3307878 pre-F prefusion 59 3307877 preF prefusion F 16 3307882 pre-F prefusion F protein 12 3307880 preF prefusion conformation 6 3307884 pre-F prefusion form 3 3307876 pre-F pre-fusion stabilized F protein 1 3307879 pre-F pre-fusion-stabilized trimeric RSV F 1 3307883 pre-F prefusion F glycoprotein 1 3307881 pre-F prefusion RSV fusion protein 1 3307875 pre-F prefusion trimeric conformation 1