Ligand binding to anti-fluorescyl antibodies: stability of the antigen binding site

A fluorescent sensor based on methyldopa drug modified γ-Fe2O3 nanoparticles for ultrasensitive detection of calf thymus DNA

We reported the study of calf thymus DNA (ct-DNA) adsorption by the polymer of methyldopa (2-amino-3-(3,4-dihydroxyphenyl)-2-methyl acid, propanoic) (PMDP), magnetofluorescent PMDP-γ-Fe2O3 nanocrystal. The method is based on the extraordinarily high quenching efficiency of ct-DNA and the specific interaction between ct-DNA and PMDP-γ-Fe2O3 via guanine base and metal coordination, probably. It was found that the designed magnetic nanoparticles can adsorb ct-DNA in nM levels in the presence of NaCl and KCl. In acetate and phosphate buffers DNA were adsorbed completely. Also, we found that pH plays an important role in DNA adsorption onto PMDP-γ-Fe2O3 nanocrystal. PMDP-γ-Fe2O3 nanocrystal is highly hydrophilic and DNA desorption wasn't observed. We believe this study will further stimulate the application of PMDP-γ-Fe2O3 nanocrystal in bioanalytical chemistry and nanotechnology. PMDP-γ-Fe2O3 nanocrystal possesses the ability to interact with ct-DNA via a partial intercalative binding mechanism, as demonstrated by fluorescence displacement experiments and a significant red shift (ca, 10nm) in UV-vis spectra.

Characterization of the 4D5Flu single-chain antibody with a stimulus-responsive elastin-like peptide linker: a potential reporter of peptide linker conformation

Single-chain antibodies (scFvs) are comprised of IgG variable light and variable heavy domains tethered together by a peptide linker whose length and sequence can affect antigen binding properties. The ability to modulate antigen binding affinity through the use of environmental triggers would be of great interest for many biotechnological applications. We have characterized the antigen binding properties of an anti-fluorescein scFv, 4D5Flu, containing stimulus-responsive short elastin-like peptide linkers and nonresponsive flexible linkers. Comparison of length-matched flexible and short elastin-like peptide linkers indicates that a stimulus-responsive linker can confer stimulus-responsive control of fluorescein binding. A linker length of either six or 10 amino acids proved to have the largest thermally induced response. Similar differences in binding free energy changes indicate a common underlying mechanism of thermal responsiveness. Contrary to the thermal behavior, the effect of salt, another elastin beta-turn-inducing stimulus, stabilized antigen binding in the six- and 10-amino-acid linkers such that elastin-like linkers became less stimulus-responsive as compared with flexible linkers. Again, the thermodynamic analysis indicates a common mechanism of salt responsiveness. Characterization of the room-temperature binding affinities and evidence indicating a dimeric state of the scFvs concomitantly suggest the major contribution to the stimulus-responsive behavior derives from the perturbation of interdomain associations, rather than the linker-constrained disruption of the intramolecular association. The ability to use stimulus-responsive peptide modules to exert a novel control over protein function will likely find application in the creation of allosteric antibodies and scFv-based biosensors, and as a platform to enable the evolution of new stimulus-responsive peptides.

Enzyme-based visualization of receptor-ligand binding in tissues

New methods of elucidating the ligand-binding activity of receptors could improve our understanding of receptor function, key events they control, and their presence in normal and pathological states. We describe a method for visualizing receptor-ligand binding in cells and tissues that substitutes fluorescein for radioactive labels, and detects receptor bound, fluoresceinated ligand with an antifluorescein/horseradish peroxidase amplification system. Receptor-bound ligand is then visualized by light microscopy against a standard hemotoxylin-stained background of cell structure. Quantitative versions of the assay provide an apparent dissociation constant and number of receptors per cell at saturation in cell or tissue specimens. Receptors examined include the folate receptor, bombesin peptide-binding receptors, the epidermal growth factor receptor, the neuropeptide Y receptor, the asialoglycoprotein receptor, and RGD peptide-binding integrins. Using fluoresceinated versions of molecules, we show the method can visualize and quantitate receptor-bound ligands in cell culture monolayers and animal tissue specimens. Ligand binding to receptors present in tissues was visualized in normal and pathological samples of human tissue microarrays. The enzyme-amplified detection of receptor-bound fluoresceinated ligand is a simple and nonradioactive-based method that provides information on the receptor activity in tissue specimens.

