Entangled polymer solutions and melts exhibit elastic, solid-like
resistance to quick deformations and a viscous, fluid-like
response to slow deformations. This viscoelastic behaviour
reflects the dynamics of individual polymer chains driven by
brownian motion1: since individual chains can only move in a
snake-like fashion through the mesh of surrounding polymer
molecules, their diffusive transport, described by reptation2’4, is
so slow that the relaxation of suddenly imposed stress is delayed.
Entangled polymer solutions and melts therefore elastically resist
deforming motions that occur faster than the stress relaxation
time. Here we show that the protein myosin II permits active
control over the viscoelastic behaviour of actin filament solutions.
We find that when each actin filament in a polymerized
actin solution interacts with at least one myosin minifilament,
the stress relaxation time of the polymer solution is significantly
shortened. We attribute this effect to myosin’s action as a
‘molecular motor’, which allows it to interact with randomly
oriented actin filaments and push them through the solution,
thus enhancing longitudinal filament motion. By superseding
reptation with sliding motion, the molecular motors thus overcome
a fundamental principle of complex fluids: that only
depolymerization makes an entangled, isotropic polymer solution
fluid for quick deformations.