Regulation of Phosphoinositide-specific Phospholipase C Isozymes

The hydrolysis of a minor membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP2), 1The abbreviations used are: PIP2, phosphatidylinositol 4,5-bisphosphate; PLC, phospholipase C; PKC, protein kinase C; PLD, phospholipase D; PI 3-kinase, phosphoinositide 3-kinase; PH, pleckstrin homology; SH, Src homology; GTPγS, guanosine 5′-3-O-(thio)triphosphate; IL, interleukin; PDGF, platelet-derived growth factor; PTK, protein tyrosine kinase; AA, arachidonic acid; PC, phosphatidylcholine; cPLA2, cytosolic phospholipase A2; PIP3, phosphatidylinositol 3,4,5-trisphosphate; α1-AR, α1-adrenergic receptor(s); PKA, cAMP-dependent protein kinase. by a specific phospholipase C (PLC) is one of the earliest key events in the regulation of various cell functions by more than 100 extracellular signaling molecules (1Rhee S.G. Choi K.D. J. Biol. Chem. 1992; 267: 12393-12396Abstract Full Text PDF PubMed Google Scholar, 2Cockcroft S. Thomas G.M.H. Biochem. J. 1992; 288: 1-14Crossref PubMed Scopus (371) Google Scholar, 3Berridge M.J. Nature. 1993; 361: 315-325Crossref PubMed Scopus (6196) Google Scholar, 4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar). This reaction produces two intracellular messengers, diacylglycerol and inositol 1,4,5-trisphosphate, which mediate the activation of protein kinase C (PKC) and intracellular Ca2+ release, respectively. Furthermore, a decrease in the amount of PIP2 itself in the cell membrane is likely an important signal because the activities of several proteins are modulated by this phospholipid (5Lee S.B. Rhee S.G. Curr. Opin. Cell Biol. 1995; 7: 183-189Crossref PubMed Scopus (285) Google Scholar). PIP2 is a cofactor for phosphatidylcholine-specific phospholipase D (PLD) and a substrate for phosphoinositide 3-kinase (PI 3-kinase), both of which are also receptor-activated effector enzymes. In addition, PIP2modulates actin polymerization by interacting with various actin-binding proteins and serves as a membrane-attachment site for many signaling proteins that contain pleckstrin homology (PH) domains. Consequently, the activity of PLC is stringently regulated in cells through several distinct mechanisms that link multiple PLC isoforms to various receptors. The 10 mammalian PLC isozymes (excluding alternatively spliced forms) identified to date are all single polypeptides and can be divided into three types, β, γ, and δ, of which four PLC-β, two PLC-γ, and four PLC-δ proteins are known (1Rhee S.G. Choi K.D. J. Biol. Chem. 1992; 267: 12393-12396Abstract Full Text PDF PubMed Google Scholar, 2Cockcroft S. Thomas G.M.H. Biochem. J. 1992; 288: 1-14Crossref PubMed Scopus (371) Google Scholar, 3Berridge M.J. Nature. 1993; 361: 315-325Crossref PubMed Scopus (6196) Google Scholar, 4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 5Lee S.B. Rhee S.G. Curr. Opin. Cell Biol. 1995; 7: 183-189Crossref PubMed Scopus (285) Google Scholar). The δ-type isozymes are smaller (M r 85,000) than the PLC-β and PLC-γ (M r 140,000–155,000) isoforms. Lower eukaryotes such as yeast and slime molds contain only δ-type isozymes, suggesting that β- and γ-type isoforms in higher eukaryotes evolved from the archetypal PLC-δ. Two regions of high sequence homology (40–60% identity), designated X and Y, constitute the PLC catalytic domain (1Rhee S.G. Choi K.D. J. Biol. Chem. 1992; 267: 12393-12396Abstract Full Text PDF PubMed Google Scholar, 2Cockcroft S. Thomas G.M.H. Biochem. J. 1992; 288: 1-14Crossref PubMed Scopus (371) Google Scholar, 3Berridge M.J. Nature. 