Molecular cloning and sequencing of the region of the rubella virus genome coding for glycoprotein E1

Mutagenic analysis of the 3' cis-acting elements of the rubella virus genome

Thermodynamically predicted secondary structure analysis of the 3'-terminal 305 nucleotides (nt) of the rubella virus (RUB) genome, a region conserved in all RUB defective interfering RNAs, revealed four stem-loop (SL) structures; SL1 and SL2 are both located in the E1 coding region, while SL3 and SL4 are within the 59-nt 3' untranslated region (UTR) preceding the poly(A) tract. SL2 is a structure shown to interact with human calreticulin (CAL), an autoantigen potentially involved in RUB RNA replication and pathogenesis. RNase mapping indicated that SL2 and SL3 are in equilibrium between two conformations, in the second of which the previously proposed CAL binding site in SL2, a U-U bulge, is not formed. Site-directed mutagenesis of the 3' UTR with a RUB infectious clone, Robo302, revealed that most of the 3' UTR is required for viral viability except for the 3'-terminal 5 nt and the poly(A) tract, although poly(A) was rapidly regenerated during subsequent replication. Maintenance of the overall SL3 structure, the 11-nt single-stranded sequence between SL3 and SL4, and the sequences forming SL4 were all important for viral viability. Studies on the interaction between host factors and the 3' UTR showed the formation of three RNA-protein complexes by gel mobility shift assay, and UV-induced cross-linking detected six host protein species, with molecular masses of 120, 80, 66, 55, 48, and 36 kDa, interacting with the 3' UTR. Site-directed mutagenesis of SL2 by nucleotide substitutions showed that maintenance of SL2 stem rather than the U-U bulge was critical in CAL binding since mutants having the U-U bulge base paired had a similar binding activity for CAL as the native structure whereas mutants having the SL2 stem destabilized had much lower binding activity. However, all of these mutations gave rise to viable viruses when introduced into Robo302, indicating that binding of CAL to SL2 is independent of viral viability.

Characterization of defective-interfering RNAs of rubella virus generated during serial undiluted passage

During serial undiluted passage of rubella virus (RUB) in Vero cells, two species of defective-interfering (DI) RNAs of approximately 7000 and 800 nucleotides (nts) in length were generated (Frey, T. K., and Hemphill, M. L., Virology 164, 22-29, 1988). In this study, these DI RNAs were characterized by molecular cloning, hybridization with probes of defined sequence, and primer extension. The 7000-nt DI RNA species were found to be authentic DI RNAs which contain a single 2500- to 2700-nt deletion in the structural protein open reading frame (ORF) region of the genome. The 800-nt RNAs were found to be subgenomic DI RNAs synthesized from the large DI RNA templates. Analysis of the extent of the deletions using a reverse-transcription-PCR protocol revealed that the 3' end of the deletions did not extend beyond the 3' terminal 244 nts of the genome. The 5' end of the deletions did not extend into the nonstructural protein ORF; however, DI RNAs in which the subgenomic start site was deleted were present. Following serial undiluted passage of seven independent stocks of RUB, this was the only pattern of DI RNAs generated. DI RNAs of 2000 to 3000 nt in length were the majority DI RNA species in a persistently infected line of Vero cells, showing that other types of RUB DI RNAs can be generated and selected. However, when supernatant from the persistently infected cells was passaged, the only DI RNAs present after two passages were 7000 nts in length, indicating that this species has a selective advantage over other types of DI RNAs during serial passage.

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.

Construction of rubella virus genome-length cDNA clones and synthesis of infectious RNA transcripts

Plasmids containing a complete cDNA copy of the rubella virus (RUB) genomic RNA were constructed. Transfection into cell culture of genome-length RNA transcribed in vitro from one of these cDNA clones, Robo102, resulted in the production of virus which preserved the genetic and phenotypic characteristics of the parental virus from which the cDNA clone was derived. Prior to construction of the RUB genome-length cDNA clones, the 5'-terminal sequence of the RUB genomic RNA was determined to be 5'CAAUGG...3' following the cap structure. Analysis of the specific infectivity of RUB genomic RNA isolated from virions revealed that in Vero cells, the specific infectivity of RUB genomic RNA is roughly equivalent to that of Sindbis virus genomic RNA. In RUB virion RNA preparations, the subgenomic RNA was detected. It was demonstrated that subgenomic RNA was packaged into RUB virions; however, the presence of the subgenomic RNA was not essential for infectivity of the genomic RNA.

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.

