Alterations at the carboxyl terminus change assembly and secretion properties of the B subunit of Escherichia coli heat-labile enterotoxin

C-terminal processing of GlyGly-CTERM containing proteins by rhombosortase in Vibrio cholerae

Vibrio cholerae and a subset of other Gram-negative bacteria, including Acinetobacter baumannii, express proteins with a C-terminal tripartite domain called GlyGly-CTERM, which consists of a motif rich in glycines and serines, followed by a hydrophobic region and positively charged residues. Here we show that VesB, a V. cholerae serine protease, requires the GlyGly-CTERM domain, the intramembrane rhomboid-like protease rhombosortase, and the type II secretion system (T2SS) for localization at the cell surface. VesB is cleaved by rhombosortase to expose the second glycine residue of the GlyGly-CTERM motif, which is then conjugated to a glycerophosphoethanolamine-containing moiety prior to engagement with the T2SS and outer membrane translocation. In support of this, VesB accumulates intracellularly in the absence of the T2SS, and surface-associated VesB activity is no longer detected when the rhombosortase gene is inactivated. In turn, when VesB is expressed without an intact GlyGly-CTERM domain, VesB is released to the extracellular milieu by the T2SS and does not accumulate on the cell surface. Collectively, our findings suggest that the posttranslational modification of the GlyGly-CTERM domain is essential for cell surface localization of VesB and other proteins expressed with this tripartite extension.

Protection Against Necrotic Enteritis in Broiler Chickens by Regulated Delayed Lysis Salmonella Vaccines

Necrotic enteritis (NE), caused by Gram-positive Clostridium perfringens type A strains, has gained more attention in the broiler industry due to governmental restrictions affecting the use of growth-promoting antibiotics in feed. To date, there is only one commercial NE vaccine available, based on the C. perfringens alpha toxin. However, recent work has suggested that the NetB toxin, not alpha toxin, is the most critical virulence factor for causing NE. These findings notwithstanding, it is clear from prior research that immune responses against both toxins can provide some protection against NE. In this study, we delivered a carboxyl-terminal fragment of alpha toxin and a GST-NetB fusion protein using a novel attenuated Salmonella vaccine strain designed to lyse after 6-10 rounds of replication in the chicken host. We immunized birds with vaccine strains producing each protein individually, a mixture of the two strains, or with a single vaccine strain that produced both proteins. Immunization with strains producing either of the single proteins was not protective, but immunization with a mixture of the two or with a single strain producing both proteins resulted in protective immunity. The vaccine strain synthesizing both PlcC and GST-NetB was able to elicit strong production of intestinal IgA, IgY, and IgM antibodies and significantly protect broilers against C. perfringens challenge against both mild and severe challenges. Although not part of our experimental plan, the broiler chicks we obtained for these studies were apparently contaminated during transit from the hatchery with group D Salmonella. Despite this drawback, the vaccines worked well, indicating applicability to real-world conditions.

Involvement of the GspAB complex in assembly of the type II secretion system secretin of Aeromonas and Vibrio species

The type II secretion system (T2SS) functions as a transport mechanism to translocate proteins from the periplasm to the extracellular environment. The ExeA homologue in Aeromonas hydrophila, GspA(Ah), is an ATPase that interacts with peptidoglycan and forms an inner membrane complex with the ExeB homologue (GspB(Ah)). The complex may be required to generate space in the peptidoglycan mesh that is necessary for the transport and assembly of the megadalton-sized ExeD homologue (GspD(Ah)) secretin multimer in the outer membrane. In this study, the requirement for GspAB in the assembly of the T2SS secretin in Aeromonas and Vibrio species was investigated. We have demonstrated a requirement for GspAB in T2SS assembly in Aeromonas salmonicida, similar to that previously observed in A. hydrophila. In the Vibrionaceae species Vibrio cholerae, Vibrio vulnificus, and Vibrio parahaemolyticus, gspA mutations significantly decreased assembly of the secretin multimer but had minimal effects on the secretion of T2SS substrates. The lack of effect on secretion of the mutant of gspA of V. cholerae (gspA(Vc)) was explained by the finding that native secretin expression greatly exceeds the level needed for efficient secretion in V. cholerae. In cross-complementation experiments, secretin assembly and secretion in an A. hydrophila gspA mutant were partially restored by the expression of GspAB from V. cholerae in trans, further suggesting that GspAB(Vc) performs the same role in Vibrio species as GspAB(Ah) does in the aeromonads. These results indicate that the GspAB complex is functional in the assembly of the secretin in Vibrio species but that a redundancy of GspAB function may exist in this genus.

Heat-labile enterotoxin: beyond G(m1) binding

Enterotoxigenic Escherichia coli (ETEC) is a significant source of morbidity and mortality worldwide. One major virulence factor released by ETEC is the heat-labile enterotoxin LT, which is structurally and functionally similar to cholera toxin. LT consists of five B subunits carrying a single catalytically active A subunit. LTB binds the monosialoganglioside G_M1, the toxin’s host receptor, but interactions with A-type blood sugars and E. coli lipopolysaccharide have also been identified within the past decade. Here, we review the regulation, assembly, and binding properties of the LT B-subunit pentamer and discuss the possible roles of its numerous molecular interactions.

No direct binding of the heat-labile enterotoxin of Escherichia coli to E. coli lipopolysaccharides

A novel carbohydrate binding site recognizing blood group A and B determinants in a hybrid of cholera toxin and Escherichia coli heat-labile enterotoxin B-subunits (termed LCTBK) has previously been described, and also the native heat-labile enterotoxin bind to some extent to blood group A/B terminated glycoconjugates. The blood group antigen binding site is located at the interface of the B-subunits. Interestingly, the same area of the B-subunits has been proposed to be involved in binding of the heat-labile enterotoxin to lipopolysaccharides on the bacterial cell surface. Binding of the toxin to lipopolysaccharides does not affect the GM1 binding capacity. The present study aimed at characterizing the relationship between the blood group A/B antigen binding site and the lipopolysaccharide binding site. However, no binding of the B-subunits to E. coli lipopolysaccharides in microtiter wells or on thin-layer chromatograms was obtained. Incubation with lipopolysaccharides did not affect the binding of the B-subunits of heat-labile enterotoxin of human isolates to blood group A-carrying glycosphingolipids, indicating that the blood group antigen site is not involved in LPS binding. However, the saccharide competition experiments showed that GM1 binding reduced the affinity for blood group A determinants and vice versa, suggesting that a concurrent occupancy of the two binding sites does not occur. The latter finding is related to a connection between the blood group antigen binding site and the GM1 binding site through residues interacting with both ligands.

