The PC motif: a novel and evolutionarily conserved sequence involved in interaction between p40phox and p67phox, SH3 domain-containing cytosolic factors of the phagocyte NADPH oxidase

The role of NADPH oxidases in infectious and inflammatory diseases

Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOX) are enzymes that generate superoxide or hydrogen peroxide from molecular oxygen utilizing NADPH as an electron donor. There are seven enzymes in the NOX family: NOX1-5 and dual oxidase (DUOX) 1-2. NOX enzymes in humans play important roles in diverse biological functions and vary in expression from tissue to tissue. Importantly, NOX2 is involved in regulating many aspects of innate and adaptive immunity, including regulation of type I interferons, the inflammasome, phagocytosis, antigen processing and presentation, and cell signaling. DUOX1 and DUOX2 play important roles in innate immune defenses at epithelial barriers. This review discusses the role of NOX enzymes in normal physiological processes as well as in disease. NOX enzymes are important in autoimmune diseases like type 1 diabetes and have also been implicated in acute lung injury caused by infection with SARS-CoV-2. Targeting NOX enzymes directly or through scavenging free radicals may be useful therapies for autoimmunity and acute lung injury where oxidative stress contributes to pathology.

Phylogenetic and Structural Analysis of NIN-Like Proteins With a Type I/II PB1 Domain That Regulates Oligomerization for Nitrate Response

Absorption of macronutrients such as nitrogen is a critical process for land plants. There is little information available on the correlation between the root evolution of land plants and the protein regulation of nitrogen absorption and responses. NIN-like protein (NLP) transcription factors contain a Phox and Bem1 (PB1) domain, which may regulate nitrate-response genes and seem to be involved in the adaptation to growing on land in terms of plant root development. In this report, we reveal the NLP phylogeny in land plants and the origin of NLP genes that may be involved in the nitrate-signaling pathway. Our NLP phylogeny showed that duplication of NLP genes occurred before divergence of chlorophyte and land plants. Duplicated NLP genes may lost in most chlorophyte lineages. The NLP genes of bryophytes were initially monophyletic, but this was followed by divergence of lycophyte NLP genes and then angiosperm NLP genes. Among those identified NLP genes, PB1, a protein-protein interaction domain was identified across our phylogeny. To understand how protein-protein interaction mediate via PB1 domain, we examined the PB1 domain of Arabidopsis thaliana NLP7 (AtNLP7) in terms of its molecular oligomerization and function as representative. Based on the structure of the PB1 domain, determined using small-angle x-ray scattering (SAXS) and site-directed mutagenesis, we found that the NLP7 PB1 protein forms oligomers and that several key residues (K867 and D909/D911/E913/D922 in the OPCA motif) play a pivotal role in the oligomerization of NLP7 proteins. The fact that these residues are all conserved across land plant lineages means that this oligomerization may have evolved after the common ancestor of extant land plants colonized the land. It would then have rapidly become established across land-plant lineages in order to mediate protein-protein interactions in the nitrate-signaling pathway.

Deep Evolutionary History of the Phox and Bem1 (PB1) Domain Across Eukaryotes

Protein oligomerization is a fundamental process to build complex functional modules. Domains that facilitate the oligomerization process are diverse and widespread in nature across all kingdoms of life. One such domain is the Phox and Bem1 (PB1) domain, which is functionally well-studied in the animal kingdom. However, beyond animals, neither the origin nor the evolutionary patterns of PB1-containing proteins are understood. While PB1 domain proteins have been found in other kingdoms including plants, it is unclear how these relate to animal PB1 proteins. To address this question, we utilized large transcriptome datasets along with the proteomes of a broad range of species. We discovered eight PB1 domain-containing protein families in plants, along with four each in Protozoa and Fungi and three families in Chromista. Studying the deep evolutionary history of PB1 domains throughout eukaryotes revealed the presence of at least two, but likely three, ancestral PB1 copies in the Last Eukaryotic Common Ancestor (LECA). These three ancestral copies gave rise to multiple orthologues later in evolution. Analyzing the sequence and secondary structure properties of plant PB1 domains from all the eight families showed their common ubiquitin β-grasp fold, despite poor sequence identity. Tertiary structural models of these plant PB1 families, combined with Random Forest based classification, indicated family-specific differences attributed to the length of PB1 domain and the proportion of β-sheets. Thus, this study not only identifies novel PB1 families, but also provides an evolutionary basis to understand their diverse functional interactions.

Arachidonic Acid and Nitroarachidonic: Effects on NADPH Oxidase Activity

Arachidonic acid (AA) is a polyunsaturated fatty acid that participates in the inflammatory response mainly through bioactive-lipids formation in macrophages and also in the phagocytic NADPH oxidase 2 (NOX2) activation. NOX2 is the enzyme responsible for a huge superoxide formation in macrophages, essential to eliminate pathogens inside the phagosome. The oxidase is an enzymatic complex comprised of a membrane-bound flavocytochrome b ₅₅₈ (gp91^phox/p22^phox), three cytosolic subunits (p47^phox, p40^phox and p67^phox) and a Rac-GTPase. The enzyme becomes active when macrophages are exposed to appropriate stimuli that trigger the phosphorylation of cytosolic subunits and its migration to plasmatic membrane to form the active complex. It is proposed that AA stimulates NOX2 activity through AA interaction with different components of the NADPH oxidase complex. In inflammatory conditions, there is an increase in reactive oxygen and nitrogen species that results in the production of nitrated derivatives of AA, such as nitroarachidonic acid (NO₂-AA). NO₂-AA is capable to inhibit NOX2 activity by interfering with p47^phox migration to the membrane without affecting phosphorylation of cytosolic proteins. Also, NO₂-AA is capable to interact with protein disulfide isomerase (PDI), which is involved on NOX2 active complex formation. It has been demonstrated that NO₂-AA forms a covalent adduct with PDI that could prevent the interaction with NOX2 and it would explain the inhibitory effects of the fatty acid upon NOX2. Together, current data indicate that AA is an important activator of NOX2 formed in the early events of the inflammatory response, leading to a massive production of oxidants that may, in turn, promote NO₂-AA formation and shutting down the oxidative burst. Hence, AA and its derivatives could have antagonistic roles on NOX2 activity regulation.

The Role of PB1 Domain Proteins in Endothelial Cell Dysfunction and Disease

SIGNIFICANCE

There are a limited number of proteins containing the Phox-Bem1 (PB1) protein interaction domain, and almost all of them play some role in endothelial cell (EC) function, health, and homeostasis.

RECENT ADVANCES

Most of these proteins have been shown to physically interact through PB1-PB1 binding and, as such, are linked together to form complexes that are responsive to hemodynamic force. These complexes range from redox regulation to inflammation to autophagy and back, and they employ multiple feedback mechanisms that are reliant on PB1 domain proteins.

CRITICAL ISSUES

Pathologic roles for PB1 domain-containing proteins have been demonstrated in multiple diseases, including vascular disease, cancer, liver disease, and myriad other concerns. Findings cited in this review show that dimerization of PB1 proteins exerts novel effects on EC function that may be important in multiple cardiovascular diseases, including atherosclerosis, thrombosis, inflammation, and hypertension.

FUTURE DIRECTIONS

As mechanistic understanding of the component pathways (redox regulation, cell polarity, inflammation, atheroprotection, and autophagy) is continually increasing, the larger picture of how these pathways interact with one another is evolving rapidly. We can now evaluate the PB1 domain proteins as a family in the context of multiple phenotypic readouts in EC function as well as evaluate them as drug targets against disease.