Identification of immunoreactive regions of rubella virus E1 and E2 envelope proteins by using synthetic peptides

Relatively large (16-33 aa) synthetic peptides (SPs) representing defined sequences of rubella virus (RV) E1 and E2 envelope proteins were used in lymphocyte stimulation and enzyme immunoassays to map immunoreactive regions recognized by peripheral blood mononuclear cells (PBMNC) and serum antibodies from healthy RV-seropositive, RV-seronegative, and RV-vaccinated adults. Five distinct immunoreactive regions were identified in RV E1 protein, spanning residues (11-39), (154-179), (199-239), (226-277), and (389-412), which stimulated cellular responses in 29-83% of the subjects tested. Two SPs, E1(213-239) and E1(258-277) containing previously-identified virus neutralizing antibody domains, reacted with serum antibodies and also stimulated lymphoproliferation suggesting that these E1 sequences contain linked or overlapping B-and T-cell antigenic sites. The frequency and magnitude of cellular responses to E2 SPs were somewhat lower. SPs encompassing E2 residues (50-72), (140-199), and (244-263) stimulated lymphocyte responses in 28-64% of the subjects tested, while to a lesser degree, SPs within residues (1-36) were also stimulatory. E2 SPs within the regions (1-36), (151-170), and (244-263) also showed low levels of antibody reactivity with sera from RV-seropositive subjects. E2(244-263) which induced the highest level of response among the E2 SPs tested, was of interest due to previous reports of sequence homology of this RV region with human myelin and its potential immunopathogenic role in demyelinating autoimmune diseases. Identification of these potentially immunodominant regions of RV envelope proteins is an important first step in the rational design of new RV vaccines.

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.

An antibody- and synthetic peptide-defined rubella virus E1 glycoprotein neutralization domain

We previously described a monoclonal antibody (MAb) library generated by infecting BALB/c mice with rubella virus (RV) and selected by an enzyme-linked immunosorbent assay (ELISA) using purified virion targets. Plasmid pARV02-01, which expresses the fusion protein RecA1-35-GIGDLGSP-E1(202)-E1(283)-GDP-LacZ9-1015 in Escherichia coli, was shown to be a ligand for MAbs E1-18 and E1-20 (J. S. Wolinsky, M. McCarthy, O. Allen-Cannady, W. T. Moore, R. Jin, S. N. Cao, A. Lovett, and D. Simmons, J. Virol. 65:3986-3994, 1991). Both of these MAbs neutralize RV infectivity. A series of five overlapping synthetic peptides was made to further explore the requirements of this MAb binding domain. One of these peptides (SP15; E1(208) to E1(239)) proved an effective ligand for both MAbs in the ELISA. Stepwise synthesis of SP15 defined the minimal amino-terminal requirement for binding MAb E1-18 as E1(221) and that of MAb E1-20 as E1(223); the minimal carboxyl-terminal requirement is uncertain but does not exceed E1(239). Immunization of mice and rabbits with SP15 induced polyvalent antibody reactive with SP15, with other overlapped and related but not unrelated synthetic peptides, and with RV. The rabbit anti-SP15 antibody showed neutralization activity to RV similar to that of MAbs E1-18 and E1-20 but lacked hemagglutination inhibition activity. These data define a neutralization domain on E1 and suggest that the RV epitopes conserved by SP15 may be critical for protective host humoral immune responses.

The cis-acting 3'-element of rubella virus RNA has DNA promoter activity

The 3'-terminal region of the rubella virus (RV) positive-strand RNA, referred to here as the cis-acting element (CAE), is implicated in the initiation of negative-strand RNA synthesis. Sequence analysis of the 3'-CAE shows that there is a putative TATA box which is surrounded by G + C-rich sequences. To determine whether this element, in a DNA form, has the capability to initiate transcription, a 3'-end 165-bp NarI-EcoRI fragment from the RV cDNA was cloned upstream from a cat reporter gene. The level of CAT activity was dependent on the presence of the 3'-CAE and the SV40 enhancer. Primer extension analysis of the CAT mRNA showed that the transcription start point is in the RV 3'-CAE, 34 bp downstream from the putative TATA box. DNA-gel shift analysis revealed that three nucleoprotein-specific complexes were formed with the 3'-CAE and the binding sites for these proteins were between bp -64 to -108. The possible promoter function of the RV 3'-CAE is discussed in context to RV persistence.

