Processing and intracellular transport of rubella virus structural proteins in COS cells

Rubella virus assembly requirements and evolutionary relationships with novel rubiviruses

Rubella virus (RuV) is an enveloped virus that usually causes mild disease in children, but can produce miscarriage or severe congenital birth defects. While in nature RuV only infects humans, the discovery of the related Ruhugu (RuhV) and Rustrela (RusV) viruses highlights the spillover potential of mammalian rubiviruses to humans. RuV buds into the Golgi, but its assembly and exit are not well understood. We identified a potential late domain motif 278PPAY281 at the C-terminus of the RuV E2 envelope protein. Such late domain motifs can promote virus budding by recruiting the cellular ESCRT machinery. An E2 Y281A mutation reduced infectious virus production by >3 logs and inhibited virus particle production. However, RuV was insensitive to inhibition by dominant-negative VPS4, and thus appeared ESCRT-independent. The E2 Y281A mutation did not significantly inhibit the production of the viral structural proteins capsid (Cp), E2, and E1, or dimerization, glycosylation, Golgi transport, and colocalization of E2 and E1. However, E2 Y281A significantly reduced glycoprotein-Cp colocalization and interaction, and inhibited Cp localization to the Golgi. Revertants of the E2 Y281A mutant contained an E2 281V substitution or the second site mutations [E2 N277I + Cp D215A]. These mutations promoted virus growth, particle production, E2/Cp colocalization and Cp-Golgi localization. Both the E2 substitutions 281V and 277I were found at the corresponding positions in the RuhV E2 protein. Taken together, our data identify a key interaction of the RuV E2 endodomain with the Cp during RuV biogenesis, and support the close evolutionary relationship between human and animal rubiviruses.

IMPORTANCE

Rubella virus (RuV) is an enveloped virus that only infects humans, where transplacental infection can cause miscarriage or congenital birth defects. We identified a potential late domain, 278PPAY281, at the C terminus of the E2 envelope protein. However, rather than this domain recruiting the cellular ESCRT machinery as predicted, our data indicate that E2 Y281 promotes a critical interaction of the E2 endodomain with the capsid protein, leading to capsid's localization to the Golgi where virus budding occurs. Revertant analysis demonstrated that two substitutions on the E2 protein could partially rescue virus growth and Cp-Golgi localization. Both residues were found at the corresponding positions in Ruhugu virus E2, supporting the close evolutionary relationship between RuV and Ruhugu virus, a recently discovered rubivirus from bats.

Molecular and Structural Insights into the Life Cycle of Rubella Virus

Rubella virus (RUBV), a rubivirus, is an airborne human pathogen that generally causes mild measles-like symptoms in children or adults. However, RUBV infection of pregnant women can result in miscarriage or congenital rubella syndrome (CRS), a collection of long-term birth defects including incomplete organ development and mental retardation. Worldwide vaccination campaigns have significantly reduced the number of RUBV infections, but RUBV continues to be a problem in countries with low vaccination coverage. Further, the recent discovery of pathogenic rubiviruses in other mammals emphasizes the spillover potential of rubella-related viruses to humans. In the last decade, our understanding of RUBV has been significantly increased by virological, biochemical, and structural studies, providing a platform to begin understanding the life cycle of RUBV at the molecular level. This review concentrates on recent work on RUBV, focusing on the virion, its structural components, and its entry, fusion, and assembly mechanisms. Important features of RUBV are compared with those of viruses from other families. We also use comparative genomics, manual curation, and protein homology modeling to highlight distinct features of RUBV that are evolutionarily conserved in the non-human rubiviruses. Since rubella-like viruses may potentially have higher pathogenicity and transmissibility to humans, we also propose a framework for utilizing RUBV as a model to study its more pathogenic cousins.

The key role of rubella virus glycoproteins in the formation of immune response, and perspectives on their use in the development of new recombinant vaccines

Rubella is a highly contagious viral disease which is mostly threatens to women of reproductive age. Existent live attenuated vaccines are effective enough, but have some drawbacks and are unusable for a certain group of people, including pregnant women and people with AIDS and other immunodeficiency. Thereby the development of alternative non-replicating, recombinant vaccines undoubtedly is needed. This review discusses the protein E1 and E2 role in formation of immune response and perspectives in development of new generation recombinant vaccines using them.

The Rubella virus capsid is an anti-apoptotic protein that attenuates the pore-forming ability of Bax

Apoptosis is an important mechanism by which virus-infected cells are eliminated from the host. Accordingly, many viruses have evolved strategies to prevent or delay apoptosis in order to provide a window of opportunity in which virus replication, assembly and egress can take place. Interfering with apoptosis may also be important for establishment and/or maintenance of persistent infections. Whereas large DNA viruses have the luxury of encoding accessory proteins whose primary function is to undermine programmed cell death pathways, it is generally thought that most RNA viruses do not encode these types of proteins. Here we report that the multifunctional capsid protein of Rubella virus is a potent inhibitor of apoptosis. The main mechanism of action was specific for Bax as capsid bound Bax and prevented Bax-induced apoptosis but did not bind Bak nor inhibit Bak-induced apoptosis. Intriguingly, interaction with capsid protein resulted in activation of Bax in the absence of apoptotic stimuli, however, release of cytochrome c from mitochondria and concomitant activation of caspase 3 did not occur. Accordingly, we propose that binding of capsid to Bax induces the formation of hetero-oligomers that are incompetent for pore formation. Importantly, data from reverse genetic studies are consistent with a scenario in which the anti-apoptotic activity of capsid protein is important for virus replication. If so, this would be among the first demonstrations showing that blocking apoptosis is important for replication of an RNA virus. Finally, it is tempting to speculate that other slowly replicating RNA viruses employ similar mechanisms to avoid killing infected cells.

Among the variety of defense systems employed by mammalian cells to combat virus infection, apoptosis or programmed cell death is the most drastic response. Some large DNA viruses encode proteins whose sole function is to block apoptosis. Conversely, very little is known about whether RNA viruses encode analogous proteins. In many cases, RNA viruses are able to replicate before cell death occurs, which may be one reason why so little thought has been given to this topic. However, a number of RNA viruses, some of which are important human pathogens, have slow replication cycles and it stands to reason that they must block apoptosis during this time period. Here we show that the multifunctional capsid protein of Rubella virus is a potent inhibitor of apoptosis. Data from reverse genetic experiments suggest that the anti-apoptotic function of a virus-encoded protein is important for replication of an RNA virus. We anticipate that other slowly replicating RNA viruses may employ similar mechanisms and, as such, these studies have implications for development of novel anti-virals and vaccines.

Rubella virus capsid protein interacts with poly(a)-binding protein and inhibits translation

During virus assembly, the capsid proteins of RNA viruses bind to genomic RNA to form nucleocapsids. However, it is now evident that capsid proteins have additional functions that are unrelated to nucleocapsid formation. Specifically, their interactions with cellular proteins may influence signaling pathways or other events that affect virus replication. Here we report that the rubella virus (RV) capsid protein binds to poly(A)-binding protein (PABP), a host cell protein that enhances translational efficiency by circularizing mRNAs. Infection of cells with RV resulted in marked increases in the levels of PABP, much of which colocalized with capsid in the cytoplasm. Mapping studies revealed that capsid binds to the C-terminal half of PABP, which interestingly is the region that interacts with other translation regulators, including PABP-interacting protein 1 (Paip1) and Paip2. The addition of capsid to in vitro translation reaction mixtures inhibited protein synthesis in a dose-dependent manner; however, the capsid block was alleviated by excess PABP, indicating that inhibition of translation occurs through a stoichiometric mechanism. To our knowledge, this is the first report of a viral protein that inhibits protein translation by sequestration of PABP. We hypothesize that capsid-dependent inhibition of translation may facilitate the switch from viral translation to packaging RNA into nucleocapsids.

