Togaviridae. Togaviridae

Description of Togaviridae adapted from ICTVdb Virion Properties Morphology Virions consist of an envelope and a nucleocapsid. During their life cycle, virions have not been observed outside a cellular environment and have a cell-associated cycle. Virus capsid is tightly enveloped by a detergent sensitive lipoprotein. Virions are spherical to pleomorphic. Virions measure 70 nm in diameter. Surface projections are distinctive glycoprotein spikes that cover evenly the surface. Capsid/nucleocapsid is round and exhibits icosahedral symmetry (T=4. The isometric capsid has a diameter of 40 nm. Capsids appear round. Physicochemical and Physical Properties The molecular mass (Mr) of virions is 52 x 106. Virions have a buoyant density in sucrose of 1.18-1.2 g cm-3. The sedimentation coefficient is 280 S20w. The thermal inactivation point (TIP) is at 58°C. The longevity in vitro (LIV) is 0.35 days (at 37°C in culture medium). Under in vitro conditions virions are stable in alkaline environment of pH 7-8. Virions are sensitive to treatment with organic solvents and detergents (which solubilzes their lipoprotein envelops). The infectivity is reduced after exposure to irradiation. Nucleic Acid The genome is not segmented and contains a single molecule of linear positive-sense, single-stranded RNA. The complete genome is 9700-11800 nucleotides long. The 5'-end of the genome has a methylated nucleotide cap. The 3'-terminus has a poly (A) tract. Proteins The viral genome encodes structural proteins and non-structural proteins. Virions consist of 3 structural protein(s), or 4 structural protein(s). Lipids Lipids are present and located in the envelope. Virions are composed of 30% lipids by weight. The composition of viral lipids and host cell membranes are similar. The lipids are of host origin and are derived from host cell membranes. Viral membranes include phosphatidyl ethanolamine, phosphatidyl choline, phosphatidyl serine, cholesterol, and sphingomyelin. Genome Organization and Replication By itself, genomic nucleic acid is infectious.