Mechanisms of guanine nucleotide exchange and Rac-mediated signaling revealed by a dominant negative trio mutant.

Rho family GTPases play important roles in a variety of cellular processes, including actin cytoskeleton reorganization, transcription activation, and DNA synthesis. Dominant negative mutants of Rho GTPases, such as T17NRac1, that block the endogenous Rho protein activation by sequestering upstream guanine nucleotide exchange factors (GEFs) have been widely used to implicate specific members of the Rho family in various signaling pathways. We show here that such an approach could produce potentially misleading results since many Rho GEFs can interact with multiple Rho proteins promiscuously, and overexpression of one dominant negative Rho protein mutant may affect the activity of other members of the Rho family. Based on the available structural information, we have identified the highly conserved amino acid pairing of Asn(1406)Trio-Asp(65)Rac1 of the GEF-Rho GTPase interaction as the critical catalytic machinery required for the Rac1 GDP/GTP exchange reaction. The N1406A/D1407A mutant of Trio acted dominant negatively in vitro by retaining Rac1 binding activity but losing GEF catalytic activity and competitively inhibited Rac1 activation by wild type Trio. It readily blocked the platelet-derived growth factor (PDGF)-induced lamellipodia formation and inhibited the wild type Trio-induced serum response factor activation. Moreover the mutant was able to selectively inhibit Dbl-induced Rac1 activation without affecting RhoA activity in cells. In contrast to the non-discriminative inhibitory effect displayed by T17NRac1, the Trio mutant was ineffective in inhibiting PDGF-stimulated DNA synthesis and Dbl-induced transformation, revealing the Rac-independent functions of PDGF and Dbl. These studies identify a conserved pair of amino acid residues of the Trio-Rac interaction that is likely to be essential to the GEF catalysis of Rho family GTPases and demonstrate that a dominant negative mutant derived from a Rho GTPase regulator constitutes a new generation of specific inhibitors of Rho GTPase signaling pathways.