An actin nucleation complex catalyzes filament formation at sites of exocytosis

Due to the local enrichment of factors that influence its formation, dynamics, and organization, the actin cytoskeleton displays different shapes and functions within the same cell. In yeast cells post-Golgi vesicles ride on long actin cables to the bud tip. The scaffold proteins Boi1 and Boi2 participate in tethering and docking these vesicles to the plasma membrane. Here we show that Boi1/2 also recruit nucleation and elongation factors to form actin filaments at sites of exocytosis. Disrupting the physical connection between Boi1/2 and the nucleation factor Bud6 impairs filament formation in the bud, reduces the directed movement of the vesicles to the tip, and shortens their tethering time at the cortex. Artificially transplanting Boi1 from the bud tip to the peroxisomal membrane partially redirects the actin cytoskeleton and the vesicular flow towards the peroxisome, and creates an alternative, rudimentary vesicle-docking zone. We conclude that Boi1/2 is sufficient to induce the formation of a cortical actin structure that receives and aligns incoming vesicles before fusing with the membrane.