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

Enhanced soluble production of cholera toxin B subunit in Escherichia coli by co-expression of SKP chaperones

The cholera toxin B subunit (CTB) is a nontoxic portion of the cholera toxin that retains mucosal adjuvant properties. Expression of CTB in Escherichia coli is difficult as CTB aggregates and accumulates as insoluble inclusion bodies. To remedy this problem, the periplasmic chaperone, SKP, was investigated as possible co-expression partner to increase the solubility of recombinant CTB (rCTB) in E. coli. The result showed co-expression of SKP enhanced the soluble expression of rCTB in E. coli. Moreover, soluble rCTB was successfully expressed and secreted into the periplasmic space through the direction of the LTB leader signal. rCTB in periplasm was purified using an immobilized d-galactose resin; GM1-ELISA experiments showed that rCTB retains strong GM1 ganglioside-binding activity. Intranasal administration of ovalbumin (OVA) with rCTB significantly induced both mucosal and humoral immune responses specific to OVA. These data indicate that co-expression of the molecular chaperone SKP with CTB increased the solubility of rCTB while maintaining its function.

A single point mutation within the coding sequence of cholera toxin B subunit will increase its expression yield

Background: Cholera toxin B subunit (CTB) has been extensively considered as an immunogenic and adjuvant protein, but its yield of expression is not satisfactory in many studies. The aim of this study was to compare the expression of native and mutant recombinant CTB (rCTB) in pQE vector. Methods: ctxB fragment from Vibrio cholerae O₁ ATCC14035 containing the substitution of mutant ctxB for amino acid S128T was amplified by PCR and cloned in pGETM-T easy vector. It was then transformed to E. coli Top 10F' and cultured on LB agar plate containing ampicillin. Sequence analysis confirmed the mature ctxB gene sequence and the mutant one in both constructs which were further subcloned to pQE-30 vector. Both constructs were subsequently transformed to E. coli M15 (pREP4) for expression of mature and mutant rCTB. Results: SDS-PAGE analysis showed the maximum expression of rCTB in both systems at 5 hours after induction and Western-blot analysis confirmed the presence of rCTB in blotting membranes. The expression of mutant rCTB was much higher than mature rCTB, which may be the result of serine-to-threonine substitution at position 128 of mature rCTB amino acid sequence created by PCR mutagenesis. The mutant rCTB retained pentameric stability and its ability to bind to anti- cholera toxin IgG antibodies. Conclusion: Point mutation in ctxB sequence resulted in over-expression of rCTB, probably due to the increase of solubility of produced rCTB. Consequently, this expression system can be used to produce rCTB in high yield.

Effect of pressure on refolding of recombinant pentameric cholera toxin B

The production of recombinant proteins is an essential tool for the expansion of modern biological research and biotechnology. The expression of heterologous proteins in Escherichia coli often results in an incomplete folding process that leads to the accumulation of inclusion bodies (IB), aggregates that hold a certain degree of native-like secondary structure. High hydrostatic pressure (HHP) impairs intermolecular hydrophobic and electrostatic interactions, leading to dissociation of aggregates under non-denaturing conditions and is therefore a useful tool to solubilize proteins for posterior refolding. Cholera toxin (CT) is composed of a non-toxic pentamer of B subunits (CTB), a useful adjuvant in vaccines, and a toxic subunit A (CTA). We studied the process of refolding of CTB using HHP. HHP was shown to be effective for dissociation of CTB monomers from IB. Posterior incubation at atmospheric pressure of concentrated CTB (1mg/ml) is necessary for the association of the monomers. Pentameric CTB was obtained when suspensions of CTB IB were compressed at 2.4kbar for 16h in the presence of Tween 20 and incubated at 1bar for 120h. Soluble and biologically active pentameric CTB was obtained, with a yield of 213mg CTB/liter of culture. The experience gained in this study can be important to improve the refolding of proteins with quaternary structure.

The hows and whys of constructing a native recombinant cholera vaccine

Emergence of different ctxB genotypes within virulent Vibrio cholerae populations accentuates the need to develop a vaccine that has the potential to protect against all cholera toxin genotypes. Oral administration of rCTB-alone and in combination with 2 dominant domestic killed whole cells of V. cholerae (O1 Ogawa El Tor and O1 Inaba El Tor) plus one standard V. cholerae (O1 Ogawa classic ATCC 14035)-has shown satisfactory protection as a potent vaccine candidate against toxigenic V. cholerae.

