Fig. 1

Conformational changes in Env during membrane fusion. a Pre-fusion and post-fusion structures of gp41. Left panel: Schematic representation of the prefusion structure of Env based on the structure of BG505 SOSIP.664, as determined by CryoEM (PDB 4TVP). The upper and lower images depict the side and bottom (from the viral membrane) views, respectively. The main chain of the gp120 subunit is shown in gray, and the gp41 subunit is shown in various colors corresponding to different α-helices. The loop between the NHR and CHR is shown in yellow. Note that only a monomer of Env is shown in the left panel, and bound antibodies PGT122 and 35O22 are removed for clarity. The N- and C- terminus of gp120 are wrapped in the center of the complex formed by gp41 [7]. Right panel: the post-fusion structures of the six-helix bundle (6HB) of gp41 (PDB 1AIK). NHR and CHR are shown in the same color assignment as in the left panel. The side and bottom views are shown. The images were generated by the University of California, San Francisco Chimera program. b The alanine insertion mutants used in this study. The insertion sites for alanine are indicated by arrowheads above the sequence of gp41. The sequence and secondary structures of HIV-1 gp41 are indicated. Both pre-fusion and post-fusion secondary structures are colored corresponding to a. Cylinders represent α-helices, and the disordered regions are indicated by “x”. The regions corresponding to N36 and C34 are also shown. Mutants are named by the position of the inserted alanine residue (such as 645+A). The numbering is based on HXB2 Env