PI(3,5)P(2) controls endosomal branched actin dynamics by regulating cortactin–actin interactions

Branched actin critically contributes to membrane trafficking by regulating membrane curvature, dynamics, fission, and transport. However, how actin dynamics are controlled at membranes is poorly understood. Here, we identify the branched actin regulator cortactin as a direct binding partner of phosphatidylinositol 3,5-bisphosphate (PI(3,5)P(2)) and demonstrate that their interaction promotes turnover of late endosomal actin. In vitro biochemical studies indicated that cortactin binds PI(3,5)P(2) via its actin filament-binding region. Furthermore, PI(3,5)P(2) competed with actin filaments for binding to cortactin, thereby antagonizing cortactin activity. These findings suggest that PI(3,5)P(2) formation on endosomes may remove cortactin from endosome-associated branched actin. Indeed, inhibition of PI(3,5)P(2) production led to cortactin accumulation and actin stabilization on Rab7(+) endosomes. Conversely, inhibition of Arp2/3 complex activity greatly reduced cortactin localization to late endosomes. Knockdown of cortactin reversed PI(3,5)P(2)-inhibitor–induced actin accumulation and stabilization on endosomes. These data suggest a model in which PI(3,5)P(2) binding removes cortactin from late endosomal branched actin networks and thereby promotes net actin turnover.