Prefusion structures of the fusion (F) proteins of PIV5 and HRSV. (A) Ectodomain structure of the F protein. Domains are as follows: domain I (DI, yellow), domain II (DII, green), domain III (DIII, orange), domain III heptad repeat A (DIII HRA, brown), fusion peptide (f.p., magenta), heptad repeat B (HRB, blue), and trimerization domain (t.d.). (B, C) Trimer and monomer structure of the PIV5 F protein in its prefusion form. (D) Monomer structure of the HRSV F protein in its prefusion form. Domains in B–D are color coded as in the domain structure (A). Structures of PIV5 F (96) and HRSV F (54) were rendered with MacPYMOL using coordinates 2B9B and 4JHW, respectively.