Avisivirus

VIRION

Non-enveloped, spherical, about 30 nm in diameter, T=pseudo3 icosahedral capsid surrounding the naked RNA genome. The capsid consists of a densely-packed icosahedral arrangement of 60 protomers, each consisting of 3 polypeptides, VP0, VP1 and VP3. There is no internal capsid protein.

GENOME

Monopartite, linear ssRNA(+) genome of about 7.5 kb, polyadenylated, composed of a single ORF encoding a polyprotein. Viral genomic RNA has a viral protein (VPg) at its 5’ end instead of a methylated nucleotide cap structure. The 5’ end contains a type II internal ribosome entry site. The P1 region encodes the structural polypeptides. The P2 and P3 regions encode the nonstructural proteins associated with replication.

GENE EXPRESSION

The virion RNA is infectious and serves as both the genome and viral messenger RNA. The IRES allows direct translation of the polyprotein that is subsequently processed into the functional products. There are three or four in-tandem 2A motifs (The Hungarian virus possesses an extra 2A motif compared to the American virus, but probably also three 2A polypeptides).
Ribosomal skipping is used at the junction of the 2A and downstream sequence.

REPLICATION

CYTOPLASMIC

  1. Attachement of the virus to host receptors mediates endocytosis of the virus into the host cell.
  2. The capsid undergoes a conformational change and possibly opens a pore in the host endosomal membrane. The viral genomic RNA penetrates into the host cell cytoplasm.
  3. VPg is removed from the viral RNA, which is then translated into a processed polyprotein.
  4. Replication occurs in viral factories made of membrane vesicles derived from the ER. A dsRNA genome is synthesized from the genomic ssRNA(+).
  5. The dsRNA genome is transcribed/replicated thereby providing viral mRNAs/new ssRNA(+) genomes.
  6. New genomic RNA is believed to be packaged into preassembled procapsids.
  7. Cell lysis and virus release.