Comparison of two recombinant systems for expression of cholera toxin B subunit from Vibrio cholerae

PURPOSE

The aim of this study was to assess the production of recombinant cholera toxin B subunit (rCTB) protein in two different expression systems (pAE_ctxB and pQE_ctxB constructs) in Escherichia coli BL21 (DE3).

MATERIALS AND METHODS

The ctxB fragment was amplified from Vibrio cholerae O 1 ATCC14035 and cloned in pGETM-T easy vector after which it was transformed to E. coli Top 10F' and grown on LB-ampicillin agar medium. Sequence analysis confirmed the complete ctxB gene sequence in the construct which was further subcloned to pQE-30 vector. The construct was subsequently transformed to E. coli M15 (pREP4). The recombinant pAE_ctxB and pQE_ctxB were transformed to competent E. coli BL21 (DE3) cells to express CTB protein.

RESULT

Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed the maximum expression of rCTB in both systems at 5 h after induction and western blot analysis confirmed the presence of recombinant CTB in blotting membranes.

CONCLUSION

Expression of rCTB in pAE_ctxB construct was more efficient (15-fold) than pQE_ctxB, and it seems that Lac UV5 in E. coli BL21 (DE3) is more compatible with the former construct. This expression system can be used to produce recombinant CTB in high yield which may enable us to study the oral tolerance or mucosal adjuvant properties of rCTB using animal models.

Lincomycin-induced over-expression of mature recombinant cholera toxin B subunit and the holotoxin in Escherichia coli

Cholera toxin (CT) B subunit (CTB) was overproduced using a novel expression system in Escherichia coli. An expression plasmid was constructed by inserting the gene encoding the full-length CTB and the Shine-Dalgarno (SD) sequence derived from CTB or from the heat-labile enterotoxin B subunit (LTB) of enterotoxigenic E. coli into the lacZalpha gene fragment in the pBluescript SK(+) vector. The E. coli strain MV1184 was transformed with each plasmid and then cultured in CAYE broth containing lincomycin. Recombinant CTB (rCTB) was purified from each cell extract. rCTB was overproduced in both transformants without obvious toxicity and was structurally and biologically identical to that of CT purified from Vibrio cholerae, indicating that the original SD and CTB signal sequences were also sufficient to express rCTB in E. coli. Lincomycin-induced rCTB expression was inhibited by mutating the lac promoter, suggesting that lincomycin affects the lactose operon. Based on these findings, we constructed a plasmid that contained the wild-type CT operon and successfully overproduced CT (rCT) using the same procedure for rCTB. Although rCT had an intact A subunit, the amino-terminal modifications and biological properties of the A and B subunits of rCT were identical to those of CT. These results suggest that this novel rCTB over-expression system would also be useful to generate both wild-type and mutant CT proteins that will facilitate further studies on the characteristics of CT, such as mucosal adjuvant activity.

B subunit of E. coli enterotoxin as adjuvant and carrier in oral and skin vaccination

Mucosal sites are one of the main natural ports of entry into the body. Stimulation of a local response by antibodies as the systemic protection may enhance the efficacy of non-living vaccines, and allow for vaccination by subunit vaccines without the need for injection. Mucosal or skin vaccination necessitates a suitable adjuvant and carrier. Escherichia coli heat-labile enterotoxin (LT) and its B subunit (LTB) have been found to be effective adjuvants. The aim of this study was to efficiently produce and purify recombinant LTB (brLTB), and examine its adjuvant and carrier properties. The gene encoding LTB was cloned and expressed in E. coli, and the product was found to have a pentameric form with the ability to bind the cell receptor, GM1 ganglioside. A one-step method for efficient purification and concentration of brLTB was developed. Both oral and intramuscular vaccination with purified brLTB yielded high antibody titers, which detected the whole toxin. In an attempt to test its adjuvant characteristics, brLTB was mixed with either BSA or a recombinant protein (rKnob of egg drop syndrome adenovirus) and delivered intramuscularly, orally or transcutaneously. The addition of brLTB significantly elevated the antibody response in groups vaccinated orally and transcutaneously, but had no influence in injected groups. Vaccination with another recombinant protein, (viral protein 2 of infectious bursal disease virus) supplemented with brLTB did not elevate the antibody response, as compared to vaccination with the antigen alone. These results demonstrate that the addition of brLTB makes oral and transcutaneous vaccination with protein antigens possible.

