Crystal structure of the breakpoint cluster region-homology domain from phosphoinositide 3-kinase p85 alpha subunit

Molecular Basis of Oncogenic PI3K Proteins

The dysregulation of phosphatidylinositol 3-kinase (PI3K) signaling plays a pivotal role in driving neoplastic transformation by promoting uncontrolled cell survival and proliferation. This oncogenic activity is primarily caused by mutations that are frequently found in PI3K genes and constitutively activate the PI3K signaling pathway. However, tumorigenesis can also arise from nonmutated PI3K proteins adopting unique active conformations, further complicating the understanding of PI3K-driven cancers. Recent structural studies have illuminated the functional divergence among highly homologous PI3K proteins, revealing how subtle structural alterations significantly impact their activity and contribute to tumorigenesis. In this review, we summarize current knowledge of Class I PI3K proteins and aim to unravel the complex mechanism underlying their oncogenic traits. These insights will not only enhance our understanding of PI3K-mediated oncogenesis but also pave the way for the design of novel PI3K-based therapies to combat cancers driven by this signaling pathway.

Dynamic molecular architecture and substrate recruitment of cullin3-RING E3 ligase CRL3KBTBD2

Phosphatidylinositol 3-kinase α, a heterodimer of catalytic p110α and one of five regulatory subunits, mediates insulin- and insulin like growth factor-signaling and, frequently, oncogenesis. Cellular levels of the regulatory p85α subunit are tightly controlled by regulated proteasomal degradation. In adipose tissue and growth plates, failure of K48-linked p85α ubiquitination causes diabetes, lipodystrophy and dwarfism in mice, as in humans with SHORT syndrome. Here we elucidated the structures of the key ubiquitin ligase complexes regulating p85α availability. Specificity is provided by the substrate receptor KBTBD2, which recruits p85α to the cullin3-RING E3 ubiquitin ligase (CRL3). CRL3KBTBD2 forms multimers, which disassemble into dimers upon substrate binding (CRL3KBTBD2-p85α) and/or neddylation by the activator NEDD8 (CRL3KBTBD2~N8), leading to p85α ubiquitination and degradation. Deactivation involves dissociation of NEDD8 mediated by the COP9 signalosome and displacement of KBTBD2 by the inhibitor CAND1. The hereby identified structural basis of p85α regulation opens the way to better understanding disturbances of glucose regulation, growth and cancer.

Class IA PI3K regulatory subunits: p110-independent roles and structures

The phosphatidylinositol 3-kinase (PI3K) pathway is a critical regulator of many cellular processes including cell survival, growth, proliferation and motility. Not surprisingly therefore, the PI3K pathway is one of the most frequently mutated pathways in human cancers. In addition to their canonical role as part of the PI3K holoenzyme, the class IA PI3K regulatory subunits undertake critical functions independent of PI3K. The PI3K regulatory subunits exist in excess over the p110 catalytic subunits and therefore free in the cell. p110-independent p85 is unstable and exists in a monomer-dimer equilibrium. Two conformations of dimeric p85 have been reported that are mediated by N-terminal and C-terminal protein domain interactions, respectively. The role of p110-independent p85 is under investigation and it has been found to perform critical adaptor functions, sequestering or influencing compartmentalisation of key signalling proteins. Free p85 has roles in glucose homeostasis, cellular stress pathways, receptor trafficking and cell migration. As a regulator of fundamental pathways, the amount of p110-independent p85 in the cell is critical. Factors that influence the monomer-dimer equilibrium of p110-independent p85 offer additional control over this system, disruption to which likely results in disease. Here we review the current knowledge of the structure and functions of p110-independent class IA PI3K regulatory subunits.

Impact of p85α Alterations in Cancer

The phosphatidylinositol 3-kinase (PI3K) pathway plays a central role in the regulation of cell signaling, proliferation, survival, migration and vesicle trafficking in normal cells and is frequently deregulated in many cancers. The p85α protein is the most characterized regulatory subunit of the class IA PI3Ks, best known for its regulation of the p110-PI3K catalytic subunit. In this review, we will discuss the impact of p85α mutations or alterations in expression levels on the proteins p85α is known to bind and regulate. We will focus on alterations within the N-terminal half of p85α that primarily regulate Rab5 and some members of the Rho-family of GTPases, as well as those that regulate PTEN (phosphatase and tensin homologue deleted on chromosome 10), the enzyme that directly counteracts PI3K signaling. We highlight recent data, mapping the interaction surfaces of the PTEN⁻p85α breakpoint cluster region homology (BH) domain, which sheds new light on key residues in both proteins. As a multifunctional protein that binds and regulates many different proteins, p85α mutations at different sites have different impacts in cancer and would necessarily require distinct treatment strategies to be effective.

Insight into the PTEN - p85α interaction and lipid binding properties of the p85α BH domain

The phosphatidylinositol 3-kinase (PI3K) pathway plays a key role in regulating cell growth and cell survival and is frequently deregulated in cancer cells. p85α regulates the p110α lipid kinase, and also stabilizes and stimulates PTEN, the lipid phosphatase that downregulates this pathway. In this report, we determined that the p85α BH domain binds several phosphorylated phosphoinositide lipids, an interaction that could help localize p85α to membranes rich in these lipids. We also identified key residues responsible for mediating PTEN – p85α complex formation. Based on these experimental results, a docking model for the PTEN – p85α BH domain complex was developed that is consistent with the known binding interactions for both PTEN and p85α. This model involves extensive side-chain and peptide backbone contacts between both the PASE and C2 domains of PTEN with the p85α BH domains. The p85α BH domain residues shown to be important for PTEN binding were p85α residues E212, Q221, K225, R228 and H234. We also verified experimentally the importance of PTEN-E91 in mediating the interaction with the p85α BH domain. These results shed new light on the mechanism of PTEN regulation by p85α.

