Actin Flow Dependent and Independent Force Transmission in Integrin-Mediated Adhesions

Abstract

Integrin-mediated adhesions are essential for multicellular life because of their ability to both apply force to and sense the mechanical properties of the extracellular matrix. These effects are attributed to the “focal adhesion clutch”, in which moving actin filaments transmit force to integrins via dynamic protein interactions. To understand mechanotransduction through integrins, we simultaneously measured force on talin together with rates of actin flow by combining a previously developed talin FRET tension sensor with quantitative actin speckle microscopy. While force on talin in small lamellipodial adhesions correlated with actin flow, force on talin in large adhesions in the lamella was mainly flow-independent. Stopping actin flow blocked force transmission in lamellipodia but not lamellar adhesions. This indicates there are two unique types of force transfer in integrin based adhesions, one that is actin flow-dependent and one that is actin flow-independent. Flow-dependent force transfer required talin's C-terminal actin binding site, ABS3, whereas flow-independent force transfer required vinculin and the central actin binding site, ABS2. Mutation of these sites identified distinct functions in force transmission, cell spreading and mechanosensing. Together, these results revealed that the focal adhesion clutch is a transient structure that mediates events at the cell edge prior to arrest of actin filaments and establishment of stable adhesions that are the primary force-transmitting structures.