The thixotropic properties of filamentous actin suspensions were examined by a step-function shearing protocol. Samples of purified framentous actin were sheared at 0.2 sec' in a cone and plate rheometer. We noted a sharp stress overshoot upon the initiation of shear, indicative of a gel state, and a nearly instantaneous drop to zero stress upon cessation of shear. Stress-overshoot recovery was almost complete after 5 min of "rest" before samples were again sheared at 0.2 sec1.Overshoot recovery increased linearly with the square root of rest time, suggesting that gel-state recovery is diffusion limited. Actin suspensions subjected to oscillatory shearing at frequencies from 0.003 to 30 radians/sec confirmed the existence of a 5-min time scale in the gel, similar to that for stress-overshoot recovery. Flow of filamentous actin was visualized by polarized light observations. Actin from 6 mg/ml to 20 mg/ml showed the "polycrystalline" texture of birefringence typical for liquid crystal structure. At shear rates <1 sec', flow occurred by the relative movement of irregular, roughly ellipsoidal actin domains 40-140 ,um long; the appearance was similar to moving ice floes. At shear rates >1 sect, domains decreased in size, possibly by frictional interactions among domains. Eventually domains flow in a "river" of actin aligned by the flow. Our observations confirm our previous domain-friction model for actin rheology. The similarities between the unusual flow properties of actin and cytoplasm argue that cytoplasm also may flow as domains.Cytoplasm and suspensions of actin have unusual and complex fluid behaviors that must play a role in such phenomena as cytoplasmic streaming, amoeboid movement, movement of vesicles through the cytoplasm, and bulk flow of polymer. Both cytoplasm and actin solutions are shear thinning; their viscosity decreases with increasing shear rate (1-8). Shear thinning of actin differs from most polymer solutions in the absence of a Newtonian viscosity plateau at very low shear rates (1-4, 7). Further, cytoplasm and cytoskeletal suspensions show a constant shear stress (force) for shear rates between 0.001 and 1 sect (1, 5). Flow rate, therefore, is not fixed by the force; no flow occurs at forces less than the constant and fluid velocity is limited by inertia alone at forces higher than the constant. We (1) called this "flow indeterminacy," which, for example, helps explain the capacity of Physarum to support very rapid cytoplasmic streaming within "walls" of cytoplasm that do not shear away: High flow rates do not necessarily require large forces and a small difference in filament concentration could account for the difference between flowing and nonflowing cytoplasm. Cytoplasm and actin suspensions are also thixotropic (3, 4, 8-10)-i.e., show time-dependent decreases in viscosity at steady shear rate and subsequent recovery when the flow is discontinued (11). Thixotropy is frequently discussed in terms of gel --sol transitions familiar to biologists and has been among the most durable ...

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