Purification of a 110-kDa phosphoinositide phospholipase C that is activated by G-protein beta gamma-subunits

General and versatile autoinhibition of PLC isozymes

Phospholipase C (PLC) isozymes are directly activated by heterotrimeric G proteins and Ras-like GTPases to hydrolyze phosphatidylinositol 4,5-bisphosphate into the second messengers diacylglycerol and inositol 1,4,5-trisphosphate. Although PLCs play central roles in myriad signaling cascades, the molecular details of their activation remain poorly understood. As described here, the crystal structure of PLC-beta2 illustrates occlusion of the active site by a loop separating the two halves of the catalytic TIM barrel. Removal of this insertion constitutively activates PLC-beta2 without ablating its capacity to be further stimulated by classical G protein modulators. Similar regulation occurs in other PLC members, and a general mechanism of interfacial activation at membranes is presented that provides a unifying framework for PLC activation by diverse stimuli.

Specificity and structural requirements of phospholipase C-beta stimulation by Rho GTPases versus G protein beta gamma dimers

Phospholipase C-beta(2) (PLC beta(2)) is activated both by heterotrimeric G protein alpha- and beta gamma- subunits and by Rho GTPases. In this study, activated Rho GTPases are shown to stimulate PLC beta isozymes with the rank order of PLC beta(2) > PLC beta(3) > or = PLC beta(1). The sensitivity of PLC beta isozymes to Rho GTPases was clearly different from that observed for G protein beta gamma dimers, which decreased in the following order: PLC beta(3) > PLC beta(2) > PLC beta(1) for beta(1)gamma(1/2) and PLC beta(2) > PLC beta(1) >>> PLC beta(3) for beta(5)gamma(2). Rac1 and Rac2 were found to be more potent and efficacious activators of PLC beta(2) than was Cdc42Hs. The stimulation of PLC beta(2) by Rho GTPases and G protein beta gamma dimers was additive, suggesting that PLC beta(2) activation can be augmented by independent regulation of the enzyme by the two stimuli. Using chimeric PLC beta(1)-PLC beta(2) enzymes, beta gamma dimers, and Rho GTPases are shown to require different regions of PLC beta(2) to mediate efficient stimulation of the enzyme. Although the catalytic subdomains X and Y of PLC beta(2) were sufficient for efficient stimulation by beta gamma, the presence of the putative pleckstrin homology domain of PLC beta(2) was absolutely required for the stimulation of the enzyme by Rho GTPases. Taken together, these results identify Rho GTPases as novel PLC beta regulators, which mediate PLC beta isozyme-specific stimulation and are potentially involved in coordinating the activation of PLC beta(2) by extracellular mediators in intact cells.

Role of the gamma subunit prenyl moiety in G protein beta gamma complex interaction with phospholipase Cbeta

The G protein betagamma complex regulates a wide range of effectors, including the phospholipase Cbeta isozymes (PLCbetas). Prenyl modification of the gamma subunit is necessary for this activity. Evidence presented here supports a direct interaction between the G protein gamma subunit prenyl group and PLCbeta isozymes. A geranylgeranylated peptide corresponding to the C-terminal region of the gamma subunit type, gamma2, strongly inhibits stimulation of PLCbeta2 and PLCbeta3 activity by the betagamma complex. This effect is specific because the same peptide has no effect on stimulation of PLCbeta by an alpha subunit type, alphaq. Prenylation of the gamma peptide is required for its inhibitory effect. When interaction of prenylated gamma subunit peptide to fluorophore-tagged PLCbeta2 was examined by fluorescence spectroscopy, prenylated but not unprenylated peptide increased PLCbeta2 fluorescence emission energy, indicating direct binding of the prenyl moiety to PLCbeta. In addition, fluorescence resonance energy transfer was detected between fluorophore tagged PLCbeta and wild type betagamma complex but not an unprenylated mutant betagamma complex. We conclude that a major function of the gamma subunit prenyl group is to facilitate direct protein-protein interaction between the betagamma complex and an effector, phospholipase Cbeta.

