Exo70 stimulates the Arp2/3 complex for lamellipodia formation and directional cell migration.

Directional cell migration requires the coordination of actin assembly and membrane remodeling. The exocyst is an octameric protein complex essential for exocytosis and plasma membrane remodeling. A component of the exocyst, Exo70, directly interacts with the Arp2/3 complex, a core nucleating factor for the generation of branched actin networks for cell morphogenesis and migration. Using in vitro actin polymerization assay and time-lapse total internal reflection fluorescence microscopy, we found that Exo70 functions as a kinetic activator of the Arp2/3 complex that promotes actin filament nucleation and branching. We further found that the effect of Exo70 on actin is mediated by promoting the interaction of the Arp2/3 complex with WAVE2, a member of the N-WASP/WAVE family of nucleation promoting factors. At the cellular level, the stimulatory effect of Exo70 on the Arp2/3 complex is required for lamellipodia formation and maintaining directional persistence of cell migration. Our findings provide a novel mechanism for regulating actin polymerization and branching for effective membrane protrusion during cell morphogenesis and migration.