Snake venom toxins. The amino acid sequence of toxin Vi2, a homologue of pancreatic trypsin inhibitor, from Dendroaspis polylepis polylepis (black mamba) venom

Evolutionary Aspects of the Structural Convergence and Functional Diversification of Kunitz-Domain Inhibitors

Kunitz-type domains are ubiquitously found in natural systems as serine protease inhibitors or animal toxins in venomous animals. Kunitz motif is a cysteine-rich peptide chain of ~ 60 amino acid residues with alpha and beta fold, stabilized by three conserved disulfide bridges. An extensive dataset of amino acid variations is found on sequence analysis of various Kunitz peptides. Kunitz peptides show diverse biological activities like inhibition of proteases of other classes and/or adopting a new function of blocking or modulating the ion channels. Based on the amino acid residues at the functional site of various Kunitz-type inhibitors, it is inferred that this 'flexibility within the structural rigidity' is responsible for multiple biological activities. Accelerated evolution of functional sites in response to the co-evolving molecular targets of the hosts of venomous animals or parasites, gene sharing, and gene duplication have been discussed as the most likely mechanisms responsible for the functional heterogeneity of Kunitz-domain inhibitors.

Synthetic peptide antigens derived from long-chain alpha-neurotoxins: Immunogenicity effect against elapid venoms

Three-finger toxins (3FTXs), especially α-neurotoxins, are the most poorly neutralized elapid snake toxins by current antivenoms. In this work, the conserved structural similarity and motif arrangements of long-chain α-neurotoxins led us to design peptides with consensus sequences. Eight long-chain α-neurotoxins (also known as Type II) were used to generate a consensus sequence from which two peptides were chemically synthesized, LCP1 and LCP2. Rabbit sera raised against them were able to generate partially-neutralizing antibodies, which delayed mice mortality in neutralization assays against Naja haje, Dendrospis polylepis and Ophiophagus hannah venoms.

Discreplasminin, a plasmin inhibitor isolated from Tityus discrepans scorpion venom

Tityus discrepans venom (TdV) produces digestive hemorrhages, disseminated intravascular coagulation, alveoli fibrin deposition and/or prothrombin and partial thromboplastin time alterations in humans. T. discrepans venom presents an in vitro tissue plasminogen activator-like (tPA-like), fibrino(geno)lytic and plasmin inhibitory activities. The plasmin inhibitor, called discreplasminin, was isolated from TdV. Discreplasminin has a pI of 8.0 and a relative molecular weight of <6,000 Da. Discreplasminin and aprotinin strongly inhibited plasmin activity and moderately tPA activity, while epsilon amino caproic acid (EACA) moderately inhibited both enzymes. In presence and absence of fibrin, the plasmin generation by tPA was completely inhibited by aprotinin and discreplasminin. EACA in the absence of fibrin partially inhibited plasmin generation (37%); however, it produced a total inhibition of plasmin generation on a fibrin surface. The tPA-clot lysis assay showed that discreplasminin acts like aprotinin inducing a slight delay in lysis time and lysis rate; in contrast, EACA presented a total inhibitory effect on fibrin lysis. These results suggest that discreplasminin presents an anti-fibrinolytic mechanism similar to aprotinin. Discreplasminin probably interacts with the active sites of plasmin and tPA. The presence of discreplasminin and other similar components in scorpion venom could partially explain the generalized fibrin deposition which was found previously in rams.

Discovery of a distinct superfamily of Kunitz-type toxin (KTT) from tarantulas

Background

Kuntiz-type toxins (KTTs) have been found in the venom of animals such as snake, cone snail and sea anemone. The main ancestral function of Kunitz-type proteins was the inhibition of a diverse array of serine proteases, while toxic activities (such as ion-channel blocking) were developed under a variety of Darwinian selection pressures. How new functions were grafted onto an old protein scaffold and what effect Darwinian selection pressures had on KTT evolution remains a puzzle.

