Determination of the three-dimensional structure of iberiotoxin in solution by 1H nuclear magnetic resonance spectroscopy

Molecular Determinants of BK Channel Functional Diversity and Functioning

Large-conductance Ca2+- and voltage-activated K+ (BK) channels play many physiological roles ranging from the maintenance of smooth muscle tone to hearing and neurosecretion. BK channels are tetramers in which the pore-forming α subunit is coded by a single gene ( Slowpoke, KCNMA1). In this review, we first highlight the physiological importance of this ubiquitous channel, emphasizing the role that BK channels play in different channelopathies. We next discuss the modular nature of BK channel-forming protein, in which the different modules (the voltage sensor and the Ca2+ binding sites) communicate with the pore gates allosterically. In this regard, we review in detail the allosteric models proposed to explain channel activation and how the models are related to channel structure. Considering their extremely large conductance and unique selectivity to K+, we also offer an account of how these two apparently paradoxical characteristics can be understood consistently in unison, and what we have learned about the conduction system and the activation gates using ions, blockers, and toxins. Attention is paid here to the molecular nature of the voltage sensor and the Ca2+ binding sites that are located in a gating ring of known crystal structure and constituted by four COOH termini. Despite the fact that BK channels are coded by a single gene, diversity is obtained by means of alternative splicing and modulatory β and γ subunits. We finish this review by describing how the association of the α subunit with β or with γ subunits can change the BK channel phenotype and pharmacology.

β1-subunit-induced structural rearrangements of the Ca2+- and voltage-activated K+ (BK) channel

Large-conductance Ca(2+)- and voltage-activated K(+) (BK) channels are involved in a large variety of physiological processes. Regulatory β-subunits are one of the mechanisms responsible for creating BK channel diversity fundamental to the adequate function of many tissues. However, little is known about the structure of its voltage sensor domain. Here, we present the external architectural details of BK channels using lanthanide-based resonance energy transfer (LRET). We used a genetically encoded lanthanide-binding tag (LBT) to bind terbium as a LRET donor and a fluorophore-labeled iberiotoxin as the LRET acceptor for measurements of distances within the BK channel structure in a living cell. By introducing LBTs in the extracellular region of the α- or β1-subunit, we determined (i) a basic extracellular map of the BK channel, (ii) β1-subunit-induced rearrangements of the voltage sensor in α-subunits, and (iii) the relative position of the β1-subunit within the α/β1-subunit complex.

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.

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.

Synthesis of an iberiotoxin derivative by chemical ligation: a method for improved yields of cysteine-rich scorpion toxin peptides

Automated and manual solid phase peptide synthesis techniques were combined with chemical ligation to produce a 37-residue peptide toxin derivative of iberiotoxin which contained: (i) substitution of Val(16) to Ala, to facilitate kinetic feasibility of native chemical ligation, and; (ii) substitution of Asp(19) to orthogonally protected Cys-4-MeOBzl for chemical conjugate derivatization following peptide folding and oxidation. This peptide ligation approach increased synthetic yields approximately 12-fold compared to standard linear peptide synthesis. In a functional inhibition assay, the ligated scorpion toxin derivative, iberiotoxin V16A/D19-Cys-4-MeOBzl, exhibited 'native-like' affinity (K(d)=1.9 nM) and specificity towards the BK Ca(2+)-activated K(+) Channel (K(Ca)1.1). This was characterized by the rapid association and slow dissociation rates (k(on)=4.59 x 10(5)M(-1)s(-1); k(off)=8.65 x 10(-4) s(-1)) as determined by inhibition of macroscopic whole-cell currents of cloned human K(Ca)1.1 channel. These results illustrate the successful application of peptide chemical ligation to improve yield of cysteine-rich peptide toxins over traditional solid phase peptide synthesis. Native chemical ligation is a promising method for improving production of biologically active disulfide containing peptide toxins, which have diverse applications in studies of ion-channel function.

