Do structural deviations between toxins adopting the same fold reflect functional differences?

A consensus recombinant elapid long-chain α-neurotoxin and how protein folding matters for antibody recognition and neutralization of elapid venoms

Antivenoms are essential in the treatment of the neurotoxicity caused by elapid snakebites. However, there are elapid neurotoxins, e.g., long-chain α-neurotoxins (also known as long-chain three-finger toxins) that are barely neutralized by commercial elapid antivenoms; so, recombinant elapid neurotoxins could be an alternative or complements for improving antibody production against the lethal long-chain α-neurotoxins from elapid venoms. This work communicates the expression of a recombinant long-chain α-neurotoxin, named HisrLcNTx or rLcNTx, which based on the most lethal long-chain α-neurotoxins reported, was constructed de novo. The gene of rLcNTx was synthesized and introduced into the expression vector pQE30, which contains a proteolytic cleavage region for exscinding the mature protein, and His residues in tandem for affinity purification. The cloned pQE30/rLcNTx was transfected into Escherichia coli Origami cells to express rLcNTx. After expression, it was found in inclusion bodies, and folded in multiple Cys-Cys structural isoforms. To observe the capability of those isoforms to generate antibodies against native long-chain α-neurotoxins, groups of rabbits were immunized with different cocktails of Cys-Cys rLcNTx isoforms. In vitro, and in vivo analyses revealed that rabbit antibodies raised against different rLcNTx Cys-Cys isoforms were able to recognize pure native long-chain α-neurotoxins and their elapid venoms, but they were unable to neutralize bungarotoxin, a classical long-chain α-neurotoxin, and other elapid venoms. The rLcNTx Cys-Cys isoform 2 was the immunogen that produced the best neutralizing antibodies in rabbits. Yet to neutralize the elapid venoms from the black mamba Dendroaspis polylepis, and the coral shield cobra Aspidelaps lubricus, it was required to use two types of antibodies, the ones produced using rLcNTx Cys-Cys isoform 2 and antibodies produced using short-chain α-neurotoxins. Expression of recombinant elapid neurotoxins as immunogens could be an alternative to improve elapid antivenoms; nevertheless, recombinant elapid neurotoxins must be well-folded to be used as immunogens for obtaining neutralizing antibodies.

Blocking of negative charged carboxyl groups converts Naja atra neurotoxin to cardiotoxin-like protein

Naja atra cobrotoxin and cardiotoxin 3 (CTX3) exhibit neurotoxicity and cytotoxicity, respectively. In the present study, we aimed to investigate whether the carboxyl groups of cobrotoxin play a role in structural constraints, thereby preventing cobrotoxin from exhibiting cytotoxic activity. Six of the seven carboxyl groups in cobrotoxin were conjugated with semicarbazide. Measurement of circular dichroism spectra and Trp fluorescence quenching showed that the gross conformation of semicarbazide-modified cobrotoxin (SEM-cobrotoxin) and cobrotoxin differed. In sharp contrast to cobrotoxin, SEM-cobrotoxin demonstrated membrane-damaging activity and cytotoxicity, which are feature more characteristic of CTX3. Furthermore, both SEM-cobrotoxin and CTX3 induced cell death through AMPK activation. Analyses of the interaction between polydiacetylene/lipid vesicles and fluorescence-labeled lipids revealed that SEM-cobrotoxin and cobrotoxin adopted different membrane-bound states. The structural characteristics of SEM-cobrotoxin were similar to those of CTX3, including trifluoroethanol (TFE)-induced structural transformation and membrane binding-induced conformational change. Conversely, cobrotoxin was insensitive to the TFE-induced effect. Collectively, the data of this study indicate that blocking negatively charged residues confers cobrotoxin with membrane-damaging activity and cytotoxicity. The findings also suggest that the structural constraints imposed by carboxyl groups control the functional properties of snake venom α-neurotoxins during the divergent evolution of snake venom neurotoxins and cardiotoxins.

