Cosecretion of chaperones and low-molecular-size medium additives increases the yield of recombinant disulfide-bridged proteins

Escherichia coli as a versatile cell factory: Advances and challenges in recombinant protein production

E. coli plays a substantial role in recombinant protein production. Its importance increased with the discovery of recombinant DNA technology and the subsequent production of the first recombinant insulin in E. coli. E. coli is a widely used and cost-effective host to produce recombinant proteins. It is also noteworthy that a significant portion of the approved therapeutic proteins have been produced in this organism. Despite these advantages, it has some disadvantages, such as toxicity and lack of eukaryotic post-translational modifications that can lead to the production of misfolded, insoluble, or dysfunctional proteins. This study focused on the challenges and engineering approaches for improved expression and solubility in recombinant protein production in E. coli. In this context, solution strategies such as strain and vector selection, codon usage, mRNA stability, expression conditions, translocation to the periplasmic region and addition of fusion tags in E. coli were discussed.

Recombinant protein expression: Challenges in production and folding related matters

Protein folding is a biophysical process by which proteins reach a specific three-dimensional structure. The amino acid sequence of a polypeptide chain contains all the information needed to determine the final three-dimensional structure of a protein. When producing a recombinant protein, several problems can occur, including proteolysis, incorrect folding, formation of inclusion bodies, or protein aggregation, whereby the protein loses its natural structure. To overcome such limitations, several strategies have been developed to address each specific issue. Identification of proper protein refolding conditions can be challenging, and to tackle this high throughput screening for different recombinant protein folding conditions can prove a sound solution. Different approaches have emerged to tackle refolding issues. One particular approach to address folding issues involves molecular chaperones, highly conserved proteins that contribute to proper folding by shielding folding proteins from other proteins that could hinder the process. Proper protein folding is one of the main prerequisites for post-translational modifications. Incorrect folding, if not dealt with, can lead to a buildup of protein misfoldings that damage cells and cause widespread abnormalities. Said post-translational modifications, widespread in eukaryotes, are critical for protein structure, function and biological activity. Incorrect post-translational protein modifications may lead to individual consequences or aggregation of therapeutic proteins. In this review article, we have tried to examine some key aspects of recombinant protein expression. Accordingly, the relevance of these proteins is highlighted, major problems related to the production of recombinant protein and to refolding issues are pinpointed and suggested solutions are presented. An overview of post-translational modification, their biological significance and methods of identification are also provided. Overall, the work is expected to illustrate challenges in recombinant protein expression.

Role of molecular and chemical chaperon in assisting refolding of BMP2 in E. coli

Bone morphogenetic protein 2 (BMP2) when expressed in bacteria forms inclusion bodies (IBs) due to its complex disulfide-rich structure. Chaperons are already well known for their role in assisting protein folding. In our studies, we have used two E. coli strains, BL21(DE3) and SHuffle® T7 cells for overexpressing BMP2 in soluble fraction. We observed that SHuffle® T7 cells successfully expressed soluble functionally active BMP2 in presence of molecular and chemical chaperones at low temperature. The combination of chemical and molecular chaperons further increases the yield of protein. The best-suited chaperon system for overexpression of BMP2 is GroES-GroEL at low temperature. The soluble functionally active BMP2 is confirmed by its binding to its receptor ALK3 through Native PAGE and ELISA assay using BMP2 specific antibody. It is possible to obtain BMP2 expression in soluble active form by using molecular and chemical chaperons which work synergistically in bacterial cells to fold disulphide-rich proteins at low temperature in easy and time saving steps (18 ̊C).

Interaction of Periplasmic Fab Production and Intracellular Redox Balance in Escherichia coli Affects Product Yield

Antibody fragments such as Fab's require the formation of disulfide bonds to achieve a proper folding state. During their recombinant, periplasmic expression in Escherichia coli, oxidative folding is mediated by the DsbA/DsbB system in concert with ubiquinone. Thereby, overexpression of Fab's is linked to the respiratory chain, which is not only immensely important for the cell's energy household but also known as a major source of reactive oxygen species. However, the effects of an increased oxidative folding demand and the consequently required electron flux via ubiquinone on the host cell have not been characterized so far. Here, we show that Fab expression in E. coli BL21(DE3) interfered with the intracellular redox balance, thereby negatively impacting host cell performance. Production of four different model Fab's in lab-scale fed-batch cultivations led to increased oxygen consumption rates and strong cell lysis. An RNA sequencing analysis revealed transcription activation of the oxidative stress-responsive soxS gene in the Fab-producing strains. We attributed this to the accumulation of intracellular superoxide, which was measured using flow cytometry. An exogenously supplemented ubiquinone analogue improved Fab yields up to 82%, indicating that partitioning of the quinone pool between aerobic respiration and oxidative folding limited ubiquinone availability and hence disulfide bond formation capacity. Combined, our results provide a more in-depth understanding of the profound effects that periplasmic Fab expression and in particular disulfide bond formation has on the host cell. Thereby, we show new possibilities to elaborate cell engineering and process strategies for improved host cell fitness and process outcome.

