Arp2/3-mediated bidirectional actin assembly by SPIN90 dimers in metazoans

Branched actin networks nucleated by the Arp2/3 complex play critical roles in various cellular processes, from cell migration to intracellular transport. However, when activated by WISH/DIP/SPIN90 family proteins, Arp2/3 nucleates linear actin filaments. Unexpectedly, we found that human SPIN90 is a dimer that can nucleate bidirectional actin filaments. To understand the basis for this, we determined a 3 [A] resolution structure of human SPIN90-Arp2/3 complex nucleating actin filaments. Our structure shows that SPIN90 dimerises via a 3-helix bundle and interacts with two Arp2/3 complexes. Each SPIN90 molecule binds both Arp2/3 complexes to promote their activation. Our analysis demonstrates that single filament nucleation by Arp2/3 is mechanistically more like branch formation than previously appreciated. The dimerisation domain in SPIN90 orthologues is conserved in metazoans, suggesting that this mode of bidirectional nucleation is a common strategy to generate anti-parallel actin filaments.