Figure 1.
Experimental scheme and representative velocity versus force measurements.
(A) Schematics of the experiment: actin polymerization is initiated at the bead surface by gelsolin, and pushes the beads apart. (B) Bright-field images of a colloid chain, aligned under a 5-mT magnetic field, at two different times. Actin filaments are not dense enough to be seen. Scale bar, 5 µm. (C) Evolution of the center-to-center distance d with time for different loading forces. The distance increases linearly with time, allowing a direct measurement of the beads relative velocity. (D) Velocity versus loading force profile. Error bars indicate estimated error (standard deviation) from the slope determination in (C). For the largest forces, the error bars are smaller than the symbols. Number of filaments per particle: NGS = 10,000.
Figure 2.
Growth of actin filaments is independent of applied force.
The bottom graph shows the evolution of the center-to-center distance d as a function of the time t for an experiment where the applied force is represented in top graph as a function of t. Circles are experimental data. In this experiment, the chain is formed at low force (f = 0.8 pN). At time t = 650 s the force is increased to a higher value (f = 39 pN), and at time t = 855 s the force is reduced to the first value. The line is the best linear fit for the points at low forces: vbead = 0.302±0.004 nm/s; the intercept is 1,132±3 nm, which is the beads' diameter. NGS = 10,000.
Figure 3.
Elastic response of filaments.
Instantaneous force-distance profile for different filament initial lengths: applied force f as a function of the distance X between the surfaces of adjacent beads. Experimental data correspond to a cycle of compression and decompression (NGS = 4,000; blue triangles, L0 = 400 nm and t0 = 1,020 s; green circles, L0 = 200 nm and t0 = 480 s). For clarity, the L0 = 400 nm data are shifted by 5 nm to the right). Solid and dash lines are predictions of our model with c = 0.2±0.1, with L = 200 nm and L = 400 nm, respectively.
Figure 4.
Orientational entropy of the filament at the bead surface.
(A) The number of active filaments is estimated from the area of the red surface and the measured filament density. (B) When the distance between black surfaces is large enough (X>L), the filament explores the whole half sphere shaded on the figure because of thermal fluctuations (left). When X<L, the accessible surface Ω decreases (right), leading to a repulsive force.
Figure 5.
Velocity versus force profiles for different filament densities.
Experimental data are discrete open symbols, while the curves are the predictions from our model (red circles, NGS = 10,000, 42 measurements; blue triangles, NGS = 4,000, 41 measurements).