Truncating the putative membrane association region circumvents the difficulty of expressing hepatitis C virus protein E1 in Escherichia coli

Purification and application of bacterially expressed chimeric protein E1E2 of hepatitis C virus

E1 and E2 glycoproteins are structural components of hepatitis C virus (HCV) virion. They are involved in cellular receptors interaction, neutralising antibodies elicitation, and viral morphogenesis. They are considered as major candidates for anti-HCV vaccine. In this report, we first expressed tandem E1E2 as well as C-terminally truncated E1 fragment and C-terminally truncated E2 fragment, respectively, in Escherichia coli cells and the proteins were purified to homogenesis. All the purified proteins can react specifically with patient sera. Both purified chimeric protein E1E2 and protein E2 can interact with a putative cellular receptor CD81, while purified protein E1 cannot interact with CD81. The sera of rabbit immunized with the E1E2 inhibited the binding of E2 protein to the major extracellular loop of human CD81 and reacted with both proteins E1 and E2, respectively. Anti-E1 and E2 antibodies can be generated simultaneously in the rabbit immunized with the E1E2, and the titers of antibodies were 63 or 56% higher than the titers induced by E1 or E2 alone, respectively. The results suggest that E1 and E2 can enhance their immunogenicity each other in chimeric protein E1E2 and the E. coli-derived chimeric protein E1E2 and corresponding antisera can be used as an useful tools in anti-HCV vaccine research.

Purification and application of C-terminally truncated hepatitis C virus E1 proteins expressed in Escherichia coli

AIM

To explore the possibility of expressing hepatitis C virus (HCV) envelope protein 1 (E1) in Escherichia coli (E. coli) and to test the purified recombinant E1 proteins for clinical and research applications.

METHODS

C-terminally truncated E1 fragments were expressed in E. coli as hexa-histidine-tagged fusion proteins. The expression products were purified under denaturing conditions using immobilized-metal affinity chromatography. Purified E1 proteins were used to immunize rabbits. Rabbit anti-sera thus obtained were reacted with both E. coli- and mammalian cell-expressed E1 glycoproteins as detected by Western blot.

RESULTS

Full-length E1 protein proved difficult to express in E. coli. C-terminally truncated E1 was successfully expressed in E. coli as hexa-histidine-tagged recombinant fusion protein and was purified under denaturing conditions on Ni(2+)-NTA agarose. Rabbit anti-sera raised against purified recombinant E1 specifically reacted with mammalian cell-expressed E1 glycoproteins in Western blot. Furthermore, E. coli-derived E1 protein was able to detect animal antibodies elicited by E1-based DNA immunization.

CONCLUSION

These results demonstrate that the prokaryotically expressed E1 proteins share identical epitopes with eukaryotically expressed E1 glycoprotein. The E. coli-derived E1 proteins and corresponding antisera can become useful tools in anti-HCV vaccine research.

Activation of dengue protease autocleavage at the NS2B-NS3 junction by recombinant NS3 and GST-NS2B fusion proteins

Dengue virus possesses a protease complex made up of the non-structural proteins NS2B and NS3. This protease complex catalyzes autocleavage (cis) at the junction between NS2A and NS2B as well as between NS2B and NS3. It also catalyzes trans cleavage at the junctions between NS3 and NS4A as well as NS4B and NS5. The cis cleavage at the NS2B-NS3 junction has been demonstrated in Escherichia coli by linking a 40-residue hydrophilic segment of NS2B to a NS3 N-terminal protease domain carrying the NS2B-NS3 cleavage site. To explore whether the hydrophilic segment could be further shortened, residues from both N- and C-termini of the NS2B hydrophilic segment were deleted. The results indicate that the four C-terminal's consecutive Glu residues could be deleted, each one leading to a further loss of activity, whereas the N-terminal boundary needed to be absolutely preserved. To examine whether an NS2B peptide could be expressed independently and added to activate the NS3 protease domain, the hydrophilic region of NS2B was fused to the C-terminus of glutathione-S-transferase (GST). This recombinant protein was soluble in bacteria and easily purified by affinity chromatography. Without removing the GST, the fusion protein activated the NS3 protease domain allowing it to function at the adjacent NS2B-NS3 junction. Thus, the findings reported below have produced a feasible alternative for the assay of dengue viral protease and this should facilitate the development of a screening method for inhibitors of dengue protease.