Structure-function analysis of protease-activated receptor 4 tethered ligand peptides. Determinants of specificity and utility in assays of receptor function

Thrombin activates protease-activated receptors (PARs) by specific cleavage of their amino-terminal exodomains to unmask a tethered ligand that binds intramolecularly to the body of the receptor to effect transmembrane signaling. Peptides that mimic such ligands are valuable as agonists for probing PAR function, but the tethered ligand peptide for PAR4, GYPGKF, lacks potency and is of limited utility. In a structure-activity analysis of PAR4 peptides, AYPGKF was approximately 10-fold more potent than GYPGKF and, unlike GYPGKF, elicited PAR4-mediated responses comparable in magnitude to those elicited by thrombin. AYPGKF was relatively specific for PAR4 in part due to the tyrosine at position 2; substitution of phenylalanine or p-fluorophenylalanine at this position produced peptides that activated both PAR1 and PAR4. Because human platelets express both PAR1 and PAR4, it might be desirable to inhibit both receptors. Identifying a single agonist for both receptors raises the possibility that a single antagonist for both receptors might be developed. The AYPGKF peptide is a useful new tool for probing PAR4 function. For example, AYPGKF activated and desensitized PAR4 in platelets and, like thrombin, triggered phosphoinositide hydrolysis but not inhibition of adenylyl cyclase in PAR4-expressing cells. The latter shows that, unlike PAR1, PAR4 couples to G(q) and not G(i).

Glutamate-stimulated activation of DNA synthesis via mitogen-activated protein kinase in primary astrocytes: involvement of protein kinase C and related adhesion focal tyrosine kinase

Glutamate is the major excitatory neurotransmitter in the CNS. Although its role in neurons has been studied extensively, little is known about its function in astrocytes. We studied the effects of glutamate on signaling pathways in primary astrocytes. We found that the tyrosine kinase related adhesion focal tyrosine kinase (RAFTK) is tyrosine phosphorylated in response to glutamate in a time- and dose-dependent manner. This phosphorylation was pertussis toxin (PTX) sensitive and could be attenuated by the depletion of Ca2+ from intracellular stores. RAFTK tyrosine phosphorylation was mediated primarily by class I/II metabotropic glutamate receptors and depends on protein kinase C (PKC) activation. Glutamate treatment of primary astrocytes also results in a significant increase in the activity of the mitogen-activated protein kinases [extracellular signal-related kinases 1/2 (ERK1/2)]. Like RAFTK phosphorylation, ERK1/2 activation is PTX sensitive and can be attenuated by the depletion of intracellular Ca2+ and by PKC inhibition, suggesting that RAFTK might mediate the glutamate-dependent activation of ERK1/2. Furthermore, we demonstrated that glutamate stimulation of primary astrocytes leads to a significant increase in DNA synthesis. Glutamate-stimulated DNA synthesis is PTX sensitive and can be inhibited by the MAP kinase kinase inhibitor PD98059, suggesting that in primary astrocytes, glutamate might signal via RAFTK and MAP kinase to promote DNA synthesis and cell proliferation.

Sites important for PLCbeta2 activation by the G protein betagamma subunit map to the sides of the beta propeller structure

The betagamma subunits of the heterotrimeric GTP-binding proteins (G proteins) that couple heptahelical, plasma membrane-bound receptors to intracellular effector enzymes or ion channels directly regulate several types of effectors, including phospholipase Cbeta and adenylyl cyclase. The beta subunit is made up of two structurally different regions: an N-terminal alpha helix followed by a toroidal structure made up of 7 blades, each of which is a twisted beta sheet composed of four anti-parallel beta strands (Wall, M. A., Coleman, D. E., Lee, E., Iñiguez-Lluhi, J. A., Posner, B. A., Gilman, A. G., and Sprang, S. R. (1995) Cell 83, 1047-1058; Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319). We have previously shown that sites for activation of PLCbeta2, PLCbeta3, and adenylyl cyclase II overlap on the "top" surface of the propeller, where Galpha also binds (Li, Y., Sternweis, P. M., Charnecki, S., Smith, T. F., Gilman, A. G., Neer, E. J., and Kozasa, T. (1998) J. Biol. Chem. 273, 16265-16272). The present study was undertaken to identify the regions on the side of the torus that might be important for effector interactions. We made mutations in each of the outer beta strands of the G protein beta1 propeller, as well as mutations in the loops that connect the outer strands to the adjacent beta strands. Our results suggest that activation of PLCbeta2 involves residues in the outer strands of blades 2, 6, and 7 of the propeller. We tested three of the mutations that most severely affected PLCbeta2 activity against two forms of adenylyl cyclase (ACI and ACII). Both inhibition of ACI and activation of ACII were unaffected by these mutations, suggesting that if ACI and ACII contact the outer strands, the sites of contact are different from those for PLCbeta2. We propose that distinct sets of contacts along the sides of the propeller will define the specificity of the interaction of betagamma with effectors.

