Solution structure of gamma 1-H and gamma 1-P thionins from barley and wheat endosperm determined by 1H-NMR: a structural motif common to toxic arthropod proteins

Solution structure of a conformationally constrained Arg-Gly-Asp-like motif inserted into the alpha/beta scaffold of leiurotoxin I

A monoclonal antibody, AC7, directed against the RGD-binding site of the GPIIIa subunit of the platelet fibrinogen receptor, interacts with activated platelet. The H3 region (H3, RQMIRGYFDV sequence) of the complementarity-determining region 3 heavy chain of AC7 inhibits platelet aggregation and fibrinogen binding to platelet. H3 contains the arginine, glycine and aspartate residues, but in an unusual order. The solution structure of the decapeptide has been studied by proton NMR. The NMR data suggested a helical equilibrium. To test whether the helical structure of H3 was biologically relevant, a conformationally constrained peptide with the RGD-like motif was designed. The sequence of a scorpion toxin (leiurotoxin I) has been modified in order to constrain the H3 sequence in a rigid helical conformation. The structure of leiurotoxin I consists of a beta-sheet and an alpha-helix, linked by three disulfide bridges. The structural feature of the chimeric peptide (H3-leiurotoxin) has been determined by standard two-dimensional NMR techniques. H3-Leiurotoxin structure closely resembles that of leiurotoxin I.

NMR structures and activity of a novel alpha-like toxin from the scorpion Leiurus quinquestriatus hebraeus

NMR structures of a new toxin from the scorpion Leiurus quinquestriatus hebraeus (Lqh III) have been investigated in conjunction with its pharmacological properties. This toxin is proposed to belong to a new group of scorpion toxins, the alpha-like toxins that target voltage-gated sodium channels with specific properties compared with the classical alpha-scorpion toxins. Electrophysiological analysis showed that Lqh III inhibits a sodium current inactivation in the cockroach axon, but induces in addition a resting depolarization due to a slowly decaying tail current atypical to other alpha-toxin action. Binding studies indicated that radiolabeled Lqh III binds with a high degree of affinity (Ki=2.2 nM) on cockroach sodium channels and that the alpha-toxin from L quinquestriatus hebraeus highly active on insects (LqhalphaIT) and alpha-like toxins compete at low concentration for its receptor binding site, suggesting that the alpha-like toxin receptor site is partially overlapping with the receptor site 3. Conversely, in rat brain, Lqh III competes for binding of the most potent anti-mammal alpha-toxin from Androctonus australis Hector venom (AaH II) only at very high concentration. The NMR structures were used for the scrutiny of the similarities and differences with representative scorpion alpha-toxins targeting the voltage-gated sodium channels of either mammals or insects. Three turn regions involved in the functional binding site of the anti-insect LqhalphaIT toxin reveal significant differences in the Lqh III structure. The electrostatic charge distribution in the Lqh III toxin is also surprisingly different when compared with the anti-mammal alpha-toxin AaH II. Similarities in the electrostatic charge distribution are, however, recognized between alpha-toxins highly active on insects and the alpha-like toxin Lqh III. This affords additional important elements to the definition of the new alpha-like group of scorpion toxins and the mammal versus insect scorpion toxin selectivities.

Solution structure of a beta-neurotoxin from the New World scorpion Centruroides sculpturatus Ewing

We report the detailed solution structure of the 7.2 kDa protein CsE-I, a beta-neurotoxin from the New World scorpion Centruroides sculpturatus Ewing. This toxin binds to sodium channels, but unlike the alpha-neurotoxins, shifts the voltage of activation toward more negative potentials causing the membrane to fire spontaneously. Sequence-specific proton NMR assignments were made using 600 MHz 2D-NMR data. Distance geometry and dynamical simulated annealing refinements were performed using experimental distance and torsion angle constraints from NOESY and pH-COSY data. A family of 40 structures without constraint violations was generated, and an energy-minimized average structure was computed. The backbone conformation of the CsE-I toxin shows similar secondary structural features as the prototypical alpha-neurotoxin, CsE-v3, and is characterized by a short 2(1/2)-turn alpha-helix and a 3-strand antiparallel beta-sheet, both held together by disulfide bridges. The RMSD for the backbone atoms between CsE-I and CsE-v3 is 1.48 A. Despite this similarity in the overall backbone folding, the these two proteins show some important differences in the primary structure (sequence) and electrostatic potential surfaces. Our studies provide a basis for unravelling the role of these differences in relation to the known differences in the receptor sites on the voltage sensitive sodium channel for the alpha- and beta-neurotoxins.

Epithelial peptide antibiotics

Surfaces of higher eukaryotes such as plants, invertebrates, and vertebrates, including humans, are normally covered with microorganisms but usually are not infected by them. The reason, apart from physical barriers, is the production of gene-encoded antimicrobial peptides by epithelial cells. Many novel antimicrobial peptides have been discovered recently in the epithelia of plants, insects, amphibians, and cattle, and, more recently, also in humans. In situ hybridization studies indicate a rather organ-specific expression of the genes for peptide antibiotics, which, due to their antimicrobial spectrum and conditions of expression, may also define the physiologic microflora. Some epithelial antimicrobial peptides are constitutively expressed; others are inducible, either by the presence of microorganisms via as of yet not well characterized elicitor receptors or by endogenous proinflammatory cytokines. Most antimicrobial peptides kill microorganisms by forming pores in the cell membrane, and the sensitivity of some peptide antibiotics towards cholesterol, a major mammalian cell membrane constituent, may indicate why these peptide antibiotics are not toxic for mammalian cells. Thus, it seems to be difficult for microorganisms to acquire resistance, making these peptides very attractive for therapeutic use as antibiotics. The first clinical studies are very promising, and after solving the problems of a large-scale biotechnical synthesis, which is more complicated due to the principally suicidal activity of these peptides, a number of new natural structure-based peptides may be developed. Furthermore, discovery of the inducibility of many antimicrobial peptides may also lead to the development of compounds that elicit epithelial defense reactions by stimulating the synthesis of endogenous peptide antibiotics.

