Engineering green fluorescent protein as a dual functional tag

A hexa-histidine (6 x His) sequence was inserted into a surface loop of the green fluorescent protein (GFP) to develop a dual functional GFP useful for both monitoring and purification of recombinant proteins. Two variants (GFP172 and GFP157), differentiated by the site of insertion of the 6xHis sequence, were developed and compared with a control variant (GFPHis) having the 6xHis sequence at its C-terminus. The variants were produced in Escherichia coli and purified using immobilized metal affinity chromatography (IMAC). The purification efficiencies by IMAC for all variants were found to be comparable. Purified GFP172 and GFP157 variants retained approximately 60% of the fluorescence compared to that of GFPHis. The reduction in the fluorescence intensity associated with GFP172 and GFP157 was attributed to the lower percentage of fluorescent GFP molecules in these variants. Nonetheless, the rates of fluorescence acquisition were found to be similar for all functional variants. Protein misfolding at an elevated temperature (37 degrees C) was found to be less profound for GFP172 than for GFP157. The dual functional properties of GFP172 were tested with maltose binding protein (MBP) as the fusion partner. The MBP-GFP172 fusion protein remained fluorescent and was purified from E. coli lysate as well as from spiked tobacco leaf extracts in a single-step IMAC. For the latter, a recovery yield of approximately 75% was achieved and MBP-GFP172 was found to coelute with a degraded product of the fusion protein at a ratio of about 4:1. The primary advantage of the chimeric GFP tag having an internal hexa-histidine sequence is that such a tag allows maximum flexibility for protein or peptide fusions since both N- and C-terminal ends of the GFP are available for fusion.     

A bifunctional chimeric protein consisting of MutS and beta-galactosidase

A bifunctional protein consisting of MutS, a mismatch binding protein and a beta-galactosidase reporter domain has been constructed. The fusion of beta-galactosidase to the MutS C-terminus was obtained by cloning the Escherichia coli lacZ gene encoding beta-galactosidase into a plasmid vector carrying the Thermus thermophilus mutS gene. Milligram amounts of this huge chimeric protein (217 kDa monomer) were purified from 1l of overexpressing E. coli cells using metal-chelate affinity chromatography. The mismatch binding properties of the fusion protein were confirmed by DNA mobility shift assay in polyacrylamide gels. Binding to biotinylated mismatched DNA immobilized on streptavidin microplates followed by colorimetric reaction with X-gal (5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside), demonstrated both mismatch recognition and beta-galactosidase activity of the chimeric protein. The activity of beta-galactosidase domain of the fusion was similar to that of the native enzyme. A colorimetric assay for beta-galactosidase activity using X-Gal supplemented with NBT (nitro blue tetrazolium) allowed detection of 50 and 500 fmol of the chimeric protein with naked eye in 45 microl volumes after 120 and 15 min incubation, respectively.     

Specific drug binding by purified lipid-reconstituted P-glycoprotein: dependence on the lipid composition.

We fused P-glycoprotein with beta-galactosidase at the C-terminus aiming to study the mechanism of drug binding of P-glycoprotein in reconstitution experiments. Expression of the fusion protein in NIH 3T3 cells conferred a multidrug-resistant phenotype, suggesting that beta-galactosidase fusion at the C-terminus does not affect the functions of P-glycoprotein. The fusion protein was partially purified by simple immunoprecipitation with anti-beta-galactosidase polyclonal antibody, and its [3H]azidopine binding property was investigated in the presence of various compositions of liposomes. The purified P-glycoprotein, after reconstitution into liposomes, was capable of binding [3H]azidopine. When the cholesterol content of liposomes was increased to a weight ratio of 20%, the specific binding activity of the partially purified fusion protein was stimulated, and when the cholesterol content was increased higher, the binding activity decreased. The binding was specifically decreased by competition with vinblastine. Stigmasterol was less effective, and ergosterol was the least effective in stimulating the specific binding.     