Snake three-finger α-neurotoxins and nicotinic acetylcholine receptors: molecules, mechanisms and medicine

Snake venom three-finger α-neurotoxins (α-3FNTx) act on postsynaptic nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction (NMJ) to produce skeletal muscle paralysis. The discovery of the archetypal α-bungarotoxin (α-BgTx), almost six decades ago, exponentially expanded our knowledge of membrane receptors and ion channels. This included the localisation, isolation and characterization of the first receptor (nAChR); and by extension, the pathophysiology and pharmacology of neuromuscular transmission and associated pathologies such as myasthenia gravis, as well as our understanding of the role of α-3FNTxs in snakebite envenomation leading to novel concepts of targeted treatment. Subsequent studies on a variety of animal venoms have yielded a plethora of novel toxins that have revolutionized molecular biomedicine and advanced drug discovery from bench to bedside. This review provides an overview of nAChRs and their subtypes, classification of α-3FNTxs and the challenges of typifying an increasing arsenal of structurally and functionally unique toxins, and the three-finger protein (3FP) fold in the context of the uPAR/Ly6/CD59/snake toxin superfamily. The pharmacology of snake α-3FNTxs including their mechanisms of neuromuscular blockade, variations in reversibility of nAChR interactions, specificity for nAChR subtypes or for distinct ligand-binding interfaces within a subtype and the role of α-3FNTxs in neurotoxic envenomation are also detailed. Lastly, a reconciliation of structure-function relationships between α-3FNTx and nAChRs, derived from historical mutational and biochemical studies and emerging atomic level structures of nAChR models in complex with α-3FNTxs is discussed.

Fulditoxin, representing a new class of dimeric snake toxins, defines novel pharmacology at nicotinic ACh receptors

BACKGROUND AND PURPOSE

Animal toxins have contributed significantly to our understanding of the neurobiology of receptors and ion channels. We studied the venom of the coral snake Micrurus fulvius fulvius and identified and characterized the structure and pharmacology of a new homodimeric neurotoxin, fulditoxin, that exhibited novel pharmacology at nicotinic ACh receptors (nAChRs).

EXPERIMENTAL APPROACH

Fulditoxin was isolated by chromatography, chemically synthesized, its structure determined by X-ray crystallography, and its pharmacological actions on nAChRs characterized by organ bath assays and two-electrode voltage clamp electrophysiology.

KEY RESULTS

Fulditoxin's distinct 1.95-Å quaternary structure revealed two short-chain three-finger α-neurotoxins (α-3FNTxs) non-covalently bound by hydrophobic interactions and an ability to bind metal and form tetrameric complexes, not reported previously for three-finger proteins. Although fulditoxin lacked all conserved amino acids canonically important for inhibiting nAChRs, it produced postsynaptic neuromuscular blockade of chick muscle at nanomolar concentrations, comparable to the prototypical α-bungarotoxin. This neuromuscular blockade was completely reversible, which is unusual for snake α-3FNTxs. Fulditoxin, therefore, interacts with nAChRs by utilizing a different pharmacophore. Unlike short-chain α-3FNTxs that bind only to muscle nAChRs, fulditoxin utilizes dimerization to expand its pharmacological targets to include human neuronal α4β2, α7, and α3β2 nAChRs which it blocked with IC₅₀ values of 1.8, 7, and 12 μM respectively.

CONCLUSIONS AND IMPLICATIONS

Based on its distinct quaternary structure and unusual pharmacology, we named this new class of dimeric Micrurus neurotoxins represented by fulditoxin as Σ-neurotoxins, which offers greater insight into understanding the interactions between nAChRs and peptide antagonists.

Short-chain consensus alpha-neurotoxin: a synthetic 60-mer peptide with generic traits and enhanced immunogenic properties

