Mapping the actin filament with myosin.

Structural studies have shown that the heads of the myosin motor molecule bind preferentially to "target zones" of favorably oriented sites on the helices of the actin filament. We present direct evidence for target zones from the interactions of a single myosin head with an actin filament held between two optically trapped beads. With compliant traps, thermal motions of the filament allow the fixed myosin-S1 to interact with at least two zones, observed as a bi-modal distribution of filament displacements due to myosin binding, whose spacing is near the 36-nm helix repeat distance. The number of binding events and the "apparent working stroke" (mean displacement with myosin bound), vary periodically as the filament is moved past the fixed myosin by displacing the traps; observed periods are close to 36 nm and the apparent stroke varies from 0-10 nm. We also observe a strong modulation at the 5.5-nm actin monomer repeat confirming that myosin interacts with a single strand and that the actin is not free to rotate. Each interaction can be assigned to an actin monomer and each active zone on the helix is made up of three actin monomers.