Full-length core sequence dependent complex-type glycosylation of hepatitis C virus E2 glycoprotein

AIM

To study HCV polyprotein processing is important for the understanding of the natural history of HCV and the design of vaccines against HCV. The purpose of this study is to investigate the affection of context sequences on hepatitis C virus (HCV) E2 processing.

METHODS

HCV genes of different lengths were expressed and compared in vaccinia virus/T7 system with homologous patient serum S94 and mouse anti-serum M( E2116) raised against E.coli -derived E2 peptide, respectively. Deglycosylation analysis and GNA ( Galanthus nivalus ) lectin binding assay were performed to study the post-translational processing of the expressed products.

RESULTS

E2 glycoproteins with different molecular weights (-75 kDa and -60 kDa) were detected using S94 and M( E2116), respectively. Deglycosylation analysis showed that this difference was mainly due to different glycosylation. Endo H resistance and its failure to bind to GNA lectin demonstrated that the higher molecular weight form (75 kDa) of E2 was complex-type glycosylated, which was readily recognized by homologous patient serum S94. Expression of complex-type glycosylated E2 could not be detected in all of the core-truncated constructs tested, but readily detected in constructs encoding full-length core sequences.

CONCLUSION

The upstream conserved full-length core coding sequence was required for the production of E2 glycoproteins carrying complex-type N-glycans which reacted strongly with homologous patient serum and therefore possibly represented more mature forms of E2. As complex-type N-glycans indicated modification by Golgi enzymes, the results suggest that the presence of full-length core might be critical for E1/E2 complex to leave ER. Our data may contribute to a better understanding of the processing of HCV structural proteins as well as HCV morphogenesis.

Immunological evaluation of Escherichia coli-derived hepatitis C virus second envelope protein (E2) variants

Two variants of the hepatitis C virus (HCV) E2 envelope protein, lacking the C-terminal domain and comprising amino acids 458-650 (E2A) and 382-605 (E2C), respectively, were efficiently produced in BL21 (DE3) Escherichia coli cells. E2A and E2C were used to immunize mice. The E2C variant induced the maximal mean antibody titer. Anti-E2C mouse sera reacted mainly with E2 synthetic peptides covering the 70 amino acid N-terminal region of the E2 protein. Moreover, a panel of anti-HCV positive human sera recognized only the E2C protein (28.2%) and the synthetic peptide covering the HVR-1 of the E2 protein (23.1%). These data indicate the existence of an immunologically relevant region in the HVR-1 of the HCV E2 protein.

Expression and membrane association of hepatitis C virus envelope 1 protein

The expression of hepatitis C virus (HCV) E1 protein is toxic for Escherichia coli cells. For this reason, we have cloned the E1 gene in the pET3a vector and analyzed the inducible expression of the protein in two strains of E. coli characterised by a different level of reduction of basal synthesis. The results indicated that synthesis of E1 was supported only by the BL21(DE3)pLysS strain which provides a tightest control of protein expression before the induction. The BL21(DE3)pLysS cells were then used for the expression of E1 gene, varying at its carboxy terminus in order to retain (E1, aa 192-383) or delete (Elt, aa 192-340) a C-terminal hydrophobic region that may be involved in membrane association. Following cell fractionation, E1 protein was found associated with the membrane fraction. By contrast, the truncated mutant E1t, was identified in the soluble phase suggesting a direct role for the C-terminal domain in E1 membrane association.

Hepatitis C virus E1 protein induces modification of membrane permeability in E. coli cells

The E1 gene of hepatitis C virus (HCV) has been cloned and expressed in BL21(DE3)pLys Escherichia coli strain by pET3a vector to analyze changes in membrane permeability produced by this protein. We showed that the expression of E1 (aa 192-383), as well as of two C-terminal fragments (aa 331-383 and aa 341-383) corresponding to the transmembrane (TM) region of this protein, induced a rapid lysis of cells. On the contrary, the expression of a mutant of E1 (aa 192-340), lacking the last 40 amino acids, did not cause cell lysis. The analysis of permeability changes revealed that modification of membrane permeability to several compounds were observed only in clones expressing E1 and C-terminal fragments, while the synthesis of the C-terminal-deleted mutant had little or no effect on permeability. These findings demonstrate that the TM domain of E1 protein has membrane-active properties that may be involved in some aspects of virus-cell interaction.

