Figure 1.

Cooperative action of polymerizing actin filaments. Calculations of the force exerted by actin filaments on a nucleating bead show its dependence on the dissociation rate of actin monomers. At low dissociation rates, the bead experiences a very small force and remains trapped in its cloud of cross-linked actin. At an optimal value of dissociation rate, the forces are sufficient to drive the bead through its actin cloud and to rocket through the cytoplasm (data from [3]).