Cloning and characterization of cDNAs that code for Na(+)-channel-blocking toxins of the scorpion Centruroides noxius Hoffmann

Evolution of alternative methodologies of scorpion antivenoms production

Scorpionism represents a serious public health problem resulting in the death of children and debilitated individuals. Scorpion sting treatment employs various strategies including the use of specific medicines such as antiserum, especially for patients with severe symptoms. In 1909 Charles Todd described the production of an antiserum against the venom of the scorpion Buthus quinquestriatus. Based on Todd's work, researchers worldwide began producing antiserum using the same approach i.e., immunization of horses with crude venom as antigen. Despite achieving satisfactory results using this approach, researchers in this field have developed alternative approaches for the production of scorpion antivenom serum. In this review, we describe the work published by experts in toxinology to the development of scorpion venom antiserum. Methods and results describing the use of specific antigens, detoxified venom or toxins, purified toxins and or venom fractions, native toxoids, recombinant toxins, synthetic peptides, monoclonal and recombinant antibodies, and alternative animal models are presented.

Transcriptome analysis of scorpion species belonging to the Vaejovis genus

Scorpions belonging to the Buthidae family have traditionally drawn much of the biochemist's attention due to the strong toxicity of their venoms. Scorpions not toxic to mammals, however, also have complex venoms. They have been shown to be an important source of bioactive peptides, some of them identified as potential drug candidates for the treatment of several emerging diseases and conditions. It is therefore important to characterize the large diversity of components found in the non-Buthidae venoms. As a contribution to this goal, this manuscript reports the construction and characterization of cDNA libraries from four scorpion species belonging to the Vaejovis genus of the Vaejovidae family: Vaejovis mexicanus, V. intrepidus, V. subcristatus and V. punctatus. Some sequences coding for channel-acting toxins were found, as expected, but the main transcribed genes in the glands actively producing venom were those coding for non disulfide-bridged peptides. The ESTs coding for putative channel-acting toxins, corresponded to sodium channel β toxins, to members of the potassium channel-acting α or κ families, and to calcium channel-acting toxins of the calcin family. Transcripts for scorpine-like peptides of two different lengths were found, with some of the species coding for the two kinds. One sequence coding for La1-like peptides, of yet unknown function, was found for each species. Finally, the most abundant transcripts corresponded to peptides belonging to the long chain multifunctional NDBP-2 family and to the short antimicrobials of the NDBP-4 family. This apparent venom composition is in correspondence with the data obtained to date for other non-Buthidae species. Our study constitutes the first approach to the characterization of the venom gland transcriptome for scorpion species belonging to the Vaejovidae family.

Mass fingerprinting of the venom and transcriptome of venom gland of scorpion Centruroides tecomanus

Centruroides tecomanus is a Mexican scorpion endemic of the State of Colima, that causes human fatalities. This communication describes a proteome analysis obtained from milked venom and a transcriptome analysis from a cDNA library constructed from two pairs of venom glands of this scorpion. High perfomance liquid chromatography separation of soluble venom produced 80 fractions, from which at least 104 individual components were identified by mass spectrometry analysis, showing to contain molecular masses from 259 to 44,392 Da. Most of these components are within the expected molecular masses for Na⁺- and K⁺-channel specific toxic peptides, supporting the clinical findings of intoxication, when humans are stung by this scorpion. From the cDNA library 162 clones were randomly chosen, from which 130 sequences of good quality were identified and were clustered in 28 contigs containing, each, two or more expressed sequence tags (EST) and 49 singlets with only one EST. Deduced amino acid sequence analysis from 53% of the total ESTs showed that 81% (24 sequences) are similar to known toxic peptides that affect Na⁺-channel activity, and 19% (7 unique sequences) are similar to K⁺-channel especific toxins. Out of the 31 sequences, at least 8 peptides were confirmed by direct Edman degradation, using components isolated directly from the venom. The remaining 19%, 4%, 4%, 15% and 5% of the ESTs correspond respectively to proteins involved in cellular processes, antimicrobial peptides, venom components, proteins without defined function and sequences without similarity in databases. Among the cloned genes are those similar to metalloproteinases.

