Expression, characterization, and purification of a phosphorylated rabies nucleoprotein synthesized in insect cells by baculovirus vectors

Confirmation of a new conserved linear epitope of Lyssavirus nucleoprotein

Bioinformatics analysis was used to predict potential epitopes of Lyssavirus nucleoprotein and highlighted some distinct differences in the quantity and localization of the epitopes disclosed by epitope analysis of monoclonal antibodies against Lyssavirus nucleoprotein. Bioinformatics analysis showed that the domain containing residues 152-164 of Lyssavirus nucleoprotein was a conserved linear epitope that had not been reported previously. Immunization of two rabbits with the corresponding synthetic peptide conjugated to the Keyhole Limpe hemocyanin (KLH) macromolecule resulted in a titer of anti-peptide antibody above 1:200,000 in rabbit sera as detected by indirect enzyme-linked immunosorbent assay (ELISA). Western blot analysis demonstrated that the anti-peptide antibody recognized denatured Lyssavirus nucleoprotein in sodium dodecylsulfonate-polyacrylate gel electrophoresis (SDS-PAGE). Affinity chromatography purification and FITC-labeling of the anti-peptide antibody in rabbit sera was performed. FITC-labeled anti-peptide antibody could recognize Lyssavirus nucleoprotein in BSR cells and canine brain tissues even at a 1:200 dilution. Residues 152-164 of Lyssavirus nucleoprotein were verified as a conserved linear epitope in Lyssavirus.

Characterization of the Ebola virus nucleoprotein-RNA complex

When Ebola virus nucleoprotein (NP) is expressed in mammalian cells, it assembles into helical structures. Here, the recombinant NP helix purified from cells expressing NP was characterized biochemically and morphologically. We found that the recombinant NP helix is associated with non-viral RNA, which is not protected from RNase digestion and that the morphology of the helix changes depending on the environmental salt concentration. The N-terminal 450 aa residues of NP are sufficient for these properties. However, digestion of the NP-associated RNA eliminates the plasticity of the helix, suggesting that this RNA is an essential structural component of the helix, binding to individual NP molecules via the N-terminal 450 aa. These findings enhance our knowledge of Ebola virus assembly and understanding of the Ebola virus life cycle.

Protein expression in the baculovirus system

Insect cell-recombinant baculovirus co-cultures offer a protein production system that complements microbial systems by providing recombinant proteins in soluble form and with most post-translational modifications. Moreover, the large size of the viral genome enables cloning of large segments of DNA and consequent expression of complex protein aggregates. This unit describes methods associated with the large-scale production of recombinant proteins in the baculovirus expression system. A method for large-scale production of viral stocks is described and methods for titration of virus are provided (a plaque assay and an end-point assay). Once viral stocks have been prepared and titered, a protocol for testing the virus in small-scale cultures is provided to determine the kinetics of expression, which allows evaluation of various cell culture and infection conditions aimed at developing optimal levels of protein production (e.g., comparisons of different host cell lines, media, and environmental parameters). Support protocols provide instructions for preparing culture samples for protein analysis by SDS-PAGE and discuss analytical methods for monitoring nutrient levels in cell culture fluids. Once optimal process parameters are identified, protocols describe production of the target protein on a large scale in fermentors using either regular batch production in bioreactors or a fed-batch procedure of production in perfusion cultures. Techniques for harvesting cultures from bioreactors are also provided.

Protein expression in the baculovirus system

Insect cell-recombinant baculovirus co-cultures offer a protein production system that complements microbial systems by providing recombinant proteins in soluble form and with most post-translational modifications. Moreover, the large size of the viral genome enables cloning of large segments of DNA and consequent expression of complex protein aggregates. This unit describes methods associated with the large-scale production of recombinant proteins in the baculovirus expression system. A method for large-scale production of viral stocks is described and methods for titration of virus are provided (a plaque assay and an end-point assay). Once viral stocks have been prepared and titered, a protocol for testing the virus in small-scale cultures is provided to determine the kinetics of expression, which allows evaluation of various cell culture and infection conditions aimed at developing optimal levels of protein production (e.g., comparisons of different host cell lines, media, and environmental parameters). Support protocols provide instructions for preparing culture samples for protein analysis by SDS-PAGE and discuss analytical methods for monitoring nutrient levels in cell culture fluids. Once optimal process parameters are identified, protocols describe production of the target protein on a large scale in fermentors using either regular batch production in bioreactors or a fed-batch procedure of production in perfusion cultures. Techniques for harvesting cultures from bioreactors are also provided.

