ClpX eats randomly

ClpXP is an ATP-powered eating machine. Its ring of six ClpX ATPase modules feeds substrates to the ClpP protease. The ClpX subunits have been suggested to fire all at once or in a sequential, piston-like sequence. But now Andreas Martin, Tania Baker, and Robert Sauer (MIT, Cambridge, MA) find that the hexamer functions even if only one subunit can hydrolyze ATP. They suggest that the the order of firing of the subunits may be not deterministic but probabalistic—all the better to manhandle half-digested protein substrates whose features protrude haphazardly.

Martin wanted to test the models by mixing and matching functional and nonfunctional ClpX subunits. But the six subunits are identical, so coexpression of wild-type and mutant subunits would yield only messy mixtures. “If you were going to make headway you needed to connect the subunits” into...