Structure of variant 2 scorpion toxin from Centruroides sculpturatus Ewing

Centruroides sculpturatus Ewing variant 2 toxin (CsE-v2) is a neurotoxin isolated from the venom of a scorpion native to the Arizona desert. The structure of CsE-v2 was solved in two different crystal forms using a combination of molecular replacement and multiple isomorphous replacement techniques. Crystals of CsE-v2 display a temperature-dependent, reversible-phase transition near room temperature. At lower temperature the space group changes from P3(2)21 to P3(1)21 with an approximate doubling of the C-axis. The small-cell structure, which has one molecule per asymmetric unit, has an R factor of 0.229 at 2.8 A resolution. The large-cell structure has two molecules per asymmetric unit and was refined at 2.2 A resolution to an R factor of 0.255. CsE-v2 is a rigid, compact structure with four intrachain disulfide bonds. The structure is similar to other long-chain beta neurotoxins, and the largest differences occur in the last six residues. The high-resolution structure of CsE-v2 corrects an error in the reported C-terminal sequence; the terminal tripeptide sequence is Ser 64-Cys 65-Ser 66 rather than Ser 64-Ser 65-Cys 66. Comparison of CsE-v2 with long-chain alpha toxins reveals four insertions and one deletion, as well as additional residues at the N and C termini. Structural alignment of alpha and beta toxins suggests that the primary distinguishing feature between the two classes is the length of the loop between the second and third strands in a three-strand beta sheet. The shorter loop in alpha toxins exposes a critical lysine side chain, whereas the longer loop in beta toxins buries the corresponding basic residue (either arginine or lysine).     

Isolation and primary structure of a potent toxin from the venom of the scorpion Centruroides sculpturatus Ewing.

A potent toxin has been purified from the venom of the scorpion Centruroides sculpturatus Ewing using the ion-exchange resin CM-Sepharose CL-6B at basic pH. The toxin, designated CsE M1, comprised 65 amino acid residues and its primary structure was established as: Lys-Glu-Gly-Tyr-Leu-Val-Asn-Ser-Tyr-Thr10-Gly-Cys-Lys-Tyr-Glu-Cys- Leu-Lys-Leu- Gly20-Asp-Asn-Asp-Tyr-Cys-Leu-Arg-Glu-Cys-Arg30-Gln-Gln-Tyr- Gly-Lys-Ser-Gly-Gly - Tyr-Cys40-Tyr-Ala-Phe-Ala-Cys-Trp-Cys-Thr-His-Leu50-Tyr-Glu- Gln-Ala-Val-Val-Trp - Pro-Leu-Pro60-Asn-Lys-Thr-Cys-Asn. CsE M1 is the most lethal protein to be identified in C. sculpturatus venom and the LD50 of the toxin, determined by subcutaneous injection into Swiss mice, is 87 micrograms/kg. CsE M1 shows strong structural similarity (92% positional identity) to the most potent beta-toxin, Css II, from the Mexican scorpion, Centruroides suffusus suffusus but is quite dissimilar to the previously characterized toxins with low potency isolated from C. sculpturatus Ewing.     

Crystals of a toxic protein from the venom of the scorpion Centruroides sculpturatus Ewing: preparation and preliminary x-ray investigation

A toxic protein from the venom of the scorpion Centruroides sculpturatus has been crystallized. The crystals are orthorhombic, with unit cell dimensions $\text{a = 52.2}\text{A}\text{?}\text{, b = 42.0}\text{A}\text{?}\text{and}\text{c = 28.5}\text{A}\text{?}$, space group P2₁2₁2₁. Density measurements indicate that there are four molecules of toxin in the unit cell.     

Amino acid sequences of neurotoxic protein variants from the venom of Centruroides sculpturatus Ewing

The venom from the scorpion, Centruroides sculpturatus Ewing (range, Southwestern United States), was fractionated into ten protein zones by chromatography on CM-cellulose. Further purification of three of these zones on DEAE Sephadex in ammonium acetate developers yielded three principal neurotoxins designated variants 1, 2, and 3. Variants 1 and 3 each consist of 65 amino acid residues, variant 2 is composed of 66 residues, and each variant is a single polypeptide chain crosslinked by four disulfide bridges. The three variants have lysine at the amino terminus, serine at the carboxyl terminus, and their sequences exhibit a high degree of homology. The complete structure of variant 2 was deduced from the sequence of its tryptic peptides and overlaps provided by its chymotryptic peptides. The sequences of most of the tryptic peptides of variants 1 and 3 were determined, and the peptides were aligned by comparison with the homologous peptides in variant 2. The results show that there are 4 differences in sequence between variants 2 and 3 and 9 differences between variants 1 and 2. Variants 1 and 3 differ from each other at 5 positions in their sequences. These differences between the three protein variants are found in the amino terminal one-third of the molecules except for the deletion of one seryl residue at the carboxyl terminal of variants 1 and 3.     

