VIRION
Enveloped, spherical, about 120 to 140nm in diameter. The tubular nucleocapsid is bend into an open torus hence the name "torovirus".
GENOME
Monopartite, linear ssRNA(+) genome of 28kb in size. Capped, and polyadenylated. Enzymes: EndoU ribonuclease
GENE EXPRESSION
The virion RNA is infectious and serves as both the genome and viral messenger RNA. Genomic RNA encodes for ORF1a, as for ORF1b, it is translated by ribosomal frameshifting. Resulting proteins pp1a and pp1ab are processed into the viral polymerase (RdRp) and other non-structural proteins involved in RNA synthesis. Structural proteins are expressed as subgenomic RNAs. Each RNA (genomic and subgenomic) is translated to yield only the protein encoded by the 5'-most ORF.
A putative 30K protein overlapping ORF1a might be expressed from an alternative CUG codon
.
ENZYMES
- RNA-directed RNA polymerase [P0C6V8]
- Nido-type capping
- GDP polyribonucleotidyltransferase, N7methyltransferase [P0C6V8]
- 2'O methylase [2'-O-methyl transferase]
- EndoU ribonuclease [Uridylate-specific endoribonuclease]
- Polyprotein major protease (Peptidase S65, S75) [RdRp]
REPLICATION
CYTOPLASMIC
- Attachement of the viral S protein (maybe also HE if present) to host receptors mediates endocytosis of the virus into the host cell.
- Synthesis and proteolytic cleavage of the replicase polyprotein.
- Fusion of virus membrane with the endosomal membrane (probably mediated by E2), ssRNA(+) genome is released into the cytoplasm.
- Synthesis and proteolytic cleavage of the replicase polyprotein.
- Replication occurs in viral factories. A dsRNA genome is synthesized from the genomic ssRNA(+).
- The dsRNA genome is transcribed/replicated thereby providing viral mRNAs/new ssRNA(+) genomes.
- Synthesis of structural proteins encoded by subgenomic mRNAs.
- Assembly and budding at membranes of the endoplasmic reticulum (ER), the intermediate compartments, and/or the Golgi complex.
- Release of new virions by exocytosis.