Mechanism of synergistic activation of Arp2/3 complex by cortactin and WASP-family proteins

Cortactin coactivates Arp2/3 complex synergistically with WASP-family nucleation-promoting factors (NPFs) and stabilizes branched networks by linking Arp2/3 complex to F-actin. It is poorly understood how cortactin performs these functions. We describe the 2.89 angstrom resolution cryo-EM structure of cortactin's N-terminal domain (Cort1-76) bound to Arp2/3 complex. Cortactin binds Arp2/3 complex through an inverted Acidic domain (D20-V29), which targets the same site on Arp3 as the Acidic domain of NPFs but with opposite polarity. Sequences N- and C-terminal to cortactin's Acidic domain do not increase its affinity for Arp2/3 complex but contribute toward coactivation with NPFs. Coactivation further increases with NPF dimerization and for longer cortactin constructs with stronger binding to F-actin. The results suggest that cortactin contributes to Arp2/3 complex coactivation with NPFs in two ways, by helping recruit the complex to F-actin and by stabilizing the short-pitch (active) conformation, which are both byproducts of cortactin's core function in branch stabilization. Arp2/3 complex is activated by nucleation promoting factors (NPFs) to form actin branches that are stabilized by cortactin. Here, the authors show that NPFs and cortactin activate Arp2/3 complex synergistically by helping recruit the complex to F-actin and by stabilizing its active conformation

Automated summary

Cortactin coactivates with NPFs to stabilize branched networks by linking Arp2/3 complex to F-actin. Cortactin's Acidic domain binds Arp2/3 complex through a reversal binding site and its N- and C-terminal sequences contribute to coactivation with NPFs.