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.

Neurotoxin-specific immunoglobulins accelerate dissociation of the neurotoxin-acetylcholine receptor complex.

Toxin isolated from cobra venom and labeled with tritium was incubated with membranes rich in acetylcholine receptors. The amount of toxin bound to the receptors was determined and the kinetics of dissociation of the receptor-toxin complex was followed. Addition of an excess of horse antiserum to the venom resulted in a significant acceleration of the dissociation reaction. Similarly, a monoclonal antibody against the toxin accelerated dissociation of the receptor-toxin complex. The results indicate that specific antibody binding destabilizes the toxin-receptor complex.

Refined structure of charybdotoxin: common motifs in scorpion toxins and insect defensins.

Conflicting three-dimensional structures of charybdotoxin (Chtx), a blocker of K+ channels, have been previously reported. A high-resolution model depicting the tertiary structure of Chtx has been obtained by DIANA and X-PLOR calculations from new proton nuclear magnetic resonance (NMR) data. The protein possesses a small triple-stranded antiparallel beta sheet linked to a short helix by two disulfides and to an extended fragment by one disulfide, respectively. This motif also exists in all known structures of scorpion toxins, irrespective of their size, sequence, and function. Strikingly, antibacterial insect defensins also adopt this folding pattern.

Tailoring new enzyme functions by rational redesign.

Site-directed mutagenesis is still a very efficient strategy to elaborate improved enzymes. Recently, advances have been made in developing rational strategies aimed at reshaping enzyme specificities and mechanisms, and at engineering biocatalysts through molecular assembling. These knowledge-based studies greatly benefit from the most recent computational analyses of enzyme structures and functions. The combination of rational and combinatorial methods opens up new vistas in the design of stable and efficient enzymes.

Molecular evolution of snake toxins: is the functional diversity of snake toxins associated with a mechanism of accelerated evolution?

Recent studies revealed that animal toxins with unrelated biological functions often possess a similar architecture. To tentatively understand the evolutionary mechanisms that may govern this principle of functional prodigality associated with a structural economy, two complementary approaches were considered. One of them consisted of investigating the rates of mutations that occur in cDNAs and/or genes that encode a variety of toxins with the same fold. This approach was largely adopted with phospholipases A2 from Viperidae and to a lesser extent with three-fingered toxins from Elapidae and Hydrophiidae. Another approach consisted of investigating how a given fold can accommodate distinct functional topographies. Thus, a number of topologies by which three-fingered toxins exert distinct functions were investigated either by making chemical modifications and/or mutational analyses or by studying the three-dimensional structure of toxin-target complexes. This review shows that, although the two approaches are different, they commonly indicate that most if not all the surface of a snake toxin fold undergoes natural engineering, which may be associated with an accelerated rate of evolution. The biochemical process by which this phenomenon occurs remains unknown.

CD and FTIR studies of an immunogenic disulphide cyclized octadecapeptide, a fragment of a snake curaremimetic toxin.

In a previous paper, the systematic epitope screening of a snake curaremimetic toxin or toxin a was described by this group using a panel of synthetic octadecapeptides. The disulphide cyclized peptide (Cys-23,40)(23-40) corresponding to loop II of the native toxin was found to elicit, with no linkage to a carrier, neutralizing antisera against the toxin. We have now undertaken the conformational study of this immunogenic disulphide cyclized peptide by CD and FTIR. The CD study of the peptide was carried in aqueous solution under various conditions (pH, temperature, presence of micelles) and in trifluoroethanol solution. Low temperature, SDS micelles and trifluoroethanol were found to induce a beta-sheet formation (16 to 39%). FTIR spectra of the peptide in the solid state (dry film) and in D2O solution or deuterated-TFE solution (hydrated film) displayed some characteristic bands indicating the presence of beta-sheet (1623 cm-1) and beta-turn (1637 cm-1; 1694 cm-1). These studies indicate that the immunogenic disulphide cyclized peptide (23-40) can adopt in solution an ordered structure.

