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) probably interacts with HN 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 (By similarity). Homotrimer of disulfide-linked F1-F2. Interacts with host LAMP1; LAMP2 and LAMP3; these interactions promote the cleavage of the viral fusion protein F. The inactive precursor F0 is glycosylated and proteolytically cleaved into F1 and F2 to be functionally active (PubMed:3617503). The cleavage is mediated by cellular proteases including host FURIN during the transport and maturation of the polypeptide (By similarity). Belongs to the paramyxoviruses fusion glycoprotein family.