1993; 361: 315-325Crossref PubMed Scopus (6196) Google Scholar, 4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 5Lee S.B. Rhee S.G. Curr. Opin. Cell Biol. 1995; 7: 183-189Crossref PubMed Scopus (285) Google Scholar) (Fig.1). A PH domain is located in the NH2-terminal region, preceding the X domain, in all three types of PLC. Whereas PLC-β and PLC-δ isozymes contain a short sequence of 50–70 amino acids that separates the X and Y regions, PLC-γ isozymes have a long sequence of ∼400 amino acids that contains Src homology (SH) (two SH2 and one SH3) domains. PLC-γ isozymes contain an additional PH domain that is split by the SH domains. PH (∼100 residues), SH2 (∼100 residues), and SH3 (∼50 residues) domains are protein modules that are shared by many signaling proteins; whereas PH domains mediate interaction with the membrane surface by binding to PIP2, SH domains mediate interactions with other proteins by binding to phosphorylated tyrosine residues (SH2) or proline-rich sequences (SH3). The three-dimensional structure of a PLC-δ1 molecule lacking the PH domain has recently been determined (6Essen L.-O. Perisic O. Cheung R. Katan M. Williams R.L. Nature. 1996; 380: 595-602Crossref PubMed Scopus (519) Google Scholar). As expected, the X and Y regions are tightly associated. The structure also revealed two accessory modules, an EF-hand domain and a C2 domain, the latter of which was previously suggested to mediate the Ca2+-dependent binding to lipid vesicles. On the basis of the structural information, a catalytic mechanism comprising two steps, tether and fix, was proposed. The PH domain of PLC-δ1 would tether the enzyme to the membrane by specific binding to PIP2, and the C2 domain would fix the catalytic domain in a productive orientation on the membrane. The EF-hand domain would serve as a flexible link between the PH domain and the rest of the enzyme. Calcium is required for the function of the C2 domain. Another Ca2+ ion located at the active site, together with His311 and His356, directly participates in catalysis, consistent with the fact that all eukaryotic PLC isozymes require Ca2+ for activity, that the two histidines equivalent to His311 and His356 are completely conserved among all PLC isoforms, and that mutation of either of the two histidine residues results in enzyme inactivation (7Smith M.R. Liu Y.-L. Matthews N.T. Rhee S.G. Sung W.K. Kung H.-F. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 6554-6558Crossref PubMed Scopus (105) Google Scholar). The multidomain structure observed with PLC-δ1 is likely to be common to all mammalian PLC isoforms (Fig. 1). However, PLC-β and PLC-γ isozymes contain additional regulatory COOH-terminal and SH domains, respectively. These regulatory domains are responsible for the fact that different PLC isozymes are linked to receptors through distinct mechanisms. Furthermore, the COOH-terminal domain of PLC-β isozymes might contribute to the tethering of the enzyme to the membrane surface, given that truncation of this domain completely blocked membrane association of PLC-β1 (8Kim C.G. Park D. Rhee S.G. J. Biol. Chem. 1996; 271: 21187-21192Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar). The SH domains of PLC-γ appear to play a critical role in mitogenic signaling independently of PLC activity; catalytically inactive mutants of PLC-γ (containing mutations at the essential His residues) elicited a mitogenic response when microinjected into NIH 3T3 cells, and mitogenic activity was localized to the SH region (7Smith M.R. Liu Y.-L. Matthews N.T. Rhee S.G. Sung W.K. Kung H.-F. Proc. Natl. Acad. Sci. U. S. A. 1994; 91: 6554-6558Crossref PubMed Scopus (105) Google Scholar, 9Smith M.R. Liu Y.-L. Kim S.R. Bae Y.S. Kim C.G. Kwon K.-S. Rhee S.G. Kung H.-F. Biochem. Biophys. Res. Commun. 1996; 222: 186-193Crossref PubMed Scopus (34) Google Scholar). The α subunits (αq, α11, α14, and α16) of all four members of the Gq subfamily of heterotrimeric G proteins activate PLC-β isozymes but not PLC-γ1 or PLC-δ1 (1Rhee S.G. Choi K.D. J. Biol. Chem. 1992; 267: 12393-12396Abstract Full Text PDF PubMed Google Scholar, 2Cockcroft S. Thomas G.M.H. Biochem. J. 1992; 288: 1-14Crossref PubMed Scopus (371) Google Scholar, 3Berridge M.J. Nature. 