The mechanical properties of actin gels. Elastic modulus and filament motions

To address large discrepancies reported in the literature, the viscoelastic properties of gels formed by purified actin filaments have been measured by five different techniques and five different instruments using actin preparations purified separately in four different laboratories. These measurements consistently showed that the elastic shear modulus of 2 mg/ml F-actin is on the order of several hundred pascals, and depends very strongly on the length of the filaments and on the history of the sample prior to measurement. Shortening of actin filaments with gelsolin and mechanical perturbations reduce the shear modulus to low values identical to some reported in the literature, indicating that such perturbations account for low shear moduli and poor responsiveness to filament modifying treatments reported previously. The structures of individual actin filaments within gels very similar or identical to those studied rheometrically were also examined by dynamic light scattering and fluorescence microscopy. Dynamic light scattering data were analyzed by a new method to confirm that actin filaments have no stable associations with each other and fluctuate in solution at a rate governed by the filament bending modulus or persistence length, determined to be approximately 10 microns. Fluorescence microscopy confirmed that applying even small shear stresses to F-actin can orient and rupture the filaments, and that in a minimally perturbed viscoelastic gel, long actin filaments are free to diffuse within a limit of constraints formed by their neighbors. These findings confirm that relatively isotropic F-actin networks are sufficiently strong to stabilize cells.