Improved mimicry of a foot-and-mouth disease virus antigenic site by a viral peptide displayed on beta-galactosidase surface

Nanoparticulate architecture of protein-based artificial viruses is supported by protein-DNA interactions

AIM & METHODS: We have produced two chimerical peptides of 10.2 kDa, each contain four biologically active domains, which act as building blocks of protein-based nonviral vehicles for gene therapy. In solution, these peptides tend to aggregate as amorphous clusters of more than 1000 nm, while the presence of DNA promotes their architectonic reorganization as mechanically stable nanometric spherical entities of approximately 80 nm that penetrate mammalian cells through arginine-glycine-aspartic acid cell-binding domains and promote significant transgene expression levels.

RESULTS & CONCLUSION

The structural analysis of the protein in these hybrid nanoparticles indicates a molecular conformation with predominance of α-helix and the absence of cross-molecular, β-sheet-supported protein interactions. The nanoscale organizing forces generated by DNA-protein interactions can then be observed as a potentially tunable, critical factor in the design of protein-only based artificial viruses for gene therapy.

Viral fitness can influence the repertoire of virus variants selected by antibodies

Minority genomes in the mutant spectra of viral quasispecies may differ in relative fitness. Here, we report experiments designed to evaluate the contribution of relative fitness to selection by a neutralizing monoclonal antibody (mAb). We have reconstructed a foot-and-mouth disease virus (FMDV) quasispecies, with two matched pairs of distinguishable mAb-escape mutants as minority genomes of the mutant spectrum. Each mutant of a pair differs from the other by 11-fold or 33-fold in relative fitness. Analysis of the mutant spectra of virus populations selected with different concentrations of antibody in infections in liquid culture medium has documented a dominance of the high fitness counterpart in the selected population. Plaque development as a function of increasing concentration of the antibody has shown that each mutant of a matched pair yielded the same number of plaques, although the high fitness mutant required less time for plaque formation, and attained a larger plaque size at any given time-point. This result documents equal intrinsic resistance to the antibody of each mutant of a matched pair, confirming previous biochemical, structural, and genetic studies, which indicated that the epitopes of each mutant pair were indistinguishable regarding reactivity with the monoclonal antibody. Thus, relative viral fitness can influence in a significant way the repertoire of viral mutants selected from a viral quasispecies by a neutralizing antibody. We discuss the significance of these results in relation to antibody selection, and to other selective forces likely encountered by viral quasispecies in vivo.

High-throughput, functional screening of the anti-HIV-1 humoral response by an enzymatic nanosensor

The impact of antibodies on the target's epitope conformation is a major determinant of HIV-1 neutralization and a potential contributor to disease progression. We explore here a conformation-sensitive enzymatic nanosensor for the high-throughput functional screening of human anti-HIV-1 antibodies in sera. When displaying a model epitope from a gp41 immunodominant region (Env residues from 579 to 613), the sensing signal quantitatively distinguishes between adaptive and non-adaptive antibody binding. By using this tool, we have identified IgG4 as the immunoglobulin subpopulation most efficient in the structural modification of the target epitope.

Insertional protein engineering for analytical molecular sensing

The quantitative detection of low analyte concentrations in complex samples is becoming an urgent need in biomedical, food and environmental fields. Biosensors, being hybrid devices composed by a biological receptor and a signal transducer, represent valuable alternatives to non biological analytical instruments because of the high specificity of the biomolecular recognition. The vast range of existing protein ligands enable those macromolecules to be used as efficient receptors to cover a diversity of applications. In addition, appropriate protein engineering approaches enable further improvement of the receptor functioning such as enhancing affinity or specificity in the ligand binding. Recently, several protein-only sensors are being developed, in which either both the receptor and signal transducer are parts of the same protein, or that use the whole cell where the protein is produced as transducer. In both cases, as no further chemical coupling is required, the production process is very convenient. However, protein platforms, being rather rigid, restrict the proper signal transduction that necessarily occurs through ligand-induced conformational changes. In this context, insertional protein engineering offers the possibility to develop new devices, efficiently responding to ligand interaction by dramatic conformational changes, in which the specificity and magnitude of the sensing response can be adjusted up to a convenient level for specific analyte species. In this report we will discuss the major engineering approaches taken for the designing of such instruments as well as the relevant examples of resulting protein-only biosensors.

Engineering the E. coli beta-galactosidase for the screening of antiviral protease inhibitors

Site-specific proteolysis is essential in many fundamental cellular and viral processes. It has been previously shown that the Escherichia coli beta-galactosidase can be useful for the high-throughput screening of human immunodeficiency virus type 1 protease inhibitors. Here, by using crystallographic and functional data of the bacterial enzyme, we have identified a new accommodation site between amino acids 581 and 582, in a solvent-exposed and flexible beta-turn of domain III. The placement of the model peptide reproducing the matrix-capsid (p17/p24) gag cleavage sequence renders a highly active and efficiently digested chimeric construct. The use of this insertion site, that increases the cleavage potential of this reporter enzyme, can improve the sensitivity and dynamic range of the antiviral drug assay. This simple and highly specific analytical test may also be extended to the screening of other specific protease inhibitors by a convenient colorimetric assay.