Significant role of PB1 and UBA domains in multimerization of Joka2, a selective autophagy cargo receptor from tobacco

Tobacco Joka2 protein is a hybrid homolog of two mammalian selective autophagy cargo receptors, p62 and NBR1. These proteins can directly interact with the members of ATG8 family and the polyubiquitinated cargoes designed for degradation. Function of the selective autophagy cargo receptors relies on their ability to form protein aggregates. It has been shown that the N-terminal PB1 domain of p62 is involved in formation of aggregates, while the UBA domains of p62 and NBR1 have been associated mainly with cargo binding. Here we focus on roles of PB1 and UBA domains in localization and aggregation of Joka2 in plant cells. We show that Joka2 can homodimerize not only through its N-terminal PB1-PB1 interactions but also via interaction between N-terminal PB1 and C-terminal UBA domains. We also demonstrate that Joka2 co-localizes with recombinant ubiquitin and sequestrates it into aggregates and that C-terminal part (containing UBA domains) is sufficient for this effect. Our results indicate that Joka2 accumulates in cytoplasmic aggregates and suggest that in addition to these multimeric forms it also exists in the nucleus and cytoplasm in a monomeric form.

Biochemistry, physiology, and pathophysiology of NADPH oxidases in the cardiovascular system

The NADPH oxidase (Nox) enzymes are critical mediators of cardiovascular physiology and pathophysiology. These proteins are expressed in virtually all cardiovascular cells, and regulate such diverse functions as differentiation, proliferation, apoptosis, senescence, inflammatory responses and oxygen sensing. They target a number of important signaling molecules, including kinases, phosphatases, transcription factors, ion channels, and proteins that regulate the cytoskeleton. Nox enzymes have been implicated in many different cardiovascular pathologies: atherosclerosis, hypertension, cardiac hypertrophy and remodeling, angiogenesis and collateral formation, stroke, and heart failure. In this review, we discuss in detail the biochemistry of Nox enzymes expressed in the cardiovascular system (Nox1, 2, 4, and 5), their roles in cardiovascular cell biology, and their contributions to disease development.

Characterization of superoxide overproduction by the D-Loop(Nox4)-Nox2 cytochrome b(558) in phagocytes-Differential sensitivity to calcium and phosphorylation events

NADPH oxidase is a crucial element of phagocytes involved in microbicidal mechanisms. It becomes active when membrane-bound cytochrome b(558), the redox core, is assembled with cytosolic p47(phox), p67(phox), p40(phox), and rac proteins to produce superoxide, the precursor for generation of toxic reactive oxygen species. In a previous study, we demonstrated that the potential second intracellular loop of Nox2 was essential to maintaining NADPH oxidase activity by controlling electron transfer from FAD to O(2). Moreover, replacement of this loop by the Nox4-D-loop (D-loop(Nox4)-Nox2) in PLB-985 cells induced superoxide overproduction. In the present investigation, we demonstrated that both soluble and particulate stimuli were able to induce this superoxide overproduction. Superoxide overproduction was also observed after phosphatidic acid activation in a purified cell-free-system assay. The highest oxidase activity was obtained after ionomycin and fMLF stimulation. In addition, enhanced sensitivity to Ca(2+) influx was shown by thapsigargin, EDTA, or BTP2 treatment before fMLF activation. Mutated cytochrome b(558) was less dependent on phosphorylation triggered by ERK1/2 during fMLF or PMA stimulation and by PI3K during OpZ stimulation. The superoxide overproduction of the D-loop(Nox4)-Nox2 mutant may come from a change of responsiveness to intracellular Ca(2+) level and to phosphorylation events during oxidase activation. Finally the D-loop(Nox4)-Nox2-PLB-985 cells were more effective against an attenuated strain of Pseudomonas aeruginosa compared to WT-Nox2 cells. The killing mechanism was biphasic, an early step of ROS production that was directly bactericidal, and a second oxidase-independent step related to the amount of ROS produced in the first step.

p47phox molecular activation for assembly of the neutrophil NADPH oxidase complex

The p47(phox) cytosolic factor from neutrophilic NADPH oxidase has always been resistant to crystallogenesis trials due to its modular organization leading to relative flexibility. Hydrogen/deuterium exchange coupled to mass spectrometry was used to obtain structural information on the conformational mechanism that underlies p47(phox) activation. We confirmed a relative opening of the protein with exposure of the SH3 Src loops that are known to bind p22(phox) upon activation. A new surface was shown to be unmasked after activation, representing a potential autoinhibitory surface that may block the interaction of the PX domain with the membrane in the resting state. Within this surface, we identified 2 residues involved in the interaction with the PX domain. The double mutant R162A/D166A showed a higher affinity for specific phospholipids but none for the C-terminal part of p22(phox), reflecting an intermediate conformation between the autoinhibited and activated forms.

Structure, regulation and evolution of Nox-family NADPH oxidases that produce reactive oxygen species

NADPH oxidases of the Nox family exist in various supergroups of eukaryotes but not in prokaryotes, and play crucial roles in a variety of biological processes, such as host defense, signal transduction, and hormone synthesis. In conjunction with NADPH oxidation, Nox enzymes reduce molecular oxygen to superoxide as a primary product, and this is further converted to various reactive oxygen species. The electron-transferring system in Nox is composed of the C-terminal cytoplasmic region homologous to the prokaryotic (and organelle) enzyme ferredoxin reductase and the N-terminal six transmembrane segments containing two hemes, a structure similar to that of cytochrome b of the mitochondrial bc(1) complex. During the course of eukaryote evolution, Nox enzymes have developed regulatory mechanisms, depending on their functions, by inserting a regulatory domain (or motif) into their own sequences or by obtaining a tightly associated protein as a regulatory subunit. For example, one to four Ca(2+)-binding EF-hand motifs are present at the N-termini in several subfamilies, such as the respiratory burst oxidase homolog (Rboh) subfamily in land plants (the supergroup Plantae), the NoxC subfamily in social amoebae (the Amoebozoa), and the Nox5 and dual oxidase (Duox) subfamilies in animals (the Opisthokonta), whereas an SH3 domain is inserted into the ferredoxin-NADP(+) reductase region of two Nox enzymes in Naegleria gruberi, a unicellular organism that belongs to the supergroup Excavata. Members of the Nox1-4 subfamily in animals form a stable heterodimer with the membrane protein p22(phox), which functions as a docking site for the SH3 domain-containing regulatory proteins p47(phox), p67(phox), and p40(phox); the small GTPase Rac binds to p67(phox) (or its homologous protein), which serves as a switch for Nox activation. Similarly, Rac activates the fungal NoxA via binding to the p67(phox)-like protein Nox regulator (NoxR). In plants, on the other hand, this GTPase directly interacts with the N-terminus of Rboh, leading to superoxide production. Here I describe the regulation of Nox-family oxidases on the basis of three-dimensional structures and evolutionary conservation.

