Assembly and priming of the VPS4B AAA+ ATPase of the human ESCRT system.

Vacuolar protein sorting-associated protein 4 (VPS4), which occurs in A and B isoforms in humans, is the only enzyme in the core endosomal sorting complex required for transport (ESCRT) machinery. Human VPS4 is considered a potential therapeutic target for activation in neurodegeneration, and for inhibition in cancer and HIV-1 infection. VPS4 assembles transiently into a hexamer, which then removes ESCRT-III subunits from polymeric assemblies by unfolding them and threading them through its central pore. The N-terminal MIT domain of VPS4 binds to C-terminal MIM motifs of ESCRT-III. Here, we determined the cryo-electron microscopy structure of the full-length human VPS4B hexamer in six-membered helical "spiral staircase" states. In one of these states, two of the six MIT domains are ordered and stabilize the hexamer by bridging the seam of the spiral staircase. Residues involved in seam-bridging contacts were found to be important for biochemical and cellular activity. The structures also revealed two modes for polypeptide occupancy of the central pore, one of which involves the MIT-AAA linker peptide. These observations suggest a mechanism for substrate-dependent hexamerization and priming of VPS4 for its ESCRT-III remodeling activity.