This Project is designed to elucidate the mechanistic bases for HIV-1 budding and its inhibition by the host factor retroCHMP3. To initiate budding, HIV-1 Gag binds ESCRT-I1-4 and ALIX5-7, two early-acting factors in the host Endosomal Sorting Complexes Required for Transport (ESCRT) pathway. These factors, in turn, recruit the late-acting ESCRT factors CHMP2, CHMP4 (ESCRT-III) and VPS4 (AAA ATPase)2,8-10 (reviewed in refs. 11-16). Numerous models for membrane constriction and fission have been proposed.17-26 We favor those in which: 1) ESCRT-III proteins form spiraling filaments that recruit VPS4 and constrict the membrane neck from the cytoplasmic face and 2) The VPS4 ATPase promotes lipid mixing by remodeling the underlying ESCRT-III scaffold. We now propose to determine the structural bases for membrane remodeling by ESCRT-III filaments and VPS4 (Aims 1 and 2), characterize how mammalian retroCHMP3 (ESCRT-III) proteins can inhibit HIV budding without inducing cellular toxicity (Aim 3), and image ESCRT assemblies within budding virions by electron cryotomography (Aim 4). These studies will reveal how HIV-1 uses the host ESCRT-III/VPS4 machinery to exit cells and how host retroCHMP3 proteins can specifically inhibit this process.