Mapping critical interactive sites within the periplasmic domain of the Vibrio cholerae type II secretion protein EpsM

The type II secretion (T2S) system is present in many gram-negative species, both pathogenic and nonpathogenic, where it supports the delivery of a variety of toxins, proteases, and lipases into the extracellular environment. In Vibrio cholerae, the T2S apparatus is composed of 12 Eps proteins that assemble into a multiprotein complex that spans the entire cell envelope. Two of these proteins, EpsM and EpsL, are key components of the secretion machinery present in the inner membrane. In addition to likely forming homodimers, EpsL and EpsM have been shown to form a stable complex in the inner membrane and to protect each other from proteolytic degradation. To identify and map the specific regions of EpsM involved in protein-protein interactions with both another molecule of EpsM and EpsL, we tested the interactions of deletion constructs of EpsM with full-length EpsM and EpsL by functional characterization and copurification as well as coimmunoprecipitation. Analysis of the truncated EpsM mutants revealed that the region of EpsM from amino acids 100 to 135 is necessary for EpsM to form homo-oligomers, while residues 84 to 99 appear to be critical for a stable interaction with EpsL.

Vaccine and adjuvant activity of recombinant subunit B of E. coli enterotoxin produced in yeast

Escherichia coli heat-labile enterotoxin (LT) and cholera toxin (CT) have been studied intensively as vaccines against diseases caused by those bacteria and as adjuvants for mucosal vaccination. Two major problems interfere with the use of these promising adjuvants: their toxicity and the residual bacterial endotoxins mixed with the desired LT. In this study, subunit B of LT was expressed in Pichia pastoris yeast cells (yrLTB) and the recombinant protein was purified and concentrated by ion-exchange chromatography. The final yield of the recombinant protein was 5-8 mg/l induction medium. The molecule is in pentameric form and binds to GM1 gangliosides. When given orally to chickens, anti-LTB antibodies were produced, exhibiting its ability to cross the digestive system and induce an immune response. The adjuvant activity of yrLTB was proven by fusing it to viral protein 2 (VP2) of infectious bursal disease virus. Birds intramuscularly vaccinated with this molecule exhibit 70-100% protection, in a dose-response-dependent manner. This method eliminated the bacterial endotoxins and enabled the production of large quantities of LTB. Expression in a eukaryotic system allows the production of fusion proteins that require post-translational modifications. This may allow oral vaccination with a protein fused to yrLTB. The approach described in this study will enable the efficient production of a non-toxic, eukaryotically expressed enterotoxin as a vaccine against the toxin itself or as a carrier or adjuvant for foreign vaccine molecules.

Production of a fusion protein consisting of the enterotoxigenic Escherichia coli heat-labile toxin B subunit and a tuberculosis antigen in Arabidopsis thaliana

Transgenic plants are potentially safe and inexpensive vehicles to produce and mucosally deliver protective antigens. However, the application of this technology is limited by the poor response of the immune system to non-particulate, subunit vaccines. Co-delivery of therapeutic proteins with carrier proteins could increase the effectiveness of the antigen. This paper reports the ability of transgenic Arabidopsis thaliana plants to produce a fusion protein consisting of the B subunit of the Escherichia coli heat-labile enterotoxin and a 6 kDa tuberculosis antigen, the early secretory antigenic target ESAT-6. Both components of the fusion protein were detected using GM1-ganglioside-dependent enzyme-linked immunosorbant assay. This suggested the fusion protein retained both its native antigenicity and the ability to form pentamers.

The Disassembly and Reassembly of Mutants of Escherichia Coli Heat-Labile Enterotoxin: Replacement of Proline 93 Does Not Abolish the Reassembly-Competent and Reassembly-Incompetent States

The carrier moiety of heat-labile enterotoxin of Escherichia coli (EtxB) is formed by the noncovalent association of identical monomeric subunits, which assemble, in vivo and in vitro, into exceptionally stable pentameric complexes. In vitro, acid disassembly followed by neutralization results in reassembly yields of between 20% and 60% depending on the identity of the salts present during the acid denaturation process. Loss of reassembly competence has been attributed to isomerization of the native cis-proline residue at position 93. To characterize this phenomenon further, two mutants of EtxB at proline 93 (P93G and P93A) were generated and purified. The proline variants reveal only minor differences in their biophysical and biochemical properties relative to wild-type protein, but major changes were observed in the kinetics of pentamer disassembly and reassembly. Additionally, a loss of assembly competence was observed following longer term acid treatment, which was even more marked than that of the wild-type protein. We present evidence that the loss of assembly competence of these mutants is best explained by a cis/trans peptidyl isomerization of the unfolded mutant subunits in acid conditions; this limited reassembly competence and the biophysical properties of the native P93 mutant pentamers imply the retention of the native cis conformation in the nonproline peptide bond between residues 92 and 93 in the mutated proteins.

Key role for DsbA in cell-to-cell spread of Shigella flexneri, permitting secretion of Ipa proteins into interepithelial protrusions

DsbA, a disulfide bond catalyst, is necessary for realization of the pathogenic potential of Shigella flexneri. Sh42, a mutant strain differing from wild-type M90TS solely because it expresses nonfunctional DsbA33G (substitution for 33C at the active site), secreted less IpaB and IpaC than M90TS in response to various stimuli in vitro. A kinetic study demonstrated that Sh42 responded more slowly to Congo red than M90TS. By modulating relative concentrations of functional and nonfunctional DsbA within bacteria, functional enzyme has been shown to be necessary for intercellular spread. By confocal microscopy, M90TS dividing in protrusions was shown to secrete Ipa proteins from the septation furrow, anticipating lysis of protrusions, while Sh42 showed minimal Ipa secretion in this location. In the light of a previous demonstration that DsbA is not necessary for entry of epithelial cells, we conclude that a role in virulence of this disulfide bond catalyst lies in facilitating secretion of Ipa proteins specifically within epithelial protrusions, in turn allowing cell-to-cell spread of S. flexneri.

Mucosal and systemic antibody responses after peroral or intranasal immunization: effects of conjugation to enterotoxin B subunits and/or of co-administration with free toxin as adjuvant

The mucosa-binding molecules cholera toxin (CT) from Vibrio cholerae and heat-labile enterotoxin (LT) from Escherichia coli have previously been used as mucosal adjuvants and carriers for many types of antigen. However, since these molecules are toxic and cannot be used in human vaccines, it is important to study whether their non-toxic mucosa-binding B subunits, CTB and LTB, can be used as alternative safe mucosal adjuvants and/or carrier molecules. We have as a model protein antigen used human gammaglobulin (HGG) for admixture with or chemical conjugation to recombinantly produced CTB and LTB, respectively, and measured antigen-specific local secretory IgA antibodies in saponin extracts from intestine and lung tissue by ELISA following intra-nasal (i.n.) or per-oral (p.o.) immunization. The results show that local antibody formation against HGG was increased after immunization with conjugated as compared to free HGG. However, while the conjugates alone gave rise to significant immune responses in the lung and also, to a lesser degree, in the intestine after i.n. immunization, co-administration of a small amount of free CT/LT as adjuvant was needed to induce a significant immune response in the intestine after p.o. immunization. We also found that following i.n. immunization, the addition of CTB to HGG, without coupling, increased the mucosal immune response to some extent, indicating that CTB by itself can work as an adjuvant by the i.n. route of immunization. A striking finding was that, as a carrier, CTB was superior to LTB when the conjugates were used by the oral but not by the i.n. route of immunization. In conclusion, conjugation of an antigen to mucosa-binding molecules such as CTB and/or LTB can dramatically increase their mucosal immunogenicity. This approach may thus be useful in the preparation of mucosal vaccines.

