Comparative interaction kinetics of two recombinant Fabs and of the corresponding antibodies directed to the coat protein of tobacco mosaic virus

Spatial Separation of Microbeads into Detection Levels by a Bioorthogonal Porous Hydrogel for Size-Selective Analysis and Increased Multiplexity

Multiplex detection techniques are emerging within the fields of life science research and medical diagnostics where it is mandatory to analyze a great number of molecules. The detection techniques need to be highly efficient but often involve complicated and expensive fabrication procedures. Here, we present the immobilization and geometric separation of fluorescence-labeled microbeads for a multiplex detection in k levels. A compound of differently sized target molecules (DNA, proteins) is channeled into the respective detection levels by making use of a hydrogel as a size selective filter. The immobilized microbeads (10-20 μm) are considerably larger than the pores of the hydrogel network and therefore stay fixed at the well bottom and in higher elevations, respectively. Small biomolecules can diffuse through the pores of the network, whereas medium-sized biomolecules pass slower and large molecules will be excluded. Besides filtering, this method discriminates the used microbeads into k levels and thereby introduces a geometric multiplexity. Additionally, the exclusion of large entities enables the simultaneous detection of two target molecules, which exhibit the same affinity interaction. The hydrogel is formed through the combination of two macromonomers. One component is a homobifunctional polyethylene glycol linker, carrying a strained alkyne (PEG-BCN) and the second component is the azide-functionalized dendritic polyglycerol (dPG-N₃). They react via the bioorthogonal strain-promoted azide alkyne cycloaddition (SPAAC). The hydrogel creates a solution-like environment for the diffusion of the investigated biomolecules all the while providing a stable, bioinert, and surface bound network.

Surface plasmon resonance for monitoring the interaction of Potato virus Y with monoclonal antibodies

Surface plasmon resonance (SPR)-based biosensors have been widely utilized for measuring interactions of a variety of molecules. Fewer examples include higher biological entities such as bacteria and viruses, and even fewer deal with plant viruses. Here, we describe the optimization of an SPR sensor chip for evaluation of the interaction of the economically relevant filamentous Potato virus Y (PVY) with monoclonal antibodies. Different virus isolates were efficiently and stably bound to a previously immobilized polyclonal antibody surface, which remained stable over subsequent injection regeneration steps. The ability of the biosensor to detect and quantify PVY particles was compared with ELISA and RT-qPCR. Stably captured virus surfaces were successfully used to explore kinetic parameters of the interaction of a panel of monoclonal antibodies with two PVY isolates representing the main viral serotypes N and O. In addition, the optimized biosensor proved to be suitable for evaluating whether two given monoclonal antibodies compete for the same epitope within the viral particle surface. The strategy proposed in this work can help to improve existing serologic diagnostic tools that target PVY and will allow investigation of the inherent serological variability of the virus and exploration for new interactions of PVY particles with other proteins.

Colorimetric protein sensing by controlled assembly of gold nanoparticles functionalized with synthetic receptors

A novel strategy is described for the colorimetric sensing of proteins, based on polypeptide-functionalized gold nanoparticles. Recognition is accomplished using a polypeptide sensor scaffold designed to specifically bind to the model analyte, human carbonic anhydrase II (HCAII). The extent of particle aggregation, induced by the Zn(2+)-triggered dimerization and folding of a second polypeptide also present on the surface of the gold nanoparticle, gives a readily detectable colorimetric shift that is dependent on the concentration of the target protein. In the absence of HCAII, particle aggregation results in a major redshift of the plasmon peak, whereas analyte binding prevented the formation of dense aggregates, significantly reducing the magnitude of the redshift. The versatility of the technique is demonstrated using a second model system based on the recognition of a peptide sequence from the tobacco mosaic virus coat protein (TMVP) by a recombinant antibody fragment (Fab57P). Concentrations down to approximately 10 nM and approximately 25 nM are detected for HCAII and Fab57P, respectively. This strategy is proposed as a generic platform for robust and specific protein analysis that can be further developed to monitor a wide range of target proteins.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

