Molecular biology
VIRION
Non-enveloped, spherical, about 30 nm in diameter, composed of a protein shell surrounding the naked RNA genome. The capsid consists of a
densely-packed icosahedral arrangement of 60 protomers, each consisting of 4 polypeptides, VP1, VP2, VP3 and VP4. VP4 is located on the internal side of the capsid.
GENOME
Monopartite, linear, ssRNA(+) genome of 7.1-8.9 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 long UTR at the 5’ end contains an internal ribosome entry site (IRES). The P1 region encodes the structural polypeptides. The P2 and P3 regions encode the nonstructural proteins associated with replication. The shorter 3’ UTR is important in (-)strand synthesis. L is an additional N-terminal leader protein present in some genera that can either be a protease (aphthoviruses, erboviruses) or have ther function (kobuvirus, cardiovirus).
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. The polyprotein is initially processed by the viral protease(s) into various precursor and mature proteins to yield the structural proteins, replicase, VPg, and a number of proteins that modify the host cell, ultimately leading to cell lysis.
REPLICATION
CYTOPLASMIC
- Virus attaches to host receptors, formation of a coated vesicle.
- Uncoating, and release of the viral genomic RNA into the cytoplasm possibly through the formation of a pore in the host cell membrane.
- VPg is removed from the viral RNA, which is then translated into a processed polyprotein.
- In entero-, rhino, and aphtoviruses, shutoff of cellular cap-dependent translation through the cleavage of the translation initiation factor eIF4G by viral protease.
- Replication of viral RNA takes place on membrane vesicles derived from the ER. A negative-sense complementary ssRNA is synthesized using the genomic RNA as a template.
- New genomic RNA synthesized using the negative-sense RNA as a template is believed to be packaged into preformed procapsids.
- Cell lysis and virus release.
- Maturation of provirions by an unknown host protease.