Anti-idiotypic and anti-anti-idiotypic responses to a monoclonal antibody directed to the acetylcholine receptor binding site of curaremimetic toxins.

Serotherapy, an approach currently used to protect humans against animal bites or stings, is often too specific. To broaden antiserum paraspecificity, use of antibodies directed against areas shared by all members of a toxin family was previously proposed. MST2 is a mAb that recognizes all long-chain curaremimetic toxins (Charpentier et al. (1990) J. Mol. Recog. 3, 74-81). It binds to toxin residues that make contact with the toxin's target, e.g., the nicotinic acetylcholine receptor (AcChoR). We now show that MST2 also recognizes (-) nicotine, an agonist of AcChoR. Binding properties of MST2 therefore mimick, at least partially, binding properties of AcChoR. Injection in rabbits of MST2 mixed with adjuvant, elicited anti-idiotypic (anti-Id) antibodies that inhibited binding of the toxin to AcChoR. A proportion of these anti-Id antibodies specifically bound AcChoR and thereby mimicked the toxin. Furthermore, rabbits immunized with MST2 elicited auto-anti-anti-Id antibodies capable of binding the toxin. Our data provide a molecular explanation for the previously reported signs of myasthenia gravis as triggered by antibodies raised against cholinergic antagonists. Implications in the design of antisera to toxic proteins are discussed.

Conformational changes in two neurotoxic proteins from snake venoms.

alpha-Neurotoxin from Naja nigricollis and erabutoxin b from Laticauda semifasciata, two homologous neurotoxic proteins, are studied by circular dichroism, ultraviolet spectroscopy and fluorescence in various water/trifluoroethanol mixtures. The data obtained show that the beta structure of alpha-neurotoxin is conserved in water as well as in the organic solvent. By contrast, erabutoxin b changes from the beta-structure in water to the helix type in trifluoroethanol. The latter induces similarly for both toxins a structural modification around tryptophan 29, a residue common to all neurotoxins known to date. The vicinity of tyrosine 25, another common amino acid, is also altered by the presence of the organic solvent as demonstrated by the sudden increase of reactivity of the phenolic ring towards iodine. The present work affords some evidence for the presence of a particular structure located around the two aromatic residues, which is common to all neurotoxins and able to rearrange independently from the rest of the molecule. Biological importance of this peculiar region is highly probable.

Purification of the nicotinic acetylcholine receptor protein by affinity chromatography using a regioselectively modified and reversibly immobilized alpha-toxin from Naja nigricollis.

A new method of affinity chromatography purification of the detergent-solubilized nicotinic acetylcholine receptor protein (nAChR) is presented, based on the reversible coupling of a chemically monomodified alpha-toxin from Naja nigricollis to a resin. The alpha-toxin was monothiolated on the epsilon-amino group of its lysine-15 by reaction with N-succinimidly-3-(2-pyridyldithio)propionate and was covalently linked in a reversible manner to a thiopropyl-activated agarose resin by thiol-disulfide exchange. We found that 50% of the immobilized toxin molecules were effective for purifying nAChR, indicating a high accessibility of resin-bound toxins to their binding sites on the receptor protein. Purified alpha-toxin/nAChR complexes were eluted with nearly 100% recovery by reduction of disulfide bridges with dithiothreitol. nAChR solutions of high purity were obtained, as shown by polyacrylamide gel electrophoresis. A comparison was made with two other procedures of affinity chromatography using: (1) alpha-bungarotoxin from Bungarus multicinctus polymodified on several amines and covalently linked to a resin in a reversible manner, and (2) a commercial agarose resin bearing irreversibly immobilized alpha-cobrotoxin from Naja naja kaouthia. We conclude that: (1) the use of a selected regioselective linking of a peptidic ligand to a chromatography resin results in an increased efficiency of protein binding, and (2) a high yield of protein recovery is obtained via reversible covalent linking.

