Current views on scorpion toxins specific for K+-channels

Assembling an arsenal, the scorpion way

Background

For survival, scorpions depend on a wide array of short neurotoxic polypeptides. The venoms of scorpions from the most studied group, the Buthida, are a rich source of small, 23–78 amino acid-long peptides, well packed by either three or four disulfide bridges that affect ion channel function in excitable and non-excitable cells.

Results

In this work, by constructing a toxin transcripts data set from the venom gland of the scorpion Buthus occitanus israelis, we were able to follow the evolutionary path leading to mature toxin diversification and suggest a mechanism for leader peptide hyper-conservation. Toxins from each family were more closely related to one another than to toxins from other species, implying that fixation of duplicated genes followed speciation, suggesting early gene conversion events. Upon fixation, the mature toxin-coding domain was subjected to diversifying selection resulting in a significantly higher substitution rate that can be explained solely by diversifying selection. In contrast to the mature peptide, the leader peptide sequence was hyper-conserved and characterized by an atypical sub-neutral synonymous substitution rate. We interpret this as resulting from purifying selection acting on both the peptide and, as reported here for the first time, the DNA sequence, to create a toxin family-specific codon bias.

Conclusion

We thus propose that scorpion toxin genes were shaped by selective forces acting at three levels, namely (1) diversifying the mature toxin, (2) conserving the leader peptide amino acid sequence and intriguingly, (3) conserving the leader DNA sequences.

Intraspecific variation in the Egyptian scorpion Scorpio maurus palmatus venom collected from different biotopes

The present study was conducted to explore the following hypotheses: (i) do scorpions (Scorpio maurus palmatus) from different biotopes exhibit intraspecific diversity in their venom? (ii) if so, is this variation associated with ecological or genetic factors, geographical distance, and/or multiple interrelated parameters? To address these questions, scorpions were collected from four geographically isolated localities in Egypt. Three of these locations are from mutually isolated pockets in the arid biotope of Southern Sinai (Wadi Sahab, El-Agramia and Rahaba plains). The fourth population was sampled from the semiarid biotope of Western Mediterranean Costal Desert (WMCD). Using reducing gel electrophoresis (SDS-PAGE), we have shown biotope-specific variation in the expression of peptides from scorpions collected from these distinct areas. WMCD sourced venom samples contain higher molecular weight protein components (219, 200, 170, 139, 116 kDa) than Southern Sinai scorpion venom samples. The Southern Sinai venom is characterized by the presence of 11 protein bands (93-0.58 kDa) that are not mirrored in the individual venom samples of WMCD. Bands of 33 and 3.4 kDa were characteristics of all individual venom samples of the scorpion populations. Even within Southern Sinai area, Sahab venom contains five fractions that are not detected in both El-Agramia and Rahaba venom samples. Moreover, male and female venom analysis revealed some sex-related proteomic similarities and differences between scorpion populations. Female venom appears to be more complicated than the male venom. Female venom samples showed bands of 219, 200, 77.5, 55.5, 45, 39, 37, 24 and 16 kDa which were absent in the male venom. The random amplified polymorphic DNA (RAPD) technique was used to estimate the genetic distance between the four scorpion populations. The RAPD data confirmed the genetic diversity at molecular level among the sampled populations. More than 77 RAPD bands (ranging in size from 125 to 15,000 bp) were defined from the four scorpion populations. Of the 77 bands, 57 (76.2%) were polymorphic and 20 were monomorphic among the populations. The similarity coefficient data of venom and DNA were used to construct separate dendrograms, which grouped together the Southern Sinai populations and these were some distance away from the WMCD population. Taken together, we suspect that a combination of local environmental conditions, geographical separation and genetic separation may play a major role in the intraspecific variation of venom of S. m. palmatus.

Molecular evolution, functional variation, and proposed nomenclature of the gene family that includes sphingomyelinase D in sicariid spider venoms

The venom enzyme sphingomyelinase D (SMase D) in the spider family Sicariidae (brown or fiddleback spiders [Loxosceles] and six-eyed sand spiders [Sicarius]) causes dermonecrosis in mammals. SMase D is in a gene family with multiple venom-expressed members that vary in functional specificity. We analyze molecular evolution of this family and variation in SMase D activity among crude venoms using a data set that represents the phylogenetic breadth of Loxosceles and Sicarius. We isolated a total of 190 nonredundant nucleotide sequences encoding 168 nonredundant amino acid sequences of SMase D homologs from 21 species. Bayesian phylogenies support two major clades that we name alpha and beta, within which we define seven and three subclades, respectively. Sequences in the alpha clade are exclusively from New World Loxosceles and Loxosceles rufescens and include published genes for which expression products have SMase D and dermonecrotic activity. The beta clade includes paralogs from New World Loxosceles that have no, or reduced, SMase D and no dermonecrotic activity and also paralogs from Sicarius and African Loxosceles of unknown activity. Gene duplications are frequent, consistent with a birth-and-death model, and there is evidence of purifying selection with episodic positive directional selection. Despite having venom-expressed SMase D homologs, venoms from New World Sicarius have reduced, or no, detectable SMase D activity, and Loxosceles in the Southern African spinulosa group have low SMase D activity. Sequence conservation mapping shows >98% conservation of proposed catalytic residues of the active site and around a plug motif at the opposite end of the TIM barrel, but alpha and beta clades differ in conservation of key residues surrounding the apparent substrate binding pocket. Based on these combined results, we propose an inclusive nomenclature for the gene family, renaming it SicTox, and discuss emerging patterns of functional diversification.

