S1 domain
The main components of the S1 domain are the N-terminal domain (NTD) and the C-terminal domain (CTD). The S1 domain acts as a major antigen on the surface of the virus[40] and has a receptor-binding domain (RBD).[25] The 18 residues of ACE-2 interact with the RBD (contain 14 amino acids) of SARS-CoV spike protein,[45] and for this contact, K341 of ACE-2 and R453 residue of RBD play the most important role. If point mutated on the D454 or R441 of RBD, it disturbs the binding activity with ACE-2.[25] The S1 domain interacts with the ACE-2 or DPP-4 receptors of the host. Anti-ACE-2 antibody blocked viral entry and replication in Vero E6 cells.[1445] One another mechanism of virus for binding to host cell is using dendritic cell-specific intercellular adhesion molecule-3 grabbing non-integrin (DC-SIGN receptor) or L-SIGN in lymph nodes or in liver.[4647] S protein has seven (109, 118, 119, 158, 227, 589, and 699) glycosylation asparagine-linked sites, which is pivotal for both L-SIGN- or DC-SIGN-based virus entry into the host.[48]