The O2- generating oxidase of B lymphocytes: Epstein-Barr virus-immortalized B lymphocytes as a tool for the identification of defective components of the oxidase in chronic granulomatous disease

Reactive oxygen species: The signal regulator of B cell

Reactive oxygen species (ROS) are indispensable for determining the fate of immune cells in both physiological and pathogenic environments, thus stimulating the interest of immunologists and clinicians. B cells are essential in maintaining immune homeostasis, and studies have indicated that ROS affect the maturation, activation and differentiation of B cells by controlling the signaling molecules in various molecular pathways. In the present review, we aimed to summarize the biological properties of ROS and the mechanisms by which ROS regulate B cell signaling pathways. We propose that ROS and their mediated signal transduction can be a new approach for manipulating B cell immune functions.

Prolonged production of reactive oxygen species in response to B cell receptor stimulation promotes B cell activation and proliferation

We have investigated the intracellular sources and physiological function of reactive oxygen species (ROS) produced in primary B cells in response to BCR stimulation. BCR stimulation of primary resting murine B cells induced the rapid production of ROS that occurred within minutes and was maintained for at least 24 h after receptor stimulation. While the early production of ROS (0-2 h) was dependent on the Nox2 isoform of NADPH oxidase, at later stages of B cell activation (6-24 h) ROS were generated by a second pathway, which appeared to be dependent on mitochondrial respiration. B cells from mice deficient in the Nox2 NADPH oxidase complex lacked detectable early production of extracellular and intracellular ROS after BCR stimulation but had normal proximal BCR signaling and BCR-induced activation and proliferation in vitro and mounted normal or somewhat elevated Ab responses in vivo. In contrast, neutralizing both pathways of BCR-derived ROS with the scavenger N-acetylcysteine resulted in impaired in vitro BCR-induced activation and proliferation and attenuated BCR signaling through the PI3K pathway at later times. These results indicate that the production of ROS downstream of the BCR is derived from at least two distinct cellular sources and plays a critical role at the later stages of B cell activation by promoting sustained BCR signaling via the PI3K pathway, which is needed for effective B cell responses to Ag.

Coupling of 6-phosphogluconate dehydrogenase with NADPH oxidase in neutrophils: Nox2 activity regulation by NADPH availability

It is well known that activation of the phagocyte NADPH oxidase requires the association of cytosolic proteins (p67-phox, p47-phox, p40-phox, and Rac) with the membrane cytochrome b(558), leading to its conformation change. Recently, the phagocyte NADPH oxidase complex was isolated in a constitutively active form. In this complex, 6-phosphogluconate dehydrogenase (6PGDH), an enzyme involved in the production of intracellular NADPH, was identified. This protein was absent from the oxidase complex isolated from B lymphocytes, suggesting a specific interaction with the neutrophil NADPH oxidase. To clarify the implication of 6PGDH in the NADPH oxidase activity, a siRNA approach was conducted in neutrophil-like PLB985 cells. NADPH oxidase activity of siRNA-transfected cells was shown to be decreased. Similar results were obtained in vitro, after reconstitution of oxidase activity with subcellular fractions isolated from siRNA-transfected cells. Interestingly, the Michaelis constant (K(m)) of Nox2 for NADPH increases in 6PGDH-depleted cells. Moreover, 6PGDH coimmunoprecipitated with oxidase cytosolic factors from cytosol of stimulated cells. Data suggested that the affinity of Nox2 for NADPH is increased in the presence of 6PGDH on cell stimulation. The present work proposes a new way of NADPH oxidase activity regulation by modulating Nox2 affinity for NADPH.

BCR-induced superoxide negatively regulates B-cell proliferation and T-cell-independent type 2 Ab responses

Superoxide and its derivatives have been implicated as secondary messenger molecules that influence signaling cascades in non-phagocytes. B lymphocytes produce superoxide after BCR ligation. We found that these ROS regulate B-cell signaling and entry into the cell cycle. B cells from mice deficient in the gp91(phox) subunit of the NADPH oxidase complex are unable to generate ROS after BCR ligation. However, after BCR stimulation, more gp91(phox) KO B cells enter the G1 stage of the cell cycle and proliferate than WT B cells. BCR ligation leads to a more rapid decrease in p27(Kip1) levels in gp91(phox) KO B cells. Gp91(phox) KO mice display enhanced T-cell-independent type 2, but normal T-dependent Ab responses. ROS-dependent regulation of BCR-induced proliferation may help modulate the size of the humoral response to T-cell-independent type 2 Ag immunization.