Specific binding of host cell proteins to the 3'-terminal stem-loop structure of rubella virus negative-strand RNA

At the 5' end of the rubella virus genomic RNA, there are sequences that can form a potentially stable stem-loop (SL) structure. The complementary negative-strand equivalent of the 5'-end SL structure of positive-strand rubella virus RNA [5' (+) SL structure] is thought to serve as a promoter for the initiation of positive-strand synthesis. We screened the negative-strand equivalent of the 5' (+) SL structure (64 nucleotides) and the adjacent region of the negative-strand RNA for their ability to bind to host cell proteins. Specific binding to the 64-nucleotide-long potential SL structure of three cytosolic proteins with relative molecular masses of 97, 79, and 56 kDa was observed by UV-induced covalent cross-linking. There was a significant increase in the binding of the 97-kDa protein from cells upon infection with rubella virus. Altering the SL structure by deleting sequences in either one of the two potential loops abolished the binding interaction. The 56-kDa protein also appeared to bind specifically to an SL derived from the 3' end of positive-strand RNA. The 3'-terminal structure of rubella virus negative-strand RNA shared the same protein-binding activity with similar structures in alphaviruses, such as Sindbis virus and eastern equine encephalitis virus. A possible role for the host proteins in the replication of rubella virus and alphaviruses is discussed.

Monoclonal antibody-defined epitope map of expressed rubella virus protein domains

An expanded library of murine monoclonal antibodies (MAbs) was generated by infecting BALB/C mice with the Therien strain of rubella virus (RV) and selecting secreting hybrids by enzyme-linked immunosorbent assay (ELISA) using purified virion targets. A panel of plasmids containing specified RV cDNA fragments was also constructed by using a variety of strategies with pGE374- and pGE374-derived expression vectors. Hybrid RecA-RV-beta-galactosidase (LacZ)- or RecA-RV-truncated LacZ-containing proteins collectively representing the entire open reading frame of the structural proteins of RV were overexpressed in Escherichia coli. Bacterial lysates were then probed by ELISA with selected MAbs and by immunoblot following separation by electrophoresis under denaturing conditions. With this approach, MAbs that appeared to react with linear determinants defined epitopes localized within the following domains: MAbs C-1, C-2, and C-8 bind epitopes within the predicted amino-terminal 21 amino acids of the capsid region C9 to C29; MAb C-9 binds to a domain bounded by C64 and C97; MAbs E2-1 through E2-6 bind to the E2 glycoprotein backbone region from E2(1) to E2(115); MAbs E1-18 and E1-20 bind to the E1 glycoprotein region from E1(202) to E1(283). MAb E1-18 neutralizes RV infectivity; MAb E1-20 neutralizes infectivity and modestly inhibits hemagglutination. Analyses with selected synthetic peptides have confirmed several of the molecular domains deduced with the expressed proteins. These plasmid constructions and peptides have proven useful in beginning to unravel the molecular organization of several antigenic sites of this human pathogen.

Detection of rubella virus gene sequences by enzymatic amplification and direct sequencing of amplified DNA

We developed a rapid and sensitive polymerase chain reaction (PCR) assay for detecting and identifying rubella virus (RV). A segment of the RV gene which encodes the E1 membrane glycoprotein of RV was selected as a target for PCR amplification. Single-stranded viral RNA, extracted from infected cells or released from virions, was used as a template for reverse transcription followed by PCR amplification with two different sets of primer pairs, one nested within the other. The amplified E1 gene sequences were detected in ethidium bromide-stained agarose minigels, and their identities were verified by restriction enzyme digestion and hybridization to a probe directed at a site within the PCR target. Single-stranded DNA generated by asymmetric amplification of the target was directly sequenced by using fluorescent dideoxy-terminators and an automated procedure in order to confirm the target sequence. This PCR assay provides a rapid confirmatory test for the detection of RV by cell culture and appears to have considerable potential for the direct detection of RV in clinical specimens. The strategy used in the development of this PCR assay should be useful for developing other diagnostic PCR assays for viruses.

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.

The E2 signal sequence of rubella virus remains part of the capsid protein and confers membrane association in vitro

The capsid (C) protein of rubella virus is translated from a 24S subgenomic mRNA as the first part of a polyprotein containing all three structural proteins of the virus. It is separated from the following protein (E2) by signal peptidase, which cleaves after the E2 signal sequence. We raised an antipeptide antiserum directed against the signal sequence and used the antiserum to show that this sequence is still a part of the C protein in the mature virion. Furthermore, we also showed that, when the C protein is synthesized by in vitro transcription and translation, the resultant protein is membrane associated. This association is not seen with a variant C protein which lacks the signal sequence, and a normally soluble protein (dihydrofolate reductase) becomes membrane associated when the signal sequence is placed at its carboxy terminus.