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.

Sites for Galpha binding on the G protein beta subunit overlap with sites for regulation of phospholipase Cbeta and adenylyl cyclase

Heterotrimeric G proteins, composed of alpha and betagamma subunits, forward signals from transmembrane receptors to intracellular effector enzymes and ion channels. Free betagamma activates downstream targets, but its action is terminated by association with GDP-liganded alpha subunits. Because alpha can inhibit activation of many effectors by betagamma, it is likely that the alpha subunit binding surfaces on betagamma overlap the surfaces necessary for effector activation. To test this hypothesis, we mutated residues on beta shown to contact alpha in the recently published crystal structures of the alphabetagamma heterotrimer (Wall, M. A., Coleman, D. E., Lee, E., Iniguez-Lluhi, J. A., Posner, B. A., Gilman, A. G., and Sprang, S. R. (1995) Cell 83, 1047-1058; Lambright, D. G., Sondek, J., Bohm, A., Skiba, N. P., Hamm, H. E., and Sigler, P. B. (1996) Nature 379, 311-319.). The alpha subunit binds to the flat, top surface of the toroidal beta subunit and also extends a helix along the side of the beta subunit at blade 1. We mutated four residues on the top surface of beta (Hbeta1[L117A], Hbeta1[D228R], Hbeta1[D246S], and Hbeta1[W332A]) and two residues on the side of beta that contacts alpha (Hbeta1[N88A/K89A]). Each of the mutant proteins was able to form beta gamma dimers, but they differed in their ability to bind alpha and to activate phospholipase C beta2 (PLCbeta2), PLCbeta3, and adenylyl cyclase II. Mutation of residues along the side of the torus at blade 1 diminish affinity for alpha but do not prevent activation of any of the effectors. Mutations on the alpha binding surface differentially affected PLCbeta2, PLCbeta3, and adenylyl cyclase II. Residues that affect PLCbeta and adenylyl cyclase II activity are found on opposite sides of the central tunnel, suggesting that PLC and adenylyl cyclase, like the alpha subunit, make many contacts on the top surface. None of the mutations affected the ability of betagamma to inhibit adenylyl cyclase I. We conclude that alpha, PLCbeta2, PLCbeta3, and adenylyl cyclase II share an interaction on the top surface of beta. The importance of individual residues is different for alpha binding and for effector activation and differs even between closely related isoforms of the same effector.

The thrombin receptor second cytoplasmic loop confers coupling to Gq-like G proteins in chimeric receptors. Additional evidence for a common transmembrane signaling and G protein coupling mechanism in G protein-coupled receptors

Thrombin activates human platelets and other cells in part by cleaving an unusual G protein-coupled receptor. Thrombin cleavage of this receptor's amino-terminal exodomain unmasks a new amino terminus. This then binds intramolecularly to the body of the receptor to trigger transmembrane signaling and activation of Gi- and Gq-like G proteins. Toward identifying the domains responsible for thrombin receptor-G protein interactions, we examined the signaling properties of chimeric receptors in which thrombin receptor cytoplasmic sequences replaced the cognate sequences in the Gs-coupled beta2-adrenergic receptor (beta2AR) or the Gi-coupled dopamine D2 receptor (D2R). In Xenopus oocytes, a chimeric beta2AR bearing the thrombin receptor second cytoplasmic (C2) loop gained the ability to trigger intracellular Ca2+ release in response to adrenergic agonist, whereas a beta2AR bearing the cognate C2 loop from the D2R did not. Similarly, in COS-7 cells, a chimeric D2R bearing the thrombin receptor C2 loop gained the ability to trigger phosphoinositide hydrolysis in response to dopaminergic agonist, apparently by coupling to a Gq-like G protein. No detectable Gs coupling was seen. Thus, the thrombin receptor C2 loop was able to confer Gq-like coupling in several different receptor contexts. These observations suggest that the thrombin receptor C2 loop specifies Gq coupling by directly contacting Gq or by contributing to a structure required for Gq coupling. The ability of the thrombin receptor C2 loop to function in the context of the D2R and beta2AR strongly suggests that the transmembrane switching and G protein activation strategies used by the thrombin receptor must be very similar to those used by the D2R and beta2AR despite the thrombin receptor's strikingly different liganding mechanism.