A peptide-based, ratiometric biosensor construct for direct fluorescence detection of a protein analyte

We present the design, synthesis, and functional evaluation of peptide-based fluorescent constructs for wavelength-ratiometric biosensing of a protein analyte. The concept was shown using the high-affinity model interaction between the 18 amino acid peptide pTMVP and a recombinant antibody fragment, Fab57P. pTMVP was functionalized in two different positions with 6-bromomethyl-2-(2-furanyl)-3-hydroxychromone, an environmentally sensitive fluorophore with a two-band emission. The equilibrium dissociation constant of the interaction between pTMVP and Fab57P was largely preserved upon labeling. The biosensor ability of the labeled peptide constructs was evaluated in terms of the relative intensity change of the emission bands from the normal (N*) and tautomer (T*) excited-state species of the fluorophore ( I(N*)/I(T*)) upon binding of Fab57P. When the peptide was labeled in the C terminus, the I(N*)/I(T*) ratio changed by 40% upon analyte binding, while labeling close to the residues most important for binding resulted in a construct that completely lacked ratiometric biosensor ability. Integrated biosensor elements for reagentless detection, where peptides and ratiometric fluorophores are combined to ensure robustness in both recognition and signaling, are expected to become an important contribution to the design of future protein quantification assays in immobilized formats.

QSAR studies applied to the prediction of antigen-antibody interaction kinetics as measured by BIACORE

The objective of this work was to investigate the potential of the quantitative structure-activity relationships (QSAR) approach for predictive modulation of molecular interaction kinetics. A multivariate QSAR approach involving modifications in peptide sequence and buffer composition was recently used in an attempt to predict the kinetics of peptide-antibody interactions as measured by BIACORE. Quantitative buffer-kinetics relationships (QBKR) and quantitative sequence-kinetics relationships (QSKR) models were developed. Their predictive capacity was investigated in this study by comparing predicted and observed kinetic dissociation parameters (k(d)) for new antigenic peptides, or in new buffers. The range of experimentally measured k(d) variations was small (300-fold), limiting the practical value of the approach for this particular interaction. However, the models were validated from a statistical point of view. In QSKR, the leave-one-out cross validation gave Q(2) = 0.71 for 24 peptides (all but one outlier), compared to 0.81 for 17 training peptides. A more precise model (Q(2) = 0.92) could be developed when removing sets of peptides sharing distinctive structural features, suggesting that different peptides use slightly different binding modes. All models share the most important factor and are informative for structure-kinetics relationships. In QBKR, the measured effect on k(d) of individual additives in the buffers was consistent with the effect predicted from multivariate buffers. Our results open new perspectives for the predictive optimization of interaction kinetics, with important implications in pharmacology and biotechnology.

Comparative properties of two peptide-antibody interactions as deduced from epitope delineation

The linear epitope recognized by three closely related antibodies specific for the E6 oncoprotein of papillomavirus type 16 was delineated by phage display, spot peptide synthesis on cellulose membranes, and kinetic measurements with antigenic variants using a BIACORE. The same approaches, recently applied to an antibody specific for tobacco mosaic virus protein, led to the clear-cut delineation of a functional epitope comprising four key positions with well defined physico-chemical properties. In contrast, the E6 system is characterized by a non-essential contribution to binding of various factors, so that combinations of alternative properties are compatible with measurable binding activity.

