Shells, rings, and networks—Determinants of actin configuration confined within VASP droplets

Actin is one of the most abundant proteins in the cell and is essential for various cellular functions such as growth, migration, and endocytosis. Growing experimental evidence suggests that several essential proteins including actin-binding proteins phase separate to form condensates. This results in spontaneous demixing, which aids in the spatial and dynamic control of cellular responses. However, the role of actin-binding protein condensates in controlling actin organization is poorly understood. Here, we use computational modeling to study how liquid droplets of vasodilator-stimulated phosphoprotein (VASP) affect actin organization. Actin selectively partitions inside the VASP droplets to form actin filaments organized into morphologies such as shells, rings, and disks (https://doi.org/10.1101/2022.05.09.491236). Describing the energy of the VASP-actin droplet in two dimensions, we show that above a threshold number of actin filaments, the bending energy of actin competes with the droplet surface energy to drive droplet deformation from a circle to an ellipse. These findings are consistent with experimental observations. To understand how various actin network morphologies emerge, we model filament growth and VASP-driven crosslinking in three dimensions using CytoSim. We show that the shell-shaped actin organization is favored under a wide range of VASP association rates. In contrast, rings are favored under a narrow subset of association rates through a kinetic trapping mechanism. We believe that the identified mechanism can also be relevant to liquid droplets of other actin crosslinking proteins.