The actin protein is one of the major component of the cytoskeleton. Actin polymerizes into filaments which forms the cortex beneath the cell membrane. The polymerisation of actin also generates forces, a process used by the cell to move, change its shape, or divide. A large number of associated proteins interacts with actin to control the length of the filaments, accelarate the polymerisation and organise the filaments into bundles or dendritic arrays. More specifically the eukaryotic cells crawl on surface using the directed polymerisation of actin via the protein complex, Arp2/3 at the leading edge of the cell: the membrane is pushed forward by the polymerisation. Some pathogens, like Listeria, are able to recruit the actin of the host cells to assemble a “comet”. This comet is used by the pathogen to move inside the cytoplasm and from a cell to another. In vitro, such comet can be obtained from the growth of a gel at the surface of beads via a mix of purified actin associated protein.

We study the mechanics of actin branched gels via a novel technique using magnetic colloids. Based on dipolar interactions that organise superparamagnetic beads in chain when a magnetic field is applied, this technique allows to apply well-controlled forces in the range of piconewtons to nanonewtons. Such forces are sufficient to deform an actin gel grown on the beads. We can thus study mechanical properties of actin gel. Compared to other existing techniques such as AFM (Chaudhuri et al. Nature 2007), we are here able to obtain easily very large statistics: each link between two beads gives an independent force distance curve, and hundreds of such characterisations can be made in one experiment. We are comparing gels made from different concentrations of branching and capping proteins, to unravel the link between the network architecture and its mechanical properties.


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