Engineering Escherichia coli to secrete heterologous gene products

Proteins adopt functionally active conformations after type III secretion

Background

Bacterial production of natively folded heterologous proteins by secretion to the extracellular space can improve protein production by simplifying purification and enabling continuous processing. In a typical bacterial protein production process, the protein of interest accumulates in the cytoplasm of the cell, requiring cellular lysis and extensive purification to separate the desired protein from other cellular constituents. The type III secretion system of Gram-negative bacteria is used to secrete proteins from the cytosol to the extracellular space in one step, but proteins must unfold during translocation, necessitating the folding of secreted proteins in the extracellular space for an efficient production process. We evaluated type III secretion as a protein production strategy by characterizing and quantifying the extent of correct folding after secretion.

Results

We probed correct folding by assaying the function after secretion of two enzymes—beta-lactamase and alkaline phosphatase—and one single-chain variable fragment of an antibody. Secreted proteins are correctly folded and functional after unfolding, secretion, and refolding in the extracellular space. Furthermore, structural and chemical features required for protein function, such as multimerization and disulfide bond formation, are evident in the secreted protein samples. Finally, the concentration of NaCl in the culture media affects the folding efficiency of secreted proteins in a protein-specific manner.

Conclusions

In the extracellular space, secreted proteins are able to fold to active conformations, which entails post-translational modifications including: folding, multimerization, acquisition of metal ion cofactors, and formation of disulfide bonds. Further, different proteins have different propensities to refold in the extracellular space and are sensitive to the chemical environment in the extracellular space. Our results reveal strategies to control the secretion and correct folding of diverse target proteins during bacterial cell culture.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0606-4) contains supplementary material, which is available to authorized users.

Use of Transcriptional Control to Increase Secretion of Heterologous Proteins in T3S Systems

Heterologous proteins can be produced in a bacterial host and purified from the cellular constituents. Secretion of the protein of interest to the extracellular space simplifies the purification process and is thought to alleviate toxicity problems associated with intracellular accumulation of the protein of interest. In this protocol, we describe a strategy to engineer protein secretion in a bacterial culture using transcriptional control. The transcription factor HilA is inducibly produced to control production of the secretion machine, and in turn signals the production and secretion of a protein of interest. This allows for high titer of secreted protein in optimized culturing conditions and the effect is observed with all proteins tested.

Using transcriptional control to increase titers of secreted heterologous proteins by the type III secretion system

The type III secretion system (T3SS) encoded at the Salmonella pathogenicity island 1 (SPI-1) locus secretes protein directly from the cytosol to the culture media in a concerted, one-step process, bypassing the periplasm. While this approach is attractive for heterologous protein production, product titers are too low for many applications. In addition, the expression of the SPI-1 gene cluster is subject to native regulation, which requires culturing conditions that are not ideal for high-density growth. We used transcriptional control to increase the amount of protein that is secreted into the extracellular space by the T3SS of Salmonella enterica. The controlled expression of the gene encoding SPI-1 transcription factor HilA circumvents the requirement of endogenous induction conditions and allows for synthetic induction of the secretion system. This strategy increases the number of cells that express SPI-1 genes, as measured by promoter activity. In addition, protein secretion titer is sensitive to the time of addition and the concentration of inducer for the protein to be secreted and SPI-1 gene cluster. Overexpression of hilA increases secreted protein titer by >10-fold and enables recovery of up to 28±9 mg/liter of secreted protein from an 8-h culture. We also demonstrate that the protein beta-lactamase is able to adopt an active conformation after secretion, and the increase in secreted titer from hilA overexpression also correlates to increased enzyme activity in the culture supernatant.

