Frequency Spectrum of F—Actin and F—Actin/Filamin Complexes as Studied by Photon Correlation Spectroscopy

Abstract

The frequency spectrum of entangled rigid rods with an exponential length distribution was calculated and compared to a spectrum obtained by an inverse Laplace transform of the first order autocorrelation function from dynamic light scattering data for F—actin, a polymerized muscle protein usually considered to be rather rigid. The experimental and model frequency distributions disagree substantially. In view of this discrepancy the effect of factors like flexibility, deviation from an exponential length distribution, interference with chemical reactions, and translational—rotational coupling as possible explanations were calculated. By comparison of the data to the model calculations or qualitative estimates all these could be excluded. Therefore we conclude that the frequency spectrum of F—actin is due to the formation of an infinite network structure. Further support to this interpretation is provided by the frequency spectra of actin/ filamin networks, that are very similar in shape, but show a shift of the main peak of the spectra to lower frequencies with increasing cross—link density. High frequency modes, which result from an increase of the elasticity constants of the network with progressive cross—link density are obviously more efficiently damped than the low ones, leading to a predominance of the slow motions in the spectra.