Novel Human Homologues of p47^phox and p67^phox Participate in Activation of Superoxide-producing NADPH Oxidases

The catalytic core of a superoxide-producing NADPH oxidase (Nox) in phagocytes is gp91^phox/Nox2, a membrane-integrated protein that forms a heterodimer with p22^phox to constitute flavocytochrome b ₅₅₈. The cytochrome becomes activated by interacting with the adaptor proteins p47^phox and p67^phox as well as the small GTPase Rac. Here we describe the cloning of human cDNAs for novel proteins homologous to p47^phox and p67^phox, designated p41 ^nox and p51^nox, respectively; the former is encoded by NOXO1 (Nox organizer 1), and the latter is encoded by NOXA1 (Nox activator 1). The novel homologue p41^nox interacts with p227^phox via the two tandem SH3 domains, as does p47^phox. The protein p51 ^nox as well as p67^phox can form a complex with p47 ^phox and with p41^nox via the C-terminal SH3 domain and binds to GTP-bound Rac via the N-terminal domain containing four tetratricopeptide repeat motifs. These bindings seem to play important roles, since p47^phox and p67^phox activate the phagocyte oxidase via the same interactions. Indeed, p41^nox and p51^nox are capable of replacing the corresponding classical homologue in activation of gp91^phox. Nox1, a homologue of gp91^phox, also can be activated in cells, when it is coexpressed with p41^nox and p51 ^nox, with p41^nox and p67^phox, or with p47 ^phox and p51^nox; in the former two cases, Nox1 is partially activated without any stimulants added, suggesting that p41 ^nox is normally in an active state. Thus, the novel homologues p41^nox and p51^nox likely function together or in combination with a classical one, thereby activating the two Nox family oxidases.