Measurements of the dynamic viscosity of various actin filament preparations under conditions of low and controlled shear: (a) confirm the shear rate dependence of F-actin viscosities and show that this dependence obeys the power law relationship observed for entangled synthetic polymers; (b) permit estimation of the extent to which shear artifact amplifies changes in the apparent viscosity of F-actin measured in a falling ball viscometer; (c) show that gel-filtration chromatography of actin and the addition of cytochalasin B to F-actin bring about small (20-40%) changes in the viscosity of the F-actin solutions. These variations are consistent with alterations in the actin-binding protein concentrations required for incipient gelation, a parameter inversely related to average filament length. Therefore: (a) changes in the viscosity of F-actin can be magnified by use of the falling ball viscometer, and may exaggerate their biological importance; (b) chromatography of actin may not be required to obtain meaningful information about the rheology of actin filaments; (c) changes in actin filament length can satisfactorily explain alterations in F-actin viscosity exerted by cytochalasin B and by chromatography, obviating the need to postulate specific interfilament interactions.

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