Patient-derived mutations within the N-terminal domains of p85α impact PTEN or Rab5 binding and regulation

The p85α protein regulates flux through the PI3K/PTEN signaling pathway, and also controls receptor trafficking via regulation of Rab-family GTPases. In this report, we determined the impact of several cancer patient-derived p85α mutations located within the N-terminal domains of p85α previously shown to bind PTEN and Rab5, and regulate their respective functions. One p85α mutation, L30F, significantly reduced the steady state binding to PTEN, yet enhanced the stimulation of PTEN lipid phosphatase activity. Three other p85α mutations (E137K, K288Q, E297K) also altered the regulation of PTEN catalytic activity. In contrast, many p85α mutations reduced the binding to Rab5 (L30F, I69L, I82F, I177N, E217K), and several impacted the GAP activity of p85α towards Rab5 (E137K, I177N, E217K, E297K). We determined the crystal structure of several of these p85α BH domain mutants (E137K, E217K, R262T E297K) for bovine p85α BH and found that the mutations did not alter the overall domain structure. Thus, several p85α mutations found in human cancers may deregulate PTEN and/or Rab5 regulated pathways to contribute to oncogenesis. We also engineered several experimental mutations within the p85α BH domain and identified L191 and V263 as important for both binding and regulation of Rab5 activity.

P85 regulates neuronal migration through affecting neuronal morphology during mouse corticogenesis

In mammalian developing embryonic cortex, projection neurons migrate from the ventricular zone to the cortical plate, guided by radial glial cells with a transformation between bipolar and multipolar morphology. Previous studies have demonstrated that the PI3K-Akt-mTOR signal plays a critical role in brain development. However, the function of P85 in cortical development is still unclear. In the present study, we found that overexpression of P85 impaired cortical neuronal migration. Using in utero electroporation, we revealed that the length of the leading process in P85 overexpressed neurons became shorter than that in the control group but with more branches. Using markers for new-born neurons, we further found that overexpression of P85 did not affect the ultimate fate of these cortical neurons. These findings indicated that the P85 subunit plays an essential role in neuronal migration and neuronal morphology during mouse corticogenesis.

New Insights into PI3K Inhibitor Design using X-ray Structures of PI3Kα Complexed with a Potent Lead Compound

Phosphatidylinositol 3-kinase α is an attractive target to potentially treat a range of cancers. Herein, we described the evolution of a reported PI3K inhibitor into a moderate PI3Kα inhibitor with a low molecular weight. We used X-ray crystallography to describe the accurate binding mode of the compound YXY-4F. A comparison of the p110α–YXY-4F and apo p110α complexes showed that YXY-4F induced additional space by promoting a flexible conformational change in residues Ser773 and Ser774 in the PI3Kα ATP catalytic site. Specifically, residue 773(S) in PI3Kα is quite different from that of PI3Kβ (D), γ (A), and δ (D), which might guide further optimization of substituents around the NH group and phenyl group to improve the selectivity and potency of PI3Kα.

Modulation of nongenomic activation of PI3K signalling by tetramerization of N-terminally-cleaved RXRα

Retinoid X receptor-alpha (RXRα) binds to DNA either as homodimers or heterodimers, but it also forms homotetramers whose function is poorly defined. We previously discovered that an N-terminally-cleaved form of RXRα (tRXRα), produced in tumour cells, activates phosphoinositide 3-kinase (PI3K) signalling by binding to the p85α subunit of PI3K and that K-80003, an anti-cancer agent, inhibits this process. Here, we report through crystallographic and biochemical studies that K-80003 binds to and stabilizes tRXRα tetramers via a ‘three-pronged’ combination of canonical and non-canonical mechanisms. K-80003 binding has no effect on tetramerization of RXRα, owing to the head–tail interaction that is absent in tRXRα. We also identify an LxxLL motif in p85α, which binds to the coactivator-binding groove on tRXRα and dissociates from tRXRα upon tRXRα tetramerization. These results identify conformational selection as the mechanism for inhibiting the nongenomic action of tRXRα and provide molecular insights into the development of RXRα cancer therapeutics.

The transcription factor retinoid X receptor-alpha (RXRα) can also form homotetramers. Here the authors show that the anti-cancer agent K-80003 selectively inhibits the nongenomic action of N-terminally-cleaved RXRα in tumour cells by stabilizing its tetramerization but not that of full-length RXRα.

Structure of the Toll/Interleukin-1 Receptor (TIR) Domain of the B-cell Adaptor That Links Phosphoinositide Metabolism with the Negative Regulation of the Toll-like Receptor (TLR) Signalosome

Ligand binding to Toll-like receptors (TLRs) results in dimerization of their cytosolic Toll/interleukin-1 receptor (TIR) domains and recruitment of post-receptor signal transducers into a complex signalosome. TLR activation leads to the production of transcription factors and pro-inflammatory molecules and the activation of phosphoinositide 3-kinases (PI3K) in a process that requires the multimodular B-cell adaptor for phosphoinositide 3-kinase (BCAP). BCAP has a sequence previously proposed as a "cryptic" TIR domain. Here, we present the structure of the N-terminal region of human BCAP and show that it possesses a canonical TIR fold. Dimeric BCAP associates with the TIR domains of TLR2/4 and MAL/TIRAP, suggesting that it is recruited to the TLR signalosome by multitypic TIR-TIR interactions. BCAP also interacts with the p85 subunit of PI3K and phospholipase Cγ, enzymes that deplete plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP2), and these interactions provide a molecular explanation for BCAP-mediated down-regulation of inflammatory signaling.

Salt-Inducible Kinase 2 Couples Ovarian Cancer Cell Metabolism with Survival at the Adipocyte-Rich Metastatic Niche

The adipocyte-rich microenvironment forms a niche for ovarian cancer metastasis, but the mechanisms driving this process are incompletely understood. Here we show that salt-inducible kinase 2 (SIK2) is overexpressed in adipocyte-rich metastatic deposits compared with ovarian primary lesions. Overexpression of SIK2 in ovarian cancer cells promotes abdominal metastasis while SIK2 depletion prevents metastasis in vivo. Importantly, adipocytes induce calcium-dependent activation and autophosphorylation of SIK2. Activated SIK2 plays a dual role in augmenting AMPK-induced phosphorylation of acetyl-CoA carboxylase and in activating the PI3K/AKT pathway through p85α-S154 phosphorylation. These findings identify SIK2 at the apex of the adipocyte-induced signaling cascades in cancer cells and make a compelling case for targeting SIK2 for therapy in ovarian cancer.

