Mimicry between receptors and antibodies. Identification of snake toxin determinants recognized by the acetylcholine receptor and an acetylcholine receptor-mimicking monoclonal antibody

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.

Did evolution create a flexible ligand-binding cavity in the urokinase receptor through deletion of a plesiotypic disulfide bond?

The urokinase receptor (uPAR) is a founding member of a small protein family with multiple Ly6/uPAR (LU) domains. The motif defining these LU domains contains five plesiotypic disulfide bonds stabilizing its prototypical three-fingered fold having three protruding loops. Notwithstanding the detailed knowledge on structure-function relationships in uPAR, one puzzling enigma remains unexplored. Why does the first LU domain in uPAR (DI) lack one of its consensus disulfide bonds, when the absence of this particular disulfide bond impairs the correct folding of other single LU domain-containing proteins? Here, using a variety of contemporary biophysical methods, we found that reintroducing the two missing half-cystines in uPAR DI caused the spontaneous formation of the corresponding consensus 7-8 LU domain disulfide bond. Importantly, constraints due to this cross-link impaired (i) the binding of uPAR to its primary ligand urokinase and (ii) the flexible interdomain assembly of the three LU domains in uPAR. We conclude that the evolutionary deletion of this particular disulfide bond in uPAR DI may have enabled the assembly of a high-affinity urokinase-binding cavity involving all three LU domains in uPAR. Of note, an analogous neofunctionalization occurred in snake venom α-neurotoxins upon loss of another pair of the plesiotypic LU domain half-cystines. In summary, elimination of the 7-8 consensus disulfide bond in the first LU domain of uPAR did have significant functional and structural consequences.

Specific membrane binding of neurotoxin II can facilitate its delivery to acetylcholine receptor

The action of three-finger snake alpha-neurotoxins at their targets, nicotinic acetylcholine receptors (nAChR), is widely studied because of its biological and pharmacological relevance. Most such studies deal only with ligands and receptor models; however, for many ligand/receptor systems the membrane environment may affect ligand binding. In this work we focused on binding of short-chain alpha-neurotoxin II (NTII) from Naja oxiana to the native-like lipid bilayer, and the possible role played by the membrane in delivering the toxin to nAChR. Experimental (NMR and mutagenesis) and molecular modeling (molecular-dynamics simulation) studies revealed a specific interaction of the toxin molecule with the phosphatidylserine headgroup of lipids, resulting in the proper topology of NTII on lipid bilayers favoring the attack of nAChR. Analysis of short-chain alpha-neurotoxins showed that most of them possess a high positive charge and sequence homology in the lipid-binding motif of NTII, implying that interaction with the membrane surrounding nAChR may be common for the toxin family.

Three-finger alpha-neurotoxins and the nicotinic acetylcholine receptor, forty years on

The discovery, about forty years ago, of alpha-bungarotoxin, a three-finger alpha-neurotoxin from Bungarus multicinctus venom, enabled the isolation of the nicotinic acetylcholine receptor (nAChR), making it one of the most thoroughly characterized receptors today. Since then, the sites of interaction between alpha-neurotoxins and nAChRs have largely been delineated, revealing the remarkable plasticity of the three-finger toxin fold that has optimally evolved to utilize different combinations of functional groups to generate a panoply of target specificities to discern subtle differences between nAChR subtypes. New facets in toxinology have now broadened the scope for the use of alpha-neurotoxins in scientific discovery. For instance, the development of short, combinatorial library-derived, synthetic peptides that bind with sub-nanomolar affinity to alpha-bungarotoxin and prevent its interaction with muscle nAChRs has led to the in vivo neutralization of experimental alpha-bungarotoxin envenomation, while the successful introduction of pharmatopes bearing "alpha-bungarotoxin-sensitive sites" into toxin-insensitive nAChRs has permitted the use of various alpha-neurotoxin tags to localize and characterize new receptor subtypes. More ambitious strategies can now be envisaged for engineering rationally designed novel activities on three-finger toxin scaffolds to generate lead peptides of therapeutic value that target the nicotinic pharmacopoeia. This review details the progress made towards achieving this goal.