Expression and characterization of rubella virus glycoprotein E1 in yeast cells

OBJECTIVES

To express E1 glycoprotein of rubella virus (RuV) strain JR23 in yeast and develop a diagnostic assay using expressed E1 protein as coating antigen in comparison with other diagnostic assays.

METHODS

cDNA of E1 open reading frame of RuV was PCR amplified using plasmid pMD18-T-E1 as template and cloned into plasmid pBluscriptII SK+. After being confirmed by PCR, restriction endonuclease digestion and sequencing, pBluscriptII SK(+)-E1 plasmid DNA was digested by restriction endonuclease EcoR I and Xba I, and a fragment of 1.5 kb was isolated and cloned into a yeast expression pGAPZ(alpha)A, resulting in pGAPZ(alpha)A-E1. After confirmation by sequencing, pGAPZ(alpha)A- E1 was transformed into yeast GS115 cells with LiCl method. E1 protein expression in GS115 was analyzed by SDS-PAGE and Western blot. An indirect ELISA was developed using the recombinant E1 protein as coating antigen for detecting RuV E1 antibodies in 90 serum samples. To compare the specificity, sensitivity and reproducibility of the assay with other methods, the same serum samples were also assayed by RuV culture medium as coating antigen (Jingmei kit and German RECI kit). Statistical analyses were performed to compare the differences among these methods and to determine which coating antigen source, the recombinant E1 protein or RuV-infected culture medium, is more suitable for the assay.

RESULTS

A fragment of 1.5 kb, corresponding to the full open reading frame of E1, was PCR amplified and cloned in yeast expression vector. The clone was confirmed by restriction digestion, PCR and sequencing. E1 as a secretive protein was successfully expressed by GS115. Its molecular weight was about 58 kDa. SDS-PAGE showed that the recombinant protein was expressed efficiently and constantly in Pichia pastoris GS115 cells. The expression level reached a peak 48 h after culturing and stabilized thereafter. E1 protein was detected in both supernatant and cells. Western blot showed that the secretive E1 protein in the supernatants could react with both the anti-RuV-positive serum and a monoclonal antibody against E1. However, E1 protein derived from cells could only react with the anti-RuV-positive serum, polyclonal antibody, but not the monoclonal antibody. Compared with the German RECI kit, the sensitivity, specificity, and accordance rate of the assay using recombinant E1 protein as coating antigen were 67.11, 71.43 and 67.78%, respectively, while those of the assay using RuV-infected culture medium as coating antigen were 50, 78.57 and 54.44%, respectively. Compared with the German RECI kit, the sensitivity, specificity, and accordance rate of the ELISA assay using the Jingmei kit were 84.71, 71.43 and 82.22%, respectively. The data indicated that recombinant E1 protein derived from the yeast expression system can serve as a better source than RuV-infected cell medium as coating antigen for detecting antibodies against RuV in the indirect ELISA assay. Statistical analysis of the data generated from two independent experiments using recombinant E1 protein as coating antigen indicated that the assay was very consistent with no statistically significant difference between the two experiments (p > 0.05). 76 out of 90 serum samples were detected positive using the German RECI kit, while 68, 55 and 41 samples were positive using the Jingmei kit, recombinant E1 and RuV-infected cell medium, respectively. Statistical analyses indicated that the positive rates were significantly different among all four assays (p < 0.05) except for one pair (German RECI kit and the Jingmei kit: p > 0.05). Comparing the positive rates obtained from the assay using recombinant E1 and that using RuV-infected cell medium, it seems that the recombinant E1 protein is a better source than RuV culture medium as coating antigen in the indirect ELISA assay for detection of RuV antibody.

CONCLUSIONS

The recombinant yeast expression vector of RuV E1 glycoprotein was constructed successfully. The E1 protein as a secretive protein was successfully expressed by GS115 and maintained its antigenicity very well. As coating antigen, the recombinant E1 protein served a better source than RuV culture medium in the indirect ELISA method for the detection of RuV antibody.

Interactions between rubella virus capsid and host protein p32 are important for virus replication

The distribution and morphology of mitochondria are dramatically affected during infection with rubella virus (RV). Expression of the capsid, in the absence of other viral proteins, was found to induce both perinuclear clustering of mitochondria and the formation of electron-dense intermitochondrial plaques, both hallmarks of RV-infected cells. We previously identified p32, a host cell mitochondrial matrix protein, as a capsid-binding protein. Here, we show that two clusters of arginine residues within capsid are required for stable binding to p32. Mutagenic ablation of the p32-binding site in capsid resulted in decreased mitochondrial clustering, indicating that interactions with this cellular protein are required for capsid-dependent reorganization of mitochondria. Recombinant viruses encoding arginine-to-alanine mutations in the p32-binding region of capsid exhibited altered plaque morphology and replicated to lower titers. Further analysis indicated that disruption of stable interactions between capsid and p32 was associated with decreased production of subgenomic RNA and, consequently, infected cells produced significantly lower amounts of viral structural proteins under these conditions. Together, these results suggest that capsid-p32 interactions are important for nonstructural functions of capsid that include regulation of virus RNA replication and reorganization of mitochondria during infection.

Phosphorylation of rubella virus capsid regulates its RNA binding activity and virus replication

Rubella virus is an enveloped positive-strand RNA virus of the family TOGAVIRIDAE: Virions are composed of three structural proteins: a capsid and two membrane-spanning glycoproteins, E2 and E1. During virus assembly, the capsid interacts with genomic RNA to form nucleocapsids. In the present study, we have investigated the role of capsid phosphorylation in virus replication. We have identified a single serine residue within the RNA binding region that is required for normal phosphorylation of this protein. The importance of capsid phosphorylation in virus replication was demonstrated by the fact that recombinant viruses encoding hypophosphorylated capsids replicated at much lower titers and were less cytopathic than wild-type virus. Nonphosphorylated mutant capsid proteins exhibited higher affinities for viral RNA than wild-type phosphorylated capsids. Capsid protein isolated from wild-type strain virions bound viral RNA more efficiently than cell-associated capsid. However, the RNA-binding activity of cell-associated capsids increased dramatically after treatment with phosphatase, suggesting that the capsid is dephosphorylated during virus assembly. In vitro assays indicate that the capsid may be a substrate for protein phosphatase 1A. As capsid is heavily phosphorylated under conditions where virus assembly does not occur, we propose that phosphorylation serves to negatively regulate binding of viral genomic RNA. This may delay the initiation of nucleocapsid assembly until sufficient amounts of virus glycoproteins accumulate at the budding site and/or prevent nonspecific binding to cellular RNA when levels of genomic RNA are low. It follows that at a late stage in replication, the capsid may undergo dephosphorylation before nucleocapsid assembly occurs.