Allosteric enzymes as biosensors for molecular diagnosis

Biosensors are hybrid analytical devices that amplify signals generated from the specific interaction between a receptor and the analyte, through a biochemical mechanism. Biosensors use tissues, whole cells, artificial membranes or cell components like proteins or nucleic acids as receptors, coupled to a physicochemical signal transducer. Allosteric enzymes exhibit a catalytic activity that is modulated by specific effectors, through binding to receptor sites that are distinct from the active site. Several enzymes, catalyzing easily measurable reactions, have been engineered to allosterically respond to specific ligands, being themselves the main constituent of new-generation biosensors. The molecular basis, robustness and application of allosteric enzymatic biosensing are revised here.

Engineering nuclear localization signals in modular protein vehicles for gene therapy

Amino acids from 126 to 135 of the SV40 virus T antigen act as efficient nuclear localization signal during infection but also when fused to recombinant proteins. This peptide has been inserted into two alternative acceptor sites of a modified Escherichia coli beta-galactosidase which also displays a DNA-binding domain, a cell-binding motif for integrin alpha(v)beta(3) targeting and cell internalization, and a cryptic nuclear targeting signal naturally present in the bacterial enzyme. In cultured cells, the presence of the SV40 peptide enhances the expression of a delivered DNA up to 30-fold. However, the DNA expression levels are largely depending on the chosen insertion site for the SV40 segment concomitant to the structural impact of peptide accommodation on the protein vehicle. The structural stability of the hybrid protein, apparently critical for efficient gene transfer, is discussed in the context of modular protein engineering to develop non-viral vectors for gene therapy.

Engineering of Escherichia coli beta-galactosidase for solvent display of a functional scFv antibody fragment

Protein engineering allows the generation of hybrid polypeptides with functional domains from different origins and therefore exhibiting new biological properties. We have explored several permissive sites in Escherichia coli beta-galactosidase to generate functional hybrid enzymes displaying a mouse scFv antibody fragment. When this segment was placed at the amino-terminus of the enzyme, the whole fusion protein was stable, maintained its specific activity and interacted specifically with the target antigen, a main antigenic determinant of foot-and-mouth disease virus. In addition, the antigen-targeted enzyme was enzymatically active when bound to the antigen and therefore useful as a reagent in single-step immunoassays. These results prove the flexibility of E. coli beta-galactosidase as a carrier for large-sized functional domains with binding properties and prompt the further exploration of the biotechnological applicability of the scFv enzyme targeting principle for diagnosis or other biomedical applications involving antigen tagging.

Enhanced response to antibody binding in engineered beta-galactosidase enzymatic sensors

Peptide display on solvent-exposed surfaces of engineered enzymes allows them to respond to anti-peptide antibodies by detectable changes in their enzymatic activity, offering a new principle for biosensor development. In this work, we show that multiple peptide insertion in the vicinity of the Escherichia coli beta-galactosidase active site dramatically increases the enzyme responsiveness to specific anti-peptide antibodies. The modified enzymes HD7872A and HT7278CA, carrying eight and 12 copies respectively of a foot-and-mouth disease peptide per enzyme molecule, show antibody-mediated activation factors higher than those previously observed in the first generation enzymatic sensors, for HT7278CA being close to 400%. The analysis of the signal transduction process with multiple inserted proteins strongly suggests a new, non-exclusive mechanism of enzymatic regulation in which the target proteins might be stabilised by the bound antibody, extending the enzyme half-life and consequently enhancing the signal-background ratio. In addition, the tested sensors are differently responsive to sera from immune farm animals, depending on the antigenic similarity between the B-cell epitopes in the immunising virus and those in the peptide used as sensing element on the enzyme surface. Altogether, these results point out the utility of these enzymatic biosensors for a simple diagnosis of foot-and-mouth disease in an extremely fast homogeneous assay.

Efficient accommodation of recombinant, foot-and-mouth disease virus RGD peptides to cell-surface integrins

The engineering of either complete virus cell-binding proteins or derived ligand peptides generates promising nonviral vectors for cell targeting and gene therapy. In this work, we have explored the molecular interaction between a recombinant, integrin-binding foot-and-mouth disease virus RGD peptide displayed on the surface of a carrier protein and its receptors on the cell surface. By increasing the number of viral segments, cell binding to recombinant proteins was significantly improved. This fact resulted in a dramatic growth stimulation of virus-sensitive BHK(21) cells but not virus-resistant HeLa cells in protein-coated wells. Surprisingly, growth stimulation was not observed in vitronectin-coated plates, suggesting that integrins other than alpha(v)beta(3) could be involved in binding of the recombinant peptide, maybe as coreceptors. On the other hand, both free and cell-linked integrins did not modify the enzymatic activity of RGD-based enzymatic sensors that contrarily, were activated by the induced fit of anti-RGD antibodies. Those findings are discussed in the context of a proper mimicry of the unusually complex architecture of this cell-binding site as engineered in multifunctional proteins.

