(1/525) Phosphorylation by protein kinase C decreases catalytic activity of avian phospholipase C-beta.
The potential role of protein kinase C (PKC)-promoted phosphorylation has been examined in the G-protein-regulated inositol lipid signalling pathway. Incubation of [32P]Pi-labelled turkey erythrocytes with either the P2Y1 receptor agonist 2-methylthioadenosine triphosphate (2MeSATP) or with PMA resulted in a marked increase in incorporation of 32P into the G-protein-activated phospholipase C PLC-betaT. Purified PLC-betaT also was phosphorylated by PKC in vitro to a stoichiometry (mean+/-S. E.M.) of 1.06+/-0.2 mol of phosphate/mol of PLC-betaT. Phosphorylation by PKC was isoenzyme-specific because, under identical conditions, mammalian PLC-beta2 also was phosphorylated to a stoichiometry near unity, whereas mammalian PLC-beta1 was not phosphorylated by PKC. The effects of PKC-promoted phosphorylation on enzyme activity were assessed by reconstituting purified PLC-betaT with turkey erythrocyte membranes devoid of endogenous PLC activity. Phosphorylation resulted in a decrease in basal activity, AlF4(-)-stimulated activity, and activity stimulated by 2MeSATP plus guanosine 5'-[gamma-thio]triphosphate in the reconstituted membranes. The decreases in enzyme activities were proportional to the extent of PKC-promoted phosphorylation. Catalytic activity assessed by using mixed detergent/phospholipid micelles also was decreased by up to 60% by phosphorylation. The effect of phosphorylation on Gqalpha-stimulated PLC-betaT in reconstitution experiments with purified proteins was not greater than that observed on basal activity alone. Taken together, these results illustrate that PKC phosphorylates PLC-betaT in vivo and to a physiologically relevant stoichiometry in vitro. Phosphorylation is accompanied by a concomitant loss of enzyme activity, reflected as a decrease in overall catalytic activity rather than as a specific modification of G-protein-regulated activity. (+info)
(2/525) Resolution of a signal transfer region from a general binding domain in gbeta for stimulation of phospholipase C-beta2.
Signaling by guanine nucleotide-binding proteins (G proteins) involves sequential protein-protein interactions. G protein-betagamma subunit (Gbetagamma) interactions with phospholipase C-beta2 (PLC-beta2) were studied to determine if all Gbeta contacts are required for signaling. A peptide encoding Gbeta amino acid residues 86 to 105 stimulated PLC-beta2. Six residues (96 to 101) within this sequence could transfer signals and thus constitute a core signal transfer region. Another peptide, encoding Gbeta amino acid residues 115 to 135, did not substantially stimulate PLC-beta2 by itself but inhibited Gbetagamma stimulation, indicating that residues 115 to 135 constitute a general binding domain. Resolution of signal transfer regions from general binding domains indicates that all protein-protein contacts are not required for signal transfer and that it may be feasible to synthesize agonists and antagonists that regulate intracellular signal flow. (+info)
(3/525) Inositides in the nucleus: presence and characterisation of the isozymes of phospholipase beta family in NIH 3T3 cells.
Previous reports from our laboratories and others have hinted that the nucleus is a site for an autonomous signalling system acting through the activation of the inositol lipid cycle. Among phospholipases (PLC) it has been shown previously that PLCbeta1 is specifically localised in the nucleus as well as at the plasma membrane. Using NIH 3T3 cells, it has been possible to obtain, with two purification strategies, in the presence or in the absence of Nonidet P-40, both intact nuclei still maintaining the outer membrane and nuclei completely stripped of their envelope. In these nuclei, we show that not only PLCbeta1 is present, but also PLCbeta2, PLCbeta3 and PLCbeta4. The more abounding isoform is PLCbeta1 followed by PLCbeta3, PLCbeta2 and PLCbeta4, respectively. All the isoforms are enriched in nuclear preparations free from nuclear envelope and cytoplasmatic debris, indicating that the actual localisation of the PLCbeta isozymes is in the inner nuclear compartment. (+info)
(4/525) Role of isoprenoid lipids on the heterotrimeric G protein gamma subunit in determining effector activation.
Post-translational prenylation of heterotrimeric G protein gamma subunits is essential for high affinity alpha-beta gamma and alpha-beta gamma-receptor interactions, suggesting that the prenyl group is an important domain in the beta gamma dimer. To determine the role of the prenyl modification in the interaction of beta gamma dimers with effectors, the CAAX (where A indicates alipathic amino acid) motifs in the gamma1, gamma2, and gamma11 subunits were altered to direct modification with different prenyl groups. Six recombinant beta gamma dimers were overexpressed in baculovirus-infected Sf9 insect cells, purified, and examined for their ability to stimulate three phospholipase C-beta isozymes and type II adenylyl cyclase. The native beta1 gamma2 dimer (gamma subunit modified with geranylgeranyl) is more potent and effective in activating phospholipase C-beta than either the beta1 gamma1 (farnesyl) or the beta1 gamma11 (farnesyl) dimers. However, farnesyl modification of the gamma subunit in the beta1 gamma2 dimer (beta1 gamma2-L71S) caused a decrement in its ability to activate phospholipase C-beta. In contrast, both the beta1 gamma1-S74L (geranylgeranyl) and the beta1 gamma11-S73L (geranylgeranyl) dimers were more active than the native forms. The beta1 gamma2 dimer activates type II adenylyl cyclase about 12-fold; however, neither the beta1 gamma1 nor the beta1 gamma11 dimers activate the enzyme. As was the case with phospholipase C-beta, the beta1gamma2-L71S dimer was less able to activate adenylyl cyclase than the native beta1 gamma2 dimer. Interestingly, neither the beta1 gamma1-S74L nor the beta1 gamma11-S73L dimers stimulated adenylyl cyclase. The results suggest that both the amino acid sequence of the gamma subunit and its prenyl group play a role in determining the activity of the beta gamma-effector complex. (+info)
(5/525) Suppression of the neoplastic phenotype by transfection of phospholipase C beta 3 to neuroendocrine tumor cells.
