Phosphoinositide binding specificity among phospholipase C isozymes as determined by photo-cross-linking to novel substrate and product analogs

Hepatic transcript profiling in beef cattle: Effects of rumen-protected niacin supplementation

The objective of our study was to assess the effect of rumen-protected niacin supplementation on the transcriptome of liver tissue in growing Angus × Simmental steers and heifers through RNA-seq analysis. Consequently, we wanted to assess the known role of niacin in the physiological processes of vasodilation, detoxification, and immune function in beef hepatic tissue. Normal weaned calves (~8 months old) were provided either a control diet or a diet supplemented with rumen-protected niacin (6 g/hd/d) for a 30-day period, followed by a liver biopsy. We observed a significant list of changes at the transcriptome level due to rumen-protected niacin supplementation. Several metabolic pathways revealed potential positive effects to the animal's liver metabolism due to administration of rumen-protected niacin; for example, a decrease in lipolysis, apoptosis, inflammatory responses, atherosclerosis, oxidative stress, fibrosis, and vasodilation-related pathways. Therefore, results from our study showed that the liver transcriptional machinery switched several metabolic pathways to a condition that could potentially benefit the health status of animals supplemented with rumen-protected niacin. In conclusion, based on the results of our study, we can suggest the utilization of rumen-protected niacin supplementation as a nutritional strategy could improve the health status of growing beef cattle in different beef production stages, such as backgrounding operations or new arrivals to a feedlot.

Comparative transcriptome analysis revealed genes involved in the sexual size dimorphisms and expressed sequence tag-Simple Sequence Repeat loci validation in Odorrana graminea

Sexual size dimorphism (SSD) is widespread among animals and is characterized by differences in body size between sexes. Previous studies suggested SSD might reflect the adaptations of particular sexes to their specific reproductive or ecological roles. The large green cascade frogs (Odorrana graminea) exhibit obvious SSD that females are nearly twice the body size of males. However, the molecular mechanisms underlying SSD of O. graminea are still unknown. In the present study, we first obtained nearly 5 Gb of the transcriptome data through Illumina sequencing, and the de novo transcriptome assembly produced 189,868 unigenes of O. graminea. A total of 774 significantly sex-differentially expressed genes (DEGs) were identified. Of which, 436 DEGs showed significantly higher expression levels in females than those in males, whereas 338 DEGs showed significantly lower expression in females than those in males. We also found 10 sex-differentially expressed genes related to energy metabolism between sexes of O. graminea, and these DEGs were related to the estrogen signaling pathway, oxidative phosphorylation, fatty acid biosynthesis, gastric acid secretion, and nitrogen metabolism. We found that the differences in energy metabolism and steroid hormone synthesis might be the main driving force leading to the sexual growth dimorphism of O. graminea. In addition, a total of 63,269 potential EST-SSR loci and 4,669 EST-SSR loci were detected and validated in different populations of O. graminea and other species within Odorrana. The assembled transcriptome will facilitate functional genomic studies of O. graminea and the developed EST-SSR markers will contribute to the population genetics of the species within Odorrana. The sex-differentially expressed genes involved in energy metabolism might provide insights into the genetic mechanisms underlying the SSD of O. graminea.

Profiling Glycosylphosphatidylinositol (GPI)-Interacting Proteins in the Cell Membrane Using a Bifunctional GPI Analogue as the Probe

Glycosylphosphatidylinositol (GPI) anchorage of cell surface proteins to the membrane is biologically important and ubiquitous in eukaryotes. However, GPIs do not contain long enough lipids to span the entire membrane bilayer. To transduce binding signals, GPIs must interact with other membrane components, but such interactions are difficult to define. Here, a new method was developed to explore GPI-interacting membrane proteins in live cell with a bifunctional analogue of the glucosaminylphosphatidylinositol motif conserved in all GPIs as a probe. This probe contained a diazirine functionality in the lipid and an alkynyl group on the glucosamine residue to respectively facilitate the cross-linkage of GPI-binding membrane proteins with the probe upon photoactivation and then the installation of biotin to the cross-linked proteins via a click reaction for affinity-based protein isolation and analysis. Profiling the proteins pulled down from the Hela cells revealed 94 unique and 18 overrepresented proteins compared to the control, and most of them are membrane proteins and many are GPI-related. The results have proved not only the concept of using the new bifunctional GPI probe to investigate GPI-binding membrane proteins but also the important role of inositol in the biological functions of GPI anchors and GPI-anchored proteins.

Upregulation of Phospholipase C Gene Expression Due to Norepinephrine-Induced Hypertrophic Response

The activation of phospholipase C (PLC) is thought to have a key role in the cardiomyocyte response to several different hypertrophic agents such as norepinephrine, angiotensin II and endothelin-1. PLC activity results in the generation of diacylglycerol and inositol trisphosphate, which are downstream signal transducers for the expression of fetal genes, increased protein synthesis, and subsequent cardiomyocyte growth. In this article, we describe the signal transduction elements that regulate PLC gene expression. The discussion is focused on the norepinephrine- α₁-adrenoceptor signaling pathway and downstream signaling processes that mediate an upregulation of PLC isozyme gene expression. Evidence is also indicated to demonstrate that PLC activities self-regulate the expression of PLC isozymes with the suggestion that PLC activities may be part of a coordinated signaling process for the perpetuation of cardiac hypertrophy. Accordingly, from the information provided, it is plausible that specific PLC isozymes could be targeted for the mitigation of cardiac hypertrophy.

Role of Phospholipase C in Catecholamine-induced Increase in Myocardial Protein Synthesis

The activation of the α1-adrenoceptor-(α1-AR) by norepinephrine results in the G-protein (Gqα) mediated increase in the phosphoinositide-specific phospholipase C (PLC) activity. The byproducts of PLC hydrolytic activity, namely, 1,2-diacylglycerol and inositol-1,4,5-trisphosphate, are important downstream signal transducers for increased protein synthesis in the cardiomyocyte and the subsequent hypertrophic response. In this article, evidence is outlined to demonstrate the role of cardiomyocyte PLC isozymes in the catecholamine-induced increase in protein synthesis by using a blocker of α1-AR and an inhibitor of PLC. The discussion will be focused on the α1-AR-Gqα-PLC-mediated hypertrophic signaling pathway from the viewpoint that it may compliment the other β1-AR-Gs protein-adenylyl cyclase signal transduction mechanisms in the early stages of cardiac hypertrophy development, but may become more relevant at the late stage of cardiac hypertrophy. From the information provided here, it is suggested that some specific PLC isozymes may potentially serve as important targets for the attenuation of cardiac hypertrophy in the vulnerable patient population at-risk for heart failure.