Regulation of B-cell entry into the cell cycle

B cells are induced to enter the cell cycle by stimuli including ligation of the B-cell receptor (BCR) complex and Toll-like receptor (TLR) agonists. This review discusses the contribution of several molecules, which act at distinct steps in B-cell activation. The adapter molecule Bam32 (B-lymphocyte adapter of 32 kDa) helps promote BCR-induced cell cycle entry, while the secondary messenger superoxide has the opposite effect. Bam32 and superoxide may fine tune BCR-induced activation by competing for the same limited resources, namely Rac1 and the plasma membrane phospholipid PI(3,4)P(2). The co-receptor CD22 can inhibit BCR-induced proliferation by binding to novel CD22 ligands. Finally, regulators of B-cell survival and death also play roles in B-cell transit through the cell cycle. Caspase 6 negatively regulates CD40- and TLR-dependent G(1) entry, while acting later in the cell cycle to promote S-phase entry. Caspase 6 deficiency predisposes B cells to differentiate rather than proliferate after stimulation. Bim, a pro-apoptotic Bcl-2 family member, exerts a positive regulatory effect on cell cycle entry, which is opposed by Bcl-2. New insights into what regulates B-cell transit through the cell cycle may lead to thoughtful design of highly selective drugs that target pathogenic B cells.

Regulation of phagocyte NADPH oxidase activity: identification of two cytochrome b558 activation states

Activation of the phagocyte NADPH oxidase (phox) requires the association of cytosolic proteins (p67-phox, p47-phox, p40-phox, and Rac1/2) with the membrane cytochrome b558, leading to a hemoprotein conformation change. To clarify this mechanism, the phagocyte NADPH oxidase complex was isolated through cytochrome b558 purification after three chromatographic steps. The purified neutrophil complex was constitutively active in the absence of an amphiphile agent with a maximum turnover (125 mol O2(-) x s(-1) x mol heme b(-1)), indicating that cytochrome b558 has been activated by cytosolic proteins and is in an "open conformation," able to transfer a maximum rate of electrons. In contrast, the phox complex prepared with B lymphocyte cytosol shows a lower constitutive turnover (approximately 50 mol O2(-) x s(-1) x mol heme b(-1)). Analysis of phox complex components by Western blot and mass spectrometry showed the presence of cytosolic factors (especially p67-phox) and structural proteins (moesin, ezrin). To investigate the difference in activity of phox complexes, we evaluated the effect of MRP8 and MRP14, specifically expressed in neutrophils, on the activity of the B lymphocyte complex. MRPs induce the switch between the partially and the fully "open" cytochrome b558 conformation. Moreover, their effect was independent of p67-phox. Data point out two potential cytochrome b558 activation states.

Genetic and mutational heterogeneity of autosomal recessive chronic granulomatous disease in Tunisia

NADPH oxidase, a multi-subunit protein consisting of cytosolic components and the membrane-bound heterodimer, plays an instrumental role in host defence mechanisms of phagocytes. Genetic deficiency of the enzymatic complex results in an inherited disorder, chronic granulomatous disease (CGD), which is characterized by an impaired phagocyte microbicidal activity. X-Linked (XL) CGD results from a mutation in the CYBB gene encoding the gp91phox subunit, while autosomal recessive (AR) CGD is associated with mutations in one of the NCF1, NCF2 and CYBA genes that encode the p47phox, p67phox and p22phox subunits, respectively. In the study reported here, we investigated genetic defects underlying CGD in 15 Tunisian patients from 14 unrelated families. Haplotype analyses and homozygosity mapping with microsatellite markers around known CGD genes assigned the genetic defect to NCF1 in four patients, to NCF2 in four patients and to CYBA in two patients. However, one family with two CGD patients seemed not to link the genetic defect to any known AR-CGD genes. Mutation screening identified two novel mutations in NCF2 and CYBA in addition to the recurrent mutation, DeltaGT, in NCF1 and a splice site mutation previously reported in a North African patient. Our results revealed the genetic and mutational heterogeneity of the AR recessive form of CGD in Tunisia.

