The NADPH oxidase complex [gp91phox, p22phox, p47phox, p40phox, and the guanosine triphosphatase (GTPase) Rac] is responsible for the production of superoxide radicals in phagocytes during killing of engulfed microorganisms. The cytochrome b559 part of the complex consisting of gp91phox and p22phox is activated by the cytosolic factors p47phox, p40phox, and Rac. This activation can be achieved in vitro with phagocyte membranes supplemented only with these cytosolic factors in the presence of an anioinic amphiphile. Prenylated Rac can substitute for the amphiphile and for p47phox in the in vitro system. Sigal et al. reconstituted the NADPH complex in vitro and determined that prenylated Rac1 bound to guanosine diphosphate (GDP) was able to stimulate oxidase activity in the presence of the functional fragment of the guanine exchange factor (GEF) Trio. However, the stimulation did not require the presence of guanosine triphosphate (GTP) or guanine exchange and the presence of nonhydrolyzable GTP analogs prevented Trio stimulation of Rac-mediated oxidase activation. Using mutants of Rac, the authors determined that Trio binding was required and that a conformational change associated with guanine exchange was required for stimulation of oxidase activity by prenylated Rac. Thus, the authors propose that Trio stimulates a conformational change, but not guanine nucleotide exchange, in Rac that alters Rac's affinity for the NADPH oxidase complex components allowing Rac to recruit the cytosolic factors to the catalytic subunits of the NADPH complex.
N. Sigal, Y. Gorzalczany, R. Sarfstein, C. Weinbaum, Y. Zheng, E. Pick, The guanine nucleotide exchange factor Trio activates the phagocyte NADPH oxidase in the absence of GDP to GTP exchange on Rac. J. Biol. Chem. 278, 4854-4861 (2003). [Abstract] [Full Text]