Molecular cloning and biochemical characterization of the first Na(+)-channel α-type toxin peptide (Acra4) from Androctonus crassicauda scorpion venom

Due to the medical importance played in Turkey by stings of the scorpion Androctonus crassicauda, its venom has been studied with more attention. In this communication we report a new toxic peptide, named Acra4, because it is the fourth peptide completely characterized from venom of this scorpion. The peptide contains 64 amino acid residues stabilized by four disulfide bridges, with a molecular weight of 6937 Da. Purification of the lethal peptide was performed by three steps of high performance liquid chromatography (HPLC) separations, and the molecular weight was determined by mass spectrometry analysis and the full amino acid sequence was obtained by direct Edman degradation in conjunction with gene cloning. The LD50 of Acra4 was 50.5 ng/20 g mouse body weight (95% confidence intervals from 48.8 to 52.2 ng/20 g mouse body weight). Additionally, from a sample of cDNA of A. crassicauda four genes were cloned displaying sequence similarities to known scorpion toxins, and are reported here as potentially toxic peptides, named Acra5 to Acra8. Electrophysiological studies of Acra4 were performed using Na(+)-channels expressed in F11 cell culture, by patch-clamp recordings. This is the first time that such peptide from A. crassicauda having a specific Na(+)-channel α-type effect is reported. Its affinity toward Na(+)-channels in F11 cell line is in the order of 1 μM concentration.

Turkish scorpion Buthacus macrocentrus: general characterization of the venom and description of Bu1, a potent mammalian Na⁺-channel α-toxin

The venom of the scorpion Buthacus macrocentrus of Turkey was fractionated by high performance liquid chromatography (HPLC) and its mass finger print analysis was obtained by spectrometry. More than 70 different fractions were obtained, allowing the determination of the molecular masses of at least 60 peptides ranging between 648 and 44,336 Da. The venom is enriched with peptides containing molecular masses between 3200-4500 Da, and 6000-7500 Da. They very likely correspond to K⁺-channel and Na⁺-channel specific peptides, respectively, as expected from venoms of scorpions of the family Buthidae, already determined for other species. The major component obtained from HPLC was shown to be lethal to mice and was further purified and characterized. It contains 65 amino acid residues maintained closely packed by 4 disulfide bridges, and shows a molecular weight of 7263 Da. Additionally, a cDNA from the venomous glands of this scorpion was used in conjunction with sequence data from Edman degradation and mass spectrometry for cloning the gene that codes for Bu1 as we named this toxin. This gene codes for a 67 amino acid residues peptide, where the two last are eliminated post-translationally for production of an amidated C-terminal arginine. Its sequence is closely related to toxins from the species Leiurus quinquestriatus, as revealed by a phylogenetic tree analysis. Electrophysiological results conducted with Bu1 using patch-clamp techniques indicate that it modifies the Na⁺ currents, in a similar way as other well known α-scorpion toxins. These results support the conclusion that this species of scorpions is dangerous to humans, having an epidemiological interest for the country.

Scorpion and spider venom peptides: gene cloning and peptide expression

This communication reviews most of the important findings related to venom components isolated from scorpions and spiders, mainly by means of gene cloning and expression. Rather than revising results obtained by classical biochemical studies that report structure and function of venom components, here the emphasis is placed on cloning and identification of genes present in the venomous glands of these arachnids. Aspects related to cDNA library construction, specific or random ESTs cloning, transcriptome analysis, high-throughput screening, heterologous expression and folding are briefly discussed, showing some numbers of species and components already identified, but also shortly mentioning limitations and perspectives of research for the future in this field.

