Fig. 2.

Inactive and active state conformations of spike protein with its receptor binding. a, b Represent the inactive state conformations in which S2 subunits (stem portion) are completely covered by the “down” position of CTD1s (head portion). It causes steric clashes and inhibits the binding between spike protein and ACE2. c Represents the active state conformation, in which one of the CTD1s is in open state (shown in red stars) and facilitates the binding between spike protein and ACE2. TM and CT stands for transmembrane domain and cytoplasmic tail, respectively