Strategies for successful recombinant expression of disulfide bond-dependent proteins in Escherichia coli

Towards a comprehensive Plasmodium falciparum merozoite cell surface and secreted recombinant protein library

BACKGROUND

Plasmodium falciparum is the aetiological agent for malaria, a deadly infectious disease for which no vaccine has yet been licensed. The proteins displayed on the merozoite cell surface have long been considered attractive vaccine targets because of their direct exposure to host antibodies; however, progress in understanding the functional role of these targets has been hindered by technical challenges associated with expressing these proteins in a functionally active recombinant form. To address this, a method that enables the systematic expression of functional extracellular Plasmodium proteins was previously developed, and used to create a library of 42 merozoite proteins.

METHODS

To compile a more comprehensive library of recombinant proteins representing the repertoire of P. falciparum merozoite extracellular proteins for systematic vaccine and functional studies, genome-wide expression profiling was used to identify additional candidates. Candidate proteins were recombinantly produced and their integrity and expression levels were tested by Western blotting and ELISA.

RESULTS

Twenty-five additional genes that were upregulated during late schizogony, and predicted to encode secreted and cell surface proteins, were identified and expressed as soluble recombinant proteins. A band consistent with the entire ectodomain was observed by immunoblotting for the majority of the proteins and their expression levels were quantified. By using sera from malaria-exposed immune adults, the immunoreactivity of 20 recombinant proteins was assessed, and most of the merozoite ligands were found to carry heat-labile epitopes. To facilitate systematic comparative studies across the entire library, multiple Plasmodium proteins were simultaneously purified using a custom-made platform.

CONCLUSIONS

A library of recombinant P. falciparum secreted and cell surface proteins was expanded by 20 additional proteins, which were shown to express at usable levels and contain conformational epitopes. This resource of extracellular P. falciparum merozoite proteins, which now contains 62 full-length ectodomains, will be a valuable tool in elucidating the function of these proteins during the blood stages of infection, and facilitate the comparative assessment of blood stage vaccine candidates.

Overview of Recent Progress in Protein-Expression Technologies for Small-Molecule Screening

Production of novel soluble and membrane-localized protein targets for functional and affinity-based screening has often been limited by the inability of traditional protein-expression systems to generate recombinant proteins that have properties similar to those of their endogenous counterparts. Such targets have often been labeled as challenging. Although biological validation of these challenging targets for specific disease areas may be strong, discovery of small-molecule modulators can be greatly delayed or completely halted due to target-expression issues. In this article, the limitations of traditional protein-expression systems will be discussed along with new systems designed to overcome these challenges. Recent work in this field has focused on two major areas for both soluble and membrane targets: construct-design strategies to improve expression levels and new hosts that can carry out the posttranslational modifications necessary for proper target folding and function. Another area of active research has been on the reconstitution of solubilized membrane targets for both structural analysis and screening. Finally, the potential impact of these new systems on the output of small-molecule screening campaigns will be discussed.

Computationally designed libraries for rapid enzyme stabilization

The ability to engineer enzymes and other proteins to any desired stability would have wide-ranging applications. Here, we demonstrate that computational design of a library with chemically diverse stabilizing mutations allows the engineering of drastically stabilized and fully functional variants of the mesostable enzyme limonene epoxide hydrolase. First, point mutations were selected if they significantly improved the predicted free energy of protein folding. Disulfide bonds were designed using sampling of backbone conformational space, which tripled the number of experimentally stabilizing disulfide bridges. Next, orthogonal in silico screening steps were used to remove chemically unreasonable mutations and mutations that are predicted to increase protein flexibility. The resulting library of 64 variants was experimentally screened, which revealed 21 (pairs of) stabilizing mutations located both in relatively rigid and in flexible areas of the enzyme. Finally, combining 10-12 of these confirmed mutations resulted in multi-site mutants with an increase in apparent melting temperature from 50 to 85°C, enhanced catalytic activity, preserved regioselectivity and a >250-fold longer half-life. The developed Framework for Rapid Enzyme Stabilization by Computational libraries (FRESCO) requires far less screening than conventional directed evolution.

Functional expression of an scFv on bacterial magnetic particles by in vitro docking

A Gram-negative, magnetotactic bacterium, Magnetospirillum magneticum AMB-1 produces nano-sized magnetic particles (BacMPs) in the cytoplasm. Although various applications of genetically engineered BacMPs have been demonstrated, such as immunoassay, ligand-receptor interaction or cell separation, by expressing a target protein on BacMPs, it has been difficult to express disulfide-bonded proteins on BacMPs due to lack of disulfide-bond formation in the cytoplasm. Here, we propose a novel dual expression system, called in vitro docking, of a disulfide-bonded protein on BacMPs by directing an immunoglobulin Fc-fused target protein to the periplasm and its docking protein ZZ on BacMPs. By in vitro docking, an scFv-Fc fusion protein was functionally expressed on BacMPs in the dimeric or trimeric form. Our novel disulfide-bonded protein expression system on BacMPs will be useful for efficient screening of potential ligands or drugs, analyzing ligand-receptor interactions or as a magnetic carrier for affinity purification.