Purification, characterization and functional expression of a new peptide with an analgesic effect from Chinese scorpion Buthus martensii Karsch (BmK AGP-SYPU1)

In this study, a new peptide named BmK AGP-SYPU1 with an analgesic effect was purified from the venom of Chinese scorpion Buthus martensi Karsch (BmK) through a four-step chromatographic process. The mouse twisting test was used to identify the target peptides in every separation step. The purified BmK AGP-SYPU1 was further qualified by RP-HPLC and HPCE. The molecular mass determined by the MALDI-4800-TOF/TOF MS for BmK AGP-SYPU1 was 7544 Da. Its primary structure of the N-terminal was obtained using Edman degradation. The gene sequence of BmK AGP-SYPU1 was cloned from the cDNA pool and genomic of scorpion glands, respectively, and then expressed in Escherichia coli. The sequence determination showed that BmK AGP-SYPU1 was composed of 66 amino acid residues with a new primary structure. The metal chelating affinity column and cation exchange chromatography were used to purify the recombinant BmK AGP-SYPU1. Consequently, the native and recombinant BmK AGP-SYPU1 showed similar analgesic effects on mice as assayed using a mouse twisting model. These results suggested that BmK AGP-SYPU1 is a new analgesic component found in the Chinese scorpion Buthus martensi Karsch.

Adaptive evolution after gene duplication in alpha-KT x 14 subfamily from Buthus martensii Karsch

A series of isoforms of alpha-KT x 14 (short chain potassium channel scorpion toxins) were isolated from the venom of Buthus martensii Karsch by RACE and screening cDNA library methods. These isoforms adding BmKK1--3 and BmSKTx1--2 together shared high homology (more than 97%) with each other. The result of genomic sequence analysis showed that a length 79 bp intron is inserted Ala codes between the first and the second base at the 17th amino acid of signal peptide. The introns of these isoforms also share high homology with those of BmKK2 and BmSKT x 1 reported previously. Sequence analysis of many clones of cDNA and genomic DNA showed that a species population or individual polymorphism of alpha-KT x 14 genes took place in scorpion Buthus martensii Karsch and accelerated evolution played an important role in the forming process of alpha-KT x 14 scorpion toxins subfamily. The result of southern hybridization indicated that alpha-KT x 14 toxin genes existed in scorpion chromosome with multicopies. All findings maybe provided an important evidence for an extensive evolutionary process of the scorpion "pharmacological factory": at the early course of evolution, the ancestor toxic gene duplicated into a series of multicopy genes integrated at the different chromosome; at the late course of evolution, subsequent functional divergence of duplicate genes was generated by mutations, deletions and insertion.

Biochemical, genetic and physiological characterization of venom components from two species of scorpions: Centruroides exilicauda Wood and Centruroides sculpturatus Ewing

Current literature concerning the taxonomic names of two possibly distinct species of scorpions from the genus Centruroides (sculpturatus and/or exilicauda) is controversial. This communication reports the results of biochemical, genetic and electrophysiological experiments conducted with C. exilicauda Wood of Baja California (Mexico) and C. sculpturatus Ewing of Arizona (USA). The chromatographic profile fractionation of the soluble venom from both species of scorpions is different. The N-terminal amino acid sequence for nine toxins of C. exilicauda was determined and compared with those from C. sculpturatus. Lethality tests conducted in mice support the idea that C. exilicauda venom should be expected to be medically less important than C. sculpturatus. Thirteen genes from the venomous glands of the scorpion C. exilicauda were obtained and compared with previously published sequences from genes of the species C. sculpturatus. Genes coding for cytochrome oxidase I and II of both species were also sequenced. A phylogenetic tree was generated with this information showing important differences between them. Additionally, the results of electrophysiological assays conducted with the venom from both species on the Ca(2+)-dependent K(+)-channels, showed significant differences. These results strongly support the conclusion that C. exilicauda and C. sculpturatus are in fact two distinct species of scorpions.

Analysis of the immune response induced by a scorpion venom sub-fraction, a pure peptide and a recombinant peptide, against toxin Cn2 of Centruroides noxius Hoffmann

Three different immunogens from the venom of the Mexican scorpion Centruroides noxius Hoffmann were used to study protective antibody response in mice and rabbits, challenged with toxin Cn2, one of the most abundant toxic peptide of this venom. The immunogens were: Cn5, a crustacean specific toxin; a recombinant protein containing the peptide Cn5 linked to the maltose transporter and a sub-fraction (F.II.5) containing 25 distinct peptides, among which is Cn5. Mice immunized with these three preparations, when directly challenged with Cn2 presented no apparent protection, whereas anti-sera produced in rabbits with these three immunogens were capable of partially neutralizing the effect of Cn2, when injected into naive mice. Cn5 rabbit anti-serum showed a better protective effect on mice, than the rabbit sera obtained against the two other antigens. The subcutaneous route of challenging mice was shown to be better than intraperitoneal injections. Comparative structural analysis of Cn5 with other toxins of this venom showed that our results are important to be taken into consideration, when choosing appropriate immunogens aimed at the production of better anti-venoms or for the rational design of possible vaccines.

