Role of Src homology 3 domains in assembly and activation of the phagocyte NADPH oxidase

The phagocyte NADPH oxidase, dormant in resting cells, is activated during phagocytosis to produce superoxide, a precursor of microbicidal oxidants. The activated oxidase is a complex of membrane-integrated cytochrome b₅₅₈, composed of 91-kDa (gp91(phox)) and 22-kDa (p22(phox)) subunits, and two cytosolic factors (p47(phox) and p67(phox)), each containing two Src homology 3 (SH3) domains. Here we show that the region of the tandem SH3 domains of p47(phox) (p47-SH3) expressed as a glutathione S-transferase fusion protein inhibits the superoxide production in a cell-free system, indicating involvement of the domains in the activation. Furthermore, we find that arachidonic acid and sodium dodecyl sulfate, activators of the oxidase in vitro, cause exposure of p47-SH3, which has probably been masked by the C- terminal region of this protein in a resting state. The unmasking of p47-SH3 appears to play a crucial role in the assembly of the oxidase components, because p47-SH3 binds to both p22(phox) and p67(phox) but fails to interact with a mutant p22(phox) carrying a Pro-156 → Gln substitution in a proline- rich region, which has been found in a patient with chronic granulomatous disease. Based on the observations, we propose a signal-transducing mechanism whereby normally inaccessible SH3 domains become exposed upon activation to interact with their target proteins.