Bacterial expression and refolding of single-chain Fv fragments with C-terminal cysteines

Tagging Recombinant Proteins to Enhance Solubility and Aid Purification

Protein fusion technology has had a major impact on the efficient production and purification of individual recombinant proteins. The use of genetically engineered affinity and solubility-enhancing polypeptide "tags" has a long history, and there is a considerable repertoire of these that can be used to address issues related to the expression, stability, solubility, folding, and purification of their fusion partner. In the case of large-scale proteomic studies, the development of purification procedures tailored to individual proteins is not practicable, and affinity tags have become indispensable tools for structural and functional proteomic initiatives that involve the expression of many proteins in parallel. In this chapter, the rationale and applications of a range of established and more recently developed solubility-enhancing and affinity tags is described.

Dual functionality of lamprey VLRB C-terminus (LC) for multimerization and cell surface display

Lamprey, one of the living representatives of jawless vertebrates, uses variable lymphocyte receptors B (VLRB) for antigen recognition, rather than immunoglobulin (Ig) based receptors as used by higher vertebrates. The C-terminus of lamprey VLRB (LC) possess a glycosylphosphatidylinositol (GPI) signal sequence and seven cysteine residues providing dual functionality of the VLRB antibody in the form of a humoral agglutinin and cell membrane receptors. Here, we show that the LC can be either secreted or be membrane anchored as a heterologous fused protein in a multimeric form comprising of eight or ten monomeric units. Using serially truncated LC variants, we showed that the LC, in which the last three amino acid "RKR" were deleted, referred to as LC7, was the most suitable domain for multimeric construction, whereas, the intact LC is more tailored for applications involving membrane anchorage. We show that an antibody specific for viral hemorrhagic septicemia virus (VHSV) (VLR43), displayed on HEK-293F cells using a PiggyBac (PB) transposase system, exhibited a dose-dependent reaction with its antigen, verifying that the LC can be applied in antibody display technology. Therefore, the present report provides valuable insight into the structure of the lamprey VLRB and highlights its potential use as a novel fusion partner for multimerization and membrane anchorage of chimeric proteins.

Tagging Recombinant Proteins to Enhance Solubility and Aid Purification

Protein fusion technology has had a major impact on the efficient production and purification of individual recombinant proteins. The use of genetically engineered affinity and solubility-enhancing polypeptide "tags" has increased greatly in recent years and there now exists a considerable repertoire of these that can be used to solve issues related to the expression, stability, solubility, folding, and purification of their fusion partner. In the case of large-scale proteomic studies, the development of purification procedures tailored to individual proteins is not practicable, and affinity tags have therefore become indispensable tools for structural and functional proteomic initiatives that involve the expression of many proteins in parallel. Here, the rationale and applications of a range of established and more recently developed solubility-enhancing and affinity tags is described.

Blocking monocyte transmigration in in vitro system by a human antibody scFv anti-CD99. Efficient large scale purification from periplasmic inclusion bodies in E. coli expression system

Migration of leukocytes into site of inflammation involves several steps mediated by various families of adhesion molecules. CD99 play a significant role in transendothelial migration (TEM) of leukocytes. Inhibition of TEM by specific monoclonal antibody (mAb) can provide a potent therapeutic approach to treating inflammatory conditions. However, the therapeutic utilization of whole IgG can lead to an inappropriate activation of Fc receptor-expressing cells, inducing serious adverse side effects due to cytokine release. In this regard, specific recombinant antibody in single chain variable fragments (scFvs) originated by phage library may offer a solution by affecting TEM function in a safe clinical context. However, this consideration requires large scale production of functional scFv antibodies and the absence of toxic reagents utilized for solubilization and refolding step of inclusion bodies that may discourage industrial application of these antibody fragments. In order to apply the scFv anti-CD99 named C7A in a clinical setting, we herein describe an efficient and large scale production of the antibody fragments expressed in E. coli as periplasmic insoluble protein avoiding gel filtration chromatography approach, and laborious refolding step pre- and post-purification. Using differential salt elution which is a simple, reproducible and effective procedure we are able to separate scFv in monomer format from aggregates. The purified scFv antibody C7A exhibits inhibitory activity comparable to an antagonistic conventional mAb, thus providing an excellent agent for blocking CD99 signaling. This protocol can be useful for the successful purification of other monomeric scFvs which are expressed as periplasmic inclusion bodies in bacterial systems.

