Liposomes comprising anionic but not neutral phospholipids cause dissociation of Rac(1 or 2)·RhoGDI complexes and support amphiphile-independent NADPH oxidase activation by such complexes

Activation of the phagocyte NADPH oxidase involves the assembly of a membrane-localized cytochrome b₅₅₉ with the cytosolic components p47^phox, p67^phox, p40^phox, and the GTPase Rac (1 or 2). In resting phagocytes, Rac is found in the cytosol as a prenylated protein in the GDP-bound form, associated with the Rho GDP dissociation inhibitor (RhoGDI). In the process of NADPH oxidase activation, Rac is dissociated from RhoGDI and translocates to the membrane, in concert with the other cytosolic components. The mechanism responsible for dissociation of Rac from RhoGDI is poorly understood. We generated Rac(1 or 2)·RhoGDI complexes in vitro from recombinant Rac(1 or 2), prenylated enzymatically, and recombinant RhoGDI, and purified these by anion exchange chromatography. Exposing Rac(1 or 2)(GDP)·RhoGDI complexes to liposomes containing four different anionic phospholipids caused the dissociation of Rac(1 or 2)(GDP) from RhoGDI and its binding to the anionic liposomes. Rac2(GDP)·RhoGDI complexes were more resistant to dissociation, reflecting the lesser positive charge of Rac2. Liposomes consisting of neutral phospholipid did not cause dissociation of Rac(1 or 2)·RhoGDI complexes. Rac1 exchanged to the hydrolysis-resistant GTP analogue, GMPPNP, associated with RhoGDI with lower affinity than Rac1(GDP) and Rac1(GMPPNP)·RhoGDI complexes were more readily dissociated by anionic liposomes. Rac1(GMPPNP)·RhoGDI complexes elicited NADPH oxidase activation in native phagocyte membrane liposomes in the presence of p67^phox, without the need for an anionic amphiphile, as activator. Both Rac1(GDP)·RhoGDI and Rac1(GMPPNP)·RhoGDI complexes elicited amphiphile-independent, p67^phox-dependent NADPH oxidase activation in phagocyte membrane liposomes enriched in anionic phospholipids but not in membrane liposomes enriched in neutral phospholipids.