The cell cortex is a dense actin network, few hundreds of nanometers thick, which localizes just below the plasma membrane of animal cells and plays a central role in cell shape control. The structural and mechanical properties of the cell cortex depend on its protein composition. Several actin bundling and cross-linking proteins localize to the cortex as well as myosin II motors that provide the cortex with its contractile ability. Myosin motors were shown to affect cortex thickness, structural organization, and mechanical properties. The cortex is nucleated at the membrane surface by actin nucleators that link the cortical network to the membrane surface and it undergoes dynamic remodeling which allows cells to rapidly transform, move, and exert forces in response to internal and extracellular signals.
A difficulty inherent in studies in vivo is that cells have many ways to control their mechanical properties and it can thus be difficult to draw conclusions about the principles of cytoskeletal organization from these experiments. For this reason reconstituted systems have become popular. In principle, these systems allow for full control of the constituents and for studying the effects of specific changes in molecular composition and the impact of the system`s geometry. This renders reconstituted systems optimal for physical analysis.
In this work we investigate the self-organization of actomyosin gels grown on flat supported membranes with the aim to reconstitute an actin cortex under well-defined and controlled conditions. The system is composed of purified proteins that include actin, actin binding proteins that regulate the rate of actin turnover, myosin motors and nucleating molecules that are confined to the membrane surface and physically link the network to it. Specifically, we varied the concentration of myosin motors in wide concentration range to explore the effect of contractility on the cortical actin gel dynamics and structural organization. We show that the addition of myosin motors affects both the thickness of the cortical actin layer as well as the density of the gel across the actin layer. Specifically we find that the thickness decreased with the increase in myosin concentration. The contractile stresses generated by the motors seem to affect density profile of actin, from exponential to a more step-like function.
