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Rubella virus (RUBV) has pleomorphic virions with the envelope proteins arranged in rows. In contrast to the alphaviruses, which are frequently arthropod-borne, RUBV transmission is airborne via aerosols. The taxonomic relationship of Rubivirus to Alphavirus is under active assessment because of significant differences in their members' virion structures and because their genome sequences are not monophyletic with respect to those of other positive-sense ssRNA viruses.
In contrast to the alphaviruses, RUBV particles are heterogenous, ranging from spherical to tube-like in shape. Particles range between 50 and 90 nm in length and width (Battisti et al., 2012). As with the alphaviruses, RUBV particles have a nucleocapsid core, a lipid bilayer, and surface glycoproteins on the particles. However, the arrangement of these proteins in the particle is different (Mangala Prasad et al., 2017, Battisti et al., 2012, Mangala Prasad et al., 2013).
The glycoproteins, E1 and E2 dimers, are arranged in rows on the particle surface. There are 4–6 rows of glycoproteins and some wrap around the particle forming a helical arrangement. This is unique to enveloped viruses, as helical arrangements have previously only observed with matrix proteins or nucleoprotein complexes, not surface glycoproteins. There is a gap between the lipid bilayer and the nucleocapsid core, as seen with flaviviruses. The internal core does not have icosahedral symmetry as do alphavirus particles. Rather the capsid protein is arranged as a homodimer in a grid like pattern, often underneath the position of the glycoprotein spikes (Mangala Prasad et al., 2017, Battisti et al., 2012, Mangala Prasad et al., 2013).
The reported sedimentation coefficients for RUBV range from 240S to 350S. The wide range is due to contamination with host proteins and lipids associated with virions during purification (Hobman 2013).
Genomes for rubiviruses range from 9.8–10.0 kb, and thus are slightly smaller than the alphavirus genomes.
The RUBV non-structural proteins are P150 and P90 and are both required for genome replication. P150 contains methyltransferase, protease, proline hinge, and X and Y domains. P90 contains the RNA-dependent RNA polymerase and helicase activity. The structural proteins are synthesized at the ER membrane and host cell signalase cleaves the polyprotein into capsid, E2, and E1. Capsid protein has been reported to have roles in viral assembly, as well as viral transcription and replication.
Capsid protein forms disulphide-linked homodimers. The C-terminus of capsid is cytoplasmic and phosphorylation is important for virus assembly, particularly for regulating viral RNA binding to the capsid protein. E2 and E1 are involved in host cell binding and membrane fusion. E1 is a class II fusion protein that requires endosomal levels of calcium for fusion (Dube et al., 2014, DuBois et al., 2013, Dube et al., 2016) which is different compared with alphavirus and flavivirus class II fusion proteins (Vaney and Rey 2011). E2 and E1 are glycosylated, both N-linked and O-linked, and both are palmitoylated (Hobman 2013, Qiu et al., 1992, Lundstrom et al., 1991). E2 is translated in two forms that are differentially glycosylated (Frey 1994).
RUBV assembly and budding occurs at the Golgi complex (Risco et al., 2003) and the plasma membrane depending on the host cell (Bardeletti et al., 1979). In the Golgi, the newly budded particles are electron-dense and considered “immature.” Virion maturation occurs in the Golgi (Bardeletti et al., 1979), the core condenses, and is no longer associated with the virion membrane and a double shell-like structure is observed (Mangala Prasad et al., 2017). These mature particles are secreted to the extracellular environment.
RUBV proteins E2 and E1 are both glycosylated. E1 has three N-linked residues and E2 has three N-linked and O-linked carbohydrate moieties (Oker-Blom et al., 1984, Lundstrom et al., 1991). Two forms of E2 have been reported to be translated, each with differing amounts of glycosylation (Frey 1994).
See discussion under family description.
Rubella virus strains show no major antigenic differences. Therefore, rubella vaccines will protect circulating strains and rubella antigens used in serological tests for screening and diagnosis will detect antibodies induced by all strains (Best et al., 1992).
The illness, known as rubella or German measles, generally consists of fever and rash; complications are rare. However, if a pregnant woman is infected with RUBV during the first trimester, there is a 20% chance of the foetus developing congenital rubella syndrome (CRS). Birth defects due to CRS include deafness, cataracts, heart defects, mental retardation, and liver and spleen damage. CRS infants shed virus for up to six months. Such persistence contrasts with alphavirus infections which are cleared within days or weeks. RUBV is endemic worldwide and is vaccine-preventable.
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