Kaliotoxin (1-37) shows structural differences with related potassium channel blockers

Peptide toxins and small-molecule blockers of BK channels

Large conductance, Ca(2+)-activated potassium (BK) channels play important roles in the regulation of neuronal excitability and the control of smooth muscle contractions. BK channels can be activated by changes in both the membrane potential and intracellular Ca(2+) concentrations. Here, we provide an overview of the structural and pharmacological properties of BK channel blockers. First, the properties of different venom peptide toxins from scorpions and snakes are described, with a focus on their characteristic structural motifs, including their disulfide bond formation pattern, the binding interface between the toxin and BK channel, and the functional consequence of the blockage of BK channels by these toxins. Then, some representative non-peptide blockers of BK channels are also described, including their molecular formula and pharmacological effects on BK channels. The detailed categorization and descriptions of these BK channel blockers will provide mechanistic insights into the blockade of BK channels. The structures of peptide toxins and non-peptide compounds could provide templates for the design of new channel blockers, and facilitate the optimization of lead compounds for further therapeutic applications in neurological disorders or cardiovascular diseases.

Scorpion toxins prefer salt solutions

There is a wide variety of ion channel types with various types of blockers, making research in this field very complicated. To reduce this complexity, it is essential to study ion channels and their blockers independently. Scorpion toxins, a major class of blockers, are charged short peptides with high affinities for potassium channels. Their high selectivity and inhibitory properties make them an important pharmacological tool for treating autoimmune or nervous system disorders. Scorpion toxins typically have highly charged surfaces and-like other proteins-an intrinsic ability to bind ions (Friedman J Phys Chem B 115(29):9213-9223, 1996; Baldwin Biophys J 71(4):2056-2063, 1996; Vrbka et al. Proc Natl Acad Sci USA 103(42):15440-15444, 2006a; Vrbka et al. J Phys Chem B 110(13):7036-43, 2006b). Thus, their effects on potassium channels are usually investigated in various ionic solutions. In this work, computer simulations of protein structures were performed to analyze the structural properties of the key residues (i.e., those that are presumably involved in contact with the surfaces of the ion channels) of 12 scorpion toxins. The presence of the two most physiologically abundant cations, Na(+) and K(+), was considered. The results indicated that the ion-binding properties of the toxin residues vary. Overall, all of the investigated toxins had more stable structures in ionic solutions than in water. We found that both the number and length of elements in the secondary structure varied depending on the ionic solution used (i.e., in the presence of NaCl or KCl). This study revealed that the ionic solution should be chosen carefully before performing experiments on these toxins. Similarly, the influence of these ions should be taken into consideration in the design of toxin-based pharmaceuticals.

Structure of a pore-blocking toxin in complex with a eukaryotic voltage-dependent K(+) channel

Pore-blocking toxins inhibit voltage-dependent K⁺ channels (K_v channels) by plugging the ion-conduction pathway. We have solved the crystal structure of paddle chimera, a K_v channel in complex with charybdotoxin (CTX), a pore-blocking toxin. The toxin binds to the extracellular pore entryway without producing discernable alteration of the selectivity filter structure and is oriented to project its Lys27 into the pore. The most extracellular K⁺ binding site (S1) is devoid of K⁺ electron-density when wild-type CTX is bound, but K⁺ density is present to some extent in a Lys27Met mutant. In crystals with Cs⁺ replacing K⁺, S1 electron-density is present even in the presence of Lys27, a finding compatible with the differential effects of Cs⁺ vs K⁺ on CTX affinity for the channel. Together, these results show that CTX binds to a K⁺ channel in a lock and key manner and interacts directly with conducting ions inside the selectivity filter.

DOI:http://dx.doi.org/10.7554/eLife.00594.001

The deadly toxins produced by many creatures, including spiders, snakes, and scorpions, work by blocking the ion channels that are essential for the normal operation of many different types of cells. Ion channels are proteins and, as their name suggests, they allow ions—usually sodium, potassium, or calcium ions—to move in and out of cells. They are especially important for cells that generate or respond to electrical signals, such as neurons and the cells in heart muscle.

Ion channels are located in the lipid membranes that surround all cells, and the ions enter or leave the cell via a pore that runs through the channel protein. They can be opened and closed (or ‘gated’) in different ways: some ion channels open and close in response to voltages, whereas others are gated by biomolecules, such as neurotransmitters, that bind to them.

Now, Banerjee et al. have used x-ray crystallography to study the structure of the complex that is formed when charybdotoxin (CTX), a toxin that is found in scorpion venom, blocks a voltage-gated potassium channel. Previous studies have shown that CTX binds to the channel on the extracellular side of the pore. Banerjee et al. show that the toxin fits into the entrance to the channel like a key into a lock, which means the toxin is preformed to fit the shape of the channel.

