Mutational analysis of the rubella virus nonstructural polyprotein and its cleavage products in virus replication and RNA synthesis

Exogenous Rubella Virus Capsid Proteins Enhance Virus Genome Replication

Enhanced replication of rubella virus (RuV) and replicons by de novo synthesized viral structural proteins has been previously described. Such enhancement can occur by viral capsid proteins (CP) alone in trans. It is not clear whether the CP in the virus particles, i.e., the exogenous CP, modulate viral genome replication. In this study, we found that exogenous RuV CP also enhanced viral genome replication, either when used to package replicons or when mixed with RNA during transfection. We demonstrated that CP does not affect the translation efficiency from genomic (gRNA) or subgenomic RNA (sgRNA), the intracellular distribution of the non-structural proteins (NSP), or sgRNA synthesis. Significantly active RNA replication was observed in transfections supplemented with recombinant CP (rCP), which was supported by accumulated genomic negative-strand RNA. rCP was found to restore replication of a few mutants in NSP but failed to fully restore replicons known to have defects in the positive-strand RNA synthesis. By monitoring the amount of RuV RNA following transfection, we found that all RuV replicon RNAs were well-retained in the presence of rCP within 24 h of post-transfection, compared to non-RuV RNA. These results suggest that the exogenous RuV CP increases efficiency of early viral genome replication by modulating the stage(s) prior to and/or at the initiation of negative-strand RNA synthesis, possibly through a general mechanism such as protecting viral RNA.

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.

Heat Shock Protein 90 Ensures the Integrity of Rubella Virus p150 Protein and Supports Viral Replication

Two viral nonstructural proteins, p150 and p90, are expressed in rubella virus (RUBV)-infected cells and mediate viral genome replication, presumably using various host machineries. Molecular chaperones are critical host factors for the maintenance of cellular proteostasis, and certain viral proteins use this chaperone system. The RUBV p150 and p90 proteins are generated from a precursor polyprotein, p200, via processing by the protease activity of its p150 region. This processing is essential for RUBV genome replication. Here we show that heat shock protein 90 (HSP90), a molecular chaperone, is an important host factor for RUBV genome replication. The treatment of RUBV-infected cells with the HSP90 inhibitors 17-allylamino-17-desmethoxygeldanamycin (17-AAG) and ganetespib suppressed RUBV genome replication. HSP90α physically interacted with p150, but not p90. Further analyses into the mechanism of action of the HSP90 inhibitors revealed that HSP90 activity contributes to p150 functional integrity and promotes p200 processing. Collectively, our data demonstrate that RUBV p150 is a client of the HSP90 molecular chaperone and that HSP90 functions as a key host factor for RUBV replication.IMPORTANCE Accumulating evidence indicates that RNA viruses use numerous host factors during replication of their genomes. However, the host factors involved in rubella virus (RUBV) genome replication are largely unknown. In this study, we demonstrate that the HSP90 molecular chaperone is needed for the efficient replication of the RUBV genome. Further, we reveal that HSP90 interacts with RUBV nonstructural protein p150 and its precursor polyprotein, p200. HSP90 contributes to the stability of p150 and the processing of p200 via its protease domain in the p150 region. We conclude that the cellular molecular chaperone HSP90 is a key host factor for functional maturation of nonstructural proteins for RUBV genome replication. These findings provide novel insight into this host-virus interaction.

Hepatitis E Virus Genome Structure and Replication Strategy

Hepatitis E virus (HEV) possesses many of the features of other positive-stranded RNA viruses but also adds HEV-specific nuances, making its virus-host interactions unique. Slow virus replication kinetics and fastidious growth conditions, coupled with the historical lack of an efficient cell culture system to propagate the virus, have left many gaps in our understanding of its structure and replication cycle. Recent advances in culturing selected strains of HEV and resolving the 3D structure of the viral capsid are filling in knowledge gaps, but HEV remains an extremely understudied pathogen. Many steps in the HEV life cycle and many aspects of HEV pathogenesis remain unknown, such as the host and viral factors that determine cross-species infection, the HEV-specific receptor(s) on host cells, what determines HEV chronicity and the ability to replicate in extrahepatic sites, and what regulates processing of the open reading frame 1 (ORF1) nonstructural polyprotein.