Characterization of LmTxLP11 and LmVP1.1 transcripts and genomic organizations: alternative splicing contributing to the diversity of scorpion venom peptides

Scorpion venoms are rich resources of bioactive peptides with extreme variability. Multiple molecular mechanisms are involved in the diversity of scorpion venom peptides. However, alternative splicing, which plays a major role in the generation of proteomic and functional diversity in metazoan organisms, hasn't been reported in genes coding for scorpion venom peptides. In the EST analysis of venom peptide transcripts from scorpion Lychas mucronatus, we reported an alternative splicing event. Transcripts of LmTxLP11 and LmVP1.1 share identical 5' region. LmVP1.1 is a novel type of scorpion venom peptides constrained by one disulfide bridge, whereas LmTxLP11 is an extended version of LmVP1.1. By transcript alignment with its genomic sequence, it is found that both transcripts are generated from a single gene by alternative poly A site and terminal exon. The gene encoding LmTxLP11 and LmVP1.1 is the first one harboring three introns ever reported from scorpion venoms. This work demonstrates for the first time that alternative splicing is involved in regulating the diversity of scorpion venom peptides.

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

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

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.

Purification and N-terminal sequence of a serine proteinase-like protein (BMK-CBP) from the venom of the Chinese scorpion (Buthus martensii Karsch)

A serine proteinase-like protein was isolated from the venom of Chinese red scorpion (Buthus martensii Karsch) by combination of gel filtration, ion-exchange and reveres-phase chromatography and named BMK-CBP. The apparent molecular weight of BMK-CBP was identified as 33 kDa by SDS-PAGE under non-reducing condition. The sequence of N-terminal 40 amino acids was obtained by Edman degradation. The sequence shows highest similarity to proteinase from insect source. When tested with commonly used substrates of proteinase, no significant hydrolytic activity was observed for BMK-CBP. The purified BMK-CBP was found to bind to the cancer cell line MCF-7 and the cell binding ability was dose-dependent.

Effect of Cu2+ on the oxidative folding of synthetic maurotoxin in vitro

Maurotoxin (MTX) is a 34-mer scorpion toxin cross-linked by four disulphide bridges that acts on various K+ channel types. It folds according to an alpha/beta scaffold, i.e., a helix connected to a two stranded beta-sheet by two disulphide bridges. In a former study, various parameters that affect the oxidation and folding of the reduced form of synthetic MTX were investigated in vitro. It was found that MTX achieves its final 3-D structure by evolving over time through a series of oxidation intermediates, from the least to the most oxidized species. MTX oxidative intermediates can be studied by iodoacetamide alkylation of free cysteine residues followed by mass spectrometry analysis. Here, we have analysed the effect of Cu2+ (0.1 to 50 mM) on the kinetics of MTX oxidative folding and found that it dramatically speeds up the formation of the four-disulphide bridged, native-like, MTX (maximal production within 30 minutes instead of > 60 hours). This catalysing effect of Cu2+ was found to be concentration-dependent, reaching a plateau at 10 mM copper ions. Cu2+ was also found to prevent the slow transition of a three disulphide-bridged MTX intermediate towards the final four disulphide-bridged product (12% of total MTX). The data are discussed in light of the potential effects of Cu2+ on MTX secondary structure formation, disulphide bridging and peptidyl prolyl cis-trans isomerization.

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

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

OdK2, a Kv1.3 channel-selective toxin from the venom of the Iranian scorpion Odonthobuthus doriae

The first Kv1.3 channel-selective toxin from the venom of the Iranian scorpion Odonthobuthus doriae (OdK2) was purified, sequenced and characterized physiologically. OdK2 consists of 38 amino acids, including six conserved cysteine and a C-terminal lysine residue, as revealed by the unique use of a quadrupole ion cyclotron resonance Fourier-transform mass spectrometer. Based on multiple sequence alignments, OdK2 was classified as alpha-KTX3.11. The pharmacological effects of OdK2 were studied on a panel of eight different cloned K(+) channels (vertebrate Kv1.1-Kv1.6, Shaker IR and hERG) expressed in Xenopus laevis oocytes. Interestingly, OdK2 selectively inhibits the currents through Kv1.3 channels with an IC50 value of 7.2+/-2.7nM.

Two novel ergtoxins, blockers of K+-channels, purified from the Mexican scorpion Centruroides elegans elegans

Voltage-gated potassium channels of the ether-a-go-go related gene (ERG) family are implicated in many important cellular processes. Three such genes have been cloned (erg1, erg2 and erg3) and shown to be expressed in the central nervous system (CNS) of mammalians. This communication describes the isolation and characterization of two isoforms of scorpion toxin (CeErg4 and CeErg5, systematic nomenclature gamma-KTx1.7 and gamma-KTx1.8, respectively) that can discriminate the various subtypes of ERG channels of human and rat. These peptides were purified from the venom of the Mexican scorpion Centruroides elegans elegans. They contain 42 amino acid residues, tightly folded by four disulfide bridges. Both peptides block in a reversible manner human and rat ERG1 channels, but have no effect on human ERG2. They also block completely and irreversibly the rat ERG2 and the human ERG3 channels hence are excellent tools for the discrimination of the various sub-types of ion-channels studied.

