Active actin and microtubule composite materials

Actin and microtubules are fundamental structural proteins within the cell. Studies of their passive mechanical properties form a basal framework for understanding cytoskeletal mechanics. It is becoming increasingly clear that the interplay between structural networks of actin, microtubules and intermediate filaments is critical to understand details of cellular physiology. Further, how the active dynamics of these structural polymers couples to the structure of actin and microtubule composite networks is not well understood. To address these questions, we have built a polymer blend of actin and active kinesin-driven microtubule bundles. We experimentally probed the structural morphology of this out-of-equilibrium composite material and measured the dynamics of the system over time. When the mechanical contribution of the passive network is small, the active microtubule bundles fluidize the system. Increasing the actin concentration disrupts this process. In spite of being composed of a few simple components, this well characterized non-equilibrium system generates rich phenomena evocative of living cells.