Heat-induced formation of a specific binding site for self-assembled Congo Red in the V domain of immunoglobulin L chain lambda

Moderate heating (40-50 degrees C) of immunoglobulins makes them accessible for binding with Congo Red and some related highly associated dyes. The binding is specific and involves supramolecular dye ligands presenting ribbon-like micellar bodies. The L chain lambda dimer, which upon heating disclosed the same binding requirement with respect to supramolecular dye ligands, was used in this work to identify the site of their attachment. Two clearly defined dye-protein (L lambda chain) complexes arise upon heating, here called complex I and complex II. The first is formed at low temperatures (up to 40-45 degrees C) and hence by a still native protein, while the formation of the second one is associated with domain melting above 55 degrees C. They contain 4 and 8 dye molecules bound per L chain monomer, respectively. Complex I also forms efficiently at high dye concentration even at ambient temperature. Complex I and its formation was the object of the present studies. Three structural events that could make the protein accessible to penetration by the large dye ligand were considered to occur in L chains upon heating: local polypeptide chain destabilization, VL-VL domain incoherence, and protein melting. Of these three possibilities, local low-energy structural alteration was found to correlate best with the formation of complex I. It was identified as decreased packing stability of the N-terminal polypeptide chain fragment, which as a result made the V domain accessible for dye penetration. The 19-amino acid N-terminal fragment becomes susceptible to proteolytic cleavage after being replaced by the dye at its packing locus. Its splitting from the dye-protein complex was proved by amino acid sequence analysis. The emptied packing locus, which becomes the site that holds the dye, is bordered by strands of amino acids numbered 74-80 and 105-110, as shown by model analysis. The character of the temperature-induced local polypeptide chain destabilization and its possible role in intramolecular antibody signaling is discussed.

Antigen-antibody interactions: binding studies with fluorescence and surface plasmon resonance exemplified by acid-traseolide as antigen

The interaction between the musk fragrance acid-traseolide and monoclonal antibodies (mAB) generated against this odorant has been investigated with two different techniques. Fluorescence spectroscopy was used to study the quenching of tryptophan fluorescence of the antibody upon binding acid-traseolide. This spectroscopic approach is based on measurements under equilibrium conditions. The second technique exploited the surface plasmon resonance (SPR) phenomenon. The acid-traseolide was immobilized in the surface matrix and upon presenting mAB changes in SPR were recorded in real time during the association reaction. The SPR approach can be considered as a kinetic method. Although having a different origin, both methods lead to comparable equilibrium dissociation constants (Kd). However, the results obtained with fluorescence spectroscopy were more accurate and reproducible. Not only the association of acid-traseolide with antibody was evaluated, also Fab fragment and peptide (H3-peptide) mimicking the heavy chain CDR3 of this antibody were included in this study. The Kd-values, determined by both methods, increase in the order mAB < Fab < H3-peptide because of diminishing recognition.

Molecular cloning, expression, and characterization of a functional single-chain Fv antibody to the mycotoxin zearalenone

The heavy-chain and kappa light-chain variable region genes of an antizearalenone hybridoma cell line (2G3-6E3-2E2) were isolated by PCR and joined by a DNA linker encoding peptide (Gly4Ser)3 as a single-chain Fv (scFv) DNA fragment. The scFv DNA fragment was cloned into a phagemid (pCANTAB5E) and expressed as a fusion protein with E tag and phage M13 p3 in Escherichia coli TG1. In the presence of helper phage M13K07, the scFv fusion protein was displayed on the surfaces of recombinant phages. High-affinity scFv phages were enriched through affinity selection in microtiter wells coated with zearalenone-ovalbumin conjugate. The selected recombinant phages were used to infect E. coli HB2151 for the production of soluble scFv antibodies. One selected clone (pQY1.5) in HB2151 secreted a soluble scFv antibody (QY1.5) with a high zearalenone-binding affinity (concentration required for 50% inhibition of binding, 14 ng/ml), similar to that of parent monoclonal antibody in a competitive indirect enzyme-linked immunosorbent assay. However, scFv QY1.5 exhibited higher cross-reactivity with zearalenone analogs and had greater sensitivity to methanol destabilization than the parent monoclonal antibody did. Nucleotide sequence analyses revealed that the light-chain portion of scFv QY1.5 had a nucleotide sequence identity of 97% to a mouse germ line gene VK23.32 in mouse kappa light-chain variable region subgroup V, whereas the heavy-chain nucleotide sequence was classified as mouse heavy-chain subgroup III (D) but without any closely related members having highly homologous complementarity-determining region sequences. The potential of soluble scFv QY1.5 for routine screening of zearalenone and its analogs was demonstrated with zearalenone-spiked corn extracts.

