Enhanced Immunoaffinity Purification of Human Neutrophil Flavocytochrome B for Structure Determination by Electron Microscopy.

Determination of the structure of human neutrophil (PMN) flavocytochrome b (Cytb) is a necessary step for the understanding of the structure-function essentials of NADPH oxidase activity. This understanding is crucial for structure-driven therapeutic approaches addressing control of inflammation and infection. Our work on purification and sample preparation of Cytb has facilitated progress toward the goal of structure determination. Here we describe exploiting immunoaffinity purification of Cytb for initial examination of its size and shape by a combination of classical and cryoelectron microscopic (EM) methods. For these evaluations, we used conventional negative-stain transmission electron microscopy (TEM) to examine both detergent-solubilized Cytb as single particles and Cytb in phosphatidylcholine reconstituted membrane vesicles as densely packed random, partially ordered, and subcrystalline arrays. In preliminary trials, we also examined single particles by cryoelectron microscopy (cryoEM) methods. We conclude that Cytb in detergent and reconstituted in membrane is a relatively compact, symmetrical protein of about 100 Å in maximum dimension. The negative stain, preliminary cryoEM, and crude molecular models suggest that the protein is probably a heterotetramer of two p22phox and gp91phox subunits in both detergent micelles and membrane vesicles. This exploratory study also suggests that high-resolution 2D electron microscopic approaches may be accessible to human material collected from single donors.

Anionic lipid-induced conformational changes in human phagocyte flavocytochrome b precede assembly and activation of the NADPH oxidase complex.

Phagocyte NADPH oxidases generate superoxide at high rates in defense against infectious agents, a process regulated by second messenger anionic lipids using incompletely understood mechanisms. We reconstituted the catalytic core of the human neutrophil NADPH oxidase, flavocytochrome b (Cyt b) in 99% phosphatidylcholine vesicles in order to correlate anionic lipid-dependent conformational changes in membrane-bound Cyt b and oxidase activity. The anionic lipid 10:0 phosphatidic acid (10:0 PA) specifically induced conformational changes in Cyt b as measured by a combination of fluorescence resonance energy transfer methods and size exclusion chromatography. The fluorescence lifetime of a complex between Cyt b and Cascade Blue-derivatized anti-p22(phox) antibody (CCB-CS9), increased after exposure to 10:PA by ∼50% of the change observed when the complex is dissociated, indicating a structural rearrangement of p22(phox) and/or the Cyt b heme prosthetic groups. Half of the quenching relaxation occurred at 10:0 PA concentrations permissive to less than 10% full NADPH oxidase activity, but saturated near the saturation in activity in a matched cell-free oxidase assay. We conclude that anionic lipids modulate the conformation of Cyt b in the membrane and suggest they may serve to modulate the structure of Cyt b as a control mechanism for the NADPH oxidase.

Invariant local conformation in p22phox p.Y72H polymorphisms suggested by mass spectral analysis of crosslinked human neutrophil flavocytochrome b.

The NADPH oxidase of phagocytic leukocytes generates superoxide that plays a critical role in innate immunity and inflammatory responses. The integral membrane protein flavocytochrome b (Cyt b, a.k.a. cytochrome b(558/559)) is the catalytic core of the complex and serves as a prototype for homologs important in regulating signaling networks in a wide variety of animal and plant cells. Our analysis identifies a naturally-occurring Tyr72/His72 polymorphism (p.Y72H) in the p22(phox) subunit of Cyt b at the protein level that has been recognized at the nucleotide level (c.214T > C, formerly C242T) and implicated in cardiovascular disease. In the present study, Cyt b was isolated from human neutrophils and reacted with chemical crosslinkers for subsequent structure analysis by MALDI mass spectrometry. Following mild chemical modification of Cyt b with two pairs of isotopically-differentiated lysine crosslinkers: BS(2)G-d(0)/d(4) and BS(3)-d(0)/d(4), the reaction mixtures were digested with trypsin and purified on C(18)ZipTips to generate samples for mass analysis. MALDI analysis of tryptic digests from each of the above reactions revealed a series of masses that could be assigned to p22(phox) residues 68-85, assuming an intra-molecular crosslink between Lys71 and Lys78. In addition to the 30 ppm mass accuracy obtained with internal mass calibration, increased confidence in the assignment of the crosslinks was provided by the presence of the diagnostic mass patterns resulting from the isotopically-differentiated crosslinking reagent pairs and the Tyr72/His72 p22(phox) polymorphisms in the crosslinked peptides. This work identifies a novel, low-resolution distance constraint in p22(phox) and suggests that the medically-relevant p.Y72H polymorphism has an invariant structural motif in this region. Because position 72 in p22(phox) lies outside regions identified as interactive with other oxidase components, the structural invariance also provides additional support for maturational differences as the source of the wide variation in observed reactive oxygen species production by cells expressing p.Y72H.

