Kinetic analysis of phospholipase C beta isoforms using phospholipid-detergent mixed micelles. Evidence for interfacial catalysis involving distinct micelle binding and catalytic steps

Human sphingomyelin synthase 1 generates diacylglycerol in the presence and absence of ceramide via multiple enzymatic activities

Sphingomyelin (SM) synthase 1 (SMS1), which is involved in lipodystrophy, deafness, and thrombasthenia, generates diacylglycerol (DG) and SM using phosphatidylcholine (PC) and ceramide as substrates. Here, we found that SMS1 possesses DG-generating activities via hydrolysis of PC and phosphatidylethanolamine (PE) in the absence of ceramide and ceramide phosphoethanolamine synthase (CPES) activity. In the presence of the same concentration (4.7 mol%) of PC and ceramide, the amounts of DG produced by SMS and PC-phospholipase C (PLC) activities of SMS1 were approximately 65% and 35% of total DG production, respectively. PC-PLC activity showed substrate selectivity for saturated and/or monounsaturated fatty acid-containing PC species. A PC-PLC/SMS inhibitor, D609, inhibited only SMS activity. Mn2+ inhibited only PC-PLC activity. Intriguingly, DG attenuated SMS/CPES activities. Our study indicates that SMS1 is a unique enzyme with PC-PLC/PE-PLC/SMS/CPES activities.

Sphingomyelin synthase-related protein generates diacylglycerol via the hydrolysis of glycerophospholipids in the absence of ceramide

Diacylglycerol (DG) is a well-established lipid second messenger. Sphingomyelin synthase (SMS)-related protein (SMSr) produces DG and ceramide phosphoethanolamine (CPE) by the transfer of phosphoethanolamine from phosphatidylethanolamine (PE) to ceramide. We previously reported that human SMSr overexpressed in COS-7 cells significantly increased DG levels, particularly saturated and/or monounsaturated fatty acid-containing DG molecular species, and provided DG to DG kinase (DGK) δ, which regulates various pathophysiological events, including epidermal growth factor-dependent cell proliferation, type 2 diabetes, and obsessive-compulsive disorder. However, mammalian SMSr puzzlingly produces only trace amounts of CPE/DG. To clarify this discrepancy, we highly purified SMSr and examined its activities other than CPE synthase. Intriguingly, purified SMSr showed a DG-generating activity via hydrolysis of PE, phosphatidic acid (PA), phosphatidylinositol (PI), and phosphatidylcholine (PC) in the absence of ceramide. DG generation through the PA phosphatase (PAP) activity of SMSr was approximately 300-fold higher than that with PE and ceramide. SMSr hydrolyzed PI ten times stronger than PI(4,5)bisphosphate (PI(4,5)P₂). The PAP and PC-phospholipase C (PLC) activities of SMSr were inhibited by propranolol, a PAP inhibitor, and by D609, an SMS/PC-PLC inhibitor. Moreover, SMSr showed substrate selectivity for saturated and/or monounsaturated fatty acid-containing PA molecular species, but not arachidonic-acid-containing PA, which is exclusively generated in the PI(4,5)P₂ cycle. We confirmed that SMSr expressed in COS-7 cells showed PAP and PI-PLC activities. Taken together, our study indicated that SMSr possesses previously unrecognized enzyme activities, PAP and PI/PE/PC-PLC, and constitutes a novel DG/PA signaling pathway together with DGKδ, which is independent of the PI(4,5)P₂ cycle.

Yeast phosphatidic acid phosphatase Pah1 hops and scoots along the membrane phospholipid bilayer

PA phosphatase, encoded by PAH1 in the yeast Saccharomyces cerevisiae, catalyzes the Mg²⁺-dependent dephosphorylation of PA, producing DAG at the nuclear/ER membrane. This enzyme plays a major role in triacylglycerol synthesis and in the regulation of phospholipid synthesis. As an interfacial enzyme, PA phosphatase interacts with the membrane surface, binds its substrate, and catalyzes its reaction. The Triton X-100/PA-mixed micellar system has been utilized to examine the activity and regulation of yeast PA phosphatase. This system, however, does not resemble the in vivo environment of the membrane phospholipid bilayer. We developed an assay system that mimics the nuclear/ER membrane to assess PA phosphatase activity. PA was incorporated into unilamellar phospholipid vesicles (liposomes) composed of the major nuclear/ER membrane phospholipids, PC, PE, PI, and PS. We optimized this system to support enzyme-liposome interactions and to afford activity that is greater than that obtained with the aforementioned detergent system. Activity was regulated by phospholipid composition, whereas the enzyme's interaction with liposomes was insensitive to composition. Greater activity was attained with large (≥100 nm) versus small (50 nm) vesicles. The fatty-acyl moiety of PA had no effect on this activity. PA phosphatase activity was dependent on the bulk (hopping mode) and surface (scooting mode) concentrations of PA, suggesting a mechanism by which the enzyme operates along the nuclear/ER membrane in vivo.

Structural basis for calcium and phosphatidylserine regulation of phospholipase C δ1

Many membrane-associated enzymes, including those of the phospholipase C (PLC) superfamily, are regulated by specific interactions with lipids. Previously, we have shown that the C2 domain of PLC δ1 is required for phosphatidylserine (PS)-dependent enzyme activation and that activation requires the presence of Ca(2+). To identify the site of interaction and the role of Ca(2+) in the activation mechanism, we mutagenized three highly conserved Ca(2+) binding residues (Asp-653, Asp-706, and Asp-708) to Gly in the C2 domain of PLC δ1. The PS-dependent Ca(2+) binding affinities of the mutant enzymes D653G, D706G, and D708G were reduced by 1 order of magnitude, and the maximal level of Ca(2+) binding was reduced to half of that of the native enzyme. The level of Ca(2+)-dependent PS binding was also reduced in the mutant enzymes. Under basal conditions, the Ca(2+) dependence and the maximal level of hydrolysis of phosphatidylinositol 4,5-bisphosphate were not altered in the mutants. However, the Ca(2+)-dependent PS stimulation was severely defective. PS reduces the K(m) of the native enzyme almost 20-fold, but far less for the mutants. Replacing Asp-653, Asp-706, and Asp-708 simultaneously with glycine in the C2 domain of PLC δ1 leads to a complete and selective loss of the stimulation and binding by PS. These results show that D653, D706, and D708 are required for Ca(2+) binding in the C2 domain and demonstrate a mechanism by which C2 domains can mediate regulation of enzyme activity by specific lipid ligands.