Principal Findings

Here we report the presence of a new superfamily of KTTs in spiders (Tarantulas: Ornithoctonus huwena and Ornithoctonus hainana), which share low sequence similarity to known KTTs and is clustered in a distinct clade in the phylogenetic tree of KTT evolution. The representative molecule of spider KTTs, HWTX-XI, purified from the venom of O. huwena, is a bi-functional protein which is a very potent trypsin inhibitor (about 30-fold more strong than BPTI) as well as a weak Kv1.1 potassium channel blocker. Structural analysis of HWTX-XI in 3-D by NMR together with comparative function analysis of 18 expressed mutants of this toxin revealed two separate sites, corresponding to these two activities, located on the two ends of the cone-shape molecule of HWTX-XI. Comparison of non-synonymous/synonymous mutation ratios (ω) for each site in spider and snake KTTs, as well as PBTI like body Kunitz proteins revealed high Darwinian selection pressure on the binding sites for Kv channels and serine proteases in snake, while only on the proteases in spider and none detected in body proteins, suggesting different rates and patterns of evolution among them. The results also revealed a series of key events in the history of spider KTT evolution, including the formation of a novel KTT family (named sub-Kuntiz-type toxins) derived from the ancestral native KTTs with the loss of the second disulfide bridge accompanied by several dramatic sequence modifications.

Conclusions/Significance

These finding illustrate that the two activity sites of Kunitz-type toxins are functionally and evolutionally independent and provide new insights into effects of Darwinian selection pressures on KTT evolution, and mechanisms by which new functions can be grafted onto old protein scaffolds.

Analgesic compound from sea anemone Heteractis crispa is the first polypeptide inhibitor of vanilloid receptor 1 (TRPV1)

Venomous animals from distinct phyla such as spiders, scorpions, snakes, cone snails, or sea anemones produce small toxic proteins interacting with a variety of cell targets. Their bites often cause pain. One of the ways of pain generation is the activation of TRPV1 channels. Screening of 30 different venoms from spiders and sea anemones for modulation of TRPV1 activity revealed inhibitors in tropical sea anemone Heteractis crispa venom. Several separation steps resulted in isolation of an inhibiting compound. This is a 56-residue-long polypeptide named APHC1 that has a Bos taurus trypsin inhibitor (BPTI)/Kunitz-type fold, mostly represented by serine protease inhibitors and ion channel blockers. APHC1 acted as a partial antagonist of capsaicin-induced currents (32 +/- 9% inhibition) with half-maximal effective concentration (EC(50)) 54 +/- 4 nm. In vivo, a 0.1 mg/kg dose of APHC1 significantly prolonged tail-flick latency and reduced capsaicin-induced acute pain. Therefore, our results can make an important contribution to the research into molecular mechanisms of TRPV1 modulation and help to solve the problem of overactivity of this receptor during a number of pathological processes in the organism.

Nanoanalysis of the arthropod neuro-toxins

Many kinds of venomous principles modulate physiological responses of mammalian signal transduction systems, on which they act selectively as enhancers, inhibitors or some other kind of effectors. These toxins become useful tools for physiological research. We have employed and characterized paralyzing toxins from the venom of spiders, insects and scorpions with a limited supply. We have developed rapid and sensitive mass spectrometric technology and applied for the identification of these toxins. Venom profiles are screened by MALDI-TOF fingerprinting analysis prior to purification of venomous components, then marked target toxins of small molecular mass (1000-5000) are characterized directly by means of mass spectrometric techniques such as Frit-FAB MS/MS, CID/PSD-TOF MS, Capil.-HPLC/Q-TOF MS/MS etc.

Molecular components and toxicity of the venom of the solitary wasp, Anoplius samariensis

The solitary spider wasp, Anoplius samariensis, is known to exhibit a unique long-term, non-lethal paralysis in spiders that it uses as a food source for its larvae. However, neither detailed venom components nor paralytic compounds have ever been characterized. In this study, we examined the components in the low molecular weight fraction of the venom and the paralytic activity of the high molecular weight fraction. The major low molecular weight components of the venom were identified as gamma-aminobutyric acid and glutamic acid by micro-liquid chromatography/electrospray ionization mass spectrometry and nuclear magnetic resonance spectrometry analysis. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis and mass analysis revealed that the A. samariensis venom contained the various proteins with weights of 4-100 kDa. A biological assay using Joro spiders (Nephila clavata) clearly showed that the high molecular weight fraction of the venom prepared by ultrafiltration exerted as potent non-lethal long-term paralysis as the whole venom, whereas the low molecular weight fraction was devoid of any paralytic activity. These results indicated that several venomous proteins in the high molecular weight fraction are responsible for the paralytic activity. Furthermore, we determined the primary structure of one component designated As-fr-19, which was a novel multiple-cysteine peptide with high sequence similarity to several sea anemone and snake toxins including dendrotoxins, rather than any insect toxic peptides identified so far. Taken together, our data showed the unprecedented molecular and toxicological profiles of wasp venoms.

