Nucleotide-Driven Triple-State Remodeling of the AAA-ATPase Channel in the Activated Human 26S Proteasome

The proteasome is a sophisticated ATP-dependent molecular machine responsible for protein degradation in all eukaryotic cells. It remains elusive how conformational changes of the AAA-ATPase unfoldase in the regulatory particle (RP) control the gating of substrate-translocation channel to the proteolytic chamber of the core particle (CP). Here we report three alternative states of the ATP-γS-bound human proteasome, in which the CP gate is asymmetrically open, visualized by cryo-EM at near-atomic resolutions. Only four nucleotides are stably bound to the AAA-ATPase ring in the open-gate states. Concerted nucleotide exchange gives rise to a back-and-forth wobbling motion of the AAA-ATPase channel, coincident with remarkable transitions of their pore loops between the spiral staircase and saddle-shaped circle topologies. Gate opening in the CP is thus controlled with nucleotide-driven remodeling of the AAA-ATPase unfoldase. These findings demonstrate an elegant mechanism of allosteric coordination among sub-machines within the holoenzyme that is crucial for substrate translocation.