Rapid two-step purification of a recombinant mouse Fab fragment expressed in Escherichia coli

We report a rapid, large-scale process for the purification of a recombinant Fab fragment specific for the tobacco mosaic virus coat protein (Fab57P). The fragment is expressed periplasmically in Escherichia coli. The expression level was optimized in 0.3-L fermentors. The highest levels were obtained using the following conditions: (1) low postinduction temperature (21 degrees C), (2) combined use of two beta-lactam antibiotics (carbenicillin and ampicillin), (3) IPTG concentration 0.1 mM, (4) regulated pH 7.2, (5) 17-h induction time, and (6) conditions that reduce mechanical stress. Optimized large-scale fermentations were done in 15- and 300-L capacity fermentors. The recombinant Fab fragment was purified by two chromatographic steps. After disruption of the bacteria using an APV Gaulin homogenizer, the crude E. coli homogenate was directly applied, without centrifugation, to an SP Sepharose Big Beads column. The recombinant Fab fragment was eluted as a single peak in a sodium chloride gradient. The fragment was further purified by affinity adsorption to a column packed with Epoxy-activated Sepharose 6B to which the antigen peptide NH(2)-CGS YNR GSF SQS SGLV-CONH(2) had been coupled through its N-terminal cysteine. The purified Fab57P fragment showed one band in SDS-PAGE. The overall purification yield was 35%.

Inhibitory activity of antibodies against the human atrial 5-HT(4)receptor

Antibodies directed against the second extracellular loop of G protein-coupled receptors have been shown to exert "agonist-like" activities. In order to test the hypothesis that this is a general phenomenon, antibodies were raised in rabbits against a synthetic peptide corresponding to the second extracellular loop of the newly sequenced human cardiac 5-HT(4)receptor. The antibodies were affinity-purified and shown to recognize the 5-HT(4)receptor in immunoblots of Chinese hamster ovary (CHO) cells expressing the receptor. The antibodies had no intrinsic effect but could depress the activation of L -type calcium channel induced by serotonin in human atrial cells. This antagonist-like effect was exerted both by intact IgG and by Fab fragments. These results are physiologically important since it has been shown that the 5-HT(4)receptor could be a target for autoantibodies in mothers at risk of giving birth to children with neonatal atrio-ventricular block.

Delineation of a linear epitope by multiple peptide synthesis and phage display

Two different approaches, the phage display technique and the Spot peptide synthesis on cellulose membranes, were used to identify sequences recognized by Fab 57P, specific for tobacco mosaic virus protein (TMVP), and define the preferred chemical composition of a functional epitope. Kinetic measurements of the interaction between peptide variants and the antibody fragment were used to further refine the molecular basis of binding activity. Our results show that the functional epitope of Fab 57P requires precise physico-chemical properties at a limited number of positions, and that residues flanking these key residues can influence binding affinity. The phage display and Spot synthesis methods allowed the straightforward localization of the binding region and the identification of residues that are essential for recognition. However, these methods yielded slightly different views of accessory factors that are able to influence antibody binding. The influence on binding activity of these factors can only be assessed through quantitative affinity measurements.

Predicting the kinetics of peptide-antibody interactions using a multivariate experimental design of sequence and chemical space

A multivariate approach involving modifications in peptide sequence and chemical buffer medium was used as an attempt to predict the kinetics of peptide-antibody interactions. Using a BIACORE system the kinetic parameters of the interaction of Fab 57P with 18 peptide analogues of an epitope of tobacco mosaic virus protein were characterized in 20 buffers of various pH values and containing different chemical additives (NaCl, urea, EDTA, KSCN and DMSO). For multivariate peptide design, three amino acid positions were selected because their modification was known to moderately affect binding, without abolishing it entirely. Predictive mathematical models were developed which related kinetic parameters (k(a) or k(d)) measured in standard buffer to the amino acid sequence of the antigen. ZZ-scales and a helix-forming-tendency (HFT) scale were used as descriptors of the physico-chemical properties of amino acids in the peptide antigen. These mathematical models had good predictive power (Q(2) = 0.49 for k(a), Q(2) = 0.73 for k(d)). For the non-essential residues under study, HFT and charge were found to be the most important factors that influenced the activity. Experiments in 19 buffers were performed to assess the sensitivity of the interactions to buffer composition. The presence of urea, DMSO and NaCl in the buffer influenced binding properties, while change in pH and the presence of EDTA and KSCN had no effect. The chemical sensitivity fingerprints were different for the various peptides. The results indicate that multivariate experimental design and mathematical modeling can be applied to the prediction of interaction kinetics.