The potassium ion channel is made up of four subunits, and the pore contains four ion-binding sites that form a ‘selectivity filter’: it is this filter that ensures that only potassium ions can pass through the channel when it is open. When CTX binds to the channel, a lysine residue poised at a critical position on the toxin is so close to the outermost ion-binding site that it prevents potassium ions binding to the site. The structure determined by Banerjee et al. explains many previous findings, including the fact that ions entering the pore from inside the cell can disrupt the binding between the toxin and the ion channel protein. It remains to be seen if the toxins that target the pore of other types of ion channels work in the same way.

DOI:http://dx.doi.org/10.7554/eLife.00594.002

Scorpion toxins specific for potassium (K+) channels: a historical overview of peptide bioengineering

Scorpion toxins have been central to the investigation and understanding of the physiological role of potassium (K⁺) channels and their expansive function in membrane biophysics. As highly specific probes, toxins have revealed a great deal about channel structure and the correlation between mutations, altered regulation and a number of human pathologies. Radio- and fluorescently-labeled toxin isoforms have contributed to localization studies of channel subtypes in expressing cells, and have been further used in competitive displacement assays for the identification of additional novel ligands for use in research and medicine. Chimeric toxins have been designed from multiple peptide scaffolds to probe channel isoform specificity, while advanced epitope chimerization has aided in the development of novel molecular therapeutics. Peptide backbone cyclization has been utilized to enhance therapeutic efficiency by augmenting serum stability and toxin half-life in vivo as a number of K⁺-channel isoforms have been identified with essential roles in disease states ranging from HIV, T-cell mediated autoimmune disease and hypertension to various cardiac arrhythmias and Malaria. Bioengineered scorpion toxins have been monumental to the evolution of channel science, and are now serving as templates for the development of invaluable experimental molecular therapeutics.

A designer ligand specific for Kv1.3 channels from a scorpion neurotoxin-based library

Venomous animals immobilize prey using protein toxins that act on ion channels and other targets of biological importance. Broad use of toxins for biomedical research, diagnosis, and therapy has been limited by inadequate target discrimination, for example, among ion channel subtypes. Here, a synthetic toxin is produced by a new strategy to be specific for human Kv1.3 channels, critical regulators of immune T cells. A phage display library of 11,200 de novo proteins is designed using the alpha-KTx scaffold of 31 scorpion toxin sequences known or predicted to bind to potassium channels. Mokatoxin-1 (moka1) is isolated by affinity selection on purified target. Moka1 blocks Kv1.3 at nanomolar levels that do not inhibit Kv1.1, Kv1.2, or KCa1.1. As a result, moka1 suppresses CD3/28-induced cytokine secretion by T cells without cross-reactive gastrointestinal hyperactivity. The 3D structure of moka1 rationalizes its specificity and validates the engineering approach, revealing a unique interaction surface supported on an alpha-KTx scaffold. This scaffold-based/target-biased strategy overcomes many obstacles to production of selective toxins.

Venom therapy in multiple sclerosis

To date many people with multiple sclerosis (MS) seek complementary and alternative medicines (CAM) to treat their symptoms as an adjunct to conventionally used therapies. Among the common CAM therapies, there is a renewed interest in the therapeutic potential of venoms in MS. The efficacy of this therapeutic method remains unclear. However, venom-based therapy using bee, snakes and scorpions venom and/or sea anemones toxin has been recently developed because current investigations have identified the various components and molecular mechanism of the effects of venoms under in vitro and in vivo conditions. The aim of this review is to describe the recent findings regarding the role of venoms and their components in treatment of MS disease and that whether venom therapy could be recommended as a complementary treatment or not.

Probing the pH-dependent structural features of alpha-KTx12.1, a potassium channel blocker from the scorpion Tityus serrulatus

Potassium channels are widespread in living cells and are involved in many diseases. The scorpion toxin alpha-KTx(12.1) interacts with various K(+) channels, suggesting its capacity to match diverse channel pores. It is recognized that tissue injuries may affect the pH at toxins site of action, thereby modulating both protein conformation and activity. To better understand its molecular mechanism of action, we studied alpha-KTx(12.1) using pH as a tool to explore its plasticity and NMR in combination with MD calculations to detect it. The toxin solution structure consists of an alpha-helix and a triple-stranded beta-sheet stabilized by four disulfide bridges. The NMR results show, in addition, that His28 possesses an unusually low pK(a) of 5.2. The best set of protein conformers is obtained at pH 4.5, while at pH 7.0, the reduced number of NOEs resulting from a faster hydrogen exchange does not allow to reach a good structural convergence. Nonetheless, MD calculations show that the toxin structure does not vary significantly in that pH range, while conformational changes and modifications of the surface charge distribution occur when His28 is fully protonated. Moreover, essential dynamics analysis reveals variations in the toxin's coherent motions. In conclusion, His28, with its low pK(a) value, provides alpha-KTx(12.1) with the ability to preserve its active conformation over a wide pH interval, thus expanding the range of cellular conditions where the toxin can fully exhibit its activity. Overall, the results further underline the role of histidine as a natural controller of proteins' functionality.