Tagging recombinant proteins to enhance solubility and aid purification

Protein fusion technology has enormously facilitated the efficient production and purification of individual recombinant proteins. The use of genetically engineered affinity and solubility-enhancing polypeptide "tags" has increased greatly in recent years and there now exists a considerable repertoire of these that can be used to solve issues related to the expression, stability, solubility, folding, and purification of their fusion partner. In the case of large-scale proteomic studies, the development of purification procedures tailored to individual proteins is not practicable, and affinity tags have therefore become indispensable tools for structural and functional proteomic initiatives that involve the expression of many proteins in parallel. Here, the rationale and applications of a range of established and more recently developed solubility-enhancing and affinity tags are outlined.

High-level periplasmic expression and purification of scFvs

The isolation of recombinant antibodies by phage display naturally leads to experiments to evaluate their biological and immunological properties. Although crude preparations may have their value in initial studies, the need often exists for highly purified protein that can be tested in vivo. This chapter describes methods to generate high yields of scFv from bacterial cultures and to purify protein to the degree of homogeneity required for the most exacting analysis.

The effect of the hexahistidine-tag in the oligomerization of HSC70 constructs

The hexahistidine is a fusion tag used for the isolation of proteins via an immobilized metal-ion affinity chromatography (IMAC). In the present study, we have purified and analyzed two constructs of the heat shock protein HSC70 in the presence or the absence of the His-tag (C30WT-His(+)/C30WT and C30DeltaL-His(+)/C30DeltaL). The oligomerization properties of the constructs were analyzed by size exclusion chromatography (SEC) and analytical ultracentrifugation (AU). Results from SEC analysis indicated that the His-tag promotes the dimerization of C30DeltaL-His(+) but has no effect on the elution profile of C30WT-His(+), compared to their respective untagged forms C30DeltaL and C30WT. These observations were also confirmed by AU analysis which indicates that C30DeltaL is stabilized in the dimeric form in the presence of the His-tag. These results emphasize the need to remove the His-tag before structural characterization of some recombinant proteins.

Modified tobacco mosaic virus particles as scaffolds for display of protein antigens for vaccine applications

Display of peptides or proteins in an ordered, repetitive array, such as on the surface of a virus-like particle, is known to induce an enhanced immune response relative to vaccination with the "free" protein antigen. The coat protein of Tobacco mosaic virus (TMV) can accommodate short peptide insertions into the primary sequence, but the display of larger protein moieties as genetic fusions to the capsid protein has not been possible. We employed a randomized library approach to introduce a reactive lysine at the externally located amino terminus of the coat protein, which facilitated biotinylation of the capsid. To characterize display of heterologous proteins on the virion surface, we bound a model antigen (green fluorescent protein (GFP)-streptavidin (SA), expressed and purified from plants) to the biotinylated TMV particles, creating a GFP-SA decorated virus particle. A GFP-SA tetramer loading of 26% was obtained, corresponding to approximately 2200 GFP moieties displayed per intact virion. We evaluated the immunogenicity of GFP decorated virions in both mice and guinea pigs and found augmented humoral IgG titers in both species, relative to unbound GFP-SA tetramer. Next, we fused an N-terminal fragment of the Canine oral papillomavirus L2 protein to streptavidin. With TMV display, the L2 protein fragment was significantly more immunogenic than uncoupled antigen when tested in mice. By demonstrating the presentation of whole proteins, this study expands the utility of TMV as a vaccine scaffold beyond that which is possible by genetic manipulation.

Covalent disulfide-linked anti-CEA diabody allows site-specific conjugation and radiolabeling for tumor targeting applications