Phosphatidylinositol 3-phosphate-dependent and -independent functions of p40phox in activation of the neutrophil NADPH oxidase

In response to bacterial infection, the neutrophil NADPH oxidase assembles on phagolysosomes to catalyze the transfer of electrons from NADPH to oxygen, forming superoxide and downstream reactive oxygen species (ROS). The active oxidase is composed of a membrane-bound cytochrome together with three cytosolic phox proteins, p40(phox), p47(phox), and p67(phox), and the small GTPase Rac2, and is regulated through a process involving protein kinase C, MAPK, and phosphatidylinositol 3-kinase. The role of p40(phox) remains less well defined than those of p47(phox) and p67(phox). We investigated the biological role of p40(phox) in differentiated PLB-985 neutrophils, and we show that depletion of endogenous p40(phox) using lentiviral short hairpin RNA reduces ROS production and impairs bacterial killing under conditions where p67(phox) levels remain constant. Biochemical studies using a cytosol-reconstituted permeabilized human neutrophil cores system that recapitulates intracellular oxidase activation revealed that depletion of p40(phox) reduces both the maximal rate and total amount of ROS produced without altering the K(M) value of the oxidase for NADPH. Using a series of mutants, p47PX-p40(phox) chimeras, and deletion constructs, we found that the p40(phox) PX domain has phosphatidylinositol 3-phosphate (PtdIns(3)P)-dependent and -independent functions. Translocation of p67(phox) requires the PX domain but not 3-phosphoinositide binding. Activation of the oxidase by p40(phox), however, requires both PtdIns(3)P binding and an Src homology 3 (SH3) domain competent to bind to poly-Pro ligands. Mutations that disrupt the closed auto-inhibited form of full-length p40(phox) can increase oxidase activity approximately 2.5-fold above that of wild-type p40(phox) but maintain the requirement for PX and SH3 domain function. We present a model where p40(phox) translocates p67(phox) to the region of the cytochrome and subsequently switches the oxidase to an activated state dependent upon PtdIns(3)P and SH3 domain engagement.

Molecular evolution of Phox-related regulatory subunits for NADPH oxidase enzymes

Background

The reactive oxygen-generating NADPH oxidases (Noxes) function in a variety of biological roles, and can be broadly classified into those that are regulated by subunit interactions and those that are regulated by calcium. The prototypical subunit-regulated Nox, Nox2, is the membrane-associated catalytic subunit of the phagocyte NADPH-oxidase. Nox2 forms a heterodimer with the integral membrane protein, p22phox, and this heterodimer binds to the regulatory subunits p47phox, p67phox, p40phox and the small GTPase Rac, triggering superoxide generation. Nox-organizer protein 1 (NOXO1) and Nox-activator 1 (NOXA1), respective homologs of p47phox and p67phox, together with p22phox and Rac, activate Nox1, a non-phagocytic homolog of Nox2. NOXO1 and p22phox also regulate Nox3, whereas Nox4 requires only p22phox. In this study, we have assembled and analyzed amino acid sequences of Nox regulatory subunit orthologs from vertebrates, a urochordate, an echinoderm, a mollusc, a cnidarian, a choanoflagellate, fungi and a slime mold amoeba to investigate the evolutionary history of these subunits.

Results

Ancestral p47phox, p67phox, and p22phox genes are broadly seen in the metazoa, except for the ecdysozoans. The choanoflagellate Monosiga brevicollis, the unicellular organism that is the closest relatives of multicellular animals, encodes early prototypes of p22phox, p47phox as well as the earliest known Nox2-like ancestor of the Nox1-3 subfamily. p67phox- and p47phox-like genes are seen in the sea urchin Strongylocentrotus purpuratus and the limpet Lottia gigantea that also possess Nox2-like co-orthologs of vertebrate Nox1-3. Duplication of primordial p47phox and p67phox genes occurred in vertebrates, with the duplicated branches evolving into NOXO1 and NOXA1. Analysis of characteristic domains of regulatory subunits suggests a novel view of the evolution of Nox: in fish, p40phox participated in regulating both Nox1 and Nox2, but after the appearance of mammals, Nox1 (but not Nox2) became independent of p40phox. In the fish Oryzias latipes, a NOXO1 ortholog retains an autoinhibitory region that is characteristic of mammalian p47phox, and this was subsequently lost from NOXO1 in later vertebrates. Detailed amino acid sequence comparisons identified both putative key residues conserved in characteristic domains and previously unidentified conserved regions. Also, candidate organizer/activator proteins in fungi and amoeba are identified and hypothetical activation models are suggested.

Conclusion

This is the first report to provide the comprehensive view of the molecular evolution of regulatory subunits for Nox enzymes. This approach provides clues for understanding the evolution of biochemical and physiological functions for regulatory-subunit-dependent Nox enzymes.

Structure and Function of the PB1 Domain, a Protein Interaction Module Conserved in Animals, Fungi, Amoebas, and Plants

Proteins containing the PB1 domain, a protein interaction module conserved in animals, fungi, amoebas, and plants, participate in diverse biological processes. The PB1 domains adopt a ubiquitin-like beta-grasp fold, containing two alpha helices and a mixed five-stranded beta sheet, and are classified into groups harboring an acidic OPCA motif (type I), the invariant lysine residue on the first beta strand (type II), or both (type I/II). The OPCA motif of a type I PB1 domain forms salt bridges with basic residues, especially the conserved lysine, of a type II PB1 domain, thereby mediating a specific PB1-PB1 heterodimerization, whereas additional contacts contribute to high affinity and specificity of the modular interaction. The canonical PB1 dimerization is required for the formation of complexes between p40(phox) and p67(phox) (for activation of the NADPH oxidase crucial for mammalian host defense), between the scaffold Bem1 and the guanine nucleotide exchange factor Cdc24 (for polarity establishment in yeasts), and between the polarity protein Par6 and atypical protein kinase C (for cell polarization in animal cells), as well as for the interaction between the mitogen-activated protein kinase kinase kinases MEKK2 or MEKK3 and the downstream target mitogen-activated protein kinase kinase MEK5 (for early cardiovascular development in mammals). PB1 domains can also mediate interactions with other protein domains. For example, an intramolecular interaction between the PB1 and PX domains of p40(phox) regulates phagosomal targeting of the microbicidal NADPH oxidase; the PB1 domain of MEK5 is likely responsible for binding to the downstream kinase ERK5, which lacks a PB1 domain; and the scaffold protein Nbr1 associates through a PB1-containing region with titin, a sarcomere protein without a PB1 domain. This Review describes various aspects of PB1 domains at the molecular and cellular levels.

A regulated adaptor function of p40phox: distinct p67phox membrane targeting by p40phox and by p47phox

In the phagocytic cell, NADPH oxidase (Nox2) system, cytoplasmic regulators (p47(phox), p67(phox), p40(phox), and Rac) translocate and associate with the membrane-spanning flavocytochrome b(558), leading to activation of superoxide production. We examined membrane targeting of phox proteins and explored conformational changes in p40(phox) that regulate its translocation to membranes upon stimulation. GFP-p40(phox) translocates to early endosomes, whereas GFP-p47(phox) translocates to the plasma membrane in response to arachidonic acid. In contrast, GFP-p67(phox) does not translocate to membranes when expressed alone, but it is dependent on p40(phox) and p47(phox) for its translocation to early endosomes or the plasma membrane, respectively. Translocation of GFP-p40(phox) or GFP-p47(phox) to their respective membrane-targeting sites is abolished by mutations in their phox (PX) domains that disrupt their interactions with their cognate phospholipid ligands. Furthermore, GFP-p67(phox) translocation to either membrane is abolished by mutations that disrupt its interaction with p40(phox) or p47(phox). Finally, we detected a head-to-tail (PX-Phox and Bem1 [PB1] domain) intramolecular interaction within p40(phox) in its resting state by deletion mutagenesis, cell localization, and binding experiments, suggesting that its PX domain is inaccessible to interact with phosphatidylinositol 3-phosphate without cell stimulation. Thus, both p40(phox) and p47(phox) function as diverse p67(phox) "carrier proteins" regulated by the unmasking of membrane-targeting domains in distinct mechanisms.