Involvement of phospholipase C in the norepinephrine-induced hypertrophic response in Cardiomyocytes

Norepinephrine (NE) is known to mediate cardiomyocyte hypertrophy through the G protein coupled a1 -adrenoceptor (a1 -AR) and the activation of the phosphoinositide-specific phospholipase C (PLC). Since the by-products of PLC activity are important downstream signal transducers for cardiac hypertrophy, the role of and the regulatory mechanisms involved in the activation of PLC isozymes in cardiac hypertrophy are highlighted in this review. The discussion is focused to underscore PLC in different experimental models of cardiac hypertrophy, as well as in isolated adult and neonatal cardiomyocytes treated with NE. Particular emphasis is laid concerning the a1 -AR-PLC-mediated hypertrophic signalling pathway. From the information provided, it is evident that the specific activation of PLC isozymes is a primary signalling event in the a1 -AR mediated response to NE as well as initiation and progression of cardiac hypertrophy. Furthermore, the possibility of PLC involvement in the perpetuation of cardiac hypertrophy is also described. It is suggested that specific PLC isozymes may serve as viable targets for the prevention of cardiac hypertrophy in patient population at-risk for the development of heart failure.

Phosphatidylinositol-4-phosphate 5-kinase type 1α attenuates Aβ production by promoting non-amyloidogenic processing of amyloid precursor protein

Alzheimer's disease (AD) is characterized by a chronic decline in cognitive function and is pathologically typified by cerebral deposition of amyloid-β peptide (Aβ). The production of Aβ is mediated by sequential proteolysis of amyloid precursor protein (APP) by β- and γ-secretases, and has been implicated as the essential determinant of AD pathology. Previous studies have demonstrated that the level of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] in the membrane may potentially modulate Aβ production. Given that PI(4,5)P2 is produced by type 1 phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks), we sought to determine whether the level of PIP5K type Iα (PIP5K1A) can affect production of Aβ by modulating the lipid composition of the membrane. Using a HEK-derived cell line that constitutively expresses yellow fluorescent protein-tagged APP (APP-YFP), we demonstrated that overexpression of PIP5K1A results in significant enhancement of non-amyloidogenic APP processing and a concomitant suppression of the amyloidogenic pathway, leading to a marked decrease in secreted Aβ. Consistently, cells overexpressing PIP5K1A exhibited a significant redistribution of APP-YFP from endosomal compartments to the cell surface. Our findings suggest that PIP5K1A may play a critical role in governing Aβ production by modulating membrane distribution of APP, and as such, the pathway may be a valuable therapeutic target for AD.

The Life of Pi Star: Exploring the Exciting and Forbidden Worlds of the Benzophenone Photophore

The widespread applications of benzophenone (BP) photochemistry in biological chemistry, bioorganic chemistry, and material science have been prominent in both academic and industrial research. BP photophores have unique photochemical properties: upon n-π* excitation at 365 nm, a biradicaloid triplet state is formed reversibly, which can abstract a hydrogen atom from accessible C-H bonds; the radicals subsequently recombine, creating a stable covalent C-C bond. This light-directed covalent attachment process is exploited in many different ways: (i) binding/contact site mapping of ligand (or protein)-protein interactions; (ii) identification of molecular targets and interactome mapping; (iii) proteome profiling; (iv) bioconjugation and site-directed modification of biopolymers; (v) surface grafting and immobilization. BP photochemistry also has many practical advantages, including low reactivity toward water, stability in ambient light, and the convenient excitation at 365 nm. In addition, several BP-containing building blocks and reagents are commercially available. In this review, we explore the "forbidden" (transitions) and excitation-activated world of photoinduced covalent attachment of BP photophores by touring a colorful palette of recent examples. In this exploration, we will see the pros and cons of using BP photophores, and we hope that both novice and expert photolabelers will enjoy and be inspired by the breadth and depth of possibilities.

Phospholipase Cβ1 induces membrane tubulation and is involved in caveolae formation

Lipid membrane curvature plays important roles in various physiological phenomena. Curvature-regulated dynamic membrane remodeling is achieved by the interaction between lipids and proteins. So far, several membrane sensing/sculpting proteins, such as Bin/amphiphysin/Rvs (BAR) proteins, are reported, but there remains the possibility of the existence of unidentified membrane-deforming proteins that have not been uncovered by sequence homology. To identify new lipid membrane deformation proteins, we applied liposome-based microscopic screening, using unbiased-darkfield microscopy. Using this method, we identified phospholipase Cβ1 (PLCβ1) as a new candidate. PLCβ1 is well characterized as an enzyme catalyzing the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2). In addition to lipase activity, our results indicate that PLCβ1 possessed the ability of membrane tubulation. Lipase domains and inositol phospholipids binding the pleckstrin homology (PH) domain of PLCβ1 were not involved, but the C-terminal sequence was responsible for this tubulation activity. Computational modeling revealed that the C terminus displays the structural homology to the BAR domains, which is well known as a membrane sensing/sculpting domain. Overexpression of PLCβ1 caused plasma membrane tubulation, whereas knockdown of the protein reduced the number of caveolae and induced the evagination of caveolin-rich membrane domains. Taken together, our results suggest a new function of PLCβ1: plasma membrane remodeling, and in particular, caveolae formation.

PLIF: A rapid, accurate method to detect and quantitatively assess protein-lipid interactions

Phosphoinositides are a type of cellular phospholipid that regulate signaling in a wide range of cellular and physiological processes through the interaction between their phosphorylated inositol head group and specific domains in various cytosolic proteins. These lipids also influence the activity of transmembrane proteins. Aberrant phosphoinositide signaling is associated with numerous diseases, including cancer, obesity, and diabetes. Thus, identifying phosphoinositide-binding partners and the aspects that define their specificity can direct drug development. However, current methods are costly, time-consuming, or technically challenging and inaccessible to many laboratories. We developed a method called PLIF (for "protein-lipid interaction by fluorescence") that uses fluorescently labeled liposomes and tethered, tagged proteins or peptides to enable fast and reliable determination of protein domain specificity for given phosphoinositides in a membrane environment. We validated PLIF against previously known phosphoinositide-binding partners for various proteins and obtained relative affinity profiles. Moreover, PLIF analysis of the sorting nexin (SNX) family revealed not only that SNXs bound most strongly to phosphatidylinositol 3-phosphate (PtdIns3P or PI3P), which is known from analysis with other methods, but also that they interacted with other phosphoinositides, which had not previously been detected using other techniques. Different phosphoinositide partners, even those with relatively weak binding affinity, could account for the diverse functions of SNXs in vesicular trafficking and protein sorting. Because PLIF is sensitive, semiquantitative, and performed in a high-throughput manner, it may be used to screen for highly specific protein-lipid interaction inhibitors.

Immune regulation by phospholipase C-β isoforms

Rapid progress has recently been made regarding how phospholipase C (PLC)-β functions downstream of G protein-coupled receptors and how PLC-β functions in the nucleus. PLC-β has also been shown to interplay with tyrosine kinase-based signaling pathways, specifically to inhibit Stat5 activation by recruiting the protein-tyrosine phosphatase SHP-1. In this regard, a new multimolecular signaling platform, named SPS complex, has been identified. The SPS complex has important regulatory roles in tumorigenesis and immune cell activation. Furthermore, a growing body of work suggests that PLC-β also participates in the differentiation and activation of immune cells that control both the innate and adaptive immune systems.

