Activation of the superoxide-producing NADPH oxidase Nox1 requires both the 
organizer protein Noxo1 and the activator protein Noxa1. Here we describe an 
alternative splicing form of Noxo1, Noxo1gamma, which is expressed in the testis 
and fetal brain. The Noxo1gamma protein contains an additional five amino acids 
in the N-terminal PX domain, a phosphoinositide-binding module; the domain plays 
an essential role in supporting superoxide production by NADPH oxidase (Nox) 
family oxidases including Nox1, gp91(phox)/Nox2, and Nox3, as shown in this 
study. The PX domain isolated from Noxo1gamma shows a lower affinity for 
phosphoinositides than that from the classical splicing form Noxo1beta. 
Consistent with this, in resting cells, Noxo1gamma is poorly localized to the 
membrane, and thus less effective in activating Nox1 than Noxo1beta, which is 
constitutively present at the membrane. On the other hand, cell stimulation with 
phorbol 12-myristate 13-acetate (PMA), an activator of Nox1-3, facilitates 
membrane translocation of Noxo1gamma; as a result, Noxo1gamma is equivalent to 
Noxo1beta in Nox1 activation in PMA-stimulated cells. The effect of the 
five-amino-acid insertion in the Noxo1 PX domain appears to depend on the type 
of Nox; in activation of gp91(phox)/Nox2, Noxo1gamma is less active than 
Noxo1beta even in the presence of PMA, whereas Noxo1gamma and Noxo1beta support 
the superoxide-producing activity of Nox3 to the same extent in a manner 
independent of cell stimulation.