VIRION
Enveloped, spherical, icosahedral, 65-70nm in diameter, capsid with a T=4 icosahedral symmetry made of 240 monomers. The envelope contains 80 trimer spikes, each spike = 3 x E1/E2 heterodimers.
GENOME
Monopartite, linear, ssRNA(+) genome of 9.7-11.8 kb. The genome is capped and polyadenylated.
GENE EXPRESSION
The virion RNA is infectious and serves as both genome and viral messenger RNA. The whole genome is translated into a non-structural polyprotein which is processed by host and viral proteases. Structural polyprotein is expressed through a subgenomic mRNA.
In alphaviruses, RdRp is expressed by suppression of termination at the end of 10% of nsP polyproteins. Moreover a ribosomal frameshifting in the 6K region induces the translation of the TF protein in alphaviruses.
ENZYMES
- Alpha-type capping [N7MTase + GTase nsp1, 2'OMTase nsp2/3]
- RNA-dependent RNA polymerase [nsp4]
- Polyprotein major protease (Peptidase C9) [nsp2]
- Capsid protease (Peptidase S3) [capsid]
REPLICATION
CYTOPLASMIC
- Attachement of the viral E glycoprotein to host receptors mediates clathrin-mediated endocytosis of virus into the host cell.
- Fusion of virus membrane with host endosomal membrane. RNA genome is released into the cytoplasm.
- The positive-sense genomic ssRNA is translated into a polyprotein, which is cleaved into non-structural proteins necessary for RNA synthesis (replication and transcription).
- Replication takes place in cytoplasmic viral factories at the surface of endosomes. A dsRNA genome is synthesized from the genomic ssRNA(+).
- The dsRNA genome is transcribed/replicated thereby providing viral mRNAs/new ssRNA(+) genomes.
- Expression of the subgenomic RNA (sgRNA) gives rise to the structural proteins.
- Capsid assembly occurs in cytoplasm.
- The capsid is envelopped by budding at the plasma membrane where the virion exits the cell.
