Molecular biology

VIRION

Non-enveloped, spherical, icosahedric capsid of 28-30 nm. Surface projections are small and surface appears rough, spikes protruding from the 12 vertices.

GENOME

Monopartite, linear, ssRNA(+) genome of 6.8-7kb in size. The 3’-terminus has a poly(A) tract.

GENE EXPRESSION

The virion RNA is infectious and serves as both the genome and viral messenger RNA. The genome contains three overlapping open reading frames (ORF1a, ORF1b, and ORF2). The nonstructural proteins are translated from the genomic RNA as two large polyproteins, nsP1a and nsP1a/1b, through a translational frameshifting mechanism. ORF1a and ORF1b encode the viral protease and polymerase respectively (and probably other proteins involved in genome replication). ORF 2 is expressed as subgenomic RNAs and encodes the capsid protein.

REPLICATION

CYTOPLASMIC

1. Virus attaches to host receptors.
2. Uncoating, and release of the viral genomic RNA into the cytoplasm.
3. Viral RNA is translated into two processed polyproteins.
4. Replication of viral RNA: a negative-sense complementary ssRNA is synthesized using the genomic RNA as a template.
5. New genomic RNA synthesized using the negative-sense RNA as a template.
6. Subgenomic RNA translation gives rise to the capsid protein precursor.
7. Virus release and maturation of the capsid by proteolytic cleavages.