The rice OsGAE1 is a novel gibberellin-regulated gene and involved in rice growth

Gibberellins (GAs) are a class of phytohormones that regulate many aspects of plant growth and development processes including stem elongation, flowering, and seed germination. A novel GA-enhanced gene, designated as OsGAE1, was identified using microarray analysis of GA-regulated genes. OsGAE1 expressed in a dose- and time course-dependent manner with minimum expression at 1 microM GA(3) and maximum expression at 50 microM GA(3) starting from 30 min and peaked at 24 h after GA(3) treatment. OsGAE1 expression was up-regulated by GA(3) at transcript level while no significant effect was observed for other hormones. OsGAE1 was expressed in Escherichia coli with N-terminal His(6) tag and the recombinant protein migrated at 38 kDa, slightly larger than the predicted 29 kDa, during SDS-PAGE. Anti-OsGAE1 antibodies immunoreacted with a protein of 40 kDa in rice leaf sheath. OsGAE1 expressed mainly in growing leaf sheath and callus compared to leaf and root. In situ hybridization and OsGAE1 promoter analysis revealed that OsGAE1 expressed in shoot apex meristem and young primary leaves. Northern blot, Western blot, and GUS activities revealed that OsGAE1 is up-regulated by GA(3). Transgenic rice expressing OsGAE1 in antisense orientation exhibited severely affected vegetative and reproductive growth. The transgenic plants were 55-70% short compared to control. These results suggest that OsGAE1 is differentially expressed in rice leaf sheath in relation to GA(3) and it encodes a functional protein which is involved in GA-regulated growth and development of rice.

Select what you need: a comparative evaluation of the advantages and limitations of frequently used expression systems for foreign genes

The expression of heterologous proteins in microorganisms using genetic recombination is still the high point in the development and exploitation of modern biotechnology. People can produce bioactive proteins from relatively cheap culture medium instead of expensive extraction. Host cell systems for the expression of heterologous genes are generally prokaryotic or eukaryotic systems, both of which have inherent advantages and drawbacks. An optimal expression system can be selected only if the productivity, bioactivity, purpose, and physicochemical characteristics of the interest protein are taken into consideration, together with the cost, convenience and safety of the system itself. Here, we concisely review the most frequently used prokaryotic, yeast, insect and mammalian expression systems, as well as expression in eukaryote individuals. The merits and demerits of these systems are discussed.

Improved secretory production of recombinant proteins by random mutagenesis of hlyB, an alpha-hemolysin transporter from Escherichia coli

Fusion proteins with an alpha-hemolysin (HlyA) C-terminal signal sequence are known to be secreted by the HlyB-HlyD-TolC translocator in Escherichia coli. We aimed to establish an efficient Hly secretory expression system by random mutagenesis of hlyB and hlyD. The fusion protein of subtilisin E and the HlyA signal sequence (HlyA(218)) was used as a marker protein for evaluating secretion efficiency. Through screening of more than 1.5 x 10(4) E. coli JM109 transformants, whose hlyB and hlyD genes had been mutagenized by error-prone PCR, we succeeded in isolating two mutants that had 27- and 15-fold-higher levels of subtilisin E secretion activity than the wild type did at 23 degrees C. These mutants also exhibited increased activity levels for secretion of a single-chain antibody-HlyA(218) fusion protein at 23 and 30 degrees C but unexpectedly not at 37 degrees C, suggesting that this improvement seems to be dependent on low temperature. One mutant (AE104) was found to have seven point mutations in both HlyB and HlyD, and an L448F substitution in HlyB was responsible for the improved secretion activity. Another mutant (AE129) underwent a single amino acid substitution (G654S) in HlyB. Secretion of c-Myc-HlyA(218) was detected only in the L448F mutant (AE104F) at 23 degrees C, whereas no secretion was observed in the wild type at any temperature. Furthermore, for the PTEN-HlyA(218) fusion protein, AE104F showed a 10-fold-higher level of secretion activity than the wild type did at 37 degrees C. This result indicates that the improved secretion activity of AE104F is not always dependent on low temperature.

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.

Expression of a soluble and activatable form of bovine procarboxypeptidase A in Escherichia coli

Bovine pancreatic procarboxypeptidase A has been overexpressed in a soluble and activatable form in Escherichia coli. When the protein was expressed under the control of bacteriophage T7 promoter in E. coli ADA494 (a thioredoxin reductase deficient bacteria), a thioredoxin fusion protein was produced at relatively high level in the cytoplasm (4 mg/L culture medium). Although the recombinant protein essentially accumulated as inclusion bodies, as much as 30% of the fusion protein was recovered in a soluble form at low growth temperature and could therefore be purified to homogeneity in a single-step procedure by metal-affinity chromatography. The recombinant precursor form of bovine carboxypeptidase A was recognized by a monoclonal antibody directed against purified bovine pancreatic carboxypeptidase A. Moreover, upon tryptic activation it gave rise to an enzyme, the N-terminal sequence, molecular size,and specific activity of which were comparable to those of the enzyme derived from the native precursor purified from bovine pancreas.

