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Living cells change shape and move in response to environmental signals. Motile processes play a pivotal role in morphogenesis, migration of embryonic and metastatic cells, angiogenesis, synaptic plasticity, immune response and interaction of the host cells with pathogens. They are generated by polarized, spatially directed actin assembly in filaments organized in specific structures. Protrusive force at the leading edge as well as tensile forces at focal contacts are produced by barbed end growth of actin filaments, locally stimulated at the membrane and globally inhibited in solution. The number of filaments and the rate of barbed end growth, fed by the treadmilling of actin filaments, determines the force. These parameters are controlled by protein machineries that initiate filaments in a spatially restricted fashion either by nucleation or by end branching, and by proteins that bind monomeric actin in a complex that has specific assembly rate parameters.
Two protein machineries are responsible for spatially directed initiation of actin filaments. They operate with different mechanisms and in distinct processes, 1) the WASP-Arp2/3 system is at the origin of the formation of a branched filament array responsible for protrusion of lamellipodium ; 2) formins, in association with profilin, catalyze the rapid processive assembly of non-branched actin filaments arranged in parallel bundles, in the cytokinetic ring and in adhesive structures. We have combined a biochemical and a biomimetic approach to understand the molecular mechanisms of these auto-organized processes. We have reconstituted the sustained actin-based movement of a N-WASP- or formin-functionalized solid particle or giant liposome in a biochemically controlled medium, which enables measurements of force production in correlation with structure and motility, and we can derive informations on the molecular mechanism of movement by single molecule measurements. The dependence of continuous or saltatory (periodic) movement on the solution components has been analyzed for solid particles and liposomes. |
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