Molecular organization of protein-DNA complexes for cell-targeted DNA delivery

Multifunctional proteins are interesting candidates for nonviral gene transfer to and expression in their target cells. Since at difference of viral vectors, the performance of these vehicles depends on their functional optimisation, a better comprehension of the molecular organisation within protein-DNA complexes would be of great help in reaching their full delivery potential. In this work, we have characterised an RGD-tagged, cell-targeted multifunctional beta-galactosidase carrying a poly-lysine-based DNA-binding domain. In solution, the engineered enzyme spontaneously forms proteinaceous particles of between 20 and 40 nm in diameter that might contain around 10 molecules of enzymatically active protein. Plasmid DNA is efficiently condensed into these particles without modification of the shape, morphology or enzymatic activity, indicative of a comfortable molecular accommodation to the DNA-binding domains. Although the RGD peptide remains equally solvent-exposed and immunoreactive at different DNA-protein ratios, an optimal expression level of cell-delivered genes and integrin-binding specificity are both achieved at 0.02 microg of DNA per microg of protein, indicative of influences of the packaged nucleic acid on the interaction between filled vehicles and the receptors of target cells.

Successful mimicry of a complex viral antigen by multiple peptide insertions in a carrier protein

The antigenic properties of a viral peptide from the surface of foot-and-mouth disease virus particles have been successfully mimicked by multiple insertion in solvent-exposed regions of Escherichia coli beta-galactosidase. By increasing the number of viral peptides per enzyme monomer, the average IC(50) of hybrid proteins in a competitive enzyme-linked immunosorbent assay) have decreased to values close to that presented by natural virions. Moreover, the antigenic diversity of these new recombinant enzymes when measured with different anti-virus antibodies has also been largely reduced, indicating a better presentation of the epitopes located in the viral peptide. Although bivalent antibody binding could have been favoured by multiple presentation, conformational modifications of the viral peptide, due to the presence of other insertions or a cooperative antibody binding cannot be excluded. In addition, a multidimensional antigenic analysis have grouped together the multiple-inserted proteins with the native virus, suggesting that increasing the number of insertions could be a good strategy to reproduce the antigenic properties of an immunoreactive peptide in a natural multimeric disposition.

Influence of three-dimensional structure on the immunogenicity of a peptide expressed on the surface of a plant virus

The influence of peptide structure on immunogenicity has been investigated by constructing a series of cowpea mosaic virus (CPMV) chimaeras expressing the 14 amino acid NIm-1A epitope from human rhinovirus 14 (HRV-14) at different positions on the capsid surface. Biochemical and crystallographic analysis of a CPMV/HRV chimaera expressing the NIm-1A epitope inserted into the betaC'-betaC" loop of the S protein revealed that, although the inserted peptide was free at its C-terminus, it adopted a conformation distinct from that previously found when a similarly cleaved peptide was expressed in the betaB-betaC loop of the S protein. Adjustment of the site of insertion within the betaB-betaC loop resulted in the isolation of a chimaera in which cleavage at the C-terminus of the epitope was much reduced. Crystallographic analysis confirmed that in this case the epitope was presented as a closed loop. Polyclonal antisera raised against the CPMV/ HRV chimaera presenting the NIm-1A epitope as a closed loop had a significantly enhanced ability to bind to intact HRV-14 particles compared with antisera raised against chimaeras presenting the same sequence as peptides with free C-termini. These results demonstrate that the mode of presentation of an epitope on a heterologous carrier can dramatically affect its immunological properties.

Position-dependent processing of peptides presented on the surface of cowpea mosaic virus

The plant virus cowpea mosaic virus (CPMV) has been developed as an epitope-presentation system. Numerous epitopes have been expressed in the betaB-betaC loop of the CPMV small coat protein, all of which undergo a cleavage reaction between their two carboxy-terminal residues. Although many peptides presented in this manner give an authentic immune response, this was not the case for the NIm-1A epitope from human rhinovirus-14. Crystallography revealed significant differences between the structure of NIm-1A on CPMV compared with its native configuration. The 3D structure of C PMV expressing NIm-1A was used to design alterations to the context of the NIm-1A graft.

The expression of recombinant genes from bacteriophage lambda strong promoters triggers the SOS response in Escherichia coli

The production of several non-related heterologous proteins in recombinant Escherichia coli cells promotes a significant transcription of recA and sfiA SOS DNA repair genes. The activation of the SOS system occurs when the expression of plasmid-encoded genes is directed by the strong lambda lytic promoters, but not by IPTG-controlled promoters either at 37 or at 42 degrees C, and it is linked to an extensive degradation of the proteins after their synthesis. The triggering signal for the SOS response could be an important arrest of cell DNA replication observed within the first hour after the induction of recombinant gene expression. The stimulation of this DNA repair system can partially account for the toxicity exhibited by recombinant proteins on actively producing E. coli cells.

