VIRION
Non-enveloped, spherical virion about 23 nm in diameter with T=3 icosahedral symmetry composed of 180 CP proteins, with some percentage containing a readthrough domain (RTD) extension located on the particle's surface.
GENOME
Monopartite, linear, ssRNA(+) genome of 5.3-5.7 kb in size with a VPg bound at the 5' end. There is no poly(A) tail or tRNA-like structure at the 3' end.
GENE EXPRESSION
The virion RNA is infectious and serves as both the genome and viral messenger RNA. The polymerase is expressed by a ribosomal frameshifting near the end of ORF1. Rap1 translation initiates about 1500 nt downstream of the 5' end of the gRNA by an unusual internal ribosome entry site (IRES).
The movement protein (MVP) and P3a are expressed by leaky scanning of the subgenomic mRNA. Suppression of termination of the CP stop codon produces CP-RTD (CP extended), the capsid subunit essential for aphid transmission.
ENZYMES
- RNA-directed RNA polymerase [RdRp]
- VPG-type capping [VPg]
- Polyprotein major protease (Peptidase S39) [Pro]
REPLICATION
CYTOPLASMIC
- Virus penetrates into the host cell.
- Uncoating, and release of the viral genomic RNA into the cytoplasm.
- The viral RNA ORF1 and ORF2 are translated to produce the RdRp fusion protein.
- Replication takes place in cytoplasmic viral factories. 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 3' coterminal sgRNAs. Traduction of these sgRNAs yields the capsid, CP-RTD and movement proteins.
- Virus assembly.
- Viral movement protein probably mediates virion cell-to-cell transfer.
