Pleckstrin inhibits phosphoinositide hydrolysis initiated by G-protein-coupled and growth factor receptors. A role for pleckstrin's PH domains

Multi-Omic Profiling of Macrophages Treated with Phospholipids Containing Omega-3 and Omega-6 Fatty Acids Reveals Complex Immunomodulatory Adaptations at Protein, Lipid and Metabolic Levels

In recent years, several studies have demonstrated that polyunsaturated fatty acids have strong immunomodulatory properties, altering several functions of macrophages. In the present work, we sought to provide a multi-omic approach combining the analysis of the lipidome, the proteome, and the metabolome of RAW 264.7 macrophages supplemented with phospholipids containing omega-3 (PC 18:0/22:6; ω3-PC) or omega-6 (PC 18:0/20:4; ω6-PC) fatty acids, alone and in the presence of lipopolysaccharide (LPS). Supplementation of macrophages with ω3 and ω6 phospholipids plus LPS produced a significant reprogramming of the proteome of macrophages and amplified the immune response; it also promoted the expression of anti-inflammatory proteins (e.g., pleckstrin). Supplementation with the ω3-PC and ω6-PC induced significant changes in the lipidome, with a marked increase in lipid species linked to the inflammatory response, attributed to several pro-inflammatory signalling pathways (e.g., LPCs) but also to the pro-resolving effect of inflammation (e.g., PIs). Finally, the metabolomic analysis demonstrated that supplementation with ω3-PC and ω6-PC induced the expression of several metabolites with a pronounced inflammatory and anti-inflammatory effect (e.g., succinate). Overall, our data show that supplementation of macrophages with ω3-PC and ω6-PC effectively modulates the lipidome, proteome, and metabolome of these immune cells, affecting several metabolic pathways involved in the immune response that are triggered by inflammation.

Crystallization and preliminary diffraction analysis of truncated human pleckstrin

Pleckstrin is a major substrate of protein kinase C in platelets and leukocytes and appears to play an important role in exocytosis through a currently unknown mechanism. Pleckstrin function is regulated by phosphorylation, which is thought to cause dissociation of pleckstrin dimers, thereby facilitating phosphoinositide interactions and membrane localization. Evidence also exists suggesting that phosphorylation causes a subtle conformational change in pleckstrin. Structural studies of pleckstrin have been initiated in order to characterize these structural changes and ultimately advance understanding of pleckstrin function. Here, the crystallization and preliminary X-ray diffraction analysis of a truncated version of pleckstrin consisting of the N-terminal PH domain, the protein kinase C phosphorylation sites and the DEP domain (NPHDEP) are reported. In addition, the oligomeric state and phospholipid-binding properties of NPHDEP were analyzed. This work demonstrates that NPHDEP behaves as a monomer in solution and suggests that all three pleckstrin domains contribute to the dimerization interface. Furthermore, based on the binding properties of NPHDEP, the C-terminal PH domain appears to increase the specificity of pleckstrin for phosphoinositides. This work represents a significant step towards determining the structure of pleckstrin.

Global gene expression profiling of somatic motor neuron populations with different vulnerability identify molecules and pathways of degeneration and protection

Different somatic motor neuron subpopulations show a differential vulnerability to degeneration in diseases such as amyotrophic lateral sclerosis, spinal muscular atrophy and spinobulbar muscular atrophy. Studies in mutant superoxide dismutase 1 over-expressing amyotrophic lateral sclerosis model mice indicate that initiation of disease is intrinsic to motor neurons, while progression is promoted by astrocytes and microglia. Therefore, analysis of the normal transcriptional profile of motor neurons displaying differential vulnerability to degeneration in motor neuron disease could give important clues to the mechanisms of relative vulnerability. Global gene expression profiling of motor neurons isolated by laser capture microdissection from three anatomical nuclei of the normal rat, oculomotor/trochlear (cranial nerve 3/4), hypoglossal (cranial nerve 12) and lateral motor column of the cervical spinal cord, displaying differential vulnerability to degeneration in motor neuron disorders, identified enriched transcripts for each neuronal subpopulation. There were striking differences in the regulation of genes involved in endoplasmatic reticulum and mitochondrial function, ubiquitination, apoptosis regulation, nitrogen metabolism, calcium regulation, transport, growth and RNA processing; cellular pathways that have been implicated in motor neuron diseases. Confirmation of genes of immediate biological interest identified differential localization of insulin-like growth factor II, guanine deaminase, peripherin, early growth response 1, soluble guanylate cyclase 1A3 and placental growth factor protein. Furthermore, the cranial nerve 3/4-restricted genes insulin-like growth factor II and guanine deaminase protected spinal motor neurons from glutamate-induced toxicity (P < 0.001, ANOVA), indicating that our approach can identify factors that protect or make neurons more susceptible to degeneration.

Loss of pleckstrin defines a novel pathway for PKC-mediated exocytosis

Pleckstrin, the platelet and leukocyte C kinase substrate, is a prominent substrate of PKC in platelets, monocytes, macrophages, lymphocytes, and granulocytes. Pleckstrin accounts for 1% of the total protein in these cells, but it is best known for containing the 2 prototypic Pleckstrin homology, or PH, domains. Overexpressed pleckstrin can affect polyphosphoinositide second messenger-based signaling events; however, its true in vivo role has been unknown. Here, we describe mice containing a null mutation within the pleckstrin gene. Platelets lacking pleckstrin exhibit a marked defect in exocytosis of delta and alpha granules, alphaIIbbeta3 activation, actin assembly, and aggregation after exposure to the PKC stimulant, PMA. Pleckstrin-null platelets aggregate normally in response to thrombin, but they fail to aggregate in response to thrombin in the presence of PI3K inhibitors, suggesting that a PI3K-dependent signaling pathway compensates for the loss of pleckstrin. Although pleckstrin-null platelets merged their granules in response to stimulation of PKC, they failed to empty their contents into the open canalicular system. This might be attributable to impaired actin assembly present in cells lacking pleckstrin. These data show that pleckstrin regulates the fusion of granules to the cell membrane and is an essential component of PKC-mediated exocytosis.

