Cellular actin networks grow by ATP-actin addition at filament barbed ends and have long been presumed to depolymerize at their pointed ends, primarily after filaments undergo “aging” (ATP hydrolysis and Pi release). The cytosol contains high levels of actin monomers, which favors assembly over disassembly, and barbed ends are enriched in ADP-Pi actin. For these reasons, the potential for a barbed end depolymerization mechanism in cells has received little attention. Here, using microfluidics-assisted TIRF microscopy, we show that mouse twinfilin, a member of the ADF-homology family, induces depolymerization of ADP-Pi barbed ends even under assembly-promoting conditions. Indeed, we observe in single reactions containing micromolar concentrations of actin monomers the simultaneous rapid elongation of formin-bound barbed ends and twinfilin-induced depolymerization of free barbed ends. The data show that twinfilin catalyzes dissociation of subunits from ADP-Pi barbed ends and thereby bypasses filament aging prerequisites to disassemble newly polymerized actin filaments.

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