G protein beta gamma subunits

Guanine nucleotide binding (G) proteins relay extracellular signals encoded in light, small molecules, peptides, and proteins to activate or inhibit intracellular enzymes and ion channels. The larger G proteins, made up of G alpha beta gamma heterotrimers, dissociate into G alpha and G beta gamma subunits that separately activate intracellular effector molecules. Only recently has the G beta gamma subunit been recognized as a signal transduction molecule in its own right; G beta gamma is now known to directly regulate as many different protein targets as the G alpha subunit. Recent X-ray crystallography of G alpha, G beta gamma, and G alpha beta gamma subunits will guide the investigation of structure-function relationships.

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.

ATP-evoked inositol phosphates formation through activation of P2U purinergic receptors in cultured astrocytes: regulation by PKC subtypes alpha, delta, and theta

ATP-induced phosphoinositide (PI) hydrolysis was studied in cultured astrocytes. To characterize the P2 purinergic receptor-mediated effects of ATP, the subtype-specific agonists 2-methylthio ATP (2-MeSATP), UTP, and alpha, beta-methylene ATP were compared. ATP, UTP, or 2-MeSATP induced a dose-dependent increase of inositol phosphates (IP) accumulation; alpha, beta-methylene ATP and adenosine had no effect. The order of potency was ATP > or = UTP >> 2-MeSATP. Cross-desensitization experiments indicated that ATP interacted with both P2U and P2Y receptors. P2U was the predominant P2 receptor in mediating PI hydrolysis in astrocytes. The effect of ATP, UTP, or 2-MeSATP was markedly inhibited by pretreatment of cells with pertussis toxin (PTX), indicating that both P2U and P2Y receptors coupled to phospholipase C through PTX-sensitive G protein. Short-term (10 min) treatment of cells with 1 microM TPA attenuated ATP, UTP, and 2-MeSATP-induced PI breakdown; however, long-term (24 h) pretreatment resulted in marked potentiation of both ATP and UTP, and restoration of 2-MeSATP responses. In a further analysis of the effect of TPA, 10 min and 1.5 h pretreatment attenuated ATP-and UTP-induced PI breakdown, but this inhibitory action was lost after 3 h of treatment. Both 6 and 24 h pretreatments resulted in a potentiation. Western blot analysis showed translocation of protein kinase C (PKC) alpha, -delta, and -theta from the cytosol to the membrane following 10 min and 1.5 h treatments, and restoration to basal levels in the membrane fraction was seen after 3 h of treatment. On the other hand, partial and complete down-regulation of these three isoforms was seen after 6 and 24 h of treatment, respectively. PKC eta was translocated but not down-regulated by TPA. These results suggested that PKC alpha, -delta, and -theta, not -eta may exert tonic inhibition on P2U receptor-mediated PI turnover in unstimulated astrocytes.

Expression and immunohistochemical localization of eight phospholipase C isoforms in adult male mouse cerebellar cortex

By means of specific polyclonal or monoclonal antibodies we have investigated the expression and the localization of phospholipase C isoforms in the adult mice cerebellar cortex. Western-blot analysis revealed that mouse cerebellum expressed eight phospholipase C isozymes: -beta 1, -beta 2, -beta 3, -beta 4, -gamma 1, -gamma 2, -delta 1, -delta 2. Immunohistochemical analysis carried out on cryosections showed a distinct pattern of expression for each of the isoforms. Purkinje cells had high levels of -beta 1, -beta 3, -gamma 2 and -delta 2 isotypes. The -gamma 2 isozyme was the only one that was identified also in the dendrites of Purkinje cells. In the molecular layer we detected mostly -beta 1 and -gamma 1 isozymes whereas in the granular layer -gamma 1 and -gamma 2 isoforms prodominated. These results indicate a heterogeneity of the phospholipase C isoforms expressed in the layers of mouse cerebellar cortex conceivably due to the fact that these enzymes are coupled to different receptors and perform selective tasks in regulating cell signalling events taking place in the cerebellar cortex of mice.