Molecular cloning, expression and characterization of three short chain alpha-neurotoxins from the venom of sea snake--Hydrophiinae Hydrophis cyanocinctus Daudin

Three different genes named sn311, sn316 and sn285 were discovered by large-scale randomly sequencing the high quality cDNA library of the venom glands from Hydrophiinae Hydrophis cyanocinctus Daudin. Sequence analysis showed that these three genes encoded three different short chain alpha-neurotoxins of 81 amino acids, which contained a signal peptide of 21 amino acids and followed by a mature peptide of 60 amino acids. Amino acid comparison reveals that mature peptides of sn311 and sn316 are highly homologous, with the only variance at position 46, which is Lys46 and Ser46, respectively. Whereas the mature peptide of sn285 lacks the most conserved amino acids in short chain alpha-neurotoxins, Asp31 and Arg33. The coding sequences of three neurotoxins were cloned into a thioredoxin (TRX) fusion expression vector (pTRX) and expressed as soluble recombinant fusion proteins in E. coli. After purification, approximately 10 mg/l recombinant proteins with the purity up to 95% were obtained. These three recombinant proteins are designated as rSN311, rSN316 and rSN285, they have a molecular weight of 6.963, 6.920 and 6.756 kDa, respectively, which are similar to those predicted from amino acid sequences. LD50 values of rSN311, rSN316 and rSN285 are 0.0827, 0.095, and 0.0647 mg/kg to mice, respectively. Studies on effects of these recombinant proteins on neuromuscular transmission were carried out, and results indicate that they all can produce prompt blockade of neuromuscular transmission, but display distinct biological activity characteristic individually. The results from UV-circular dichroism (CD) spectra indicate that they share similar secondary structure compared to other identified alpha-neurotoxins, and no significant structural differences in these recombinant proteins are observed.

Candoxin, a novel toxin from Bungarus candidus, is a reversible antagonist of muscle (alphabetagammadelta ) but a poorly reversible antagonist of neuronal alpha 7 nicotinic acetylcholine receptors

In contrast to most short and long chain curaremimetic neurotoxins that produce virtually irreversible neuromuscular blockade in isolated nerve-muscle preparations, candoxin, a novel three-finger toxin from the Malayan krait Bungarus candidus, produced postjunctional neuromuscular blockade that was readily and completely reversible. Nanomolar concentrations of candoxin (IC(50) = approximately 10 nm) also blocked acetylcholine-evoked currents in oocyte-expressed rat muscle (alphabetagammadelta) nicotinic acetylcholine receptors in a reversible manner. In contrast, it produced a poorly reversible block (IC(50) = approximately 50 nm) of rat neuronal alpha7 receptors, clearly showing diverse functional profiles for the two nicotinic receptor subsets. Interestingly, candoxin lacks the helix-like segment cyclized by the fifth disulfide bridge at the tip of the middle loop of long chain neurotoxins, reported to be critical for binding to alpha7 receptors. However, its solution NMR structure showed the presence of some functionally invariant residues involved in the interaction of both short and long chain neurotoxins to muscle (alphabetagammadelta) and long chain neurotoxins to alpha7 receptors. Candoxin is therefore a novel toxin that shares a common scaffold with long chain alpha-neurotoxins but possibly utilizes additional functional determinants that assist in recognizing neuronal alpha7 receptors.

Mimicking the nicotinic receptor binding site by a single chain Fv selected by competitive panning from a synthetic phage library

We have developed a novel competitive method to select from a phage display library a single chain Fv which is able to mimic the alpha-bungarotoxin binding site of the muscle nicotinic receptor. The single chain Fv was selected from a large synthetic library using alpha-bungarotoxin-coated magnetic beads. Toxin-bound phages were then eluted by competition with affinity purified nicotinic receptor. Recognition of the toxin by the anti-alpha-bungarotoxin single chain Fv was very similar to that of the receptor, such as indicated by the epitope mapping of alpha-bungarotoxin through overlapping synthetic peptides. Moreover, several positively charged residues located in the toxin second loop and in the C-terminal region were found to be critical, to a similar extent, for toxin recognition by the single chain Fv and the receptor. However, although the anti-alpha-bungarotoxin single chain Fv seems to mimic the toxin binding site of the nicotinic receptor, it does not bind other nicotinic agonists or antagonists. Our results suggest that competitive selection of anti-ligand antibody phages can allow the production of receptor-mimicking molecules directly and exclusively targeted at one specific ligand. Since physiologically and pharmacologically different ligands can produce opposite effects on receptor functions, such selective ligand decoys can have important therapeutic applications.

