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 plasma cell membrane fusion, the heptad repeat (HR) regions 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 plasma cell membranes. Directs fusion of viral and cellular membranes leading to delivery of the nucleocapsid into the cytoplasm. This fusion is pH independent and occurs directly at the outer cell membrane. The trimer of F1-F2 (F protein) interacts with HN tetramer at the virion surface. Upon HN binding to its cellular receptor, the hydrophobic fusion peptide is unmasked and interacts with the cellular membrane, inducing the fusion between cell and virion membranes. Later in infection, F proteins expressed at the plasma membrane of infected cells could mediate fusion with adjacent cells to form syncytia, a cytopathic effect that could lead to tissue necrosis. Homotrimer of disulfide-linked F1-F2. Interacts with HN and M proteins. Folded in the endoplasmic reticulum by the human CANX and HSPA5 chaperones. The cytoplasmic region mediates the interaction with HN and M proteins. In natural infection, inactive F0 is matured into F1 and F2 outside the cell by one or more trypsin-like, arginine-specific endoprotease secreted by the bronchial epithelial cells. One identified protease that may be involved in this process is tryptase Clara. Unlike most paramyxoviruses, Sendai F0 processing occurs on the cell surface and induces a conformational change in the protein that unmasks the fusion peptide. F0 maturation is a primary determinant for organ tropism and pathogenicity. F1 and F2 display interchain and intrachain disulfide bonds, that are necessary for correct folding and intracellular transport. N-glycosylated; glycans consist of a mixture of high mannose-type oligosaccharides and of complex-type oligosaccharides. Glycosylation at Asn-245 is essential for membrane localization and F0 cleavage. Sendai virus or recombinant F protein are widely used in cellular biology to fuse cells. Belongs to the paramyxoviruses fusion glycoprotein family.