The systems biochemistry of actin network assembly

Eukaryotic cells determine their shape and internal organization through an actin cytoskeleton. Actin is the most abundant protein in the cytoplasm in most eukaryotic cells. My lab combines in vitro reconstitution with structural and cell biology uncover the mechanisms that control actin dynamics under cell-like conditions. We have recently reconstituted actin assembly from physiological actin concentrations for the first time and identified profilin as a major regulator, which i) inhibits spontaneous nucleation by tight monomer binding and ii) limits elongation by transiently capping growing filaments. We find that actin polymerases can modulate the rate of profilin release from the filament end. By obtaining the first high resolution structures of filament ends, we identify the molecular mechanism that allows these formin polymerases to remain bound to the growing filament end and accelerate profilin release in this process. These results reveal how profilin and formin establish a biochemical module that can locally control actin assembly, while globally maintaining a metastable pool of actin.