A Model System for the Reconstitution of the Cellular Actin Cortex

The cell cortex is a dense actin network, few hundreds of nanometers thick, which localizes just below the plasma membrane of animal cells. Actin nucleators 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. 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. 

A difficulty inherent in studies in vivo is that cells perform many processes to control their mechanical properties and it can thus be difficult to draw conclusions about the principles of cytoskeletal organization from these experiments. Reconstituted systems, on the other hand, allow for full control of the constituents and as such to obtain a direct relation between function and composition.

This work investigates 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. 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.