Flexibility of F-actin in aqueous solution: a study on filaments of different average lengths

Abstract

We have investigated the dynamics of F-actin with different average filament lengths in dilute and semidilute solution by polarized dynamic light scattering. The average degree of polymerization, (X), of F-actin was controlled by the actin-binding protein gelsolin. By variation of the molar ratio of actin/ gelsolin, we obtained a homologous series of filaments with 50 < (X) < 2000, the polydispersity of the system being characterized by an exponential distribution. Comparison of the electric field autocorrelation function decay profile in the dilute regime with model calculations for rigid and semiflexible rodlike polymers revealed that the microfilaments are rather stiff molecules. From measurements on the homologous series of different filament lengths the persistence length was determined to P=7.5+/-2.5 mu m. The contribution of internal bending motions to the correlation function was clearly discernible for longer filaments and at larger scattering vector, respectively. To characterize the concentration effect on the polarized dynamic structure factor, we studied solutions with (X) = 350 from 2.2 to 105 mu M actin. The transition from dilute to semidilute regime was detected at c(L)(3) approximate to 15. This is very similar to findings on other polymer systems with a narrower size distribution. While at low scattering angle the apparent diffusion coefficient is independent on concentration, we observed a decrease of D-app at theta=90 degrees. From this we concluded that the rotational motion of the rodlike molecules is restricted at higher C(L)(3). In addition, with increasing concentration a slow mode appeared in the experimental correlation functions. The amplitude decreased with increasing scattering vector. The nature of the slow relaxation process, also observed in other polymer systems, is still under debate.