(A) Schematic of SARS-CoV-2 spike (S) protein primary structure colored by domain. Domains that were excluded from the ectodomain expression construct or could not be visualized in the final map are colored white. SS: signal sequence; S2': S2' protease cleavage site; FP: fusion peptide; HR1: heptad repeat 1; CH: central helix; CD: connector domain; HR2: heptad repeat 2; TM: transmembrane domain; CT: cytoplasmic tail. Arrows denote protease cleavage sites.
(B) Side and top views of the prefusion structure of the SARS-CoV-2 spike (S) protein with a single receptor binding domain (RBD) in the up conformation. The two RBD down protomers are shown as cryo-electron-microscopy density in either white or gray and the RBD up protomer is shown in ribbons colored corresponding to the schematic in (A).
