FIGURE 5.

Ensemble-averaged MSDs of particles moving in untreated extract (□) as well as extracts that have been treated with 30 μM latrunculin B (○), 10 μM nocodazole (▿), 10 μM taxol (⋄), and 500 nM taxol (▵), after a 10 min (open symbols) or 30 min incubation (solid symbols) at room temperature. Our data evolve slightly subdiffusively with lag time, indicating that the cytosol is not a pure fluid on these length scales. The solid line represents a slope of 1, as expected for a pure viscous fluid, and the dotted line a slope of 0.85. Although not a simple fluid, the material is predominantly viscous with viscosity in the range of 10–30 mPa-s. We measure no significant change in particle dynamics upon disassembly of either the actin or microtubule networks, or the stabilization of the microtubule network, and observe no significant changes in viscosity for waiting times of up to 30 min. At 30 min, the extracts are still dominated by viscous relaxation, with viscosity in the range of 10–30 mPa-s.