Thrombin primes responsiveness of selective chemoattractant receptors at a site distal to G protein activation

To define the molecular basis of human chemoattractant receptor regulation, rat basophilic leukemia RBL-2H3 cells, which are thrombin-responsive, were transfected to stably express epitope-tagged receptors for C5a, interleukin-8 (IL-8), formylpeptides (e.g. N-formyl-methionyl-leucyl-phenylalanine (fMLP)), and platelet-activating factor (PAF). Here we demonstrate that both thrombin and a synthetic peptide ligand for the thrombin receptor (sequence SFLLRN) caused phosphorylation and heterologous desensitization of the receptors for C5a, IL-8, and PAF but not that for formylpeptides as measured by agonist-stimulated [35S]guanosine 5'-3-O-(thio)triphosphate binding to membranes. Consistent with the PAF receptor phosphorylation, both thrombin and thrombin receptor peptide inhibited phosphoinositide hydrolysis, Ca2+ mobilization, and degranulation stimulated by PAF. Unexpectedly, despite heterologous desensitization at the level of receptor/G protein activation, there was enhancement ("priming") by thrombin of subsequent activities stimulated by C5a and IL-8 as well as fMLP. The priming effect of thrombin was blocked by its inhibitor, hirudin. However, two other activators of the thrombin receptor, the peptide SFLLRN and trypsin, stimulated Ca2+ mobilization in RBL-2H3 cells but did not cause priming. In addition, SFLLRN and the thrombin receptor antagonist peptide FLLRN both inhibited thrombin-induced Ca2+ mobilization but not priming. Furthermore, the proteolytically active gamma-thrombin, which does not stimulate the tethered ligand thrombin receptor and caused little or no Ca2+ mobilization in RBL-2H3 cells, effectively primed the response to fMLP. These data demonstrate that heterologous receptor phosphorylation and attenuation of G protein activation are not, by themselves, sufficient for the inhibition of biological responses mediated by C5a and IL-8. Moreover, thrombin appears to utilize mechanism(s) independent of its tethered ligand receptor to selectively prime phospholipase C-mediated biological responses of the C5a, IL-8, and formylpeptide receptors but not PAF. Because C5a, IL-8, and formylpeptide activate phospholipase Cbeta2, whereas PAF stimulates a different phospholipase C, the striking selectivity of thrombin's priming may be mediated via its ability to enhance receptor-mediated activation of phospholipase Cbeta2.

The inositol phosphate response to thrombin in rat right atria differs from the response to noradrenaline

Addition of thrombin to isolated [3H]inositol-labelled rat right atria stimulated the release of 3H-labelled inositol phosphates. The thrombin response was smaller than the response to noradrenaline and generated a different spectrum of inositol phosphates. Unlike the inositol phosphate response to noradrenaline, the thrombin response was inhibited by pertussis toxin treatment and by the phospholipase C inhibitor U-73122 (1-(6-((17 beta-3- methoxyestra-1,3,5(10)-trien-17-yl)amino)amino)hexyl)-1H- pyrrole-2, 5-dione). The data indicate that the thrombin stimulation involves different G-proteins and phospholipase C isoforms from those which couple alpha 1-adrenoceptors in the myocardium.

Potentiation of Gi-mediated phospholipase C activation by retinoic acid in HL-60 cells. Possible role of G gamma 2

Differentiated HL-60 cells acquire responsiveness to fMet-Leu-Phe (fMLP), which activates phospholipase C and O2- generation in a pertussis toxin-sensitive manner. Addition of retinoic acid (RA) for the last 24 h during dimethyl sulfoxide (Me2SO)-induced differentiation enhanced fMLP-dependent signals and interaction between fMLP receptor and G(i). RA modifies both the function and subunit composition of G(i)2, the predominant G(i) of HL-60 membranes, as shown by comparing purified G(i)2 from membranes of Me2SO-treated cells (D-G(i)2) to G(i)2 from membranes of cells treated with both Me2SO and RA (DR-G(i)2). As compared to D-G(i)2, DR-G(i)2 induced more fMLP binding when added to membranes of pertussis toxin-treated HL-60 cells and, in the presence of GTP gamma S, stimulated beta gamma-sensitive phospholipase C in extracts of HL-60 cells to a much greater extent at a lower concentrations. Immunoblasts revealed that RA induced expression of the gamma 2 subunit, which was otherwise undetectable in G(i)2 purified from HL-60 cells or in HL-60 membranes. Possibly by inducing expression of gamma 2, RA alters two functions of the G(i) beta gamma subunit, modulation of fMLP receptor-G(i)2 coupling and activation of the effector, Phospholipase C.

