Molecular basis for actin polymerization kinetics modulated by solution crowding

Bryan Demosthene, Myeongsang Lee, Ryan R. Marracino, James B. Heidings, Ellen H. Kang

NanoScience Technology Center, Burnett School of Biomedical Sciences, Department of Physics, Department of Materials Science and Engineering,
University of Central Florida, Orlando, FL 32826, USA

Actin polymerization drives cell movement and provides cells with structural integrity. Intracellular environments contain high concentrations of solutes including organic compounds, macromolecules, and proteins. Macromolecular crowding has been shown to affect actin filament stability and bulk polymerization kinetics. However, the molecular mechanism behind how crowding influences individual actin filament assembly is not well understood. In this study, we investigate how crowding modulates filament assembly kinetics using total internal reflection fluorescence (TIRF) microscopy imaging and pyrene fluorescence assays. The elongation rates of individual actin filaments analyzed from TIRF imaging depended on the types of crowding agents (polyethylene glycol, bovine serum albumin, and sucrose) as well as their concentrations. Further, we utilized the all-atom molecular dynamics (MD) simulations to evaluate the effects of crowding on diffusion coefficients of actin monomers during assembly. Taken together, our data suggest that solution crowding can regulate actin assembly kinetics at the molecular level.