A novel short-chain peptide BmKX from the chinese scorpion Buthus martensi karsch, sequencing, gene cloning and structure determination

Scorpion venom is a rich source of bioactive peptides. From the venom of Chinese scorpion Buthus martensi Karsch (BmK), a novel short chain peptide BmKX of 31-amino acid residues was purified, and its amino acid sequence and gene structure were determined. The gene of BmKX was composed of two exons interrupted by an 86-bp intron at the codon-7 upstream of the mature peptide. Although its gene structure is similar to those of other known scorpion toxins, its amino acid sequence, especially the cysteine framework, is different from those of all other known subfamilies of short-chain scorpion toxins. The solution structure of BmKX, determined with two-dimensional NMR spectroscopy, shows that BmKX also forms a typical cysteine-stabilized alpha/beta scaffold adopted by most short-chain scorpion toxins, consisting of a short 3(10)-helix and a two-stranded antiparallel beta-sheet, and the short N-terminal segment forms a pseudo-strand of the beta-sheet. However, the orientation between the helix and the beta-sheet is significantly different from the others, which might be the reason for its unique but still unclear physiological function.

Computational simulations of interactions of scorpion toxins with the voltage-gated potassium ion channel

Based on a homology model of the Kv1.3 potassium channel, the recognitions of the six scorpion toxins, viz. agitoxin2, charybdotoxin, kaliotoxin, margatoxin, noxiustoxin, and Pandinus toxin, to the human Kv1.3 potassium channel have been investigated by using an approach of the Brownian dynamics (BD) simulation integrating molecular dynamics (MD) simulation. Reasonable three-dimensional structures of the toxin-channel complexes have been obtained employing BD simulations and triplet contact analyses. All of the available structures of the six scorpion toxins in the Research Collaboratory for Structural Bioinformatics Protein Data Bank determined by NMR were considered during the simulation, which indicated that the conformations of the toxin significantly affect both the molecular recognition and binding energy between the two proteins. BD simulations predicted that all the six scorpion toxins in this study use their beta-sheets to bind to the extracellular entryway of the Kv1.3 channel, which is in line with the primary clues from the electrostatic interaction calculations and mutagenesis results. Additionally, the electrostatic interaction energies between the toxins and Kv1.3 channel correlate well with the binding affinities (-logK(d)s), R(2) = 0.603, suggesting that the electrostatic interaction is a dominant component for toxin-channel binding specificity. Most importantly, recognition residues and interaction contacts for the binding were identified. Lys-27 or Lys-28, residues Arg-24 or Arg-25 in the separate six toxins, and residues Tyr-400, Asp-402, His-404, Asp-386, and Gly-380 in each subunit of the Kv1.3 potassium channel, are the key residues for the toxin-channel recognitions. This is in agreement with the mutation results. MD simulations lasting 5 ns for the individual proteins and the toxin-channel complexes in a solvated lipid bilayer environment confirmed that the toxins are flexible and the channel is not flexible in the binding. The consistency between the results of the simulations and the experimental data indicated that our three-dimensional models of the toxin-channel complex are reasonable and can be used as a guide for future biological studies, such as the rational design of the blocking agents of the Kv1.3 channel and mutagenesis in both toxins and the Kv1.3 channel. Moreover, the simulation result demonstrates that the electrostatic interaction energies combined with the distribution frequencies from BD simulations might be used as criteria in ranking the binding configuration of a scorpion toxin to the Kv1.3 channel.

Solution structure of BmKK2, a new potassium channel blocker from the venom of chinese scorpion Buthus martensi Karsch

A natural K+ channel blocker, BmKK2 (a member of scorpion toxin subfamily alpha-KTx 14), which is composed of 31 amino acid residues and purified from the venom of the Chinese scorpion Buthus martensi Karsch, was characterized using whole-cell patch-clamp recording in rat hippocampal neurons. The three dimensional structure of BmKK2 was determined with two-dimensional NMR spectroscopy and molecular modelling techniques. In solution this toxin adopted a common alpha/beta-motif, but showed distinct local conformation in the loop between alpha-helix and beta-sheet in comparison with typical short-chain scorpion toxins (e.g., CTX and NTX). Also, the alpha helix is shorter and the beta-sheet element is smaller (each strand consisted only two residues). The unusual structural feature of BmKK2 was attributed to the shorter loop between the alpha-helix and beta-sheet and the presence of two consecutive Pro residues at position 21 and 22 in the loop. Moreover, two models of BmKK2/hKv1.3 channel and BmKK2/rSK2 channel complexes were simulated with docking calculations. The results demonstrated the existence of a alpha-mode binding between the toxin and the channels. The model of BmKK2/rSK2 channel complex exhibited favorable contacts both in electrostatic and hydrophobic, including a network of five hydrogen bonds and bigger interface containing seven pairs of inter-residue interactions. In contrast, the model of BmKK2/hKv1.3 channel complex, containing only three pairs of inter-residue interactions, exhibited poor contacts and smaller interface. The results well explained its lower activity towards Kv channel, and predicted that it may prefer a type of SK channel with a narrower entryway as its specific receptor.

