Liquid behavior and motor-induced division of actin droplets: a theoretical model*

Self-organized liquid droplets of biomaterial that grow and divide are intriguing models for cell behavior besides being novel instances of active matter. Recent experiments at the Gardel laboratory show that cross-linked, short actin filaments form elongated liquid-like domains in vitro, which are susceptible to shape change and division under myosin motor activity. We attribute the characteristic spindle shape of these droplets to the anisotropic interfacial tension originating from the underlying nematic ordering of the constituent actin filaments. Based on such a continuum picture and available experimental evidence, we propose a simple explanation for the observed droplet division by modeling self-organized clusters of motors as defect-inducing centers within the droplet of actin. This “colloid-in-nematic” type of model incorporates the interplay of droplet geometry and the binding affinity of myosin to actin. Consistent with experimental observation, the model indicates a critical strength and size of motor clusters for droplet division. We next indicate how the shape instabilities can arise from motor-induced active stresses and explore connections to a more complete hydrodynamic theory.