, and competitors.

BIOLOGICAL SIGNIFICANCE

Proteomic analysis of milked venom combined with analysis of a tentacle transcriptome revealed the full extent of the venom arsenal of the sea anemone Stichodactyla haddoni. This combined approach led to the discovery of 12 entirely new families of disulfide-rich peptides and proteins in a genus of anemones that have been studied for over a century.

The enigma of the near-symmetry of proteins: Domain swapping

The majority of proteins form oligomers which have rotational symmetry. Literature has suggested many functional advantages that the symmetric packing offers. Yet, despite these advantages, the vast majority of protein oligomers are only nearly symmetric. A key question in the field of proteins structure is therefore, if symmetry is so advantageous, why do oligomers settle for aggregates that do not maximize that structural property? The answer to that question is apparently multi-parametric, and involves distortions at the interaction zones of the monomer units of the oligomer in order to minimize the free energy, the dynamics of the protein, the effects of surroundings parameters, and the mechanism of oligomerization. The study of this problem is in its infancy: Only the first parameter has been explored so far. Here we focus on the last parameter-the mechanism of formation. To test this effect we have selected to focus on the domain swapping mechanism of oligomerization, by which oligomers form in a mechanism that swaps identical portions of monomeric units, resulting in an interwoven oligomer. We are using continuous symmetry measures to analyze in detail the oligomer formed by this mechanism, and found, that without exception, in all analyzed cases, perfect symmetry is given away, and we are able to identify that the main burden of distortion lies in the hinge regions that connect the swapped portions. We show that the continuous symmetry analysis method clearly identifies the hinge region of swapped domain proteins-considered to be a non-trivial task. We corroborate our conclusion about the central role of the hinge region in affecting the symmetry of the oligomers, by a special probability analysis developed particularly for that purpose.

Venom-derived peptide inhibitors of voltage-gated potassium channels

Voltage-gated potassium channels play a key role in human physiology and pathology. Reflecting their importance, numerous channelopathies have been characterised that arise from mutations in these channels or from autoimmune attack on the channels. Voltage-gated potassium channels are also the target of a broad range of peptide toxins from venomous organisms, including sea anemones, scorpions, spiders, snakes and cone snails; many of these peptides bind to the channels with high potency and selectivity. In this review we describe the various classes of peptide toxins that block these channels and illustrate the broad range of three-dimensional structures that support channel blockade. The therapeutic opportunities afforded by these peptides are also highlighted. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

[Structural Features of Cysteine-Stabilized Polypeptides from Sea Anemones Venoms]

Nowadays, venom-based drug discovery becomes popular again: pharmaceutical companies evaluate animal venom potential as a combinatory library of biologically-active compounds. Collaborations with research groups from academia are intensified, new toxins are being investigated, among which polypeptides are of paramount importance. Sea anemones produce the most diversified, from structural point of view, polypep- tide venom components among other animals. This particular review considers known polypeptide toxins from sea anemones, basically taking into account its classification by primary structural features. The most important functional characteristics are analyzed in each structural class.

The near-symmetry of proteins

The majority of protein oligomers form clusters which are nearly symmetric. Understanding of that imperfection, its origins, and perhaps also its advantages requires the conversion of the currently used vague qualitative descriptive language of the near-symmetry into an accurate quantitative measure that will allow to answer questions such as: "What is the degree of symmetry deviation of the protein?," "how do these deviations compare within a family of proteins?," and so on. We developed quantitative methods to answer this type of questions, which are capable of analyzing the whole protein, its backbone or selected portions of it, down to comparison of symmetry-related specific amino-acids, and which are capable of visualizing the various levels of symmetry deviations in the form of symmetry maps. We have applied these methods on an extensive list of homomers and heteromers and found that apparently all proteins never reach perfect symmetry. Strikingly, even homomeric protein clusters are never ideally symmetric. We also found that the main burden of symmetry distortion is on the amino-acids near the symmetry axis; that it is mainly the more hydrophilic amino-acids that take place in symmetry-distortive interactions; and more. The remarkable ability of heteromers to preserve near-symmetry, despite the different sequences, was also shown and analyzed. The comprehensive literature on the suggested advantages symmetric oligomerizations raises a yet-unsolved key question: If symmetry is so advantageous, why do proteins stop shy of perfect symmetry? Some tentative answers to be tested in further studies are suggested in a concluding outlook.