Analysis of cDNAs encoding the two subunits of crotoxin, a phospholipase A2 neurotoxin from rattlesnake venom: the acidic non enzymatic subunit derives from a phospholipase A2-like precursor.

We report the sequences of three cDNAs encoding the two subunits (CA and CB) of crotoxin, a neurotoxic phospholipase A2 from the venom of the South-American rattlesnake Crotalus durissus terrificus. CB is a basic and toxic phospholipase A2 and CA is an acidic, non toxic and non enzymatic three chain containing protein which enhances the lethal potency of CB. Two cDNAs encoding precursors of CB isoforms have been isolated from a cDNA library prepared from one venom gland. Both precursors are made of the same 16 residues signal peptide followed by a polypeptide of 122 amino acid residues. The two mature sequences differ from each other at eight positions and are in good agreement with the previous polypeptide sequence reported for CB. In the case of CA, the cDNA encodes a signal peptide identical to those found in CB precursors, followed by a polypeptide of 122 amino acids clearly homologous to phospholipases A2 and including three regions which correspond to the three chains of mature CA. This demonstrates that CA is generated from a phospholipase A2-like precursor, called pro-CA, by the removal of three peptides, leaving unchanged the molecule core cross-linked by disulfide bridges. The 5'-untranslated tracts of cDNAs encoding CA and CB are nearly identical and the 3'-untranslated tracts are very similar, suggesting that the mRNAs encoding the two crotoxin subunits may result from the alternative splicing of a single gene or from the existence of a recent gene conversion. Data have been analysed in light of recent results on other phospholipases A2 from different origins.

Molecular dynamics of two homologous neurotoxins revealed by 1H-2H exchange: an infrared spectrometry study.

Temperature effects on the hydrogen exchange kinetics and the infrared spectra of two homologous snake neurotoxins (Laticauda semifasciata erabutoxin b and Naja nigricollis toxin alpha) were investigated between 10 and 40 degrees C, at their isoionic pH. (1) Erabutoxin b is more accessible to the solvent than toxin alpha. (2) With increasing temperature, both toxin molecules undergo a global transition affecting the most accessible as well as the most buried hydrogens: the overall accessibility changes are more important for erabutoxin b than for toxin alpha. The different conformational stabilities of the toxins are also qualitatively supported by the temperature-induced shifts which affect the infrared amide I band of toxin alpha only. The existence of two conformer families could be responsible for the different conformational stability of these proteins.

Surface activity of polypeptide toxins isolated from Anemonia sulcata.

Circular dichroism spectra and surface activities are reported for toxins II and III isolated from Anemonia sulcata. Toxin II is highly surface active and as a result possesses a synergistic effect with phospholipase A2. In complete contrast, toxin III lacks detectable surface activity. The presence of sodium dodecyl sulfate (2.5 mg.ml-1) failed to trigger large conformation changes in both toxins II and III. From a comparison of the sequences of toxins II and III it is suggested that the hydrophobic region 17-27 is responsible for the surface activity of toxin II.

Complete amino-acid sequence of the beta-subunit of VTX from venom of the stonefish (Synanceia verrucosa) as identified from cDNA cloning experiments.

A cDNA encoding a subunit of the verrucotoxin (VTX) has been identified from a cDNA library derived from stonefish venom glands. It encodes a polypeptide of 708 amino-acid residues, followed by a 3'-untranslated region of 895 bp long. The ORF contains the complete mature sequence of the beta-subunit of the VTX, as inferred from both the presence of an identical N-terminus sequence and 96% homology among the 506 amino terminus residues found in the partial sequence of the beta-subunit of the stonustoxin from Synanceia horrida (Ghadessy, F.J., Jeyaseelan, K., Chung, M.C.M., Khoo, H.E., and Yuen, R. (1994) Toxicon 32, 1684-1688). Upstream the mature sequence, we noticed the presence of an incomplete peptide of a 13 amino acids, whose unusual primary structure supports the idea of the existence of a propeptide and/or of a new secretion signal.

