Togaviridae
Baltimore Classification
Higher order taxa
Viruses; ssRNA positive-strand viruses, no DNA stage; Togaviridae
Genera
Alphavirus, Rubivirus
Description and Significance
Genome Structure
The togavirus genome is non-segmented and contains a molecule of linear, positive-sense, single-stranded RNA. The complete genome is 9700-11800 nucleotides long. The 5'-terminus has a methylated nucleotide cap. (source: ICTVdB)
Virion Structure of a Togavirus
Togavirus virions consist of an envelope and a nucleocapsid. The virions are spheical to pleomorphic in shape, and about 70nm in diameter. Glycoprotein spikes evenly cover the surface of the virion. The genome is contained in the T=4 icosahedral capsid, which measures 40 nm in diameter. (sources: Garoff, ICTVdB)
Reproduction Cycle of a Togavirus in a Host Cell
Togaviruses attach to the surface of a cell using the glycoprotein spikes that cover the virion surface, and are absorbed into the cell. According to Novoa et al., the togavirus creates "factories" for virus reproduction around the host cell's endosomes and lysosomes, a site for viral replication unique to togaviruses. The replication complexes of the togaviruses are not derived from the endosomes or lysosomes, however; instead, it appears that the complexes are derived from the endoplasmic reticulum.
RNA transcription is similart to that of other positive-sense RNA viruses. The positive-sense RNA functions as the mRNA and is used to create a negative-sense RNA template. The negative-sense RNA template is used to create new positive-sense RNA genomes, which are packaged into nucleocapsids. Rubellavirus virions are assembled and mature in the Golgi complex, while alphavirus virions are present in the cytoplasm before budding and mature almost simultaneously with budding. The virions bud from the host cell, gaining their envelopes as they exit.
Viral Ecology & Pathology
The ecology and pathology of togaviruses varies greatly between the two genera. Rubella, the only species in Rubivirus, is very similar to measles. Humans are the only reservoir for the virus, and transmission is airborne.
References
Garoff, Henrik, Mathilda Sjöberg and R. Holland Cheng. "Budding of Alphaviruses." Virus Research 106.2 (2004): 103-116.
ICTVdB - The Universal Virus Database, version 4. http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/