Myosin II filament size and activity influences localization in nematic actin droplets

Soft, active materials self-organize from macromolecules in cells to form precisely structured assemblies, such as the mitotic spindle, that orchestrate specific biological functions. A central question is how these self-organized assemblies arise from their macromolecular components. We investigate mechanisms of self-organization in structured biopolymer assemblies, using a minimal model system of biopolymer droplets constructed from cross-linked actin filaments. These droplets have nematic structure, which arises from the actin filaments. Myosin II motor proteins, which form filaments that bind to and translocate actin filaments, spatially self-organize in these actin droplets. We find that motors large compared to the characteristic structure of the nematic liquid self-organize to the center of the droplet, evocative of mitotic spindle configurations. In contrast, motors small compared to the liquid are dispersed throughout the droplet. We investigate the influence of motor size and activity on the dynamics and localization within the droplet and capture the spatial localization with a continuum model based on liquid crystal theory. Our results reveal potential physical mechanisms of self-organization in biological assemblies and bio-inspired soft materials design.