Assembly and Molecular Architecture of the Phosphoinositide 3-Kinase p85α Homodimer

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that are activated by growth factor and G-protein-coupled receptors and propagate intracellular signals for growth, survival, proliferation, and metabolism. p85α, a modular protein consisting of five domains, binds and inhibits the enzymatic activity of class IA PI3K catalytic subunits. Here, we describe the structural states of the p85α dimer, based on data from in vivo and in vitro solution characterization. Our in vitro assembly and structural analyses have been enabled by the creation of cysteine-free p85α that is functionally equivalent to native p85α. Analytical ultracentrifugation studies showed that p85α undergoes rapidly reversible monomer-dimer assembly that is highly exothermic in nature. In addition to the documented SH3-PR1 dimerization interaction, we identified a second intermolecular interaction mediated by cSH2 domains at the C-terminal end of the polypeptide. We have demonstrated in vivo concentration-dependent dimerization of p85α using fluorescence fluctuation spectroscopy. Finally, we have defined solution conditions under which the protein is predominantly monomeric or dimeric, providing the basis for small angle x-ray scattering and chemical cross-linking structural analysis of the discrete dimer. These experimental data have been used for the integrative structure determination of the p85α dimer. Our study provides new insight into the structure and assembly of the p85α homodimer and suggests that this protein is a highly dynamic molecule whose conformational flexibility allows it to transiently associate with multiple binding proteins.

Regulation of the PI3K pathway through a p85α monomer-homodimer equilibrium

The canonical action of the p85α regulatory subunit of phosphatidylinositol 3-kinase (PI3K) is to associate with the p110α catalytic subunit to allow stimuli-dependent activation of the PI3K pathway. We elucidate a p110α-independent role of homodimerized p85α in the positive regulation of PTEN stability and activity. p110α-free p85α homodimerizes via two intermolecular interactions (SH3:proline-rich region and BH:BH) to selectively bind unphosphorylated activated PTEN. As a consequence, homodimeric but not monomeric p85α suppresses the PI3K pathway by protecting PTEN from E3 ligase WWP2-mediated proteasomal degradation. Further, the p85α homodimer enhances the lipid phosphatase activity and membrane association of PTEN. Strikingly, we identified cancer patient-derived oncogenic p85α mutations that target the homodimerization or PTEN interaction surface. Collectively, our data suggest the equilibrium of p85α monomer-dimers regulates the PI3K pathway and disrupting this equilibrium could lead to disease development.

Identification of the GTPase-activating protein DEP domain containing 1B (DEPDC1B) as a transcriptional target of Pitx2

Pitx2 is a bicoid-related homeobox transcription factor implicated in regulating left-right patterning and organogenesis. However, only a limited number of Pitx2 downstream target genes have been identified and characterized. Here we demonstrate that Pitx2 is a transcriptional repressor of DEP domain containing 1B (DEPDC1B). The first intron of the human and mouse DEP domain containing 1B genes contains multiple consensus DNA-binding sites for Pitx2. Chromatin immunoprecipitation assays revealed that Pitx2, along with histone deacetylase 1, was recruited to the first intron of Depdc1b. In contrast, RNAi-mediated depletion of Pitx2 not only enhanced the acetylation of histone H4 in the first intron of Depdc1b, but also increased the protein level of Depdc1b. Luciferase reporter assays also showed that Pitx2 could repress the transcriptional activity mediated by the first intron of human DEPDC1B. The GAP domain of DEPDC1B interacted with nucleotide-bound forms of RAC1 in vitro. In addition, exogenous expression of DEPDC1B suppressed RAC1 activation and interfered with actin polymerization induced by the guanine nucleotide exchange factor TRIO. Moreover, DEPDC1B interacted with various signaling molecules such as U2af2, Erh, and Salm. We propose that Pitx2-mediated repression of Depdc1b expression contributes to the regulation of multiple molecular pathways, such as Rho GTPase signaling.

A putative role of the Sup35p C-terminal domain in the cytoskeleton organization during yeast mitosis

Based on structural analysis of several effectors and partners, Sup35pC is proposed to serve as actin modulator during mitosis.

Identification of mutations in distinct regions of p85 alpha in urothelial cancer

Bladder cancers commonly show genetic aberrations in the phosphatidylinositol 3-kinase signaling pathway. Here we have screened for mutations in PIK3R1, which encodes p85α, one of the regulatory subunits of PI3K. Two hundred and sixty-four bladder tumours and 41 bladder tumour cell lines were screened and 18 mutations were detected. Thirteen mutations were in C-terminal domains and are predicted to interfere with the interaction between p85α and p110α. Five mutations were in the BH domain of PIK3R1. This region has been implicated in p110α-independent roles of p85α, such as binding to and altering the activities of PTEN, Rab4 and Rab5. Expression of these mutant BH-p85α forms in mouse embryonic fibroblasts with p85α knockout indicated that all forms, except the truncation mutants, could bind and stabilize p110α but did not increase AKT phosphorylation, suggesting that BH mutations function independently of p110α. In a panel of 44 bladder tumour cell lines, 80% had reduced PIK3R1 mRNA expression relative to normal urothelial cells. This, along with mutation of PIK3R1, may alter BH domain functioning. Our findings suggest that mutant forms of p85α may play an oncogenic role in bladder cancer, not only via loss of ability to regulate p110α but also via altered function of the BH domain.

Dynamic steps in receptor tyrosine kinase mediated activation of class IA phosphoinositide 3-kinases (PI3K) captured by H/D exchange (HDX-MS)

The catalytic subunits of all class IA phosphoinositide 3-kinases (PI3Ks) associate with identical p85-related subunits and phosphorylate PIP2 yielding PIP3, but they can vary greatly in the signaling pathways in which they participate. The binding of the p85 subunit to the p110 catalytic subunits is constitutive, and this inhibits activity, but some of the inhibitory contacts are reversible and subject to regulation. Interaction with phosphotyrosine-containing peptides (RTK-pY) releases a subset of these inhibitory contacts. Hydrogen/deuterium exchange mass spectrometry (HDX-MS) provides a map of the dynamic interactions unique to each of the isotypes. RTK-pY binding exposes the p110 helical domains for all class IA enzymes (due to release of the nSH2 contact) and exposes the C-lobe of the kinase domains of p110β and p110δ (resulting from release of the cSH2 contact). Consistent with this, our in vitro assays show that all class IA isoforms are inhibited by the nSH2, but only p110β and p110δ are inhibited by the cSH2. While a C2/iSH2 inhibitory contact exists in all isoforms, HDX indicates that p110β releases this contact most readily. The unique dynamic relationships of the different p110 isozymes to the p85 subunit may facilitate new strategies for specific inhibitors of the PI3Ks.

