During my thesis I have developed different projects in order to tackle the problem of actin network dynamics and organization as well as the molecular mechanism at the origin of force production in biomimetic reconstituted systems. My first interest concerned the spatiotemporal organization of actin networks and actin-binding proteins during actin based motility of nucleation promoting factor-coated particles (Achard et al, Current Biology, 2010 and Reymann et al, accepted at MBOC). I observed in real time the incorporation of two actin regulators (Capping protein et ADF/cofilin) and showed that their biochemical control of actin dynamics also governs its mechanical properties. To further characterize the mechanical properties of expanding actin networks, I used an innovative micropatterning set-up allowing a reproducible spatial control of actin nucleation sites. It allowed me to show that geometrical boundaries, such as those encountered in cells, affect the dynamic formation of highly ordered actin structures and hence control the location of force production (Reymann et al, Nature Materials, 2010). Finally the addition of molecular motors on this tunable system allowed me to study implications for myosin-induced contractility. In particular, HMM-MyosinVI selectively interact with the different actin network architectures (parallel, anti-parallel organization or entangled networks) and leads to a selective three-phase process of tension, deformation of actin networks tightly coupled to massive filament disassembly. This phenomenon being highly dependent on actin network architecture could therefore play an essential role in the spatial regulation of expanding and contracting regions of actin cytoskeleton in cells.