Activities of Thrombin and Factor Xa Are Essential for Replication of Hepatitis E Virus and Are Possibly Implicated in ORF1 Polyprotein Processing

Hepatitis E virus (HEV) is a clinically important positive-sense RNA virus. The ORF1 of HEV encodes a nonstructural polyprotein of 1,693 amino acids. It is not clear whether the ORF1 polyprotein (pORF1) is processed into distinct enzymatic domains. Many researchers have attempted to understand the mechanisms of pORF1 processing. However, these studies gave various results and could never convincingly establish the mechanism of pORF1 processing. In this study, we demonstrated the possible role of thrombin and factor Xa in pORF1 processing. We observed that the HEV pORF1 polyprotein bears conserved cleavage sites of thrombin and factor Xa. Using a reverse genetics approach, we demonstrated that an HEV replicon having mutations in the cleavage sites of either thrombin or factor Xa could not replicate efficiently in cell culture. Further, we demonstrated in vitro processing when we incubated recombinant pORF1 fragments with thrombin, and we observed the processing of pORF1 polyprotein. The treatment of a liver cell line with a serine protease inhibitor as well as small interfering RNA (siRNA) knockdown of thrombin and factor Xa resulted in significant reduction in the replication of HEV. Thrombin and factor Xa have been well studied for their roles in blood clotting. Both of these proteins are believed to be present in the active form in the blood plasma. Interestingly, in this report, we demonstrated the presence of biologically active thrombin and factor Xa in a liver cell line. The results suggest that factor Xa and thrombin are essential for the replication of HEV and may be involved in pORF1 polyprotein processing of HEV.IMPORTANCE Hepatitis E virus (HEV) causes a liver disorder called hepatitis in humans, which is mostly an acute and self-limiting infection in adults. A high mortality rate of about 30% is observed in HEV-infected pregnant women in developing countries. There is no convincing opinion about HEV ORF1 polyprotein processing owing to the variability of study results obtained so far. HEV pORF1 has cleavage sites for two host cellular serine proteases, thrombin and factor Xa, that are conserved among HEV genotypes. For the first time, this study demonstrated that thrombin and factor Xa cleavage sites on HEV pORF1 are obligatory for HEV replication. Intracellular biochemical activities of the said serine proteases are also essential for efficient HEV replication in cell culture and must be involved in pORF1 processing. This study sheds light on the presence and roles of clotting factors with respect to virus replication in the cells.

Analysis of the temperature sensitivity of Japanese rubella vaccine strain TO-336.vac and its effect on immunogenicity in the guinea pig

The marker of Japanese domestic rubella vaccines is their lack of immunogenicity in guinea pigs. This has long been thought to be related to the temperature sensitivity of the viruses, but supporting evidence has not been described. In this study, we generated infectious clones of TO-336.vac, a Japanese domestic vaccine, TO-336.GMK5, the parental virus of TO-336.vac, and their mutants, and determined the molecular bases of their temperature sensitivity and immunogenicity in guinea pigs. The results revealed that Ser(1159) in the non-structural protein-coding region was responsible for the temperature sensitivity of TO-336.vac dominantly, while the structural protein-coding region affected the temperature sensitivity subordinately. The findings further suggested that the temperature sensitivity of TO-336.vac affected the antibody induction in guinea pigs after subcutaneous inoculation.

Proteolytic processing of the porcine reproductive and respiratory syndrome virus replicase

