Molecular crowding modulates actin filament mechanics and structure*

The cellular environment is crowded with macromolecules that reduce accessible volume for biomolecule interactions and protein assembly. Actin filament assembly and mechanics play critical roles in many cellular functions including structural support, cell movement, and force generation. Although the effects of molecular crowding on actin assembly have been shown, how crowded environments affect filament conformations and mechanical properties remain unclear. Here, we investigate the effects of molecular crowding on actin filament mechanics and structure both in vitro and in silico. Direct visualization of filaments in the presence of crowders allows for the quantification of filament thermal bending dynamics and mechanics. Biophysical analysis show that molecular crowding enhances filament's effective bending stiffness and reduces average filament lengths. Utilizing molecular dynamics simulations, we demonstrate that molecular crowding leads to changes in filament conformations and inter-subunit contacts affecting filament mechanics. This work suggests that the interplay between excluded volume effects and non-specific interactions induced by molecular crowding may modulate actin filament mechanics and structure.