Mechanism of NADPH oxidase activation by the Rac/Rho-GDI complex

The low molecular weight GTP binding protein Rac is essential to the activation of the NADPH oxidase complex, involved in pathogen killing during phagocytosis. In resting cells, Rac exists as a heterodimeric complex with Rho GDP dissociation inhibitor (Rho-GDI). Two types of interactions exist between Rac and Rho-GDI: a protein - lipid interaction, implicating the polyisoprene of the GTPase, as well as protein - protein interactions. Using the two-hybrid system, we show that nonprenylated Rac1 interacts very weakly with Rho-GDI, pointing to the predominant role of protein-isoprene interaction in complex formation. In the absence of this strong interaction, we demonstrate that three sites of protein - protein interaction, Arg66_Rac-LeU67_Rac, His103_Rac, and the C-terminal polybasic region Arg183_Rac-Lys188_Rac, are involved and cooperate in complex formation. When Rac1 mutants are prenylated by expression in insect cells, they all interact with Rho-GDI. Rho-GDI is able to exert an inhibitory effect on the GDP/GTP exchange reaction except in the complex in which Rac1 has a deletion of the polybasic region (Arg 183_Rac - Lys 188_Rac). This complex is, most likely, held together through protein-lipid interaction only. Although able to function as GTPases, the mutants of Racl that failed to interact with Rho-GDI also failed to activate the NADPH oxidase in a cell-free assay after loading with GTP. Mutant Leu119_Rac-Gln could both interact with Rho-GDI and activate the NADPH oxidase. The Rac1/Rho-GDI and Rac1-(Leu119Gln)/Rho-GDI complexes, in which the GTPases were bound to GDP, were found to activate the oxidase efficiently. These data suggest that Rho-GDI stabilizes Rac in an active conformation, even in the GDP-bound state, and presents it to its effector, the p67phox component of the NADPH oxidase.