The cholera toxin A1(3) subdomain is essential for interaction with ADP-ribosylation factor 6 and full toxic activity but is not required for translocation from the endoplasmic reticulum to the cytosol

Cholera toxin (CT) moves from the plasma membrane to the endoplasmic reticulum (ER) by retrograde vesicular traffic. In the ER, the catalytic CTA1 polypeptide dissociates from the rest of the toxin and enters the cytosol by a process that involves the quality control mechanism of ER-associated degradation (ERAD). The cytosolic CTA1 then ADP ribosylates Gsalpha, resulting in adenylate cyclase activation and intoxication of the target cell. It is hypothesized that the C-terminal A1(3) subdomain of CTA1 plays two crucial roles in the intoxication process: (i) it contains a hydrophobic domain that triggers the ERAD mechanism and (ii) it facilitates interaction with the cytosolic ADP-ribosylation factors (ARFs) that serve as allosteric activators of CTA1. In this study, we examined the role(s) of the CTA1(3) subdomain in CT intoxication. Full-length CTA1 constructs and truncated CTA1 constructs lacking the A1(3) subdomain were generated and used to conduct two-hybrid studies of interactions with ARF6, in vitro enzyme assays, in vivo toxicity assays, and in vivo processing/degradation assays. Direct, plasmid-mediated expression of CTA1 constructs in the ER or cytosol of transfected CHO cells was used to perform the in vivo assays. With these methods, we found that the A1(3) subdomain of CTA1 is important both for interaction with ARF6 and for full expression of enzyme activity in vivo. Surprisingly, however, the A1(3) subdomain was not required for ERAD-mediated passage of CTA1 from the ER to the cytosol. A possible alternative trigger for CTA1 to activate the ERAD mechanism is discussed.

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.

Crystal structure of the catalytic domain of MMP-16/MT3-MMP: characterization of MT-MMP specific features

Membrane-type matrix metalloproteinases (MT-MMPs) have attracted strong attention, because four of them can activate a key player in the tumor scenario, proMMP-2/progelatinase A. In addition to this indirect effect on the cellular environment, these MT-MMPs degrade extracellular matrix proteins, and their overproduction is associated with tumor growth. We have solved the structure of the catalytic domain (cd) of MT3-MMP/MMP-16 in complex with the hydroxamic acid inhibitor batimastat. CdMT3-MMP exhibits a classical MMP-fold with similarity to MT1-MMP. Nevertheless, it also shows unique properties such as a modified MT-specific loop and a closed S1' specificity pocket, which might help to design specific inhibitors. Some MT-MMP-specific features, derived from the crystal structures of MT-1-MMP determined previously and MT3-MMP, and revealed in recent mutagenesis experiments, explain the impaired interaction of the MT-MMPs with TIMP-1. Docking experiments with proMMP-2 show some exposed loops including the MT-loop of cdMT3-MMP involved in the interaction with the proMMP-2 prodomain in the activation encounter complex. This model might help to understand the experimentally proven importance of the MT-loop for the activation of proMMP-2.

A mucosally administered recombinant fusion protein vaccine against schistosomiasis protecting against immunopathology and infection

We have constructed and efficiently produced and purified a candidate vaccine against schistosomiasis consisting of a novel hybrid protein in which two dominant T- and B-cell epitopes from Schistosoma mansoni 28 kDa glutathione-S-transferase (Sm28GST) antigen (a.a 24-43 and 191-212) are fused to cholera toxin B subunit (CTB). Intranasal treatment of S. mansoni-infected mice with the hybrid protein, which similar to native CTB was assembled into receptor binding pentamers, significantly reduced total worm burden and liver egg counts due to the induction of Sm28GST-specific antibodies. Immunopathologic granuloma formation in the liver was also significantly suppressed and there was an almost complete suppression of delayed-type hypersensitivity reactions to both Sm28GST and to total soluble egg antigen in infected animals. The results suggest that this type of hybrid protein could be used as a combined anti-immunopathology and anti-infection vaccine against schistosomiasis.

