The Arp1/11 minifilament of dynactin primes the endosomal Arp2/3 complex.

Dendritic actin networks develop from a first actin filament through branching by the Arp2/3 complex. At the surface of endosomes, the WASH complex activates the Arp2/3 complex and interacts with the capping protein for unclear reasons. Here, we show that the WASH complex interacts with dynactin and uncaps it through its FAM21 subunit. In vitro, the uncapped Arp1/11 minifilament elongates an actin filament, which then primes the WASH-induced Arp2/3 branching reaction. In dynactin-depleted cells or in cells where the WASH complex is reconstituted with a FAM21 mutant that cannot uncap dynactin, formation of branched actin at the endosomal surface is impaired. Our results reveal the importance of the WASH complex in coordinating two complexes containing actin-related proteins.