Orientation of alpha-neurotoxin at the subunit interfaces of the nicotinic acetylcholine receptor

The alpha-neurotoxins are three-fingered peptide toxins that bind selectively at interfaces formed by the alpha subunit and its associating subunit partner, gamma, delta, or epsilon of the nicotinic acetylcholine receptor. Because the alpha-neurotoxin from Naja mossambica mossambica I shows an unusual selectivity for the alpha gamma and alpha delta over the alpha epsilon subunit interface, residue replacement and mutant cycle analysis of paired residues enabled us to identify the determinants in the gamma and delta sequences governing alpha-toxin recognition. To complement this approach, we have similarly analyzed residues on the alpha subunit face of the binding site dictating specificity for alpha-toxin. Analysis of the alpha gamma interface shows unique pairwise interactions between the charged residues on the alpha-toxin and three regions on the alpha subunit located around residue Asp(99), between residues Trp(149) and Val(153), and between residues Trp(187) and Asp(200). Substitutions of cationic residues at positions between Trp(149) and Val(153) markedly reduce the rate of alpha-toxin binding, and these cationic residues appear to be determinants in preventing alpha-toxin binding to alpha 2, alpha 3, and alpha 4 subunit containing receptors. Replacement of selected residues in the alpha-toxin shows that Ser(8) on loop I and Arg(33) and Arg(36) on the face of loop II, in apposition to loop I, are critical to the alpha-toxin for association with the alpha subunit. Pairwise mutant cycle analysis has enabled us to position residues on the concave face of the three alpha-toxin loops with respect to alpha and gamma subunit residues in the alpha-toxin binding site. Binding of NmmI alpha-toxin to the alpha gamma interface appears to have dominant electrostatic interactions not seen at the alpha delta interface.

Critical residues of epitopes recognized by several anti-p53 monoclonal antibodies correspond to key residues of p53 involved in interactions with the mdm2 protein

The aim of this work was to study the reactivity of antibodies directed against the N-terminus of p53 protein. First, we analysed the cross-reactivity of anti-p53 antibodies from human, mouse and rabbit sera with peptides derived from human, mouse and Xenopus p53. Next, we characterized more precisely a series of monoclonal antibodies directed against the N-terminal part of p53 and produced by immunizing mice with either full length human or Xenopus p53. For each of these mAbs we localized the epitope recognized on human p53 by the Spot method of multiple peptide synthesis, defined critical residues on p53 involved in the interaction by alanine scanning replacement experiments and determined kinetic parameters using real-time interaction analysis. These antibodies could be divided into two groups according to their epitopic and kinetic characteristics and their cross-reactivity with murine p53. Our results indicate that critical residues involved in the interaction of some of these mAbs with p53 correspond to key residues on p53 involved in its interaction with the mdm2 protein. These antibodies could, therefore, represent powerful tools for the study of p53 regulation.

Molecular determinants by which a long chain toxin from snake venom interacts with the neuronal alpha 7-nicotinic acetylcholine receptor

Long chain curarimimetic toxins from snake venom bind with high affinities to both muscular type nicotinic acetylcholine receptors (AChRs) (K(d) in the pm range) and neuronal alpha 7-AChRs (K(d) in the nm range). To understand the molecular basis of this dual function, we submitted alpha-cobratoxin (alpha-Cbtx), a typical long chain curarimimetic toxin, to an extensive mutational analysis. By exploring 36 toxin mutants, we found that Trp-25, Asp-27, Phe-29, Arg-33, Arg-36, and Phe-65 are involved in binding to both neuronal and Torpedo (Antil, S., Servent, D., and Ménez, A. (1999) J. Biol. Chem. 274, 34851-34858) AChRs and that some of them (Trp-25, Asp-27, and Arg-33) have similar binding energy contributions for the two receptors. In contrast, Ala-28, Lys-35, and Cys-26-Cys-30 selectively bind to the alpha 7-AChR, whereas Lys-23 and Lys-49 bind solely to the Torpedo AChR. Therefore, alpha-Cbtx binds to two AChR subtypes using both common and specific residues. Double mutant cycle analyses suggested that Arg-33 in alpha-Cbtx is close to Tyr-187 and Pro-193 in the alpha 7 receptor. Since Arg-33 of another curarimimetic toxin is close to the homologous alpha Tyr-190 of the muscular receptor (Ackermann, E. J., Ang, E. T. H., Kanter, J. R., Tsigelny, I., and Taylor, P. (1998) J. Biol. Chem. 273, 10958-10964), toxin binding probably occurs in homologous regions of neuronal and muscular AChRs. However, no coupling was seen between alpha-Cbtx Arg-33 and alpha 7 receptor Trp-54, Leu-118, and Asp-163, in contrast to what was observed in a homologous situation involving another toxin and a muscular receptor (Osaka, H., Malany, S., Molles, B. E., Sine, S. M., and Taylor, P. (2000) J. Biol. Chem. 275, 5478-5484). Therefore, although occurring in homologous regions, the detailed modes of toxin binding to alpha 7 and muscular receptors are likely to be different. These data offer a molecular basis for the design of toxins with predetermined specificities for various members of the AChR family.