The three-fingered toxin family and more precisely short-chain α-neurotoxins (also known as Type I α-neurotoxins) are crucial in defining the elapid envenomation process, but paradoxically, they are barely neutralized by current elapid snake antivenoms. This work has been focused on the primary structural identity among Type I neurotoxins in order to create a consensus short-chain α-neurotoxin with conserved characteristics. A multiple sequence alignment considering the twelve most toxic short-chain α-neurotoxins reported from the venoms of the elapid genera Acanthophis, Oxyuranus, Walterinnesia, Naja, Dendroaspis and Micrurus led us to propose a short-chain consensus α-neurotoxin, here named ScNtx. The synthetic ScNtx gene was de novo constructed and cloned into the expression vector pQE30 containing a 6His-Tag and an FXa proteolytic cleavage region. Escherichia coli Origami cells transfected with the pQE30/ScNtx vector expressed the recombinant consensus neurotoxin in a soluble form with a yield of 1.5 mg/L of culture medium. The 60-amino acid residue ScNtx contains canonical structural motifs similar to α-neurotoxins from African elapids and its LD₅₀ of 3.8 µg/mice is similar to the most toxic short-chain α-neurotoxins reported from elapid venoms. Furthermore, ScNtx was also able to antagonize muscular, but not neuronal, nicotinic acetylcholine receptors (nAChR). Rabbits immunized with ScNtx were able to immune-recognize short-chain α-neurotoxins within whole elapid venoms. Type I neurotoxins are difficult to isolate and purify from natural sources; therefore, the heterologous expression of molecules such ScNtx, bearing crucial motifs and key amino acids, is a step forward to create common immunogens for developing cost-effective antivenoms with a wider spectrum of efficacy, quality and strong therapeutic value.

The three-finger toxin fold: a multifunctional structural scaffold able to modulate cholinergic functions

Three-finger fold toxins are miniproteins frequently found in Elapidae snake venoms. This fold is characterized by three distinct loops rich in β-strands and emerging from a dense, globular core reticulated by four highly conserved disulfide bridges. The number and diversity of receptors, channels, and enzymes identified as targets of three-finger fold toxins is increasing continuously. Such manifold diversity highlights the specific adaptability of this fold for generating pleiotropic functions. Although this toxin superfamily disturbs many biological functions by interacting with a large diversity of molecular targets, the most significant target is the cholinergic system. By blocking the activity of the nicotinic and muscarinic acetylcholine receptors or by inhibiting the enzyme acetylcholinesterase, three-finger fold toxins interfere most drastically with neuromuscular junction functioning. Several of these toxins have become powerful pharmacological tools for studying the function and structure of their molecular targets. Most importantly, since dysfunction of these receptors/enzyme is involved in many diseases, exploiting the three-finger scaffold to create novel, highly specific therapeutic agents may represent a major future endeavor. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Central loop of non-conventional toxin WTX from Naja kaouthia is important for interaction with nicotinic acetylcholine receptors

'Three-finger' toxin WTX from Naja kaouthia interacts with nicotinic and muscarinic acetylcholine receptors (nAChRs and mAChRs). Mutagenesis and competition experiments with (125)I-α-bungarotoxin revealed that Arg31 and Arg32 residues from the WTX loop II are important for binding to Torpedo californica and human α7 nAChRs. Computer modeling suggested that loop II occupies the orthosteric binding site at α7 nAChR. The similar toxin interface was previously described as a major determinant of allosteric interactions with mAChRs.

Identification and structural characterization of a new three-finger toxin hemachatoxin from Hemachatus haemachatus venom

Snake venoms are rich sources of biologically active proteins and polypeptides. Three-finger toxins are non-enzymatic proteins present in elapid (cobras, kraits, mambas and sea snakes) and colubrid venoms. These proteins contain four conserved disulfide bonds in the core to maintain the three-finger folds. Although all three-finger toxins have similar fold, their biological activities are different. A new three-finger toxin (hemachatoxin) was isolated from Hemachatus haemachatus (Ringhals cobra) venom. Its amino acid sequence was elucidated, and crystal structure was determined at 2.43 Å resolution. The overall fold is similar to other three-finger toxins. The structure and sequence analysis revealed that the fold is maintained by four highly conserved disulfide bonds. It exhibited highest similarity to particularly P-type cardiotoxins that are known to associate and perturb the membrane surface with their lipid binding sites. Also, the increased B value of hemachotoxin loop II suggests that loop II is flexible and may remain flexible until its interaction with membrane phospholipids. Based on the analysis, we predict hemachatoxin to be cardiotoxic/cytotoxic and our future experiments will be directed to characterize the activity of hemachatoxin.