Structural analysis of the carboxy terminal PH domain of pleckstrin bound to D-myo-inositol 1,2,3,5,6-pentakisphosphate

BACKGROUND

Pleckstrin homology (PH) domains are one of the most prevalent domains in the human proteome and represent the major phosphoinositide-binding module. These domains are often found in signaling proteins and function predominately by targeting their host proteins to the cell membrane. Inositol phosphates, which are structurally similar to phosphoinositides, are not only known to play a role as signaling molecules but are also capable of being bound by PH domains.

RESULTS

In the work presented here it is shown that the addition of commercial myo-inositol hexakisphosphate (IP6) inhibited the binding of the carboxy terminal PH domain of pleckstrin (C-PH) to phosphatidylinositol 3,4-bisphosphate with an IC50 of 7.5 muM. In an attempt to characterize this binding structurally, C-PH was crystallized in the presence of IP6 and the structure was determined to 1.35 A. Examination of the resulting electron density unexpectedly revealed the bound ligand to be D-myo-inositol 1,2,3,5,6-pentakisphosphate.

CONCLUSION

The discovery of D-myo-inositol 1,2,3,5,6-pentakisphosphate in the crystal structure suggests that the inhibitory effects observed in the binding studies may be due to this ligand rather than IP6. Analysis of the protein-ligand interaction demonstrated that this myo-inositol pentakisphosphate isomer interacts specifically with protein residues known to be involved in phosphoinositide binding. In addition to this, a structural alignment of other PH domains bound to inositol phosphates containing either four or five phosphate groups revealed that the majority of phosphate groups occupy conserved locations in the binding pockets of PH domains. These findings, taken together with other recently reported studies suggest that myo-inositol pentakisphosphates could act to regulate PH domain-phosphoinositide interactions by directly competing for binding, thus playing an important role as signaling molecules.

p42IP4/Centaurin α1, a Brain-specific PtdIns(3,4,5)P3/Ins(1,3,4,5)P4-binding Protein: Membrane Trafficking Induced by Epidermal Growth Factor is Inhibited by Stimulation of Phospholipase C-coupled Thrombin Receptor

The brain-specific 42-kDa protein, p42(IP4), contains a N-terminal zinc finger (ZF) motif and a tandem of two pleckstrin homology (PH) domains. p42(IP4) binds in vitro the second messengers phosphatidylinositol(3,4,5)trisphosphate (PtdIns(3,4,5)P3) and inositol(1,3,4,5)tetrakisphosphate (Ins(1,3,4,5)P4). We observed by confocal microscopy in live HEK 293 cells the GFP-p42(IP4), a chimera of human p42(IP4) and green fluorescence protein (GFP). There, we studied the influence of thrombin, which raises Ins(1,3,4,5)P4, on membrane translocation of GFP-p42(IP4), induced by epidermal growth factor (EGF). Thrombin in the presence of LiCl inhibited the EGF-induced membrane recruitment of GFP-p42(IP4). In the absence of LiCl, thrombin weakened the EGF-mediated membrane recruitment of GFP-p42(IP4). Furthermore, the participation of p42(IP4) protein domains on the EGF-mediated membrane translocation was analyzed. We used several p42(IP4) variants, in which one of the domains was deleted. Alternatively, single p42(IP4) domain-GFP fusion proteins were generated. Only the p42(IP4) variant lacking the ZF domain showed a very weak membrane translocation in response to EGF stimulation, but all the other p42(IP4) variants did not translocate. Thus, we conclude that the combination of both PH domains with ZF is required for membrane translocation of p42(IP4).

Cloning, expression and chromosomal assignment of human pleckstrin 2

We report the isolation of human pleckstrin 2 cDNA. The cDNA contains a 1059 bp open reading frame encoding a polypeptide of 353 amino acid residues. The deduced amino acid sequence indicates that pleckstrin 2 contains two pleckstrin homology domains and a DEP (dishvelled, egl-10, and pleckstrin) domain and had a 95% identity with the sequence of mouse pleckstrin 2. Northern blot and a reverse transcription-coupled polymerase chain reaction analysis revealed that pleckstrin 2 mRNA is widely expressed in a variety of cell lines. The chromosomal location of the mouse pleckstrin 2 gene was on the D3 band of chromosome 12, as determined by fluorescence in situ hybridization and the human pleckstrin 2 gene was mapped to chromosome 14q24.1 by a bioinformatics analysis.

Control of pre-BCR signaling by Pax5-dependent activation of the BLNK gene

The developmental progression from pro-B to pre-B cells is controlled by pre-B cell receptor (pre-BCR) signaling which depends on BLNK (SLP-65) for coupling the Syk kinase to its downstream effector pathways. Here we identified BLNK as a direct target of the transcription factor Pax5 (BSAP). Restoration of BLNK expression in Ig(mu) transgenic Pax5(-/-) pro-B cells resulted in constitutive pre-BCR signaling and increased cell proliferation without inducing progression to the pre-B cell stage. Ig(mu)(+) Pax5(-/-) pro-B cells expressing a BLNK-estrogen receptor fusion protein initiated signaling immediately upon hormone addition, which facilitated analysis of pre-BCR-induced gene expression changes. The pre-BCR was shown to execute its checkpoint function by regulating genes involved in cell proliferation, intracellular signaling, growth factor responsiveness, and V(D)J recombination.