Synthesis of a biotin derivative of iberiotoxin: binding interactions with streptavidin and the BK Ca2+-activated K+ channel expressed in a human cell line

Iberiotoxin (IbTx) is a scorpion venom peptide that inhibits BK Ca2+-activated K+ channels with high affinity and specificity. Automated solid-phase synthesis was used to prepare a biotin-labeled derivative (IbTx-LC-biotin) of IbTx by substitution of Asp19 of the native 37-residue peptide with N--(D-biotin-6-amidocaproate)-L-lysine. Both IbTx-LC-biotin and its complex with streptavidin (StrAv) block single BK channels from rat skeletal muscle with nanomolar affinity, indicating that the biotin-labeled residue, either alone or in complex with StrAv, does not obstruct the toxin binding interaction with the BK channel. IbTx-LC-biotin exhibits high affinity (KD = 26 nM) and a slow dissociation rate (koff = 5.4 x 10(-4) s(-1)) in a macroscopic blocking assay of whole-cell current of the cloned human BK channel. Titration of IbTx-LC-biotin with StrAv monitored by high performance size exclusion chromatography is consistent with a stoichiometry of two binding sites for IbTx-LC-biotin per StrAv tetramer, indicating that steric interference hinders simultaneous binding of two toxin molecules on each of the two biotin-binding faces of StrAv. In combination with fluorescent conjugates of StrAv or anti-biotin antibody, IbTx-LC-biotin was used to image the surface distribution of BK channels on a transfected cell line. Fluorescence microscopy revealed a patch-like surface distribution of BK channel protein. The results support the feasibility of using IbTx-LC-biotin and similar biotin-tagged K+ channel toxins for diverse applications in cellular neurobiology. .

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.

Interaction of agitoxin2, charybdotoxin, and iberiotoxin with potassium channels: selectivity between voltage-gated and Maxi-K channels

To gain insight into the molecular determinants that define the specificity of interaction of pore-blocking peptides, such as agitoxin 2 (AgTX2), charybdotoxin (ChTX), and iberiotoxin (IbTX) with the Shaker-type voltage-gated potassium channel Kv1.3, or the large-conductance Ca(2+)-activated K(+) (Maxi-K) channel, homology models of these channels were generated based on the crystal structure of the bacterial, KcsA, potassium channel. Peptide-channel complexes were analyzed to evaluate the predicted interaction interfaces between the peptides and the channels' outer vestibules. The docking model, for either AgTX2 or ChTX with the Kv1.3 channel, predicts a novel hydrogen bonding interaction between the Asn30 side-chain of the peptide and the Asp381 side-chain of the channel. This interaction is consistent with the >500-fold decreased potency of both AgTX2 and ChTX mutants at position 30 for the Shaker channel [(Ranganathan et al., Neuron 1996;16:131-139); (Goldstein et al., Neuron 1994;12:1377-1388)]. This hydrogen bonding interaction also suggests that Gly30 in IbTX may be the critical determinant for its lack of activity against Shaker Kv channels. The model of the Maxi-K channel reveals a narrower and more structurally restrained outer vestibule in which the aromatic residues Phe266 and Tyr294 may stabilize binding of IbTX and ChTX by pi-pi stacking with the aromatic residues Trp14 and Tyr36 of the peptides. This study also suggests that the extra net negative charge of IbTX is not related to the selectivity of this peptide for the Maxi-K channel.

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.

Glycine 30 in iberiotoxin is a critical determinant of its specificity for maxi-K versus K(V) channels

Iberiotoxin (IbTX) is a remarkably selective alpha-K toxin peptide (alpha-KTx) inhibitor of the maxi-K channel. In contrast, the highly homologous charybdotoxin inhibits both the maxi-K and K(V)1.3 channels with similar high affinity. The present study investigates the molecular basis for this specificity through mutagenesis of IbTX. The interactions of mutated peptides with maxi-K and K(V)1.3 channels were monitored through dose-dependent displacement of specifically bound iodinated alpha-KTx peptides from membranes expressing these channels. Results of these studies suggest that the presence of a glycine at position 30 in IbTX is a major determinant of its specificity while the presence of four unique acidic residues in IbTX is not.

Purification, characterization and biosynthesis of parabutoxin 3, a component of Parabuthus transvaalicus venom