Functional and structural features of gamma-zeathionins, a new class of sodium channel blockers

Gamma1- and gamma2-zeathionins (gamma1-Z and gamma2-Z) are members of a family of small and basic peptides involved in plant protection. These plant defensins exhibit remarkable structural similarity to scorpion neurotoxins and insect defensins. In the present report, we used the whole-cell patch clamp technique to investigate the inhibition of the sodium current (I(Na)) by gamma1-Z and gamma2-Z in the GH3 cell line. Both gamma1-Z and gamma2-Z rapidly and reversibly inhibited I(Na) without changing the kinetics or voltage dependence of activation or inactivation. To our knowledge, this is the first example of a plant protein that inhibits the sodium channel. From structural comparisons with the mu-conotoxins, a family of peptides that block the sodium channel, we detected some similar features that could provide the basis of inhibition of sodium channels by gamma-zeathionins.

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.

Membrane potential modulators: a thread of scarlet from plants to humans

The preservation along evolution of specific core motifs in proteins of diverse functions and taxonomic origins pinpoints a possible developmental advantage at the structural level. Such a preservation was observed in a group of membrane potential modulators including plant gamma-thionins, scorpion toxins, insect and scorpion defensins, bee venom apamin and MCD peptide, snake sarafotoxins, and human endothelins. These substances are short polypeptides of various lengths and nonhomologous sequences that affect organisms of distant phyla. Despite the structural differences, comparative analysis reveals commonality at three levels: 1) effect on membrane potential; 2) a common cysteine-stabilized alpha-helical (CSH) motif; and 3) similar gene organization (except for insect defensins), i.e., an intron that splits a codon toward the end of the leader sequence. We thus propose that these modulators, divided into two groups differing in their CSH motif orientation, have either diverged from two independent ancestors or have evolved by gene diversification via exon shuffling and subsequent modifications. To enforce protein synthesis through the secretory pathway and enable disulfide bond formation and secretion, insertion sites downstream of preexisting leader sequences have been a prerequisite. What seems advantageous for evolution, may also be exploited in attempts to 'accelerate evolution' by protein design using the conserved CSH core as a suitable scaffold for reshaping molecular exteriors.

Functional architectures of animal toxins: a clue to drug design?

Toxic proteins are produced by a diversity of venomous animals from various phyla. They are often of small size, possess a large density of disulfide bonds and exert multiple functions directed toward a variety of molecular targets, including a diversity of enzymes and ion channels. The aim of this brief and non-exhaustive review is three-fold. First, the structural context associated with the functional diversity of animal toxins is presented. Among various situations, it is shown that toxins with a similar fold can exert different functions and that toxins with unrelated folds can exert similar functions. Second, the functional sites of some animal toxins are presented. Their comparison shed light on how (i) distinct functions can be exerted by similarly folded toxins and (ii) similar functions can be shared by structurally distinct toxins. Third, it is shown that part of the functional site of foreign proteins can be grafted on an animal toxin scaffold, opening new perspectives in the domain of protein engineering.

Novel defensin subfamily from spinach (Spinacia oleracea)

Antimicrobial peptides (So-D1-7) were isolated from a crude cell wall preparation from spinach leaves (Spinacia oleracea cv. Matador) and, judged from their amino acid sequences, six of them (So-D2-7) represented a novel structural subfamily of plant defensins (group IV). Group-IV defensins were also functionally distinct from those of groups I-III. They were active at concentrations < 20 microM against Gram-positive (Clavibacter michiganensis) and Gram-negative (Ralstonia solanacearum) bacterial pathogens, as well as against fungi, such as Fusarium culmorum, F. solani, Bipolaris maydis, and Colletotrichum lagenarium. Fungal inhibition occurred without hyphal branching. Group-IV defensins were preferentially distributed in the epidermal cell layer of leaves and in the subepidermal region of stems.

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.

Evidence for a new class of scorpion toxins active against K+ channels

cDNAs encoding novel long-chain scorpion toxins (64 amino acid residues, including only six cysteines) were isolated from cDNA libraries produced from the venom glands of the scorpions Androctonus australis from Old World and Tityus serrulatus from New World. The encoded peptides were very similar to a recently identified toxin from T. serrulatus, which is active against the voltage-sensitive 'delayed-rectifier' potassium channel, but they were completely different from the long-chain and short-chain scorpion toxins already characterised. However, there was some sequence similarity (42%) between these new toxins, Aa TX Kbeta and Ts TX Kbeta, and scorpion defensins purified from the hemolymph of Buthidae scorpions Leiurus quinquestriatus and A. australis. Thus, according to a multiple sequence alignment using CLUSTAL, these new toxins seem to be related to the scorpion defensins.

