Synthesis and sequence-specific proteolysis of a hybrid protein (colicin A::growth hormone releasing factor) produced in Escherichia coli

Homologous expression and purification of mutants of an essential protein by reverse epitope-tagging

Purification of mutant enzymes is a prime requirement of biophysical and biochemical studies. Our investigations on the essential Escherichia coli enzyme glutaminyl-tRNA synthetase demand mutant enzymes free of any wild-type protein contamination. However, as it is not possible to express noncomplementing mutant enzymes in an E. coli glnS-deletion strain, we developed a novel strategy to address these problems. Instead of following the common tactic of epitope-tagging the mutant protein of interest on an extrachromosomal genetic element, we fused a reporter epitope to the 5' end of the chromosomal glnS-gene copy: this is referred to as 'reverse epitope-tagging.' The corresponding strain, E. coli HAPPY101, displays a normal phenotype, and glutaminyl-tRNA synthetase is exclusively present as an epitope-tagged form in cell-free extracts. Here we report the use of E. coli HAPPY101 to express and purify a number of mutant glutaminyl-tRNA synthetases independently of their enzymatic activity. In this process, epitope-tagged wild-type protein is readily separated from mutant enzymes by conventional chromatographic methods. In addition, the absence of wild-type can be monitored by immunodetection using a monoclonal antibody specific for the epitope. The strategy described here for expression and purification of an essential enzyme is not restricted to glutaminyl-tRNA synthetase and should be applicable to any essential enzyme that retains sufficient activity to sustain growth following reverse epitope-tagging.

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.

Characterization of inclusion bodies in recombinant Escherichia coli producing high levels of porcine somatotropin

The protein composition of inclusion bodies (IBs) formed in recombinant Escherichia coli producing high levels of porcine somatotropin (pST) was analyzed by one- and two-dimensional protein gel electrophoresis. Recombinant pST is exclusively recovered from the insoluble cell fraction. Results indicate that, in addition to the main species of pST, subspecies with different isoelectric points and degradative fragments are contained within IBs. The presence of outer membrane proteins in IB fractions results from coprecipitation of cell debris during IB preparation and not from specific in vivo or in vitro interaction of these proteins with IBs. Cells producing pST contain up to three IBs located in the cytoplasm. The implication of high level gene expression on the uniformity of the desired product is discussed.

An evaluation of different enzymatic cleavage methods for recombinant fusion proteins, applied on des(1-3)insulin-like growth factor I

Different enzymatic methods for cleavage of recombinant fusion proteins were compared. To find an efficient cleavage method, five different fusion proteins were produced. The fusion proteins differed only in the linker region between the fusion partner and the desired product, human des(1-3)insulin-like growth factor I. A cleavage study was performed with enterokinase, plasmin, thrombin, urokinase, and recombinant H64A subtilisin. Significant cleavage was obtained using thrombin, H64A subtilisin, and enterokinase. Thrombin cleavage was studied on a larger scale and des(1-3)IGF-I was recovered at a final yield of 3 mg/L growth medium. Thrombin and enterokinase were also studied as immobilized proteases and they cleaved the fusion proteins with retained activity. To further improve thrombin cleavage, a continuous reactor was constructed, consisting of a closed system with a thrombin column and an ion exchange column in series. Here, the fusion protein circulated while free des(1-3)IGF-I was bound to the ion exchange column after release from the fusion protein. In the reactor, thrombin was as efficient as the free enzyme but gave a diminished rate of product degradation.