A novel peptidyl inhibitor of voltage-gated K+ channels, named parabutoxin 3 (PBTx3), has been purified to homogeneity from the venom of Parabuthus transvaalicus. This scorpion toxin contains 37 residues, has a mass of 4274 Da and displays 41% identity with charybdotoxin (ChTx, also called 'alpha-KTx1.1'). PBTx3 is the tenth member (called 'alpha-KTx1.10') of subfamily 1 of K+ channel-blocking peptides known thus far. Electrophysiological experiments using Xenopus laevis oocytes indicate that PBTx3 is an inhibitor of Kv1 channels (Kv1.1, Kv1.2, Kv1.3), but has no detectable effects on Kir-type and ERG-type channels. The dissociation constants (Kd) for Kv1.1, Kv1.2 and Kv1.3 channels are, respectively, 79 microm, 547 nm and 492 nm. A synthetic gene encoding a PBTx3 homologue was designed and expressed as a fusion protein with the maltose-binding protein (MBP) in Escherichia coli. The recombinant protein was purified from the bacterial periplasm compartment using an amylose affinity resin column, followed by a gel filtration purification step and cleavage by factor Xa (fXa) to release the recombinant toxin peptide (rPBTx3). After final purification and refolding, rPBTx3 was shown to be identical to the native PBTx3 with respect to HPLC retention time, mass spectrometric analysis and functional properties. The three-dimensional structure of PBTx3 is proposed by homology modelling to contain a double-stranded antiparallel beta sheet and a single alpha-helix, connected by three disulfide bridges. The scaffold of PBTx3 is homologous to most other alpha-KTx scorpion toxins.

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.

Slotoxin, alphaKTx1.11, a new scorpion peptide blocker of MaxiK channels that differentiates between alpha and alpha+beta (beta1 or beta4) complexes

A novel peptide from Centruroides noxius Hoffmann scorpion venom was isolated and sequenced. The 37 amino acid peptide belongs to the charybdotoxin sub-family (alphaKTx1) and was numbered member 11. alphaKTx1.11 has 75% sequence identity with iberiotoxin and 54% with charybdotoxin. alphaKTx1.11 revealed specificity for mammalian MaxiK channels (hSlo), thus, was named slotoxin. Slotoxin blocks the MaxiK pore-forming alpha subunit reversibly (K(d)=1.5 nM). Slotoxin association with alpha+beta (beta1 or beta4) channels was approximately 10 times slower than iberiotoxin and charybdotoxin, leading to a lack of effect on alpha+beta4 when tested at 100 nM for 5 min. Thus, slotoxin is a better tool to distinguish MaxiK alpha+beta complexes.

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.

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.

Solution structure of potassium channel-inhibiting scorpion toxin Lq2

Lq2 is a unique scorpion toxin. Acting from the extracellular side, Lq2 blocks the ion conduction pore in not only the voltage- and Ca2+ -activated channels, but also the inward-rectifier K+ channels. This finding argues that the three-dimensional structures of the pores in these K+ channels are similar. However, the amino acid sequences that form the external part of the pore are minimally conserved among the various classes of K+ channels. Because Lq2 can bind to all the three classes of K+ channels, we can use Lq2 as a structural probe to examine how the non-conserved pore-forming sequences are arranged in space to form similar pore structures. In the present study, we determined the three-dimensional structure of Lq2 using nuclear magnetic resonance (NMR) techniques. Lq2 consists of an alpha-helix (residues S10 to L20) and a beta-sheet, connected by an alphabeta3 loop (residues N22 to N24). The beta-sheet has two well-defined anti-parallel strands (residues G26 to M29 and residues K32 to C35), which are connected by a type I' beta-turn centered between residues N30 and K31. The N-terminal segment (residues Z1 to T8) appears to form a quasi-third strand of the beta-sheet.

Scorpion toxins as tools for studying potassium channels

The search for peptidyl inhibitors of K+ channels is a very active area of investigation. In addition to scorpion venoms, other venom sources have been investigated; all of these sources have yielded novel peptides with interesting properties. For instance, spider venoms have provided peptides that block other families of K+ channels (e.g., Kv2 and Kv4) that act via mechanisms which modify the gating properties of these channels. Such inhibitors bind to a receptor on the channel that is different from the pore region in which the peptides discussed in this chapter bind. In fact, it is possible to have a channel occupied simultaneously by both inhibitor types. It is expected that many of the methodologies concerning peptidyl inhibitors from scorpion venom, which have been developed in the past and outlined above, will be extended to the new families of K+ channel blockers currently under development.