Immunological characterization of a non-toxic peptide conferring protection against the toxic fraction (AahG50) of the Androctonus australis hector venom

KAaH1 and KAaH2 are non-toxic peptides, isolated from the venom of the Androctonus australis hector (Aah) scorpion. In a previous study, we showed these peptides to be the most abundant (approximately 10% each) in the toxic fraction (AahG50) of the Aah venom. KAaH1 and KAaH2 showed high sequence identities (approximately 60%) with birtoxin-like peptides, which likewise are the major peptidic components of Parabuthus transvaalicus scorpion venom. Here, we report the immunological characterization of KAaH1 and KAaH2. These peptides were found to be specifically recognized by polyclonal antibodies raised against AahII, the most toxic peptide of Aah venom, and represents the second antigenic group, including toxins from different scorpion species in the world. Moreover, KAaH1 partially inhibits AahII binding to its specific antibody, suggesting some common epitopes between these two peptides. The identification of possible key antigenic residues in KAaH1 was deduced from comparison of its 3-D model with the experimental structure of AahII. Two clusters of putative antigenically important residues were found at the exposed surface; one could be constituted of V3 and D53, the other of D10, T15 and Y16. Polyclonal antibodies raised against KAaH1 in mice were found to cross-react with both AahII and AahG50, and neutralizing 5LD(50)/ml of the toxic fraction. Mice vaccinated with KAaH1 were protected against a challenge of 2LD(50) of AahG50 fraction. All these data suggest that KAaH1 has clear advantages over the use of the whole or part of the venom. KAaH1 is not toxic and could produce sera-neutralizing scorpion toxins, not only from Aah venom, but also toxins of other venoms from Buthus, Leiurus, or Parabuthus scorpion species presenting antigenically related toxins.

Molecular cloning and electrophysiological studies on the first K(+) channel toxin (LmKTx8) derived from scorpion Lychas mucronatus

LmKTx8, the first toxic gene isolated from the venom of scorpion Lychas mucronatus by constructing cDNA library method, was expressed and characterized physiologically. The mature peptide has 40 residues including six conserved cysteines, and is classified as one of alpha-KTx11 subfamily. Using patch-clamp recording, the recombinant LmKTx8 (rLmKTx8) was used to test the effect on voltage-gated K(+) channels (Kv1.3) stably expressed in COS7 cells and large conductance-Ca(2+)-activated K(+) (BK) channels expressed in HEK293. The results of electrophysiological experiments showed that the rLmKTx8 was a potent inhibitor of Kv1.3 channels with an IC(50)=26.40+/-1.62nM, but 100nM rLmKTx8 did not block the BK currents. LmKTx8 or its analogs might serve as a potential candidate for the development of new drugs for autoimmune diseases.

Purification and characterization of a novel short-chain insecticidal toxin with two disulfide bridges from the venom of the scorpion Liocheles australasiae

Scorpion venoms contain a variety of peptides toxic to mammals, insects and crustaceans. Most of the scorpion toxins have been isolated from the venoms of scorpions in the family Buthidae, but little interest has been paid to non-Buthidae scorpions. In this study, we isolated a short-chain insecticidal toxin (LaIT1) from the venom of the scorpion Liocheles australasiae belonging to the Hemiscorpiidae family. This toxin showed insect toxicity against crickets at a dose of 1.0 microg/insect, but no toxicity was observed against mice even after injection of 1.0 microg of LaIT1 via the intracerebroventricular route, suggesting that the effect of the toxin is insect-selective. Edman sequencing and mass spectrometric analysis revealed that the toxin is composed of 36 amino acid residues and cross-linked by only two disulfide bridges. The pattern of the disulfide bridges was assigned by LC/MS analysis after enzymatic digestion. LaIT1 shows no sequence homology to any other known toxins, suggesting that this toxin represents a novel structural motif class.

Cloning and characterization of BmK86, a novel K+-channel blocker from scorpion venom

Scorpion venom represents a tremendous hitherto unexplored resource for understanding ion channels. BmK86 is a novel K+ -channel toxin gene isolated from a cDNA library of Mesobuthus martensii Karsch, which encodes a signal peptide of 22 amino acid residues and a mature toxin of 35 residues with three disulfide bridges. The genomic sequence of BmK86 consists of two exons disrupted by an intron of 72 bp. Comparison with the other scorpion toxins BmK86 shows low sequence similarity. The GST-BmK86 fusion protein was successfully expressed in Escherichia coli. The fusion protein was cleaved by enterokinase and the recombinant BmK86 was purified by HPLC. Using whole-cell patch-clamp recording, the recombinant BmK86 was found to inhibit the potassium current of mKv1.3 channel expressed in COS7 cells. These results indicated that BmK86 belongs to a representative member of a novel subfamily of alpha-KTxs. The systematic number assigned to BmK86 is alpha-KTx26.1.