Dynamics of in vivo protein aggregation: building inclusion bodies in recombinant bacteria

Time-dependent aggregation of a plasmid-encoded beta-galactosidase fusion protein, VP1LAC, has been carefully monitored during its high-rate synthesis in Escherichia coli. Immediately after recombinant gene induction, the full-length form of the protein steadily accumulates into rapidly growing cytoplasmic inclusion bodies. Their volume increases during at least 5 h at a rate of 0.4 micron3 h-1, while the average density remains constant. Protein VP1LAC accounts for about 90% of the aggregated protein throughout the building process. Minor components, such as DnaK and GroEL chaperones, have been identified in variable, but low concentrations. The homogeneous distribution of inclusion bodies among the cell population and the coexistence of large, still growing bodies with newly appearing aggregates indicate that the aggregation cores are mutually exclusive, this fact being a main determinant of the in vivo dynamics of protein aggregation.

Distinct mechanisms of antibody-mediated enzymatic reactivation in beta-galactosidase molecular sensors

The antibody-mediated reactivation of engineered Escherichia coli beta-galactosidases [Benito et al. (1996) J. Biol. Chem. 271, 21251-21256] has been thoughtfully investigated in three recombinant molecular sensors. Proteins M278VP1, JX772A and JX795A display the highly antigenic G-H loop peptide segment of foot-and-mouth disease virus VP1 protein, accommodated in different solvent-exposed loops of the assembled tetramer. These chimaeric enzymes exhibit a significant increase in enzymatic activity upon binding of either monoclonal antibodies or sera directed against the inserted viral peptide. In JX772A but not in M278VP1, the Fab 3E5 antibody fragment promotes reactivation to the same extent as the complete antibody. On the other hand, M278VP1 Km is reduced by more than 50% in the presence of activating serum, this parameter remains invariable in JX772A and it is only slightly modified in JX795A. In these last two proteins, significant k(cat) variations can account for the increased enzymatic activity. Alternative reactivation mechanisms in the different beta-galactosidase probes are discussed in the context of the bacterial enzyme structure and its tolerance to antibody-induced conformational modifications.

Conformational flexibility in a highly mobile protein loop of foot-and-mouth disease virus: distinct structural requirements for integrin and antibody binding

The G-H loop of foot-and-mouth disease virus VP1 protein is a highly mobile peptide, that extends from the capsid surface and that in native virions is invisible by X-ray crystallography. In serotype C, this segment contains a hypervariable region with several continuous, overlapping, B-cell epitopes that embrace the conserved Arg-Gly-Asp (RGD) cell attachment motif. The solvent-exposed positioning of this peptide by selective insertion into different structural frameworks of E. coli beta-galactosidase, generates a spectrum of antigenic variants which react distinctively with a panel of anti-VP1 monoclonal antibodies and exhibit different efficiencies as cell ligands. The cell attachment efficiency is much less restricted by the different positioning of the viral segment at the insertion sites. A molecular model of an inserted stretch reveals a highest flexibility of the RGD tripeptide segment compared with the flanking sequences, that could allow a proper accommodation to integrin receptors even in poorly antigenic conformations. The non-converging structural requirements for RGD-mediated integrin binding and antibody recognition, explains the dynamism of the generation of neutralisation-resistant antigenic variants in the viral quasi-species, arising from a conformational space of integrin-binding competent peptides. This might be of special relevance for foot-and-moth disease virus evolution, since unlike in other picornaviruses, the cell binding motif and the major neutralising B-cell epitopes overlap in a solvent-exposed peptide accessible to the host immune system, in a virion lacking canyons and similar hiding structures.

Engineering of solvent-exposed loops in Escherichia coli beta-galactosidase

The Escherichia coli beta-galactosidase is a high molecular mass tetrameric enzyme extensively used as a molecular marker. Despite its proven utility as a partner in fusion proteins, previous attempts to generate insertional mutants rendered inactive or poorly active enzymes, hampering its further engineering for the construction of multifunctional enzymes. We have explored several solvent-exposed loops on the tetramer, namely those spanning residues 246-254, 271-287, 578-584, 770-773, and 793-806, as acceptor sites to accommodate functional protein segments on the surface of active beta-galactosidase enzymes. An RGD-containing antigenic peptide positioned in these sites interacts efficiently with specific monoclonal antibodies as well as target integrins on the surface of mammalian cells. The resulting chimeric enzymes are soluble, stable, produced in high yields and enzymatically active. Moreover, the identified insertion sites could be appropriated for the design of promising beta-galactosidase-based molecular sensors.

