Energy conversion mechanism revealed by ATP-free and ATP-dependent walking of myosin V motor

The mechanical processes of ATPase protein motors are coupled to their ATPase reactions. The free energy released at a given step(s) of the ATPase cycle is converted into mechanical work. Therefore, it has long been thought that ATP is the energy source for mechanical work. Here, we show that the double-headed myosin V (M5) motor can perform ATP-free walking. In the presence of ADP, M5 bound to F-actin by both heads makes a forward step when the trailing head is detached by an external force. This forward step harnesses the strain energy in the leading (L) head, provided from the strong binding energy between the L-head and actin. Consequently, strain generation reduces the binding affinity, which manifests as sporadic, brief L-head detachment. Strain generation and brief L-head detachment also occur during ATP-dependent walking to similar extents as during ATP-free walking. These results suggest that the strong binding energy is also converted into mechanical work during ATP-dependent walking.