An engineered anti-carcinoembryonic antigen (CEA) diabody (scFv dimer, 55 kDa) was previously constructed from the murine anti-CEA T84.66 antibody. Tumor targeting, imaging and biodistribution studies in nude mice bearing LS174T xenografts with radiolabeled anti-CEA diabody demonstrated rapid tumor uptake and fast blood clearance, which are favorable properties for an imaging agent. Current radiolabeling approaches result in random modification of the protein surface, which may impair immunoreactivity especially for smaller antibody fragments. Site-specific conjugation approaches can direct modifications to reactive groups located away from the binding site. Here, cysteine residues were introduced into the anti-CEA diabody at three different locations, to provide specific thiol groups for chemical modification. One version (with a C-terminal Gly-Gly-Cys) existed exclusively as a disulfide-bonded dimer. This cysteine-modified diabody (Cys-diabody) retained high binding to CEA and demonstrated tumor targeting and biodistribution properties identical to the non-covalent diabody. Furthermore, following reduction of the disulfide bond, the Cys-diabody could be chemically modified using a thiol-specific bifunctional chelating agent, for radiometal labeling. Thus, the Cys-diabody provides a covalently linked alternative to conventional diabodies, which can be reduced and modified site-specifically. This format will provide a versatile platform for targeting a variety of agents to CEA-positive tumors.

Improving selection of αIIbβ3-binding phage antibodies with increased reactivity derived from immunized donors

Although many studies of the immune response in polytransfused Glanzmann thrombasthenia (GT) patients and in autoimmune thrombocytopenic purpura (AITP) have demonstrated the frequent development of Abs directed against the alphaIIbbeta3 integrin, little is known about the induced anti-alphaIIbbeta3 autoantibodies at the molecular level. Phage display is a powerful technology for selecting and engineering mAbs expressed on the surface of filamentous bacteriophage. Combinatorial libraries of single-chain IgG were constructed from splenocytes from two patients with AITP and one patient with GT. In a previous study, activated platelets or alphaIIbbeta3-expressing CHO cells selection was performed to isolate human IgG anti-alphaIIbbeta3 binding fragments using combinatorial libraries created from the B cells of a GT and an AITP patient. However, we have experienced practical problems such as enrichment of truncated antibodies during selection. We decided to test prolonged treatments with elution agents after screening on the purified form of the alphaIIbbeta3 integrin activated with the RGD peptide. We obtained a higher percentage of clones with full-size antibody fragments as well as an enrichment of more specific alphaIIbbeta3-binding phage-Abs. Some of them, recognizing the activated form of the integrin, would be interesting to further study as potential diagnostic or therapeutic agents in acute coronary syndromes. Sequencing of selected phage-Abs revealed that they used different VH and VL genes with, for the majority of them, a high level of extensive hypermutations in the complementarity determining regions, indicating the diversity of the antigen-driven immune response that occurred in GT and AITP patients.

Effect of domain order on the activity of bacterially produced bispecific single-chain Fv antibodies

Bispecific single-chain Fv antibodies comprise four covalently linked immunoglobulin variable (VH and VL) domains of two different specificities. Depending on the order of the VH and VL domains and on the length of peptides separating them, the single-chain molecule either forms two single-chain Fv (scFv) modules from the adjacent domains of the same specificity, a so-called scFv-scFv tandem [(scFv)(2)], or folds head-to-tail with the formation of a diabody-like structure, a so-called bispecific single-chain diabody (scBsDb). We generated a number of four-domain constructs composed of the same VH and VL domains specific either for human CD19 or CD3, but arranged in different orders. When expressed in bacteria, all (scFv)(2) variants appeared to be only half-functional, binding to CD19 and demonstrating no CD3-binding activity. Only the diabody-like scBsDb could bind both antigens. Comparison of the scBsDb with a structurally similar non-covalent dimer (diabody) demonstrated a stabilizing effect of the linker in the middle of the scBsDb molecule. We demonstrated that the mechanism of inactivation of CD19xCD3 diabody under physiological conditions is initiated by a dissociation of the weaker (anti-CD3) VH/VL interface followed by domain swapping with the formation of non-active homodimers. The instability of one homodimer makes the process of diabody dissociation/reassociation irreversible, thus gradually decreasing the fraction of active molecules. The structural parameters influencing the formation of functional bispecific single-chain antibodies are indicated and ways of making relatively stable bispecific molecules are proposed.