Escherichia coli as an antibody expression host for the production of diagnostic proteins: significance and expression

This review article concerns the production of recombinant antibody fragments for applications mainly in the diagnostic sector. The so-called "point of care diagnostics" is very important for timely diagnosis and treatment, thus being able to save lives and resources. There is intense pressure for more accurate and less expensive rapid diagnostic tests, with a value preferably <$1. Thus, the large-scale cost-effective production of recombinant antibodies is vital. The importance of Escherichia coli toward the production of inexpensive rapid tests will be explained in this review paper. Details about the different strains of E. coli, the strategies used for the insertion and the expression of recombinant proteins, and the challenges that still exist are provided. Afterward, the importance of the expression scale and culture parameters in the final yield of the antibodies are examined. From this analysis, it appears that for good yields of recombinant antibodies, aside from appropriate gene transfer and expression, the culturing parameters are of paramount importance. Larger scale production is more favorable, mainly due to the higher cell densities that can be achieved. Yields of functional Fab fragments in the range of 10-20 mg/L are considered good in shake flasks, whereas in bioreactors can be up to 1-2 g/L. An amount of 10-500 mg of such antibody per million rapid tests is required. Despite the substantial importance of the production of the antibodies and their fragments, their downstream processing should be appropriately considered from the beginning for achieving the target value of the final rapid diagnostic tests.

Chaperones Promote Remarkable Solubilization of Salmonella enterica serovar Enteritidis Flagellin Expressed in Escherichia coli

BACKGROUND

Flagellin of Salmonella enterica serovar Enteritidis (SEF) stimulates immune responses to both itself and coapplied antigens. It is therefore used in vaccine development and immunotherapy. Removal of pathogenic S. enterica ser. Enteritidis from SEF production process is advantageous due to the process safety improvement. The protein solubility analysis using SDS-PAGE indicated that 53.49% of SEF expressed in Escherichia coli formed inclusion bodies. However, the protein recovery from inclusion bodies requires a complex process with a low yield.

OBJECTIVE

We thus aim to study possibility of enhancing SEF expression in E. coli in soluble form using chemical and molecular chaperones.

METHODS

Chemical chaperones including arginine, sorbitol, trehalose, sodium chloride and benzyl alcohol were used as cultivation medium additives during SEF expression. SEF solubilization by coexpression of molecular chaperones DnaK, DnaJ, and GrpE was also investigated.

RESULTS

All of the chemical chaperones were effective in improving SEF solubility. However, sorbitol showed the most profound effect. SEF solubilization by molecular chaperones was slightly better than that using sorbitol and this approach enhanced noticeably SEF soluble concentration and SEF solubility percentage to almost two folds and 96.37% respectively. Results of limited proteolysis assay and native PAGE indicated similar conformational states and proper folding for SEF obtained without using chaperones and for those obtained using sorbitol and the molecular chaperones. However, the molecular chaperones based system was less costly than the sorbitol based system.

CONCLUSION

The coexpression of molecular chaperones was then considered as the most appropriate approach for soluble SEF production. Therefore, SEF production for medical purposes is expected to be facilitated.

QbD Based Media Development for the Production of Fab Fragments in E. coli

Ranibizumab is a biotherapeutic Fab fragment used for the treatment of age-related macular degeneration and macular oedema. It is currently expressed in the gram-negative bacterium, Escherichia coli. However, low expression levels result in a high manufacturing cost. The protein expression can be increased by manipulating nutritional requirements (carbon source, nitrogen source, buffering agent), process parameters (pH, inducer concentration, agitation, temperature), and the genetic make-up of the producing strain. Further, understanding the impact of these factors on product quality is a requirement as per the principles of Quality by Design (QbD). In this paper, we examine the effect of various media components and process parameters on the expression level and quality of the biotherapeutic. First, risk analysis was performed to shortlist different media components based on the literature. Next, experiments were performed to screen these components. Eight components were identified for further investigation and were examined for their effect and interactions using a Fractional Factorial experimental design. Sucrose, biotin, and pantothenate were found to have the maximum effect during Fab production. Furthermore, cyanocobalamin glutathione and biotin-glutathione were the most significant interactions observed. Product identification was performed with Liquid Chromatography–Mass Spectrometry (LC-MS), the expression level was quantified using Bio-layer Interferometry, Reverse Phase-HPLC, and SDS-PAGE, and product quality were measured by RP-HPLC. Overall, a five-fold enhancement of the target protein titer was obtained (from 5 mg/L to 25 mg/L) using the screened medium components vis-a-vis the basal medium, thereby demonstrating the efficacy of the systematic approach purported by QbD.