Antibodies directed to envelope proteins of hepatitis C virus outside of hypervariable region 1

The relatively high variability of the hepatitis C virus (HCV) envelope proteins E1 and E2 suggests that parts of these proteins other than the hypervariable region 1 (HVR1) might be involved in the induction of virus neutralizing antibodies. To test this hypothesis, two HCV proteins, pE1 and pE2 delta, were generated by in vitro translation. They represent amino acids 174-337 of E1 and 411-688 of E2, respectively, of isolate HCV-AD78; the protein pE2 delta contained no HVR1. As a control, protein pG.HVR1, which represents amino acids 384-410 of HVR1 of isolate HCV-AD78, was expressed separately. These three proteins were used in an immunoprecipitation assay to detect the presence of antiviral antibodies in sera of patients infected with the same isolate of HCV (HCV-AD78). Sera were obtained 4-8 months postinfection from patients who later resolved an acute infection or developed chronic liver disease. A high prevalence of antibodies (up to 85.7%) against pE1 and pE2 delta could be detected in both groups of patients, suggesting that these forms of the HCV envelope proteins contain B-cell epitopes. The antibody responses against proteins pE1 and pE2 delta did not differ significantly between patients with resolving or chronic infection, whereas antibodies against protein pG.HVR1 were associated with resolution of infection. Rabbit antisera raised against pE1 and pE2 delta were tested for their ability to neutralize the binding of HCV to susceptible cells in tissue cultures. The results suggested that although a few B-cell epitopes outside of HVR1 can induce virus neutralizing antibodies, these antibodies are probably not associated with the resolution of infection.

Evaluation of hepatitis C virus envelope proteins expressed in E. coli and insect cells for use as tools for antibody screening

BACKGROUND/METHODS

The two envelope proteins of hepatitis C virus, E1 and E2, were expressed in E. coli and, as secretory proteins, in Sf9 insect cells using recombinant baculoviruses. Co-infection of insect cells with E1 and E2-recombinant baculoviruses was performed, which has been shown to result in formation of E1-E2 dimers. All envelope proteins were purified by Ni2+-NTA chromatography and used for screening of serum samples in a HCV EIA assay. Serum samples of normal blood donors, chronically HCV-infected patients, a mixed titer panel and several seroconversion panels were screened and compared to test results with Cobas Core Anti-HCV EIA.

RESULTS

Screening of the sera of chronically HCV-infected patients (100% positive in Cobas Core Anti-HCV EIA) revealed 10-40% anti-E1 positive sera using different Sf9-expressed, glycosylated proteins and 93% using E. coli-expressed, non-glycosylated E1 protein. When the same sera were tested with different E2 proteins expressed in Sf9 cells and in E. coli, about 70-73% showed anti-E2 reactivity. When the proteins from Sf9 cells co-infected with E1- and E2-recombinant baculoviruses were tested, 70-80% of the same sera showed anti-envelope reactivity.

CONCLUSIONS

Testing of these patient antisera, and those from the well-characterized mixed titer panel BBI-PHV203, showed that recombinant E1 expressed in E. coli and co-expressed E1 and E2 proteins from Sf9 cells could be used as additional tools for anti-HCV antibody screening.

A monoclonal antibody reacts with maltose-binding protein of Escherichia coli and related enteric bacteria

Maltose-binding protein (MBP) encoded by malE is essential for the energy-dependent translocation of maltose through the cytoplasmic membrane of bacteria. Its property of specific binding to maltose has been used in constructing fusion proteins for easy affinity purification. A monoclonal antibody named MAb SC1D7 was produced against Escherichia coli MBP. This MAb also bound to MBP-containing recombinant proteins in both Western blotting and immunoprecipitation analysis. As a result, this MAb can be a useful probe for tracing MBP-fusion proteins in various applications. Furthermore, intrinsic MBPs from E. coli, Shigella dysenteriae, Salmonella typhimurium, Enterobacter cloacae, and Klebsiella pneumoniae were also detected by this MAb. No reaction was observed with the total proteins from Serratia marcescens, Aeromonas hydrophila and Plesiomonas shigelloides. These observations suggest that the MAb SC1D7-defined epitope is conserved among some enteric bacteria, but not the others. The results strengthen the phylogenetic positions of these closely related bacteria previously placed by other means.