Regulation of reactive oxygen species (ROS) production by C18 fatty acids in Jurkat and Raji cells

In the present study, the effects of C18 fatty acids with different numbers of double bonds, SA (stearic acid; C18:0), OA (oleic acid; C18:1), LA (linoleic acid; C18:2) and gamma-LNA (gamma-linolenic acid; C18:3), on ROS (reactive oxygen species) production by Jurkat (a human T-lymphocyte-derived cell line) and Raji (a human B-lymphocyte-derived cell line) cells were investigated. ROS production was determined by NBT (Nitro Blue Tetrazolium) reduction (intracellular and extracellular ROS production) and by dihydroethidium oxidation using flow cytometry (intracellular ROS production). The effectiveness on ROS production was gamma-LNA<SA<OA<LA in Jurkat cells and SA<gamma-LNA<OA<LA in Raji cells. LA (found in corn, soya bean and sunflower oils) was more potent than OA (found in olive oil) in stimulating ROS production in both Raji and Jurkat cells. The lower ROS production by OA compared with LA may be one of the benefits of olive oil consumption. As SA and gamma-LNA acids had little or no effect, further studies on the site of ROS production in these cells were carried out with OA and LA only. Activation of NADPH oxidase via PKC (protein kinase C) was found to be the major mechanism of ROS production induced by OA and LA in Jurkat and Raji cells.

Changing the conformation state of cytochrome b558 initiates NADPH oxidase activation: MRP8/MRP14 regulation

Phagocyte NADPH oxidase generates O2. for defense mechanisms and cellular signaling. Myeloid-related proteins MRP8 and MRP14 of the S100 family are EF-hand calcium-binding proteins. MRP8 and MRP14 were co-isolated from neutrophils on an anti-p47phox matrix with oxidase cytosolic factors and identified by mass spectrometry. MRP8 and MRP14 are absent from Epstein-Barr virus-immortalized B lymphocytes, and, coincidentally, these cells display weak oxidase activity compared with neutrophils. MRP8/MRP14 that was purified from neutrophils enhanced oxidase turnover of B cells in vitro, suggesting that MRP8/MRP14 is involved in the activation process. This was confirmed ex vivo by co-transfection of Epstein-Barr virus-transformed B lymphocytes with genes encoding MRP8 and MRP14. In a semi-recombinant cell-free assay, recombinant MRP8/MRP14 increased the affinity of p67phox for cytochrome b558 synergistically with p47phox. Moreover, MRP8/MRP14 initiated oxidase activation on its own, through a calcium-dependent specific interaction with cytochrome b558 as shown by atomic force microscopy and a structure-function relationship investigation. The data suggest that the change of conformation in cytochrome b558, which initiates the electron transfer, can be mediated by effectors other than oxidase cytosolic factors p67phox and p47phox. Moreover, MRP8/MRP14 dimer behaves as a positive mediator of phagocyte NADPH oxidase regulation.

Increased free radical production in hypertension due to increased expression of the NADPH oxidase subunit p22(phox) in lymphoblast cell lines

OBJECTIVES

To confirm increased production of reactive oxygen species (ROS) in hypertension, to demonstrate the source of ROS and to analyse NADPH oxidase subcomponent expression in hypertension.

DESIGN

A lymphoblast model was used, as this has previously been used in the study of hypertension and of NADPH oxidase. Chemiluminescence (CL) was chosen to assay ROS production, as it is simple and sensitive.

METHODS

Lymphocytes from 12 hypertensive patients (HT), and 12 age- and sex-matched normotensive (NT) subjects, were immortalized. Luminol, isoluminol and Cypridina luciferin analogue (CLA) CL were used to assay ROS production. NADPH oxidase subunits were measured by Western blot analysis.

RESULTS

Stimulation with 50 micromol/l arachidonic acid (AA) resulted in increased ROS production in HT cell lines with luminol, CLA and isoluminol CL. Stimulation with 500 nmol/l 12-O-tetradecanoylphorbol-13-acetate (TPA) produced a detectable increase in HT ROS production with luminol and with CLA, whereas there was no significant difference with isoluminol. The ROS production was abolished by diphenyleneiodonium chloride (DPI) but not by rotenone, indicating that a non-mitochondrial flavoprotein such as NADPH oxidase is the source of ROS. Analysis of NADPH oxidase subcomponents revealed an increase in p22(phox) in HT subjects.