Multivalent Display Using Hybrid Virus Nanoparticles

Many important biological interactions are multivalent and often sensitive to spatial organization. Nonenveloped viruses are a natural source of scaffolds for building multivalent ligands to probe these types of interactions which avoid complex synthetic schemes required for other types of scaffolds. The coat protein (CP) of bacteriophage Qβ can be fused to protein domains and coexpressed with the unfused CP to produce hybrid nanoparticles with high exterior loading of xenogenic protein domains. These hybrid nanoparticles are simple to produce in large quantity. Starting from cDNAs for the virus CP and a codon-optimized ligand domain of interest, bulk purification can be completed in as little as 3 weeks. Major phases of the work involve the cloning of cDNAs into plasmid vectors, test expressions for hybrid nanoparticle formation, and purification by selective precipitation and ultracentrifugation. For uncomplicated protein domains, laboratory culture yields as high as 50 mg/L and 30 protein domains per particle have been routinely achieved.

Optimizing antibody expression: The nuts and bolts

Antibodies are extensively utilized entities in biomedical research, and in the development of diagnostics and therapeutics. Many of these applications require high amounts of antibodies. However, meeting this ever-increasing demand of antibodies in the global market is one of the outstanding challenges. The need to maintain a balance between demand and supply of antibodies has led the researchers to discover better means and methods for optimizing their expression. These strategies aim to increase the volumetric productivity of the antibodies along with the reduction of associated manufacturing costs. Recent years have witnessed major advances in recombinant protein technology, owing to the introduction of novel cloning strategies, gene manipulation techniques, and an array of cell and vector engineering techniques, together with the progress in fermentation technologies. These innovations were also highly beneficial for antibody expression. Antibody expression depends upon the complex interplay of multiple factors that may require fine tuning at diverse levels to achieve maximum yields. However, each antibody is unique and requires individual consideration and customization for optimizing the associated expression parameters. This review provides a comprehensive overview of several state-of-the-art approaches, such as host selection, strain engineering, codon optimization, gene optimization, vector modification and process optimization that are deemed suitable for enhancing antibody expression.

Structural and Functional Characterization of Recombinant Interleukin-10 from Indian Major Carp Labeo rohita

Interleukin-10, an important regulator of both the innate and adaptive immune systems, is a multifunctional major cytokine. Though it is one of the major cytokines, IL-10 from the Indian major carp, Labeo rohita, has not yet been characterized. In the present study, we report large scale production and purification of biologically active recombinant IL-10 of L. rohita (rLrIL-10) using a heterologous expression system and its biophysical and functional characterization. High yield (~70 mg/L) of soluble rLrIL-10 was obtained at shake flask level. The rLrIL-10 was found to exist as a dimer. Far-UV CD spectroscopy showed presence of predominantly alpha helices. The tertiary structure of the purified rLrIL-10 was verified by fluorescence spectroscopy. Two-dimensional gel analysis revealed the presence of six isoforms of the rLrIL-10. The rLrIL-10 was biologically active and its administration significantly reduced serum proinflammatory cytokines, namely, interleukin 1β, TNFα, and IL-8, and augmented the NKEF transcript levels in spleen of L. rohita. Anti-inflammatory role of the rLrIL-10 was further established by inhibition of phagocytosis using NBT reduction assay in vitro. The data indicate that the dimeric alpha helical structure and function of IL-10 of L. rohita as a key regulator of anti-inflammatory response have remained conserved during evolution.

Production of recombinant proteins from Plasmodium falciparum in Escherichia coli

INTRODUCTION

The production of recombinant proteins is essential for the characterization and functional study of proteins from Plasmodium falciparum. However, the proteins of P. falciparum are among the most challenging to express, and when expression is achieved, the recombinant proteins usually fold incorrectly and lead to the formation of inclusion bodies. 

OBJECTIVE

To obtain and purify four recombinant proteins and to use them as antigens to produce polyclonal antibodies. The production efficiency and solubility were evaluated as the proteins were expressed in two genetically modified strains of Escherichia coli to favor the production of heterologous proteins (BL21-CodonPlus (DE3)-RIL and BL21-pG-KJE8). 

MATERIALS AND METHODS

The four recombinant P. falciparum proteins corresponding to partial sequences of PfMyoA (Myosin A) and PfGAP50 (gliding associated protein 50), and the complete sequences of PfMTIP (myosin tail interacting protein) and PfGAP45 (gliding associated protein 45), were produced as glutathione S-transferase-fusion proteins, purified and used for immunizing mice. 

RESULTS

The protein expression was much more efficient in BL21-CodonPlus, the strain that contains tRNAs that are rare in wild-type E. coli, compared to the expression in BL21-pG-KJE8. In spite of the fact that BL21-pG-KJE8 overexpresses chaperones, this strain did not minimize the formation of inclusion bodies. 