Comparison of sea anemone and scorpion toxins binding to Kv1 channels: an example of convergent evolution

Comparison of data from functional mapping carried out on scorpion and sea anemones toxins blocking currents through voltage-gated potassium channels revealed that, despite their different 3D structures, the binding cores of these toxins displayed some similarities. Further molecular modeling studies suggested that these similarities reflect the use by these toxins of a common binding mode to exert their blocking function. Therefore, scorpion and sea anemone toxins offer an example of mechanistic convergent evolution.

cDNA sequence and in vitro folding of GsMTx4, a specific peptide inhibitor of mechanosensitive channels

The peptide GsMTx4 from the tarantula venom (Grammostola spatulata) inhibits mechanosensitive ion channels. In this work, we report the cDNA sequence encoding GsMTx4. The gene is translated as a precursor protein of 80 amino acids. The first 21 amino acids are a predicted signal sequence and the C-terminal residues are a signal for amidation. An arginine residue adjacent to the N-terminal glycine of GsMTx4 is the cleavage site for release. The resulting peptide is 34 amino acids in length with a C-terminal phenylalanine and not a serine-alanine previously identified [J. Gen. Physiol. 115 (2000) 583]. We chemically synthesized this peptide and folded it in 0.1 M Tris, pH 7.9 with oxidized/reduced glutathione (1/10). Properties of the synthetic peptide were identical to the wild type for high performance liquid chromatography (HPLC), mass spectrometry, CD, and NMR. We also cloned GsMTx4 in a thioredoxin fusion protein system containing six histidines. Nickel affinity columns allowed rapid purification and folding occurred in conditions described above with 0.5 M guanidiniumHCl present. Thrombin cleavage liberated GsMTx4 with three extra amino acids at the N-terminus. The retention time in HPLC analysis and the CD spectrum was similar to wild type. Both the synthetic and cloned peptides were active in the patch clamp assay.

BTK-2, a new inhibitor of the Kv1.1 potassium channel purified from Indian scorpion Buthus tamulus

A novel inhibitor of voltage-gated potassium channel was isolated and purified to homogeneity from the venom of the red scorpion Buthus tamulus. The primary sequence of this toxin, named BTK-2, as determined by peptide sequencing shows that it has 32 amino acid residues with six conserved cysteines. The molecular weight of the toxin was found to be 3452 Da. It was found to block the human potassium channel hKv1.1 (IC(50)=4.6 microM). BTK-2 shows 40-70% sequence similarity to the family of the short-chain toxins that specifically block potassium channels. Multiple sequence alignment helps to categorize the toxin in the ninth subfamily of the K+ channel blockers. The modeled structure of BTK-2 shows an alpha/beta scaffold similar to those of the other short scorpion toxins. Comparative analysis of the structure with those of the other toxins helps to identify the possible structure-function relationship that leads to the difference in the specificity of BTK-2 from that of the other scorpion toxins. The toxin can also be used to study the assembly of the hKv1.1 channel.

Isolation and characterization of a novel lepidopteran-selective toxin from the venom of South Indian red scorpion, Mesobuthus tamulus

BACKGROUND

Scorpion venom contains insect and mammal selective toxins. We investigated the venom of the South Indian red scorpion, Mesobuthus tamulus for the purpose of identifying potent insecticidal peptide toxins.

RESULTS

A lepidopteran-selective toxin (Buthus tamulus insect toxin; ButaIT) has been isolated from this venom. The primary structure analysis reveals that it is a single polypeptide composed of 37 amino acids cross-linked by four disulfide bridges with high sequence homology to other short toxins such as Peptide I, neurotoxin P2, Lqh-8/6, chlorotoxin, insectotoxin I5A, insect toxin 15 and insectotoxin I1. Three dimensional modeling using Swiss automated protein modeling server reveals that this toxin contains a short alpha-helix and three antiparallel beta-strands, similar to other short scorpion toxins. This toxin is selectively active on Heliothis virescens causing flaccid paralysis but was non-toxic to blowfly larvae and mice.

CONCLUSION

This is the first report of a Heliothine selective peptide toxin. Identification of diverse insect selective toxins offer advantages in employing these peptides selectively for pest control.

Solution structure of a K(+)-channel blocker from the scorpion Tityus cambridgei

A new K(+)-channel blocking peptide identified from the scorpion venom of Tityus cambridgei (Tc1) is composed of 23 amino acid residues linked with three disulfide bridges. Tc1 is the shortest known toxin from scorpion venom that recognizes the Shaker B K(+) channels and the voltage-dependent K(+) channels in the brain. Synthetic Tc1 was produced using solid-phase synthesis, and its activity was found to be the same as that of native Tc1. The pairings of three disulfide bridges in the synthetic Tc1 were identified by NMR experiments. The NMR solution structures of Tc1 were determined by simulated annealing and energy-minimization calculations using the X-PLOR program. The results showed that Tc1 contains an alpha-helix and a 3(10)-helix at N-terminal Gly(4)-Lys(10) and a double-stranded beta-sheet at Gly(13)-Ile(16) and Arg(19)-Tyr(23), with a type I' beta-turn at Asn(17)-Gly(18). Superposition of each structure with the best structure yielded an average root mean square deviation of 0.26 +/- 0.05 A for the backbone atoms and of 1.40 +/- 0.23 A for heavy atoms in residues 2 to 23. The three-dimensional structure of Tc1 was compared with two structurally and functionally related scorpion toxins, charybdotoxin (ChTx) and noxiustoxin (NTx). We concluded that the C-terminal structure is the most important region for the blocking activity of voltage-gated (Kv-type) channels for scorpion K(+)-channel blockers. We also found that some of the residues in the larger scorpion K(+)-channel blockers (31 to 40 amino acids) are not involved in K(+)-channel blocking activity.