Production of an anti-prostate-specific antigen single-chain antibody fragment from Pichia pastoris

Prostate-specific antigen (PSA) is a widely used marker for screening and monitoring prostate cancer. Because PSA levels are normally quite low, an antibody-based assay must be used to detect PSA. However, not all PSA-specific antibodies bind equally well to PSA or to its different isoforms. Therefore, a better understanding of how PSA interacts with PSA-specific antibodies is of considerable clinical interest. B80.3 is a widely used murine monoclonal anti-PSA antibody (IgG), which has very high affinity for both free and alpha-anti-chymotrypsin complexed PSA. More importantly, its gene sequence is known-making it one of only two anti-PSA antibodies that has been fully cloned and sequenced. To better elucidate the interaction between PSA and B80.3, a single-chain antibody fragment, derived from the variable domain of B80.3 (scFvB80), was cloned into a pPIC9 vector and expressed in Pichia pastoris. The secreted protein was purified using a three-step protocol beginning with a 50% ammonium sulfate precipitation step, followed by a T-gel thio-affinity step and concluding with a simple anion-exchange (DE52) filtration step. NMR studies indicate the protein is correctly folded while competitive enzyme-linked immunosorbant assays show that the purified scFvB80 has approximately 20% of the activity of the full-length B80.3 antibody. The protocol described here provides a quick and convenient route to prepare large quantities of very pure anti-PSA antibody fragments (15-20 mg/L culture medium) for detailed structural and biophysical characterization.

Expression of recombinant zeta-crystallin in Escherichia coli with the help of GroEL/ES and its purification

zeta-Crystallin is a taxon-specific crystallin found in the eye lens of guinea pig and other hystricomorph rodents and camelids. It is an NADPH:quinone oxidoreductase and is also present in low amounts in other tissues where it might act as a detoxifying enzyme. A lens-specific promoter confers lens-specific expression of the gene in high amounts where it is speculated to play a structural role in maintaining the transparency of the lens ensemble. A deletion mutation leads to autosomal dominant congenital cataract and also results in the loss of NADPH binding. In order to perform structural studies with the protein with an aim to delineate the cause of cataract in these mutant guinea pigs, recombinant zeta-crystallin was cloned and expressed in Escherichia coli. The overexpression of the protein in E. coli resulted in a major fraction of it partitioning into inclusion bodies. The co-overexpression of the bacterial chaperone system GroEL/ES along with zeta-crystallin could significantly enhance the yield of soluble protein. Active zeta-crystallin could then be purified from the E. coli using Mono Q anion exchange FPLC and was found to be identical to the native zeta-crystallin isolated from the guinea pig lens with respect to size, spectral properties, and activity.

Export and folding of signal-sequenceless Bacillus licheniformis beta-lactamase in Escherichia coli

Two genetically engineered variants of the Bacillus licheniformis beta-lactamase gene were expressed in Escherichia coli. One variant coded for the exo-small mature enzyme without the signal peptide. The other coded for the exo-large mature enzyme preceded by 10, mostly polar, residues from an incomplete heterologous signal. As observed following the extraction by a lysozyme-EDTA treatment, the signal-less variant was exported to the periplasm with nearly 20% efficiency, whereas the variant with the N-terminal extension was translocated to a lesser degree; interestingly, nearly all of the former and half of the latter were extracted by osmotic shock, which may be of importance for our understanding of cellular compartments. The fact that a signal-less protein is translocated with substantial yields raises questions about the essential role of signal peptides for protein export. As folding and export are related processes, we investigated the folding in vitro of the two variants. No differences were found between them. In the absence of denaturant, they are completely folded, fully active and have a large DeltaG of unfolding. Under partially denaturing conditions they populate several partially folded states. The absence of significant amounts of a non-native state under native conditions makes a thermodynamic partitioning between folding and export less likely. In addition, kinetic measurements indicated that these B. licheniformis lactamases fold much faster than E. coli beta-lactamase. This behavior suggests that they are exported by a kinetically controlled process, mediated by one or more still unidentified interactions that slow folding and allow a folding intermediate to enter the export pathway.

