Insertion of a disulfide-containing neurotoxin into E. coli alkaline phosphatase: the hybrid retains both biological activities

Development of an Immunoassay for Chloramphenicol Based on the Preparation of a Specific Single-Chain Variable Fragment Antibody

Specific antibodies are essential for the immune detection of small molecule contaminants. In the present study, the heavy and light variable regions (V(H )and V(L)) of the immunoglobulin genes from a hybridoma secreting a chloramphenicol (CAP)-specific monoclonal antibody (mAb) were cloned and sequenced. In addition, the light and heavy chains obtained from the monoclonal antibody were separated using SDS-PAGE and analyzed using Orbitrap mass spectrometry. The results of DNA sequencing and mass spectrometry analysis were compared, and the V(H) and V(L) chains specific for CAP were determined and used to construct a single-chain variable fragment (scFv). This fragment was recombinantly expressed as a soluble scFv-alkaline phosphatase fusion protein and used to develop a direct competitive ELISA. Compared with the parent mAb, scFv exhibits lower sensitivity but better food matrix resistance. This work highlights the application of engineered antibodies for CAP detection.

Production and characterization of a single-chain variable fragment linked alkaline phosphatase fusion protein for detection of O,O-diethyl organophosphorus pesticides in a one-step enzyme-linked immunosorbent assay

A single-chain variable fragment (scFv) linked alkaline phosphatase (AP) fusion protein for detection of O,O-diethyl organophosphorus pesticides (O,O-diethyl OPs) was produced and characterized. The scFv gene was prepared by cloning V(L) and V(H) genes from hybridoma cells secreting monoclonal antibody with broad specificity for O,O-diethyl OPs. The amplified V(L) and V(H) regions were assembled using a linker (Gly(4)Ser)(3) by means of splicing overlap extension polymerase chain reaction to obtain the scFv gene, which was cloned into the expression vector pLIP6/GN containing an AP gene to produce the scFv-AP fusion protein in Escherichia coli strain BL21. The protein was purified by antigen-conjugated immunoaffinity chromatography and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Western blotting, and competitive direct enzyme-linked immunosorbent assay (cdELISA). The fusion protein is bifunctional, retaining both antigen binding specificity and AP enzymatic activity. Analysis of spiked and blind river water and Chinese cabbage samples demonstrated that the fusion protein based cdELISA(FP) exhibited good sensitivity and reproducibility.

Recombinant single-chain Fv antibody fragment–alkaline phosphatase conjugate: A novel in vitro tool to estimate rabies viral glycoprotein antigen in vaccine manufacture

The purpose of this study was to design a novel in vitro tool by using recombinant protein technology to qualify the whole reagent preparation procedure, to be used to quantify rabies viral antigen preparation in a simple and rapid format for potency control of rabies vaccines. 50AD1 is a neutralizing monoclonal antibody directed against the rabies virus glycoprotein that binds to native conformational antigenic site III. In the present study, the DNA fragments encoding the variable domains of 50AD1 were inserted into a prokaryotic expression vector so as to produce a single-chain Fv antibody fragment (scFv) genetically fused to the bacterial alkaline phosphatase (AP). The recombinant fusion protein preserved both the AP enzymatic activity and the antigen-binding activity against the rabies virus glycoprotein nearly identical to the parental antibody, and was used successfully in different assays including ELISA, dot-blot and cell culture tests. The present study shows that the genetic fusion protein provides a new tool for one-step rabies virus immunodetection, which can be produced in homogeneous bifunctional reagent, easily, quickly and reproducibly. In addition, this recombinant immunoconjugate is a promising alternative reagent for applications involving immunodetection, it presents a similar sensitivity and specificity to that obtained with classical reagents.

