In studying a magnetic bead's creep response to force pulses in an entangled actin network we have found a novel regime where the bead motion obeys a power law x(t) approximately t(1/2) over two decades in time. It is flanked by a short-time regime with x(t) approximately t(3/4) and a viscous with x(t)approximately t. In the intermediate regime the creep compliance depends on the actin concentration c as c(-beta) with beta approximately 1.1 +/- 0.3. We explain this behavior in terms of osmotic restoring force generated by the piling up of filaments in front of the moving bead. A model based on this concept predicts intermediate x(t) approximately t(1/2) and long-time regimes x(t) approximately t in which the compliance varies as c(-4/3), in agreement with experiment.

We present a study on filamentous actin solutions containing heavy meromyosin subfragments of myosin II motor molecules. We focus on the viscoelastic phase behavior and internal dynamics of such networks during ATP depletion. Upon simultaneously using micro-rheology and fluorescence microscopy as complementary experimental tools, we find a sol-gel transition accompanied by a sudden onset of directed filament motion. We interpret the sol-gel transition in terms of myosin II enzymology, and suggest a "zipping" mechanism to explain the filament motion in the vicinity of the sol-gel transition.PACS numbers: 87.16. Ka, 87.15.La, 87.15.Nn, 87.16.Nn Eucaryotic cells show an amazing versatility in their mechanical properties. Not only can they sustain stresses ranging from some tenths to hundreds of Pascals, but they can equally well perform such complex processes as cytokinesis and cell locomotion. A vital role for these and other cellular functions is played by the cytoskeleton, the structural framework of the cell composed of a network of protein filaments. A major component is filamentous actin (F-Actin), whose physical properties are by now well characterized [1]. The length distribution, spatial arrangement and connectivity of these filaments is controlled by a great variety of regulatory proteins.An important family of these regulatory proteins are cross-linkers, which can be further classified as passive or active. The function of passive cross-linkers, e.g. α-actinin, is mainly determined by their molecular structure and the on-off kinetics of their binding sites to actin. Upon changing the association-dissociation equilibrium and hence the degree of cross-linking by varying the temperature the network can be driven from a sol into a gel state [2]. Depending on both the concentration and the affinity of these cross-linkers for F-actin there is a tendency to either form random networks or bundles [3]. Motor proteins of the myosin family can also act as active cross-linkers. When both of its functional head groups are bound to two different filaments they can use the energy of adenosine-triphosphate (ATP) hydrolysis to exert relative forces and motion between them. However, such an event is very unlikely under physiological conditions and ATP saturation, because then myosin II spends only a short fraction of its chemomechanical cycle attached to the filament (duty ratio: r 0.02 [4]). Active relative transport yet becomes possible due to the concerted action of several motors if in vitro myosin II proteins assemble into multimeric minifilaments [5].In this letter we study actin networks containing the heavy meromyosin (HMM) subfragment lacking the light meromyosin domain responsible for myosin II assembly. Our focus is on the viscoelastic phase behavior and internal dynamics of such networks during ATP depletion. We use an experimental setup combining microrheology with fluorescent microscopy of labeled filaments. This allows us to identify a sol-gel transition accompanied by a sudden onset of directed filament...

We studied the viscoelastic response of entangled actin networks using embedded microbeads driven by force pulses with amplitudes in the range from 3 to 120 pN and durations up to 60 s. We distinguished three regimes in the time dependence of the compliance J(t) of the network. These were characterized by specific power laws J(t) approximately t(alpha)(i) (i=1, 2, 3). In the short-time regime (i=1), we observed the exponent alpha1 approximately 0.75. In the long-time regime (i=3), we find that alpha3 approximately 1. For the intermediate-time interval (i=2), we observed a novel dynamic regime: for all actin concentrations and all applied forces, it was characterized by the exponent alpha3 approximately 0.5. In both regimes i=2 and i=3, the compliance depended upon the actin concentration c, such as J approximately c(-gamma)(i) with gamma2 approximately 1.1 and gamma 3 approximately 1.4. Using these results, we calculated the shear modulus in the frequency domain and found that the intermediate-time regime in the t domain corresponds to its plateau behavior.

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