A technique for isolation of rubella virus-like particles by sucrose gradient ultracentrifugation using Coomassie brilliant blue G crystals

An improved method for the isolation of rubella virus-like particles (RVLP) from cell culture supernatant of transfected Chinese hamster ovary (CHO24S) cells is described. It employs a combination of membrane filtration with sucrose gradient ultracentrifugation. It was found that staining the RVLP band with Coomassie brilliant blue G (CBB) resulted in the CBB crystals adsorbing RVLP. After ultracentrifugation (25,000 rpm, 3h, 4 degrees C) a sharp blue band with crystals (diameter 30-40 microm) was observed (at a density of 1.250 g/ml at 25 degrees C) in a 30-60% sucrose gradient. Using a combination of SDS-PAGE and Western blotting techniques, E1 rubella virus structural protein was detected only in the solutions derived from the sharp blue band. A decrease in crystal concentration a few millimeters above or below the main band was associated with a decrease in protein concentration. By dilution with a saturated ice-cold 30% sucrose solution it was possible to pellet the crystals by centrifugation (15,000 rpm, 10 min). SDS-PAGE showed a much higher concentration of RVLP structural protein in the pellet than in the supernatant. This RVLP-containing material is especially suitable for the preparation of rubella virus immunoblot stripes.

Effect of site-directed asparagine to isoleucine substitutions at the N-linked E1 glycosylation sites on rubella virus viability

The role of three N-linked glycosylation sites in rubella virus (RV) E1 protein on virion release was analyzed by transfecting Vero 76 cells with infectious RV RNA (Robo302WT) containing isoleucine substitutions at N76, N177, and N209 (individually and in combinations). RV RNAs were detected and found to retain substitutions in the transfected cells, but RV capsid indicative of infection was undetectable, except for in Robo302WT and Robo302-N177I transfected cells. Only culture supernatants of Robo302WT and Robo302-N177I RNA transfected cells were positive for RV, suggestive of the virion release into the culture medium. Further, detection of intracellular RV E1 and newly released virion-associated E1 was possible only from cells previously incubated with Robo302-N177I and Robo302WT culture supernatants, suggesting that N177I substituted virus retained infectivity. These results suggest that while glycosylation at N177 is not critical, N76I and N209I mutations are lethal to RV viability.

Rubella virus E2 signal peptide is required for perinuclear localization of capsid protein and virus assembly

The rubella virus (RV) structural proteins capsid, E2, and E1 are synthesized as a polyprotein precursor. The signal peptide that initiates translocation of E2 into the lumen of the endoplasmic reticulum remains attached to the carboxy terminus of the capsid protein after cleavage by signal peptidase. Among togaviruses, this feature is unique to RV. The E2 signal peptide has previously been shown to function as a membrane anchor for the capsid protein. In the present study, we demonstrate that this domain is required for RV glycoprotein-dependent localization of the capsid protein to the juxtanuclear region and subsequent virus assembly at the Golgi complex.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.

Rubella virus capsid protein induces apoptosis in transfected RK13 cells

Rubella virus is an enveloped positive-strand RNA virus that can cause mild to severe birth defects or death in an infected fetus. RV induction of programmed cell death, demonstrated in cell culture, has been implicated in the pathogenesis. The timing of apoptosis, 48 h p.i., suggested that accumulation of RV structural proteins might induce cell death in infected cells. Expression of RV structural proteins, capsid, envelope glycoproteins E1 and E2, in transiently transfected RK13 cells was as potent an inducer of cell death as RV infection. Immunofluorescence microscopy revealed that RV structural protein transfected cells exhibited the condensed nuclei typical of apoptotic cell death. Transfection with the capsid protein construct, but not E2 and E1, resulted in as much cell death as joint expression of all three RV structural proteins. Capsid required a membrane-anchoring domain to induce cell death, but a heterologous polypeptide fused to the capsid membrane anchor did not cause apoptosis. Deletion mutants demonstrated that the apoptosis-inducing activity resides in the N-terminal 170 amino acids of capsid. Though apoptosis-inducing capsid constructs appear to have an ER sub-cellular localization, disruption of the ER calcium storage capacity does not correlate with cell death. Mechanisms consistent with these results are discussed.

Mutations in the E1 hydrophobic domain of rubella virus impair virus infectivity but not virus assembly

Rubella virus (RV) virions contain three structural proteins, a capsid protein that interacts with viral genomic RNA to form a nucleocapsid and two membrane glycoproteins, E2 and E1. We found that substitution of either an aspartic acid residue at Gly93 (G93D) or a glycine residue at Pro104 (P104G) in the internal hydrophobic domain of E1 affected virus infectivity but not virus assembly. Viruses carrying G93D and P104G mutations had impaired infectivity, reduced 1,000-fold and 10-fold, respectively. A revertant was isolated from the G93D mutant. Sequencing analysis showed that the substituted aspartic acid residue in G93D mutant had reverted to the original glycine residue, suggesting the involvement of Gly93 in membrane fusion during viral entry.

Neutralizing monoclonal antibody to the E1 glycoprotein epitope of rubella virus mediates virus arrest in VERO cells

The best-known mechanism of action of antibody-mediated virus neutralization is to impede the entrance of viruses to host cells, as determined by neutralization assays. Antibodies may also inhibit the exit of rubella virus (RV) from infected host cells; in this case, the interaction of the antibodies with their domains must occur on the plasma membrane, because antibodies cannot enter the cells. In the present study, we were able to block temporally the exit of virions from RV-infected cells by the binding of monoclonal antibody (mAb) H3 to their surface. The objective was accomplished in three steps: first, we determined the duration of the viral replication cycle; then we established the kinetics of the presence of the domains defined by our mAbs in the cytoplasm of RV-infected VERO cells; and, finally, we assessed the release of viral particles to the supernatant of infected VERO cells in the presence or absence of mAbs or positive and negative mice sera. RV-specific mice sera and mAb H3, which binds to the amino acid sequence 208-239 of the RV-E1 glycoprotein, were able to delay for 24 hours the release of virions from infected cultures, suggesting that the reaction of mAb H3 with its epitope may arrest any change necessary for the assembly and/or release of virions. In conclusion, the neutralizing domain recognized by mAb induces antibodies that can block the viral replication by several mechanisms of action, such as the obstruction of virus entry into cells and the delay of viral release. All of these mechanisms are intimately involved in the critical virus-host cell interactions that allow self-limitation of the infection.

Rubella virus capsid associates with host cell protein p32 and localizes to mitochondria

Togavirus nucleocapsids have a characteristic icosahedral structure and are composed of multiple copies of a capsid protein complexed with genomic RNA. The assembly of rubella virus nucleocapsids is unique among togaviruses in that the process occurs late in virus assembly and in association with intracellular membranes. The goal of this study was to identify host cell proteins which may be involved in regulating rubella virus nucleocapsid assembly through their interactions with the capsid protein. Capsid was used as bait to screen a CV1 cDNA library using the yeast two-hybrid system. One protein that interacted strongly with capsid was p32, a cellular protein which is known to interact with other viral proteins. The interaction between capsid and p32 was confirmed using a number of different in vitro and in vivo methods, and the site of interaction between these two proteins was shown to be at the mitochondria. Interestingly, overexpression of the rubella virus structural proteins resulted in clustering of the mitochondria in the perinuclear region. The p32-binding site in capsid is a potentially phosphorylated region that overlaps the viral RNA-binding domain of capsid. Our results are consistent with the possibility that the interaction of p32 with capsid plays a role in the regulation of nucleocapsid assembly and/or virus-host interactions.

