Binding and entry: gp41
After binding, membrane fusion takes place, mediated by the gp41 component of the viral envelope spike. Fusion makes the lipid bilayer of the viral particle continuous with that of the infection target cell, so that the viral core complex can enter into the cytoplasm. Rearrangements in the gp41 protein to form the six helix bundle structure are thought to mediate membrane fusion.
For review see: 
- Wilen CB, Tilton JC, Doms RW. 2012. HIV: Cell Binding and Entry. in HIV From Biology to Prevention and Treatment, pp. 23 Cold Spring Harbor Laboratory Press, Cold Spring Harbor.