The porcine reproductive and respiratory syndrome virus (PRRSV) replicase polyproteins pp1a and pp1ab are proteolytically processed by four proteases encoded in ORF1a. In this study, a large set of PRRSV replicase cleavage products were identified and pp1a cleavage sites were verified by using a combination of bioinformatics, proteomics, immunoprecipitation, and site-directed mutagenesis. For genotype 1 PRRSV (isolate SD01-08), proteomic analysis identified H180/S181, G385/A386, and G1446/A1447 as the cleavage sites separating nsp1α/1β, nsp1β/nsp2, and nsp2/nsp3, respectively. Transient expression of nsp2-8, nsp3-8, nsp4-8, nsp5-8 (using the recombinant vaccinia virus/T7 RNA polymerase system) and immunoprecipitation identified the cleavage end products nsp2, nsp3, nsp4, nsp7α and nsp7β, and various processing intermediates. Our studies also revealed the existence of alternative proteolytic processing pathways for the processing of the nsp3-8 region, depending on the presence or absence of nsp2 as a co-factor. The identity of most cleavage products was further corroborated by site-directed mutagenesis of individual cleavage sites in constructs expressing nsp3-8 or nsp4-8. This study constitutes the first in-depth experimental analysis of PRRSV replicase processing and the data are discussed against the background of the processing scheme previously derived for the arterivirus prototype, the distantly related equine arteritis virus (EAV). Despite several differences between the two viruses, of which the functional significance remains to be studied, our study demonstrates the general conservation of the replicase pp1a processing scheme between EAV and PRRSV, and likely also the other members of the arterivirus family.

Short self-interacting N-terminal region of rubella virus capsid protein is essential for cooperative actions of capsid and nonstructural p150 proteins

Nucleocapsid formation is a primary function of the rubella virus capsid protein, which also promotes viral RNA synthesis via an unknown mechanism. The present study demonstrates that in infected cells, the capsid protein is associated with the nonstructural p150 protein via the short self-interacting N-terminal region of the capsid protein. Mutational analyses indicated that hydrophobic amino acids in this N-terminal region are essential for its N-terminal self-interaction, which is critical for the capsid-p150 association. An analysis based on a subgenomic replicon system demonstrated that the self-interacting N-terminal region of the capsid protein plays a key role in promoting viral gene expression. Analyses using a virus-like particle (VLP) system also showed that the self-interacting N-terminal region of the capsid protein is not essential for VLP production but is critical for VLP infectivity. These results demonstrate that the close cooperative actions of the capsid protein and p150 require the short self-interacting N-terminal region of the capsid protein during the life cycle of the rubella virus.

IMPORTANCE

The capsid protein of rubella virus promotes viral RNA replication via an unknown mechanism. This protein interacts with the nonstructural protein p150, but the importance of this interaction is unclear. In this study, we demonstrate that the short N-terminal region of the capsid protein forms a homo-oligomer that is critical for the capsid-p150 interaction. These interactions are required for the viral-gene-expression-promoting activity of the capsid protein, allowing efficient viral growth. These findings provide information about the mechanisms underlying the regulation of rubella virus RNA replication via the cooperative actions of the capsid protein and p150.

Development of an improved RT-LAMP assay for detection of currently circulating rubella viruses

Rubella virus is the causative agent of rubella. The symptoms are usually mild, and characterized by a maculopapular rash and fever. However, rubella infection in pregnant women sometimes can result in the birth of infants with congenital rubella syndrome (CRS). Global efforts have been made to reduce and eliminate CRS. Although a reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay for detection of rubella virus has been reported, the primers contained several mismatched nucleotides with the genomes of currently circulating rubella virus strains. In the present study, a new RT-LAMP assay was established. The detection limit of this assay was 100-1000PFU/reaction of viruses for all rubella genotypes, except for genotype 2C, which is not commonly found in the current era. Therefore, the new RT-LAMP assay can successfully detect all current rubella virus genotypes, and does not require sophisticated devices like TaqMan real-time PCR systems. This assay should be a useful assay for laboratory diagnosis of rubella and CRS.

[The life cycle of Rubella Virus]

Rubella virus (RV), an infectious agent of rubella, is the sole member of the genus Rubivirus in the family of Togaviridae. RV has a positive-stranded sense RNA as a genome. A natural host of RV is limited to human, and rubella is considered to be a childhood disease in general. When woman is infected with RV during early pregnancy, her fetus may develop severe birth defects known as congenital rubella syndrome. In this review, the RV life cycle from the virus entry to budding is illustrated in comparison with those of member viruses of the genus alphavirus in the same family. The multiple functions of the RV capsid protein are also introduced.