Structural organization of the neutrophil NADPH oxidase: phosphorylation and translocation during priming and activation

The reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is part of the microbicidal arsenal used by human polymorphonuclear neutrophils (PMNs) to eradicate invading pathogens. The production of a superoxide anion (O2-) into the phagolysosome is the precursor for the generation of more potent products, such as hydrogen peroxide and hypochlorite. However, this production of O2- is dependent on translocation of the oxidase subunits, including gp91phox, p22phox, p47phox, p67phox, p40phox, and Rac2 from the cytosol or specific granules to the plasma membrane. In response to an external stimuli, PMNs change from a resting, nonadhesive state to a primed, adherent phenotype, which allows for margination from the vasculature into the tissue and chemotaxis to the site of infection upon activation. Depending on the stimuli, primed PMNs display altered structural organization of the NADPH oxidase, in that there is phosphorylation of the oxidase subunits and/or translocation from the cytosol to the plasma or granular membrane, but there is not the complete assembly required for O2- generation. Activation of PMNs is the complete assembly of the membrane-linked and cytosolic NADPH oxidase components on a PMN membrane, the plasma or granular membrane. This review will discuss the individual components associated with the NADPH oxidase complex and the function of each of these units in each physiologic stage of the PMN: rested, primed, and activated.

p40phox: The last NADPH oxidase subunit

The phagocytic NADPH-oxidase is a multiprotein system activated during the inflammatory response to produce superoxide anion (O2-), which is the substrate for formation of additional reactive oxygen species (ROS). The importance of this system for innate immunity is established by chronic granulomatous disease (CGD), a primary immunodeficiency caused by defects in the NADPH oxidase. In this review, we present and discuss recent knowledge about p40phox, the last NADPH oxidase component to be identified. Furthermore, its interaction with cellular pathways outside of the NADPH oxidase is discussed. Described in this review is evidence that p40phox participates in NADPH oxidase dynamics within cells, what is known about its role in the oxidase, the possibility that p40phox participates in non-NADPH oxidase processes in phagocytic and non-phagocytic cells and whether p40phox could mediate a similar function in other NADPH oxidases. An improved understanding of p40phox should provide new insights about NADPH oxidase, the physiology of phagocytic cells and the innate immune system.

Superoxide production in Galleria mellonella hemocytes: identification of proteins homologous to the NADPH oxidase complex of human neutrophils

The insect immune response has a number of structural and functional similarities to the innate immune response of mammals. The objective of the work presented here was to establish the mechanism by which insect hemocytes produce superoxide and to ascertain whether the proteins involved in superoxide production are similar to those involved in the NADPH oxidase-induced superoxide production in human neutrophils. Hemocytes of the greater wax moth (Galleria mellonella) were shown to be capable of phagocytosing bacterial and fungal cells. The kinetics of phagocytosis and microbial killing were similar in the insect hemocytes and human neutrophils. Superoxide production and microbial killing by both cell types were inhibited in the presence of the NADPH oxidase inhibitor diphenyleneiodonium chloride. Immunoblotting of G. mellonella hemocytes with antibodies raised against human neutrophil phox proteins revealed the presence of proteins homologous to gp91phox, p67phox, p47phox, and the GTP-binding protein rac 2. A protein equivalent to p40phox was not detected in insect hemocytes. Immunofluorescence analysis localized insect 47-kDa and 67-kDa proteins throughout the cytosol and in the perinuclear region. Hemocyte 67-kDa and 47-kDa proteins were immunoprecipitated and analyzed by matrix-assisted laser desorption ionization--time of flight analysis. The results revealed that the hemocyte 67-kDa and 47-kDa proteins contained peptides matching those of p67phox and p47phox of human neutrophils. The results presented here indicate that insect hemocytes phagocytose and kill bacterial and fungal cells by a mechanism similar to the mechanism used by human neutrophils via the production of superoxide. We identified proteins homologous to a number of proteins essential for superoxide production in human neutrophils and demonstrated that significant regions of the 67-kDa and 47-kDa insect proteins are identical to regions of the p67phox and p47phox proteins of neutrophils.

The solution structure of an N-terminally truncated version of the yeast CDC24p PB1 domain shows a different β-sheet topology

Phox and Bem1 (PB1) domains mediate protein-protein interactions via the formation of homo- or hetero-dimers. The C-terminal PB1 domain of yeast cell division cycle 24 (CDC24p), a guanine-nucleotide exchange factor involved in cell polarity establishment, is known to interact with the PB1 domain occurring in bud emergence MSB1 interacting 1 (BEM1p) during the regulation of the yeast budding process via its OPR/PC/AID (OPCA) motif. Here, we present the structure of an N-terminally truncated version of the Sc CDC24p PB1 domain. It shows a different topology of the beta-sheet than the long form. However, the C-terminal part of the structure shows the conserved PB1 domain features including the OPCA motif with a slight rearrangement of helix alpha1. Residues which are important for the heterodimerization with BEM1p are structurally preserved.

Effects of p47 C Terminus Phosphorylations on Binding Interactions with p40 and p67

The neutrophil NADPH oxidase produces superoxide anions in response to infection. This reaction is activated by association of cytosolic factors, p47phox and p67phox, and a small G protein Rac with the membranous flavocytochrome b558. Another cytosolic factor, p40phox, is associated to the complex and is reported to play regulatory roles. Initiation of the NADPH oxidase activation cascade has been reported as consecutive to phosphorylation on serines 359/370 and 379 of the p47phox C terminus. These serines surround a polyproline motif that can interact with the Src homology 3 (SH3) module of p40phox (SH3p40) or the C-terminal SH3 of p67phox (C-SH3p67). The latter one presents a higher affinity in the resting state for p47phox. A change in SH3 binding preference following phosphorylation has been postulated earlier. Here we report the crystal structures of SH3p40 alone or in complex with a 12-residue proline-rich region of p47phox at 1.46 angstrom resolution. Using intrinsic tryptophan fluorescence measurements, we compared the affinity of the strict polyproline motif and the whole C terminus peptide with both SH3p40 and C-SH3p67. These data reveal that SH3p40 can interact with a consensus polyproline motif but also with a noncanonical motif of the p47phox C terminus. The electrostatic surfaces of both SH3 are very different, and therefore the binding preference for C-SH3p67 can be attributed to the polyproline motif recognition and particularly to the Arg-368p47 binding mode. The noncanonical motif contributes equally to interaction with both SH3. The influence of serine phosphorylation on residues 359/370 and 379 on the affinity for both SH3 domains has been checked. We conclude that contrarily to previous suggestions, phosphorylation of Ser-359/370 does not modify the SH3 binding affinity for both SH3, whereas phosphorylation of Ser-379 has a destabilizing effect on both interactions. Other mechanisms than a phosphorylation induced switch between the two SH3 must therefore take place for NADPH oxidase activation cascade to start.

