Folding of Eukaryotic Proteins Produced in Escherichia Coli

Oxidative refolding from inclusion bodies

This protocol describes the growth and purification of bacterial inclusion body proteins with an option to selenomethionine label the targeted protein through feedback inhibition of methionine biosynthesis in common (non-auxotrophic) strains of E. coli. The method includes solubilization of inclusion body proteins by chemical denaturation and disulfide reduction, renaturation of the solubilized material through rapid dilution by pulsed injection into refolding buffer containing arginine and a mixture of oxidized and reduced glutathione, recovery of the recombinant protein using a stirred cell concentrator, and removal of the aggregated or misfolded fraction by passage over size-exclusion chromatography. The quality of the resulting protein can be assessed by SDS-PAGE.

Temperature, media, and point of induction affect the N-terminal processing of interleukin-1β

The expression of recombinant proteins in bacterial hosts may alter the biophysical properties of the protein of interest as a result of differences in post-translational processing from that observed when produced in the native cell. For example, recombinant human interleukin-1beta (IL-1beta) is produced as three isoforms when expressed in the Escherichia coli strain BL-21(DE3). These isoforms are produced by the non-homogeneous processing of the N-terminal methionine residue by the endogenous bacterial aminopeptidase and differ in the first residue (1-met, 1-ala, and 1-pro). Importantly, these isoforms have significantly different binding affinities for the IL receptor protein. Varying the temperature, media composition, and point of induction affects this N-terminal processing to favor one of the three isoforms of IL-1beta. We found changes in media composition and/or point of induction affected the abundance of the isoforms by as much as 15-fold. The 1-pro isoform decreased from 60.9 to 4.7% in Luria broth (LB) and minimal media (MM), respectively, when protein expression was induced at an OD600 of 0.9. Conversely, the abundance of the 1-met isoform is much higher in MM than in LB showing the reverse effect (2.6 and 50.7% in LB and MM, respectively, at an OD600 of 0.9), and the degree to which they are favored depends significantly upon the induction point. Our results show that it is possible to favor the expression of various N-terminal isoforms of IL-1beta by adjusting the environmental growth conditions. Given that the initiator methionine residue is necessary for expression in bacterial hosts and is known to affect the stability of other recombinant proteins our approach may be a useful general method for determining the optimal conditions for expressing and purifying pure, homogenous samples of recombinant proteins for structural and biological studies.

High-level production and isotope labeling of snake neurotoxins, disulfide-rich proteins

The aim of this work was to produce and to label snake neurotoxins, disulfide-rich proteins. A mutant of a snake toxin, erabutoxin a, was used as a model. Its N-terminal part was fused to ZZ, a synthetic IgG-binding domain of protein A (B. Nilsson et al., 1987, Protein Eng. 1, 107-113), thus preventing degradation in the bacterial cytoplasm and providing a simple affinity-purification method on IgG Sepharose. A soluble fusion protein was obtained with a yield of 60 mg/L, corresponding to 20 mg/L toxin. The toxin moiety was folded on the column while the hybrid was still bound. The oxidoreducing conditions for the refolding were optimized and were found to be oxidative but with a need for reducing molecules. The concentration of the hybrid bound to the column could be increased up to 3.3 mg/ml without significantly altering the folding process. CNBr cleavage of the fusion protein followed by a purification step yielded about 2 mg of biologically active toxin mutant per gram of dry cell weight. This procedure was applied to produce 55 mg of a toxin uniformly labeled with 15N.

Expression of Erythrina corallodendron lectin in Escherichia coli

The cDNA of the Erythrina corallodendron lectin (ECorL) has been expressed in Escherichia coli. For this purpose, an NcoI site was inserted into the cDNA coding for the lectin precursor [Arango, R., Rozenblatt, S. & Sharon, N. (1990) FEBS Lett. 264, 109-112] immediately before the codon GTG (103-105) which codes for the N-terminal valine of the mature lectin. This introduced an ATG codon for a methionine preceding the valine. The mutated cDNA was ligated into pUC-8, then subcloned into the expression vector pET-3d, which carries a strong promoter derived from gene 10 of the phage T7. The recombinant plasmid was introduced into the E. coli lysogenic strain BL21(DE3). Recombinant ECorL was expressed by growing the bacteria in the presence of isopropyl beta-D-thiogalactopyranoside. Most of the recombinant lectin was found in an insoluble aggregated form as inclusion bodies and only a small part was in the culture medium in a soluble active form. Functional recombinant lectin was recovered from the inclusion bodies by solubilization with 6 M urea in cyclohexylaminopropane sulfonate pH 10.5, renaturation by 10-fold dilution in the same buffer and further adjustment of the pH to 8.0. The recombinant lectin, obtained at a yield of 4-7 mg/l culture, had, by gel filtration, a slightly lower molecular mass (56 kDa) than the native lectin, and was devoid of covalently linked carbohydrate; it was, however, essentially indistinguishable from native ECorL by other criteria, including its dimeric structure, Western blot analysis with anti-ECorL polyclonal and monoclonal antibodies, and Ouchterlony double-diffusion analysis with polyclonal antibodies, as well as hemagglutinating activity and specificity for mono- or disaccharides.

Purification of recombinant antigenic epitopes of the human 68-kDa (U1) ribonucleoprotein antigen using the expression system pH6EX3 followed by metal chelating affinity chromatography

A novel plasmid expression vector (pH6EX3) that directs the synthesis of a fusion protein with a histidine hexapeptide at its N-terminus and a foreign protein at its C-terminus was constructed. The fusion gene is controlled by a strong tac promoter, leading to high-level expression of recombinant protein in several bacterial strains; the protein is deposited mainly as an insoluble mass in inclusion bodies. The fusion protein can be purified from the insoluble cell fraction by one-step affinity chromatography based on the selective interaction between the histidine hexapeptide and a metal chelating matrix charged with Ni2+ ions. The principle of this new system was tested by expressing and purifying antigenic epitopes of the human 68-kDa (U1) ribonucleoprotein autoantigen. With the use of column chromatography and pH gradient elution, about 25 micrograms recombinant protein/ml of bacterial culture was obtained.