Direct activation of human phospholipase C by its well known inhibitor u73122

Phospholipase C (PLC) enzymes are an important family of regulatory proteins involved in numerous cellular functions, primarily through hydrolysis of the polar head group from inositol-containing membrane phospholipids. U73122 (1-(6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione), one of only a few small molecules reported to inhibit the activity of these enzymes, has been broadly applied as a pharmacological tool to implicate PLCs in diverse experimental phenotypes. The purpose of this study was to develop a better understanding of molecular interactions between U73122 and PLCs. Hence, the effects of U73122 on human PLCβ3 (hPLCβ3) were evaluated in a cell-free micellar system. Surprisingly, U73122 increased the activity of hPLCβ3 in a concentration- and time-dependent manner; up to an 8-fold increase in enzyme activity was observed with an EC50=13.6±5 μm. Activation of hPLCβ3 by U73122 required covalent modification of cysteines as evidenced by the observation that enzyme activation was attenuated by thiol-containing nucleophiles, l-cysteine and glutathione. Mass spectrometric analysis confirmed covalent reaction with U73122 at eight cysteines, although maximum activation was achieved without complete alkylation; the modified residues were identified by LC/MS/MS peptide sequencing. Interestingly, U73122 (10 μm) also activated hPLCγ1 (>10-fold) and hPLCβ2 (∼2-fold); PLCδ1 was neither activated nor inhibited. Therefore, in contrast to its reported inhibitory potential, U73122 failed to inhibit several purified PLCs. Most of these PLCs were directly activated by U73122, and a simple mechanism for the activation is proposed. These results strongly suggest a need to re-evaluate the use of U73122 as a general inhibitor of PLC isozymes.

A specific phospholipase C activity regulates phosphatidylinositol levels in lung surfactant of patients with acute respiratory distress syndrome

Lung surfactant (LS) is a lipid-rich material lining the inside of the lungs. It reduces surface tension at the liquid/air interface and thus, it confers protection of the alveoli from collapsing. The surface-active component of LS is dipalmitoyl-phosphatidylcholine, while anionic phospholipids such as phosphatidylinositol (PtdIns) and primarily phosphatidylglycerol are involved in the stabilization of the LS monolayer. The exact role of PtdIns in this system is not well-understood; however, PtdIns levels change dramatically during the acute respiratory distress syndrome (ARDS) evolution. In this report we present evidence of a phosphoinositide-specific phospholipase C (PI-PLC) activity in bronchoalveolar lavage (BAL) fluid, which may regulate PtdIns levels. Characterization of this extracellular activity showed specificity for PtdIns and phosphatidylinositol 4,5-bisphosphate, sharing the typical substrate concentration-, pH-, and calcium-dependencies with mammalian PI-PLCs. Fractionation of BAL fluid showed that PI-PLC did not co-fractionate with large surfactant aggregates, but it was found mainly in the soluble fraction. Importantly, analysis of BAL samples from control subjects and from patients with ARDS showed that the PI-PLC specific activity was decreased by 4-fold in ARDS samples concurrently with the increase in BAL PtdIns levels. Thus, we have identified for the first time an extracellular PI-PLC enzyme activity that may be acutely involved in the regulation of PtdIns levels in LS.

Regulation of PLCβ1a membrane anchoring by its substrate phosphatidylinositol (4,5)-bisphosphate

Basic knowledge as to the subcellular location and dynamics of PLCβ isozymes is scant. Here, we report on the subcellular location of GFP-PLCβ1a and the use of total internal reflection fluorescence (TIRF) microscopy to examine the dynamics of GFP-PLCβ1a at the plasma membrane upon stimulation of Gq-coupled receptors. Using this technique, we observed PLCβ1a dissociation from the plasma membrane upon addition of agonist. An increase in intracellular calcium and a decrease in PtdIns(4,5)P2 both coincided with a translocation of PLCβ1a from the plasma membrane into the cytosol. In order to differentiate between calcium and PtdIns(4,5)P2, rapamycin-induced heterodimerization of FRB and FKBP12 fused to 5-phosphatase IV was used to instantaneously convert PtdIns(4,5)P2 into PtdIns(4)P. Addition of rapamycin caused PLCβ1a to dissociate from the plasma membrane, indicating that removal of PtdIns(4,5)P2 is sufficient to cause translocation of PLCβ1a from the plasma membrane. In conclusion, PLCβ1a localization is regulated by its own substrate.

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.

The sec14 homology module of neurofibromin binds cellular glycerophospholipids: mass spectrometry and structure of a lipid complex

Neurofibromin is the protein product of the tumor suppressor gene NF1, alterations of which are responsible for the pathogenesis of the common disorder Neurofibromatosis type I (NF1). The only well-characterized function of neurofibromin is its RasGAP activity, contained in the central GAP related domain (GRD). By solving the crystal structure of a 31 kDa fragment at the C-terminal end of the GRD we have recently identified a novel bipartite lipid-binding module composed of a Sec14 homologous and a previously undetected pleckstrin homology (PH)-like domain. Using lipid exchange assays along with mass spectrometry we show here that the Sec14-like portion binds to 1-(3-sn-phosphatidyl)-sn-glycerol (PtdGro), (3-sn-phosphatidyl)-ethanolamine (PtdEtn) and -choline (PtdCho) and to a minor extent to (3-sn-phosphatidyl)-l-serine (PtdSer) and 1-(3-sn-phosphatidyl)-d-myo-inositol (PtdIns). Phosphorylated PtdIns (PtdInsPs) are not detected as binders in the mass spectrometry assay, but their soluble inositol-phosphate headgroups and related compounds can inhibit the lipid exchange reaction. We also present here the crystal structure of this module with the Sec14 portion bound to a cellular glycerophospholipid ligand. Our structure has model character for the substrate-bound form of yeast Sec14p, of which only detergent bound structures are available so far. To assess potential regulation of the lipid exchange reaction in detail, we present a novel strategy using nanospray mass spectrometry. Ion intensities of initial phospholipids and exchanged deuterated analogues bound by the protein module allow the quantitative analysis of differences in the exchange activity under various conditions.

Changes of enzyme activity in lipid signaling pathways related to substrate reordering

The static fluid mosaic model of biological membranes has been progressively complemented by a dynamic membrane model that includes phospholipid reordering in domains that are proposed to extend from nanometers to microns. Kinetic models for lipolytic enzymes have only been developed for homogeneous lipid phases. In this work, we develop a generalization of the well-known surface dilution kinetic theory to cases where, in a same lipid phase, both domain and nondomain phases coexist. Our model also allows understanding the changes in enzymatic activity due to a decrease of free substrate concentration when domains are induced by peptides. This lipid reordering and domain dynamics can affect the activity of lipolytic enzymes, and can provide a simple explanation for how basic peptides, with a strong direct interaction with acidic phospholipids (such as beta-amyloid peptide), may cause a complex modulation of the activities of many important enzymes in lipid signaling pathways.

