Fig. 2: Bending and twisting of mother filaments in branch junctions.
a Definition of the subunit coordinate frame and use of this frame to follow the path of a distorted filament14 (see Methods). b Ribbon diagram of the ADP branch junction viewed from the pointed end with a 10° offset along the axis of the mother filament (red) with Arp2/3 complex and daughter filament in gray. The green disk is the plane fit by least-squares superposition of the subunit origin coordinates oi along the bent mother filament through Arp2/3 complex, roughly parallel with the daughter filament axis (solid black line). Arp2/3 complex is on the convex side of the mother filament (see panels c–d). c–g Paths of subunits in bent actin filaments, viewed from the pointed end. Note that the vertical deflection of the paths (vertical scale bar in c) is highly exaggerated compared with the end-to-end distance (horizontal scale bar in c). Subunits are represented by coordinate frame vectors ni, depicted as red three-dimensional rods. Semitransparent green surfaces are bending planes through the path centers oi (fit by least squares). The staggered frame vectors in bent filaments (c–g) but not straight actin filaments, indicate shearing between neighboring subunits compared with the straight reference structure. c, d Mother filament subunit paths in our branch junctions with ADP BeFx- and ADP-Arp2/3 complex. e Path of mother filament subunits in branch junctions with bovine Arp2/3 complex (PDB 7TPT) is within 5° of those in c, d. f, g Paths of actin filament subunits in bent actin filaments with ADP-Pi (PDB 8d16) and ADP (PDB 8d15) subunits20. h–i In-plane bending angles between subunits n and n + 1 in actin filaments. h Mother filaments in branch junctions (c–e) have a one-degree bend between subunits M3 and M5. i Bent ADP-Pi- and ADP-actin filaments20 have approximately constant curvatures. j, k Excess end-to-end twist, or ‘net twist difference’18 between the first and nth subunits (inclusive) as a function of n. (See Supplemental Fig. 2 and supplemental notes) j ADP BeFx- and ADP-branch junctions. k Bent ADP-Pi- and ADP-actin filaments20. Source data for panels h–k are provided as a Source Data file.
