Rescue of stalled clathrin-mediated endocytosis by asymmetric Arp2/3-mediated actin assembly

Actin assembly facilitates vesicle formation in several trafficking pathways. Clathrin-mediated endocytosis (CME) shows elevated actin assembly dependence under high membrane tension. Why actin assembly at CME sites occurs heterogeneously even within the same cell, and how assembly forces are harnessed, are not fully understood. Here, endocytic dynamics, actin presence, and geometry of CME proteins from three different functional modules, were analyzed using three-dimensional (3D) super-resolution microscopy, live-cell imaging, and machine-learning-based computation. When hundreds of CME events were compared, sites with actin assembly showed a distinct signature, a delay between completion of coat expansion and vesicle scission, indicating that actin assembly occurs preferentially at stalled CME sites. N-WASP is recruited to one side of CME sites where it is positioned to stimulate asymmetric actin assembly. We propose that asymmetric actin assembly rescues stalled CME sites by pulling vesicles into the cell much like a bottle opener pulls off a bottle cap.