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Hydrodynamics of active permeating gels. 
Auteur(s): Callan-Jones A. , Jülicher Frank
(Article) Publié:
New Journal Of Physics, vol. 13 p.093027 (2011)
Texte intégral en Openaccess : 
Ref HAL: hal-00624806_v1
DOI: 10.1088/1367-2630/13/9/093027
WoS: 000296646200002
Exporter : BibTex | endNote
44 Citations
Résumé: We develop a hydrodynamic theory of active permeating gels with viscoelasticity in which a polymer network is embedded in a background fluid. This situation is motivated by active processes in the cell cytoskeleton in which motor molecules generate elastic stresses in the network, which can drive permeation flows of the cytosol. Our approach differs from earlier ones by considering the elastic strain in the polymer network as a slowly relaxing dynamical variable. We first present the general ideas for the case of a passive, isotropic gel and then extend this description to a polar, active gel. We discuss two specific cases to illustrate the role of permeation in active gels: self-propulsion of a thin slab of gel relative to a substrate driven by filament polymerization and depolymerization; and non-equilibrium deswelling of a gel driven by molecular motors.
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Curvature-driven lipid sorting needs proximity to a demixing point and is aided by proteins
Auteur(s): Sorre Benoît, Callan-Jones A., Manneville Jean-Baptiste, Nassoy Pierre, Joanny Jean-François, Prost Jacques, Bassereau P.
(Article) Publié:
Proceedings Of The National Academy Of Sciences Of The United States Of America, vol. 106 p.5622-5626 (2009)
Texte intégral en Openaccess : 
Ref HAL: hal-00821354_v1
DOI: 10.1073/pnas.0811243106
WoS: 000264967500032
Exporter : BibTex | endNote
303 Citations
Résumé: Sorting of lipids and proteins is a key process allowing eukaryotic cells to execute efficient and accurate intracellular transport and to maintain membrane homeostasis. It occurs during the formation of highly curved transport intermediates that shuttle between cell compartments. Protein sorting is reasonably well described, but lipid sorting is much less understood. Lipid sorting has been proposed to be mediated by a physical mechanism based on the coupling between membrane composition and high curvature of the transport intermediates. To test this hypothesis, we have performed a combination of fluorescence and force measurements on membrane tubes of controlled diameters pulled from giant unilamellar vesicles. A model based on membrane elasticity and nonideal solution theory has also been developed to explain our results. We quantitatively show, using 2 independent approaches, that a difference in lipid composition can build up between a curved and a noncurved membrane. Importantly, and consistent with our theory, lipid sorting occurs only if the system is close to a demixing point. Remarkably, this process is amplified when even a low fraction of lipids is clustered upon cholera toxin binding. This can be explained by the reduction of the entropic penalty of lipid sorting when some lipids are bound together by the toxin. Our results show that curvature-induced lipid sorting results from the collective behavior of lipids and is even amplified in the presence of lipid-clustering proteins. In addition, they suggest a generic mechanism by which proteins can facilitate lipid segregation in vivo.
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