Hexameric ring-shaped ATPases of the AAA + (for ATPases associated with various cellular activities) superfamily power cellular processes in which macromolecular structures and complexes are dismantled or denatured, but the mechanisms used by these machine-like enzymes are poorly understood. By covalently linking active and inactive subunits of the ATPase ClpX to form hexamers, here we show that diverse geometric arrangements can support the enzymatic unfolding of protein substrates and translocation of the denatured polypeptide into the ClpP peptidase for degradation. These studies indicate that the ClpX power stroke is generated by ATP hydrolysis in a single subunit, rule out concerted and strict sequential ATP hydrolysis models, and provide evidence for a probabilistic sequence of nucleotide hydrolysis. This mechanism would allow any ClpX subunit in contact with a translocating polypeptide to hydrolyse ATP to drive substrate spooling into ClpP, and would prevent stalling if one subunit failed to bind or hydrolyse ATP. Energy-dependent machines with highly diverse quaternary architectures and molecular functions could operate by similar asymmetric mechanisms.
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We thank S. Bell, D. Bolon, R. Burton, P. Chien, C. Farrell, G. Hersch, J. Kenniston, K. McGinness, S. Moore, F. Solomon and K. Wang for help and discussions. T.A.B. is an employee of the Howard Hughes Medical Institute. This work was supported by grants from the NIH.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
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