Translocation of pleckstrin requires its phosphorylation and newly formed ligands

Pleckstrin is the major substrate of protein kinase C (PKC) in platelets. We sought to determine whether pleckstrin phosphorylation is sufficient to target the soluble protein to binding sites. Permeabilization of platelets by streptolysin O (SLO) was used to separate bound and soluble pleckstrin. Platelets were incubated with phorbol 12-myristate 13-acetate (PMA) and/or guanosine 5'-[gamma-thio]triphosphate (GTP[S]) in the presence of [gamma-(32)P]ATP and SLO. PMA stimulated pleckstrin phosphorylation, but this pleckstrin diffused from permeabilized platelets. Addition of GTP[S] with PMA caused up to 40-50% of pleckstrin to be retained within platelets and enhanced secretion of platelet 5-hydroxytryptamine. PKC alpha pseudosubstrate peptide inhibited pleckstrin phosphorylation, the binding of pleckstrin and secretion. After extraction of permeabilized platelets containing bound pleckstrin with Triton X-100, the protein was solubilized. Thus, phosphorylated pleckstrin was retained in platelets only after activation of GTP-binding proteins that stimulate the formation of membrane-bound pleckstrin ligands. Translocation of pleckstrin may facilitate the associated secretion.

G-protein-coupled receptor activation induces the membrane translocation and activation of phosphatidylinositol-4-phosphate 5-kinase I alpha by a Rac- and Rho-dependent pathway

Phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) mediates cell motility and changes in cell shape in response to extracellular stimuli. In platelets, it is synthesized from PI4P by PIP5K in response to stimulation of a G-protein-coupled receptor by an agonist, such as the thrombin. In the present study, we have addressed the pathway that induces PIP5K I alpha activation following the addition of thrombin. Under resting condition expressed PIP5K I alpha was predominantly localized in a perinuclear distribution. After stimulation of the thrombin receptor, PAR1, or overexpression of a constitutively active variant of G alpha(q), PIP5K I alpha translocated to the plasma membrane. Movement of PIP5K I alpha to the cell membrane was dependent on both GTP-bound Rac and Rho, but not Arf, because: 1) inactive GDP-bound variants of either Rac or Rho blocked the translocation induced by constitutively active G alpha(q), 2) constitutively GTP-bound active variants of Rac or Rho induced PIP5K I alpha translocation in the absence of other stimuli, and 3) constitutively active variants of Arf1 or Arf6 failed to induce membrane translocation of PIP5K I alpha. In addition, a dominant negative variant of Rho blocked the PIP5K I alpha membrane translocation induced by constitutively active Rac, whereas dominant negative variants of either Rac or Arf6 failed to block PIP5K I alpha membrane translocation induced by constitutively active Rho. This implies that the effect on PIP5K I alpha by Rac is indirect, and requires the activation of Rho. In contrast to the findings with PIP5K I alpha, the related lipid kinase PIP4K failed to undergo translocation after stimulation by small GTP-binding proteins Rac or Rho. We also tested whether membrane localization of PIP5K I alpha correlated with an increase in its lipid kinase activity and found that co-expressing of PIP5K I alpha with either constitutively active G alpha(q), Rac, or Rho led to a 5- to 7-fold increase in PIP5K I alpha activity. Thus, these findings suggest that stimulation of a G-protein-coupled receptor (PAR1) leads to the sequential activation of G alpha(q), Rac, Rho, and PIP5K I alpha. Once activated and translocated to the cell membrane, PIP5K I alpha becomes available to phosphorylate PI4P to generate PI4,5P(2) on the plasma membrane.

cDNA cloning and mapping of mouse pleckstrin (Plek), a gene upregulated in transformation-resistant cells

Changes that occur during tumor promotion, the rate-limiting phase of multistep carcinogenesis, may offer the best targets for prevention of cancer or reversal of early disease. The murine epidermal JB6 promotion-sensitive (P+) and -resistant (P-) cell lines provide a cell culture model for tumor promoter-induced neoplastic transformation ideally suited to the identification of molecular events that mediate or inhibit transformation. A differential display comparison of P+ and P- cell mRNAs yielded seven differentially expressed sequences. One of the sequences preferentially expressed in P- cells identified an approximately 3. 6-kb message that was induced to higher levels in P- cells following exposure to the tumor promoter 12-O-tetradecanoylphorbol acetate than in P+ cells. The message was detected in mRNA from heart, lung, and spleen. cDNA cloning of the P- preferential sequence revealed a high degree of identity to human pleckstrin (PLEK), the major PKC substrate in platelets (Tyers et al., 1988, Nature 333: 470). We report the complete mouse cDNA sequence of pleckstrin and the localization of the gene to chromosome 11, its expression in a nonhematopoetic cell line, and its potential role in blocking neoplastic transformation.

Pleckstrin induces cytoskeletal reorganization via a Rac-dependent pathway

Pleckstrin homology (PH) domains are present in over one hundred signaling molecules, where they are thought to mediate membrane targeting by binding to phosphoinositides. They were initially defined at the NH(2) and COOH termini of the molecule, pleckstrin, a major substrate for protein kinase C in platelets. We have previously reported that pleckstrin associates with the plasma membrane, where it induces the formation of villous and ruffled structures from the surface of transfected cells (1). We now show that overexpression of pleckstrin results in reorganization of the actin cytoskeleton. This pleckstrin effect is regulated by its phosphorylation and requires the NH(2)-terminal, but not the COOH-terminal, PH domain. Overexpression of the NH(2)-terminal PH domain alone of pleckstrin is sufficient to induce the cytoskeletal effects. Pleckstrin-induced actin rearrangements are not inhibited by pharmacologic inhibition of phosphatidylinositol 3-kinase, nor are they blocked by co-expression of a dominant negative phosphatidylinositol 3-kinase. The cytoskeletal effects of pleckstrin can be blocked by co-expression of a dominant negative Rac1 variant, but not wild-type Rac and not a dominant negative Cdc42 variant. These data indicate that the NH(2)-terminal PH domain of pleckstrin induces reorganization of the actin cytoskeleton via a pathway dependent on Rac but independent of Cdc42 and phosphatidylinositol 3-kinase.