CONCLUSION

The use of genetically modified strains of E. coli was essential to achieve high expression levels of the four evaluated P. falciparum proteins and lead to improved solubility of two of them. The approach used here allowed us to obtain and purify four P. falciparum proteins in enough quantity to produce polyclonal antibodies in mice, and a fair amount of two pure and soluble recombinant proteins for future assays.

Cellular disulfide bond formation in bioactive peptides and proteins

Bioactive peptides play important roles in metabolic regulation and modulation and many are used as therapeutics. These peptides often possess disulfide bonds, which are important for their structure, function and stability. A systematic network of enzymes--a disulfide bond generating enzyme, a disulfide bond donor enzyme and a redox cofactor--that function inside the cell dictates the formation and maintenance of disulfide bonds. The main pathways that catalyze disulfide bond formation in peptides and proteins in prokaryotes and eukaryotes are remarkably similar and share several mechanistic features. This review summarizes the formation of disulfide bonds in peptides and proteins by cellular and recombinant machinery.

Effect of culture medium, host strain and oxygen transfer on recombinant Fab antibody fragment yield and leakage to medium in shaken E. coli cultures

Background

Fab antibody fragments in E. coli are usually directed to the oxidizing periplasmic space for correct folding. From periplasm Fab fragments may further leak into extracellular medium. Information on the cultivation parameters affecting this leakage is scarce, and the unpredictable nature of Fab leakage is problematic regarding consistent product recovery. To elucidate the effects of cultivation conditions, we investigated Fab expression and accumulation into either periplasm or medium in E. coli K-12 and E. coli BL21 when grown in different types of media and under different aeration conditions.

Results

Small-scale Fab expression demonstrated significant differences in yield and ratio of periplasmic to extracellular Fab between different culture media and host strains. Expression in a medium with fed-batch-like glucose feeding provided highest total and extracellular yields in both strains. Unexpectedly, cultivation in baffled shake flasks at 150 rpm shaking speed resulted in higher yield and accumulation of Fabs into culture medium as compared to cultivation at 250 rpm. In the fed-batch medium, extracellular fraction in E. coli K-12 increased from 2-17% of total Fab at 250 rpm up to 75% at 150 rpm. This was partly due to increased lysis, but also leakage from intact cells increased at the lower shaking speed. Total Fab yield in E. coli BL21 in glycerol-based autoinduction medium was 5 to 9-fold higher at the lower shaking speed, and the extracellular fraction increased from ≤ 10% to 20-90%. The effect of aeration on Fab localization was reproduced in multiwell plate by variation of culture volume.

Conclusions

Yield and leakage of Fab fragments are dependent on expression strain, culture medium, aeration rate, and the combination of these parameters. Maximum productivity in fed-batch-like conditions and in autoinduction medium is achieved under sufficiently oxygen-limited conditions, and lower aeration also promotes increased Fab accumulation into extracellular medium. These findings have practical implications for screening applications and small-scale Fab production, and highlight the importance of maintaining consistent aeration conditions during scale-up to avoid changes in product yield and localization. On the other hand, the dependency of Fab leakage on cultivation conditions provides a practical way to manipulate Fab localization.

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.

Reducing conditions are the key for efficient production of active ribonuclease inhibitor in Escherichia coli

Background

The eukaryotic RNase ribonuclease/angiogenin inhibitors (RI) are a protein group distinguished by a unique structure - they are composed of hydrophobic leucine-rich repeat motifs (LRR) and contain a high amount of reduced cysteine residues. The members of this group are difficult to produce in E. coli and other recombinant hosts due to their high aggregation tendency.

Results

In this work dithiothreitol (DTT) was successfully applied for improving the yield of correctly folded ribonuclease/angiogenin inhibitor in E. coli K12 periplasmic and cytoplasmic compartments. The feasibility of the in vivo folding concepts for cytoplasmic and periplasmic production were demonstrated at batch and fed-batch cultivation modes in shake flasks and at the bioreactor scale.

Firstly, the best secretion conditions of RI in the periplasmic space were evaluated by using a high throughput multifactorial screening approach of a vector library, directly with the Enbase fed-batch production mode in 96-well plates. Secondly, the effect of the redox environment was evaluated in isogenic dsbA⁺ and dsbA^- strains at the various cultivation conditions with reducing agents in the cultivation medium. Despite the fusion to the signal peptide, highest activities were found in the cytoplasmic fraction. Thus by removing the signal peptide the positive effect of the reducing agent DTT was clearly proven also for the cytoplasmic compartment. Finally, optimal periplasmic and cytoplasmic RI fed-batch production processes involving externally added DTT were developed in shake flasks and scaled up to the bioreactor scale.

