Fig. 1. (A) Schematic drawing of the measurement for the displacement of a myosin head along an actin filament. Single myosin head was captured on the tip of a probe through a biotin–streptavidin bond at its light chain. Actin filament was immobilized on the surface of a coverslip in the presence of ATP. (B) Typical time course of the probe capturing a single myosin head (upper) and three traces of the rising phase of the displacement in an expanded time scale (lower). In the lower panel, multiple steps occurring stochastically were detected within the rising phase.

Fig. 2. Steric restriction between actin and myosin interaction in the scanning probe experiment. Movement of a myosin head captured on the tip of a probe is interpreted as Brownian movement under a potential created by the interaction between actin and myosin. The potential slope in a half helical pitch of an actin filament reflects the directional movement of myosin heads due to the orientation compatibility between an actin filament and a myosin head.

Fig. 3. Schematic drawing of myosin and actin filaments in skeletal muscle. On the myosin filament, the myosin heads protruded at interval of 14.3 nm are aligned in a helical structure with a pitch of 42.9 nm. The actin filament also has a helical structure with a half pitch of 36 nm. In skeletal muscle, the actin and myosin filaments are interdigitated in a hexagonal lattice and three myosin filaments surround one actin filament. An actin filament and a myosin filament are anchored in a Z-line and a M-line, respectively.

Fig. 4. The model describing the cooperative actions of myosin heads coupled with ATPase reaction. Myosin heads that interact with a particular actin filament are connected by elastic joint on a line of myosin filament. The potential formed between actin and myosin is assumed that the potential slope within a half helical pitch of actin filament depicted in Fig. 2 cyclically repeats in a line.

Fig. 5. (A) Typical simulated sample paths of the movements between an actin and a myosin filament. Arrows denote the binding of an ATP molecule to the rigor head, open triangles the movement caused by the rewind of an actin filament, and closed triangles the trailed movements by newly rebinding myosin heads to the actin filament. Sliding distance was defined as the maximum displacement caused by the cooperative action between heads after the binding of ATP molecule. (B) Simulated trace of a myosin head along an actin filament. Symbols denote the same meaning mentioned above. (C) The distribution of the sliding distance during a single ATPase process. The backward displacement was not included (see text for detail). The mean sliding distance of actin filament for the forward displacement was 53.8 ± 1.4 nm per ATP.

The values of the parameters used for simulation are listed