Protein condensates induce biopolymer filament bundling and network remodeling via capillary interactions

Many cellular processes are regulated by biopolymers that are self-assembled into higher order structures by protein interactions. The actin cytoskeleton, which forms the mechanical structure of cells, consists of actin filaments that are assembled into networks and bundles by protein cross-linkers. Specific network and bundle microstructures are determined by the type of cross-linker and support different physiological functions. Recently, there is also evidence that protein condensates can nucleate bundle formation with cytoskeletal filaments. Here, we find that protein condensates interact with pre-polymerized actin filaments to form networks of bundles. The condensates absorb on actin bundles and relax into barrel shaped droplets, evocative of drops of simple liquids on fibers. We investigate the condensate spreading and measure contact angle that condensates make with bundles. Intriguingly, condensates at the intersection of bundles cause capillary bridges which induce network remodeling. Our results suggest that network formation, bundling, and remodeling in biopolymer assemblies could be induced by capillary interactions due to condensates.