An imposed load on an actin network leads to exuberant growth when the pressure is released, say Sapun Parekh, Ovijit Chaudhuri, Julie Theriot, and Daniel Fletcher (University of California, Berkeley, CA). Thus, history matters when it comes to actin growth dynamics.
Parekh et al. slowly increased the load pushing against a polymerizing actin network and measured network growth velocity along the way. Velocity remained constant for a while and then slowed as the load approached a stall-inducing maximum.
The authors then returned the system to a lighter load and found that the network grew much faster—even faster than it did previously at this lighter load. A sudden burst of polymerization like that might help a migrating cell push quickly through a weak spot in the surrounding tissue.
To explain how multiple velocities can exist for a given force, Fletcher suggests, “maybe the network is adapting to different loads.” Force-mediated activation of Arp2/3, for instance, would increase branching. “With higher load,” he explains, “you'd get a net addition in the number of filaments that are pushing the load. When the load is reduced, you now have many more filaments to push the smaller load.” Still to be explained, however, is why the higher growth rate under the lightened load was long lasting, i.e., why Arp2/3 activity did not lessen and thereby reduce the number of filaments over time.