Amino-terminal truncation of procalcitonin, a marker for systemic bacterial infections, by dipeptidyl peptidase IV (DP IV)

Increased concentrations of procalcitonin (PCT) are found in the plasma of patients with thermal injury and in patients with sepsis and severe infection, making this molecule important as a diagnostic and prognostic marker in these diseases. Interestingly, only the truncated form of PCT, PCT(3-116), is present in the plasma of these patients. The enzyme responsible for this truncation is unknown as yet. Here, using capillary zone electrophoresis, mass spectrometry and Edman sequence analysis, we demonstrate that dipeptidyl peptidase IV (DP IV, EC 3.4.14.5) is capable of catalyzing the hydrolysis of PCT(1-116), releasing the N-terminal dipeptide Ala-Pro. We hypothesize that PCT(3-116) is the result of the hydrolysis of PCT(1-116) by soluble DP IV of the blood plasma or by DP IV expressed on the surface of cells.

Expression of chymotrypsin(ogen) in the thioredoxin reductase deficient mutant strain of Escherichia coli AD494(DE3) and purification via a fusion product with a hexahistidine-tail

A reliable protocol was designed for fast expression and purification of recombinant chymotrypsin(ogen). The zymogen was overexpressed in soluble form as a (His)6-fusion construct in the cytoplasm of the thioredoxin reductase deficient Escherichia coli strain AD494(DE3). This allowed purification of chymotrypsinogen in a highly selective affinity chromatography capture step using a Ni-NTA column. After activation with enterokinase, the enzymatically active chymotrypsin was purified in a polishing step using a modified soybean trypsin inhibitor agarose column. This expression system and the use of affinity chromatography for capture and polishing, offers an easier and faster route to recombinant chymotrypsin(ogen) than the previously described use of Saccharomyces cerevisiae.

Strategies for optimizing heterologous protein expression in Escherichia coli

The many advantages of Escherichia coli have ensured that it remains a valuable host for the efficient, cost-effective and high-level production of heterologous proteins. Here, we describe the current status of this prokaryotic expression system and focus on strategies designed to maximize the yields of recombinant proteins. Major challenges facing this expression system are also outlined.

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

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

Different consequences of incorporating chloroplast ribosomal proteins L12 and S18 into the bacterial ribosomes of Escherichia coli

We have incorporated chloroplast ribosomal proteins (R-proteins) L12 and S18 into Escherichia coli ribosomes and examined the hybrid ribosomes for their ability to form polysomes in vivo and perform poly(U)-dependent poly(Phe) synthesis in vitro. The rye chloroplast S18 used for the experiment is a highly divergent protein (170 amino acid residues; E. coil S18, 74 residues), containing a repeating, chloroplast-specific, heptapeptide motif, and has amino acid sequence identity of only 35% to E. coli S18. When expressed in E. coli, chloroplast S18 was assembled in E. coli ribosomes. The latter formed polysomes in vivo at about the same rate as the host ribosomes, indicating that the replacement of E. coli S18 with its chloroplast homologue has only a minor, if any, effect on function. The L12 protein is much more conserved in sequence and chain length, and is known to have a very important function. The Arabidopsis chloroplast L12 used in the experiment was incorporated into E. coli 50S subunits that associated with the 30S subunits to form ribosomes, but the latter were unable to form polysomes. This result indicates functional inactivation of E. coil ribosomes by a chloroplast R-protein. To further confirm this result, we overproduced chloroplast L12 through the use of a secretion vector and purified the protein to homogeneity. Chloroplast L12 could be efficiently incorporated in vitro into L7/12-lacking E. coli ribosomes, but the hybrid ribosomes were totally inactive in poly(U)-dependent poly(Phe) synthesis. Computer modeling of the spatial structure of all known chloroplast L12 proteins (using E. coli L12 coordinates) indicated a 'chloroplast loop' present only in chloroplast L12. The presence of this loop might have a role in the observed inactivation. Taken together with previously reported results (summarized in this paper), it would appear that the features of chloroplast R-proteins concerned with specific functions are more divergent than their assembly properties. We have previously described methods suitable for overproduction and purification of chloroplast R-proteins that are encoded in organellar DNA (approximately 20), but that gave poor yield for those encoded in the nuclear DNA (approximately 45). Here we describe a method that overcomes this problem and allows the purification of nucleus-encoded chloroplast R-proteins in milligram quantities.