Expression, purification, and characterization of an aminopeptidase (Xac2987) with broad specificity from Xanthomonas axonopodis pv. citri

We report here, the cloning, expression, and purification of a broad specificity aminopeptidase from Xanthomonas campestris pv. citri in fusion with a hexa-histidine tag at the N-terminal portion of the protein to facilitate purification. The protein was expressed in the soluble fraction and could be purified in one step by IMAC, yielding approximately 50mg pure protein per liter of cells. We show that the protein is folded and presents aminopeptidase activity against synthetic substrates. Also, we present the characterization of its specificity, showing that the protein was, indeed, able to catalyze the removal of N-terminal residues from synthetic substrates.     

Use of a phoB'-'lacZ fusion gene to determine the N-terminal amino acid sequence of the phoB protein and to prepare antiserum against the protein

phoB is a positive regulatory gene of the phosphate regulon of Escherichia coli. A plasmid carrying a phoB'-'lacZ fusion gene was constructed by in vitro recombination. A PhoB-LacZ hybrid protein was purified from the cells carrying the plasmid by monitoring beta-galactosidase activity. The amino-terminal amino acid sequence of the PhoB protein was determined by utilizing the hybrid protein. Antiserum against the PhoB protein was prepared by immunizing rabbits with the hybrid protein. The serum thus prepared showed specificity for both the PhoB protein and beta-galactosidase.     

A comparative study of the human T-cell leukemia virus type 2 integrase expressed in and purified from Escherichia coli and Pichia pastoris

The human T-cell leukemia virus type-2 (HTLV-2) integrase (IN) catalyzes the insertion of the viral genome into the host chromosome. HTLV-2 IN was expressed as an N-terminal hexa-histidine tagged protein in the methylotrophic yeast Pichia pastoris and as a C-terminal hexa-histidine fusion in Escherichia coli. Maximal IN expression was observed at 48h post-induction for the yeast system and 2h post-induction for E. coli. Effective purification strategies were developed using non-ionic and zwitterionic detergents for initial protein extraction, followed by a one-step nickel-chelating chromatography purification. IN from both sources was routinely greater than 90% pure with yields exceeding 1.5mg of purified IN per liter of culture for P. pastoris. The relative pI was defined for both INs, pH 5.0-5.4, by 2D-gel electrophoresis. Specific activities for IN purified from E. coli and P. pastoris were calculated from in vitro 3(') processing assays and were comparable. In vitro IN assays were also performed to optimize reaction buffer pH and metal concentrations for both 3(') processing and strand transfer assays. Strand transfer was optimal from pH 6.2-6.8, more than 1.5 pH units below the optimal 3(') processing pH of 8.3. IN from both sources showed no enhancement in activity with MnCl(2) concentrations greater than 5mM. The specific activity of P. pastoris purified IN was 0.35 product (pmol)/h/microg IN, and E. coli produced IN was 0.48 product (pmol)/h/microg IN.     

Expression and purification of an antitumor-analgesic peptide from the venom of Mesobuthus martensii Karsch by small ubiquitin-related modifier fusion in Escherichia coli

To prevent protein aggregation, some proteins are usually expressed as fusion proteins from which target proteins can be released by proteolytic or chemical reagents. In this report, small ubiquitin-related modifier (SUMO) linked with a hexa-histidine tag was used as a fusion partner for the antitumor-analgesic peptide from the venom of Buthus martensii (Karsch) scorpion (AGAP). The optimal expression level of the soluble fusion protein, SUMO-AGAP, was up to 40% of the total cellular protein. The fusion protein was purified by Ni-NTA affinity chromatography and cleaved by a SUMO-specific protease (Ulp1) to obtain the recombinant AGAP (rAGAP), which was further purified by Ni-NTA affinity chromatography. The purified final product was >95% pure by SDS-PAGE stained with Coomassie brilliant blue R-250. Mass spectroscopic analysis indicated the protein to be 7142.63 Dalton, which equaled the theoretically expected mass. N-terminal sequencing of rAGAP showed the sequence corresponded to the native protein. MTT assay indicated the rAGAP could significantly inhibit the proliferation of Jurkat and Hut 78 T lymphoma cell lines. The further writhing experiment showed that the rAGAP had an intensive analgesic effect. The expression strategy presented in this study allows convenient high yield and easy purification of the rAGAP with native sequences.     