Activation and assembly of the NADPH oxidase: a structural perspective

The NADPH oxidase of professional phagocytes is a crucial component of the innate immune response due to its fundamental role in the production of reactive oxygen species that act as powerful microbicidal agents. The activity of this multi-protein enzyme is dependent on the regulated assembly of the six enzyme subunits at the membrane where oxygen is reduced to superoxide anions. In the resting state, four of the enzyme subunits are maintained in the cytosol, either through auto-inhibitory interactions or through complex formation with accessory proteins that are not part of the active enzyme complex. Multiple inputs are required to disrupt these inhibitory interactions and allow translocation to the membrane and association with the integral membrane components. Protein interaction modules are key regulators of NADPH oxidase assembly, and the protein-protein interactions mediated via these domains have been the target of numerous studies. Many models have been put forward to describe the intricate network of reversible protein interactions that regulate the activity of this enzyme, but an all-encompassing model has so far been elusive. An important step towards an understanding of the molecular basis of NADPH oxidase assembly and activity has been the recent solution of the three-dimensional structures of some of the oxidase components. We will discuss these structures in the present review and attempt to reconcile some of the conflicting models on the basis of the structural information available.

Structure of a Cell Polarity Regulator, a Complex between Atypical PKC and Par6 PB1 Domains

A complex of atypical PKC and Par6 is a common regulator for cell polarity-related processes, which is an essential clue to evolutionary conserved cell polarity regulation. Here, we determined the crystal structure of the complex of PKCiota and Par6alpha PB1 domains to a resolution of 1.5 A. Both PB1 domains adopt a ubiquitin fold. PKCiota PB1 presents an OPR, PC, and AID (OPCA) motif, 28 amino acid residues with acidic and hydrophobic residues, which interacts with the conserved lysine residue of Par6alpha PB1 in a front and back manner. On the interface, several salt bridges are formed including the conserved acidic residues on the OPCA motif of PKCiota PB1 and the conserved lysine residue on the Par6alpha PB1. Structural comparison of the PKCiota and Par6alpha PB1 complex with the p40phox and p67phox PB1 domain complex, subunits of neutrophil NADPH oxidase, reveals that the specific interaction is achieved by tilting the interface so that the insertion or extension in the sequence is engaged in the specificity determinant. The PB1 domain develops the interaction surface on the ubiquitin fold to increase the versatility of molecular interaction.

Free radicals in cell biology

In aerobic cells, free radicals are constantly produced mostly as reactive oxygen species. Once produced, free radicals are removed by antioxidant defenses including enzyme catalase, glutathione peroxidase, and superoxide dismutase. Reactive oxygen species, including nitric oxide and related species, commonly exert a series of useful physiological effects. However, imbalance between prooxidant and antioxidant defenses in favor of prooxidants results in oxidative stress associated with the oxidative modification of biomolecules such as lipids, proteins, and nucleic acids. Alone or in combination with primary ethiological factors, free radicals are involved in a pathogenesis of more than a hundred diseases. This chapter reviews the basic science of some of the potential sources and characteristics of free radicals, as well as antioxidant enzymes. Special attention is paid to the role of free radicals in the pathogenesis of atherosclerosis and immunology-mediated inflammatory reaction.

Assembly of the phagocyte NADPH oxidase

Stimulated phagocytes undergo a burst in respiration whereby molecular oxygen is converted to superoxide anion through the action of an NADPH-dependent oxidase. The multicomponent phagocyte oxidase is unassembled and inactive in resting cells but assembles at the plasma or phagosomal membrane upon phagocyte activation. Oxidase components include flavocytochrome b558, an integral membrane heterodimer comprised of gp91phox and p22phox, and three cytosolic proteins, p47phox, p67phox, and Rac1 or Rac2, depending on the species and phagocytic cell. In a sense, the phagocyte oxidase is spatially regulated, with critical elements segregated in the membrane and cytosol but ready to undergo nearly immediate assembly and activation in response to stimulation. To achieve such spatial regulation, the individual components in the resting phagocyte adopt conformations that mask potentially interactive structural domains that might mediate productive intermolecular associations and oxidase assembly. In response to stimulation, post-translational modifications of the oxidase components release these constraints and thereby render potential interfaces accessible and interactive, resulting in translocation of the cytosolic elements to the membrane where the functional oxidase is assembled and active. This review summarizes data on the structural features of the phagocyte oxidase components and on the agonist-dependent conformational rearrangements that contribute to oxidase assembly and activation.

Structure and regulation of the neutrophil respiratory burst oxidase: comparison with nonphagocyte oxidases

Neutrophils play an essential role in the body's innate defense against pathogens and are one of the primary mediators of the inflammatory response. To defend the host, neutrophils use a wide range of microbicidal products, such as oxidants, microbicidal peptides, and lytic enzymes. The generation of microbicidal oxidants by neutrophils results from the activation of a multiprotein enzyme complex known as the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which is responsible for transferring electrons from NADPH to O2, resulting in the formation of superoxide anion. During oxidase activation, cytosolic oxidase proteins translocate to the phagosome or plasma membrane, where they assemble around a central membrane-bound component known as flavocytochrome b. This process is highly regulated, involving phosphorylation, translocation, and multiple conformational changes. Originally, it was thought that the NADPH oxidase was restricted to phagocytes and used solely in host defense. However, recent studies indicate that similar NADPH oxidase systems are present in a wide variety of nonphagocytic cells. Although the nature of these nonphagocyte NADPH oxidases is still being defined, it is clear that they are functionally distinct from the phagocyte oxidases. It should be noted, however, that structural features of many nonphagocyte oxidase proteins do seem to be similar to those of their phagocyte counterparts. In this review, key structural and functional features of the neutrophil NADPH oxidase and its protein components are described, including a consideration of transcriptional and post-translational regulatory features. Furthermore, relevant details about structural and functional features of various nonphagocyte oxidase proteins will be included for comparison.

Solution Structure of Atypical Protein Kinase C PB1 Domain and Its Mode of Interaction with ZIP/p62 and MEK5

Atypical protein kinase C (aPKC) has been implicated in several signaling pathways such as cell polarity, cell survival, and cell differentiation. In contrast to other PKCs, aPKC is unique in having the PB1 (Phox and Bem 1) domain in the N terminus. The aPKC PB1 domain binds with ZIP/p62, Par6, or MEK5 through a PB1-PB1 domain interaction that controls the localization of aPKC. Here, we determined the three-dimensional structure of the PB1 domain of PKCiota by NMR and found that the PB1 domain adopts a ubiquitin fold. The OPCA (OPR, PC, and AID) motif inserted into the ubiquitin fold was presented as a betabetaalpha fold in which the side chains of conserved Asp residues were oriented to the same direction to form an acidic surface. This structural feature suggested that the acidic surface of the PKCiota PB1 domain interacted with the basic surface of the target PB1 domains, and this was confirmed in the case of the PKCiota-ZIP/p62 complex by mutational analysis. Interestingly, in the PKCiota PB1 domain a conserved lysine residue was located on the side opposite to the OPCA motif-presenting surface, suggesting dual roles for the PKCiota PB1 domain in that it could interact with either the conserved lysine residue or the acidic residues on the OPCA motif of the target PB1 domains.

The NADPH oxidase of professional phagocytes--prototype of the NOX electron transport chain systems

The NADPH oxidase is an electron transport chain in "professional" phagocytic cells that transfers electrons from NADPH in the cytoplasm, across the wall of the phagocytic vacuole, to form superoxide. The electron transporting flavocytochrome b is activated by the integrated function of four cytoplasmic proteins. The antimicrobial function of this system involves pumping K+ into the vacuole through BKCa channels, the effect of which is to elevate the vacuolar pH and activate neutral proteases. A number of homologous systems have been discovered in plants and lower animals as well as in man. Their function remains to be established.

