Extensile to contractile transition in active networks of microtubule and molecular motors

Active materials are far-from-equilibrium materials composed of energy-consuming building blocks. They self-organize into spontaneously moving structures larger than their microscopic components, where material-scale mechanics emerge from non-equilibrium interactions between active units. Rationalizing why active materials self-organize into various far-from-equilibrium states often relies on determining what symmetries are conserved or broken at the mesoscopic scale. However, coarse-graining the microscopic interactions is often challenging, in particular for living systems and biomimetic active matter where molecular interactions are either unknown, non-linear, or too complex to be reduced to a single physical mechanism. Here, we investigate the microscopic origin of the extensile to contractile transition in active gels composed of microtubule bundles and multivalent clusters of kinesin-1 motors. We combined bulk assays, single microtubule experiments, and theory to explain why decreasing ATP concentration or increasing motor concentration leads in extensile gels leads to the formation of contractile asters. Our results suggest that competition between nematic alignment and polar sorting by molecular motors controls the extensile-to-contractile transition.