Expression of the cell-binding domain of human fibronectin in E. coli. Identification of sequences promoting full to minimal adhesive function

Two cDNA subfragments containing the cell-attachment site of human fibronectin (FN) were expressed as beta-galactosidase fusion proteins in E. coli. The products were purified to homogeneity by monoclonal antibody affinity chromatography and assayed for activity in a standard cell-adhesion assay. A fusion protein containing an 80 kDa fragment of human FN appeared functionally equivalent to intact FN purified from human plasma, whereas a truncated fusion protein of 33 kDa still containing a previously postulated cell-attachment site was approx. 50-fold less active. Our study establishes a system for analyzing adhesive protein function by DNA manipulation, rules out any major role for eukaryotic post-translational modifications in FN adhesive function, and localizes additional functional activity to a 1.3 kb region.     

A deoxyinosine specific endonuclease from hyperthermophile, Archaeoglobus fulgidus: a homolog of Escherichia coli endonuclease V

Deoxyadenosine undergoes spontaneous deamination to deoxyinosine in DNA. Based on amino acids sequence homology, putative homologs of endonuclease V were identified in several organisms including archaebacteria, eubacteria as well as eukaryotes. The translated amino acid sequence of the Archaeoglobus fulgidus nfi gene shows 39% identity and 55% similarity to the E. coli nfi gene. A. fulgidus endonuclease V was cloned and expressed in E. coli as a C-terminal hexa-histidine fusion protein. The C-terminal fusion protein was purified to apparent homogeneity by a combination of Ni(++) affinity and MonoS cation exchange liquid chromatography. The purified C-terminal fusion protein has a molecular weight of about 25kDa and showed endonuclease activity towards DNA containing deoxyinosine. A. fulgidus endonuclease V has an absolute requirement for Mg(2+) and an optimum reaction temperature at 85 degrees C. However, in contrast to E. coli endonuclease V, which has a wide substrate spectrum, endonuclease V from A. fulgidus recognized only deoxyinosine. These data suggest that the deoxyinosine cleavage activity is a primordial activity of endonuclease V and that multiple enzymatic activities of E. coli endonuclease V were acquired later during evolution.     

Rapid, high-throughput purification of HIV-1 integrase using microtiter plate technology

The human immunodeficiency virus type-1 (HIV-1) integrase (IN) catalyzes the insertion of the retroviral genome into the chromosome of an infected host cell. HIV-1 IN was expressed as a N-terminal hexa-histidine fusion in Escherichia coli. A high-throughput purification strategy was developed using denaturing methods for the initial protein extraction, followed by a one-step nickel-chelating chromatography purification and step-wise refolding. IN was routinely greater than 90% pure with yields exceeding 14 microg of purified IN per ml of E. coli culture. In vitro 3' processing and strand transfer assays showed the enzyme preparations to be highly active. The specific activity of the purified IN was 2.65 pmol/h/microg IN, which is very similar to the activity of IN routinely produced by large-scale column chromatographic methods. This high-throughput platform should be of general utility to those interested in defining the structure-function relationship of proteins and enzymes.     

Overexpression and purification of an immunologically reactive His-BIV capsid fusion protein

The gene of the capsid protein of bovine immunodeficiency virus (BIV) was linked to a sequence encoding for six histidines and expressed as the (His)6 p26 capsid fusion protein. The fusion protein was strongly expressed as both soluble and insoluble forms after induction by isopropylthio-beta-d-galactoside. Purification was based on interaction of the hexa-histidine polypeptide with metal ions. Expression could represent 11% of the total protein in Escherichia coli, allowing more than 20 mg of highly purified protein to be obtained per liter of bacterial culture. The (His)6 p26 capsid fusion protein purified by immobilized metal affinity chromatography reacted specifically in Western blot with sera from cattle experimentally infected by BIV, as well as with two monoclonal antibodies directed against different epitopes of the Gag protein. The ease of expression, purification, and specificity of this fusion protein should permit a thorough study of prevalence of BIV infection in large-scale serological studies of field samples.     

