Structural basis for regulation of Arp2/3 complex by GMF

Arp2/3 complex mediates formation of complex cellular structures such as lamellapodia by nucleating branched actin filaments. Arp2/3 complex activity is precisely controlled by more than a dozen regulators, yet the structural mechanism by which regulators interact with the complex is unknown. GMF is a recently discovered regulator of Arp2/3 complex that can inhibit nucleation and dissemble branches. We solved the structure of the 240 kDa complex of Mus musculus GMF and Bos taurus Arp2/3 and found GMF binds to the barbed end of Arp2, overlapping with the proposed binding site of WASP family proteins. The structure suggests GMF can bind branch junctions like cofilin binds filament sides, consistent with a modified cofilin-like mechanism for debranching by GMF. The GMF-Arp2 interface reveals how the ADF-H actin-binding domain in GMF is exploited to specifically recognize Arp2/3 complex and not actin.