Rho/RacGAPs: embarras de richesse?

Regulatory proteins such as guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs) determine the activity of small GTPases. In the Rho/Rac family, the number of GEFs and GAPs largely exceeds the number of small GTPases, raising the question of specific or overlapping functions. In our recent study we investigated the first time ARHGAP25 at the protein level, determined its activity as RacGAP and showed its involvement in phagocytosis. With the discovery of ARHGAP25, the number of RacGAPs described in phagocytes is increased to six. We provide data that indicate the specific functions of selected Rho/RacGAPs and we show an example of differential regulation of a Rho/Rac family GAP by different kinases. We propose that the abundance of Rho/Rac family GAPs is an important element of the fine spatiotemporal regulation of diverse cellular functions.

Small G proteins and their regulators in cellular signalling

Small molecular weight GTPases (small G proteins) are essential in the transduction of signals from different plasma membrane receptors. Due to their endogenous GTP-hydrolyzing activity, these proteins function as time-dependent biological switches controlling diverse cellular functions including cell shape and migration, cell proliferation, gene transcription, vesicular transport and membrane-trafficking. This review focuses on endocrine diseases linked to small G proteins. We provide examples for the regulation of the activity of small G proteins by various mechanisms such as posttranslational modifications, guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs) or guanine nucleotide dissociation inhibitors (GDIs). Finally we summarize endocrine diseases where small G proteins or their regulatory proteins have been revealed as the cause.

Physiological mechanisms of signal termination in biological systems

Studies on the regulation of cellular activity mainly focus on signal generation, but termination of signalling is an equally important factor, which prevents inappropriate activity. This paper reviews the mechanisms, which can cause termination of signalling, and provides examples that illustrate the importance of these processes. Inactivation of voltage-gated Na(+) channels and the photoactivated rhodopsin molecule is caused by rapid conformational rearrangements. Negative feedback can also contribute to the termination of signalling for various mechanisms, including plasma membrane ion channels or cAMP signal generation. In immune cells, the tyrosine-based inhibitory motif (ITIM)-containing molecules are essential negative regulatory components. Desensitization of G-protein-coupled receptors can occur with homologous and heterologous mechanisms, mediated by β-arrestin molecules and second messenger-induced kinases respectively. In NF-κB signalling, resynthetized IκB and other enzymes form negative feedback loops. GTPase-activating proteins are also dedicated to termination of signalling, because they can switch off the small G proteins by increasing their endogenous GTP hydrolysis. In many systems, signal termination is a result of a combined action of several different mechanisms, which underlines the importance of these processes.

Inhibition and Termination of Physiological Responses by GTPase Activating Proteins

Physiological processes are strictly organized in space and time. However, in cell physiology research, more attention is given to the question of space rather than to time. To function as a signal, environmental changes must be restricted in time; they need not only be initiated but also terminated. In this review, we concentrate on the role of one specific protein family involved in biological signal termination. GTPase activating proteins (GAPs) accelerate the endogenously low GTP hydrolysis rate of monomeric guanine nucleotide-binding proteins (GNBPs), limiting thereby their prevalence in the active, GTP-bound form. We discuss cases where defective or excessive GAP activity of specific proteins causes significant alteration in the function of the nervous, endocrine, and hemopoietic systems, or contributes to development of infections and tumors. Biochemical and genetic data as well as observations from human pathology support the notion that GAPs represent vital elements in the spatiotemporal fine tuning of physiological processes.

Multiple roles for the p85α isoform in the regulation and function of PI3K signalling and receptor trafficking

The p85α protein is best known as the regulatory subunit of class 1A PI3Ks (phosphoinositide 3-kinases) through its interaction, stabilization and repression of p110-PI3K catalytic subunits. PI3Ks play multiple roles in the regulation of cell survival, signalling, proliferation, migration and vesicle trafficking. The present review will focus on p85α, with special emphasis on its important roles in the regulation of PTEN (phosphatase and tensin homologue deleted on chromosome 10) and Rab5 functions. The phosphatidylinositol-3-phosphatase PTEN directly counteracts PI3K signalling through dephosphorylation of PI3K lipid products. Thus the balance of p85α–p110 and p85α–PTEN complexes determines the signalling output of the PI3K/PTEN pathway, and under conditions of reduced p85α levels, the p85α–PTEN complex is selectively reduced, promoting PI3K signalling. Rab5 GTPases are important during the endocytosis, intracellular trafficking and degradation of activated receptor complexes. The p85α protein helps switch off Rab5, and if defective in this p85α function, results in sustained activated receptor tyrosine kinase signalling and cell transformation through disrupted receptor trafficking. The central role for p85α in the regulation of PTEN and Rab5 has widened the scope of p85α functions to include integration of PI3K activation (p110-mediated), deactivation (PTEN-mediated) and receptor trafficking/signalling (Rab5-mediated) functions, all with key roles in maintaining cellular homoeostasis.

ARHGAP25, a novel Rac GTPase-activating protein, regulates phagocytosis in human neutrophilic granulocytes

Members of the Rac/Rho family of small GTPases play an essential role in phagocytic cells in organization of the actin cytoskeleton and production of toxic oxygen compounds. GTPase-activating proteins (GAPs) decrease the amount of the GTP-bound active form of small GTPases, and contribute to the control of biologic signals. The number of potential Rac/RhoGAPs largely exceeds the number of Rac/Rho GTPases and the expression profile, and their specific role in different cell types is largely unknown. In this study, we report for the first time the properties of full-length ARHGAP25 protein, and show that it is specifically expressed in hematopoietic cells, and acts as a RacGAP both in vitro and in vivo. By silencing and overexpressing the protein in neutrophil model cell lines (PLB-985 and CosPhoxFcγR, respectively) and in primary macrophages, we demonstrate that ARHGAP25 is a negative regulator of phagocytosis acting probably via modulation of the actin cytoskeleton.

