IQGAP proteins reveal an atypical phosphoinositide (aPI) binding domain with a pseudo C2 domain fold.

Class I phosphoinositide (PI) 3-kinases act through effector proteins whose 3-PI selectivity is mediated by a limited repertoire of structurally defined, lipid recognition domains. We describe here the lipid preferences and crystal structure of a new class of PI binding modules exemplified by select IQGAPs (IQ motif containing GTPase-activating proteins) known to coordinate cellular signaling events and cytoskeletal dynamics. This module is defined by a C-terminal 105-107 amino acid region of which IQGAP1 and -2, but not IQGAP3, binds preferentially to phosphatidylinositol 3,4,5-trisphosphate (PtdInsP(3)). The binding affinity for PtdInsP(3), together with other, secondary target-recognition characteristics, are comparable with those of the pleckstrin homology domain of cytohesin-3 (general receptor for phosphoinositides 1), an established PtdInsP(3) effector protein. Importantly, the IQGAP1 C-terminal domain and the cytohesin-3 pleckstrin homology domain, each tagged with enhanced green fluorescent protein, were both re-localized from the cytosol to the cell periphery following the activation of PI 3-kinase in Swiss 3T3 fibroblasts, consistent with their common, selective recognition of endogenous 3-PI(s). The crystal structure of the C-terminal IQGAP2 PI binding module reveals unexpected topological similarity to an integral fold of C2 domains, including a putative basic binding pocket. We propose that this module integrates select IQGAP proteins with PI 3-kinase signaling and constitutes a novel, atypical phosphoinositide binding domain that may represent the first of a larger group, each perhaps structurally unique but collectively dissimilar from the known PI recognition modules.

A screen for novel phosphoinositide 3-kinase effector proteins.

Class I phosphoinositide 3-kinases exert important cellular effects through their two primary lipid products, phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2)). As few molecular targets for PtdIns(3,4)P(2) have yet been identified, a screen for PI 3-kinase-responsive proteins that is selective for these is described. This features a tertiary approach incorporating a unique, primary recruitment of target proteins in intact cells to membranes selectively enriched in PtdIns(3,4)P(2). A secondary purification of these proteins, optimized using tandem pleckstrin homology domain containing protein-1 (TAPP-1), an established PtdIns(3,4)P(2) selective ligand, yields a fraction enriched in proteins of potentially similar lipid binding character that are identified by liquid chromatography-tandem MS. Thirdly, this approach is coupled to stable isotope labeling with amino acids in cell culture using differential isotope labeling of cells stimulated in the absence and presence of the PI 3-kinase inhibitor wortmannin. This provides a ratio-metric readout that distinguishes authentically responsive components from copurifying background proteins. Enriched fractions thus obtained from astrocytoma cells revealed a subset of proteins that exhibited ratios indicative of their initial, cellular responsiveness to PI 3-kinase activation. The inclusion among these of tandem pleckstrin homology domain containing protein-1, three isoforms of Akt, switch associated protein-70, early endosome antigen-1 and of additional proteins expressing recognized lipid binding domains demonstrates the utility of this strategy and lends credibility to the novel candidate proteins identified. The latter encompass a broad set of proteins that include the gene product of TBC1D2A, a putative Rab guanine nucleotide triphosphatase activating protein (GAP) and IQ motif containing GAP1, a potential tumor promoter. A sequence comparison of the former protein indicates the presence of a pleckstrin homology domain whose lipid binding character remains to be established. IQ motif containing GAP1 lacks known lipid interacting components and a preliminary analysis here indicates that this may exemplify a novel class of atypical phosphoinositide (aPI) binding domain.

The control of phosphatidylinositol 3,4-bisphosphate concentrations by activation of the Src homology 2 domain containing inositol polyphosphate 5-phosphatase 2, SHIP2.