An ERG channel inhibitor from the scorpion Buthus eupeus

The isolation of the peptide inhibitor of M-type K(+) current, BeKm-1, from the venom of the Central Asian scorpion Buthus eupeus has been described previously (Fillipov A. K., Kozlov, S. A., Pluzhnikov, K. A., Grishin, E. V., and Brown, D. A. (1996) FEBS Lett. 384, 277-280). Here we report the cloning, expression, and selectivity of BeKm-1. A full-length cDNA of 365 nucleotides encoding the precursor of BeKm-1 was isolated using the rapid amplification of cDNA ends polymerase chain reaction technique from mRNA obtained from scorpion telsons. Sequence analysis of the cDNA revealed that the precursor contains a signal peptide of 21 amino acid residues. The mature toxin consists of 36 amino acid residues. BeKm-1 belongs to the family of scorpion venom potassium channel blockers and represents a new subgroup of these toxins. The recombinant BeKm-1 was produced as a Protein A fusion product in the periplasm of Escherichia coli. After cleavage and high performance liquid chromatography purification, recombinant BeKm-1 displayed the same properties as the native toxin. Three BeKm-1 mutants (R27K, F32K, and R27K/F32K) were generated, purified, and characterized. Recombinant wild-type BeKm-1 and the three mutants partly inhibited the native M-like current in NG108-15 at 100 nm. The effect of the recombinant BeKm-1 on different K(+) channels was also studied. BeKm-1 inhibited hERG1 channels with an IC(50) of 3.3 nm, but had no effect at 100 nm on hEAG, hSK1, rSK2, hIK, hBK, KCNQ1/KCNE1, KCNQ2/KCNQ3, KCNQ4 channels, and minimal effect on rELK1. Thus, BeKm-1 was shown to be a novel specific blocker of hERG1 potassium channels.

NMR solution structure of butantoxin

The NMR structure of a new toxin, butantoxin (BuTX), which is present in the venoms of the three Brazilian scorpions Tityus serrulatus, Tityus bahiensis, and Tityus stigmurus, has been investigated. This toxin was shown to reversibly block the Shaker B potassium channels (K(d) approximately 660 nM) and inhibit the proliferation of T-cells and the interleukin-2 production of antigen-stimulated T-helper cells. BuTX is a 40 amino acid basic protein stabilized by the four disulfide bridges: Cys2-Cys5, Cys10-Cys31, Cys16-Cys36, and Cys20-Cys38. The latter three are conserved among all members of the short-chain scorpion toxin family, while the first is unique to BuTX. The three-dimensional structure of BuTX was determined using (1)H-NMR spectroscopy. NOESY, phase sensitive COSY (PH-COSY), and amide hydrogen exchange data were used to generate constraints for molecular modeling calculations. Distance geometry and simulated annealing calculations were performed to generate a family of 49 structures free of constraint violations. The secondary structure of BuTX consists of a short 2(1/2) turn alpha-helix (Glu15-Phe23) and a beta-sheet. The beta-sheet is composed of two well-defined antiparallel strands (Gly29-Met32 and Lys35-Cys38) connected by a type-I' beta-turn (Asn33-Asn34). Residues Cys5-Ala9 form a quasi-third strand of the beta-sheet. The N-terminal C2-C5 disulfide bridge unique to this toxin does not appear to confer stability to the protein.

Novel miniproteins engineered by the transfer of active sites to small natural scaffolds

Small multidisulfide-containing proteins are attractive structural templates to produce a biologically active conformation that mimics the binding surface of natural large proteins. In particular, the structural motif that is evolutionary conserved in all scorpion toxins has a small size (30-40 amino acid residues), a great structural stability, and high permissiveness for sequence mutation. This motif is composed of a beta-sheet and an alpha-helix bridged in the interior core by three disulfides. We have used this motif successfully to transfer within its beta-sheet new functional sites, including the curaremimetic loop of a snake neurotoxin and the CDR2-like site of human CD4. Accumulated evidence indicated that the two miniproteins produced, the curaremimetic miniprotein and the CD4 mimetic, contain the alpha/beta fold that is characteristic of the scaffold used and bind respectively to the acetylcholine receptor and to the envelope gp120 of HIV-1. Furthermore, the latter was shown to prevent viral infection of lymphocytes. These examples illustrate that, by the transfer of active sites to small and stable natural scaffolds, it is possible to engineer miniproteins reproducing, in part, the function of much larger proteins. Such miniproteins may be of great utility as tools in structure-function studies and as leads in drug design.

