Mechanisms of Dynamic Stability in the Actomyosin Cytoskeleton*

The actomyosin cytoskeleton is an active semi-flexible polymer network whose non-equilibrium behaviors coordinate both cell elasticity and fluidity to maintain or change cell shape. Unlike the induction of contractile flows, the maintenance of dynamic stability in highly labile yet internally pre-stressed active materials remains unknown. To this end, we synthesize a biomimetic active nematic liquid crystal from long semi-flexible actin filaments driven out-of-equilibrium by myosin motor activity. We identify diverse actomyosin interactions that govern the dynamic architecture and mechanical response of the network to active stresses. These responses include dynamic steady states, in which myosin reversibly bends actin filaments, whose curvatures show anomalous and strongly coupled fluctuations across a broad spectrum of filament bending modes. These fluctuations break detailed balance, enhance network elasticity, while maintaining dynamic stability. Furthermore, the actomyosin interactions that maintain dynamic stability are fundamentally distinct from those that drive contractile flows of actomyosin networks.