Protease power strokes force proteins to unfold

Abstract

ATP-dependent proteases degrade proteins in the cytosol of cells. Two recent articles, by Aubin-Tam et al. (2011) and Maillard et al. (2011 [this issue]), use single-molecule optical tweezers to show directly that these molecular machines use the energy derived from ATP hydrolysis to mechanically unfold and translocate its substrates into the proteolytic chamber.

Figure 1. Force-Induced Protein Unfolding by ATP-Dependent Proteases

(A) In the standard geometry for single-protein force spectroscopy experiments, a stretching force is applied to the two termini of a protein. (B) The ATP-dependent protease ClpXP generates a force FM, which pulls degradation-targeted substrates into its central pore. A normal force, FN, arises as the folded substrate is pinned against the narrow opening of ClpXP. These opposing forces trigger unfolding of the substrate in vivo. In the optical tweezer setup used by Maillard et al. (2011) and Aubin-Tam et al. (2011), a probe anchored to one end of the substrate introduces a third force in the system, FP, which counteracts the pull of ClpXP. Changing FP shifts the balance between FN and FP, according to the equation shown. (C) Mechanical unfolding of the substrate reduces its resilience and enables degradation. However, in vivo, the substrate probably remains collapsed. Refolding from this collapsed state allows the protein to survive in an ongoing struggle against degradation.