Vav2 activates Rac1, Cdc42, and RhoA downstream from growth factor receptors but not beta 1 integrins

The Rho family of GTPases plays a major role in the organization of the act in cytoskeleton. These G proteins are activated by guanine nucleotide excha nge factors that stimulate the exchange of bound CDP for GTP. In their GTP- bound state, these G proteins interact with downstream effecters. Vav2 is a n exchange factor for Rho family GTPases. It is a ubiquitously expressed ho mologue of Vav1, and like Vav1, it has previously been shown to be activate d by tyrosine phosphorylation. Because Vav1 becomes tyrosine phosphorylated and activated following integrin engagement in hematopoietic cells, we inv estigated the tyrosine phosphorylation of Vav2 in response to integrin-medi ated adhesion in fibroblasts and epithelial cells. However, no tyrosine pho sphorylation of Vav2 was detected in response to integrin engagement. In co ntrast, treating cells with either epidermal growth factor or platelet-deri ved growth factor stimulated tyrosine phosphorylation of Vav2. We have exam ined the effects of overexpressing either wild-type or amino-terminally tru ncated (constitutively active) forms of Vav2 as fusion proteins with green fluorescent protein. Overexpression of either wild-type or constitutively a ctive Vav2 resulted in prominent membrane ruffles and enhanced stress fiber s. These cells revealed elevated rates of cell migration that were inhibite d by expression of dominant negative forms of Rac1 and Cdc42. Using a bindi ng assay to measure the activity of Rac1, Cdc42, and RhoA, we found that ov erexpression of Vav2 resulted in increased activity of each of these G prot eins. Expression of a carboxy-terminal fragment of Vav2 decreased the eleva tion of Rac1 activity induced by epidermal growth factor, consistent with V av2 mediating activation of Rac1 downstream from growth factor receptors.