Solution structure of the thermostable sweet-tasting protein brazzein

The fruit of Pentadiplandra brazzeana Baillon contains a small, sweet-tasting protein named brazzein. The structure of brazzein in solution was determined by proton nuclear magnetic resonance spectroscopy at pH 5.2 and 22 degrees C. The brazzein fold, which contains one alpha-helix and three strands of antiparallel beta-sheet, does not resemble that of either of the other two sweet-tasting proteins with known structures, monellin and thaumatin. Instead, the structure of brazzein resembles those of plant gamma-thionins and defensins and arthropod toxins. Sequence comparisons predict that members of a newly-identified family of serine proteinase inhibitors share the brazzein fold.

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.

Cecropin A-derived peptides are potent inhibitors of fungal plant pathogens

Cecropins are naturally occurring peptides that play an important role in the immune response of insects. Cecropin A-derived and cecropin A-melittin hybrid peptides, all smaller than the natural compound cecropin A, were synthesized and tested for their ability to inhibit growth of several agronomically important fungal pathogens. We found that an 11-amino-acid sequence, corresponding to the N-terminal amphipathic alpha-helix domain of cecropin A, exhibited antifungal activity. Differences in susceptibility of the various pathogens were observed, Phytophthora infestans being particularly sensitive to the shortened cecropin A peptides (IC50 = 2 x 10(-6) M). Biotoxicity of the shortest cecropin A-derived peptide was variously affected by the presence of proteins extracted from leaves of tobacco and tomato plants, either total extracts or intercellular fluids (ICFs). Overall, there was a greater tolerance to tomato protein extracts than to tobacco extracts. These findings suggest that tobacco should not be used as a model for testing the possible protective effects of transgenically expressed, cecropin-based genes. The feasibility of tailoring cecropin A genes to enhance crop protection in particular plant/fungus combinations is discussed.

Cationic peptides: a new source of antibiotics

Antimicrobial cationic peptides are an important component of the innate defenses of all species of life. Different peptides may have antibacterial, antiendotoxic, antibiotic-potentiating or antifungal properties, and so they are being developed for use as a novel class of antimicrobial agents and as the basis for making transgenic disease-resistant plants and animals.

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.

Snake venom cardiotoxins-structure, dynamics, function and folding

Snake cardiotoxins are highly basic (pI > 10) small molecular weight (approximately 6.5 kDa), all beta-sheet proteins. They exhibit a broad spectrum of interesting biological activities. The secondary structural elements in these toxins include antiparallel double and triple stranded beta-sheets. The three dimensional structures of these toxins reveal an unique asymmetric distribution of the hydrophobic and hydrophilic amino acids. The 3D structures of closely related snake venom toxins such as neurotoxins and cardiotoxin-like basic proteins (CLBP) fail to show similar pattern(s) in the distribution of polar and nonpolar residues. Recently, many novel biological activities have been reported for cardiotoxins. However, to-date, there is no clear structure-function correlation(s) available for snake venom cardiotoxins. The aim of this comprehensive review is to summarize and critically evaluate the progress in research on the structure, dynamics, function and folding aspects of snake venom cardiotoxins.

Solution structure of drosomycin, the first inducible antifungal protein from insects

Drosomycin is the first antifungal protein characterized recently among the broad family of inducible peptides and proteins produced by insects to respond to bacterial or septic injuries. It is a small protein of 44 amino acid residues extracted from Drosophila melanogaster that exhibits a potent activity against filamentous fungi. Its three-dimensional structure in aqueous solution was determined using 1H 2D NMR. This structure, involving an alpha-helix and a twisted three-stranded beta-sheet, is stabilized by three disulfide bridges. The corresponding Cysteine Stabilized alpha beta (CS alpha beta) motif, which was found in other defense proteins such as the antibacterial insect defensin A, short- and long-chain scorpion toxins, as well as in plant thionins and potent antifungal plant defensins, appears as remarkably persistent along evolution.

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 natural larger form of the antifungal protein (AFP) from Aspergillus giganteus

Two major proteins, alpha-sarcin and an antifungal polypeptide (AFP), are secreted by the mould Aspergillus giganteus MDH 18894 when it is cultured for 70-80 h. A third major protein is also found in the extracellular medium at 48-60 h, but it disappears as the culture proceeds. This protein has been isolated and characterized in terms of apparent molecular mass, electrophoretic and chromatographic behaviour, NH2-terminal primary structure, amino acid content, spectroscopical features, reactivity against anti-AFP antibodies, and antifungal activity. Based on the obtained results it would be an extracellular inactive precursor form of AFP, designated as the large form of AFP (lf-AFP). Its amino acid composition is identical to that of AFP but containing six extra residues. NH2-terminal sequence analysis of the first eight amino acid residues of this polypeptide revealed that the extra residues can be perfectly accommodated within the DNA-deduced sequence of the precursor form of AFP. Its alignment with precursor sequences of different proteins, secreted by a variety of Aspergillus spp., reveals the existence of a common tetrapeptide at the carboxy-terminal end of their leader peptides. This sequence would be Ile/Leu-Xaa-Yaa-Arg, being mostly Xaa and Yaa an acid residue (Asp/Glu) and alanine, respectively. The presence of lf-AFP as an extracellular protein would be in perfect agreement with the existence of this tetrapeptide motif, that can be involved in the protein secretion mechanisms of filamentous fungi.

