Purification and characterization of the recombinant human prostaglandin H synthase-2 expressed in Pichia pastoris

Prostaglandin H synthase-1 and -2 (PGHS-1 and PGHS-2, EC 1.14.99.1) are membrane associated glycoproteins that catalyze the first two steps in prostaglandin synthesis. As the enzymes play an important regulatory role in several physiological and pathophysiological processes, recombinant PGHS isoforms are widely used in biomedical research. In the present study, we expressed human PGHS-2 (hPGHS-2) with and without a six histidine sequence tag (His(6) tag) near the amino- or carboxy-terminus of the protein in the Pichia pastoris (P. pastoris) expression system using native or yeast signal sequences. The recombinant His(6) tagged hPGHS-2 was purified using Ni-affinity and anion exchange chromatography, whereas the purification of the C-terminally His(6) tagged hPGHS-2 was more efficient. K(m), k(cat) and IC(50) values were determined to characterize the protein. The data obtained indicate that both the N- and C-terminally His(6) tagged hPGHS-2 are functional and the catalytic properties of the recombinant protein and the enzyme produced in other expression systems are comparable. As the yeast culture is easy to handle, the P. pastoris system could serve as an alternative to the most commonly used baculovirus-insect cell expression system for the production of the recombinant PGHS-2.     

Effect of surface histidine mutations and their number on the partitioning and refolding of recombinant human granulocyte-colony stimulating factor (Cys17Ser) in aqueous two-phase systems containing chelated metal ions

High-level expression of recombinant proteins in Escherichia coli frequently leads to the formation of insoluble protein aggregates, termed inclusion bodies. In order to recover a native protein from inclusion bodies, various protein refolding techniques have been developed. Column-based refolding methods and refolding in aqueous two-phase systems are often an attractive alternative to dilution refolding due to simultaneous purification and improved refolding yields. In this work, the effect of surface histidine mutations and their number on the partitioning and refolding of recombinant human granulocyte-colony stimulating factor Cys17Ser variant (rhG-CSF (C17S)) from solubilized inclusion bodies in aqueous two-phase systems polyethylene glycol (PEG)-dextran, containing metal ions, chelated by dye Light Resistant Yellow 2KT (LR Yellow 2KT)-PEG derivative, was investigated. Human G-CSF is a growth factor that regulates the production of mature neutrophilic granulocytes from the precursor cells. Initially, the role of His156 and His170 residues in the interaction of rhG-CSF (C17S) with Cu(II), Ni(II) and Hg(II) ions, chelated by LR Yellow 2KT-PEG, was investigated at pH 7.0 by means of affinity partitioning of purified, correctly folded rhG-CSF (C17S) mutants. It was determined that both His156 and His170 mutations reduced the affinity of rhG-CSF (C17S) for chelated Cu(II) ions at pH 7.0. His170 mutation significantly reduced the affinity of protein for chelated Ni(II) ions. However, histidine mutations had only a small effect on the affinity of protein for Hg(II) ions. The influence of His156 and His170 mutations on the refolding of rhG-CSF (C17S) from solubilized inclusion bodies in aqueous two-phase systems PEG-dextran, containing chelated Ni(II) and Hg(II) ions, was investigated. Reversible interaction of protein mutants with chelated metal ions was used for refolding in aqueous two-phase systems. Both histidine mutations resulted in a significant decrease of protein refolding efficiency in two-phase systems containing chelated Ni(II) ions, while in the presence of chelated Hg(II) ions their effect on protein refolding was negligible. Refolding studies of rhG-CSF variants with different number of histidine mutations revealed that a direct correlation exists between the number of surface histidine residues and refolding efficiency of rhG-CSF variant in two-phase systems containing chelated Ni(II) ions. This method of protein refolding in aqueous two-phase systems containing chelated metal ions should be applicable to other recombinant proteins that contain accessible histidine residues.     

