Notes on Genus: Betacoronavirus
enveloped spheres
Type member: Murine coronavirus
Contents
General Description
This is one of the genera in the subfamily
Coronavirinae of the family
Coronaviridae. Within this subfamily, viruses have spherical virions with club-shaped surface projections and a core shell. The name is from the Latin
corona = crown, reflecting the appearance of these projections in electron microscopy. This genus contains what were previously considered phylogroup 2 coronaviruses.
Morphology
Virions enveloped and spherical 120-160 nm in diameter, with a core shell of about 65nm. The large surface projections are formed by glycoproteins and a trimers of the spike protein.
Genome
Monopartite positive sense single-stranded RNA 29-31 kb in length and with a 3'-polyA tail.
Genus Genomic Organization
Two large, overlapping ORFs at the 5'-end of the genome encode the major non-structural proteins and are expressed as a fusion protein by ribosomal frameshift. These contain regions with protease, helicase and RNA polymerase motifs. Downstream are up to 9 other genes, most of which encode structural proteins, and these are expressed from a 3'-coterminal nested set of subgenomic RNAs. The major characterised proteins occur in a conserved order and are:
- Hemagglutinin-esterase protein HE (only in some members)
- Spike protein S
- Two to three small non-structural proteins
- Small envelope protein E
- Membrane protein M
- Coat protein N
- In some viruses there are additional small non-structural proteins between genes M and N or downstream of N
Type Member Genomic Organization
The RNA of 31.3kb is translated into the following products:
- Replication protein 1a of 491 kDa
- Polymerase 1b of 309 kDa, expressed as a fusion protein with 1a by ribosomal frameshift
- Non-structural protein 2a of 31 kDa
- Hemagglutinin-esterase protein HE of 11 kDa
- Spike protein S of 150 kDa
- Non-structural protein 5a of 13 kDa
- Small envelope protein E of 10 kDa
- Membrane protein M of 26 kDa
- Coat protein N of 49 kDa