Structural insights into phospholipase C-β function

Phospholipase C (PLC) enzymes convert phosphatidylinositol-4,5-bisphosphate into the second messengers diacylglycerol and inositol-1,4,5-triphosphate. The production of these molecules promotes the release of intracellular calcium and activation of protein kinase C, which results in profound cellular changes. The PLCβ subfamily is of particular interest given its prominent role in cardiovascular and neuronal signaling and its regulation by G protein-coupled receptors, as PLCβ is the canonical downstream target of the heterotrimeric G protein Gαq. However, this is not the only mechanism regulating PLCβ activity. Extensive structural and biochemical evidence has revealed regulatory roles for autoinhibitory elements within PLCβ, Gβγ, small molecular weight G proteins, and the lipid membrane itself. Such complex regulation highlights the central role that this enzyme plays in cell signaling. A better understanding of the molecular mechanisms underlying the control of its activity will greatly facilitate the search for selective small molecule modulators of PLCβ.

Phospholipase C as a potential target for cardioprotection during oxidative stressThis review is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

Cardiac dysfunction due to ischemia–reperfusion (I/R) is associated with marked changes in membrane function and subsequent Ca2+-handling abnormalities in cardiomyocytes. The membrane abnormalities in hearts subjected to I/R arise primarily from oxidative stress as a consequence of increased formation of reactive oxygen species and other oxidants, as well as reduced antioxidant defenses. Little is known, however, about the nature and mechanisms of the sarcolemmal membrane changes with respect to phospholipase C (PLC)-related signaling events. In addition, the mechanisms involved in protection of the postischemic myocardium and in ischemic preconditioning with respect to PLC function need to be established. Accordingly, this article reviews the historical and current information on PLC-mediated signal transduction mechanisms in I/R, as well as outlining future directions that should be addressed. Such information will extend our knowledge of ischemic heart disease and help improve its therapy.

Phospholipid-mediated signaling and heart disease

Cardiac hypertrophy, congestive heart failure, diabetic cardiomyopathy and myocardial ischemia-reperfusion injury are associated with a disturbance in cardiac sarcolemmal membrane phospholipid homeostasis. The contribution of the different phospholipases and their related signaling mechanisms to altered function of the diseased myocardium is not completely understood. Resolution of this issue is essential for both the understanding of the pathophysiology of heart disease and for determining if components of the phospholipid signaling pathways could serve as appropriate therapeutic targets. This review provides an outline of the role of phospholipase A2, C and D and subsequent signal transduction mechanisms in different cardiac pathologies with a discussion of their potential as targets for drug development for the prevention/treatment of heart disease.

Stimulation of phospholipase Cbeta by membrane interactions, interdomain movement, and G protein binding--how many ways can you activate an enzyme?

Signaling proteins are usually composed of one or more conserved structural domains. These domains are usually regulatory in nature by binding to specific activators or effectors, or species that regulate cellular location, etc. Inositol-specific mammalian phospholipase C (PLC) enzymes are multidomain proteins whose activities are controlled by regulators, such as G proteins, as well as membrane interactions. One of these domains has been found to bind membranes, regulators, and activate the catalytic region. The recently solved structure of a major region of PLC-beta2 together with the structure of PLC-delta1 and a wealth of biochemical studies poises the system towards an understanding of the mechanism through which their regulations occurs.

Photoaffinity labeling and its application in structural biology

This review contains a brief consideration of some theoretical aspects of photoaffinity (photoreactive) labeling (PAL), and the most widely used photoreactive groups, such as arylazide, benzophenone, and 3-(trifluoromethyl)-3-phenyldiazirine, are characterized in comparison. Experimental methodology is described, including modern approaches of mass spectrometry for analysis of cross-linking products between the photoreactive probes and biomolecules. Examples of PAL application in diverse fields of structural biology during the last five-ten years are presented. Potential drug targets, transport processes, stereochemistry of interaction of G-protein-coupled receptors with ligands, as well as structural changes in nicotinic acetylcholine receptor are considered. Applications of photoaffinity ganglioside and phospholipid probes for studying biological membranes and of nucleotide probes in investigations of replicative and transcriptional complexes, as well as photoaffinity glycoconjugates for detecting carbohydrate-binding proteins are covered. In combination with modern techniques of instrumental analysis and computer-aided modeling, PAL remains the most important approach in studies on the organization of biological systems.

Regulation of phospholipase C isozymes by ras superfamily GTPases

The physiological effects of many extracellular stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid-signaling pathways. These signaling responses include the classically described conversion of PtdIns(4,5)P(2) to the Ca(2+)-mobilizing second messenger Ins(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. Here we discuss how the family of PLCs elaborates a minimal catalytic core typified by PLC-delta to confer multiple modes of regulation on their phospholipase activities. Although PLC-dependent signaling is prominently regulated by direct interactions with heterotrimeric G proteins or tyrosine kinases, the existence of at least 13 divergent PLC isozymes promises a diverse repertoire of regulatory mechanisms for this class of important signaling proteins. We focus here on the recently realized and extensive regulation of inositol lipid signaling by Ras superfamily GTPases directly acting on PLC isozymes and conclude by considering the biological and pharmacological ramifications of this regulation.

Identification of interaction sites of protein kinase Cα on phospholipase D1

Phospholipase D (PLD) is regulated by many factors, including protein kinase C (PKC) and small G-proteins of the Rho and ADP-ribosylation factor families. Previous studies revealed that the activation of PLD1 by phorbol ester is associated with the binding of PKCalpha to a site in the N-terminus of PLD1. The purpose of the present study was to determine this site more precisely. Immunoprecipitation with a series of four PLD1 deletion mutants confirmed that PKCalpha strongly interacted with the amino acid sequence 1-318 at the N-terminus of PLD1 and weakly with the sequence 841-1036 at the C-terminus. Further immunoprecipitation studies with deletion mutants of the 1-318 and 1-215 PLD1 fragments revealed that there were binding sites in the 1-49 N-terminal sequence and also in the 216-318 sequence containing the PH domain. Studies of N-terminal deletion mutants of full-length PLD1 confirmed the presence of a binding site in the 1-49 sequence and a further site in the 1-318 sequence. Both deletion mutants showed impaired activation by PKCalpha in vivo, but unchanged activation by active V(14)RhoA. These findings identify the 1-49 sequence is a major binding/activation site for PKCalpha on PLD1, but also indicate involvement of the PH domain.

