Structural changes in the actin filament associated with tension sensing

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Molecular and biophysical mechanisms, including how mechanical forces are sensed, are totally unknown. ADF/cofilin proteins are the most preferable candidate for the stress fiber disassembly induced by tension decline in the fiber. Here, we propose a hypothesis that tension in an actin filament prevents the filament from being severed by cofilin. To test this, we prepared single actin filaments tensed with optical tweezers. When the fiber was tensed, it was severed after the application of cofilin with a significantly larger delay in comparison with the control filaments. Direct measurement of the torsional fluctuation of a single actin filament was made under different stresses, which indicated that the standard deviation of the fluctuation was reduced by an applied force of ca. 5 pN. These results demonstrate that tension in the actin filament reduces the binding of cofilin, resulting in a decrease in the effective severing activity of cofilin.