Capillary-induced bundling of biopolymer networks via protein condensates

Within the cell, biopolymers form self-organized assemblies that regulate cellular processes. These assemblies can be constructed through either protein interactions or phase separation. It is known that actin filaments, which form the mechanical structure of cells, are assembled into networks and bundles by protein cross-linkers. Different network and bundle microstructures support different physiological functions. Recently, there is evidence that protein condensates interact with biopolymers to create bundles. Here, we show that protein condensates colocalize with actin filaments and form networks of bundles. The condensates absorb on actin bundles and relax into a barrel shaped droplet on bundles, evocative of drops of simple liquids on fibers. We investigate the condensate spreading and measure contact angle that condensates make with bundles. 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. Understanding this bundling mechanism could expand our toolkit for making self-assembled fiber-based soft materials.