From noxiustoxin to scorpine and possible transgenic mosquitoes resistant to malaria

Scorpion venom contains different types of peptides toxic to a variety of organisms whose molecular targets have been described as mainly ion-channels of excitable cells where they cause impairment of function. Based on mouse, cricket, and crustacean bioassays, specific toxins for each group of animals have been found. Chromatographic techniques were used to isolate and chemically characterize these peptides. One of the best-studied peptides is noxiustoxin, a 39-amino acid residue-long peptide specific for K(+)-channels. Hadrurin is another scorpion venom peptide whose activity was shown to be bactericidal to a variety of species. Structural similarities of a newly discovered peptide (scorpine) with those of defensins and cecropins showed that scorpion venom contains peptides toxic to microorganisms and malaria parasites. Scorpine was shown to disrupt the sporogonic development of Plasmodium berghei. Using this system as a model for malaria, we introduced the gene of scorpine into a vector for generation of transgenic flies resistant to the infection by Plasmodium. The final aim of this work is to incorporate this gene under the promoter of proteolytic enzymes of digestive tract of mosquitoes for synthesis and liberation of toxic peptide(s) into stomach of freshly fed mosquitoes potentially carrying Plasmodium gametes. In this manner, a putative transgenic mosquito with these characteristics would secrete a toxic peptide with digestive enzymes into midgut, impairing proper development of Plasmodium, hence controlling malaria, one of the most important tropical diseases worldwide.

Genes and peptides from the scorpion Centruroides sculpturatus Ewing, that recognize Na(+)-channels

Sixteen different genes were cloned from the venomous glands of Centruroides sculpturatus Ewing using RNA extracted from scorpions collected in Tucson, Arizona. Based on the amino acid sequence similarities of the proteins coded by these genes, all together there are 22 different structural components in this venom, thought to be specific for Na(+)-channels. The genes reported contain signal peptides with 19 amino acid residues followed by mature peptides of 63-66 amino acid residues in length. One of them correspond to toxin I (CsEI), a known scorpion toxin specific for Na(+)-channels. Four different genes are almost identical to variant 1 (Csv1), presenting only one amino acid change from the original protein. For variant 2 (Csv2) four related genes were found, with only one amino acid change in their primary sequences. Another gene resembles to variant 3 (Csv3, the best known Centruroides sculpturatus toxin), with only three amino acid changes in their primary sequences. Additionally, two genes show variations only on the nucleotide sequence at level of the signal peptides, and several genes clearly show sequences that suggest post-transcriptional modifications, during the maturation process. A phylogenetic tree was generated with the primary structures available and three main divergent branches were found.

A new insect neurotoxin AngP1 with analgesic effect from the scorpion Buthus martensii Karsch: purification and characterization

An insect toxin named BmK AngP1 was purified from the venom of the scorpion Buthus martensii Karsch (BmK). It also shows an evident analgesic effect on mice, but is interestingly devoid of mammalian toxicity. Bioassay showed that the CPU value of AngP1 was 0.01 microg/body ( approximately 30 mg) for the excitatory insect toxicity and 43.0% inhibition efficiency for analgesia at a dose of 5 mg/kg. However, even at the dosage of 10 mg/kg no detectable toxicity on mice could be found. The isoelectric point (pI) value for AngP1 was 4.0, and its molecular mass analyzed by MALDI-TOF MS was 8141.0. The first 15 N-terminal residues of AngP1 were determined by Edman degradation and showed high similarity to that of other excitatory scorpion insect toxins. The circular dichroism spectroscopy measured on a JASCO J-720 system showed that there were 10.4% alpha-helix, 46.2% beta-strand and 14.1% turn structure in this peptide. Under two conditions single crystals of AngP1 were obtained.