From agonist to antagonist: Fab fragments of an agonist-like monoclonal anti-beta(2)-adrenoceptor antibody behave as antagonists

We previously demonstrated that the monoclonal antibody Mab6H8 raised against the second extracellular loop of the beta(2)-adrenoceptor (beta(2)-AR) had an agonist-like activity, mediated by the activation of L-type Ca(2+) channels by protein kinase A through the adenylyl cyclase pathway. We suggested that this Mab acts by stabilizing an active dimeric conformation of the beta(2)-AR. To substantiate this hypothesis, we prepared monomeric Fab fragments of Mab6H8. Comparison of the physicochemical parameters of antigen interaction with both the Mab and its Fab fragments were determined by surface plasmon resonance, showing a 5- to 10-fold lower affinity of the fragments compared with the bivalent antibody. We determined the biological activity of antibody and Fab fragments in two systems: spontaneous beating neonatal rat cardiomyocytes to study the chronotropic effects and isolated guinea pig cardiomyocytes to study L-type Ca(2+) channel activation. Fab fragments as such had no "agonist-like" effects in both systems but inhibited receptor activation with the beta(2)-specific agonist clenbuterol. Addition of a cross-linking rabbit anti-mouse IgG restored the agonist-like effect of the Fab fragments. These results suggest that Fab fragments induce a conformational change in the receptor, inhibiting the accessibility of the pharmacophore pocket to clenbuterol. Dimerization of this receptor conformation induces an agonist-like effect. Antireceptor antibodies can thus act both as agonist in the dimeric state and as antagonist in the monomeric state.

Measurement of antigen-antibody interactions with biosensors

The introduction in 1990 of a new biosensor technology based on surface plasmon resonance has revolutionized the measurement of antigen-antibody binding interactions. In this technique, one of the interacting partners is immobilized on a sensor chip and the binding of the other is followed by the increase in refractive index caused by the mass of bound species. The following immunochemical applications of this new technology will be described: (1) functional mapping of epitopes and paratopes by mutagenesis; (2) analysis of the thermodynamic parameters of the interaction; (3) measurement of the concentration of biologically active molecules; (4) selection of diagnostic probes.

Effects on interaction kinetics of mutations at the VH-VL interface of Fabs depend on the structural context

The influence of framework residues belonging to VH and VL modules of antibody molecules on antigen binding remains poorly understood. To investigate the functional role of such residues, we have performed semi-conservative amino acid replacements at the VH-VL interface. This work was carried out with (i) variants of the same antibody and (ii) with antibodies of different specificities (Fab fragments 145P and 1F1h), in order to check if functional effects are additive and/or similar for the two antibodies. Interaction kinetics of Fab mutants with peptide and protein antigens were measured using a BIACORE instrument. The substitutions introduced at the VH-VL interface had no significant effects on k(a) but showed small, significant effects on k(d). Mutations in the VH module affected k(d) not only for the two different antibodies but also for variants of the same antibody. These effects varied both in direction and in magnitude. In the VL module, the double mutation F(L37)L-Q(L38)L, alone or in combination with other mutations, consistently decreased k(d) about two-fold in Fab 145P. Other mutations in the VL module had no effect on k(d) in 145P, but always decreased k(d) in 1F1h. Moreover, in both systems, small-magnitude non-additive effects on k(d) were observed, but affinity variations seemed to be limited by a threshold. When comparing functional effects in antibodies of different specificity, no general rules could be established. In addition, no clear relationship could be pointed out between the nature of the amino acid change and the observed functional effect. Our results show that binding kinetics are affected by alteration of framework residues remote from the binding site, although these effects are unpredictable for most of the studied changes.

