Actin-depolymerizing factor (ADF)/cofilins contribute to cytoskeletal dynamics by promoting rapid actin filament disassembly. In the classical view, ADF/cofilin sever filaments, and capping proteins block filament barbed ends whereas pointed ends depolymerize, at a rate that is still debated. Here, by monitoring the activity of the three mammalian ADF/cofilin isoforms on individual skeletal muscle and cytoplasmic actin filaments, we directly quantify the reactions underpinning filament severing and depolymerization from both ends. We find that, in the absence of monomeric actin, soluble ADF/cofilin can associate with bare filament barbed ends to accelerate their depolymerization. Compared to bare filaments, ADF/cofilin-saturated filaments depolymerize faster from their pointed ends and slower from their barbed ends, resulting in similar depolymerization rates at both ends. This effect is isoform specific because depolymerization is faster for ADF-than for cofilin-saturated filaments. We also show that, unexpectedly, ADF/cofilin-saturated filaments qualitatively differ from bare filaments: their barbed ends are very difficult to cap or elongate, and consequently undergo depolymerization even in the presence of capping protein and actin monomers. Such depolymerizing ADF/cofilin-decorated barbed ends are produced during 17% of severing events. They are also the dominant fate of filament barbed ends in the presence of capping protein, because capping allows growing ADF/cofilin domains to reach the barbed ends, thereby promoting their uncapping and subsequent depolymerization. Our experiments thus reveal how ADF/cofilin, together with capping protein, control the dynamics of actin filament barbed and pointed ends. Strikingly, our results propose that significant barbed-end depolymerization may take place in cells.

The actin cytoskeleton powers membrane deformation during many cellular processes, such as migration, morphogenesis, and endocytosis. Membrane phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate [PI(4,5)P-2], regulate the activities of many actinbinding proteins (ABPs), including profilin, cofilin, Dia2, N-WASP, ezrin, and moesin, but the underlying molecular mechanisms have remained elusive. Moreover, because of a lack of available methodology, the dynamics of membrane interactions have not been experimentally determined for any ABP. Here, we applied a combination of biochemical assays, photobleaching/activation approaches, and atomistic molecular dynamics simulations to uncover the molecular principles by which ABPs interact with phosphoinositide-rich membranes. We show that, despite using different domains for lipid binding, these proteins associate with membranes through similar multivalent electrostatic interactions, without specific binding pockets or penetration into the lipid bilayer. Strikingly, our experiments reveal that these proteins display enormous differences in the dynamics of membrane interactions and in the ranges of phosphoinositide densities that they sense. Profilin and cofilin display transient, low-affinity interactions with phosphoinositide-rich membranes, whereas F-actin assembly factors Dia2 and N-WASP reside on phosphoinositide-richmembranes for longer periods to performtheir functions. Ezrin and moesin, which link the actin cytoskeleton to the plasma membrane, bindmembranes with very high affinity and slow dissociation dynamics. Unlike profilin, cofilin, Dia2, and N-WASP, they do not require high "stimulus-responsive" phosphoinositide density for membrane binding. Moreover, ezrin can limit the lateral diffusion of PI(4,5)P-2 along the lipid bilayer. Together, these findings demonstrate that membrane-interaction mechanisms of ABPs evolved to precisely fulfill their specific functions in cytoskeletal dynamics.

Eukaryotic cells have diverse protrusive and contractile actin filament structures, which compete with one another for a limited pool of actin monomers. Numerous actin-binding proteins regulate the dynamics of actin structures, including tropomodulins (Tmods), which cap the pointed end of actin filaments. In striated muscles, Tmods prevent actin filaments from overgrowing, whereas in non-muscle cells, their function has remained elusive. Here, we identify two Tmod isoforms, Tmod1 and Tmod3, as key components of contractile stress fibers in non-muscle cells. Individually, Tmodl and Tmod3 can compensate for one another, but their simultaneous depletion results in disassembly of actin-tropomyosin filaments, loss of force-generating stress fibers, and severe defects in cell morphology. Knockout-rescue experiments reveal that Tmod's interaction with tropomyosin is essential for its role in the stabilization of actin-tropo-myosin filaments in cells. Thus, in contrast to their role in muscle myofibrils, in non-muscle cells, Tmods bind actin-tropomyosin filaments to protect them from depolymerizing, not elongating. Furthermore, loss of Tmods shifts the balance from linear actin-tropomyosin filaments to Arp2/3 complex-nucleated branched networks, and this phenotype can be partially rescued by inhibiting the Arp2/3 complex. Collectively, the data reveal that Tmods are essential for the maintenance of contractile actomyosin bundles and that Tmod-dependent capping of actin-tropomyosin filaments is critical for the regulation of actin homeostasis in non-muscle cells.