Spatiotemporal coordination of the GTP-binding activity of Rac1 and RhoA initiates and reinforces cell membrane protrusions and retractions during cell migration1-7. However, while protrusions and retractions form cycles that cells use to efficiently probe their microenvironment8-10, the control of their finite lifetime remains unclear. To examine if Rac1 or RhoA may also control protrusion and retraction lifetimes, we here define the relation of their spatiotemporal GTP-binding levels to key protrusion and retraction events, as well as to cell-ECM mechanical forces in fibrosarcoma cells grown on collagen of physiologically relevant stiffness. We identified temporal Rac1-GTP nadirs and peaks at the maximal edge velocity of local membrane protrusions and retractions, respectively, followed by declined edge velocity. Moreover, increased local Rac1-GTP consistently preceded increased cell-ECM traction force. This suggests that Rac1-GTP nadirs and peaks may restrain the lifetime of protrusions and retractions, possibly involving the regulation of local traction forces. Functional testing by optogenetics validated this notion, since local Rac1-GTP elevation applied early in the process prolonged protrusions and restrained retractions, while local Rac1-GTP inhibition acted in reverse. Optogenetics also defined Rac1-GTP as a promotor of local traction force. Together, we show that Rac1 plays a fundamental role in restricting the size and durability of protrusions and retractions, plausibly in part through controlling traction forces.