N-terminus of mature heat-labile enterotoxin chain B is critical for its extracellular secretion in Vibrio cholerae

The effects of addition of a few amino acids to the amino- and carboxy-terminal regions of the mature portion of the heat-labile enterotoxin chain B (LTB) of Escherichia coli on protein export, secretion and assembly were investigated. In E. coli, LTB (secretory protein) with or without the extension at the N- or C-terminus accumulated in the periplasmic fraction. For Vibrio cholerae, LTB with the extension at the C-terminus was exported to the periplasm followed by secretion to the extracellular milieu. However, LTB with the N-terminus extension was exported to the periplasm only. Our findings suggest that in the case of V. cholerae, the N-terminus of the mature LTB plays an important role in its secretion to the extracellular milieu.

Immunogenicity of Actinobacillus ApxIA toxin epitopes fused to the E. coli heat-labile enterotoxin B subunit

Peptides KDYGASTGSSL (Epil). SLLRRRRNGEDVSV (Epi3) and DDEIYGNDGHP (Epi6), predicted to constitute immunogenic epitopes of the hemolysin-cytotoxin ApxIA of Actinobacillus pleuropneumoniae were inserted into a surface-exposed loop of the B subunit of the E. coli heat-labile enterotoxin (EtxB). The resulting chimeric proteins were recognized by monospecific antibodies against purified native ApxI and by convalescent sera of pigs that were positive for A. pleuropneumoniae serotype 1. Mice anti-sera against chimeric proteins EtxB::ApxIAEpi3 and EtxB::ApxIAEpi6 reacted with purified ApxI. These results indicate that Epi3 and Epi6 regions constitute linear epitopes of the structural ApxIA protein toxin. Epitope Epi6 which is located in the structure of the glycine rich repeats in ApxI elicits the formation of hemolysin neutralizing antibodies when introduced into mice in the form of a chimeric EtxB fusion protein. We suggest that fusion of peptide sequences to EtxB is a useful tool for the analysis of epitopes of complex proteins such as RTX toxins.

Fusions to the cholera toxin B subunit: influence on pentamerization and GM1 binding

The cholera toxin B (CTB) subunit has been used extensively in vaccine research as a carrier for peptide immunogens due to its immunopotentiating properties, where coupling has been obtained either by genetic fusion or chemical conjugation. For genetically fused immunogens both N- and C-terminal fusions have been used. Only shorter extensions have previously been evaluated and in some reports these fusions have impaired the biological functions of CTB, such as the ability to form pentamers and to adhere to its cell receptor, the GM1 ganglioside. Here we report the first systematic study where the same fusion partner has been used for either C-terminal, N-terminal or dual fusions to CTB. The serum albumin binding region (BB, approximately 25 kDa) from streptococcal protein G, which is known to fold independently of N- or C-terminal fusions, was selected as fusion partner. The three fusion proteins CTB-BB, BB-CTB and BB-CTB-BB were expressed in Escherichia coli, where they were efficiently secreted to the periplasmic space, and could be purified by affinity chromatography on human serum albumin (HSA) columns. The CTB fusion proteins were compared for their ability to form pentamers, by gel electrophoresis and size-exclusion chromatography, and it was concluded that all three fusion proteins were able to pentamerize. Interestingly, the C-terminal fusion to CTB showed most efficient pentamerization, while the dual fusion was much less efficient. Purified pentamer fractions from all three fusions where found to react to a monoclonal antibody described to react only to pentameric forms of CTB. In addition, the purified pentamer fractions were analyzed in an enzyme-linked immunosorbent assay (ELISA) for their ability to bind GM1, and it was found that the C-terminal fusion (CTB-BB) showed significant GM1-binding, but that also the N-terminal and dual CTB fusion proteins bound GM1, although less efficiently. The implications of the results for the design and use of CTB fusion proteins as subunit vaccines are discussed.

Role of receptor binding in toxicity, immunogenicity, and adjuvanticity of Escherichia coli heat-labile enterotoxin

The role of receptor binding in the toxicity, immunogenicity, and adjuvanticity of the heat-labile enterotoxin of Escherichia coli (LT) was examined by comparing native LT and LT(G33D), a B-subunit receptor binding mutant, with respect to the ability to bind to galactose and to GM1, toxicity on mouse Y-1 adrenal tumor cells, the ability to stimulate adenylate cyclase in Caco-2 cells, enterotoxicity in the patent mouse model, and oral immunogenicity and adjuvanticity. In contrast to native LT, LT(G33D) was unable to bind to the galactosyl moiety of Sepharose 4B or GM1 but did retain the lectin-like ability to bind to immobilized galactose on 6% agarose beads. LT(G33D) had no enterotoxicity in the patent mouse model but exhibited residual toxicity on mouse Y-1 adrenal tumor cells and had an ability equivalent to that of native LT to stimulate adenylate cyclase in Caco-2 cells (5,000 versus 6,900 pmol per mg of protein). In addition, LT(G33D) was unable to serve as an effective oral adjuvant for induction of immunoglobulin G or A directed against a coadministered antigen. Furthermore, LT(G33D) elicited negligible serum and mucosal antibody responses against itself. These data indicate that the toxicity, immunogenicity, and oral adjuvanticity of LT are dependent upon binding of the B subunit to ganglioside GM1.

General secretion pathway (eps) genes required for toxin secretion and outer membrane biogenesis in Vibrio cholerae

The general secretion pathway (GSP) of Vibrio cholerae is required for secretion of proteins including chitinase, enterotoxin, and protease through the outer membrane. In this study, we report the cloning and sequencing of a DNA fragment from V. cholerae, containing 12 open reading frames, epsC to -N, which are similar to GSP genes of Aeromonas, Erwinia, Klebsiella, Pseudomonas, and Xanthomonas spp. In addition to the two previously described genes, epsE and epsM (M. Sandkvist, V. Morales, and M. Bagdasarian, Gene 123: 81-86, 1993; L. J. Overbye, M. Sandkvist, and M. Bagdasarian, Gene 132:101-106, 1993), it is shown here that epsC, epsF, epsG, and epsL also encode proteins essential for GSP function. Mutations in the eps genes result in aberrant outer membrane protein profiles, which indicates that the GSP, or at least some of its components, is required not only for secretion of soluble proteins but also for proper outer membrane assembly. Several of the Eps proteins have been identified by use of the T7 polymerase-promoter system in Escherichia coli. One of them, a pilin-like protein, EpsG, was analyzed also in V. cholerae and found to migrate as two bands on polyacrylamide gels, suggesting that in this organism it might be processed or otherwise modified by a prepilin peptidase. We believe that TcpJ prepilin peptidase, which processes the subunit of the toxin-coregulated pilus, TcpA, is not involved in this event. This is supported by the observations that apparent processing of EpsG occurs in a tcpJ mutant of V. cholerae and that, when coexpressed in E. coli, TcpJ cannot process EpsG although the PilD peptidase from Neisseria gonorrhoeae can.