Identification of factors impeding the production of a single-chain antibody fragment in Escherichia coliby comparing in vivo and in vitro expression

The atrazine-specific single-chain variable antibody fragments (scFv) K411B was produced by expression in either the cytoplasm or the periplasm of Escherichia coli BL21(DE3). For periplasmic production, the pelB leader was N-terminally fused to scFv, whereas the unfused variant resulted in cytoplasmic expression. The extent of protein accumulation differed significantly. Expression of scFv with leader was 2.3 times higher than that of the protein without leader. This was further investigated by generating the respective translation profiles using coupled in vitro transcription/translation assays, the results of which were in agreement. This comparative approach was also applied to functionality: Periplasmic expression and in vitro expression resulted in only 10% correctly folded scFv, indicating that the oxidizing environment of the periplasm did not increase proper folding. Thus, the data obtained in vitro confirmed the findings observed in vivo and suggested that the discrepancy in expression levels was due to different translation efficiencies. However, the in vivo production of scFv with enhanced green fluorescent protein (EGFP) fused C-terminally (scFv-EGFP) was only successful in the cytoplasm, although in vitro the expression with and without the leader rendered the same production profile as for scFv. This indicated that neither the translation efficiency nor the solubility but other factors impeded periplasmic expression of the fusion protein.

Bacterial expression and purification of recombinant bovine Fab fragments

We have previously described a recombinant phagemid expression vector, pComBov, designed for the production of native sequence bovine monoclonal antibodies (mAb) generated by antibody phage display. Bovine mAb Fab fragments isolated from libraries constructed using pComBov in Escherichia coli strain XL1-Blue, which is routinely used for antibodies expressed on the surface of phage, were expressed at very low yields. Therefore, a study was undertaken to determine optimal growth conditions for maximal expression of bovine Fab fragments in E. coli. By varying the E. coli strain, and the temperature and length of the culture growth, we were able to substantially increase the yield of soluble Fab fragments. A high yield of Fab fragments was found in the culture growth medium, which enabled us to devise a rapid and simple single-step method for the purification of native (nondenatured) Fabs based on immobilized metal affinity chromatography against a six-histidine amino acid carboxyl-terminal extension of the heavy-chain constant region. Using these methods we were able to express and purify antigen-specific bovine Fab fragments from E. coli.

Molecular immunolabeling with recombinant single-chain variable fragment (scFv) antibodies designed with metal-binding domains

To study the molecular structure and function of gene products in situ, we developed a molecular immunolabeling technology. Starting with cDNA from hybridomas producing monoclonal antibodies against biotin, catalase, and superoxide dismutase, we bioengineered recombinant single-chain variable fragment antibodies (scFv) and their derivatives containing metal-binding domains (scFv:MBD). As tested with surface plasmon resonance and enzyme-linked immunosorbent assay, affinity binding constants of the scFv (5.21 x 10(6) M(-1)) and scFv:MBD (4.17 x 10(6) M(-1)) were close to those of Fab proteolytic fragments (9.78 x 10(6) M(-1)) derived from the parental IgG antibodies. After saturation of MBD with nickel or cobalt, scFv:MBD was imaged with electron spectroscopic imaging at each element's specific energy loss, thus generating the element's map. Immunolabeling with scFv:MBD resulted in a significant improvement of the labeling fidelity over that obtained with Fab or IgG derivatives, as it produced a much heavier specific labeling and label-free background. As determined with radioimmunoassay, labeling effectiveness with scFv:MBD was nearly the same as with scFv, but much higher than with scFv conjugated to colloidal gold, Nanogold, or horseradish peroxidase. This technology opens possibilities for simultaneous imaging of multiple molecules labeled with scFv:MBD at the molecular resolution within the same sample with electron spectroscopic imaging. Moreover, the same scFv:MBD can also be imaged with fluorescence resonance energy transfer and lifetime imaging as well as positron emission tomography and magnetic resonance imaging. Therefore, this technology may serve as an integrative factor in life science endeavors.

Noncompetitive immunoassay of small analytes at the femtomolar level by affinity probe capillary electrophoresis: direct analysis of digoxin using a uniform-labeled scFv immunoreagent

A general method for noncompetitive immunoassay of small analytes using affinity probe capillary electrophoresis (APCE) is demonstrated using digoxin as a model analyte. A uniform immunoreagent was prepared from a single-chain antibody (scFv) gene specific for digoxin. Site-directed mutagenesis introduced a unique cysteine residue for uniform labeling with a thiol-reactive fluorochrome. After expression in E. coli, the scFv was purified by immobilized metal affinity chromatography (IMAC) using an added C-terminal 6-histidine sequence. The protein was renatured and labeled while immobilized on the IMAC resin. After 0.02-microm filtration to remove microaggregates, the resulting reagent was highly uniform and stable at -12 degrees C for at least 1 year. Three formats of APCE using the scFv reagent were explored. A "mix-and-inject" assay optimized for low detection limits demonstrated analysis of 10 pM digoxin in aqueous standard solutions in 10 min. A rapid mix-and-inject format in a short capillary allowed detection of 1 nM digoxin in 1 min. Digoxin samples in serum and urine were injected directly after 10-fold dilution. In combination with solid-phase extraction, 400 fM digoxin was detected in 1 mL of serum. Including solid-phase extraction, reproducibility was within 2.5%, and the linear range was 3 orders of magnitude. The strategy adopted in this paper should be of general use in the low-level analysis of small analytes.