Activation of class Ia PI3K (phosphoinositide 3-kinase) produces PtdInsP3, a vital intracellular mediator whose degradation generates additional lipid signals. In the present study vanadate analogues that inhibit PTPs (protein tyrosine phosphatases) were used to probe the mechanisms which regulate the concentrations of these molecules allowing their independent or integrated function. In 1321N1 cells, which lack PtdInsP3 3-phosphatase activity, sodium vanadate or a cell permeable derivative, bpV(phen) [potassium bisperoxo(1,10-phenanthroline)oxovanadate (V)], increased the recruitment into anti-phosphotyrosine immunoprecipitates of PI3K activity and of the p85 and p110a subunits of class Ia PI3K and enhanced the recruitment of PI3K activity stimulated by PDGF (platelet-derived growth factor). However, neither inhibitor much increased cellular PtdInsP3 concentrations, but both diminished dramatically the accumulation of PtdInsP3 stimulated by PDGF or insulin and markedly increased the control and stimulated concentrations of PtdIns(3,4)P2. These actions were accounted for by the ability of PTP inhibitors to stimulate the activity of endogenous PtdInsP3 5-phosphatase(s), particularly SHIP2 (Src homology 2 domain containing inositol polyphosphate 5-phosphatase 2) and to inhibit types I and II PtdIns(3,4)P2 4-phosphatases. Thus bpV(phen) promoted the translocation of SHIP2 from the cytosol to a Triton X-100-insoluble fraction and induced a marked (5-10-fold) increase in SHIP2 specific activity mediated by enhanced tyrosine phosphorylation. The net effect of these inhibitors was, therefore, to switch the signal output of class I PI3K from PtdInsP3 to PtdIns(3,4)P2. A key component controlling this shift in the balance of lipid signals is the activation of SHIP2 by increased tyrosine phosphorylation, an effect observed in HeLa cells in response to both PTP inhibitors and epidermal growth factor.

Muscarinic-receptor-mediated inhibition of insulin-like growth factor-1 receptor-stimulated phosphoinositide 3-kinase signalling in 1321N1 astrocytoma cells.

In 1321N1 astrocytoma cells, stimulation of the IGF-1 (insulin-like growth factor-1) receptor increased the association of PI3K [phosphoinositide (PI) 3-kinase] activity with IRS-1 (insulin re-ceptor substrate 1), and increased the cellular concentration of PtdIns(3,4,5)P3. Carbachol, acting on M3 muscarinic receptors, inhibited insulin-, but not PDGF (platelet-derived growth factor)-, stimulated responses by approximately 50%. The inhibition of IRS-1-associated PI3K activity by carbachol (i) was rapid (<1 min), persistent (> or =60 min) and potent (half-maximal concentration approximately 1 microM); (ii) was reproduced by stimuli for several phospholipase-C-coupled receptors; (iii) was prevented by the inhibition of protein kinase C, but not by chelation of intracellular Ca2+; and (iv) was not blocked or reproduced by inhibitors or stimuli respectively of mitogen-activated protein kinase, PI3K, protein kinase B or the mammalian target of rapamycin. However, the effects of carbachol were prevented by sodium vanadate, a protein tyrosine phosphatase inhibitor, and were accompanied by reduced insulin-stimulated IRS-1 tyrosine phosphorylation and recruitment of the 85 kDa regulatory subunit of PI3K to IRS-1, but not by reduced IGF-1 receptor kinase activity. The inhibitory effect of carbachol was reproduced by okadaic acid, a protein serine/threonine phosphatase inhibitor, but not by PDGF, yet all three agents stimulated the serine phosphorylation of IRS-1 at residues Ser312, Ser616 and Ser636/639, albeit to different extents. Thus muscarinic receptors may inhibit insulin signalling by promoting IRS-1 tyrosine dephosphorylation and/or by uncoupling IRS-1 from the stimulated IGF-1 receptor by stimulating IRS-1 serine phosphorylation. However, the proportion of IRS-1 molecules phosphorylated at a particular site or the phosphorylation of additional IRS-1 serine residues other than those noted above must be important.

The tumour-suppressor function of PTEN requires an N-terminal lipid-binding motif.

The PTEN (phosphatase and tensin homologue deleted on chromosome 10) tumour-suppressor protein is a phosphoinositide 3-phosphatase which antagonizes phosphoinositide 3-kinase-dependent signalling by dephosphorylating PtdIns(3,4,5)P3. Most tumour-derived point mutations of PTEN induce a loss of function, which correlates with profoundly reduced catalytic activity. However, here we characterize a point mutation at the N-terminus of PTEN, K13E from a human glioblastoma, which displayed wild-type activity when assayed in vitro. This mutation occurs within a conserved polybasic motif, a putative PtdIns(4,5)P2-binding site that may participate in membrane targeting of PTEN. We found that catalytic activity against lipid substrates and vesicle binding of wild-type PTEN, but not of PTEN K13E, were greatly stimulated by anionic lipids, especially PtdIns(4,5)P2. The K13E mutation also greatly reduces the efficiency with which anionic lipids inhibit PTEN activity against soluble substrates, supporting the hypothesis that non-catalytic membrane binding orientates the active site to favour lipid substrates. Significantly, in contrast to the wild-type enzyme, PTEN K13E failed either to prevent protein kinase B/Akt phosphorylation, or inhibit cell proliferation when expressed in PTEN-null U87MG cells. The cellular functioning of K13E PTEN was recovered by targeting to the plasma membrane through inclusion of a myristoylation site. Our results establish a requirement for the conserved N-terminal motif of PTEN for correct membrane orientation, cellular activity and tumour-suppressor function.

