NOXO1, regulation of lipid binding, localization, and activation of Nox1 by the Phox homology (PX) domain.

NOXO1 (Nox organizing protein 1) and NOXA1 (Nox activating protein 1) are homologs of p47phox and p67phox. p47phox functions in phagocytes as an essential organizing protein mediating the binding of other regulatory proteins during activation of the phagocyte oxidase, and its translocation to the membrane is triggered upon cell activation by hyperphosphorylation, which relieves autoinhibition of SH3 and PX domains. NOXO1 lacks an autoinhibitory region and phosphorylation sites that are present in p47phox. Co-transfection of Nox1, NOXO1, and NOXA1 reconstitutes ROS (reactive oxygen species) generation in HEK293 cells in the absence of cell stimulation. NOXO1 binds to the phosphatidylinositol (PtdIns) lipids PtdIns 3,5-P2, PtdIns 5-P, and PtdIns 4-P. Unlike p47phox, which is located in the cytosol of resting cells and translocates to the plasma membrane where gp91phox is located, NOXO1 co-localizes with Nox1 in the membranes of resting cells. This localization of NOXO1 is dictated by its PX domain, since this domain but not the remainder of the molecule localizes to membranes. A point mutation in the PX domain of holo-NOXO1 decreases lipid binding resulting in cytosolic localization and also inhibits NOXO1-activation of Nox1. Thus, in transfected HEK293 cells, NOXO1 and NOXA1 activate Nox1 without the need for agonist activation, and this is mediated in part by binding of the NOXO1 PX domain to membrane lipids.