Fits of unmodified and modified crystal structures in the reconstruction of branch junctions. (A) Best fit of the Arp2/3 complex crystal structure. Arrows indicate mismatches with the reconstruction. The magenta and red arrows indicate the movement of ARPC3 and Arp2 upon remodeling. (B) Best fit of the remodeled Arp2/3 complex. (C and D) Comparisons between mother filament density in the branch reconstruction and a filament model viewed from the side opposite to the origin of the daughter filament. (C) Superimposition of the models fit to optimize the top portion of the reconstruction (orange) and the bottom portion of the reconstruction (dark pink). The gray arrow points to the area where a break in symmetry occurs at subunit M4. The pink and orange arrows indicate the movement necessary to fit the density. (D) Fit after replacing M4 with a monomer conformation. (E and F) Surface contours of the reconstruction at ∼70% of the mass. (E) Fit of the subdomains 2 of M2 (light pink) and M4 (dark gray) in the original filament conformation. The arrow points at subdomain 2 of actin subunit M4. (F) Improved fit with modified subdomains 2. (G) Close-up stereo view of the branch junction model inside the reconstruction displayed at the 100% mass contour level. The tethered helix of APC1 (green helix in the foreground) is near the groove between subdomains 1 and 3 of mother filament subunit M4.