p67phoxbinds to a newly identified site in Nox2 following the disengagement of an intramolecular bond—Canaan sighted?

Clinical and molecular significance of flow cytometric analysis for reactive oxygen species production and residual p67phox expression in p67phox-deficient chronic granulomatous disease

Chronic granulomatous disease (CGD) is a primary immunodeficiency disease caused by molecular defects in nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. p67phox-CGD is an autosomal recessive CGD, which is caused by a defect in the cytosolic components of NADPH oxidase, p67phox, encoded by NCF2. We previously established a flow cytometric analysis for p67phox expression, which allows accurate assessment of residual protein expression in p67phox-CGD. We evaluated the correlation between oxidase function and p67phox expression, and assessed the relevancy to genotypes and clinical phenotypes in 11 patients with p67phox-CGD. Reactive oxygen species (ROS) production by granulocytes was evaluated using dihydrorhodamine-1,2,3 (DHR) assays. p67phox expression was evaluated in the monocyte population. DHR activity and p67phox expression were significantly correlated (r = 0.718, p < 0.0162). Additionally, DHR activity and p67phox expression were significantly higher in patients carrying one missense variant in combination with one nonsense or frameshift variant in the NCF2 gene than in patients with only null variants. The available clinical parameters of our patients (i.e., age at disease onset, number of infectious episodes, and each infection complication) were not linked with DHR activity or p67phox expression levels. In summary, our flow cytometric analysis revealed a significant correlation between residual ROS production and p67phox expression. More deleterious NCF2 genotypes were associated with lower levels of DHR activity and p67phox expression. DHR assays and protein expression analysis by using flow cytometry may be relevant strategies for predicting the genotypes of p67phox-CGD.

Structural basis for EROS binding to human phagocyte NADPH oxidase NOX2

Essential for reactive oxygen species (EROS) protein is a recently identified molecular chaperone of NOX2 (gp91phox), the catalytic subunit of phagocyte NADPH oxidase. Deficiency in EROS is a recently identified cause for chronic granulomatous disease, a genetic disorder with recurrent bacterial and fungal infections. Here, we report a cryo-EM structure of the EROS-NOX2-p22phox heterotrimeric complex at an overall resolution of 3.56Å. EROS and p22phox are situated on the opposite sides of NOX2, and there is no direct contact between them. EROS associates with NOX2 through two antiparallel transmembrane (TM) α-helices and multiple β-strands that form hydrogen bonds with the cytoplasmic domain of NOX2. EROS binding induces a 79° upward bend of TM2 and a 48° backward rotation of the lower part of TM6 in NOX2, resulting in an increase in the distance between the two hemes and a shift of the binding site for flavin adenine dinucleotide (FAD). These conformational changes are expected to compromise superoxide production by NOX2, suggesting that the EROS-bound NOX2 is in a protected state against activation. Phorbol myristate acetate, an activator of NOX2 in vitro, is able to induce dissociation of NOX2 from EROS with concurrent increase in FAD binding and superoxide production in a transfected COS-7 model. In differentiated neutrophil-like HL-60, the majority of NOX2 on the cell surface is dissociated with EROS. Further studies are required to delineate how EROS dissociates from NOX2 during its transport to cell surface, which may be a potential mechanism for regulation of NOX2 activation.

Structure of the core human NADPH oxidase NOX2

NOX2 is the prototypical member of the NADPH oxidase NOX superfamily and produces superoxide (O₂^•-), a key reactive oxygen species (ROS) that is essential in innate and adaptive immunity. Mutations that lead to deficiency in NOX2 activity correlate with increased susceptibility to bacterial and fungal infections, resulting in chronic granulomatous disease. The core of NOX2 is formed by a heterodimeric transmembrane complex composed of NOX2 (formerly gp91) and p22, but a detailed description of its structural architecture is lacking. Here, we present the structure of the human NOX2 core complex bound to a selective anti-NOX2 antibody fragment. The core complex reveals an intricate extracellular topology of NOX2, a four-transmembrane fold of the p22 subunit, and an extensive transmembrane interface which provides insights into NOX2 assembly and activation. Functional assays uncover an inhibitory activity of the 7G5 antibody mediated by internalization-dependent and internalization-independent mechanisms. Overall, our results provide insights into the NOX2 core complex architecture, disease-causing mutations, and potential avenues for selective NOX2 pharmacological modulation.