A single-amino-acid substitution of a tyrosine residue in the rubella virus E1 cytoplasmic domain blocks virus release

Rubella virus particles, consisting of a nucleocapsid surrounded by a lipid envelope in which two virus-encoded glycoproteins E1 and E2 are embedded, assemble on intracellular membranes and are secreted from cells, possibly via the cellular secretory pathway. We have recently demonstrated that the cytoplasmic domain of E1 (residues 469 to 481, KCLYYLRGAIAPR) is required for virus release. Alteration of cysteine 470 to alanine did not affect virus release, whereas mutation of leucine 471 to alanine reduced virus production by 90%. In the present study, substitutions of remaining amino acids in the E1 cytoplasmic domain were made in order to investigate the role of each amino acid in regulating rubella virus release. Generated mutants were analyzed in the context of infectious full-length cDNA clone and virus-like particles using combined genetic, biochemical, and electron microscopic approaches. Substitution of a single residue of tyrosine 472 to alanine or tyrosine 473 to serine resulted in a block in virus release without affecting protein transport and virus budding into the lumen of the Golgi complexes. Infectious RNA transcripts bearing these mutations were incapable of forming plaques. Mutants with substitutions at the amino-terminal region (leucine 474, arginine 475, and glycine 476) in the E1 cytoplasmic domain had reduced virus release and small-plaque phenotype, while mutants with substitutions at the carboxy-terminal region (alanine 477, isoleucine 478, alanine 479, proline 480, and arginine 481) had only marginal defects in virus release. Plaque-forming revertants could be isolated from mutants Y472A and Y473S. Sequencing analysis revealed that the substituted serine residue in mutant Y473S reverted to the original tyrosine residue, whereas the substituted alanine residue in mutant Y472A was retained. These results indicate that the E1 cytoplasmic domain modulates virus release in a sequence-dependent manner and that the tyrosine residues are critical for this function. We postulate that residues YYLRG constitute a domain in the E1 tail that may interact with other proteins and this interaction is involved in regulating virus release.

Cell surface monkey CD9 antigen is a coreceptor that increases diphtheria toxin sensitivity and diphtheria toxin receptor affinity

Monkey (Mk) CD9 antigen has been shown previously to increase the diphtheria toxin (DT) sensitivity of cells when co-expressed with Mk proHB-EGF (DT receptor). We have elucidated here the mechanism whereby Mk CD9 influences Mk proHB-EGF and present evidence that Mk CD9 is a coreceptor for DT. We observed that Mk CD9 not only increased the DT sensitivity but also increased the DT receptor affinity of cells. Furthermore, the higher the Mk CD9/Mk proHB-EGF ratio, the higher the affinity. In contrast, mouse (Ms) CD9 did not increase the toxin sensitivity or receptor affinity of cells when co-expressed with Mk proHB-EGF. Using Mk/Ms chimeric CD9 molecules, we determined that the second extracellular domain of Mk CD9 is responsible for both increased sensitivity and receptor affinity. This domain of Mk CD9 also interacts with Mk proHB-EGF in a yeast two-hybrid system. Our findings thus suggest that Mk CD9 has a direct physical interaction with Mk proHB-EGF to form a DT receptor complex and that this contact may change the conformation of the receptor to increase DT binding affinity and consequently increase toxin sensitivity. We thus propose that Mk CD9 is a coreceptor for DT.

Localization of rubella virus core particles in vero cells

Rubella virus (RV) infection induces a variety of morphological changes in the host cell including the modification of lysosomes to produce "replication complexes" and the alteration of mitochondrial morphology and distribution. The morphogenesis of RV was further characterized with particular emphasis on the localization of RV core particles. Thin-section electron microscopy (TSEM) studies indicated that RV core-like particles, measuring approximately 33 nm in diameter, were found associated with RV replication complexes. Immunogold-labeling electron microscopy (EM) using monoclonal antibodies to RV capsid proteins confirmed that these particles were viral cores. RV core particles were also detected in association with mitochondria as observed by TSEM and immunogold-labeling EM using monoclonal antibodies to capsid or polyclonal antibodies to RV virions. The results of this study indicate that the localization of RV core particles in relation to replication complexes is similar to that found for the alphaviruses. However, the association of RV core particles with mitochondria appears unique within the family Togaviridae.

Mutational analysis, using a full-length rubella virus cDNA clone, of rubella virus E1 transmembrane and cytoplasmic domains required for virus release

We report on the construction of a full-length cDNA clone, pBRM33, derived from wild-type rubella virus M33 strain. The RNA transcripts synthesized in vitro from pBRM33 are highly infectious, and the viruses produced retain the phenotypic characteristics of the parental M33 virus in growth rate and plaque size. This cDNA clone was used to study the role of E1 transmembrane and cytoplasmic domains in virus assembly by site-directed mutagenesis. Three different alanine substitutions were introduced in the transmembrane domain of E1. These included substitution of leucine 464, cysteine 466, cysteine 467, and both cysteines 466 and 467 to alanine. In the E1 cytoplasmic domain, cysteine 470 and leucine 471 were altered to alanine. We found that these mutations did not significantly affect viral RNA replication, viral structural protein synthesis and transport, or E2/E1 heterodimer formation. Except for the substitution of cysteine 470, these mutations did, however, lead to a reduction in virus release. Substitution of cysteine 467 in the transmembrane region and of leucine 471 in the cytoplasmic domain dramatically reduced virus yield, resulting in the production of only 1 and 10% of the parental virus yield, respectively, in a parallel infection. These data show that E1 transmembrane and cytoplasmic domains play an important role in late stages of virus assembly, possibly during virus budding, consistent with earlier studies indicating that the E1 cytoplasmic domain may interact with nucleocapsids and that this interaction drives virus budding.

Role of rubella virus glycoprotein domains in assembly of virus-like particles

Rubella virus is a small enveloped positive-strand RNA virus that assembles on intracellular membranes in a variety of cell types. The virus structural proteins contain all of the information necessary to mediate the assembly of virus-like particles in the Golgi complex. We have recently identified intracellular retention signals within the two viral envelope glycoproteins. E2 contains a Golgi retention signal in its transmembrane domain, whereas a signal for retention in the endoplasmic reticulum has been localized to the transmembrane and cytoplasmic domains of E1 (T. C. Hobman, L. Woodward, and M. G. Farquhar, Mol. Biol. Cell 6:7-20, 1995; T. C. Hobman, H. F. Lemon, and K. Jewell, J. Virol. 71:7670-7680, 1997). In the present study, we have analyzed the role of these retention signals in the assembly of rubella virus-like particles. Deletion or replacement of these domains with analogous regions from other type I membrane glycoproteins resulted in failure of rubella virus-like particles to be secreted from transfected cells. The E1 transmembrane and cytoplasmic domains were not required for targeting of the structural proteins to the Golgi complex and, surprisingly, assembly and budding of virus particles into the lumen of this organelle; however, the resultant particles were not secreted. In contrast, replacement or alteration of the E2 transmembrane or cytoplasmic domain, respectively, abrogated the targeting of the structural proteins to the budding site, and consequently, no virion formation was observed. These results indicate that the transmembrane and cytoplasmic domains of E2 and E1 are required for early and late steps respectively in the viral assembly pathway and that rubella virus morphogenesis is very different from that of the structurally similar alphaviruses.

