Active microphase separation in mixtures of microtubules and tip-accumulating molecular motors

TL;DR

This study maps the non-equilibrium phase diagram of microtubules and tip-accumulating kinesin-4 motors, revealing diverse dynamic behaviors and complex microphase separated structures driven by concentration-dependent active stresses.

Contribution

It provides a comprehensive phase diagram and uncovers new active phases and kinetic pathways in microtubule-motor mixtures, highlighting the role of concentration and spatial arrangement.

Findings

Kinesin-4 induces contractions or turbulence depending on concentrations.

Identification of multiple non-equilibrium phases like asters, chains, bilayers, and foams.

Microphase separation arises from frustration between filament orientation and motor positioning.

Abstract

Mixtures of microtubules and molecular motors form active materials with diverse dynamical behaviors that vary based on their constituents' molecular properties. We map the non-equilibrium phase diagram of microtubules and tip-accumulating kinesin-4 molecular motors. We find that kinesin-4 can drive either global contractions or turbulent-like extensile dynamics, depending on the concentrations of both microtubules and a bundling agent. We also observe a range of spatially heterogeneous non-equilibrium phases, including finite-sized radial asters, 1D wormlike chains, extended 2D bilayers, and system-spanning 3D active foams. Finally, we describe intricate kinetic pathways that yield microphase separated structures and arise from the inherent frustration between the orientational order of filamentous microtubules and the positional order of tip-accumulating molecular motors. Our work shows that the form of active stresses and phases in cytoskeletal networks are not solely dictated by the properties of individual motors and filaments, but are also contingent on the constituent's concentrations and spatial arrangement.