Insight into molecular interactions between two PB1 domains

Specific protein-protein interactions play crucial roles in the regulation of any biological process. Recently, a new protein-protein interaction domain termed PB1 (Phox and Bem1) was identified, which is conserved throughout evolution and present in diverse proteins functioning in signal transduction, cell polarity and survival. Here, we investigated the structure and molecular interactions of the PB1 heterodimer complex composed of the PB1 domains of the yeast proteins Bem1 and Cdc24. A structural model of the Cdc24 PB1 was built by homology modeling and molecular dynamics simulations, and experimentally validated by 15N nuclear Overhauser effect spectroscopy (NOESY)-heteronuclear single quantum coherence (HSQC) analysis. Residues at the interface of the complex for both proteins were identified by NMR titration experiments. A model of the heterodimer was obtained by docking of the two PB1 domains with HADDOCK, which applies ambiguous interaction restraints on residues at the interface to drive the docking procedure. The refined model was validated by site-directed mutagenesis on both Bem1 and Cdc24. Finally, the docking was repeated from the newly published NMR structure of Cdc24, allowing us to assess the performance of homology-based docking. Our results provide insight into the molecular structure of the Bem1-Cdc24 PB1-mediated heterodimer, which allowed identification of critical residues at the binding interface.

Binding of FAD to cytochrome b558 is facilitated during activation of the phagocyte NADPH oxidase, leading to superoxide production

The superoxide-producing phagocyte NADPH oxidase can be reconstituted in a cell-free system. The activity of NADPH oxidase is dependent on FAD, but the physiological status of FAD in the oxidase is not fully elucidated. To clarify the role of FAD in NADPH oxidase, FAD-free full-length recombinant p47(phox), p67(phox), p40(phox), and Rac were prepared, and the activity was reconstituted with these proteins and purified cytochrome b(558) (cyt b(558)) with different amounts of FAD. A remarkably high activity, over 100 micromol/s/micromol heme, was obtained in the oxidase with purified cyt b(558), ternary complex (p47-p67-p40(phox)), and Rac. From titration with FAD of the activity of NADPH oxidase reconstituted with purified FAD-devoid cyt b, the dissociation constant K(d) of FAD in cyt b(558) of reconstituted oxidase was estimated as nearly 1 nm. We also examined addition of FAD on the assembly process in reconstituted oxidase. The activity was remarkably enhanced when FAD was present during assembly process, and the efficacy of incorporating FAD into the vacant FAD site in purified cyt b(558) increased, compared when FAD was added after assembly processes. The absorption spectra of reconstituted oxidase under anaerobiosis showed that incorporation of FAD into cyt b(558) recovered electron flow from NADPH to heme. From both K(d) values of FAD and the amount of incorporated FAD in cyt b(558) of reconstituted oxidase, in combination with spectra, we propose the model in which the K(d) values of FAD in cyt b(558) is changeable after activation and FAD binding works as a switch to regulate electron transfer in NADPH oxidase.

The PB1 domain and the PC motif-containing region are structurally similar protein binding modules

The PC motif is evolutionarily conserved together with the PB1 domain, a binding partner of the PC motif-containing protein. For interaction with the PB1 domain, the PC motif-containing region (PCCR) comprising the PC motif and its flanking regions is required. Because the PB1 domain and the PCCR are novel binding modules found in a variety of signaling proteins, their structural and functional characterization is crucial. Bem1p and Cdc24p interact through the PB1-PCCR interaction and regulate cell polarization in budding yeast. Here, we determined a tertiary structure of the PCCR of Cdc24p by NMR. The tertiary structure of the PCCR is similar to that of the PB1 domain of Bem1p, which is classified into a ubiquitin fold. The PC motif portion takes a compact betabetaalpha-fold, presented on the ubiquitin scaffold. Mutational studies indicate that the PB1-PCCR interaction is mainly electrostatic. Based on the structural information, we group the PB1 domains and the PCCRs into a novel family, named the PB1 family. Thus, the PB1 family proteins form a specific dimer with each other.

Molecular recognition in dimerization between PB1 domains

The PB1 (Phox and Bem 1) domain is a recently identified module that mediates formation of a heterodimeric complex with other PB1 domain, e.g. the complexes between the phagocyte oxidase activators p67phox and p40phox and between the yeast polarity proteins Bem1p and Cdc24p. These PB1 domains harbor either a conserved lysine residue on one side or an acidic OPCA (OPR/PC/AID) motif around the other side; the lysine of p67phox or Bem1p likely binds to the OPCA of p40phox or Cdc24p, respectively, via electrostatic interactions. To further understand molecular recognition by PB1 domains, here we investigate the interactions mediated by proteins presenting both the lysine and OPCA on a single PB1 domain, namely Par6, atypical protein kinase C (aPKC), and ZIP. Par6 and aPKC form a complex via the interaction of the Par6 lysine with aPKC-OPCA but not via that between the aPKC lysine and Par6-OPCA, thereby localizing to the tight junction of epithelial cells. aPKC also uses its OPCA to interact with ZIP, another protein that has a PB1 domain presenting both the lysine and OPCA, whereas aPKC binds via the conserved lysine to MEK5 in the same manner as ZIP interacts with MEK5. In addition, ZIP can form a homotypic complex via the conserved electrostatic interactions. Thus the PB1 domain appears to be a protein module that fully exploits its two mutually interacting elements in molecular recognition to expand its repertoire of protein-protein interactions.

PB1 domains of MEKK2 and MEKK3 interact with the MEK5 PB1 domain for activation of the ERK5 pathway

MEKK2 and MEKK3 are MAPK kinase kinases that activate the ERK5 pathway by phosphorylating and activating the MAPK kinase, MEK5. Activated MEK5 then phosphorylates and activates ERK5. PB1 domains were first defined in the p67phox and Bem1p proteins and have been shown to mediate protein-protein heterodimerization. A PB1 domain is encoded within the N-terminal portion of MEKK2, MEKK3, and MEK5. Herein, we analyze the functional role of MEKK2, MEKK3, and MEK5 PB1 domains in the ERK5 activation pathway. The PB1 domains of MEKK2 and MEKK3 bind the PB1 domain of MEK5 but do not significantly homo- or heterodimerize with one another in vitro. Furthermore, co-immunoprecipitation of MEKK2 and MEK5 from cell lysates shows that they form a complex in vivo. Deletion or mutation of the MEKK2 PB1 domain abolishes MEKK2-MEK5 complexes, demonstrating that the PB1 domain interaction is required for MEKK2-MEK5 interactions. Expression in cells of the MEKK2 or MEKK3 PB1 domain inhibits ERK5 activation, whereas expression of a mutant MEKK2 unable to bind the MEK5 PB1 domain or expression of the p67phox PB1 domain has no effect on ERK5 activation. These findings demonstrate that the PB1 domain mediates the association of MEKK2 and MEKK3 with MEK5 and that the respective PB1 domains of these kinases are critical for regulation of the ERK5 pathway. The free PB1 domain of MEKK2 or MEKK3 functions effectively to inhibit the ERK5 pathway but not the p38 or JNK pathways, demonstrating the specific and unique requirement of the MEKK2 and MEKK3 PB1 domain in regulating ERK5 activation.