Molecular and structural basis of the specificity of a neutralizing acetylcholine receptor-mimicking antibody, using combined mutational and molecular modeling analyses

The antagonist activity of short-chain toxins from snake venoms toward the nicotinic acetylcholine receptor (nAChR) is neutralized upon binding to a toxin-specific monoclonal antibody called Malpha2-3 (1). To establish the molecular basis of this specificity, we predicted from both mutational analyses and docking procedures the structure of the Malpha2-3-toxin complex. From knowledge of the functional paratope and epitope, and using a double-mutation cycle procedure, we gathered evidence that Asp(31) in complementarity determining region 1H is close to, and perhaps interacts with, Arg(33) in the antigen. The use of this pair of proximate residues during the selection procedure yielded three models based on docking calculations. The selected models predicted the proximity of Tyr(49) and/or Tyr(50) in the antibody to Lys(47) in the toxin. This was experimentally confirmed using another round of double-mutation cycles. The two models finally selected were submitted to energy minimization in a CHARMM22 force field, and were characterized by a root mean square deviation of 7.0 +/- 2.9 A. Both models display most features of antibody-antigen structures. Since Malpha2-3 also partially mimics some binding properties of nAChR, these structural features not only explain its fine specificity of recognition, but may also further clarify how toxins bind to nAChR.

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

Three-finger proteins form a structurally related family of compounds that exhibit a great variety of biological properties. To address the question of the prediction of functional areas on their surfaces, we tentatively conferred the acetylcholinesterase inhibitory activity of fasciculins on a short-chain curaremimetic toxin. For this purpose, we assimilated the three-dimensional structure of fasciculin 2 with the one of toxin alpha. This comparison revealed that the tips of the first and second loops, together with the C terminus residue, deviated most. A first recombinant fasciculin/toxin alpha chimera was designed by transferring loop 1 in its entirety together with the tip of loop 2 of fasciculin 2 into the toxin alpha scaffold. A second chimera (rChII) was obtained by adding the point Asn-61 --> Tyr substitution. Comparison of functional and structural properties of both chimeras show that rChII can accommodate the imposed modifications and displays nearly all the acetylcholinesterase-blocking activities of fasciculins. The three-dimensional structure of rChII demonstrates that rChII adopts a typical three-fingered fold with structural features of both parent toxins. Taken together, these results emphasize the great structural flexibility and functional adaptability of that fold and confirm that structural deviations between fasciculins and short-chain neurotoxins do indeed reflect functional diversity.

Structural and functional consequences of peptide-carbohydrate mimicry. Crystal structure of a carbohydrate-mimicking peptide bound to concanavalin A

The functional consequences of peptide-carbohydrate mimicry were analyzed on the basis of the crystal structure of concanavalin A (ConA) in complex with a carbohydrate-mimicking peptide, DVFYPYPYASGS. The peptide binds to the non-crystallographically related monomers of two independent dimers of ConA in two different modes, in slightly different conformations, demonstrating structural adaptability in ConA-peptide recognition. In one mode, the peptide has maximum interactions with ConA, and in the other, it shows relatively fewer contacts within this site but significant contacts with the symmetry-related subunit. Neither of the peptide binding sites overlaps with the structurally characterized mannose and trimannose binding sites on ConA. Despite this, the functional mimicry between the peptide and carbohydrate ligands was evident. The peptide-inhibited ConA induced T cell proliferation in a dose-dependent manner. The effect of the designed analogs of the peptide on ConA-induced T cell proliferation and their recognition by the antibody response against alpha-d-mannopyranoside indicate a role for aromatic residues in functional mimicry. Although the functional mimicry was observed between the peptide and carbohydrate moieties, the crystal structure of the ConA-peptide complex revealed that the two peptide binding sites are independent of the methyl alpha-d-mannopyranoside binding site.