Nuclear Translocation of Phospholipase C-δ1 Is Linked to the Cell Cycle and Nuclear Phosphatidylinositol 4,5-Bisphosphate

Nuclear phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate, fluctuate throughout the cell cycle and are linked to proliferation and differentiation. Here we report that phospholipase C-delta(1) accumulates in the nucleus at the G(1)/S boundary and in G(0) phases of the cell cycle. Furthermore, as wild-type protein accumulated in the nucleus, nuclear phosphatidylinositol 4,5-bisphosphate levels were elevated 3-5-fold, whereas total levels were decreased compared with asynchronous cultures. To test whether phosphatidylinositol 4,5-bisphosphate binding is important during this process, we introduced a R40D point mutation within the pleckstrin homology domain of phospholipase C-delta(1), which disables high affinity phosphatidylinositol 4,5-bisphosphate binding, and found that nuclear translocation was significantly reduced at G(1)/S and in G(0). These results demonstrate a cell cycle-dependent compartmentalization of phospholipase C-delta(1) and support the idea that relative levels of phosphoinositides modulate the portioning of phosphoinositide-binding proteins between the nucleus and other compartments.

Dissection of the steps of phospholipase C beta 2 activity that are enhanced by G beta gamma subunits

Phosphatidylinositol-specific phospholipase C (PLC) enzymes catalyze the hydrolysis of phosphatidylinositol 4,5 bisphosphate in a two step reaction that involves a cyclic intermediate. The PLCbetafamily are activated by both the alpha and betagamma subunits of heterotrimeric G proteins. To determine which catalytic step is affected by Gbetagamma subunits, we compared the change in PLCbeta(2) activity catalysis toward monomeric short-chain phosphatidylinositol (PI) substrates and monomeric water-soluble cyclic inositol phosphates as well as long-chain PI in bilayer and micellar interfaces in the absence and presence of Gbetagammasubunits. Unlike other PLC enzymes, no cyclic products were detected for either wild-type PLCbeta(2) or a chimeric protein composed of the PH domain of PLCbeta(2) and the catalytic domain of PLCdelta(1). Using cIP as a substrate to examine the second step of the reaction, we found that the presence of Gbetagamma subunits stimulated this step by a higher level than that for the overall reaction (k(cat) 1.5-fold (cIP) as opposed to 1.20-fold for soluble diC(4)PI). Detergents above their CMC can generate the same kinetic activation of PLCbeta(2) as Gbetagamma, suggesting that hydrophobic compounds stabilize the activated state of the enzyme. The most pronounced effect of Gbetagamma is that it relieves competitive product inhibition. Taken together, our results show that activation of PLCbeta(2) occurs through enhancement in the catalytic rate of hydrolysis of the cyclic intermediate and increased product release, and that hydrophobic interactions play a key role.

Phospholipase Cβ2 Binds to and Inhibits Phospholipase Cδ1

Phospholipase Cbeta (PLCbeta) isoforms, which are under the control of Galphaq and Gbetagamma subunits, generate Ca2+ signals induced by a broad array of extracellular agonists, whereas PLCdelta isoforms depend on a rise in cytosolic Ca2+ for their activation. Here we find that PLCbeta2 binds strongly to PLCdelta1 and inhibits its catalytic activity in vitro and in living cells. In vitro, this PLC complex can be disrupted by increasing concentrations of free Gbetagamma subunits. Such competition has consequences for signaling, because in HEK293 cells PLCbeta2 suppresses elevated basal [Ca2+] and inositol phosphates levels and the sustained agonist-induced elevation of Ca2+ levels caused by PLCdelta1. Also, expression of both PLCs results in a synergistic release of [Ca2+] upon stimulation in A10 cells. These results support a model in which PLCbeta2 suppresses the basal catalytic activity of PLCdelta1, which is relieved by binding of Gbetagamma subunits to PLCbeta2 allowing for amplified calcium signals.

The C-terminal basic tail of RhoG assists the guanine nucleotide exchange factor trio in binding to phospholipids

The multidomain protein Trio regulates among others neuronal outgrowth and axonal guidance in vertebrates and invertebrates. Trio contains two Dbl-homology/pleckstrin homology (DH/PH) tandem domains that activate several RhoGTPases. Here, we present the x-ray structure of the N-terminal DH/PH, hereafter TrioN, refined to 1.7-A resolution. We show that the relative orientations of the DH and PH domains of TrioN and free Dbs are similar. However, this relative orientation is dissimilar to Dbs in the Dbs/Cdc42 structure. In vitro nucleotide exchange experiments catalyzed by TrioN show that RhoG is approximately 3x more efficiently exchanged than Rac and support the conclusion that RhoG is likely the downstream target of TrioN. Residues 54 and 69, which are not conserved between the two GTPases, are responsible for this specificity. Dot-blot assay reveals that the TrioN-PH domain does not detectably bind phosphatidylinositol 3,4-bisphosphate, PtdIns(3,4)P(2), or other phospholipids. This finding is supported by our three-dimensional structure and affinity binding experiments. Interestingly, the presence of RhoG but not Rac or a C-terminal-truncated RhoG mutant allows TrioN to bind PtdIns(3,4)P(2) with a micromolar affinity constant. We conclude the variable C-terminal basic tail of RhoG specifically assists the recruitment of the TrioN-PH domain to specific membrane-bound phospholipids. Our data suggest a role for the phosphoinositide 3-kinase, PI 3-kinase, in modulating the Trio/RhoG signaling pathway.

Phosphoinositide Signaling

Lipid signaling by phosphoinositides (PIP(n)s) involves an array of proteins with lipid recognition, kinase, phosphatase, and phospholipase functions. Understanding PIP(n) pathway signaling requires identification and characterization of PIP(n)-interacting proteins. Moreover, spatiotemporal localization and physiological function of PIP(n)-protein complexes must be elucidated in cellular and organismal contexts. For protein discovery to functional elucidation, reporter-linked phosphoinositides or tethered PIP(n)s have been essential. The phosphoinositide 3-kinase (PI 3-K) signaling pathway has recently emerged as an important source of potential "druggable" therapeutic targets in human pathophysiology in both academic and pharmaceutical environments. This review summarizes the chemistry of PIP(n) affinity probes and their use in identifying macromolecular targets. The process of target validation will be described, i.e., the use of tethered PIP(n)s in determining PIP(n) selectivity in vitro and in establishing the function of PIP(n)-protein complexes in living cells.

Dual role for phosphoinositides in regulation of yeast and mammalian phospholipase D enzymes

Phospholipase D (PLD) generates lipid signals that coordinate membrane trafficking with cellular signaling. PLD activity in vitro and in vivo is dependent on phosphoinositides with a vicinal 4,5-phosphate pair. Yeast and mammalian PLDs contain an NH2-terminal pleckstrin homology (PH) domain that has been speculated to specify both subcellular localization and regulation of PLD activity through interaction with phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2). We report that mutation of the PH domains of yeast and mammalian PLD enzymes generates catalytically active PI(4,5)P2-regulated enzymes with impaired biological functions. Disruption of the PH domain of mammalian PLD2 results in relocalization of the protein from the PI(4,5)P2-containing plasma membrane to endosomes. As a result of this mislocalization, mutations within the PH domain render the protein unresponsive to activation in vivo. Furthermore, the integrity of the PH domain is vital for yeast PLD function in both meiosis and secretion. Binding of PLD2 to model membranes is enhanced by acidic phospholipids. Studies with PLD2-derived peptides suggest that this binding involves a previously identified polybasic motif that mediates activation of the enzyme by PI(4,5)P2. By comparison, the PLD2 PH domain binds PI(4,5)P2 with lower affinity but sufficient selectivity to function in concert with the polybasic motif to target the protein to PI(4,5)P2-rich membranes. Phosphoinositides therefore have a dual role in PLD regulation: membrane targeting mediated by the PH domain and stimulation of catalysis mediated by the polybasic motif.

