Domain compliance and elastic power transmission in rotary FOF1-ATPase

Abstract

The 2 nanomotors of rotary ATP synthase, ionmotive FO and chemically active F1, are mechanically coupled by a central rotor and an eccentric bearing. Both motors rotate, with 3 steps in F1 and 10–15 in FO. Simulation by statistical mechanics has revealed that an elastic power transmission is required for a high rate of coupled turnover. Here, we investigate the distribution in the FOF1 structure of compliant and stiff domains. The compliance of certain domains was restricted by engineered disulfide bridges between rotor and stator, and the torsional stiffness (κ) of unrestricted domains was determined by analyzing their thermal rotary fluctuations. A fluorescent magnetic bead was attached to single molecules of F1 and a fluorescent actin filament to FOF1, respectively. They served to probe first the functional rotation and, after formation of the given disulfide bridge, the stochastic rotational motion. Most parts of the enzyme, in particular the central shaft in F1, and the long eccentric bearing were rather stiff (torsional stiffness κ > 750 pNnm). One domain of the rotor, namely where the globular portions of subunits γ and ε of F1 contact the c-ring of FO, was more compliant (κ ≅ 68 pNnm). This elastic buffer smoothes the cooperation of the 2 stepping motors. It is located were needed, between the 2 sites where the power strokes in FO and F1 are generated and consumed.