Figure 1.

Deformation of an actin bundle at a velocity of 1 μm/s. (a) The bundle, attached to two beads trapped in optical tweezers, is held at zero force. (b) One trap is then moved closer to the second one (yellow area), consecutively compressing (steep slope after t = 10 s), buckling (shoulder at t = 12 s), and finally bending the actin bundle (slight increase of force). (c) The traps are moved away from each other, reversing the deformation (blue area, negative slope of force) until they reach their initial positions. The sign of the force, positive or negative, indicates directionality, pointing outward or inward, respectively. (a–d) The red and black data curves are the force parallel and perpendicular to the motion, respectively. The nonzero perpendicular force originates from the off-center fixation of the bundle on the bead. The dashed line indicates the distance between the traps, whereas the actual distance between the beads is shown as the blue line. The phase contrast images (e–i) illustrate the aspect of the bundle at different positions in the cycle, indicated by the time in the lower-right corner. The scale bar represents 15 μm.