Notes on Genus: Alphacoronavirus

enveloped spheres

Type member: Alphacoronavirus 1

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 1 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 27-29 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 7 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
One or two small non-structural proteins
Small envelope protein E
Membrane protein M
Coat protein N
In some viruses there is an additional small non-structural protein downstream of N

Type Member Genomic Organization

The RNA of 28.6 kb is translated into the following products:

Replication protein 1a of 447 kDa
Polymerase 1b, expressed as a 749 kDa fusion protein with 1a by ribosomal frameshift
Spike protein S of 160 kDa
Non-structural protein 3a of 8 kDa
Non-structural protein 3b of 28 kDa
Small envelope protein E of 9 kDa
Membrane protein M of 30 kDa
Coat protein N of 44 kDa
Non-structural protein 7 of 9 kDa