Strategies for achieving high-level expression of genes in Escherichia coli

Progress in our understanding of several biological processes promises to broaden the usefulness of Escherichia coli as a tool for gene expression. There is an expanding choice of tightly regulated prokaryotic promoters suitable for achieving high-level gene expression. New host strains facilitate the formation of disulfide bonds in the reducing environment of the cytoplasm and offer higher protein yields by minimizing proteolytic degradation. Insights into the process of protein translocation across the bacterial membranes may eventually make it possible to achieve robust secretion of specific proteins into the culture medium. Studies involving molecular chaperones have shown that in specific cases, chaperones can be very effective for improved protein folding, solubility, and membrane transport. Negative results derived from such studies are also instructive in formulating different strategies. The remarkable increase in the availability of fusion partners offers a wide range of tools for improved protein folding, solubility, protection from proteases, yield, and secretion into the culture medium, as well as for detection and purification of recombinant proteins. Codon usage is known to present a potential impediment to high-level gene expression in E. coli. Although we still do not understand all the rules governing this phenomenon, it is apparent that "rare" codons, depending on their frequency and context, can have an adverse effect on protein levels. Usually, this problem can be alleviated by modification of the relevant codons or by coexpression of the cognate tRNA genes. Finally, the elucidation of specific determinants of protein degradation, a plethora of protease-deficient host strains, and methods to stabilize proteins afford new strategies to minimize proteolytic susceptibility of recombinant proteins in E. coli.

Bacteriocin release proteins: mode of action, structure, and biotechnological application

The mechanism by which Gram-negative bacteria like Escherichia coli secrete bacteriocins into the culture medium is unique and quite different from the mechanism by which other proteins are translocated across the two bacterial membranes, namely through the known branches of the general secretory pathway. The release of bacteriocins requires the expression and activity of a so-called bacteriocin release protein and the presence of the detergent-resistant phospholipase A in the outer membrane. The bacteriocin release proteins are highly expressed small lipoproteins which are synthesized with a signal peptide that remains stable and which accumulates in the cytoplasmic membrane after cleavage. The combined action of these stable, accumulated signal peptides, the lipid-modified mature bacteriocin release proteins (BRPs) and phospholipase A cause the release of bacteriocins. The structure and mode of action of these BRPs as well as their application in the release of heterologous proteins by E. coli is described in this review.

Physiological and biochemical analysis of the effects of alkaline phosphatase overproduction in Escherichia coli

Overexpression of the Escherichia coli phoA gene, coding for alkaline phosphatase (PhoA), on multicopy plasmids caused a severe defect in the precursor processing (secretion) of PhoA, beta-lactamase, and the outer membrane protein OmpA. This secretion defect continued even after the repression of phoA expression, indicating that protein secretion was irreversibly impaired in cells. Among the secretory proteins, only OmpA gradually secreted posttranslationally. The inverted inner membrane vesicles prepared from cells with the secretion defect showed appreciably reduced translocation activity in vitro. But the membrane vesicles retained the ability to generate a proton motive force which, together with ATP, is essential as an energy source for the efficient secretion of proteins in E. coli. An appreciable amount of incompletely translocated PhoA molecules was detected in the inner membranes of cells with the secretion defect.

Expression of the pheromone 3-encoding gene of Euplotes octocarinatus using a novel bacterial secretion vector

The pheromone 3-encoding gene (phr3) of Euplotes octocarinatus was expressed in Escherichia coli using a novel expression-secretion vector. The vector, pExSec1, contains a strong and tightly regulated T7 promoter, the corresponding Shine-Dalgarno sequence and the T7 terminator region. Translation starts at the protein A leader sequence followed by the synthetic ZZ sequence of protein A. The expression-secretion modules are embedded in the high-copy-number plasmid vector, pUK21, which carries a kanamycin-resistance marker (KmR). The produced ZZ-pheromone 3 (Phr3) fusion protein was secreted into the culture medium of the host cells. It was isolated by affinity chromatography and was further purified by gel filtration. After refolding with protein disulfide isomerase (PDI), the fusion protein exhibited the same high activity as the native pheromone.