Determinants in the maturation of rubella virus p200 replicase polyprotein precursor

Rubella virus (RUBV), a positive-strand RNA virus, replicates its RNA within membrane-associated replication complexes (RCs) in the cytoplasm of infected cells. RNA synthesis is mediated by the nonstructural proteins (NSPs) P200 and its cleavage products, P150 and P90 (N and C terminal within P200, respectively), which are processed by a protease residing at the C terminus of P150. In this study of NSP maturation, we found that early NSP localization into foci appeared to target the membranes of the endoplasmic reticulum. During maturation, P150 and P90 likely interact within the context of P200 and remain in a complex after cleavage. We found that P150-P90 interactions were blocked by mutational disruption of an alpha helix at the N terminus (amino acids [aa] 36 to 49) of P200 and that these mutations also had an effect on NSP targeting, processing, and membrane association. While the P150-P90 interaction also required residues 1700 to 1900 within P90, focus formation required the entire RNA-dependent RNA polymerase (aa 1700 to 2116). Surprisingly, the RUBV capsid protein (CP) rescued RNA synthesis by several alanine-scanning mutations in the N-terminal alpha helix, and packaged replicon assays showed that rescue could be mediated by CP in the virus particle. We hypothesize that CP rescues these mutations as well as internal deletions of the Q domain within P150 and mutations in the 5' and 3' cis-acting elements in the genomic RNA by chaperoning the maturation of P200. CP's ability to properly target the otherwise aggregated plasmid-expressed P200 provides support for this hypothesis.

Lack of processing of the expressed ORF1 gene product of hepatitis E virus

Background

Proteolytic processing is a common mechanism among plus strand RNA viruses and the replicases of all plus strand RNA viruses of animals thus far characterized undergo such processing. The replicase proteins of hepatitis E virus (HEV) are encoded by ORF1. A previous report published by our group [1] provided data that processing potentially occurred when ORF1 (Burma strain; genotype 1) was expressed using a vaccinia virus-based expression system.

Findings

To further test for processing and to rule out artifacts associated with the expression system, ORF1 was re-expressed using a plasmid-based expression vector with the result that the previous processing profile could not be confirmed. When ORF1 from an HEV infectious cDNA clone (US swine strain; genotype 3) was expressed using the plasmid-based system, the only species detected was the 185 kDa precursor of ORF1. A putative papain-like cysteine protease [2] had been predicted within ORF1 using the original HEV genomic sequence. However, analysis of subsequent ORF1 sequences from a large number of HEV isolates reveals that this protease motif is not conserved.

Conclusions

The expressed HEV ORF1 gene product does not undergo proteolytic processing, indicating that the replicase precursor of HEV is potentially unique in this regard.

Rubella virus-induced superinfection exclusion studied in cells with persisting replicons

For the first time, homologous superinfection exclusion was documented for rubella virus (RUB) by using Vero cells harbouring persisting RUB replicons. Infection with wild-type RUB was reduced by tenfold, whereas Sindbis virus infection was unaffected. Replication following infection with packaged replicons and transfection with replicon transcripts was also restricted in these cells, indicating that restriction occurred after penetration and entry. Translation of such 'supertransfecting' replicon transcripts was not impaired, but no accumulation of supertransfecting replicon RNA could be detected. We tested the hypothesis favoured in the related alphaviruses that superinfection exclusion is mediated by cleavage of the incoming non-structural precursor by the pre-existing non-structural (NS) protease, resulting in an inhibition of minus-strand RNA synthesis. However, cleavage of a precursor translated from a supertransfecting replicon transcript with an NS protease catalytic-site mutation was not detected and the event in the replication cycle at which superinfection exclusion is executed remains to be elucidated.