Purification, characterization and primary structure of a chymotrypsin inhibitor from Naja atra venom

A chymotrypsin inhibitor, designated NA-CI, was isolated from the venom of the Chinese cobra Naja atra by three-step chromatography. It inhibited bovine alpha-chymotrypsin with a Ki of 25 nM. The molecular mass of NA-CI was determined to be 6403.8 Da by matrix-assisted laser-desorption ionization time-of-flight (MALDI-TOF) analysis. The complete amino acid sequence was determined after digestion of S-carboxymethylated inhibitor with Staphylococcus aureus V8 protease and porcine trypsin. NA-CI was a single polypeptide chain composed of 57 amino acid residues. The main contact site with the protease (P1) has a Phe, showing the specificity of the inhibitor. NA-CI shared great similarity with the chymotrypsin inhibitor from Naja naja venom (identities=89.5%) and other snake venom protease inhibitors.

Primary structures of four trypsin inhibitor E homologs from venom of Dendroaspis angusticeps: structure-function comparisons with other dendrotoxin homologs

Four trypsin inhibitor homologs, the first known from Dendroaspis angusticeps venom, were characterized using a combination of gel filtration, cation exchange, reverse-phase liquid chromatography, Edman degradation and mass spectrometry. The four toxins comprise two 57 residue and two 59 residue isoforms. The long toxins possess a Lys-Gln N-terminal extension lacked by the short toxins. The only other structural difference is an Arg/His replacement at position 55. The long Arg55 variant is identical to trypsin inhibitor E from the venom of Dendroaspis polylepis. The name epsilon-dendrotoxin is suggested so as to follow the nomenclature of Benishin, C.G., Sorensen, R.G., Brown, W.E., Krueger, B.K., Blaustein, M.P., 1988. Four polypeptide components of green mamba venom selectively block certain potassium channels in rat brain synaptosomes. Mol. Pharmacol. 34, 152-159. Among snake venom protease inhibitors, the epsilon-dendrotoxins are structurally most like the delta-dendrotoxins, with which they share only 64% of their residues. In addition, the epsilon-dendrotoxins display hydropathy profiles more like those of the alpha- and delta-dendrotoxins, than those of the trypsin inhibitors from snake venoms. Given the strong protease inhibitory activity of trypsin inhibitor E and the recently demonstrated weak K(+) channel inhibitory activity of two of these variants (Tytgat, J., Vandenberghe, I., Ulens, C., Van Beeumen, J., 2001. New polypeptide components purified from mamba venom. FEBS Lett. 491, 217-221), the epsilon-dendrotoxins represent structural and functional intermediates between the facilitatory toxins and the protease inhibitors.

Textilinins from Pseudonaja textilis textilis. Characterization of two plasmin inhibitors that reduce bleeding in an animal model

The incidence of vein-graft occlusion associated with myocardial infarction and thrombosis following the use of the plasmin inhibitor, aprotinin, to reduce blood loss during vascular surgery has prompted the isolation of an alternative kinetically distinct inhibitor of plasmin from the venom of Pseudonaja textilis. This inhibitor has been called textilinin (Txln) and two distinct forms have been isolated from the Brown-snake venom (molecular weight, 6688 and 6692). A comparison of plasmin inhibitor constants for aprotinin and the Txlns 1 and 2 indicated that the former bound very tightly (inhibitor constant, Ki approximately 10(-11) mol/l), while both of the latter bound less tightly (Ki approximately 10(-9) mol/l). Homogeneity of Txlns 1 and 2 was confirmed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and mass spectrometry. A sequence difference of six amino acids was observed between the two forms of Txln. Txln 1 and 2 showed, respectively, 45 and 43% homology with aprotinin, while there was 58 and 55% homology, respectively, with a plasmin inhibitor from the venom of eastern Taipan, Oxyuranus scutellatus. Both Txlns have six cysteines, like other inhibitors of this group, and homology was determined by alignment of these cysteines. Both have been shown to reduce blood loss by about 60% in a murine tail vein bleeding model. It is proposed that the kinetic profiles of Txln 1 and 2 for plasmin allow the arrest of haemorrhage without the possible threat of thrombosis.

