Filament transport supports contractile steady states of actin networks

In all eukaryotic cells, the actin cytoskeleton is maintained in a dynamic steady-state. Actin filaments are continuously displaced from cell periphery, where they assemble, towards the cell's center, where they disassemble. Despite this constant flow and turnover, cellular networks maintain their overall architecture constant. How such a flow of material can support dynamic yet steady cellular architectures remains an open question. To investigate the role of myosin-based forces in contractile steady-states of actin networks, we used a reconstituted in vitro system based on a minimal set of purified proteins, namely actin, myosin and actin regulators. We found that, contrary to previous bulk experiments, when confined in microwells, the actin network could self-organize into ordered arrangements of contractile bundles, flowing continuously without collapsing. This was supported by three-dimensional fluxes of actin filaments, spatially separated yet balancing each other. Unexpectedly, maintaining these fluxes did not depend on filament nucleation or elongation, but solely on filament transport. Ablation of the contractile bundles abolished the flux balance and led to network collapse. These findings demonstrate that the dynamic steady state of actin networks can be sustained by filament displacement and recirculation, independently of filament assembly and disassembly.