Mechanical forces alter zyxin unbinding kinetics within focal adhesions of living cells

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Abstract
The formation of focal adhesions that mediate alterations of cell shape and movement is controlled by a mechanochemical mechanism in which cytoskeletal tensional forces drive changes in molecular assembly; however, little is known about the molecular biophysical basis of this response. Here, we describe a method to measure the unbinding rate constant kOFF of individual GFP-labeled focal adhesion molecules in living cells by modifying the fluorescence recovery after photobleaching (FRAP) technique and combining it with mathematical modeling. Using this method, we show that decreasing cellular traction forces on focal adhesions by three different techniques—chemical inhibition of cytoskeletal tension generation, laser incision of an associated actin stress fiber, or use of compliant extracellular matrices—increases the kOFF of the focal adhesion protein zyxin. In contrast, the kOFF of another adhesion protein, vinculin, remains unchanged after tension dissipation. Mathematical models also demonstrate that these force-dependent increases in zyxin's kOFF that occur over seconds are sufficient to quantitatively predict large-scale focal adhesion disassembly that occurs physiologically over many minutes. These findings demonstrate that the molecular binding kinetics of some, but not all, focal adhesion proteins are sensitive to mechanical force, and suggest that force-dependent changes in this biophysical parameter may govern the supramolecular events that underlie focal adhesion remodeling in living cells. J. Cell. Physiol. 207: 187–194, 2006.