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Living cells exhibit an important out-of-equilibrium mechanical activity,mainly due to the forces generated by molecular motors. These motor proteins,acting individually or collectively on the cytoskeleton, contribute to theviolation of the fluctuation-dissipation theorem in living systems. In thiswork we probe the cytoskeletal out-of-equilibrium dynamics by performingsimultaneous active and passive microrheology experiments, using the samemicron-sized probe specifically bound to the actin cortex. The free motion ofthe probe exhibits a constrained, subdiffusive behavior at short time scales (t< 2s), and a directed, superdiffusive behavior at larger time scales, while, inresponse to a step force, its creep function presents the usual weak power lawdependence with time. Combining the results of both experiments, we preciselymeasure for the first time the power spectrum of the force fluctuations exertedon this probe, which lies more than one order of magnitude above the spectrumexpected at equilibrium, and greatly depends on frequency. We retrieve aneffective temperature Teff of the system, as an estimate of the departure fromthermal equilibrium. This departure is especially pronounced on long timescales, where Teff bears the footprint of the cooperative activity of motorspulling on the actin network. ATP depletion reduces the fluctuating forceamplitude and results in a sharp decrease of Teff towards equilibrium.

Power spectrum of out-of-equilibrium forces in living cells: amplitude and frequency dependence