10.1 Structure
Spike proteins S are present on surface of coronaviruses and facilitate in viral entry for infection.76 The entry of cronavirus into the host cells is mediated by spike glycoproteins (S) that form homo-trimers that extend above the surface of virus77 (Figure 8a). S consists of two functional subunits which are present for attachment with the receptor of host cell (S1 subunit) and fusion of the viral and cellular membranes (S2 subunit). For most CoVs, at the boundary between the S1 and S2 subunits, S is cleaved which remain non-covalently bound in the prefusion conformation.52,54,62,70,78-80 The diatal S1 unit consists of receptor binding domains and participates in the stability of the prefusion state of the membrane-anchored S2 subunit which consists of the fusion machinery52,54,55,58,60,81,82(Figure 8b).
Trimers are being formed by spike molecules, which must be cleaved by cellular proteases, so that fusion peptide can aid in fusion of viral membrane with infected cells. The proteases produce two subunits S1 and S2 from the spike molecule, and S1 subunit consists of critical RBD receptor binding domains to bind with ACE2 of host cells. The receptor binding motif which is present in receptor binding domain forms direct contact with ACE2. Several others critical structures of spike proteins including Central Helix (CH) and heptad repeat 1 and 2 (HR1 and HR2) domains also help in fusion of virus with host cells51(Figure 9).

10.2 Spike proteins and conformational changes

For all CoVs, the so-called S2 site located immediately upstream of the fusion peptide is being used for further cleavage of S.62,83 This cleavage has been reported to trigger conformational changes that activate the proteins for membrane fusion via extensive irreversible changes.60-62,70Consequently, entry of coronavirus into susceptible cells is a complicated activity that requires coherent action of receptor binding and proteolytic processing of the S protein to enhance virus-cell fusion. After getting entry into the host cell, usually through contact with contaminated surfaces and aerosolized viral particles, the virus needs to undergo biological cycle. Spike proteins which are encoded by S gene on open reading frame on viral genome need to interact with viral receptors on the surface of host cells. Coronavirus spike binds with angiotensin-converting enzyme-2 (ACE-2), which are present in epithelial cells of the lungs.84,85 This is one of the major reasons why coronaviruses often cause respiratory disorders. ACE-2 receptors may also be highly expressed in intestinal tissues leading to diarrhea, as detected in few cases of SARS COV-2 patients.86 After binding to ACE-2 receptors via the RBD of the S1 and S2 domains of the spike protein, the viral envelope fuses with the host cell membrane and is further internalized. Genetic material approximately of the size 20-32 kb, is released into the cytoplasm for replication51 (Figure 10).