Conclusions

DTT highly improved both, periplasmic and cytoplasmic accumulation and activity of RI at low synthesis rate, i.e. in constructs harbouring weak recombinant synthesis rate stipulating genetic elements together with cultivation at low temperature. In a stirred bioreactor environment RI folding was strongly improved by repeated pulse addition of DTT at low aeration conditions.

Expression of active recombinant human tissue-type plasminogen activator by using in vivo polyhydroxybutyrate granule display

Recombinant human tissue plasminogen activator (rPA) is a truncated version of tissue plasminogen activator (tPA), which contains nine disulfide bonds and is prone to forming inactive inclusion bodies when expressed in bacteria. To obtain functional rPA expression, we displayed the rPA on the surface of polyhydroxybutyrate (PHB) granules using phasin as the affinity tag. rPA was fused to the N terminus of the phasin protein with a thrombin cleavage site as the linker. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblot analysis showed that rPA fusion was successfully displayed on the surface of PHB granules. An activity assay indicated that the rPA fusion is active. The in vivo surface display strategy for functional rPA expression in Escherichia coli is distinct for its efficient folding and easier purification and may be expanded to the expression of other eukaryotic proteins with complex conformation.

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

Bacteria are simple and cost effective hosts for producing recombinant proteins. However, their physiological features may limit their use for obtaining in native form proteins of some specific structural classes, such as for instance polypeptides that undergo extensive post-translational modifications. To some extent, also the production of proteins that depending on disulfide bridges for their stability has been considered difficult in E. coli.

Both eukaryotic and prokaryotic organisms keep their cytoplasm reduced and, consequently, disulfide bond formation is impaired in this subcellular compartment. Disulfide bridges can stabilize protein structure and are often present in high abundance in secreted proteins. In eukaryotic cells such bonds are formed in the oxidizing environment of endoplasmic reticulum during the export process. Bacteria do not possess a similar specialized subcellular compartment, but they have both export systems and enzymatic activities aimed at the formation and at the quality control of disulfide bonds in the oxidizing periplasm.

This article reviews the available strategies for exploiting the physiological mechanisms of bactera to produce properly folded disulfide-bonded proteins.

Use of folding modulators to improve heterologous protein production in Escherichia coli

Despite the fundamental importance of E. coli in the manufacture of a wide range of biotechnological and biomedical products, extensive process and/or target optimisation is routinely required in order to achieve functional yields in excess of low mg/l levels. Molecular chaperones and folding catalysts appear to present a panacea for problems of heterologous protein folding in the organism, due largely to their broad substrate range compared with, e.g., protein-specific mutagenesis approaches. Painstaking investigation of chaperone overproduction has, however, met with mixed - and largely unpredictable - results to date. The past 5 years have nevertheless seen an explosion in interest in exploiting the native folding modulators of E. coli, and particularly cocktails thereof, driven largely by the availability of plasmid systems that facilitate simultaneous, non-rational screening of multiple chaperones during recombinant protein expression. As interest in using E. coli to produce recombinant membrane proteins and even glycoproteins grows, approaches to reduce aggregation, delay host cell lysis and optimise expression of difficult-to-express recombinant proteins will become even more critical over the coming years. In this review, we critically evaluate the performance of molecular chaperones and folding catalysts native to E. coli in improving functional production of heterologous proteins in the bacterium and we discuss how they might best be exploited to provide increased amounts of correctly-folded, active protein for biochemical and biophysical studies.

Role of the chaperone DnaK in protein solubility and conformational quality in inclusion body-formingEscherichia colicells

Misfolding-prone proteins produced in bacteria usually fail to adopt their native conformation and aggregate. In cells producing folding-reluctant protein species, folding modulators are supposed to be limiting, a fact that enhances protein deposition. Therefore, coproducing DnaK or other main chaperones along with the target protein has been a common approach to gain solubility, although with very inconsistent and often discouraging results. In an attempt to understand the reason for this inconsistency, the impact of exogenous DnaK (encoded in an accompanying plasmid) on two protein features observed as indicators of protein quality, namely solubility and functionality, has been analysed here through the specific fluorescence emission of a reporter Green Fluorescent Protein (GFP). Intriguingly, GFP solubility is strongly dependent on its own yield but poorly affected by DnaK levels. On the contrary, the specific fluorescence of both soluble and insoluble GFP populations is simultaneously modulated by the availability of DnaK, with a profile that is clearly dissimilar to that shown by protein solubility. Therefore, solubility, not being coincident with the biological activity of the target protein, might not be a robust indicator of protein quality.