1993; 361: 315-325Crossref PubMed Scopus (6196) Google Scholar, 4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 10Sternweis P.C. Smrcka A.V. Trends Biochem. Sci. 1992; 17: 502-506Abstract Full Text PDF PubMed Scopus (175) Google Scholar) (Fig.2). The receptors that activate this Gqα-PLC-β pathway include those for thromboxane A2, bradykinin, bombesin, angiotensin II, histamine, vasopressin, acetylcholine (muscarinic m1 and m3), α1-adrenergic agonists, thyroid-stimulating hormone, C-C and C-X-C chemokines, and endothelin-1 (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 11Kuang Y. Wu Y. Jiang H. Wu D. J. Biol. Chem. 1996; 271: 3975-3978Abstract Full Text Full Text PDF PubMed Scopus (176) Google Scholar). The GTPγS-activated Gαq or Gα11 subunits stimulate PLC-β isoforms with the rank order of potency PLC-β1 ≥ PLC-β3 > PLC-β2 (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 5Lee S.B. Rhee S.G. Curr. Opin. Cell Biol. 1995; 7: 183-189Crossref PubMed Scopus (285) Google Scholar). PLC-β4 is also activated by Gqα subunits; however, because the basal activity of this enzyme is inhibited by ribonucleotides, including GTPγS, accurate estimation of the extent of activation is difficult (12Lee C.W. Lee K.H. Lee S.B. Park D. Rhee S.G. J. Biol. Chem. 1994; 269: 25335-25338Abstract Full Text PDF PubMed Google Scholar). All four Gqα members are palmitoylated at residues Cys9 and Cys10 (13Hepler J.R. Biddlecome G.H. Kleuss C. Camp L.A. Hofmann S.L. Ross E.M. Gilman A.G. J. Biol. Chem. 1996; 271: 496-504Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar). Removal of the two palmitate groups affects neither the capacity of the proteins to activate PLC-β1 nor their association with the cell membrane. Gα16, which is detected only in hematopoietic cells and is distantly related to the more widely expressed Gαq(amino acid sequence identity of 55%), activates PLC-β1, -β2, and -β3 in a manner essentially indistinguishable from that of Gαq (14Kozasa T. Hepler J.R. Smrcka A.V. Simon M.I. Rhee S.G. Sternweis P.C. Gilman A.G. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 9176-9180Crossref PubMed Scopus (71) Google Scholar). However, the α subunits can be discriminated by certain receptors (11Kuang Y. Wu Y. Jiang H. Wu D. J. Biol. Chem. 1996; 271: 3975-3978Abstract Full Text Full Text PDF PubMed Scopus (176) Google Scholar). The receptor-mediated activation of PLC-β has been studied in detail by reconstituting the m1 muscarinic acetylcholine receptor, G protein, and PLC-β in lipid vesicles (15Nakamura F. Kato M. Kameyama K. Nakada T. Haga T. Kato H. Takenawa T. Kikkawa U. J. Biol. Chem. 1995; 270: 6246-6253Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar, 16Biddlecome G.H. Bernstein G. Ross E.M. J. Biol. Chem. 1996; 271: 7999-8007Abstract Full Text Full Text PDF PubMed Scopus (213) Google Scholar). The muscarinic agonist carbachol stimulated PLC activity 90-fold, and each member of the Gqα family mediated this activation. The intrinsic GTPase activity of purified Gαq was low but was stimulated >50-fold by the presence of PLC-β1, that is PLC-β1 is a GTPase-activating protein for Gαq (16Biddlecome G.H. Bernstein G. Ross E.M. J. Biol. Chem. 1996; 271: 7999-8007Abstract Full Text Full Text PDF PubMed Scopus (213) Google Scholar). In the reconstituted system, PLC-β1 also increased the rate of GTP hydrolysis by Gαq up to 60-fold in the presence of carbachol, which alone stimulated activity 6–10-fold (16Biddlecome G.H. Bernstein G. Ross E.M. J. Biol. Chem. 1996; 271: 7999-8007Abstract Full Text Full Text PDF PubMed Scopus (213) Google Scholar). These results indicate that the receptor and PLC-β1 coordinately regulate the amplitude of the PLC signal and the rate of signal termination. The Gβγ dimer also activates PLC-β isozymes (1Rhee S.G. Choi K.D. J. Biol. Chem. 1992; 267: 12393-12396Abstract Full Text PDF PubMed Google Scholar, 2Cockcroft S. Thomas G.M.H. Biochem. J. 1992; 288: 1-14Crossref PubMed Scopus (371) Google Scholar, 3Berridge M.J. Nature. 