Figure 1.

(a) An actin filament held taut by optical trapping of beads attached to both ends. (b) Schematic representation of the elastic constraints in the system, with Hookean traps and assumed Hookean-compliant connections between the optically trapped beads and the filament. The compliance of the filament itself (10) is about 1,000 times less than that of the bead-to-bead linkage we observe, indicating the presence in series of weaker elements, probably the bead–filament linkages. The motion of each bead depends on trap strength, viscous drag, and the motion of the other bead “felt” through a compliant attachment. (c) Diffusion correlation coefficients between diffusive motion of the two beads along the filament axis measured every 5 ms for a 1.7-s interval. Despite statistical fluctuation, the values remain centered around 0.4 to 0.5. (d) A surface-attached myosin binds to the filament, schematically illustrated in e. Because the actin filament center is now linked to the myosin-bound surface through a relatively stiff contact, motion at one filament end propagates only weakly to the other. Consistent with this prediction, we show in f observed transient drops in correlation between diffusion of the two beads. We interpret these drops to reflect myosin binding to the actin filament.