Solution structure of an insect-specific neurotoxin from the New World scorpion Centruroides sculpturatus Ewing.

We report the high-resolution solution structure of the 6.3 kDa neurotoxic protein CsE-v5 from the scorpion Centruroides sculpturatus Ewing (CsE, range southwestern U.S.). This protein is the second example of an Old World-like neurotoxin isolated from the venom of this New World scorpion. However, unlike CsE-V, which is the first Old World-like toxin isolated and shows both anti-insect and anti-mammal activity, CsE-v5 shows high specificity for insect sodium channels. Sequence-specific proton NMR assignments and distance and angle constraints were obtained from 600 MHz 2D-NMR data. Distance geometry and dynamical simulated annealing refinements were performed to produce a final family of 20 structures without constraint violations, along with an energy-minimized average structure. The protein structure is well-defined (0.66 and 0.97 D rmsd for backbone and all heavy atoms, respectively) with a compact hydrophobic core and several extending loops. A large hydrophobic patch, containing four aromatic rings and other aliphatic residues, makes up a large area of one side of the protein. CsE-v5 shows secondary structural features characteristic of long-chain scorpion toxins: a two and a half-turn alpha-helix, a three-strand antiparallel beta-sheet, and four beta-turns. Among the proteins studied to date from the CsE venom, CsE-v5 is the most compact protein with nearly 50% of the amide protons having long exchange lifetimes, but CsE-v5 is unusual in that it has loop structures similar to both Old and New World toxins. Further, it also lacks prolines in its C-terminal 14 residues. It shows some important differences with respect to CsE-V not only in its primary sequence, but also in its electrostatic potential surface, especially around areas in register with residues 8, 9, 17, 18, 32, 43, and 57. The loss of anti-mammal activity in CsE-v5 and the differences in its anti-insect activity compared to that of other proteins such as CsE-V, v1, and v3 from this New World scorpion may be related to residue variations at these locations.     

Structure of variant-3 scorpion neurotoxin from Centruroides sculpturatus Ewing, refined at 1.8 A resolution

The three-dimensional structure of the variant-3 protein neurotoxin from the scorpion Centruroides sculpturatus Ewing has been determined by X-ray diffraction data. The initial model for the 65-residue protein was obtained at 3 A resolution by multiple-isomorphous-replacement methods. The structure was refined at 1.8 A resolution by restrained difference-Fourier methods, and by free-atom, block-diagonal least-squares. Considering the 4900 reflections for which d = 1.8-7 A and Fo greater than 2.5 sigma (Fo), the final R-index is 0.16 for the restrained model, and 0.14 for the free-atom model. Average estimated errors in atomic co-ordinates are about 0.1 A. The refined structure includes 492 protein atoms; one molecule of 2-methyl-2,4-pentanediol, which is tightly bound in a hydrophobic pocket on the surface of the protein; and 72 additional solvent sites. The major secondary structural features are two and a half turns of alpha-helix and a three-strand stretch of antiparallel beta-sheet. The helix is connected to the middle strand of the beta-sheet by two disulfide bridges, and a third disulfide bridge is located nearby. Several loops extend out of this dense core of secondary structure. The protein displays several reverse turns and a highly contorted proline-rich, COOH-terminal segment. One of the proline residues (Pro59) assumes a cis-conformation. The structure involves 44 intramolecular hydrogen bonds. The crystallographic results suggest two major corrections in the published primary structure; one of these has been confirmed by new chemical sequence data. The protein displays a large flattened surface that contains a high concentration of hydrophobic residues, along with most of the conserved amino acids that are found in the scorpion neurotoxins.     