Solution structure of Lqh-8/6, a toxin-like peptide from a scorpion venom--structural heterogeneity induced by proline cis/trans isomerization

Lqh-8/6 is a minor fraction isolated from the venom of the scorpion Leiurus quinquestriatus hebraeus. Here we describe the purification, amino acid sequencing and solution structure determination by NMR and molecular modeling of this peptide. Lqh-8/6 is a small polypeptide (38 residues) which contains 8 half-cystines and is highly similar to another venom component, chlorotoxin. Standard homonuclear methods were used to sequentially assign the proton NMR spectra and to collect spatial restraints for structure determination. Two populations, identified early in the assignment step, are in slow interconversion on the NMR timescale. The two conformers were shown to originate from a cis/trans peptidyl-prolyl isomerization. Using a distance geometry program and simulated annealing protocol under the NMR restraints we obtained 10 final structures for the major conformation (trans isomer). None of the structures showed NOE violations larger than 0.05 nm, and the rmsd value relative to the mean structure (considering the main chain atoms in well-defined secondary structure) is 0.07 nm. The three-dimensional structure contains a short alpha-helix strapped on a small antiparallel beta-strand and an N-terminal extended fragment. The sequence/structure and structure/function relationships of the new scorpion toxin-like peptide are discussed in the context of the present structure determination. This toxin shows a stable, highly populated cis conformer of a peptidyl-prolyl peptide bond.

Fabatins: new antimicrobial plant peptides

Two new antimicrobial peptides related to the gamma-thionine family have been isolated by acid extraction from the broad bean Vicia faba. The extract was separated by ion exchange chromatography, and a fraction showing antibacterial activity was further purified by reverse-phase HPLC. Material from a single HPLC peak was sequenced and revealed the presence of two peptides differing by one amino acid. The peptides were named fabatins. They are 47 amino acids long, have an overall positive charge and contain 8 cysteines that probably form 4 disulfide bridges characteristic of the gamma-thionins. Fabatins were active against both Gram-negative and Gram-positive bacteria, but were inactive against the yeasts Saccharomyces cerevisiae and Candida albicans.

Mutational analysis of a plant defensin from radish (Raphanus sativus L.) reveals two adjacent sites important for antifungal activity

Mutational analysis of Rs-AFP2, a radish antifungal peptide belonging to a family of peptides referred to as plant defensins, was performed using polymerase chain reaction-based site-directed mutagenesis and yeast as a system for heterologous expression. The strategy followed to select candidate amino acid residues for substitution was based on sequence comparison of Rs-AFP2 with other plant defensins exhibiting differential antifungal properties. Several mutations giving rise to peptide variants with reduced antifungal activity against Fusarium culmorum were identified. In parallel, an attempt was made to construct variants with enhanced antifungal activity by substituting single amino acids by arginine. Two arginine substitution variants were found to be more active than wild-type Rs-AFP2 in media with high ionic strength. Our data suggest that Rs-AFP2 possesses two adjacent sites that appear to be important for antifungal activity, namely the region around the type VI beta-turn connecting beta-strands 2 and 3, on the one hand, and the region formed by residues on the loop connecting beta-strand 1 and the alpha-helix and contiguous residues on the alpha-helix and beta-strand 3, on the other hand. When added to F. culmorum in a high ionic strength medium, Rs-AFP2 stimulated Ca2+ uptake by up to 20-fold. An arginine substitution variant with enhanced antifungal activity caused increased Ca2+ uptake by up to 50-fold, whereas a variant that was virtually devoid of antifungal activity did not stimulate Ca2+ uptake.

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

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

Endogenous vertebrate antibiotics. Defensins, protegrins, and other cysteine-rich antimicrobial peptides

Although newly recognized, endogenous cystine-stabilized beta-sheet antimicrobial peptides have ancient origins. These peptides can arm circulating phagocytes and cells of the gastrointestinal, respiratory, and genitourinary tracts to resist invasion by bacteria, mycobacteria, fungi, and enveloped viruses. Defensins and protegrin-like peptides are likely to play a considerable role in innate immunity and may provide molecular templates that can be used to generate novel antibiotics for topical and systemic use.

Primary structure of omega-hordothionin, a member of a novel family of thionins from barley endosperm, and its inhibition of protein synthesis in eukaryotic and prokaryotic cell-free systems

A new sulfur-rich basic polypeptide, so called omega-hordothionin, has been isolated from barley endosperm by extractions with NaCl and ammonium bicarbonate followed by reverse-phase high performance liquid chromatography. Purified omega-hordothionin was found to be homogeneous by SDS/polyacrylamide gel electrophoresis, N-terminal amino-acid sequencing and electrospray-ionization mass spectrometric analysis. The complete primary structure of omega-hordothionin was determined by automatic degradation of the intact molecule and peptides obtained by proteolytic cleavage. Omega-hordothionin consists of a single polypeptide chain of 48 amino acids with a molecular mass of 5508 Da deduced from its amino acid sequence, which fully coincides with the 5508.2 Da determined by electrospray-ionization mass spectrometry. The isolated polypeptide showed a characteristic composition with a high content of basic amino acids (five arginine residues, two lysine residues and six histidine residues) and eight cysteine residues, and has strong sequence identity (66%) with the sorghum SI alpha 1 alpha-amylase inhibitor. Omega-hordothionin, like gamma-hordothionin, exhibited translation inhibitory activity on both eukaryotic cell-free systems from mammalian (rat liver and rabbit reticulocyte lysates) and prokaryotic cell-free systems (Escherichia coli). However, in contrast to gamma-hordothionin, omega-hordothionin did not inhibit plant systems such as Triticum aestivum, Cucumis sativus, Vicia sativa and Hordeum vulgare. Gamma-hordothionin also inhibited the alpha-amylase activity from human saliva, while omega-hordothionin and the other different genetic variants of thionins, alpha-hordothionin and beta-hordothionin, failed to show any inhibitory effect.

