Application of an alkaline phosphatase fusion protein system suitable for efficient screening and production of Fab-enzyme conjugates in Escherichia coli

Enhancement of binding avidity by bivalent binding enables PrPSc-specific detection by anti-PrP monoclonal antibody 132

Anti-prion protein (PrP) monoclonal antibody 132, which recognizes mouse PrP amino acids 119–127, enables us to reliably detect abnormal isoform prion protein (PrP^Sc) in cells or frozen tissue sections by immunofluorescence assay, although treatment with guanidinium salts is a prerequisite. Despite the benefit of this mAb, the mechanism of PrP^Sc-specific detection remains unclear. Therefore, to address this mechanism, we analyzed the reactivities of mono- and bivalent mAb 132 to recombinant mouse PrP (rMoPrP) by enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR). In ELISA, binding of the monovalent form was significantly weaker than that of the bivalent form, indicating that bivalent binding confers a higher binding stability to mAb 132. Compared with other anti-PrP mAbs tested, the reactivity of bivalent mAb 132 was easily affected by a decrease in antigen concentration. The binding kinetics of mAb 132 assessed by SPR were consistent with the results of ELISA. The dissociation constant of the monovalent form was approximately 260 times higher than that of the bivalent form, suggesting that monovalent binding is less stable than bivalent binding. Furthermore, the amount of mAb 132 that bound to rMoPrP decreased if the antigen density was too low to allow bivalent binding. If two cellular PrP (PrP^C) are close enough to allow bivalent binding, mAb 132 binds to PrP^C. These results indicate that weak monovalent binding to monomeric PrP^C diminishes PrP^C signals to background level, whereas after exposure of the epitope, mAb 132 binds stably to oligomeric PrP^Sc in a bivalent manner.

Zipbodyzyme: Development of new antibody-enzyme fusion proteins

A new antibody-enzyme fusion protein, named Zipbodyzyme, composed of a Fab antibody (i.e., an antigen-binding fragment of an antibody) and an enzyme, has been successfully produced in the cytoplasm of Escherichia coli. Zipbodyzymes have a leucine zipper (LZ) pair at the C-termini of the heavy chain (Hc) and the light chain (Lc) of Fab, to promote the association of the Hc and the Lc in E. coli cytoplasm, adjoining a fused enzyme. A Zipbodyzyme containing mouse-derived anti-E. coli O157 Fab and a luciferase or a green fluorescent protein retained both the antigen-binding and an enzymatic activity/fluorescence. The bifunctional proteins were applicable in direct enzyme-linked immunosorbent assay (ELISA) without the need for a secondary antibody, indicating that the utility of the antibody enzyme bifunctional fusion protein will be expanded.

Expression of antibody variable region-human alkaline phosphatase fusion proteins in mammalian cells

Antibody fragments and their fusion proteins are indispensable tools as immunoassay reagents in diagnostics and molecular/cellular biotechnology. However, bacterial expression of cloned antibody genes with correct tertiary structure is not always guaranteed because of the lack of proper folding machinery and/or post-translational modifications. In addition, frequently used bacterial alkaline phosphatase as a fusion partner generally shows lower specific activity than the mammalian enzyme, which hampers its wider use as a detection reagent. Here we tried to express the fusion proteins of antibody variable region(s) and secreted human placental alkaline phosphatase (SEAP) using mammalian cell culture. As a result, functional V(H)-SEAP and single-chain Fv-SEAP fusion proteins were successfully obtained from COS-1 cells, which was confirmed by ELISA and Western blotting. This system will be applicable to the rapid production of various antibody-enzyme fusions suitable for ELISA and open-sandwich ELISA that utilizes antigen-dependent V(H)/V(L) interaction for antigen quantitation.

