Activation of the phagocyte NADPH oxidase by Rac guanine nucleotide exchange factors in conjunction with ATP and nucleoside diphosphate kinase

Activation of the phagocyte NADPH oxidase is the consequence of the assembly of membranal cytochrome b(559) with the cytosolic components p47(phox), p67(phox), and the GTPase Rac and is mimicked by a cell-free system comprising these components and an activator. We designed a variant of this system, consisting of membranes, p67(phox), prenylated Rac1-GDP, and the Rac-specific guanine nucleotide exchange factor (GEF) Trio, in which oxidase activation is induced in the absence of an activator and p47(phox). We now show that: 1) Trio and another Rae GEF (Tiam1) act by inducing GDP to GTP exchange on prenylated Rac1-GDP and that our earlier assertion that activation is GTP-independent is explained by contamination of p67(phox) preparations with GTP and/or ATP. 2) Oxidase activation by Rae GEFs is supported not only by GTP but also by ATP. 3) Non-hydrolysable GTP analogs are active, whereas ATP analogs, incapable of gamma-phosphoryl transfer, are inactive. 4) The ability of ATP to support GEF-induced oxidase activation is explained by ATP serving as a gamma-phosphoryl donor for a membrane-localized nucleoside diphosphate kinase (NDPK), converting GDP to GTP. 5) The existence of a NDPK in macrophage membranes is proven by functional, enzymatic, and immunologic criteria. 6) NDPK acts on free GDP, and the newly formed GTP is bound again to Rac. 7) Free GDP is derived exclusively by dissociation from prenylated Rac1-GDP, mediated by GEF. NDPK and GEF appear to be functionally linked in the sense that the availability of GDP, serving as substrate for NDPK, is dependent on the level of activity of GEF.