The Rho family GTPase Cdc42 regulates the activation of Ras/MAP kinase by the exchange factor Ras-GRF

The Ras guanine-nucleotide exchange factor Ras-GRF/Cdc25(Mn) harbors a comp lex array of structural motifs that include a Dbl-homology (DH) domain, usu ally found in proteins that interact functionally with the Rho family GTPas es, and the role of which is not yet fully understood. Here, we present evi dence that Ras-GRF requires its DH domain to translocate to the membrane, t o stimulate exchange on Ras, and to activate mitogen-activated protein kina se (MAPK), In an unprecedented fashion, we have found that these processes are regulated by the Rho family GTPase Cdc42. We show that GDP- but not GTP -bound Cdc42 prevents Ras-GRF recruitment to the membrane and activation of Ras/MAPK, although no direct association of Ras-GrRF with Cdc42 was detect ed, We also demonstrate that catalyzing GDP/GTP exchange on Cdc42 facilitat es Ras-GRF-induced MAPK activation. Moreover, we show that the potentiating effect of ionomycin on Ras-GRF-mediated MAPK stimulation is also regulated by Cdc42. These results provide the first evidence for the involvement of a Rho family G protein in the control of the activity of a Ras exchange fac tor.