Physical study of a reconstituted actomyosin cortex

The actomyosin cortex, a thin network of branched and unbranched actin filaments and myosin motors, that lies beneath the plasma membrane of cells and mechanically attached to it, endows cells with the ability to reshape. Within the cortex, the myosin motors crosslink the actin filaments and actively generate contractile stresses fueled by ATP hydrolysis, which together with filament turnover, provide the cortex its active non-equilibrium character. Currently, many features of the actomyosin cortex remain unknown, notably, how the thickness is regulated and what are the impact of myosin motors contractility on tension built up.

We recreate under well-defined and controlled conditions actin cortices of finite thickness that dynamically turnover with time like the actin cortex in cells. As far as we are aware of, this is the first time that turnover conditions were recreated in vitro using purified proteins. We show how the interplay between actin nucleation, branching, and filament elongation rates at the cortex front regulate cortical network self-organization and steady state thickness. Furthermore, we demonstrate how cortex disassembly at the rear plays a crucial role in achieving steady state conditions.

Beyond the experimental achievement, defining the conditions for recreating fast turnover was one of the major obstacles to overcome before a synthetic motile cell could be created.