Steady-state contractile actin flow in Xenopus egg extract droplets

The actin cytoskeleton of eukaryotic cells is a highly dynamic viscoelastic “active material”. A typical cell maintains a cortex lining that supports the cell membrane, a polymer network consisting of actin, myosin motors and a plethora of regulatory proteins. Actin turns over between polymeric and monomeric forms on a time scale of minutes. Myosin motors generate active contractile stresses that can induce large-scale actin flow, which is essential for the transport of cytoplasmic components, locomotion as well as shape changes of cells. How exactly so many interacting biochemical processes result in static or dynamic steady states is unclear. Using water-in-oil droplet containing cytoplasmic extract of Xenopus laevis eggs as a model system for an active cytoskeleton, we could produce radially convergent continuous flow of polymerized actin that persist over time scales much longer than the turn-over time of a single actin filament. We mapped the spatiotemporal distribution of this contractile persistent actin flow. Interestingly, we found that macromolecular cargo present in the extract gets transported into the center of the droplet and compacted into a jammed state. We demonstrated this by tracking embedded IR fluorescent single-walled carbon nanotubes as mechanical probes.