Production and biological activity of hybrid growth hormone-releasing hormone propeptides

Growth hormone-releasing hormone (GHRH), a hypothalamic hormone that stimulates the synthesis and release of growth hormone (GH) from anterior pituitary cells, has been previously produced by synthetic peptide chemistry and recombinant DNA procedures. GHRH is capable of stimulating growth as well as eliciting other anabolic effects on animals and thus may have potential applications in agriculture and human medicine. However, economical production of GHRH by recombinant DNA process has been difficult since GHRH is degraded rapidly by endogenous E. coli proteases. We report here an efficient process to produce hybrid GHRH analogs of higher molecular weight. These hybrid GHRH propeptides (proGHRH) are comprised of an analog of GHRH (44 aa) and the human GHRH carboxy-terminal peptide (33 aa). In E. coli K-12 RV308, the expression levels of the proGHRH analogs were estimated to be 10% of the total cellular protein. An in vitro assay to measure the release of rat growth hormone by GHRH analogs using crude E. coli lysates was also developed. This assay showed that the proGHRH analogs produced in E. coli efficiently stimulated GH release from rat anterior pituitary cells. One proGHRH analog, [alao]-proGHRH, was purified ans shown to efficiently elevate plasma GH levels in wether lambs. Our data indicate that the hybrid proGHRH peptides, unlike other hormone propeptides such as proinsulin, are remarkably bioactive.

Colicin A and colicin E1 lysis proteins differ in their dependence on secA and secY gene products

The export of colicin A and of colicin E1 is not equally affected in both secA and secY mutants of Escherichia coli: release of colicin A occurs slowly while that of colicin E1 is blocked. Processing and functioning of Cal, the colicin A lysis protein, seem to be slightly or not at all modified in these mutants, whereas synthesis and assembly of CelA, the colicin E1 lysis protein, are highly inhibited. These variations observed in the dependence of the two lysis proteins on secA and secY gene products are interpreted as being either the cause or the consequence of the differences observed in their rate of biogenesis.

Synthesis of a gene for human serum albumin and its expression in Saccharomyces cerevisiae

A 1761 base pairs long artificial gene coding for human serum albumin (HSA) has been prepared by a newly developed synthetic approach, resulting in the largest synthetic gene so far described. Oligonucleotides corresponding to only one strand of the HSA gene were prepared by chemical synthesis, while the complementary strand was obtained by a combination of enzymatic and cloning steps. 24 synthetic, 69-85 nucleotides long oligonucleotides covering the major part of the HSA gene (41-1761 nucleotides) were used as building blocks. Generally, four groups of 6-6 such oligonucleotides were successively cloned in pUC19 Escherichia coli vector to obtain about quarters of the gene as large fragments. Joining of these four fragments resulted in a cloned DNA coding for the 13-585 amino acid region of HSA, which was further supplemented with a double-stranded linker sequence coding for the amino terminal 12 amino acids. The completed structural gene composed of frequently used codons in the highly expressed yeast genes was then supplied with yeast regulatory sequences and the HSA expression cassette so obtained was inserted into an Escherichia coli-Saccharomyces cerevisiae shuttle vector. This vector was shown to direct the expression in Saccharomyces cerevisiae of correctly processed, mature HSA which was recognized by antiserum to HSA, and possessed the correct N-terminal amino acid sequence.

High-level expression of the colicin A lysis protein

Two plasmids that overproduce the colicin A lysis proteins, Cal, are described. Plasmid AT1 was constructed by a deletion in the colicin A operon, which placed the cal gene near a truncated caa gene in such a way that both gene products wer synthesized at high levels following induction. Plasmid CK4 was constructed by insertion of the cal gene downstream from the tac promoter of an expression vector. Overproduction of Cal was obtained after mitomycin C induction of pAT1 cells and after IPTG induction of pCK4 cells. The kinetics of Cal synthesis were examined with [35S] methionine and [2-3H] glycerol in lpp or lpp+ host strains. Each of the steps of the lipid modification and maturation pathway of Cal was demonstrated. The modified precursor form of overproduced Cal was not chased as efficiently as when it is produced in pColA cells. After treatment with globomycin, a significant amount of this modified precursor form accumulated and was degraded with time into smaller acylated proteins, but without release of the signal peptide. Release of cellular proteins and quasi-lysis were observed after about 1 hour of induction for cells containing either plasmid. In addition, in Cal-overproducing cells, the rate of quasi-lysis was increased but not its extent. In pldA cells, quasi-lysis was reduced but not abolished. Lethality of the Cal induction in the overproducing cells was in th same range as that in wild-type cells.