Construction and characterization of a set of E. coli strains deficient in all known loci affecting the proteolytic stability of secreted recombinant proteins

Even though secretion offers numerous advantages for the production of proteins in Escherichia coli, the expression of many heterologous proteins is severely limited by degradation in the periplasmic space. We found that mutations in rpoH, the RNA polymerase sigma factor responsible for heat shock protein synthesis, affect the stability of heterologous secreted proteins. A particularly dramatic increase in expression was further observed in rpoH degP double mutants. To minimize proteolytic degradation, we constructed a family of 25 isogenic strains deficient in all known cell envelope proteases (DegP, Protease III, Tsp(Prc), and OmpT), as well as the rpoH15 mutant allele, and characterized their growth in both shake flasks and fermentors. The availability of this set of strains permits the selection of a suitable host based on the optimal combination between the optimum reduction in protease activity and acceptable growth properties.

Effect of modification of connecting peptide of proinsulin on its export

The production of a high yield of proinsulin using a secretion vector has been difficult, even with such modifications to the vector as a strong promoter and a good ribosome binding site. This investigation was therefore initiated to see whether modification of the connecting peptide of proinsulin has any effect on the export of proinsulin. We constructed three types of proinsulin secretion vectors: (a) pEZZ18-PI, by inserting the proinsulin gene into pEZZ18 vector; (b) pEZZ18-PI-C, by modifying ZZ-proinsulin by addition of the carboxy terminal peptide region of human insulin-like growth factor I (hIGFI) to the carboxy terminal end of proinsulin; and (c) pEZZ18-PI analogues, by sequentially deleting the connecting peptide region of proinsulin. The highest export yield of proinsulin was obtained when the connecting peptide region of the proinsulin was similar in size to that of hIGFI, or when most of the connecting peptide region of the proinsulin was deleted. The amount of exported ZZ-proinsulin analogues in these clones was over 25-times higher than that of ZZ-proinsulin with an unmodified connecting peptide in the secretion/expression vector pEZZ18-PI. On the basis of these observations, we conclude that modification of the mature domain of proinsulin is a critical factor for determination of the export of proinsulin.

Translational regulation of a recombinant operon containing human platelet-derived growth factor (PDGF)-encoding genes in Escherichia coli: genetic titration of the peptide chains of the heterodimer AB

A new strategy is described for the production of recombinant heteromultimeric proteins using Escherichia coli as host. A recombinant operon was constructed containing modified cDNA sequences encoding the mature A and B chains of human platelet-derived growth factor (PDGF). The relative expression rates of the PDGF genes were varied over a range equivalent to A:B ratios from 0.8 to 3.7 by means of translational regulation. This was achieved using two different translational initiation sequences (TIS) upstream from the respective coding regions, one derived from the E. coli atpE translational initiation region, and the other containing a sequence with extended complementarity to the 3' end of the 16S rRNA. The generation of mature PDGF A and B chains in different relative amounts in E. coli provided the basis for developing a novel procedure for the production of the biologically active PDGF heterodimer AB in large quantities. The general strategy is applicable to the preparation of a wide range of heteromultimeric complexes. Moreover, the described PDGF operon constitutes a compact and versatile model system for studies of the posttranscriptional regulation of gene expression.

Expression of the human UDP-glucuronosyltransferase UGT1*6 in Escherichia coli. Influence of bacterial signal peptides on the production and localization of the recombinant protein

The membrane-bound human liver UDP-glucuronosyltransferase UGT1*6 was expressed in Escherichia coli. Exchange of the natural signal peptide by the bacterial signal peptides of pclB or OmpT proteins considerably increased the level of expression and, as the natural signal peptide, targeted the protein to the membranes. The extent of maturation of SpelB-UGT1*6 precursor was about 30%. No processing of sOmpT-UGT1*6 occurred but the processing rate of this precursor could be significantly increased by mutagenesis of the first two amino acid residues of the mature sequence. These expression vectors allowed us to produce high levels of recombinant mature UGT1*6 required for further structural studies.