Purification and primary structure of low molecular mass peptides from scorpion (Buthus sindicus) venom

The primary structures of four low molecular mass peptides (Bs 6, 8, 10 and 14) from scorpion Buthus sindicus were elucidated via combination of Edman degradation and matrix-assisted laser desorption ionization mass spectrometry. Bs 8 and 14 are cysteine-rich, thermostable peptides composed of 35-36 residues with molecular weights of 3.7 and 3.4 kDa, respectively. These peptides show close sequence homologies (55-78%) with other scorpion chlorotoxin-like short-chain neurotoxins (SCNs) containing four intramolecular disulfide bridges. Despite the sequence variation between these two peptides (37% heterogeneity) their general structural organization is very similar as shown by their clearly related circular dichroism spectra. Furthermore, Bs6 is a minor component, composed of 38 residues (4.1 kDa) containing six half-cystine residues and having close sequence identities (40-80%) with charybdotoxin-like SCNs containing three disulfide bridges. The non-cysteinic, bacic and thermolabile Bs10 is composed of 34 amino acid residues (3.7 kDa), and belongs to a new class of peptides, with no sequence resemblance to any other so far reported sequence isolated from scorpions. Surprisingly, Bs10 shows some limited sequence analogy with oocyte zinc finger proteins. Results of these studies are discussed with respect to their structural similarities within the scorpion LCNs, SCNs and other biologically active peptides.

Synthesis and structure-function study about tenecin 1, an antibacterial protein from larvae of Tenebrio molitor

Tenecin 1, an inducible antibacterial protein secreted in the larvae of Tenebrio molitor, has a long N-terminal loop and common structural feature of insect defensin family corresponding to cysteine stabilized alpha/beta motif. To study the function of the N-terminal loop and disulfide bridges, N-terminal loop deleted tenecin 1, reduced tenecin 1 and tenecin 1 were chemically synthesized and their activities were measured. N-terminal loop deleted tenecin and reduced tenecin 1 did not show antibacterial activity. Circular dichroism (CD) spectroscopy data revealed that the alpha-helical content of tenecin 1 and the other proteins increased in the presence of 50% (v/v) trifluoroethanol (TFE) and the alpha-helical content of tenecin 1 was much higher than that of the other proteins in buffer with or without 50% (v/v) TFE. These results suggest that disulfide bridges are necessary for the activity structure and the N-terminal loop plays an important role in the increase of alpha-helix in the membrane mimetic environment and the activity.

Scorpion toxins: tools for studying K+ channels

Over the last period of time, a large number of scorpion toxins have been characterized. These peptidyl inhibitors of K+ channels have been very useful as probes for determining the molecular architecture of these channels, for purifying channels from native tissue and determining their subunit composition, for developing the pharmacology of K+ channels, and for determining the physiologic role that K+ channels play in target tissues. The large knowledge that we have developed regarding K+ channel function would not have been possible without the discovery of these peptidyl inhibitors. It is expected that as more novel peptides are discovered, our understanding of K+ channel structure and function will be further enhanced.

1H NMR structure of an antifungal gamma-thionin protein SIalpha1: similarity to scorpion toxins

The three-dimensional structure of the Sorghum bicolor seed protein gamma-thionin SIalpha1 has been determined by 2D 1H nuclear magnetic resonance (NMR) spectroscopy. The secondary structure of this 47-residue antifungal protein with four disulphide bridges consists of a three-stranded antiparallel sheet and one helix. The helix is tethered to the sheet by two disulphide bridges which link two successive turns of the helix to alternate residues i, i+2 in one strand. Possible binding sites for antifungal activity are discussed. The same fold has been observed previously in several scorpion toxins.

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.

Rapid purification and molecular modeling of AaIT peptides from venom of Androctonus australis

As recombinant viruses expressing scorpion toxins are moving closer toward the market, it is important to obtain large amounts of pure toxin for biochemical characterization and the evaluation of biological activity in nontarget organisms. In the past, we purified a large amount of Androctonus australis anti-insect toxin (AaIT) present in the venom of A. australis with an analytical reversed-phase column by repeated runs of crude sample. We now report 20 times improved efficiency and speed of the purification by employing a preparative reversed-phase column. In just two consecutive HPLC steps, almost 1 mg of AaIT was obtained from 70 mg crude venom. Furthermore, additional AaIT was obtained from side fractions in a second HPLC run. Recently discovered insect selective toxin, AaIT5, was isolated simultaneously from the same venom batch. It shows different biological toxicity symptoms than the known excitatory and depressant insect toxins. AaIT5 gave 100% mortality with a dose of less than 1.3 micrograms against fourth-instar tobacco budworms Heliothis virescens 24 h after injection. During the purification process, we implemented mass spectrometry in addition to bioassays to monitor the presence of AaIT and AaIT5 in the HPLC fractions. Mass spectrometric screening can unambiguously follow the purification process and can greatly facilitate and expedite the downstream purification of AaIT and AaIT5 eliminating the number of bioassays required. Further, electrospray ionization was compared with matrix-assisted desorption/ionization and evaluated as a method of choice for mass spectrometric characterization of fractions from the venom purification for it provided higher mass accuracy and relative quantitation capability. Molecular models were built for AaIT5, excitatory toxin AaIT4, and depressant toxin LqhIT2. Three-dimensional structure of AaIT5 was compared with structures of the other two toxins, suggesting that AaIT5 is similar to depressant toxins.