Regulation of inositol phospholipid binding and signaling through syndecan-4

Syndecan-4 is a transmembrane heparan sulfate proteoglycan that can regulate cell-matrix interactions and is enriched in focal adhesions. Its cytoplasmic domain contains a central region unlike that of any other vertebrate or invertebrate syndecan core protein with a cationic motif that binds inositol phospholipids. In turn, lipid binding stabilizes the syndecan in oligomeric form, with subsequent binding and activation of protein kinase C. The specificity of phospholipid binding and its potential regulation are investigated here. Highest affinity of the syndecan-4 cytoplasmic domain was seen with phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5P)(2)) and phosphatidylinositol 4-phosphate, and both promoted syndecan-4 oligomerization. Affinity was much reduced for 3-phosphorylated inositides while no binding of diacylglycerol was detected. Syndecan-2 cytoplasmic domain had negligible affinity for any lipid examined. Inositol hexakisphosphate, but not inositol tetrakisphosphate, also had high affinity for the syndecan-4 cytoplasmic domain and could compete effectively with PtdIns(4,5)P(2). Since inositol hexaphosphate binding to syndecan-4 does not promote oligomer formation, it is a potential down-regulator of syndecan-4 signaling. Similarly, phosphorylation of serine 183 in syndecan-4 cytoplasmic domain reduced PtdIns(4,5)P(2) binding affinity by over 100-fold, although interaction could still be detected by nuclear magnetic resonance spectroscopy. Only protein kinase Calpha was up-regulated in activity by the combination of syndecan-4 and PtdIns(4,5)P(2), with all other isoforms tested showing minimal response. This is consistent with the codistribution of syndecan-4 with the alpha isoform of protein kinase C in focal adhesions.

Lateral sequestration of phosphatidylinositol 4,5-bisphosphate by the basic effector domain of myristoylated alanine-rich C kinase substrate is due to nonspecific electrostatic interactions

A peptide corresponding to the basic (+13), unstructured effector domain of myristoylated alanine-rich C kinase substrate (MARCKS) binds strongly to membranes containing phosphatidylinositol 4,5-bisphosphate (PIP(2)). Although aromatic residues contribute to the binding, three experiments suggest the binding is driven mainly by nonspecific local electrostatic interactions. First, peptides with 13 basic residues, Lys-13 and Arg-13, bind to PIP(2)-containing vesicles with the same high affinity as the effector domain peptide. Second, removing basic residues from the effector domain peptide reduces the binding energy by an amount that correlates with the number of charges removed. Third, peptides corresponding to a basic region in GAP43 and MARCKS effector domain-like regions in other proteins (e.g. MacMARCKS, adducin, Drosophila A kinase anchor protein 200, and N-methyl-d-aspartate receptor) also bind with an energy that correlates with the number of basic residues. Kinetic measurements suggest the effector domain binds to several PIP(2). Theoretical calculations show the effector domain produces a local positive potential, even when bound to a bilayer with 33% monovalent acidic lipids, and should thus sequester PIP(2) laterally. This electrostatic sequestration was observed experimentally using a phospholipase C assay. Our results are consistent with the hypothesis that MARCKS could reversibly sequester much of the PIP(2) in the plasma membrane.

Synthesis of hybrid lipid probes: derivatives of phosphatidylethanolamine-extended phosphatidylinositol 4,5-bisphosphate (Pea-PIP(2))

The total asymmetric synthesis of a novel hybrid lipid possessing a 2,3-diacylthreitol backbone, rather than a 1,2-diacylglycerol backbone, is described. The title compound, Pea-PIP(2), possesses a phosphatidylethanolamine (PE) headgroup at the 1-position and a phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) headgroup at the 4-position. Reporters (biotin, fluorophores, spin label) were covalently attached to the free amino group of the PE, such that these reporters were targeted to the lipid-water interface. The diacyl moieties allow incorporation of Pea-PIP(2) into a lipid bilayer, while the PtdIns(4,5)P(2) moiety in the aqueous layer was specifically recognized by PtdIns(4,5)P(2)-specific binding proteins.

Myristoylated alanine-rich C kinase substrate (MARCKS) sequesters spin-labeled phosphatidylinositol 4,5-bisphosphate in lipid bilayers

The myristoylated alanine-rich protein kinase C substrate (MARCKS) may function to sequester phosphoinositides within the plane of the bilayer. To characterize this interaction with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), a novel spin-labeled derivative, proxyl-PIP(2), was synthesized and characterized. In the presence of molecules known to bind PI(4,5)P(2) the EPR spectrum of this label exhibits an increase in line width because of a decrease in label dynamics, and titration of this probe with neomycin yields the expected 1:1 stoichiometry. Thus, this probe can be used to quantitate the interactions made by the PI(4,5)P(2) head group within the bilayer. In the presence of a peptide comprising the effector domain of MARCKS the EPR spectrum broadens, but the changes in line shape are modulated by both changes in label correlation time and spin-spin interactions. This result indicates that at least some proxyl-PIP(2) are in close proximity when bound to MARCKS and that MARCKS associates with multiple PI(4,5)P(2) molecules. Titration of the proxyl-PIP(2) EPR signal by the MARCKS-derived peptide also suggests that multiple PI(4,5)P(2) molecules interact with MARCKS. Site-directed spin labeling of this peptide shows that the position and conformation of this protein segment at the membrane interface are not altered significantly by binding to PI(4,5)P(2). These data are consistent with the hypothesis that MARCKS functions to sequester multiple PI(4,5)P(2) molecules within the plane of the membrane as a result of interactions that are driven by electrostatic forces.

Synthesis of photoreactive phosphatidic acid analogues displaying activatory properties on cyclic AMP-phosphodiesterases. Photoaffinity labeling of an isoform of phosphodiesterase

We have previously shown that phosphatidic acid (PA) is a specific activator of some isoforms of type 4 cyclic nucleotide phosphodiesterases (PDE 4) and that accumulation of endogenous PA can, in this way, influence the cAMP signaling pathway in different cell types. Enzyme activation depends on direct binding of the effector to specific sites carried by the enzyme. To identify the binding domain, photoactivatable phosphatidic acid analogues 1-azidoPA (12) and 2-azidoPA (7 and 15), potentially suitable for covalent labeling of PDE4, have been synthesized. The ability of phospholipases A(2) and D to hydrolyze unnatural phospholipids has been considered in this paper. The effect of 1-azidoPA (12) and 2-azidoPA (7 and 15) on the activity of a recombinant PA-sensitive isoform PDE4D3 was evaluated. The three compounds were able to activate the enzyme with different efficiencies. A tritiated analogue of 15 was synthesized and used in PDE4D3 labeling experiments, which showed that this PA analogue was specifically and covalently linked to the enzyme after UV irradiation. Photoactivatable analogues thus appear as suitable tools for the characterization of PA binding sites.