A cell adhesion peptide from foot-and-mouth disease virus can direct cell targeted delivery of a functional enzyme

The G-H loop of foot-and-mouth disease virus is a disordered protrusion of the VP1 protein exposed on the virion surface. This short stretch includes an arginine-glycine-aspartic acid tripeptide, a recognized integrin-binding motif, which is responsible for cell attachment and infection. Eight copies of a peptide reproducing the amino acid sequence of this FMDV ligand have been displayed in solvent-exposed regions on an enzymatically active recombinant beta-galactosidase. This viral peptide segment enables the chimeric enzyme to bind mammalian cell lines with different efficiencies, probably depending on the number of suitable cell receptors present on each of them. Moreover, it also promotes the internalization of the attached enzyme, which is transiently active inside the cells. These results suggest further exploration of the potential use of short adhesion peptides of viral origin as cell attachment tags to direct the targeted delivery of both genes and enzymes, instead of whole, infectious viruses.

Display-induced antigenic variation in recombinant peptides

Peptide display on solvent-exposed surfaces of carrier proteins is a promising approach pursuing the identification and improvement of reactive amino acid sequences. However, the contribution of the molecular environment where the peptide is inserted on its interactive properties remains essentially unexplored. By an exhaustive antigenic analysis of the same peptide displayed on 20 structurally distinct frameworks, we show that peptide accommodation into the acceptor site has dramatic effects on its immunoreactivity. Conformational constraints can modulate the molecular recognition properties of the insert within a surprisingly wide range, probably by affecting the positioning of critical contact residues. The observed display-induced antigenic variation prompts a careful consideration of the molecular context when evaluating output amino acid sequences from screening of peptide libraries or application of directed molecular evolution technologies.

Optimized release of recombinant proteins by ultrasonication of E. coli cells

The release kinetics of beta-galactosidase protein have been determined during small-scale ultrasonication of E. coli cells. Among several studied parameters, ionic strength and cell concentration have the least influence on the rate of protein recovery, whereas sample volume and acoustic power dramatically affect the final yield of soluble protein in the cell-free fraction. The analysis of these critical parameters has prompted us to propose a simple model for E. coli disintegration that only involves acoustic power and sample volume, and which allows prediction of optimal sonication times to recover significant amounts of both natural and recombinant proteins in a given set of relevant conditions.

Biosensor characterization of antigenic site A of foot-and-mouth disease virus presented in different vector systems

The region 141-160 of the VP1 protein of foot-and-mouth disease virus known as site A is an immunodominant region that has been extensively studied for the purpose of developing a synthetic vaccine. In the present study, site A of foot-and-mouth disease virus was inserted in three different loops of the maltose-binding protein and its antigenicity was compared with site A presented as a conjugated synthetic peptide or inserted in beta-galactosidase. The affinity of antibodies elicited against the site A synthetic peptide was also compared with that of antibodies raised against the site A inserted within the two carrier proteins. Using biosensor technology it was possible to estimate the concentration of site A antibodies present in the various antisera and to show that site A fused to maltose-binding protein was a slightly better mimic of the epitope present in the virus particle than the synthetic peptide or the beta-galactosidase recombinant construct.

Insertional mutagenesis in the tailspike protein of bacteriophage P22

The tailspike protein (TSP) of bacteriophage P22 is a homotrimeric multifunctional protein responsible for recognition and hydrolysis of Salmonella typhimurium host receptors. Once properly folded, TSP shows an unusual stability to temperature and detergent denaturation, prompting the analysis of TSP as a framework for the positioning of heterologous protein segments. We have explored the flexibility of inner sites and both amino and carboxy termini to accommodate foreign peptides for phage display. In the examined inner sites, TSP is extremely sensitive to minor sequence modifications, the folding intermediates being rapidly degraded. However, both the amino and carboxy termini are tolerant to peptide fusions, rendering stable and functional chimeric proteins. Surprisingly, the amino terminus, which connects the tail to the neck structure, can accept large peptide fusions, and the foreign amino acid stretches are solvent-exposed and highly antigenic on assembled, infectious virus particles.

A similar pattern of interaction for different antibodies with a major antigenic site of foot-and-mouth disease virus: implications for intratypic antigenic variation

The three-dimensional structures of the Fab fragment of a neutralizing antibody raised against a foot-and-mouth disease virus (FMDV) of serotype C1, alone and complexed to an antigenic peptide representing the major antigenic site A (G-H loop of VP1), have been determined. As previously seen in a complex of the same antigen with another antibody which recognizes a different epitope within antigenic site A, the receptor recognition motif Arg-Gly-Asp and some residues from an adjacent helix participate directly in the interaction with the complementarity-determining regions of the antibody. Remarkably, the structures of the two antibodies become more similar upon binding the peptide, and both undergo considerable induced fit to accommodate the peptide with a similar array of interactions. Furthermore, the pattern of reactivities of five additional antibodies with versions of the antigenic peptide bearing amino acid replacements suggests a similar pattern of interaction of antibodies raised against widely different antigens of serotype C. The results reinforce the occurrence of a defined antigenic structure at this mobile, exposed antigenic site and imply that intratypic antigenic variation of FMDV of serotype C is due to subtle structural differences that affect antibody recognition while preserving a functional structure for the receptor binding site.