Mapping of the VP40-binding regions of the nucleoprotein of Ebola virus

Expression of Ebola virus nucleoprotein (NP) in mammalian cells leads to the formation of helical structures, which serve as a scaffold for the nucleocapsid. We recently found that NP binding with the matrix protein VP40 is important for nucleocapsid incorporation into virions (T. Noda, H. Ebihara, Y. Muramoto, K. Fujii, A. Takada, H. Sagara, J. H. Kim, H. Kida, H. Feldmann, and Y. Kawaoka, PLoS Pathog. 2:e99, 2006). To identify the region(s) on the NP molecule required for VP40 binding, we examined the interaction of a series of NP deletion mutants with VP40 biochemically and ultrastructurally. We found that both termini of NP (amino acids 2 to 150 and 601 to 739) are essential for its interaction with VP40 and for its incorporation into virus-like particles (VLPs). We also found that the C terminus of NP is important for nucleocapsid incorporation into virions. Of interest is that the formation of NP helices, which involves the N-terminal 450 amino acids of NP, is dispensable for NP incorporation into VLPs. These findings enhance our understanding of Ebola virus assembly and in so doing move us closer to the identification of targets for the development of antiviral compounds to combat Ebola virus infection.

Association of rabies virus nominal phosphoprotein (P) with viral nucleocapsid (NC) is enhanced by phosphorylation of the viral nucleoprotein (N)

We investigated possible role(s) of N protein phosphorylation in the rabies virus replication process. A large amount of P proteins are associated with the viral nucleocapsid (NC) in the infected cell, the amount which was greatly decreased by phosphatase-treatment of the isolated NC, indicating that the phosphate group of N and/or P proteins is essential for their stable association with the NC. Immunoprecipitation studies were performed on the coexpressed normal N or phosphorylation deficient N(S389A) and P proteins, demonstrating that the P protein associated with phosphorylation-deficient NC-like structures was much less in amount than that associated with the wild type NC. Similar results were also obtained with a mutant P protein, PDeltaN19, which lacked the N-terminal 19 amino acids and was capable of binding to the NC-like structures but incapable of forming the RNA-free N-P complexes. Immunoprecipitation studies with mAb #402-13 further suggested that the NC-specific linear 402-13 epitope was exposed even on the P proteins which were associated with the phosphorylation-deficient NC-like structures, but such association was very weak as demonstrated by greatly decreased amounts of coprecipitated NC-like structures. From these results, we assume that the phosphorylation of N protein enhances the association between the 402-13 epitope-positive P protein and the NC probably by stabilizing such P-NC binding.

Interactions amongst rabies virus nucleoprotein, phosphoprotein and genomic RNA in virus-infected and transfected cells

Previous in vitro studies have indicated that rabies virus (RV) phosphoprotein (P), by interacting with the nucleoprotein (N), confers the specificity of genomic RNA encapsidation by N. In this study, interactions amongst N, P and the genomic RNA in virus-infected as well as in transfected cells were studied. The results showed that when N was expressed alone, it bound non-specific RNA, particularly the N mRNA. When N and P were co-expressed, they formed N-P complexes that did not bind to non-specific RNA. When N and P were co-expressed together with (mini-)genomic RNA, N-P complexes preferentially bound the (mini-)genomic RNA. This demonstrated that RV P, by binding to N, does indeed confer specificity of genomic RNA encapsidation by N in vivo. Furthermore, the role of N phosphorylation in the N, P and RNA interactions was investigated. It was found that only N that bound to RNA was phosphorylated, while N in the N-P complex prior to RNA encapsidation was not, suggesting that RV P, by binding to nascent N, prevents the immediate phosphorylation of de novo-synthesized N. However, mutation at the phosphorylation site of N did not alter the pattern of N-P and N-RNA interactions, indicating that N phosphorylation per se does not play a direct role in N-P interaction and RNA encapsidation.

