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The Interplay between Viscoelastic and Thermodynamic Properties Determines the Birefringence of F-Actin Gels

Emmanuèle Helfer ^*, Pierre Panine , Marie-France Carlier ^* and Patrick Davidson 

^* Dynamique du Cytosquelette, Laboratoire d'Enzymologie et Biochimie Structurales, UPR 9063 Centre National de la Recherche Scientifique, Gif-sur-Yvette, France; European Synchrotron Radiation Facility, Grenoble, France; and Laboratoire de Physique des Solides, UMR 8502 Centre National de la Recherche Scientifique, Université Paris-Sud, Orsay, France

Correspondence: Address reprint requests to Emmanuèle Helfer, LEBS, UPR 9063 CNRS, 1 Avenue de la Terrasse, F-91198, Gif-sur-Yvette, France. Tel.: 33-0-1-698-23480; Fax: 33-0-1-698-23129; E-mail: helfer{at}lebs.cnrs-gif.fr.

F-actin gels of increasing concentrations (25–300 µM) display in vitro a progressive onset of birefringence due to orientational ordering of actin filaments. At F-actin concentrations <100 µM, 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 100 µM, 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 µM to S = 0.4 at 25 µM. 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 µM.