Alphacoronavirus
Alphacoronaviruses (Alpha-CoV) are members of the first of the four genera ("Alpha"-, "Beta-", "Gamma-", and "Delta-") of coronaviruses. They are positive-sense, single-stranded RNA viruses that infect mammals, including humans. They have spherical virions with club-shaped surface projections formed by trimers of the spike protein, and a viral envelope. Alphacoronaviruses are in the subfamily "Orthocoronavirinae" of the family "Coronaviridae". Both the "Alpha"- and "Betacoronavirus" lineages descend from the bat viral gene pool. Alphacoronaviruses were previously known as "phylogroup 1 coronaviruses". Etymology. The name alphacoronavirus is derived from Ancient Greek ἄλφα (álpha, "the first 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. Structure. The virion is enveloped and spherical measuring 120–160 nm in diameter and a core shell of about 65 nm. Glycoproteins and trimers form large surface projections which create the appearance of solar corona. This genus, like other coronaviruses, has a spike protein with a type I fusion machine (S2) and a receptor-binding domain (S1). It assembles into a trimer. Unlike beta- and gammacoronaviruses, this protein is not cleaved into two halves. Genome . The genome is positive-sense, single-stranded RNA with a length of 27 to 29 kilobases and a 3’-polyA tail. Two large, overlapping ORFs at the 5'-end of the genome encode the major non-structural proteins expressed as a fusion protein by ribosomal frameshift. These include regions with protease, helicase and RNA polymerase motifs. There are seven other genes downstream which encode structural proteins. These are expressed from a 3'-coterminal nested set of subgenomic mRNAs. Recombination. Both types of "Alphacoronavirus 1", feline coronavirus (FCoV) and canine coronavirus (CCoV), are known to exist in two serotypes. Serotype II targets Aminopeptidase N , while the receptor for Serotype I is unknown. The difference is due to a different spike protein. There is a common ancestor for FCoV and CCoV. This ancestor gradually evolved into FCoV I and CCoV I. An S protein from an unknown virus was recombined into the ancestor and gave rise to CCoV II. CCoV II once again recombined with FCoV to create FCoV II. CCoV II gradually evolved into TGEV. A spike deletion in TGEV creates PRCV. All these viruses are sorted into the subgenus "Tegacovirus". Classification. The following subgenera and species are recognized: