Spike protein S1: attaches the virion to the cell membrane by interacting with host receptor, initiating the infection.
Spike protein S2: Acts as a viral fusion peptide which is unmasked following S2 cleavage occurring upon virus endocytosis.
Spike protein S2: mediates fusion of the virion and cellular membranes by acting as a class I viral fusion protein. Under the current model, the protein has at least three conformational states: pre-fusion native state, pre-hairpin intermediate state, and post-fusion hairpin state. During viral and target cell membrane fusion, the coiled coil regions (heptad repeats) assume a trimer-of-hairpins structure, positioning the fusion peptide in close proximity to the C-terminal region of the ectodomain. The formation of this structure appears to drive apposition and subsequent fusion of viral and target cell membranes.
Within the host cell endocytic vesicle, SARS-CoV-1 Spike (S) protein is cleaved between residues 797 and 798 by cathepsin L1 (CTSL) (Huang et al. 2006). The roles of S protein in viral binding to the host cell membrane and fusion of viral and host cell membranes and thus the central role of S protein in determining the host range and tissue tropisms of the virus are reviewed by Belouzard et al. (2012).