Construction of single chain variable fragment (ScFv) and BiscFv-alkaline phosphatase fusion protein for detection of Bacillus anthracis

This paper describes an attempt for convenient and sensitive detection of Bacillus anthracis with single chain variable fragment (scFv)-based protein chip. Phage display technology was employed to generate scFv by using the protective antigen (PA) of B. anthracis for immunization. V(H) and V(L) genes of the scFv were amplified separately by reverse transcriptase-PCR from mRNA of immunized mice and then assembled into scFv gene with a linker DNA sequence. The scFv gene was inserted into a phagemid vector pCANTAB-5E and then transformed into Escherichia coli TG1 to yield recombinant phages after infection with helper phage M13KO7. After six rounds of panning with PA, the phage clones displaying scFv fragments of the antibody were selected by ELISA. One phage clone scFv-6w10 showing the strongest positive signal in ELISA was selected. To enhance the affinity of the scFv-6w10, a recombinant bivalent single-chain Fv antibody (biscFv-6w10) directed against PA was constructed and tested in functional assays. The affinity of the biscFv-6w10 was much higher than that of scFv-6w10 and reached 6.5 x 10(9) M(-1). An expression system was constructed for the production of E. coli alkaline phosphatase (EAP) labeled biscFv-6w10 (biscFv-6w10-EAP) in E. coli cells. The expressed fusion protein retained both antigen-specific binding and enzymatic activity and thus directly served as an enzyme-labeled antibody. Detections of PA and bacterial cells of B. anthracis using biscFv-6w10-EAP and Cy3-labeled biscFv-6w10 were performed on a protein chip. The fusion protein (biscFv-6w10-EAP) chip could detect 10 pg of PA and 500-1000 bacterial cells in approximately 2 h, while the sensitivity of Cy3-labeled protein chip reached 1 pg of PA and 50-100 cells within 2 h.

Bacterial expression of a human monoclonal antibody-alkaline phosphatase conjugate specific for Entamoeba histolytica

We previously produced human monoclonal antibody Fab fragments specific to Entamoeba histolytica in Escherichia coli. In order to use these Fab fragments for diagnostic purposes, an expression vector to produce a fusion protein of Fab and alkaline phosphatase (PhoA) in E. coli was designed and constructed. The E. coli PhoA gene was fused to the 3' terminus of the gene encoding the heavy-chain Fd region. The kappa and Fd genes from a previously prepared antibody clone, CP33, which is specific for the 260-kDa lectin of E. histolytica, were used as human antibody genes. When the fusion protein of CP33 and PhoA was incubated with paraformaldehyde-fixed trophozoites of E. histolytica and developed with a substrate, the trophozoites appeared to be stained. These results demonstrate the feasibility of bacterial expression of a human monoclonal antibody-PhoA conjugate specific for E. histolytica and that the antibody can be used to detect E. histolytica antigen without the use of chemically conjugated secondary antibodies.

Affibody-beta-galactosidase immunoconjugates produced as soluble fusion proteins in the Escherichia coli cytosol

Recombinant immunoconjugates constitute a novel class of immunoassay reagents produced by genetic fusion between an antigen recognizing moiety and a reporter enzyme or fluorescent protein, obviating the need for chemical coupling. In this work, we describe the construction, Escherichia coli production and characterization of recombinant beta-galactosidase (beta-gal)-based immunoconjugates directed to human immunoglobulin A (IgA). As the antigen recognizing moieties, either monovalent or dimeric (head-to-tail) versions of an IgA-specific affibody (Z(IgA1)) were used, previously selected in vitro from a protein library based on combinatorial engineering of a single staphylococcal protein A domain. To increase the likelihood of proper presentation on the assembled homotetrameric enzyme surface, the affibody moieties were linked to the N-terminus of the enzyme subunits via a heptapeptide linker sequence. The two resulting immunoconjugates Z(IgA1)-beta-gal and (Z(IgA1))(2)-beta-gal, containing four and eight affibody moieties per enzyme, respectively, could be expressed as soluble and proteolytically stable proteins intracellularly in E. coli from where they were purified to high purity by a single anion exchange chromatography step. The yields of immunoconjugates were in the range 200-400 mg/l culture. Biosensor-binding studies showed that both the Z(IgA1)-beta-gal and (Z(IgA1))(2)-beta-gal immunoconjugates were capable of selective IgA-recognition, but with an apparent higher binding affinity for the variant containing divalent affibody moieties, presumably due to avidity effects. The applicability of this class of recombinant immunoconjugates was demonstrated by IgA detection in enzyme-linked immunosorbent assay (ELISA) and dot-blot analyses. In addition, using human kidney biopsy samples from a nephropathy patient, IgA depositions in glomeruli could be detected by immunohistochemistry with low background staining of tissue.

Single-chain Fv antibody-alkaline phosphatase fusion proteins produced by one-step cloning as rapid detection tools for ELISA

A system was constructed for the production of alkaline phosphatase (aP)-labeled antibody single-chain Fv (scFv) fragments in Escherichia coli. The expression vector pASK75 was modified by sequentially inserting the E. coli aP coding region and the scFv cloning cassette. Engineering the cloning sites SfiI and NotI located at the 5' and 3' end of the scFv gene provides an easy means to insert scFv fragments. These cloning sites are widely used in recombinant antibody technology and, thus, enable the one-step cloning of scFv fragments derived from corresponding antibody phage libraries into the expression vector. An expressed herbicide-specific scFv aP fusion protein retained both, analyte binding and enzymatic activity, as determined by ELISA. Therefore, this system permits the production of scFv-aP conjugates in E. coli, which can replace conventionally prepared aP-labeled antibodies in immunoassays. These fusion proteins are designed to accelerate the immunochemical detection of analytes, since the assay duration is essentially reduced by omitting the use of enzyme labeled secondary antibodies.

Construction and characterization of affibody-Fc chimeras produced in Escherichia coli

Affibody-Fc chimeras were constructed by genetic fusion between different affibody affinity proteins with prescribed specificities and an Fc fragment derived from human IgG. Using affibody ligands previously selected for binding to respiratory syncytial virus (RSV) surface protein G and Thermus aquaticus (Taq) DNA polymerase, respectively, affibody-Fc fusion proteins showing spontaneous Fc fragment-mediated homodimerization via disulfide bridges were produced in Escherichia coli and affinity purified on protein A Sepharose from bacterial periplasms at yields ranging between 1 and 6 mg/l culture. Further characterization of the chimeras using biosensor technology showed that the affibody moieties have retained high selectivities for their respective targets after fusion to the Fc fragment. Avidity effects in the target binding were observed for the affibody-Fc chimeras compared to monovalent affibody fusion proteins, indicating that both affibody moieties in the chimeras were accessible and contributed in the binding. Fusion of a head-to-tail dimeric affibody moiety to the Fc fragment resulted in tetravalent affibody constructs which showed even more pronounced avidity effects. In addition, the Fc moiety of the chimeras was demonstrated to be specifically recognized by anti-human IgG antibody enzyme conjugates. One application for this class of "artificial antibodies" was demonstrated in a western blotting experiment in which one of the anti-RSV surface protein G affibody-Fc chimeras was demonstrated to be useful for specific detection of the target protein in a complex background consisting of a total E. coli lysate. The results show that through the replacement of the Fab portion of an antibody for an alternative binding domain based on a less complicated structure, chimeric proteins compatible with bacterial production routes containing both antigen recognition domains and Fc domains can be constructed. Such "artificial antibodies" should be interesting alternatives to, for example, whole antibodies or scFv-Fc fusions as detection devices and in diagnostic or therapeutic applications.

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.

