Molecular mechanism for activation of the phagocyte NADPH oxidase involued in host defense

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The enzymatic core of the phagocyte NADPH oxidase, that participates in host defense, is cytochrome b_<558> in the membrane. Activation of this enzyme requires stimulus-induced membrane translocation of SH3 domain containing cytosolic proteins, namely p47^<phox>, p67^<phox>, and p40^<phox> to interact with cytochrome b_<558>. We studied mechanism for the oxidase activation obtained the following results.(1) We determined the three-dimensional structure of the PX/PB2 domain a novel module that we had found in the N-termini of p47^<phox> and p40^<phox>. Furthermore, we found that the PX/PB2 domain has an activity to bind to phosphoinositides, and that the PX domain of p47^<phox> is essential for membrane translocation of this protein and activation of the NADPH oxidase. In addition, we also clarified that the lipid-binding activity of the p47^<phox> PX domain is negatively regulated by intramolecular interaction with its SH3 domains.(2) We found that p40^<phox> associates with p67^<phox> in resting phagocytes via a novel protein-protein interaction : the PB1 domain of p67^<phox> recognizes and binds to the PC motif of p40^<phox>. We also clarified that this modular interaction also occurs in Bem1p and Cdc24p, signaling proteins in the budding yeast, and plays a crucial role in polarity establishment, and determined the three-dimensional structure of the PB1 domain of Bem1p. Although the role of p40^<phox> had remained unknown, our study revealed that p40^<phox> enhances membrane translocation of p67^<phox> and P47^<phox> via the PB1-PC interaction with p67^<phox>, thereby positively regulating activation of the phagocyte NADPH oxidase.