Engineering of three-finger fold toxins creates ligands with original pharmacological profiles for muscarinic and adrenergic receptors

Protein engineering approaches are often a combination of rational design and directed evolution using display technologies. Here, we test “loop grafting,” a rational design method, on three-finger fold proteins. These small reticulated proteins have exceptional affinity and specificity for their diverse molecular targets, display protease-resistance, and are highly stable and poorly immunogenic. The wealth of structural knowledge makes them good candidates for protein engineering of new functionality. Our goal is to enhance the efficacy of these mini-proteins by modifying their pharmacological properties in order to extend their use in imaging, diagnostics and therapeutic applications. Using the interaction of three-finger fold toxins with muscarinic and adrenergic receptors as a model, chimeric toxins have been engineered by substituting loops on toxin MT7 by those from toxin MT1. The pharmacological impact of these grafts was examined using binding experiments on muscarinic receptors M1 and M4 and on the α_1A-adrenoceptor. Some of the designed chimeric proteins have impressive gain of function on certain receptor subtypes achieving an original selectivity profile with high affinity for muscarinic receptor M1 and α_1A-adrenoceptor. Structure-function analysis supported by crystallographic data for MT1 and two chimeras permits a molecular based interpretation of these gains and details the merits of this protein engineering technique. The results obtained shed light on how loop permutation can be used to design new three-finger proteins with original pharmacological profiles.

Structure, function and evolution of three-finger toxins: mini proteins with multiple targets

Snake venoms are complex mixtures of pharmacologically active peptides and proteins. These protein toxins belong to a small number of superfamilies of proteins. Three-finger toxins belong to a superfamily of non-enzymatic proteins found in all families of snakes. They have a common structure of three beta-stranded loops extending from a central core containing all four conserved disulphide bonds. Despite the common scaffold, they bind to different receptors/acceptors and exhibit a wide variety of biological effects. Thus, the structure-function relationships of this group of toxins are complicated and challenging. Studies have shown that the functional sites in these 'sibling' toxins are located on various segments of the molecular surface. Targeting to a wide variety of receptors and ion channels and hence distinct functions in this group of mini proteins is achieved through a combination of accelerated rate of exchange of segments as well as point mutations in exons. In this review, we describe the structural and functional diversity, structure-function relationships and evolution of this group of snake venom toxins.

Structural and functional characterization of a novel homodimeric three-finger neurotoxin from the venom of Ophiophagus hannah (king cobra)

Snake venoms are a mixture of pharmacologically active proteins and polypeptides that have led to the development of molecular probes and therapeutic agents. Here, we describe the structural and functional characterization of a novel neurotoxin, haditoxin, from the venom of Ophiophagus hannah (King cobra). Haditoxin exhibited novel pharmacology with antagonism toward muscle (alphabetagammadelta) and neuronal (alpha(7), alpha(3)beta(2), and alpha(4)beta(2)) nicotinic acetylcholine receptors (nAChRs) with highest affinity for alpha(7)-nAChRs. The high resolution (1.5 A) crystal structure revealed haditoxin to be a homodimer, like kappa-neurotoxins, which target neuronal alpha(3)beta(2)- and alpha(4)beta(2)-nAChRs. Interestingly however, the monomeric subunits of haditoxin were composed of a three-finger protein fold typical of curaremimetic short-chain alpha-neurotoxins. Biochemical studies confirmed that it existed as a non-covalent dimer species in solution. Its structural similarity to short-chain alpha-neurotoxins and kappa-neurotoxins notwithstanding, haditoxin exhibited unique blockade of alpha(7)-nAChRs (IC(50) 180 nm), which is recognized by neither short-chain alpha-neurotoxins nor kappa-neurotoxins. This is the first report of a dimeric short-chain alpha-neurotoxin interacting with neuronal alpha(7)-nAChRs as well as the first homodimeric three-finger toxin to interact with muscle nAChRs.

