The core of the respiratory syncytial virus fusion protein is a trimeric coiled coil

Entry into the host cell by enveloped viruses is mediated by fusion (F) or transmembrane glycoproteins. Many of these proteins share a fold comprising a trimer of antiparallel coiled-coil heterodimers, where the heterodimers are formed by two discontinuous heptad repeat motifs within the proteolytic ally processed chain. The F protein of human respiratory syncytial virus (R SV; the major cause of lower respiratory tract infections in infants) conta ins two corresponding regions that are predicted to form coiled coils (HR1 and HR2), together with a third predicted heptad repeat (HR3) located in a nonhomologous position. In order to probe the structures of these three dom ains and ascertain the nature of the interactions between them, we have stu died the isolated HR1, HR2, and HR3 domains of RSV F by using a range of bi ophysical techniques, including circular dichroism, nuclear magnetic resona nce spectroscopy, and sedimentation equilibrium. HR1 forms a symmetrical, t rimeric coiled coil in solution (K-3 approximate to 2.2 x 10(11) M-2) Which interacts with HR2 to form a 3:3 hexamer, The HR1-BR2 interaction domains have been mapped using limited proteolysis, reversed-phase high-performance liquid chromatography, and electrospray-mass spectrometry. HR2 in isolatio n exists as a largely unstructured monomer, although it exhibits a tendency to form aggregates with beta-sheet-like characteristics. Only a small incr ease in or-helical content was observed upon the formation of the hexamer. This suggests that the RSV F glycoprotein contains a domain that closely re sembles the core structure of the simian parainfluenza virus 5 fusion prote in (K. A. Baker, R.E, Dutch, R. k Lamb, and T, S. Jardetzky, Mel. Cell 3:30 9-319, 1999). Finally, HR3 forms weak or-helical homodimers that do not app ear to interact with HR1, HR2, or the HR1-HR2 complex. The results of these studies support the idea that viral fusion proteins have a common core arc hitecture.