Soluble expression of disulfide bond containing proteins FGF15 and FGF19 in the cytoplasm of Escherichia coli

Fibroblast growth factor 19 (FGF19) is the human ortholog of mouse FGF15, and both proteins function as an endocrine signal to regulate various liver functions. FGF15/FGF19 protein contains two disulfide bonds. It is unfavorable to form disulfide bonds in Escherichia coli (E. coli) cytoplasm because of the bacterial cytoplasmic reducing environment. Modification of the cytoplasmic reducing environment and/or co-expression of protein chaperones are common strategies to express disulfide bond containing proteins in E. coli. In the current study, we report a method to produce soluble FGF15/FGF19 protein in cytoplasm of E. coli. Several commercial available strains with the disruption of thiol-redox pathways, and/or co-expression of redoxase or refolding chaperones were used to develop this novel method for expression of FGF15/FGF19 in E. coli. Mutation of the thiol-disulfide bond reducing pathway in E. coli or N-terminal fusion of thioredox (TRX) alone is not enough to support disulfide bond formation in FGF15/19 proteins. However, TRX fusion protein improved FGF19 solubility in strains of thiol-redox system mutants. In addition, DsbC co-expressed in thiol-redox system mutants alone improved and further enhanced FGF19 solubility with combination of TRX fusion tag. The soluble FGF19 proteins were easily purified through Ni-NTA affinity chromatography and anion exchange chromatography, and the purified protein maintained its biological activities, confirmed by suppressing hepatic Cyp7a1 gene transcription in mice and by activating ERK1/2 signaling pathway in HepG2 cells. In contrast, soluble FGF15 protein in cytoplasm remained very low using these strategies. In summary, we have successfully developed a method to express functional FGF19 protein in prokaryotic cells, and this strategy may be adapted for the expression of other disulfide-containing proteins.

Recombinant production of mecasermin in E. coli expression system

Human Insulin-like growth factor 1 (hIGF-1) consists of 70 amino acids in a single chain with three intermolecular disulfide bridges possessing valuable therapeutic effects. To date, numerous variants of specifically engineered hIGF-1 have been produced so as to improve hIGF-1 biological activity, stability and stronger binding to IGF-1 receptor. Mecasermin is one of the modified variants with one amino acid substitution near the N-terminal (T4I) approved for the treatment of growth failure diabetes, wound healing, amyotrophic lateral sclerosis and severe primary IGF-1 deficiency. No scientific report for recombinant production of mecasermin in Escherichia coli (E. coli) expression system has been sofar reported. In the present study, we therefore investigated the overexpression of mecasermin in two different E. coli strains in order to obtain higher yield of recombinant protein. To achieve this goal, mecasermin DNA encoding sequence was designed based on polypeptide sequence, optimized according to E. coli codon preference, and cloned in pET15b. Recombinant vector, pET15-mecasermin, transferred into two E. coli strains rigami B (DE3) and BL21 (DE3) and induced for expression in a small scale. Results revealed the E. coli Origami B (DE3) expression system was a preferable host for mecasermin production due to its high expression level being around twice as much as BL21 (DE3). Large scale mecasermin production was performed in batch culture and produced recombinant protein specifically confirmed by western blotting and mass spectroscopy. Since major part of recombinant mecasermin was expressed as inclusion body, isolation and refolding was accomplished through developed purification procedure, and finally recombinant protein was successfully purified by gel filtration chromatography.

Enhanced production of human full-length immunoglobulin G1 in the periplasm of Escherichia coli

Monoclonal antibodies are currently the most important pharmaceutical proteins, and the economic production of functional immunoglobulin G (IgG) is an important issue in biotechnology. Recent successes in the development of aglycosylated IgG variants that do not require glycosylation for effector functions have increased the use of Escherichia coli as an alternative host for economic production of IgG, instead of traditional mammalian host expression systems. Here, we have developed a new E. coli host-vector system for the high-level production of full-length IgG1 by examining (1) E. coli strains, (2) modification of 5' untranslated region sequences, and (3) co-expression of periplasmic foldase. With the engineered host-vector system, fed-batch cultivations were conducted at two different conditions, and under optimized conditions, up to 362 mg/L of full-length IgG1 could be produced in a relatively short-time (22 h) cultivation. The fully assembled IgG1 from fed-batch cultivation was purified with high purity and yield. With the purified IgG1, the specific bindings to an antigen, anthrax toxin PA, and to human neonatal Fc receptor were successfully demonstrated.

Recombinant polypeptide production in E. coli: towards a rational approach to improve the yields of functional proteins

The development of complementary technologies enabled the successful production of recombinant polypeptides in bacteria and opened to biology researchers new avenues as obtaining suitable amounts of proteins necessary for their experimental work became easy, fast, and inexpensive. Nevertheless, the recombinant approach remained somehow unpredictable, since many constructs resisted to apparent production or accumulated as aggregates.

Several factors and physical/chemical conditions that could improve the accumulation of native-like protein were identified. At the same time, it was acknowledged that the outcome of most of them was erratic and that almost any protein required its own specific optimized set of conditions to achieve its correct folding. The attempt to understand the critical points specific for recombinant protein production missed the goal of setting universally useful protocols, but contributed to the increase of the rate of success by proposing always new empiric combinations.

Nevertheless, the results published in the recent literature allow for a better comprehension of some key mechanisms controlling protein production in E. coli and could enable the elaboration of rational methodologies for improving the quantitative and qualitative features of the produced polypeptides. This result will be achieved when the identification of the limiting step that impairs the accomplishment of the native folding for any single construct will become straightforward. This minireview will discuss how factors such as the expression rate, the folding machinery, and the secretion efficiency may impact the final protein yields.

High-yield production of functional soluble single-domain antibodies in the cytoplasm of Escherichia coli

Background

For their application in the area of diagnosis and therapy, single-domain antibodies (sdAbs) offer multiple advantages over conventional antibodies and fragments thereof in terms of size, stability, solubility, immunogenicity, production costs as well as tumor uptake and blood clearance. Thus, sdAbs have been identified as valuable next-generation targeting moieties for molecular imaging and drug delivery in the past years. Since these probes are much less complex than conventional antibody fragments, bacterial expression represents a facile method in order to produce sdAbs in large amounts as soluble and functional proteins.