Irditoxin, a novel covalently linked heterodimeric three-finger toxin with high taxon-specific neurotoxicity

A novel heterodimeric three-finger neurotoxin, irditoxin, was isolated from venom of the brown treesnake Boiga irregularis (Colubridae). Irditoxin subunit amino acid sequences were determined by Edman degradation and cDNA sequencing. The crystal structure revealed two subunits with a three-finger protein fold, typical for "nonconventional" toxins such as denmotoxin, bucandin, and candoxin. This is the first colubrid three-finger toxin dimer, covalently connected via an interchain disulfide bond. Irditoxin showed taxon-specific lethality toward birds and lizards and was nontoxic toward mice. It produced a potent neuromuscular blockade at the avian neuromuscular junction (IC(50)=10 nM), comparable to alpha-bungarotoxin, but was three orders of magnitude less effective at the mammalian neuromuscular junction. Covalently linked heterodimeric three-finger toxins found in colubrid venoms constitute a new class of venom peptides, which may be a useful source of new neurobiology probes and therapeutic leads.

Plethodontid modulating factor, a hypervariable salamander courtship pheromone in the three-finger protein superfamily

The soluble members of the three-finger protein superfamily all share a relatively simple 'three-finger' structure, yet perform radically different functions. Plethodontid modulating factor (PMF), a pheromone protein produced by the lungless salamander, Plethodon shermani, is a new and unusual member of this group. It affects female receptivity when delivered to the female's nares during courtship. As with other plethodontid pheromone genes, PMF is hyperexpressed in a specialized male mental (chin) gland. Unlike other plethodontid pheromone genes, however, PMF is also expressed at low levels in the skin, liver, intestine and kidneys of both sexes. The PMF sequences obtained from all tissue types were highly variable, with 103 unique haplotypes identified which averaged 35% sequence dissimilarity (range 1-60%) at the protein level. Despite this variation, however, all PMF sequences contained a conserved approximately 20-amino-acid secretion signal sequence and a pattern of eight cysteines that is also found in cytotoxins and short neurotoxins from snake venoms, as well as xenoxins from Xenopus. Although they share a common cysteine pattern, PMF isoforms differ from other three-finger proteins in: (a) amino-acid composition outside of the conserved motif; (b) length of the three distinguishing 'fingers'; (c) net charge at neutral pH. Whereas most three-finger proteins have a net positive charge at pH 7.0, PMF has a high net negative charge at neutral pH (pI range of most PMFs 3.5-4.0). Sequence comparisons suggest that PMF belongs to a distinct multigene subfamily within the three-finger protein superfamily.

A model study of interactions between TcAChE peripheral site segment Tyr70Val71 and loop 1 of Fasciculin 2

The continuous chain of residues (Thr7 to Ala12) of Loop1 of Fas2 (F1) and its interaction with the peripheral binding sites (Tyr70-Val71) of AChE (P1) has been studied. Our results suggest that the flexibility of Loop1 might be caused by either the partially protonated guanidine group of Arg11 under experimental conditions or by the interaction with the negatively charged center of substrates. The binding energy of F1-P1 is predicted to be -16.6 kcal/mol at the B3LYP/6-311G(d,p) level, which is assumed to originate from one isolated O7...HN10 H-bond, one possible O10...HC71 unconventional O...HC type H-bonding, and the improved pi-bonding cooperativity around the peptide group of the AChE segment Tyr70-Val71. The classical Kitaura-Morokuma energy decomposition analysis, the NPA charge analysis, and the AIM analysis consistently reveal that the peptide group in segment P1 is more polarizable, which might play the key role in the interactions between F1 and P1. The PCM solvent effect corrected results reveal decrease of the interaction energy of the considered model. The importance of Thr8 of Fas2 in the P-site binding of AChE is also concluded. Site-directed mutations on either the Fas2 residue of Thr8 or the AChE residue of Tyr70 are expected to alter the binding behavior of the Loop1 of Fas2 with AChE.

Novel genes encoding six kinds of three-finger toxins in Ophiophagus hannah (king cobra) and function characterization of two recombinant long-chain neurotoxins