Limited in vivo proteolysis of aggregated proteins

Degradation pathways of insoluble proteins have been analyzed in Escherichia coli by using a N-terminal beta-galactosidase fusion protein (VP1LAC) that aggregates immediately after its synthesis. In recombinant E. coli cells, lower molecular mass products, antigenically related to the entire fusion, accumulate together with the entire fusion. In absence of protein synthesis, the insoluble intact protein declines, suggesting that degradation of the recombinant protein also affects aggregated protein. Time course analysis of both soluble and insoluble cell fractions has revealed a limited proteolysis of the insoluble protein that removes the heterologous domain and permits the resulting beta-galactosidase fragments to refold and solubilize. Further extensive degradation occurs exclusively on soluble protein. The restricted proteolysis of misfolded, insoluble protein is the initiating event of a subsequent degradative pathway in which rate-limiting steps permit the accumulation of stable degradative intermediates.

Conformational preferences of a peptide corresponding to the major antigenic determinant of foot-and-mouth disease virus: implications for peptide-vaccine approaches

The conformational preferences in solution of a peptide corresponding to the GH loop of the VP1 capsid protein from the foot-and-mouth disease virus were examined by proton nuclear magnetic resonance and circular dichroism. The GH loop is the major antigenic determinant of the virus and participates in cell attachment through an integrin-like Arg-Gly-Asp sequence. The synthetic peptide, corresponding to residues Gly132 to Ser162 of the VP1 capsid protein of the serotype O, is largely disordered in aqueous solution as shown by the absence of long- and medium-range NOE contacts and by random-like chemical shifts values. Helical contents in aqueous solution were estimated to be less than 10%, as determined by extrapolation of trifluoroethanol titration from CD measurements, in good agreement with estimations from NMR experiments. In the presence of 40% trifluoroethanol an alpha-helix, flanked by two proline residues between Asn12 (Asn143 in the intact protein) and Leu28 (159), is induced. This contrasts with the 3(10) helix observed between residues Leu148 and Val155 in the crystal structure of the dithiothreitol-reduced virus, indicating that the cosolvent does not stabilize a residual, low-populated structure, similar to that in the intact virus. Several algorithms also fail to predict the structure found in the intact virus because these are based mainly on local sequence information. The lack of structure of the peptide in aqueous solution strongly suggests that the conformational determinants sufficient for the structure stabilization of this highly immunogenic antigen are mostly dictated by interactions of the loop with other regions of the virus structure, and do not arise from local amino acid sequence information. The ability of designed GH-VP1 peptides to neutralize anti-virus antibodies is likely to arise from antibody-induced conformation of the peptide and its application as peptide vaccines is not straightforward. Similarly, insertion of these peptides in carriers or macromolecular assemblies as vaccine vectors would depend on the conformation adopted at the insertion site and its success cannot be predicted.

Modulating the immunological properties of a linear B-cell epitope by insertion into permissive sites of the MalE protein

In a previous study, a set of positions in the MalE protein from Escherichia coli were identified, which tolerated short insertions or deletions without compromising the maltose binding activity of the protein. It is now shown that these sites accommodate an insert of 13 amino acids and are, therefore, permissive. Eleven sites were used, including eight permissive sites, to display a linear neutralization B-cell epitope of poliovirus (C3 epitope) at different positions on the surface of MalE. The affinity of a monoclonal neutralizing anti-poliovirus antibody (anti-C3 mAb) for the hybrid proteins varied from undetectable, to more than 1000 times higher than for the synthetic peptide. Therefore, some MalEC3 proteins mimic interactions of the viral epitope with the monoclonal antibody more efficiently than the free peptide. The results are interpreted in terms of the mobility of the insert and its flanking regions. It was further shown that some of the purified hybrid proteins are able to induce high titer anti-C3-peptide antibodies in mice. A strong correlation exists between the capacity of a MalEC3 protein to induce anti-C3-peptide antibodies and the antigenicity of the inserted peptide, measured with a polyclonal serum raised against the synthetic peptide.

A recombinant, arginine-glycine-aspartic acid (RGD) motif from foot-and-mouth disease virus binds mammalian cells through vitronectin and, to a lower extent, fibronectin receptors

The cell-binding abilities of a recombinant, RGD-containing peptide from foot-and-mouth disease virus (FMDV) have been characterized in HeLa and BHK cells. This peptide represents the aa sequence of the solvent-exposed G-H loop of protein VP1 which is involved in cell recognition and infection. The efficiency of the viral motif in promoting cell attachment and spreading is comparable to that shown by fibronectin or vitronectin. Cell binding is inhibited by a monoclonal antibody directed against a viral, RGD-involving B-cell epitope and also by sera against vitronectin (alpha V beta 3/beta 5) and fibronectin (alpha 5 beta 1) receptors. In addition, a synthetic RGD peptide, which is a ligand for both integrins, prevents the cell binding mediated by the FMDV domain. These data demonstrate that the FMDV RGD motif is a potent ligand for cell-receptor integrins and sufficient to promote cell attachment to susceptible cells mainly through the vitronectin receptor.