Experimental mapping of soluble protein domains using a hierarchical approach

Exploring the function and 3D space of large multidomain protein targets often requires sophisticated experimentation to obtain the targets in a form suitable for structure determination. Screening methods capable of selecting well-expressed, soluble fragments from DNA libraries exist, but require the use of automation to maximize chances of picking a few good candidates. Here, we describe the use of an insertion dihydrofolate reductase (DHFR) vector to select in-frame fragments and a split-GFP assay technology to filter-out constructs that express insoluble protein fragments. With the incorporation of an IPCR step to create high density, focused sublibraries of fragments, this cost-effective method can be performed manually with no a priori knowledge of domain boundaries while permitting single amino acid resolution boundary mapping. We used it on the well-characterized p85α subunit of the phosphoinositide-3-kinase to demonstrate the robustness and efficiency of our methodology. We then successfully tested it onto the polyketide synthase PpsC from Mycobacterium tuberculosis, a potential drug target involved in the biosynthesis of complex lipids in the cell envelope. X-ray quality crystals from the acyl-transferase (AT), dehydratase (DH) and enoyl-reductase (ER) domains have been obtained.

A high-throughput immobilized bead screen for stable proteins and multi-protein complexes

We describe an in vitro colony screen to identify Escherichia coli expressing soluble proteins and stable, assembled multiprotein complexes. Proteins with an N-terminal 6His tag and C-terminal green fluorescent protein (GFP) S11 tag are fluorescently labeled in cells by complementation with a coexpressed GFP 1-10 fragment. After partial colony lysis, the fluorescent soluble proteins or complexes diffuse through a supporting filtration membrane and are captured on Talon(®) resin metal affinity beads immobilized in agarose. Images of the fluorescent colonies convey total expression and the level of fluorescence bound to the beads indicates how much protein is soluble. Both pieces of information can be used together when selecting clones. After the assay, colonies can be picked and propagated, eliminating the need to make replica plates. We used the method to screen a DNA fragment library of the human protein p85 and preferentially obtained clones expressing the full-length 'breakpoint cluster region-homology' and NSH2 domains. The assay also distinguished clones expressing stable multi-protein complexes from those that are unstable due to missing subunits. Clones expressing stable, intact heterotrimeric E.coli YheNML complexes were readily identified in libraries dominated by complexes of YheML missing the N subunit.

Baculovirus production of fully-active phosphoinositide 3-kinase alpha as a p85alpha-p110alpha fusion for X-ray crystallographic analysis with ATP competitive enzyme inhibitors

Phosphoinositide 3-kinases have been targeted for therapeutic research because they are key components of a cell signaling cascade controlling proliferation, growth, and survival. Direct activation of the PI3Kalpha pathway contributes to the development and progression of solid tumors in breast, endometrial, colon, ovarian, and gastric cancers. In the context of a drug discovery effort, the availability of a robust crystallographic system is a means to understand the subtle differences between ATP competitive inhibitor interactions with the active site and their selectivity against other PI3Kinase enzymes. To generate a suitable recombinant design for this purpose, a p85alpha-p110alpha fusion system was developed which enabled the expression and purification of a stoichiometrically homogeneous, constitutively active enzyme for structure determination with potent ATP competitive inhibitors (Raha et al., in preparation) [56]. This approach has yielded preparations with activity and inhibition characteristics comparable to those of the full-length PI3Kalpha from which X-ray diffracting crystals were grown with inhibitors bound in the active site.

The regulation of class IA PI 3-kinases by inter-subunit interactions

Phosphoinositide 3-kinases (PI 3-kinases) are activated by growth factor and hormone receptors, and regulate cell growth, survival, motility, and responses to changes in nutritional conditions (Engelman et al. 2006). PI 3-kinases have been classified according to their subunit composition and their substrate specificity for phosphoinositides (Vanhaesebroeck et al. 2001). The class IA PI 3-kinase is a heterodimer consisting of one regulatory subunit (p85α, p85β, p55α, p50α, or p55γ) and one 110-kDa catalytic subunit (p110α, β or δ). The Class IB PI 3-kinase is also a dimer, composed of one regulatory subunit (p101 or p87) and one catalytic subunit (p110γ) (Wymann et al. 2003). Class I enzymes will utilize PI, PI[4]P, or PI[4,5]P2 as substrates in vitro, but are thought to primarily produce PI[3,4,5]P3 in cells.The crystal structure of the Class IB PI 3-kinase catalytic subunit p110γ was solved in 1999 (Walker et al. 1999), and crystal or NMR structures of the Class IA p110α catalytic subunit and all of the individual domains of the Class IA p85α regulatory subunit have been solved (Booker et al. 1992; Günther et al. 1996; Hoedemaeker et al. 1999; Huang et al. 2007; Koyama et al. 1993; Miled et al. 2007; Musacchio et al. 1996; Nolte et al. 1996; Siegal et al. 1998). However, a structure of an intact PI 3-kinase enzyme has remained elusive. In spite of this, studies over the past 10 years have lead to important insights into how the enzyme is regulated under physiological conditions. This chapter will specifically discuss the regulation of Class IA PI 3-kinase enzymatic activity, focusing on regulatory interactions between the p85 and p110 subunits and the modulation of these interactions by physiological activators and oncogenic mutations. The complex web of signaling downstream from Class IA PI 3-kinases will be discussed in other chapters in this volume.

Somatic mutations in PI3Kalpha: structural basis for enzyme activation and drug design

The PI3K pathway is a communication hub coordinating critical cell functions including cell survival, cell growth, proliferation, motility and metabolism. Because PI3Kalpha harbors recurrent somatic mutations resulting in gains of function in human cancers, it has emerged as an important drug target for many types of solid tumors. Various PI3K isoforms are also being evaluated as potential therapeutic targets for inflammation, heart disease, and hematological malignancies. Structural biology is providing insights into the flexibility of the PI3Ks, and providing basis for understanding the effects of mutations, drug resistance and specificity.

