Proposed model for CAP induced formation of the gp41 six-helix bundles within HIV-1 particles. In the native state ((1)), HIV-1 envelope glycoprotein spikes correspond to a trimeric gp120(pink)/gp41(yellow) complex, the fusion peptide from the gp41-N terminus not being shown. The relative positions of the C-helix (yellow) and N-helix in the native envelope spike are not known and only the C-helix is depicted. Following interaction of the gp120 portion of the spikes with cellular receptors or CAP, the gp120/gp41 trimeric complex undergoes a conformational change, in which the fusion peptide (red) becomes exposed and the N-terminal half of gp41 (green) becomes a trimeric coiled coil. In this configuration ((2)) the C-peptide portion of gp41 (yellow) is not yet associated with the N-peptide coiled coil. This intermediate conformation is converted into a six-helix bundle when the C-peptide region binds to the N-peptide coiled coil region. This structure ((3)), in the absence of target cell membranes, corresponds to a nonfunctional spike, which cannot initiate anymore fusion with target cells. The position of fusion peptides in the model for "dead-end" spikes is not shown.