Biological applications of a chimeric probe for the assessment of galectin-3 ligands

Beta1-6 branching of N-linked oligosaccharides has been correlated with the progression of different cancers. The leukoagglutinins of Phaseolus vulgaris (L-PHA) have been used to study this pattern of glycosylation whose biological significance is incompletely understood. The animal lectin, galectin-3, also binds to structures recognized by L-PHA. To develop a functional tool for the in situ identification of this pattern of glycosylation, human galectin-3 was fused to bacterial alkaline phosphatase (gal3/AP). Gal3/AP recognized both A and B blood group saccharides (B>A) and lactosamine derivatives. Gal3/AP recognition depended at least in part on the N-linked oligosaccharides of different glycoproteins. The presence and distribution of galectin-3 ligands were analyzed in both murine and human normal and tumor samples. Loss of apical expression of galectin-3 ligands was commonly found in carcinomas. Endothelial and inflammatory cells were enriched in galectin-3 ligands as compared with tumor cells; thus, gal3/AP is a suitable tool for studying tumor microenvironments. Comparative analysis of both gal3/AP and L-PHA binding patterns indicated that although similar, these patterns are not identical. The probe developed was useful for several immunoenzymatic assays and will allow the physiological and clinical significance of the expression pattern of galectin-3 ligands to be established. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

BgK, a disulfide-containing sea anemone toxin blocking K+ channels, can be produced in Escherichia coli cytoplasm as a functional tagged protein

BgK, a sea anemone peptide consisting of 37 amino acid residues and 3 disulfide bonds, blocks voltage-gated potassium (Kv1) channels. Here, we report a method for producing tagged BgK in Escherichia coli, as a soluble cytoplasmic protein. First, using peptidic synthesis, we show that addition of a 15 residue peptide (S.Tag) at the BgK C-terminus does not affect its biological activity. Then, a synthetic DNA sequence encoding BgK was constructed and cloned to produce a BgK-S.Tag hybrid in the cytoplasm of E. coli. The presence of S.Tag did not only facilitate detection, quantification, and purification of the recombinant protein, but also increased the production yield by more than two orders of magnitude. Moreover, use of an E. coli OrigamiB(DE3)pLacI strain also increased production; up to 5.8-7.5mg of BgK-S.Tag or mutated BgK(F6A)-S.Tag was produced per liter of culture and could be functionally characterized in crude extracts. Using a two-step purification procedure (affinity chromatography and RP-HPLC), we obtained 1.8-2.8mg of purified recombinant protein per liter of culture. The recombinant peptides displayed functional properties similar to those of native BgK or BgK(F6A).

Cloning, expression, and characterization of a bi-functional disintegrin/alkaline phosphatase hybrid protein

Integrins are transmembrane heterodimeric glycoproteins responsible for cellular communication; therefore, they play an essential role in many physiological events. Viper snake venoms contain integrin antagonists called disintegrins which bind and inhibit integrin function. They present a loop containing an RGD motif responsible for integrin binding. The engineering of disintegrins fused to a reporter enzyme will be an interesting approach to build integrin markers. Even more, the disintegrin scaffold could be used to present other protein binding motifs. In this work, we have obtained alkaline phosphatase (APv) tagged eristostatin (Er) by cloning and expressing eristostatin DNA into the pLIP6-GN vector. Eristostatin, a 49 residue disintegrin, binds selectively to alphaIIbbeta3 integrin, inhibiting its binding to fibrinogen. The resulting fusion protein Er/APv was identified by SDS-PAGE and by Western blotting using both anti-Er and anti-AP antibodies. This fusion protein showed enzymatic AP activity similar to that of wild APv and its potential use for an alphaIIbbeta3 integrin assay was tested in a one-step dot blot using immobilized cells incubated with the marker and developed by AP substrate. Er/APv showed selectivity towards platelets and alphaIIbbeta3 integrin transfected cells and reacted with the same region as unlabeled Er, as analyzed in competition assays. Our data present a novel tool, Er/APv, with potential use as molecular marker in processes where the alphaIIbbeta3 integrin is involved.