Purification and reconstitution of an osmosensor: transporter ProP of Escherichia coli senses and responds to osmotic shifts

The ProP protein of Escherichia coli is an osmoregulatory H+-compatible solute cotransporter. ProP is activated by an osmotic upshift in both whole cells and membrane vesicles. We are using biochemical and biophysical techniques to explore the osmosensory and catalytic mechanisms of ProP. We now report the purification and reconstitution of the active transporter. Protein purification was facilitated by the addition of six histidine (His) codons to the 3' end of proP. The recombinant gene was overexpressed from the E. coli galP promoter, and ProP-(His)6 was shown to be functionally equivalent to wild-type ProP by enzymatic assay of whole cells. ProP-(His)6, purified by Ni2+ (NTA) affinity chromatography, cross-reacted with antibodies raised against the ProP protein. ProP-(His)6 was reconstituted into Triton X-100 destabilized liposomes prepared with E. coli phospholipid. The reconstituted transporter mediated proline accumulation only if (1) a membrane potential was generated by valinomycin-mediated K+ efflux and (2) the proteoliposomes were subjected to an osmotic upshift (0.6 M sucrose). Activity was also stimulated by DeltapH. Pure ProP acts, in the proteoliposome environment, as sensor, transducer, and respondent to a hyperosmotic shift. It is the first such osmosensor to be isolated.     

Conferring cadmium resistance to mature tobacco plants through metal-adsorbing particles of tomato mosaic virus vector

Tomato mosaic virus vectors were designed that produced, by a translational readthrough, a fusion protein consisting of coat protein and metal-binding peptide, as a result of which particles were expected to present the metal-binding peptides on their surface. When inoculated in plants, they were expected to replicate and form a metal-adsorbing artificial sink in the cytoplasm, so as to reduce metal toxicity. Vectors were constructed harbouring sequences encoding various lengths of polyhistidine as a metal-binding peptide. One of the vectors, TLRT6His, which contains a 6 x histidine sequence, moved systemically in tobacco plants, and its particles were shown to retain cadmium ions by an in vitro assay. When a toxic amount of cadmium was applied, the toxic effect was much reduced in TLRT6His-inoculated tobacco plants, probably as a result of cadmium adsorption by TLRT6His particles in the cytosol. This shows the possible use of an artificial sink for metal tolerance and the advantage of employing a plant viral vector for phytoremediation.     

Cloning,overexpression, purification,and matrix-assisted refolding of DevS (Rv 3132c) histidine protein kinase of Mycobacterium tuberculosis

The devR-devS (Rv 3133c-Rv 3132c) two-component system of Mycobacterium tuberculosis was identified in our laboratory by RNA subtractive hybridization. This genetic system was predicted to encode a response regulator and histidine protein kinase, respectively. The putative histidine kinase protein DevS was overexpressed to high levels in Escherichia coli as a fusion protein with a hexahistidine tag, His(6)-DevS201, in the form of inclusion bodies. Here we report a "redox-based" method of matrix-bound renaturation of DevS protein. The refolded protein was biochemically active in an autophosphorylation reaction characteristic of histidine kinases and was suitable for the generation of polyclonal antibodies and as an antigen in ELISA.     

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.     

Production and characterization of clinical grade Escherichia coli derived Plasmodium falciparum 42 kDa merozoite surface protein 1 (MSP1(42)) in the absence of an affinity tag