The expression of phospholipase C beta 3 (PLCB3) is low or absent in several neuroendocrine neoplasias. To investigate the role of PLCB3 in the neuroendocrine tumorigenesis, we transfected a PLCB3 construct to three neuroendocrine tumor cell lines with a low PLCB3 expression. The growth rate and tumorigenicity were assessed in vitro by [3H]thymidine incorporation and cell counting, in vivo, by xenografting to nude mice. In vitro, PLCB3 expressing clones showed a significant growth inhibition. The tumor weight was reduced for one of the two xenografted PLCB3-transfected cell lines and in both, a reduced number of proliferating (Ki-67 positive) cells was observed. This study implies an essential role for PLCB3 in the neuroendocrine tumorigenesis. (+info)
(6/525) Identification of the G protein-activating domain of the natriuretic peptide clearance receptor (NPR-C).
We have shown recently that the 37-amino acid intracellular domain of the single-transmembrane, natriuretic peptide clearance receptor, NPR-C, which is devoid of kinase and guanylyl cyclase activities, activates selectively Gi1 and Gi2 in gastric and tenia coli smooth muscle. In this study, we have used synthetic peptide fragments of the N-terminal, C-terminal, and middle regions of the cytoplasmic domain of NPR-C to identify the G protein-activating sequence. A 17-amino acid peptide of the middle region (Arg469-Arg485), denoted Peptide 4, which possesses two N-terminal arginine residues and a C-terminal B-B-X-X-B motif (where B and X are basic and non-basic residues, respectively) bound selectively to Gi1 and Gi2, activated phospholipase C-beta3 via the betagamma subunits, inhibited adenylyl cyclase, and induced smooth muscle contraction, in similar fashion to the selective NPR-C ligand, cANP4-23. A similar sequence (Peptide 3), but with a partial C-terminal motif, had minimal activity. Sequences which possessed either the N-terminal basic residues (Peptide 1) or the C-terminal B-B-X-X-B motif (Peptide 2) were inactive. Peptide 2, however, inhibited G protein activation and cellular responses mediated by the stimulatory Peptide 4 and by cANP4-23, suggesting that the B-B-X-X-B motif mediated binding but not activation of G protein, thus causing Peptide 2 to act as a competitive inhibitor of G protein activation. (+info)
(7/525) A lobster phospholipase C-beta that associates with G-proteins in response to odorants.
A cDNA clone encoding a protein of 1116 amino acids with significant homology to beta-isoforms of phospholipase C was isolated from lobster olfactory organ cDNA libraries and named lobPLCbeta. This cDNA hybridized predominantly to a 9 kb transcript in RNA from olfactory organ, pereiopod, brain, and eye-eyestalk and to several smaller minor transcripts only in eye-eyestalk. An antiserum raised to the C terminus of lobPLCbeta detected immunoreactivity in a single 130 kDa band in olfactory aesthetasc hairs, olfactory organ, pereiopod, dactyl, and brain. In eye-eyestalk this 130 kDa band was abundant, and minor bands of 100, 79, and 57 kDa also were detected. In cross sections of the aesthetasc hairs, immunoreactivity was detected in the outer dendritic segments of the olfactory receptor neurons, the site of olfactory transduction. A complex odorant caused lobPLCbeta immunoreactivity to increase in membrane fractions and decrease in soluble fractions of homogenates of aesthetasc hairs. The odorant also increased the amount of lobPLCbeta in immunoprecipitates of Galphaq and Gbeta from homogenates of aesthetasc hairs. These results support the conclusion that lobPLCbeta mediates olfactory transduction. (+info)
(8/525) Phospholipase C-beta1 directly accelerates GTP hydrolysis by Galphaq and acceleration is inhibited by Gbeta gamma subunits.
Phospholipase C-beta, the principal effector protein regulated by Galphaq, has been shown to increase the agonist-stimulated, steady-state GTPase activity of Gq in proteoliposomes that contain both heterotrimeric Gq and m1 muscarinic receptor. We now use a moderately stable complex of R183C Galphaq bound to GTP to show that PLC-beta1 acts directly as a GTPase-activating protein (GAP) for isolated Galphaq in a membrane-free system. PLC-beta1 accelerated the hydrolysis of GalphaqR183C.GTP up to 20-fold. The Km was 1.5 nM, which is similar both to the EC50 with which R183C and wild type Galphaq activate PLC-beta1 and to the EC50 with which PLC-beta1 acts as a Gq GAP in the vesicle-based assay. The Galphaq GAP activity of RGS4 can also be quantitated by this assay; it accelerated hydrolysis of bound GTP about 100-fold. The Gq GAP activities of both PLC-beta1 and RGS4 are blocked by Gbeta gamma subunits, probably by a competitive mechanism. These data suggest either that the Gbeta gamma subunits are not continuously required for receptor-catalyzed GDP/GTP exchange during steady-state GTP hydrolysis or that GAPs, either PLC-beta or RGS proteins, can substitute for Gbeta gamma in this set of reactions. (+info)