Measles virus (MV) nucleoprotein binds to a novel cell surface receptor distinct from FcgammaRII via its C-terminal domain: role in MV-induced immunosuppression

During acute measles virus (MV) infection, an efficient immune response occurs, followed by a transient but profound immunosuppression. MV nucleoprotein (MV-N) has been reported to induce both cellular and humoral immune responses and paradoxically to account for immunosuppression. Thus far, this latter activity has been attributed to MV-N binding to human and murine FcgammaRII. Here, we show that apoptosis of MV-infected human thymic epithelial cells (TEC) allows the release of MV-N in the extracellular compartment. This extracellular N is then able to bind either to MV-infected or uninfected TEC. We show that recombinant MV-N specifically binds to a membrane protein receptor, different from FcgammaRII, highly expressed on the cell surface of TEC. This new receptor is referred to as nucleoprotein receptor (NR). In addition, different Ns from other MV-related morbilliviruses can also bind to FcgammaRII and/or NR. We show that the region of MV-N responsible for binding to NR maps to the C-terminal fragment (N(TAIL)). Binding of MV-N to NR on TEC triggers sustained calcium influx and inhibits spontaneous cell proliferation by arresting cells in the G(0) and G(1) phases of the cell cycle. Finally, MV-N binds to both constitutively expressed NR on a large spectrum of cells from different species and to human activated T cells, leading to suppression of their proliferation. These results provide evidence that MV-N, after release in the extracellular compartment, binds to NR and thereby plays a role in MV-induced immunosuppression.

Cross-reactive antigenicity of nucleoproteins of lyssaviruses recognized by a monospecific antirabies virus nucleoprotein antiserum on paraffin sections of formalin-fixed tissues

Diagnosis of rabies is routinely confirmed by detection of rabies virus antigens in acetone-fixed frozen brain tissues or imprint smears using an immunofluorescence method with commercial antirabies virus antibodies. Since recent molecular analyses disclosed wide heterogeneity in the genome sequences of rabies virus strains and related lyssaviruses, it is necessary to confirm the presence of common epitopes in these lyssaviruses. In this study we confirmed the presence of cross-reactive antigens of various lyssaviruses in paraffin sections of formalin-fixed tissue using a monospecific rabbit antiserum prepared by immunization with a recombinant nucleoprotein of rabies virus. By immunohistochemical application, the antigen was detected predominantly in the cytoplasm of neurons in the brains of mice infected with rabies virus, Duvenhage virus, Mokola virus and European bat lyssavirus-1, while no cross-reaction was observed in uninfected humans and animals including dogs, bats, and raccoons. In addition, we examined one autopsy case that was infected in a rabies-endemic nation and developed the clinical manifestation of rabies after returning to Japan in 1970, and found that the antigen was well preserved in paraffin sections of formalin-fixed tissues. Thus, this suggests that the lyssavirus-specific antigen is recognized by the monospecific antibody against rabies virus nucleoprotein, and that this cross-reactive antigen is detectable on formalin-fixed paraffin-embedded tissues by immunohistochemical analysis.

Rabies virus nucleoprotein is phosphorylated by cellular casein kinase II

It has been reported that phosphorylation of rabies virus N plays an important role in the process of viral transcription and replication. Rabies virus N is phosphorylated when expressed alone, indicating that cellular kinase phosphorylates rabies virus N. To identify what cellular kinase phosphorylates rabies virus N, the N was expressed in Escherichia coli and purified by metal affinity chromatography. The recombinant N was phosphorylated by BHK cellular extracts and by purified CK-II. In addition, the phosphorylation of the recombinant N in vitro can be blocked by a CK-II inhibitor, heparin. Furthermore, N phosphorylation in the virus-infected cells can be inhibited by a CK-II specific inhibitor, 5,6-dichloro-beta-D-ribofuranosyl benzimidazole. However, PKC did not phosphorylate the recombinant N in vitro; nor did staurosporine, a PKC and other kinase inhibitor, prevent rabies virus N from phosphorylation. Thus, our data demonstrate that cellular CK-II phosphorylates rabies virus N.