Delineation of the functional site of alpha-dendrotoxin. The functional topographies of dendrotoxins are different but share a conserved core with those of other Kv1 potassium channel-blocking toxins

We identified the residues that are important for the binding of alpha-dendrotoxin (alphaDTX) to Kv1 potassium channels on rat brain synaptosomal membranes, using a mutational approach based on site-directed mutagenesis and chemical synthesis. Twenty-six of its 59 residues were individually substituted by alanine. Substitutions of Lys5 and Leu9 decreased affinity more than 1000-fold, and substitutions of Arg3, Arg4, Leu6, and Ile8 by 5-30-fold. Substitution of Lys5 by norleucine or ornithine also greatly altered the binding properties of alphaDTX. All of these analogs displayed similar circular dichroism spectra as compared with the wild-type alphaDTX, indicating that none of these substitutions affect the overall conformation of the toxin. Substitutions of Ser38 and Arg46 also reduced the affinity of the toxin but, in addition, modified its dichroic properties, suggesting that these two residues play a structural role. The other residues were excluded from the recognition site because their substitutions caused no significant affinity change. Thus, the functional site of alphaDTX includes six major binding residues, all located in its N-terminal region, with Lys5 and Leu9 being the most important. Comparison of the functional site of alphaDTX with that of DTX-K, another dendrotoxin (Smith, L. A., Reid, P. F., Wang, F. C., Parcej, D. N., Schmidt, J. J., Olson, M. A., and Dolly, J. O. (1997) Biochemistry 36, 7690-7696), reveals that they only share the predominant lysine and probably a leucine residue; the additional functional residues differ from one toxin to the other. Comparison of the functional site of alphaDTX with those of structurally unrelated potassium channel-blocking toxins from venomous invertebrates revealed the common presence of a protruding key lysine with a close important hydrophobic residue (Leu, Tyr, or Phe) and few additional residues. Therefore, irrespective of their phylogenetic origin, all of these toxins may have undergone a functional convergence. The functional site of alphaDTX is topographically unrelated to the "antiprotease site" of the structurally analogous bovine pancreatic trypsin inhibitor.

Characterization of three endogenous peptide inhibitors for multiple metalloproteinases with fibrinogenolytic activity from the venom of Taiwan habu (Trimeresurus mucrosquamatus)

Three small peptide components were isolated and purified from the venom of Taiwan habu (Trimeresurus mucrosquamatus), which show specific activity to inhibit the strong proteolytic activity of multiple metalloproteinases present in the crude venom. Using multiple chromatographies coupled with successive ultrafiltrations, three inhibitors, i.e. pyroglutamate-lysine-tryptophan (pyroGlu-Lys-Trp), pyroglutamate-asparagine-tryptophan (pyroGlu-Asn-Trp) and pyroglutamate-glutamine-tryptophan (pyroGlu-Gln-Trp) were obtained in good yields and high homogeneity. The yields of these peptide fractions were estimated to be about 0.65 mg, 0.55 mg and 0.42 mg from 250 mg total lyophilized crude venom, which corresponded to the approximate concentrations of 8.4 mM, 7.3 mM and 5.4 mM respectively in venom secretion. Detailed and unambiguous structural determination was established by amino acid analyses, mass spectrometry and microsequencing of purified peptides. Further functional characterization of these three tripeptides showed that they could weakly inhibit three metalloproteinases previously isolated from the same venom. The inhibitory activities were similar among these tripeptides and their IC50 (concentration for 50% inhibition) were estimated in a range of 0.20-0.95 mM, which is much more effective than citrate, another venom protease inhibitor of low molecular-weight component. Since these tripeptides are the endogenous peptide inhibitors present in the lumen of venom glands, it is conceivable that they may act as a self-defensive mechanism against the auto-digestive deleterious effect of the strong metalloproteinases in vivo, particularly several zinc-dependent metalloproteinases present in crotalid and viperid venoms.

