Togavirus transcription and translation   Chikungunya virus. Colorized transmission EM  CDC

The genus Alphaviridae is part of the Togaviridae family

• Virion size: 65-70nm. Spherical
• RNA: single positive strand. Polyadenylated. 5' methyl nucleotide cap. RNA is infectious as it serves both as genome and viral mRNA
• Envelope: Yes. The virus buds through and the membrane comes from host cell endoplasmic reticulum and Golgi body. The virus leaves the cell via the secretory pathway.
• Replication site: Entirely cytoplasmic (as is the case with most RNA viruses). Virus replication occurs on the surface of endosomes in viral "factories"
• Symmetry: T4 Icosahedral. Each virus has 240 glycoprotein monomers arranged in 80 trimers
• Genome size: 11-12kB
• Cell receptor: ?
• Name: Togaviridae comes from the fact that they are enveloped (cloaked in a toga-like membrane)

The virus is taken up by endocytosis through clathrin-coated pits after binding to a cell surface receptor. In the cytoplasm the positive strand RNA is translated to a large polyprotein. This is then self-cleaved to four non-structural proteins that are the enzymes that will be needed for RNA replication via a double strand (+/-) intermediate and the transcription of more positive strand RNA.  Later, in the second phase, a subgenomic mRNA corresponding to the 3' end of the genome is transcribed and this is translated to the structural proteins. These are the nucleocapsid protein (C) and the membrane proteins (P62 and E1 which form a dimer). P62 is further cleaved to E2 and E3 after embedding in the membrane.