From Molecules to Movement: In Vitro Reconstitution of Self-Organized Actin-based Motile Processes

Abstract

This chapter shows how a bottom-up approach has been instrumental in understanding and reconstituting actin-based movement from a minimum set of 5 individual purified proteins. We delineate the essential thermodynamic features that support actin-based movement as follows. We demonstrate first that the regulated treadmilling of actin filaments fuels the growth of barbed ends to develop protrusive or propulsive force against a functionalized soft or solid particle. Second, global inhibition of actin assembly in the medium and local stimulation of actin assembly restricts filament growth to specific sites and controls the filament size and its transient growth period against the particle; hence, stationary propulsion of the particle results from a balance between creation of new filaments and death of these filaments. Third, transient or permanent links between the particle-immobilized actin-nucleating machinery play an important role in maintaining the morphology of the force-producing actin meshwork and the velocity of the propulsion. These principles allow reconstitution of propulsive branched actin networks and formin-based processive growth of actin bundles. Finally upgrading the complexity of the bottom-up approach offers perspectives in analyzing more complex actin-based processes that occur in axis patterning and morphogenesis, as well as understanding the physical-chemical basis for the coordinated turnover of various actin structures during cell migration.