Analysis of oxysterol binding protein homologue Kes1p function in regulation of Sec14p-dependent protein transport from the yeast Golgi complex

Oxysterol binding proteins (OSBPs) comprise a large conserved family of proteins in eukaryotes. Their ubiquity notwithstanding, the functional activities of these proteins remain unknown. Kes1p, one of seven members of the yeast OSBP family, negatively regulates Golgi complex secretory functions that are dependent on the action of the major yeast phosphatidylinositol/phosphatidylcholine Sec14p. We now demonstrate that Kes1p is a peripheral membrane protein of the yeast Golgi complex, that localization to the Golgi complex is required for Kes1p function in vivo, and that targeting of Kes1p to the Golgi complex requires binding to a phosphoinositide pool generated via the action of the Pik1p, but not the Stt4p, PtdIns 4-kinase. Localization of Kes1p to yeast Golgi region also requires function of a conserved motif found in all members of the OSBP family. Finally, we present evidence to suggest that Kes1p may regulate adenosine diphosphate-ribosylation factor (ARF) function in yeast, and that it may be through altered regulation of ARF that Kes1p interfaces with Sec14p in controlling Golgi region secretory function.

Conformation, localization, and integrin binding of talin depend on its interaction with phosphoinositides

Talin is a structural component of focal adhesion sites and is thought to be engaged in multiple protein interactions at the cytoplasmic face of cell/matrix contacts. Talin is a major link between integrin and the actin cytoskeleton and was shown to play an important role in focal adhesion assembly. Consistent with the view that talin must be activated at these sites, we found that phosphatidylinositol 4-monophosphate and phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) bound to talin in cells in suspension or at early stages of adhesion, respectively. When phosphoinositides were associated with phospholipid bilayer, talin/phosphoinositide association was restricted to PI4,5P(2). This association led to a conformational change of the protein. Moreover, the interaction between integrin and talin was greatly enhanced by PI4,5P(2)-induced talin activation. Finally, sequestration of PI4,5P(2) by a specific pleckstrin homology domain confirms that PI4,5P(2) is necessary for proper membrane localization of talin and that this localization is essential for the maintenance of focal adhesions. Our results support a model in which PI4,5P(2) exposes the integrin-binding site on talin. We propose that PI4,5P(2)-dependent signaling modulates assembly of focal adhesions by regulating integrin-talin complexes. These results demonstrate that activation of the integrin-binding activity of talin requires not only integrin engagement to the extracellular matrix but also the binding of PI4,5P(2) to talin, suggesting a possible role of lipid metabolism in organizing the sequential assembly of focal adhesion components.

Microelectrophoresis of a bilayer-coated silica bead in an optical trap: application to enzymology

We describe an apparatus that combines microelectrophoresis and laser trap technologies to monitor the activity of phosphoinositide-specific phospholipase C-delta1 (PLC-delta) on a single bilayer-coated silica bead with a time resolution of approximately 1 s. A 1-microm-diameter bead was coated with a phospholipid bilayer composed of electrically neutral phosphatidylcholine (PC) and negatively charged phosphatidylinositol 4,5-bisphosphate (2% PIP2) and captured in a laser trap. When an AC field was applied (160 Hz, 20 V/cm), the electrophoretic force produced a displacement of the bead, Delta(x), from its equilibrium position in the trap; Delta(x), which was measured using a fast quadrant diode detector, is proportional to the zeta potential and thus to the number of PIP2 molecules on the outer leaflet (initially, approximately 10(5)). When a solution containing PLC-delta flows past the bead, the enzyme adsorbs to the surface and hydrolyzes PIP2 to form the neutral lipid diacylglycerol. We observed a nonexponential decay of PIP2 on the bead with time that is consistent with a model based on the known structural properties of PLC-delta.

The role and relevance of phospholipase D1 during growth and dimorphism of Candida albicans

The phosphatidylcholine-specific phospholipase D1 (PLD1) in Saccharomyces cerevisiae is involved in vesicle transport and is essential for sporulation. The gene encoding the homologous phospholipase D1 from Candida albicans (PLD1) was used to study the role of PLD1 in this pathogenic fungus. In vitro and in vivo expression studies using Northern blots and reverse transcriptase-PCR showed low PLD1 mRNA levels in defined media supporting yeast growth and during experimental infection, while enhanced levels of PLD1 transcripts were detected during the yeast to hyphal transition. To study the relevance of PLD1 during yeast and hyphal growth, an essential part of the gene was deleted in both alleles of two isogenic strains. In vitro PLD1 activity assays showed that pld1 mutants produced no detectable levels of phosphatidic acid, the hydrolytic product of PLD1 activity, and strongly reduced levels of diacylglycerol, the product of lipid phosphate phosphohydrolase, suggesting no or a negligible background PLD1 activity in the pld1 mutants. The pld1 mutants showed no growth differences compared to the parental wild-type in liquid complex and minimal media, independent of the growth temperature. In addition, growth rates of pld1 mutants in media with protein as the sole source of nitrogen were similar to growth rates of the wild-type, indicating that secretion of proteinases was not reduced. Chlamydospore formation was normal in pld1 mutants. When germ tube formation was induced in liquid media, pld1 mutants showed similar rates of yeast to hyphal transition compared to the wild-type. However, no hyphae formation was observed on solid Spider medium, and cell growth on cornmeal/Tween 80 medium indicated aberrant morphogenesis. In addition, pld1 mutants growing on solid media had an attenuated ability to invade the agar. In a model of oral candidosis, pld1 mutants showed no attenuation of virulence. In contrast, the mutant was less virulent in two different mouse models. These data suggest that PLD1 is not essential for growth and oral infections. However, they also suggest that a prominent part of the phosphatidic acid and diacylglycerol pools is produced by PLD1 and that the level of these components is important for morphological transitions under certain conditions in C. albicans.

The effector domain of myristoylated alanine-rich C kinase substrate binds strongly to phosphatidylinositol 4,5-bisphosphate

Both the myristoylated alanine-rich protein kinase C substrate protein (MARCKS) and a peptide corresponding to its basic effector domain, MARCKS-(151-175), inhibit phosphoinositide-specific phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP(2)) in vesicles (Glaser, M., Wanaski, S., Buser, C. A., Boguslavsky, V., Rashidzada, W., Morris, A., Rebecchi, M., Scarlata, S. F., Runnels, L. W., Prestwich, G. D., Chen, J., Aderem, A., Ahn, J., and McLaughlin, S. (1996) J. Biol. Chem. 271, 26187-26193). We report here that adding 10-100 nm MARCKS-(151-175) to a subphase containing either PLC-delta or -beta inhibits hydrolysis of PIP(2) in a monolayer and that this inhibition is due to the strong binding of the peptide to PIP(2). Two direct binding measurements, based on centrifugation and fluorescence, show that approximately 10 nm PIP(2), in the form of vesicles containing 0.01%, 0.1%, or 1% PIP(2), binds 50% of MARCKS-(151-175). Both electrophoretic mobility measurements and competition experiments suggest that MARCKS-(151-175) forms an electroneutral complex with approximately 4 PIP(2). MARCKS-(151-175) binds equally well to PI(4,5)P(2) and PI(3,4)P(2). Local electrostatic interactions of PIP(2) with MARCKS-(151-175) contribute to the binding energy because increasing the salt concentration from 100 to 500 mm decreases the binding 100-fold. We hypothesize that the effector domain of MARCKS can bind a significant fraction of the PIP(2) in the plasma membrane, and release the bound PIP(2) upon interaction with Ca(2+)/calmodulin or phosphorylation by protein kinase C.

