TI - Translocation of the Rac1 guanine nucleotide exchange factor Tiam1 induced by platelet-derived growth factor and lysophosphatidic acid . AB - Several guanine nucleotide exchange factors for the Rho family of GTPases that induce activation by exchanging GDP for GTP have been identified . One of these is the tumor invasion gene product Tiam1 , which acts on Rac1 . In this study , we demonstrate that platelet-derived growth factor ( PDGF ) and lysophosphatidic acid induce the translocation of Tiam1 to the membrane fraction of NIH 3T3 fibroblasts in a time -dependent manner . Previously , we have shown that Tiam1 is phosphorylated by protein kinase C ( PKC ) and calcium/calmodulin kinase II ( CaMK II ) after stimulation with agonists . Here we show , by pretreatment of cells with kinase inhibitors , that CaMK II , but not PKC , is involved in the membrane translocation of Tiam1 . Addition of the calcium ionophore ionomycin alone induced the translocation of Tiam1 . However , the cell -permeable diacylglycerol oleoylacetylglycerol was without effect and did not enhance the effect of ionomycin . These data further indicated a role for CaMK II and not PKC . Inhibition of phosphoinositide 3-kinase by wortmannin had little effect on the translocation of Tiam1 . The role of phosphorylation was further studied by comparing the phosphorylation pattern of Tiam1 in the membranes versus whole cell Tiam1 . PDGF -induced phosphorylation of membrane-associated Tiam1 occurred more rapidly than that of the total Tiam1 pool , and CaMK II , but not PKC , played a significant role in this process . Furthermore , by using the p21-binding domain of PAK-3 , we show that PDGF , but not lysophosphatidic acid , activates Rac1 in vivo and that this activation involves CaMK II and PKC , but not 3-phosphoinositides . Our results indicate that Tiam1 is translocated to and phosphorylated at membranes after agonist stimulation and that CaMK II , but not PKC , is involved in this process . Also , these kinases are involved in the activation of Rac in vivo .