  Actin is one of the most studied cytoskeleton proteins showing a very rich
span of structures. It can self-assemble actively into dynamical structures
that govern the mechanical properties of the cell, its motility and its
division. However, only very few studies characterize the kinetics of the
active actin self-assembly process beyond the formation of an entangled
network. Here, we follow actin polymerization kinetics and organization into
entangled networks using time resolved passive microrheology. We establish a
relationship between the initial concentration of monomers, the active
polymerization and network formation kinetics, and the viscoelastic properties
from the onset of actin polymerization upto the formation of a steady state
entangled network. Surprisingly, we find that at high enough initial monomer
concentrations the elastic modulus of the forming actin networks overshoots and
then relaxes with a -2/5 power law, that we attribute to rearrangements of the
network into a steady state structure.
