Pulling to the protease

Before destroying proteins, proteases such as bacterial ClpXP and the related eukaryotic proteasome must denature them. Jon Kenniston, Robert Sauer (MIT, Cambridge, MA), and colleagues now show that ClpX denatures by repeatedly applying a uniform unfolding force. “The way the enzyme works is to keep trying,” says Sauer.

This strategy works because proteins fluctuate around an average structure over time. Even a very stable protein will, very infrequently, be surprised in a relatively susceptible state. When this happens, the standard pulling force by ClpX is enough to unravel the protein. The enzyme then quickly threads the denatured protein through a narrow hole, toward the protease active site, before the substrate can refold.

The MIT group discovered the repetitive pulling phenomenon by studying stability variants of the muscle protein titin. This allowed “us to deconvolute how much ATP...