Starting-NOX2-Up: Rac unrolls p67phox

Discussion on the molecular mechanism of phagocyte NADPH oxidase activation.

The molecular basis of Rac-GTP action-promoting binding of p67phox to Nox2 by disengaging the β hairpin from downstream residues

p67^phox fulfils a key role in the assembly/activation of the NADPH oxidase by direct interaction with Nox2. We proposed that Rac-GTP serves both as a carrier of p67^phox to the membrane and an inducer of a conformational change enhancing its affinity for Nox2. This study provides evidence for the latter function: (i) oxidase activation was inhibited by p67^phox peptides (106-120) and (181-195), corresponding to the β hairpin and to a downstream region engaged in intramolecular bonds with the β hairpin, respectively; (ii) deletion of residues 181-193 and point mutations Q115R or K181E resulted in selective binding of p67^phox to Nox2 peptide (369-383); (iii) both deletion and point mutations led to a change in p67^phox , expressed in increased apparent molecular weights; (iv) p67^phox was bound to p67^phox peptide (181-195) and to a cluster of peptides (residues 97-117), supporting the participation of selected residues within these sequences in intramolecular bonds; (v) p67^phox failed to bind to Nox2 peptide (369-383), following interaction with Rac1-GTP, but a (p67^phox -Rac1-GTP) chimera exhibited marked binding to the peptide, similar to that of p67^phox deletion and point mutants; and (vi) size exclusion chromatography of the chimera revealed its partition in monomeric and polymeric forms, with binding to Nox2 peptide (369-383) restricted to polymers. The molecular basis of Rac-GTP action entails unmasking of a previously hidden Nox2-binding site in p67^phox , following disengagement of the β hairpin from more C-terminal residues. The domain in Nox2 binding the "modified" p67^phox comprises residues within the 369-383 sequence in the cytosolic dehydrogenase region.

p67phox -derived self-assembled peptides prevent Nox2 NADPH oxidase activation by an auto-inhibitory mechanism

Activation of the Nox2-dependent NADPH oxidase is the result of a conformational change in Nox2 induced by interaction with the cytosolic component p67^phox . In preliminary work we identified a cluster of overlapping 15-mer synthetic peptides, corresponding to p67^phox residues 259-279, which inhibited oxidase activity in an in vitro, cell-free assay, but the results did not point to a competitive mechanism. We recently identified an auto-inhibitory intramolecular bond in p67^phox , one extremity of which was located within the 259-279 sequence, and we hypothesized that inhibition by exogenous peptides might mimic intrinsic auto-inhibition. In this study, we found that: (i) progressive N- and C-terminal truncation of inhibitory p67^phox peptides, corresponding to residues 259-273 and 265-279, revealed that inhibitory ability correlated with the presence of residues ²⁶⁵ NIVFVL²⁷⁰ , exposed at either the N- or C-termini of the peptides; (ii) inhibition of oxidase activity was associated exclusively with self-assembled peptides, which pelleted upon centrifugation at 12,000 ×g; (iii) self-assembled p67^phox peptides inhibited oxidase activity by specific binding of p67^phox and the ensuing depletion of this component, essential for interaction with Nox2; and (iv) peptides subjected to scrambling or reversing the order of residues in NIVFVL retained the propensity for self-assembly, oxidase inhibitory ability, and specific binding of p67^phox , indicating that the dominant parameter was the hydrophobic character of five of the six residues. This appears to be the first description of inhibition of oxidase activity by self-assembled peptides derived from an oxidase component, acting by an auto-inhibitory mechanism.