A unique approach for high level expression and production of a recombinant cobra neurotoxin in Escherichia coli

In this report, we describe a simple approach to produce a large quantity of a recombinant cobra neurotoxin containing four pairs of disulfide bonds. A cDNA encoding the toxin was fused, in frame, to the carboxyl termini of thioredoxin via a linker sequence encoding two amino acids, Asp and Pro. Due to the presence of thioredoxin, a soluble form of the fusion protein was expressed in a compartment, sensitive to osmotic pressure, in Escherichia coli. The fusion protein was released into the solution with low ionic strength under an osmotic shock treatment, and purified in a single step using an ion exchange chromatography column. The purified protein was treated in diluted hydrochloric acid to induce hydrolysis of the protein at the Asp-Pro linker site. Then, the recombinant neurotoxin was purified by gel filtration of the acid-treated sample. When the biological activity of the purified toxin was assayed, it was as potent as the natural toxin. Using this protocol, approximately 12 mg of pure recombinant neurotoxin can be produced from one liter of bacterial culture. More importantly, this protocol can be easily used for the production of the toxin at a larger scale with low cost. The approach outlined in this report will be suitable for the production of other recombinant proteins especially those of the 'three-finger' family.

Artificial evolution of an enzyme active site: structural studies of three highly active mutants of Escherichia coli alkaline phosphatase

The crystal structure of three mutants of Escherichia coli alkaline phosphatase with catalytic activity (k(cat)) enhancement as compare to the wild-type enzyme is described in different states. The biological aspects of this study have been reported elsewhere. The structure of the first mutant, D330N, which is threefold more active than the wild-type enzyme, was determined with phosphate in the active site, or with aluminium fluoride, which mimics the transition state. These structures reveal, in particular, that this first mutation does not alter the active site. The second mutant, D153H-D330N, is 17-fold more active than the wild-type enzyme and activated by magnesium, but its activity drops after few days. The structure of this mutant was solved under four different conditions. The phosphate-free enzyme was studied in an inactivated form with zinc at site M3, or after activation by magnesium. The comparison of these two forms free of phosphate illustrates the mechanism of the magnesium activation of the catalytic serine residue. In the presence of magnesium, the structure was determined with phosphate, or aluminium fluoride. The drop in activity of the mutant D153H-D330N could be explained by the instability of the metal ion at M3. The analysis of this mutant helped in the design of the third mutant, D153G-D330N. This mutant is up to 40-fold more active than the wild-type enzyme, with a restored robustness of the enzyme stability. The structure is presented here with covalently bound phosphate in the active site, representing the first phosphoseryl intermediate of a highly active alkaline phosphatase. This study shows how structural analysis may help to progress in the improvement of an enzyme catalytic activity (k(cat)), and explains the structural events associated with this artificial evolution.

Improving Escherichia coli alkaline phosphatase efficacy by additional mutations inside and outside the catalytic pocket

We describe a strategy that allowed us to confer on a bacterial (E. coli) alkaline phosphatase (AP) the high catalytic activity of the mammalian enzyme while maintaining its high thermostability. First, we identified mutations, at positions other than those occupied by essential catalytic residues, which inactivate the bacterial enzyme without destroying its overall conformation. We transferred concomitantly into the bacterial enzyme four residues of the mammalian enzyme, two being in the catalytic pocket and two being outside. Second, the gene encoding the inactive mutant was submitted to random mutagenesis. Enzyme activity was restored upon the single mutation D330N, at a position that is 12 A away from the center of the catalytic pocket. Third, this mutation was combined with other mutations previously reported to increase AP activity slightly in the presence of magnesium. As a result, at pH 10.0 the phosphatase activity of both mutants D330N/D153H and D330N/D153G was 17-fold higher than that of the wild-type AP. Strikingly, although the two individual mutations D153H and D153G destabilize the enzyme, the double mutant D330N/D153G remained highly stable (T(m)=87 degrees C). Moreover, when combining the phosphatase and transferase activities, the catalytic activity of the mutant D330N/D153G increased 40-fold (k(cat)=3200 s-1) relative to that of the wild-type enzyme (k(cat)=80 s-1). Due to the simultaneous increase in K(m), the resulting k(cat)/K(m) value was only increased by a factor of two. Therefore, a single mutation occurring outside a catalytic pocket can dramatically control not only the activity of an enzyme, but also its thermostability. Preliminary crystallographic data of a covalent D330N/D153G enzyme-phosphate complex show that the phosphate group has significantly moved away from the catalytic pocket, relative to its position in the structure of another mutant previously reported.

