Dynamic and static components power unfolding in topologically closed rings of a AAA+ proteolytic machine

In the E. coli ClpXP protease, a hexameric ClpX ring couples ATP binding and hydrolysis to mechanical protein unfolding and translocation into the ClpP degradation chamber. Rigid-body packing between the small AAA+ domain of each ClpX subunit and the large AAA+ domain of its neighbor stabilizes the hexamer. By connecting the parts of each rigid-body unit with disulfide bonds or linkers, we created covalently closed rings that retained robust activity. A single-residue insertion in the hinge that connects the large and small AAA+ domains and forms part of the nucleotide-binding site uncoupled ATP hydrolysis from productive unfolding. We propose that ATP hydrolysis drives changes in the conformation of one hinge and its flanking domains, which are propagated around the AAA+ ring via the topologically constrained set of rigid-body units and hinges to produce coupled ring motions that power substrate unfolding.