Molecular biology
VIRION
Enveloped, spherical, about 120 nm in diameter. The RNA genome is associated with the N protein to form the nucleocapsid (helical for the genus coronavirus, and tubular for the genus torovirus). see Neuman BW et al. for virion cryo-electron microscopy analysis.
GENOME
Monopartite, linear, positive sense, ssRNA genome of 27-32kb in size (the largest of all RNA virus genomes). Capped, and polyadenylated. The leader RNA (65-89 bp) at the 5’ end of the genome is also present at the end of each subgenomic RNAs.
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
- Virus attaches to host receptors through the S protein (maybe also through HE if present) and is endocytosed into vesicles in the host cell.
- Fusion of virus membrane with the vesicle membrane (probably mediated by E2), ssRNA(+) genome is released into the cytoplasm.
- 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.
- 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.