Isolation and characterisation of the rabies virus N degrees-P complex produced in insect cells

When the nucleoprotein (N) of nonsegmented negative-strand RNA viruses is expressed in insect cells, it binds to cellular RNA and forms N-RNA complexes just like viral nucleocapsids. However, in virus-infected cells, N is prevented from binding to cellular RNA because a soluble complex is formed between N and the viral phosphoprotein (P), the N degrees -P complex. N is only released from this complex for binding to newly made viral or complementary RNA. We coexpressed rabies virus N and P proteins in insect cells and purified the N degrees -P complex. Characterisation by gel filtration, polyacrylamide gel electrophoresis, analytical ultracentrifugation, native mass spectroscopy, and electron microscopy showed that the complex consists of one N protein plus two P proteins, i.e., an N degrees -P(2) complex.

Morphology of Marburg virus NP-RNA

When Marburg virus (MBGV) nucleoprotein (NP) is expressed in insect cells, it binds to cellular RNA and forms NP-RNA complexes such as insect cell-expressed nucleoproteins from other nonsegmented negative-strand RNA viruses. Recombinant MBGV NP-RNA forms loose coils that resemble rabies virus N-RNA. MBGV NP monomers are rods that are spaced along the coil similar to the nucleoprotein monomers of the rabies virus N-RNA. High salt treatment induces tight coiling of the MBGV NP-RNA, again a characteristic observed for other nonsegmented negative-strand virus N-RNAs. Electron microscopy of fixed Marburg virus particles shows that the viral nucleocapsid has a smaller diameter than the free, recombinant NP-RNA. This difference in helical parameters could be caused by the interaction of other viral proteins with the NP-RNA. A similar but opposite phenomenon is observed for rhabdovirus nucleocapsids that are condensed by the viral matrix protein upon which they acquire a larger diameter. Finally, there appears to be an extensive and regular protein scaffold between the viral nucleocapsid and the membrane that seems not to exist in the other negative-strand RNA viruses.

Structure of recombinant rabies virus nucleoprotein-RNA complex and identification of the phosphoprotein binding site

Rabies virus nucleoprotein (N) was produced in insect cells, in which it forms nucleoprotein-RNA (N-RNA) complexes that are biochemically and biophysically indistinguishable from rabies virus N-RNA. We selected recombinant N-RNA complexes that were bound to short insect cellular RNAs which formed small rings containing 9 to 11 N monomers. We also produced recombinant N-RNA rings and viral N-RNA that were treated with trypsin and that had lost the C-terminal quarter of the nucleoprotein. Trypsin-treated N-RNA no longer bound to recombinant rabies virus phosphoprotein (the viral polymerase cofactor), so the presence of the C-terminal part of N is needed for binding of the phosphoprotein. Both intact and trypsin-treated recombinant N-RNA rings were analyzed with cryoelectron microscopy, and three-dimensional models were calculated from single-particle image analysis combined with back projection. Nucleoprotein has a bilobed shape, and each monomer has two sites of interaction with each neighbor. Trypsin treatment cuts off part of one of the lobes without shortening the protein or changing other structural parameters. Using negative-stain electron microscopy, we visualized phosphoprotein bound to the tips of the N-RNA rings, most likely at the site that can be removed by trypsin. Based on the shape of N determined here and on structural parameters derived from electron microscopy on free rabies virus N-RNA and from nucleocapsid in virus, we propose a low-resolution model for rabies virus N-RNA in the virus.