Converging antigenic structure of a recombinant viral peptide displayed on different frameworks of carrier proteins

A peptide reproducing the G-H loop amino acid sequence of foot-and-mouth disease virus VP1 protein was fused to the solvent-exposed C-terminus of the bacteriophage P22 tailspike protein [Carbonell and Villaverde (1996) Gene, in press], a homotrimeric polypeptide with a strong beta-helical structure. This fusion does not interfere with the biological activities of the phage tail. The antigenic profile of the complex antigenic site A within the G-H loop has been determined by competitive ELISA with a panel of monoclonal antibodies directed against different overlapping B-cell epitopes. The antigenic data have been compared with those obtained with a set of 12 chimeric beta-galactosidases displaying the G-H loop on different exposed regions. A high coincidence has been evidenced between the antigenicity of the viral peptide fused to the phage protein and that of some peptides inserted in an exposed loop of the activating interface of beta-galactosidase. This indicates that completely different structural frameworks of carrier proteins can provide similar constraints that allow the recombinant peptide to successfully mimic the antigenicity, and probably conformational features, of the natural peptide on the virion surface.

Antigenicity of a viral peptide displayed on beta-galactosidase fusion proteins is influenced by the presence of the homologous partner protein

Several beta-galactosidase fusion proteins have been constructed containing the entire VP1 protein from foot-and-mouth disease virus (FMDV) [Corchero et al. (1996) J. Biotechnol. in press]. The antigenicity of the major immunodominant site A (13 amino acids in length) within the VP1 protein has been studied in competitive ELISA using a panel of seven monoclonal antibodies elicited against the whole virus and recognizing B-cell epitopes within this site. None of the fusion proteins is able to reproduce the antigenic profile of FMDV, all of them being less immunoreactive than the virus particles. On the other hand, significant differences in the reactivity of site A are displayed on the different fusion proteins, being for some antibodies about 10-fold. This indicates that the reactivity of a small peptide included in its natural place inside the heterologous domain can be significantly influenced by the position of the homologous partner in the fusion protein.

Peptide display on functional tailspike protein of bacteriophage P22

The tailspike protein (TSP) of Salmonella typhimurium P22 bacteriophage is a multifunctional homotrimer, 6 copies of which are non-covalently attached to the capsid to form the virion tail in the last reaction of phage assembly. An antigenic peptide of foot-and-mouth disease virus (FMDV), aa 134-156 of protein VP1, has been joined to the carboxy terminus of TSP, and produced as a fusion protein in Escherichia coli directed by the trp promoter. The resulting fusion protein is soluble, stable, non-toxic, and can be easily purified by standard procedures. Moreover, both the endorhamnosidase and capsid assembly activities of the TSP are conserved, permitting the fusion protein to reconstitute infectious viruses by in vitro association with tailless particles. In both free TSP and P22 chimeric virions, the foreign peptide is solvent-exposed and highly antigenic, indicating that P22 TSP could be an appropriate carrier protein for multimeric peptide display.

Beta-galactosidase enzymatic activity as a molecular probe to detect specific antibodies

The main antigenic region of foot-and-mouth disease virus serotype C1, also called site A, has been inserted in zones of the beta-galactosidase important for the stabilization of the active site, causing important changes in the Km and the specific activity of the resulting enzymes. The peptide is displayed at the surface of the recombinant proteins and, in all the cases, presents a good antigenicity. Among the recombinant proteins constructed, in proteins M278VP1 and M275SVP1 the peptide is inserted in a large loop of the beta-galactosidase (amino acids 272-288) involved in the formation of the activating interface. In these constructs, the binding of the specific antibodies directed to the foreign peptide causes an increase of the beta-galactosidase activity up to about 200%. This phenomenon has been proved using monoclonal antibodies and also using polyclonal sera generated against the peptide. Different hypothesis of the mechanism of modulation upon antibody binding are discussed. This insertion site seems to be sensitive enough to enzymatic modulation mediated by antibody binding. We propose further exploring this insertion site as a tool for a rapid detection of specific antibodies in a quick and simple homogeneous assay based on the colorimetric determination of beta-galactosidase activity.

The position of the heterologous domain can influence the solubility and proteolysis of beta-galactosidase fusion proteins in E. coli

The VP1 protein (23 kDa) of the foot-and-mouth disease virus has been produced in MC1061 and BL21 E. coli strains as beta-galactosidase fusion proteins, joined to either the amino and/or the carboxy termini of the bacterial enzyme. In BL21, devoid of La protease, all the recombinant fusion proteins are produced at higher yields than in MC1061, and occur mainly as inclusion bodies. The fusion of VP1 at the carboxy terminus yields a protease-sensitive protein whose degradation releases a stable, enzymatically active polypeptide indistinguishable from the native beta-galactosidase. On the contrary, when the same viral domain is fused to the amino terminus, the resulting chimeric protein is resistant to proteolysis even in the soluble form. These data demonstrate that the position of the heterologous domain in beta-galactosidase fusion proteins would not be irrelevant since it can dramatically influence properties of biotechnological interest such as solubility and proteolytic resistance.

