The GTP hydrolases of the Rho/Rac family participate in the generation of coordinated cellular responses to extracellular stimuli (52, 153). Their actions are essential to promote the formation of cytoskeletal structures that contribute to changes in cell shape and motility, the activation of lipid and protein kinase cascades, and the induction of patterns of gene expression required for both developmental and proliferative decisions (52, 153). The functions of these proteins are also associated with a number of human disorders. Thus, hereditary diseases, such as immunodeficiencies (Wiscott-Aldrich syndrome), certain types of X-linked mental retardation, and developmental abnormalities (Aarskog-Scott syndrome), have been linked to null mutations in either upstream or downstream elements of the Rho/Rac pathways (2, 113, 144). The activities of these proteins have also been associated with cellular transformation and oncogenesis, either by enhancing the metastatic properties of transformed cells or by serving as ancillary factors that contribute to the transforming activities of oncoproteins such as Ras (153, 172). Rho/Rac family members are found amplified in some tumors but, unlike Ras, no evidence has been found for their activation by gain-of-function mutations (40, 143). This indicates that the constitutive activation of their pathways is mediated by the deregulation of upstream signals in most cancer cells. Due to these observations, the manipulation of the activation-deactivation cycles of these proteins has received special attention as a potential point of pharmacological intervention to stop the growth of cancer cells.

The key step in this activation cycle is the differential binding of guanosine nucleotides (7). In quiescent cells, these proteins are locked in an inactive state maintained by the presence of bound GDP molecules. In this state, Rho/Rac proteins bind to negative regulators (Rho GDP dissociation inhibitors) that keep them sequestered in the cytosol and block the intrinsic release of bound GDP (7). After cell simulation, there is an exchange of GDP for GTP molecules, resulting in the release of the inhibitory molecules, the translocation of the GTPases to the plasma membrane, and their interaction with their effector molecules (7). Eventually, the action of the GTPaseactivating proteins leads to the hydrolysis of the bound GTP molecules and the reversion of the GTPases to the inactive, GDP-bound state (7). Since the intrinsic exchange rate of these proteins is low under normal physiological conditions, the stimulation of Rho/Rac proteins requires the participation of regulatory molecules known as guanosine nucleotide exchange factors (GEFs)(153). To date, two different families of Rho/