What is the characteristic angle between the old and new filaments formed by the Arp2/3 complex?
Arp2/3 nucleation geometry defines dendritic array organization. Single particle cryo EM and platinum replica electron tomography of lamellipodia show mother filament and daughter filament meet at Y junction with outer angle about 70 degrees, interior acute angle 110 degrees due to filament twist. Daughter pointed end capped by Arp2/3 complex remaining at junction while barbed end grows outward facing membrane. Angle determined by binding mode: ARPC2 ARPC4 subunits clamp side of mother, Arp2 Arp3 arranged in short pitch to emulate filament barbed end, orienting daughter at defined angle constrained by helical rise per subunit 2.7 nanometer. Seventy degree optimizes mechanical force transmission, allowing many barbed ends to push membrane synchronously while mesh remains crosslinked preventing buckling. Branches at 30 45 90 degrees seen with bundling proteins fascin actinin not Arp2/3. Cortactin stabilizes 70 degree junction, coronin promotes debranching. This characteristic angle widely used as signature of Arp2/3 mediated network in cells and in reconstitution assays.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Arp2/3 Branch Angle 70 Degrees.