Evidence that the transition of HIV-1 gp41 into a six-helix bundle, not the bundle configuration, induces membrane fusion

Many viral fusion proteins exhibit a six-helix bundle as a core structure. HIV Env-induced fusion was studied to resolve whether membrane merger was d ue to the transition into the bundle configuration or occurred after bundle formation. Suboptimal temperature was used to arrest fusion at an intermed iate stage. When bundle formation was prevented by adding inhibitory peptid es at this stage, membranes did not merge upon raising temperature. Inverse ly, when membrane merger was prevented by incorporating lysophosphatidylcho line (LPC) into cell membranes at the intermediate, the bundle did not form upon optimizing temperature. In the absence of LPC, the six-helix bundle d id not form when the temperature of the intermediate was raised for times t oo short to promote fusion. Kinetic measures showed that after the temperat ure pulse, cells had not advanced further toward fusion. The latter results indicate that bundle formation is the rate-limiting step between the arres ted intermediate and fusion. Electrical measures showed that the HIV Env-in duced pore is initially large and grows rapidly. It is proposed that bundle formation and fusion are each contingent on the other and that movement of Env during its transition into the six-helix bundle directly induces the l ipid rearrangements of membrane fusion, Because peptide inhibition showed t hat, at the intermediate stage, the heptad repeats of gp41 have become stab ly exposed, creation of the intermediate could be of importance in drug and /or vaccine development.