Results

By the combined use of high cell density cultivation media with a genetically engineered E. coli mutant strain designed for the cytoplasmic formation of proper disulfide bonds, we achieved high level of intracellular sdAb production (up to 200 mg/L). Due to a carboxyterminal hexahistidine epitope, the soluble recombinant sdAbs could be purified by one-step immobilized metal affinity chromatography to apparent homogeneity and easily radiolabeled with ^99mTc within 1 h. The intradomain disulfide bridge being critical for the stability and functionality of the sdAb molecule was shown to be properly formed in ~96% of the purified proteins. In vitro binding studies confirmed the high affinity and specificity of the expressed sdAb 7C12 towards its molecular target.

Conclusions

Our study demonstrates an efficient cultivation and expression strategy for the production of substantial amounts of soluble and functional sdAbs, which may be adopted for high-yield production of other more complex proteins with multiple disulfides as well.

Functional expression of thyroid-stimulating hormone receptor on nano-sized bacterial magnetic particles in Magnetospirillum magneticum AMB-1

The measurement of autoantibodies to thyroid-stimulating hormone receptor (TSHR) is important for the diagnosis of autoimmune thyroid disease such as Graves' disease (GD). Although TSHR from porcine thyroid membrane is commonly used for the measurement of TSHR autoantibodies (TRAb), recombinant human TSHR (hTSHR) remains ideal in terms of stable supply and species identity. Here we set out to express recombinant hTSHR on the lipid-bilayer surface of magnetic nanoparticles from a magnetotactic bacterium, Magnetospirillum magneticum AMB-1. Using a tetracycline-inducible expression system, we successfully overexpressed functional hTSHR on bacterial magnetic particles (BacMPs) in AMB-1 via an anchor protein specific for BacMPs. The overexpressed hTSHR was membrane integrated and possessed both ligand and autoantibody binding activity. Our data suggest that hTSHR-displayed BacMPs have potential as novel tools for ligand-receptor interaction analysis or for TRAb immunoassay in GD patients.

Functional recombinant extra membrane loop of human CD20, an alternative of the full length CD20 antigen

BACKGROUND

Targeting of CD20 antigen with monoclonal antibodies has become the mainstay in the treatment of non-Hodgkin's lymphomas and immunotherapeutic depletion of malignant B cells. Accessibility of antigen is one of the crucial factors in development of monoclonal antibodies against this antigen. One major problem in expression of full length CD20 is aggregation and misfolding. Therefore, production of an alternative polypeptide is easer and favorable comparing to that of a full length transmembrane protein CD20.

METHODS

In this study, we expressed the extra membrane loop of hCD20 (exCD20) consisting of a non-glycosylated 47-amino acids region. The exCD20 coding sequence was amplified by PCR and cloned in pET32a(+) expression vector. The desired protein was expressed in fusion with thioredoxin and 6× His tag in E. coli Origami strain. ELISA and Western-blotting data were performed to indicate the functionality of this protein.

RESULTS

We have obtained the exCD20 recombinant protein which can be detected in ELISA and Western-blot experiments. This recombinant fusion protein was soluble and stable without aggregation and misfolding problems.

CONCLUSION

The recombinant extra membrane loop of human CD20 protein in fusion with thioredoxin (exCD20) can be used in function assays and some applications such as ELISA, immuneblotting, affinity purification, immunization, screening, and development of anti-CD20 antibodies.

High throughput screening identifies disulfide isomerase DsbC as a very efficient partner for recombinant expression of small disulfide-rich proteins in E. coli

Background

Disulfide-rich proteins or DRPs are versatile bioactive compounds that encompass a wide variety of pharmacological, therapeutic, and/or biotechnological applications. Still, the production of DRPs in sufficient quantities is a major bottleneck for their complete structural or functional characterization. Recombinant expression of such small proteins containing multiple disulfide bonds in the bacteria E. coli is considered difficult and general methods and protocols, particularly on a high throughput scale, are limited.

Results

Here we report a high throughput screening approach that allowed the systematic investigation of the solubilizing and folding influence of twelve cytoplasmic partners on 28 DRPs in the strains BL21 (DE3) pLysS, Origami B (DE3) pLysS and SHuffle® T7 Express lysY (1008 conditions). The screening identified the conditions leading to the successful soluble expression of the 28 DRPs selected for the study. Amongst 336 conditions tested per bacterial strain, soluble expression was detected in 196 conditions using the strain BL21 (DE3) pLysS, whereas only 44 and 50 conditions for soluble expression were identified for the strains Origami B (DE3) pLysS and SHuffle® T7 Express lysY respectively. To assess the redox states of the DRPs, the solubility screen was coupled with mass spectrometry (MS) to determine the exact masses of the produced DRPs or fusion proteins. To validate the results obtained at analytical scale, several examples of proteins expressed and purified to a larger scale are presented along with their MS and functional characterization.

Conclusions

Our results show that the production of soluble and functional DRPs with cytoplasmic partners is possible in E. coli. In spite of its reducing cytoplasm, BL21 (DE3) pLysS is more efficient than the Origami B (DE3) pLysS and SHuffle® T7 Express lysY trxB^-/gor^- strains for the production of DRPs in fusion with solubilizing partners. However, our data suggest that oxidation of the proteins occurs ex vivo. Our protocols allow the production of a large diversity of DRPs using DsbC as a fusion partner, leading to pure active DRPs at milligram scale in many cases. These results open up new possibilities for the study and development of DRPs with therapeutic or biotechnological interest whose production was previously a limitation.

