Figure 5.
Twisting of Arp2 closes the barbed end groove and weakens its interactions with the daughter filament.A, ribbon diagram showing the interaction of the D-loop of actin D2 with the barbed end groove of Arp2. Arp2 and actin D2 subdomains are labeled 1 to 4. PE: pointed end, BE: barbed end. Right panel shows closeup of the interaction with distances measured in B. Actin D2 from 0.67 μs (transparent light blue ribbon) in the branch junction simulation was placed by overlaying Arp2 from the 0.67 μs frame in the trajectory with Arp2 from 7TPT. BEG: Barbed-end groove. Start: position of actin D-loop at the beginning of the simulation. B, plot of twisting/flattening angle of Arp2 (φ) in the branch junction and the branch junction without daughter filament (no daughter) simulations. Arp2 twisting/flattening angles from inactive (4JD2) and active Arp2/3 complex structures (7TPT) are shown in dotted or dashed lines, as indicated. C, surface representation of branch junction model (7TPT) showing the interface between Arp2 and Arp3 and the pointed end of the nucleated daughter filament. D, plot of the W-loop opening (x1) and D-loop to Arp2 distance (x2) in the branch junction simulation. See panel A for definition of x1 and x2. Distances x1 and x2 in the branch junction structure are plotted for reference. E, plot of W-loop distance (x1) in Arp2 subunits for all unbiased simulations. Distance x1 in the branch junction (7TPT) and inactive Arp2/3 complex structure (4JD2) is plotted as dashed or dotted lines, as indicated. F, identical to E, except x1 for Arp3 from each simulation is plotted. Arp2/3 complex, actin-related protein 2/3 complex.