FIG. 4.

Schematic of coronavirus spike proteins. Shown are spike proteins representative of those of all group I to III coronaviruses and of SARS-CoV. The coronavirus spike protein is synthesized as a precursor, cotranslationally glycosylated, and, in some cases, cleaved in the approximate middle into S1 and S2 subunits at a site with dibasic amino acids (BBXBB). S1 forms the external domain containing the receptor binding domain (RBD) at its 5′ end, followed by, in the case of MHV, a hypervariable domain (HVR). A short signal sequence in cleaved from the 5′ end of the mature protein. S2 is the transmembrane subunit containing two heptad repeats (HR1 and HR2) and the transmembrane (TM) domain.