A high-capacity alkaline phosphatase reporter system for the rapid analysis of specificity and relative affinity of peptides from phage-display libraries

We describe a novel reporter enzyme cassette system which enables the analysis of large numbers of linear and cyclic peptides in terms of their binding to a specific target molecule. In this system, peptides selected for target binding from random peptide phage-display libraries are expressed as cloned fusion proteins with bacterial alkaline phosphatase. The binding specificity and relative affinity of each peptide-enzyme fusion protein is then evaluated in a target-specific ELISA. This strategy enables direct identification of the highest affinity peptides, specific for a given target, which can then be sequenced at the DNA level to derive their peptide sequences. This eliminates the need to sequence large numbers of clones to establish consensus sequences for binding peptides. This approach also eliminates the need for peptide synthesis or phage ELISA to determine relative binding affinities, which can be technically difficult. Identification of binding peptides based on specificity and relative affinity, rather than conforming to an amino acid consensus sequence, enables the rapid evaluation of hundreds of candidate peptides and identification of rare (non-consensus) binding peptides which may otherwise be missed.

A genetically engineered fusion protein with horseradish peroxidase as a marker enzyme for use in competitive immunoassays

Horseradish peroxidase is one of the most widely used marker enzymes in immunoassays. Several disadvantages are encountered upon chemical conjugation of peroxidase with antibodies or antigens, as are low reproducibility and undefined stoichiometry. We here describe for the first time the production of a recombinant fusion of a protein analyte with horseradish peroxidase in Escherichia coli, employing refolding of inclusion bodies and reconstitution with heme. The genetic fusion approach enables preparation of conjugates with 1:1 stoichiometry and defined structure. As a protein analyte, the human heart fatty acid binding protein (H-FABP) was chosen, which is a new and sensitive marker for acute myocardial infarction. The recombinant conjugate was fully active [650 U/mg with 2,2-azino-bis(3-ethyl-thiazoline-6-sulfonate) as substrate] and obtained in a yield of 12 mg/L of E. coli culture, which is better than that for recombinant peroxidase alone. The competitive immunoassay that was developed with the recombinant conjugate requires fewer incubation steps than the traditional sandwich ELISA format. It permitted the detection of H-FABP directly in plasma in the range of 10-1500 ng/mL which is the relevant range for clinical decision-making.

Recombinant single-chain Fv antibody fragment-alkaline phosphatase conjugate for one-step immunodetection in molecular hybridization

Using phage-display technology, a recombinant single-chain Fv antibody fragment (scFv) was rapidly generated from the K16-16 hybridoma secreting mouse monoclonal antibody (MAb) that binds to acetylaminofluorene-labeled DNA (AAF-DNA). The selected A4 phage-scFv specifically bound to AAF-DNA. The anti-AAF scFv gene was then recloned into a fusion vector for the production of a hybrid protein comprising the antibody fragment fused to a potent bacterial alkaline phosphatase variant (PhoAv). The anti-AAF scFv-PhoAv hybrid protein was bifunctional and possessed both antigen binding capacity and PhoA activity. The recombinant conjugate was directly used, without further purification, for one-step immunodetection in dot-blot hybridization. The detection limit was identical and the test was quicker than the conventional two-step procedure with the purified anti-AAF MAb revealed with a secondary enzyme-labeled antibody. To assess the value of this new reagent for the immunodetection of genomic nucleic acids, genomic DNAs of Campylobacter jejuni and Campylobacter coli were then one-step immunodetected with non-purified recombinant scFv-PhoAv conjugate in a Southern-blot hybridization experiment. The present study shows that the genetic fusion with PhoAv provides a new tool for immunodetection which presents easier and quicker production and use with the same sensitivity and specificity as classical reagents. The recombinant anti-AAF scFv-PhoAv conjugate is a promising alternative reagent for applications involving the immunodetection of specific DNA or RNA sequences, such as the detection and characterization of microorganisms.

