S1 attaches the virion to the cell membrane by interacting with cell receptors, initiating the infection. Homotrimer; each monomer consists of a S1 and a S2 subunit. The resulting peplomers protrude from the virus surface as spikes (By similarity). Accumulates in the endoplasmic reticulum-Golgi intermediate compartment, where it participates in virus particle assembly. S1 is not anchored to the viral envelope, but associates with the extravirion surface through its binding to S2 (By similarity). 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. Cleavage is not necessary for virus-cell fusion (By similarity). Belongs to the coronaviruses spike protein family.