1.G.2.1.3
The respiratory syncytial virus (RSV) fusion (F) glycoprotein.  The crystal strcuture is available (McLellan et al. 2013).  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 which leads to delivery of the nucleocapsid into the cytoplasm. Fusion is pH independent and occurs directly at the outer cell membrane. The trimer of F1-F2 (protein F) interacts with glycoprotein G at the virion surface. Upon binding of G to heparan sulfate, the hydrophobic fusion peptide is unmasked and interacts with the cellular membrane, inducing the fusion between host cell and virion membranes. RSV fusion protein is able to interact directly with heparan sulfate and therefore actively participates in virus attachment.