Structure of SARS-CoV-2 S protein. (a) Schematics of SARS-CoV-2 spike (S) protein domain organization. NTD, N-terminal domain; RBD, receptor-binding domain; FP, fusion peptide; HR1 and HR2, heptad region 1 and 2; TM: transmembrane domain; IC: intracellular tail. (b) Cryo-EM structure of SARS-CoV-2 S protein trimer (PDB 6VYB) with closed up views of the S1-NTD and RBD. Three copies of the S protein in the trimer are colored in green, orange and purple, respectively. (c) Crystal structure of SARS-CoV-2 RBD in complex with the host cell receptor angiotensin-converting enzyme 2 (ACE2) (PDB 6 M0J). SARS-CoV-2 RBD is colored in green and ACE2 is in orchid.