Figure 3.

Internal actin subunit configuration changes abruptly at the actin/cofilactin boundary. (Inset) Ribbon diagram of an actin monomer with beads corresponding to the center of mass of actin’s four primary subdomains (SD), as defined in Ref. (35). Cleft distance d and the flatness angle ϕ (an effective torsion angle with respect to the centers of the four subdomains) can characterize different actin configurations. (Main figure) Scatter plot of ϕ versus d for subunits in the slow-severing (top, red) and fast-severing (bottom, blue) filament models (final 50 ns of simulation). ϕ changes abruptly at the boundary (see also Fig. S3) between actin-like range (black dashed line = mean, one SD is shaded) and a cofilactin-like range (magenta). NB: The flattening angle ϕ is not to be confused with the third Euler angle, commonly denoted with the same Greek letter ‘φ’, used in cryo-EM image analysis to define the “twist” (axial orientation of a helical subunit with respect to the filament axis).