Figure 16–28 Direct evidence for the motor activity of the myosin head. In this experiment, purified myosin heads were attached to a glass slide, and then actin filaments labeled with fluorescent phalloidin were added and allowed to bind to the myosin heads. (A) When ATP was added, the actin filaments began to glide along the surface, owing to the many individual steps taken by each of the dozens of myosin heads bound to each filament. The video frames shown in this sequence were recorded about 0.6 second apart; the two actin filaments shown (one red and one green) were moving in opposite directions at a rate of about 4 μm/sec. (B) Diagram of the experiment. The large red arrows indicate the direction of actin filament movement (Movie 16.2). (A, courtesy of James Spudich.)
ATTACHED At the start of the cycle shown in this fgure, plus a myosin head lacking a bound nucleotide is locked end tightly onto an actin flament in a rigor confguration (so named because it is responsible for rigor mortis, the rigidity of death). In an actively contracting muscle, this state is very short-lived, being rapidly terminated by the binding of a molecule of ATP.
RELEASED A molecule of ATP binds to the large cleft on the “back” of the head (that is, on the side furthest from the actin flament) and immediately causes a slight change in the conformation of the actin-binding site, reducing the affnity of the head for actin and allowing it to move along the flament. (The space drawn here between the head and actin emphasizes this change, although in reality the head probably remains very close to the actin.)
COCKED The cleft closes like a clam shell around the ATP molecule, triggering a movement in the lever arm that causes the head to be displaced along the flament by a distance of about 5 nm. Hydrolysis of ATP occurs, but the ADP and inorganic phosphate (Pi) remain tightly bound to the protein.
FORCE-GENERATING Weak binding of the myosin head to a new site on the actin flament causes release of the inorganic phosphate produced by ATP hydrolysis, concomitantly with the tight binding of the head to actin. This release triggers the power stroke—the force-generating change in shape during which the head regains its original conformation. In the course of the power stroke, the head loses its bound ADP, thereby returning to the start of a new cycle.
ATTACHED At the end of the cycle, the myosin head is again locked tightly to the actin flament in a rigor confguration. Note that the head has moved to a new position on the actin flament.
Figure 16–29 The cycle of structural changes used by myosin II to walk along an actin filament. In the myosin II cycle, the head remains bound to the actin filament for only about 5% of the entire cycle time, allowing many myosins to work together to move a single actin filament (Movie 16.3). (Based on I. Rayment et al., Science 261:50–58, 1993.) movements such as heart pumping and gut peristalsis depend on the contraction of cardiac muscle and smooth muscle, respectively. All these forms of muscle contraction depend on the ATP-driven sliding of highly organized arrays of actin filaments against arrays of myosin II filaments.