Relative position of the hexahistidine tag effects binding properties of a tumor-associated single-chain Fv construct

Hexahistidine tag (His-tag) is the most widely used tag for affinity purification of recombinant proteins for their structural and functional analysis. In the present study, single chain Fv (scFv) constructs were engineered form the monoclonal antibody (MAb) CC49 which is among the most extensively studied MAb for cancer therapy. For achieving efficient purification of scFvs by immobilized metal-ion affinity chromatography (IMAC), a His-tag was placed either at the C-terminal (scFv-His6) or N-terminal (His6-scFv) of the coding sequence. Solid-phase radioimmunoassay for scFv-His6 showed only 20-25% binding whereas both His6-scFv and scFv (no His-tag) showed 60-65% binding. Surface plasmon resonance studies by BIAcore revealed the binding affinity constant (KA) for His6-scFv and scFv as 1.19 x 10(6) M(-1) and 3.27 x 10(6) M(-1), respectively. No K(A) value could be calculated for scFv-His6 due to poor binding kinetics (kon and koff). Comparative homology modeling for scFv and scFv-His6 showed that the C-terminal position of the His-tag partially covered the antigen-binding site of the protein. The study demonstrates that the C-terminal position of His-tag on the CC49 scFv adversely affects the binding properties of the construct. The results emphasize the importance of functional characterization of recombinant proteins expressed with purification tags.

Effects of unpaired cysteines on yield, solubility and activity of different recombinant antibody constructs expressed in E. coli

New E. coli vectors based on the pOPE/pSTE vector system [Gene 128 (1993) 97] were constructed to express a single-chain Fv antibody fragment (scFv), a scFv-streptavidin fusion protein and two disulfide bond-stabilized Fv antibody fragments (dsFvs) utilizing different side chain positions for disulfide stabilization. All of these constructs encoded fusion proteins carrying five C-terminal histidine residues preceded by an unpaired cysteine. The influence of this cysteine, which was originally introduced to allow the chemical modification of the fusion proteins, was assessed by exchanging the two amino acids CysIle in front of the carboxy terminal His-tag to SerHis in all constructs. Yield and antigen-binding activity of the antibody constructs were compared after standard lab-scale periplasmic expression in Escherichia coli. The removal of the unpaired cysteine resulted in a significant increase in antigen-binding activity of the crude periplasmic extracts. Further, a three-five fold increase of yield and a significantly improved purity were observed after immobilized metal affinity chromatography (IMAC) with all four constructs.

Three-step purification of bacterially expressed human single-chain Fv antibodies for clinical applications

We have obtained a cell line which secretes a human monoclonal IgM (B7) reacting with the myosin heavy chain of human heart. We have constructed single-chain fragments (scFv) of B7. The scFv may be useful for the imaging of myocardial necrosis after myocarditis, cardiac drug toxicosis or graft rejection. The aim of our work was to purify the scFv for immunoscintigraphy. We describe several purification steps including immobilized metal affinity chromatography (IMAC), anti-c-myc monoclonal antibody affinity chromatography, size-exclusion chromatography with Superdex 75 HR 10/30 and ion-exchange chromatography (mini Q TM 30Q).

