Fig. 5

Schematic representation of the virus and cell fusion. a Prefusion structure of gp41. Side (left panel) and bottom (right upper panel) views of gp41 are shown. The four-helix collar (α6–α9) of gp41 encircled gp120 (gp120 is indicated as a dotted circle). Close-up view of gp41 around residue 647 is shown in the right lower panel. The distances between residues 647 and residues 592 (4.786 Å), 595 (3.199 Å), and 596 (4.829 Å) are indicated. b Schematic description of conformational changes in gp41 during membrane fusion based on our data. After the fusion peptide (FP) reached the host-cell membrane, packing of CHR into the NHR grooves was initiated near the connecting loop region between the CHR and NHR. Pairing of residues in the CHR located before residue 647 may be sufficient for generation of initial unstable fusion pores. Residues beyond 647 of the CHR needed to interact with the NHR to achieve pore enlargement. The cytoplasmic (CT) domain, transmembrane (TM) domains, fusion peptide (FP), membrane proximal external region (MPER), NHR, CHR, and loop region are indicated