The residence time of focal adhesion kinase (FAK) and paxillin at focal adhesions in renal epithelial cells is determined by adhesion size, strength and life cycle status

Focal adhesions (FAs) are specialized membrane-associated multi-protein complexes that link the cell to the extracellular matrix and enable cell proliferation, survival and motility. Despite the extensive description of the molecular composition of FAs, the complex regulation of FA dynamics is unclear. We have used photobleaching assays of whole cells to determine the protein dynamics in every single focal adhesion. We identified that the focal adhesion proteins FAK and paxillin exist in two different states: a diffuse cytoplasmic pool and a transiently immobile FA-bound fraction with variable residence times. Interestingly, the average residence time of both proteins increased with focal adhesion size. Moreover, increasing integrin clustering by modulating surface collagen density increased residence time of FAK but not paxillin. Finally, this approach was applied to measure FAK and paxillin dynamics using nocodazole treatment followed by washout. This revealed an opposite residence time of FAK and paxillin in maturing and disassembling FAs, which depends on the ventral and peripheral cellular position of the FAs.