Cloning and expression of mamba toxins

Mamba venoms contain pharmacologically active proteins that interfere with neuromuscular transmission by binding to and altering the normal functioning of neuronal proteins involved, directly or indirectly, with regulating nerve transmission. Of the mamba toxins studied to date, many act on voltage-sensitive K+ channels, nicotinic or muscarinic acetylcholine receptors, or acetylcholinesterase. In an attempt to clone, characterize, and express the genes encoding these toxins, as well as other genes specifying activities not completely elucidated as yet, a cDNA library was constructed from mRNA isolated from the glands of the black mamba. Clones from the library harboring sequences encoding 14 different mamba toxins were isolated and characterized by nucleotide sequence analysis. Genes coding for three proteins, dendrotoxins (DTX) K, I, and E, were expressed as maltose-binding (MBP) fusion proteins in the periplasmic space of Escherichia coli. The DTXK-MBP fusion protein was affinity purified, cleaved from its chaperon, and the recombinant DTXK purified from MBP. Recombinant DTXK was shown to be identical to native DTXK in its N-terminal sequence, chromatographic behavior, convulsion-inducing activity, and binding to voltage-activated K+ channels in bovine synaptic membranes. Computer modeling was employed to create three-dimensional structures of DTXK and DTX1 from the X-ray crystal structure of alpha-DTX utilizing both structural and sequence homologies. Comparisons were made between the three toxins, providing a framework for site-directed mutagenesis.

Inhibition of metalloproteinases in Bothrops asper venom by endogenous peptides

Bothrops asper venom contains a variety of degradative enzymes, including metal-ion dependent proteinases as well as low molecular weight peptides. Two of these peptides, pyroglutamate-glutamine-tryptophan (pEQW) and pyroglutamate-asparagine-tryptophan are present in crude venom at concentrations of about 4.5 and 1 mM, respectively. Proteinase fractions from B. asper are inhibited from digesting oxidized insulin B-chain in vitro by both of these tripeptides with an IC50 for pEQW of approximately 0.3 mM. Digestion of purified myotoxin MIII from B. asper venom is also inhibited in vitro by pEQW, suggesting that similar inhibition of proteinase activities probably occurs in the venom gland. Inhibitory peptides present in venom allow snakes to be protected from their own toxic proteinases and inhibit hydrolysis of venom proteins during storage in the venom gland. Upon dilution, such as when venom is injected into prey, peptide inhibitors dissociate from the proteinase and allow their activation. A simple procedure for isolation of these inhibitory peptides is described.

The characterization of snake venoms using capillary electrophoresis in conjunction with electrospray mass spectrometry: Black Mambas

Capillary electrophoresis has been used in conjunction with electrospray mass spectrometry using both full-scan and selected ion monitoring modes to supply as much information as possible about the venom of Dendroaspis polylepis polylepis (Black Mamba). As an example of the application of capillary electrophoresis/electrospray mass spectrometry (CE/ESI/MS) to the analysis of a complex mixture of small proteins, we have analyzed the venom of Dendroaspis polylepis polylepis using the combined techniques. Both full-scan and selected ion monitoring modes were employed. CE/ESI/MS provides a rapid and extremely sensitive method for molecular weight determination, particularly when selected ion monitoring is employed. It has been utilized to provide sequence confirmation for those toxins which have already been described in the literature. Our methodology indicates the presence of at least 70 peptides in the molecular weight range 6000-9000.

Snake venoms. The amino-acid sequence of trypsin inhibitor E of Dendroaspis polylepis polylepis (Black Mamba) venom

Trypsin inhibitor E from black mamba venom comprises 59 amino acid residues in a single polypeptide chain, cross-linked by three intrachain disulphide bridges. The complete primary structure of inhibitor E was elucidated. The sequence is homologous with trypsin inhibitors from different sources. Unique among this homologous series of proteinase inhibitors, inhibitor E has an affinity for transition metal ions, exemplified here by Cu2 and Co2+.