 Once gp120 binds to the coreceptor, gp41 undergoes a conformational change that releases it from its once-metastable position.This change causes the hydrophobic N-terminus of the gp41 protein, also known as the fusion domain, to insert into the host cell membrane and anchor the virus into place.The insertion of gp41 into the target cell causes a subtle rearrangement in the gp41 protein that brings together two trimeric coiled coils, HR1 and HR2, to form a six-helix bundle.