Cross-talk between Rac and Rap GTPases in migrating cells.

To enable effective cell migration, local cell protrusion has to be coordinated with local cell attachment. Here, we investigate spatiotemporal activity patterns of key regulators of cell protrusion and adhesion, the small GTPases Rac and Rap, in migrating cells. These analyses show that Rac activity correlates very tightly with instantaneous cell protrusion events, while the Rap activity stays elevated for prolonged time periods after protrusion and is also detectable before cell protrusion. Direct analysis of activity cross-talk in living cells via light-based perturbation methods revealed that Rap can efficiently activate Rac; however, reciprocal cross-talk from Rac to Rap was not detectable. These findings suggest that Rap plays an instructive role in the generation of cell protrusions by its ability to activate Rac. Furthermore, prolonged Rap activity suggests that this molecule also plays a role in maintenance or stabilization of cell protrusions. Indeed, analysis of Rap1-depleted A431 cells revealed a significant reduction of cell attachment, suggesting that Rap-stimulated cell adhesion can stabilize newly formed protrusions. Taken together, our study suggests a mechanism, by which cell protrusion is coupled to cell adhesion via unidirectional cross-talk that connects the activity of the small GTPases Rap and Rac.