  The composite cytoskeleton, comprising interacting networks of semiflexible
actin and rigid microtubules, actively generates forces and restructures using
motor proteins such as myosins to enable key mechanical processes including
cell motility and mitosis. Yet, how motor-driven activity alters the mechanics
of cytoskeleton composites remains an open challenge. Here, we perform optical
tweezers microrheology on actin-microtubule composites driven by myosin II
motors to show that motor activity increases the linear viscoelasticity and
elastic storage of the composite by active restructuring to a network of
tightly-packed filament clusters and bundles. Our nonlinear microrheology
measurements performed hours after cessation of activity show that the
motor-contracted structure is stable and robust to nonlinear forcing. Unique
features of the nonlinear response include increased mechanical stiffness,
memory and heterogeneity, coupled with suppressed filament bending following
motor-driven restructuring. Our results shed important new light onto the
interplay between viscoelasticity and non-equilibrium dynamics in active
polymer composites such as the cytoskeleton.
