Schematic illustration on the role of FP and TMD in the late stages of HA2-mediated fusion. (1) In the pre-hairpin stage, FP inserts into the target membrane following disengagement of HA1 from HA2. The inner leaflet of the bilayer is minimally disrupted by FP with an oblique insertion angle. Note the loose FP self-assembly and tight self-association of TMD in the membrane. (2) Low pH-induced refolding of HR1 and HR2 regions of the HA2 driven by strong interactions between them. The two apposing membranes are pulled in proximity and bulged-out to facilitate the merge. (3) Driven by the energy liberated by HR1-HR2 association and additional force provided by the polar, conformationally plastic linker segment downstream of FP and the membranetropic pre-TM region, the two fusing membranes undergo dehydration, deformation and coalescence of the outer leaflets, causing hemifusion. In the process, the compact TMD homo-trimer approaches the loose FP aggregate and may be interspersed with FP molecules, gradually forming the TMD-FP complex, which is not specific per se, with TMD in the inner core. Nonetheless, the interaction is sufficiently strong to align FP with TMD to a certain extent and deepen FP penetration into the inner leaflet, further destabilizing the bilayer. (4) Partly as a result of the complex formation-enhanced perturbation of both leaflets of the effector and target membranes, the hemifusion diaphragm transits to an inceptive fusion pore, concomitant with the six-helix bundle formation of HR1 and HR2. By this stage, the recruitment of adjacent TMD:FP triplex subunits cooperatively stabilizes the initial pore and its dilation to facilitate the mixing of cytoplasmic contents.