Reduced nicotinamide adenine dinucleotide phosphate oxidase-independent resistance to Aspergillus fumigatus in alveolar macrophages.

The fungal pathogen Aspergillus fumigatus is responsible for increasing numbers of fatal infections in immune-compromised humans. Alveolar macrophages (AM) are important in the innate defense against aspergillosis, but little is known about their molecular responses to fungal conidia in vivo. We examined transcriptional changes and superoxide release by AM from C57BL/6 and gp91(phox)(-/-) mice in response to conidia. Following introduction of conidia into the lung, microarray analysis of AM showed the transcripts most strongly up-regulated in vivo to encode chemokines and additional genes that play a critical role in neutrophil and monocyte recruitment, indicating that activation of phagocytes represents a critical early response of AM to fungal conidia. Of the 73 AM genes showing > or = 2-fold changes, 8 were also increased in gp91(phox)(-/-) mice by conidia and in C57BL/6 mice by polystyrene beads, suggesting a common innate response to particulate matter. Ingenuity analysis of the microarray data from C57BL/6 mice revealed immune cell signaling and gene expression as primary mechanisms of this response. Despite the well-established importance of phagocyte NADPH oxidase in resisting aspergillosis, we found no evidence of this mechanism in AM following introduction of conidia into the mouse lung using transcriptional, luminometry, or NBT staining analysis. In support of these findings, we observed that AM from C57BL/6 and gp91(phox)(-/-) mice inhibit conidial germination equally in vitro. Our results indicate that early transcription in mouse AM exposed to conidia in vivo targets neutrophil recruitment, and that NADPH oxidase-independent mechanisms in AM contribute to inhibition of conidial germination.

Identification of a conserved Rac-binding site on NADPH oxidases supports a direct GTPase regulatory mechanism.

The NADPH oxidases (Noxs) are a family of superoxide-generating enzymes implicated in a variety of biological processes. Full activity of Nox1, -2, and -3 requires the action of a Rac GTPase. A direct regulatory interaction of Rac with Nox2 has been proposed as part of a two-step mechanism for regulating electron transfer during superoxide formation. Using truncation analysis of Rac binding to the cytoplasmic tail of Nox2, along with peptides derived from this region in cell-free assays, we identify a Rac interaction site within amino acids 419-430 of Nox2. This region is required for binding Rac2 but not p47(phox) or p67(phox) cytosolic regulatory factors. A cell-permeant version of the peptide encompassing amino acids 419-430 specifically inhibits NADPH oxidase activation in intact human neutrophils. Mutational analysis of the putative Rac-binding site revealed specific residues, particularly Lys-421, Tyr-425, and Lys-426, individually required for Rac-dependent NADPH oxidase activity that are conserved in the Rac-regulated Nox1, Nox2, and Nox3 enzymes but not in Nox4 or Nox5. Mutation of the conserved residues in the Rac-binding site of Nox1 also result in the loss of Rac-dependent activity. Our data identify a functional Rac interaction site conserved in Rac-dependent Noxs and support a direct regulatory interaction of Rac GTPases to promote activation of these NADPH oxidases.

Single-step immunoaffinity purification and functional reconstitution of human phagocyte flavocytochrome b.

