– Inactive precursor that is cleaved to give rise to the mature F1 and F2 fusion glycoproteins.; 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 coiled coil 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 cellular membranes leading to delivery of the nucleocapsid into the cytoplasm. This fusion is pH independent and occurs at the plasma or endosomal membrane. The trimer of F1-F2 (F protein) also facilitates the attachment to host cell by binding to host heparan sulfate.; Major determinant of the species specificity of RSV infection. The trimer of F1-F2 (F protein) also facilitates the attachment to host cell by binding to host heparan sulfate.