X-ray structure at 1.55 A of toxin gamma, a cardiotoxin from Naja nigricollis venom. Crystal packing reveals a model for insertion into membranes.

The crystal structure of toxin gamma from Naja nigricollis has been solved and refined to 1.55 A resolution. The final R-factor, computed with all X-ray data available, is 17.9%. The three-dimensional structure is characterized by a core formed by two beta-sheets organized in three extended loops. It is similar to that of cardiotoxin V4II from Naja mossambica mossambica, with the exception of the hydrophobic loop I. The flexibility and variability of the loops contrast sharply with the rigidity of the molecular core and its high degree of structural conservation among the cardiotoxin family. The most flexible loop II adopts different conformations in the three monomers forming the crystal asymmetric unit. These monomers form a trimer around an approximate 3-fold axis, with conserved hydrophobic side-chains on the outside and hydrophilic residues in the central channel or involved in interactions with the other molecules. The trimer thus resembles a membrane protein with a central channel that could allow the passage of small ions. It is proposed as a model for the insertion of cardiotoxin into a membrane.

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.

Proton NMR studies of the structural and dynamical effect of chemical modification of a single aromatic side-chain in a snake cardiotoxin. Relation to the structure of the putative binding site and the cytolytic activity of the toxin.

This paper presents the comparative comprehensive analysis of NMR structural parameters (NOEs, scalar coupling, chemical shifts) of toxin gamma, a cardiotoxin isolated from the venom of Naja nigricollis, and three chemical derivatives, i.e. the 2-nitrophenylsulphonyl (NPS)-Trp11, 3-nitro-Tyr22 and 3-nitro-Tyr51 derivatives. In previous work, the chemical modifications of single side chains have suggested that these aromatic residues, in association with several lysine residues, contributed to the cytotoxicity of toxin gamma. Analysis of these results based on the refined solution structure of the toxin has resulted in the proposal of a conserved phospholipid binding site through which cardiotoxins are likely to interact with the membrane of target cells. The present work shows that modifications of either the tryptophan residue or the tyrosine residues, which are within or near the proposed binding site, have no influence on the three-dimensional structure of the protein. On the other hand, the proton exchange study of the backbone amides indicates that the structural core of the protein is destabilized in the three derivatives. This corresponds to a decrease of the overall stability of the protein as indicated by the comparative solvent denaturation study of the unmodified toxin gamma and the Trp11 derivative. More specifically, the dynamics of the three-stranded beta sheet, a part of the structural core, are highly perturbed by the chemical modifications. This sheet was previously proposed as a part of the phospholipid binding site of cardiotoxins. The dynamical perturbation of this site appears to be correlated with the decrease in toxicity of the chemical derivatives.

Artificial evolution of an enzyme active site: structural studies of three highly active mutants of Escherichia coli alkaline phosphatase.

The crystal structure of three mutants of Escherichia coli alkaline phosphatase with catalytic activity (k(cat)) enhancement as compare to the wild-type enzyme is described in different states. The biological aspects of this study have been reported elsewhere. The structure of the first mutant, D330N, which is threefold more active than the wild-type enzyme, was determined with phosphate in the active site, or with aluminium fluoride, which mimics the transition state. These structures reveal, in particular, that this first mutation does not alter the active site. The second mutant, D153H-D330N, is 17-fold more active than the wild-type enzyme and activated by magnesium, but its activity drops after few days. The structure of this mutant was solved under four different conditions. The phosphate-free enzyme was studied in an inactivated form with zinc at site M3, or after activation by magnesium. The comparison of these two forms free of phosphate illustrates the mechanism of the magnesium activation of the catalytic serine residue. In the presence of magnesium, the structure was determined with phosphate, or aluminium fluoride. The drop in activity of the mutant D153H-D330N could be explained by the instability of the metal ion at M3. The analysis of this mutant helped in the design of the third mutant, D153G-D330N. This mutant is up to 40-fold more active than the wild-type enzyme, with a restored robustness of the enzyme stability. The structure is presented here with covalently bound phosphate in the active site, representing the first phosphoseryl intermediate of a highly active alkaline phosphatase. This study shows how structural analysis may help to progress in the improvement of an enzyme catalytic activity (k(cat)), and explains the structural events associated with this artificial evolution.