Expression of the Protein Kinase C Substrate Pleckstrin in Macrophages: Association with Phagosomal Membranes

Despite evidence suggesting that protein kinase C (PKC) isoforms are important in phagocytosis by Fcγ receptors, the mechanisms by which the substrates of these kinases act are largely unknown. We have investigated the role of one PKC substrate, pleckstrin, in cells of the monocyte/macrophage lineage. Pleckstrin expression in mouse macrophages was induced severalfold in response to bacterial LPS and IFN-γ. In unstimulated cells, the protein was largely confined to the cytosol. Upon ingestion of IgG-opsonized zymosan particles (OPZ), however, pleckstrin accumulated on the phagosomal membrane. This association was transient, being maximal after 15 min and declining thereafter. Similar kinetics of association was also seen for both filamentous actin and the δ isoform of PKC. Ingestion of OPZ was found to induce phosphorylation of pleckstrin. To examine whether phosphorylation was required for phagosomal association, pleckstrin was expressed in CHO-IIA cells that stably express the FcγRIIA receptor and are competent for phagocytosis of OPZ. In these cells, both wild-type pleckstrin and mutants in which the phosphoacceptor sites had been mutated to either alanine (nonphosphorylatable) or glutamine (pseudophosphorylated) were found to accumulate on OPZ phagosomes. Thus, association of pleckstrin with phagosomes is independent of its phosphorylation. Our findings suggest that pleckstrin may serve as an intracellular adaptor/targeting protein in response to particulate stimuli. By targeting interacting ligands to the phagosomal compartment, pleckstrin may serve to regulate phagocytosis and/or early steps during maturation of the phagosome.

Pleckstrin 2, a widely expressed paralog of pleckstrin involved in actin rearrangement

We have identified a cDNA for pleckstrin 2 that is 39% identical and 65% homologous to the original pleckstrin. Like the original pleckstrin 1, this protein contains a pleckstrin homology (PH) domain at each end of the molecule as well as a DEP (Dishevelled, Egl-10, and pleckstrin) domain in the intervening sequence. A Northern blot probed with the full-length cDNA reveals that this homolog is ubiquitously expressed and is most abundant in the thymus, large bowel, small bowel, stomach, and prostate. Unlike pleckstrin 1, this newly discovered protein does not contain obvious sites of PKC phosphorylation, and in transfected Cos-7 cells, it is a poor substrate for phosphorylation, even after PMA stimulation. Cells expressing pleckstrin 2 undergo a dramatic shape change associated with actin rearrangement, including a loss of central F-actin and a redistribution of actin toward the cell cortex. Overexpression of pleckstrin 2 causes large lamellipodia and peripheral ruffle formation. A variant of pleckstrin 2 lacking both PH domains still had some membrane binding but did not efficiently induce lamellipodia, suggesting that the PH domains of pleckstrin 2 contribute to lamellipodia formation. This work describes a novel, widely expressed, membrane-associating protein and suggests that pleckstrin 2 may help orchestrate cytoskeletal arrangement.

Phosphatidylinositol 3-kinase-dependent membrane association of the Bruton's tyrosine kinase pleckstrin homology domain visualized in single living cells

Phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) has been proposed to act as a second messenger to recruit regulatory proteins to the plasma membrane via their pleckstrin homology (PH) domains. The PH domain of Bruton's tyrosine kinase (Btk), which is mutated in the human disease X-linked agammaglobulinemia, has been shown to interact with PI(3,4,5)P3 in vitro. In this study, a fusion protein containing the PH domain of Btk and the enhanced green fluorescent protein (BtkPH-GFP) was constructed and utilized to study the ability of this PH domain to interact with membrane inositol phospholipids inside living cells. The localization of expressed BtkPH-GFP in quiescent NIH 3T3 cells was indistinguishable from that of GFP alone, both being cytosolic as assessed by confocal microscopy. In NIH 3T3 cells coexpressing BtkPH-GFP and the epidermal growth factor receptor, activation of epidermal growth factor or endogenous platelet-derived growth factor receptors caused a rapid (<3 min) translocation of the cytosolic fluorescence to ruffle-like membrane structures. This response was not observed in cells expressing GFP only and was completely inhibited by treatment with the PI 3-kinase inhibitors wortmannin and LY 292004. Membrane-targeted PI 3-kinase also caused membrane localization of BtkPH-GFP that was slowly reversed by wortmannin. When the R28C mutation of the Btk PH domain, which causes X-linked agammaglobulinemia, was introduced into the fluorescent construct, no translocation was observed after stimulation. In contrast, the E41K mutation, which confers transforming activity to native Btk, caused significant membrane localization of BtkPH-GFP with characteristics indicating its possible binding to PI(4,5)P2. This mutant, but not wild-type BtkPH-GFP, interfered with agonist-induced PI(4,5)P2 hydrolysis in COS-7 cells. These results show in intact cells that the PH domain of Btk binds selectively to 3-phosphorylated lipids after activation of PI 3-kinase enzymes and that losing such binding ability or specificity results in gross abnormalities in the function of the enzyme. Therefore, the interaction with PI(3,4,5)P3 is likely to be an important determinant of the physiological regulation of Btk and can be utilized to visualize the dynamics and spatiotemporal organization of changes in this phospholipid in living cells.