Structure of the Malpha2-3 toxin alpha antibody-antigen complex: combination of modelling with functional mapping experimental results

Modelled structures of the acetylcholine receptor-mimicking antibody, Malpha2-3, both free and bound to its antigen, toxin alpha, are assessed in the light of new experimental mutational data from functional mapping of the paratopic region of Malpha2-3. The experimental results are consistent with the previously-predicted structure of the free antibody, and also demonstrate that structural particularities of the Malpha2-3 combining site that were identified in the models play a role in the protein association. The modelled conformations of the hypervariable loops are discussed in the context of recent new data and analyses. The new mutational data allow several previously-considered modelled structures of the complex to be rejected. Two quite similar models now remain.

Chemical engineering of a three-fingered toxin with anti-alpha7 neuronal acetylcholine receptor activity

Though it possesses four disulfide bonds the three-fingered fold is amenable to chemical synthesis, using a Fmoc-based method. Thus, we synthesized a three-fingered curaremimetic toxin from snake with high yield and showed that the synthetic and native toxins have the same structural and biological properties. Both were characterized by the same 2D NMR spectra, identical high binding affinity (K(d) = 22 +/- 5 pM) for the muscular acetylcholine receptor (AChR) and identical low affinity (K(d) = 2.0 +/- 0.4 microM) for alpha7 neuronal AchR. Then, we engineered an additional loop cyclized by a fifth disulfide bond at the tip of the central finger. This loop is normally present in longer snake toxins that bind with high affinity (K(d) = 1-5 nM) to alpha7 neuronal AchR. Not only did the chimera toxin still bind with the same high affinity to the muscular AchR but also it displayed a 20-fold higher affinity (K(d) = 100 nM) for the neuronal alpha7 AchR, as compared with the parental short-chain toxin. This result demonstrates that the engineered loop contributes, at least in part, to the high affinity of long-chain toxins for alpha7 neuronal receptors. That three-fingered proteins with four or five disulfide bonds are amenable to chemical synthesis opens new perspectives for engineering new activities on this fold.

Cloning and characterization of an alpha-neurotoxin-type protein specific for the coral snake Micrurus corallinus

During the cloning of abundant cDNAs expressed in the Micrurus corallinus coral snake venom gland, we cloned an alpha-neurotoxin homologue cDNA (nxh1). Two others isoforms were also cloned (nxh3 and nxh7, respectively). The nxh1 cDNA codes for a potential coral snake toxin with a signal peptide of 21 amino acids plus a predicted mature peptide with 57 amino acids. The deduced protein is highly similar to known toxic three-finger alpha-neurotoxins, with four deduced S-S bridges at the same conserved positions. This is the first cDNA coding for a three-finger related protein described so far for coral snakes. However, the predicted protein does not possess some of the important amino acids for the nicotinic acetylcholine receptor interaction. This protein was expressed in Escherichia coli as a His-tagged protein that allowed the rapid purification of the recombinant protein. This protein was used to generate antibodies which recognized the recombinant protein in Western blot and also a single band present in the M. corallinus venom, but not in the venom of 10 other Micrurus species.

Variability among the sites by which curaremimetic toxins bind to torpedo acetylcholine receptor, as revealed by identification of the functional residues of alpha-cobratoxin

alpha-Cobratoxin, a long chain curaremimetic toxin from Naja kaouthia venom, was produced recombinantly (ralpha-Cbtx) from Escherichia coli. It was indistinguishable from the snake toxin. Mutations at 8 of the 29 explored toxin positions resulted in affinity decreases for Torpedo receptor with DeltaDeltaG higher than 1.1 kcal/mol. These are R33E > K49E > D27R > K23E > F29A >/= W25A > R36A >/= F65A. These positions cover a homogeneous surface of approximately 880 A(2) and mostly belong to the second toxin loop, except Lys-49 and Phe-65 which are, respectively, on the third loop and C-terminal tail. The mutations K23E and K49E, and perhaps R33E, induced discriminative interactions at the two toxin-binding sites. When compared with the short toxin erabutoxin a (Ea), a number of structurally equivalent residues are commonly implicated in binding to muscular-type nicotinic acetylcholine receptor. These are Lys-23/Lys-27, Asp-27/Asp-31, Arg-33/Arg-33, Lys-49/Lys-47, and to a lesser and variable extent Trp-25/Trp-29 and Phe-29/Phe-32. In addition, however, the short and long toxins display three major differences. First, Asp-38 is important in Ea in contrast to the homologous Glu-38 in alpha-Cbtx. Second, all of the first loop is insensitive to mutation in alpha-Cbtx, whereas its tip is functionally critical in Ea. Third, the C-terminal tail may be specifically critical in alpha-Cbtx. Therefore, the functional sites of long and short curaremimetic toxins are not identical, but they share common features and marked differences that might reflect an evolutionary pressure associated with a great diversity of prey receptors.