Effects of mutations in the rubella virus E1 glycoprotein on E1-E2 interaction and membrane fusion activity

Rubella virus (RV) virions contain two glycosylated membrane proteins, E1 and E2, that exist as a heterodimer and form the viral spike complexes on the virion surface. Formation of an E1-E2 heterodimer is required for transport of E1 out of the endoplasmic reticulum lumen to the Golgi apparatus and plasma membrane. To investigate the nature of the E1-E2 interaction, we have introduced mutations in the internal hydrophobic region (residues 81 to 109) of E1. Substitution of serine at Cys82 (mutant C82S) or deletion of this hydrophobic domain (mutant dt) of E1 resulted in a disruption of the E1 conformation that ultimately affected E1-E2 heterodimer formation and cell surface expression of both E1 and E2. Substitution of either aspartic acid at Gly93 (G93D) or glycine at Pro104 (P104G) was found to impair neither E1-E2 heterodimer formation nor the transport of E1 and E2 to the cell surface. Fusion of RV-infected cells is induced by a brief treatment at a pH below 6. 0. To test whether this internal hydrophobic domain is involved in the membrane fusion activity of RV, transformed BHK cell lines expressing either wild-type or mutant spike proteins were exposed to an acidic pH and polykaryon formation was measured. No fusion activity was observed in the C82S, dt, and G93D mutants; however, the wild type and the P104G mutant exhibited fusogenic activities, with greater than 60% and 20 to 40% of the cells being fused, respectively, at pH 4.8. These results suggest that it is likely that the region of E1 between amino acids 81 and 109 is involved in the membrane fusion activity of RV and that it may be important for the interaction of that protein with E2 to form the E1-E2 heterodimer.

Characterization of an endoplasmic reticulum retention signal in the rubella virus E1 glycoprotein

Rubella virus contains three structural proteins, capsid, E2, and E1. E2 and E1 are type I membrane glycoproteins that form a heterodimer in the endoplasmic reticulum (ER) before they are transported to and retained in the Golgi complex, where virus assembly occurs. The bulk of unassembled E2 and E1 subunits are not transported to the Golgi complex. We have recently shown that E2 contains a Golgi-targeting signal that mediates retention of the E2-E1 complex (T. C. Hobman, L. Woodward, and M. G. Farquhar, Mol. Biol. Cell 6:7-20, 1995). The focus of this study was to determine if E1 glycoprotein also contains intracellular targeting information. We constructed a series of chimeric reporter proteins by fusing domains from E1 to the ectodomains of two other type I membrane proteins which are normally transported to the cell surface, vesicular stomatitis virus G protein (G) and CD8. Fusion of the E1 transmembrane and cytoplasmic regions, but not analogous domains from two control membrane proteins, to the ectodomains of G and CD8 proteins caused the resulting chimeras to be retained in the ER. Association of the ER-retained chimeras with known ER chaperone proteins was not detected. ER localization required both the transmembrane and cytoplasmic regions of E1, since neither of these domains alone was sufficient to retain the reporter proteins. Increasing the length of the E1 cytoplasmic domain by 10 amino acids completely abrogated ER retention. This finding also indicated that the chimeras were not retained as a result of misfolding. In summary, we have identified a new type of ER retention signal that may function to prevent unassembled E1 subunits and/or immature E2-E1 dimers from reaching the Golgi complex, where they could interfere with viral assembly. Accordingly, assembly of E2 and E1 would mask the signal, thereby allowing transport of the heterodimer from the ER.

Characterization of an overlapping CD8+ and CD4+ T-cell epitope on rubella capsid protein

A synthetic peptide corresponding to rubella virus capsid protein residues 263 to 275 which contains an epitope recognized by a cloned CD4+ cytotoxic T-lymphocyte (CTL) line was used to induce CD8+ T-cell lines specific to this peptide. A peptide-specific CD8+ CTL clone was derived and characterized. This peptide-specific CD8+ CTL clone exhibited cytotoxicity against target cells infected by a vaccinia recombinant virus expressing rubella virus capsid protein, but not by target cells infected by vaccinia recombinant virus expressing rubella virus E1 or E2 envelope proteins. Analysis of HLA class I restriction of the CD8+ CTL clone revealed that A11 and A3 were restrictive elements. Fine mapping with truncated and overlapping peptide analogs revealed a nonamer sequence, C(264-272), as the T-cell epitope eliciting stronger cytotoxicity. Two anchor residues for binding to HLA A11 and A3 were identified at position 2 (isoleucine) and at position 9 (histidine) or at position 8 (arginine) of the epitope sequence. The identification of overlapping CD4+ and CD8+ T-cell epitopes within the capsid protein sequence C(263-275) implicates a strategy for using such epitopes in a candidate peptide-based rubella vaccine.

Synthesis of soluble rubella virus spike proteins in two lepidopteran insect cell lines: large scale production of the E1 protein

The two envelope glycoproteins of rubella virus (RV), E1 of 58 kDa and E2 of 42-47 kDa, were individually expressed in lepidopteran Spodoptera frugiperda as well as in Trichoplusia ni insect cells using baculovirus vectors. The authentic signal sequences of E1 and E2 were replaced with the honeybee melittin signal sequence, allowing efficient entrance into the secretory pathway of the insect cell. In addition, the hydrophobic transmembrane anchors at the carboxyl termini of E1 and E2 proteins were removed to enable secretion rather than maintenance in the cellular membranes. Synthesis of the recombinant proteins in the absence and presence of tunicamycin revealed that both E1 and E2 were glycosylated with apparent molecular weights of 52 kDa and 37 kDa, respectively. Recombinant E2 appeared to be partially secreted, whereas E1 was essentially found inside the infected insect cell. The E1 protein was produced in large scale using a 10-1 bioreactor and serum-free medium (SFM). Purification of the recombinant protein product was performed from cytoplasmic extracts by ammonium sulphate precipitation followed by Concanavalin A affinity chromatography. This type of purified recombinant viral glycoproteins may be useful not only in diagnostic medicine or for immunization, but should enable studies designed to solve the structure of the virus particle.

Detection of rubella virus-specific immunoglobulin M antibodies with a baculovirus-expressed E1 protein

The structural proteins of rubella virus (RV) were expressed in insect cells by using the baculovirus expression vector system. The recombinant E1 envelope glycoprotein was purified by immunoaffinity chromatography and used to detect RV-specific immunoglobulin M antibodies in a time-resolved fluoroimmunoassay. Correlation analysis between the reactivities of antibodies against this recombinant E1 and the reactivities against authentic RV antigen shows that purified E1 can detect RV antibodies of the immunoglobulin M type.

Envelopment of varicella-zoster virus: targeting of viral glycoproteins to the trans-Golgi network

Previous studies suggested that varicella-zoster virus derives its final envelope from the trans-Golgi network (TGN) and that envelope glycoproteins (gps) are transported to the TGN independently of nucleocapsids. We tested the hypothesis that gpI is targeted to the TGN as a result of a signal sequence or patch encoded in its cytosolic domain. cDNAs encoding gpI wild type (wt) and a truncated mutant gpI(trc) lacking transmembrane and cytosolic domains were cloned by using the PCR. Cells transfected with cDNA encoding gpI(wt) or gpI(trc) synthesized and N glycosylated the proteins. gpI(wt) accumulated in the TGN, some reached the plasmalemma, but none was secreted. In contrast, gpI(trc) was retained and probably degraded in the endoplasmic reticulum; none was found on cell surfaces, but some was secreted. The distribution of gpI(trc) was not affected by deletion of potential glycosylation sites. To locate a potential gpI-targeting sequence, cells were transfected with cDNA encoding chimeric proteins in which the ectodomain of a plasmalemmal marker, the interleukin-2 receptor (tac), was fused to different domains of gpI. A chimeric protein in which tac was fused with the transmembrane and cytoplasmic domains of gpI was targeted to the TGN. In contrast, a chimeric protein in which tac was fused only with the gpI transmembrane domain passed through the TGN and concentrated in endosomes. We conclude that gpI is targeted to the TGN as a result of a targeting sequence or patch in its cytosolic domain.

