Unbinding force of a single motor molecule of muscle measured using optical tweezers

THE unbinding and rebinding of motor proteins and their substrate filaments are the main components of sliding movement¹. We have measured the unbinding force between an actin filament and a single motor molecule of muscle, myosin, in the absence of ATP, by pulling the filament with optical tweezers². The unbinding force could be measured repeatedly on the same molecule, and was independent of the number of measurements and the direction of the imposed loads within a range of ±90°. The average unbinding force was 9.2 ± 4.4 pN, only a few times larger than the sliding force3"5 but an order of magnitude smaller than other intermolecular forces^6,7. From its kinetics⁸ we suggest that unbinding occurs sequentially at the molecular interface, which is an inherent property of motor molecules.