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

Abstract

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.

DOI: 10.1529/biophysj.104.050245

Cite this paper

@article{Helfer2005TheIB, title={The interplay between viscoelastic and thermodynamic properties determines the birefringence of F-actin gels.}, author={Emmanu{\`e}le Helfer and Pierre Panine and Marie-France Carlier and Patrick Davidson}, journal={Biophysical journal}, year={2005}, volume={89 1}, pages={543-53} }