Synthesis of cholera toxin B subunit gene: cloning and expression of a functional 6XHis-tagged protein in Escherichia coli

Cholera toxin B subunit (CTB) has been extensively studied as immunogen, adjuvant, and oral tolerance inductor depending on the antigen conjugated or coadministered. It has been already expressed in several bacterial and yeast systems. In this study, we synthesized a versatile gene coding a 6XHis-tagged CTB (359bp). The sequence was designed according to codon usage of Escherichia coli, Lactobacillus casei, and Salmonella typhimurium. The gene assembly was based on a polymerase chain reaction, in which the polymerase extends DNA fragments from a pool of overlapping oligonucleotides. The synthetic gene was amplified, cloned, and expressed in E. coli in an insoluble form, reaching levels about 13 mg of purified active pentameric rCTB per liter of induced culture. Western blot and ELISA analyses showed that recombinant CTB is strongly and specifically recognized by polyclonal antibodies against the cholera toxin. The ability to form the functional pentamers was observed in cell culture by the inhibition of cholera toxin activity on Y1 adrenal cells in the presence of recombinant CTB. The 6XHis-tagged CTB provides a simple way to obtain functional CTB through Ni(2+)-charged resin after refolding and also free of possible CTA contaminants as in the case of CTB obtained from Vibrio cholerae cultures.

Cholera toxin B subunit as a carrier molecule promotes antigen presentation and increases CD40 and CD86 expression on antigen-presenting cells

Cholera toxin B subunit (CTB) is an efficient mucosal carrier molecule for the generation of mucosal antibody responses and/or induction of systemic T-cell tolerance to linked antigens. CTB binds with high affinity to GM1 ganglioside cell surface receptors. In this study, we evaluated how conjugation of a peptide or protein antigen to CTB by chemical coupling or genetic fusion influences the T-cell-activating capacity of different antigen-presenting cell (APC) subsets. Using an in vitro system in which antigen-pulsed APCs were incubated with antigen-specific, T-cell receptor-transgenic T cells, we found that the dose of antigen required for T-cell activation could be decreased >10,000-fold using CTB-conjugated compared to free antigen. In contrast, no beneficial effects were observed when CTB was simply admixed with antigen. CTB conjugation enhanced the antigen-presenting capacity not only of dendritic cells and B cells but also of macrophages, which expressed low levels of cell surface major histocompatibility complex (MHC) class II and were normally poor activators of naive T cells. Enhanced antigen-presenting activity by CTB-linked antigen resulted in both increased T-cell proliferation and increased interleukin-12 and gamma interferon secretion and was associated with up-regulation of CD40 and CD86 on the APC surface. These results imply that conjugation to CTB dramatically lowers the threshold concentration of antigen required for immune cell activation and also permits low-MHC II-expressing APCs to prime for a specific immune response.

Evidence of rainbow trout prolactin interaction with its receptor through unstable homodimerisation

This study aims to characterise Prolactin receptor (PRLR) in rainbow trout for which no information is available despite the availability of Salmonid PRL preparations. By screening a freshwater rainbow trout intestine cDNA library with a probe corresponding to the extracellular domain (ECD) of tilapia PRLR, we have cloned a 2.5 kb insert coding for the PRLR. The mature protein of 614 amino acid residues is similar to PRLR isolated in tilapia and also the long form of mammalian PRLR. Analysis of PRLR gene expression in osmoregulatory organs revealed the presence of a unique transcript, thus confirming the involvement of this hormone in the control of osmoregulation in this fish species. By using surface plasmon resonance (SPR) technology, kinetic measurement of interaction between trout PRL and its receptor ECD was studied. This approach allowed us to demonstrate the formation of a transient, unstable homodimeric complex. This unstability could explain the inability to perform binding experiments using homologous PRL. In contrast, heterologous lactogenic ligands were able to interact through a more stable complex. Whether these characteristics of PRL-receptor interaction in rainbow trout are different to what occurs in tilapia where a homologous radioreceptor assay was developed would require further studies.

Lys13 plays a crucial role in the functional adaptation of the thermophilic triose-phosphate isomerase from Bacillus stearothermophilus to high temperatures