Enhanced production of full-length immunoglobulin G via the signal recognition particle (SRP)-dependent pathway in Escherichia coli

Because of the lack of post-translational glycosylation, Escherichia coli is not a preferable host for immunoglobulin G (IgG) production. However, recent successes in the developments of aglycosylated IgG variants that do not require glycosylation for effector functions have increased the likelihood of using E. coli for IgG production. Here, we have developed a new E. coli host-vector system for enhanced production of recombinant IgG using: (i) a combination of SRP/Sec-dependent pathways for the efficient secretion of heavy and light chains in the periplasm; (ii) co-expression of periplasmic foldase (DsbC) for efficient assembly of IgG in the periplasm; and (iii) co-expression of Ffh for enhancing the SRP machinery. Finally, with engineered host-vector system, fed-batch cultivations were conducted at four different conditions, and under an optimized condition, up to 62 mg/L of active full-length IgG was produced during a 28-h cultivation.

Optimizing heterologous protein production in the periplasm of E. coli by regulating gene expression levels

Background

In Escherichia coli many heterologous proteins are produced in the periplasm. To direct these proteins to the periplasm, they are equipped with an N-terminal signal sequence so that they can traverse the cytoplasmic membrane via the protein-conducting Sec-translocon. For poorly understood reasons, the production of heterologous secretory proteins is often toxic to the cell thereby limiting yields. To gain insight into the mechanism(s) that underlie this toxicity we produced two secretory heterologous proteins, super folder green fluorescent protein and a single-chain variable antibody fragment, in the Lemo21(DE3) strain. In this strain, the expression intensity of the gene encoding the target protein can be precisely controlled.

Results

Both SFGFP and the single-chain variable antibody fragment were equipped with a DsbA-derived signal sequence. Producing these proteins following different gene expression levels in Lemo21(DE3) allowed us to identify the optimal expression level for each target gene. Too high gene expression levels resulted in saturation of the Sec-translocon capacity as shown by hampered translocation of endogenous secretory proteins and a protein misfolding/aggregation problem in the cytoplasm. At the optimal gene expression levels, the negative effects of the production of the heterologous secretory proteins were minimized and yields in the periplasm were optimized.

Conclusions

Saturating the Sec-translocon capacity can be a major bottleneck hampering heterologous protein production in the periplasm. This bottleneck can be alleviated by harmonizing expression levels of the genes encoding the heterologous secretory proteins with the Sec-translocon capacity. Mechanistic insight into the production of proteins in the periplasm is key to optimizing yields in this compartment.

Recombinant production of human interleukin 6 in Escherichia coli

In this study, we compared basic expression approaches for the efficient expression of bioactive recombinant human interleukin-6 (IL6), as an example for a difficult-to-express protein. We tested these approaches in a laboratory scale in order to pioneer the commercial production of this protein in Escherichia coli (E. coli). Among the various strategies, which were tested under Research and Development (R&D) conditions, aggregation-prone IL6 was solubilized most effectively by co-expressing cytoplasmic chaperones. Expression of a Glutathion-S-Transferase (GST) fusion protein was not efficient to increase IL6 solubility. Alteration of the cultivation temperature significantly increased the solubility in both cases, whereas reduced concentrations of IPTG to induce expression of the T7lac-promotor only had a positive effect on chaperone-assisted expression. The biological activity was comparable to that of commercial IL6. Targeting the expressed protein to an oxidizing environment was not effective in the generation of soluble IL6. Taken together, the presence of chaperones and a lowered cultivation temperature seem effective to isolate large quantities of soluble IL6. This approach led to in vivo soluble, functional protein fractions and reduces purification and refolding requirements caused by downstream purification procedures. The final yield of soluble recombinant protein averaged approximately 2.6 mg IL6/liter of cell culture. These findings might be beneficial for the development of the large-scale production of IL6 under the conditions of current good manufacturing practice (cGMP).

High-level transient expression of ER-targeted human interleukin 6 in Nicotiana benthamiana

Tobacco plants can be used to express recombinant proteins that cannot be produced in a soluble and active form using traditional platforms such as Escherichia coli. We therefore expressed the human glycoprotein interleukin 6 (IL6) in two commercial tobacco cultivars (Nicotiana tabacum cv. Virginia and cv. Geudertheimer) as well as the model host N. benthamiana to compare different transformation strategies (stable vs. transient expression) and subcellular targeting (apoplast, endoplasmic reticulum (ER) and vacuole). In T(0) transgenic plants, the highest expression levels were achieved by ER targeting but the overall yields of IL6 were still low in the leaves (0.005% TSP in the ER, 0.0008% in the vacuole and 0.0005% in the apoplast). The apoplast variant accumulated to similar levels in leaves and seeds, whereas the ER-targeted variant was 1.2-fold more abundant in seeds and the vacuolar variant was 6-fold more abundant in seeds. The yields improved in subsequent generations, with the best-performing T(2) plants producing the ER-targeted IL6 at 0.14% TSP in both leaves and seeds. Transient expression of ER-targeted IL6 in leaves using the MagnICON system resulted in yields of up to 7% TSP in N. benthamiana, but only 1% in N. tabacum cv. Virginia and 0.5% in cv. Geudertheimer. Although the commercial tobacco cultivars produced up to threefold more biomass than N. benthamiana, this was not enough to compensate for the lower overall yields. The recombinant IL6 produced by transient and stable expression in plants was biologically active and presented as two alternative bands matching the corresponding native protein.