The 42 kDa cleavage product from the carboxyl end of the Plasmodium falciparum merozoite surface protein 1 (MSP1(42)) is an important blood-stage malaria vaccine target. Several recombinant protein expression systems have been used for production of MSP1(42) including yeast (Saccharomyces cerevisiae and Pichia pastoris), Escherichia coli, baculovirus and transgenic animals. To date, all of the reported recombinant proteins include a 6 x His affinity tag to facilitate purification, including three MSP1(42) clinical grade proteins currently in human trials. Under some circumstances, the presence of the 6 x His tag may not be desirable. Therefore, we were interested to produce clinical grade MSP1(42) without a 6 x His affinity tag from E. coli inclusion bodies. We produced a recombinant MSP1(42) with a P. falciparum FUP (Uganda-Palo Alto) phenotype which accounts for a substantial proportion of the MSP1(42) protein observed in African isolates. EcMSP1(42)-FUP was produced in E. coli inclusion bodies by high cell mass induction with IPTG using 5 L and 60 L bioreactors. Isolated inclusion bodies were solubilized in 8M guanidine-HCl and the EcMSP1(42)-FUP protein refolded by rapid dilution. Refolded EcMSP1(42)-FUP was purified using hydrophobic interaction chromatography, anion exchange chromatography, and size exclusion chromatography, and subject to biochemical characterization for integrity, identity, and purity. Endotoxin and host cell protein levels were within acceptable limits for human use. The process was successfully transferred to pilot-scale production in a cGMP environment. A final recovery of 87.8 mg of clinical-grade material per liter of fermentation broth was achieved. The EcMSP1(42)-FUP clinical antigen is available for preclinical evaluation and human studies.     

Histidine-aromatic interactions in barnase. Elevation of histidine pKa and contribution to protein stability.

Aromatic side-chains are found in the vicinity of histidine residues in many proteins and protein complexes. We have studied the interaction between a histidine residue (His18) and aromatic residues at position 94 in barnase. Three different techniques have been applied to show that Trp94 interacts more strongly with the protonated form of His18. The aromatic-histidine interaction stabilizes the protonated form of histidine by 0.8 to 1 kcal mol-1 relative to the unprotonated and, thereby, increases its pKa value. This was shown indirectly from the pH dependence of the stability of the wild-type protein and the mutant Trp94----Leu; and directly from the difference in pKa of His18 between wild-type barnase and the same mutant protein, and from double-mutant cycles that measure the total interaction energy of Trp94 with His18 at both low and high pH. When Trp94 is replaced by other aromatic amino acids, the strength of the interaction decreases in the series His-Trp greater than His-Tyr greater than His-Phe. The interaction is not masked by high salt concentrations. The raising of the pKa value of His18 by interaction with Trp94 is shown to be consistent with solution studies with model compounds. The histidine-aromatic interaction could have implications in binding and catalysis for modulation of the histidine pKa value.     

Putidaredoxin reductase,a new function for an old protein

Properties of recombinant wild type (WT) and six-histidine tag-fused (His(6)) putidaredoxin reductase (Pdr), a FAD-containing component of the soluble cytochrome P450cam monooxygenase system from Pseudomonas putida, have been studied. Both WT and His(6) Pdr were found to undergo a monomer-dimer association-dissociation and were partially present as an NAD(+)-bound form. Although molecular, spectral, and electron transferring properties of recombinant His(6) Pdr to artificial and native electron acceptors were similar to those of the WT protein, the presence of eight additional C-terminal amino acid residues, Pro-Arg-His-His-His-His-His-His, had a crucial effect on the enzyme interaction with oxidized pyridine nucleotide. Under anaerobic conditions, NAD(+) induced in His(6) Pdr spectral changes indicative of flavin reduction and formation of the charge transfer complex between the reduced FAD and NAD(+). The reaction proceeded considerably faster in the presence of free histidine and thiol-reducing agents, such as dithiothreitol and reduced glutathione. In the presence of any of these three reagents, NAD(+) was capable of inducing reduction of the flavin in WT Pdr. Free thiol groups were identified as an internal source of electrons in the enzyme. The results showed that WT and His(6) Pdr were able to function as NAD(H)-dependent dithiol/disulfide oxidoreductases catalyzing both forward and reverse reactions, NAD(+)-dependent oxidation of thiols, and NADH-dependent reduction of disulfides. This function of the flavoprotein can be dissociated from electron transfer to putidaredoxin. Similarity of Pdr to the enzymes of the glutathione reductase family is discussed.     