Regulation of Class IA PI 3-kinases: C2 domain-iSH2 domain contacts inhibit p85/p110alpha and are disrupted in oncogenic p85 mutants

We previously proposed a model of Class IA PI3K regulation in which p85 inhibition of p110alpha requires (i) an inhibitory contact between the p85 nSH2 domain and the p110alpha helical domain, and (ii) a contact between the p85 nSH2 and iSH2 domains that orients the nSH2 so as to inhibit p110alpha. We proposed that oncogenic truncations of p85 fail to inhibit p110 due to a loss of the iSH2-nSH2 contact. However, we now find that within the context of a minimal regulatory fragment of p85 (the nSH2-iSH2 fragment, termed p85ni), the nSH2 domain rotates much more freely (tau(c) approximately 12.7 ns) than it could if it were interacting rigidly with the iSH2 domain. These data are not compatible with our previous model. We therefore tested an alternative model in which oncogenic p85 truncations destabilize an interface between the p110alpha C2 domain (residue N345) and the p85 iSH2 domain (residues D560 and N564). p85ni-D560K/N564K shows reduced inhibition of p110alpha, similar to the truncated p85ni-572(STOP). Conversely, wild-type p85ni poorly inhibits p110alphaN345K. Strikingly, the p110alphaN345K mutant is inhibited to the same extent by the wild-type or truncated p85ni, suggesting that mutation of p110alpha-N345 is not additive with the p85ni-572(STOP) mutation. Similarly, the D560K/N564K mutation is not additive with the p85ni-572(STOP) mutant for downstream signaling or cellular transformation. Thus, our data suggests that mutations at the C2-iSH2 domain contact and truncations of the iSH2 domain, which are found in human tumors, both act by disrupting the C2-iSH2 domain interface.

Crystal structure of the GTPase-activating protein-related domain from IQGAP1

IQGAP1 is a 190-kDa molecular scaffold containing several domains required for interaction with numerous proteins. One domain is homologous to Ras GTPase-activating protein (GAP) domains. However, instead of accelerating hydrolysis of bound GTP on Ras IQGAP1, using its GAP-related domain (GRD) binds to Cdc42 and Rac1 and stabilizes their GTP-bound states. We report here the crystal structure of the isolated IQGAP1 GRD. Despite low sequence conservation, the overall structure of the GRD is very similar to the GAP domains from p120 RasGAP, neurofibromin, and SynGAP. However, instead of the catalytic "arginine finger" seen in functional Ras GAPs, the GRD has a conserved threonine residue. GRD residues 1099-1129 have no structural equivalent in RasGAP and are seen to form an extension at one end of the molecule. Because the sequence of these residues is highly conserved, this region likely confers a functionality particular to IQGAP family GRDs. We have used isothermal titration calorimetry to demonstrate that the isolated GRD binds to active Cdc42. Assuming a mode of interaction similar to that displayed in the Ras-RasGAP complex, we created an energy-minimized model of Cdc42.GTP bound to the GRD. Residues of the GRD that contact Cdc42 map to the surface of the GRD that displays the highest level of sequence conservation. The model indicates that steric clash between threonine 1046 with the phosphate-binding loop and other subtle changes would likely disrupt the proper geometry required for GTP hydrolysis.

SH3 domain of the phosphatidylinositol 3-kinase regulatory subunit is responsible for the formation of a sequestration complex with insulin receptor substrate-1

Class IA phosphatidylinositol 3-kinase (PI 3-kinase), which is composed of a 110kDa catalytic subunit and a regulatory subunit, plays a key role in most insulin dependent cellular responses. To date, five mammalian regulatory subunit isoforms have been identified, including two 85kDa proteins (p85alpha and p85beta), two 55kDa proteins (p55gamma and p55alpha), and one 50kDa protein (p50alpha). In the present study, we overexpressed these recombinant proteins, tagged with green fluorescent proteins (GFP), in CHO-IR cells and investigated intracellular localizations in both the presence and the absence of insulin stimulation. Interestingly, in response to insulin, only p85alpha and p85beta redistributed to isolated foci in the cells, while both were present throughout the cytoplasm in quiescent cells. In contrast, p55s accumulated in the perinuclear region irrespective of insulin stimulation, while p50alpha behaved similarly to control GFP. Immunofluorescent antibodies against endogenous IRS-1 revealed IRS-1 to be co-localized in the p85 foci in response to insulin. As both insulin receptors and p110alpha catalytic subunits were absent from these foci on immunofluorescence study, only p85 and IRS-1 were suggested to form a sequestration complex in response to insulin. To determine the domain responsible for IRS-1 complex formation, we prepared and overexpressed the SH3 domain deletion mutant of p85alpha in CHO-IR cells. This mutant failed to form foci, suggesting the SH3 domain of regulatory subunits to be responsible for formation of the p85-IRS-1 sequestration complex. In conclusion, our study revealed the SH3 domain of PI 3-kinase to play a critical role in intracellular localizations, including formation of foci with IRS-1 in response to insulin.

The p85alpha regulatory subunit of phosphoinositide 3-kinase potentiates c-Jun N-terminal kinase-mediated insulin resistance

Insulin resistance is a defining feature of type 2 diabetes and the metabolic syndrome. While the molecular mechanisms of insulin resistance are multiple, recent evidence suggests that attenuation of insulin signaling by c-Jun N-terminal kinase (JNK) may be a central part of the pathobiology of insulin resistance. Here we demonstrate that the p85alpha regulatory subunit of phosphoinositide 3-kinase (PI3K), a key mediator of insulin's metabolic actions, is also required for the activation of JNK in states of insulin resistance, including high-fat diet-induced obesity and JNK1 overexpression. The requirement of the p85alpha regulatory subunit for JNK occurs independently of its role as a component of the PI3K heterodimer and occurs only in response to specific stimuli, namely, insulin and tunicamycin, a chemical that induces endoplasmic reticulum stress. We further show that insulin and p85 activate JNK by via cdc42 and MKK4. The activation of this cdc42/JNK pathway requires both an intact N terminus and functional SH2 domains within the C terminus of the p85alpha regulatory subunit. Thus, p85alpha plays a dual role in regulating insulin sensitivity and may mediate cross talk between the PI3K and stress kinase pathways.

Combinatorial Domain Hunting: An effective approach for the identification of soluble protein domains adaptable to high-throughput applications

Exploitation of potential new targets for drug and vaccine development has an absolute requirement for multimilligram quantities of soluble protein. While recombinant expression of full-length proteins is frequently problematic, high-yield soluble expression of functional subconstructs is an effective alternative, so long as appropriate termini can be identified. Bioinformatics localizes domains, but doesn't predict boundaries with sufficient accuracy, so that subconstructs are typically found by trial and error. Combinatorial Domain Hunting (CDH) is a technology for discovering soluble, highly expressed constructs of target proteins. CDH combines unbiased, finely sampled gene-fragment libraries, with a screening protocol that provides "holistic" readout of solubility and yield for thousands of protein fragments. CDH is free of the "passenger solubilization" and out-of-frame translational start artifacts of fusion-protein systems, and hits are ready for scale-up expression. As a proof of principle, we applied CDH to p85alpha, successfully identifying soluble and highly expressed constructs encapsulating all the known globular domains, and immediately suitable for downstream applications.