Biochemical and electrophysiological characteristics of toxins isolated from the venom of the scorpion Centruroides sculpturatus

Recent progress in biochemical, structural and physiological studies has revealed several interesting properties of the toxins from the American scorpion, Centruroides sculpturatus. These toxins, together with similar toxins from other species of scorpions, comprise a unique family of homologous proteins with phylogenetically related structural differences. There is now evidence from both binding and electrophysiological studies that two distinct classes of toxins are present in the venom of C. sculpturatus. One class of toxins markedly slows inactivation of the sodium permeability but has no demonstrable effect on activation, whereas the second class induces a transient shift in the voltage-dependence of activation. Both groups make inactivation incomplete.     

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.     

NMR studies of the variant-3 neurotoxin from Centruroides sculpturatus Ewing

We report a preliminary high-resolution proton nuclear magnetic resonance characterization of the variant-3 toxin from the scorpion Centruroides sculpturatus Ewing (range Southwestern USA). This toxin assumes a well defined folded conformation in aqueous solutions at room temperature and undergoes reversible thermal denaturation. A number of amide hydrogens exhibit exchange life times varying from several minutes to several hours. A few tentative assignments of the low field aromatic CH resonances has been made on the basis of 2D-COSY and NOE experiments. The upfield shifts exhibited by Trp-47 suggest a unique microenvironment for this residue. The NMR data suggest that there is some degree of correlation between the solution structure of the variant-3 toxin and its crystallographic structure. Our studies provide a basis for a detailed elucidation of the structure-function relationships of these interesting scorpion toxins which bind to the sodium channels of excitable membranes and delay sodium current inactivation.     

Voltage-dependent calcium and potassium conductances in striated muscle fibers from the scorpion,Centruroides sculpturatus

Ionic currents responsible for the action potential in scorpion muscle fibers were characterized using a three-intracellular microelectrode voltage clamp applied at the fiber ends (8–12°C). Large calcium currents (I _Ca) trigger contractile activation in physiological saline (5 mm Ca) but can be studied in the absence of contractile activation in a low Ca saline (2.5 mm). Barium (Ba) ions (1.5–3 mm) support inward current but not contractile activation.Ca conductance kinetics are fast (time constant of 3 msec at 0 mV) and very voltage dependent, with steady-state conductance increasing e-fold in approximately 4 mV. Half-activation occurs at –25 mV. Neither I _Ca nor I _Ba show rapid inactivation, but a slow, voltage-dependent inactivation eliminates I _Ca at voltages positive to –40 mV. Kinetically, scorpion channels are more similar to L-type Ca channels in vertebrate cardiac muscle than to those in skeletal muscle.Outward K currents turn on more slowly and with a longer delay than do Ca currents, and K conductance rises less steeply with voltage (e-fold change in 10 mV; half-maximal level at 0 mV). K channels are blocked by externally applied tetraethylammonium and 3,4 diaminopyridine.This work was supported by a grant from the NIH (NS-17510) to W.F.G. and a NRSA award to T.S. (GM-09921).     

Proton nuclear magnetic resonance characterization of the aromatic residues in the variant-3 neurotoxin from Centruroides sculpturatus Ewing

The amino acid sequence for the variant-3 (CsE-v3) toxin from the venom of the scorpion Centruroides sculpturatus Ewing contains eight aromatic residues. By use of 2D NMR spectroscopic methods, the resonances from the individual protons (NH, C alpha H, C beta H',H", and the ring) for each of the individual aromatic residues have been completely assigned. The spatial arrangement of the aromatic ring systems with respect to each other has been qualitatively analyzed by 2D-NOESY techniques. The results show that Trp-47, Tyr-4, and Tyr-42 are in close spatial proximity to each other. The NOESY contacts and the ring current induced shifts in the resonances of the individual protons of Tyr-4 and Trp-47 suggest that the aromatic ring planes of these residues are in an orthogonal arrangement. In addition, the spatial proximity of the rings in the pairs Tyr-4, Tyr-58; Tyr-42, Tyr-40; and Tyr-40, Tyr-38 has also been established. A comparison with the published crystal structure suggests that there is a minor rearrangement of the aromatic rings in the solution phase. No 2D-NOESY contacts involving Phe-44 and Tyr-14 to any other aromatic ring protons have been observed. The pH dependence of the aromatic ring proton chemical shifts has also been studied. These results suggest that the Tyr-58 phenolic group is experiencing a hydrogen-bonding interaction with a positively charged group, while Tyr-4, -14, -38, and -40 are experiencing through-space interactions with proximal negatively charged groups. The Trp-47 indole NH is interacting with the carboxylate groups of two proximal acidic residues. These studies define the microenvironment of the aromatic residues in the variant-3 neurotoxin in aqueous solution.     