Oligomers of the cytoplasmic domain of the p62/E2 membrane protein of Semliki Forest virus bind to the nucleocapsid in vitro

We analyzed the interaction between the nucleocapsid and synthetic peptides corresponding to the complete or truncated cytoplasmic protein domain of the Semliki Forest virus p62/E2 glycoprotein. We found that the peptide corresponding to the full-length domain efficiently bound nucleocapsids when coupled to a solid matrix via specific antibodies, whereas the shorter one did not. In solution, a substantial fraction of the full-length peptide associated into oligomers. Binding studies showed that it was mostly these oligomers, rather than the monomeric form of the peptide, which were able to interact with the nucleocapsid. Thus, our findings demonstrate a direct interaction between the spike proteins and the viral nucleocapsid. Furthermore, they suggest that this interaction is directed through formation of complexes containing several p62 or E2 subunits.

Structure of rubella E1 glycoprotein epitopes established by multiple peptide synthesis

Essential parts of epitopes have been identified on rubella virion envelope glycoprotein E1, by scanning with overlapping octapeptides situated between amino acids 243-286 in a previously determined antigenic domain.

Sequence of the genome RNA of rubella virus: evidence for genetic rearrangement during togavirus evolution

The nucleotide sequence of the rubella virus (RUB) genomic RNA was determined. The RUB genomic RNA is 9757 nucleotides in length [excluding the poly(A) tail] and has a G/C content of 69.5%, the highest of any RNA virus sequenced to data. The RUB genomic RNA contains two long open reading frames (ORFs), a 5'-proximal ORF of 6656 nucleotides and a 3'-proximal ORF of 3189 nucleotides which encodes the structural proteins. Thus, the genomic organization of RUB is similar to that of alphaviruses, the other genus of the Togavirus family, and the 5'-proximal ORF of RUB therefore putatively codes for the nonstructural proteins. Sequences homologous to three regions of nucleotide sequence highly conserved among alphaviruses (a stem-and-loop structure at the 5' end of the genome, a 51-nucleotide conserved sequence near the 5' end of the genome, and a 20-nucleotide conserved sequence at the subgenomic RNA start site) were found in the RUB genomic RNA. Amino acid sequence comparisons between the nonstructural ORF of RUB and alphaviruses revealed only one short (122 amino acids) region of significant homology, indicating that these viruses are only distantly related. This region of homology is located at the NH2 terminus of nsP3 in the alphavirus genome. The RUB nonstructural protein ORF contains two global amino acid motifs conserved in a large number of positive-polarity RNA viruses, a motif indicative of helicase activity and a motif indicative of replicase activity. The order of the helicase motif and the nsP3 homology region in the RUB genome is reversed with respect to the alphavirus genome indicating that a genetic rearrangement has occurred during the evolution of these viruses.

Hybrids between rubella virus and a latent virus of baby hamster kidney cell line BHK21: characterization of rubella virus and type 2 hybrid virus genomes and determination of their physical homology

The biochemical nature of rubella virus and type 2 hybrid virus, which is a recombinant between rubella virus and a latent retrovirus of BHK21 cells, has been characterized. Type 2 hybrid virus carries DNA polymerase able to copy exogenous DNA. However, disrupted type 2 hybrid virions do not synthesize detectable amounts of DNA using the endogenous viral RNA or synthetic poly(rA)/oligo(dT) primed as a template. Thus, the type 2 hybrid virus DNA polymerase has no detectable reverse transcriptase activity. Rubella virus and type 2 hybrid virus RNA can serve as templates for avian myeloblastosis virus (AMV) reverse transcriptase, although they are inefficient. The addition of oligo(dT) to these viral RNA showed no significant stimulation of their template activity for AMV reverse transcriptase. The oligo(dT)-cellulose affinity column bound neither rubella virus nor type 2 hybrid virus RNA. This suggests that both RNA genomes have a very short poly(A) tail at their 3' end. Thus, complementary DNA (cDNA) synthesis by AMV reverse transcriptase using oligo(dT) primers showed no preferential reverse transcription from the genomic 3' terminus and produced only short cDNA fragments (about 200 nucleotides). We cross-hybridized these short cDNA fragments with their viral RNA, assuming that they are copies of random sites of the genome. These cDNA-RNA hybridization analyses of physical homology between type 2 hybrid virus and rubella virus genomes revealed that about 70% of the type 2 hybrid virus genome is derived from about an 85% portion of the rubella virus genome. These values indicate that the size of the type 2 hybrid virus genome is about 21% larger than that of the rubella virus genome. Co-sedimentation studies of these viral RNA by sucrose density gradient centrifugation confirmed that the molecular weight of type 2 hybrid virus RNA is 20% higher than that of rubella virus RNA. We propose a genomic structure of the type 2 hybrid virus taking into account both physical and biochemical data.

