Structure and mechanism of Vps4, the enzyme that drives membrane fission in the ESCRT pathway
Many cellular membrane fission reactions are driven by ESCRT pathways, which culminate in remodeling and disassembly of ESCRT-III polymers by the AAA ATPase Vps4. HIV-1 and many other viruses recruit an ESCRT pathway in order to bud from cells. Recent advances in understanding of the budding machinery will be summarized, with special emphasis on HIV and findings from our 3.2 Å resolution cryo-EM structure of the active Vps4 hexamer in complex with its cofactor Vta1, ADP·BeFx, and an ESCRT-III substrate peptide. Five Vps4 subunits form a helix, with interfaces between the first four of these subunits apparently bound to ADP.BeFx (ATP) and the interface between the fourth and fifth subunit bound to ADP, as if it is just commencing dissociation from the helix. The final Vps4 subunit completes a notched-washer configuration as if transitioning between the ends of the helix. The ESCRT-III peptide binds in an extended (beta-strand) conformation against the five helical subunits. Two classes of side chain binding pockets are formed primarily by Vps4 pore loop 1 residues, with four copies of each pocket propagating along the highly solvated pore through the Vps4 hexamer. The pockets accommodate a wide range of residues, while main chain hydrogen bonds help dictate substrate-binding orientation. The structure supports a ‘conveyor belt’ model of translocation in which ATP binding allows a Vps4 subunit to join the growing end of the helix and engage the substrate, while hydrolysis and release promotes helix disassembly and substrate disengagement at the lagging end. In this manner Vps4 may disassemble ESCRT-III to reveal a metastable membrane configuration that resolves by fission and virus budding. This model likely applies to other ESCRT pathways and may be generally applicable to multiple other protein-translocating AAA ATPases.