1993; 361: 315-325Crossref PubMed Scopus (6196) Google Scholar, 4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 15Nakamura F. Kato M. Kameyama K. Nakada T. Haga T. Kato H. Takenawa T. Kikkawa U. J. Biol. Chem. 1995; 270: 6246-6253Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). The sensitivity of PLC-β isozymes to Gβγ subunits differs from that to Gqα and decreases in the order PLC-β3 > PLC-β2 > PLC-β1 (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 5Lee S.B. Rhee S.G. Curr. Opin. Cell Biol. 1995; 7: 183-189Crossref PubMed Scopus (285) Google Scholar). The ability of Gβγ subunits to activate PLC-β2 in response to ligation of the luteinizing hormone receptor, V2 vasopressin receptor, β1- and β2-adrenergic receptors, m2 muscarinic acetylcholine receptor, and the receptors for the chemoattractants interleukin 8 (IL-8), formyl-Met-Leu-Phe, and complementation factor 5a was demonstrated using a cotransfection assay system in COS cells (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 17Zhu X. Birnbaumer L. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 2827-2831Crossref PubMed Scopus (91) Google Scholar,18Jiang H. Kuang Y. Wu Y. Smrcka A. Simon M.I. Wu D. J. Biol. Chem. 1996; 271: 13430-13434Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar). These receptors also stimulate PLC-β through Gqα subunits. Although the concentrations of Gβγ required for maximal activation of PLC-β isoforms in vitro are much larger than those of Gqα subunits, the final extents of activation are similar. Thus, both Gqα and Gβγ likely are transducers in PLC signaling. However, it was recently suggested that Gβγ is the predominant transducer in the activation ofXenopus oocyte PLC and that the role of Gqα subunits is to specify the receptor coupled to the enzyme (19Stehno-Bittel L. Krapivinsky G. Krapivinsky L. Perez-Terzic C. Clapham D.E. J. Biol. Chem. 1995; 270: 30068-30074Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar). The region of PLC-β that interacts with Gqα differs from that responsible for interaction with Gβγ; whereas the COOH-terminal region downstream of the Y domain is essential for the activation of PLC-β1 and PLC-β2 by Gqα (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 5Lee S.B. Rhee S.G. Curr. Opin. Cell Biol. 1995; 7: 183-189Crossref PubMed Scopus (285) Google Scholar), the site of interaction of PLC-β2 with Gβγ was localized to the region spanning Glu435 to Val641 (20Kuang Y. Wu Y. Smrcka A. Jiang H. Wu D. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 2964-2968Crossref PubMed Scopus (69) Google Scholar). Thus, Gqα and Gβγ subunits may independently modulate a single PLC-β molecule concurrently. Several positively charged residues important for interaction with Gqα have been identified in the COOH-terminal region of PLC-β1 (8Kim C.G. Park D. Rhee S.G. J. Biol. Chem. 1996; 271: 21187-21192Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar). The COOH-terminal 14 residues of the Gβ subunit were also shown to be important for PLC-β activation (21Zhang S. Coso O.A. Collins R. Gutkind J.S. Simonds W.F. J. Biol. Chem. 1996; 271: 20208-20212Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar). Mammalian cDNAs that encode five distinct Gβ subunits and 11 Gγ subunits have been isolated. Although certain subunits are expressed only in specific tissues and there is some selectivity in the interaction of β and γ, Gβγ subunits