Visualization of phosphoinositides that bind pleckstrin homology domains: calcium- and agonist-induced dynamic changes and relationship to myo-[3H]inositol-labeled phosphoinositide pools

Phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P2) pools that bind pleckstrin homology (PH) domains were visualized by cellular expression of a phospholipase C (PLC)delta PH domain-green fluorescent protein fusion construct and analysis of confocal images in living cells. Plasma membrane localization of the fluorescent probe required the presence of three basic residues within the PLCdelta PH domain known to form critical contacts with PtdIns(4, 5)P2. Activation of endogenous PLCs by ionophores or by receptor stimulation produced rapid redistribution of the fluorescent signal from the membrane to cytosol, which was reversed after Ca2+ chelation. In both ionomycin- and agonist-stimulated cells, fluorescent probe distribution closely correlated with changes in absolute mass of PtdIns(4,5)P2. Inhibition of PtdIns(4,5)P2 synthesis by quercetin or phenylarsine oxide prevented the relocalization of the fluorescent probe to the membranes after Ca2+ chelation in ionomycin-treated cells or during agonist stimulation. In contrast, the synthesis of the PtdIns(4,5)P2 imaged by the PH domain was not sensitive to concentrations of wortmannin that had been found inhibitory of the synthesis of myo-[3H]inositol- labeled PtdIns(4,5)P2. Identification and dynamic imaging of phosphoinositides that interact with PH domains will further our understanding of the regulation of such proteins by inositol phospholipids.

Pleckstrin homology domains: a common fold with diverse functions

Pleckstrin homology (PH) motifs are approximately 100 amino-acid residues long and have been identified in nearly 100 different eukaryotic proteins, many of which participate in cell signaling and cytoskeletal regulation. Despite minimal sequence homology, the three-dimensional structures are remarkably conserved. This review gives an overview of the PH domain architecture and examines the best-studied examples in an attempt to understand their function.

Protein kinase C-dependent and Ca2+-dependent mechanisms of secretion from streptolysin O-permeabilized platelets: effects of leakage of cytosolic proteins

Human platelets containing dense granules labelled with 5-hydroxy[14C]tryptamine ([14C]5-HT) were permeabilized by exposure to streptolysin O (SLO) in the presence of 4 mM [gamma-32P]ATP. Addition of either 100 nM phorbol 12-myristate 13-acetate (PMA) or of Ca2+ (pCa 5) at the same time as SLO induced secretion of dense-granule [14C]5-HT and the phosphorylation of pleckstrin by protein kinase C (PKC). Ca2+ also induced phosphorylation of myosin P-light chains. Guanosine 5'-[gamma-thio]triphosphate (GTP[S], 100 microM) did not stimulate secretion from SLO-permeabilized platelets in the absence of Ca2+ (pCa>9), but greatly potentiated secretion in the presence of low PMA (10 nM) or low Ca2+ (pCa 6). However, GTP[S] did stimulate myosin P-light-chain phosphorylation in the absence of Ca2+, an effect that was associated with morphological changes, including granule centralization. Inhibition of PKC and of pleckstrin phosphorylation by Ro 31-8220 blocked secretion induced by PMA or by GTP[S] and PMA in the absence of Ca2+, but did not prevent the GTP[S]-induced phosphorylation of myosin P-light chains or secretion induced by Ca2+ at pCa 5. When the time period between exposure of platelets to SLO and challenge at pCa>9 with PMA or with GTP[S] and PMA was increased, there were rapid and parallel decreases in the secretion and pleckstrin phosphorylation responses, which were lost after 3-5 min. In contrast, the responsiveness of secretion to Ca2+ (pCa 5) or to GTP[S] and Ca2+ (pCa 6) persisted for at least 10 min after exposure of platelets to SLO, although the ability of pleckstrin to undergo phosphorylation was still lost after 3-5 min. Both PKC and pleckstrin were undetectable within platelets after 5 min exposure to SLO. The results suggest that the loss of responsiveness to PMA or to GTP[S] and PMA is attributable to the leakage of PKC (and possibly pleckstrin) from the platelets, whereas secretion stimulated by Ca2+ or by GTP[S] and Ca2+ utilizes membrane-associated Ca2+- and GTP-binding proteins and occurs independently of PKC activation.

Regulatory recruitment of signalling molecules to the cell membrane by pleckstrinhomology domains

Pleckstrin-homology (PH) domains are small protein modules found in more than 100 proteins, most of which require association with the cell membrane to mediate their biological functions. Recent studies have demonstrated that some PH domains bind specifically to phosphoinositides, and that PH-domain-mediated recruitment of certain proteins to the cell membrane is important in regulation of their activities or functions. This provides the cell with a simple and efficient mechanism for linking growth-factor-induced changes in the levels of specific membrane phosphoinositides with other signalling pathways that control diverse processes such as protein synthesis, DNA synthesis and cell adhesion.

Pleckstrin associates with plasma membranes and induces the formation of membrane projections: requirements for phosphorylation and the NH2-terminal PH domain

Pleckstrin homology (PH) domains are sequences of approximately 100 amino acids that form "modules" that have been proposed to facilitate protein/protein or protein/lipid interactions. Pleckstrin, first described as a substrate for protein kinase C in platelets and leukocytes, is composed of two PH domains, one at each end of the molecule, flanking an intervening sequence of 147 residues. Evidence is accumulating to support the hypothesis that PH domains are structural motifs that target molecules to membranes, perhaps through interactions with G betagamma or phosphatidylinositol 4,5-bisphosphate (PIP2), two putative PH domain ligands. In the present studies, we show that pleckstrin associates with membranes in human platelets. We further demonstrate that, in transfected Cos-1 cells, pleckstrin associates with peripheral membrane ruffles and dorsal membrane projections. This association depends on phosphorylation of pleckstrin and requires the presence of its NH2-terminal, but not its COOH-terminal, PH domain. Moreover, PH domains from other molecules cannot effectively substitute for pleckstrin's NH2-terminal PH domain in directing membrane localization. Lastly, we show that wild-type pleckstrin actually promotes the formation of membrane projections from the dorsal surface of transfected cells, and that this morphologic change is similarly PH domain dependent. Since we have shown previously that pleckstrin-mediated inhibition of PIP2 metabolism by phospholipase C or phosphatidylinositol 3-kinase also requires pleckstrin phosphorylation and an intact NH2-terminal PH domain, these results suggest that: (a) pleckstrin's NH2-terminal PH domain may regulate pleckstrin's activity by targeting it to specific areas within the cell membrane; and (b) pleckstrin may affect membrane structure, perhaps via interactions with PIP2 and/or other membrane-bound ligands.