Expression of the rubella virus structural proteins by an infectious Sindbis virus vector

To assess the potential of an infectious Sindbis virus vector (dsSIN) as a eukaryotic expression vector, dsSIN recombinants which contain each of the rubella virus (RUB) structural proteins (C, E2, and E1) individually were constructed. Expression of the RUB proteins by each dsSIN recombinant was robust and processing was similar to that observed when these proteins were expressed using other vectors. The C and E2 recombinants, each of which contains a cassette of less than 1,000 nts, were stable through low multiplicity amplification; however, the E1 recombinant, which contains a 1700 nt cassette, was not. Therefore, use of the E1 recombinant is restricted to stock derived from the original transfection. The replication rate of dsSIN and the C and E2 recombinants was similar, however, the replication rate of the E1 recombinant was slower. No phenotypic mixing of any of the RUB proteins in recombinant dsSIN virions could be detected.

The two major envelope proteins of equine arteritis virus associate into disulfide-linked heterodimers

In a coimmunoprecipitation assay with monospecific antisera, the two major envelope proteins GL and M of equine arteritis virus were found to occur in heteromeric complexes in virions and infected cells. While the GL protein associated with M rapidly and efficiently, newly synthesized M protein was incorporated into complexes at a slower rate, which implies that it interacts with GL molecules synthesized earlier. Analysis under nonreducing conditions revealed that the GL/M complexes consist of disulfide-linked heterodimeric structures. Pulse-chase experiments showed that virtually all GL monomers ended up in heterodimers, whereas a fraction of the M protein persisted as monomers. The M protein also formed covalently linked homodimers, but only the heterodimers were incorporated into virus particles.

Synthesis and processing of the rubella virus p110 polyprotein precursor in baculovirus-infected Spodoptera frugiperda cells

In order to study the processing of rubella virus (RV) structural proteins (capsid protein, of 33 kDa; E2 of 42-47 kDa; and E1 of 58 kDa) in Spodoptera frugiperda (fall armyworm) cells, a 24S cDNA encoding the polyprotein precursor, p110, was inserted under the transcriptional regulation of the polyhedrin gene promoter of the Autographa californica nuclear polyhedrosis virus (AcNPV) and expressed during viral infection. By immunoblot analysis using antibodies directed against whole RV and the individual structural proteins, evidence is presented that polypeptides similar to those synthesized in RV-infected B-Vero cells are expressed in this lepidopteran insect cell line infected with the recombinant baculovirus, VL1392-RV24S. The identity of the recombinant proteins was further confirmed using human convalescent sera. By expressing the recombinant proteins in the presence and absence of tunicamycin, we have further demonstrated that the 24S transcription-translation unit of RV, is expressed and proteolytically cleaved similarly, if not identically, in Sf9 cells as compared to B-Vero cells.

Targeting of a heterodimeric membrane protein complex to the Golgi: rubella virus E2 glycoprotein contains a transmembrane Golgi retention signal

Rubella virus (RV) envelope glycoproteins, E2 and E1, form a heterodimeric complex that is targeted to medial/trans-Golgi cisternae. To identify the Golgi targeting signal(s) for the E2/E1 spike complex, we constructed chimeric proteins consisting of domains from RV glycoproteins and vesicular stomatitis virus (VSV) G protein. The location of the chimeric proteins in stably transfected Chinese hamster ovary cells was determined by immunofluorescence, immunoelectron microscopy, and by the extent of processing of their N-linked glycans. A trans-dominant Golgi retention signal was identified within the C-terminal region of E2. When the transmembrane (TM) and cytoplasmic (CT) domains of VSV G were replaced with those of RV E2, the hybrid protein (G-E2TMCT+) was retained in the Golgi. Transport of G-E2TMCT+ to the Golgi was rapid (t1/2 = 10-20 min). The G-E2TMCT+ protein was determined to be distal to or within the medial Golgi based on acquisition of endo H resistance but proximal to the trans-Golgi network since it lacked sialic acid. Deletion analysis revealed that only the TM domain of E2 was required for Golgi targeting. Although the cytoplasmic domain of E2 was not necessary for Golgi retention, it was required for efficient transport of VSV G-RV chimeras out of the endoplasmic reticulum. When assayed in sucrose velocity sedimentations gradients, the Golgi-retained G-E2TMCT+ protein behaved as a dimer. Unlike virtually all other Golgi targeting signals, the E2 TM domain does not contain any polar amino acids. The TM and CT domains of E1 were not required for targeting of E2 and E1 to the Golgi indicating that a heterodimer of two integral membrane proteins can be retained in the Golgi by a single retention signal.

Expression and characterization of a soluble rubella virus E1 envelope protein

Individual specific antigenic rubella virus (RV) structural proteins are required for accurate serological diagnosis of acute and congenital rubella infections as well as rubella immune status. The RV envelope glycoprotein E1 is the major target antigen and plays an important role in viral-specific immune responses. The native virion is difficult to produce in large quantities and the protein subunits are also difficult to isolate without loss of antigenicity. The production of a soluble RV E1 (designated E1 delta Tm) using the baculovirus-insect cell expression system is described. In contrast to wild-type RV E1, the genetically engineered E1 delta Tm protein lacks a transmembrane anchor. It behaved as a secretory protein and was secreted abundantly from insect cells. Pulse-chase studies were used to examine the synthesis, glycosylation, and secretion of E1 delta Tm by the insect cells. The secreted E1 delta Tm protein was purified from serum-free medium by one-step immunochromatography. The purified E1 delta Tm protein retained full antigenicity and may be a convenient source of E1 protein for use in diagnostic assay and rubella vaccine development.

Expression and characterization of virus-like particles containing rubella virus structural proteins

Rubella virus (RV) virions contain two envelope glycoproteins (E1 and E2) and a capsid protein (C). Noninfectious RV-like particles (VLPs) containing three structural proteins were expressed in a BHK cell line (BHK-24S) by using an inducible promoter. These VLPs were found to resemble RV virons in terms of their size, their morphology, and some biological activities. In immunoblotting studies, VLPs were found to bind similarly to native RV virions with 10 of a panel of 12 RV-specific murine monoclonal antibodies. Immunization of mice with VLPs induced specific antibody responses against RV structural proteins as well as virus-neutralizing and hemagglutination-inhibiting antibodies. After immunization of mice with VLPs, in vitro challenge of isolated lymphocytes with inactivated RV and individual RV structural proteins stimulated proliferation. Our data suggest the possibility of using VLPs as immunogens for serodiagnostic assays and RV vaccines.

Expression of soluble forms of rubella virus glycoproteins in mammalian cells

Rubella virus (RV) virions contain two envelope glycoproteins, E1 and E2. Removal of hydrophobic regions in their carboxyl termini by genetic engineering caused them to be secreted rather than maintained in cell membranes of transfected COS cells. Truncated E2 was secreted in the absence of E1, whereas E1 lacking its transmembrane domain required coexpression of E2 for export from the cell. Secreted E2 was found to contain both O-linked and N-linked complex glycans, whereas secreted E1 retained virus neutralization and hemagglutination epitopes, suggesting the possibility of using soluble RV antigens as subunit vaccines and for serodiagnostic purposes. Stable Chinese hamster ovary cell lines secreting RV E1 were constructed for large scale preparation of recombinant E1.