The Rift Valley fever virus nonstructural protein NSs is phosphorylated at serine residues located in casein kinase II consensus motifs in the carboxy-terminus

The S segment of Rift Valley fever virus (Bunyaviridae, Phlebovirus) codes for two proteins, the nucleoprotein N and the nonstructural protein NSs. The NSs protein is a phosphoprotein of unknown function that is localized in the cytoplasm and the nuclei of infected cells where it forms filamentous structures. To characterize further the protein expressed in VC10 cells infected with the MP12 strain, we analyzed its phosphorylation states and showed that phosphorylated forms were found in both compartments. Cytoplasmic and nuclear NSs were phosphorylated only at serine residues. Phosphopeptide mapping and molecular analysis of mutants obtained by site-directed mutagenesis allowed us to map the major phosphorylation sites of nuclear and cytoplasmic forms of NSs to serine residues 252 and 256, located at the carboxy-terminus in consensus sequences for casein kinase II. A similar map was obtained when the protein was purified from mosquito cells infected with MP12. In addition, we showed that the purified unphosphorylated NSs protein expressed from pET-NSs plasmid in a coupled transcription-translation reaction containing Escherichia coli S30 extracts did not possess autophosphorylation activity but was phosphorylated in vitro after incubation with recombinant casein kinase II.

Rabies vaccine. Developments employing molecular biology methods

Rabies vaccines produced by means of molecular biology are described. Recombinant vaccines employing either viruses as vectors (vaccinia, adenovirus, poxvirus, baculovirus, plant viruses) or a plasmid vector carrying the rabies virus glycoprotein gene are discussed. Synthetic peptide technology directed to rabies vaccine production is also presented.

Production and purification of recombinant African swine fever virus attachment protein p12

The conditions for cultivation of Spodoptera frugiperda (Sf9) insect cells for production of recombinant baculoviruses have been studied, to scale-up and improve the efficiency of the process for production of the African swine fever virus attachment protein p12 in the baculovirus expression system. It was shown that the total virus and recombinant protein production in insect cells infected with the Acp12 recombinant baculovirus were slightly dependent on cell density, but largely dependent on the serum concentration, in the case of suspended cells, but not in static monolayer cultures. The yield of recombinant protein p12 exceeded 50 mg per 1 of 2 x 10(9) cells, representing more than 10% of total cell proteins, a level > 20-fold higher than that observed with other eukaryotic expression systems. The presence of p12 in the cytoplasmic fraction of infected cells has allowed the purification of the protein by a simple two-step procedure of aqueous phase partition and octyl-glucoside solubilization. The recombinant protein p12 was able to inhibit, in a dose-dependent manner, the African swine fever virus production in swine macrophages infected with a number of different virus isolates, including attenuated, virulent, highly passaged on tissue culture, and non-haemadsorbing strains, indicating a fundamental role for p12 in the early interaction of the virus with the natural target cell receptors. However, pigs immunized with purified recombinant p12 did not develop protective immunity against African swine fever.

A liquid-phase blocking ELISA for the detection of antibodies to rabies virus

A liquid-phase blocking ELISA was adapted to the detection and titration of antibodies to principally the nucleoprotein of rabies virus. Sera from animals that had either been vaccinated against rabies or inoculated with street rabies viruses, as well as sera from animals that had no recorded contact with rabies, were tested. These included sera from people, cattle, sheep, goats, dogs, laboratory mice, rabbits, yellow mongooses, wild dogs and lions. Where possible, the results were compared with those obtained with a commercial kit incorporating an indirect ELISA that measures antibody to the rabies glycoprotein. There was a high correlation (r = 0.79) between the two tests. The blocking ELISA provides a single test suitable for the rapid detection of antibodies against rabies virus in the sera of any animal species and for that reason is particularly apt for epidemiological investigations in regions where species diversity is important, as in southern Africa.

Respiratory syncytial virus nucleocapsid protein (N) expressed in insect cells forms nucleocapsid-like structures

The gene coding for the N protein of RSV strain Long has been cloned and sequenced. It was introduced behind the polyhedrin promoter of the shuttle vector pVL941 and baculoviruses containing the N gene were constructed by homologous recombination. Infection of Spodoptera frugiperda 9 cells resulted in the production of large amounts of a protein similar in size and antigenicity to the authentic N protein. The baculovirus expressed N protein was concentrated in the cytoplasm of the insect cells and could be extracted at low salt concentration. Nucleocapsid structures similar to those purified from RSV-infected cells could be observed by electron microscopy after negative staining of cellular extracts.