A T cell receptor V alpha domain expressed in bacteria: does it dimerize in solution?

To evaluate the potential for dimerization through a particular T cell receptor (TCR) domain, we have cloned the cDNA encoding a TCR V alpha from a hybridoma with specificity for the human immunodeficiency virus (HIV) envelope glycoprotein 120-derived peptide P18-110 (RGPGRAFVTI) bound to the murine major histocompatibility complex (MHC) class I molecule, H-2Dd. This cDNA was then expressed in a bacterial vector, and protein, as inclusion bodies, was solubilized, refolded, and purified to homogeneity. Yield of the refolded material was from 10 to 50 mg per liter of bacterial culture, the protein was soluble at concentrations as high as 25 mg/ml, and it retained a high level of reactivity with an anti-V alpha 2 monoclonal antibody. This domain was monomeric both by size exclusion gel chromatography and by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Circular dichroism spectra indicated that the folded V alpha domain had secondary structure similar to that of single immunoglobulin or TCR domains, consisting largely of beta sheet. Conditions for crystallization were established, and at least two crystal geometries were observed: hexagonal bipyramids that failed to diffract beyond approximately 6 A, and orthorhombic crystals that diffracted to 2.5 A. The dimerization of the V alpha domain was investigated further by solution nuclear magnetic resonance spectroscopy, which indicated that dimeric and monomeric forms of the protein were about equally populated at a concentration of 1 mM. Thus, models of TCR-mediated T cell activation that invoke TCR dimerization must consider that some V alpha domains have little tendency to form homodimers or multimers.

Anti-metatype antibody stabilization of Fv 4-4-20 variable domain dynamics

Anti-metatype (anti-Met) antibodies are immunoglobulins that specifically recognize and stabilize antibodies in their liganded or metatypic state, but lack specificity for either the hapten or the unliganded antibody. Autologous anti-Met antibodies were previously observed in vivo, suggesting that a metatypic autoantibody response could play a physiological role in the immune network, e.g. controlling the clearance of immune complexes from circulation. The first elicited anti-Met antibodies were against the fluorescein-liganded high affinity murine anti-fluorescein monoclonal antibody 4-4-20. The fluorescein-hapten system has proved to be an invaluable tool for both the recognition and characterization of the metatypic response by utilization of its spectral properties. In this investigation, hydrostatic pressure measurements, in conjunction with fluorescence spectroscopy, were performed on the recombinant Fv derivative (Fv 4-4-20) of the high affinity anti-fluorescein monoclonal antibody 4-4-20 complexed to anti-Met antibodies to study the influence of anti-Met antibodies of Fv 4-4-20 intervariable domain interactions. Anti-Met antibodies bound to liganded Fv 4-4-20 were observed to cause a change in the fluorescence properties of fluorescein that was not observed when anti-Met antibodies were bound to the liganded parent immunoglobulin. The variation of these spectral properties upon addition of anti-Met antibodies was shown to be correlated with dissociation of the variable domains in Fv 4-4-20 in response to its interaction with the anti-Met antibody. The ability to cause variable domain dissociation was dependent on whether monoclonal or polyclonal anti-Met antibodies were bound to the metatype. A model was proposed that elucidated the interaction of anti-Met antibodies, polyclonal and monoclonal, with variable domains of the primary anti-antigen antibody.

