Active matter from living polymers: The FtsZ paradigm

This review reframes the bacterial cytokinetic protein FtsZ as a living-polymer route to motor-free active matter whose mechanical output is active stress rather than directed stepping. FtsZ, a tubulin homolog, self-assembles into dynamic treadmilling filaments that consume GTP to inject stress into soft environments and, through interfacial coupling, bias deformation and flow. Experiments across interfaces (membrane-bound assemblies, monolayers) and bulk composites (polymer matrices of hydrogels) show that turnover-driven activity can produce extensile remodeling, strain-dependent softening, and activity-controlled fluidization, with mechanical polarity selected by confinement and boundary conditions. On the theory side, ideas from polymer physics, lever-like geometric transduction, instability and bifurcation frameworks, and soft glassy rheology connect filament turnover to emergent metamaterial response, including regimes of negative mechanical susceptibility. Together, these advances position FtsZ as a minimal, biochemically tunable actuator for designing reconfigurable nonequilibrium mechanics in active soft materials and biohybrid matter. Stress-first lever logic of FtsZ-based active matter. GTP-driven treadmilling of FtsZ protofilaments sustains a distributed internal active stress rather than directed transport (left). This turnover-generated stress is projected at interfaces as tractions and torques, t = σ act · n, with geometry and coupling acting as a continuum lever that selects the dominant projection mode: Stress focusing (constriction), stress spreading, or dilative relaxation (center). At the macroscopic level, this projection renormalizes the mechanical response, yielding activity-controlled stiffening or extensile softening captured by a stress-first SGR–Landau framework through an effective susceptibility and mechanical permittivity (right). Together, the scheme illustrates how living-polymer turnover, interfacial projection, and geometry jointly program motor-free active mechanics across membranes and bulk soft materials • FtsZ operates as a minimal, motor-free active system that injects stress through GTP-driven filament turnover. • Mechanical polarity (contractile vs. extensile) is selected by geometry, confinement, and interfacial coupling, not by molecular motors. • A stress-first “lever” framework unifies membrane reconstitution and bulk hydrogel experiments across scales. • Interfacial stress projection provides a direct link between turnover- sustained activity and measurable rheological response. • A response-based SGR–Landau model captures extensile softening, fluidization, and negative mechanical permittivity in FtsZ composites.