Abstract
BACKGROUND: Depolymerization of actin filaments is vital for the morphogenesis of dynamic cytoskeletal arrays and actin-dependent cell motility. Cofilin is necessary for actin disassembly in cells, and it severs filaments most efficiently at low cofilin to actin ratios, whereas higher concentrations of cofilin suppress severing. However, the cofilin concentration in thymocytes is too high to allow the severing of single-actin filaments. RESULTS: We observed that filaments sever efficiently in thymus cytosol. We identified Aip1 as a critical factor responsible for the severing and destabilization of actin filaments even in the presence of high amounts of cofilin. By fluorescence resonance energy transfer (FRET)-based spectroscopy and single-filament imaging of actin, we show that, besides driving the rapid severing of cofilin-actin filaments, Aip1 also augments the monomer dissociation rate at both the barbed and pointed ends of actin. Our results also demonstrate that Aip1 does not cap the barbed ends of actin filaments, as was previously thought. CONCLUSIONS: Our results indicate that Aip1 is a cofilin-dependent actin depolymerization factor and not a barbed-end-capping factor as was previously thought. Aip1 inverts the rules of cofilin-mediated actin disassembly such that increasing ratios of cofilin to actin now result in filament destabilization through faster severing and accelerated monomer loss from barbed and pointed ends. Aip1 therefore offers a potential control point for disassembly mechanisms in cells to switch from a regime of cofilin-saturation and stabilization to one that favors fast disassembly and destabilization.