Inositol phospholipids regulate the guanine-nucleotide-exchange factor Tiam1 by facilitating its binding to the plasma membrane and regulating GDP/GTP exchange on Rac1.

Binding of the Rac1-specific guanine-nucleotide-exchange factor, Tiam1, to the plasma membrane requires the N-terminal pleckstrin homology domain. In the present study, we show that membrane-association is mediated by binding of PtdIns(4,5)P(2) to the pleckstrin homology domain. Moreover, in 1321N1 astrocytoma cells, translocation of Tiam1 to the cytosol, following receptor-mediated stimulation of PtdIns(4,5)P(2) breakdown, correlates with decreased Rac1-GTP levels, indicating that membrane-association is required for GDP/GTP exchange on Rac1. In addition, we show that platelet-derived growth factor activates Rac1 in vivo by increasing PtdIns(3,4,5)P(3) concentrations, rather than the closely related lipid, PtdIns(3,4)P(2). Finally, the data demonstrate that PtdIns(4,5)P(2) and PtdIns(3,4,5)P(3) bind to the same pleckstrin homology domain in Tiam1 and that soluble inositol phosphates appear to compete with lipids for this binding. Together, these novel observations provide strong evidence that distinct phosphoinositides regulate different functions of this enzyme, indicating that local concentrations of signalling lipids and the levels of cytosolic inositol phosphates will play crucial roles in determining its activity in vivo.

Cross talk between the Akt and p38α pathways in macrophages downstream of Toll-like receptor signaling.

The stimulation of Toll-like receptors (TLRs) on macrophages by pathogen-associated molecular patterns (PAMPs) results in the activation of intracellular signaling pathways that are required for initiating a host immune response. Both phosphatidylinositol 3-kinase (PI3K)-Akt and p38 mitogen-activated protein kinase (MAPK) signaling pathways are activated rapidly in response to TLR activation and are required to coordinate effective host responses to pathogen invasion. In this study, we analyzed the role of the p38-dependent kinases MK2/3 in the activation of Akt and show that lipopolysaccharide (LPS)-induced phosphorylation of Akt on Thr308 and Ser473 requires p38α and MK2/3. In cells treated with p38 inhibitors or an MK2/3 inhibitor, phosphorylation of Akt on Ser473 and Thr308 is reduced and Akt activity is inhibited. Furthermore, BMDMs deficient in MK2/3 display greatly reduced phosphorylation of Ser473 and Thr308 following TLR stimulation. However, MK2/3 do not directly phosphorylate Akt in macrophages but act upstream of PDK1 and mTORC2 to regulate Akt phosphorylation. Akt is recruited to phosphatidylinositol 3,4,5-trisphosphate (PIP3) in the membrane, where it is activated by PDK1 and mTORC2. Analysis of lipid levels in MK2/3-deficient bone marrow-derived macrophages (BMDMs) revealed a role for MK2/3 in regulating Akt activity by affecting availability of PIP3 at the membrane. These data describe a novel role for p38α-MK2/3 in regulating TLR-induced Akt activation in macrophages.

Binding of influenza A virus NS1 protein to the inter-SH2 domain of p85 suggests a novel mechanism for phosphoinositide 3-kinase activation.

