Actin(g) on phase separation

F-actin networks play a crucial role for cellular integrity and induction of shape changes during development and homeostasis. Actin polymerization is highly regulated by multiple pathways involving nucleating and sequestering factors, or mechanisms that regulate local monomer concentration. Recently, actin partitioning into biomolecular condensates has been reported as an additional mechanism to enhance polymerization kinetics. We reconstitute phase separated droplets of the components wsp-1 and the arp2/3 complex in vitro and study their interaction with actin. We find that actin partitions inside phase separated drops, where it polymerizes into fibers which extrude. We use this assay to study the effect of locally enhanced concentration on branched actin polymerization, the dynamics of active condensates and the mechanical interplay between established actin networks and biomolecular condensates. Our aim is to recapitulate the complex spectrum of kinetic behaviors observed for actomyosin in vivo.