Kinetic analysis of Arabidopsis phospholipase Ddelta. Substrate preference and mechanism of activation by Ca2+ and phosphatidylinositol 4,5-biphosphate

Phospholipase D (PLD) is a major plant phospholipase family involved in many cellular processes such as signal transduction, membrane remodeling, and lipid degradation. Five classes of PLDs have been identified in Arabidopsis thaliana, and Ca(2+) and polyphosphoinositides have been suggested as key regulators for these enzymes. To investigate the catalysis and regulation mechanism of individual PLDs, surface-dilution kinetics studies were carried out on the newly identified PLDdelta from Arabidopsis. PLDdelta activity was dependent on both bulk concentration and surface concentration of substrate phospholipids in the Triton X-100/phospholipid mixed micelles. V(max), K(s)(A), and K(m)(B) values for PLDdelta toward phosphatidylcholine or phosphatidylethanolamine were determined; phosphatidylethanolamine was the preferred substrate. PLDdelta activity was stimulated greatly by phosphatidylinositol 4,5-bisphosphate (PIP(2)). Maximal activation was observed at a PIP(2) molar ratio around 0.01. Kinetic analysis indicates that PIP(2) activates PLD by promoting substrate binding to the enzyme, without altering the bulk binding of the enzyme to the micelle surface. Ca(2+) is required for PLDdelta activity, and it significantly decreased the interfacial Michaelis constant K(m)(B). This indicates that Ca(2+) activates PLD by promoting the binding of phospholipid substrate to the catalytic site of the enzyme.

A model of activation of the protein tyrosine phosphatase SHP-2 by the human leptin receptor

Signalling through the leptin receptor has been shown to activate the SH2 domain-containing tyrosine phosphatase SHP-2 through tyrosine phosphorylation. The human leptin receptor contains five tyrosine residues in the cytoplasmic domain that may become phosphorylated. We show here using BIAcore studies, wherein binding of peptides to SHP-2 was detected, that peptides corresponding to sequences containing phosphotyrosines 974 and 986 (LR974P and LR986P, respectively) from the leptin receptor cytoplasmic domain were the only two peptides that bound to the enzyme. Binding of LR974P to SHP-2 was inhibited in a dose-dependent fashion by orthovanadate, whereas binding of LY986P was not, indicating that the enzyme binds to these peptides through different sites. Only the leptin receptor-derived peptide corresponding to tyrosine 974 was dephosphorylated by recombinant purified SHP-2. Time courses of the reaction were complex, and fitted a two exponent rate equation. Preincubation of SHP-2 with LR986P markedly activated the enzyme at early time points and time courses of the activated enzyme fitted a single exponential first order rate equation. We propose that LR974P binds to the active site of SHP-2, whereas LR986P may bind to the N- and C-terminal SH2 domains of SHP-2, thus activating the phosphatase activity. These data support a model in which SHP-2 binds to phosphotyrosine 986 in the activated leptin receptor and is activated to dephosphorylate phosphotyrosine 974, downregulating signalling events emanating from SH2 domain-containing proteins that bind here.

Midportion antibodies stimulate glycosylphosphatidylinositol-specific phospholipase D activity

Limited information is known regarding the regulation, structural features, and functional domains of glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD, EC 3. 1.4.50). Previous studies demonstrated that trypsin cleavage of GPI-PLD at or near Arg325 and/or Arg589 in bovine serum GPI-PLD was associated with an increase in enzymatic activity. Since the Arg325 is predicted to be in a region between the catalytic domain and predicted beta-propeller structure in the C-terminal portion of GPI-PLD (T. A. Springer, Proc. Natl. Acad. Sci. USA 94, 65-72, 1997), we hypothesized that this connecting region is important for catalytic activity. Trypsin cleavage of human serum GPI-PLD, which has an Arg325 but lacks the Arg589 present in bovine serum GPI-PLD, also increased GPI-PLD activity. Peptide-specific antibodies to residues 275-296 (anti-GPI-PLD(275)) and a monoclonal antibody, 191, with an epitope encompassing Arg325, also stimulated GPI-PLD activity. Pretreating human GPI-PLD with trypsin demonstrated that anti-GPI-PLD(275) only stimulated the activity of intact GPI-PLD. These results suggest that trypsin activation and anti-GPI-PPLD(275) may have similar effects on GPI-PLD. Consistent with this is the observation that both manipulations decreased the affinity of GPI-PLD for mixed micelle substrates. These results indicate that the midportion region of GPI-PLD is important in regulating enzymatic activity.

Kinetic analysis of phospholipase C from catharanthus roseus transformed roots using different assays

The properties of phospholipase C (PLC) partially purified from Catharanthus roseus transformed roots were analyzed using substrate lipids dispersed in phospholipid vesicles, phospholipid-detergent mixed micelles, and phospholipid monolayers spread at an air-water interface. Using [(33)P]phosphatidylinositol 4,5-bisphosphate (PIP(2)) of high specific radioactivity, PLC activity was monitored directly by measuring the loss of radioactivity from monolayers as a result of the release of inositol phosphate and its subsequent dissolution on quenching in the subphase. PLC activity was markedly affected by the surface pressure of the monolayer, with reduced activity at extremes of initial pressure. The optimum surface pressure for PIP(2) hydrolysis was 20 mN/m. Depletion of PLC from solution by incubation with sucrose-loaded PIP(2) vesicles followed by ultracentrifugation demonstrated stable attachment of PLC to the vesicles. A mixed micellar system was established to assay PLC activity using deoxycholate. Kinetic analyses were performed to determine whether PLC activity was dependent on both bulk PIP(2) and PIP(2) surface concentrations in the micelles. The interfacial Michaelis constant was calculated to be 0.0518 mol fraction, and the equilibrium dissociation constant of PLC for the lipid was 45.5 &mgr;M. These findings will add to our understanding of the mechanisms of regulation of plant PLC.

Activation of phospholipase C delta1 through C2 domain by a Ca(2+)-enzyme-phosphatidylserine ternary complex

The concentration of free Ca(2+) and the composition of nonsubstrate phospholipids profoundly affect the activity of phospholipase C delta1 (PLCdelta1). The rate of PLCdelta1 hydrolysis of phosphatidylinositol 4,5-bisphosphate was stimulated 20-fold by phosphatidylserine (PS), 4-fold by phosphatidic acid (PA), and not at all by phosphatidylethanolamine or phosphatidylcholine (PC). PS reduced the Ca(2+) concentration required for half-maximal activation of PLCdelta1 from 5.4 to 0.5 microM. In the presence of Ca(2+), PLCdelta1 specifically bound to PS/PC but not to PA/PC vesicles in a dose-dependent and saturable manner. Ca(2+) also bound to PLCdelta1 and required the presence of PS/PC vesicles but not PA/PC vesicles. The free Ca(2+) concentration required for half-maximal Ca(2+) binding was estimated to be 8 microM. Surface dilution kinetic analysis revealed that the K(m) was reduced 20-fold by the presence of 25 mol % PS, whereas V(max) and K(d) were unaffected. Deletion of amino acid residues 646-654 from the C2 domain of PLCdelta1 impaired Ca(2+) binding and reduced its stimulation and binding by PS. Taken together, the results suggest that the formation of an enzyme-Ca(2+)-PS ternary complex through the C2 domain increases the affinity for substrate and consequently leads to enzyme activation.