Proteomic analysis of the venom from the scorpion Tityus stigmurus: biochemical and physiological comparison with other Tityus species

The venom from the Brazilian scorpion Tityus stigmurus was fractionated by high performance liquid chromatography (HPLC) and the corresponding components were used for molecular mass determination using electrospray ion trap mass spectrometry. One hundred distinct components were clearly assigned showing molecular masses from 216.5 to 44,800.0 Da. Fifteen new components were isolated and sequenced, four of them to completion: Tst-3 (similar to Na(+) channel specific scorpion toxins), Tst-17 (a K(+) channel blocking peptide similar to Tc1), Tst beta KTx (a peptide with identical sequence as that of TsTX-K beta toxin earlier described to exist in T. serrulatus venom) and finally a novel proline-rich peptide of unknown function. Among the eleven components partially sequenced were two enzymes: hyaluronidase and lysozyme. The first enzyme has a molecular mass of 44,800.0 Da. This enzyme showed high activity against the substrate hyaluronan in vitro. Amino acid sequence of the second enzyme showed that it is similar to other known lysozymes, with similar molecular mass and sequence to that of bona fide lysozymes reported in public protein data banks. Finally, this communication reports a correlation among HPLC retention times and molecular masses of folded scorpion toxins as well as a comparative structural and physiological analysis of components from the venom of several species of the genus Tityus.

Transcriptome analysis of the venom gland of the Mexican scorpion Hadrurus gertschi (Arachnida: Scorpiones)

Background

Scorpions like other venomous animals posses a highly specialized organ that produces, secretes and disposes the venom components. In these animals, the last postabdominal segment, named telson, contains a pair of venomous glands connected to the stinger. The isolation of numerous scorpion toxins, along with cDNA-based gene cloning and, more recently, proteomic analyses have provided us with a large collection of venom components sequences. However, all of them are secreted, or at least are predicted to be secretable gene products. Therefore very little is known about the cellular processes that normally take place inside the glands for production of the venom mixture. To gain insights into the scorpion venom gland biology, we have decided to perform a transcriptomic analysis by constructing a cDNA library and conducting a random sequencing screening of the transcripts.

Results

From the cDNA library prepared from a single venom gland of the scorpion Hadrurus gertschi, 160 expressed sequence tags (ESTs) were analyzed. These transcripts were further clustered into 68 unique sequences (20 contigs and 48 singlets), with an average length of 919 bp. Half of the ESTs can be confidentially assigned as homologues of annotated gene products. Annotation of these ESTs, with the aid of Gene Ontology terms and homology to eukaryotic orthologous groups, reveals some cellular processes important for venom gland function; including high protein synthesis, tuned posttranslational processing and trafficking. Nonetheless, the main group of the identified gene products includes ESTs similar to known scorpion toxins or other previously characterized scorpion venom components, which account for nearly 60% of the identified proteins.

Conclusion

To the best of our knowledge this report contains the first transcriptome analysis of genes transcribed by the venomous gland of a scorpion. The data were obtained for the species Hadrurus gertschi, belonging to the family Caraboctonidae. One hundred and sixty ESTs were analyzed, showing enrichment in genes that encode for products similar to known venom components, but also provides the first sketch of cellular components, molecular functions, biological processes and some unique sequences of the scorpion venom gland.

Characterization of peptide components in the venom of the scorpion Liocheles australasiae (Hemiscorpiidae)

Scorpion venoms are composed of a number of neurotoxic peptides. A variety of toxins have been isolated from the venoms of scorpions of the family Buthidae, however, little interest has been paid to non-Buthidae scorpions. In this study, we examined the toxicity of the venom of Liocheles australasiae (Hemiscorpiidae) to mice and crickets, and characterized the peptide components by HPLC and mass spectrometry. Over 200 components were detected in the L. australasiae venom by LC/MS analysis, with components of molecular masses ranging from 500 to 5000 Da being particularly abundant. A number of peptides contained two to four disulfide bridges, which was estimated based on the mass difference after derivatization of Cys residues. A peptide having a monoisotopic molecular mass of 7781.6 Da and four disulfide bridges was isolated from the venom. The peptide has a primary structure similar in terms of the position of eight Cys residues to those observed in several peptides found from scorpions, ticks and insects, although biological roles of these peptides are unknown.

Isolation of the first toxin from the scorpion Buthus occitanus israelis showing preference for Shaker potassium channels

We have purified BoiTx1, the first toxin from the venom of the Israeli scorpion, Buthus occitanus israelis, and studied its activity and genomic organization. BoiTx1 is a 37 amino acid-long peptide contained six conserved cysteines, and is classified as an alpha-KTx3.10 toxin. The pharmacological effects of BoiTx1 were studied on various cloned K(+) channels expressed in Xenopus laevis oocytes. BoiTx1 inhibited currents through Drosophila Shaker channels with an IC(50) value of 3.5+/-0.5nM, yet had much lesser effect on its mammalian orthologs. Thus, BoiTx1 is the first member of the alpha-KTx3 family that preferentially affects insect potassium channels.

Improved fusion protein expression of EGFP via the mutation of both Kozak and the initial ATG codon

Since its discovery, green fluorescence protein (GFP) has been used as a reporter in a broad range of applications, including the determination of gene expresion in diverse organisms, and subcellular protein localization. pEGFP-N1 is a eukayotic expression vector encoding EGFP, the MCS of which locates at the N terminus of EGFP. In this study, the cDNA sequence of scorpion toxin BmKK2 was inserted into the XhoI-HindIII cut of pEGFP-N1 to construct a toxin-EGFP fusion gene (named pEGFP-BmKK2). Fluorescence imaging revealed that HEK 293T cells that were transfected by pEGFP-BmKK2 emitted green fluorescence. Transcription of pEGFP-BmKK2 was confirmed by RT-PCR. However, western blotting analysis showed that the transfected HEK 293T cells expressed mostly EGFP, but little toxin-EGFP fusion protein, implying that pEGFP-N1 cannot be used as a fusion expression vector for subcellular protein localization for the BmKK2 gene. Consequently, two modified recombinant vectors (pEGFP-BmKK2-M1 and pEGFP-BmKK2-M2) were constructed based on pEGFP-BmKK2. This greatly improved the expression of toxin-EGFP fusion protein from pEGFP-BmKK2-M2.

