Actin pool controls the scaling of dynamic actin networks

Scaling is a fundamental property of living systems, allowing cells to match the size of their internal components to their volume. The size and shape of cells depend largely on the actin cytoskeleton, whose dynamic self-assembling structures are central to this adaptation. Yet how they self-assemble in a volume-dependent manner remains unresolved. To address this question, we used a biomimetic system consisting of beads propelled by actin polymerization in microwells of different volumes, which allowed us to work with a limited and defined number of components. This approach enabled the first reconstitution, from purified proteins, of dynamic cytoskeletal structures that adapt to the volume of their environment. The scalability of these structures depends on how strongly they deplete monomers from a shared pool, and tuning the disassembly rate is sufficient to shift their scaling regime. Scaling principles also hold for multiple competing structures, with greater competition imposing stronger constraints on the monomer pool. Together, these results establish a mechanistic framework in which a structure's capacity to adapt is set by how much it draws down shared resources as it grows, a principle that may extend to other intracellular structures that scale with the cells that build them.