Regeneration of actin filament branches from the same Arp2/3 complex

Branched actin filaments are found in many key cellular structures. Branches are nucleated by the Arp2/3 complex activated by nucleation-promoting factor (NPF) proteins and bound to the side of pre-existing ‘mother’ filaments. Over time, branches dissociate from their mother filament, leading to network reorganization and turnover, but this mechanism is less understood. Here, using microfluidics and purified proteins, we examined the dissociation of individual branches under controlled biochemical and mechanical conditions. We observe that Arp2/3 remains bound to the mother filament after most debranching events, even when accelerated by force. Unexpectedly, this mother-remaining Arp2/3 readily nucleates a new actin filament branch, without being activated anew by an NPF: it simply needs to exchange its nucleotide and bind an actin monomer. The protein GMF, which accelerates debranching, prevents branch re-nucleation. Our results suggest that actin filament re-nucleation can provide a self-repair mechanism, helping branched networks to sustain mechanical stress in cells over extended periods of time.