Benzophenone photoprobes for phosphoinositides, peptides and drugs

Benzophenones (BP) and related aryl ketone photophores have become established as the photoactivatable group of choice for high-efficiency covalent modification of hydrophobic regions of binding proteins, including enzymes and receptors that recognize peptide hormones, (oligo) nucleotides and nucleosides, phosphoinositides, inositol polyphosphates and a wide variety of therapeutic molecules. This review presents the advantages of BP as photoaffinity labels and provides specific examples from the last 3 years of applications of BP-containing ligands used in biochemistry.

Phosphorylation of platelet pleckstrin activates inositol polyphosphate 5-phosphatase I

Pleckstrin is the major substrate phosphorylated on serine and threonine in response to stimulation of human platelets by thrombin (Abrams, C. S., Zhao, W., Belmonte, E., and Brass, L. F. (1995) J. Biol. Chem. 270, 23317-23321). We now show that pleckstrin in platelets is in a complex with inositol polyphosphate 5-phosphatase I (5-phosphatase I). This enzyme hydrolyzes the 5-phosphate from inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate and thus serves as a calcium signal-terminating enzyme, since the substrates but not the products mobilize intracellular calcium. Pleckstrin co-immunoprecipitates with 5-phosphatase I in homogenates of platelets. Platelet homogenates fractionated by anion exchange chromatography show co-elution of pleckstrin and 5-phosphatase I. Fractions containing phosphorylated pleckstrin have 7-fold greater 5-phosphatase activity than those containing unphosphorylated pleckstrin. Mixing experiments with recombinant 5-phosphatase I and pleckstrin in vitro show that they form a stoichiometric complex. A mutant form of pleckstrin, in which the serine and threonine residues that are phosphorylated by protein kinase C are substituted with glutamic acid (pseudophosphorylated pleckstrin), activates recombinant 5-phosphatase I 2-3-fold while native unphosphorylated pleckstrin does not stimulate the enzyme. Thus pleckstrin functions to terminate calcium signaling in platelets when it is phosphorylated by binding to and activating 5-phosphatase I.

A site of interaction between pleckstrin's PH domains and G beta gamma

Pleckstrin is a 40 kDa substrate for protein kinase C found in platelets and neutrophils. Based upon its sequence, pleckstrin contains two of the recently-described PH domains that are thought to be binding motifs for phosphatidyl 4,5-bisphosphate (PIP2) and/or G protein beta gamma heterodimers (G beta gamma). In the present studies we have examined the interaction between pleckstrin and G beta gamma by incubating pleckstrin fusion proteins with lysates from human platelets. In this analysis, both the N-terminal and C-terminal PH domains from pleckstrin bound G beta gamma in vitro, as did peptides containing as little as the first 30 residues of the C-terminal pleckstrin PH domain. Introduction of a point mutation into this region, analogous to the mutation in the Btk PH domain that causes X-linked immunodeficiency disease (XID) in mice, dramatically disrupted this interaction. We propose that pleckstrin may interact with G beta gamma, and that one potential site for this interaction involves the first 30 residues of pleckstrin's C-terminal PH domain.

Phosphopleckstrin inhibits gbetagamma-activable platelet phosphatidylinositol-4,5-bisphosphate 3-kinase

Pleckstrin, the prototypic protein containing two copies of the pleckstrin homology domain, is a prominent substrate of protein kinase C in platelets and neutrophils. Both cell types have p85 subunit-containing phosphoinositide 3-kinase (p85/PI3K) and non-p85-containing PI3K (PI3Kgamma) that is activated by betagamma subunits of heterotrimeric GTP-binding proteins. We have shown that a PI3K product, phosphatidylinositol (PI) 3,4,5-trisphosphate, promotes pleckstrin phosphorylation in platelets. Since pleckstrin homology domains are thought to interact with Gbetagamma heterodimers and/or PI(4,5)P2, we have examined the effects of recombinant pleckstrins on platelet PI3Kgamma and p85/PI3K activities. Depending upon its phosphorylation/charged state, pleckstrin inhibits PI3Kgamma, but not p85/PI3K. Pleckstrin-mediated inhibition of PI3Kgamma is overcome by excess Gbetagamma and is restricted to PI(4,5)P2 as substrate, i.e. pleckstrin does not inhibit phosphorylation of PI()P or PI. Consistent with this, activation of protein kinase C by exposure of platelets to beta-phorbol diester (to increase endogenous pleckstrin phosphorylation) prior to platelet lysis causes inhibition of Gbetagamma-stimulatable PI3K activity only with respect to PI(4,5)P2 substrate. This phosphopleckstrin-mediated inhibition is overcome by increasing concentrations of Gbetagamma. We propose that phosphorylation of pleckstrin may constitute an important inhibitory mechanism for PI3Kgamma-mediated cell signaling.