Expression, purification and activities of the entire family of intact membrane sensor kinases from Enterococcus faecalis

Two-component signal transduction systems are the main mechanism by which bacteria sense and respond to their environment, and their membrane-located histidine protein kinases generally constitute the sensory components of these systems. Relatively little is known about their fundamental mechanisms and precise nature of the molecular signals sensed, because of the technical challenges of producing sufficient quantities of these hydrophobic membrane proteins. This study evaluated the heterologous production, purification and activities of the 16 intact membrane sensor kinases of Enterococcus faecalis. Following the cloning of the genes into expression plasmid pTTQ18His, all but one kinase was expressed successfully in Escherichia coli inner membranes. Purification of the hexa-histidine 'tagged' recombinant proteins was achieved for 13, and all but one were verified as intact. Thirteen intact kinases possessed autophosphorylation activity with no added signal when assayed in membrane vesicles or as purified proteins. Signal testing of two functionally-characterized kinases, FsrC and VicK, was successful examplifying the potential use of in vitro activity assays of intact proteins for systematic signal identification. Intact FsrC exhibited an approximately 10-fold increase in activity in response to a two-fold molar excess of synthetic GBAP pheromone, whilst glutathione, and possibly redox potential, were identified for the first time as direct modulators of VicK activity in vitro. The impact of DTT on VicK phosphorylation resulted in increased levels of phosphorylated VicR, the downstream response regulator, thereby confirming the potential of this in vitro approach for investigations of modulator effects on the entire signal transduction process of two-component systems.     

Expression of His-tagged Shigella IpaC in Arabidopsis

Although the expression of histidine (His)-tagged proteins in bacteria is routine, few His-tagged proteins have been expressed in plants, and no His-tagged proteins from bacterial pathogens have been expressed in plants, to our knowledge. Here, we demonstrate expression of the Shigella flexneri invasion plasmid antigen, IpaC, in Arabidopsis thaliana. S. flexneri is the causitive trigger for bacillary dysentery, and IpaC is essential for bacterial entry into epithelial cells. IpaC, attached to a 5' leader containing six tandem His codons, was cloned into a pBI121 vector. This clone was introduced into Agrobacterium tumefaciens and Arabidopsis plants were then transformed. T1 and T2 plant generations were obtained. Total plant proteins were extracted from T2 leaves; the Bradford assay was used to determine protein concentrations. A nickel-coated ELISA plate method, using both anti-His and anti-IpaC 1 degrees antibodies, was used to detect and quantify IpaC in transgenic Arabidopsis plants. Between 1.9 and 2.3 microg IpaC/mg total plant protein was obtained; this equals 0.2% of total protein, an amount comparable to other recombinant protein estimates in plants. Expressing His-tagged proteins from bacterial pathogens, in plants, is important because plant material could ultimately be fed or applied intranasally to animals that are "at risk" for infection by such bacterial pathogens, thus causing them to raise antibodies against the pathogens--functioning as a vaccine.     

