Direct observation of cortactin protecting Arp2/3-actin filament branch junctions from GMF-mediated destabilization

Two ligands of Arp2/3 complex, yeast coronin and GMF, interact and synergize in pruning branched actin networks

The rapid turnover of branched actin networks underlies key in vivo processes such as lamellipodial extension, endocytosis, phagocytosis, and intracellular transport. However, our understanding of the mechanisms used to dissociate, or "prune," branched filaments has remained limited. Glia maturation factor (GMF) is a cofilin family protein that binds to the Arp2/3 complex and catalyzes branch dissociation. Here, we show that another ligand of Arp2/3 complex, Saccharomyces cerevisiae coronin (Crn1), enhances Gmf1-mediated debranching by 8- to 10-fold, and that these effects depend on Arp2/3-binding "C" and "A" motifs in Crn1. Further, we show that Crn1 directly binds with high affinity (KD = 1.4 nM) to S. cerevisiae GMF (Gmf1), and together they form a stable ternary Crn1-Gmf1-Arp2/3 complex in solution. Using single-molecule analysis, we show that Gmf1 binds transiently and multiple times to F-actin branch junctions prior to debranching. These and other results suggest a mechanism of mutual recruitment, in which Crn1 increases the on-rate of Gmf1 for branch junctions and Gmf1 blocks Crn1 binding to actin filament sides, increasing its availability to bind branch junctions. Taken together, these observations reveal an unanticipated mechanism in which two distinct ligands of the Arp2/3 complex bind to each other and synergize to prune actin branches.

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

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 is important for actin branch stabilization, but the mechanism by which this occurs is unclear. Given this, we determined the structure of vertebrate cortactin-stabilized Arp2/3 actin branches using cryogenic 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. It 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. The preference of cortactin for activated Arp3 explains its retention at the actin branch and accounts for its synergy with other nucleation-promoting factors in regulating branched actin network dynamics.

The stabilization of Arp2/3 complex generated actin filaments

The Arp2/3 complex, which generates both branched but also linear actin filaments via activation of SPIN90, is evolutionarily conserved in eukaryotes. Several factors regulate the stability of filaments generated by the Arp2/3 complex to maintain the dynamics and architecture of actin networks. In this review, we summarise recent studies on the molecular mechanisms governing the tuning of Arp2/3 complex nucleated actin filaments, which includes investigations using microfluidics and single-molecule imaging to reveal the mechanosensitivity, dissociation and regeneration of actin branches. We also discuss the high-resolution cryo-EM structure of cortactin bound to actin branches, as well as the differences and similarities between the stability of Arp2/3 complex nucleated branches and linear filaments. These new studies provide a clearer picture of the stabilisation of Arp2/3 nucleated filaments at the molecular level. We also identified gaps in our understanding of how different factors collectively contribute to the stabilisation of Arp2/3 complex-generated actin networks.