Cells attach to substrates, spread and move by exerting traction forces at multi-protein complexes called focal adhesions. While mechanical forces generated through the actin cytoskeleton control adhesion assembly, the molecular mechanisms of this response are unclear. We tested the hypothesis that force alters the kinetics of protein-protein interactions inside focal adhesions formed by living capillary endothelial cells. Using femtosecond laser ablation to cut stress fibers anchored into adhesions, and fluorescence photobleaching to measure protein escape times, we found that the dissociation constant of zyxin, a focal adhesion protein, is sensitive to force, unlike another adhesion molecule vinculin. Our findings provide a molecular basis for tensional control of focal adhesion assembly.

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