The Translation of Actin Dynamics into Traction Force via Focal Adhesions in Migrating Cells

Forces are generated in the actin cytoskeleton by myosin-II motors and transmitted to the extracellular matrix (ECM) via dynamic macromolecular assemblies called focal adhesions (FA). To explore how forces are transmitted from the contractile actomyosin network to the ECM, we combine traction force microscopy and fluorescent speckle microscopy (FSM) of FAs and actin cytoskeleton in Ptk1 epithelial cells. We find that the relationship between intracellular actin flow and traction force is spatially segregated within individual focal adhesions. Near the leading edge, actin flow is inversely related to force, while towards the cell center, there is a positive correlation. This change is regulated by small GTPase signal transduction pathways and myosin II motor based contraction. Thus, the FA is a molecular clutch that exhibits regulatory switching between different coupling mechanisms.