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

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.

A single-point mutation in the rubella virus E1 glycoprotein promotes rescue of recombinant vesicular stomatitis virus

Rubella virus (RuV) is an enveloped plus-sense RNA virus and a member of the Rubivirus genus. RuV infection in pregnant women can lead to miscarriage or an array of severe birth defects known as congenital rubella syndrome. Novel rubiviruses were recently discovered in various mammals, highlighting the spillover potential of other rubiviruses to humans. Many features of the rubivirus infection cycle remain unexplored. To promote the study of rubivirus biology, here, we generated replication-competent recombinant VSV-RuV (rVSV-RuV) encoding the RuV transmembrane glycoproteins E2 and E1. Sequencing of rVSV-RuV showed that the RuV glycoproteins acquired a single-point mutation W448R in the E1 transmembrane domain. The E1 W448R mutation did not detectably alter the intracellular expression, processing, glycosylation, colocalization, or dimerization of the E2 and E1 glycoproteins. Nonetheless, the mutation enhanced the incorporation of RuV E2/E1 into VSV particles, which bud from the plasma membrane rather than the RuV budding site in the Golgi. Neutralization by E1 antibodies, calcium dependence, and cell tropism were comparable between WT-RuV and either rVSV-RuV or RuV containing the E1 W448R mutation. However, the E1 W448R mutation strongly shifted the threshold for the acid pH-triggered virus fusion reaction, from pH 6.2 for the WT RuV to pH 5.5 for the mutant. These results suggest that the increased resistance of the mutant RuV E1 to acidic pH promotes the ability of viral envelope proteins to generate infectious rVSV and provide insights into the regulation of RuV fusion during virus entry and exit.IMPORTANCERubella virus (RuV) infection in pregnant women can cause miscarriage or severe fetal birth defects. While a highly effective vaccine has been developed, RuV cases are still a significant problem in areas with inadequate vaccine coverage. In addition, related viruses have recently been discovered in mammals, such as bats and mice, leading to concerns about potential virus spillover to humans. To facilitate studies of RuV biology, here, we generated and characterized a replication-competent vesicular stomatitis virus encoding the RuV glycoproteins (rVSV-RuV). Sequence analysis of rVSV-RuV identified a single-point mutation in the transmembrane region of the E1 glycoprotein. While the overall properties of rVSV-RuV are similar to those of WT-RuV, the mutation caused a marked shift in the pH dependence of virus membrane fusion. Together, our studies of rVSV-RuV and the identified W448R mutation expand our understanding of rubivirus biology and provide new tools for its study.

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.

Animal and human RNA viruses: genetic variability and ability to overcome vaccines

RNA viruses, in general, exhibit high mutation rates; this is mainly due to the low fidelity displayed by the RNA-dependent polymerases required for their replication that lack the proofreading machinery to correct misincorporated nucleotides and produce high mutation rates. This lack of replication fidelity, together with the fact that RNA viruses can undergo spontaneous mutations, results in genetic variants displaying different viral morphogenesis, as well as variation on their surface glycoproteins that affect viral antigenicity. This diverse viral population, routinely containing a variety of mutants, is known as a viral 'quasispecies'. The mutability of their virions allows for fast evolution of RNA viruses that develop antiviral resistance and overcome vaccines much more rapidly than DNA viruses. This also translates into the fact that pathogenic RNA viruses, that cause many diseases and deaths in humans, represent the major viral group involved in zoonotic disease transmission, and are responsible for worldwide pandemics.

Rubella virus-like replicon particles: analysis of encapsidation determinants and non-structural roles of capsid protein in early post-entry replication