Highly specific anti-estradiol antibodies: structural characterisation and binding diversity.

Subtle modulation of antibody-binding properties by protein engineering often lies with an accurate structural and energetic description of how an antigen is recognised. Thus, with the intent to increase the affinity and add a bias in favour of natural estradiol compared with its chemically modified immunogen, we have determined the crystal structure of two anti-estradiol monoclonal antibodies, 10G6D6 and 17E12E5. Although generated against the same estradiol derivative, these antibodies share little sequence identity, which is reflected in dissimilar binding pockets and in different positioning of the steroid. In both antibodies the characteristic 17-hydroxyl group is buried deeply at the bottom of hydrophobic pockets and stabilised by hydrogen bonds. Apart from this similarity, the steroid is oriented differently in the respective binding pockets. The high specificity of both antibodies has been mapped out, and even closely related steroids show low cross-reactivity. The structural studies of the complex formed between 10G6D6 and 6-CMO-estradiol have identified contacts between the 6-CMO coupling linker and an arginine residue from the heavy chain CDR2 segment. This segment is now being targeted by random mutagenesis to select mutants with a preference for natural estradiol compared to the branched hapten.

A monoclonal antibody recognizing a conserved epitope in a group of phospholipases A2.

Notexin and nigexine are monomeric phospholipases A2(PLA2s) from the venoms of Notechis scutatus scutatus and Naja nigricollis, respectively. Polyclonal antibodies raised in mice against these antigenic proteins displayed non-reciprocal cross-reactivity; anti-notexin antibodies recognized notexin but not nigexine, whereas anti-nigexine antibodies recognized both antigens. Polyclonal antibodies raised by successive immunization with nigexine and notexin contained cross-reacting antibodies with affinities for both antigens that differed from those of antibodies present in anti-nigexine antiserum. A monoclonal antibody has been obtained from a mouse immunized with both PLA2s. This monoclonal antibody, called MN1, recognized notexin and nigexine with comparable high affinity (Kd = 10(-9) M). It also recognized most purified PLA2s from elapid snake venoms and all PLA2-containing venoms from cobras and sea-snakes. This offers the first demonstration that most PLA2s from cobras and sea-snakes share a fine structure which is not restricted to the common catalytic site.

Fine chemical modifications at N- and C-termini enhance peptide presentation to T cells by increasing the lifespan of both free and MHC-complexed peptides.

We investigated the effect of modifying the N- and/or C-termini of the snake toxin peptide 24-36 on its presentation to T cells. Acetylation at the N-terminus as well as amidation at the C-terminus enhanced the capacity of the peptide to activate T cells. Simultaneous modifications further increased the stimulating activity, the peptide becoming approximately 100-fold more potent than the unmodified peptide. Clearly, the introduced modifications increased the lifetime of the peptide free in solution, by decreasing its proteolytic degradation, during the T cell stimulation assays. Paradoxically, however, at similar concentrations of free peptides, the modified ones, especially those having an acetylated N-terminus, were much more active than the unmodified peptide, irrespective of the experimental conditions. These observations suggested that components other than protection from proteolytic degradation should be associated with the higher stimulating activities of the modified peptides. Accordingly, chasing experiments with APC revealed that acetylation at N-terminus caused a higher persistence of the peptides at APC surface. Together, our data indicate that (i) the T cell stimulating capacity of a peptide is associated with its lifespans in the free and MHC II bound states; and (ii) these lifespans can be greatly enhanced by introducing fine chemical modifications at N- and C-termini. These data may have some implications in designing more potent peptidic immunomodulators.