Three-finger toxins are a family of low-molecular-mass toxins (<10 kDa) having very similar three-dimensional structures. In the present study, 19 novel cDNAs coding three-finger toxins were cloned from the venom gland of Ophiophagus hannah (king cobra). Alignment analysis showed that the putative peptides could be divided into six kinds of three-finger toxins: LNTXs (long-chain neurotoxins), short-chain neurotoxins, cardiotoxins (CTXs), weak neurotoxins, muscarinic toxins and a toxin with a free SH group. Furthermore, a phylogenetic tree was established on the basis of the toxin cDNAs and the previously reported similar nucleotide sequences from the same source venom. It indicated that three-finger-toxin genes in O. hannah diverged early in the course of evolution by long- and short-type pathways. Two LNTXs, namely rLNTX1 (recombinant LNTX1) and rLNTX3, were expressed and showed cytolytic activity in addition to their neurotoxic function. By comparing the functional residues, we offer some possible explanations for the differences in their neurotoxic function. Moreover, a plausible elucidation of the additonal cytolytic activity was achieved by hydropathy-profile analysis. This, to our knowledge, is the first observation that recombinant long chain alpha-neurotoxins have a CTX-like cytolytic activity.

Denmotoxin, a three-finger toxin from the colubrid snake Boiga dendrophila (Mangrove Catsnake) with bird-specific activity

Boiga dendrophila (mangrove catsnake) is a colubrid snake that lives in Southeast Asian lowland rainforests and mangrove swamps and that preys primarily on birds. We have isolated, purified, and sequenced a novel toxin from its venom, which we named denmotoxin. It is a monomeric polypeptide of 77 amino acid residues with five disulfide bridges. In organ bath experiments, it displayed potent postsynaptic neuromuscular activity and irreversibly inhibited indirectly stimulated twitches in chick biventer cervicis nerve-muscle preparations. In contrast, it induced much smaller and readily reversible inhibition of electrically induced twitches in mouse hemidiaphragm nerve-muscle preparations. More precisely, the chick muscle alpha(1)betagammadelta-nicotinic acetylcholine receptor was 100-fold more susceptible compared with the mouse receptor. These data indicate that denmotoxin has a bird-specific postsynaptic activity. We chemically synthesized denmotoxin, crystallized it, and solved its crystal structure at 1.9 A by the molecular replacement method. The toxin structure adopts a non-conventional three-finger fold with an additional (fifth) disulfide bond in the first loop and seven additional residues at its N terminus, which is blocked by a pyroglutamic acid residue. This is the first crystal structure of a three-finger toxin from colubrid snake venom and the first fully characterized bird-specific toxin. Denmotoxin illustrates the relationship between toxin specificity and the primary prey type that constitutes the snake's diet.

Snake postsynaptic neurotoxins: gene structure, phylogeny and applications in research and therapy

Snake venoms are complex mixtures of biologically active polypeptides that target a variety of vital physiological functions in mammals. alpha-Neurotoxins, toxins that cause paralysis by binding to the nicotinic receptors at the postsynaptic region of the neuromuscular junction have been widely studied in terms of their structure-function relationships as well as gene structure, organization and expression. In this review, we describe the structure of alpha-neurotoxin genes and discuss their evolutionary relationships. Almost all members of neurotoxins have been found to exhibit a common evolutionary origin. The importance of alpha-neurotoxins in therapy and research has also been discussed to highlight their potential applications especially in the area of drug discovery.

Structurally conserved α-neurotoxin genes encode functionally diverse proteins in the venom of Naja sputatrix 1

The structure and organization of the genes encoding the long-chain neurotoxins and four other isoforms of weak neurotoxins in the venom of Naja sputatrix are reported. The genes contained three exons interrupted by two introns, a structure similar to other members of the three-finger toxin family. The proteins encoded by these genes, however, show varied affinity towards nicotinic acetylcholine receptors. Phylogenetic analysis of these genes showed that the weak neurotoxin gene is confined to a distinct group. We also observe that specific mutations of the gene provide the diversity in function in these toxins while maintaining a common structural scaffold. This forms the first report where the molecular basis of evolution of postsynaptic neurotoxins from an ancestral gene can be demonstrated using the same species of snake.

