Cortactin stabilizes actin branches by bridging activated Arp2/3 to its nucleated actin filament

Abstract

Regulation of the assembly and turnover of branched actin filament networks nucleated by the Arp2/3 complex is essential during many cellular processes including cell migration and membrane trafficking. Cortactin plays a key role in stabilizing actin filament branches by interacting with the Arp2/3 complex and actin filaments via its N-terminal Acidic domain (NtA) and 6.5 central unstructured 37 amino acid repeats, respectively 1, but the mechanism of this is unclear. We determined the structure of cortactin-stabilized Arp2/3 actin branches using cryo-electron microscopy. We find that cortactin interacts with the new daughter filament nucleated by the Arp2/3 complex at the branch site rather than the initial mother actin filament. Cortactin preferentially binds activated Arp3 in contrast to other nucleation promoting factors (NPFs) 2,3. Cortactin also stabilizes the F-actin-like interface of activated Arp3 with the first actin subunit of the new filament, and its central repeats extend along successive daughter filament subunits. Cortactin binding to Arp3 is incompatible with NPF interaction and its preference for activated Arp3 explains why it is retained at the actin branch. Our data have uncovered why cortactin displaces NPFs, while at the same time promoting synergy to regulate branched actin network dynamics.