Figure 2.

Myosin, thick filament structure, and actomyosin power stroke. A. Myosin is composed of three paired molecules, the heavy chain and the essential and regulatory light chains. Part of the heavy chains form a coiled coil tail; the remainder of the heavy chains and the two light chains form two globular heads, each of which can independently bind the thin (actin) filament. Modified from Rayment and Holden (1994). B. Thick filaments can be end or side polarized. In end polarized filaments the heads on each half of the filament have the same orientation and the filament thus has a central zone bare of heads. As a result of this orientation, each end of the filament ‘pulls’ the actin filaments with which it interacts toward the central bare zone (arrows). In side polarized filaments the heads on each side of the filament all have the same orientation. Modified from Xu et al. (1996). C. The actomyosin power stroke. 1. A myosin head with bound ADP-Pi approaches an actin binding site. 2. The head become strongly bound. 3. The head rotates about a hinge, and the actin filament is displaced. During this step the Pi disassociates. 4. The ADP also disassociates, ATP binds to the myosin head, and the head dissociates from the actin filament, thus allowing the cycle to repeat. Blue is head catalytic core; yellow and red are, respectively, the pre and post stroke lever arm of the head. Modified from Vale and Milligan (2000).