Human neutrophil flavocytochrome b (Cyt b) is a heterodimeric, integral membrane protein that generates high levels of superoxide in the multisubunit NADPH oxidase complex. Since Cyt b is currently isolated in limited quantities, improved methods for purification from low levels of starting membranes (from both neutrophils and other expressing cell types) are important for the analysis of structure and catalytic mechanism. In the present study, the epitope-mapped monoclonal antibody CS9 was coupled to Sepharose beads and used as an affinity matrix for single-step immunoaffinity purification of Cyt b. Following solubilization of both human neutrophil and PLB-985 membrane fractions in the nonionic detergent octylglucoside, Cyt b was absorbed on the CS9-Sepharose affinity matrix and purified protein was eluted under non-denaturing conditions with an epitope-mimicking peptide. The high efficiency of this isolation procedure allowed Cyt b to be reproducibly purified from readily obtainable levels of starting membrane fractions (9x10(8) cell equivalents of neutrophil membranes and 2x10(9) cell equivalents of PLB-985 membranes). Since Cyt b could be affinity-purified in the detergent octylglucoside, high-level functional reconstitution was carried out directly on elution fractions by simple addition of solubilized phospholipid and subsequent dialysis for detergent removal. To our knowledge, this study describes the most efficient method for generating purified, functionally-reconstituted Cyt b and should facilitate analyses that require a highly-defined NADPH oxidase system.

Characterization of surface structure and p47phox SH3 domain-mediated conformational changes for human neutrophil flavocytochrome b.

The heterodimeric, integral membrane protein flavocytochrome b (Cyt b) is the catalytic core of the phagocyte NADPH oxidase and generates superoxide which plays a critical role in host defense. To better define the activation of superoxide production by this multisubunit enzyme complex, Cyt b-specific monoclonal antibodies (mAbs) and the p47phox SH3 domains (p47SH3AB) were used in the present study as probes to map surface structure and conformational dynamics in human neutrophil Cyt b. In pull-down and co-immunoprecipitation studies with detergent-solubilized Cyt b, the oxidase-inhibitory mAb CS9 was shown to share an overlapping binding site with p47SH3AB on the C-terminal region of the p22phox subunit. Similar studies demonstrated a surprising lack of overlap between the mAb 44.1 and CS9/p47SH3AB binding sites, and they indicated that the oxidase-inhibitory mAb NL7 binds a region physically separated from the p22phox C-terminal domain. Resonance energy transfer and size exclusion chromatography confirmed the above results for functionally reconstituted Cyt b and provided evidence that binding of both mAb CS9 and p47SH3AB altered the conformation of Cyt b. Further support that binding of the p47phox SH3 domains modulates the structure of Cyt b was obtained using a cell-free assay system where p47SH3AB enhanced superoxide production in the presence of a p67phox (1-212)-Rac1(Q61L) fusion protein. Taken together, this study further characterizes the structure of human neutrophil Cyt b in both detergent micelles and reconstituted membrane bilayers, and it provides evidence that the cytosolic regulatory subunit p47phox modulates the conformation of Cyt b (in addition to serving as an adapter protein) during oxidase activation.

C-terminal tail phosphorylation of N-formyl peptide receptor: differential recognition of two neutrophil chemoattractant receptors by monoclonal antibodies NFPR1 and NFPR2.