Signal-transducing protein phosphorylation cascades mediated by Ras/Rho proteins in the mammalian cell: the potential for multiplex signalling

The features of three distinct protein phosphorylation cascades in mammalian cells are becoming clear. These signalling pathways link receptor-mediated events at the cell surface or intracellular perturbations such as DNA damage to changes in cytoskeletal structure, vesicle transport and altered transcription factor activity. The best known pathway, the Ras-->Raf-->MEK-->ERK cascade [where ERK is extracellular-signal-regulated kinase and MEK is mitogen-activated protein (MAP) kinase/ERK kinase], is typically stimulated strongly by mitogens and growth factors. The other two pathways, stimulated primarily by assorted cytokines, hormones and various forms of stress, predominantly utilize p21 proteins of the Rho family (Rho, Rac and CDC42), although Ras can also participate. Diagnostic of each pathway is the MAP kinase component, which is phosphorylated by a unique dual-specificity kinase on both tyrosine and threonine in one of three motifs (Thr-Glu-Tyr, Thr-Phe-Tyr or Thr-Gly-Tyr), depending upon the pathway. In addition to activating one or more protein phosphorylation cascades, the initiating stimulus may also mobilize a variety of other signalling molecules (e.g. protein kinase C isoforms, phospholipid kinases, G-protein alpha and beta gamma subunits, phospholipases, intracellular Ca2+). These various signals impact to a greater or lesser extent on multiple downstream effectors. Important concepts are that signal transmission often entails the targeted relocation of specific proteins in the cell, and the reversible formation of protein complexes by means of regulated protein phosphorylation. The signalling circuits may be completed by the phosphorylation of upstream effectors by downstream kinases, resulting in a modulation of the signal. Signalling is terminated and the components returned to the ground state largely by dephosphorylation. There is an indeterminant amount of cross-talk among the pathways, and many of the proteins in the pathways belong to families of closely related proteins. The potential for more than one signal to be conveyed down a pathway simultaneously (multiplex signalling) is discussed. The net effect of a given stimulus on the cell is the result of a complex intracellular integration of the intensity and duration of activation of the individual pathways. The specific outcome depends on the particular signalling molecules expressed by the target cells and on the dynamic balance among the pathways.

Phorbol esters stimulate phosphatidylinositol 3,4,5-trisphosphate production in 3T3-L1 adipocytes: implications for stimulation of glucose transport

The effects of insulin and phorbol 12-myristate 13-acetate (PMA) on the levels of cellular phosphoinositides were investigated in 3T3-L1 adipocytes. Stimulation for 4 min with PMA (1 microM) or insulin (10 nM) increased levels of PtdIns(3,4,5)P3 approx. 2-fold and 6-fold respectively. PMA also had a small effect on the cellular levels of PtdIns4P, whereas insulin had no effect on PtdIns4P levels; levels of PtdIns(4,5)P2 and PtdIns3P were not significantly affected by either agent. Insulin increased the levels of the p85 alpha subunit of phosphoinositide (PI) 3-kinase associated with membranes, whereas PMA decreased levels of membrane-associated p85 alpha. PMA did not increase PI 3-kinase activity in anti-phosphotyrosine or anti-p85 immunoprecipitates. The stimulation of glucose transport by insulin or PMA was blocked by 100 nM wortmannin or 10 ng/ml LY294002, indicating that PI 3-kinase is essential for stimulation by both agents. In summary, these results demonstrate: (1) that PMA and insulin stimulate PtdIns(3,4,5)P3 production by distinct mechanisms in 3T3-L1 adipocytes, and (2) that stimulation of PtdIns(3,4,5)P3 production by PMA is likely to be important in signalling pathways leading from PMA stimulation to end-point responses such as glucose transport.

Chemical cross-linking of pleckstrin in human platelets: evidence for oligomerization of the protein and its dissociation by protein kinase C

The major substrate of protein kinase C(PKC) in platelets is the 40 kDa protein, pleckstrin. Addition of the homobifunctional reagent, bis(sulphosuccinimidyl)suberate (BS3), to platelet lysate, cytosol fraction or to electropermeabilized platelets resulted in cross-linking of pleckstrin to give higher-molecular-mass complexes of 68 kDa, 90 kDa and 100-120 kDa respectively, which were visualized by immunoblotting with an anti-pleckstrin antibody. Higher levels of cross-linking were observed in permeabilized platelets than in platelet lysates. The yields of the cross-linked complexes were much reduced after dilution of platelet lysate or lysis of electropermeabilized platelets and, in the case of the 90 kDa and 100-120 kDa species, after activation of PKC by phorbol 12-myristate 13-acetate. Similar experiments with purified pleckstrin indicated that the 90 kDa and 100-120 kDa species consist, at least in part, of pleckstrin dimers and higher oligomers. After incubation of purified pleckstrin (0.45 mg/ml) for 1 h with 2 mM BS3, about 25% of the protein was present in cross-linked species. The results indicate that pleckstrin undergoes a reversible self-association that can be prevented by phosphorylation of the protein, and also interacts with an unidentified platelet protein of about 28 kDa.

Phosphorylation of human pleckstrin on Ser-113 and Ser-117 by protein kinase C

During platelet activation, receptor-coupled phospholipid hydrolysis stimulates protein kinase C (PKC) and results in the phosphorylation of several proteins, the most prominent being pleckstrin. Pleckstrin is composed of two repeated domains, now called pleckstrin homology (PH) domains, separated by a spacer region that contains several consensus PKC phosphorylation sites. To determine the role of PKC-dependent phosphorylation in pleckstrin function, we mapped the phosphorylation sites in vivo of wild-type and site-directed mutants of pleckstrin expressed in COS cells. Phosphorylation was found to occur almost exclusively on Ser-113 and Ser-117 within the sequence 108-KFARKS*TRRS*IRL-120. Phosphorylation of these sites was confirmed by phosphorylation of the corresponding wild-type and mutant synthetic peptides in vitro.