Caenorhabditis elegans degenerins and vertebrate ENaC ion channels contain an extracellular domain related to venom neurotoxins

The DEG/ENaC (DEGenerin/Epithelial Na+ Channel) superfamily includes closely related ion channel subunits from divergent species ranging from the simple nematode Caenorhabditis elegans to humans. Members of this protein group play roles in several important processes including transduction of mechanical stimuli, sodium re-absorption and blood pressure regulation. Structure/function relationships in members of this superfamily are just beginning to be elaborated. Using a bio-informatics approach, we identified a novel structural element in the extracellular region of DEG/ENaC proteins that exhibits significant similarity to venom neurotoxins. Since venom neurotoxins bind to sodium channels at high affinity, we suggest that the related domain embedded in DEG/ENaC channels may interact with other regions of the channel or channel complex to modulate channel function.

Scorpion toxins specific for Na+-channels

Na+-channel specific scorpion toxins are peptides of 60-76 amino acid residues in length, tightly bound by four disulfide bridges. The complete amino acid sequence of 85 distinct peptides are presently known. For some toxins, the three-dimensional structure has been solved by X-ray diffraction and NMR spectroscopy. A constant structural motif has been found in all of them, consisting of one or two short segments of alpha-helix plus a triple-stranded beta-sheet, connected by variable regions forming loops (turns). Physiological experiments have shown that these toxins are modifiers of the gating mechanism of the Na+-channel function, affecting either the inactivation (alpha-toxins) or the activation (beta-toxins) kinetics of the channels. Many functional variations of these peptides have been demonstrated, which include not only the classical alpha- and beta-types, but also the species specificity of their action. There are peptides that bind or affect the function of Na+-channels from different species (mammals, insects or crustaceans) or are toxic to more than one group of animals. Based on functional and structural features of the known toxins, a classification containing 10 different groups of toxins is proposed in this review. Attempts have been made to correlate the presence of certain amino acid residues or 'active sites' of these peptides with Na+-channel functions. Segments containing positively charged residues in special locations, such as the five-residue turn, the turn between the second and the third beta-strands, the C-terminal residues and a segment of the N-terminal region from residues 2-11, seems to be implicated in the activity of these toxins. However, the uncertainty, and the limited success obtained in the search for the site through which these peptides bind to the channels, are mainly due to the lack of an easy method for expression of cloned genes to produce a well-folded, active peptide. Many scorpion toxin coding genes have been obtained from cDNA libraries and from polymerase chain reactions using fragments of scorpion DNAs, as templates. The presence of an intron at the DNA level, situated in the middle of the signal peptide, has been demonstrated.

Molecular characterization of a new excitatory insect neurotoxin with an analgesic effect on mice from the scorpion Buthus martensi Karsch

Besides the neurotoxins active on mammals, a new excitatory insect selective toxin with a mice analgesic activity was found and purified from the venom of the scorpion Buthus martensi Karsch (BmK) (Ji, Y.H., Mansuelle, P., Terakawa, S., Kopeyan, C., Yanaihara, N., Hsu, K., Rochat, H., 1996. Toxicon 34, 987; Luo, M.J., Xiong, Y.M., Wang, M., Wang, D.C., Chi, C.W., 1997. Toxicon 35, 723.). This peptide (designated as BmK IT-AP) is composed of 72 amino acid residues. Its primary structure was determined by automated Edman degradation of the N-terminal part of the reduced and S-carboxamidemethylated protein and its lysylendopeptidase degraded fragments. Based on the determined sequence, the gene specific primers were designed and synthesized for 3' and 5' RACE (rapid amplification of cDNA ends). Their partial cDNA fragments obtained by 3' and 5' RACEwere cloned and sequenced and the full length cDNA sequence of BmK IT-AP was then completed by overlapping their two partial cDNA sequences. It encodes a precursor of 90 amino acid residues: a signal peptide of 18 residues and a mature peptide of 72 residues which are consistent with the determined protein sequence of BmK IT-AP. The genomic DNA of the peptide was also amplified by PCR from the scorpion genomic DNA and sequenced, which is a first report on the genomic structure of a scorpion toxin specific for insects. Its sequence revealed an intron of 590 bp inserted in the end part of the signal peptide. The peptide caused a fast excitatory contraction paralysis on house fly larvae. Furthermore, the peptide also showed an obvious analgesic effect on mice, as assayed by using a twisting test model. This effect of BmK IT-AP well characterized at molecular level is first reported among the known scorpion insect neurotoxins.