KTX3, the kaliotoxin from Buthus occitanus tunetanus scorpion venom: one of an extensive family of peptidyl ligands of potassium channels

A new ligand of the K+ channels sensitive to KTX was purified from the venom of Buthus occitanus tunetanus, using two steps of high-performance-liquid-chromatography and by following its ability to compete with [125I]-KTX for binding to the KTX receptor on rat brain synaptosomes. Amino-acid analysis, amino acid sequencing and mass spectroscopy defined this new ligand. KTX3, as a 37-amino acid peptide, with three disulfide bridges. Its sequence shares 76% identity with KTX. The main differences between the two peptides are in the N-terminal region and the residue position 34 located in the region involved in channel recognition. These differences may explain the 5-fold lower binding affinity of KTX3, IC50=50 pM, than KTX to rat brain synaptosomes. Specific antibodies raised against KTX (1-37) were not able to recognize KTX3.

Determination of the three-dimensional solution structure of Raphanus sativus antifungal protein 1 by 1H NMR

Raphanus sativus Antifungal Protein 1 (Rs-AFP1) is a 51 amino acid residue plant defensin isolated from radish (Raphanus sativus L.) seeds. The three-dimensional structure in aqueous solution has been determined from two-dimensional 1H NMR data recorded at 500 MHz using the DIANA/REDAC calculation protocols. Experimental constraints consisted of 787 interproton distances extracted from NOE cross-peaks, 89 torsional constraints from 106 vicinal interproton coupling constants and 32 stereospecific assignments of prochiral protons. Further refinement by simulated annealing resulted in a set of 20 structures having pairwise root-mean-square differences of 1.35(+/- 0.35) A over the backbone heavy atoms and 2.11(+/- 0.46) A over all heavy atoms. The molecule adopts a compact globular fold comprising an alpha-helix from Asn18 till Leu28 and a triple-stranded beta-sheet (beta 1 = Lys2-Arg6, beta 2 = His33-Tyr38 and beta 3 = His43-Pro50). The central strand of this beta-sheet is connected by two disulfide bridges (Cys21-Cys45 and Cys25-Cys47) to the alpha-helix. The connection between beta-strand 2 and 3 is formed by a type VIa beta-turn. Even the loop (Pro7 to Asn17) between beta-strand 1 and the alpha-helix is relatively well defined. The structure of Raphanus sativus Antifungal Protein 1 features all the characteristics of the "cysteine stabilized alpha beta motif". A comparison of the complete structure and of the regions important for interaction with the fungal receptor according to a mutational study, is made with the structure of gamma-thionin, a plant defensin that has no antifungal activity. It is concluded that this interaction is both electrostatic and specific, and some possible scenarios for the mode of action are given.

Influence of a NH2-terminal extension on the activity of KTX2, a K+ channel blocker purified from Androctonus australis scorpion venom

A cDNA encoding a short polypeptide blocker of K+ channels, kaliotoxin 2 (KTX2), from the venom of the North African scorpion Androctonus australis was expressed in the periplasmic space of Escherichia coli. KTX2 was produced as a fusion protein with the maltose binding protein followed by the recognition site for factor Xa or enterokinase preceding the first amino acid residue of the toxin. The fully refolded recombinant KTX2 (rKTX2) was obtained (0.15-0.30 mg/l of culture) and was indistinguishable from the native toxin according to chemical and biological criteria. An N-extended analogue of KTX2 exhibiting three additional residues was also expressed. This analogue had 1000-fold less affinity for the 125I-kaliotoxin binding site on rat brain synaptosomes than KTX2. Conformational models of KTX2 and its mutant were designed by amino acid replacement using the structure of agitoxin 2 from Leiurus quinquestriatus as template, to try to understand the decrease in affinity for the receptor.

3D structure of kaliotoxin: is residue 34 a key for channel selectivity?

Kaliotoxin (KTX) is a natural peptide blocker of voltage-dependent K+ channels. The 3D structure of a truncated analogue of KTX (Fernandez et al. (1994) Biochemistry 33, 14256-14263) was determined by NMR spectroscopy and showed significant differences from structures established for other related scorpion toxins. A recent publication with the structure of the complete toxin (Aiyar et al. (1995) Neuron 15, 1169-1181) did not confirm these differences. In this communication we report NMR data for KTX at pH 3.0, 5.5 and 7.2 and the 3D structure obtained from data at pH = 5.5. Complete KTX displays a folding similar to that of other toxins with an alpha-helix and a beta-sheet linked by two disulphide bonds. The pKa of His 34 is anomalously low (4.7-5.2 depending on the buffer) owing to its interaction with two Lys residues (including the essential Lys 27), the charged N-terminus and the side chain of Met 29. Charged residues are placed symmetrically with respect to an axis that approximately coincides with one of the principal components of the moment of inertia of the toxin. His 34, which occupies a well-defined position between two conserved Cys, is located on the centre of a layer of charged groups. Positively and negatively charged residues are found at the same position in related toxins. It is suggested that electrostatic effects modulate the distances between positive charges in flexible side chains, contributing to the fine tuning of the selectivity toward different channel subclasses and that the approximate coincidence between the moment of inertia and the charge axis facilitate the approach of the toxin to the channel. The very low pKa of His 34 implies that it will be completely unprotonated at physiological pH.