Understanding the molecular basis of toxin promiscuity: the analgesic sea anemone peptide APETx2 interacts with acid-sensing ion channel 3 and hERG channels via overlapping pharmacophores

The sea anemone peptide APETx2 is a potent and selective blocker of acid-sensing ion channel 3 (ASIC3). APETx2 is analgesic in a variety of rodent pain models, but the lack of knowledge of its pharmacophore and binding site on ASIC3 has impeded development of improved analogues. Here we present a detailed structure-activity relationship study of APETx2. Determination of a high-resolution structure of APETx2 combined with scanning mutagenesis revealed a cluster of aromatic and basic residues that mediate its interaction with ASIC3. We show that APETx2 also inhibits the off-target hERG channel by reducing the maximal current amplitude and shifting the voltage dependence of activation to more positive potentials. Electrophysiological screening of selected APETx2 mutants revealed partial overlap between the surfaces on APETx2 that mediate its interaction with ASIC3 and hERG. Characterization of the molecular basis of these interactions is an important first step toward the rational design of more selective APETx2 analogues.

Sea anemone (Cnidaria, Anthozoa, Actiniaria) toxins: an overview

The Cnidaria phylum includes organisms that are among the most venomous animals. The Anthozoa class includes sea anemones, hard corals, soft corals and sea pens. The composition of cnidarian venoms is not known in detail, but they appear to contain a variety of compounds. Currently around 250 of those compounds have been identified (peptides, proteins, enzymes and proteinase inhibitors) and non-proteinaceous substances (purines, quaternary ammonium compounds, biogenic amines and betaines), but very few genes encoding toxins were described and only a few related protein three-dimensional structures are available. Toxins are used for prey acquisition, but also to deter potential predators (with neurotoxicity and cardiotoxicity effects) and even to fight territorial disputes. Cnidaria toxins have been identified on the nematocysts located on the tentacles, acrorhagi and acontia, and in the mucous coat that covers the animal body. Sea anemone toxins comprise mainly proteins and peptides that are cytolytic or neurotoxic with its potency varying with the structure and site of action and are efficient in targeting different animals, such as insects, crustaceans and vertebrates. Sea anemones toxins include voltage-gated Na⁺ and K⁺ channels toxins, acid-sensing ion channel toxins, Cytolysins, toxins with Kunitz-type protease inhibitors activity and toxins with Phospholipase A2 activity. In this review we assessed the phylogentic relationships of sea anemone toxins, characterized such toxins, the genes encoding them and the toxins three-dimensional structures, further providing a state-of-the-art description of the procedures involved in the isolation and purification of bioactive toxins.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Crotamine pharmacology revisited: novel insights based on the inhibition of KV channels

Crotamine, a 5-kDa peptide, possesses a unique biological versatility. Not only has its cell-penetrating activity become of clinical interest but, moreover, its potential selective antitumor activity is of great pharmacological importance. In the past, several studies have attempted to elucidate the exact molecular target responsible for the crotamine-induced skeletal muscle spasm. The aim of this study was to investigate whether crotamine affects voltage-gated potassium (K(V)) channels in an effort to explain its in vivo effects. Crotamine was studied on ion channel function using the two-electrode voltage clamp technique on 16 cloned ion channels (12 K(V) channels and 4 Na(V) channels), expressed in Xenopus laevis oocytes. Crotamine selectively inhibits K(V)1.1, K(V)1.2, and K(V)1.3 channels with an IC(50) of ∼300 nM, and the key amino acids responsible for this molecular interaction are suggested. Our results demonstrate for the first time that the symptoms, which are observed in the typical crotamine syndrome, may result from the inhibition of K(V) channels. The ability of crotamine to inhibit the potassium current through K(V) channels unravels it as the first snake peptide with the unique multifunctionality of cell-penetrating and antitumoral activity combined with K(V) channel-inhibiting properties. This new property of crotamine might explain some experimental observations and opens new perspectives on pharmacological uses.

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.

Peptide fingerprinting of the neurotoxic fractions isolated from the secretions of sea anemones Stichodactyla helianthus and Bunodosoma granulifera. New members of the APETx-like family identified by a 454 pyrosequencing approach

Sea anemones are known to contain a wide diversity of biologically active peptides, mostly unexplored according to recent peptidomic and transcriptomic studies. In the present work, the neurotoxic fractions from the exudates of Stichodactyla helianthus and Bunodosoma granulifera were analyzed by reversed-phase chromatography and mass spectrometry. The first peptide fingerprints of these sea anemones were assessed, revealing the largest number of peptide components (156) so far found in sea anemone species, as well as the richer peptide diversity of B. granulifera in relation to S. helianthus. The transcriptomic analysis of B. granulifera, performed by massive cDNA sequencing with 454 pyrosequencing approach allowed the discovery of five new APETx-like peptides (U-AITX-Bg1a-e - including the full sequences of their precursors for four of them), which together with type 1 sea anemone sodium channel toxins constitute a very distinguishable feature of studied sea anemone species belonging to genus Bunodosoma. The molecular modeling of these new APETx-like peptides showed a distribution of positively charged and aromatic residues in putative contact surfaces as observed in other animal toxins. On the other hand, they also showed variable electrostatic potentials, thus suggesting a docking onto their targeted channels in different spatial orientations. Moreover several crab paralyzing toxins (other than U-AITX-Bg1a-e), which induce a variety of symptoms in crabs, were isolated. Some of them presumably belong to new classes of crab-paralyzing peptide toxins, especially those with molecular masses below 2kDa, which represent the smallest peptide toxins found in sea anemones.