Pandinus imperator scorpion venom blocks voltage-gated K+ channels in human lymphocytes

Using the patch-clamp technique, we determined that Pandinus imperator scorpion venom blocked whole-cell n-type K+ currents in human peripheral blood lymphocytes in a dose-dependent manner with Kd = 0.02 microgram/ml. K+ channel block was instantaneous and removable by washing with venom-free extracellular solution. The venom-induced block was independent of membrane potential. The venom did not influence activation and inactivation kinetics of the K+ channels, however, accelerated recovery from inactivation. Purified peptides Pi1, Pi2, and Pi3 from the P. imperator venom powerfully blocked Kv1.3 channels in human lymphocytes with Kd values of 9.7 nM, 50 pM, and 0.5 nM, respectively. Flow cytometric membrane potential measurements with the oxonol dye showed that Pi2, the most effective peptide toxin of the P. imperator venom, depolarizes human lymphocytes in accordance with its K+ channel blocking effect.

Novel insecticidal peptides from Tegenaria agrestis spider venom may have a direct effect on the insect central nervous system

Fractionation of venom from an agelenid spider, Tegenaria agrestis, resulted in the isolation of a family of three peptides with potent insecticidal activity. These peptide toxins, TaITX-1, -2, and -3, whose sequences were revealed from cloned cDNAs, each consist of 50 amino acid residues, six of which are cysteines. They appear to be amidated at their C-termini and exhibit greater than 90% sequence identity. Unlike other reported spider toxins, the TaI toxins are processed from precursors containing no propeptide sequences. In lepidopteran larvae and corn rootworm beetles, the insecticidal Tegenaria toxins caused an unusual excitatory symptomatology with 50% paralytic doses ranging from 0.23 to 2.6 nmol/g. In a series of electrophysiological experiments performed in house fly larvae, these toxins caused an elevated rate of firing from central nervous system neurons. No significant effects were found when any peripheral sensory or motor systems were examined. Thus, it appears that the TaI toxins may act in a fashion not previously reported for insecticidal peptide toxins; they may act directly on the insect central nervous system.

Toxins and genes isolated from scorpions of the genus Tityus

Scorpion venoms contain a variety of low mol. wt peptides toxic to different organisms. These peptides have been intensively studied because they represent excellent models for investigating structure-function relationships and they are also fine probes for studying ionic channel functions. This review deals with the biological and chemical aspects of toxic peptides that affect Na+ or K+ channels and the cloning of the cDNAs and genes encoding the main alpha and beta neurotoxins present in the venom of the three most dangerous species of Brazilian scorpion, Tityus bahiensis, Tityus stigmurus and Tityus serrulatus, and the Venezuelan scorpion Tityus discrepans. At least 16 different peptides specific for Na+ channels and five affecting K+ channels were isolated and characterized from the venom of these scorpions. The isolation of cDNAs and genes encoding four distinct toxins has permitted the elucidation of their nucleotide sequences as well as their genomic organization. Venoms and isolated toxins from scorpions of the genus Tityus were shown to enhance the secretory activity of the pancreas. Antisera obtained against venom of T. serrulatus show cross-reactivity with other species of the Brazilian scorpions.

