RhoGDI-3 regulates RhoG and targets this protein to the Golgi complex through its unique N-terminal domain.

Guanine nucleotide dissociation inhibitors (GDIs) regulate both GDP/GTP and membrane association/dissociation cycles of Rho/Rac and Rab proteins.RhoGDI-3 is distinguishable from other rhoGDI proteins by its partial association with a detergent-resistant subcellular fraction. Here, we investigate the activity of this unusual rhoGDI using confocal laser scanning microscopy, immuno-isolation, and rhoGDI-3 mutants. We establish that the noncytosolic fraction of rhoGDI-3 is associated with the Golgi apparatus. The domain involved in this association is the unique N-terminal segment of rhoGDI-3 predicted to form an amphipathic alpha helix. This peptide is indispensable for Golgi association of rhoGDI-3 and sufficient to address a green fluorescent protein to the Golgi apparatus. Site-directed mutations, decreasing the hydrophobic surface of the helix, localize rhoGDI-3 into the cytoplasm. We establish that rhoGDI-3 is able to inhibit activation of the RhoG protein and to target this protein to the Golgi apparatus. Furthermore, we demonstrate the importance of the rhoGDI-3 N-terminal segment for both Golgi targeting and stability of the cytoplasmic RhoG/rhoGDI-3 complex. RhoGDI-3 is the first example of a GDI directly involved in the delivery of a Rho protein to a specific subcellular compartment.