Fig. 4.
Rapid cofilin-mediated turnover is a single-timescale mode of stress relaxation and dominates reptation. All microrheology measurements are of steady-state entangled actin solutions polymerized from 11.9 µM Mg–ATP–actin (0% or 5% Oregon-Green labeled) with Rp = 3 (35.7 µM) and Rf and Rc as indicated, except for the sample denoted by triangles in A and C, where Rp = 0. (A) Ensemble-averaged MSD for samples with similar values of the phase angle (Φ) evaluated at 0.1 Hz. (B) Collapse of MSD curves at long times to a reference sample of short stable filaments (A and B, black; C, circle) after rescaling lag time and MSD by shift factors a and b, respectively. (C) Dependence of relaxation time, estimated from the shift factor a, on filament length with (orange) and without (blue) cofilin. Data points represent individual samples. Error bars represent estimated uncertainty in relaxation time determination. The blue circle denotes the reference sample. The solid curve is the relaxation time predicted for reptation alone. The dashed line is a fit of the length-independent severing-based model to the cofilin-containing data. (D) Dependence of relaxation time on Rc for samples with Rp = 3 and Rf = 0.0092. Data points represent individual samples. Error bars represent estimated uncertainty in relaxation time determination. The dashed line is the relaxation time dependence on cofilin concentration predicted by the severing-based model using the severing rate inferred from the fit in C, an association constant for cofilin:F-actin binding of 1/Ka = 10 µM and a cofilin-binding cooperativity parameter of ω = 7.5 (17).