ER contact sites define the position of endosome bud fission during actin-dependent cargo sorting. Disrupting endosomal actin structures prevents retrograde cargo movement; however, how actin affects ER contact site formation and endosome fission is not known. Here we show that in contrast with the WASH complex, actin, its nucleator ARP2/3, and COR1C form a contained structure at the bud neck that defines the site of bud fission. We found that actin confinement is facilitated by type I coronins. Depletion of type I coronins allows actin to extend along the length of the bud in an ARP2/3-dependent manner. We demonstrate that extension of branched actin prevents ER recruitment and stalls buds before fission. Finally, our structure-function studies show that the COR1C's coiled-coil domain is sufficient to restore actin confinement, ER recruitment, and endosome fission. Together, our data reveal how the dynamics of endosomal actin and activity of actin regulators organize ER-associated bud fission.
© 2022 Striepen and Voeltz.