Before being coated with nitrocellulose, the glass coverslips were sprayed with 4.5 μm polystyrene microspheres (171355; Polybead; Polysciences) to create pedestals (Roman et al., 2014 (link)). Trapping was performed with 3-μm amino polystyrene microspheres (171455; Polybead; Polysciences) coated with gelsolin according to Suzuki et al. (1996) (link), except that the time of suspension/gentle mixing of the microspheres in glutaraldehyde was increased to 15 h and the amount of protein dissolved in 0.5 ml of solution B was 27 μg gelsolin, 424 μg BSA, and 27 μg G-actin. Proteins and solutions were prepared and perfused in the assay chamber in the same manner as for the in vitro motility assay described above, with the exception that (gelsolin-coated) microsphere-tailed actin filaments were added to the laser trap assay buffer. The microsphere-tailed actin filaments were prepared according to Suzuki et al. (1996) (link) with the following modifications: 5 μM TRITC-labeled actin was used, and the suspension was diluted 1:10 with laser trap assay buffer to be perfused in the assay chamber.