Macromolecular crowding modulates actin filament bending dynamics and mechanics*

Cellular environment is crowded with high concentrations of macromolecules such as polysaccharides and proteins limiting the amount of unoccupied space within eukaryotic cells. Actin filaments are semiflexible protein biopolymers that play a dynamic role in cellular structural support, movement, and intracellular transport. Although the effects of molecular crowding on actin filament assembly have been shown, how crowded environments affect filament mechanics remains to be elucidated. In this study, we investigate how macromolecular crowding affects the mechanical properties and bending dynamics of actin filaments in vitro. We simulate crowded cellular environments by using inert/polymer molecular crowders, sucrose and polyethylene glycol. Visualization of filaments in crowded environments by fluorescence microscopy allows for the quantification of filament bending modes and stiffness. Our finding suggests that molecular crowding enhances filament mechanics and modulates thermal bending dynamics, thereby potentially regulating interactions with actin binding proteins in vivo.