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. 2007 Dec 7;94(6):2082–2094. doi: 10.1529/biophysj.107.121988

FIGURE 2.

FIGURE 2

Steady-state dynamics of actin filaments. (A) Filament length time course for different actin monomer concentrations. We assumed that depolymerization at the pointed end is multiplied by a factor of up to 25 and that ADF/cofilin has no severing activity (14). The actin monomer concentrations used in the simulations are (from bottom to top) 0.63 (○), 0.71 (□), 0.81 (•), 0.92 (▪), and 1.05 (Inline graphic) μM. (B) ADF/cofilin-mediated large amplitude fluctuations. Increasing concentrations of ADF/cofilin reduce the average filament length and the amplitude of the fluctuations. We used a single actin monomer concentration of 1 μM; no qualitative changes have been observed for different actin monomer concentrations. Black curves 0.1 μM ADF/cofilin, dark gray 1 μM ADF/cofilin, and light gray 10 μM ADF/cofilin. (C) Increasing severing activity decreases both the average and the variance of the distribution of filament length for different values of ADF/cofilin (light gray 10 μM; gray 1 μM, and black 0.1 μM) and [Actin] = 1 μM. The model prediction from the balance equation (Eq. A5, solid curve) matches the empirical distribution based on the molecule-based model (points). (Inset) Model prediction (solid curve) and empirical (points) distributions for the filament length with different concentrations of actin monomers (light gray curve 0.5 μM, gray curve 1 μM, and black curve 2 μM). The ADF/cofilin is fixed at 1 μM. (D) ADF/cofilin controls the average filament length (solid curve) and the average fragment size (dashed curve); [Actin] = 1 μM. Note that for ADF/cofilin above 0.2 μM, the fragment size is <0.5 μM, the resolution limit in light microscopy, as indicated by the shaded area. (Inset) The average filament length increases with the concentration of actin monomers. ADF/cofilin is held constant at 1 μM. Parameters for A are vB = 11.6[Actin] s−1, vP = 1.3[Actin]-6.75 s−1, r1 = 0.3 s−1, r2 = 0.0019s−1, r3 = 0 s−1, r4 = 0 s−1, and r5 = 0 s−1. Parameters for B–D are vB = 11.6[Actin] s−1, vP = 1.3[Actin]-0.27 s−1, r1 = 0.3 s−1, r2 = 0.035 s−1, r3 = (0.0085[ADF/cofilin]-0.005) s−1, r4 = (0.075[ADF/cofilin]-0.005) s−1, and r5 = 0.012 s−1.