The internal motion of F-actin in the time range from 10-6 to 10-3 second has been explored by measuring the transient absorption anisotropy of eosin-labeled F-actin using laser flash photolysis. The transient absorption anisotropy of eosin-F-actin at 20 °C has a component that decays in the submicrosecond time scale to an anisotropy of about 0.3. This anisotropy then decays with a relaxation time of about 450 μs to a residual anisotropy of about 0.1 after 2 ms. When the concentration of eosin-F-actin was varied in the range from 7 to 28 μm, the transient absorption anisotropy curves obtained were almost indistinguishable from each other. These results show that the anisotropy decay arises from internal motion of eosin-F-actin. Analysis of the transient absorption anisotropy curves indicates that the internal motion detected by the decay in anisotropy is primarily a twisting of actin protomers in the F-actin helix; bending of the actin filament makes a minor contribution only to the measured decay. The torsional rigidity calculated from the transient absorption anisotropy is 0.2 × 10-17 dyn cm2 at 20 °C, which is about an order of magnitude smaller than the flexural rigidity determined from previous studies. Thus, we conclude that F-actin is more flexible in twisting than in bending. The calculated root-mean-square fluctuation of the torsional angle between adjacent actin protomers in the actin helix is about 4 ° at 20 °C. We also found that the torsional rigidity is approximately constant in the temperature range from 5 to ~35 °C, and that the binding of phalloidin does not appreciably affect the torsional motion of F-actin.
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