Attaches the virion to the host cell membrane by interacting with sialic acids, initiating the infection. 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 3 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. Acts as a viral fusion peptide after S2 cleavage occurring upon virus endocytosis. Homotrimer; each monomer consists of a S1 and a S2 subunit. The resulting peplomers protrude from the virus surface as spikes. Accumulates in the endoplasmic reticulum-Golgi intermediate compartment, where it participates in virus particle assembly. Some S oligomers may be transported to the plasma membrane, where they may mediate cell-cell fusion. Accumulates in the endoplasmic reticulum-Golgi intermediate compartment, where it participates in virus particle assembly. Some S oligomers may be transported to the plasma membrane, where they may mediate cell-cell fusion. S1 is not anchored to the viral envelope, but associates with the extravirion surface through its binding to S2. The di-lysine motif confers endoplasmic reticulum localization for type I membrane proteins. Specific enzymatic cleavages in vivo yield mature proteins. The precursor is processed into S1 and S2 by host cell furin or furin-like protease to yield the mature S1 and S2 proteins. The cleavage site between S1 and S2 requires the optimal sequence [KR]-X-[KR]-R. Additionally, a second cleavage leads to the release of a fusion peptide after viral attachment to host cell receptor. Belongs to the gammacoronaviruses spike protein family.