Characterization of PO1, a new peptide ligand of the apamin-sensitive Ca2+ activated K+ channel

A new peptide ligand of the small conductance Ca2+ activated K+ channels has been purified from the venom (obtained by manual rather than electrical stimulation of the scorpion Androctonus mauretanicus mauretanicus), by following the inhibition of the 125I-apamin binding to its receptor on rat brain synaptosomes. Only one step on a C18 reversed-phase high-performance liquid chromatography column was necessary to obtain PO1. Its K0.5 for the apamin binding site was 100 nM. The amino acid sequence of PO1 is different from those of leiurotoxin and PO5. For the first time the same peptide was also purified from the venoms of two other species of North African scorpions, Androctonus australis and Buthus occitanus tunetanus. PO1 was chemically synthesized by the solid-phase technique and fully characterized. A model of PO1 was constructed by amino acid replacement using PO5 nuclear magnetic resonance studies as the starting model. Structure-activity relationships between these toxins and their receptor are discussed.

Noxiustoxin 2, a novel K+ channel blocking peptide from the venom of the scorpion Centruroides noxius Hoffmann

A novel peptide called Noxiustoxin 2 (NTX2) was purified from the venom of the scorpion Centruroides noxius and characterized chemically and functionally. It is composed of 38 amino acid residues linked by three disulfide bridges and its primary structure is 61% identical to that of Noxiustoxin (NTX). It is not toxic to mice (using up to 200 micrograms/20 g mouse weight) and crustaceans (up to 30 micrograms/g of crayfish), but has a paralysing effect on crickets (30 micrograms/g animal). It displaces the binding of [125I]NTX to rat brain synaptosome membranes with a Ki of 0.1 microM, in comparison NTX has a Ki of 100 pM. Similarly, using single Ca2+ activated K+ channels of small conductance obtained from cultured bovine aortic endothelial cells it was shown that NTX2 is over two logarithm units less potent than NTX in producing 50% blockade of the probability of opening the channels. NTX2 is not recognized by a panel of six distinct monoclonal antibodies against NTX, however it is recognized by polyclonal antibodies raised in mouse, with native NTX. Primary structure comparison of both NTX and NTX2 suggests that the N-terminal segments of these peptides are important for channel affinity.

Changing the structural context of a functional beta-hairpin. Synthesis and characterization of a chimera containing the curaremimetic loop of a snake toxin in the scorpion alpha/beta scaffold

An approach to obtain new active proteins is the incorporation of all or a part of a well defined active site onto a natural structure acting as a structural scaffold. According to this strategy we tentatively engineered a new curaremimetic molecule by transferring the functional central loop of a snake toxin, sequence 26-37, sandwiched between two hairpins, onto the structurally similar beta-hairpin of the scorpion toxin charybdotoxin, stabilized by a short helix. The resulting chimeric molecule, only 31 amino acids long, was produced by solid phase synthesis, refolded, and purified to homogeneity. As shown by structural analysis performed by CD and NMR spectroscopy, the chimera maintained the expected alpha/beta fold characteristic of scorpion toxins and presented a remarkable structural stability. The chimera competitively displaces the snake curaremimetic toxin alpha from the acetylcholine receptor at 10(-5) M concentrations. Antibodies, elicited in rabbits against the chimera, recognize the parent snake toxin and prevent its binding to the acetylcholine receptor, thus neutralizing its toxic function. All these data demonstrate that the strategy of active site transfer to the charybdotoxin scaffold has general applications in the engineering of novel ligands for membrane receptors and in vaccine design.

1H-NMR-derived secondary structure and the overall fold of the potent anti-mammal and anti-insect toxin III from the scorpion Leiurus quinquestriatus quinquestriatus

We describe the secondary structure and the overall fold of toxin III from the venom of the scorpion Leiurus quinquestriatus quinquestriatus determined using two-dimensional-1H-NMR spectroscopy. This protein, which contains 64 amino acids and 4 disulfide bridges, belongs to the long-chain toxin category and is highly toxic to both mammals and insects. The overall fold was determined on the basis of 1208 inter-proton-distance restraints derived from NOE measurements and 90 psi, phi dihedral-angle restraints derived from NOE connectivities and 3JNH-alphaH coupling constants using the HABAS program. This fold, which mainly consists of an alpha-helix packed against a small antiparallel three-stranded beta-sheet, and of several turns and loops, is similar to that of other long-chain scorpion toxins. Aromatic and non-polar residues form several patches on the surface of the protein which alternate with patches of charged and polar residues. Such a topology should be important in the interactions of toxin III with sodium channels in membranes. Two weakly constrained loops introduce some flexibility to the structure which could be related to the activity of this toxin. The central core of toxin III is compared with the cysteine-stabilized alpha beta motif (an alpha-helix connected to a beta-sheet through two disulfide bridges) found in insect defensins and plant thionins. Defensins and thionins are small proteins (approximately 40--50 amino acid residues) containing three or four disulfide bridges, respectively. This comparison confirms that the cysteine-stabilized alpha beta motif is a common core to a number of small proteins from different origins and having different activities.