Selection of phage-displayed anti-guinea pig C5 or C5a antibodies and their application in xenotransplantation

Xenogeneic liver transplantation in the discordant guinea pig (gp) to rat model results in hyperacute rejection within a few minutes, which is due to activation of the complement system. Currently no antibodies against gp complement factors are available, which allow activation of the gp complement system in serum or complement deposition in tissue to be detected. To close this gap, we started developing single chain Fvs (scFvs) against gpC5 and gpC5a. We generated a combinatorial library of scFv antibodies comprising the variable heavy and light chain repertoire from mice immunized with gpC5. Out of this library we selected several antibodies against gpC5 and C5a after four and six rounds of biopanning, respectively. Selected gpC5-specific scFvs were purified by metal affinity chromatography followed by size exclusion chromatography or by affinity chromatography using Protein L. Purified scFvs were able to inhibit gp complement system in a hemolytic assay and to detect gpC5 deposition in tissue. A surface plasmon resonance based assay on BIAcore was established, with which the C5 concentration in gp serum was determined to 240 microg/ml. As at least 0.04% of the normal gpC5 concentration can be detected, the test provides a powerful tool to investigate the development and the consequence of a hybrid complement system after liver xenotransplantation from gp to rat.

Efficient recombination of Lym-1 scFv gene using multiple doubly-restricted DNA fragments

In order to improve radioimmunotherapy of lymphoma, a Lym-1 single-chain antigen-binding (scFv) protein molecule was produced. Because the commonly used polymerase chain reaction (PCR) method frequently causes unexpected mutations, we developed a non-PCR method for scFv gene assembly. The method involved a stepwise linkage of doubly-restricted DNA fragments and re-digestion of the resultant concatamers. Using this strategy, the Lym-1 scFv expression gene was readily constructed without mutations. The recombinant gene was cloned into an expression vector and scFv protein was expressed. The method can be used for other genes or DNA recombination.

Fine mapping of the antigen-antibody interaction of scFv215, a recombinant antibody inhibiting RNA polymerase II from Drosophila melanogaster

A bacterially expressed single chain antibody (scFv215) directed against the largest subunit of drosophila RNA polymerase II was analysed. Structure and function of the antigen binding site in scFv215 were probed by chain shuffling and by site-specific mutagenesis. The entire variable region of either the heavy or light chain was replaced by an unrelated heavy or light chain. Both replacements resulted in a total loss of binding activity suggesting that the antigen binding site is contributed by both chains. The functional contributions of each complementarity determining region (CDR) were investigated by site specific mutagenesis of each CDR separately. Mutations in two of the CDRs, CDR1 of light chain and CDR2 of heavy chain, reduced the binding activity significantly. Each of the amino acids in these two CDRs was replaced individually by alanine (alanine walking). Seven amino acid substitutions in the two CDRs were found to reduce the binding activity by more than 50%. The data support a computer model of scFv215 which fits an epitope model based on a mutational analysis of the epitope suggesting an alpha-helical structure for the main contact area.

Procaryotic expression of single-chain variable-fragment (scFv) antibodies: secretion in L-form cells of Proteus mirabilis leads to active product and overcomes the limitations of periplasmic expression in Escherichia coli

Recently it has been demonstrated that L-form cells of Proteus mirabilis (L VI), which lack a periplasmic compartment, can be efficiently used in the production and secretion of heterologous proteins. In search of novel expression systems for recombinant antibodies, we compared levels of single-chain variable-fragment (scFv) production in Escherichia coli JM109 and P. mirabilis L VI, which express four distinct scFvs of potential clinical interest that show differences in levels of expression and in their tendencies to form aggregates upon periplasmic expression. Production of all analyzed scFvs in E. coli was limited by the severe toxic effect of the heterologous product as indicated by inhibition of culture growth and the formation of insoluble aggregates in the periplasmic space, limiting the yield of active product. In contrast, the L-form cells exhibited nearly unlimited growth under the tested production conditions for all scFvs examined. Moreover, expression experiments with P. mirabilis L VI led to scFv concentrations in the range of 40 to 200 mg per liter of culture medium (corresponding to volume yields 33- to 160-fold higher than those with E. coli JM109), depending on the expressed antibody. In a translocation inhibition experiment the secretion of the scFv constructs was shown to be an active transport coupled to the signal cleavage. We suppose that this direct release of the newly synthesized product into a large volume of the growth medium favors folding into the native active structure. The limited aggregation of scFv observed in the P. mirabilis L VI supernatant (occurring in a first-order-kinetics manner) was found to be due to intrinsic features of the scFv and not related to the expression process of the host cells. The P. mirabilis L VI supernatant was found to be advantageous for scFv purification. A two-step chromatography procedure led to homogeneous scFv with high antigen binding activity as revealed from binding experiments with eukaryotic cells.