Prototypic α-genus coronaviruses include human coronavirus NL63(HCoV-NL63), porcine transmissible gastroenteritis coronavirus (TGEV), and porcine respiratory coronavirus (PRCV). Prototypic β-genus coronaviruses include severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East Respiratory Syndrome coronavirus (MERS-CoV), mouse hepatitis coronavirus (MHV), and bovine coronavirus (BCoV). Prototypic γ-genus coronaviruses include avian infectious bronchitis virus (IBV).
Coronaviruses belong to the Coronaviridae family in the order of Nidovirales. They can be classified into at least three major genera, α, β, γ (formerly group 1, 2, and 3, respectively).
Coronaviruses are a group of common, ancient, and diverse viruses. They infect many mammalian and avian species and cause respiratory, gastrointestinal and central nervous system diseases. Coronavirus virions contain an envelope, a helical capsid, and single-stranded and positive-sense RNA genome. The length of their genomes, which are the largest among all RNA viruses, typically ranges between 27 and 32 kb. They were named “coronaviruses” because the protruding spike proteins on their envelope that give the virions a crown-like shape (“corona” in Latin means crown).
The receptor-binding S1 subunit of coronavirus spike proteins contain two distinctive domains, the N-terminal domain (S1-NTD) and the C-terminal domain (S1-CTD), both of which can function as receptor-binding domains (RBDs). S1-NTDs and S1-CTDs from three major coronavirus genera recognize at least four protein receptors and three sugar receptors and demonstrate a complex receptor recognition pattern. For example, highly similar coronavirus S1-CTDs within the same genus can recognize the same receptor. Moreover, coronavirus S1-NTDs can recognize either protein or sugar receptor.
Receptor recognition by viruses is the first and essential step of viral infection of host cells. It is important determinant of viral host range and cross-species infection and a primary target for antiviral intervention. Coronaviruses recognize a variety of host receptors, infect many hosts, and are health threats to humans and animals.
With human activity increasingly overlapping the habitats of bats, diseases outbreaks resulted from spillover of bat coronaviruses will continue to occur in the future despite the fact that direct transmission of bat coronaviruses to humans appears to be rare. To better prepare ourselves in predicting and preventing the next emergence of coronovirus disease, it is necessary to maintain our vigilance in long-term coronavirus surveillance studies in bats as well as in other wildlife and livestock.
The spike protein and host receptor are key factors of cross-species transmission of coronaviruses, characterization of the receptor and key binding sites of the spike protein will be important in estimating host tropism of bat coronaviruses and predicting spillover risk.