Insight into odorant perception: the crystal structure and binding characteristics of antibody fragments directed against the musk odorant traseolide

Monoclonal antibodies were elicited against the small hydrophobic hapten traseolide, a commercially available musk fragrance. Antibody variable region sequences were found to belong to different sequence groups, and the binding characteristics of the corresponding antibody fragments were investigated. The antibodies M02/01/01 and M02/05/01 are highly homologous and differ in the binding pocket only at position H93. M02/05/01 (H93 Val) binds the hapten traseolide about 75-fold better than M02/01/01 (H93 Ala). A traseolide analog, missing only one methyl group, does not have the characteristic musk odorant fragrance. The antibody M02/05/01 binds this hapten analog about tenfold less tightly than the original traseolide hapten, and mimics the odorant receptor in this respect, while the antibody M02/01/01 does not distinguish between the analog and traseolide. To elucidate the structural basis for the fine specificity of binding, we determined the crystal structure of the Fab fragment of M02/05/01 complexed with the hapten at 2.6 A resolution. The crystal structure showed that only van der Waals interactions are involved in binding. The somatic Ala H93 Val mutation in M02/05/01 fills up an empty cavity in the binding pocket. This leads to an increase in binding energy and to the ability to discriminate between the hapten traseolide and its derivatives. The structural understanding of odorant specificity in an antibody gives insight in the physical principles on how specificity for such hydrophobic molecules may be achieved.

Molecular mimicry between a monoclonal antibody and one subunit of crotoxin, a heterodimeric phospholipase A2 neurotoxin

Crotoxin is a heterodimeric phospholipase A2 neurotoxin formed by the non-covalent association of an acidic and non-toxic subunit, CA, and a basic and weakly toxic phospholipase A2, CB. The two subunits behave in a synergistic manner. CA enhances the lethal potency of CB by increasing its selectivity of action. The mAb A-56.36, directed against the non-toxic subunit CA, was previously shown to neutralize crotoxin toxicity by dissociating the crotoxin complex. In the present report, a polypeptide sequence similarity was observed between some CDRs of mAb A-56.36 and two regions of CB (pos. 60-80 and 95-110). Phage displayed peptides corresponding to VH2 and VH3 of mAb A-56.36 and to their homologous sequences in CB bind CA to different extents. This observation shows that mAb A-56.36 interacts with a region of CA involved in its interaction with CB, therefore mimicking the binding of CB to CA. A similar approach was used to determine the regions of ammodytoxin A and of agkistrodotoxin, two phospholipase A2 neurotoxins similar to CB, which are involved in the formation of heterocomplexes with CA. The analysis of these data contributes to the determination of stretches of amino acids which could constitute the paratope of mAb A-56.36, as well as the region of association of CB with CA in crotoxin.

Presentation of antigen in immune complexes is boosted by soluble bacterial immunoglobulin binding proteins

Using a snake toxin as a proteic antigen (Ag), two murine toxin-specific monoclonal antibodies (mAbs), splenocytes, and two murine Ag-specific T cell hybridomas, we showed that soluble protein A (SpA) from Staphylococcus aureus and protein G from Streptococcus subspecies, two Ig binding proteins (IBPs), not only abolish the capacity of the mAbs to decrease Ag presentation but also increase Ag presentation 20-100-fold. Five lines of evidence suggest that this phenomenon results from binding of an IBP-Ab-Ag complex to B cells possessing IBP receptors. First, we showed that SpA is likely to boost presentation of a free mAb, suggesting that the IBP-boosted presentation of an Ag in an immune complex results from the binding of IBP to the mAb. Second, FACS analyses showed that an Ag-Ab complex is preferentially targeted by SpA to a subpopulation of splenocytes mainly composed of B cells. Third, SpA-dependent boosted presentation of an Ag-Ab complex is further enhanced when splenocytes are enriched in cells containing SpA receptors. Fourth, the boosting effect largely diminishes when splenocytes are depleted of cells containing SpA receptors. Fifth, the boosting effect occurs only when IBP simultaneously contains a Fab and an Fc binding site. Altogether, our data suggest that soluble IBPs can bridge immune complexes to APCs containing IBP receptors, raising the possibility that during an infection process by bacteria secreting these IBPs, Ag-specific T cells may activate IBP receptor-containing B cells by a mechanism of intermolecular help, thus leading to a nonspecific immune response.

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.

Camel single-domain antibody inhibits enzyme by mimicking carbohydrate substrate

Whereas antibodies have demonstrated the ability to mimic various compounds, classic heavy/light-chain antibodies may be limited in their applications. First, they tend not to bind enzyme active site clefts. Second, their size and complexity present problems in identifying key elements for binding and in using these elements to produce clinically valuable compounds. We have previously shown how cAb-Lys3, a single variable domain fragment derived from a lysozyme-specific camel antibody naturally lacking light chains, overcomes the first limitation to become the first antibody structure observed penetrating an enzyme active site. We now demonstrate how cAb-Lys3 mimics the oligosaccharide substrate functionally (inhibition constant for lysozyme, 50 nM) and structurally (lysozyme buried surface areas, hydrogen bond partners, and hydrophobic contacts are similar to those seen in sugar-complexed structures). Most striking is the mimicry by the antibody complementary determining region 3 (CDR3) loop, especially Ala104, which mimics the subsite C sugar 2-acetamido group; this group has previously been identified as a key feature in binding lysozyme. Comparative simplicity, high affinity and specificity, potential to reach and interact with active sites, and ability to mimic substrate suggest that camel heavy-chain antibodies present advantages over classic antibodies in the design, production, and application of clinically valuable compounds.

Identification of pairwise interactions in the alpha-neurotoxin-nicotinic acetylcholine receptor complex through double mutant cycles

alpha-Neurotoxins are potent inhibitors of the nicotinic acetylcholine receptor (nAChR), binding with high affinity to the two agonist sites located on the extracellular domain. Previous site-directed mutagenesis had identified three residues on the alpha-neurotoxin from Naja mossambica mossambica (Lys27, Arg33, and Lys47) and four residues on the mouse muscle nAChR alpha-subunit (Val188, Tyr190, Pro197, and Asp200) as contributing to binding. In this study, thermodynamic mutant cycle analysis was applied to these sets of residues to identify specific pairwise interactions. Amino acid variants of alpha-neurotoxin from N. mossambica mossambica at position 33 and of the nAChR at position 188 showed strong energetic couplings of 2-3 kcal/mol at both binding sites. Consistently smaller yet significant linkages of 1.6-2.1 kcal/mol were also observed between variants at position 27 on the toxin and position 188 on the receptor. Additionally, toxin residue 27 coupled to the receptor residues 190, 197, and 200 at the alphadelta binding site with observed coupling energies of 1.5-1.9 kcal/mol. No linkages were found between toxin residue Lys47 and the receptor residues studied here. These results provide direct evidence that the two conserved cationic residues Arg33 and Lys27, located on loop II of the toxin structure, are binding in close proximity to the alpha-subunit region between residues 188-200. The toxin residue Arg33 is closer to Val188, where it is likely stabilized by adjacent negative or aromatic residues on the receptor structure. Lys27 is positioned closer to Tyr190, Pro197, and Asp200, where it is likely stabilized through electrostatic interaction with Asp200 and/or cation/pi interactions with Tyr190.

Functional determinants by which snake and cone snail toxins block the alpha 7 neuronal nicotinic acetylcholine receptors

Snakes and cone snails produce toxins which block muscular and/or neuronal nicotinic acetylcholine receptors (AChRs). This paper mostly focuses on the determinants by which a snake long chain curaremimetic toxin and the cone snail toxin ImI bind specifically to the alpha 7 neuronal receptor. In both cases, the site involves a small turn-like structure constrained by two half-cystines.