Delineation of regions important for heteromeric association of hepatitis C virus E1 and E2

Hepatitis C virus (HCV) is the major causative agent of blood-borne non-A non-B hepatitis. The persistence of HCV infection is believed to reflect escape from the host immunosurveillance system by mutations in hypervariable region 1 (HVR1) of the envelope protein 2 (E2). Two envelope proteins of HCV, E1 and E2, have been reported to form a heteromeric complex but the exact organization of the viral envelope proteins remains uncertain. We examined the interaction of E1 and E2 by far- Western blotting using the bacterial recombinant proteins and also by pull-down assay using mammalian expressed proteins. The major E1-interacting site of E2 was mapped within the N-terminal part of E2 (NCD1) (aa 415 to 500 of the polyprotein). Both HVR1 and HVR2, located at the N-terminal part of E2, were dispensable for the interaction. Although several discontinuous regions within NCD1 seemed to contribute to the strong binding to E1, the highly conserved amino acid sequences flanking HVR2 had the most significant effect. The amino acid residues "WHY" from 489 to 491 of E2 played an especially crucial role since the constructs with the internal deletion or substitution of the residues showed severely impaired E1-binding. The N-terminal part of E1 is important for the E2-binding as determined by far-Western blotting using the mammalian- and bacterial-expressed E2 proteins as probes. The mammalian-expressed, glycosylated forms of the E1 and E2 proteins exhibited E1-E2 binding activities similar to those of the bacterial-expressed, nonglycosylated forms in pull-down assays, suggesting that glycosylation is not prerequisite for the heteromeric complex formation of E1 and E2.

Characterization of monoclonal antibodies to the 26-kDa glutathione S-transferase of Schistosoma japonicum

Six monoclonal antibodies (MAbs) were raised in mice against the 26-kDa glutathione S-transferase (GST) of the parasite Schistosoma japonicum. These MAbs were originally selected for their specific binding to the recombinant GST (r-GST) generated in E. coli by an enzyme-linked immunosorbent assay. A further study demonstrated that all these MAbs bound to plate-coated GST affinity-purified from the parasite Schistosoma japonicum. However, in Western blotting analysis only a single monoclonal antibody (MAb Y3D7) yielded positive binding. The binding of MAb Y3D7 on Western blotting was further characterized; specific binding was found on other GST fusion proteins and on the authentic 26-kDa GST but not the 28-kDa GST in the total soluble worm proteins from Schistosoma japonicum. Using protein-A-mediated immunoprecipitation, MAbs Y3D7 and Y5D5 precipitated r-GST while in parallel experiments the remaining MAbs did not generate r-GST precipitation. In an alternative co-precipitation experiment, r-GST was first bound to glutathione (GSH) Sepharose beads and subsequently tested for interaction with the MAbs. In this manner, all MAbs except MAb Y5D5 were co-precipitated with the complexes. Thus, these select MAbs readily reacted with GST although their binding characteristics were different. Because GST has been widely used in the generation of fusion proteins for various purposes and is a potential vaccine candidate in controlling schistosomiasis, these MAbs should prove valuable for their application to molecular biology and parasitology.

Deletion or alteration of hydrophobic amino acids at the first and the third transmembrane domains of hepatitis B surface antigen enhances its production in Escherichia coli

To investigate the failure of high-level production of hepatitis B viral (HBV) surface antigen (HBsAg), including three authentic forms, large (L), middle (M) and major/small (S) HBsAg, in Escherichia coli, we employed the high-expression vector pGEX containing the glutathione S-transferase-encoding gene (GST) to study HBsAg production. Different fragments of HBV DNA containing the entire pre-S1/pre-S2/S region (for L protein), or partial pre-S1, pre-S2, pre-S1/pre-S2 and pre-S2/S region (for M protein), were fused downstream from the GST gene, in order to obtain five plasmids which encode GST-HBsAg fusion proteins. SDS-PAGE analyses revealed that cells containing plasmids with a full-length S region (pGLS and pGMS) produced undetectable GST-HBsAg fusion proteins, in contrast to those cells harboring plasmids without the S region (pGS1, pGS2 and pGS1S2), which synthesized fusion proteins in 3-10% of the total cellular protein. Using an immunoblot method to screen HBsAg production in cells which harbored plasmids derived from exonuclease BAL 31-digested pGLS, we obtained eight positive clones. Nucleotide sequence analyses of plasmids from the positive clones revealed that termination, deletion or frameshift occurred at the regions encoding either the first or the third transmembrane domain of the major HBsAg. Correlation between the production level of GST-HBsAg fusion proteins and their constituent and arrangement of amino acids (aa) at the last 20 aa among 15 clones suggested that the fusion protein ended with a longer stretch of or a higher ratio of hydrophobic aa had a lower production in E. coli.