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.

Wide phylogenetic distribution of Scorpine and long-chain beta-KTx-like peptides in scorpion venoms: identification of "orphan" components

Scorpine and toxins specific for potassium channels of the family beta (beta-Ktx) are two types of structurally related scorpion venom components, characterized by an unusually long extended N-terminal segment, followed by a Cys-rich domain with some resemblance to other scorpion toxins. In this communication, we report evidence supporting the ubiquitous presence of Scorpine and beta-KTx-like polypeptides and their precursors in scorpions of the genus Tityus of the family Buthidae, but also included is the first example of such peptides in scorpions from the family Iuridae. Seven new beta-KTxs or Scorpine-like peptides and precursors are reported: five from the genus Tityus (T. costatus, T. discrepans and T. trivittatus) and two from Hadrurus gertschi. The cDNA precursors for all of these peptides were obtained by molecular cloning and their presence in the venoms were confirmed for various peptides. Analysis of the sequences revealed the existence of at least three distinct groups: (1) beta-KTx-like peptides from buthids; (2) Scorpine-like peptides from scorpionid and iurid scorpions; (3) heterogeneous peptides similar to BmTXKbeta of buthids and iurids. The biological function for most of these peptides is not well known; that is why they are here considered "orphan" peptides.

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

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

Molecular characterization of a new scorpion venom lipolysis activating peptide: Evidence for disulfide bridge-mediated functional switch of peptides

Venoms from scorpions contain extremely rich bioactive peptides that often carry diverse functions and are presumably needed to achieve synergistic effects for rapidly immobilizing prey and defending themselves. BotLVP1 is a unique heterodimer protein recently found in the scorpion Buthus occitanus tunetanus venom that is structurally related to scorpion toxins affecting sodium channels (NaScTxs) but exhibits adipocyte lipolysis activity. We have isolated and identified two cDNA clones encoding subunits alpha and beta of a BotLVP1-like peptide (named BmLVP1) from the Chinese scorpion Buthus martensii venom gland and determined the first complete gene structure of this subfamily. These results highlight a genetic link between these lipolysis activating peptides and NaScTxs. Comparison of cDNA and genomic sequences combined with protein structural and functional analysis provides evidence supporting the existence of RNA editing mechanism in scorpion venom glands, which could mediate functional switch of BmLVP1 gene, from adipocyte lipolysis to neurotoxicity, by altering the wrapper disulfide bridge (WDB) pattern of the peptides.

The Universal Protein Resource (UniProt)

The ability to store and interconnect all available information on proteins is crucial to modern biological research. Accordingly, the Universal Protein Resource (UniProt) plays an increasingly important role by providing a stable, comprehensive, freely accessible central resource on protein sequences and functional annotation. UniProt is produced by the UniProt Consortium, formed in 2002 by the European Bioinformatics Institute (EBI), the Protein Information Resource (PIR) and the Swiss Institute of Bioinformatics (SIB). The core activities include manual curation of protein sequences assisted by computational analysis, sequence archiving, development of a user-friendly UniProt web site and the provision of additional value-added information through cross-references to other databases. UniProt is comprised of three major components, each optimized for different uses: the UniProt Archive, the UniProt Knowledgebase and the UniProt Reference Clusters. An additional component consisting of metagenomic and environmental sequences has recently been added to UniProt to ensure availability of such sequences in a timely fashion. UniProt is updated and distributed on a bi-weekly basis and can be accessed online for searches or download at .

The first potassium channel toxin from the venom of the Iranian scorpion Odonthobuthus doriae

The very first member of K(+) channels toxins from the venom of the Iranian scorpion Odonthobuthus doriae (OdK1) was purified, sequenced and characterized physiologically. OdK1 has 29 amino acids, six conserved cysteines and a pI value of 4.95. Based on multiple sequence alignments, OdK1 was classified as alpha-KTx 8.5. The pharmacological effects of OdK1 were studied on six different cloned K(+) channels (vertebrate Kv1.1-Kv1.5 and Shaker IR) expressed in Xenopus laevis oocytes. Interestingly, OdK1 selectively inhibited the currents through Kv1.2 channels with an IC50 value of 183+/-3 nM but did not affect any of the other channels.

Reversible inhibition of the platelet procoagulant response through manipulation of the Gardos channel

The platelet procoagulant response requires a sustained elevation of the intracellular Ca2+ concentration, [Ca2+]i, causing exposure of phosphatidylserine (PS) at the outer surface of the plasma membrane. An increased [Ca2+]i also activates Ca2+-dependent K+ channels. Here, we investigated the contribution of the efflux of K+ ions on the platelet procoagulant response in collagen-thrombin–activated platelets using selective K+ channel blockers. The Gardos channel blockers clotrimazol, charybdotoxin, and quinine caused a similar decrease in prothrombinase activity as well as in the number of PS-exposing platelets detected by fluorescence-conjugated annexin A5. Apamin and iberiotoxin, inhibitors of other K+ channels, were without effect. Only clotrimazol showed a significant inhibition of the collagen-plus-thrombin–induced intracellular calcium response. Clotrimazol and charybdotoxin did not inhibit aggregation and release under the conditions used. Inhibition by Gardos channel blockers was reversed by valinomycin, a selective K+ ionophore. The impaired procoagulant response of platelets from a patient with Scott syndrome was partially restored by pretreatment with valinomycin, suggesting a possible defect of the Gardos channel in this syndrome. Collectively, these results provide evidence for the involvement of efflux of K+ ions through Ca2+-activated K+ channels in the procoagulant response of platelets, opening potential strategies for therapeutic interventions.

