Biophysical Properties of Actin Filaments: Theory and Light Scattering Experiments

Actins adopt a variety of conformations ranging from globular actin (G-actin) monomers, and actin filaments (F-actin), to higher-order filament formations, such as bundles, networks, and sheets. These structures are essential for various biological activities in eukaryotic cellular processes. A significant challenge in biophysics is to elucidate the role of the polyelectrolyte properties of the filaments, their interactions with the biological environment, and external force fields on their biological functions. Due to a lack of appropriate methodologies, the biophysical principles governing the polyelectrolyte and hydrodynamic F-actin properties remain unclear.



Thus, we present a unique and novel approach that combines dynamic and electrophoresis light scattering experiments, an extended semiflexible worm-like chain model (WLC), and an asymmetric polymer length distribution theory to characterize wild-type actin filament’s biophysical properties in aqueous electrolyte solutions. We also present a novel experimental approach based on bio-statistical tools to minimize errors and assure reproducibility in our results. Our experimental techniques and protocols consider several G-actin and polymerization buffers to elucidate the impact of their chemical composition, reducing agents, pH values, and ionic strengths on the filament's properties such as translational diffusion coefficient, electrophoretic mobility, asymmetric length distribution, effective filament diameter, electric charge, and semi-flexibility.



Some of our findings revealed a lower value of the effective G-actin charge and a more significant value of the effective filament diameter, compared to older molecular models, due to the formation of the double layer of the electrolyte surrounding the filaments. Contrary to the data usually reported from electron micrographs, the lower values of our persistence length and average contour filament length agree with the significant difference in the association rates at the filament ends that shift to sub-micro lengths, which is the maximum of the length distribution. Future work involves the extension of these theories and experimental protocols to elucidate the role of linker proteins and filament mutations on higher-order structure properties of actin filaments.