Fig. 6. The proposed mechanism of ACE2-induced conformational transitions of SARS-CoV-2 S trimer.

Conformational transitions from the closed ground prefusion state (with packed FP, in red) to the transiently open state (step 1) with an untwisting motion (highlighted in dark gray arrow) associated with a downward movement of S1 (red arrow), from the open state to the dynamic ACE2 engaged state (step 2), and then all the way to the refolded postfusion state (step 3). The continuous swing motions of ACE2-RBD within S trimer are indicated by red arrows. The S trimer associated with ACE2 dimer (third panel) was generated by aligning the ACE2 of our S-ACE2 structure with the available full-length ACE2 dimer structure (PDB: 6M1D). The postfusion state was illustrated as a cartoon (PDB: 6XRA).