Characterization of Y1, Y2 and Y5 subtypes of the neuropeptide Y (NPY) receptor in rabbit kidney. Sensitivity of ligand binding to guanine nucleotides and phospholipase C inhibitors

The binding of two peptide YY/neuropeptide Y analogues selective for major subtypes of neuropeptide Y (NPY) receptors was compared in particulates from rabbit kidney cortex employing modulators of activity of G-proteins, phospholipase enzymes, and ion channels. The binding of (Leu31,Pro34)human peptide YY resembled the patterns observed previously for the brain tissue Y1 receptor, exhibiting a high sensitivity to monovalent cations, disulfide disruptors, guanosine polyphosphates and phospholipase C inhibitors. However, this binding was bimodal in response to human pancreatic polypeptide and to peptides selective for the Y2 subtype of the NPY receptor, displaying a large component pharmacologically similar to the brain Y5 receptor. This kidney Y5-like binding largely shared the sensitivity to monovalent cations, guanine nucleotides and phospholipase C inhibitors found for either the kidney or the brain Y1 receptor, and also was activated by Ca2+ ion. Both Y1- and Y5-like binding in the kidney displayed a uniformly low reactivity to a nonpeptidic Y1 antagonist, BIBP-3226, and to a receptor peptide mimetic, mastoparan analogue MAS-7. The kidney Y2 binding shared the low sensitivity to ionic environment observed for the brain Y2 subtype, and was only partially sensitive to guanine nucleotides or to MAS-7. The Y2 liganding had a sensitivity to phospholipase C inhibitors similar to the Y1/Y5 binding. This reactivity was retained in the fraction of the Y2 receptor persisting detergent solubilization in a high-affinity form, which, however, was activated rather than inhibited by G-protein agonists.

Direct immobilization of gangliosides onto gold-carboxymethyldextran sensor surfaces by hydrophobic interaction: applications to antibody characterization

We describe a novel immobilization technique to investigate interactions between immobilized gangliosides (GD3, GM1, and GM2) and their respective antibodies, antibody fragments, or binding partners using an optical biosensor. Immobilization was performed by direct injection onto a carboxymethyldextran sensor chip and did not require derivatization of the sensor surface or the ganglioside. The ganglioside appeared to bind to the sensor surface by hydrophobic interaction, leaving the carbohydrate epitope available for antibody or, in the case of GM1, cholera toxin binding. The carboxyl group of the dextran chains on the sensor surface did not appear to be involved in the immobilization as evidenced by equivalent levels of immobilization following conversion of the carboxyl groups into acyl amino esters, but rather the dextran layer provided a hydrophilic coverage of the sensor chip which was essential to prevent nonspecific binding. This technique gave better reactivity and specificity for anti-ganglioside monoclonal antibodies (anti-GD3: KM871, KM641, R24; and anti-GM2: KM966) than immobilization by hydrophobic interaction onto a gold sensor surface or photoactivated cross-linking onto carboxymethydextran. This rapid immobilization procedure has facilitated detailed kinetic analysis of ganglioside/antibody interactions, with the surface remaining viable for a large number of cycles (>125). Kinetic constants were determined from the biosensor data using linear regression, nonlinear least squares and equilibrium analysis. The values of kd, ka, and KAobtained by nonlinear analysis (KAKM871 = 1.05, KM641 = 1.66, R24 = 0.14, and KM966 = 0.65 x 10(7) M-1) were essentially independent of concentration and showed good agreement with data obtained by other analytical methods.

Catalytic domain of phosphoinositide-specific phospholipase C (PLC). Mutational analysis of residues within the active site and hydrophobic ridge of plcdelta1

Structural studies of phospholipase C delta1 (PLCdelta1) in complexes with the inositol-lipid headgroup and calcium identified residues within the catalytic domain that could be involved in substrate recognition, calcium binding, and catalysis. In addition, the structure of the PLCdelta1 catalytic domain revealed a cluster of hydrophobic residues at the rim of the active site opening (hydrophobic ridge). To assess a role of each of these residues, we have expressed, purified, and characterized enzymes with the point mutations of putative active site residues (His311, Asn312, Glu341, Asp343, His356, Glu390, Lys438, Lys440, Ser522, Arg549, and Tyr551) and residues from the hydrophobic ridge (Leu320, Phe360, and Trp555). The replacements of most active site residues by alanine resulted in a great reduction (1,000-200,000-fold) of PLC activity analyzed in an inositol lipid/sodium cholate mixed micelle assay. Measurements of the enzyme activity toward phosphatidylinositol, phosphatidylinositol 4-monophosphate, and phosphatidylinositol 4, 5-bis-phosphate (PIP2) identified Ser522, Lys438, and Arg549 as important for preferential hydrolysis of polyphosphoinositides, whereas replacement of Lys440 selectively affected only hydrolysis of PIP2. When PLC activity was analyzed at different calcium concentrations, substitutions of Asn312, Glu390, Glu341, and Asp343 resulted in a shift toward higher calcium concentrations required for PIP2 hydrolysis, suggesting that all these residues contribute toward Ca2+ binding. Mutational analysis also confirmed the importance of His311 ( approximately 20,000-fold reduction) and His356 ( approximately 6,000-fold reduction) for the catalysis. Mutations within the hydrophobic ridge, which had little effect on PIP2 hydrolysis in the mixed-micelles, resulted in an enzyme that was less dependent on the surface pressure when analyzed in a monolayer. This systematic mutational analysis provides further insights into the structural basis for the substrate specificity, requirement for Ca2+ ion, catalysis, and surface pressure/activity dependence, with general implications for eukaryotic phosphoinositide-specific PLCs.

Membrane-binding properties of phospholipase C-beta1 and phospholipaseC-beta2: role of the C-terminus and effects of polyphosphoinositides, G-proteins and Ca2+

We have studied the binding of two G-protein-regulated phospholipase C (PLC) enzymes, PLCs-beta1 and -beta2, to membrane surfaces using sucrose-loaded bilayer phospholipid vesicles of varying compositions. Neither enzyme binds appreciably to pure phosphatidylcholine vesicles at lipid concentrations up to 10(-3) M. PLC-beta1 and PLC-beta2 bind vesicles composed of phosphatidylcholine, phosphatidylserine and phosphatidylethanolamine (molar ratio 1:1:1) with an approximate Kd of 10(-5) M. Inclusion of 2% PtdIns(4,5)P2 in these vesicles had no effect on the affinity of this interaction. As reported by others, removal of the C-terminus of PLC-beta1 and PLC-beta2 produces catalytically active fragments. The affinity of these truncated proteins for phospholipid vesicles is dramatically reduced suggesting that this region of the proteins contains residues important for membrane binding. Inclusion of G-protein alpha- and betagamma-subunit activators in the phospholipid vesicles does not increase the binding of PLC-beta1 or PLC-beta2, and the magnitude of G-protein-mediated PLC activation observed at low phospholipid concentrations (10(-6) M) is comparable to that observed at concentrations at which the enzymes are predominantly membrane-bound (10(-3) M). PLC-beta1 and -beta2 contain C2 domains but Ca2+ does not enhance binding to the vesicles. Our results indicate that binding of these enzymes to membranes involves the C-temini of the proteins and suggest that activation of these enzymes by G-proteins results from a regulated interaction between the membrane-bound proteins rather than G-protein-dependent recruitment of soluble enzymes to a substrate-containing phospholipid surface.