Periplasmic production of native human proinsulin as a fusion to E. coli ecotin

Native proinsulin belongs to the class of the difficult-to-express proteins in Escherichia coli. Problems mainly arise due to its small size, a high proteolytic decay, and the necessity to form a native disulfide pattern. In the present study, human proinsulin was produced in the periplasm of E. coli as a fusion to ecotin, which is a small periplasmic protein of 16 kDa encoded by the host, containing one disulfide bond. The fusion protein was secreted to the periplasm and native proinsulin was determined by ELISA. Cultivation parameters were studied in parallel batch mode fermentations using E. coli BL21(DE3)Gold as a host. After improvement of fed-batch high density fermentation conditions, 153 mg fusion protein corresponding to 51.5mg native proinsulin was obtained per L. Proteins were extracted from the periplasm by osmotic shock treatment. The fusion protein was purified in one step by ecotin affinity chromatography on immobilized trypsinogen. After thrombin cleavage of the fusion protein, the products were separated by Ni-NTA chromatography. Proinsulin was quantified by ELISA and characterized by mass spectrometry. To evaluate the influence of periplasmic proteases, the amount of ecotin-proinsulin was determined in E. coli BL21(DE3)Gold and in a periplasmic protease deficient strain, E. coli SF120.

Biotechnology applications of amino acids in protein purification and formulations

Amino acids are widely used in biotechnology applications. Since amino acids are natural compounds, they can be safely used in pharmaceutical applications, e.g., as a solvent additive for protein purification and as an excipient for protein formulations. At high concentrations, certain amino acids are found to raise intra-cellular osmotic pressure and adjust to the high salt concentrations of the surrounding medium. They are called "compatible solutes", since they do not affect macromolecular function. Not only are they needed to increase the osmotic pressure, they are known to increase the stability of the proteins. Sucrose, glycerol and certain amino acids were used to enhance the stability of unstable proteins after isolation from natural environments. The mechanism of the action of these protein-stabilizing amino acids is relatively well understood. On the contrary, arginine was accidentally discovered as a useful reagent for assisting in the refolding of recombinant proteins. This effect of arginine was ascribed to its ability to suppress aggregation of the proteins during refolding, thereby increasing refolding efficiency. By the same mechanism, arginine now finds much wider applications than previously anticipated in the research and development of proteins, in particular in pharmaceutical applications. For example, arginine solubilizes proteins from loose inclusion bodies, resulting in efficient production of active proteins. Arginine suppresses protein-protein interactions in solution and also non-specific adsorption to gel permeation chromatography columns. Arginine facilitates elution of bound proteins from various column resins, including Protein-A or dye affinity columns and hydrophobic interaction columns. This review covers various biotechnology applications of amino acids, in particular arginine.

Dual expression system suitable for high-throughput fluorescence-based screening and production of soluble proteins

Many studies that aim to characterize the proteome structurally or functionally require the production of pure protein in a high-throughput format. We have developed a fast and flexible integrated system for cloning, protein expression in Escherichia coli, solubility screening and purification that can be completely automated in a 96-well microplate format. We used recombination cloning in custom-designed vectors including (i) a (His)(6) tag-encoding sequence, (ii) a variable solubilizing partner gene, (iii) the DNA sequence corresponding to the TEV protease cleavage site, (iv) the gene (or DNA fragment) of interest, (v) a suppressible amber stop codon, and (vi) an S.tag peptide-encoding sequence. First, conditions of bacterial culture in microplates (250 microL) were optimized to obtain expression and solubility patterns identical to those obtained in a 1-L flask (100-mL culture). Such conditions enabled the screening of various parameters in addition to the fusion partners (E. coli strains, temperature, inducer...). Second, expression of fusion proteins in amber suppressor strains allowed quantification of soluble and insoluble proteins by fluorescence through the detection of the S.tag. This technique is faster and more sensitive than other commonly used methods (dot blots, Western blots, SDS-PAGE). The presence of the amber suppressor tRNA was shown to affect neither the expression pattern nor the solubility of the target proteins. Third, production of the most interesting soluble fusion proteins, as detected by our screening method, could be performed in nonsuppressor strains. After cleavage with the TEV protease, the target proteins were obtained in a native form with a unique additional N-terminal glycine.

Twin-arginine translocation of active human tissue plasminogen activator in Escherichia coli

When eukaryotic proteins with multiple disulfide bonds are expressed at high levels in Escherichia coli, the efficiency of thiol oxidation and isomerization is typically not sufficient to yield soluble products with native structures. Even when such proteins are secreted into the oxidizing periplasm or expressed in the cytoplasm of cells carrying mutations in the major intracellular disulfide bond reduction systems (e.g., trxB gor mutants), correct folding can be problematic unless a folding modulator is simultaneously coexpressed. In the present study we explored whether the bacterial twin-arginine translocation (Tat) pathway could serve as an alternative expression system for obtaining appreciable levels of recombinant proteins which exhibit complex patterns of disulfide bond formation, such as full-length human tissue plasminogen activator (tPA) (17 disulfides) and a truncated but enzymatically active version of tPA containing nine disulfides (vtPA). Remarkably, targeting of both tPA and vtPA to the Tat pathway resulted in active protein in the periplasmic space. We show here that export by the Tat translocator is dependent upon oxidative protein folding in the cytoplasm of trxB gor cells prior to transport. Whereas previous efforts to produce high levels of active tPA or vtPA in E. coli required coexpression of the disulfide bond isomerase DsbC, we observed that Tat-targeted vtPA and tPA reach a native conformation without thiol-disulfide oxidoreductase coexpression. These results demonstrate that the Tat system may have inherent and unexpected benefits compared with existing expression strategies, making it a viable alternative for biotechnology applications that hinge on protein expression and secretion.