Gene construction, expression and functional testing of an inotropic peptide from the venom of the black scorpion Hottentotta judaicus

Anti-insect depressant toxins represent a subfamily of scorpion venom-derived β-toxins that are polypeptides composed of 61-65 amino acid residues stabilized by four disulfide bridges. These toxins affect the activation of voltage-sensitive sodium channels (NaScTx) and exhibit the preferential ability to induce flaccid paralysis in insect larvae. Here we demonstrate the recombinant expression of the novel cardiac inotropic peptide (Bj-IP) that was classified as an anti-insect depressant βNaScTx isolated from the venom of Hottentotta judaicus. By using "splicing by overlap extension" (SOE)-PCR, allowing for the first time one step de novo synthesis of long-chain scorpion toxin genes, we generated a codon-optimized DNA fragment of Bj-IP for cloning into the Escherichia coli vector pQE30. Moreover, the gene of interest was fused to a 6xHis coding DNA sequence. Subsequent recombinant expression was performed in E. coli KRX. The purification of the polypeptide was achieved by a combination of NiNTA agarose columns and RP (C(18)) high-performance liquid chromatography. The purified fusion protein was digested with factor Xa resulting in the elution of Bj-IP. The yield of recombinant Bj-IP expression was approximately 4.5 mg per liter of culture. Mass spectrometry confirmed the theoretical total mass of Bj-IP (6608 Da). Tag-free Bj-IP was refolded in guanidine chloride buffer with a glutathione redox system which was supplemented with different additives at 16 °C. Supplementation with 10% glycerol produced Bj-IP folding forms that exhibited reproducible biological activity in mouse cardiomyocytes. Cell contractility was increased by almost 3-fold and decay kinetics were hasten by 47% after administration of Bj-IP. Taken together, here we show the recombinant expression of the functionally active cardiac inotropic peptide Bj-IP, a new βNaScTx from H. judaicus, for promising pharmacological applications. Furthermore, our data suggest that the use of SOE-PCR may help to facilitate in future the high throughput of cloning and/or modification of scorpion toxin genes.

Recent contributions in the field of the recombinant expression of disulfide bonded proteins in bacteria

The production of heterologous disulfide bonded proteins in bacteria remains a biotechnological challenge. A rapid literature survey results in the identification of some interesting proposals, such as the option of producing functional proteins in the cytoplasm in the presence of sulfhydryl oxidases and isomerases. Furthermore, an ever-increasing number of applications refers to recombinant proteins displayed at the bacterial surface. Time will tell whether these developments will lead to universally accepted laboratory protocols.

Cell-free expression of disulfide-containing eukaryotic proteins for structural biology

We describe Escherichia coli based cell-free production of milligram quantities of eukaryotic proteins containing native disulfide bonds. Using a previously described expression system, we systematically investigated the influence of redox potential variation in the reaction mixture and the impact of adding disulfide bond catalysts on soluble protein production. It is then shown that the optimized reaction conditions for native disulfide bond formation can be combined with the use of N-terminal fusion constructs with the GB1 domain for increased expression yields. The resulting cell-free system is suitable for stable-isotope labeling and does not require chemical pretreatment of the cell extract to stabilize the redox potential. For the human doppel protein, the mouse doppel protein and mouse interleukin-22 we obtained 0.3-0.7 mg of purified native protein per milliliter of reaction mixture. Formation of disulfide bonds was validated using the Ellman assay, and native folding of the three proteins was monitored by NMR and CD spectroscopy.

Evaluation of the immunogenicity and vaccine potential of recombinant Plasmodium falciparum merozoite surface protein 8

The C-terminal 19-kDa domain of merozoite surface protein 1 (MSP1₁₉) is the target of protective antibodies but alone is poorly immunogenic. Previously, using the Plasmodium yoelii murine model, we fused P. yoelii MSP1₁₉ (PyMSP1₁₉) with full-length P. yoelii merozoite surface protein 8 (MSP8). Upon immunization, the MSP8-restricted T cell response provided help for the production of high and sustained levels of protective PyMSP1₁₉- and PyMSP8-specific antibodies. Here, we assessed the vaccine potential of MSP8 of the human malaria parasite, Plasmodium falciparum. Distinct from PyMSP8, P. falciparum MSP8 (PfMSP8) contains an N-terminal asparagine and aspartic acid (Asn/Asp)-rich domain whose function is unknown. Comparative analysis of recombinant full-length PfMSP8 and a truncated version devoid of the Asn/Asp-rich domain, PfMSP8(ΔAsn/Asp), showed that both proteins were immunogenic for T cells and B cells. All T cell epitopes utilized mapped within rPfMSP8(ΔAsn/Asp). The dominant B cell epitopes were conformational and common to both rPfMSP8 and rPfMSP8(ΔAsn/Asp). Analysis of native PfMSP8 expression revealed that PfMSP8 is present intracellularly in late schizonts and merozoites. Following invasion, PfMSP8 is found distributed on the surface of ring- and trophozoite-stage parasites. Consistent with a low and/or transient expression of PfMSP8 on the surface of merozoites, PfMSP8-specific rabbit IgG did not inhibit the in vitro growth of P. falciparum blood-stage parasites. These studies suggest that the further development of PfMSP8 as a malaria vaccine component should focus on the use of PfMSP8(ΔAsn/Asp) and its conserved, immunogenic T cell epitopes as a fusion partner for protective domains of poor immunogens, including PfMSP1₁₉.

Disulfide bond formation and activation of Escherichia coli β-galactosidase under oxidizing conditions

Escherichia coli β-galactosidase is probably the most widely used reporter enzyme in molecular biology, cell biology, and biotechnology because of the easy detection of its activity. Its large size and tetrameric structure make this bacterial protein an interesting model for crystallographic studies and atomic mapping. In the present study, we investigate a version of Escherichia coli β-galactosidase produced under oxidizing conditions, in the cytoplasm of an Origami strain. Our data prove the activation of this microbial enzyme under oxidizing conditions and clearly show the occurrence of a disulfide bond in the β-galactosidase structure. Additionally, the formation of this disulfide bond is supported by the analysis of a homology model of the protein that indicates that two cysteines located in the vicinity of the catalytic center are sufficiently close for disulfide bond formation.

