Fig. 4.

Force generation correlates with presence of actin at the tip of the filopodium. (A) Schematics of the attachment of the filopodium to the trapped bead. A small patch of membrane adheres to the bead substrate, thus allowing actin to bind to a fixed membrane at the tip. Binding of actin to the side walls of the tubular and fluid membrane results in a frictional dissipative force during rotation and directed flow of actin. (B) Images of the F-actin (magenta, labeled with Lifeact-GFP) at the tip of the filopodium overlaid with the reflection signal from the trapped bead (white). Correlation of successive detachment and reattachment of the actin to the bead with the forces exerted reveal a load-and-fail behavior of the actin filaments at the tip. Yellow arrows show the direction of movement of the optically trapped bead. See also Movie S4. (C) Quantification of the results shown in B. The blue curve shows the intensity of actin at the tip of the filopodium (see Movie S5 and SI Appendix, Fig. S11, for details). The green curve shows the change in the pulling force on the bead in the optical trap. The red and blue asterisks mark two time points; red, just before detachment of actin from the tip; blue, just after detachment of actin from the tip. The corresponding images (Inset) reveal that bending of the actin (shown by the solid line, which is the tracked skeleton of the actin shaft) is present both before and after release of tension.