Fungal membrane responses induced by plant defensins and thionins

Treatment of hyphae of Neurospora crassa with antifungal plant defensins, i.e. Rs-AFP2 and Dm-AMP1 isolated from radish and dahlia seed, respectively, induced a rapid K+ efflux, Ca2+ uptake, and alkalinization of the incubation medium. The Rs-AFP2-induced alkalinization of the incubation medium could be inhibited with G-protein inhibitors. alpha-Hordothionin, an antifungal thionin from barley seed, caused a sustained increased Ca2+ uptake at subinhibitory concentrations but only a transient increased uptake at inhibitory concentrations. alpha-Hordothionin also caused increased K+ efflux and alkalinization of the medium, but these fluxes occurred more rapidly compared to those caused by plant defensins. Furthermore, alpha-hordothionin caused permeabilization of fungal hyphae to the non-metabolite alpha-aminoisobutyric acid and, in addition, altered the electrical properties of artificial lipid bilayers, consistently leading to rupture of the lipid bilayers. The plant defensins did not form ion-permeable pores in artificial membranes and did not exhibit substantial hyphal membrane permeabilization activity. Our results are consistent with the notion that thionins inhibit fungal growth as a result of direct protein-membrane interactions, whereas plant defensins might act via a different, possibly receptor-mediated, mechanism.

Purification and sequencing of multiple forms of Brassica napus seed napin large chains that are calmodulin antagonists and substrates for plant calcium-dependent protein kinase

Six napin large (L) chains (as well as six napin small chains) were resolved from the seeds of kohlrabi (Brassica napus var. rapifera) by a procedure involving extraction, batchwise elution from carboxymethylcellulose (CM52) and reversed-phase HPLC after treatment with guanidine hydrochloride and 2-mercaptoethanol. The precise average molecular masses of the circa 4.5 kDa small subunits and the circa 10 kDa large subunits were determined by electrospray ionisation mass spectrometry (ESMS). Of six large subunits resolved (L1A), L1B, L1C, L2A, L2B and L2C), the complete amino acid sequences of four (L1A, L2A, L2B and L2C) and the near-complete sequences of two (L1B and L1C) were deduced from the ESMS-based masses of tryptic fragments, Edman sequencing and previously published data. The deduced structures are precisely consistent with this data and with the ESMS-based average molecular masses of these polypeptides. ESMS analysis of unreduced napin extract revealed only seven circa 14.5 kDa complexes, the observed masses being in close agreement with those calculated for 1:1 complexes of particular small and large subunits assuming four disulfides in each napin complex. The structures of the napin large subunits (86-91 residues) are very similar and all amino acid differences observed are confined to only 25 positions. The L2A, L2B AND L2C large chains (but not the L1A, L1B and L1C large chains) are phosphorylated well by plant Ca2+-dependent protein kinase (CDPK). The CDPK-catalyzed phosphorylation site on the large chain L2A is inferred to be S57 within the sequence LQQVIS57RIYQT (the site being S60 within the same sequence in L2B and L2C). The napin-containing basic protein fraction from B. napus seeds largely abolishes the Ca2+-dependent fluorescence enhancement of dansyl-calmodulin and also inhibits calmodulin (CaM)-dependent myosin light chain kinase (MLCK). The resolved napin long chains also inhibit MLCK. Each kohlrabi large chain contains 2 sequences (corresponding to L(10)-Q(20) and Q(51)-L(64) of L1A) which have the potential to form amphipathic alpha-helices. Each large chain also contains a Q-rich 19 amino acid sequence (corresponding to L(30)-Q(48) of L1A) which has the potential to form a '2-sided' alpha-helix with basic residues confined to one side. These structural elements may be involved in the inferred interaction of these proteins with CaM and may be relevant to the biological activity of antifungal proteins of this kind.

Purification and sequencing of multiple forms of Brassica napus seed napin small chains that are calmodulin antagonists and substrates for plant calcium-dependent protein kinase

Six napin small (S) subunits and six napin large (L) subunits were resolved from the seeds of kohlrabi (Brassica napus var. rapifera) by a procedure involving extraction, batchwise elution from carboxymethylcellulose (CM52) and reverse-phase HPLC after treatment with guanidine hydrochloride and 2-mercaptoethanol. The precise average molecular masses of the ca. 4.5 kDa small subunits and the ca. 10 kDa large subunits were determined by electrospray ionisation mass spectrometry (ESMS). The amino-acid sequences of six small subunits (S1A, S1B, S2, S3A, S3B and S4) were deduced from the ESMS-based masses of tryptic fragments, Edman sequencing and previously published data. The deduced structures were precisely consistent with this data and with the ESMS-based average molecular masses of these polypeptides. The structures of the small subunits (39-41 residues) are very similar with variations involving single substitutions at or near the N-terminus and 1 to 3 changes within the last 7 amino acids. Particular B. napus small and large chains are phosphorylated by plant Ca2+-dependent protein kinase (CDPK). The best site of phosphorylation on small chains is inferred to be either S34 or S39 of S1B. The napin-containing basic protein fraction from B. napus seeds largely abolishes the Ca2+-dependent fluorescence enhancement of dansyl-calmodulin and also inhibits calmodulin (CaM)-dependent myosin light chain kinase (MLCK). The resolved napin small chains also inhibit MLCK. All of the kohlrabi napin small chains, as well as homologous Brassicaceae small chains, have a central 23 amino-acid sequence that can potentially form an alpha-helix in which all the basic residues are located on one side. This structural element may be involved in the interaction of these proteins with CaM and the biological activity of antifungal proteins of this kind.