A novel expression system for production of soluble prion proteins in E. coli

Expression of eukaryotic proteins in Escherichia coli is challenging, especially when they contain disulfide bonds. Since the discovery of the prion protein (PrP) and its role in transmissible spongiform encephalopathies, the need to obtain large quantities of the recombinant protein for research purposes has been essential. Currently, production of recombinant PrP is achieved by refolding protocols. Here, we show that the co-expression of two different PrP with the human Quiescin Sulfhydryl OXidase (QSOX), a human chaperone with thiol/disulfide oxidase activity, in the cytoplasm of E. coli produces soluble recombinant PrP. The structural integrity of the soluble PrP has been confirmed by nuclear magnetic resonance spectroscopy, demonstrating that properly folded PrP can be easily expressed in bacteria. Furthermore, the soluble recombinant PrP produced with this method can be used for functional and structural studies.

Antigen presentation for the generation of binding molecules

In the last few decades, several new methods have been established to isolate full antibodies and fragments thereof, some even using alternative scaffolds from in vivo and in vitro sources. These methods encompass robust techniques including immunization and hybridoma technology or phage display and also more laborious and novel approaches including ribosome display or B-cell immortalization. All methodologies are dependent upon proper antigen presentation for isolation, screening, and further characterization of the selected binding molecules. Here, antigens are classes of molecules including soluble or membrane proteins, part or domains thereof (extracellular domains of GPCRs), peptides, carbohydrates, and small-molecular-weight moieties. Presentation of the antigen in a functional state or perhaps even mimicking the intended application is crucial for successful isolation of useful binding molecules. Moreover, it is also necessary to consider the expression host and any posttranslational modifications of target proteins. The increasing demand to target more complex antigens, for instance, receptors and ion channels, is leading to the development of alternative procedures to present these proteins appropriately, for example by the use of virus-like particles and DNA immunization. This chapter describes in general approaches for the preparation of different forms of immunogens including synthetic peptides, proteins, cell-based antigens for immunization and in vitro display systems and in detail the preparation of a soluble protein as antigen.

User-friendly expression plasmids enable the fusion of VHHs to application-specific tags

One of the advantages of using recombinant instead of conventional antibodies is that these can be easily manipulated by means of standard molecular biology techniques. Therefore this opportunity can be exploited to prepare fusion constructs composed of VHHs and suitable tags. According to the applications in which the antibodies will be applied, molecules such as fluorescent probes, biotin, and PEG can be either covalently or non-covalently linked to the antibodies. Within this chapter, practical tips for the choice and expression of the most appropriate among the available plasmids are listed, keeping in mind the experimental conditions in which usually the fusion antibodies will be applied.

Using folding promoting agents in recombinant protein production: a review

Recombinant production has become an invaluable tool for supplying research and therapy with proteins of interest. The target proteins are not in every case soluble and/or correctly folded. That is why different production parameters such as host, cultivation conditions and co-expression of chaperones and foldases are applied in order to yield functional recombinant protein. There has been a constant increase and success in the use of folding promoting agents in recombinant protein production. Recent cases are reviewed and discussed in this chapter. Any impact of such strategies cannot be predicted and has to be analyzed and optimized for the corresponding target protein. The in vivo effects of the agents are at least partially comparable to their in vitro mode of action and have been studied by means of modern systems approaches and even in combination with folding/activity screening assays. Resulting data can be used directly for experimental planning or can be fed into knowledge-based modelling. An overview of such technologies is included in the chapter in order to facilitate a decision about the potential in vivo use of folding promoting agents.

Human platelet concentrates: a source of solvent/detergent-treated highly enriched brain-derived neurotrophic factor

BACKGROUND

Human blood platelets (PLTs) contain brain-derived neurotrophic factor (BDNF), a neurotrophin that binds to neurotrophic tropomyosin-related kinase B (TrkB) receptor on central nervous system cells. This binding promotes neural synaptic plasticity and memory and prevents neuronal degeneration. Alterations in BDNF homeostasis are associated with aging and are found in several neurodegenerative conditions such as Alzheimer's, Huntington's, and Parkinson's diseases and multiple sclerosis. We have developed PLT viral inactivation and chromatographic fractionation processes and decided here to identify fractions enriched in BDNF.

STUDY DESIGN AND METHODS

PLT concentrates (PCs) were treated by solvent/detergent (S/D), extracted by oil, and subjected to fractionation (C18, sulfopropyl [SP]-Sepharose, diethylaminoethyl [DEAE]-Sepharose, or activated charcoal). BDNF and pro-BDNF were evaluated by enzyme-linked immunosorbent assay, and Western blot. TrkB was studied by Western blot. Tri-n-butyl phosphate (TnBP) was quantified by high-performance liquid chromatography, and Triton X-45 by gas chromatography.

RESULTS

The mean BDNF content of 2.9 ± 0.7 ng/mL in PC was noted to increase to 56.2 ± 2.4 ng/mL after S/D treatment and remained stable during oil extraction. Approximately 70% of the BDNF content was recovered after C18 chromatography. BDNF did not bind to DEAE-Sepharose and was almost completely adsorbed by charcoal. Chromatography on SP-Sepharose yielded a highly enriched 13-kDa mature BDNF fraction that was more than 170-fold purified, with a mean of 137 ± 29.4 ng/mL and 82% chromatographic recovery, devoid of detectable TnBP and Triton X-45. Pro-BDNF and TrkB proteins were not detected in the PLT extracts.

CONCLUSION

We obtained a S/D-treated, highly enriched mature PLT-derived BDNF fraction that could help unveil the pharmacokinetics, pharmacodynamic, and potential therapeutic applications of the BDNF neurotrophin.

