We have measured the effects of cofilin on the conformation and dynamics of actin filaments labeled at Cys374 with erythrosin-iodoacetemide (ErIA), using time-resolved phosphorescence anisotropy (TPA). Cofilin quenches the phosphorescence intensity of actin-bound ErIA, indicating that binding changes the local environment of the probe. The cofilin concentration-dependence of the phosphorescence intensity is sigmoidal, consistent with cooperative actin filament binding. Model-independent analysis of the anisotropies indicates that cofilin increases the rates of the microsecond rotational motions of actin. In contrast to the reduction in phosphorescence intensity, the changes in the rates of rotational motions display non-nearest-neighbor cooperative interactions and saturate at substoichiometric cofilin binding densities. Detailed analysis of the TPA decays indicates that cofilin decreases the torsional rigidity (C) of actin, increasing the thermally driven root-mean-square torsional angle between adjacent filament subunits from ∼4° (C=2.30×10-27 Nm2 radian-1) to ∼17° (C=0.13×10 -27 Nm2 radian-1) at 25°C. We favor a mechanism in which cofilin binding shifts the equilibrium between thermal ErIA-actin filament conformers, and facilitates two distinct structural changes in actin. One is local in nature, which affects the structure of actin's C terminus and is likely to mediate nearest-neighbor cooperative binding and filament severing. The second is a change in the internal dynamics of actin, which displays non-nearest-neighbor cooperativity and increases the torsional flexibility of filaments. The long-range effects of cofilin on the torsional dynamics of actin may accelerate Pi release from filaments and modulate interactions with other regulatory actin filament binding proteins.
Bibliographical noteFunding Information:
We thank Dr Wenxiang Cao for engaging discussions, Adrian O. Olivares and James P. Robblee for assistance with some of the cofilin preparations used in this study, and Brian Tucker for assistance with the kinetics of cofilin binding to pyrene actin filaments. This work was supported by a Hellman Family Fellowship (to E.M.D.L.C.), grants from the American Heart Association (0235203N to E.M.D.L.C.), the National Science Foundation (MCB-0216834 to E.M.D.L.C.), and the NIH (AR32961 to D.D.T.).
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- Torsional rigidity