The molecular mechanism of load adaptation by branched actin networks

Branched actin networks are self-assembling molecular motors that move biological membranes and drive many important cellular processes. Load forces slow the growth and increase the density of these networks, but the molecular mechanisms governing this force response are not well understood. Here we use single-molecule imaging and AFM cantilever deflection to measure how applied forces affect each step in branched actin network assembly. Unexpectedly, force slows the rate of filament nucleation by promoting the interaction of nucleation promoting factors with actin filament ends, limiting branch formation. This inhibition is countered by an even larger force-induced drop in the rate of filament capping, resulting in a shift in the balance between nucleation and capping that increases network density. Remarkably, the force dependence of capping is identical to that of filament elongation because they require the same size gap to appear between the filament and load for insertion. These results provide direct evidence that Brownian Ratchets generate force and govern the load adaptation of branched actin networks.