Fig. 1.

(A) Composite optical microscopy image of a typical experiment. Here, actin polymerizes from primer coated beads (black) against fluorescently labeled probe beads (red) under a magnetic force of 4.6 pN (2 μM actin and 2 μM fascin). Cyan arrows indicate the pairs considered for analysis. After 11 min of polymerization, the average surface-to-surface distance is 0.30 μm. (B and C) Differences in the organization of actin filaments caused by fascin. Fluorescent filaments growing on a nonfluorescent bead without magnetic field are observed by confocal microscopy. Images are taken in the equatorial plane of the bead after 3 h of polymerization of (B) 6 μM actin; (C) 6 μM actin and 2 μM fascin. (D and E) Schemes of the organization of the filaments (green) anchored to the surface of magnetic beads by flexible primers (pink) corresponding, respectively, to B and C.