Molecular biology

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, positive sense ssRNA genome of 28kb in size. Capped, and polyadenylated.

GENE EXPRESSION

The virion RNA is infectious and serves as both the genome and viral messenger RNA. Genomic RNA encodes for ORF1a, and ORF1b is translated by a frameshifting mechanism. 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.

REPLICATION

CYTOPLASMIC

1. Virus attaches to host receptors through the S protein and is endocytosed into vesicles in the host cell.
2. Fusion of virus membrane with the vesicle membrane, ssRNA(+) genome is released into the cytoplasm.
3. Synthesis and proteolysis of replicase polyproteins. Replicase produces first the complementary ssRNA(-), which will serve as a template for the synthesis of genomic RNA and subgenomic mRNAs.
4. Synthesis of structural proteins encoded by subgenomic mRNAs.
5. Assembly and budding at membranes of the endoplasmic reticulum (ER), the intermediate compartments, and/or the Golgi complex.
6. Release of new virions.