Speaker
Description
The 20S proteasome is a compartmentalized protease that ensures controlled proteolysis and safeguards cell protein homeostasis in all kingdoms of Life. Proteasome activity is mainly regulated by ATPase activators that cap its extremities, although alternative energy-independent activators have been described and shown to play important roles. Here, we report the discovery of a novel ATP-independent activator in the hyperthermophile Pyrococcus abyssi, which we named Archaeal proteasome activator (APA). In vitro assays show that APA interacts directly with the 20S proteasome complex, stimulates proteasome-mediated substrate degradation and exhibits an independent chaperone function. The structural characterization of APA reveals a ring-shaped homoheptameric complex with no structural similarity to previously described activators. Using single-particle cryo-EM, we solved the structure of the APA-20S proteasome complex at 3.1 Å resolution, showing an uncanonical proteasome binding and gate-opening mode in which APA is tightly clamped by the C-terminal regions of the 20S proteasome α-subunits. Collectively, our findings provide a novel mechanistic insight into ATP-independent proteasome activation and suggest a wider variety of molecular actors in proteasome regulation.