Increasing recombinant protein production in Escherichia coli through metabolic and genetic engineering

Different hosts have been used for recombinant protein production, ranging from simple bacteria, such as Escherichia coli and Bacillus subtilis, to more advanced eukaryotes as Saccharomyces cerevisiae and Pichia pastoris, to very complex insect and animal cells. All have their advantages and drawbacks and not one seems to be the perfect host for all purposes. In this review we compare the characteristics of all hosts used in commercial applications of recombinant protein production, both in the area of biopharmaceuticals and industrial enzymes. Although the bacterium E. coli remains a very often used organism, several drawbacks limit its possibility to be the first-choice host. Furthermore, we show what E. coli strains are typically used in high cell density cultivations and compare their genetic and physiological differences. In addition, we summarize the research efforts that have been done to improve yields of heterologous protein in E. coli, to reduce acetate formation, to secrete the recombinant protein into the periplasm or extracellular milieu, and to perform post-translational modifications. We conclude that great progress has been made in the incorporation of eukaryotic features into E. coli, which might allow the bacterium to regain its first-choice status, on the condition that these research efforts continue to gain momentum.

Post-production protein stability: trouble beyond the cell factory

Being protein function a conformation-dependent issue, avoiding aggregation during production is a major challenge in biotechnological processes, what is often successfully addressed by convenient upstream, midstream or downstream approaches. Even when obtained in soluble forms, proteins tend to aggregate, especially if stored and manipulated at high concentrations, as is the case of protein drugs for human therapy. Post-production protein aggregation is then a major concern in the pharmaceutical industry, as protein stability, pharmacokinetics, bioavailability, immunogenicity and side effects are largely dependent on the extent of aggregates formation. Apart from acting at the formulation level, the recombinant nature of protein drugs allows intervening at upstream stages through protein engineering, to produce analogue protein versions with higher stability and enhanced therapeutic values.

Biotechnological applications of recombinant single-domain antibody fragments

BACKGROUND

Single-domain antibody fragments possess structural features, such as a small dimension, an elevated stability, and the singularity of recognizing epitopes non-accessible for conventional antibodies that make them interesting for several research and biotechnological applications.

RESULTS

The discovery of the single-domain antibody's potentials has stimulated their use in an increasing variety of fields. The rapid accumulation of articles describing new applications and further developments of established approaches has made it, therefore, necessary to update the previous reviews with a new and more complete summary of the topic.

CONCLUSIONS

Beside the necessary task of updating, this work analyses in detail some applicative aspects of the single-domain antibodies that have been overseen in the past, such as their efficacy in affinity chromatography, as co-crystallization chaperones, protein aggregation controllers, enzyme activity tuners, and the specificities of the unconventional single-domain fragments.

Strain engineering for improved expression of recombinant proteins in bacteria

Protein expression in Escherichia coli represents the most facile approach for the preparation of non-glycosylated proteins for analytical and preparative purposes. So far, the optimization of recombinant expression has largely remained a matter of trial and error and has relied upon varying parameters, such as expression vector, media composition, growth temperature and chaperone co-expression. Recently several new approaches for the genome-scale engineering of E. coli to enhance recombinant protein expression have been developed. These methodologies now enable the generation of optimized E. coli expression strains in a manner analogous to metabolic engineering for the synthesis of low-molecular-weight compounds. In this review, we provide an overview of strain engineering approaches useful for enhancing the expression of hard-to-produce proteins, including heterologous membrane proteins.

Tuning different expression parameters to achieve soluble recombinant proteins in E. coli: advantages of high-throughput screening

Proteins are the main reagents for structural, biomedical, and biotechnological studies; however, some important challenges remain concerning protein solubility and stability. Numerous strategies have been developed, with some success, to mitigate these challenges, but a universal strategy is still elusive. Currently, researchers face a plethora of alternatives for the expression of the target protein, which generates a great diversity of conditions to be evaluated. Among these, different promoter strength, diverse expression host and constructs, or special culture conditions have an important role in protein solubility. With the arrival of automated high-throughput screening (HTS) systems, the evaluation of hundreds of different conditions within reasonable cost and time limits is possible. This technology increases the chances to obtain the target protein in a pure, soluble, and stable state. This review focuses on some of the most commonly used strategies for the expression of recombinant proteins in the enterobacterium Escherichia coli, including the use of HTS for the production of soluble proteins.

An approach to the production of soluble protein from a fungal gene encoding an aggregation-prone xylanase in Escherichia coli

The development of new procedures and protocols that allow researchers to obtain recombinant proteins is of fundamental importance in the biotechnology field. A strategy was explored to overcome inclusion-body formation observed when expressing an aggregation-prone fungal xylanase in Escherichia coli. pHsh is an expression plasmid that uses a synthetic heat-shock (Hsh) promoter, in which gene expression is regulated by an alternative sigma factor (σ(32)). A derivative of pHsh was constructed by fusing a signal peptide to xynA2 gene to facilitate export of the recombinant protein to the periplasm. The xylanase was produced in a soluble form. Three factors were essential to achieving such soluble expression of the xylanase: 1) the target gene was under the control of the Hsh promoter, 2) the gene product was exported into the periplasm, and 3) gene expression was induced by a temperature upshift. For the first time we report the expression of periplasmic proteins under the control of an Hsh promoter regulated by σ(32). One unique feature of this approach was that over 200 copies of the Hsh promoter in an E. coli cell significantly increased the concentration of σ(32). The growth inhibition of the recombinant cells corresponded to an increase in the levels of soluble periplasmic protein. Therefore, an alternative protocol was designed to induce gene expression from pHsh-ex to obtain high levels of active soluble enzymes.