The functional architecture of an acetylcholine receptor-mimicking antibody

Malpha2-3 is a monoclonal antibody that partially mimics the nicotinic acetylcholine receptor (AChR). Its three-dimensional structure has been previously predicted by molecular modeling, suggesting that 29 complementarity determining region (CDR) residues and 2 framework residues are exposed to solvent. To identify the antibody residues that bind to the antigen, i.e. snake toxin that binds specifically to AChR, we (i) produced the scFv form of Malpha2-3 fused to alkaline phosphatase, in the periplasmic space of Escherichia coli; (ii) submitted approximately 75% of exposed residues of the fused scFv to individual or combined mutations, and (iii) identified the residues whose mutations affect scFv binding to the toxin, using a sensitive enzyme-linked immunosorbent assay. 11 critical residues were identified, including 8 heavy chain residues, 2 framework residues, and 1 light chain residue. They cover a surface of approximately 800 A2, with a subset of most critical residues (VHD31, VHY32, and VHG101) and several aromatic residues. This functional architecture not only constitutes a plausible complementary binding surface for the snake toxin but also offers a structural basis to ultimately understand the capacity of the antibody to partially mimic AChR.

A simple screen for permissive sites in proteins: analysis of Escherichia coli lac permease

Proteins can be remarkably tolerant of major mutational changes. Sites that accomodate large insertions without loss of function ("permissive" sites) appear generally to correspond to surface regions at which the added sequences do not disrupt overall folding. The identification of such sites can aid in the engineering of functional derivatives of a protein with novel properties. To screen for permissive sites, we developed a simple two-step procedure for generating 31-codon insertions in cloned genes. In a first step, a beta-galactosidase or alkaline phosphatase gene fusion is generated by insertion of a transposon derivative into the target gene. Requiring beta-galactosidase or alkaline phosphatase activity fixes the translational reading frame of the transposon relative to the target gene. In a second step, most of the transposon sequences are excised in vitro, leaving the in-frame insertion. Insertions may be targeted either to cytoplasmic or exported protein sequences, and the inserted sequence acts as an epitope in a variety of proteins. As a test case, a set of 31-codon insertions in the Escherichia coli lac permease gene was generated. The lactose transport activities of the mutant proteins followed a simple pattern: most of the proteins (10/12) with insertions in sequences thought to face the cytoplasm or periplasm were at least partially active, whereas all proteins (9/9) with insertions in membrane-spanning sequences were inactive. The only exceptions were two inactive proteins with insertions in the third cytoplasmic region. Most of the inactive proteins were detected at reduced levels in cells, presumably due to proteolytic breakdown. These studies thus illustrate the use of the new method to identify permissive sites and help document the remarkable sequence flexibility of many of the hydrophilic loops in lac permease. In addition to screening for permissive sites, 31-codon insertion mutagenesis may be useful in epitope-tagging proteins at multiple internal positions, in analyzing membrane protein topology, and in dissecting structure-function relationships in proteins.

Metal-recognition by repeating polypeptides

Attachment of proteins to metal surfaces has the potential to improve our understanding of protein adhesion and has applications in sensor technology. Repeating polypeptides able to bind to metallic gold or chromium were selected from a population of approximately 5 million different polypeptides. Each polypeptide contained several direct repeats of identical peptide units 14 or 28 amino acids long. The metal-recognizing polypeptides were found to retain their binding properties when freed from the protein used to select them. One gold-binding polypeptide's avidity for gold was found to be dependent on the number of repeats and the presence of salt.

Periplasmic expression of part of the major rotavirus capsid protein VP7 containing all the three antigenic regions in Escherichia coli

Part of the porcine rotavirus outer capsid protein VP7 containing all the three antigenic regions was expressed as a chimeric protein with bacterial alkaline phosphatase (AP) in E. coli. The construct contains an ompF promoter, the DNA encoding the signal sequence and the first 12 amino acids of mature OmpF, part of vp7 and the DNA encoding mature AP. The chimeric protein is stable, retains the biological property of AP and ability to react with polyclonal antiserum against the virus, and can be exported through the bacterial inner membrane into the periplasm.