In vitro and in vivo expression of rubella virus glycoprotein E2: the signal peptide is contained in the C-terminal region of capsid protein

The 24 subgenomic mRNA of rubella virus (RV) specifies a polyprotein which is post-translationally processed to three structural protein species E1, E2, and capsid. E1 and E2 are membrane glycoproteins forming the virion spikes. In the polyprotein, E2 and E1 are both preceded by stretches of uncharged, mainly nonpolar amino acids which probably function as signal peptides mediating translocation into the endoplasmic reticulum. We have previously shown that translocation of E1 is reinitiated by a signal peptide located in the carboxy-terminus of E2 (Hobman et al., 1988, J. Virol. 62, 4259-4264). A cDNA from RV encoding the entire E2 gene fused to the capsid N-terminus has been constructed, allowing expression of RV E2 in vitro and in vivo. The resulting protein is efficiently translocated into canine pancreatic microsomes and is glycosylated when expressed in vitro. In vivo some of the N-linked sugars are processed to complex types. Cell surface immunofluorescence indicates that RV E2 is transported to the plasma membrane in COS cells. Oligonucleotide-directed mutagenesis was used to create a cDNA lacking 163 nucleotides immediately 5' to the E2 coding region. This deletion mutant failed undergo translocation into microsomes in vitro and was unstable when expressed in COS cells. The results imply that a signal peptide domain for RV E2 is contained in the carboxyl terminus of the capsid.

Nucleotide sequence of the 24S subgenomic messenger RNA of a vaccine strain (HPV77) of rubella virus: comparison with a wild-type strain (M33)

A full-length cDNA clone for the 24S subgenomic mRNA of the vaccine strain (HPV77) of rubella virus has been isolated from a cDNA library made from the RNAs of infected cells. Starting from the first Met start codon, the 24S mRNA codes for a precursor protein of 1063 amino acids (aa). This precursor encodes a capsid protein of 300 aa, and two envelope proteins, E1 (481 aa) and E2 (282 aa). Both the E1 and E2 proteins are preceded by a stretch of 21 hydrophobic aa, characteristic of a signal peptide, and each has three putative glycosylation sites in the polypeptide chains. Comparison between the structural proteins of the vaccine and the wild-type (wt; M33) strains of rubella virus, revealed that the E2 protein of the vaccine strain differs, in its apparent Mr, by approx. 3 kDa, from the wt strain. The difference could be due to decreased glycosylation of the vaccine strain E2 protein, as revealed by [3H]mannose incorporation studies. Five single-aa changes in the structural proteins occurred during the attenuation process, one each in the capsid and the E1 protein and three in the E2 protein. The change of Thr-412----Ile in the E2 protein results in the loss of a putative glycosylation site at Asn-410, which offers a plausible explanation for decreased glycosylation of the E2 protein from the vaccine strain of rubella virus.

The 3'terminus of lactate dehydrogenase-elevating virus genome RNA does not contain togavirus or flavivirus conserved sequences

Lactate dehydrogenase-elevating virus (LDV) is currently considered to be an unclassified togavirus. The 3' terminus of the genome RNA of the C-strain of LDV was cloned and sequenced. A synthetic DNA oligomer complementary to the 3' portion of this cloned sequence was then used to prime dideoxy sequencing from the LDV-C genome RNA as well as from the genome RNAs of three additional LDV isolates. A high degree of sequence conservation was observed in the 3' terminal region among the four LDV isolates analyzed. Comparison of the LDV 3' sequence with those of the alpha togaviruses, rubella virus, and the flaviviruses showed that the LDV genome does not contain conserved 3' sequences characteristic of these viruses.