Interaction codes within the family of mammalian Phox and Bem1p domain-containing proteins

The Phox and Bem1p (PB1) domain constitutes a recently recognized protein-protein interaction domain found in the atypical protein kinase C (aPKC) isoenzymes, lambda/iota- and zeta PKC; members of mitogen-activated protein kinase (MAPK) modules like MEK5, MEKK2, and MEKK3; and in several scaffold proteins involved in cellular signaling. Among the last group, p62 and Par6 (partitioning-defective 6) are involved in coupling the aPKCs to signaling pathways involved in cell survival, growth control, and cell polarity. By mutation analyses and molecular modeling, we have identified critical residues at the interaction surfaces of the PB1 domains of aPKCs and p62. A basic charge cluster interacts with an acidic loop and helix both in p62 oligomerization and in the aPKC-p62 interaction. Subsequently, we determined the abilities of mammalian PB1 domain proteins to form heteromeric and homomeric complexes mediated by this domain. We report several novel interactions within this family. An interaction between the cell polarity scaffold protein Par6 and MEK5 was found. Furthermore, p62 interacts both with MEK5 and NBR1 in addition to the aPKCs. Evidence for involvement of p62 in MEK5-ERK5 signaling is presented.

PB1 domain-mediated heterodimerization in NADPH oxidase and signaling complexes of atypical protein kinase C with Par6 and p62

Maximal activation of NADPH oxidase requires formation of a complex between the p40(phox) and p67(phox) subunits via association of their PB1 domains. We have determined the crystal structure of the p40(phox)/p67(phox) PB1 heterodimer, which reveals that both domains have a beta grasp topology and that they bind in a front-to-back arrangement through conserved electrostatic interactions between an acidic OPCA motif on p40(phox) and basic residues in p67(phox). The structure enabled us to identify residues critical for heterodimerization among other members of the PB1 domain family, including the atypical protein kinase C zeta (PKC zeta) and its partners Par6 and p62 (ZIP, sequestosome). Both Par6 and p62 use their basic "back" to interact with the OPCA motif on the "front" of the PKC zeta. Besides heterodimeric interactions, some PB1 domains, like the p62 PB1, can make homotypic front-to-back arrays.

The Cdc42 binding and scaffolding activities of the fission yeast adaptor protein Scd2

The small GTP-binding protein Cdc42, the guanine nucleotide exchange factor Scd1, the p21-activated kinase Shk1, and the adaptor protein Scd2 are involved in the Cdc42-dependent signaling cascade in fission yeast. In the present study, we analyzed the Cdc42 binding and scaffolding activities of Scd2 by co-precipitation assays. We found that two SH3-containing regions, amino acid residues 1-87 (CB1 (Cdc42-binding region 1)) and 110-266 (CB2), of Scd2 can bind to the GTP-bound form of Cdc42. CB2 is cryptic because of the intramolecular binding between the SH3 domain in CB2 (SH3(C)) and the PX domain and binds to Cdc42 only when the Scd2 PB1 domain binds to the PC motif-containing region (residues 760-872) of Scd1. This CB2.Cdc42 association, which would stabilize the open configuration of Scd2, enables the SH3(C) domain to bind to the polyproline motif of Shk1. We also found that the GTP-bound form of Cdc42 binds to the CRIB motif of Shk1 more strongly than to Scd2. Thus, Scd2 functions as a scaffold to form a protein complex, and the GTP-bound Cdc42 might be transferred effectively from the upstream activator Scd1 to the downstream effector Shk1 via Scd2.

The adaptor protein p40(phox) as a positive regulator of the superoxide-producing phagocyte oxidase

Activation of the superoxide-producing phagocyte NADPH oxidase, crucial in host defense, requires the cytosolic proteins p67(phox) and p47(phox). They translocate to the membrane upon cell stimulation and activate flavocytochrome b(558), the membrane-integrated catalytic core of this enzyme system. The activators p67(phox) and p47(phox) form a ternary complex together with p40(phox), an adaptor protein with unknown function, comprising the PX/PB2, SH3 and PC motif- containing domains: p40(phox) associates with p67(phox) via binding of the p40(phox) PC motif to the p67(phox) PB1 domain, while p47(phox) directly interacts with p67(phox) but not with p40(phox). Here we show that p40(phox) enhances membrane translocation of p67(phox) and p47(phox) in stimulated cells, which leads to facilitated production of superoxide. The enhancement cannot be elicited by a mutant p40(phox) carrying the D289A substitution in PC or a p67(phox) with the K355A substitution in PB1, each being defective in binding to its respective partner. Thus p40(phox) participates in activation of the phagocyte oxidase by regulating membrane recruitment of p67(phox) and p47(phox) via the PB1-PC interaction with p67(phox).

Architecture of the p40-p47-p67phox complex in the resting state of the NADPH oxidase. A central role for p67phox

The phagocyte NADPH oxidase is a multiprotein enzyme whose subunits are partitioned between the cytosol and plasma membrane in resting cells. Upon exposure to appropriate stimuli multiple phosphorylation events in the cytosolic components take place, which induce rearrangements in a number of protein-protein interactions, ultimately leading to translocation of the cytoplasmic complex to the membrane. To understand the molecular mechanisms that underlie the assembly and activation process we have carried out a detailed study of the protein-protein interactions that occur in the p40-p47-p67(phox) complex of the resting oxidase. Here we show that this complex contains one copy of each protein, which assembles to form a heterotrimeric complex. The apparent high molecular weight of this complex, as observed by gel filtration studies, is due to an extended, non-globular shape rather than to the presence of multiple copies of any of the proteins. Isothermal titration calorimetry measurements of the interactions between the individual components of this complex demonstrate that p67(phox) is the primary binding partner of p47(phox) in the resting state. These findings, in combination with earlier reports, allow us to propose a model for the architecture of the resting complex in which p67(phox) acts as the bridging molecule that connects p40(phox) and p47(phox).

The respiratory burst response of surface-adhering leukocytes. A key to tissue engineering

Biomaterials implanted into tissue will participate in the complex signalling between cells during wound healing. Recent studies have revealed that crucial cellular signalling pathways are regulated by the extra- and intracellular redox states and that reactive oxygen species function as intercellular signal molecules. Biomaterials have been shown to affect the respiratory burst response of surface-adhering leukocytes, thus interfering with major regulatory functions of cells also in surrounding tissues. The respiratory burst of surface-adhering leukocytes may thus be a key event in the understanding of biomaterial interaction with tissues, and the aim of this review is to highlight this field of research.

The PX domain as a novel phosphoinositide- binding module

The phox (phagocyte oxidase) homology (PX) domain occurs in the mammalian phox proteins p40(phox) and p47(phox), the polarity establishment protein Bem1p in budding yeast, and a variety of proteins involved in membrane trafficking. Here we show that the PX domains of p40(phox) and p47(phox) directly bind to phosphoinositides: p40(phox) prefers Ptdlns(3)P, while p47(phox) does Ptdlns(4)P and Ptdlns(3,4)P(2). In addition, the Bem1p PX domain also interacts with Ptdlns(4)P. When the p40(phox) PX domain is expressed as a fusion to green fluorescent protein in HeLa cells, it exists at early endosomes where Ptdlns(3)P is enriched. Furthermore, a mutant p40(phox) PX carrying the substitution of Lys for Arg105 only weakly binds to phosphoinositides in vitro, and fails to locate to early endosomes. Thus the PX domain functions as a novel phosphoinositide-binding module and likely participates in targeting of proteins to membranes.