Intracellular delivery of phosphoinositides and inositol phosphates using polyamine carriers

Phosphoinositide signaling regulates events in endocytosis and exocytosis, vesicular trafficking of proteins, transduction of extracellular signals, remodeling of the actin cytoskeleton, regulation of calcium flux, and apoptosis. Obtaining mechanistic insights in living cells is impeded by the membrane impermeability of these anionic lipids. We describe a carrier system for intracellular delivery of phosphoinositide polyphosphates (PIP(n)s) and fluorescently labeled PIP(n)s into living cells, such that intracellular localization can be directly observed. Preincubation of PIP(n)s or inositol phosphates with carrier polyamines produced complexes that entered mammalian, plant, yeast, bacterial, and protozoal cells in seconds to minutes via a nonendocytic mechanism. Time-dependent transit of both PIP(n)s and the carrier to specific cytosolic and nuclear compartments was readily visualized by fluorescence microscopy. Platelet-derived growth factor treatment of NIH 3T3 fibroblasts containing carrier-delivered phosphatidylinositol 4,5-bisphosphate [PtdIns(4, 5)P(2)]-7-nitrobenz-2-oxa-1,3-diazole resulted in the redistribution of the fluorescent signal, suggesting that fluorescent PtdIns(4, 5)P(2) was a substrate for phospholipase C. We also observed a calcium flux in NIH 3T3 cells when complexes of carrier and PtdIns(4, 5)P(2) or inositol 1,4,5-trisphosphate were added extracellularly. This simple intracellular delivery system allows for the efficient translocation of biologically active PIP(n)s, inositol phosphates, and their fluorescent derivatives into living cells in a physiologically relevant context.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

The role of the pleckstrin homology domain in membrane targeting and activation of phospholipase Cbeta(1)

Current studies involve an investigation of the role of the pleckstrin homology (PH) domain in membrane targeting and activation of phospholipase Cbeta(1) (PLCbeta(1)). Here we report studies on the membrane localization of the isolated PH domain from the amino terminus of PLCbeta(1) (PLCbeta(1)-PH) using fluorescence microscopy of a green fluorescent protein fusion protein. Whereas PLCbeta(1)-PH does not localize to the plasma membrane in serum-starved cells, it undergoes a rapid but transient migration to the plasma membrane upon stimulation of cells with serum or lysophosphatidic acid (LPA). Regulation of the plasma membrane localization of PLCbeta(1)-PH by phosphoinositides was also investigated. PLCbeta(1)-PH was found to bind phosphatidylinositol 3-phosphate most strongly, whereas other phosphoinositides were bound with lower affinity. The plasma membrane localization of PLCbeta(1)-PH induced by serum and LPA was blocked by wortmannin pretreatment and by LY294002. In parallel, activation of PLCbeta by LPA was inhibited by wortmannin, by LY294002, or by the overexpression of PLCbeta(1)-PH. Microinjection of betagamma subunits of G proteins in serum-starved cells induced the translocation of PLCbeta(1)-PH to the plasma membrane. These results demonstrate that a cooperative mechanism involving phosphatidylinositol 3-phosphate and the Gbetagamma subunit regulates the plasma membrane localization and activation of PLCbeta(1)-PH.

The pleckstrin homology domain of phospholipase C-beta(2) links the binding of gbetagamma to activation of the catalytic core

Pleckstrin homology (PH) domains are membrane tethering devices found in many signal transducing proteins. These domains also couple to the betagamma subunits of GTP binding proteins (G proteins), but whether this association transmits allosteric information to the catalytic core is unclear. To address this question, we constructed protein chimeras in which the PH domain of phospholipase C-beta(2) (PLC-beta(2)), which is regulated by Gbetagamma, replaces the PH domain of PLC-delta(1) which binds to, but is not regulated by, Gbetagamma. We found that attachment of the PH domain of PLC-beta(2) onto PLC-delta(1) not only causes the membrane-binding properties of PLC-delta(1) to become similar to those of PLC-beta(2), but also results in a Gbetagamma-regulated enzyme. Thus, PH domains are more than simple tethering devices and mediate regulatory signals to the host protein.

Using photolabile ligands in drug discovery and development

Photoactivatable ligands are important tools used in drug discovery and drug development. These ligands enable researchers to identify the targets of drugs, to determine the affinity and selectivity of the drug-target interaction, and to identify the binding site on the target. Examples are presented from three fundamentally different approaches: (1) photoaffinity labeling of target macromolecules; (2) photoactivation and release of 'caged ligands'; and (3) photoimmobilization of ligands onto surfaces.

Mammalian phosphoinositide-specific phospholipase C

Mammalian phosphoinositide-specific phospholipases C (PI-PLCs) are involved in most receptor-mediated signal transduction pathways. The mammalian isozymes employ a modular arrangement of domains to achieve a regulated production of two key second messengers. The roles of the PH, EF hand, C2, SH2 and SH3 modules in regulation of these enzymes and in interactions with membranes and other proteins is becoming apparent from recent structural and functional studies.

Changes in sarcolemmal PLC isoenzymes in postinfarct congestive heart failure: partial correction by imidapril

We have examined the changes in quantity and activity of cardiac sarcolemmal (SL) phosphoinositide-phospholipase C (PLC)-beta(1), -gamma(1), and -delta(1) in a model of congestive heart failure (CHF) secondary to large transmural myocardial infarction (MI). We also instituted a late in vivo monotherapy with imidapril, an ANG-converting enzyme (ACE) inhibitor, to test the hypothesis that its therapeutic action is associated with the functional correction of PLC isoenzymes. SL membranes were purified from the surviving left ventricle of rats in a moderate stage of CHF at 8 wk after occlusion of the left anterior descending coronary artery. SL PLC isoenzymes were examined in terms of protein mass and hydrolytic activity. CHF resulted in a striking reduction (to 6-17% of controls) of the mass and activity of gamma(1)- and delta(1)-isoforms in combination with a significant increase of both PLC beta(1) parameters. In vivo treatment with imidapril (1 mg/kg body wt, daily, initiated 4 wk after coronary occlusion) improved the contractile function and induced a partial correction of PLCs. The mass of SL phosphatidylinositol 4,5-bisphosphate and the activities of the enzymes responsible for its synthesis were significantly reduced in post-MI CHF and partially corrected by imidapril. The results indicate that profound changes in the profile of heart SL PLC-beta(1), -gamma(1), and -delta(1) occur in CHF, which could alter the complex second messenger responses of these isoforms, whereas their partial correction by imidapril may be related to the mechanism of action of this ACE inhibitor.

