Extended Data Fig. 7: Flattening of Arps and SPIN90’s interaction with Arp3.

a, Overlay (overlayed on SD3 and SD4) of twisted Arp3 (white colored, from inactive Arp2/3 complex (PDB: 4JD2)) on flattened Arp3 (orange, active Arp2/3 complex) in the left side of the image, and similarly for Arp2 (twisted in white, and flattened in red) is shown on the right side of the image. The dihedrals between the subdomains are shown as green bonds connecting the centre of masses of each subdomain (shown as green spheres). The direction of flattening during activation is shown by the curved black arrows for both the Arps. b, SPIN90’s insertion loop does not make any contact with Arp3 in the inactive complex, compared to the insertion of this loop into Arp3’s pointed end cleft in the active complex. The dashed bold yellow lines indicate the separation distances between the Cα backbone atoms for W⁴⁰¹ of SPIN90 and R¹⁹⁷ of Arp3 respectively c, Reconstructed density between W⁴⁰¹ of SPIN90 and R¹⁹⁸ of Arp3 emphasizing the cation-pie interaction. d, The SPIN90 insertion loop penetrates more deeply into the Arp3 pointed end cleft than the loop from the filamentous actin subunit in the branch junction (PDB: 7TPT). The dashed box highlighting the distance between Cα backbone atom of D¹⁷² of Arp3 to N³⁹⁷ of SPIN90 in the SPIN90-Arp2/3 complex activated structure (left side), and N³⁶⁰ of actin subunit from the mother filament in the branched actin junction structure (PDB: 7TPT) (right side).