are available in many different combinations. Among several permitted combinations tested, all except β1γ1 activated purified PLC-β3 with similar potencies (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 5Lee S.B. Rhee S.G. Curr. Opin. Cell Biol. 1995; 7: 183-189Crossref PubMed Scopus (285) Google Scholar). In cells, however, not all the available Gqα members and Gβγ combinations appear to be utilized to activate PLC. Experiments with antisense oligonucleotides directed against the mRNAs encoding various G protein subunits suggested that m1 muscarinic acetylcholine receptor interacts only with the G protein complexes composed of the subunits αq, α11, β1, β4, and γ4 to activate PLC in RBL-2H3 cells, despite the fact that the subunits α14, β2, β3, γ2, γ3, γ5, and γ7 are also expressed in the cell (22Dippel E. Kalkbrenner F. Wittig B. Schultz G. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 1391-1396Crossref PubMed Scopus (70) Google Scholar). Polypeptide growth factors, such as platelet-derived growth factor (PDGF), epidermal growth factor, fibroblast growth factor, nerve growth factor, and hepatocyte growth factor, induce PIP2 turnover by activating PLC-γ in a wide variety of cells. Binding of these growth factors to their receptors results in activation of the intrinsic protein tyrosine kinase (PTK) activity of the receptor and the consequent tyrosine phosphorylation of numerous proteins, including the receptor itself and PLC-γ (1Rhee S.G. Choi K.D. J. Biol. Chem. 1992; 267: 12393-12396Abstract Full Text PDF PubMed Google Scholar, 2Cockcroft S. Thomas G.M.H. Biochem. J. 1992; 288: 1-14Crossref PubMed Scopus (371) Google Scholar, 3Berridge M.J. Nature. 1993; 361: 315-325Crossref PubMed Scopus (6196) Google Scholar, 4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar) (Fig. 3). Receptor autophosphorylation creates high affinity binding sites for several SH2 domain-containing proteins, including PLC-γ1. A specific autophosphorylated site (for example, Tyr1021 of the β-type PDGF receptor) is recognized by one of the SH2 domains of PLC-γ. Mutation of the PLC-γ-binding Tyr residue to Phe in the receptors for PDGF, epidermal growth factor, and nerve growth factor prevents association of the receptor with PLC-γ and abolishes the growth factor-dependent production of inositol 1,4,5-trisphosphate (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar). Phosphorylation of PLC-γ1 by all growth factor receptors occurs at identical sites: tyrosines 771, 783, and 1254. Phe substitution at Tyr783 completely blocks the activation of PLC by PDGF in NIH 3T3 cells (1Rhee S.G. Choi K.D. J. Biol. Chem. 1992; 267: 12393-12396Abstract Full Text PDF PubMed Google Scholar, 2Cockcroft S. Thomas G.M.H. Biochem. J. 1992; 288: 1-14Crossref PubMed Scopus (371) Google Scholar, 3Berridge M.J. Nature. 1993; 361: 315-325Crossref PubMed Scopus (6196) Google Scholar, 4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 5Lee S.B. Rhee S.G. Curr. Opin. Cell Biol. 1995; 7: 183-189Crossref PubMed Scopus (285) Google Scholar). Tyrosine phosphorylation of PLC-γ1 appears to promote its association with unidentified components of the cytoskeleton; the SH3 domain of PLC-γ1 is responsible for targeting the enzyme to the actin microfilament network. Whether this cytoskeletal association serves to bring the enzyme into contact with its substrate or whether it promotes interaction with another protein component essential for its activation is unknown. Autophosphorylation of growth factor receptors and subsequent tyrosine phosphorylation of substrate proteins, including PLC-γ1, require the presence of H2O2, whose concentration increases transiently and which appears to function as an intracellular messenger in growth factor-stimulated cells (23Bae Y.S. Kang S.W. Seo M.S. Baines