Role of protein-phospholipid interactions in the activation of ARF1 by the guanine nucleotide exchange factor Arno

Arno is a 47-kDa human protein recently identified as a guanine nucleotide exchange factor for ADP ribosylation factor 1 (ARF1) with a central Sec7 domain responsible for the exchange activity and a carboxyl-terminal pleckstrin homology (PH) domain (Chardin, P., Paris, S., Antonny, B., Robineau, S., Béraud-Dufour, S., Jackson, C. L., and Chabre, M. (1996) Nature 384, 481-484). Binding of the PH domain to phosphatidylinositol 4,5-bisphosphate (PIP2) greatly enhances Arno-mediated activation of myristoylated ARF1. We show here that in the absence of phospholipids, Arno promotes nucleotide exchange on [Delta17]ARF1, a soluble mutant of ARF1 lacking the first 17 amino acids. This reaction is unaffected by PIP2, which suggests that the PIP2-PH domain interaction does not directly regulate the catalytic activity of Arno but rather serves to recruit Arno to membranes. Arno catalyzes the release of GDP more efficiently than that of GTP from [Delta17]ARF1, and a stable complex between Arno Sec7 domain and nucleotide-free [Delta17]ARF1 can be isolated. In contrast to [Delta17]ARF1, full-length unmyristoylated ARF1 is not readily activated by Arno in solution. Its activation requires the presence of phospholipids and a reduction of ionic strength and Mg2+ concentration. PIP2 is strongly stimulatory, indicating that binding of Arno to phospholipids is involved, but in addition, electrostatic interactions between phospholipids and the amino-terminal portion of unmyristoylated ARF1GDP seem to be important. We conclude that efficient activation of full-length ARF1 by Arno requires a membrane surface and two distinct protein-phospholipid interactions: one between the PH domain of Arno and PIP2, and the other between amino-terminal cationic residues of ARF1 and anionic phospholipids. The latter interaction is normally induced by insertion of the amino-terminal myristate into the bilayer but can also be artificially facilitated by decreasing Mg2+ and salt concentrations.

Regulation of effectors by G-protein alpha- and beta gamma-subunits. Recent insights from studies of the phospholipase c-beta isoenzymes

Both the alpha- and beta gamma-subunits of heterotrimeric guanine nucleotide-dependent regulatory proteins (G-proteins) couple members of the heptahelical class of cell-surface receptors to a diverse range of signal-generating effectors including retinal cyclic GMP phosphodiesterase, ion channels, adenylylcyclases, phosphoinositide 3-kinase, and members of the beta-class of inositol lipid-specific phospholipases C. Although the molecular details of the G-protein-regulated phospholipase C system were elucidated comparatively recently, these enzymes have become an important model for investigations of the process of G-protein effector coupling. A combination of molecular biological, biochemical, and structural studies using the phospholipase C-beta enzymes has provided some important insights into the interplay between G-proteins and their effectors and promises to reveal the mechanisms by which G-protein alpha- and beta gamma-subunits selectively associate with and activate effectors.

Protein kinase C and phospholipase C: bilayer interactions and regulation

Protein kinase C and phospholipase C are interfacially active modular enzymes that contain multiple membrane-binding domains. During the past two years, 3D structures and functional data have been reported for the key domains: pleckstrin homology, protein kinase C homology-1 and -2, and the phospholipase C catalytic domain. Roles for membrane bilayer structure and lipid microdomains have become clearly domains has shown how the domains work together to coordinate regulation.

A single amino acid substitution in the pleckstrin homology domain of phospholipase C delta1 enhances the rate of substrate hydrolysis

The pleckstrin homology (PH) domain has been postulated to serve as an anchor for enzymes that operate at a lipid/water interface. To understand further the relationship between the PH domain and enzyme activity, a phospholipase C (PLC) delta1/PH domain enhancement-of-activity mutant was generated. A lysine residue was substituted for glutamic acid in the PH domain of PLC delta1 at position 54 (E54K). Purified native and mutant enzymes were characterized using a phosphatidylinositol 4,5-bisphosphate (PI(4, 5)P2)/dodecyl maltoside mixed micelle assay and kinetics measured according to the dual phospholipid model of Dennis and co-workers (Hendrickson, H. S., and Dennis, E. A. (1984) J. Biol. Chem. 259, 5734-5739; Carmen, G. M., Deems, R. A., and Dennis, E. A. (1995) J. Biol. Chem. 270, 18711-18714). Our results show that both PLC delta1 and E54K bind phosphatidylinositol bisphosphate cooperatively (Hill coefficients, n = 2.2 +/- 0.2 and 2.0 +/- 0.1, respectively). However, E54K shows a dramatically increased rate of (PI(4, 5)P2)-stimulated PI(4,5)P2 hydrolysis (interfacial Vmax for PLC delta1 = 4.9 +/- 0.3 micromol/min/mg and for E54K = 31 +/- 3 micromol/min/mg) as well as PI hydrolysis (Vmax for PLC delta1 = 27 +/- 3.4 nmol/min/mg and for E54K = 95 +/- 12 nmol/min/mg). In the absence of PI(4,5)P2 both native and mutant enzyme hydrolyze PI at similar rates. E54K also has a higher affinity for micellar substrate (equilibrium dissociation constant, Ks = 85 +/- 36 microM for E54K and 210 +/- 48 microM for PLC delta1). Centrifugation binding assays using large unilamelar phospholipid vesicles confirm that E54K binds PI(4,5)P2 with higher affinity than native enzyme. E54K is more active even though the interfacial Michaelis constant (Km) for E54K (0.034 +/- 0.01 mol fraction PI(4,5)P2) is higher than the Km for native enzyme (0.012 +/- 0.002 mol fraction PI(4,5)P2). D-Inositol trisphosphate is less potent at inhibiting E54K PI(4,5)P2 hydrolysis compared with native enzyme. These results demonstrate that a single amino acid substitution in the PH domain of PLC delta1 can dramatically enhance enzyme activity. Additionally, the marked increase in Vmax for E54K argues for a direct role of PH domains in regulating catalysis by allosteric modulation of enzyme structure.

