A contracting cytoskeletal network organizes into a self-centering swimmer

Collective cytoskeletal flow patterns drive many processes in eukaryotic cells. The physical mechanisms underlying the emergence of mesoscale flow of the cytoskeleton are still not well known. We here reconstituted an active cytoskeleton in water-in-oil emulsion droplets filled with Xenopus Laevis egg extract to study a model system that includes the full molecular complexity of the cytoskeleton and its non-equilibrium dynamics. The actin network remained isotropic, while a 3D radially convergent steady-state flow emerged, driven by actomyosin contraction and maintained by continuous actin turnover. We present a hydrodynamic computational model that treats the actin network as an isotropic active viscoelastic gel and suggests that connectivity percolation of actin filaments is essential for the observed flow velocity and density profiles. We introduce the concept of the cytoskeletal network as an exotic active swimmer that can sense boundaries without being physically attached, which leads to the observed robust centering of phase-separated inclusions.