Proteolytic processing of turnip yellow mosaic virus replication proteins and functional impact on infectivity

Turnip yellow mosaic virus (TYMV), a positive-strand RNA virus belonging to the alphavirus-like supergroup, encodes its nonstructural replication proteins as a 206K precursor with domains indicative of methyltransferase (MT), proteinase (PRO), NTPase/helicase (HEL), and polymerase (POL) activities. Subsequent processing of 206K generates a 66K protein encompassing the POL domain and uncharacterized 115K and 85K proteins. Here, we demonstrate that TYMV proteinase mediates an additional cleavage between the PRO and HEL domains of the polyprotein, generating the 115K protein and a 42K protein encompassing the HEL domain that can be detected in plant cells using a specific antiserum. Deletion and substitution mutagenesis experiments and sequence comparisons indicate that the scissile bond is located between residues Ser879 and Gln880. The 85K protein is generated by a host proteinase and is likely to result from nonspecific proteolytic degradation occurring during protein sample extraction or analysis. We also report that TYMV proteinase has the ability to process substrates in trans in vivo. Finally, we examined the processing of the 206K protein containing native, mutated, or shuffled cleavage sites and analyzed the effects of cleavage mutations on viral infectivity and RNA synthesis by performing reverse-genetics experiments. We present evidence that PRO/HEL cleavage is critical for productive virus infection and that the impaired infectivity of PRO/HEL cleavage mutants is due mainly to defective synthesis of positive-strand RNA.

Expression and processing of the Hepatitis E virus ORF1 nonstructural polyprotein

Background

The ORF1 of hepatitis E virus (HEV) encodes a nonstructural polyprotein of ~186 kDa that has putative domains for four enzymes: a methyltransferase, a papain-like cysteine protease, a RNA helicase and a RNA dependent RNA polymerase. In the absence of a culture system for HEV, the ORF1 expressed using bacterial and mammalian expression systems has shown an ~186 kDa protein, but no processing of the polyprotein has been observed. Based on these observations, it was proposed that the ORF1 polyprotein does not undergo processing into functional units. We have studied ORF1 polyprotein expression and processing through a baculovirus expression vector system because of the high level expression and post-translational modification abilities of this system.

Results

The baculovirus expressed ORF1 polyprotein was processed into smaller fragments that could be detected using antibodies directed against tags engineered at both ends. Processing of this ~192 kDa tagged ORF1 polyprotein and accumulation of lower molecular weight species took place in a time-dependent manner. This processing was inhibited by E-64d, a cell-permeable cysteine protease inhibitor. MALDI-TOF analysis of a 35 kDa processed fragment revealed 9 peptide sequences that matched the HEV methyltransferase (MeT), the first putative domain of the ORF1 polyprotein. Antibodies to the MeT region also revealed an ORF1 processing pattern identical to that observed for the N-terminal tag.

Conclusion

When expressed through baculovirus, the ORF1 polyprotein of HEV was processed into smaller proteins that correlated with their proposed functional domains. Though the involvement of non-cysteine protease(s) could not be be ruled out, this processing mainly depended upon a cysteine protease.

The involvement of survival signaling pathways in rubella-virus induced apoptosis

Rubella virus (RV) causes severe congenital defects when acquired during the first trimester of pregnancy. RV cytopathic effect has been shown to be due to caspase-dependent apoptosis in a number of susceptible cell lines, and it has been suggested that this apoptotic induction could be a causal factor in the development of such defects. Often the outcome of apoptotic stimuli is dependent on apoptotic, proliferative and survival signaling mechanisms in the cell. Therefore we investigated the role of phosphoinositide 3-kinase (PI3K)-Akt survival signaling and Ras-Raf-MEK-ERK proliferative signaling during RV-induced apoptosis in RK13 cells. Increasing levels of phosphorylated ERK, Akt and GSK3β were detected from 24–96 hours post-infection, concomitant with RV-induced apoptotic signals. Inhibition of PI3K-Akt signaling reduced cell viability, and increased the speed and magnitude of RV-induced apoptosis, suggesting that this pathway contributes to cell survival during RV infection. In contrast, inhibition of the Ras-Raf-MEK-ERK pathway impaired RV replication and growth and reduced RV-induced apoptosis, suggesting that the normal cellular growth is required for efficient virus production.

