Notes on Genus: Gammacoronavirus

enveloped spheres
enveloped spheres

Type member: Avian 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 3 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 27-32 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 12 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:
  • 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 27.6kb is translated into the following products:
  • Replication protein 1a of 441 kDa
  • Polymerase 1b of 305 kDa, expressed as a fusion protein with 1a by ribosomal frameshift
  • Spike protein S of 128 kDa
  • Non-structural protein 3a of 7 kDa
  • Non-structural protein 3b of 7 kDa
  • Small envelope protein E of 12 kDa
  • Membrane protein M of 25 kDa
  • Non-structural protein 5a of 7 kDa
  • Non-structural protein 5b of 9 kDa
  • Coat protein N of 45 kDa