Open sandwich ELISA with V(H)-/V(L)-alkaline phosphatase fusion proteins

The Sandwich ELISA is a widely used technique to measure antigen concentration. Recently, a novel ELISA based on the interchain interaction of separated V(H) and V(L) chains from a single antibody variable region (Fv) was proposed (Open Sandwich ELISA). Since it employs a single antibody recognizing one epitope, the assay requires, in essence, only one cycle of incubation and washing steps. To demonstrate this directly, we have constructed a recombinant gene fusion encoding the V(H) chain of an anti-hen egg lysozyme (HEL) antibody HyHEL-10 and Escherichia coli alkaline phosphatase (V(H)-PhoA). The same type of gene fusion using V(L) chain instead of V(H) chain (V(L)-PhoA) was also constructed and the proteins were obtained with an E. coli expression/secretion system. Open Sandwich ELISAs were performed using microtiter plates with immobilized V(L) or V(H) fragment, and V(H)-PhoA or V(L)-PhoA, respectively, as the detection reagent which was simultaneously added to each well with samples. As a result, HEL concentrations in the samples were determined after one round of incubation and washing steps, with a signal generated in a direct relationship to the concentration of HEL added to the reaction mixture. The minimum detectable HEL concentration was approximately 10 ng/ml, which was almost equal to the value previously obtained with plate-immobilized V(L) and V(H) fragment displayed on M13 phage. When the active-site mutant V(H)-PhoA(D101S) was employed instead of V(H)-PhoA and reacted at an optimum pH of 10, a significant enhancement in signal was attained.

Production and characterization of a bivalent single chain Fv/alkaline phosphatase conjugate specific for the hemocyanin of the scorpion Androctonus australis

A102 is a monoclonal antibody raised against the hemocyanin of the Tunisian scorpion Androctonus australis. It is directed against the subunit Aa6 and does not cross-react when tested against a variety of similar scorpion hemocyanins. Here, we report the construction of a plasmid encoding a recombinant enzyme-linked antigen-binding protein with the antigen-binding specificity of antibody A102. The DNA fragments encoding the variable domains of A102 were inserted into a prokaryotic expression vector so as to produce a single chain antibody variable fragment (scFv) fused to the bacterial alkaline phosphatase. The fusion protein preserved the IgG binding and alkaline phosphatase activities. Immunoelectron microscopic analysis showed that the recombinant protein bound antigen bivalently as is the case for natural antibodies. Crude preparations containing the conjugate were used in a rapid visual immunoassay for the specific detection of A. australis hemocyanin, using a droplet of hemolymph removed from live animals by puncture. The simplicity of the test made it suitable for the direct identification of animals belonging to this species. It could be useful in areas where A. australis, the most dangerous African scorpion, is found with other species from which it is not easy to distinguish using morphological criteria.

Single-chain Fv fusion proteins suitable as coating and detecting reagents in a double antibody sandwich enzyme-linked immunosorbent assay

A recombinant enzyme-linked immunosorbent assay (ELISA) system, entirely based on antibody fragments, is described here as an attractive alternative to assays using polyclonal antisera or monoclonal antibodies. Two expression vectors have been developed for cloning and production of single-chain Fv (scFv) fusion proteins suitable as coating and detecting reagents, respectively, in a highly sensitive double antibody sandwich ELISA. The coating reagent is produced from the vector pZIP1, as a bivalent miniantibody with a leucine zipper for dimerization. The detection reagent is a fusion protein, in which a modified Escherichia coli alkaline phosphatase with increased specific activity is attached to the carboxy-terminus of the scFv. This conjugate is produced using pDAP2/S as cloning and expression vector. Optimized bacteria expression and simple one-step purification by hexahistidine tag-mediated metal chelate affinity chromatography yielded milligrams of ELISA reagent per liter of bacterial culture in both cases. A double antibody sandwich ELISA for the detection of beet necrotic yellow vein virus, the causal agent of sugarbeet rhizomania, was developed using fusion proteins obtained by means of pZIP1 and pDAP2/S. The plant pathogen was detected with a sensitivity higher than that reached in a conventional ELISA employing polyclonal antisera. Both plasmid vectors are compatible to phage display vectors such as pHEN1, pCOCK, and the pCANTAB series, allowing simple subcloning after isolation of scFv from phage display libraries. It is therefore easy to develop and produce an ELISA entirely by using bacterial recombination and expression techniques.