In vivo biotinylated recombinant antibodies: high efficiency of labelling and application to the cloning of active anti-human IgG1 Fab fragments

In vivo biotinylation of antibody fragments with a gene fusion approach is a realistic alternative to conventional in vitro chemical labelling. We have previously reported the construction of a vector system suitable for the bacterial expression of the binding fragment of antibody (Fab) genetically linked to the C-terminal domain of Escherichia Coli biotin carboxy carrier protein (BCCP*). A minor fraction of the expressed hybrids was biotinylated in vivo and therefore able to interact with streptavidin. We now show that the large majority of bacterially-expressed Fab-BCCP* fusions are labelled with biotin when plasmid-encoded biotin holoenzyme synthetase (BirA) is co-expressed. The yield of biotinylated Fab is maximal when overexpression of BirA is driven by a second compatible plasmid. We took advantage of this property to develop a novel filter assay for the rapid identification of recombinant Fab reacting with immunoglobulin. Starting with total RNA of two newly established murine hybridoma cell lines producing anti-human IgG1 antibodies, we selected in a single experiment the bacterial clones that expressed in vivo biotinylated anti-IgG1 Fab. Sequence analysis of the isolated Fabs showed that they did not derive from a single B clone. In addition, we found that these recombinant Fabs labelled with biotin in vivo are useful for the specific detection of human IgG1 by a solid-phase immunoassay.

The antigenicity of tobacco mosaic virus

The antigenic properties of the tobacco mosaic virus (TMV) have been studied extensively for more than 50 years. Distinct antigenic determinants called neotopes and cryptotopes have been identified at the surface of intact virions and dissociated coat protein subunits, respectively, indicating that the quaternary structure of the virus influences the antigenic properties. A correlation has been found to exist between the location of seven to ten residue-long continuous epitopes in the TMV coat protein and the degree of segmental mobility along the polypeptide chain. Immunoelectron microscopy, using antibodies specific for the bottom surface of the protein subunit, showed that these antibodies reacted with both ends of the stacked-disk aggregates of viral protein. This finding indicates that the stacked disks are bipolar and cannot be converted directly into helical viral rods as has been previously assumed. TMV epitopes have been mapped at the surface of coat protein subunits using biosensor technology. The ability of certain monoclonal antibodies to block the cotranslational disassembly of virions during the infection process was found to be linked to the precise location of their complementary epitopes and not to their binding affinity. Such blocking antibodies, which act by sterically preventing the interaction between virions and ribosomes may, when expressed in plants, be useful for controlling virus infection.

Kinetic analysis of the effect on Fab binding of identical substitutions in a peptide and its parent protein

Monoclonal antibody 57P, which was raised against tobacco mosaic virus protein, cross-reacts with a peptide corresponding to residues 134-146 of this protein. Previous studies using peptide variants suggested that the peptide in the antibody combining site adopts a helical configuration that mimics the structure in the protein. In this study, we carried out a detailed comparison of Fab-peptide and Fab-protein interactions. The same five amino acid substitutions were introduced in the peptide (residues 134-151) and the parent protein, and the effect of these substitutions on antibody binding parameters have been measured with a Biacore instrument. Fabs that recognize epitopes located away from the site of mutations were used as indirect probes for the conformational integrity of protein antigens. Their interaction kinetics with all proteins were similar, suggesting that the substitutions had no drastic effect on their conformation. The five substitutions introduced in the peptide and the protein had minor effects on association rate constants (ka) and significant effects on dissociation rate constants (kd) of the antigen-Fab 57P interactions. In four out of five cases, the effect on binding affinity of the substitutions was identical when the epitope was presented in the form of a peptide or a protein antigen, indicating that antibody binding specifity was not affected by epitope presentation. However, ka values were about 10 times larger and kd values about 5 times larger for the peptide-Fab compared to the protein-Fab interaction, suggesting a different binding mechanism. Circular dichroism measurements performed for three of the peptides showed that they were mainly lacking structure in solution. Differences in conformational properties of the peptide and protein antigens in solution and/or in the paratope could explain differences in binding kinetics. Our results demonstrate that the peptides were able to mimic correctly some but not all properties of the protein-Fab 57P interaction and highlight the importance of quantitative analysis of both equilibrium and kinetic binding parameters in the design of synthetic vaccines and drugs.