Motions and structural variability within toxins: implication for their use as scaffolds for protein engineering

Animal toxins are small proteins built on the basis of a few disulfide bonded frameworks. Because of their high variability in sequence and biologic function, these proteins are now used as templates for protein engineering. Here we report the extensive characterization of the structure and dynamics of two toxin folds, the "three-finger" fold and the short alpha/beta scorpion fold found in snake and scorpion venoms, respectively. These two folds have a very different architecture; the short alpha/beta scorpion fold is highly compact, whereas the "three-finger" fold is a beta structure presenting large flexible loops. First, the crystal structure of the snake toxin alpha was solved at 1.8-A resolution. Then, long molecular dynamics simulations (10 ns) in water boxes of the snake toxin alpha and the scorpion charybdotoxin were performed, starting either from the crystal or the solution structure. For both proteins, the crystal structure is stabilized by more hydrogen bonds than the solution structure, and the trajectory starting from the X-ray structure is more stable than the trajectory started from the NMR structure. The trajectories started from the X-ray structure are in agreement with the experimental NMR and X-ray data about the protein dynamics. Both proteins exhibit fast motions with an amplitude correlated to their secondary structure. In contrast, slower motions are essentially only observed in toxin alpha. The regions submitted to rare motions during the simulations are those that exhibit millisecond time-scale motions. Lastly, the structural variations within each fold family are described. The localization and the amplitude of these variations suggest that the regions presenting large-scale motions should be those tolerant to large insertions or deletions.

Structure and stability of an acidic fibroblast growth factor from Notophthalmus viridescens

The three-dimensional solution structure of an acidic fibroblast growth factor (nFGF-1) from the newt (Notophthalmus viridescens) is determined using multidimensional NMR techniques. Complete assignment of all the atoms ((1)H, (15)N, and (13)C) has been achieved using a variety of triple resonance experiments. 50 structures were calculated using hybrid distance geometry-dynamical simulated annealing technique with a total of 1359 constraints. The atomic root mean square distribution for the backbone atoms in the structured region is 0.60 A. The secondary structural elements include 12 beta-strands arranged antiparallely into a beta-barrel structure. The protein (nFGF-1) exists in a monomeric state upon binding to the ligand, sucrose octa sulfate (SOS), in a stoichiometric ratio of 1:1. The SOS binding site consists of a dense cluster of positively charged residues located at the C-terminal end of the molecule. The conformational stabilities of nFGF-1 and its structural and functional homologue from the human source (hFGF-1) are drastically different. The differential stabilities of nFGF-1 and hFGF-1 are attributed to the differences in the number of hydrogen bonds and the presence of solvent inaccessible cavities in the two proteins.

Molecular moulds with multiple missions: functional sites in three-finger toxins

1. Snake venoms are complex mixtures of pharmacologically active peptides and proteins. 2. These protein toxins belong to a small number of superfamilies of proteins. The present review describes structure-function relationships of three-finger toxins. 3. All toxins share a common structure of three beta-stranded loops extending from a central core. However, they bind to different receptors/acceptors and exhibit a wide variety of biological effects. 4. Thus, the structure-function relationships of this group of toxins are complicated and challenging. 5. Studies have shown that the functional sites in these "sibling" toxins are located on various segments of the molecular surface.

Cytotoxic potency of cardiotoxin from Naja sputatrix: development of a new cytolytic assay

The possible involvement of specific regions/loops of cardiotoxin from Naja sputatrix venom in mediating its cytolytic activity is evaluated using a new cytolytic assay. In this assay, the amount of chloramphenicol acetyltransferase (CAT) that is released upon lysis of the cellular membranes by the cytotoxin has been measured as an index of cytolysis. This newly developed CAT system is more sensitive than the traditional haemolysis method utilizing red blood cells or the lactate dehydrogenase assay for cytolysis. Series of chimaeric toxin molecules have been constructed by swapping the loops between highly hydrophilic neurotoxin and highly hydrophobic cardiotoxin molecules from Naja sputatrix, which are known to exhibit structural similarity (three-finger conformation) but to have different functional properties. Comparison of the cytolytic activities of the recombinant chimaeric toxins demonstrated the possible involvement of all three loops of cardiotoxin in its cytolytic potency. However, the first two loops of the protein appear to make the major contribution to its lytic activity. cDNAs encoding cardiotoxin and the chimaeric toxins, when expressed in transfected cultured Chinese hamster ovary cells, resulted in cell lysis, indicating that these cDNAs can be developed as useful cytolytic agents.