Solution structure of TsKapa, a charybdotoxin-like scorpion toxin from Tityus serrulatus with high affinity for apamin-sensitive Ca(2+)-activated K+ channels

TsKapa (TsK), purified from the Buthidae Tityus serrulatus is a very high potent ligand for small-conductance apamin-sensitive calcium-activated potassium channels (SK). It is able to efficiently compete with apamin for binding on this channel (K0.5 = 0.3 nM) [Legros, C. et al., FEBS Lett. 390:81-84, 1996]. The solution structure of TsK has been determined by 2D-NMR techniques, which led to the full description of its 3D conformation: a short alpha helix from residues 14 to 20 and a three-stranded antiparallel beta sheet (residues 2-3, 27-29, and 32-34). The interaction of TsK with the SK potassium channel has been modeled according to the charge anisotropy of the ligand. The resulting dipole moment orientates TsK so that it presents toward the receptor, a surface, mainly basic, encompassing residues K18 and K19 on one side and R9 and Y8 on the other. Despite its three-dimensional structure that is related with scorpion toxins active on voltage-gated potassium channels such as charybdotoxin, the pharmacological activity and specificity of TsK is related with shorter scorpion toxins (i.e., possessing an only two-stranded beta sheet) such as scyllatoxin (also named leiurotoxin I) or P05.

Electrostatic binding of proteins to membranes. Theoretical predictions and experimental results with charybdotoxin and phospholipid vesicles

We previously applied the Poisson-Boltzmann equation to atomic models of phospholipid bilayers and basic peptides to calculate their electrostatic interactions from first principles (Ben-Tal, N., B. Honig, R. M. Peitzsch, G. Denisov, and S. McLaughlan. 1996. Binding of small basic peptides to membranes containing acidic lipids. Theoretical models and experimental results. Biophys. J. 71:561-575). Specifically, we calculated the molar partition coefficient, K (the reciprocal of the lipid concentration at which 1/2 the peptide is bound), of simple basic peptides (e.g., pentalysine) with phospholipid vesicles. The theoretical predictions agreed well with experimental measurements of the binding, but the agreement could have been fortuitous because the structure(s) of these flexible peptides is not known. Here we use the same theoretical approach to calculate the membrane binding of two small proteins of known structure: charybdotoxin (CTx) and iberiotoxin (IbTx); we also measure the binding of these proteins to phospholipid vesicles. The theoretical model describes accurately the dependence of K on the ionic strength and mol % acidic lipid in the membrane for both CTx (net charge +4) and IbTx (net charge +2). For example, the theory correctly predicts that the value of K for the binding of CTx to a membrane containing 33% acidic lipid should decrease by a factor of 10(5) when the salt concentration increases from 10 to 200 mM. We discuss the limitations of the theoretical approach and also consider a simple extension of the theory that incorporates nonpolar interactions.

Refined solution structure of the anti-mammal and anti-insect LqqIII scorpion toxin: comparison with other scorpion toxins

The solution structure of the anti-mammal and anti-insect LqqIII toxin from the scorpion Leiurus quinquestriatus quinquestriatus was refined and compared with other long-chain scorpion toxins. This structure, determined by 1H-NMR and molecular modeling, involves an alpha-helix (18-29) linked to a three-stranded beta-sheet (2-6, 33-39, and 43-51) by two disulfide bridges. The average RMSD between the 15 best structures and the mean structure is 0.71 A for C alpha atoms. Comparison between LqqIII, the potent anti-mammal AaHII, and the weakly active variant-3 toxins revealed that the LqqIII three-dimensional structure is closer to that of AaHII than to the variant-3 structure. Moreover, striking analogies were observed between the electrostatic and hydrophobic potentials of LqqIII and AaHII. Several residues are well conserved in long-chain scorpion toxin sequences and seem to be important in protein structure stability and function. Some of them are involved in the CS alpha beta (Cysteine Stabilized alpha-helix beta-sheet) motif. A comparison between the sequences of the RII rat brain and the Drosophila extracellular loops forming scorpion toxin binding-sites of Na+ channels displays differences in the subsites interacting with anti-mammal or anti-insect toxins. This suggests that hydrophobic as well as electrostatic interactions are essential for the binding and specificity of long-chain scorpion toxins.