Rubella virus capsid protein modulates viral genome replication and virus infectivity

The structural proteins (SP) of the Togaviridae can be deleted in defective interfering RNAs. The dispensability of viral SP has allowed construction of noninfectious viral expression vectors and replicons from viruses of the Alphavirus and Rubivirus genera. Nevertheless, in this study, we found that the SP of rubella virus (RUB) could enhance expression of reporter genes from RUB replicons in trans. SP enhancement required capsid protein (CP) expression and was not due to RNA-RNA recombination. Accumulation of minus- and plus-strand RNAs from replicons was observed in the presence of SP, suggesting that SP specifically affects RNA synthesis. By using replicons containing an antibiotic resistance gene, we found 2- to 50-fold increases in the number of cells surviving selection in the presence of SP. The increases depended significantly on the amount of transfected RNA. Small amounts of RNA or templates that replicated inefficiently showed more enhancement. The infectivity of infectious RNA was increased by at least 10-fold in cells expressing CP. Moreover, virus infectivity was greatly enhanced in such cells. In other cells that expressed higher levels of CP, RNA replication of replicons was inhibited. Thus, depending on conditions, CP can markedly enhance or inhibit RUB RNA replication.

Analysis of the 3' cis-acting elements of rubella virus by using replicons expressing a puromycin resistance gene

A rubella virus (RUB) replicon, RUBrep/PAC, was constructed and used to map the 3' cis-acting elements (3' CSE) of the RUB genome required for RUB replication. The RUBrep/PAC replicon had the structural protein open reading frame partially replaced by a puromycin acetyltransferase (PAC) gene. Cells transfected with RUBrep/PAC transcripts expressed the PAC gene from the subgenomic RNA, were rendered resistant to puromycin, and thus survived selection with this drug. The relative survival following puromycin selection of cells transfected with transcripts from RUBrep/PAC constructs with mutations in the 3' CSE varied. The 3' region necessary for optimal relative survival consisted of the 3' 305 nucleotides (nt), a region conserved in RUB defective-interfering RNAs, and thus this region constitutes the 3' CSE. Within the 3' CSE, deletions in the approximately 245 nt that overlap the 3' end of the E1 gene resulted in reduced relative survivals, ranging from 20 to <1% of the parental replicon survival level while most mutations within the approximately 60-nt 3' untranslated region (UTR) were lethal. None of the 3' CSE mutations affected in vitro translation of the nonstructural protein open reading frame (which is 5' proximal in the genome and encodes the enzymes involved in virus RNA replication). In cells transfected with replicons with 3' CSE mutations that survived antibiotic selection (i.e., those with mutations in the region of the 3' CSE that overlaps the E1 coding region), the amount of replicon-specific minus-strand RNA was uniform; however, the accumulation of both plus-strand RNA species, genomic and subgenomic, varied widely, indicating that this region of the RUB 3' CSE affects plus-strand RNA accumulation rather than minus-strand RNA synthesis.

Complementation of a deletion in the rubella virus p150 nonstructural protein by the viral capsid protein

Rubella virus (RUB) replicons with an in-frame deletion of 507 nucleotides between two NotI sites in the P150 nonstructural protein (DeltaNotI) do not replicate (as detected by expression of a reporter gene encoded by the replicon) but can be amplified by wild-type helper virus (Tzeng et al., Virology 289:63-73, 2001). Surprisingly, virus with DeltaNotI was viable, and it was hypothesized that this was due to complementation of the NotI deletion by one of the virion structural protein genes. Introduction of the capsid (C) protein gene into DeltaNotI-containing replicons as an in-frame fusion with a reporter gene or cotransfection with both DeltaNotI replicons and RUB replicon or plasmid constructs containing the C gene resulted in replication of the DeltaNotI replicon, confirming the hypothesis that the C gene was the structural protein gene responsible for complementation and demonstrating that complementation could occur either in cis or in trans. Approximately the 5' one-third of the C gene was necessary for complementation. Mutations that prevented translation of the C protein while minimally disturbing the C gene sequence abrogated complementation, while synonymous codon mutations that changed the C gene sequence without affecting the amino acid sequence at the 5' end of the C gene had no effect on complementation, indicating that the C protein, not the C gene RNA, was the moiety responsible for complementation. Complementation occurred at a basic step in the virus replication cycle, because DeltaNotI replicons failed to accumulate detectable virus-specific RNA.

