Liquid-like VASP condensates drive actin polymerization and dynamic bundling

The organisation of actin filaments into bundles is required for cellular processes such as motility, morphogenesis and cell division. A network of actin-binding proteins, some of which can undergo liquid–liquid phase separation, controls filament bundling. However, it remains unclear how these liquid-like condensates contribute to filament bundling. Here we show that the processive actin polymerase and bundling protein VASP forms liquid-like droplets under physiological conditions. As actin polymerizes within VASP droplets, elongating filaments partition to the edges of the droplet to minimize filament curvature, forming an actin-rich ring within the droplet. The rigidity of this ring is balanced by the droplet’s surface tension. However, as the ring grows thicker, its rigidity increases and eventually overcomes the surface tension, deforming into a linear bundle. The fluid nature of the droplets is critical for bundling, as more solid droplets resist deformation and therefore prevent filaments from rearranging into bundles. This droplet-based bundling mechanism may be relevant to the assembly of cellular architectures rich in bundled actin filaments such as filopodia, stress fibres and focal adhesions. The protein VASP can undergo liquid–liquid phase separation. The interplay between the surface tension of the VASP droplet and actin polymerization controls the bundling of actin filaments, a necessary step for many cellular processes.