Endogenous cleavage of phospholipase C-beta 3 by agonist-induced activation of calpain in human platelets

Two membrane-associated phosphoinositide-specific phospholipase Cs (mPI-PLC-1 and mPI-PLC-2) and a cytosolic enzyme (cPI-PLC) that were activated by brain G-protein beta gamma subunits have been isolated from human platelets. The truncation of mPI-PLC-1 that was mediated by mu-calpain induced much higher activation by beta gamma subunits (Banno, Y., Asano, T., and Nozawa, Y. (1994) FEBS Lett. 340, 185-188). On the basis of size and immunological cross-reactivity, mPI-PLC-1 (155 kDa) was PLC-beta 3, and mPI-PLC-2 (100 kDa) was its truncated form. The cPI-PLC (140 kDa) was recognized by the antibody selective for internal sequences of PLC-beta 3 but not by the antibody raised against its carboxyl terminus, indicating that it may be related to PLC-beta 3. Treatment of human platelets with A23187 and dibucaine, activators of calpain, caused cleavage of actin-binding protein and talin in a time-dependent manner. At the same time, decrease of PLC-beta 3 (155 and 140 kDa) and concomitant increase of the 100-kDa product of cleavage were observed on immunoblots with the antibody to internal sequences of PLC-beta 3. Furthermore, stimulation of platelets by natural agonists, thrombin and collagen, caused the cleavage of PLC-beta 3 (155 and 140 kDa) and an increase of 100 kDa PLC-beta 3 in a time- and dose-dependent manner. The cleavage of these PLC-beta 3 enzymes was completely blocked by calpain inhibitor, calpeptin, indicating that the PLC-beta 3 modification may be a consequence of platelet activation leading to activation of calpain. This is the first demonstration that PLC-beta 3 is indeed cleaved by calpain upon platelet activation by physiological agonists. The cleavage of PLC-beta 3 evoked by thrombin and collagen but not ADP was correlated with irreversible aggregation, suggesting that the PLC-beta 3 modification may play a role in secondary irreversible aggregation in agonist-stimulated human platelets.

Homologous and heterologous regulation of receptor-stimulated phosphoinositide hydrolysis

Signal transduction at a diverse range of pharmacologically distinct receptors is effected by the enhanced turnover of inositol phospholipids, with the attendant formation of inositol 1,4,5-trisphosphate and diacylglycerol. Although considerable progress has been made in recent years towards the identification and characterization of the individual components of this pathway, much less is known of mechanisms that may underlie its regulation. In this review, evidence is presented for the potential regulation of inositol lipid turnover at the level of receptor, phosphoinositide-specific phospholipase C and substrate availability in response to either homologous or heterologous stimuli. Available data indicate that the extent of receptor-stimulated inositol lipid hydrolysis is regulated by multiple mechanisms that operate at different levels of the signal transduction pathway.

Proteolytic modification of membrane-associated phospholipase C-beta by mu-calpain enhances its activation by G-protein beta gamma subunits in human platelets

Membrane-associated phosphoinositide-phospholipase C (PI-PLC)-beta (150 kDa) and its truncated forms (100 kDa and 45 kDa) were purified from human platelets. The 100 kDa PI-PLC-beta was found to be activated to a greater extent by brain G-protein beta gamma subunits compared to the intact 150 kDa enzyme. Furthermore, treatment with mu-calpain of the intact PI-PLC-beta (150 kDa) caused a marked augmentation of its activation by beta gamma subunits. This enhanced PLC activation by beta gamma subunits was due to truncation by mu-calpain, producing a 100 kDa PI-PLC, but not by another protease, thrombin.