Actin-related protein 2 and 3 (Arp2/3) complex forms a dendritic network of actin filaments during endocytosis and cellular locomotion by nucleating branches on the sides of preexisting actin filaments. Reconstructions of electron tomograms of branch junctions show how Arp2/3 complex anchors the branch, with Arp2 and Arp3 serving as the first two subunits of the branch. Our aim was to characterize the massive conformational change that moves Arp2 ∼30 Å from its position in crystal structures of inactive Arp2/3 complex to its position in branch junctions. Starting with the inactive crystal structure, we used atomistic-scale molecular dynamics simulations to drive Arp2 toward the position observed in branch junctions. When we applied forces to Arp2 while restraining Arp3, one block of structure (Arp2, subunit ARPC1, the globular domain of ARPC4 and ARPC5) rotated counterclockwise by 30° around a pivot point in an α-helix of ARPC4 (Glu⁸¹-Asn¹⁰⁰) to align Arp2 next to Arp3 in a second block of structure including ARPC3 and the globular domains of ARPC2. This active structure buried more surface area than the inactive conformation. The complex was stable in all simulations. In most simulations, collisions of subdomain 2 of Arp2 with Arp3 impeded the movement of Arp2.
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