ATP-driven myosin II activity remodels actin networks and drives cytoskeletal matter out of thermal equilibrium, but how ATP concentration controls these dynamics remains difficult to isolate in vivo. Here, we reconstitute minimal actomyosin networks from purified components and combine passive microrheology with mean back relaxation (MBR) analysis to quantify ATP-dependent nonequilibrium fluctuations. Nonequilibrium activity is strongest at intermediate ATP concentrations ( $0.2-0.5 mathrm{mM}$ ) and decreases at higher ATP. While single-bead van Hove distributions are approximately Gaussian, pooled distributions display apparent tails caused mainly by bead-to-bead heterogeneity rather than frequent active bursts. MBR, however, reveals clear time-irreversible dynamics by distinguishing restoring relaxation from persistent active motion. Comparing activity with network stiffness suggests a trade-off between ATP-dependent stiffening and myosin-driven remodeling. A minimal active Langevin simulation reproduces the observed MBR phenomenology, supporting a picture in which rare myosin-driven cage rearrangements generate detectable nonequilibrium signatures. These results establish MBR as a sensitive probe of active matter behavior in actomyosin networks.
Cicek, N., Kim, S., Geil, B., Aghaei, Z., Janshoff, A.
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