A pH-dependent conformational change in the B-subunit pentamer of Escherichia coli heat-labile enterotoxin: structural basis and possible functional role for a conserved feature of the AB5 toxin family

The non-covalently associated B-subunit moieties of AB5 toxins, such as cholera toxin and related diarrheagenic enterotoxins, exhibit exceptional pH stability and remain pentameric at pH values as low as 2.0. Here, we investigate the structural basis of a pH-dependent conformational change which occurs within the B5 structure of Escherichia coli heat-labile enterotoxin (EtxB) at around pH 5.0. The use of far-UV CD and fluorescence spectroscopy showed that EtxB pentamers undergo a fully reversible pH-dependent conformational change with a pKa of 4.9 +/- 0.1 (R2 = 0.999) or 5.13 +/- 0.01 (R2 = 0.999), respectively. This renders the pentamer susceptible to SDS-mediated disassembly and decreases its thermal stability by 18 degrees C. A comparison of the pH-dependence of the structural change in EtxB5, with that of a mutant containing a Ser substitution at His 57, revealed that the pKa of the conformational change was shifted from ca. 5.1 to 4.4. This finding suggests that protonation of the imidazole side chain of His 57 might facilitate disruption of a spatially adjacent salt bridge, located between Glu 51 and Lys 91 in each B-subunit, thus triggering the conformational change in the pentameric structure. The pH-dependent conformational change was found to be inhibited when B-subunits bound to monosialoganglioside, GMI; and to have no effect on the stability of interaction between A- and B-subunits within the AB5 complex. This suggests that the conformational change is unlikely to have a direct involvement in toxicity. Conservation of the pH-dependent conformational change in the AB5 toxin family, combined with the potential exposure of the hydrophobic core of beta-barrel in the monomeric units, leads to the proposal that the conformational change may be the common feature that ensures the secretion of these proteins from the Vibrionaceae.

Expression of ribonucleolytic toxin restrictocin in Escherichia coli: purification and characterization

Restrictocin is a toxin produced by the fungus Aspergillus restritus. The DNA coding for restrictocin was isolated from the host by polymerase chain reaction and cloned into a T7 promoter-based expression vector. The protein was overproduced in Escherichia coli and remained insoluble in the cell in the form of inclusion bodies. Recombinant restrictocin was purified in large amounts, by a simple denaturation-renaturation protocol involving a redox system, with typical yields of 45 mg/l of original culture. Restrictocin could be secreted into the bacterial medium using ompA, pelB and LTB signal sequences. Among the three signal sequences, ompA was found to be the most efficient in secreting the recombinant protein. The protein secreted into the extracellular medium was properly processed as evident by the amino-terminal sequencing. Recombinant restrictocin was readily purified to homogeneity from either the medium or inclusion bodies by simple chromatographic techniques and was found to be functionally as active as the native fungal protein in inhibiting the eukaryotic translation.

Assembly of the B subunit pentamer of Escherichia coli heat-labile enterotoxin. Kinetics and molecular basis of rate-limiting steps in vitro

The B subunits of Escherichia coli heat-labile enterotoxin (EtxB) and cholera toxin (CtxB) assemble in vivo into exceptionally stable homopentameric complexes, which maintain their quaternary structure in a range of conditions that would normally be expected to cause protein denaturation. Recently, we showed that the simultaneous protonation of two of the COOH-terminal carboxylates in pentameric EtxB was required to cause its disassembly at pH values below 2.0 (Ruddock, L., Ruston, S. P., Kelly, S. M., Price, N. C., Freedman, R. B., and Hirst, T. R.(1995) J. Biol. Chem. 270, 29953-29958). Here, we investigate the influence of environmental parameters on the kinetics of reassembly of acid-generated EtxB monomers in vitro. Such monomers were found to undergo a further acid-mediated conformational change, with an activation energy of 76 +/- 2 J.mol-1.K-1, consistent with isomerization of the cis-proline residue at position 93, and which prevented subsequent EtxB reassembly. By using rapid neutralization of acid-generated monomers, a high proportion of the B-subunits adopted an assembly-competent conformation, which resulted in up to 75% of the protein reassembling into a stable pentameric complex, indistinguishable from native EtxB pentamers. The rate-limiting step in reassembly, over a concentration range of 50-200 microg/ml, was shown to be due to an intramolecular event, which exhibited a pH dependence with a pKa of 7.0. Modification of EtxB with amine-specific probes revealed that the protonation state of the NH2-terminal alanine residue was responsible for the pH dependence of reassembly. The implications of these findings for the biogenesis of Escherichia coli enterotoxin and related enterotoxins in vivo, are considered.

Construction, purification and immunogenicity of antigen-antibody-LTB complexes

An oligonucleotide, encoding a short epitope peptide tag, termed Pk, was inserted at the 3'-end of the gene coding B-subunit of Escherichia coli heat-labile enterotoxin (LTB). The presence of the Pk epitope on LTB-Pk was used to construct novel macromolecular assemblies comprising LTB-Pk, an anti-Pk mAb, (mAb SV5-P-k) and Pk-linked recombinant SIV proteins. The 1:1:1 stoichiometry of such complexes was ensured by binding LTB-Pk to one arm of mAb SV5-P-k and an SIV-Pk antigen to the other arm of the antibody. Such SIV-mAb-LTB macromolecular complexes bound to GM1-ganglioside in vitro, and when immunized systemically into mice were highly immunogenic, inducing both humoral and cell-mediated responses to the recombinant SIV antigens.

Synthesis of hybrid molecules between heat-labile enterotoxin and cholera toxin B subunits: potential for use in a broad-spectrum vaccine

Three variants of the cholera toxin B subunit (CTB) were generated by site-specific mutagenesis in which regions of the mature protein were altered to the composition found at the corresponding positions of the closely related B subunit of the heat-labile enterotoxin of enterotoxigenic Escherichia coli (LTB). The mutant proteins were expressed in Vibrio cholerae and purified from the growth medium. In the first of the mutant proteins, the first 25 amino acids corresponded to the sequence found in LTB, and in the second, changes were made at positions 94 and 95 of the mature protein. The third mutant protein combined the changes made in the first two. Analysis of the immunological properties of these novel proteins by using monoclonal antibodies and absorbed polyclonal antiserum demonstrated that they had acquired LTB-specific epitopes. Immunizations with the mutant proteins resulted in antisera containing LTB-specific as well as CTB-specific and cross-reactive antibodies. The sera were also found to be more strongly cross-reactive in the in vitro neutralization of both cholera toxin and heat-labile enterotoxin than were antisera raised against either CTB or LTB. The results suggest that such hybrid CTB-LTB proteins may be useful in a broad-spectrum vaccine against enterotoxin-induced diarrhea.