The thermophilic triose-phosphate isomerases (TIMs) of Bacillus stearothermophilus (bTIM) and Thermotoga maritima (tTIM) have been found to possess a His12-Lys13 pair instead of the Asn12-Gly13 pair normally present in mesophilic TIMs. His12 in bTIM was proposed to prevent deamidation at high temperature, while the precise role of Lys13 is unknown. To investigate the role of the His12 and Lys13 pair in the enzyme's thermoadaptation, we reintroduced the "mesophilic residues" Asn and Gly into both thermophilic TIMs. Neither double mutant displayed diminished structural stability, but the bTIM double mutant showed drastically reduced catalytic activity. No similar behavior was observed with the tTIM double mutant, suggesting that the presence of the His12 and Lys13 cannot be systematically correlated to thermoadaptation in TIMs. We determined the crystal structure of the bTIM double mutant complexed with 2-phosphoglycolate to 2.4-A resolution. A molecular dynamics simulation showed that upon substitution of Lys13 to Gly an increase of the flexibility of loop 1 is observed, causing an incorrect orientation of the catalytic Lys10. This suggests that Lys13 in bTIM plays a crucial role in the functional adaptation of this enzyme to high temperature. Analysis of bTIM single mutants supports this assumption.

Opposing actions of intact and N-terminal fragments of the human prolactin/growth hormone family members on angiogenesis: an efficient mechanism for the regulation of angiogenesis

Angiogenesis, the process of development of a new microvasculature, is regulated by a balance of positive and negative factors. We show both in vivo and in vitro that the members of the human prolactin/growth hormone family, i.e., human prolactin, human growth hormone, human placental lactogen, and human growth hormone variant are angiogenic whereas their respective 16-kDa N-terminal fragments are antiangiogenic. The opposite actions are regulated in part via activation or inhibition of mitogen-activated protein kinase signaling pathway. In addition, the N-terminal fragments stimulate expression of type 1 plasminogen activator inhibitor whereas the intact molecules have no effect, an observation consistent with the fragments acting via separate receptors. The concept that a single molecule encodes both angiogenic and antiangiogenic peptides represents an efficient model for regulating the balance of positive and negative factors controlling angiogenesis. This hypothesis has potential physiological importance for the control of the vascular connection between the fetal and maternal circulations in the placenta, where human prolactin, human placental lactogen, and human growth hormone variant are expressed.

A cloning vector for efficient generation of cholera toxin B gene fusions for epitope screening

Gene fusion proteins with epitopes attached to the amino end of cholera toxin B subunit (CTB) are useful to raise immunological responses. We describe a cloning vector, designated pCTBtet, carrying a tetracycline resistance gene (TetR) between the leader peptide and mature CTB. Removal of TetR to insert oligonucleotides encoding fusion epitopes allowed for screening of tetracycline-sensitive clones. Restoration of the correct CTB reading phase was subsequently used to choose gene fusion candidate colonies. The use of pCTBtet permitted the rapid construction of 8 fusion proteins carrying 9-24 aa from Salmonella typhi OmpC and 6 hybrids with 7-31 aa from Escherichia coli colonization factor CFAI.

Characterisation of cholera toxin by liquid chromatography--electrospray mass spectrometry

Cholera toxin, one of the toxins that may be generated by various strains of the bacterium Vibrio cholerae, can be considered as a substance possibly used in biological warfare. The possibilities of characterising the toxin by liquid chromatography electrospray mass spectrometry (LC-ES-MS) were investigated. The toxin can be detected by flow-injection (FIA) ES-MS of a dialysed solution and observation of the charge envelope signals of its A-unit and B-chain protein; sufficient information for identification by the molecular mass of either protein could be obtained for quantities in the order of 10 fmol. Confirmatory analysis was carried out by 2-mercaptoethanol reduction and FIA-ES-MS detection of the product proteins or by tryptic digest LC-ES-MS with ion chromatogram detection of most of the tryptic fragments of the A-unit and B-chain from the singly, doubly or triply charged ion signals. The confirmatory tryptic digest LC-ES-MS analysis could be achieved with quantities as low as 1 pmol. Possible biovariations in the toxin can mostly be determined by sequencing, where the amino acid composition of tryptic fragments of the A1-chain, T5 and T15, and of the B-chain, T1, T4 and T5, cover all known biovariations. Partial sequencing of cholera toxin, originating from a classical strain, O1/569B, was achieved by LC-ES-MS/MS of most tryptic fragments larger than three amino acid residues.