Molecular biology of rubella virus

This chapter summarizes the present medical significance of rubella virus. Rubella virus infection is systemic in nature and the accompanying symptoms are generally benign, the most pronounced being a mild rash of short duration. The most common complication of rubella virus infection is transient joint involvement such as polyarthralgia and arthritis. The primary health impact of rubella virus is that it is a teratogenic agent. The vaccination strategy is aimed at elimination of rubella and includes both universal vaccination of infants at 15 months of age with the trivalent measles, mumps, rubella (MMR) vaccine and specific targeting with the rubella vaccine of seronegative women planning pregnancy and seronegative adults who could come in contact with women of childbearing age, although it is recommended that any individual over the age of 12 months without evidence of natural infection or vaccination be vaccinated. Medically, the current challenge posed by rubella virus is to achieve complete vaccination coverage to prevent resurgences.

Expression of virus-encoded proteinases: functional and structural similarities with cellular enzymes

Many viruses express their genome, or part of their genome, initially as a polyprotein precursor that undergoes proteolytic processing. Molecular genetic analyses of viral gene expression have revealed that many of these processing events are mediated by virus-encoded proteinases. Biochemical activity studies and structural analyses of these viral enzymes reveal that they have remarkable similarities to cellular proteinases. However, the viral proteinases have evolved unique features that permit them to function in a cellular environment. In this article, the current status of plant and animal virus proteinases is described along with their role in the viral replication cycle. The reactions catalyzed by viral proteinases are not simple enzyme-substrate interactions; rather, the processing steps are highly regulated, are coordinated with other viral processes, and frequently involve the participation of other factors.

Targeting of viral glycoproteins to the Golgi complex

Certain enveloped viruses are known to assemble on membranes of the Golgi complex. Intracellular budding is facilitated by targeting of the viral glycoproteins to this organelle. It is likely that these viral glycoproteins are retained in the Golgi by the same means as are endogenous Golgi proteins. Consequently, the study of Golgi-specific viral proteins has provided important clues to the nature of Golgi retention signals.

The rubella virus E2 and E1 spike glycoproteins are targeted to the Golgi complex

Rubella virus (RV) has been reported to bud from intracellular membranes in certain cell types. In this study the intracellular site of targeting of RV envelope E2 and E1 glycoproteins has been investigated in three different cell types (CHO, BHK-21 and Vero cells) transfected with a cDNA encoding the two glycoproteins. By indirect immunofluorescence, E2 and E1 were localized to the Golgi region of all three cell types, and their distribution was disrupted by treatment with BFA or nocodazole. Immunogold labeling demonstrated that E2 and E1 were localized to Golgi cisternae and indicated that the glycoproteins were distributed across the Golgi stack. Analysis of immunoprecipitates obtained from stably transfected CHO cells revealed that E2 and E1 become endo H resistant and undergo sialylation without being transported to the cell surface. Transport of RV glycoproteins to the Golgi complex was relatively slow (t1/2 = 60-90 min). Coprecipitation experiments indicated that E2 and E1 form a heterodimer in the RER. E1 was found to fold much more slowly than E2, suggesting that the delay in transport of the heterodimer to the Golgi may be due to the slow maturation of E1 in the ER. These results indicate that RV glycoproteins behave as integral membrane proteins of the Golgi complex and thus provide a useful model to study targeting and turnover of type I membrane proteins in this organelle.

Human T- and B-cell epitopes of E1 glycoprotein of rubella virus

The identification of T- and B-cell sites recognized frequently by human populations could provide the basis for selecting the candidate T- and B-cell epitopes for the development of an effective synthetic vaccine against rubella. Rubella virus E1 glycoprotein has been shown to be the predominant antigen to which the majority of human populations develop lymphocyte proliferative and antibody responses. To define the T- and B-cell epitopes of E1 glycoprotein of rubella virus, 23 overlapping synthetic peptides corresponding to more than 90% of the amino acid sequence of E1 were synthesized and tested for their capacities to induce proliferative and antibody responses of 10 seropositive individuals. The most frequently recognized T-cell epitopes were EP19 (residues 324-343), with 7 of 10 responders, and both EP12 (residues 207-226) and EP17 (residues 289-308), with 6 of 10 responders, respectively. Two immunodominant linear B-cell epitopes were mapped to residues 157 to 176 (EP9, 8/10) and 374 to 390 (EP22, 6/10) by using peptide-specific enzyme linked immunosorbent assay.

The influence of N-linked glycosylation on the antigenicity and immunogenicity of rubella virus E1 glycoprotein

Rubella virus E1 glycoprotein contains three functional N-linked glycosylation sites. The role of N-linked glycosylation on the antigenicity and immunogenicity of E1 glycoprotein was studied using vaccinia recombinants expressing E1 glycosylation mutants. Expressed E1 glycosylation mutant proteins were recognized by a panel of E1-specific monoclonal antibodies in radioimmunoprecipitation, immunofluorescence, and immunoblotting, indicating that carbohydrate side chains on E1 are not involved in the constitution of epitopes recognized by these monoclonal antibodies. This observation was further supported by the fact that removal of oligosaccharides on E1 by glycosidase digestion did not significantly change the antigenicity of E1. All the glycosylation mutants were capable of eliciting anti-RV E1 antibodies. The single glycosylation mutants (G1, G2, and G3), but not the double mutant (G23) or the triple mutant (G123), were found to be capable of inducing virus neutralizing antibodies. Among the single glycosylation mutants, only G2 and G3 were active in producing hemagglutination inhibition antibodies in mice. Our findings suggest that although carbohydrate on E1 is not directly involved in the antigenic structures of E1, it is important in maintaining proper protein folding and stable conformation for expression of immunological epitopes on E1.

Cellular and humoral immune responses to rubella virus structural proteins E1, E2, and C

Better understanding of cell-mediated immune responses to rubella virus would provide the basis for the development of safe and effective vaccines against rubella and would aid in analysis of the pathophysiology of congenital rubella syndrome. We have expressed individual rubella virus structural proteins, E1, E2 and C, via vaccinia virus recombinants. Using the expressed recombinant proteins as antigens, we were able to demonstrate antigen-specific lymphocyte proliferative responses in control individuals and individuals with congenital rubella syndrome. Among the two human groups studied, E1 glycoprotein proved to be a better immunogen than E2 or C. For the control individuals, significant differences in proliferative responses to the structural proteins E1, E2, and C were observed. These differences were not significant in individuals with congenital rubella syndrome. In parallel to the lymphoproliferative responses, immunoglobulin G responses were also found directed mainly to the E1 glycoprotein. These results suggest that E1 may be the most important rubella virus antigen to study in determining the domains required for constructing subunit vaccines against rubella.

The rubella virus E1 glycoprotein is arrested in a novel post-ER, pre-Golgi compartment

Evidence is accumulating that a distinct compartment(s) exists in the secretory pathway interposed between the rough ER (RER) and the Golgi stack. In this study we have defined a novel post-RER, pre-Golgi compartment where unassembled subunits of rubella virus (RV) E1 glycoprotein accumulate. When RV E1 is expressed in CHO cells in the absence of E2 glycoprotein, transport of E1 to the Golgi complex is arrested. The compartment in which E1 accumulates consists of a tubular network of smooth membranes which is in continuity with the RER but has distinctive properties from either the RER, Golgi, or previously characterized intermediate compartments. It lacks RER and Golgi membrane proteins and is not disrupted by agents which disrupt either the RER (thapsigargin, ionomycin) or Golgi (nocodazole and brefeldin A). However, luminal ER proteins bearing the KDEL signal have access to this compartment. Kinetically the site of E1 arrest lies distal to or at the site where palmitylation occurs and proximal to the low temperature 15 degrees C block. Taken together the findings suggest that the site of E1 arrest corresponds to, or is located close to the exit site from the ER. This compartment could be identified morphologically because it is highly amplified in cells overexpressing unassembled E1 subunits, but it may have its counterpart among the transitional elements of non-transfected cells. We conclude that the site of E1 arrest may represent a new compartment or a differentiated proximal moiety of the intermediate compartment.