Smooth muscle sarcolemma-associated phospholipase C-beta 2; agonist-evoked translocation

About 25% of the total cellular PLC beta 2 content was found to be associated with a sarcolemmal fraction (SARC) isolated from unstimulated porcine trachealis smooth muscle. SARC-associated PLC beta 2 was located within two compartments, a detergent-extractable compartment and a nondetergent extractable compartment. SARC PLC beta 2 was measured after extraction with 0.6 M KCI; therefore, PLC beta 2 was not bound solely by electrostatic forces within either of these compartments. PLC beta 2 was shown to translocate from cytosol to SARC during a 20-sec activation of intact muscle with a muscarinic agonist, carbachol (CARB); i.e., cytosolic total PLC beta 2 content decreased significantly to 73 +/- 7% of control and SARC total PLC beta 2 content increased to 180 +/- 15% of control value. This translocation was maintained at 5 min of CARB. CARB-evoked translocation occurred into the detergent-extractable SARC fraction, and PLC beta 2 content in this fraction increased 300% compared with that in unstimulated muscle. After CARB, SARC PLC beta 2 content accounted for > 50% of total cellular PLC beta 2 content. CARB-evoked increase in PLC activity in SARC paralleled the increase in PLC beta 2 content. CARB-induced translocations of PLC beta 2 from the cytosol to SARC were of a similar magnitude as occurred with phorbol ester-induced translocations of PKC alpha.

Purification and characterization of Gbetagamma-responsive phosphoinositide 3-kinases from pig platelet cytosol

A G-protein betagamma subunit (Gbetagamma)-responsive phosphoinositide 3-kinase (PI 3-kinase) was purified approximately 5000-fold from pig platelet cytosol. The enzyme was purified by polyethylene glycol precipitation of the cytosol followed by column chromatography on Q-Sepharose fast flow, gel filtration, heparin-Sepharose, and hydroxyapatite. The major Gbetagamma-responsive PI 3-kinase is distinct from p85 containing PI 3-kinase as the activities can be distinguished chromatographically and immunologically and is related to p110gamma as it cross-reacts with anti-p110gamma-specific antibodies. The p110gamma-related PI 3-kinase cannot be activated by G-protein alphai/o subunits, and it has an apparent native molecular mass of 210 kDa. The p110gamma-related PI 3-kinase phosphorylates phosphatidylinositol (PtdIns), phosphatidylinositol 4-phosphate (PtdIns4P), and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2). The apparent Km values for ATP were found to be 25 microM with PtdIns, 44 microM with PtdIns4P, and 37 microM with PtdIns(4,5)P2 as the substrate. Gbetagamma subunits did not alter the Km of the enzyme for ATP; however, Vmax increased 2-fold with PtdIns as substrate, 3.5-fold with PtdIns4P, and 10-fold with PtdIns(4,5)P2. Under basal conditions the apparent Km values for lipid substrates were 64, 10, and 15 microM for PtdIns, PtdIns4P, and PtdIns(4,5)P2, respectively. In the presence of Gbetagamma subunits the dependence of PI 3-kinase activity on the concentrations of lipid substrates became complex with the highest level of stimulation occurring at high substrate concentration, suggesting that the binding of Gbetagamma and lipid substrate (particularly PtdIns(4,5)P2) may be mutually cooperative. Wortmannin and LY294002 inhibit the Gbetagamma-responsive PI 3-kinase activity with IC50 values of 10 nM and 2 microM, respectively. Unlike the p85 containing PI 3-kinase in platelets, the p110gamma-related PI 3-kinase is not associated with a PtdIns(3,4,5)P3 specific 5-phosphatase. The p85-associated PI 3-kinase was not activated by Gbetagamma alone but could be synergistically activated by Gbetagamma and phosphotyrosyl platelet-derived growth factor receptor peptides. This may represent a form of coincidence detection through which the effects of tyrosine kinase and G-protein-linked receptors might be coordinated.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Structural views of phosphoinositide-specific phospholipase C: signalling the way ahead

Recent structural studies of mammalian phosphoinositide-specific phospholipase C (PI-PLC) have begun to shed light on the mechanism whereby this family of effector enzymes is able to hydrolyze phospholipid substrates to yield second messengers. PI-PLC isozymes employ a variety of modules (PH domain, EF-hand domain, SH2 domain, SH3 domain and C2 domain) that are common in proteins involved in signal transduction to reversibly interact with membranes and protein components of the signalling pathways.

Phosphatidylinositol 4,5-bisphosphate binding to the pleckstrin homology domain of phospholipase C-delta1 enhances enzyme activity

The pleckstrin homology (PH) domain is a newly recognized protein module believed to play an important role in signal transduction. While the tertiary structures of several PH domains have been determined, some co-complexed with ligands, the function of this domain remains elusive. In this report, the PH domain located in the N terminus of human phospholipase C-delta1 (PLCdelta1) was found to regulate enzyme activity. The hydrolysis of phosphatidylinositol (PI) was stimulated by phosphatidylinositol 4,5-bisphosphate (PIP2) in a dose-dependent manner with an EC50 = 1 microM (0.3 mol%), up to 9-fold higher when 5 microM (1.5 mol%) of PIP2 was incorporated into the PI/phosphatidylserine (PS)/phosphatidylcholine (PC) vesicles (30 microM of PI with a molar ratio of PI:PS:PC = 1:5:5). Stimulation was specific for PIP2, since other anionic phospholipids including phosphatidylinositol 4-phosphate had no stimulatory effect. PIP2-mediated stimulation was, however, inhibited by inositol 1,4, 5-triphosphate (IP3) in a dose-dependent manner, suggesting a modulatory role for this inositol. When a nested set of PH domain deletions up to 70 amino acids from the N terminus of PLCdelta1 were constructed, the deletion mutant enzymes all catalyzed the hydrolysis of the micelle forms of PI and PIP2 with specific activities comparable with those of the wild type enzyme. However, the stimulatory effect of PIP2 was greatly diminished when more than 20 amino acid residues were deleted from the N terminus. To identify the specific residues involved in PIP2-mediated enzyme activation, amino acids with functional side chains between residues 20 and 40 were individually changed to glycine. While all these mutations had little effect on the ability of the enzyme to catalyze the hydrolysis of PI or PIP2 micelles, the catalytic activity of mutants K24G, K30G, K32G, R38G, or W36G was markedly unresponsive to PIP2. Analysis of PIP2-stimulated PI hydrolysis by a dual substrate binding model of catalysis revealed that the micellar dissociation constant (Ks) of PLCdelta1 for the PI/PS/PC vesicles was reduced from 558 microM to 53 microM, and the interfacial Michaelis constant (Km) was reduced from 0.21 to 0.06 by PIP2. The maximum rate of PI hydrolysis (Vmax) was not affected by PIP2. These results demonstrate that a major function of the PH domain of PLCdelta1 is to modulate enzyme activity. Further, our results identify PIP2 as a functional ligand for a PH domain and suggest a general mechanism for the regulation of other proteins by PIP2.

