Fig. 1.

Schematic and structure of the SARS-CoV-2 spike (S) protein. (A) Schematic of the SARS-CoV-2 S protein. The receptor binding domain (RBD) is colored in yellow. NTD: N-terminal domain; SD: subdomain (1 and 2); FP, fusion peptide; HR1, heptad repeat 1; HR2, heptad repeat 2; TM, transmembrane domain; CT, cytoplasmic tail. Arrows denote protease cleavage sites. (B) Structure of the SARS-CoV-2 S protein in the prefusion conformation with one RBD in an up conformation and two RBDs in the down conformation (PDB ID: 6VSB) [46]. For the S1 domain, RBD (in an up conformation) and NTD are highlighted in yellow and green, respectively, with the remainder of S1 and S2 shown in white and grey. For clarity, only one protomer is represented in tubes and the other two subunits are represented by a white molecular surface with their RBDs in a down conformation in pink.