Solution structure of P01, a natural scorpion peptide structurally analogous to scorpion toxins specific for apamin-sensitive potassium channel

The venom of the North African scorpion Androctonus mauretanicus mauretanicus possesses numerous highly active neurotoxins that specifically bind to various ion channels. One of these, P05, has been found to bind specifically to calcium-activated potassium channels and also to compete with apamin, a toxin extracted from bee venom. Besides the highly potent ones, several of these peptides (including that of P01) have been purified and been found to possess only a very weak, although significant, activity in competition with apamin. The amino acid sequence of P01 shows that it is shorter than P05 by two residues. This deletion occurs within an alpha-helix stretch (residues 5-12). This alpha-helix has been shown to be involved in the interaction of P05 with its receptor via two arginine residues. These two arginines are absent in the P01 sequence. Furthermore, a proline residue in position 7 of the P01 sequence may act as an alpha-helix breaker. We have determined the solution structure of P01 by conventional two-dimensional 1H nuclear magnetic resonance and show that 1) the proline residue does not disturb the alpha-helix running from residues 5 to 12; 2) the two arginines are topologically replaced by two acidic residues, which explains the drop in activity; 3) the residual binding activity may be due to the histidine residue in position 9; and 4) the overall secondary structure is conserved, i.e., an alpha-helix running from residues 5 to 12, two antiparallel stretches of beta-sheet (residues 15-20 and 23-27) connected by a type I' beta-turn, and three disulfide bridges connecting the alpha-helix to the beta-sheet.

A strongly interacting pair of residues on the contact surface of charybdotoxin and a Shaker K+ channel

Charybdotoxin, a peptide neurotoxin of known molecular structure, blocks Shaker K+ channels by binding to a receptor at the outer opening of the ion conduction pathway. Analysis of variants of CTX at position 29 and of Shaker at position 449 shows that these two residues interact closely in the channel-toxin complex. The CTX mutation M29I leads to a slight strengthening of block when tested on Shaker-449T; the same CTX mutation weakens block 1700-fold when tested on Shaker-449F. The known position of CTX-29 on the toxin's interaction surface thus locates Shaker-449 within 5 A of the pore axis of the closed channel. All four subunits must carry the 449F mutation to produce a highly toxin-insensitive channel.

Topology of the pore-region of a K+ channel revealed by the NMR-derived structures of scorpion toxins

The architecture of the pore-region of a voltage-gated K+ channel, Kv1.3, was probed using four high affinity scorpion toxins as molecular calipers. We established the structural relatedness of these toxins by solving the structures of kaliotoxin and margatoxin and comparing them with the published structure of charybdotoxin; a homology model of noxiustoxin was then developed. Complementary mutagenesis of Kv1.3 and these toxins, combined with electrostatic compliance and thermodynamic mutant cycle analyses, allowed us to identify multiple toxin-channel interactions. Our analyses reveal the existence of a shallow vestibule at the external entrance to the pore. This vestibule is approximately 28-32 A wide at its outer margin, approximately 28-34 A wide at its base, and approximately 4-8 A deep. The pore is 9-14 A wide at its external entrance and tapers to a width of 4-5 A at a depth of approximately 5-7 A from the vestibule. This structural information should directly aid in developing topological models of the pores of related ion channels and facilitate therapeutic drug design.

Solution structure of the potassium channel inhibitor agitoxin 2: caliper for probing channel geometry

The structure of the potassium channel blocker agitoxin 2 was solved by solution NMR methods. The structure consists of a triple-stranded antiparallel beta-sheet and a single helix covering one face of the beta-sheet. The cysteine side chains connecting the beta-sheet and the helix form the core of the molecule. One edge of the beta-sheet and the adjacent face of the helix form the interface with the Shaker K+ channel. The fold of agitoxin is homologous to the previously determined folds of scorpion venom toxins. However, agitoxin 2 differs significantly from the other channel blockers in the specificity of its interactions. This study was thus focused on a precise characterization of the surface residues at the face of the protein interacting with the Shaker K+ channel. The rigid toxin molecule can be used to estimate dimensions of the potassium channel. Surface-exposed residues, Arg24, Lys27, and Arg31 of the beta-sheet, have been identified from mutagenesis studies as functionally important for blocking the Shaker K+ channel. The sequential and spatial locations of Arg24 and Arg31 are not conserved among the homologous toxins. Knowledge on the details of the channel-binding sites of agitoxin 2 formed a basis for site-directed mutagenesis studies of the toxin and the K+ channel sequences. Observed interactions between mutated toxin and channel are being used to elucidate the channel structure and mechanisms of channel-toxin interactions.