Proton nuclear magnetic resonance studies on the variant-3 neurotoxin from Centruroides sculpturatus Ewing: sequential assignment of resonances

We report the sequential assignment of resonances to specific residues in the proton nuclear magnetic resonance spectrum of the variant-3 neurotoxin from the scorpion Centruroides sculpturatus Ewing (range southwestern U.S.A.). A combination of two-dimensional NMR experiments such as 2D-COSY, 2D-NOESY, and single- and double-RELAY coherence transfer spectroscopy has been employed on samples of the protein dissolved in D2O and in H2O for assignment purposes. These studies provide a basis for the determination of the solution-phase conformation of this protein and for undertaking detailed structure-function studies of these neurotoxins that modulate the flow of sodium current by binding to the sodium channels of excitable membranes.     

Balance between pro- and anti-inflammatory cytokines in mice treated with Centruroides noxius scorpion venom

CSV consists of a very complex of molecules and demonstrates significant cellular activities capable of stimulating immune functions in vivo. The purpose of this study was to analyze the effects of CSV on sex, weight, route of injection and the balance of pro- and anti-inflammatory cytokines in mice. The susceptibility and route of injection were analyzed by lethal (LD(50)) determination. The effects of CSV were also analyzed in blood from immunized mice using detection by means of antibodies and mediators production. Several functional bioassays were employed: TNF activity was assayed by measuring its cytotoxic activity in L929 cells, and other cytokines were assayed by enzyme-linked immunosorbent assay, whereas nitric oxide levels were detected by Griess colorimetric reactions in sera from BALB/c mice. After injecting subcutaneously, the LD(50) presented an increase of the CSV correlation and similar levels of susceptibility were obtained for female and male from BALB/c mice. Significant differences were observed in the time-course of cytokine levels. The balance of pro- and anti-inflammatory cytokines TNF/IL-10 and IL-6/IL-10 ratios were significantly higher in injected mice group when compared with those obtained for non-injected group. The CSV is poor in antigenic composition and it is difficult to get antibodies specific to neutralizing the lethal factor. The effect of immunization with 0.5 LD(50) of CSV on the balance of pro- and anti-inflammatory cytokines was measured. The maximum levels of TNF and IL-6, IFN-gamma and NO were observed on days 7 and 21 after immunization, respectively. IL-10 levels peaked between days 21 and 28 after immunization with CSV. With respect, to balance of pro- and anti-inflammatory cytokines it was possible to observe that negative correlation between serum levels of IL-6/IL-10 and TNF/IL-10 exists. These ratios may possibly reflect the balance of pro- and anti-inflammatory cytokines in serum, which may by manifested in the inflammatory status during the envenoming processes. In conclusion, an increase in the serum levels of TNF and IL-6 may be a useful marker for scorpion envenomation.     

Functional and immuno-reactive characterization of a previously undescribed peptide from the venom of the scorpion Centruroides limpidus

A previously undescribed toxic peptide named Cl13 was purified from the venom of the Mexican scorpion Centruroides limpidus. It contains 66 amino acid residues, including four disulfide bonds. The physiological effects assayed in 7 different subtypes of voltage gated Na⁺-channels, showed that it belongs to the β-scorpion toxin type. The most notorious effects were observed in subtypes Nav1.4, Nav1.5 and Nav1.6. Although having important sequence similarities with two other lethal toxins from this scorpion species (Cll1m and Cll2), the recently developed single chain antibody fragments (scFv) of human origin were not capable of protecting against Cl13. At the amino acid sequence level, in 3 stretches of peptide Cl13 (positions 7–9, 30–38 and 62–66) some differences with respect to other similar toxins are observed. Some of these differences coincide with contact points with the human antibody fragments.     

