  Cells make use of semi-flexible biopolymers such as actin or intermediate
filaments to control their local viscoelastic response by dynamically adjusting
the concentration and type of cross-linker molecules. The microstructure of the
resulting networks mainly determines their mechanical properties. It remains an
important challenge to relate structural transitions to both the molecular
properties of the cross-linking molecules and the mechanical response of the
network. This can be achieved best by well-defined in vitro model systems in
combination with microscopic techniques. Here, we show that with increasing
concentrations of the cross-linker HMM (heavy meromyosin) a transition in the
mechanical network response occurs. At low cross-linker densities the network
elasticity is dominated by the entanglement length of the polymer, while at
high HMM densities the cross-linker distance determines the elastic behavior.
Using microrheology the formation of heterogeneous networks is observed at low
cross-linker concentrations. Micro- and macrorheology both report the same
transition to a homogeneous cross-linked phase. This transition is set by a
constant average cross-linker distance. Thus, the micro- and macromechanical
properties of isotropically cross-linked in vitro actin networks are determined
by only one intrinsic network parameter.