Engineering of a recombinant colorimetric fusion protein for immunodiagnosis of insulin

A synthetic DNA encoding human proinsulin was inserted in frame in the bacterial alkaline phosphatase gene. A homogeneous recombinant human proinsulin-alkaline phosphatase conjugate was obtained directly from the periplasm of Escherichia coli transformed with a plasmid carrying the hybrid gene. The recombinant conjugate was stable and could be produced in the bacteria. The immunological properties of the recombinant conjugate and those of the human insulin and human proinsulin were compared using a panel of six different human insulin-specific monoclonal antibodies. Three immunological groups were thus distinguished and one of them indiscriminately recognized all of the insulin-like molecules. One monoclonal antibody from this group was used in combination with the recombinant conjugate to develop successfully a competitive immunoenzymatic assay for detecting insulin.

Recombinant antibody-alkaline phosphatase conjugates for diagnosis of human IgGs: application to anti-HBsAg detection

We have designed an expression vector permitting the production in the periplasm of Escherichia coli of a fusion protein comprising a dimer of bacterial alkaline phosphatase (PhoA) and two Fab or scFv fragments of a monoclonal antibody directed against human IgG. Each hybrid protein expressed both high specificity for the antigen and full PhoA activity. We show that crude periplasmic extracts containing these conjugates can be used as such in enzyme immunoassays for the detection of human IgG, as exemplified in the case of anti-hepatitis B immunoglobulin.

Analyses of p53 antibodies in sera of patients with lung carcinoma define immunodominant regions in the p53 protein

Antibodies specific for human p53 were analysed in sera of lung cancer patients. We detected p53 antibodies in the sera of 24% (10/42) of patients with lung carcinoma. The distribution was as follows: 4/9 small-cell lung carcinomas (SCLCs), 2/18 squamous cell lung carcinomas (SCCs), 2/10 adenocarcinomas (ADCs) and 2/5 large-cell lung carcinomas (LCCs). p53 antibodies were always present at the time of diagnosis and did not appear during progression of the disease. Using an original peptide-mapping procedure, we precisely localised the p53 epitopes recognised by p53 antibodies. Immunodominant epitopes reacting with antibodies were localised in the amino and carboxy termini of the protein, similar to those found in breast carcinoma patients or in animals immunised with p53. In light of these data, we suggest that p53 antibodies occur via a self-immunisation process that is the consequence of p53 accumulation in tumour cells. p53 antibodies were also detected in two patients without detected malignant disease. One of these patients died 6 months later of lung carcinoma, suggesting that p53 antibodies may be a precocious marker of p53 alteration.

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

We report a novel vector system suitable for the efficient preparation of alkaline phosphatase (PhoA)-labelled antibody Fab fragments in Escherichia coli. The previously described pGE20 vector used for the functional expression of truncated heavy (Fd) and light (L) chains of Fab into the bacterial culture medium was modified by inserting the E. coli PhoA coding region 3' to the Fd cloning sites. The secreted Fd-PhoA fusions and L proteins were found to be disulfide linked and Fab-PhoA complexes, prepared with IgG1 antibodies recognizing specifically human tumor necrosis factor alpha, were shown to be useful for the rapid detection of antigen. When an additional short peptide was interposed between the Fd and PhoA domains, nearly all of the recombinant Fab-PhoA conjugates present in the culture supernatant retained both antigen binding and enzymatic activity. A method for the detection and selection of bacterial colonies expressing bifunctional hybrid molecules of desired antigen specificity was also developed. Taken together, the systems described permit the generation and production of Fab-PhoA conjugates in E. coli, which can replace conventionally prepared PhoA-labelled antibodies in appropriate immunoassays.