PI3K signaling controls cell fate at many points in B lymphocyte development and activation

Many receptors on diverse cell types activate phosphoinositide 3-kinase (PI3K). The lipid products of PI3K, termed 3-phosphoinositides, regulate numerous cellular processes by recruiting specific proteins to membrane signaling complexes. In the B lymphocyte lineage, PI3K activation is a critical control point at various stages of development, proliferation and differentiation. PI3K signaling is promoted by stimulatory receptors such as surface immunoglobulin, CD40, Toll-like receptors and cytokine receptors, and opposed by the inhibitory receptor FcgammaRIIB1. Genetic dissection of the PI3K pathway in mice has indicated that certain B cell functions are regulated by a limited set of PI3K isoforms and downstream effectors. Here we review our current understanding of how signals are relayed to and from PI3K in B cells.

Structural Mechanism for Lipid Activation of the Rac-Specific GAP, β2-Chimaerin

The lipid second messenger diacylglycerol acts by binding to the C1 domains of target proteins, which translocate to cell membranes and are allosterically activated. Here we report the crystal structure at 3.2 A resolution of one such protein, beta2-chimaerin, a GTPase-activating protein for the small GTPase Rac, in its inactive conformation. The structure shows that in the inactive state, the N terminus of beta2-chimaerin protrudes into the active site of the RacGAP domain, sterically blocking Rac binding. The diacylglycerol and phospholipid membrane binding site on the C1 domain is buried by contacts with the four different regions of beta2-chimaerin: the N terminus, SH2 domain, RacGAP domain, and the linker between the SH2 and C1 domains. Phospholipid binding to the C1 domain triggers the cooperative dissociation of these interactions, allowing the N terminus to move out of the active site and thereby activating the enzyme.

The p85alpha subunit of phosphatidylinositol 3'-kinase binds to and stimulates the GTPase activity of Rab proteins

Rab5 and Rab4 are small monomeric GTPases localized on early endosomes and function in vesicle fusion events. These Rab proteins regulate the endocytosis and recycling or degradation of activated receptor tyrosine kinases such as the platelet-derived growth factor receptor (PDGFR). The p85alpha subunit of phosphatidylinositol 3'-kinase contains a BH domain with sequence homology to GTPase activating proteins (GAPs), but has not previously been shown to possess GAP activity. In this report, we demonstrate that p85alpha has GAP activity toward Rab5, Rab4, Cdc42, Rac1 and to a lesser extent Rab6, with little GAP activity toward Rab11. Purified recombinant Rab5 and p85alpha can bind directly to each other and not surprisingly, the p85alpha-encoded GAP activity is present in the BH domain. Because p85alpha stays bound to the PDGFR during receptor endocytosis, p85alpha will also be localized to the same early endosomal compartment as Rab5 and Rab4. Taken together, the physical co-localization and the ability of p85alpha to preferentially stimulate the down-regulation of Rab5 and Rab4 GTPases suggests that p85alpha regulates how long Rab5 and Rab4 remain in their GTP-bound active state. Cells expressing BH domain mutants of p85 show a reduced rate of PDGFR degradation as compared with wild type p85 expressing cells. These cells also show sustained activation of the mitogen-activated protein kinase and Akt pathways. Thus, the p85alpha protein may play a role in the down-regulation of activated receptors through its temporal control of the GTPase cycles of Rab5 and Rab4.

A vascular cell-restricted RhoGAP, p73RhoGAP, is a key regulator of angiogenesis

Angiogenesis is a major therapeutic target. Ideal drug targets are genes expressed only in endothelial cells (ECs) or only during the angiogenic process. Here, we describe a gene, p73RhoGAP (p73), that has both of these properties. By using a PCR-based subtraction-hybridization approach to clone cDNAs from ECs undergoing capillary-tube formation, we identified a RhoGAP member, p73. p73 displays GTPase activity to Rho but not to Rac or Cdc42. Knockdown of p73 protein, achieved by adenovirus delivery of p73 antisense and by small interfering RNA into ECs, demonstrated the importance of this protein in EC function. Under such conditions, EC migration, proliferation, and capillary-tube formation were inhibited. Furthermore, angiogenesis in vivo was also inhibited by antisense p73. A mutant R82A alteration achieved a similar phenotype in vitro to the antisense, demonstrating the importance of the GTPase-activating protein activity to p73 function. Expression profiling of p73 shows that it is vascular cell-selective, being highly expressed in ECs and smooth-muscle cells but not in other cell types. Finally, we show that the mRNA of p73 is up-regulated in an angiogenic milieu with little or no regulation seen under nonangiogenic conditions. p73, a vascular cell-specific GTPase-activating protein, is an important modulator of angiogenesis and displays many of features that make it worthy of being a drug target.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

Cdc42 is a Rho GTPase family member that can mediate insulin signaling to glucose transport in 3T3-L1 adipocytes

We investigated the role of cdc42, a Rho GTPase family member, in insulin-induced glucose transport in 3T3-L1 adipocytes. Microinjection of anti-cdc42 antibody or cdc42 siRNA led to decreased insulin-induced and constitutively active G(q) (CA-G(q); Q209L)-induced GLUT4 translocation. Adenovirus-mediated expression of constitutively active cdc42 (CA-cdc42; V12) stimulated 2-deoxyglucose uptake to 56% of the maximal insulin response, and this was blocked by treatment with the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, wortmannin, or LY294002. Both insulin and CA-G(q) expression caused an increase in cdc42 activity, showing that cdc42 is activated by insulin and is downstream of G alpha(q/11) in this activation pathway. Immunoprecipitation experiments showed that insulin enhanced a direct association of cdc42 and p85, and both insulin treatment and CA-cdc42 expression stimulated PI3-kinase activity in immunoprecipitates with anti-cdc42 antibody. Furthermore, the effects of insulin, CA-G(q), and CA-cdc42 on GLUT4 translocation or 2-deoxyglucose uptake were inhibited by microinjection of anti-protein kinase C lambda (PKC lambda) antibody or overexpression of a kinase-deficient PKC lambda construct. In summary, activated cdc42 can mediate 1) insulin-stimulated GLUT4 translocation and 2) glucose transport in a PI3-kinase-dependent manner. 3) Insulin treatment and constitutively active G(q) expression can enhance the cdc42 activity state as well as the association of cdc42 with activated PI3-kinase. 4) PKC lambda inhibition blocks CA-cdc42, CA-G(q), and insulin-stimulated GLUT4 translocation. Taken together, these data indicate that cdc42 can mediate insulin signaling to GLUT4 translocation and lies downstream of G alpha(q/11) and upstream of PI3-kinase and PKC lambda in this stimulatory pathway.