Purification and reconstitution of Escherichia coli proline carrier using a site specifically cleavable fusion protein

We previously constructed a bifunctionally active membrane-bound fusion protein, in which Escherichia coli proline carrier (the product of the putP gene) was linked with beta-galactosidase (the product of the lacZ gene) through a collagen linker (Hanada, K., Yamato, I., and Anraku, Y. (1987) J. Biol. Chem. 262, 14100-14104). The proline carrier was purified from this site specifically cleavable fusion protein. Cytoplasmic membranes overproducing the fusion protein were solubilized with dodecylmaltoside, and the solubilized fraction was subjected to anti-beta-galactosidase IgG-Sepharose chromatography. The fusion protein was specifically adsorbed to the immunoaffinity resin and then treated with collagenase for splitting the proline carrier moiety of the fusion protein from the beta-galactosidase moiety. The collagenase used for the collagenolysis was then removed by anti-collagenase IgG-Sepharose chromatography. In this way, the proline carrier was purified to more than 95% homogeneity of the protein. Proline transport in proteoliposomes reconstituted with the purified carrier was dependent on the membrane potential and the chemical gradient of Na+ across the membrane with apparent Michaelis constants for proline and for Na+ stimulation of 3.6 microM and 31 microM, respectively. These results indicated that the proline carrier mediates electrogenic Na+/proline symport.     

Expression,purification and functional characterization of recombinant Zucchini yellow mosaic virus HC-Pro

HC-Pro is a helper component-proteinase which acts as a multifunctional protein in the potyviral life cycle. Apart from its proteolytic activity, HC-Pro has the capacity to bind duplex small RNAs (sRNAs). To investigate HC-Pro-mediated sRNA binding in vitro, high amounts of purified protein are required. For this purpose, the Zucchini yellow mosaic virus (ZYMV) HC-Pro was expressed as a fusion with hexa-histidine (6xHis) or maltose-binding protein (MBP) in Escherichia coli. The expressed fusion proteins were purified by affinity chromatography. 6xHis:HC-Pro and MBP:HC-Pro were partially soluble. Electrophoretic mobility-shift assays demonstrated that only MBP:HC-Pro exhibits the sRNA binding activity. The recombinant HC-Pro bound 21 bp siRNAs as well as 19 bp and 24 bp siRNAs. A point mutation in the highly conserved FRNK box produced the HC-Pro^FINK protein, previously shown to be associated with reduced viral symptoms and weak sRNA binding. In this study, sRNA binding of the MBP:HA-HC-Pro^FINK was not detectable. The high yield of purified HC-Pro offers the possibility to study the biochemistry of the protein in detail.     

Overexpression and Purification of an Immunologically Reactive His–BIV Capsid Fusion Protein

The gene of the capsid protein of bovine immunodeficiency virus (BIV) was linked to a sequence encoding for six histidines and expressed as the (His)₆p26 capsid fusion protein. The fusion protein was strongly expressed as both soluble and insoluble forms after induction by isopropylthio-β- -galactoside. Purification was based on interaction of the hexa-histidine polypeptide with metal ions. Expression could represent 11% of the total protein inEscherichia coli,allowing more than 20 mg of highly purified protein to be obtained per liter of bacterial culture. The (His)₆p26 capsid fusion protein purified by immobilized metal affinity chromatography reacted specifically in Western blot with sera from cattle experimentally infected by BIV, as well as with two monoclonal antibodies directed against different epitopes of the Gag protein. The ease of expression, purification, and specificity of this fusion protein should permit a thorough study of prevalence of BIV infection in large-scale serological studies of field samples.     

Mutation prlF1 relieves the lethality associated with export of beta-galactosidase hybrid proteins in Escherichia coli.

The 42-1 lamB-lacZ gene fusion confers a conditionally lethal, export-dependent phenotype known as maltose sensitivity. A maltose-resistant mutant showing decreased beta-galactosidase activity of the hybrid protein, designated prlF1 (protein localization), was unlinked to the lamB-lacZ fusion. This mutation mapped at 70 min on the Escherichia coli linkage map and conferred maltose resistance, a 30-fold reduction in beta-galactosidase activity, and a 30% decrease in cellular growth rate at 30 degrees C that was independent of the presence of a gene fusion. prlF1 also decreased the beta-galactosidase activity and relieved the maltose sensitivity conferred by fusions of lacZ to the gene specifying the periplasmic maltose-binding protein, malE. The decrease in beta-galactosidase activity, however, was specific for exported hybrid proteins. When export of the hybrid protein was blocked by a signal sequence mutation, prlF1 decreased the beta-galactosidase activity only 2.5-fold. Similarly, prlF1 did not affect the beta-galactosidase activity of fusions of lacZ to a gene specifying a nonexported protein, malK.     