CONCLUSIONS

We have shown there is increased ROS production in lymphoblasts derived from hypertensive subjects, probably originating from NADPH oxidase. As the ROS production persists in transformed cells, this suggests a genetic predisposition to increased ROS production. Increased expression of p22(phox) in HT lymphoblasts may account for some of the increased ROS.

Protein delivery by Pseudomonas type III secretion system: Ex vivo complementation of p67(phox)-deficient chronic granulomatous disease

Bacterial type III secretion system drives the translocation of virulence factors into the cystosol of host target cells. In phagocytes and in Epstein-Barr virus immortalized B lymphocytes, NADPH oxidase generates O(-2) through an electron transfer chain the activity of which depends on the assembly of three, p67(phox), p47(phox) and p40(phox) cytosolic activating factors with Rac 1/2 and a membrane redox component, cytochrome b(558). In p67(phox) deficient chronic granulomatous disease (CGD) patients, p67-phox is missing and NADPH oxidase activity is abolished. ExoS is a virulence factor of Pseudomonas aeruginosa which is secreted via the type III secretion system: it was fused with p67(phox). Pseudomonas aeruginosa synthesized and translocated the hybrid ExoS-p67(phox) fusion protein into the cytosol of B lymphocytes via the type III secretion system. Purified ExoS-p67(phox) hybrid protein was as efficient as normal recombinant p67(phox) in cell-free reconstitution of NADPH oxidase activity. Therefore, ExoS-p67(phox) was transferred via the type III secretion system of Pseudomonas aeruginosa into the cytosol of B lymphocytes from a p67(phox)-deficient CGD patient and functionally reconstituted NADPH oxidase activity. In the complementation process, ExoS acted as a molecular courier for protein delivery: the reconstitution of an active NADPH oxidase complex suggests type III secretion system to be a new approach for cellular therapy.

Superoxide generation and tyrosine kinase

NADPH oxidase is a multi-subunit enzyme complex responsible for superoxide generation in many cells, for example, B-lymphocytes and osteoclasts. NADPH oxidase is localized on the cell surface and generates superoxide extracellularly. After synthesis, components of this oxidase are transported to the cell membrane where the functional NADPH oxidase complex is assembled. The mechanism by which the membrane-bound components are transported to the cell surface of osteoclasts remains unclear. In this study, we examined the role of tyrosine kinase activity in the transport of NADPH oxidase components. When B-lymphocytes and osteoclasts were treated with herbimycin A, a specific inhibitor of tyrosine kinase, superoxide production was significantly decreased. The amount of p91, the catalytic subunit of NADPH oxidase, was decreased in the cellular membrane of herbimycin A treated cells compared to untreated controls. Similar results were obtained for the movement of a regulatory subunit of the NADPH oxidase complex, p47, in B-lymphocytes. Thus, inhibition of tyrosine kinase decreases superoxide production by disrupting the translocation of the NADPH oxidase complex.

The Ku70 autoantigen interacts with p40phox in B lymphocytes

Ku70, a regulatory component of the DNA-dependent protein kinase, was identified by a yeast two-hybrid screen of a B lymphocyte cDNA library as a partner of p40phox, a regulatory component of the O2--producing NADPH oxidase. Truncated constructs of p40phox and Ku70 were used to map the interacting sites. The 186 C-terminal amino acids (aa) of Ku70 were found to interact with two distinct regions of p40phox, the central core region (aa 50–260) and the C-terminal extremity (aa 260–339). In complementary experiments, it was observed that Ku70 binds to immobilized recombinant p40phox fusion protein and that p40phox and Ku70 from a B lymphocyte cell extract comigrate in successive chromatographies on Q Separose, Superose 12 and hydroxylapatite columns. Moreover, we report that Ku70 and p40phox colocalize in B lymphocytes and in transfected Cos-7 cells. We also show that the two NADPH oxidase activating factors, p47phox and p67phox are substrates for DNA-PK in vitro and that they are present together with p40phox in the nucleus of B cells. These results may help solve the paradox that the phox protein triad, p40phox, p47phox and p67phox, is expressed equally in B lymphocytes and neutrophils, whereas the redox component of the NADPH oxidase, a flavocytochrome b, which is well expressed in neutrophils, is barely detectable in B lymphocytes.