GAPs galore! A survey of putative Ras superfamily GTPase activating proteins in man and Drosophila

Typical members of the Ras superfamily of small monomeric GTP-binding proteins function as regulators of diverse processes by cycling between biologically active GTP- and inactive GDP-bound conformations. Proteins that control this cycling include guanine nucleotide exchange factors or GEFs, which activate Ras superfamily members by catalyzing GTP for GDP exchange, and GTPase activating proteins or GAPs, which accelerate the low intrinsic GTP hydrolysis rate of typical Ras superfamily members, thus causing their inactivation. Two among the latter class of proteins have been implicated in common genetic disorders associated with an increased cancer risk, neurofibromatosis-1, and tuberous sclerosis. To facilitate genetic analysis, I surveyed Drosophila and human sequence databases for genes predicting proteins related to GAPs for Ras superfamily members. Remarkably, close to 0.5% of genes in both species (173 human and 64 Drosophila genes) predict proteins related to GAPs for Arf, Rab, Ran, Rap, Ras, Rho, and Sar family GTPases. Information on these genes has been entered into a pair of relational databases, which can be used to identify evolutionary conserved proteins that are likely to serve basic biological functions, and which can be updated when definitive information on the coding potential of both genomes becomes available.

Human RhoGAP domain-containing proteins: structure, function and evolutionary relationships

Proteins containing a RhoGAP (Rho GTPase activating protein) domain usually function to catalyze the hydrolysis of GTP that is bound to Rho, Rac and/or Cdc42, inactivating these regulators of the actin cytoskeleton. Using database searches, at least 53 distinct RhoGAP domain-containing proteins are likely to be encoded in human DNA. Phylogenetic analysis of only the RhoGAP domains divides these proteins into distinct families that appear to be functionally related. We also review the current understanding of the structure and likely functions of these human proteins. The presence of RhoGAP domains in a number of different human proteins suggests that cytoskeletal changes, regulated by Rho GTPase, may be integrated with many different signaling pathways.

The Rho-GAP Bem2p plays a GAP-independent role in the morphogenesis checkpoint

The Saccharomyces cerevisiae morphogenesis checkpoint delays mitosis in response to insults that impair actin organization and/or bud formation. The delay is due to accumulation of the inhibitory kinase Swe1p, which phosphorylates the cyclin-dependent kinase Cdc28p. Having screened through a panel of yeast mutants with defects in cell morphogenesis, we report here that the polarity establishment protein Bem2p is required for the checkpoint response. Bem2p is a Rho-GTPase activating protein (GAP) previously shown to act on Rho1p, and we now show that it also acts on Cdc42p, the GTPase primarily responsible for establishment of cell polarity in yeast. Whereas the morphogenesis role of Bem2p required GAP activity, the checkpoint role of Bem2p did not. Instead, this function required an N-terminal Bem2p domain. Thus, this single protein has a GAP-dependent role in promoting cell polarity and a GAP-independent role in responding to defects in cell polarity by enacting the checkpoint. Surprisingly, Swe1p accumulation occurred normally in bem2 cells, but they were nevertheless unable to promote Cdc28p phosphorylation. Therefore, Bem2p defines a novel pathway in the morphogenesis checkpoint.

Structural insight into substrate specificity and regulatory mechanisms of phosphoinositide 3-kinases

Phosphoinositide 3-kinases (PI3Ks) are implicated in a variety of fundamental cellular processes. These enzymes catalyse phosphorylation of the 3'-OH position of myo-inositol lipids that serve as secondary messengers. The catalytic subunit for one of the family members, PI3K gamma, has been structurally characterized, independently, in complexes with kinase inhibitors and with the p21(Ras) GTPase. These atomic structures provide a basis for the rationalization of some PI3K substrate specificities and regulatory mechanisms, establishing links to functional and cellular data. Ongoing comprehensive structural and functional studies are essential to realize the promise of PI3K isozyme-specific therapeutic agents.

Chemoattractant-stimulated NF-kappaB activation is dependent on the low molecular weight GTPase RhoA

Chemoattractants bind to seven transmembrane-spanning, G-protein-coupled receptors on monocytes and neutrophils and induce a variety of functional responses, including activation of the transcription factor NF-kappaB. The signaling mechanisms utilized by chemoattractants to activate NF-kappaB in human peripheral blood monocytes are poorly defined. We previously demonstrated that fMet-Leu-Phe (fMLP) stimulates NF-kappaB activation, and this function of fMLP requires phosphatidylinositol 3-kinase (PI3K). Here we present evidence that fMLP activates RhoA and that fMLP-induced NF-kappaB activation requires this small GTPase. Stimulation of monocytes with fMLP rapidly activated RhoA as well as NF-kappaB, and their activation was markedly reduced by pertussis toxin treatment. Pretreatment of monocyte with a RhoA inhibitor, C3 transferase from Clostridium botulinum, effectively blocked fMLP-induced NF-kappaB activation as well as interleukin-1beta gene expression. A dominant negative form of RhoA (T19N) also inhibited fMLP-stimulated reporter gene expression in a kappaB-dependent manner. Cotransfection of the monocytic THP1 cells with a constitutively active form of RhoA (Q63L) with the promoter reporter plasmid results in a marked increase in NF-kappaB-mediated reporter gene expression. Furthermore, the PI3K inhibitors wortmannin and LY294002 block RhoA activation induced by fMLP. These results demonstrate that low molecular weight GTPase RhoA is a novel signal transducer for fMLP-induced NF-kappaB activation and Galpha(i) or Galpha(o) class of heterotrimeric G proteins likely mediate RhoA activation via PI3K in human peripheral blood monocytes.