Figure 2.

Measurement of the rotational Brownian motion of the bead and its actin tail. (a) Schematic representation of the measurement (not drawn to scale, see text for details). Optical tweezers act as a frictionless bearing for a trapped bead, so that free rotation of the actin filament can be observed while the filament is maintained under moderate tension (see Fig. 1). (b) Fluorescent images of the bead, tagged with fluorescent microbeads and bearing an actin filament tail, undergoing motion. The plane of projection is parallel to that of the coverslip, i.e., perpendicular to the actin filament axis (see Materials and Methods). Images taken every 2 s are shown.