Togaviridae

Togaviridae refers to two genera of enveloped viruses (Latin toga: coat) with positive-stranded RNA genome. This family includes some major human pathogenic species such as:

• Rubivirus (the genus Rubivirus knows only one species, Rubellavirus. This causes the relatively harmless infectious disease rubella, which occurs mainly in children and adolescents.)
• Alphavirus (the genus Alphavirus comprises about 25 insect-borne virus species that cause diseases in humans and animals). Sindbis viruses and their relatives have the greatest distribution).

The spherical virions of the Togaviridae are about 70 nm in sizeand surrounded by a lipid membrane (viral envelope). Very closely attached to this lipid membrane, an icosahedral capsid of about 40 nm in diameter is found inside the virion, which is formed from homodimers of the capsid protein. The fixed ratio of envelope proteins to capsid proteins (1:1) is particularly important in the assembly of the capsids of alphaviruses. The capsid encases the viral genome. This consists of a single-stranded, linear RNA with positive polarity ((+)ssRNA) and is 9.7 to 11.8 kb in size.

The envelope of the virion contains the two proteins E1 and E2, organized in heterodimers, which in turn are assembled into trimers. The E1 glycoprotein is required for adsorption to the target cell.

Replication cycle: A uniform cellular receptor for viral adsorption is not known to date. After adsorption, the virus is taken up via endocytosis. Acidification of the endosome translocates the E1/E2 proteins and mediates fusion of the viral envelope with the endosomal membrane. The capsid is released into the cytoplasm. How the capsid proteins are destabilized to release the viral genome is not known.

1. Hof H et al (2019) Medical Microbiology. Thieme Publishing House Stuttgart S 209-210