Mechanism of actin filament branch formation by Arp2/3 complex revealed by a high-resolution cryo-EM structure of the branch junction

We reconstructed the structure of actin filament branch junctions formed by fission yeast Arp2/3 complex at 3.5 angstrom resolution from images collected by electron cryo-microscopy. During specimen preparation, all of the actin subunits and Arp3 hydrolyzed their bound adenosine triphosphate (ATP) and dissociated the gamma-phosphate, but Arp2 retained the.-phosphate. Binding tightly to the side of the mother filament and nucleating the daughter filament growing as a branch requires Arp2/3 complex to undergo a dramatic conformational change where two blocks of structure rotate relative to each other about 25 degrees to align Arp2 and Arp3 as the first two subunits in the branch. During branch formation, Arp2/3 complex acquires more than 8,000 angstrom(2) of new buried surface, accounting for the stability of the branch. Inactive Arp2/3 complex binds only transiently to the side of an actin filament, because its conformation allows only a subset of the interactions found in the branch junction.