Structural Determinants of Myosin I Mechanosensing: The N Terminal Region

essential regulatory protein cofilin severs actin filaments, which accelerates network remodeling by increasing the concentration of filament ends available for elongation and subunit exchange. Although cofilin effects on actin filament assembly dynamics have been extensively studied, the molecular mechanism of cofilin-induced filament severing is not understood. Here we demonstrate that actin filament severing by vertebrate cofilin is driven by the linked dissociation of a single cation per bound cofilin, which alters filament mechanical properties. Vertebrate cofilin only weakly severs Saccharomyces cerevisiae actin filaments lacking this ‘‘stiffness cation’’ unless a stiffness cation-binding site is engineered into the actin molecule. Vertebrate cofilin rescues the viability of a Saccharomyces cerevisiae cofilin deletion mutant only when the stiffness cation site is simultaneously introduced into actin, demonstrating that filament severing is the essential function of cofilin in cells. The work presented here reveals that site-specific interactions with cations serve a key regulatory function in actin filament fragmentation and dynamics.