Effect of different G protein-coupled receptor kinases on phosphorylation and desensitization of the alpha1B-adrenergic receptor

The alpha1B-adrenergic receptor (alpha1BAR), its truncated mutant T368, different G protein-coupled receptor kinases (GRK) and arrestin proteins were transiently expressed in COS-7 or HEK293 cells alone and/or in various combinations. Coexpression of beta-adrenergic receptor kinase (betaARK) 1 (GRK2) or 2 (GRK3) could increase epinephrine-induced phosphorylation of the wild type alpha1BAR above basal as compared to that of the receptor expressed alone. On the other hand, overexpression of the dominant negative betaARK (K220R) mutant impaired agonist-induced phosphorylation of the receptor. Overexpression of GRK6 could also increase epinephrine-induced phosphorylation of the receptor, whereas GRK5 enhanced basal but not agonist-induced phosphorylation of the alpha1BAR. Increasing coexpression of betaARK1 or betaARK2 resulted in the progressive attenuation of the alpha1BAR-mediated response on polyphosphoinositide (PI) hydrolysis. However, coexpression of betaARK1 or 2 at low levels did not significantly impair the PI response mediated by the truncated alpha1BAR mutant T368, lacking the C terminus, which is involved in agonist-induced desensitization and phosphorylation of the receptor. Similar attenuation of the receptor-mediated PI response was also observed for the wild type alpha1BAR, but not for its truncated mutant, when the receptor was coexpressed with beta-arrestin 1 or beta-arrestin 2. Despite their pronounced effect on phosphorylation of the alpha1BAR, overexpression of GRK5 or GRK6 did not affect the receptor-mediated response. In conclusion, our results provide the first evidence that betaARK1 and 2 as well as arrestin proteins might be involved in agonist-induced regulation of the alpha1BAR. They also identify the alpha1BAR as a potential phosphorylation substrate of GRK5 and GRK6. However, the physiological implications of GRK5- and GRK6-mediated phosphorylation of the alpha1BAR remain to be elucidated.

Signal transduction and actin filament organization

Small GTP-binding proteins of the Rho family appear to integrate extracellular signals from diverse receptor types and initiate rearrangements of F-actin. Active members of the Rho family, Rho and Rac, are now joined by Cdc42 which induces filopodia. Downstream of the Rho family proteins, actin polymerization may be induced by an increase in the availability of actin filament barbed ends. Actin organization may be affected by exposure of actin-binding sites on proteins such as vinculin and ezrin.

The pleckstrin homology domain: an intriguing multifunctional protein module

Pleckstrin homology (PH) domains are a family of compact protein modules defined by sequences of roughly 100 amino acids. These domains are common in vertebrate, Drosophila, C. elegans and yeast proteins, suggesting an early origin and fundamental importance to eukaryotic biology. Many enzymes which have important regulatory functions contain PH domains, and mutant forms of several such proteins are implicated in oncogenesis and developmental disorders. Numerous recent studies show that PH domains bind various proteins and inositolphosphates. Here I discuss PH domains in detail and conclude that they form a versatile family of membrane binding and protein localization modules.

Phosphorylation of the platelet p47 phosphoprotein is mediated by the lipid products of phosphoinositide 3-kinase

Platelet stimulation by thrombin or the thrombin receptor activating peptide (TRAP) results in the activation of phosphoinositide 3-kinase and the production of the novel polyphosphoinositides phosphatidylinositol 3,4-bisphosphate (PtdIns-3,4-P2) and phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P3). We have shown previously that these lipids activate calcium-independent protein kinase C (PKC) isoforms in vitro (Toker, A., Meyer, M., Reddy, K. K., Falck, J. R., Aneja, R., Aneja, S., Parra, A., Burns, D. J., Ballas, L. M. and Cantley, L. C. (1994) J. Biol. Chem. 269, 32358-32367). Activation of platelet PKC in response to TRAP is detected by the phosphorylation of the major PKC substrate in platelets, the p47 phosphoprotein, also known as pleckstrin. Here we provide evidence for two phases of pleckstrin phosphorylation in response to TRAP. A rapid phase of pleckstrin phosphorylation (< 1 min) precedes the peak of PtdIns-3,4-P2 production and is unaffected by concentrations of wortmannin (10-100 nM) that block production of this lipid. However prolonged phosphorylation of pleckstrin (> 2 min) is inhibited by wortmannin concentrations that block PtdIns-3,4-P2 production. Phorbol ester-mediated pleckstrin phosphorylation was not affected by wortmannin and wortmannin had no effect on purified platelet PKC activity. Phosphorylation of pleckstrin could be induced using permeabilized platelets supplied with exogenous gamma-32P[ATP] and synthetic dipalmitoyl PtdIns-3,4,5-P3 and dipalmitoyl PtdIns-3,4-P2 micelles, but not with dipalmitoyl phosphatidylinositol 3-phosphate or phosphatidylinositol 4,5-bisphosphate. These results suggest two modes of stimulating pleckstrin phosphorylation: a rapid activation of PKC (via diacylglycerol and calcium) followed by a slower activation of calcium-independent PKCs via PtdIns-3,4-P2.

Protein kinase C regulates pleckstrin by phosphorylation of sites adjacent to the N-terminal pleckstrin homology domain

Pleckstrin is a substrate for protein kinase C in activated platelets that contains at its N and C termini two of the pleckstrin homology (PH) domains that have been proposed to mediate protein-protein and protein-lipid interactions. We have recently shown that pleckstrin can inhibit agonist-induced phosphoinositide hydrolysis and that this inhibition requires an intact N-terminal PH domain (residues 6 to 99). In the present studies, we have identified the sites of phosphorylation in pleckstrin and examined their contribution to pleckstrin function. In human platelets activated with thrombin or phorbol esters, and in COS-1 cells expressing pleckstrin, a combination of phosphopeptide analysis and site-directed mutagenesis shows that three residues in the intervening sequence between the two pleckstrin PH domains become phosphorylated: Ser113, Thr114, and Ser117. Replacing all three of these sites with glycine decreased phosphorylation by > 90% and reduced pleckstrin's ability to inhibit phosphoinositide hydrolysis by as much as 80%. Replacing the phosphorylation sites with alanine residues had a similar effect, while substitution with aspartate, glutamate, or lysine residues produced pleckstrin variants that were fully active even in the absence of phosphorylation. These results suggest that phosphorylation enhances pleckstrin's activity by introducing a cluster of charges into a region adjacent to, but not within, the N-terminal PH domain. This may have an allosteric effect on the N-terminal PH domain, regulating its interaction with other molecules necessary for the inhibition of phosphoinositide hydrolysis.