Localization of the virus neutralizing and hemagglutinin epitopes of E1 glycoprotein of rubella virus

Current serological assays using whole rubella virus (RV) as a target antigen for detecting RV-specific antibodies fail to define specific RV proteins and antigenic determinants such as hemagglutinin (HA) and virus-neutralizing (VN) epitopes of rubella virus. A panel of E1 deletion mutants and a subset of E1-specific monoclonal antibodies (MAb) were used for the initial analysis of HA and VN epitopes of E1 glycoprotein. A peptide region (E1(193) to E1(269)) was found to contain HA and VN epitopes. Using both overlapping synthetic peptides and truncated fusion proteins within this region, the HA epitope defined by MAb 3D9F mapped to amino acid residues E1(214) to E1(240), while two VN epitopes defined by MAb 21B9H and MAb 16A10E mapped to amino acid residues E1(214) to E1(233) and E1(219) to E1(233), respectively. The epitopes defined in this study are recognized by antibody whether or not the epitopes are glycosylated.

Role of N-linked oligosaccharides in processing and intracellular transport of E2 glycoprotein of rubella virus

The role of N-linked glycosylation in processing and intracellular transport of rubella virus glycoprotein E2 has been studied by expressing glycosylation mutants of E2 in COS cells. A panel of E2 glycosylation mutants were generated by oligonucleotide-directed mutagenesis. Each of the three potential N-linked glycosylation sites was eliminated separately as well as in combination with the other two sites. Expression of the E2 mutant proteins in COS cells indicated that in rubella virus M33 strain, all three sites are used for the addition of N-linked oligosaccharides. Removal of any of the glycosylation sites resulted in slower glycan processing, lower stability, and aberrant disulfide bonding of the mutant proteins, with the severity of defect depending on the number of deleted carbohydrate sites. The mutant proteins were transported to the endoplasmic reticulum and Golgi complex but were not detected on the cell surface. However, the secretion of the anchor-free form of E2 into the medium was not completely blocked by the removal of any one of its glycosylation sites. This effect was dependent on the position of the deleted glycosylation site.

Diagnostic potential of baculovirus-expressed rubella virus envelope proteins

The envelope glycoproteins E1 and E2 of rubella virus were abundantly expressed in Spodoptera frugiperda Sf9 insect cells by using a baculovirus expression vector. The recombinant protein products were purified by immunoaffinity chromatography and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and enzyme immunoassay (EIA). The purified recombinant antigen consisted of the envelope polypeptides, corresponding to the viral E1 and E2 proteins, and a polyprotein precursor (molecular mass, 90 to 95 kDa). The antigen was reactive with human convalescent-phase sera in immunoblot analysis, and the reactivity correlated well (r = 0.861) with that of a whole-virus antigen when tested by EIA by using a total of 106 rubella virus immunoglobulin G-positive and -negative serum specimens. When the sera from patients with recent rubella virus infection were tested with the recombinant glycoproteins by EIA, the correlation was not as close (r = 0.690). However, all of the 26 serum specimens were reactive with the recombinant antigen. The results demonstrate that these bioengineered antigens have a potential for use in routine diagnostic assays of rubella virus immunity and recent infection.

Vaccinia-vectored expression of the rubella virus structural proteins and characterization of the E1 and E2 glycosidic linkages

The maturation of rubella virus (RV) glycoprotein E2 from the single intracellular species (E2i; MW = 40 kDa) to the heterodisperse virion species (E2v; MW = 42 to 47 kDa), was studied by pulse-chase radiolabeling in Vero cells infected with RV or with recombinant vaccinia viruses (VVs) which express the entire RV structural protein open reading frame (VV-CE2E1) or glycoprotein E2 independently (VV-E2). The RV proteins expressed by the recombinant VVs comigrated with authentic RV intracellular proteins. In pulse-chase experiments, performed in both RV- and VV-CE2E1-infected cells, the amount of pulse-labeled E2i was substantially reduced during a 3- to 4-hr chase; during the same chase the amount of pulse-labeled E1 and C did not change. The concomitant appearance of the E2v forms was not observed. In contrast, in VV-E2-infected cells, no reduction in the amount of E2i occurred after as long as a 10-hr chase. Western blots using anti-E2 monoclonal antibodies showed that E2i was the predominant E2 species in cells infected with RV, VV-CE2E1, and VV-E2. However, minor amounts of three discrete species which comigrated within the extent of the E2v smear were also detected in cells infected with all three viruses, indicating that some degree of intracellular processing to E2v did occur. The disappearance of E2i during pulse-chase radiolabeling without the concomitant appearance of detectable E2v and the predominance of this labile form under steady-state conditions as revealed by Western blot analysis suggested that E2i was selectively turned over in both RV- and VV-CE2E1-infected cells. Such turnover was not apparent in VV-E2-infected cells, indicating that association with C and E1 was necessary for turnover to occur. Endoglycosidase digestion experiments and glycan differentiation assays revealed that E2v contained O-linked glycans. The presence of O-glycans on E2v accounted for part of the difference in size between E2v and E2i. Both virion E1 and E2 were found to contain high-mannose, hybrid-type, and complex-type N-glycans. Heterogeneity existed in the extent of processing of these glycans among individual E1 and E2 molecules.

The influence of capsid protein cleavage on the processing of E2 and E1 glycoproteins of rubella virus

The structural polyprotein of rubella virus is cotranslationally processed by host cell signal peptidase. Oligonucleotide-directed mutagenesis was used to alter the cleavage site between capsid and E2 proteins and to examine the importance of this cleavage for the transport and processing of E2 and E1 glycoproteins. The in vitro and in vivo expression of the cleavage site mutant revealed that the E2 polypeptide can cross the endoplasmic reticulum membrane without the cleavage of its signal peptide, while the transport of E2 beyond the endoplasmic reticulum requires the cleavage of E2 from capsid. We have shown that capsid protein does not appear to undergo further proteolytic processing after it is cleaved from E2 by signal peptidase. Some of the requirements for the cleavage by signal peptidase between capsid and E2 were examined by the in vitro analysis of wild-type and mutant cDNAs.

Analysis of rubella virus E1 glycosylation mutants expressed in COS cells

cDNA clones encoding the envelope glycoprotein E1 of rubella virus (RV) were altered by site-directed mutagenesis at consensus sites for addition of N-linked glycans. The resulting plasmids were introduced into COS cells and the mutant E1 proteins were analyzed by indirect immunofluorescence, radioimmunoprecipitation, and immunoblotting. We found that RV E1 contains three N-linked oligosaccharides, each approximately 2 kDa in size. Although lack of glycosylation did not appear to affect targeting of E1 to the Golgi region, mutants lacking N-linked glycans at Asn 177 and Asn 209 failed to bind anti-E1 antibodies under nonreducing conditions. Our results suggest that glycosylation may be important for expression of important immunologic epitopes on RV E1.