TRANSLOCATION OF CORTACTIN TO THE CELL PERIPHERY IS MEDIATED BY THE SMALL GTPASE RAC1

Small GTPases of the Rho family regulate signaling pathways that contr ol actin cytoskeletal structures, In Swiss 3T3 cells, RhoA activation leads to stress fiber and focal adhesion formation, Rad to lamellipoda and membrane ruffles, and Cdc42 to microspikes and filopodia, Several downstream molecules mediating these effects have been recently ident ified. In this report we provide evidence that the intracellular local ization of the actin binding protein cortactin, a Src kinase substrate , is regulated by the activation of Rad. Cortactin redistributes from the cytoplasm into membrane ruffles as a result of growth factor-induc ed Rad activation, and this translocation is blocked by expression of dominant negative Rac1N17. Expression of constitutively active Rac1L61 evoked the translocation of cortactin from cytoplasmic pools into per ipheral membrane ruffles. Expression of mutant forms of the serine/thr eonine kinase PAK1, a downstream effector of Rad and Cdc42 recently de monstrated to trigger cortical actin polymerization and membrane ruffl ing, also led to the translocation of cortactin to the cell cortex, al though this was effectively blocked by coexpression of Rac1N17, Collec tively these data provide evidence for cortactin as a putative target of Rad-induced signal transduction events involved in membrane rufflin g and lamellipodia formation.