Immobilization and purification of enzymes with staphylococcal protein A gene fusion vectors.

Two improved plasmid vectors, containing the gene coding for staphylococcal protein A and adapted for gene fusions, have been constructed. These vectors allow fusion of any gene to the protein A moiety, giving fusion proteins which can be purified, in a one-step procedure by IgG affinity chromatography. One vector, pRIT2, is designed for temperature-inducible expression of intracellular fusion proteins in Escherichia coli and the other pRIT5, is a shuttle vector designed for secretion. The latter gives a periplasmatic fusion protein in E. coli and an extracellular protein in Gram-positive hosts such as Staphylococcus aureus. The usefulness of these vectors is exemplified by fusion of the protein A gene and the E. coli genes encoding the enzymes beta-galactosidase and alkaline phosphatase. High amounts of intact fusion protein are produced which can be immobilized on IgG-Sepharose in high yield (95-100%) without loss of enzymatic activity. Efficient secretion in both E. coli and S. aureus, was obtained for the alkaline phosphatase hybrid, in contrast to beta-galactosidase which was only expressed efficiently using the intracellular system. More than 80% of the protein A alkaline-phosphatase hybrid protein can be eluted from IgG affinity columns without loss of enzymatic activity.     

Coupling of chemical extraction and expanded-bed adsorption for simplified inclusion-body processing: optimization using surface plasmon resonance

Integration of the chemical extraction of recombinant inclusion-body protein from Escherichia coli, and its recovery by metal-affinity expanded-bed adsorption (IMAC-EBA) under denaturing conditions, was investigated. The viral coat protein L1 with a hexa-histidine tag was expressed in Escherichia coli HMS174(DE3) as a model protein. Interference of released host DNA with adsorbent fluidization in the EBA step was solved by selective precipitation using spermine and low-speed centrifugation. However, the capacity and selectivity of the adsorbent for L1 remained lower than anticipated. The binding of L1 to immobilized Ni(2+) was therefore studied in detail using surface plasmon resonance (SPR). The Tris buffer and ethylene-diamine tetraacetic acid (EDTA) used in the extraction mixture were found to interfere significantly with the L1-Ni(2+) interaction. The SPR studies suggest that L1 binding could be improved by replacing the Tris buffer with HEPES and by adding CaCl(2) to inactivate the EDTA. The modified chemical extraction conditions resulted in effective L1 extraction from cytoplasmic inclusion bodies, at high cell density (OD(600 )= 80) and without the use of reducing agent, into a medium optimized for subsequent IMAC recovery. The modified buffer conditions resulted in an improved binding capacity and a good L1 purification factor (12.7) and recovery yield (71%). This work demonstrates that it is possible to reduce the complexity and hence the cost associated with traditional processes used to prepare purified denatured protein, ready for refolding, from cytoplasmic inclusion bodies.     

A strategy for obtaining active mammalian enzyme from a fusion protein expressed in bacteria using phospholipase A2 as a model

An active preparation of human phospholipase A2 (PLA2) was made after expression as an insoluble fusion protein in Escherichia coli. The new key elements required for PLA2 isolation were the maintenance of the fusion protein in solution after the initial solubilization and the use of a tryptophan cleavage procedure for regeneration of native PLA2 from the fusion protein. The fusion protein was composed of a beta-galactosidase leader peptide incorporating six consecutive threonine residues to aid in insoluble inclusion body formation, followed by a tryptophan adjacent to the N-terminus of PLA2. The fusion protein was purified from cell lysates, and the leader peptide was cleaved on the C-terminal side of the tryptophan residue with N-chlorosuccinimide. The released PLA2 was refolded and renatured to produce an enzyme with activity comparable to that of other phospholipases A2.