Single-molecule analysis of the actomyosin motor using nano-manipulation

The elementary events in energy transduction by the actomyosin motor, driven by ATP hydrolysis, were directly recorded from multiple and single molecules using a recently developed technique for nano-manipulation of single actin filaments by a microneedle. In order to avoid the effects of random orientation of myosin and association of myosin with an artificial substrate in the surface motility assay, we used single myosin-rod cofilaments with various ratios. Distinct actomyosin attachment, force generation (the power stroke) and detachment events were detected at a very low myosin: rod ratio. At high load, one power stroke generated 5-6 pN peak force and 2.3 pN force averaged over the cycle, which were compatible with those deduced from noise analysis of force fluctuations caused by multiple molecules. As the load was reduced, the length of the power stroke increased. At near zero load, the length of a power stroke was approximately 17 nm. The results suggested that an ATPase cycle produces one power stroke at high load and many ones at low load.