  We have studied the elastic response of actin networks with both compliant
and rigid crosslinks by modeling molecular motors as force dipoles. Our finite
element simulations show that for compliant crosslinkers such as filamin A, the
network can be stiffened by two orders of magnitude while stiffening achieved
with incompliant linkers such as scruin is significantly smaller, typically a
factor of two, in excellent agreement with recent experiments. We show that the
differences arise from the fact that the motors are able to stretch the
compliant crosslinks to the fullest possible extent, which in turn causes to
the deformation of the filaments. With increasing applied strain, the filaments
further deform leading to a stiffened elastic response. When the crosslinks are
incompliant, the contractile forces due to motors do not alter the network
morphology in a significant manner and hence only small stiffening is observed.