Structures associated with the expression of rabies virus structural genes in insect cells

When rabies virus structural genes were expressed in insect cells, major observed alterations were a high level of cytoplasmic vacuolization caused by the matrix protein M2 and the glycoprotein G. Ring-like structures, 16 nm in diameter, were observed in cell-free extracts from insect cells that expressed the N protein alone. Hexagonally shaped structures, 16-20 nm in diameter, and regular lattice aggregates of the same structures appeared on co-expression of N and M1 proteins. Co-expression of the four structural proteins led to the formation of cell surface blebs containing the structures corresponding to N and M1 proteins.

Evidence for a viral superantigen in humans

Superantigens bind class II major histocompatibility proteins and stimulate powerful proliferative responses of T lymphocytes bearing particular V beta sequences as part of their alpha beta antigen receptor. Exogenous bacterial superantigens are responsible for food poisoning and toxic shock syndrome. Murine virus-encoded self-superantigens induce clonal deletion of T lymphocytes. Although superantigen-like properties have been suggested for human immunodeficiency virus-1, no viral superantigen has been identified in humans. Here we report that the nucleocapsid of the rabies virus is an exogenous superantigen specific for V beta 8 human T lymphocytes which binds to HLA class II alpha-chains.

Baculovirus-expressed rabies virus M1 protein is not phosphorylated: it forms multiple complexes with expressed rabies N protein

The rabies N, M1, M2, and G antigens have been expressed in Spodoptera frugiperda cells from single gene expression vectors or dual gene vectors (N/M1 or M2/G) using the baculovirus system. Although N protein was phosphorylated, no evidence for M1 phosphorylation was obtained. N-M1 complexes were formed in vivo using dual infections or the coexpression vectors, as well as in vitro in mixing experiments. The free or complexed rabies N and M1 proteins reacted with available monoclonal and polyclonal antibodies. By sedimentation analyses the N-M1 complexes were shown to exist in multiple configurations.

Sickness and recovery of dogs challenged with a street rabies virus after vaccination with a vaccinia virus recombinant expressing rabies virus N protein

Dogs were vaccinated intradermally with vaccinia virus recombinants expressing the rabies virus glycoprotein (G protein) or nucleoprotein (N protein) or a combination of both proteins. The dogs vaccinated with either the G or G plus N proteins developed virus-neutralizing antibody titers, whereas those vaccinated with only the N protein did not. All dogs were then challenged with a lethal dose of a street rabies virus, which killed all control dogs. Dogs vaccinated with the G or G plus N proteins were protected. Five (71%) of seven dogs vaccinated with the N protein sickened, with incubation periods 3 to 7 days shorter than that of the control dogs; however, three (60%) of the five rabid dogs recovered without supportive treatment. Thus, five (71%) of seven vaccinated with the rabies N protein were protected against a street rabies challenge. Our data indicate that rabies virus N protein may be involved in reducing the incubation period in dogs primed with rabies virus N protein and then challenged with a street rabies virus and, of more importance, in subsequent sickness and recovery.

Rabies virus nucleoprotein expressed in and purified from insect cells is efficacious as a vaccine

A cDNA copy of the RNA gene that encodes the nucleoprotein N of rabies virus Evelyn-Rokitnicki-Abelseth strain was cloned into baculovirus. The recombinant baculovirus expressed the N protein abundantly in Spodoptera frugiperda cells. The N protein was extracted from infected Spodoptera frugiperda cells and purified to near homogeneity by affinity chromatography. The purified N protein reacted with 31 of 32 monoclonal antibodies that recognize native rabies virus ribonucleoprotein. Like the ribonucleoprotein, the purified N protein was a major antigen capable of inducing virus-specific helper T cells. Priming of mice with the purified N protein prior to a booster inoculation with inactivated Evelyn-Rokitnicki-Abelseth virus vaccine resulted in a 20-fold increase in the production of virus-neutralizing antibodies. After immunization with the purified N protein, mice developed a strong anti-ribonucleoprotein antibody response and were protected against a lethal challenge of rabies virus. These data indicate that the N protein expressed in insect cells is antigenically and immunogenically comparable to the authentic rabies virus ribonucleoprotein and therefore represents a potential source of an effective and economical vaccine for large-scale immunization of humans and animals against rabies.