Reproductive investiment in the scorpion Centruroides vittatus

Among invertebrates, scorpions possess a relatively unique set of reproductive traits. The interrelationships of these traits may have important implications for life history theory, yet there have been few studies of these traits in scorpions. Our data indicate that larger female Centruroides vittatus produce more offspring and have a higher total litter mass than smaller females. There was, however, no significant relationship between offspring size and female or litter size. Mean offspring mass increased with increases in total litter mass and within litter variation in offspring size (coefficients of variation) decreased with increasing total litter mass. These results suggest that large female scorpions with a larger investment in reproduction produced more offspring that were more uniform in size, but not significantly larger, than small females with less investment. The fractional clutch principle and physiological and functional constraints on size and number of offspring are suggested as possible explanations for the relationships we found among offspring size, variation in offspring size and total investment in offspring in C. vittatus.     

The mitochondrial genome sequence of the scorpion Centruroides limpidus (Karsch 1879) (Chelicerata; Arachnida)

The mitochondrial genome of the scorpion Centruroides limpidus (Chelicerata; Arachnida) has been completely sequenced and is 14519 bp long. The genome contains 13 protein-encoding genes, two ribosomal RNA genes, 21 transfer RNA genes and a large non-coding region related to the control region. The overall A + T composition is the lowest among the complete mitochondrial sequences published within the Chelicerata subphylum. Gene order and gene content differ slightly from that of Limulus polyphemus (Chelicerata: Xiphosura): i.e., the lack of the trnD gene, and the translocation–inversion of the trnI gene. Preliminary phylogenetic analysis of some Chelicerata shows that scorpions (C. limpidus and Mesobuthus gibbosus) make a tight cluster with the spiders (Arachnida; Araneae). Our analysis does not support that Scorpiones order is the sister group to all Arachnida Class, since it is closer to Araneae than to Acari orders.     

A toxin to nervous, cardiac, and endocrine ERG K+ channels isolated from Centruroides noxius scorpion venom.

Toxins isolated from a variety of venoms are tools for probing the physiological function and structure of ion channels. The ether-a-go-go-related genes (erg) codify for the K+ channels (ERG), which are crucial in neurons and are impaired in human long-QT syndrome and Drosophila 'seizure' mutants. We have isolated a peptide from the scorpion Centruroides noxius Hoffmann that has no sequence homologies with other toxins, and demonstrate that it specifically inhibits (IC50=16+/-1 nM) only ERG channels of different species and distinct histogenesis. These results open up the possibility of investigating ERG channel structure-function relationships and novel pharmacological tools with potential therapeutic efficacy.     

Developmental changes in the embryo, pronymph, and first molt of the scorpion Centruroides vittatus (scorpiones: buthidae)

For the first time the scanning electron microscope was used to compare developmental changes in scorpion embryos and the first and second stadia. In the buthid species of this study, Centruroides vittatus, and all other scorpions, the newborn climb up on their mother's back and remain there without feeding for several days. At this location, they undergo their first molt and in a few days they disperse, fully capable of foraging in the terrestrial environment. The results here support earlier suggestions that the first stadium (pronymph) is a continuation and extension of embryological development. The first molt results in a nymph with exoskeletal features much like those in the adult. In the first molt the metasoma becomes relatively longer, and the sting (aculeus) becomes sharp and functional. The metasomal segments are modified for dorsal flexion and sting use. The embryos and the pronymphs have spiracles that open into an invagination near the posterior margin of flap-like abdominal plates in segments 4-7 of the ventral mesosoma. The second instars have spiracles that lead to book lungs farther anterior in sternites. Tubular legs with cylindrical segments in embryos and pronymphs become more sculptured and oval in the transverse plane. Each leg in the pronymph has a blunt, cup-shaped tip while distal claws (ungues, dactyl) are present in the second instar and subsequent stages. There are some sharp bristles and primordial sensilla in the pronymphs, but the second stadium has adult-like surface features: rows of knobs or granulations (carinae), serrations on the inner surfaces of cheliceral and pedipalpal claws, filtering hairs at the mouthparts, peg sensilla on the pectines, and mechano- and chemoreceptor sensilla on the body and appendages. Scorpion embryos and pronymphs have some structures like fossil scorpions thought to have been aquatic. There is a gradual development of features that appear to be terrestrial adaptations. Evidence is provided for the formation of the sternum from third and fourth leg coxal primordia and possibly from the first abdominal segment. This study is the first to provide evidence for a forward shift of the gonopore along with other structures in the anterior abdomen.