Rubella virus (RUBV) contains a plus-strand RNA genome with two ORFs, one encoding the non-structural replicase proteins (NS-ORF) and the second encoding the virion structural proteins (SP-ORF). This study describes development and use of a trans-encapsidation system for the assembly of infectious RUBV-like replicon particles (VRPs) containing RUBV replicons (self replicating genomes with the SP-ORF replaced with a reporter gene). First, this system was used to map signals within the RUBV genome that mediate packaging of viral RNA. Mutations within a proposed packaging signal did not significantly affect relative packaging efficiency. The insertion of various fragments derived from the RUBV genome into Sindbis virus replicons revealed that there are several regions within the RUBV genome capable of enhancing encapsidation of heterologous replicon RNAs. Secondly, the trans-encapsidation system was used to analyse the effect of alterations within the capsid protein (CP) on release of VRPs and subsequent initiation of replication in newly infected cells. Deletion of the N-terminal eight amino acids of the CP reduced VRP titre significantly, which could be partially complemented by native CP provided in trans, indicating that this mutation affected an entry or post-entry event in the replication cycle. To test this hypothesis, the trans-encapsidation system was used to demonstrate the rescue of a lethal deletion within P150, one of the virus replicase proteins, by CP contained within the virus particle. This novel finding substantiated the functional role of CP in early post-entry replication.

Histidine at position 1042 of the p150 region of a KRT live attenuated rubella vaccine strain is responsible for the temperature sensitivity

The Japanese live attenuated KRT rubella vaccine strain has a temperature sensitivity (ts) phenotype. The objective of this study is to identify the region responsible for this phenotype. Genomic sequences of the KRT strain and the wild-type strain (RVi/Matsue.JPN/68) with the non-ts phenotype were investigated and reverse genetic systems (RG) for these strains were developed. The ts phenotype of KRT varied drastically on replacement of the p150 gene (encoding a methyltransferase and a nonstructural protease). Analysis of four chimeric viruses showed the region responsible for the ts phenotype to be located between Bsm I and Nhe I sites (genome position 2803-3243). There were two amino acid differences at positions 1007 and 1042. Mutations were introduced into the KRT cDNA clone, designated G1007D, H1042Y and G1007D-H1042Y. H1042Y and G1007D-H1042Y grew well at a restrictive temperature with a 100-fold higher titer than G1007D and the KRT strain, but a 10-fold lower titer than RVi/Matsue.JPN/68. Since the growth of H1042Y was not completely the same as that of the wild-type strain at the restrictive temperature, we also assessed whether other genomic regions have an additive effect with H1042Y on the ts phenotype. H1042Y-RViM SP having structural proteins of RVi/Matsue.JPN/68 grew better than H1042Y, similar to RVi/Matsue.JPN/68. Thus, we concluded that one mutation, of the histidine at position 1042 of p150, was essential for the ts phenotype of the KRT strain, and structural proteins of KRT had an additive effect with H1042Y on the ts phenotype.

Analysis of rubella virus capsid protein-mediated enhancement of replicon replication and mutant rescue

The rubella virus capsid protein (C) has been shown to complement a lethal deletion (termed deltaNotI) in P150 replicase protein. To investigate this phenomenon, we generated two lines of Vero cells that stably expressed either C (C-Vero cells) or C lacking the eight N-terminal residues (Cdelta8-Vero cells), a construct previously shown to be unable to complement DeltaNotI. In C-Vero cells but not Vero or Cdelta8-Vero cells, replication of a wild-type (wt) replicon expressing the green fluorescent protein (GFP) reporter gene (RUBrep/GFP) was enhanced, and replication of a replicon with deltaNotI (RUBrep/GFP-deltaNotI) was rescued. Surprisingly, replicons with deleterious mutations in the 5' and 3' cis-acting elements were also rescued in C-Vero cells. Interestingly, the Cdelta8 construct localized to the nucleus while the C construct localized in the cytoplasm, explaining the lack of enhancement and rescue in Cdelta8-Vero cells since rubella virus replication occurs in the cytoplasm. Enhancement and rescue in C-Vero cells were at a basic step in the replication cycle, resulting in a substantial increase in the accumulation of replicon-specific RNAs. There was no difference in translation of the nonstructural proteins in C-Vero and Vero cells transfected with the wt and mutant replicons, demonstrating that enhancement and rescue were not due to an increase in the efficiency of translation of the transfected replicon transcripts. In replicon-transfected C-Vero cells, C and the P150 replicase protein associated by coimmunoprecipitation, suggesting that C might play a role in RNA replication, which could explain the enhancement and rescue phenomena. A unifying model that accounts for enhancement of wt replicon replication and rescue of diverse mutations by the rubella virus C protein is proposed.