Isolation, characterization and comparison of a novel crustacean toxin with a mammalian toxin from the venom of the scorpion Centruroides noxius Hoffmann

A novel crustacean-specific toxin, Cn5, containing 66 amino acid residues was isolated from the venom of the scorpion Centruroides noxius Hoffmann. It is stabilized by four disulfide bridges, formed between Cys12-Cys65, Cys16-Cys41, Cys25-Cys46 and Cys29-Cys48. Toxicity tests revealed that Cn5 is a toxin that affects arthropods but not mammals. However, at high concentrations, Cn5 does displace the mammal-specific toxin Cn2 from rat brain synaptosomes. The concentration of Cn5 that produces half-maximal inhibition (IC50) was estimated to be 100 microM. Sequence comparison of Cn5 with toxin Cn2, a mammal-specific toxin from the same scorpion, showed the presence of two sequence stretches, at positions 30 to 38 and 49 to 58, where the majority of the differences are concentrated. On the three-dimensional structure of Cn5 it is demonstrated that these two sequence stretches form a continuous surface region near the site thought to bind to the sodium channel. We assume that this region might be implicated in determining species specificity.

An insect-specific toxin from Centruroides noxius Hoffmann. cDNA, primary structure, three-dimensional model and electrostatic surface potentials in comparison with other toxin variants

Scorpion toxins acting on sodium channels differ in their specificity. Toxic peptides specific towards mammals and arthropods (insects and/or crustaceans) have been described. Because of the similar three-dimensional fold of these peptides, the molecular base of their specificity is thought to reside in certain differences at the level of amino acid residues especially within or near the binding site of the toxin to the particular ion channel. The cDNA, amino acid sequence and biological activity of an insect-specific toxin, Cn10, from the scorpion Centruroides noxius Hoffmann is reported. The electrostatic potential surface around a three-dimensional model of Cn10 was calculated. It revealed that residues Tyr4, Lys13, Ile18, Leu19, Gly20, Lys43, Leu44, Thr57, Tyr58, Pro59, Thr64 and Cys65, situated at the side of the toxin proposed in the literature to bind to the sodium channel, constitute a positive surface region. Therefore, they may form the site that binds to the channel. Cn10 was included in a comparative analysis of two groups of natural variants, highly similar peptides of the genus Centruroides with specificities towards mammals or arthropods. A number of surface-accessible residues, consistently different between the two groups and situated near the putative binding site, may be of importance for the specificity of the analyzed toxins.

Structural and functional comparison of toxins from the venom of the scorpions Centruroides infamatus infamatus, Centruroides limpidus limpidus and Centruroides noxius

Two novel toxins containing 66 amino acid residues each were isolated from the venom of the scorpions Centruroides infamatus infamatus and Centruroides limpidus limpidus, respectively. Their full amino acid sequences were determined. Comparison of primary structures showed that they share 97% similarity among themselves and 83% to that of toxin 2 from Centruroides noxius. The three toxins studied compete with each other for the same binding sites on membranes prepared from rat brain synaptosomes, suggesting that they are all beta-scorpion toxins. Toxin action was assayed into the microI-2 rat skeletal muscle Na+ channel heterologously expressed into Xenopus oocytes. All three toxins block this Na+ channel in a similar fashion, without affecting inactivation, and showed IC50 values in the micromolar concentration range.

Toxic peptides and genes encoding toxin gamma of the Brazilian scorpions Tityus bahiensis and Tityus stigmurus

Seven toxic peptides from the venom of Tityus bahiensis and Tityus stigmurus was isolated and sequenced, five of them to completion. The most abundant peptide from each of these two species of scorpion was 95% identical with that of toxin gamma from the venom of Tityus serrulatus. They were consequently named gamma-b and gamma-st respectively. The genes encoding these new gamma-like peptides were cloned and sequenced by utilizing oligonucleotides synthesized according to known cDNA sequences of toxin gamma, and amplified by PCR on templates of DNA purified from both T. bahiensis and T. stigmurus. They contain an intron of approx. 470 bp. Possible mechanisms of processing and expressing these peptides are discussed, in view of the fact that glycine is the first residue of the N-terminal sequence of T. stigmurus, whereas lysine is the residue at position 1 of toxin gamma from T. serrulatus and T. bahiensis. In addition, chemical characterization of the less abundant toxic peptides showed the presence of at least four distinct families of peptides in all three species of the genus Tityus studied. There is a large degree of similarity among peptides from different venoms of the same family. By using specific horse and rabbit antisera, the venoms of T. bahiensis, T. serrulatus and T. stigmurus were compared. They showed an extended degree of cross-reactivity. Thus these three species of scorpion have similar toxic components, the genes of which are similarly organized, processed and expressed.

