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, and fusion probably occurs at the host plasma membrane (By similarity).