Baculovirus polyhedrin promoter-directed expression of rubella virus envelope glycoproteins, E1 and E2, in Spodoptera frugiperda cells

To study the capability of Spodoptera frugiperda (fall armyworm; Sf9) cells to synthesize and process mature rubella virus (RV) proteins, a cDNA encoding the structural envelope glycoproteins, E1 (58 kDa) and E2 (42-47 kDa) were inserted into the genome of Autographa californica nuclear polyhedrosis virus (AcNPV) and expressed during infection under the transcriptional regulation of the polyhedrin gene promoter. By immunoblot analysis with antibodies directed against purified RV, the individual proteins E1 and E2, and human convalescent serum, a polyprotein precursor migrating with an apparent molecular weight of 90-95 kDa was identified in Sf9 cells infected with the recombinant baculovirus, Ac701-RVE. In addition, two proteins migrating somewhat faster than authentic viral E1 and E2 were resolved. Pulse-chase labeling experiments in the absence and presence of tunicamycin, as well as treatment of the recombinant proteins with endo-beta-N-acetyl-D-glucosaminidase H indicated that the recombinant proteins are glycosylated and that the E1 and E2 apoproteins, respectively, were similar in size as compared to their in vitro synthesized counterparts. The recombinant protein products were further detected by some monoclonal antibodies directed against RV. The results presented here indicate that a polyprotein containing the envelope glycoproteins of RV is expressed and proteolytically cleaved in lepidopteran insect cells to form two proteins which resemble authentic E1 and E2. The baculovirus may therefore be suitable for the abundant expression of RV antigen.

Reverse transcription and subsequent DNA amplification of rubella virus RNA

A method is described whereby rubella virus RNA was reverse transcribed and the resulting cDNA enzymatically amplified using Taq polymerase. The reactions were carried out in a single reaction vessel, with only minor modifications to the buffer conditions between the reverse transcription and the subsequent amplification step. Using an oligonucleotide probe to the E1 glycoprotein region and limited restriction endonuclease mapping, the resulting amplified products were shown to be specific for rubella virus. This method was also successfully applied to crude cell lysates, without the need for RNA purification. The possible applications of the polymerase chain reaction as applied to RNA sequences are discussed.

Rubella virus: mechanism of attenuation in the vaccine strain (HPV77)

The vaccine type (HPV77 strain) of rubella virus replicates slower and manifests a delayed appearance of cytopathic effect in Vero-76 cells as compared to wild-type virus (M33). The change in cytopathic effect coincides with the delayed appearance of both genomic and subgenomic RNA as well as viral structural proteins in the cell. The delay in the appearance of the viral proteins in the cells was also evident when the cells infected with the vaccine-type virus were treated with the lysosomotropic agent such as chloroquine. Binding studies using [35S]methionine-labeled virus showed that the vaccine-type virus bound to the cells poorly and the binding was not completely competed out with the cold virus.

Expression of rubella virus cDNA encoding the E1 structural protein

cDNAs synthesized from the 40S RNA genome of rubella virus were cloned into the lambda gt10 bacteriophage. A clone (pRV # 1475) which contains a truncated version of the E1 envelope glycoprotein (amino acid residues 17-481) and the 3' non-coding region was constructed from two overlapping cDNAs. pRV # 1475 was inserted into a eukaryotic expression vector under the control of the cytomegalovirus immediate early promoter. After transfection of 293 cells, a protein with intact antigenic domains could be detected.

A bio-engineered rubella E1 antigen

The major rubella envelope protein, E1, and a segment of it, comprising amino acids 207-353, have been separately expressed as fusion proteins with the IgG binding region of Staphylococcus aureus protein A in Escherichia coli. The proteins carry E1-specific antigenicity recognized by monoclonal antibodies raised against whole virus confirming that correct glycosylation is not required for antigenicity. The use of these bioengineered antigens in immunoassays for diagnosis of rubella infection and for immunization in experimental animals is described.

Identification of the 5' end of the rubella virus subgenomic RNA

The 5' end of the subgenomic RNA of rubella virus was determined by primer extension. The Maxam-Gilbert sequence ladder of the primer extension product contained a determinable sequence which was colinear with the complement of the sequence of the genomic RNA through nucleotide 3325 from the 3' end of the genomic RNA and a pair of bands in every lane above the determinable sequence. These results indicated that synthesis of the subgenomic RNA is initiated internally on the minus-polarity genome-length RNA template and that the length of subgenomic RNA is 3327 nucleotides excluding the poly(A) tail. There are thus 77 nucleotides between the 5' end of the subgenomic RNA and the first AUG, which is the initiation codon for the structural protein open reading frame. The initiation site of rubella virus subgenomic RNA synthesis is 20 nucleotides downstream from a block of 28 nucleotides which shares homology with the nucleotide sequence which is conserved at the subgenomic RNA initiation site in the Alphaviruses, the other genus of Togaviruses.