Purification and mass spectrometry-based sequencing of yellow mustard (Sinapis alba L.) 6 kDa proteins. Identification as antifungal proteins

Three basic proteins, M1, M2A and M2B, that are substrates for plant Ca(2+)-dependent protein kinase (CDPK) were purified from seeds of yellow mustard (Sinapis alba L.) by a protocol involving batchwise chromatography on carboxymethylcellulose (CM52), cation-exchange HPLC on an SP5PW column and reversed-phase HPLC on a C18 column. The complete amino-acid sequences of these proteins have been determined employing Edman sequencing and electrospray ionization mass spectrometry (ESMS) applied to the proteins and their tryptic and chymotryptic fragments. M1 (observed mass 5676.8 +/- 1.0 Da; calculated mass 5677.57 Da), M2A (observed mass 5704.8 +/- 0.8 Da; calculated mass 5704.60 Da) and M2B (observed mass 5839.5 +/- 1.2 Da; calculated mass 5838.78 Da) have been identified as gamma-thionins, which are potent antifungal proteins. M1, M2A and M2B are phosphorylated by plant CDPK on Ser residues, the site of phosphorylation on M2A being S8 as directly confirmed by Edman sequencing and mass spectrometry of the chymotryptically generated phosphopeptide CQRPS(HPO3)GTW11. M1 and M2A have apparent calmodulin (CaM) antagonist activity with IC50 values of 4.8 +/- 1.3 microM and 5.5 +/- 1.5 microM, respectively, for inhibition of CaM-dependent myosin light chain kinase (MLCK). M2A and/or M2B interacts with dansyl-CaM in both the presence and absence of calcium.

Isolation and characterization of a cDNA encoding a plant defensin-like protein from roots of Norway spruce

We are studying the interaction between the roots of Norway spruce seedlings (Picea abies L. Karst) and a highly pathogenic isolate of Pythium dimorphum. Here, we report the isolation of a cDNA from spruce roots encoding a protein with high sequence similarity to plant defensins, designated as SPI1 (Spruce Pathogen Induced No.1). The transcript hybridizing to the SPII cDNA probe is highly induced in uninfected roots when the seedlings are transferred from solid to liquid incubating malt medium (hypoxic conditions). However, when the seedlings are transferred from solid to liquid malt media containing a saturating amount of P. dimorphum hyphae, the amount of transcript is unchanged the first day after infection, but then decreases on day 1, and is no longer detectable 2 days after infection. Since plant defensins may play a role in plant defence, their negative regulation upon infection might reflect a strategy employed by this pathogenic fungus to evade the effect of toxic gene products.

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.

Determination of the three-dimensional structure of hordothionin-alpha by nuclear magnetic resonance

The high-resolution three-dimensional solution structure of the plant toxin hordothionin-alpha obtained from korean barley was determined by using two-dimensional NMR techniques combined with distance geometry and restrained molecular dynamics. Experimentally derived restraints including 292 interproton distances from nuclear Overhauser effect measurements, 16 hydrogen bond restraints together with four disulphide bridge restraints were used as input to calculations of distance geometry and restrained molecular dynamics. Also included in the calculations were 36 phi and 17 chi 1 torsion angles obtained from 33JHN alpha and 3J alpha beta coupling constants in double quantum filtered COSY and primitive exclusive COSY experiments, respectively. The overall protein fold is similar to crambin and purothionin-alpha 1. Two alpha-helices running in opposite directions are found on the basis of 3JHN alpha and 3J alpha beta and deuterium exchange rates for backbone NH protons, and encompass residues 7-18 and 22-28. These two helices are connected by a turn and form a 'helix-turn-helix' motif. A short stretch of an anti-parallel beta-sheet exists between residues 1-4 and 31-34. the two protein termini of hordothionin-alpha are 'well-anchored'; the N-terminus of the protein is immobilized by this short beta-sheet whereas the C-terminus is 'pasted' to the carbonyl group of Cys-4 by a very stable hydrogen bond. The average root-mean-square differences for the backbone and heavy atoms after the restrained molecular dynamics calculations are 0.62 and 1.16 A respectively. These numbers represent a significant improvement over the corresponding values for the previous NMR structures of other thionins. The distance violation from the experimental interproton distances for the final structures is 0.14 for all atoms.