Evaluation of the process for superoxide production by NADPH oxidase in human neutrophils: evidence for cytoplasmic origin of superoxide

We present an up-to-date insight into the function of NADPH oxidase in human neutrophils, the signalling pathways involved in activation of this enzyme and the process of association of its components with the cytoskeleton. We also discuss the functional implications of morphological studies revealing localization of the sites of NADPH oxidase activity. An original model of the process of superoxide (O2*-) production in human neutrophils is shown. Organization of NADPH oxidase is associated with several components. Upon stimulation, tri-phox cytosolic components of NADPH oxidase (p40-phox, p47-phox and p67-phox) bind to actin filaments. This process involves other actin-binding proteins, such as cofilin and coronin. Activated protein kinase C, translocated from the plasma membrane, phosphorylates cytosolic components at a scaffold of cytoskeleton. Subsequently, p40-phox, responsible for maintaining the resting state of NADPH oxidase, is separated from other two cytosolic phox proteins following an attachment of the active form of small GTP-binding protein Rac to p67-phox. Cytosolic duo-phox proteins (p47-phox and p67-phox) conjugate with membrane components (gp91-phox, p22-phox and Rapla) of NADPH oxidase residing within membranes of intracellular compartments. This chain of events triggers production of O2*-. Then, oxidant-producing intracellular compartments associate with the plasma membrane. Eventually, intracellularly produced O2*- is released to the extracellular environment through the orifice formed by fusion of oxidant-producing compartments with the plasma membrane. Intracellular movement of the oxidant-producing compartments may be regulated by myosin light chain kinase. The review emphasizes that functional assembly of NADPH oxidase and, therefore, generation of O2*- is accomplished essentially within the intracellular compartments. Upon neutrophil stimulation, intracellularly generated O2*- is transported to the plasma membrane to be released and to ensure host defense against infection.

Novel modular domain PB1 recognizes PC motif to mediate functional protein-protein interactions

Modular domains mediating specific protein-protein interactions play central roles in the formation of complex regulatory networks to execute various cellular activities. Here we identify a novel domain PB1 in the budding yeast protein Bem1p, which functions in polarity establishment, and mammalian p67(phox), which activates the microbicidal phagocyte NADPH oxidase. Each of these specifically recognizes an evolutionarily conserved PC motif to interact directly with Cdc24p (an essential protein for cell polarization) and p40(phox) (a component of the signaling complex for the oxidase), respectively. Swapping the PB1 domain of Bem1p with that of p67(phox), which abolishes its interaction with Cdc24p, confers on cells temperature- sensitive growth and a bilateral mating defect. These phenotypes are suppressed by a mutant Cdc24p harboring the PC motif-containing region of p40(phox), which restores the interaction with the altered Bem1p. This domain-swapping experiment demonstrates that Bem1p function requires interaction with Cdc24p, in which the PB1 domain and the PC motif participate as responsible modules.

Structure and ligand recognition of the PB1 domain: a novel protein module binding to the PC motif

PB1 domains are novel protein modules capable of binding to target proteins that contain PC motifs. We report here the NMR structure and ligand-binding site of the PB1 domain of the cell polarity establishment protein, Bem1p. In addition, we identify the topology of the PC motif-containing region of Cdc24p by NMR, another cell polarity establishment protein that interacts with Bem1p. The PC motif-containing region is a structural domain offering a scaffold to the PC motif. The chemical shift perturbation experiment and the mutagenesis study show that the PC motif is a major structural element that binds to the PB1 domain. A structural database search reveals close similarity between the Bem1p PB1 domain and the c-Raf1 Ras-binding domain. However, these domains are functionally distinct from each other.

Structure-function relationship in the interaction of mastoparan analogs with neutrophil NADPH oxidase

Mastoparan, an amphiphilic cationic tetradecapeptide was previously shown to block activation of the NADPH oxidase in the cell-free system presumably by association with a cytosolic component/s of the enzyme. Blockade of oxidase activation was now demonstrated in the semirecombinant NADPH oxidase system. The structural basis of the inhibitory effect of MP on oxidase assembly was explored employing a variety of truncated and specifically substituted synthetic peptide analogs. The data indicated that an alpha helical fold, positive net charge, hydrophobicity and amphiphilicity were essential for the inhibitory potency and that peptide analogs below eleven residues were inactive. To identify the MP-binding oxidase subunit three different binding assays were carried out utilizing free or immobilized recombinant p47-phox, p67-phox, p40-phox and Rac1 in conjunction with immobilized MP or soluble (125)I-tyr-MP, respectively. The data implicated p67-phox as the main MP-binding component. The binding site on the p67-phox was localized to the 1-238 aminoterminal fragment of the molecule. NADPH oxidase activation supported by this fragment was inhibitable by MP. In addition, SH3 domains of p47-phox and p40-phox and the carboxyterminal SH3 domain of p67-phox exhibited a low affinity towards MP.

Budding Yeast GCN1 Binds the GI Domain to Activate the eIF2α Kinase GCN2

When starved for a single amino acid, the budding yeast Saccharomyces cerevisiae activates the eukaryotic initiation factor 2alpha (eIF2alpha) kinase GCN2 in a GCN1-dependent manner. Phosphorylated eIF2alpha inhibits general translation but selectively derepresses the synthesis of the transcription factor GCN4, which leads to coordinated induction of genes involved in biosynthesis of various amino acids, a phenomenon called general control response. We recently demonstrated that this response requires binding of GCN1 to the GI domain occurring at the N terminus of GCN2 (Kubota, H., Sakaki, Y., and Ito, T. (2000) J. Biol. Chem. 275, 20243-20246). Here we provide the first evidence for the involvement of GCN1-GCN2 interaction in activation of GCN2 per se. We identified a C-terminal segment of GCN1 sufficient to bind the GI domain and used a novel dual bait two-hybrid method to identify mutations rendering GCN1 incapable of interacting with GCN2. The yeast bearing such an allele, gcn1-F2291L, fails to display derepression of GCN4 translation and hence general control response, as does a GI domain mutant, gcn2-Y74A, defective in association with GCN1. Furthermore, we demonstrated that phosphorylation of eIF2alpha is impaired in both mutants. Since GCN2 is the sole eIF2alpha kinase in yeast, these findings indicate a critical role of GCN1-GCN2 interaction in activation of the kinase in vivo.

Small angle neutron scattering and gel filtration analyses of neutrophil NADPH oxidase cytosolic factors highlight the role of the C-terminal end of p47phox in the association with p40phox

The NADPH oxidase of phagocytic cells is regulated by the cytosolic factors p47(phox), p67(phox), and p40(phox) as well as by the Rac1-Rho-GDI heterodimer. The regulation is a consequence of protein-protein interactions involving a variety of protein domains that are well characterized in signal transduction. We have studied the behavior of the NADPH oxidase cytosolic factors in solution using small angle neutron scattering and gel filtration. p47(phox), two truncated forms of p47(phox), namely, p47(phox) without its C-terminal end (residues 1-358) and p47(phox) without its N-terminal end (residues 147-390), and p40(phox) were found to be monomeric in solution. The dimeric form of p67(phox) previously observed by gel filtration experiments was confirmed. Our small angle neutron scattering experiments show that p40(phox) binds to the full-length p47(phox) in solution in the absence of phosphorylation. We demonstrated that the C-terminal end of p47(phox) is essential in this interaction. From the comparison of the presence or absence of interaction with various truncated forms of the proteins, we confirmed that the SH3 domain of p40(phox) interacts with the C-terminal proline rich region of p47(phox). The radii of gyration observed for p47(phox) and the truncated forms of p47(phox) (without the C-terminal end or without the N-terminal end) show that all these molecules are elongated and that the N-terminal end of p47(phox) is globular. These results suggest that the role of amphiphiles such as SDS or arachidonic acid or of p47(phox) phosphorylation in the elicitation of NADPH oxidase activation could be to disrupt the p40(phox)-p47(phox) complex rather than to break an intramolecular interaction in p47(phox).