Structures of sea anemone toxins

Sea anemones produce a variety of toxic peptides and proteins, including many ion channel blockers and modulators, as well as potent cytolysins. This review describes the structures that have been determined to date for the major classes of peptide and protein toxins. In addition, established and emerging methods for structure determination are summarized and the prospects for modelling newly described toxins are evaluated. In common with most other classes of proteins, toxins display conformational flexibility which may play a role in receptor binding and function. The prospects for obtaining atomic resolution structures of toxins bound to their receptors are also discussed.

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.

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.

Toxins interacting with ether-à-go-go-related gene voltage-dependent potassium channels

The critical role that ether-à-go-go-related gene (erg) K(+) channels play in mating in Caenorhabditis elegans, neuronal seizures in Drosophila and cardiac action potential repolarization in humans has been well documented. Three erg genes (erg1, erg2 and erg3) have been identified and characterized. A structurally diverse number of compounds block these channels, but do not display specificity among the different channel isoforms. In this review we describe the blocking properties of several peptides, purified from scorpion, sea anemone and spider venoms, which are selective for certain members of the ERG family of channels. These peptides do not behave as classical pore blockers and appear to modify the gating properties of the channel. Genomic studies predict the existence of many other novel peptides with the potential of being more selective for ERG channels than those discussed here.

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.

Species diversity and peptide toxins blocking selectivity of ether-a-go-go-related gene subfamily K+ channels in the central nervous system

The ether-à-go-go-related gene (erg) K+ channels are known to be crucial for life in Caenorhabditis elegans (mating), Drosophila melanogaster (seizure), and humans (LQT syndrome). The erg genes known to date (erg1, erg2, and erg3) are highly expressed in various areas of the rat and mouse central nervous system (CNS), and ERG channel blockers alter firing accommodation. To assign physiological roles to each isoform, it is necessary to design pharmacological strategies to distinguish individual currents. To this purpose, we have investigated the blocking properties of specific peptide inhibitors of hERG1 channels on the human and rat isoforms. In particular, we have tested ErgTx1 (from the scorpion Centruroides noxious), BeKm-1 (from the scorpion Buthus eupeus), and APETx1 (from the sea anemone Anthopleura elegantissima). Because these peptides had different species-specific effects on the six different channels, we have also carried out a biophysical characterization of hERG2 and hERG3 channels that turned out to be different from the rat homologs. It emerged that APETx1 is exquisitely selective for ERG1 and does not compete with the other two toxins. BeKm-1 discriminates well among the three rat members. ErgTx1 is unable to block hERG2, but blocks rERG2 and has the lowest KD for hERG3. BeKm-1 and ErgTx1 compete for hERG3 but not for rERG2 blockade. Our findings should be helpful for structure-function studies and for novel CNS ERG-specific drug design.

Solution structure of APETx2, a specific peptide inhibitor of ASIC3 proton-gated channels

Acid-sensing ion channels (ASIC) are proton-gated sodium channels that have been implicated in pain transduction associated with acidosis in inflamed or ischemic tissues. APETx2, a peptide toxin effector of ASIC3, has been purified from an extract of the sea anemone Anthopleura elegantissima. APETx2 is a 42-amino-acid peptide cross-linked by three disulfide bridges. Its three-dimensional structure, as determined by conventional two-dimensional 1H-NMR, consists of a compact disulfide-bonded core composed of a four-stranded beta-sheet. It belongs to the disulfide-rich all-beta structural family encompassing peptide toxins commonly found in animal venoms. The structural characteristics of APETx2 are compared with that of PcTx1, another effector of ASIC channels but specific to the ASIC1a subtype and to APETx1, a toxin structurally related to APETx2, which targets the HERG potassium channel. Structural comparisons, coupled with the analysis of the electrostatic characteristics of these various ion channel effectors, led us to suggest a putative channel interaction surface for APETx2, encompassing its N terminus together with the type I-beta turn connecting beta-strands III and IV. This basic surface (R31 and R17) is also rich in aromatic residues (Y16, F15, Y32, and F33). An additional region made of the type II'-beta turn connecting beta-strands I and II could also play a role in the specificity observed for these different ion effectors.