Recombinant protein secretion in Escherichia coli

The secretory production of recombinant proteins by the Gram-negative bacterium Escherichia coli has several advantages over intracellular production as inclusion bodies. In most cases, targeting protein to the periplasmic space or to the culture medium facilitates downstream processing, folding, and in vivo stability, enabling the production of soluble and biologically active proteins at a reduced process cost. This review presents several strategies that can be used for recombinant protein secretion in E. coli and discusses their advantages and limitations depending on the characteristics of the target protein to be produced.

Evaluation of bottlenecks in proinsulin secretion by Escherichia coli

This work evaluates three potential bottlenecks in recombinant human proinsulin secretion by Escherichia coli: protein stability, secretion capacity and the effect of molecular size on secretion efficiency. A maximum secretion level of 7.2 mg g(-1) dry cell weight was obtained in the periplasm of E. coli JM109(DE3) host cells. This value probably represents an upper limit in the transport capacity of E. coli cells secreting ZZ-proinsulin and similar proteins with the protein A signal peptide. A selective deletion study was performed in the fusion partner and no effect of the molecular size (17-24 kDa) was detected on secretion efficiency. The protective effect against proteolysis provided by the ZZ domain was thoroughly demonstrated in the periplasm of E. coli and it was also shown that a single Z domain is able to provide the same protection level without compromising the downstream processing. The use of this shorter fusion partner enables a 1.6-fold increase in the recovery of the target protein after cleavage of the affinity handle.

Strategies for the recovery of active proteins through refolding of bacterial inclusion body proteins

Recent advances in generating active proteins through refolding of bacterial inclusion body proteins are summarized in conjunction with a short overview on inclusion body isolation and solubilization procedures. In particular, the pros and cons of well-established robust refolding techniques such as direct dilution as well as less common ones such as diafiltration or chromatographic processes including size exclusion chromatography, matrix- or affinity-based techniques and hydrophobic interaction chromatography are discussed. Moreover, the effect of physical variables (temperature and pressure) as well as the presence of buffer additives on the refolding process is elucidated. In particular, the impact of protein stabilizing or destabilizing low- and high-molecular weight additives as well as micellar and liposomal systems on protein refolding is illustrated. Also, techniques mimicking the principles encountered during in vivo folding such as processes based on natural and artificial chaperones and propeptide-assisted protein refolding are presented. Moreover, the special requirements for the generation of disulfide bonded proteins and the specific problems and solutions, which arise during process integration are discussed. Finally, the different strategies are examined regarding their applicability for large-scale production processes or high-throughput screening procedures.

Studies of single-chain antibody expression in quiescent Escherichia coli

Quiescent Escherichia coli cells are generated by overexpressing the Rcd transcript in an hns-205 mutant host. The resulting nongrowing, metabolically active cells were used here to express a single-chain antibody fragment (scFv) in shake flask and fermentor cultures. The expression system is based on two plasmids; one carries the product gene expressed from lambdaP(L) under the control of the cI857 temperature-sensitive repressor, while the second expresses Rcd from lambdaP(R). Shifting the culture from 30 to 42 degrees C induces Rcd expression and product expression simultaneously. Our scFv carried a PelB leader, and 90% of the protein was secreted into the culture supernatant. In a batch culture, the supernatant concentration of scFv in the quiescent-cell culture (optical density at 600 nm [OD(600)] of 3.5) was 37 mg x liter(-1), compared to a maximum of 13 mg x liter(-1) in the control culture (final OD(600) of 20). In a fed-batch fermentor culture, quiescent cells were held at an OD(600) of 20 for 24 h and the extracellular scFv concentration reached a maximum of 150 mg x liter(-1). A control culture with a similar feed reached an OD(600) of 80, but despite the higher density, the extracellular scFv concentration did not exceed 35 mg x liter(-1). Quiescent cells at an OD(600) of 50 exhibited a small decline in the specific product formation rate, but nevertheless, an extracellular scFv concentration of 160 mg x liter(-1) was achieved in 8 h. The rate of extracellular accumulation was 10-fold greater in the quiescent culture than in the control culture. This study demonstrates that it is possible to establish high-density quiescent E. coli cultures that are capable of efficient synthesis, folding, and export of proteins.