Antibodies to ribosomal P proteins of Trypanosoma cruzi in Chagas disease possess functional autoreactivity with heart tissue and differ from anti-P autoantibodies in lupus

Anti-P antibodies present in sera from patients with chronic Chagas heart disease (cChHD) recognize peptide R13, EEEDDDMGFGLFD, which encompasses the C-terminal region of the Trypanosoma cruzi ribosomal P1 and P2 proteins. This peptide shares homology with the C-terminal region (peptide H13 EESDDDMGFGLFD) of the human ribosomal P proteins, which is in turn the target of anti-P autoantibodies in systemic lupus erythematosus (SLE), and with the acidic epitope, AESDE, of the second extracellular loop of the beta1-adrenergic receptor. Anti-P antibodies from chagasic patients showed a marked preference for recombinant parasite ribosomal P proteins and peptides, whereas anti-P autoantibodies from SLE reacted with human and parasite ribosomal P proteins and peptides to the same extent. A semi-quantitative estimation of the binding of cChHD anti-P antibodies to R13 and H13 using biosensor technology indicated that the average affinity constant was about 5 times higher for R13 than for H13. Competitive enzyme immunoassays demonstrated that cChHD anti-P antibodies bind to the acidic portions of peptide H13, as well as to peptide H26R, encompassing the second extracellular loop of the beta1 adrenoreceptor. Anti-P antibodies isolated from cChHD patients exert a positive chronotropic effect in vitro on cardiomyocytes from neonatal rats, which resembles closely that of anti-beta1 receptor antibodies isolated from the same patient. In contrast, SLE anti-P autoantibodies have no functional effect. Our results suggest that the adrenergic-stimulating activity of anti-P antibodies may be implicated in the induction of functional myocardial impairments observed in cChHD.

Kinetics and energetics of specific intermolecular interactions

Taking into account the energy vs. distance functions of the aspecific (macroscopic) repulsion that usually prevails between antigen (Ag) and antibody (Ab) molecules in polar media, as well as the specific (microscopic) attraction between epitope and paratope of Ag and Ab, it proved possible to determine the kinetic constants (von Smoluchowski, 1917; Hammes, 1978) of Ag-Ab interactions, from the surface properties of Ag, Ab and the aqueous medium. The kinetic constants thus found correlate well with experimentally determined kinetic constants in comparable systems, and confirm the importance of the influence of the concentration of one of the reagents (e.g. the Ab) on the kinetic association constant (Van Regenmortel et al., 1994), which is largely due to steric hindrance. Applying the same energy vs. distance approach to the influence of temperature (T) on Ag-Ab reactions, it ensues that the familiar occurrence of an apparent 'enthalpy-entropy compensation' in aqueous media is in fact the relatively gratuitous outcome of a complex set of effects caused by an increase in T, on the total free energy, the hydration energy and, as a result, on the inter-epitope-paratope distance. A close correlation exists between the outcome of these surface-thermodynamic analyses and experimental results.

Concentration measurement of unpurified proteins using biosensor technology under conditions of partial mass transport limitation

Using biosensor technology, it is possible to measure protein concentration when the binding of the protein to an appropriate ligand immobilized on the sensor surface is totally limited by diffusion and mass transport, a condition difficult to achieve in practice. In such a case, the observed binding rate does not reflect the intrinsic binding capacity of the molecular partners, but is simply proportional to the concentration of the protein analyte that is introduced in a continuous flow over the ligand. We describe here a more general biosensor method for measuring protein concentration which is applicable to conditions where mass transport is not totally but only partially rate limiting. The proposed method, which is based on measurements at different flow rates, does not require a standard of known protein concentration and can be used with unpurified proteins. The method is applicable to ligand-analyte pairs with an association rate constant as low as 10(3) M-1 s-1 and requires only knowledge of the molecular weight and diffusion coefficient of the analyte. The method was used successfully to measure the concentration of monoclonal antibodies, monoclonal antibody fragments (Fab) obtained by papain cleavage, and recombinant Fab fragments of widely different affinities in crude Escherichia coli extracts.