Kv4 channels sensitive to BmTX3 in rat nervous system: autoradiographic analysis of their distribution during brain ontogenesis

The binding site distribution of sBmTX3, a chemically synthesized toxin originally purified from the venom of the scorpion Buthus martensi, was investigated in adult and developing rat brain, using patch-clamp experiments and quantitative autoradiography. The molecular basis of these sBmTX3 sites was analysed by electrophysiology on transient Kv currents recorded in mammalian transfected cells. The rapidly activating and inactivating Kv4.1 current was inhibited by sBmTX3 (IC50, 105 nM). The inhibition was less effective on Kv4.2 and Kv4.3 channels and the toxin did not affect other transient currents such as Kv1.4 and Kv3.4. The distribution of the 125I-sBmTX3 binding sites was heterogeneous, with a 113-fold difference between the highest and the lowest densities in adult rat brain. The site density was particularly important in the caudate-putamen and accumbens nucleus, thalamus, hippocampal formation and cerebellum. The affinity of sBmTX3 remained constant during brain ontogenesis. The level of sBmTX3 binding sites was very low in prenatal and postnatal stages to postnatal day (P)12 but drastically increased from P15 in the major part of the studied structures except in the CA3 hippocampal field where the density was very high from P6. Thus, the distribution of sBmTX3 binding sites in rat brain and its electrophysiological characteristics suggest that sBmTX3 specifically binds to the Kv4 subfamily of K channels.

Proteomic analysis of Tityus discrepans scorpion venom and amino acid sequence of novel toxins

The Venezuelan scorpion Tityus discrepans is known to cause human fatalities. We describe the first complete proteomic analysis of its venom. By HPLC 58 different fractions were obtained and 205 different components were identified by MS analysis. Components having molecular masses from 272 to 57 908 amu were found. Forty homogeneous components had their N-terminal amino acid sequence determined by Edman degradation, from which two new peptides named TdK2 and TdK3 (meaning T. discrepans (Td) K(+) channel toxins 2 and 3) were fully characterized. The first contains 34 amino acid residues with a molecular mass of 3451 amu, and the second has 36 amino acids with 3832 amu. Both peptides are tightly bound by three disulfide bridges. TdK2 was shown to block reversibly the Shaker B K(+)-channel expressed heterologously in Sf9 cells. The systematic number assigned to TdK2 is alpha-KTx-18.2 and that of TdK3 is alpha-KTx-18.3. Comparative analysis of the amino acid sequences found suggests that this venom contains peptides highly similar to those that block K(+) channels, as well as those that modify the gating mechanisms of Na(+) channels, found in other scorpions. Additionally, peptides similar to defensins were also identified.

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.

Characterization of venom components from the scorpion Androctonus crassicauda of Turkey: Peptides and genes

The soluble venom from the scorpion Androctonus crassicauda was fractionated by high performance liquid chromatography. At least 44 different sub-fractions were resolved and collected for finger print mass analysis using an electrospray mass spectrometer. This analysis revealed the presence of 80 distinct molecular mass components, from which five were further characterized. A peptide, named Acra1 was fully sequenced. It contains 58 amino acid residues cross-bridged by six cysteines forming three disulfide pairs, with a molecular mass of 6497 Da. A second purified peptide named Acra2 was partially sequenced with a molecular mass of 7849 Da. Acra1 is toxic and Acra2 is lethal to mice, at the dose assayed. Additionally, a cDNA library of the venomous gland of one specimen was prepared and several clones were obtained among which is one that codes for Acra1. Three analog gene sequences were found with point mutations either in the section that corresponds to the mature peptide or to the signal peptide. The signal peptide is 22 amino acid residues long. Several other gene sequences obtained suggest the presence in this venom of three distinct groups of peptides, among which are peptides similar to known Na(+)-channel specific toxins of other scorpions. A new type of peptide was identified with odd number of cysteines (seven), allowing the formation of heterodimers with molecular masses in the range of 16,000 atomic mass units (a.m.u.).

Molecular dissection of venom from Chinese scorpion Mesobuthus martensii: identification and characterization of four novel disulfide-bridged venom peptides

Scorpion venom is composed of a large repertoire of biologically active polypeptides. However, most of these peptides remain to be identified and characterized. In this paper, we report the identification and characterization of four novel disulfide-bridged venom peptides (named BmKBTx, BmKITx, BmKKx1 and BmKKx2, respectively) from the Chinese scorpion, Mesobuthus martensii (also named Buthus martensii Karsch). BmKBTx is composed of 58 amino acid residues and cross-linked by three disulfide bridges. The sequence of BmKBTx shows some similarities to that of the toxin, birtoxin, and its analogs. It is likely that BmKBTx is a beta-toxin active on Na+ channels, which is toxic to either insects or mammals. BmKITx is composed of 71 amino acid residues with four disulfide bridges. It is the longest venom peptide identified from M. martensii so far. BmKITx shows little sequence identity with scorpion alpha-toxins toxic to insects. It is likely that BmKITx is a new type of Na+ -channel specific toxin active on both insects and mammals. BmKKx1 contains 38 amino acid residues cross-linked by three disulfide bridges and shows 84% sequence identity with BmTx3, an inhibitor of A-type K+ channel and HERG currents. BmKKx1 has been classified as alpha-KTx-15.8. BmKKx2 is composed of 36 residues and stabilized by three disulfide bridges. BmKKx2 is a new member of the gamma-K+ -channel toxin subfamily (classified as gamma-KTx 2.2). The venoms of scorpions thus continue to provide novel toxins with potential novel actions on targets.