Potent immunogenicity of the B subunits of Escherichia coli heat-labile enterotoxin: receptor binding is essential and induces differential modulation of lymphocyte subsets

The importance of receptor binding in the potent immunogenicity of Escherichia coli heat-labile enterotoxin B subunit (EtxB) was tested by comparing its immunogical properties with those of a receptor binding mutant, EtxB(G33D). Subcutaneous immunization of EtxB(G33D) resulted in 160-fold reduction in antibody titer compared with wild-type EtxB, whereas its oral delivery failed to provoke any detectable secretory or serum anti-B subunit responses. Moreover, the two proteins induced strikingly different effects on lymphocyte cultures in vitro. EtxB, in comparison with EtxB(G33D), caused an increase in the proportion of B cells, many of which were activated (CD25+); the complete depletion of CD8+ T cells; an increase in the activation of CD4+ T cells; and an increase in interleukin 2 and a decrease in interferon gamma. These data indicate that EtxB exerts profound effects on immune cells, suggesting that its potent immunogenicity is dependent not only on efficient receptor-mediated uptake, but also on direct receptor-mediated immunomodulation of lymphocyte subsets.

Kinetics of acid-mediated disassembly of the B subunit pentamer of Escherichia coli heat-labile enterotoxin. Molecular basis of pH stability

The B-subunit pentamer of Escherichia coli heat-labile enterotoxin (EtxB) is highly stable, maintaining its quaternary structure in a range of conditions that would normally be expected to cause protein denaturation. In this paper the structural stability of EtxB has been studied as a function of pH by electrophoretic, immunochemical, and spectroscopic techniques. Disassembly of the cyclic pentameric structure of human EtxB occurs only below pH 2. As determined by changes in intrinsic fluorescence this process follows first-order kinetics, with the rate constant for disassembly being proportional to the square of the H+ ion concentration, and with an activation energy of 155 kJ mol-1. A C-terminal deletion mutant, hEtxB214, similarly shows first-order kinetics for disassembly but with a higher pH threshold, resulting in disassembly being seen at pH 3.4 and below. These findings are consistent with the rate-limiting step for disassembly of human EtxB being the simultaneous disruption of two interfaces by protonation of two C-terminal carboxylates. By comparison, disassembly of the B-subunit of cholera toxin (CtxB), a protein which shows 80% sequence identity with EtxB, exhibits a much lower stability to acid conditions; with disassembly of CtxB occurring below pH 3.9, with an activation energy of 81 kJ mol-1. Reasons for the observed differences in acid stability are discussed, and the implications of these findings to the development of oral vaccines using EtxB and CtxB are considered.

Studies on plasmid stability and LTB production by recombinant Vibrio cholerae in batch and chemostat cultures: a lesson for optimizing conditions for chemical induction

Plasmid content, its stability and the expression of B-subunit of Escherichia coli heat-labile enterotoxin (LTB) in Vibrio cholerae/r-pMMB68 system have been studied in batch as well as in chemostat cultures. Upon induction with isopropyl-beta-D-thiogalactopyranoside (IPTG), cultures secreted LTB into the extracellular milieu. Highest specific LTB production rate of 7.3 mg mg-1h-1 was achieved in batch culture induced at the late exponential growth phase. The plasmid pMMB68 was fairly stable up to 20 generations, even in the absence of selection pressure. Instability of the plasmid was accelerated in the presence of IPTG and at higher dilution rates. Maximum productivity of 2.1 mg l-1 h-1 was achieved in continuous culture, which remained constant at a range of dilution rates from 0.20 to 0.35 h-1.

Generation of a monoclonal antibody that recognizes the amino-terminal decapeptide of the B-subunit of Escherichia coli heat-labile enterotoxin. A new probe for studying toxin assembly intermediates

Cholera toxin and the related Escherichia coli heat-labile enterotoxin are hexameric proteins comprising one A-subunit and five B-subunits. In this paper we report the generation and characterization of a monoclonal antibody, designated LDS47, that recognizes and precipitates in vivo assembly intermediates of the B-subunit (EtxB) of E. coli heat-labile enterotoxin. The monoclonal antibody is unable to precipitate native B-subunit pentamers, thus making LDS47 a useful probe for studying the early stages of enterotoxin biogenesis. The use of LDS47 to monitor the in vivo turnover of newly synthesized B-subunits in the periplasm of E. coli demonstrated that (i) the turnover of unassembled B-subunits followed an apparent first order process and (ii) it occurred concomitantly with the assembly of native B-pentamers (k = 0.317 +/- 0.170 min-1; t1/2 = 2.2 min). No other proteins were co-precipitated with the newly synthesized B-subunits; a finding that implies that unassembled B-subunits do not stably associate with other periplasmic proteins prior to their assembly into a macromolecular complex. The use of overlapping synthetic peptides corresponding to the entire EtxB polypeptide demonstrated that the epitope recognized by LDS47 is located within the amino-terminal decapeptide of the B-subunit. From the x-ray structural analysis of the toxin (Sixma, T., Kalk, K., van Zanten, B., Dauter, Z., Kingma, J., Witholt, B., and Hol, W. G. J. (1993) J. Mol. Biol. 230, 890-918), this region appears to resemble a curved finger that clasps the adjacent B-subunit. Thus, this region might be expected to be exposed in the unfolded or unassembled subunit, but to become partially buried upon assembly and thus inaccessible to recognition by the monoclonal antibody.

Specificity of the protein secretory apparatus: secretion of the heat-labile enterotoxin B subunit pentamers by different species of gram- bacteria

The B-subunit pentamer(s) (EtxBp) of Escherichia coli heat-labile enterotoxin (LT) are secreted from Vibrio cholerae via the general secretion pathway (GSP), but remain periplasmic in E. coli. In order to determine if other Gram- bacteria were also able to secrete the ExtBp, the etxB gene, which encodes EtxB was introduced into different bacteria. Of the bacteria examined, most species of Vibrio and Aeromonas were able to secrete this protein through the outer membrane; other Gram- genera, including Erwinia, Klebsiella and Xanthomonas were not, even though they encode GSP genes homologous to those of V. cholerae. Thus, the ability to recognize the EtxBp as a secretable protein is confined to bacteria that were identified as being closely related to V. cholerae by examination of their 5S rRNA [MacDonell and Colwell, Syst. Appl. Microbiol. 6 (1985) 171-182].