The N-formyl peptide receptor (FPR), a G protein-coupled receptor that binds proinflammatory chemoattractant peptides, serves as a model receptor for leukocyte chemotaxis. Recombinant histidine-tagged FPR (rHis-FPR) was purified in lysophosphatidyl glycerol (LPG) by Ni(2+)-NTA agarose chromatography to >95% purity with high yield. MALDI-TOF mass analysis (>36% sequence coverage) and immunoblotting confirmed the identity as FPR. The rHis-FPR served as an immunogen for the production of 2 mAbs, NFPR1 and NFPR2, that epitope map to the FPR C-terminal tail sequences, 305-GQDFRERLI-313 and 337-NSTLPSAEVE-346, respectively. Both mAbs specifically immunoblotted rHis-FPR and recombinant FPR (rFPR) expressed in Chinese hamster ovary cells. NFPR1 also recognized recombinant FPRL1, specifically expressed in mouse L fibroblasts. In human neutrophil membranes, both Abs labeled a 45-75 kDa species (peak M(r) approximately 60 kDa) localized primarily in the plasma membrane with a minor component in the lactoferrin-enriched intracellular fractions, consistent with FPR size and localization. NFPR1 also recognized a band of M(r) approximately 40 kDa localized, in equal proportions to the plasma membrane and lactoferrin-enriched fractions, consistent with FPRL1 size and localization. Only NFPR2 was capable of immunoprecipitation of rFPR in detergent extracts. The recognition of rFPR by NFPR2 is lost after exposure of cellular rFPR to f-Met-Leu-Phe (fMLF) and regained after alkaline phosphatase treatment of rFPR-bearing membranes. In neutrophils, NFPR2 immunofluorescence was lost upon fMLF stimulation. Immunoblotting approximately 60 kDa species, after phosphatase treatment of fMLF-stimulated neutrophil membranes, was also enhanced. We conclude that the region 337-346 of FPR becomes phosphorylated after fMLF activation of rFPR-expressing Chinese hamster ovary cells and neutrophils.

Localization of hCAP-18 on the surface of chemoattractant-stimulated human granulocytes: analysis using two novel hCAP-18-specific monoclonal antibodies.

The well-described antimicrobial and immunoregulatory properties of human cathelicidin antimicrobial protein 18 (hCAP-18) derive in part from the ability of its proteolytic fragment, LL-37 (a.k.a. CAP-37), to associate with activated immune and epithelial cells during inflammation. We now show a stable association between hCAP-18 and the cell surface of formyl-Met-Leu-Phe (fMLF)-stimulated neutrophils using two novel hCAP-18-specific mAb, H7 and N9, which recognize a single 16-kDa band, identified by N-terminal sequencing and mass spectrometry as hCAP-18. Phage display analysis of epitope-binding sites showed that both mAb probably recognize a similar five amino acid sequence near the C terminus of the prodomain. Immunoblot analysis of degranulated neutrophil supernatants resulted in mAb recognition of the 14-kDa prodomain of hCAP-18. Subcellular fractionation of unstimulated neutrophils on density gradients showed expected cosedimentation of hCAP-18 with specific granule lactoferrin (LF). fMLF stimulation resulted in an average 25% release of specific granule hCAP-18, with approximately 15% of the total cellular hCAP-18 recovered from culture media, and approximately 10% and approximately 75%, respectively, codistributing with plasma membrane alkaline phosphatase and specific granule LF. Surface association of hCAP-18 on fMLF-stimulated neutrophils was confirmed by immunofluorescence microscopy and flow cytometry analysis, which also suggested a significant up-regulation of surface hCAP-18 on cytochalasin B-pretreated, fully degranulated neutrophils. hCAP-18 surface association was labile to 10 mM NaOH treatment but resistant to 1 M NaCl and also partitioned into the detergent phase following Triton X-114 solubilization, possibly suggesting a stable association with one or more integral membrane proteins. We conclude that fMLF stimulation promotes redistribution of hCAP-18 to the surface of human neutrophils.

Cloning, sequence analysis and confirmation of derived gene sequences for three epitope-mapped monoclonal antibodies against human phagocyte flavocytochrome b.

The integral membrane protein flavocytochrome b (Cyt b) is the catalytic core of the NADPH oxidase complex, a multicomponent enzyme system that initiates a cascade of reactive oxygen species that play a critical role in innate immunity and vascular physiology. Epitope-mapped, monoclonal antibodies (mAb) that recognize the large (gp91phox) and small (p22phox) subunits of Cyt b provide valuable reagents that have been used to examine structural and mechanistic aspects of oxidase function. In the present study, the heavy and light chain variable region genes of the Cyt b-specific mAbs 44.1, NS5, and NL7 have been amplified by RT-PCR, cloned and subject to DNA sequence analysis. Since the 5' degenerate primer sets used for mAb gene amplification were observed to introduce extensive heterogeneity into the heavy and light chain FR1 regions, N-terminal protein sequence analysis was also conducted to obtain the correct amino acid sequence of this region. In order to confirm the identity of the cloned genes, intact mAbs were resolved by two-dimensional electrophoresis and subject to in-gel tryptic digestion for analysis by both MALDI and nanospray LC-MS/MS. Databases searches using the derived mAb sequences predicted residues comprising CDR loops, identified candidate germline genes, and showed the respective germline genes to accurately predict the N-terminal amino acid residues for each variable region. The above studies report the amino acid sequence of Cyt b-specific mAb variable region genes with high confidence and provide essential information for future efforts at Cyt b structure analysis by resonance energy transfer and X-ray crystallography.

