Self-organization in active, anisotropic biopolymer droplets

Biological assemblies self-organize from macromolecules into assemblies with distinctive spatial structure that have cellular function rooted in spatial asymmetry. The anisotropy of the assemblies is derived from the anisotropy of the components, such as biopolymer filaments. We present a model system of the cross-linked biopolymer filament, actin, to experimentally investigate how anisotropy and enzymatic activity drive self-organization and shape changes in biological materials. When actin filaments are short, cross-linker induces the formation of anisotropic liquid droplets, or tactoids. The cross-linker acts as a cohesion and controls the tactoid shape and dynamics. Polymers of myosin, which are molecular motors that bind to and translocate actin filaments, localize to actin tactoids. We find that myosin polymers self-organize within the tactoid, cluster, and drive tactoid shape changes. We present simple continuum model of an adhesive particle imposing alignment in an anisotropic droplet to describe the experimentally observed motor organization and tactoid deformation. Our results demonstrate how anisotropy in biological materials directs self-organization and shape changes.