Insertion of a HIV-1-neutralizing epitope in a surface-exposed internal region of the cholera toxin B-subunit

The non-toxic B-subunit of cholera toxin (CTB) is a powerful immunogen and has been investigated as a carrier for foreign peptide epitopes, with peptides genetically fused to either the N- or C terminus of CTB. In the present study, we have constructed a plasmid encoding a novel intrachain CTB fusion protein with a peptide epitope inserted into an internal region of CTB: eight amino acids (aa) in CTB (56-63) were substituted with a 10-aa peptide from the third variable (V3) loop of the HIV-1 envelope protein gp120. The resulting chimeric protein retained important functional characteristics of the native CTB including pentamerization and GM1 ganglioside receptor binding. The internal hybrid protein was also shown to be resistant to proteolytic degradation during production in Vibrio cholerae, whereas a terminal hybrid protein, where the same gp120-epitope was fused to the N terminus of CTB, was rapidly cleaved during culture. The inserted epitope, which is known to give rise to HIV-1 neutralizing Ab, could be detected with a V3 loop-specific monoclonal Ab when the chimeric protein was analyzed in ELISA and immunoblot, indicating that the epitope inserted at this site is presented on the surface of the protein. Consistent with these observations, immunization of mice with the CTB::HIV hybrid protein elicited a high titered serum Ab response to the CTB moiety and also, in some but not all animals, a detectable response to the inserted gp120 epitope.

Construction, expression, and immunogenicity of the Schistosoma mansoni P28 glutathione S-transferase as a genetic fusion to tetanus toxin fragment C in a live Aro attenuated vaccine strain of Salmonella

A vector has been constructed to allow genetic fusions of guest antigens via a hinge domain to the C terminus of the highly immunogenic C fragment of tetanus toxin. A fusion has been constructed with the gene encoding the protective 28-kDa glutathione S-transferase (EC 2.5.1.18) from Schistosoma mansoni. The recombinant vector has been electroporated into the nonvirulent Salmonella typhimurium aroA live vaccine strain SL3261. The corresponding chimeric protein is stably expressed in a soluble form in Salmonella as evaluated by Western blotting with fragment C and glutathione S-transferase antisera. Mice immunized intravenously with a single dose of the live recombinant bacteria elicit antibodies to both fragment C and glutathione S-transferase as detected by enzyme-linked immunosorbent assays. Furthermore, all of the mice were solidly protected when challenged with lethal doses of either tetanus toxin or the virulent Salmonella typhimurium strain C5. Mice have also elicited antibodies to fragment C and glutathione S-transferase after oral immunization. It may be that a live trivalent vaccine against typhoid, tetanus, and schistosomiasis is feasible.

Specific inhibition of herpes virus replication by receptor-mediated entry of an antiviral peptide linked to Escherichia coli enterotoxin B subunit

Mimetic peptides capable of selectively disrupting protein-protein interactions represent potential therapeutic agents for inhibition of viral and cellular enzymes. This approach was first suggested by the observation that the peptide YAGAVVNDL, corresponding to the carboxyl-terminal 9 amino acids of the small subunit of ribonucleotide reductase of herpes simplex virus, specifically inhibited the viral enzyme in vitro. Evaluation and use of this peptide as a potential antiviral agent has, however, been thwarted by its failure to inhibit virus replication in vivo, presumably because the peptide is too large to enter eukaryotic cells unaided. Here, we show that the nontoxic B subunit of Escherichia coli heat-labile enterotoxin can be used as a recombinant carrier for the receptor-mediated delivery of YAGAVVNDL into virally infected cells. The resultant fusion protein specifically inhibited herpes simplex virus type 1 replication and ribonucleotide reductase activity in quiescent Vero cells. Preincubation of the fusion protein with soluble GM1 ganglioside abolished this antiviral effect, indicating that receptor-mediated binding to the target cell is necessary for its activity. This provides direct evidence of the usefulness of carrier-mediated delivery to evaluate the intracellular efficacy of a putative antiviral peptide.

Efficient extracellular production of hybrid E. coli heat-labile enterotoxin B subunits in a marine Vibrio

Escherichia coli heat-labile enterotoxin B subunit (EtxB) has been proposed as a potential protein carrier for the delivery of heterologous peptides to target cells, particularly for the oral delivery of epitopes to the mucosal immune system. In this study, two extensions to the C-terminus of EtxB were genetically engineered that correspond to a well-characterized neutralising epitope of glycoprotein D from herpes simplex virus (EtxB-gD) and to the C-terminal nine amino acids from the 38 kDa subunit of HSV-encoded ribonucleotide reductase (EtxB-R2). Here we describe the extracellular secretion of the two hybrid EtxBs from a marine Vibrio harbouring a broad-host range inducible expression vector containing the hybrid genes. Large amounts of intact fusion proteins (15-20 mg per liter of culture) were secreted into the medium upon induction. These hybrid proteins maintained the receptor-binding activity of the native toxin as well as being cross-reactive with anti-EtxB and anti-heterologous peptide monoclonal antibodies.

Suppression of temperature-sensitive assembly mutants of heat-labile enterotoxin B subunits

Deletions or substitutions of amino acids at the carboxyl-terminus of the heat-labile enterotoxin B subunit (EtxB) affect its assembly into pentamers in a temperature-dependent manner. At 42 degrees C, the mutations prevent the B subunits from achieving their final pentameric structure resulting in membrane association of the monomers. However, mutant B subunits produced at 30 degrees C assemble, in the periplasm, into pentamers that remain stable when transferred to 42 degrees C, indicating that the mutant pentamers are stable under conditions where their formation is inhibited. The mutant pentamers are, similarly to wild-type pentamers, SDS-resistant and stable, in vitro, at temperatures up to 65 degrees C. This suggests that although the C-terminal amino acids are part of the subunit interface, they appear not to contribute significantly to the stability of the final pentameric complex, but are instead essential for the formation or stabilization of an assembly intermediate in the pentamerization process. Single second site mutations suppress the assembly defect of mutant EtxB191.5, which carries substitutions at its C-terminus. The Thr-->Ile replacement at position 75 in the alpha 2-helix probably restores the van der Waals contact between residues 75 and 101, which had been greatly reduced by the Met-->Leu substitution at position 101 in the beta 6-strand of EtxB191.5. Interaction between the alpha 2-helix and beta 6-strand which contains the C-terminus probably stabilizes a conformation essential for assembly and is therefore required for the formation of pentamers.

