Direct visualization of human myosin II force generation using DNA origami-based thick filaments

Abstract

Muscle contraction can be explained by the swinging lever-arm model. However, the dynamic features of how the myosin head swings the lever-arm and its initial interactions with actin are not well understood even though they are essential for the muscle force generation, contraction speed, heat production, and response to mechanical perturbations. This is because myosin heads during force generation have not been directly visualized. Here, we engineered thick filaments composed of DNA origami and recombinant human muscle myosin, and directly visualized the heads during force generation using nanometer-precision single-molecule imaging. We found that when the head diffuses, it weakly interacts with actin filaments and then strongly binds preferentially to the forward region as a Brownian ratchet. Upon strong binding, the head two-step lever-arm swing dominantly halts at the first step and occasionally reverses direction. These results can explain all mechanical characteristics of muscle contraction and suggest that our DNA origami-based assay system can be used to dissect the mechanistic details of motor proteins.