Biochemical and immunochemical properties of B lymphocyte cytochrome b558

Like neutrophils, Epstein-Barr virus (EBV)-immortalized B lymphocytes express all constituents of the NADPH oxidase complex necessary to generate superoxide anion O2-. The NADPH oxidase activity in EBV-B lymphocytes is only 5% of that measured in neutrophils upon PMA stimulation. Cytochrome b558 is the sole redox membrane component of NADPH oxidase; it is the protein core around which cytosolic factors assemble in order to mediate oxidase activity. In the present study, we have compared the structural and functional properties of cytochrome b558 from EBV-B lymphocytes and neutrophils. Cytochrome b558 from EBV-B lymphocyte plasma membrane, like that from neutrophils, is characterized by a heterodimeric structure with a highly glycosylated beta subunit, known as gp91-phox. While the amount of cytochrome b558 recovered after purification from EBV-B lymphocytes (approximately 0.24 nmol from 1010 cells) was low compared to that recovered from neutrophils (approximately 10 nmol), the biochemical properties of purified cytochrome b558 from both EBV-B lymphocytes and neutrophils were quite similar with respect to their differential spectra, redox potential, and FAD binding site. Once cytochrome b558 was extracted from the EBV-B lymphocyte membrane, it was able to mediate, in a reconstituted system of O2- production the same oxidase turnover as that found for cytochrome b558 extracted from neutrophils. A comparison between membrane bound and soluble cytochrome b558 suggested that the weak oxidase activity measured in intact EBV-B cells might be the result not only of the small amount of expressed cytochrome b558, but also of a defect of the activation process in lymphocyte membrane.

Characteristics and responses of EBV immortalized B cells from periodontal disease patients

OBJECTIVE

This study was designed to examine human B cell responses to Actinobacillus actinomycetemcomitans (Aa). The general hypothesis to be tested was that Epstein-Barr virus (EBV) immortalized B cells could be used to investigate variations in B cell responsiveness of periodontitis patients to periodontal pathogens, and that B cells derived from the peripheral blood of periodontal disease patients infected with Aa demonstrate differences in in vitro activities compared to periodontally healthy subjects.

DESIGN

EBV-transformed B cell lines were used to analyze immunoglobulin and Aa-specific antibody responses, as well as to determine the frequencies of cells producing immunoglobulin (Ig) of a specific isotype and detect clones secreting antibodies specific for Aa. Lymphoblastoid cells lines (LCL) were derived by clonal transformation of peripheral blood lymphocytes from 10 Aa-infected patients with adult periodontitis (Aa-AP) and seven normal subjects.

METHODS

The B cells were incubated in Aa-coated polystyrene plates to separate adherent and non-adherent cells, and stimulate the cells with the whole bacteria. In addition, the B cells were stimulated with Aa LPS, E. coli LPS, or the polyclonal B cell activators (PBAs), pokeweed mitogen (PWM) and Staphylococcus aureus protein A (SpA). Both adherent and non-adherent cell populations were cultured for up to 15 days.

MAIN OUTCOME MEASURE

Total immunoglobulins (Igs) and antibody (IgG, IgA, IgM) levels to Aa in the culture supernatants were assessed using an ELISA. The distribution of IgG, IgA, IgM and Aa-specific antibody producing cells was analyzed by a double immunoenzymatic staining technique.

RESULTS

IgM levels produced by the LCLs were significantly increased vs IgG and IgA (P < 0.001). Three days after Aa stimulation, a marked increase in the level of total Igs and Aa-specific antibody was observed in adherent cells from Aa-AP (P < 0.05-0.03). Aa-specific antibody levels were significantly higher in the supernatants from Aa-AP vs normals throughout the culture interval (P < 0.03). There was also a significant increase in Aa-specific antibody levels after stimulation with Aa LPS or E. coli LPS (P < 0.05), whereas PWM and SpA had no significant effect on antibody to Aa. There was a predominance of IgM cells compared to IgG and IgA isotypes (P < 0.04) in LCLs from Aa-infected patients. After stimulation with Aa, a significant increase in the number of IgA (111%) and IgG (48%) secreting cells was observed, concomitant with a 74% decrease in the Ig-negative cell population. Total Aa+ cells increased significantly after stimulation (P < 0.001), predominated by Aa-specific IgG and IgM antibody producing cells.

