Twinfilin is a non-processive depolymerase which synergizes with formin to dramatically accelerate actin filament uncapping by 300-fold.

Cellular actin networks assemble by actin filament elongation at barbed ends and are thought to disassemble primarily by depolymerization at filament pointed ends. Contrary to this conventional understanding of actin dynamics, twinfilin was recently shown to promote barbed-end depolymerization. Twinfilin has additionally been suggested to sequester monomers and cap as well as uncap filament barbed ends. As a result, the exact mechanisms by which twinfilin affects barbed-end dynamics remain controversial. Using multicolor single-molecule microscopy, we show that both mouse and yeast twinfilin are non-processive depolymerases that interact only transiently with barbed ends (~0.2-0.5 s). Each twinfilin binding event, on average, results in the removal of one or two actin subunits. At CP-capped barbed ends, twinfilin synergizes with formin to accelerate uncapping by up to ~320-fold. We find that uncapping by twinfilin, alone and together with formin, depends on the nucleotide state of the filament, with the two proteins causing a much more modest enhancement of uncapping of newly assembled filaments. Our study thus establishes twinfilin as a multifunctional barbed-end binding protein capable of non-processively depolymerizing, transiently capping, and synergizing with formin to rapidly uncap actin filament barbed ends.