Temperature and pH dependence of fluorescein binding within the monoclonal antibody 9-40 active site as monitored by hydrostatic pressure

In a comparative study, the thermodynamic parameter, DeltaV, was obtained using hydrostatic pressure-induced dissociation of fluorescein (Fl) from the active site of monoclonal antibody (mAb) 9-40 and its mutant and native derivatives equilibrated at six pH values (8.0, 7.5, 7.0, 6.5, 6.0, and 5.5) and four temperatures (35, 25, 15, and 5 degrees C). mAb 9-40 and its Fab and single-chain Fv (scFv) derivatives at pH 8.0 were found to have identical Fl dissociation behavior under pressure as a function of temperature. The pressure dissociation at 25 degrees C as a function of pH showed a sigmoidal dependence of DeltaV with a midpoint value at pH 7.4 for mAb 9-40. Comparison of experimental results for scFv 9-40/212 with its mutant scFv 9-40/212Arg-34L indicated that the pH dependence of mAb 9-40 was due to the titration of His-34L in the active site. Iodide quenching of bound Fl showed that the hapten in this active site was solvent accessible. Imperfect packing, which leads to increased conformational dynamics, was determined as a possible cause of the low affinity for mAb 9-40.

Comparative properties of the single chain antibody and Fv derivatives of mAb 4-4-20. Relationship between interdomain interactions and the high affinity for fluorescein ligand

Recombinant Fv derivative of the high affinity murine anti-fluorescein monoclonal antibody 4-4-20 was constructed and expressed in high yields, relative to the single chain antibody (SCA) derivative (2 3-fold), in Escherichia coli. Both variable heavy (VH) and variable light (VL) domains, that accumulated as insoluble inclusion bodies, were isolated, denatured, mixed, refolded, and affinity-purified to yield active Fv 4-4-20. Affinity-purified Fv 4-4-20 showed identical ligand binding properties compared with the SCA construct, both were slightly lower than the affinities expressed by Fab or IgG 4-4-20. Proper protein folding was shown to be domain-independent by in vitro mixing of individually refolded variable domains to yield functional Fv protein. In solid phase and solution phase assays, Fv 4-4-20 closely approximated the SCA derivative in terms of both idiotype and metatype, confirming identical active site structures and conformations. The equilibrium dissociation constant (Kd) for the VL/VH association (1.43 x 10(-7) M), which was determined using the change in fluorescein spectral properties upon ligand binding, was relatively low considering the high affinity displayed by the Fv protein for fluorescein (Kd, 2.9 x 10(-10) M). Thus, domain-domain stability in the Fv and SCA 4-4-20 proteins cannot be the sole cause of reduced affinity (2-3-fold) for fluorescein as compared with the Fab or IgG form of 4-4-20. With their identical ligand binding and structural properties, the decreased SCA or Fv affinity for fluorescein must be an ultimate consequence of deletion of the CH1 and CL constant domains. Collectively, these results verify the importance of constant domain interactions in antibody variable domain structure-function analyses and future antibody engineering endeavors.

Rapid assay of phage-derived recombinant human fabs as bispecific antibodies

Specific anti-tumor and anti-viral activities can be conferred on lymphocytic and myeloid effector cells by retargeting them with bispecific antibodies. These are antibodies which possess an anti-target binding region and a region capable of binding specific effector cell surface markers. For the rapid evaluation of recombinant human Fabs as bispecific antibodies, we have constructed a vector that allows for the conversion of Fabs into protein A fusion proteins. These can be used to generate bispecific antibodies when complexed to appropriate anti-effector cell immunoglobulins. As a model system, a protein A fusion derivative of a human recombinant anti-herpes simplex virus (HSV) Fab was constructed and complexed to OKT3, a T cell-activating antibody specific for CD3. This complex reduced HSV-2 yields in infected cells by about three logs relative to controls when incubated on HSV-2-infected cell monolayers in the presence of IL-2-activated lymphocytes. The system described allows for the rapid evaluation of recombinant human Fabs as bispecific antibodies for therapeutic applications. In addition, Fab-protein A fusion proteins can be used in ELISA and other immuno-assays with increased sensitivity.