A chimeric scorpion alpha-toxin displays de novo electrophysiological properties similar to those of alpha-like toxins

BotXIV and LqhalphaIT are two structurally related long chain scorpion alpha-toxins that inhibit sodium current inactivation in excitable cells. However, while LqhalphaIT from Leiurus quinquestriatus hebraeus is classified as a true and strong insect alpha-toxin, BotXIV from Buthus occitanus tunetanus is characterized by moderate biological activities. To assess the possibility that structural differences between these two molecules could reflect the localization of particular functional topographies, we compared their sequences. Three structurally deviating segments located in three distinct and exposed loops were identified. They correspond to residues 8-10, 19-22, and 38-43. To evaluate their functional role, three BotXIV/LqhalphaIT chimeras were designed by transferring the corresponding LqhalphaIT sequences into BotXIV. Structural and antigenic characterizations of the resulting recombinant chimera show that BotXIV can accommodate the imposed modifications, confirming the structural flexibility of that particular alpha/beta fold. Interestingly, substitution of residues 8-10 yields to a new electrophysiological profile of the corresponding variant, partially comparable to that one of alpha-like scorpion toxins. Taken together, these results suggest that even limited structural deviations can reflect functional diversity, and also that the structure-function relationships between insect alpha-toxins and alpha-like scorpion toxins are probably more complex than expected.

Slow folding of three-fingered toxins is associated with the accumulation of native disulfide-bonded intermediates

Snake neurotoxins are short all-beta proteins that display a complex organization of the disulfide bonds: two bonds connect consecutive cysteine residues (C43-C54, C55-C60), and two bonds intersect when bridging (C3-C24, C17-C41) to form a particular structure classified as "disulfide beta-cross". We investigated the oxidative folding of a neurotoxin variant, named alpha62, to define the chemical nature of the three-disulfide intermediates that accumulate during the process in order to describe in detail its folding pathway. These folding intermediates were separated by reverse-phase HPLC, and their disulfide bonds were identified using a combination of tryptic hydrolysis, manual Edman degradation, and mass spectrometry. Two dominant intermediates containing three native disulfide bonds were identified, lacking the C43-C54 and C17-C41 pairing and therefore named des-[43-54] and des-[17-41], respectively. Both species were individually allowed to reoxidize under folding conditions, showing that des-[17-41] was a fast-forming nonproductive intermediate that had to interconvert into the des-[43-54] isomer before forming the native protein. Conversely, the des-[43-54] intermediate appeared to be the immediate precursor of the oxidized neurotoxin. A kinetic model for the folding of neurotoxin alpha62 which fits with the observed time-course accumulation of des-[17-41] and des-[43-54] is proposed. The effect of turn 2, located between residues 17 and 24, on the overall kinetics is discussed in view of this model.

Stability of a structural scaffold upon activity transfer: X-ray structure of a three fingers chimeric protein

Fasciculin 2 and toxin alpha proteins belong to the same structural family of three-fingered snake toxins. They act on different targets, but in each case the binding region involves residues from loops I and II. The superimposition of the two structures suggests that these functional regions correspond to structurally distinct zones. Loop I, half of loop II and the C-terminal residue of fasciculin 2 were therefore transferred into the toxin alpha. The inhibition constant of the resulting chimera is only 15-fold lower than that of fasciculin 2, and as expected the potency of binding to the toxin alpha target has been lost. In order to understand the structure-function relationship between the chimera and its "parent" molecules, we solved its structure by X-ray crystallography. The protein crystallized in space group P3(1)21 with a=b=58.5 A, and c=62.3 A. The crystal structure was solved by molecular replacement and refined to 2.1 A resolution. The structure belongs to the three-fingered snake toxin family with a core of four disulphide bridges from which emerge the three loops I, II and III. Superimposition of the chimera on fasciculin 2 or toxin alpha revealed an overall fold intermediate between those of the two parent molecules. The regions corresponding to toxin alpha and to fasciculin 2 retained their respective geometries. In addition, the chimera protein displayed a structural behaviour similar to that of fasciculin 2, i.e. dimerization in the crystal structure of fasciculin 2, and the geometry of the region that binds to acetylcholinesterase. In conclusion, this structure shows that the chimera retains the general structural characteristics of three-fingered toxins, and the structural specificity of the transferred function.