Abstract
The F(1) 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 F(1)βE190D mutant to demonstrate that the stall torque was significantly smaller than the wild-type F(1), 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 α(3)β(3)-subcomplex revealed the unstable structure of the βE190D mutant. Our results suggest that the tight chemomechanical coupling of the F(1) motor relies on the structural stability of F(1). We also discuss the difference between the stepping torque and the stall torque.