Supplemental Information

There is a simple molecular-level explanation for the relaxation time scale being on the scale of seconds: the actin network behaves like an elastic solid while individual elastic filaments maintain contact with neighboring filaments. After that time, when the neighboring filaments loose contact and creep past each other, the network behaves as a fluid. In the crosslinked actin gel, this characteristic time is of the order of the inverse dissociation rate of crosslinking proteins, which is in the second(s) range (see [1, 2] and references therein). In the gel without crosslinks, this is the time needed for the neighboring filaments to disentangle (the so called reptation time), which is of the order of 0.1 − 1 sec [1]. We used the following argument to estimate this parameter from the available data. Response of a viscoelastic material to a mechanical per-turbation with frequency ω can be characterized by the so called storage and loss moduli [3], G ′ = αη λω