The cytoskeletal proteins actin and myosin II are main determinants of cell motility. In eukaryotic cells they form ATP-consuming contractile fibers and networks of actin filaments (F-actin) crosslinked by myosin II and other actin binding proteins. During cell migration stress fibers mostly dissolve, therefore this study focuses on isotropic gels of F-actin and myosin II. By utilizing rheology, single molecule fluorescence microscopy, and electron microscopy plain entangled F-actin networks were compared to networks in the presence of active or inactive myosin II and biotin-avidin as an additional crosslinker. Electron micrographs reveal that myosin II forms so-called myosin filaments at concentrations of 0.4 mg/ml. These filaments crosslink actin filaments in their inactive state (i.e. in the absence of ATP) and thus enhance the storage modulus of F-actin (i.e. the elastic strength) by nearly a factor of two. In the presence of ATP, which activates myosin II as a molecular motor, the myosin filaments significantly lowered the storage modulus in reference to plain entangled F-actin networks. Optical measurements of the diffusion coefficient of single actin filaments reveal that the active molecular motors enhance diffusion drastically by "pushing" the filaments through the entangled polymer solution. Thus, shear stresses can relax quicker and the measured shear modulus appears to be smaller. In contrast to the finding that in actin-myosin-networks the active motors liquefy the polymer network, motor activity increases the storage modulus in case of F-actin networks which are additionally crosslinked by biotin-avidin. In this case the active myosin II locally shears the actin gel because the actin filaments are hold back by biotin-avidin crosslinks when "pushed" through the network. Thus, the actin network is pre-strained and the measured storage modulus increases. In summary, molecular motors provide a novel method to modulate the strength of polymer networks unknown in conventional polymer science
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