幽门螺杆菌致病岛CagL重组抗原的可溶性表达及其多克隆抗体的制备和分析*

目的:利用原核表达和蛋白质纯化技术获得高纯度的幽门螺杆菌致病岛CagL重组抗原(rCagL),利用其制备anti-CagL多克隆抗体,并分析抗体的特异性。方法:通过生物信息学软件分析rCagL的抗原结构;利用PCR长片段DNA合成技术合成不含有信号肽序列的幽门螺杆菌致病岛CagL基因,将其插入表达质粒pCzn1中,构建重组质粒pCzn1-rCagL。然后,将pCzn1-rCagL转入大肠杆菌Arctic Express中,经IPTG诱导表达后,通过Ni-IDA镍离子亲和层析纯化重组抗原rCagL,利用Western blot鉴定rCagL与His标签抗体和Anti-H. pylori抗体的免疫反应性;最后,通过rCagL辅以弗氏佐剂免疫BALB/c小鼠,制备anti-CagL多克隆抗血清,通过ELISA方法分析抗血清的特异性。结果:生物信息学软件表明重组抗原rCagL具有较好的抗原性质;重组质粒pCzn1-rCagL经双酶切和基因测序等技术鉴定,证实rCagL核苷酸序列与理论序列完全一致;基因工程菌株pCzn1-rCagL/Arctic Express在低温11℃条件经IPTG诱导表达。 SDS-PAGE实验结果证实:rCagL可实现相对高效地可溶性蛋白表达,可溶性蛋白约占包涵体的62.07%。经Ni-IDA亲和层析柱纯化,可获得高纯度rCagL,纯度约为96.6%。Western blot结果证实:重组抗原rCagL可特异性与His标签抗体和Anti-H. pylori抗体结合。ELISA结果证实:经rCagL免疫小鼠制备的多克隆抗体anti-CagL可特异性识别rCagL和H. pylori裂解物,具有较高的抗体特异性。结论:重组抗原rCagL在低温条件下可实现可溶性表达,经纯化可获得高纯度抗原蛋白;rCagL具有较好的抗原性,制备的多克隆抗体具有较好的免疫特异性,为发展H. pylori相关诊断试剂奠定了实验基础。     

Baculovirus-Mediated Large-Scale Expression and Purification of a Polyhistidine-Tagged Rubella Virus Capsid Protein

The capsid protein of rubella virus was produced in baculovirus-infectedSpodoptera frugiperdainsect cells, with a polyhistidine affinity tag at the carboxy terminus. The RV capsid recombinant protein was produced in a 10-liter bioreactor and purified, under nondenaturing conditions, using immobilized metal–ion affinity chromatography. Immunoblot analyses indicated that the purified recombinant protein was intact and migrated with the expected molecular weight. The final yield was 5 mg of purified protein per liter of cell culture. Surface plasmon resonance was used to investigate the antigenic potential of the histidine tagged capsid protein in an antigen–antibody interaction study. A specific interaction between the two proteins was shown. Our results suggest that this strategy should be useful in interaction studies of other virus-specific proteins and antibodies.     

Expression,purification and activities of the entire family of intact membrane sensor kinases from Enterococcus faecalis

Two-component signal transduction systems are the main mechanism by which bacteria sense and respond to their environment, and their membrane-located histidine protein kinases generally constitute the sensory components of these systems. Relatively little is known about their fundamental mechanisms and precise nature of the molecular signals sensed, because of the technical challenges of producing sufficient quantities of these hydrophobic membrane proteins. This study evaluated the heterologous production, purification and activities of the 16 intact membrane sensor kinases of Enterococcus faecalis. Following the cloning of the genes into expression plasmid pTTQ18His, all but one kinase was expressed successfully in Escherichia coli inner membranes. Purification of the hexa-histidine ‘tagged’ recombinant proteins was achieved for 13, and all but one were verified as intact. Thirteen intact kinases possessed autophosphorylation activity with no added signal when assayed in membrane vesicles or as purified proteins. Signal testing of two functionally-characterized kinases, FsrC and VicK, was successful examplifying the potential use of in vitro activity assays of intact proteins for systematic signal identification. Intact FsrC exhibited an approximately 10-fold increase in activity in response to a two-fold molar excess of synthetic GBAP pheromone, whilst glutathione, and possibly redox potential, were identified for the first time as direct modulators of VicK activity in vitro. The impact of DTT on VicK phosphorylation resulted in increased levels of phosphorylated VicR, the downstream response regulator, thereby confirming the potential of this in vitro approach for investigations of modulator effects on the entire signal transduction process of two-component systems.     