Translocation of rubella virus glycoprotein E1 into the endoplasmic reticulum

Rubella virus (RV) contains four structural proteins, C (capsid), E2a, E2b, and E1, which are derived from posttranslational processing of a single polyprotein precursor, p110. C protein is nonglycosylated and is thought to interact with RV RNA to form a nucleocapsid. E1 and E2 are membrane glycoproteins that form the spike complexes located on the virion exterior. Two different E1 cDNAs were used to analyze the requirements for translocation of E1 into the endoplasmic reticulum. Analysis of expression of these cDNAs both in vivo and in vitro showed that RV E1 was stably expressed and glycosylated in COS cells and correctly targeted into microsomes in the absence of E2 glycoprotein. The results provide experimental evidence that translocation of RV E1 glycoprotein into the endoplasmic reticulum is mediated by a signal peptide contained within the 69 carboxyl-terminal residues of E2.

Localization of the rubella E1 epitopes

Three epitopes which react with haemagglutination inhibition and neutralizing antibodies have been located between amino acids 245-285 in the predicted amino acid sequence of rubella envelope glycoprotein E1.

Conformational change of rubella virus spike proteins induced by 2-mercaptoethanol

Hemagglutinating (HA) activity of rubella virus was inactivated with 2-mercaptoethanol (2ME) in a dose-dependent manner. But even low concentrations of 2ME, which had little effect on HA activity by themselves, greatly increased the sensitivity of spike polypeptides to the subsequent trypsin treatment. Increased trypsin sensitivity was shown by an enhanced reduction of HA activity and an enhanced proteolytic removal of both E1 and E2 polypeptides from the surface of the virion. The findings indicate that 2ME causes an extensive disruption in the conformation of spikes composed of E1 and E2 polypeptides.

Expression of rubella virus cDNA coding for the structural proteins

A cDNA clone encoding the precursor polypeptide (Mr 115,000) to the nucleocapsid C (Mr 30,000) and two envelope glycoproteins E1 (Mr 58,000) and E2 (Mr 42,000-47,000) of rubella virus was inserted into a simian virus 40-derived eukaryotic expression vector. When the plasmid was introduced into COS cells, three proteins were synthesized. The expressed proteins were antigenically similar and identical in size to the authentic structural proteins of rubella virus. Expression in the presence of tunicamycin confirmed that E1 and E2 are glycoproteins. Unglycosylated E1 and E2 had Mrs of about 53,000 and 30,000, respectively. The mobility of the nucleocapsid protein was unaffected by tunicamycin. The locations of the translation start and stop codons for synthesis of the precursor to the structural proteins of rubella virus were determined by in vitro and in vivo expression studies. It was found that the first AUG codon at the 5' end of the rubella virus 24S cDNA acts as a start codon for translation. The stop codon was found to be 3183 bp from the start codon.

Rubella virus replication: effect of interferons and actinomycin D

The effect of alpha and gamma interferon (IFN alpha, IFN gamma) and actinomycin D on the expression of wild type rubella virus in African green monkey kidney cells (Vero 76) was studied. Viral protein synthesis in the infected cells was significantly reduced upon treatment of the cells with IFN alpha or IFN gamma, which is accompanied by the reduction in the level of both the (+) stranded and the (-) stranded viral RNAs. The residual rubella viral RNA from interferon-treated cells, however, was structurally intact as judged by Northern blot analysis and in vitro translation. These results suggest that the effect of IFN alpha and IFN gamma on rubella viral protein synthesis is both at the transcriptional and the translational level. The effect of actinomycin D on rubella virus replication was found to be time-dependent. It is much more pronounced during the eclipse phase of the viral growth (first 4 h) than after 8 h at which time actinomycin D had lesser effect. A similar effect on rubella virus replication was observed when alpha-amanitin was used instead of actinomycin D. These results were taken to indicate that during the viral infection, host cell DNA directs the synthesis of a cellular factor(s) which is essential for the viral replication. When the synthesis of this cellular factor(s) is terminated at an early stage of viral infection by actinomycin D or by alpha-amanitin, viral replication is impaired.