Triose-phosphate isomerase (TIM) of the psychrophilic bacterium Vibrio marinus. Kinetic and structural properties

The purification and characterization of triose-phosphate isomerase from the psychrophilic bacterium Vibrio marinus (vTIM) is described. Crystal structures of the vTIM-sulfate complex and the vTIM-2-phosphoglycolate complex (at a 2.7-A resolution) are also presented. The optimal growth temperature of Vibrio marinus is 15 degrees C. Stability studies show that vTIM is an unstable protein with a half-life of only 10 min at 25 degrees C. The vTIM sequence is most closely related to the sequence of Escherichia coli TIM (eTIM) (66% identity), and several unique structural features described for eTIM are also seen in vTIM, but eTIM is considerably more stable. The Td values of vTIM and eTIM, determined by calorimetric studies, are 41 and 54 degrees C, respectively. Amino acid sequence comparison reveals that vTIM has an alanine in loop 8 (at position 238), whereas all other TIM sequences known to date have a serine. The vTIM mutant A238S was produced and characterized. Compared with wild type, the catalytic efficiency of the A238S mutant is somewhat reduced, and its stability is considerably increased.

Construction and characterization of versatile cloning vectors for efficient delivery of native foreign proteins to the periplasm of Escherichia coli

Induction of the wild type cholera toxin operon (ctxAB) from multicopy clones in Escherichia coli inhibited growth and resulted in low yields of cholera toxin (CT). We found that production of wild type CT or its B subunit (CT-B) as a periplasmic protein was toxic for E. coli, but by replacing the native signal sequences of both CT-A and CT-B with the signal sequence from the B subunit of E. coli heat-labile enterotoxin LTIIb we succeeded for the first time in producing CT holotoxin in high yield in E. coli. Based on these findings, we designed and constructed versatile cloning vectors that use the LTIIb-B signal sequence to direct recombinant native proteins with high efficiency to the periplasm of E. coli. We confirmed the usefulness of these vectors by producing two other secreted recombinant proteins. First, using phoA from E. coli, we demonstrated that alkaline phosphatase activity was 17-fold greater when the LTIIb-B signal sequence was used than when the native leader for alkaline phosphatase was used. Second, using the pspA gene that encodes pneumococcal surface protein A from Streptococcus pneumoniae, we produced a 299-residue amino-terminal fragment of PspA in E. coli in large amounts as a soluble periplasmic protein and showed that it was immunoreactive in Western blots with antibodies against native PspA. The vectors described here will be useful for further studies on structure-function relationships and vaccine development with CT and PspA, and they should be valuable as general tools for delivery of other secretion-competent recombinant proteins to the periplasm in E. coli.

Production of dog calcyphosine in bacteria and lack of phosphorylation by the catalytic subunit of protein kinase A in vitro

Calcyphosine is a calcium-binding protein containing four EF-hand domains that is found in several epithelia and in some cells of the central nervous system. In thyroid follicular cells, calcyphosine is synthesized and phosphorylated in response to stimulation by thyrotropin and cAMP agonists. The cDNA coding for dog calcyphosine has been expressed in bacteria under the control of the T7 promoter. Recombinant calcyphosine was purified from crude bacterial lysates by a combination of anion-exchange and hydrophobic interaction chromatography. Phosphorylation assays using the purified catalytic subunit of protein kinase A and the recombinant or the native calcyphosine revealed that, contrary to a previous report, calcyphosine is not significantly phosphorylated by this enzyme in vitro.

A comparison of natural and recombinant cholera toxin B subunit as stimulatory factors in intranasal immunization

Cholera toxin B (CTB) is often envisaged and used as an immune stimulating agent in protocols for mucosal immunization. However, the nature of the CTB used (natural vs recombinant) is frequently not taken in consideration. This is important since the usage of natural CTB in mucosal immunization regimen and the mucosal response resulting from such an immunization can be effected by the presence of the CTA subunit in commercial CTB preparations. To clarify this, we have compared natural vs recombinant CTB in an intranasal (i.n.) mucosal immunization procedure using ovalbumin (OVA) as antigen. The results show that recombinant CTB induces similar immune responses like natural CTB. Furthermore, our experiments show that covalent coupling of OVA to CTB is not required for the induction of OVA specific mucosal and systemic immune responses upon i.n. immunization.

Construction of CTB fusion proteins for screening of monoclonal antibodies against Salmonella typhi OmpC peptide loops

Mice were immunized with resin-bound peptides whose sequences have been proposed to be part of exposed loops in Salmonella typhi outer membrane protein OmpC. To screen hybridomas for monoclonal antibodies against those epitopes, we designed fusion proteins where the candidate peptide sequence was attached to the amino end of cholera toxin B-subunit (CTB). The constructed fusion proteins allowed the efficient selection of positive clones by GM1-ELISA. Selected antibodies recognized purified OmpC and whole Salmonella bacteria. This suggests a native structure of our genetically attached peptides in agreement with immunological properties reported for previous CTB recombinant fusion proteins. In a more general context, CTB hybrids could be used to screen for antibodies towards immunogenic epitopes in other systems. This might turn out to be particularly useful when producing antibodies against peptide sequences in microorganisms whose handling is difficult or that pose inherent health risks.