Residue Phe266 in S5-S6 loop is not critical for Charybdotoxin binding to Ca2+-activated K+ (mSlo1) channels

AIM

To gain insight into the interaction between the Charybdotoxin (ChTX) and BK channels.

METHODS

Site-directed mutagenesis was used to make two mutants: mSlo1-F266L and mSlo1-F266A. The two mutants were then expressed in Xenopus oocytes and their effects were tested on ChTX by electrophysiology experiments.

RESULTS

We demonstrate an equilibrium dissociation constant Kd=3.1-4.2 nmol/L for both the mutants mSlo1-F266L and mSlo1-F266A similar to that of the wild-type mSlo1 Kd=3.9 nmol/L.

CONCLUSION

The residue Phe266 does not play a crucial role in binding to ChTX, which is opposed to the result arising from the simulation of peptide-channel interaction.

The solution structure of BmTx3B, a member of the scorpion toxin subfamily alpha-KTx 16

This article reports the solution structure of BmTx3B (alpha-KTx16.2), a potassium channel blocker belonging to the subfamily alpha-KTx16, purified from the venom of the Chinese scorpion Buthus martensi Karsch. In solution, BmTx3B assumes a typical CSalphabeta motif, with an alpha-helix connected to a triple-stranded beta-sheet by 3 disulfide bridges, which belongs to the first structural group of short-chain scorpion toxins. On the other hand, BmTx3B is quite different from other toxins (such as ChTx and AgTx2) of this group in terms of the electrostatic and hydrophobic surface distribution. The functional surface (beta-face) of the molecule is characterized by less basic residues (only 2: Lys28 and Arg35) and extra aromatic residues (Phe1, Phe9, Trp15, and Tyr37). The peptide shows a great preference for the Kca1.1 channel over the Kv channel (about a 10(3)-fold difference). The model of BmTx3B/Kca1.1 channel complex generated by docking and dynamic simulation reveals that the stable binding between the BmTx3B and Kca1.1 channel is favored by a number of aromatic pi-pi stacking interactions. The influences of these structural features on the kinetic behavior of the toxin binding to Kca1.1 channel are also discussed.

Role of disulfide bonds in modulating internal motions of proteins to tune their function: molecular dynamics simulation of scorpion toxin Lqh III

A series of 1-ns MD simulations were performed on the scorpion toxin Lqh III in native and disulfide bond broken states. The removal of disulfide bonds has caused hydrogen bond network alteration in the five-residue turn, the long loop, the alpha-helix, the loop connecting strands II and III, and the C-terminal region. In addition and more importantly, it has influenced the amplitude of the fluctuations of five-residue turn, loops, and C-terminal region with a minor effect on the fluctuations of the cysteines in the broken bond sites. These findings suggest that disulfide bonds are not the most important factors in rigidifying their own locations, while they have more important effects at a global scale. Furthermore, our results reveal that disulfide bonds have considerable influence on the functionally important essential modes of motions and the correlations between the motions of the binding site residues. Therefore, we can conclude that disulfide bonds have a crucial role in modulating the function via adjusting the dynamics of scorpion toxin molecules. Although this conclusion cannot be generalized to all peptides and proteins, it demonstrates the importance of more investigations on this aspect of disulfide bond efficacy.

Cloning and characterization of a novel calcium channel toxin-like gene BmCa1 from Chinese scorpion Mesobuthus martensii Karsch

Many studies have been carried on peptides and genes encoding scorpion toxins from the venom of Mesobuthus martensii Karsch (synonym: Buthus martensii Karsch, BmK), such as Na+, K+ and Cl- channel modulators. In this study, a novel calcium channel toxin-like gene BmCa1 was isolated and characterized from the venom of Mesobuthus martensii Karsch. First, a partial cDNA sequence of the Ca2+ channel toxin-like gene was identified by random sequencing method from a venomous gland cDNA library of Mesobuthus martensii Karsch. The full-length sequence of BmCa1 was then obtained by 5'RACE technique. The peptide deduced from BmCa1 precursor nucleotide sequence contains a 27-residue signal peptide and a 37-residue mature peptide. Although BmCa1 and other scorpion toxins are different at the gene and protein primary structure levels, BmCa1 has the same precursor nucleotide organization and cysteine arrangement as that of the first subfamily members of calcium channel scorpion toxins. Genomic DNA sequence of BmCa1 was also cloned by PCR. Sequence analysis showed that BmCa1 gene consists of three exons separated by two introns of 72 bp and 1076 bp in length, respectively. BmCa1 is the first calcium channel toxin-like gene cloned from the venom of Mesobuthus martensii Karsch and potentially represents a novel class of calcium channel toxins in scorpion venoms.