Characterization of a new family of toxin-like peptides from the venom of the scorpion Leiurus quinquestriatus hebraeus. 1H-NMR structure of leiuropeptide II

To extend our knowledge about the structural features of short scorpion toxins, the ion-exchange fractions obtained from Leiurus quinquestriatus hebraeus venom were investigated by plasma desorption mass spectrometry in order to select low molecular mass polypeptides. Three toxin-like peptides with molecular mass close to 3 kDa, named leiuropeptides I, II and III, were purified and found devoid of any significant toxicity against mammals and insects. Their amino acid sequences revealed a cysteine pattern analogous to that of short-chain scorpion toxins. The solution structure of leiuropeptide II was determined by 2D 1H-NMR spectroscopy and indicated the presence of a helix accommodating a proline, connected to a two-standard beta-sheet by three disulfide bonds. The overall fold of leiuropeptide II is found to be similar to that of leiurotoxin I, a 31-residue toxin present in the same scorpion venom which acts on K+ channels. In order to rationalize the absence of toxicity, the electrostatic potential of leiuropeptide II was compared to that of leiurotoxin I. The peptide is characterized by a large negative zone around Glu4, Asp5 and Asp8 residues, beginning in the neighbourhood of the beta-turn and extending along the helix. In the same area, leiurotoxin I exhibits a positive surface, around Arg6 and Arg13 basic residues, which are essential for its receptor affinity.

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.

Synthesis and biological evaluation of an iodinated iberiotoxin analogue, [mono-iodo-Tyr5, Phe36]-iberiotoxin

The synthesis and iodination of a structural analogue of the specific large-conductance calcium-activated potassium (BK) channel blocker, iberiotoxin (IbTX), a 37-amino acid scorpion neurotoxin, is reported. The synthesis of this analogue, [Tyr5, Phe36]-IbTX, was accomplished using standard solid-phase Fmoc (9-fluorenylmethoxycarbonyl) chemistry protocols. The linear peptide was cyclized via the formation of three intramolecular disulfide bridges and subsequently iodinated at the Tyr5 position. Upon purification, the iodinated analogue, [mono-iodo-Tyr5, Phe36]-IbTX, exhibited comparable biological activity to native IbTX in blocking BK-mediated currents. These findings suggest the synthesis and use of an 125I labelled IbTX analogue for BK channel localization in autoradiography experiments.

High-conductance calcium-activated potassium channels; structure, pharmacology, and function

High-conductance calcium-activated potassium (maxi-K) channels comprise a specialized family of K+ channels. They are unique in their dual requirement for depolarization and Ca2+ binding for transition to the open, or conducting, state. Ion conduction through maxi-K channels is blocked by a family of venom-derived peptides, such as charybdotoxin and iberiotoxin. These peptides have been used to study function and structure of maxi-K channels, to identify novel channel modulators, and to follow the purification of functional maxi-K channels from smooth muscle. The channel consists of two dissimilar subunits, alpha and beta. The alpha subunit is a member of the slo Ca(2+)-activated K+ channel gene family and forms the ion conduction pore. The beta subunit is a structurally unique, membrane-spanning protein that contributes to channel gating and pharmacology. Potent, selective maxi-K channel effectors (both agonists and blockers) of low molecular weight have been identified from natural product sources. These agents, together with peptidyl inhibitors and site-directed antibodies raised against alpha and beta subunit sequences, can be used to anatomically map maxi-K channel expression, and to study the physiologic role of maxi-K channels in various tissues. One goal of such investigations is to determine whether maxi-K channels represent novel therapeutic targets.

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.

