Liquid crystal domains and thixotropy of filamentous actin suspensions.
The thixotropic properties of filamentous actin suspensions were examined by a step-function shearing protocol. Samples of purified filamentous actin were sheared at 0.2 sec-1 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 sec-1. 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 less than 1 sec-1, flow occurred by the relative movement of irregular, roughly ellipsoidal actin domains 40-140 microns long; the appearance was similar to moving ice floes. At shear rates greater than 1 sec-1, 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.
Kerst
A
A
Department of Physiology, Michigan State University, East Lansing 48824.
Chmielewski
C
C
Livesay
C
C
Buxbaum
R E
RE
Heidemann
S R
SR
D000199:Actins; D000818:Animals; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003461:Crystallography; D004548:Elasticity; D066298:In Vitro Techniques; D011817:Rabbits; D012212:Rheology; D014783:Viscosity
D000199:Actins