The interplay between viscoelastic and thermodynamic properties determines the birefringence of F-actin gels

¹ Laboratoire d'Enzymologie et Biochimie Structurales
² European Synchrotron Radiation Facility
³ Laboratoire de Physique des Solides

^* To whom correspondence should be addressed. E-mail: helfer{at}lebs.cnrs-gif.fr.

Submitted on July 23, 2004
Revised on September 29, 2004
Accepted on 26 April 2005

	Abstract

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 is instead 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 SAXS patterns are quite anisotropic. These patterns show no sign of filament positional short-range order and could be modelled by averaging their 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 SAXS 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 reach values of a few 100 µM.

Key Words: SAXS, actin gels, birefringence, isotropic-nematic transition, viscoelasticity