We have investigated the microsecond rotational dynamics of F-actin with transient phosphorescence anisotropy (TPA) spectroscopy, and analyzed the data to determine the relative contributions from rigid-body rotations and from intrafilament bending and twisting, using a theoretical model developed for DNA dynamics by Schurr and co-workers. The fits of the data to the model were constrained by independently determining the orientation of the dye's absorption dipole (by transient absorption anisotropy, TAA) and the actin filament length distribution (by electron microscopy). We conclude that (1) the Schurr theory enables calculation of the torsional flexibility of actin independent of any contribution from rigid body rotations of the whole filament, (2) the TPA decays cannot be explained by rigid-body or bending rotations, but reflect primarily twisting motions within actin filaments, and (3) the dynamic properties of actin filaments are best ascribed to a continuous elasticity. This analysis establishes a firm methodological foundation for future studies of the effects of perturbations of the dynamics of actin on its functional properties.
Bibliographical noteFunding Information:
We thank Drs J. M. Schurr, M. Tirion and R. Ludescher for helpful discussions, Drs E. Egelman and A. Orlova for technical advice and the use of a digitizing tablet, M. Witzman for writing the Cray program implementing the full expression of the twist correlation function, and R. Bennett and N. Cornea for excellent technical assistance. This work was supported by NIH grant AR32961 (to D.D.T.), the American Heart Association, Minn. Affiliate (E.P.), and the Minnesota Supercomputer Institute. E.H. was supported by predoctoral training grants from NSF and NIH.