Physics of actin networks. I. Rheology of semi-dilute F-actin

The mechanical properties of cytoplasm are considered to be of underlying importance in the mechanism of cell movement and are to a large extent determined by an actin-containing cytoskeleton. Several laboratories have begun to accumulate data on the mechanical or rheologic properties of protein systems derived from the actin cytoskeleton. The focus of this manuscript is to attempt to reproduce the experimentally determined mechanical properties of non-cross-linked F-actin from theoretical considerations. It was found that a mechanical spectrum for 1 mg/ml F-actin could be calculated, which approximated experimental data, from a relaxation spectrum consisting of a long range rotational diffusion motion and short range bending motion, assuming an exponential distribution of filament lengths with a weight average length of 4 mu. The calculated spectrum underestimated the dynamic moduli at high frequencies, suggesting that a more complex actin structure is present that enhances the high frequency component.