1H-nmr studies on the structure of a new thionin from barley endosperm

A new thionin from barley, omega-hordothionin, has been shown to exist in aqueous solution as a mixture of two different isoforms in a 3:2 ratio, as revealed by a complete analysis of its two-dimensional 1H-nmr spectra. The conformational heterogeneity arises from cis-trans isomerism about the Phe 12-Pro 13 peptide bond, where the major form corresponds to the cis conformation. The complete assignment of chemical shifts and nuclear Overhauser effects (NOEs) of the two isoforms allows a detailed comparative analysis of their conformational properties, even though a complete calculation of their solution structures is not possible because of a somewhat limited number of NOE constraints. Structures for the two isomers could be modeled, however, on the basis of the high structural homology between omega-hordothionin and related gamma-thionins, and under the conditions of satisfying all observed experimental data. The two isoforms adopt practically identical global folds and the structural changes imposed by cis-trans isomerization are confined to the region proximal to Pro 13. The cis-trans isomerism occurs in a conserved loop connecting the first beta-strand of the triple-stranded antiparallel beta-sheet and the alpha-helix. A comparative analysis of the sequences of this loop in the different thionins suggests that the cis-trans equilibrium about the X-Pro peptide bond depends on the size of the side chain of X (X = Gly in gamma-thionins and Phe in omega-thionin). The structural homology of this new thionin with gamma-thionins as well as with some scorpion toxins and insect defensins suggests that these proteins may share a common mode of functional activity.

Scorpion toxins as natural scaffolds for protein engineering

A compact, well-organized, and natural motif, stabilized by three disulfide bonds, is proposed as a basic scaffold for protein engineering. This motif contains 37 amino acids only and is formed by a short helix on one face and an antiparallel triple-stranded beta-sheet on the opposite face. It has been adopted by scorpions as a unique scaffold to express a wide variety of powerful toxic ligands with tuned specificity for different ion channels. We further tested the potential of this fold by engineering a metal binding site on it, taking the carbonic anhydrase site as a model. By chemical synthesis we introduced nine residues, including three histidines, as compared to the original amino acid sequence of the natural charybdotoxin and found that the new protein maintains the original fold, as revealed by CD and 1H NMR analysis. Cu2+ ions are bound with Kd = 4.2 x 10(-8) M and other metals are bound with affinities in an order mirroring that observed in carbonic anhydrase. The alpha/beta scorpion motif, small in size, easily amenable to chemical synthesis, highly stable, and tolerant for sequence mutations represents, therefore, an appropriate scaffold onto which polypeptide sequences may be introduced in a predetermined conformation, providing an additional means for design and engineering of small proteins.

Nature and regulation of pistil-expressed genes in tomato

The specialized reproductive functions of angiosperm pistils are dependent in part upon the regulated activation of numerous genes expressed predominantly in this organ system. To better understand the nature of these pistil-predominant gene products we have analyzed seven cDNA clones isolated from tomato pistils through differential hybridization screening. Six of the seven cDNAs represent sequences previously undescribed in tomato, each having a unique pistil- and/or floral-predominant expression pattern. The putative protein products encoded by six of the cDNAs have been identified by their similarity to sequences in the database of previously sequenced genes, with a seventh sequence having no significant similarity with any previously reported sequence. Three of the putative proteins appear to be targeted to the endomembrane system and include an endo-beta-1,4-glucanase which is expressed exclusively in pistils at early stages of development, and proteins similar in sequence to gamma-thionin and miraculin which are expressed in immature pistils and stamens, and in either sepals or petals, respectively. Two other clones, similar in sequence to each other, were expressed primarily in immature pistils and stamens and encode distinct proteins with similarity to leucine aminopeptidases. An additional clone, which encodes a protein similar in sequence to the enzyme hyoscyamine 6-beta-hydroxylase and to other members of the family of Fe2+/ascorbate-dependent oxidases, was expressed at high levels in pistils, stamens and sepals, and at detectable levels in some vegetative organs. Together, these observations provide new insight into the nature and possible functional roles of genes expressed during reproductive development.

Refined three-dimensional solution structure of insect defensin A

BACKGROUND

Insect defensin A is a basic 4 kDa protein secreted by Phormia terranovae larvae in response to bacterial challenges or injuries. Previous biological tests suggest that the bacterial cytoplasmic membrane is the target of defensin A. The structural study of this protein is the first step towards establishing a structure-activity relationship and forms the basis for understanding its antibiotic activity at the molecular level.

RESULTS

We describe a refined model of the three-dimensional structure of defensin A derived from an extensive analysis of 786 inter-proton nuclear Overhauser effects. The backbone fold involves an N-terminal loop and an alpha-helical fragment followed by an antiparallel beta-structure. The helix and the beta-structure are connected by two of the three disulphide bridges present in defensin A, forming a so-called 'cysteine-stabilized alpha beta' (CS alpha beta) motif. The N-terminal loop, which is locally well defined, can occupy different positions with respect to the other moieties of the molecule.

CONCLUSIONS

The CS alpha beta motif, which forms the core of the defensin A structure, appears to be a common organization for several families of small proteins with toxic properties. The distribution of amino acid side chains in the protein structure creates several hydrophobic or hydrophilic patches. This leads us to propose that the initial step in the action of positively charged defensin A molecules with cytoplasmic membranes may involve interactions with acidic phospholipids.

A common structural motif incorporating a cystine knot and a triple-stranded beta-sheet in toxic and inhibitory polypeptides

A common structural motif consisting of a cystine knot and a small triple-stranded beta-sheet has been defined from comparison of the 3-dimensional structures of the polypeptides omega-conotoxin GVIA (Conus geographus), kalata BI (Oldenlandia affinis DC), and CMTI-I (Curcurbita maxima). These 3 polypeptides have diverse biological activities and negligible amino acid sequence identity, but each contains 3 disulfide bonds that give rise to a cystine knot. This knot consists of a ring formed by the first 2 bonds (1-4 and 2-5) and the intervening polypeptide backbone, through which the third disulfide (3-6) passes. The other component of this motif is a triple-stranded, anti-parallel beta-sheet containing a minimum of 10 residues, XXC2, XC5X, XXC6X (where the numbers on the half-cysteine residues refer to their positions in the disulfide pattern). The presence in these polypeptides of both the cysteine knot and antiparallel beta-sheet suggests that both structural features are required for the stability of the motif. This structural motif is also present in other protease inhibitors and a spider toxin. It appears to be one of the smallest stable globular domains found in proteins and is commonly used in toxins and inhibitors that act by blocking the function of larger protein receptors such as ion channels or proteases.