Increasing the molecular contacts between maurotoxin and Kv1.2 channel augments ligand affinity

Scorpion toxins interact with their target ion channels through multiple molecular contacts. Because a "gain of function" approach has never been described to evaluate the importance of the molecular contacts in defining toxin affinity, we experimentally examined whether increasing the molecular contacts between a toxin and an ion channel directly impacts toxin affinity. For this purpose, we focused on two scorpion peptides, the well-characterized maurotoxin with its variant Pi1-like disulfide bridging (MTX(Pi1)), used as a molecular template, and butantoxin (BuTX), used as an N-terminal domain provider. BuTX is found to be 60-fold less potent than MTX(Pi1) in blocking Kv1.2 (IC(50) values of 165 nM for BuTX versus 2.8 nM for MTX(Pi1)). Removal of its N-terminal domain (nine residues) further decreases BuTX affinity for Kv1.2 by 5.6-fold, which is in agreement with docking simulation data showing the importance of this domain in BuTX-Kv1.2 interaction. Transfer of the BuTX N-terminal domain to MTX(Pi1) results in a chimera with five disulfide bridges (BuTX-MTX(Pi1)) that exhibits 22-fold greater affinity for Kv1.2 than MTX(Pi1) itself, in spite of the lower affinity of BuTX as compared to MTX(Pi1). Docking experiments performed with the 3-D structure of BuTX-MTX(Pi1) in solution, as solved by (1)H-NMR, reveal that the N-terminal domain of BuTX participates in the increased affinity for Kv1.2 through additional molecular contacts. Altogether, the data indicate that acting on molecular contacts between a toxin and a channel is an efficient strategy to modulate toxin affinity.

The impact of the fourth disulfide bridge in scorpion toxins of the alpha-KTx6 subfamily

Animal toxins are highly reticulated and structured polypeptides that adopt a limited number of folds. In scorpion species, the most represented fold is the alpha/beta scaffold in which an helical structure is connected to an antiparallel beta-sheet by two disulfide bridges. The intimate relationship existing between peptide reticulation and folding remains poorly understood. Here, we investigated the role of disulfide bridging on the 3D structure of HsTx1, a scorpion toxin potently active on Kv1.1 and Kv1.3 channels. This toxin folds along the classical alpha/beta scaffold but belongs to a unique family of short-chain, four disulfide-bridged toxins. Removal of the fourth disulfide bridge of HsTx1 does not affect its helical structure, whereas its two-stranded beta-sheet is altered from a twisted to a nontwisted configuration. This structural change in HsTx1 is accompanied by a marked decrease in Kv1.1 and Kv1.3 current blockage, and by alterations in the toxin to channel molecular contacts. In contrast, a similar removal of the fourth disulfide bridge of Pi1, another scorpion toxin from the same structural family, has no impact on its 3D structure, pharmacology, or channel interaction. These data highlight the importance of disulfide bridging in reaching the correct bioactive conformation of some toxins.

SCORPION2: A database for structure–function analysis of scorpion toxins

Scorpion toxins are important experimental tools for characterization of vast array of ion channels and serve as scaffolds for drug design. General public database entries contain limited annotation whereby rich structure-function information from mutation studies is typically not available. SCORPION2 contains more than 800 records of native and mutant toxin sequences enriched with binding affinity and toxicity information, 624 three-dimensional structures and some 500 references. SCORPION2 has a set of search and prediction tools that allow users to extract and perform specific queries: text searches of scorpion toxin records, sequence similarity search, extraction of sequences, visualization of scorpion toxin structures, analysis of toxic activity, and functional annotation of previously uncharacterized scorpion toxins. The SCORPION2 database is available at http://sdmc.i2r.a-star.edu.sg/scorpion/.

Genetic mechanisms of scorpion venom peptide diversification

The diversity of scorpion venom peptides is well shown by the presence of about 400 such polypeptides with or without disulfide bonds. Scorpion toxins with disulfide bonds present a variety of sequence features and pharmacological functions by affecting different ion channels, while the venom peptides without disulfide bonds represent a new subfamily, having much lower sequence homology among each other and different functions (e.g. bradykinin-potentiating, antimicrobial, molecular cell signal initiating and immune modulating). Interestingly, all scorpion venom peptides with divergent functions may have evolved from a common ancestor gene. Over the lengthy evolutionary time, the diversification of scorpion venom peptides evolved through polymorphism, duplication, trans-splicing, or alternative splicing at the gene level. In order to completely clarify the diversity of scorpion toxins and toxin-like peptides, toxinomics (genomics and proteomics of scorpion toxins and toxin-like peptides) are expected to greatly advance in the near future.

A novel toxin from the venom of the scorpionTityus trivittatus, is the first member of a new α-KTX subfamily

The first example of a new sub-family of toxins (alpha-KTx20.1) from the scorpion Tityus trivittatus was purified, sequenced and characterized physiologically. It has 29 amino acid residues, three disulfide bridges assumed to adopt the cysteine-stabilized alpha/beta scaffold with a pI value of 8.98. The sequence identities with all the other known alpha-KTx are less than 40%. Its effects were verified using seven different cloned K(+) channels (vertebrate Kv1.1-1.5, Shaker IR and hERG) expressed in Xenopus leavis oocytes. The toxin-induced effects show large differences among the different K(+) channels and a preference towards Kv1.3 (EC50=7.9+/-1.4 nM).