Escherichia coli heat-labile toxin subunit B fusions with Streptococcus sobrinus antigens expressed by Salmonella typhimurium oral vaccine strains: importance of the linker for antigenicity and biological activities of the hybrid proteins

A set of vectors possessing the genes for aspartate semialdehyde dehydrogenase (asd) and the B subunit of the heat-labile enterotoxin of Escherichia coli (LT-B) has been developed. These vectors allow operon or gene fusions of foreign gene epitopes at the C-terminal end of LT-B. Two groups of vectors have been constructed with and without leader sequences to facilitate placing of the foreign antigen in different cell compartments. Two Streptococcus sobrinus genes coding for principal colonization factors, surface protein antigen A (SpaA), and dextranase (Dex), have been fused into the 3' end of the LT-B gene. Resulting protein fusions of approximately 120 to 130 kDa are extremely well recognized by antibodies directed against both SpaA and Dex as well as against LT-B domains and retain the enzymatic activity of dextranase and the biological activity of LT-B in that they bind to GM1 gangliosides. Maximum antigenicity was obtained with the vector possessing an intervening linker of at least six amino acids with two proline residues. Some of the fusion proteins also exhibited another property of LT-B in that they were exported into the periplasm where they oligomerized. LT-B-SpaA and LT-B-Dex hybrid proteins are expressed stably and at a high level in avirulent Salmonella typhimurium vaccine strains which are being used to investigate their immunogenicity and types of induced immune responses. The fusion vectors will also be useful for production and purification of LT-B fusion antigens to be used and evaluated in other vaccine compositions.

Recombinant enterotoxins as vaccines against Escherichia coli-mediated diarrhoea

A fusion protein, comprising the B subunit of the heat-labile enterotoxin and a portion of the precursor to the heat-stable enterotoxin of Escherichia coli, has been created by recombinant genetic techniques. It is exported successfully to the bacterial periplasm and assembles into pentamers which retain the ability to bind to GM1 ganglioside. Native toxin epitopes are displayed and the molecule can be easily purified from periplasmic extracts of cells expressing the gene fusion. Although the protein carries the natural sequence of the heat-stable enterotoxin, it is greatly attenuated in toxicity. Systemic immunization of mice or oral administration of the fusion elicits antibody responses against both classes of E. coli enterotoxin.

Intermolecular interactions between the A and B subunits of heat-labile enterotoxin from Escherichia coli promote holotoxin assembly and stability in vivo

Cholera toxin and the related heat-labile enterotoxin (LT) produced by Escherichia coli consist of a holotoxin of one A subunit and five B subunits (AB5). Here we investigate the domains of the A subunit (EtxA) of E. coli LT which influence the events of B-subunit (EtxB) oligomerization and the formation of a stable AB5 holotoxin complex. We show that the C-terminal 14 amino acids of the A subunit comprise two functional domains that differentially affect oligomerization and holotoxin stability. Deletion of the last 14 amino acids (-14) from the A subunit resulted in a molecule that was significantly impaired in its capacity to promote the assembly of a mutant B subunit, EtxB191.5. In contrast, deletion of the last four amino acids (-4) from the A subunit gave a molecule that retained such a capacity. This suggests that C-terminal residues within the -14 to -4 region of the A subunit are important for promoting the oligomerization of EtxB. In addition, we demonstrate that the truncated A subunit lacking the last 4 amino acids was unable to form a stable AB5 holotoxin complex even though it promoted B-subunit oligomerization. This suggests that the last 4 residues of the A subunit function as an "anchoring" sequence responsible for maintaining the stability of A/B subunit interaction during holotoxin assembly. These data represent an important example of how intermolecular interactions between polypeptides in vivo can modulate the folding and assembly of a macromolecular complex.

A homologue of the Escherichia coli DsbA protein involved in disulphide bond formation is required for enterotoxin biogenesis in Vibrio cholerae

A strain of Vibrio cholerae, which had been engineered to express high levels of the non-toxic B subunit (EtxB) of Escherichia coli heat-labile enterotoxin, was subjected to transposon (TnphoA) mutagenesis. Two chromosomal TnphoA insertion mutations of the strain were isolated that showed a severe defect in the amount of EtxB produced. The loci disrupted by TnphoA in the two mutant derivatives were cloned and sequenced, and this revealed that the transposon had inserted at different sites in the same gene. The open reading frame of the gene predicts a 200-amino-acid exported protein, with a Cys-X-X-Cys motif characteristic of thioredoxin, protein disulphide isomerase, and DsbA (a periplasmic protein required for disulphide bond formation in E. coli). The V. cholerae protein exhibited 40% identity with the DsbA protein of E. coli, including 90% identity in the region of the active-site motif. Introduction of a plasmid encoding E. coli DsbA into the V. cholerae TnphoA derivatives was found to restore enterotoxin formation, whilst expression of Etx or EtxB in a dsbA mutant of E. coli confirmed that DsbA is required for enterotoxin formation in E. coli. These results suggest that, since each EtxB subunit contains a single intramolecular disulphide bond, a transient intermolecular interaction with DsbA occurs during toxin subunit folding which catalyses formation of the disulphide in vivo.

Targeting and assembly of an oligomeric bacterial enterotoxoid in the endoplasmic reticulum of Saccharomyces cerevisiae

A hybrid protein consisting of the Escherichia coli lipoprotein signal sequence attached to the mature sequence of the B subunit of heat-labile enterotoxin (Lipo-EtxB) was expressed in yeast and E. coli. Analyses of cell lysates from Saccharomyces cerevisiae and E. coli expressing the protein revealed that both organisms were able to assemble Lipo-EtxB into oligomers that were (i) stable in the presence of sodium dodecyl sulphate, (ii) resistant to proteinase K degradation, and (iii) able to bind to GM1-ganglioside receptors. Each of these properties are characteristic of the wild-type B subunit pentamer produced in E. coli. Assembly of Lipo-EtxB was found to be unaffected in a sec18 mutant of S. cerevisiae, which possesses a temperature-sensitive defect in protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus, but was found not to assemble in a sec53 mutant, which causes the misfolding of proteins targeted to the ER. A kar2-1 mutation with a defect in the yeast homologue of BiP caused an 18-fold reduction in Lipo-EtxB assembly at the non-permissive temperature in S. cerevisiae. However, introduction of the wild-type KAR2 gene on a plasmid into the kar2-1 mutant completely suppressed the inhibition of Lipo-EtxB assembly. This provides the first evidence that KAR2 facilitates the assembly of an oligomeric protein in yeast and thus implicates KAR2 as a 'molecular chaperone'. The possible mechanisms of enterotoxoid assembly in E. coli and S. cerevisiae are discussed.

