The interplay between viscoelastic and thermodynamic properties determines the birefringence of F-actin gels.
F-actin gels of increasing concentrations (25-300 microM) display in vitro a progressive onset of birefringence due to orientational ordering of actin filaments. At F-actin concentrations <100 microM, this birefringence can be erased and restored at will by sonication and gentle flow, respectively. Hence, the orientational ordering does not result from a thermodynamic transition to a nematic phase but instead is due to mechanical stresses stored in the gels. In contrast, at F-actin concentrations > or =100 microM, gels display spontaneous birefringence recovery, at rest, which is the sign of true nematic ordering, in good agreement with statistical physics models of the isotropic/nematic transition. Well-aligned samples of F-actin gels could be produced and their small-angle x-ray scattering patterns are quite anisotropic. These patterns show no sign of filament positional short-range order and could be modeled by averaging the form factor with the Maier-Saupe nematic distribution function. The derived nematic order parameter S of the gels ranged from S = 0.7 at 300 microM to S = 0.4 at 25 microM. Both birefringence and small-angle x-ray scattering data indicate that, even in absence of cross-linking proteins, spontaneous cooperative alignment of actin filaments may arise in motile regions of living cells where F-actin concentrations can reach values of a few 100 microM.
Helfer
Emmanuèle
E
Dynamique du Cytosquelette, Laboratoire d'Enzymologie et Biochimie Structurales, UPR 9063 Centre National de la Recherche Scientifique, Gif-sur-Yvette, France. helfer@lebs.cnrs-gif.fr
Panine
Pierre
P
Carlier
Marie-France
MF
Davidson
Patrick
P
D000199:Actins / Q000737:chemistry*; D000818:Animals; D016880:Anisotropy; D001703:Biophysics / Q000379:methods*; D003432:Cross-Linking Reagents / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004548:Elasticity; D005782:Gels; D015233:Models, Statistical; D011817:Rabbits; D012542:Scattering, Radiation; D013010:Sonication; D013314:Stress, Mechanical; D013816:Thermodynamics; D014965:X-Rays
D000199:Actins; D003432:Cross-Linking Reagents; D005782:Gels