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. 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.