Cofilin concentration controls cofilactin gel stress response*

F-actin is a scaffold protein that provides structural support for the cell and mediates its mechanical behaviors. Actin binding proteins (ABP), such as a-actinin and fascin, link F-actin into a polymer network gel. The increase in crosslinking leads to a general increase in rigidity. As a traditional-actin severing protein, cofilin has been shown to oligomerize at low pH and induce F-actin crosslinking through the formation of di-sulfide bonds. Given its known role in softening F-actin in bending, we sought to explore how F-actin crosslinking and F-actin softening contribute to the network-scale mechanical properties of cofilactin. We therefore perform rheology in the linear and nonlinear regimes of F-actin networks crosslinked by cofilin and compare them to well-known F-actin crosslinkers.  Unlike a-actinin, which is known to strain-stiffen, we find that cofilin crosslinked networks strain-stiffen at low concentration while strain-soften at saturating concentration, suggesting a novel role for cofilin in mediating the mechanics of the F-actin cytoskeleton.