Immunoaffinity purification of human phagocyte flavocytochrome b and analysis of conformational dynamics.

The heterodimeric integral membrane protein flavocytochrome b (Cyt b) is the catalytic core of the phagocyte NADPH oxidase, an enzyme complex that initiates a cascade of reactive oxygen species critical for the elimination of infectious agents. Many fundamental questions remain concerning the structure and catalytic mechanism of Cyt b, largely because of the inability to isolate this protein in quantities required for both biochemical analysis and meaningful attempts at high-resolution structure determination. In order to facilitate the direct analysis of Cyt b, the following method describes a rapid and efficient procedure for the immunoaffinity purification of Cyt b (under nondenaturing conditions) from neutrophil membrane fractions. The protocol presented here contains a number of steps that have been optimized and improved since the original description of this Cyt b isolation method. In order to address questions concerning the mechanism of superoxide generation by the NADPH oxidase complex, methods are additionally presented for analysis of conformational dynamics of immunoaffinity-purified Cyt b by resonance energy transfer.

Analysis of human phagocyte flavocytochrome b(558) by mass spectrometry.

The catalytic core of the phagocyte NADPH oxidase is a heterodimeric integral membrane protein (flavocytochrome b (Cyt b)) that generates superoxide and initiates a cascade of reactive oxygen species critical for the host inflammatory response. In order to facilitate structural characterization, the present study reports the first direct analysis of human phagocyte Cyt b by matrix-assisted laser desorption/ionization and nanoelectrospray mass spectrometry. Mass analysis of in-gel tryptic digest samples provided 73% total sequence coverage of the gp91(phox) subunit, including three of the six proposed transmembrane domains. Similar analysis of the p22(phox) subunit provided 72% total sequence coverage, including assignment of the hydrophobic N-terminal region and residues that are polymorphic in the human population. To initiate mass analysis of Cyt b post-translational modifications, the isolated gp91(phox) subunit was subject to sequential in-gel digestion with Flavobacterium meningosepticum peptide N-glycosidase F and trypsin, with matrix-assisted laser desorption/ionization and liquid chromatography-mass spectrometry/mass spectrometry used to demonstrate that Asn-132, -149, and -240 are genuinely modified by N-linked glycans in human neutrophils. Since the PLB-985 cell line represents an important model system for analysis of the NADPH oxidase, methods were developed for the purification of Cyt b from PLB-985 membrane fractions in order to confirm the appropriate modification of N-linked glycosylation sites on the recombinant gp91(phox) subunit. This study reports extensive sequence coverage of the integral membrane protein Cyt b by mass spectrometry and provides analytical methods that will be useful for evaluating posttranslational modifications involved in the regulation of superoxide production.

Evaluation of two anti-gp91phox antibodies as immunoprobes for Nox family proteins: mAb 54.1 recognizes recombinant full-length Nox2, Nox3 and the C-terminal domains of Nox1-4 and cross-reacts with GRP 58.

Progress in the study of Nox protein expression has been impeded because of the paucity of immunological probes. The large subunit of human phagocyte flavocytochrome b558 (Cytb), gp91phox, is also the prototype member of the recently discovered family of NADPH oxidase (Nox) proteins. In this study, we have evaluated the use of two anti-gp91phox monoclonal antibodies, 54.1 and CL5, as immunoprobes for Nox family proteins. Sequence alignment of gp91phox with Nox1, Nox3 and Nox4 identified regions of the Nox proteins that correspond to the gp91phox epitopes recognized by mAb 54.1 and CL5. Antibody 54.1 produced positive immunoblots of recombinant C-terminal fragments of these homologous proteins expressed in E. coli. Furthermore, only mAb 54.1 recognized full-length murine and human Nox3 expressed in HEK-293 cells, in immunoblots of alkali-stripped or detergent-solubilized membranes. 54.1 recognized Nox3 expression-specific proteins with Mr 30,000, 50,000, 65,000 and 88,000 for the murine protein and Mr of 38,000-58,000, 90,000, 100,000-130,000 and a broad species of higher than 160,000 for the human protein. We conclude that mAb 54.1 can serve as a probe of Nox3 and possibly other Nox proteins, if precautions are taken to remove GRP 58 and other crossreactive membrane-associated or detergent-insoluble proteins from the sample to be probed.