I.C. Tekle E. Chock P.B. Rhee S.G. J. Biol. Chem. 1997; 272: 217-221Abstract Full Text Full Text PDF PubMed Scopus (1103) Google Scholar). This requirement that the activation of a receptor by the binding of a growth factor is to the of protein tyrosine of protein tyrosine by is also also and activate PLC-γ isozymes in response to the ligation of certain cell surface receptors. receptors include the cell receptor, membrane the high affinity receptor, the receptors, the receptor, the receptor, and several receptors for such as factor, factor, and (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar, C. J. J. 1996; Google Scholar, G. Biochem. J. 1995; PubMed Scopus Google Scholar). These receptors, of which multiple not activity, but activate a wide variety of such as the members of and The activated one of the receptor components to which PLC-γ its SH2 domains and phosphorylated by the PLC-γ1 directly with Src and in cells, and in vitro it is phosphorylated by various including and (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, 5Lee S.B. Rhee S.G. Curr. Opin. Cell Biol. 1995; 7: 183-189Crossref PubMed Scopus (285) Google Scholar). Tyrosine phosphorylation of PLC-γ1 has also been shown to be in cells that that are or that are to Liu J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar, M. Y. Liu J. M. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar, S.H. Park Kim Y.S. Biochem. Biophys. Res. Commun. 1993; PubMed Scopus Google Scholar). Tyrosine phosphorylation of PLC-γ has also been observed in response to the ligation of several G receptors, including muscarinic acetylcholine receptor in cells, the angiotensin and receptors in cells, and factor (4Noh D.-Y. Shin S.H. Rhee S.G. Biochim. Biophys. Acta. 1995; 1242: 99-114Crossref PubMed Scopus (256) Google Scholar, B. E. Bernstein J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar, M.S. J. Biol. Chem. 1995; 270: Full Text Full Text PDF PubMed Scopus Google Scholar). Src appears to be responsible for the phosphorylation of PLC-γ1 in cells and of to Src inhibited the tyrosine phosphorylation of PLC-γ1 elicited by angiotensin or Although activation of Src family in response to of a variety of G receptors has been demonstrated J. S. J. 1996; PubMed Scopus Google Scholar), the mechanism by which the are coupled to the receptors is not mechanism is through a member of the recently identified proline-rich of receptors coupled to the G proteins or Gq in cells in tyrosine phosphorylation of binding of the SH2 domain of Src to the phosphorylated and activation of Src G. S. J. Nature. 1996; PubMed Scopus Google Scholar). PLC-γ isozymes can be activated directly by several in the of tyrosine acid by the of activates purified PLC-γ1 by as an G. J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar). PLC-γ isozymes are also stimulated by arachidonic acid in the presence of the protein or proteins D.-Y. Bae Y.S. Kim Rhee S.G. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). The of and was specific to PLC-γ isozymes and was inhibited by These that the activation of PLC-γ1 by or proteins might be by a decrease in concentration and an in both of which in cells activation of an cytosolic phospholipase This enzyme is coupled to various receptors and activation of PLC-γ isozymes may to receptor-mediated activation of Several are consistent with the that of PLC by occurs in cells. of a variety of receptors results in the activation of PI 3-kinase, which the of PIP2 to phosphatidylinositol activates purified PLC-γ isozymes by interacting with their SH2 domains. S. L. G. K.