Sequence of the region coding for virion proteins C and E2 and the carboxy terminus of the nonstructural proteins of rubella virus: comparison with alphaviruses

The sequence of the 3' 4508 nucleotides (nt) of the genomic RNA of the Therien strain of rubella virus (RV) was determined for cDNA clones. The sequence contains a 3189-nt open reading frame (ORF) which codes for the structural proteins C, E2 and E1. C is predicted to have a length of 300 amino acids (aa). The N-terminal half of the C protein is highly basic and hydrophilic in nature, and is putatively the region of the protein which interacts with the virion RNA. At the C terminus of the C protein is a stretch of 20 hydrophobic aa which also serves as the signal sequence for E2, indicating that the cleavage of C from the polyprotein precursor may be catalyzed by signalase in the lumen of the endoplasmic reticulum. E2 is 282 aa in length and contains four potential N-linked glycosylation sites and a putative transmembrane domain near its C terminus. The sequence of E1 has been previously described [Frey et al., Virology 154 (1986) 228-232]. No homology could be detected between the amino acid sequence of the RV structural proteins and the amino acid sequence of the alphavirus structural proteins. From the position of a region of 30 nt in the RV genomic sequence which exhibited significant homology with the sequence in the alphavirus genome at which subgenomic RNA synthesis is initiated, the RV subgenomic RNA is predicted to be 3346 nt in length and the nontranslated region from the 5' end of the subgenomic RNA to the structural protein ORF is predicted to be 98 nt. In a different translation frame beginning at the 5' end of the RV nt sequence reported here is a 1407 nt ORF which is the C terminal region of the nonstructural protein ORF. This ORF overlaps the structural protein ORF by 149 nt. A low level of homology could be detected between the predicted amino acid sequence of the C-terminus of the RV nonstructural protein ORF and the replicase proteins of several positive RNA viruses of animals and plants, including nsp4 of the alphaviruses, the protein encoded by the C-terminal region of the alphavirus nonstructural ORF. However, the overall homology between RV and the alphaviruses in this region of the genome was only 18%, indicating that these two genera of the Togavirus family are only distantly related. Intriguingly, there is a 2844-nt ORF present in the negative polarity orientation of the RV sequence which could encode a 928-aa polyprotein.

Diagnosis of fetal rubella infection by nucleic acid hybridization

The efficacy of nucleic acid hybridization for the diagnosis of rubella infection in experimental and clinical materials was compared with immunoblot and virus isolation techniques. Our results showed that nucleic acid hybridization is specific and rapid but gives false-negative results when compared with conventional virus isolation in some experimental although not in clinical materials so far examined. For this reason, a failure to demonstrate rubella virus in fetal specimens by this method alone cannot yet be taken as a sole criterion for ruling out fetal rubella infection.

Time course of virus-specific macromolecular synthesis during rubella virus infection in Vero cells

Virus specific macromolecular synthesis was studied in Vero cells infected with plaque-purified rubella virus under one-step multiplication conditions. Under these conditions, the rate of virus production was found to increase rapidly until 24 hr postinfection after which time the rate of virus production rose more slowly, reaching a peak level at 48 hr postinfection. This peak rate of virus production was maintained through 72 hr postinfection. A majority of the cells remained alive through 96 hr postinfection, although a 20 to 30% decrease in the number of living cells occurred between 24 and 48 hr postinfection, the time period at which cytopathic effect was first observed. The virus structural proteins were first detected intracellularly at 16 hr postinfection. The rate of synthesis of these proteins was already maximal at 16 hr postinfection and remained constant through 48 hr postinfection. By immunofluorescence, cells expressing virus proteins were first observed at 12 hr postinfection. At 24 hr postinfection, 35 to 50% of the cells in the infected culture were exhibiting immunofluorescence, at 36 hr postinfection, 65 to 90% of the cells were exhibiting immunofluorescence, and at 48 hr postinfection, all of the cells were exhibiting immunofluorescence. The virus genomic and subgenomic RNA species were first detectable by 12 hr postinfection. The rate of synthesis of both of these species peaked at 26 hr postinfection. Rubella virus infection was found to have no effect on total cell RNA synthesis. However, a modest inhibition of total cell protein synthesis which reached 40% by 48 hr postinfection was observed. When Northern analysis of RNA extracted from infected cells was performed, a negative-polarity, virus-specific RNA probe hybridized only to the virus genomic and subgenomic RNA species. A positive-polarity, virus-specific RNA probe hybridized predominantly to a negative-polarity RNA of genome length indicating that both the genomic and subgenomic RNAs are synthesized from a genome-length negative-polarity template. Defective interfering (DI) RNAs were not detected in infected cells through 96 hr postinfection or in cells onto which virus released through 96 hr postinfection was passaged. Thus, the generation of DI particles by rubella virus appears to play no role in the slow, noncytopathic replication of this virus or in the ability of rubella virus-infected cells to survive for extended periods of time.