ICTV Virus Taxonomy Profile: Togaviridae

1Togavirus particles consist of a nucleocapsid core, a lipid bilayer and surface glycoproteins. However, there are some striking differences between alphaviruses and rubella virions. Alphaviruses are spherical and about 70 nm in diameter. Particles contain a distinct icosahedral core and an icosahedral glycoprotein layer. In contrast, rubella virus particles are pleomorphic, often tube-like and do not have icosahedral symmetry. Rubella virus capsid proteins form homodimers in a grid-like pattern and the glycoproteins are arranged in rows on the virion surface (Table 1, Fig. 1).

2The virus genome is unsegmented RNA of 9.7–12 kb (alphaviruses) or 9.8–10 kb (rubella virus). Non-structural and structural polyproteins are encoded by separate ORFs that are flanked by 5′- and 3′-terminal non-coding regions and separated by an internal non-coding region (Fig. 2). The subgenomic promoter overlaps with the 3′-end of the non-structural ORF and an internal non-coding region.

3Replication complexes (spherules) are formed on the plasma membrane. Non-structural polyproteins are cleaved in an ordered manner by a viral cysteine protease to form (1) a short-lived replication complex for negative-sense RNA synthesis and later (2) a stable replication complex for synthesis of positive-sense RNA genomes and subgenomic mRNAs. Replication enzymes also include a methyl-guanylyl transferase, an RNA helicase and an RNA-dependent RNA polymerase. The orders of these conserved functional motifs in non-structural polyproteins are different between the two genera.

4Structural polyproteins are processed by cellular enzymes and, for alphaviruses, also by autoprotease activity of the capsid protein. The capsid protein assembles with the viral genomic RNA to form viral nucleocapsids. The glycoproteins are co-translationally inserted into the endoplasmic reticulum and translocated to the plasma membrane (alphaviruses) or to membranes of the Golgi apparatus (rubella virus). From these respective sites, alpha- and rubella viruses bud to form virions with lipid envelopes.

5This genus includes >30 species. Most alphaviruses are mosquito-borne, transmitting alternatively between mosquito vectors and vertebrate hosts including humans, non-human primates, equids, birds, amphibians, reptiles, rodents and pigs. Chikungunya virus is an important human pathogen; most other alphaviruses cannot develop sufficient viraemia in humans to infect mosquitoes. Eilat virus, and probably Taï Forest alphavirus, are insect-specific. The aquatic alphaviruses, southern elephant seal virus and salmon pancreas disease virus, appear to be transmitted horizontally.

6Members of the single species in this genus, Rubella virus, infect only humans and are transmitted via respiratory routes. 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.

7Full ICTV Online (10th) Report: www.ictv.global/report/togaviridae.