Expression of functional Raphanus sativus antifungal protein in yeast

Rs-AFP2 is a 51 amino acid cysteine-rich peptide isolated from radish (Raphanus sativus) seeds that exhibits potent inhibitory activity against filamentous fungi. A cDNA clone encoding the Rs-AFP2 preprotein was modified by recombinant DNA methods to allow expression in the yeast Saccharomyces cerevisiae. This peptide was expressed in yeast as a fusion protein carrying at its N-terminus the prepro-sequences derived from the precursor of the yeast pheromone mating factor alpha 1. These sequences allow secretion of the biologically active peptide in a correctly processed form. Deletion of the mating factor alpha 1 pro-peptide drastically reduced the expression level of the peptide.

Production of active, insect-specific scorpion neurotoxin in yeast

A cDNA encoding the Androctonus australis Hector insect toxin 1 (AaH IT1) was expressed in yeast leading to secretion of fully biologically active protein. Three different multicopy plasmids were constructed using PCR. Expression was directed by the strong PGK1 promoter of the yeast vector pMA 91. Plasmid pMA 91-AaH IT1 encodes AaH IT1 and its own signal peptide. In the two other constructions, the cDNA encoding the mature part of AaH IT1 is fused to the prepro-signal sequence of the yeast alpha-mating-factor precursor; the pBAL 7-alpha-KREAEA-AaH IT1 includes the cDNA sequence encoding the KR(EAEA) processing sequence of the alpha-mating factor, and pBAL 7-alpha-KR-AaH IT1 encodes the KR fused directly to the AaH IT1 gene. The yeast alpha-mating-factor signal peptide launched the pro-alpha-mating-factor-AaH IT1 fusion protein into the secretory pathway. The fusion proteins are expected to be cleaved in the Golgi by the KEX2 endopeptidase and the STE13 dipeptidyl aminopeptidase, leading to release of mature AaH IT1. Pulse/chase labelling of transformed yeast protoplasts, followed by SDS/PAGE analysis of proteins immunoprecipitated from either the lysate or the extracellular fluid, showed that AaH IT1 was produced. The highest concentration of recombinant AaH IT1 in the culture medium, as determined using a 125I-AaH IT1 specific radioimmunoassay, was 4 micrograms/l (0.5 nM). The recombinant toxin was fully biologically active against cockroaches as assessed by injection and comparison to native AaH IT1. Moreover, it competed with radiolabelled native toxin for its receptor on the voltage-sensitive Na+ channel with a dissociation constant of 0.5 nM.

Solution structure of the variant-3 neurotoxin from Centruroides sculpturatus Ewing

The solution structure of the CsE-v3 neurotoxin from the venom of the North American scorpion Centruroides sculpturatus Ewing (CsE) has been determined by a hybrid refinement procedure that employed distance geometry and dynamical simulated annealing. Distance constraints deduced from the nuclear Overhauser effect spectroscopy data and torsion angle constraints deduced from the vicinal coupling constant data were used in the refinement procedure. A family of simulated annealing structures that showed no constraint violations was generated. The energy-minimized average structure exhibited root-mean-square deviations of 0.121 nm for the backbone and 0.182 nm for all atoms, with respect to this family. These studies confirm the previously qualitative NMR findings about the secondary structural features, viz. the presence of a short alpha-helix composed of residues 23-31 and an antiparallel beta-sheet composed of the strands of residues 1-5, 45-50 and 36-42. A cluster of aromatic ring systems is located on one side of the protein. The solution and crystal structures have similar overall features, but show some minor differences.

Oxidative folding of cystine-rich peptides vs regioselective cysteine pairing strategies

The methodology of regioselective cysteine pairings in synthetic multiple-cystine peptides has progressed in the past years to an efficiency that allows for at least three specific inter- and intrachain disulfide bridgings. Conformational studies on various multiple-cystine peptides like hormones, protease inhibitors, and toxins revealed that these bioactive peptides, generated by posttranslational processing of precursor proteins, are folded into miniprotein-like compact globular structures of remarkable stability. This strongly suggests protein domain or subdomain properties of these families of peptides, and thus sufficient sequence-encoded information for correct oxidative refolding under appropriate experimental conditions. From intensive research on the mechanisms and pathways of oxidative refolding of proteins in vivo and in vitro, the efficient methods have emerged for simulating nature in the regeneration of native folds not only for intact proteins, but also for protein domains and subdomains. In fact, the results obtained in the oxidative folding of excised protein fragments and of relatively low mass products of posttranslational processings show that this procedure is indeed a simple way of preparing peptides with several disulfide bonds, if optimization of reaction conditions is performed in terms of redox buffer, temperature, and additives capable of disrupting aggregates and of stabilizing nascent secondary structures. Moreover, with increased knowledge about stable, small natural cystine frameworks, their use instead of artificial templates should facilitate engineering of synthetic miniproteins with specific conformation and tailored functions.