Synthesis and structural characterisation of analogues of the potassium channel blocker charybdotoxin

Charybdotoxin is a 37-residue polypeptide toxin from scorpion venom, which acts by blocking voltage-gated and Ca(2+)-activated K+ channels. We have synthesized charybdotoxin and three mono-substituted analogues using an Fmoc-tBu protocol. The Phe-2 --> Tyr analogues was chosen to introduce a site for Tyr iodination which was distinct from the K+ channel binding surface, while the Glu-12 --> Gln and Arg-19 --> His analogues were studied to probe the roles of charged residues at these positions in the structure and activity of the toxin. The synthetic native molecule was equipped with natural toxin in inhibiting the human erythrocyte Ca(2+)-dependent K+ channel. The affinities of all three analogues for the erythrocyte K+ channel were slightly reduced, with the Arg-19 --> His analogue showing the greatest increase in IC50 (2.30-fold). Two-dimensional 1H-NMR studies of these analogues showed that the Glu-12 to Gln substitution, which appeared to destabilise the N-terminal half of the alpha-helix, possibly due to the weakening of an N-terminal helix capping interaction which is apparent from our NMR data. His-21 has a pKa more than one unit below the value for a non-interacting histidine. Possible reasons for this are that the imidazolium side chain is partly buried and is located near positively charged moieties. Thus, His-21 would be neutral at physiological pH, where charybdotoxin binds to the potassium channel.

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.

Spatial localization of the K+ channel selectivity filter by mutant cycle-based structure analysis

The structurally well-characterized scorpion toxin Agitoxin2 inhibits ion permeation through Shaker K+ channels by binding to the external pore entryway. Scanning mutagenesis identified a set of inhibitor residues critical for making energetic contacts with the channel. Using thermodynamic mutant cycle analysis, we have mapped channel residues relative to the known inhibitor structure. This study constrains the position of multiple channel residues within the pore-forming loops; in one stretch, we have been able to map five out of seven contiguous residues to the inhibitor interaction surface, including those involved in ion selectivity. One interaction in particular, that of K27M on the inhibitor with Y445F on the channel, is unique in that it depends on the K+ ion concentration. These results reveal a shallow vestibule formed by the pore loops at the K+ channel entryway. The selectivity filter is located at the center of the vestibule close to (approximately 5 A) the extracellular solution.

Beta-amyloid peptide blocks the fast-inactivating K+ current in rat hippocampal neurons

Deposition of beta-amyloid peptide (A beta) in senile plaques is a hallmark of Alzheimer disease neuropathology. Chronic exposure of neuronal cultures to synthetic A beta is directly toxic, or enhances neuronal susceptibility to excitotoxins. Exposure to A beta may cause a loss of cellular calcium homeostasis, but the mechanism by which this occurs is uncertain. In this work, the acute response of rat hippocampal neurons to applications of synthetic A beta was measured using whole-cell voltage-clamp techniques. Pulse application of A beta caused a reversible voltage-dependent decrease in membrane conductance. A beta selectively blocked the voltage-gated fast-inactivating K+ current, with an estimated KI < 10 microM. A beta also blocked the delayed rectifying current, but only at the highest concentration tested. The response was independent of aggregation state or peptide length. The dynamic response of the fast-inactivating current to a voltage jump was consistent with a model whereby A beta binds reversibly to closed channels and prevents their opening. Blockage of fast-inactivating K+ channels by A beta could lead to prolonged cell depolarization, thereby increasing Ca2+ influx.

Synthesis of a metal binding protein designed on the alpha/beta scaffold of charybdotoxin

The alpha/beta scaffold of the scorpion toxin charybdotoxin has been used for the engineering of a metal binding site. Nine substitutions, including three histidines as metal ligands, have been introduced into the original toxin sequence. The newly designed sequence, 37 amino acids long, has been assembled by solid-phase synthesis and HBTU (2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate) coupling of Fmoc-protected amino acids. Formation of the three disulfide bonds occurred efficiently and rapidly in the presence of glutathione, and this post-synthesis modification has facilitated the purification task enormously. The process of synthesis and purification was performed in less than a week with an overall 10.2% yield. Circular dichroism analysis showed that the newly designed protein is folded in a alpha/beta structure, similarly to the parent toxin. Electronic absorption spectroscopy, circular dichroism and gel filtration experiments have been used to show that Cu2+ and Zn2+ ions bind with high affinity to the newly engineered protein. These results demonstrate that the alpha/beta fold, common to all scorpion toxins, is a very versatile basic structure, tolerant for substitutions and able to present new sequences in a predetermined conformation. The chemical approach is shown to be effective, rapid and practical for the production of novel designed small proteins.

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.