Intranasal immunization using the B subunit of the Escherichia coli heat-labile toxin fused to an epitope of the Bordetella pertussis P.69 antigen

The plasmid pBRD026, which directs expression of the B subunit of the Escherichia coli heat-labile toxin (LTB), was modified so that DNA encoding epitopes could be inserted at the 3' end of the gene. An oligonucleotide linker containing restriction sites for BglII and SpeI was inserted at the SpeI site at the 3' end of the LTB gene to form plasmid pFV1. This linker also encodes the amino acid sequence Gly-Pro-Gly-Pro which we propose acts as a 'hinge' between the LTB and the foreign epitope. Oligonucleotides specifying an epitope from the Bordetella pertussis P.69 outer membrane protein were cloned into pFV1 to form pFV169. The resultant fusion protein (LTB69) was partially purified from the periplasm of E. coli strains in a soluble pentameric form which could bind GM1 gangliosides. Mice immunized intranasally with purified LTB69 produced antibodies against both LTB and the P.69 protein. In addition, ELISPOT assays demonstrated the presence of LTB-specific and P.69-specific antibody-secreting cells in the lungs of immunized mice.

Crystal structure of a cholera toxin-related heat-labile enterotoxin from E. coli

Examination of the structure of Escherichia coli heat-labile enterotoxin in the AB5 complex at a resolution of 2.3A reveals that the doughnut-shaped B pentamer binds the enzymatic A subunit using a hairpin of the A2 fragment, through a highly charged central pore. Putative ganglioside GM1-binding sites on the B subunits are more than 20A removed from the membrane-crossing A1 subunit. This ADP-ribosylating (A1) fragment of the toxin has structural homology with the catalytic region of exotoxin A and hence also to diphtheria toxin.

Synthesis in Vibrio cholerae and secretion of hepatitis B virus antigens fused to Escherichia coli heat-labile enterotoxin subunit B

A simple and effective electroporation method for the transformation of Vibrio cholerae with nonmobilizable plasmids is described. Expression plasmids directing the synthesis of fusion proteins with the subunit B of Escherichia coli heat-labile enterotoxin B (LT-B) were transformed into nontoxinogenic V. cholerae vaccine strains. A protein consisting of two overlapping immunodominant antibody-binding sites of the hepatitis B virus (HBV) middle surface antigen fused to the C terminus of full-length LT-B was secreted into the supernatant of V. cholerae cultures, whereas two other LT-B/HBV fusion proteins were mostly retained within the cells or rapidly degraded in the culture supernatant. While the secretion of fusion proteins with cholera toxin subunit B (CT-B) from V. cholerae has been described, this is to our knowledge the first report describing extracellular secretion of defined foreign epitopes fused to LT-B in V. cholerae. The fusion of guest epitopes to LT-B or CT-B and secretion in V. cholerae could be an interesting system to rapidly produce pure fusion proteins for immunisation, functional studies or diagnostic procedures. An LT-B/pre-S2 fusion protein purified from the supernatant of recombinant V. cholerae induced serum IgG antibodies against LT-B and against the HBV middle surface antigen in mice after parenteral and oral immunisation.

Recombinant cholera toxin B subunit and gene fusion proteins for oral vaccination

The B subunit portion of cholera toxin (CTB) is a safe and effective oral immunizing agent in humans, affording protection against both cholera and diarrhoea caused by enterotoxigenic Escherichia coli producing heat-labile toxin (LT) (Clemens et al., 1986; 1988). CTB may also be used as a carrier of various "foreign" antigens suitable for oral administration. To facilitate large-scale production of CTB for vaccine development purposes, we have constructed recombinant overexpression systems for CTB proteins in which the CTB gene is under the control of strong foreign (non-cholera) promoters and in which it is also possible to fuse oligonucleotides to the CTB gene and thereby achieve overexpression of hybrid proteins (Sanchez and Holmgren, 1989; Sanchez et al., 1988). We here expand these findings by describing overexpression of CTB by a constitutive tacP promoter as well as by the T7 RNA-polymerase promoter, and also by describing gene fusions leading to overexpression of several hybrid proteins between heat-stable E. coli enterotoxin (STa)-related peptides to either the amino or carboxy ends of CTB. Each of the hybrid proteins, when tested as immunogens in rabbits, stimulated significant anti-STa as well as anti-CTB antibody formation, although the anti-STa antibody levels attained (c.a. 1-15 micrograms/ml specific anti-STa immunoglobulin) were too low to give more than partial neutralization of STa intestinal challenge in baby mice. The hybrid proteins also had a near-native conformation, as apparent from their oligomeric nature and their strong reactivity with both a neutralizing antibody against the B subunit and a neutralizing monoclonal antibody (mAb) against STa.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression in Escherichia coli of two mutated genes encoding the cholera toxin B subunit

To allow subsequent genetically mediated fusion of foreign antigens to cholera toxin B subunit (CTB), two mutated CTB encoding genes (ctxB) were constructed and overexpressed in Escherichia coli. The signal peptide coding sequence was deleted and restriction sites were created at both ends of the modified sequence. Both synthesized CTBs contain additional amino acid(s) at the N terminus (one and three). They were purified as insoluble products and refolded into the natural pentameric CTB structure by a denaturation-renaturation cycle. After renaturation, both recombinant proteins recovered CTB antigenicity and the ability to bind to GM1 gangliosides, as shown by in vitro analysis. Preliminary data indicated that both properties were unaltered by fusion of a foreign peptide to the mutated CTBs.

Recombinant system for overexpression of cholera toxin B subunit in Vibrio cholerae as a basis for vaccine development

We have constructed an overexpression system in which the gene encoding the B subunit of cholera toxin (CTB) was placed under the control of the strong tacP promoter in a wide host range plasmid. Recombinant nontoxigenic classical and E1 Tor Vibrio cholerae strains of different serotypes harboring this plasmid excreted 10- to 100-fold higher amounts of CTB than any other wild-type or recombinant strain tested and may therefore be useful killed oral vaccine strains. The manipulations to place the CTB gene under tacP also included, by design, the introduction of single enzyme restriction sites for gene fusions to the CTB amino terminus. Cloning into these sites allows construction of CTB-derived hybrid proteins carrying various putative vaccine peptide antigens.

Genetic fusion of a non-toxic heat-stable enterotoxin-related decapeptide antigen to cholera toxin B-subunit

A decapeptide highly homologous to the STa Escherichia coli heat-stable enterotoxin and to several other heat-stable enterotoxins was fused genetically to the amino-end of the B-subunit of cholera toxin (CTB) and the hybrid protein gene expressed from a tacP overexpression system. The STa-related decapeptide used, which was encoded by a synthetic oligodeoxynucleotide, contained a single mutation which substituted a disulfide-linked cysteine by alanine. After its fusion to CTB the decapeptide was able to both react with and to give rise to anti-STa antibodies. Expression of the decapeptide-CTB hybrid by non-toxigenic Vibrio cholerae resulted in its full secretion into the extracellular milieu from where it could then be readily purified by single-step affinity chromatography using immobilized GM1 ganglioside. Bacteria producing this non-toxic, immunogenic decapeptide-CTB toxoid might be useful for the development of oral vaccines against diarrhea caused by E. coli and other bacteria producing immunologically related heat-stable enterotoxins, and as a source of immunoreagents for methods used to diagnose disease caused by these bacteria.