Rescue of rubella virus replication-defective mutants using vaccinia virus recombinant expressing rubella virus nonstructural proteins

The genome of rubella virus (RV) is translated into a polyprotein precusor, p200, of the nonstructural proteins (NSPs). This is proteolytically processed by a viral-encoded protease into two mature products, p150 and p90. p150 contains sequence corresponding to the predicted methyltransferase and protease activities, while p90 has sequence for the proposed helicase and RNA-dependent RNA polymerase activities. Processing of p200 is essential for RV viral replication. RV NSPs are responsible for viral RNA replication, in which a full-length negative-strand RNA serves as the intermediate for the replication of positive-strand genomic RNA and the transcription of subgenomic RNA. Previously we demonstrated that p200 synthesizes negative- but not positive-strand RNA, and that cleavage products p150/p90 are required for efficient production of positive-strand RNA. To determine whether p150 or p90 alone or together is involved in positive-strand RNA synthesis, vaccinia virus recombinants expressing individual NSPs were constructed and characterized. These were used in in vivo rescue experiments to complement replication-defective mutants in virus replication. A protease-inactive mutant was rescued by p200 or p150 provided in trans by using vaccinia virus recombinants. Thus this protease can function in trans. Rescue of cleavage-defective mutant by either p200 alone, or p150 plus p90 but not by p150 or p90 alone suggests that p150 and p90 function together as a replication complex in positive-strand RNA synthesis.

Mutations in the GDD motif of rubella virus putative RNA-dependent RNA polymerase affect virus replication

Rubella virus (RV) nonstructural proteins are translated as a p200 polyprotein that undergoes proteolytic cleavage into p150 and p90. From conserved amino acid sequence motifs in polypeptides, p90 has been proposed to be the RV RNA-dependent RNA polymerase (RdRp). To test whether the conserved GDD motif is involved in RdRp catalytic activity, three different alanine substitutions were introduced into it. Substitution of glycine by alanine (G1966A) resulted in impaired virus infectivity. Alteration of either aspartate residue completely abolished virus replication. A fully infectious variant was isolated from the G1966A mutant. Sequencing analysis showed that the alanine residue substituted in G1966A mutant had reverted to glycine in this variant. Complementation experiments were carried out to rescue the replication-defective RNA carrying G1966A, D1967A, or D1968A mutations. The defective RNA with G1966A mutation in p90 replicated efficiently when the helper genome that supplied a wild-type p90 was provided in trans. However, the replication-defective RNA with D1967A or D1968A was not rescued by supplementation of p90 in trans. Our studies support the idea that the GDD motif is critical for RV replication and p90 function as RV RdRp.

Rubella virus RNA replication is cis-preferential and synthesis of negative- and positive-strand RNAs is regulated by the processing of nonstructural protein

Rubella virus (RV) genome encodes nonstructural protein (NSP) in a large open reading frame at its 5' end. It is translated into p200 and further processed into p150 and p90. The NSPs are responsible for viral RNA replication, during which a full-length negative-strand RNA serves as the intermediate for the replication of positive-strand genomic RNA and the transcription of subgenomic RNA. Using complementation experiments, we demonstrated that RV negative-strand RNA is synthesized preferentially in cis while positive-strand RNAs can be synthesized both in cis and in trans but with higher efficiency in cis. During virus infection, negative-strand RNA accumulates until 10 hours postinfection (hpi) and remains nearly constant thereafter. In contrast, positive-strand RNAs (both genomic and subgenomic RNA) do not increase much before 10 hpi and accumulate rapidly thereafter. Previously we demonstrated that p200 synthesizes negative- but not positive-strand RNA, whereas cleavage products p150/p90 are required for efficient production of positive-strand RNAs. In this study, we present evidence demonstrating that a higher concentration of p150/p90 is associated with lower production of negative-strand RNA. Our data support the hypothesis that p200 is the principal replicase for negative-strand RNA, as is p150/p90 for positive-strand RNA. The switch from the synthesis of negative- to positive-strand RNA is thus regulated by NSP processing, which not only activates the efficient production of positive-strand RNA, but also disables negative-strand RNA synthesis. A mechanism for NSP translation, processing, and regulation of RV RNA synthesis is proposed.