Primary structure and functional scFv antibody expression of an antibody against the human protooncogen c-myc

The immunoglobulin heavy- and light-chain variable region (Vh and Vl) genes were isolated from Myc1-9E10 hybridoma cells, which secreted monoclonal antibody against human oncogen c-myc. The expression vector pOPE52-c-myc was constructed for the recombinant production in E. coli. A 30 kDa single chain fragment (scFv) expression product was found in the periplasmic space by SDS-PAGE and immunoblotting. A significant fraction was processed correctly as demonstrated with an antiserum recognizing the processed aminoterminus only. The specific binding of the scFv fragment to the peptide epitope of the maternal monoclonal antibody was demonstrated and the primary sequence of the variable regions was determined. Sequence comparison with previously published partial Vh and Vl sequences from this hybridoma cell line revealed a genetic heterogeneity for the light chain variable region. The potential use of this scFv as a new tool for detection and purification of tagged proteins, for adding costimulatory signals to the surface of cancer cells as well as for analyzing c-myc function in the living cell by cytoplasmic expression is discussed.

Examining the specificity of Src homology 3 domain--ligand interactions with alkaline phosphatase fusion proteins

Sixteen-amino-acid-long peptides, corresponding to the optimal ligand preferences of the Src homology 3 (SH3) domains of Abl, Cortactin, Crk, p53BP2, and Src, were fused to the N-terminus of Escherichia coli alkaline phosphatase (AP). These secreted fusion proteins have been used as one-step detection probes of peptide ligand-SH3 domain interactions on microtiter plates and membranes. The binding of both the class I and II SH3 ligand-AP fusion proteins to their targets is robust and specific in comparison to chemically synthesized biotinylated peptides, used either in monovalent or tetravalent formats. p53BP2 and Cortactin SH3 ligand-AP fusions have been used to screen a mouse embryo lambda cDNA expression library and resulted in the cloning of p53BP2 and several known proteins with SH3 domains similar to that of Cortactin, respectively. In addition, the approximately 60-amino-acid-long SH3 domains of Src and Abl were fused to AP and the resulting fusion proteins were found to bind specifically to their respective peptide ligands in microtiter plates and proteins containing proline-rich regions in screens of a lambda cDNA expression library. Thus, SH3 peptide ligand- and SH3 domain-AP fusion proteins are convenient and sensitive reagents for examining the specificity of SH3 domain-ligand interactions, identifying potentially interacting proteins, and establishing high-throughput screens of combinatorial chemical libraries.

pDAP2: a vector for construction of alkaline phosphatase fusion-proteins

BACKGROUND

Expression of enzymatically active protein fusions in Escherichia coli could facilitate the analysis of proteins and even replace some reagents frequently used in immunology such as chemically produced antibody-enzyme conjugates. For this purpose there is up to now no system of general utility available.

OBJECTIVES

The vector pDAP2 has been designed for simplified fusion of single-chain Fv fragments to the N-terminus of E. coli alkaline phosphatase. The resulting immunoconjugates can be produced by expression in E. coli and purified in a single step via metal affinity chromatography.

STUDY DESIGN

Several different single-chain Fv genes as well as peptide coding oligonucleotides have been cloned into pDAP2 and tested for expression levels, purification properties and usefulness in ELISA and immunowestern blotting.

RESULTS

The fusion proteins from pDAP2 can be prepared at levels of several milligrams per liter culture from the periplasma of the cells. The proteins work well in different immunoassay formats such as ELISA or western blotting.

CONCLUSION

pDAP2 is compatible to phage display vectors such as pHEN1, pCOCK and the pCANTAB-series (Pharmacia, Sweden). Genes of single-chain antibody fragments can be simply swapped from these vectors into pDAP2. Furthermore, we demonstrate that it is possible to produce alkaline phosphatase-active peptides with this vector by inserting synthetic coding oligonucleotides. This allows simple analysis of the binding properties of peptides and may have some advantages over other systems such as fusion to glutathione-S-transferase.