The domain organization of p67 phox, a protein required for activation of the superoxide-producing NADPH oxidase in phagocytes.

The phagocyte NADPH oxidase, crucial for innate immunity, is dormant in resting cells, but becomes activated during phagocytosis to produce superoxide, a precursor of microbicidal oxidants. In activation of the oxidase, the multidomain protein p67(phox)plays a central role: it translocates to the membrane as a ternary complex with p47(phox)and p40(phox), and interacts with the small GTPase Rac to assemble with the membrane-integrated catalytic protein gp91(phox), leading to superoxide production. Here we show, using small-angle X-ray scattering (SAXS) analysis, that p67(phox)adopts an elongated conformation when it exists not only as a monomer but also as the heterotrimer. Although p67(phox)harbors an N-terminal TPR domain for binding to Rac and a p40(phox)-interacting PB1 domain, followed by an SH3 domain that associates with p47(phox), the present model suggests that no or few apparent associations occur between the domains. The positions of the protein-interaction domains in p67(phox)contribute to activation of the phagocyte NADPH oxidase: the first SH3 domain that is located between the TPR and PB1 domains positively regulates oxidase activation only when it is present at the correct position; the PB1 domain placed at this SH3 domain position inhibits the oxidase by interacting with p40(phox).