Topology of the membrane protein LamB by epitope tagging and a comparison with the X-ray model

We previously developed a genetic approach to study, with a single antibody, the topology of the outer membrane protein LamB, an Escherichia coli porin with specificity towards maltodextrins and a receptor for bacteriophage lambda. Our initial procedure consisted of inserting at random the same reporter epitope (the C3 neutralization epitope from poliovirus) into permissive sites of LamB (i.e., sites which tolerate insertions without deleterious effects on the protein activities or the cell). A specific monoclonal antibody was then used to examine the position of the inserted epitope with respect to the protein and the membrane. In the present work, we set up a site-directed procedure to insert the C3 epitope at new sites in order to distinguish between two-dimensional folding models. This allowed us to identify two new surface loops of LamB and to predict another periplasmic exposed region. The results obtained by random and directed epitope tagging are analyzed in light of the recently published X-ray structure of the LamB protein. Study of 23 hybrid LamB-C3 proteins led to the direct identification of five of the nine external loops (L4, L5, L6, L7, and L9) and led to the prediction of four periplasmic loops (I1, I4, I5, and I8) of LamB. Nine of the hybrid proteins did not lead to topological conclusions, and none led to the wrong predictions or conclusions. The comparison indicates that parts of models based on secondary structure predictions alone are not reliable and points to the importance of experimental data in the establishment of outer membrane protein topological models. The advantages and limitations of genetic foreign epitope insertion for the study of integral membrane proteins are discussed.

Use of macromolecular assemblies as expression systems for peptides and synthetic vaccines

The past decade has witnessed the development of numerous systems for the presentation of antigens on the surface of self-assembling macromolecules. Although the sites for insertion were initially chosen empirically, the determination of the three-dimensional structures of a number of carrier macromolecules has enabled structure-based insertional mutagenesis to be used increasingly. Furthermore, it is now feasible to determine the structure of an inserted sequence as presented in a heterologous environment, making it possible to correlate the detailed structure of a peptide with its immunological properties.

The major subunit ClpG of Escherichia coli CS31A fibrillae as an expression vector for different combinations of two TGEV coronavirus epitopes

Previously, two B-cell epitopes from the entero-pathogenic transmissible gastroenteritis virus (TGEV), namely the C epitope (TGEV-C) amino acids (aa) 363-371 and the A epitope (TGEV-A) aa 522-531 of the spike S protein (TGEV-S), have been separately expressed on the CS31A fibrillae at the surface of Escherichia coli following insertion into a same region of ClpG. However, the resulting chimeras induced a marginal TGEV-neutralizing antibody (Ab) response in mice. Here, with the view to improving this response, we introduced TGEV-C alone or in different tandem association with TGEV-A (A::C or C::A) in twelve putatively exposed regions of ClpG. Among the 28 resulting engineered proteins only 15, carrying up to 51 extra aa, had not essentially disturbed the correct CS31A fibrillae formation process. Six partially permissive sites accepting only TGEV-C and three highly permissive sites tolerating A::C or C::A tandem peptide, were identified throughout ClpG. Intact bacteria or extracted CS31A hybrid fibrillae expressing TGEV epitopes at any of the permissive sites, were recognized by Ab directed against the foreign parent protein, providing a direct argument for exposure of the corresponding CIpG region at the cell surface and for antigenicity of the epitopes in the polymeric CS31A fibrillae context. The potential of CS31A fibrillae as carriers of the TGEV peptides indicates that there may be three positions (N terminus, aa 202-204 and 202-218) in ClpG which may turn out to be important fusion sites and therefore be relevant for the eventual design of TGEV vaccines. Unexpectedly, TGEV-A, whatever its position in ClpG, mediated the partial proteolytic degradation of the hybrid proteins, suggesting that it functions as a substrate for a cellular protease, and thereby that its suitability as a vaccine antigen candidate is doubtful.

Antibody recognition of picornaviruses and escape from neutralization: a structural view

Escape of picornaviruses from neutralization by monoclonal antibodies is mediated by substitutions of very few, defined amino acid residues of the capsid, generally located on the tip of some surface-exposed loops. Substitutions at the same positions are possibly of major relevance to antigenic variation of picornaviruses in the field. Such residues tend to cluster in discrete areas, termed antigenic sites. The structure of virus-antibody and peptide-antibody complexes, determined by cryoelectron microscopy and X-ray crystallography, combined with studies using site-directed mutagenesis, are beginning to reveal new features of picornavirus epitopes. This information complements and expands the view on picornavirus antigenicity previously provided by analyses of antibody-escape mutants. In addition to amino acids found replaced in escape mutants, other surface residues which remain invariant in spite of immune pressure also participate in contacts with the antibody molecule. Some invariant residues are even critical for the antigen-antibody interaction. Escape mutations occur at the subset of antigenically critical residues which are tolerant to change because they are not essentially involved in capsid structure or function. Restrictions to variation differ among epitopes; this may contribute to explain the different number of serotypes among picornaviruses, and the frequency at which antigenically highly divergent variants occur in the field.