Serological detection of Plasmodium vivax malaria using recombinant proteins corresponding to the 19-kDa C-terminal region of the merozoite surface protein-1

BACKGROUND: Serological tests to detect antibodies specific to Plasmodium vivax could be a valuable tool for epidemiological studies, for screening blood donors in areas where the malaria is not endemic and for diagnosis of infected individuals. Because P. vivax cannot be easily obtained in vitro, ELISA assays using total or semi-purified antigens are rarely used. Based on this limitation, we tested whether recombinant proteins representing the 19 kDa C-terminal region of the merozoite surface protein-1 of P. vivax (MSP119) could be useful for serological detection of malaria infection. METHODS: Three purified recombinant proteins produced in Escherichia coli (GST-MSP119, His6-MSP119 and His6-MSP119-PADRE) and one in Pichia pastoris (yMSP119-PADRE) were compared for their ability to bind to IgG antibodies of individuals with patent P. vivax infection. The method was tested with 200 serum samples collected from individuals living in the north of Brazil in areas endemic for malaria, 53 serum samples from individuals exposed to Plasmodium falciparum infection and 177 serum samples from individuals never exposed to malaria. RESULTS: Overall, the sensitivity of the ELISA assessed with sera from naturally infected individuals was 95%. The proportion of serum samples that reacted with recombinant proteins GST-MSP119, His6-MSP119, His6-MSP119-PADRE and yMSP119-PADRE was 90%, 93.5%, 93.5% and 93.5%, respectively. The specificity values of the ELISA determined with sera from healthy individuals and from individuals with other infectious diseases were 98.3% (GST-MSP119), 97.7% (His6-MSP119 and His6-MSP119-PADRE) or 100% (yMSP119-PADRE). CONCLUSIONS: Our study demonstrated that for the Brazilian population, an ELISA using a recombinant protein of the MSP119 can be used as the basis for the development of a valuable serological assay for the detection of P. vivax malaria.     

High-level expression and one-step purification of recombinant dengue virus type 2 envelope domain III protein in Escherichia coli

Dengue virus infection poses a serious global public health threat for which there is currently no therapy or a licensed vaccine. The domain III of the dengue virus encoded envelope protein, which carries multiple conformation-dependent neutralizing epitopes, is critical for virus infectivity. We have expressed and purified recombinant domain III of dengue virus type-2 envelope, without the aid of a carrier protein in Escherichia coli. A 6x His tag was inserted at the N terminus to facilitate its one-step purification. The protein was overexpressed in the form of insoluble inclusion bodies, which were solubilized under highly denaturing conditions and then subjected to a previously optimized arginine-mediated renaturation protocol. We purified recombinant domain III protein to near homogeneity by Ni-NTA affinity chromatography and obtained yields of approximately 30 mg/L. The purified protein was recognized in Western analyses by monoclonal antibodies specific for the 6x His tag as well as the 3H5 neutralizing epitope known to reside in domain III. The authenticity of the recombinant protein was also verified in a sandwich ELISA designed to specifically and simultaneously identify the 6x His tag and the 3H5 epitope. In addition, murine and human polyclonal sera also recognized the recombinant protein. The in vitro refolded recombinant protein preparation was biologically functional. It could effectively protect cells in culture against dengue virus type-2 infection, apparently by blocking the virus from binding to host cells. This expression/purification strategy has the potential for inexpensive scale-up and may prove to be useful for dengue diagnostics and vaccine development efforts.     