Ligand for EPH-related kinase (LERK) 7 is the preferred high affinity ligand for the HEK receptor

HEK is a member of the EPH-like receptor tyrosine kinase family, which appear to have roles in development and oncogenesis. Recently, we purified a soluble HEK ligand which is also a ligand (AL1) for the HEK-related receptor EHK1. Promiscuity appears to be a characteristic feature of interactions between the EPH-like receptors and their ligands, termed ligands for EPH-related kinases (LERKs). This prompted us to analyze the interactions between the HEK exodomain and fusion proteins comprising candidate LERKs and the Fc portion of human IgG1 (Fc) or a FLAGTM-peptide tag by surface plasmon resonance, size exclusion high performance liquid chromatography, sedimentation equilibrium, and transphosphorylation. Our results indicate that AL1/LERK7 is the preferred high-affinity ligand for HEK, forming a stable 1:1 complex with a dissociation constant of 12 nM. As expected the apparent affinities of bivalent fusion proteins of LERKs and the Fc portion of human IgG1 had significantly reduced dissociation rates compared with their monovalent, FLAGTM-tagged derivatives. High-avidity binding of monovalent ligands can be achieved by antibody-mediated cross-linking of monovalent ligands and with LERK7 results in specific phosphorylation of the receptor. By extrapolation, our findings indicate that some of the reported LERK-receptor interactions are a consequence of the use of bivalent ligand or receptor constructs and may be functionally irrelevant.

Cooperative effects of mutations in a recombinant Fab on the kinetics of antigen binding

Recombinant Fabs, 57P and 174P, recognizing peptide 134-151 of the coat protein of tobacco mosaic virus, differ by 15 amino acid changes in the sequence of their variable region. Kinetic analysis using BIAcore showed that they recognized five peptide variants in the same ranking order, but that Fab 174P consistently dissociated faster from the peptides compared to Fab 57P. In order to identify amino acid substitutions that are responsible for differences in dissociation rates of the two Fabs, six hybrid Fabs have been constructed by exchanging three DNA segments. Four single and five multiple mutants were obtained by site-directed mutagenesis. All Fabs recognized variant peptides in a similar ranking order. The high precision of biosensor measurements made it possible to detect small contributions to dissociation kinetics of at least five substitutions, as well as the presence of small-magnitude non-additive effects of multiple substitutions. Our results demonstrate the cooperative influence on dissociation kinetics of amino acid residues located away from each other and away from the Fab combining site.

Uses of biosensors in the study of viral antigens

The introduction in 1990 of a new biosensor technology based on surface plasmon resonance has greatly simplified the measurement of binding interactions in biology. This new technology known as biomolecular interaction analysis makes it possible to visualize the binding process as a function of time by following the increase in refractive index that occurs when one of the interacting partners binds to its ligand immobilized on the surface of a sensor chip. None of the reactants needs to be labelled, which avoids the artefactual changes in binding properties that often result when the molecules are labelled. Biosensor instruments are well-suited for the rapid mapping of viral epitopes and for identifying which combinations of capturing and detector Mabs will give the best results in sandwich assays. Biosensor binding data are also useful for selecting peptides to be used in diagnostic solid-phase immunoassays. Very small changes in binding affinity can be measured with considerable precision which is a prerequisite for analyzing the functional effect and thermodynamic implications of limited structural changes in interacting molecules. On-rate (ka) and off-rate (kd) kinetic constants of the interaction between virus and antibody can be readily measured and the equilibrium affinity constant K can be calculated from the ratio ka/kd = K.