Expression of cGMP-dependent protein kinase in Escherichia coli

Cyclic GMP-dependent protein kinase (cGMP kinase) is involved in the relaxation of smooth muscle. The enzyme has been cloned and expressed in eukaryotic cell lines but so far not in prokaryotic cells. Three vectors were constructed for the expression of I alpha cGMP kinase in Escherichia coli. Transformation with the pET3a/cgk vector which uses the T7 RNA polymerase/promotor system resulted in efficient accumulation of cGMP kinase. Most of the protein was in an insoluble and catalytic inactive form. Various solubilization and refolding conditions did not yield an active enzyme. A small fraction of the cGMP kinase was present in the soluble cell extract. This fraction bound cGMP with high affinity but had no cGMP stimulated kinase activity. To prevent aggregation two additional vectors were constructed. (I) A bacterial leader sequence, which directs the export of proteins into the periplasmic space, was fused to the amino-terminus of the cGMP kinase. (II) A gram/gram+ shuttle vector for expression under the control of the tac promotor was used. Both constructs directed the synthesis of an insoluble and inactive cGMP kinase. These results suggest that large amounts of cGMP kinase can be expressed in E. coli, but mainly in an insoluble and inactive form. In contrast to eukaryotic cells, bacteria may lack systems for correct protein folding and/or posttranslational modification that are crucial for the productive folding and/or activation of cGMP kinase.

A model system for the investigation of heterologous protein secretion pathways in Lactococcus lactis

The capacity of recombinant strains of Lactococcus lactis to secrete a heterologous protein was investigated by constructing two expression-secretion vectors (pLET2 and pLET3) for use with a lactococcal gene expression system driven by the highly active T7 RNA polymerase. The vectors incorporated different lactococcal secretion leaders and translation initiation sequences. When tetanus toxin fragment C (TTFC) was used as a test protein, the quantities of TTFC produced by the pLET2-TTFC strain exceeded the rate of secretion of TTFC into the growth medium. However, nearly all of the soluble TTFC associated with the cell (3.4%) was translocated through the cell membrane. The pLET3-TTFC strain did not accumulate TTFC intracellularly and exhibited growth characteristics and viability identical to the growth characteristics and viability of the control strain. This strain secreted approximately 2.9 mg of TTFC per liter into the growth medium after 6 h of growth under test tube conditions. Our results indicate that L. lactis is capable of secreting substantial amounts of heterologous protein and also confirm the findings of other workers that the cell wall may serve as a functional barrier to the diffusion of some secreted proteins into the growth medium.

Efficient bacterial export of a eukaryotic cytoplasmic cytochrome

The soluble core domain of cytochrome b5 of liver endoplasmic reticulum was appended at its amino terminus to full-length alkaline phosphatase secretory signal sequence including the ribosomal binding site. The chimeric precursor gene was placed under the transcriptional control of the native pho promoter in a prokaryotic expression vector. Induction of Escherichia coli by growth in a phosphate-limited medium resulted in abundant synthesis of cytochrome b5 as detected spectrophotometrically and by visual transformation of the bacteria to a pink color. The signal-appended cytochrome b5, but not the corresponding signal-deficient derivative, was translocated across the bacterial inner membrane and processed to yield authentic, haem-assembled cytochrome b5 within the periplasm. The eventual processing of the chimeric cytochrome b5 precursor was unusual regarding the known reaction specificity of signal peptidase. The exported, mature haemoprotein was biochemically indistinguishable from its native mammalian counterpart. At peak induction, approximately 6 mg of correctly matured cytochrome b5 per liter of culture was exported. This amount of cytochrome b5 constituted 6% (w/w) of the periplasmic protein. The appearance of the exported apo-cytochrome b5 preceded the formation of holo-protein. Thus the eukaryotic cytoplasmic protein was efficiently exported from E. coli and post-translocationally modified to generate a functional haemoprotein in the periplasm.