Phospholipase C beta2 association with phospholipid interfaces assessed by fluorescence resonance energy transfer. G protein betagamma subunit-mediated translocation is not required for enzyme activation

Phospholipase C beta2 (PLC beta2) is activated by G protein betagamma subunits and calcium. The enzyme is soluble and its substrate, phosphatidylinositol 4,5-bisphosphate (PIP2), is present in phospholipid membranes. A potential mechanism for regulation of this enzyme is through influencing the equilibrium association of the enzyme with membrane surfaces. In this paper we describe a fluorescence resonance energy transfer (FRET) method for measuring the association of PLC beta2 with phospholipid bilayers. The method allows equilibrium measurements to be made under a variety of conditions, including those that support enzymatic activity and ability to be regulated by G proteins. Using this method it was found that PLC beta2 bound to vesicles containing anionic lipids and demonstrated a selective and unique interaction with PIP2-containing vesicles. The FRET data were corroborated with a centrifugation based method for estimating the affinity of PLC beta2 for vesicles. Apparently different modes of association of PLC beta2 with vesicles of different composition can be distinguished based on alterations in resonance energy transfer efficiency. Association of PLC beta2 with PIP2 vesicles requires an intact lipid bilayer, is blocked by neomycin, and is not affected by D-myo-inositol 1,4,5-trisphosphate (D-IP3). G protein betagamma subunits do not alter the affinity of PLC beta2 for lipid bilayers and at the PIP2 concentrations used to measure betagamma-dependent stimulation of PLC activity, the majority of the PLC beta2 is already associated with the vesicle surface. Furthermore, under conditions where betagamma subunits strongly activate PLC activity, the extent of association with vesicles is unaffected by betagamma subunits or calcium. These results indicate that activation of PLC beta2 by G protein betagamma subunits or Ca2+ in vitro does not involve translocation to the vesicle surface.

Positive charge at position 549 is essential for phosphatidylinositol 4,5-bisphosphate-hydrolyzing but not phosphatidylinositol-hydrolyzing activities of human phospholipase C delta1

Point mutagenesis, phosphatidylinositol (PI), and phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis assays and equilibrium centrifugation PIP2 assays were used to study the functional roles of four highly conserved arginine residues in the Y region of human phospholipase C delta1 (PLCdelta1) (Arg-527, -549, -556, -701). Most of the mutant enzymes were either partially defective or fully active in their abilities to catalyze the hydrolysis of PI or PIP2. However, upon substitution of Arg-549 by glycine or histidine, the mutant enzyme was defective in its ability to catalyze the hydrolysis of PIP2, but it is still able to hydrolyze PI. Replacing Arg-549 with lysine had little effect on the level of PI and PIP2 hydrolytic activities of the mutant enzyme. The residual PIP2 hydrolyzing activity of R549H is highly dependent on pH. R549H showed 5-10% of the PIP2-hydrolyzing activity of the native enzyme between pH 5 and 7 and nondetectable PIP2-hydrolyzing activity at pH 8. The PIP2-hydrolyzing activity of R549G was not detectable at all pH values. Kinetic analysis of PLCdelta1-catalyzed PIP2 hydrolysis revealed that the micellar dissociation constant Ks and interfacial Michaelis constant Km were similar in the native, R549K, and R549H enzymes; but the specific activity at the saturated substrate mole fraction and infinite level of substrate (Vmax) of the R549H mutant were reduced by a factor of 15. PIP2 competitively inhibits the native enzyme to hydrolyze PI at both pH 7 and 8. However, PIP2 inhibits R549H only at pH 7.0 and does not inhibit R549G at either pH. Taken together, these results suggest that positive charge at position 549 of PLCdelta1 protein is essential for the enzyme to recognize and catalyze the hydrolysis of PIP2 but not PI.

The role of carboxyl-terminal basic amino acids in Gqalpha-dependent activation, particulate association, and nuclear localization of phospholipase C-beta1

The phospholipase C (PLC)-beta isozymes differ from the PLC-gamma and PLC-delta isozymes in that they possess a long COOH-terminal sequence downstream of their catalytic domain, are activated by alpha subunits of the Gq class of G proteins, associate with the particulate subcellular fraction, and are present in the nucleus. Most of the COOH-terminal domain of PLC-beta isozymes is predicted to be helical, and three regions in this domain, PLC-beta1 residues 911-928 (region 1), 1055-1072 (region 2), and 1109-1126 (region 3), contain a high proportion of basic residues that are highly conserved. Projection of the sequences of these three regions in helical wheels reveals clustering of the basic residues. The role of the COOH terminus and the clustered basic residues in PLC-beta1 was investigated by either truncating the entire COOH-terminal domain (mutant DeltaC) or replacing two or three clustered basic residues with isoleucine (or methionine), and expressing the mutant enzymes in CV-1, Rat-2, or Swiss 3T3 cells. The DeltaC mutant no longer showed the ability to be activated by Gqalpha, to translocate to the nucleus, or to associate with the particulate fraction. Substitution of clusters of basic residues in regions 1 and 2 generally reduced the extent of activation by Gqalpha, whereas substitution of a basic cluster in region 3 had no effect. Substitution of the cluster of lysine residues 914, 921, and 925 in region 1 had the most marked effect, reducing Gqalpha-dependent activity to 10% of that of wild type. All substitution mutants, with the exception of that in which lysine residues 1056, 1063, and 1070 in region 2 were substituted with isoleucine, behaved like the wild-type enzyme in showing an approximately equal distribution between cytoplasm and nucleus; only 12% of the region 2 mutant was present in the nucleus. None of the basic clusters appeared critical for particulate association; however, replacement of each cluster reduced the amount of PLC-beta1 in the particulate fraction by some extent, suggesting that all the basic residues contribute to the association, presumably by interacting with acidic residues in the particulate fraction. Membrane localization of PLC-beta isozymes is therefore likely mediated by both the COOH-terminal domain and the pleckstrin homology domain, the latter of which is known to bind phosphatidylinositol 4,5-biphosphate.

Activation of phospholipase C-gamma by the concerted action of tau proteins and arachidonic acid

Phospholipase C-gamma (PLC-gamma) isozymes are thought to be activated by receptor-induced tyrosine phosphorylation. Proteins that activate PLC-gamma1 have now been purified from bovine brain and identified as members of the tau family of microtubule-associated proteins. Activation of PLC-gamma by tau was enhanced in the presence of unsaturated fatty acids such as arachidonic acid, saturated fatty acids being ineffective. Maximal (15-20-fold) activation was apparent in the presence of 0.15 microM tau and 25 microM arachidonic acid (AA). The effect of tau and AA was specific to PLC-gamma isozymes in the presence of submicromolar concentrations of Ca2+ and was markedly inhibited by phosphatidylcholine. These results suggest that in cells that express tau, receptors coupled to cytosolic phospholipase A2 may activate PLC-gamma isozymes indirectly in the absence of tyrosine phosphorylation through the hydrolysis of phosphatidylcholine to generate AA.

