DPP4 and infection: As we mentioned earlier, the RBD located in S1 subunit binds to DPP4 to initiate infection, and the HR1 and HR2 motifs in S2 subunit facilitate membrane fusion, resulting in the release of the viral genetic material into the host cell cytoplasm.43 Based on the crystallographic study, the RBD of the MERS-CoV S1 subunit ranges from residues 367 to 606 and can be divided into a core and an external subdomain.22 The receptor-binding motif (V484 to L567) of RBD is located in the external subdomain.32 The core subdomain contains a five-stranded antiparallel β-sheet in the center.