A single amino acid in VP2 is critical for the attachment of infectious bursal disease subviral particles to immobilized metal ions and DF-1 cells

VP2 protein is the primary host-protective immunogen of infectious bursal disease virus (IBDV). His249 and His253 are two surface histidine residues in IBDV subviral particles (SVP), which is formed by twenty VP2 trimers when the VP2 protein of a local isolate is expressed. Here, a systemic study was performed to investigate His249 or/and His253 on self-assembly, cell attachment and immunogenicity of SVP. Point-mutagenesis of either or both histidine residues to alanine did not affect self-assembly of the SVP, but the SVP lost its Ni-NTA binding affinity when the His253 was mutated. Indirect immunofluorescence assays and inhibitory experiments also showed that His253 is essential for SVP to attach onto the DF-1 cells and to inhibit IBDV infection of DF-1 cells. Finally, enzyme-linked immunosorbent assays and chicken protection assays demonstrated that SVP with a mutation of His253 to alanine induced comparable neutralizing antibody titers in chickens as the wild-type SVP did. It was concluded that VP2's His253, a site not significant for the overall immunogenicity induced by SVP, is crucial for the binding affinity of SVP to Ni-NTA and the attachment of an IBDV host cell line. This is the first paper to decipher the role of His253 played in receptor interaction and immunogenicity.     

Comparison of N-terminal affinity fusion domains: effect on expression level and product heterogeneity of recombinant restriction endonuclease EcoRV

The influence of different N-terminal affinity fusion domains on the product heterogeneity of recombinant proteins expressed in Escherichia coli was investigated. N-Terminal extended forms of the restriction endonuclease EcoRV with either glutathione-S-transferase [GST], histidine hexapeptide [(His)₆], or a combination of GST and (His)₆ [GST-(His)₆] were compared to native EcoRV with respect to expression level, susceptability to inclusion body formation and protein fragmentation. Fingerprinting of product heterogeneity was done by using two-dimensional (2-D) non equilibrium pH-gradient electrophoresis with subsequent immunoblotting. Fusion proteins containing GST were poorly expressed compared to native EcoRV. In addition, GST fusion proteins were highly susceptible to invivo aggregation and fragmentation and displayed more heterogeneity on 2-D immunoblots. However, the sole presence of oligohistidine at the N-terminus of EcoRV proved to be advantageous. Fragmentation of (His)₆-EcoRV was not observed and 2-D immunoblots did not show heterogenous forms of the recombinant protein. In addition, fusion of the histidine-hexapeptide to the N-terminus of native EcoRV increased the expression level of the recombinant protein twofold compared to native EcoRV. Inclusion body formation of the (His)₆-EcoRV fusion protein was intensive when cells were grown at 37°C but not at 30°C. The advantage of oligohistidine fusion to EcoRV was finally demonstrated by purifying soluble (His)₆-EcoRV in a single-step procedure from crude cell lysates using immobilized metal chelate affinity chromatography.     

Expression, purification, and characterization of His-tagged Staphylococcus xylosus lipase wild-type and its mutant Asp 290 Ala

The gene encoding the extracellular lipase of Staphylococcus xylosus (SXL) was cloned using PCR technique. The sequence corresponding to the mature lipase was subcloned in the pET-14b expression vector, with a strong T7 promoter, to construct a recombinant lipase protein containing six histidine residues at the N-terminal. High level expression of the lipase by Escherichia coli BL21 (DE3) cells harbouring the lipase gene containing expression vector was observed upon induction with 0.4 mM IPTG at 37 degrees C. One-step purification of the recombinant lipase was achieved with Ni-NTA resin. The specific activity of the purified His-tagged SXL was 1500 or 850 U/mg using tributyrin or olive oil emulsion as substrate, respectively. It has been proposed that the region near the residue Asp290 could be involved in the selection of the substrate. Therefore, we also mutated the residue Asp 290 by Ala using site-directed mutagenesis. The mutant SXL-D290A was overexpressed in E. coli BL21 (DE3) and purified with the same nickel metal affinity column. The specific activity of the purified His-tagged SXL-D290A mutant was 1000 U/mg using either tributyrin or olive oil emulsion as substrate. A comparative study of the wild type (His(6)-SXL) and the mutant (His(6)-SXL-D290A) proteins was carried out. Our results confirmed that Asp290 is important for the chain length specificity and catalytic efficiency of the enzyme.