  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 the
violation of the fluctuation-dissipation theorem in living systems. In this
work we probe the cytoskeletal out-of-equilibrium dynamics by performing
simultaneous active and passive microrheology experiments, using the same
micron-sized probe specifically bound to the actin cortex. The free motion of
the probe exhibits a constrained, subdiffusive behavior at short time scales (t
< 2s), and a directed, superdiffusive behavior at larger time scales, while, in
response to a step force, its creep function presents the usual weak power law
dependence with time. Combining the results of both experiments, we precisely
measure for the first time the power spectrum of the force fluctuations exerted
on this probe, which lies more than one order of magnitude above the spectrum
expected at equilibrium, and greatly depends on frequency. We retrieve an
effective temperature Teff of the system, as an estimate of the departure from
thermal equilibrium. This departure is especially pronounced on long time
scales, where Teff bears the footprint of the cooperative activity of motors
pulling on the actin network. ATP depletion reduces the fluctuating force
amplitude and results in a sharp decrease of Teff towards equilibrium.
