|Murine coronavirus (MHV) virion electron micrograph, schematic structure, and genome|
|Murine coronavirus (MHV)|
|Subgenera and species|
Betacoronaviruses (β-CoVs or Beta-CoVs) are one of four genera (Alpha-, Beta-, Gamma-, and Delta-) of coronaviruses. They are enveloped, positive-sense, single-stranded RNA viruses that infect humans and mammals. The natural reservoir for betacoronaviruses are bats and rodents. Rodents are the reservoir for the subgenus Embecovirus, while bats are the reservoir for the other subgenera.
The coronavirus genera are each composed of varying viral lineages with the betacoronavirus genus containing four such lineages: A, B, C, D. In older literature, this genus is also known as "group 2 coronaviruses". The genus is in the subfamily Orthocoronavirinae in the family Coronaviridae, of the order Nidovirales.
The betacoronaviruses of the greatest clinical importance concerning humans are OC43 and HKU1 (which can cause the common cold) of lineage A, SARS-CoV and SARS-CoV-2 (which causes the disease COVID-19) of lineage B, and MERS-CoV of lineage C. MERS-CoV is the first betacoronavirus belonging to lineage C that is known to infect humans.
The name "betacoronavirus" is derived from Ancient Greek βῆτα (bē̂ta, "the second letter of the Greek alphabet"), and κορώνη (korṓnē, “garland, wreath”), meaning crown, which describes the appearance of the surface projections seen under electron microscopy that resemble a solar corona. This morphology is created by the viral spike (S) peplomers, which are proteins that populate the surface of the virus and determine host tropism. The order Nidovirales is named for the Latin nidus, which means 'nest'. It refers to this order's production of a 3′-coterminal nested set of subgenomic mRNAs during infection.
Several structures of the spike proteins have been resolved. The receptor binding domain in the alpha- and betacoronavirus spike protein is cataloged as InterPro: IPR018548. The spike protein, a type 1 fusion machine, assembles into a trimer ( ); its core structure resembles that of paramyxovirus F (fusion) proteins. The receptor usage is not very conserved; for example, among Sarbecovirus, only a sub-lineage containing SARS share the ACE2 receptor.
The viruses of subgenera Embecovirus differ from all others in the genus in that they have an additional shorter (8 nm) spike-like protein called hemagglutinin esterase (HE) ( ). It is believed to have been acquired from influenza C virus.
Coronaviruses have a large genome size that ranges from 26 to 32 kilobases. The overall structure of β-CoV genome is similar to that of other CoVs, with an ORF1ab replicase polyprotein (rep, pp1ab) preceding other elements. This polyprotein is cleaved into 16 nonstructural proteins (see UniProt annotation of SARS rep, ).
Genetic recombination can occur when two or more viral genomes are present in the same host cell. The dromedary camel Beta-CoV HKU23 exhibits genetic diversity in the African camel population. Contributing to this diversity are several recombination events that had taken place in the past between closely related betacoronaviruses of the subgenus Embecovirus. Also the betacoronavirus, Human SARS-CoV, appears to have had a complex history of recombination between ancestral coronaviruses that were hosted in several different animal groups.
Alpha- and betacoronaviruses mainly infect bats, but they also infect other species like humans, camels, and rodents. Betacoronaviruses that have caused epidemics in humans generally induce fever and respiratory symptoms. They include:
Within the genus Betacoronavirus (Group 2 CoV), four lineages (A, B, C, and D) are commonly recognized.
- Lineage A (subgenus Embecovirus) includes HCoV-OC43 and HCoV-HKU1 (various species)
- Lineage B (subgenus Sarbecovirus) includes SARSr-CoV (which includes all its strains such as SARS-CoV, SARS-CoV-2, and Bat SL-CoV-WIV1)
- Lineage C (subgenus Merbecovirus) includes Tylonycteris bat coronavirus HKU4 (BtCoV-HKU4), Pipistrellus bat coronavirus HKU5 (BtCoV-HKU5), and MERS-CoV (various species)
- Lineage D (subgenus Nobecovirus) includes Rousettus bat coronavirus HKU9 (BtCoV-HKU9)
The four lineages have also been named using Greek letters or numerically. A further subgenus is Hibecovirus including Bat Hp-betacoronavirus Zhejiang2013. All recognized subgenera and species are listed hereafter:
- Embecovirus (Lineage A)
- Merbecovirus (Lineage C)
- Nobecovirus (Lineage D)
- Sarbecovirus (Lineage B)
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