Picosecond to hour time scale dynamics of a "three finger" toxin: correlation with its toxic and antigenic properties

Toxin alpha from Naja nigricollis (61 amino acids, four disulfide bridges) belongs to the "three finger" fold family, which contains snake toxins with various biological activities and nontoxic proteins from different origins. In this paper, we report an extensive 1H and 15N NMR study of the dynamics of toxin alpha in solution. 15N relaxation, 1H off-resonance ROESY, and H-D exchange experiments allowed us to probe picosecond to hour motions in the protein. Analysis of these NMR measurements demonstrates that toxin alpha exhibits various time scale motions, i.e., particularly large amplitude picosecond to nanosecond motions at the tips of the loops, observable microsecond to millisecond motions around two disulfide bridges, second time scale motions around the C-N bonds of asparagine and glutamine side chains which are more or less rapid depending on their amino acid solvent accessibility, and minute to hour motions in the beta-sheet structure. The less well-defined regions of toxin alpha solution structures are subject to important picosecond to nanosecond motions. The toxic site is organized around residues belonging to the rigid core of the molecule but also comprises residues exhibiting dynamics on various time scales. The Malpha1 epitope is subject to large picosecond to millisecond motions, which are probably modified by the interaction with the antibody. This phenomenon could be linked to the neutralizing properties of the antibody.

Nonidentity of the alpha-neurotoxin binding sites on the nicotinic acetylcholine receptor revealed by modification in alpha-neurotoxin and receptor structures

alpha-Neurotoxins constitute a large family of polypeptides that bind with high affinity to the nicotinic acetylcholine receptor (nAChR). Using a recombinant DNA-derived alpha-neurotoxin (Naja mossambica mossambica, NmmI) and mouse muscle nAChR expressed transiently on the surface of HEK 293 cells, we have delineated residues involved in the binding interaction on both the alpha-neurotoxin and the receptor interface. Several of the studied NmmI mutations, including two residues conserved throughout the alpha-neurotoxin family (K27 and R33), resulted in substantial decreases in the binding affinity. We have also examined 23 mutations located on the receptor alpha subunit and have identified 4 positions that appear to be important to NmmI recognition. These determinants represent a conserved aromatic residue (Y190), two positions where neuronal and muscle receptors differ (V188 and P197), and a negatively charged residue (D200). Unlike many of the nAChR agonists and antagonists which bind to the alphadelta and alphagamma binding sites on the receptor with different affinities, the wild-type NmmI-wild-type nAChR interaction showed a single affinity. However, by mutating critical toxin or receptor residues, we were able to produce site-selectivity between the alphagamma and alphadelta interfaces. These results suggest a nonequivalence in the binding interaction at the two sites, sensitive to discrete structural changes at key contact points on either the toxin or the receptor protein, and underscore the importance of delta and gamma receptor subunits in governing binding affinity.

The functional architecture of an acetylcholine receptor-mimicking antibody

Malpha2-3 is a monoclonal antibody that partially mimics the nicotinic acetylcholine receptor (AChR). Its three-dimensional structure has been previously predicted by molecular modeling, suggesting that 29 complementarity determining region (CDR) residues and 2 framework residues are exposed to solvent. To identify the antibody residues that bind to the antigen, i.e. snake toxin that binds specifically to AChR, we (i) produced the scFv form of Malpha2-3 fused to alkaline phosphatase, in the periplasmic space of Escherichia coli; (ii) submitted approximately 75% of exposed residues of the fused scFv to individual or combined mutations, and (iii) identified the residues whose mutations affect scFv binding to the toxin, using a sensitive enzyme-linked immunosorbent assay. 11 critical residues were identified, including 8 heavy chain residues, 2 framework residues, and 1 light chain residue. They cover a surface of approximately 800 A2, with a subset of most critical residues (VHD31, VHY32, and VHG101) and several aromatic residues. This functional architecture not only constitutes a plausible complementary binding surface for the snake toxin but also offers a structural basis to ultimately understand the capacity of the antibody to partially mimic AChR.

On the convergent evolution of animal toxins. Conservation of a diad of functional residues in potassium channel-blocking toxins with unrelated structures

BgK is a K+ channel-blocking toxin from the sea anemone Bunodosoma granulifera. It is a 37-residue protein that adopts a novel fold, as determined by NMR and modeling. An alanine-scanning-based analysis revealed the functional importance of five residues, which include a critical lysine and an aromatic residue separated by 6.6 +/- 1.0 A. The same diad is found in the three known homologous toxins from sea anemones. More strikingly, a similar functional diad is present in all K+ channel-blocking toxins from scorpions, although these toxins adopt a distinct scaffold. Moreover, the functional diads of potassium channel-blocking toxins from sea anemone and scorpions superimpose in the three-dimensional structures. Therefore, toxins that have unrelated structures but similar functions possess conserved key functional residues, organized in an identical topology, suggesting a convergent functional evolution for these small proteins.