Monoclonal antibody CL5 recognizes the amino terminal domain of human phagocyte flavocytochrome b558 large subunit, gp91phox.

Human phagocyte flavocytochrome b558 (Cytb) is a heterodimeric integral membrane protein that serves as the electron transferase of the beta-nicotinamide adenine dinucleotidephosphate, reduced (NADPH)-oxidase, an enzyme complex important in the host defense function of phagocytic cells. In this study, we report the characterization of monoclonal antibody (mAb) CL5 that is specific for the large subunit, gp91phox, of the oxidase protein. This antibody recognizes gp91phox by immunoblot analysis of membrane extracts and samples of the immunopurified gp91phox/p22phox heterodimer, prepared on anti-p22phox affinity matrices. Phage display analysis confirmed this specificity, indicating that the CL5 epitope contains the region 135-DPYSVALSELGDR of gp91phox. The antibody was used to probe for the presence of gp91phox in membrane preparations from neutrophils of patients with nine genetically distinct forms of X-linked chronic granulomatous disease (CGD). The causative mutations included missense errors as well as nonsense errors that result in premature termination of gp91phox synthesis. Analysis of the CGD samples by immunoblotting indicated that CL5 recognizes only the full-length wild-type and two missense mutations, consistent with the absence of stable short gp91phox peptide expression in CGD neutrophils. Interestingly, CL5 was also shown to be cross-reactive with cytosolic and membrane-bound gelsolin, identified by purification, mass spectrometry and immunoblot analysis. CL5 probably cross-reacts with the sequence 771-DPLDRAMAEL in the C-terminus of gelsolin. We conclude that mAb CL5 is a useful probe for detection of full length and possibly truncated N-terminal fragments of gp91phox from membranes of Cytb-producing cells.

Unusual polyclonal anti-gp91 peptide antibody interactions with X-linked chronic granulomatous disease-derived human neutrophils are not from compensatory expression of Nox proteins 1, 3, or 4.

To obtain topological information about human phagocyte flavocytochrome b558 (Cytb), rabbit anti-peptide antibodies were raised against synthetic peptides mimicking gp91(phox) regions: 1-9 (MGN), 30-44 (YRV), 150-159 (ESY), 156-166 (ARK), 247-257 (KIS-1, KIS-2). Following affinity purification on immobilized peptide matrices, all antibodies but not prebleed controls recognized purified detergent-solubilized Cytb by enzyme-linked immunosorbent assay (ELISA). Affinity-purified antibodies recognizing KIS, ARK and ESY but not YRV, MGN or prebleed IgG specifically detected gp91(phox) in immunoblot analysis. Antibodies recognizing MGN, ESY, ARK and KIS but not YRV or the prebleed IgG fraction labeled intact normal neutrophils. Surprisingly, all antibodies, with the exception of YRV and pre-immune IgG controls, bound both normal and Cytb-negative neutrophils from the obligate heterozygous mother of a patient with X-linked chronic granulomatous disease (X-CGD) and all neutrophils from another patient lacking the gp91(phox) gene. Further immunochemical examination of membrane fractions derived from nine genetically unrelated patients with X-CGD, using an antibody that recognizes other Nox protein family members, suggests that the unusual reactivity observed does not reflect the compensatory expression of gp91(phox) homologs Nox1, 3 or 4. These results suggest that an unusual surface reactivity exists on neutrophils derived from X-linked chronic granulomatous disease patients that most likely extends to normal neutrophils as well. The study highlights the need for caution in interpreting the binding of rabbit polyclonal antipeptide antibodies to human neutrophils in general and, in the specific case of antibodies directed against Cytb, the need for Cytb-negative controls.