Pre-expression of a sulfhydryl oxidase significantly increases the yields of eukaryotic disulfide bond containing proteins expressed in the cytoplasm of E.coli

Background

Disulfide bonds are one of the most common post-translational modifications found in proteins. The production of proteins that contain native disulfide bonds is challenging, especially on a large scale. Either the protein needs to be targeted to the endoplasmic reticulum in eukaryotes or to the prokaryotic periplasm. These compartments that are specialised for disulfide bond formation have an active catalyst for their formation, along with catalysts for isomerization to the native state. We have recently shown that it is possible to produce large amounts of prokaryotic disulfide bond containing proteins in the cytoplasm of wild-type bacteria such as E. coli by the introduction of catalysts for both of these processes.

Results

Here we show that the introduction of Erv1p, a sulfhydryl oxidase and a disulfide isomerase allows the efficient formation of natively folded eukaryotic proteins with multiple disulfide bonds in the cytoplasm of E. coli. The production of disulfide bonded proteins was also aided by the use of an appropriate fusion protein to keep the folding intermediates soluble and by choice of media. By combining the pre-expression of a sulfhydryl oxidase and a disulfide isomerase with these other factors, high level expression of even complex disulfide bonded eukaryotic proteins is possible

Conclusions

Our results show that the production of eukaryotic proteins with multiple disulfide bonds in the cytoplasm of E. coli is possible. The required exogenous components can be put onto a single plasmid vector allowing facile transfer between different prokaryotic strains. These results open up new avenues for the use of E. coli as a microbial cell factory.

Disruption of reducing pathways is not essential for efficient disulfide bond formation in the cytoplasm of E. coli

Background

The formation of native disulfide bonds is a complex and essential post-translational modification for many proteins. The large scale production of these proteins can be difficult and depends on targeting the protein to a compartment in which disulfide bond formation naturally occurs, usually the endoplasmic reticulum of eukaryotes or the periplasm of prokaryotes. It is currently thought to be impossible to produce large amounts of disulfide bond containing protein in the cytoplasm of wild-type bacteria such as E. coli due to the presence of multiple pathways for their reduction.

Results

Here we show that the introduction of Erv1p, a sulfhydryl oxidase and FAD-dependent catalyst of disulfide bond formation found in the inter membrane space of mitochondria, allows the efficient formation of native disulfide bonds in heterologously expressed proteins in the cytoplasm of E. coli even without the disruption of genes involved in disulfide bond reduction, for example trxB and/or gor. Indeed yields of active disulfide bonded proteins were higher in BL21 (DE3) pLysSRARE, an E. coli strain with the reducing pathways intact, than in the commercial Δgor ΔtrxB strain rosetta-gami upon co-expression of Erv1p.

Conclusions

Our results refute the current paradigm in the field that disruption of at least one of the reducing pathways is essential for the efficient production of disulfide bond containing proteins in the cytoplasm of E. coli and open up new possibilities for the use of E. coli as a microbial cell factory.

Strategies to optimize protein expression in E. coli

Recombinant protein expression in Escherichia coli (E. coli) is simple, fast, inexpensive, and robust, with the expressed protein comprising up to 50 percent of the total cellular protein. However, it also has disadvantages. For example, the rapidity of bacterial protein expression often results in unfolded/misfolded proteins, especially for heterologous proteins that require longer times and/or molecular chaperones to fold correctly. In addition, the highly reductive environment of the bacterial cytosol and the inability of E. coli to perform several eukaryotic post-translational modifications results in the insoluble expression of proteins that require these modifications for folding and activity. Fortunately, multiple, novel reagents and techniques have been developed that allow for the efficient, soluble production of a diverse range of heterologous proteins in E. coli. This overview describes variables at each stage of a protein expression experiment that can influence solubility and offers a summary of strategies used to optimize soluble expression in E. coli.

Nanotechnology, bionanotechnology and microbial cell factories

Nanotechnology is increasingly using both materials and nano-objects synthesized by living beings, most of them produced by microbial cells. Emerging technologies and highly integrative approaches (such as 'omics and systems biology), that have been largely proven successful for the production of proteins and secondary metabolites are now expected to become fully adapted for the improved biological production of nanostructured materials with tailored properties. The so far underestimated potential of microbial cell factories in nanotechnology and nanomedicine is expected to emerge, in the next years, in the context of novel needs envisaged in the nanoscience universe. This should prompt a careful revisiting of the microbial cell factories as the most versatile biological platforms to supply functional materials for nanotechnological applications.

Enhancing the secretory yields of leech carboxypeptidase inhibitor in Escherichia coli: influence of trigger factor and signal recognition particle

The signal recognition particle (SRP) dependent secretion pathway is as an attractive alternative to Sec-dependent export for the production of disulfide-bonded and/or fast-folding recombinant proteins in the Escherichia coli periplasm. SRP, which shares a ribosomal attachment site with the molecular chaperone trigger factor (TF), recognizes highly hydrophobic signal sequence as they emerge from the ribosome and delivers ribosome nascent chain complexes to FtsY for subsequent cotranslational translocation of target proteins across the SecYEG pore. However, like in the case of Sec-dependent export, secretory yields can be limited by the accumulation of precursor proteins in the cytoplasm. Using leech carboxypeptidase inhibitor (LCI) fused to the SRP-dependent DsbA signal sequence as a model system, we show that a null mutation in the gene encoding TF (Deltatig) or SRP co-expression reduce pre-LCI accumulation by half, and that quantitative export can be achieved by combining the two strategies. Interestingly, enhanced precursor processing did not alter periplasmic LCI levels but increased the amount of protein excreted in the growth medium. All mature LCI was nearly fully active and an 80% increase in productivity was achieved in Deltatig cells alone due to their faster growth. Our results show that competition between SRP and TF can interfere with efficient export of recombinant proteins targeted to the SRP pathway and establish TF-deficient strains and SRP co-expression as a simple solution to improve yields.