Molecular cloning, expression and regulatory activity of G alpha 11- and beta gamma-subunit-stimulated phospholipase C-beta from avian erythrocytes

A turkey erythrocyte phospholipase C (PLC) has been instrumental in delineating the role of G-proteins in receptor-regulated inositol lipid signalling. This isoenzyme is uniquely regulated both by alpha-subunits of the Gq family and by G-protein beta gamma-subunits. A 4819 bp cDNA encoding this PLC has been cloned from a turkey erythrocyte cDNA library. The open reading frame of this cDNA encodes a 1211-amino-acid protein (calculated molecular mass 139050 Da) that contains amino acid sequences of 16 peptides sequenced from the turkey erythrocyte PLC. The predicted sequence of the turkey PLC shows considerable similarity with the sequences of previously cloned members of the PLC-beta family, with the highest identity (71%) shared with PLC-beta 2 and lesser identities observed with PLC-beta 1 (49%), PLC-beta 3 (46%) and PLC-beta 4 (37%). The largest differences in sequence between the turkey PLC-beta and other PLC-beta isoenzymes occur in the C-terminal domain and in the region between the X- and Y-domains. The turkey isoenzyme and PLC-beta 2, which differ in their regulation by G-protein alpha-subunits, are only 44% similar across the approx. 400 amino acid residues of the C-terminal domain that has been implicated in alpha q activation of these proteins. Recombinant turkey PLC-beta was purified to homogeneity following expression from a recombinant baculovirus in Sf9 insect cells. The immunoreactivity and mobility on SDS/PAGE of the recombinant enzyme were the same as observed with native turkey erythrocyte PLC-beta. Moreover, the catalytic activities of the recombinant enzyme were indistinguishable from those of native turkey erythrocyte PLC-beta in assays carried out in the presence of cholate and Ca2+, or in assays of activity after reconstitution with G alpha 11 or G-protein beta gamma-subunits. The turkey PLC-beta was more sensitive to activation by G alpha 11 than was PLC-beta 2, and was more sensitive to activation by beta gamma-subunits than either PLC-beta 2 or PLC-beta 1.

Specific binding of the Akt-1 protein kinase to phosphatidylinositol 3,4,5-trisphosphate without subsequent activation

Recent evidence has suggested that activation of phosphoinositide 3-kinase (PI 3-kinase) is required for the activation of Akt-1 by growth factors and insulin. Here we demonstrate by two independent methods that Akt-1 from L6 myotubes binds to PtdIns(3,4,5)P3, PtdIns(3,4)P2 and PtdIns(4,5)P2 when presented against a background of phosphatidylserine (PtdSer) or a 1:1 mixture of PtdSer and phosphatidylcholine (PtdCho). No binding was observed with the lipids PtdIns(3,5)P2, PtdIns4P and PtdIns3P or background lipids. Activated, hyperphosphorylated forms of Akt-1 from insulin-stimulated L6 myotubes bound to PtdIns(3,4,5)P3 in a similar manner as inactive Akt-1. Quantitative analysis using surface plasmon resonance showed that the equilibrium association constant for the binding of Akt-1 to PtdIns(3,4,5)P3 was submicromolar and that PtdIns(3,4)P2 and PtdIns(4,5)P2 bound to Akt-1 with 3- and 6-fold lower affinities respectively. Interaction of Akt-1 with PtdIns(3,4,5)P3 did not activate the protein kinase activity, either before or after incubation with MgATP. A model is presented in which PtdIns(3,4,5)P3 may prime Akt-1 for activation by another protein kinase, perhaps by recruiting it to the plasma membrane.

Time-dependent inhibition of phospholipase C beta-catalysed phosphoinositide hydrolysis: a comparison of different assays

The properties of three different beta-isoforms of phospholipase C (PLC) were analysed using substrate lipids dispersed in phospholipid vesicles, phospholipid-detergent mixed micelles and phospholipid monolayers spread at an air-water interface. Phosphatidylinositol 4,5-bisphosphate hydrolysis went virtually to completion in monolayers, but inositol trisphosphate production was curtailed prematurely in vesicular and micellar assays. Assays were linear for less than 2 min with vesicles; the linear portion could be significantly extended in micelles by increasing the ratio of micelles to enzyme molecules. However, onset of a second lower rate of substrate hydrolysis always occurred when < or = 10% of PtdIns(4,5)P(2) had been utilized. This was not due to enzyme inactivation in the micellar interface, determined by addition of fresh substrate or fresh enzyme after the slow phase of activity had started, nor was it due to overt product inhibition of PLC or apparent entrapment of PLC at the micelle surface. These results are similar to those seen in assays using bacterial PLC and we suggest that the biphasic kinetics may be due to product-dependent changes in the presentation of substrate lipic to PLC in lamellar assays, leading to reduced activity.

Concentration of enzyme-dependent activation of PLC-beta 1 and PLC-beta 2 by G alpha 11 and beta gamma-subunits

Differential regulation of PLC-beta 1 and -beta 2 by the G-protein alpha-subunit, G alpha 11, and by G-protein beta gamma-subunits was studied utilizing recombinant PLC-beta 1 and -beta 2. Rat PLC-beta 1 and human PLC-beta 2 were purified after recombinant baculovirus-mediated expression in Sf9 cells. The catalytic properties of the purified recombinant isoenzymes were directly compared to PLC-beta 1 purified from bovine brain and PLC-beta 2 partially purified from HL60 polymorphonuclear neutrophils. The recombinant isoenzymes were indistinguishable from the native isoenzymes with respect to dependence of reaction velocity on bulk PtdIns(4,5)P2 substrate concentration, pH, and free Ca2+ concentration. Marked AlF(4-)-dependent activation was observed upon reconstitution of rPLC-beta 1 with the G-protein alpha-subunit, G alpha 11. Activation occurred with a concentration dependence on G alpha 11 for activation and elevation in reaction velocity that was similar to that of native PLC-beta 1. In contrast, G alpha 11 promoted only a small elevation in the catalytic rate of recombinant PLC-beta 2, which was also typical of the native isoenzyme. Maximal reaction rates with respect to PLC-beta isoenzyme concentration were achieved and indicated that rPLC-beta 2 required 10-fold greater concentrations of both G alpha 11 and of rPLC-beta 2 for activation than did rPLC-beta 1. rPLC-beta 1 and rPLC-beta 2 were also differentially regulated by beta gamma-subunits. This differential activation was not the result of different concentration dependencies on beta gamma-subunit for activation, but rather, the result of the greater degree to which the catalytic rate of PLC-beta 2 was elevated by beta gamma-subunits when compared to PLC-beta 1.