Site-specific inhibitors of NADPH oxidase activity and structural probes of flavocytochrome b: characterization of six monoclonal antibodies to the p22phox subunit.

The integral membrane protein flavocytochrome b (Cyt b) is the catalytic core of the human phagocyte NADPH oxidase, an enzyme complex that initiates a cascade of reactive oxygen species important in the elimination of infectious agents. This study reports the generation and characterization of six mAbs (NS1, NS2, NS5, CS6, CS8, and CS9) that recognize the p22(phox) subunit of the Cyt b heterodimer. Each of the mAbs specifically detected p22(phox) by Western blot analysis but did not react with intact neutrophils in FACS studies. Phage display mapping identified core epitope regions recognized by mAbs NS2, NS5, CS6, CS8, and CS9. Fluorescence resonance energy transfer experiments indicated that mAbs CS6 and CS8 efficiently compete with Cascade Blue-labeled mAb 44.1 (a previously characterized, p22(phox)-specific mAb) for binding to Cyt b, supporting phage display results suggesting that all three Abs recognize a common region of p22(phox). Energy transfer experiments also suggested the spatial proximity of the mAb CS9 and mAb NS1 binding sites to the mAb 44.1 epitope, while indicating a more distant proximity between the mAb NS5 and mAb 44.1 epitopes. Cell-free oxidase assays demonstrated the ability of mAb CS9 to markedly inhibit superoxide production in a concentration-dependent manner, with more moderate levels of inhibition observed for mAbs NS1, NS5, CS6, and CS8. A combination of computational predictions, available experimental data, and results obtained with the mAbs reported in this study was used to generate a novel topology model of p22(phox).

Anionic amphiphile and phospholipid-induced conformational changes in human neutrophil flavocytochrome b observed by fluorescence resonance energy transfer.

The integral membrane protein flavocytochrome b (Cyt b) comprises the catalytic core of the human phagocyte NADPH oxidase complex and serves to initiate a cascade of reactive oxygen species that participate in the elimination of infectious agents. Superoxide production by the NADPH oxidase complex has been shown to be specifically regulated by the enzymatic generation of lipid second messengers following phagocyte activation. In the present study, a Cyt b-specific monoclonal antibody (mAb 44.1) was labeled with Cascade Blue (CCB) and used in resonance energy transfer (RET) studies probing the effects of a panel of lipid species on the structure of Cyt b. The binding of CCB-mAb 44.1 to immunoaffinity-purified Cyt b was both highly specific and resulted in significant quenching of the steady state donor fluorescence. Titration of the CCB-mAb 44.1:Cyt b complex with the anionic amphiphile lithium dodecyl sulfate (LDS) resulted in a saturable relaxation of fluorescence quenching due to conformational changes in Cyt b at concentrations of the amphiphile required for maximum rates of superoxide production by Cyt b in cell-free assays. Similar results were observed for the anionic amphiphile arachidonic acid (AA), although no relaxation of fluorescence quenching was observed for arachidonate methyl ester (AA-ME). Saturable relaxation of fluorescence quenching was also observed with the anionic, 18:1 phospholipids phosphatidic acid (DOPA) and phosphatidylserine (DOPS), while no relaxation was observed upon addition of the neutral 18:1 lipids phosphatidylcholine (DOPC), phosphatidylethanolamine (DOPE) or diacylglycerol (DAG) at similar levels. Further examination of a variety of phosphatidic acid (PA) species demonstrated DOPA to both potently induce conformational changes in Cyt b and to cause more dramatic conformational changes than PA species with shorter, saturated acyl chains. The data presented in this study support the hypothesis that second messenger lipids, such as AA and PA, directly bind to flavocytochrome b and modulate conformational states relevant to the activation of superoxide production.