Cytoplasmic and periplasmic production of human apolipoprotein E in Escherichia coli using natural and bacterial signal peptides

To understand the toxicity of high levels of heterologous human serum apolipoprotein E (ApoE) in Escherichia coli, as well as to prepare a system for producing the structural domains of this protein, plasmids were constructed in which the coding sequence of the N-terminal domain or all of ApoE followed E. coli or human apolipoprotein signal peptides (SP) or the N-terminal eleven amino acids (f10) of the gene 10-encoded protein of phage T7. High levels of production of the 22-kDa N-terminal domain (22K) of ApoE were obtained either as an f10::22K fusion protein, or using the natural SP, or SP derived from the periplasmic protein, alkaline phosphatase (PhoA), or from the outer membrane protein A (OmpA). Microsequencing showed that the SP of sPhoA::22K and sOmpA::22K, but not sApoE::22K, were correctly processed and, in the former cases, the protein could be released from the cells by osmotic shock. The extent of maturation of sPhoA::22K depended upon the host strain; with JM109, about 50% of the protein was not processed. Microsequencing of the f10::22K fusion protein, which could easily be purified following lysis of the cells, showed that the N-terminal methionine had been removed in agreement with the length parameter rule. Although considerable levels of the f10::ApoE fusion protein could be produced in the cytoplasm, production was markedly less using the PhoA signal peptide and the protein was not easily isolated following osmotic shock. The recombinant protein was biologically active after reconstitution with lipids in spite of the N-terminal modifications introduced.(ABSTRACT TRUNCATED AT 250 WORDS)

A thioredoxin gene fusion expression system that circumvents inclusion body formation in the E. coli cytoplasm

We have developed a versatile Escherichia coli expression system based on the use of E. coli thioredoxin (trxA) as a gene fusion partner. The broad utility of the system is illustrated by the production of a variety of mammalian cytokines and growth factors as thioredoxin fusion proteins. Although many of these cytokines previously have been produced in E. coli as insoluble aggregates or "inclusion bodies", we show here that as thioredoxin fusions they can be made in soluble forms that are biologically active. In general we find that linkage to thioredoxin dramatically increases the solubility of heterologous proteins synthesized in the E. coli cytoplasm, and that thioredoxin fusion proteins usually accumulate to high levels. Two additional properties of E. coli thioredoxin, its ability to be specifically released from the E. coli cytoplasm by osmotic shock or freeze/thaw treatments and its intrinsic thermal stability, are retained by some fusions and provide convenient purification steps. We also find that the active-site loop of E. coli thioredoxin can be used as a general site for small peptide insertions, allowing for the high level production of soluble peptides in the E. coli cytoplasm.

Secretion of mammalian ribonucleases from Escherichia coli using the signal sequence of murine spleen ribonuclease

A nucleotide sequence identical with that of the recently identified murine pancreatic ribonuclease (RNAase) was isolated from a murine spleen cDNA library. Active RNAase was expressed and secreted from Escherichia coli lon-htpr- transformed with a plasmid containing the E. coli trp promoter followed by the murine RNAase gene sequence, including the original eukaryotic 26-amino-acid signal sequence. Approx. 1 mg of properly matured RNAase protein/litre was secreted into the medium of a fermentor culture after the promotor was induced by tryptophan starvation. When the signal sequence was deleted from the plasmid, intracellular RNAase activity was very low and there was no significant supernatant RNAase activity. Even higher RNAase yields were obtained with a synthetic gene for bovine pancreatic ribonuclease cloned after the signal sequence of the murine gene. About 2 mg of correctly processed RNAase A/litre was isolated from the growth medium, and a further 8-10 mg of correctly processed RNAase/litre could be isolated from the soluble fraction of the cells. Thus this eukaryotic signal sequence is both recognized by the E. coli transport and processing apparatus and gives efficient secretion, as well as export, of active, mature mammalian RNAases.

Secretion of Active Bovine Somatotropin in Escherichia coli

We have expressed a chimeric protein, comprising the LamB secretion signal sequence fused to mature bovine somatotropin (bST), in Escherichia coli. Plasmid constructs with the recA promoter showed significant protein accumulation prior to induction and cell lysis occurred after induction. In contrast, the lacUV5 promoter was tightly regulated. With the lacUV5 promoter, temperature and inducer concentration had significant effects on the total amount of recombinant protein produced and the fraction processed to mature bST. Quantitation of bST from shake flask cultures showed that 1-2 micrograms/ml/OD550 could be released from the periplasm by osmotic shock. N-terminal sequence analysis of the purified protein indicated that the majority of the secreted bST was correctly processed. The bST present in the osmotic shock fraction was judged to be correctly folded by comigration with oxidized methionyl-bST standard on a non-reducing polyacrylamide gel and activity in a bovine liver radioreceptor assay. These results provide a rapid method to produce bST for use in structure-function studies.