Ability of Lactococcus lactis to export viral capsid antigens: a crucial step for development of live vaccines

The food grade bacterium Lactococcus lactis is a potential vehicle for protein delivery in the gastrointestinal tract. As a model, we constructed lactococcal strains producing antigens of infectious bursal disease virus (IBDV). IBDV infects chickens and causes depletion of B-lymphoid cells in the bursa of Fabricius and subsequent immunosuppression, morbidity, or acute mortality. The two major IBDV antigens, i.e., VP2 and VP3, that form the viral capsid were expressed and targeted to the cytoplasm, the cell wall, or the extracellular compartment of L. lactis. Whereas VP3 was successfully targeted to the three compartments by the use of relevant expression and export vectors, VP2 was recalcitrant to export, thus confirming the difficulty of translocating naturally nonsecreted proteins across the bacterial membrane. This defect could be partly overcome by fusing VP2 to a naturally secreted protein (the staphylococcal nuclease Nuc) that carried VP2 through the membrane. Lactococcal strains producing Nuc-VP2 and VP3 in various bacterial compartments were administered orally to chickens. The chickens did not develop any detectable immune response against VP2 and VP3 but did exhibit an immune response against Nuc when Nuc-VP2 was anchored to the cell wall of lactococci.

The N-terminal stem region of bovine and human beta1,4-galactosyltransferase I increases the in vitro folding efficiency of their catalytic domain from inclusion bodies

Many recombinant proteins overexpressed in Escherichia coli are generally misfolded, which then aggregate and accumulate as inclusion bodies. The catalytic domain (CD) of bovine and human beta1,4-galactosyltransferase (beta4Gal-T), expressed in E. coli, it also accumulates as inclusion bodies. We studied the effect of the fusion of the stem region (SR), as an N-terminal extension of the catalytic domain, on the in vitro folding efficiencies of the inclusion bodies. The stem region fused to the catalytic domain (SRCD) increases the folding efficiency of recombinant protein with native fold compared to the protein that contains only the CD. During in vitro folding, also promotes considerably the solubility of the misfolded proteins, which do not bind to UDP-agarose columns and exhibit no galactosyltransferase activity. In contrast, the misfolded proteins that consist of only the CD are insoluble and precipitate out of solution. It is concluded that a protein domain that is produced in a soluble form does not guarantee the presence of the protein molecules in a properly folded and active form. The stem domain has a positive effect on the in vitro folding efficiency of the catalytic domain of both human and bovine beta4Gal-T1, suggesting that the stem region acts as a chaperone during protein folding. Furthermore, investigation of the folding conditions of the sulphonated inclusion bodies resulted in identifying a condition in which the presence of PEG-4000 and L-arginine, compared to their absence, increased the yields of native CD and SRCD 7- and 3-fold, respectively.

Optimal conditions for the expression of a single-chain antibody (scFv) gene in Pichia pastoris

A Pichia pastoris system was used to express a single-chain antibody (scFv) targeted against Mamestra configurata (bertha armyworm) serpins. To improve scFv production we examined parameters such as proteinase activity, temperature, cell density, osmotic stress, medium composition, pH, and reiterative induction. P. pastoris was found to express several proteases; however, adjustment of medium pH to limit their activity did not correlate with increased scFv recovery. Induction medium pH values of 6.5-8.0 were most conducive to scFv production, despite significant differences in cell growth rates. Increasing inoculum density limited growth potential but gave rise to higher levels of scFv production. Three factors, medium composition, pre-induction osmotic stress, and temperature, had the greatest effects on protein production. Supplementation of the induction medium with arganine, casamino acids, or EDTA increased scFv production several fold, as did cultivation under osmotic stress conditions during pre-induction biomass accumulation. Incubation at 15 versus 30 degrees C extended the period whereby cells were capable of producing scFv from 1 to 7 days. Under optimal conditions, yeast cultures yielded 25 mg/L of functional scFv and could be subject to five reiterative inductions.

Recombinant expression and in vitro folding of proinsulin are stimulated by the synthetic dithiol Vectrase-P

Recombinant production of native proinsulin in the periplasm of Escherichia coli is limited by formation of the correct disulfide bonds and inclusion body formation. These limitations can be overcome during in vitro folding of proinsulin by using a redox system and also protein disulfide isomerase. Here, we added a redox active substance, Vectrase-P, to the cultivation medium of E. coli cells producing proinsulin. We show that this synthetic dithiol partially mimicking the redox activity of protein disulfide isomerase provides an improved redox situation in the periplasm and, therefore, provides optimum conditions for folding of proinsulin in that cell compartment resulting in an increase in yield of 60%. The in vivo results were confirmed by analyzing in vitro folding of proinsulin in the presence of the dithiol Vectrase-P.