Cloning and characterization of the genomic region encoding toxin IV-5 from the scorpion Tityus serrulatus Lutz and Mello

By means of PCR and using synthetic oligonucleotides designed from the reported cDNA, we amplified the gene that codes for toxin IV-5 from the Brazilian scorpion Tityus serrulatus. The analysis of the nucleotide sequence shows that the amplified genomic region is composed of 659 base pairs (bp) comprising two exons (28 and 284 bp) and an intron of 347 bp interrupting the region that encodes the signal peptide of the precursor toxin. Based on these findings a model for the structural organization of scorpion toxin genes is proposed.

Cloning and characterization of the cDNAs encoding Na+ channel-specific toxins 1 and 2 of the scorpion Centruroides noxius Hoffmann

Using a cDNA library prepared from venomous glands of the Mexican scorpion Centruroides noxius Hoffmann the genes that encode toxins 1 and 2 were identified, cloned and sequenced. In view of the proposed mechanism for processing the mature peptides coded by these two genes, the corresponding peptide-toxins were sequenced de novo. Mass spectrometric and 1H-NMR analyses of the C-terminal peptide produced by enzymatic digestion of both toxins indicated that the last residue is serine-amide. Sequence comparison revealed that these two genes have a similarity of 56% and 80% at the amino acid and nucleotide levels, respectively. Small corrections to the published primary structures were introduced: Cn toxin 1 has an extra serine residue at position 65 and the residue in position 60 is a proline, while the amino acids at positions 34 and 35 of Cn 2 are, respectively, tyrosine and glycine. Sequence comparison of toxins from the genus Centruroides suggests the presence of at least three classes of distinct peptides in these venoms.

The disulfide bridges of toxin 2 from the scorpion Centruroides noxius Hoffmann and its three-dimensional structure calculated using the coordinates of variant 3 from Centruroides sculpturatus

The disulfide bridges of toxin 2 from the venom of the scorpion Centruroides noxius Hoffmann were found by amino acid sequence determination of fragments of native toxin, produced by enzymatic cleavage and separated by high-performance liquid chromatography (HPLC). They are: Cys12-Cys65, Cys16-Cys41, Cys25-Cys46 and Cys29-Cys48. The coordinates of the X-ray diffraction structure of toxin variant 3 of C. sculpturatus [(1980) Proc. Natl. Acad. Sci. USA 77, 6496-6500] were used to construct a three-dimensional model of toxin 2. All the amino acid replacements were easily accommodated, and the modeled structure reveals a clustered pattern of sequence variation, which may help to identify residues responsible for functional differences among toxins of mammals and insects.

Biochemical, pharmacological and genomic characterisation of Ts IV, an alpha-toxin from the venom of the South American scorpion Tityus serrulatus

The venom of the scorpion, Tityus serrulatus, was fractionated to investigate the chemical and pharmacological properties of its alpha-toxin content. Three alpha-toxins (Ts III, Ts IV and Ts V) were purified by conventional chromatography (gel filtration and ion-exchange chromatography), followed by immunoaffinity chromatography. Competition experiments using reference alpha- and beta-toxins suggested that these alpha-toxins were contaminated with around 0.01% of beta-toxin. The sequence of the first 30 amino acids of Ts IV was established. Using an oligonucleotide probe, a cDNA encoding its precursor was cloned from a venom gland cDNA library. The primary structure deduced from the cDNA nucleotide sequence provides possible explanations for the polymorphism of these three molecules.

The genomic region encoding toxin gamma from the scorpion Tityus serrulatus contains an intron

The gene encoding toxin gamma from the scorpion, Tityus serrulatus, was amplified by PCR from genomic DNA employing synthetic oligonucleotides designed from the reported cDNA sequence. The nucleotide sequence of this gene reveals the presence of an intron of 475 base pairs (bp) which interrupts the region that encodes the signal peptide of the precursor toxin. A comparison of the intron boundary sequences of the gamma toxin gene with ones from other arachnid genes is also presented.