Efficient production of Thermus protease aqualysin I in Escherichia coli: effects of cloned gene structure and two-stage culture

The DNA sequence encoding Thermus protease aqualysin I was inserted downstream from a bacteriophage T7 promoter in an expression vector. In the T7 expression system, using a strain lacking an F' episome, aqualysin I was produced in soluble form without chemical induction. The deletions of part (30 amino acid residues) or all (105 residues) of the C-terminal pro-sequence from the C terminus significantly affected both cellular growth and the production of the enzyme. Complete deletion adversely affected both. In contrast, the 30-residue deletion markedly improved productivity by approximately four times compared to non-deletion, and shortened the time needed for the activation of a precursor to active enzyme. The concentration of inducer isopropyl beta-D-thiogalactopyrano-side (IPTG) was varied to examine its effects, and it was found that a low concentration of IPTG improved aqualysin I production. To avoid the inhibitory effects of acetic acid accumulation in the culture medium, the use of other carbon sources besides glucose was examined. When cells were cultivated with glycerol, the acetic acid level remained relatively low, and both good cellular growth and a high level of production were attained. The aqualysin I productivity for a fed-batch culture using two carbon sources, glucose and glycerol, reached more than 150 kU/ml enzymatically active aqualysin I.

Crystal structure of cholera toxin B-pentamer bound to receptor GM1 pentasaccharide

Cholera toxin (CT) is an AB5 hexameric protein responsible for the symptoms produced by Vibrio cholerae infection. In the first step of cell intoxication, the B-pentamer of the toxin binds specifically to the branched pentasaccharide moiety of ganglioside GM1 on the surface of target human intestinal epithelial cells. We present here the crystal structure of the cholera toxin B-pentamer complexed with the GM1 pentasaccharide. Each receptor binding site on the toxin is found to lie primarily within a single B-subunit, with a single solvent-mediated hydrogen bond from residue Gly 33 of an adjacent subunit. The large majority of interactions between the receptor and the toxin involve the 2 terminal sugars of GM1, galactose and sialic acid, with a smaller contribution from the N-acetyl galactosamine residue. The binding of GM1 to cholera toxin thus resembles a 2-fingered grip: the Gal(beta 1-3)GalNAc moiety representing the "forefinger" and the sialic acid representing the "thumb." The residues forming the binding site are conserved between cholera toxin and the homologous heat-labile enterotoxin from Escherichia coli, with the sole exception of His 13. Some reported differences in the binding affinity of the 2 toxins for gangliosides other than GM1 may be rationalized by sequence differences at this residue. The CTB5:GM1 pentasaccharide complex described here provides a detailed view of a protein:ganglioside specific binding interaction, and as such is of interest not only for understanding cholera pathogenesis and for the design of drugs and development of vaccines but also for modeling other protein:ganglioside interactions such as those involved in GM1-mediated signal transduction.

Inducible expression of a toxic poliovirus membrane protein in Escherichia coli: comparative studies using different expression systems based on T7 promoters

The poliovirus 3AB gene has been cloned and overproduced in T7 expression vectors using different approaches to allow reduction of basal levels of expression. Expression of the poliovirus 3AB gene is highly toxic for E. coli cells, due to drastic changes induced in membrane permeability of the bacteria that lead to cell lysis when the T7 lysozyme is present. The best production of 3AB was achieved with the T7/lac system in cells lacking T7 lysozyme, where this toxic protein was synthesized to high levels and during several hours in the absence of cell lysis. These results show the efficient synthesis of a highly damaging membrane protein and open the possibility to apply heterologous gene expression in E. coli to other lytic proteins.

Nucleotide sequence analysis of the CT operon of the Vibrio cholerae classical strain 569B

The complete nucleotide sequence of the Vibrio cholerae classical strain 569B was determined. The results prove the exactness of the amino acid CT B sequence published by Takao et al. (1985, Eur. J. Biochem. 146, 503-508). A comparison is made with already reported CT genes.