The most critical feature of myosin is its ability to generate a force that powers movements. Researchers have used optical traps to measure the forces generated by single myosin molecules (Figure 17-28a). In this approach, myosin is immobilized on beads at a low density. An actin filament, held between two optical traps, is lowered toward the bead until it contacts a myosin molecule on the bead. When ATP is added, the myosin pulls on the actin filament. Using a mechanical feedback mechanism controlled by a computer, one can measure the distance pulled and the forces and duration of the movement. The results of optical trap studies show that myosin II takes discrete steps, which average out to about 8 nm (Figure 17-28b), and generates 3–5 piconewtons (pN) of force, approximately the same force as that exerted by gravity on a single bacterium. We can also see that myosin II does not interact with the actin filament continuously, but rather binds, moves, and releases it (see Figure 17-28b). In fact, myosin II spends on average only about 10 percent of each ATPase cycle in contact with F-actin—it is said to have a duty ratio of 10 percent. This observation will be important later when we consider that in contracting muscle, hundreds of myosin heads pull on actin filaments, so that at any one time, 10 percent of the heads are engaged to provide a smooth contraction.

Now let us examine how myosin V moves. Scientists have managed to attach a fluorescent probe to just one of the two neck regions of a myosin V molecule and watch the fluorescent image as the molecule moves along an actin filament (Figure 17-29a). The labeled head takes many 72-nm steps without releasing from the actin—it is said to move processively (Figure 17-29b). This step size is twice the 36-nm length between helical repeats in the actin filament (see Figures 17-5b and 17-29a). So each 36-nm helical repeat site on the filament presents binding sites as each head alternately takes 72-nm steps—like someone walking on stepping stones across a river and placing each foot on every other stone. Notice that although the step size for each individual head is 72 nm, a cargo attached to the myosin V tail region will move 36 nm per step as the head moves from behind the cargo to in front of it. Thus the step size of the motor overall is said to be 36 nm, as indicated in Figure 17-29. Myosin V has presumably evolved to have a long neck domain—the lever arm—to take large steps to match the size of the helical repeat of the filament. Moreover, its ATPase cycle has been modified to have a much higher duty ratio (>70 percent) by slowing the rate of ADP release; thus the head remains in contact with the actin filament for a much larger percentage of the cycle. Since a single myosin V molecule has two heads, a duty ratio greater than 50 percent ensures that it maintains contact at all times as it moves down an actin filament, so that it does not fall off. These are exactly the properties one would expect for a motor designed to transport cargo along an actin filament.

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