CONCLUSIONS

These results showed that LCLs from Aa-infected patients were polyclonal with respect to isotype distribution. Further stimulation with Aa revealed a shift to cytoplasmic IgG and IgA expression, as well as increases in the Aa-specific B cell population. In contrast, the PBAs stimulated the LCLs to synthesize primarily IgM. Additionally, the findings indicated that: (1) without T cells, polyclonal activation of B cells may lead to elevated Aa-specific B cell populations; and (2) the presence of previously sensitized B cells is required to exert an antigen specific antibody response in the LCL. We conclude that secondary activation of primed B cells by oral bacteria or their products in advanced periodontal lesions may contribute to the local accumulation of significant numbers of Ig-producing cells. This report also suggested that EBV-mediated transformation can be used to probe B cell-bacterial interactions in studies of periodontitis.

Characterization of neutrophil NADPH oxidase activity reconstituted in a cell-free assay using specific monoclonal antibodies raised against cytochrome b558

The immunochemical characterization of NADPH oxidase activity of cytochrome b558 purified from human neutrophils was determined after reconstitution in a cell-free assay using the native hemoprotein and recombinant purified cytosolic activating factors. The oxidase activity showed a strict dependence on the heme content at each step of the hemoprotein purification process. The immunochemical properties of the reconstituted oxidase made use of monoclonal antibodies raised against membrane-bound and octyl-glucoside-extracted cytochrome b. From nine specific monoclonal antibodies reacting with gp91-phox cytochrome b558, two were selected, both of which were found to bind to the beta subunit of cytochrome b558 and to inhibit superoxide formation in the oxidase reconstituted cell-free assay. The extent of inhibition was dependent on the phospholipid environment. Neutrophil membrane extracts from X-linked chronic granulomatous disease patients did not produce O2- in the reconstituted system and did not bind to the antibodies.

Low NADPH oxidase activity in Epstein-Barr-virus-immortalized B-lymphocytes is due to a post-transcriptional block in expression of cytochrome b558

The NADPH oxidase of phagocytes is known to be expressed in Epstein-Barr-virus-transformed B-lymphocytes, albeit at levels only approx. 5% of those found in neutrophils. We have investigated the basis of this low level of expression and find that all four specific components of the NADPH oxidase are expressed in B-lymphocytes, but only p47-phox protein attains levels equivalent with those found in neutrophils. This component was shown to phosphorylate and translocate to the membrane normally on activation. The other cytosolic component, p67-phox, did show a deficit, and by supplementing a B-cell cytosol extract with recombinant p67-phox, this was shown to account for the somewhat reduced activity of B-cell cytosol in a cell-free oxidase system. The cell-free analysis also clearly located the major deficiency in superoxide-generating capacity of B-lymphocytes to the membrane. Western blotting of membrane proteins revealed major reductions in the amount of cytochrome b558. Analysis of the levels of mRNA for both subunits of cytochrome b558, however, showed levels greater than expected. Significantly more mRNA for gp91-phox was present in B-cells than in undifferentiated HL60 cells, although it was not quite as abundant as in differentiated HL60 cells, which are capable of producing large amounts of superoxide. We conclude that the failure of B-lymphocytes to generate amounts of superoxide equivalent to those generated by neutrophils is primarily due to a post-transcriptionally determined block to the accumulation of cytochrome b558.

Small G proteins and the neutrophil NADPH oxidase

The NADPH oxidase of phagocytic cells is a multimeric enzyme complex activated during phagocytosis. It catalyzes the production of the superoxide anion, precursor of many toxic oxygen metabolites involved in the defense against microorganisms. The enzyme becomes active after assembly on a membrane bound flavocytochrome b of cytosolic factors p47 phox, p67 phox and p40 phox and of low molecular mass GTP binding proteins. This paper reviews recent results concerning the role of two small G proteins, Rac and Rap 1A in oxidase activation. Native prenylated small G proteins are either in the form of a complex in which the GDP bound G protein is associated with a guanine nucleotide dissociation inhibitor, GDI, or in an active GTP bound form able to trigger the activity of its effector. Rac and Rho share a common GDI. As chemotaxis, under Rho control, and oxidase activation, under Rac control, show mutually exclusive signalling pathways, we propose a model where the GDI would switch from one pathway to the other by sequestering either Rac or Rho.