Functional epitope on human neutrophil flavocytochrome b558.

mAb NL7 was raised against purified flavocytochrome b(558), important in host defense and inflammation. NL7 recognized the gp91(phox) flavocytochrome b(558) subunit by immunoblot and bound to permeabilized neutrophils and neutrophil membranes. Epitope mapping by phage display analysis indicated that NL7 binds the (498)EKDVITGLK(506) region of gp91(phox). In a cell-free assay, NL7 inhibited in vitro activation of the NADPH oxidase in a concentration-dependent manner, and had marginal effects on the oxidase substrate Michaelis constant (K(m)). mAb NL7 did not inhibit translocation of p47(phox), p67(phox), or Rac to the plasma membrane, and bound its epitope on gp91(phox) independently of cytosolic factor translocation. However, after assembly of the NADPH oxidase complex, mAb NL7 bound the epitope but did not inhibit the generation of superoxide. Three-dimensional modeling of the C-terminal domain of gp91(phox) on a corn nitrate reductase template suggests close proximity of the NL7 epitope to the proposed NADPH binding site, but significant separation from the proposed p47(phox) binding sites. We conclude that the (498)EKDVITGLK(506) segment resides on the cytosolic surface of gp91(phox) and represents a region important for oxidase function, but not substrate or cytosolic component binding.

Single-step immunoaffinity purification and characterization of dodecylmaltoside-solubilized human neutrophil flavocytochrome b.

Flavocytochrome b (Cyt b) is a heterodimeric, integral membrane protein that serves as the central component of an electron transferase system employed by phagocytes for elimination of bacterial and fungal pathogens. This report describes a rapid and efficient single-step purification of Cyt b from human neutrophil plasma membranes by solubilization in the nonionic detergent dodecylmaltoside (DDM) and immunoaffinity chromatography. A similar procedure for isolation of Cyt b directly from intact neutrophils by a combination of heparin and immunoaffinity chromatography is also presented. The stability of Cyt b was enhanced in DDM relative to previously employed solubilizing agents as determined by both monitoring the heme spectrum in crude membrane extracts and assaying resistance to proteolytic degradation following purification. Gel filtration chromatography and dynamic light scattering indicated that DDM maintains a predominantly monodisperse population of Cyt b following immunoaffinity purification. The high degree of purity obtained with this isolation procedure allowed for direct determination of a 2:1 heme to protein stoichiometry, confirming previous structural models. Analysis of the isolated heterodimer by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry allowed for accurate mass determination of p22(phox) as indicated by the gene sequence. Affinity-purified Cyt b was functionally reconstituted into artificial bilayers and demonstrated that catalytic activity of the protein was efficiently retained throughout the purification procedure.

Structural changes are induced in human neutrophil cytochrome b by NADPH oxidase activators, LDS, SDS, and arachidonate: intermolecular resonance energy transfer between trisulfopyrenyl-wheat germ agglutinin and cytochrome b(558).

Anionic amphiphiles such as sodium- and lithium dodecyl sulfate (SDS, LDS), or arachidonate (AA) initiate NADPH oxidase and proton channel activation in cell-free systems and intact neutrophils. To investigate whether these amphiphiles exert allosteric effects on cytochrome b, trisulfopyrenyl-labeled wheat germ agglutinin (Cascade Blue-wheat germ agglutinin, CCB-WGA) was used as an extrinsic fluorescence donor for resonance energy transfer (RET) to the intrinsic heme acceptors of detergent-solubilized cytochrome b. In solution, cytochrome b complexed with the CCB-WGA causing a rapid, saturable, carbohydrate-dependent quenching of up to approximately 55% of the steady-state fluorescence. Subsequent additions of SDS, LDS, or AA to typical cell-free oxidase assay concentrations completely relaxed the fluorescence quenching. The relaxation effects were specific, and not caused by dissociation of the CCB-WGA-cytochrome b complex or alterations in the spectral properties of the chromophores. In contrast, addition of the oxidase antagonist, arachidonate methyl ester, caused an opposite effect and was able to partially reverse the activator-induced relaxation. We conclude that the activators induce a cytochrome b conformation wherein the proximity or orientation between the hemes and the extrinsic CCB fluorescence donors has undergone a significant change. These events may be linked to NADPH oxidase assembly and activation or proton channel induction.