Recombinant technology in the preparation of immunogen and enzymatic tracer. Application to the development of an enzyme immunoassay for rat prolactin

A competitive enzyme immunoassay for rat prolactin using an immunogen and a tracer obtained by recombinant DNA technology is described. Polyclonal antibodies were raised in rabbits immunized with a purified chimeric protein consisting of rat prolactin fused with two synthetic immunoglobulin G binding domains derived from staphylococcal Protein A. The enzymatic tracer was obtained using an expression system which permits insertion of rPRL DNA sequence in the alkaline phosphatase gene. Antibodies and tracer were used to develop a solid-phase competitive immunoassay for the measurement of rat prolactin in plasma with a minimal detectable concentration of 0.5 ng/ml. The mean intra- and inter-assay coefficients of variation were 7.8 and 13.2%, respectively. Rat plasma concentrations measured with this assay correlated well with those obtained with a conventional enzyme immunoassay (r = 0.993, slope = 1.037, n = 24).

Recombinant colorimetric antibodies: construction and characterization of a bifunctional F(ab)2/alkaline phosphatase conjugate produced in Escherichia coli

We have designed a vector which allows the synthesis in Escherichia coli of bifunctional F(ab)2-alkaline phosphatase conjugates. The vector contains a di-cistronic operon encoding truncated heavy chain (Fd or VH-CH1) of an IgG inserted between residues +6 and +7 of bacterial alkaline phosphatase (PhoA), and the light chain of the same IgG. We demonstrate the utility of this approach with the heavy and light chain domains of a snake toxin-specific monoclonal antibody, M alpha 2-3. We show that the VH-CH1-PhoA hybrid and VL-CL are concomitantly expressed and exported to the periplasm of E. coli where they form a disulfide-linked chimeric protein. The hybrid has the same affinity as M alpha 2-3 for the snake toxin antigen and possesses PhoA enzymatic activity.

Production of recombinant notechis 11'2L, an enzymatically active mutant of a phospholipase A2 from Notechis scutatus scutatus venom, as directly generated by cleavage of a fusion protein produced in Escherichia coli

We have constructed an expression vector to produce, in Escherichia coli, a fusion protein containing successively two IgG binding domains from staphyloccocal protein A, a nine-amino-acid linker peptide terminating in a methionine residue and the phospholipase A2 notechis 11'2L, an isoform of notexin of Notechis scutatus scutatus venom. Notechis 11'2L is a mutant of the naturally occurring notechis 11'2 [Bouchier, C., Boyot, P., Tesson, F., Trémeau, O., Bouet, F., Hodgson, D., Boulain, J. C. & Ménez, A. (1991) Eur. J. Biochem. 202, 493-500] in which Met8 has been replaced by Leu. The fusion protein was recovered in the periplasmic extract with a yield of 0.25 mg/l culture. It was hydrolyzed with cyanogen bromide, yielding a protein having the molecular mass, amino acid composition and N-terminal sequence of notechis 11'2L. Notechis 11'2L and the wild notechis 11'2 displayed identical circular dichroic spectra and shared similar enzymatic, myotoxic and antigenic properties, suggesting that the recombinant notechis 11'2L was directly generated in a correctly folded form.

Membrane topology of MalG, an inner membrane protein from the maltose transport system of Escherichia coli

In Escherichia coli, the binding protein-dependent transport system for maltose and maltodextrins is composed of five proteins--LamB, MalE, MalF, MalG and MalK--located in the three layers of the bacterial envelope. Proteins MalF and MalG are hydrophobic inner membrane components mediating the energy-dependent translocation of substrates into the cytoplasm. In this paper, we analyse the topology of the MalG protein by using methods based on the properties of fusions between malG and 'phoA, a truncated gene encoding alkaline phosphatase lacking its translation initiation and exportation signals. Fusions were obtained by using either phage lambda TnphoA or by constructing in vitro fusions located randomly within the malG gene. The deduced topological model suggests that MalG spans the membrane six times and has its amino- and carboxy-termini in the cytoplasm. These results will be helpful for the interpretation of the phenotypes of mutants in malG.