Tight Chemomechanical Coupling of the F₁ Motor Relies on Structural Stability

The F₁ motor is a rotating molecular motor that ensures a tight chemomechanical coupling between ATP hydrolysis/synthesis reactions and rotation steps. However, the mechanism underlying this tight coupling remains to be elucidated. In this study, we used electrorotation in single-molecule experiments using an F₁ βE190D mutant to demonstrate that the stall torque was significantly smaller than the wild-type F₁, indicating a loose coupling of this mutant, despite showing similar stepping torque as the wild-type. Experiments on the ATPase activity after heat treatment and gel filtration of the α₃β₃-subcomplex revealed the unstable structure of the βE190D mutant. Our results suggest that the tight chemomechanical coupling of the F₁ motor relies on the structural stability of F₁. We also discuss the difference between the stepping torque and the stall torque.