Influenza A virus NS1 protein stimulates host-cell phosphoinositide 3-kinase (PI3K) signaling by binding to the p85beta regulatory subunit of PI3K. Here, in an attempt to establish a mechanism for this activation, we report further on the functional interaction between NS1 and p85beta. Complex formation was found to be independent of NS1 RNA binding activity and is mediated by the C-terminal effector domain of NS1. Intriguingly, the primary direct binding site for NS1 on p85beta is the inter-SH2 domain, a coiled-coil structure that acts as a scaffold for the p110 catalytic subunit of PI3K. In vitro kinase activity assays, together with protein binding competition studies, reveal that NS1 does not displace p110 from the inter-SH2 domain, and indicate that NS1 can form an active heterotrimeric complex with PI3K. In addition, it was established that residues at the C terminus of the inter-SH2 domain are essential for mediating the interaction between p85beta and NS1. Equivalent residues in p85alpha have previously been implicated in the basal inhibition of p110. However, such p85alpha residues were unable to substitute for those in p85beta with regards NS1 binding. Overall, these data suggest a model by which NS1 activates PI3K catalytic activity by masking a normal regulatory element specific to the p85beta inter-SH2 domain.

Muscarinic receptors mediate phospholipase C-dependent activation of protein kinase B via Ca2+, ErbB3, and phosphoinositide 3-kinase in 1321N1 astrocytoma cells.

In 1321N1 astrocytoma cells, heterotrimeric G-protein-coupled receptors that activate phosphoinositide-specific phospholipase Cbeta (PLCbeta) isoforms via G(q), induced a prolonged activation of protein kinase B (PKB) after a short delay. For example, the effect of carbachol acting on M3 muscarinic receptors is blocked by wortmannin, suggesting it is mediated via a phosphoinositide 3-kinase (PI 3-kinase). In support of this, carbachol increased PI 3-kinase activity in PI 3-kinase (p85) immunoprecipitates. The pathway linking PLC-coupled receptors to PI 3-kinase was deduced to involve phosphoinositide hydrolysis and Ca2+-dependent ErbB3 transactivation but not protein kinase C on the basis of the following evidence: (i) inhibition of carbachol stimulated PLC by pretreatment with the phorbol ester phorbol 12-myristate 13-acetate concomitantly reduced PKB activity, whereas stimulation of other PLC-coupled receptors also activated PKB; (ii) Ca2+ ionophores and thapsigargin stimulated PKB activity in a wortmannin-sensitive manner, whereas bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid blocked carbachol-stimulated PKB activity; (iii) phorbol 12-myristate 13-acetate alone did not activate PKB, whereas a protein kinase C inhibitor did not prevent the activation of PKB by carbachol; and (iv) carbachol stimulated ErbB3-tyrosine phosphorylation and association with p85, and both these and PKB activity were blocked by tyrphostin AG1478, an epidermal growth factor receptor-tyrosine kinase inhibitor. These experiments define a novel pathway linking G(q)-coupled G-protein-coupled receptors to the activation of PI 3-kinase and PKB.

Nonradioactive methods for the assay of phosphoinositide 3-kinases and phosphoinositide phosphatases and selective detection of signaling lipids in cell and tissue extracts.

We describe a novel approach to quantitation of phosphoinositides in cell extracts and in vitro enzyme-catalyzed reactions using suitably tagged and/or labeled pleckstrin homology (PH) domains as probes. Stable complexes were formed between the biotinylated target lipid and an appropriate PH domain, and phosphoinositides present in samples were detected by their ability to compete for binding to the PH domain. Complexes were detected using AlphaScreen technology or time-resolved FRET. The assay procedure was validated using recombinant PI 3-kinase gamma with diC8PtdIns(4,5)P(2) as substrate and general receptor for phosphoinositides-1 (GRP1) PH domain as a PtdIns(3,4,5)P(3)-specific probe. This PI 3-kinase assay was robust, was suitable for high-throughput screening platforms, and delivered expected IC(50) values for reference compounds. The approach is adaptable to a wide range of enzymes as demonstrated by assays of the tumor suppressor protein, PTEN, a phosphoinositide 3-phosphatase, which was measured using the same reagents but with diC8PtdIns(3,4,5)P(3) as substrate. PtdIns(3,4,5)P(3) present in lipid extracts of Swiss 3T3 and HL60 cells stimulated with platelet-derived growth factor and fMLP, respectively, was also detectable at picomole sensitivity. The versatility and general utility of this approach were demonstrated by exchanging the GRP1 PH domain for that of TAPP1 (which binds PtdIns(3,4)P(2) and not PtdIns(3,4,5)P(3)). This system was used to monitor the accumulation of PtdIns(3,4)P(2) in Swiss 3T3 cells exposed to an oxidative stress. It is therefore proposed that similar procedures should be capable of measuring any known phosphoinositide present in cell and tissue extracts or produced in kinase and phosphatase assays by using one of several well-characterized protein domains with appropriate phosphoinositide-binding specificity.