Phospholipase C-delta3 binds with high specificity to phosphatidylinositol 4,5-bisphosphate and phosphatidic acid in bilayer membranes

In order to acquire an understanding of phospholipase C-delta3 (PLC-delta3) action on substrate localized in lipid membrane we have studied the binding of human recombinant PLC-delta3 to large, unilamellar phospholipid vesicles (LUVs). PLC-delta3 bound weakly to vesicles composed of phosphatidylcholine (PtdCho) or PtdCho plus phosphatidylethanolamine (PtdEtn) or phosphatidylinositol (PtdIns). The enzyme bound strongly to LUVs composed of PtdEtn + PtdCho and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). The binding affinity (molar partition coefficient) of PLC-delta3 to PtdEtn + PtdCho + PtdInsP2 vesicles was 7.7 x 105 m-1. High binding of PLC-delta3 was also observed for LUVs composed of phosphatidic acid (PA). Binding of PLC-delta3 to phosphatidylserine (PtdSer) vesicles was less efficient. Calculated molar partition coefficient for binding of PLC-delta3 to PA and PtdSer vesicles was 1.6 x 104 m-1 and 9.4 x 102 m-1, respectively. Presence of PA in the LUVs containing PtdInsP2 considerably enhanced the binding of PLC-delta3 to the phospholipid membrane. Binding of PLC-delta3 to phospholipid vesicles was not dependent on Ca2+ presence. In the liposome assay PA caused a concentration-dependent increase in activity of PLC-delta3. The stimulatory effect of PA on PLC-delta3 was calcium-dependent. At Ca2+ concentrations lower than 1 microm, no effect of PA on the activity of PLC-delta3 was observed. PA enhanced PLC-delta3 activity by increasing the Vmax and lowering Km for PtdInsP2. As the mol fraction of PA increased from 0-40 mol% the enzyme Vmax increased 2.3-fold and Km decreased threefold. Based on the results presented, we assume that PA supports binding of PLC-delta3 to lipid membranes by interaction with the PH domain of the enzyme. The stimulatory effect of PA depends on calcium-dependent interaction with the C2 domain of PLC-delta3. We propose that binding of PLC-delta3 to PA may serve as a mechanism for dynamic membrane association and modulation of PLC-delta3 activity.

Rac homologues and compartmentalized phosphatidylinositol 4, 5-bisphosphate act in a common pathway to regulate polar pollen tube growth

Pollen tube cells elongate based on actin- dependent targeted secretion at the tip. Rho family small GTPases have been implicated in the regulation of related processes in animal and yeast cells. We have functionally characterized Rac type Rho family proteins that are expressed in growing pollen tubes. Expression of dominant negative Rac inhibited pollen tube elongation, whereas expression of constitutive active Rac induced depolarized growth. Pollen tube Rac was found to accumulate at the tip plasma membrane and to physically associate with a phosphatidylinositol monophosphate kinase (PtdIns P-K) activity. Phosphatidylinositol 4, 5-bisphosphate (PtdIns 4, 5-P2), the product of PtdIns P-Ks, showed a similar intracellular localization as Rac. Expression of the pleckstrin homology (PH)-domain of phospholipase C (PLC)-delta1, which binds specifically to PtdIns 4, 5-P2, inhibited pollen tube elongation. These results indicate that Rac and PtdIns 4, 5-P2 act in a common pathway to control polar pollen tube growth and provide direct evidence for a function of PtdIns 4, 5-P2 compartmentalization in the regulation of this process.

Regulation of phospholipase C isozymes: activation of phospholipase C-gamma in the absence of tyrosine-phosphorylation

Activation of PLC-gamma isozymes in response to various agonists involves tyrosine phosphorylation of the effector enzymes. Recent evidence indicates that PLC-gamma isozymes are additionally activated by phosphatidic acid, phosphatidylinositol 3,4,5-trisphosphate and arachidonic acid in the absence of PLC-gamma tyrosine phosphorylation. These lipid-derived messengers are the immediate products of phospholipase D, phosphatidylinositol 3-kinase, and phospholipase A2, enzymes which are often stimulated along with PLC-gamma in response to an agonist. Furthermore, phosphatidylinositol 4,5-bisphosphate acts as a substrate for both PLC-gamma and phosphatidylinositol 3-kinase and as an activator for phospholipase D and phospholipase A2. These results reveal an elaborate mechanism of cross-talk and mutual regulation between four effector enzymes that participate in receptor signaling by acting on phospholipids.

A novel phospholipase C delta4 (PLCdelta4) splice variant as a negative regulator of PLC

It has been reported that there are two alternatively spliced variants of phospholipase C-delta4 (PLCdelta4), termed ALT I and II, that contain an additional 32 and 14 amino acids in their respective sequences in the linker region between the catalytic X and Y domains (Lee, S. B., and Rhee, S. G. (1996) J. Biol. Chem. 271, 25-31). We report here the isolation and characterization of a novel alternative splicing isoform of PLCdelta4, termed ALT III, as a negative regulator of PLC. In ALT III, alternative splicing occurred in the catalytic X domain, i.e. 63 amino acids (residues 424-486) containing the C-terminal of the X domain and linker region were substituted for 32 amino acids corresponding to the insert sequence of ALT I. Although the expression level of ALT III was found to be much lower in most tissues and cells compared with that of PLCdelta4, it was significantly higher in some neural cells, such as NIE-115 cells and p19 cells differentiated to neural cells by retinoic acid. Interestingly, recombinant ALT III protein did not retain enzymatic activity, and the activity of PLCdelta4 overexpressed in COS7 cells was markedly decreased by the co-expression of ALT III but not by ALT I or II. Moreover, N-terminal pleckstrin homology domain (PH domain) of ALT III alone could inhibit the increase of inositol-1,4, 5-trisphosphate levels in PLCdelta4-overexpressing NIH3T3 cells, whereas a PH domain deletion mutant could not, indicating that the PH domain is necessary and sufficient for its inhibitory effect. The ALT III PH domain specifically bound to phosphatidylinositol (PtdIns)-4,5-P2 and PtdIns-3,4,5-P3 but not PtdIns, PtdIns-4-P, or inositol phosphates, and the mutant R36G, which retained only weak affinity for PtdIns-4,5-P2, could not inhibit the activity of PLCdelta4. These results indicate that PtdIns-4,5-P2 binding to PH domain is essential for the inhibitory effect of ALT III. ALT III also inhibited PLCdelta1 activity and partially suppressed PLCgamma1 activity, but not PLCbeta1 in vitro; it did inhibit all types of isozymes tested in vivo. Taken together, our results indicate that ALT III is a negative regulator of PLC that is most effective against the PLC delta-type isozymes, and its PH domain is essential for its function.