-S. and S. G. for In addition, of NIH 3T3 cells with in a in the intracellular an that was blocked in the presence of a PLC Thus, receptors coupled to PLD, cPLA2, or PI 3-kinase may activate PLC-γ isozymes in the of tyrosine through the of (Fig. Although four distinct PLC-δ isoforms are the mechanism by which these isozymes are coupled to membrane receptors A of protein, and α and β subunits, has been shown to be with α1-adrenergic receptors The a protein that also activity H. T. A. K. 1994; PubMed Scopus Google Scholar), activates purified PLC-δ1 and a with PLC-δ1 in cells stimulated Rhee S.G. J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). Furthermore, of in COS cells the activation of PLC by ligation of the H. T. A. K. 1994; PubMed Scopus Google Scholar). These results that directly to is not known whether other PLC-δ isozymes are also activated by other receptors to and the activity of is related to its The GTPase-activating protein for the protein also activates purified PLC-δ1 activation was suggested to downstream of activation Y. Y. J. 1995; PubMed Scopus Google Scholar). All PLC isozymes are activated by Ca2+ in but PLC-δ isozymes are more to Ca2+ with the other Furthermore, PLC-δ can be to its PH in the of other in the intracellular concentration of Ca2+ to a to fix the C2 domain of PLC-δ might its activation. Thus, activation of PLC-δ isozymes might to receptor-mediated activation of other PLC isozymes or Ca2+ PLC signaling also appears to in the 1995; Full Text PDF PubMed Scopus Google Scholar). PLC-β1 is the PLC that has been detected in the of various cells. The amount of PLC-β1 protein, which appears to be activated independently of its membrane by an increases cell growth and decreases L. L. Nature. 1992; PubMed Scopus Google Scholar, J. Biol. Chem. 1994; 269: Full Text PDF PubMed Google Scholar, S. A. A. C. L. J. Biol. Chem. 1994; 269: Full Text PDF PubMed Google Scholar, Rhee S.G. Biochem. J. 1995; PubMed Scopus Google Scholar). The in the amount of PLC-β1 with in the amount of PIP2 in the of cells lacking PLC-β1 as a of revealed that it is essential for the of in response to growth factor A. R. S. and L. in Scholar). The COOH-terminal region downstream of the Y domain was also shown to be for of PLC-β1 to the (8Kim C.G. Park D. Rhee S.G. J. Biol. Chem. 1996; 271: 21187-21192Abstract Full Text Full Text PDF PubMed Scopus (193) Google Scholar). The activation of or cAMP-dependent protein kinase the PLC signaling pathway in a variety of cells. The for phosphorylation by these include cell surface receptors, G proteins, and PLC PLC-β1 is phosphorylated in cells with and is phosphorylated at by in however, phosphorylation on either the basal or activities of PLC-β1 (1Rhee S.G. Choi K.D. J. Biol. Chem. 1992; 267: 12393-12396Abstract Full Text PDF PubMed Google Scholar). In cells, activation of or results in an in phosphorylation of and a decrease in the tyrosine phosphorylation of PLC-γ1, the latter of which might be responsible for the PLC activity in cells with or (1Rhee S.G. Choi K.D. J. Biol. Chem. 1992; 267: 12393-12396Abstract Full Text PDF PubMed Google Scholar). The interaction of PLC and was studied in COS cells with cDNAs encoding G protein subunits, and M. Simon M.I. Nature. 1996; PubMed Scopus Google Scholar). of the catalytic subunit of inhibited Gβγ of PLC-β2 activity, activation. The of was not by Furthermore, directly phosphorylated residues of PLC-β2 both in and in The δ-type PLC, which is the only known PLC in yeast and slime has been in these eukaryotes by targeting J.S. J. Biol. 1993; PubMed Scopus Google Scholar, J. J. 1994; PubMed Scopus Google Scholar). mutants were Whereas the yeast increased sensitivity to various the slime including with to such as and for 10 PLC and 8 were determined M.S. Park Rhee S.G. 1996; 7: PubMed Scopus Google Scholar). The encoding PLC-β1, and PLC-γ1 have been in The mutants for PLC-β1 or PLC-β4 are but the PLC-β4 mutants also a in and S. D. D. S. S. B. S. H. and The PLC-γ1 mutation was at K.D. D. R. G. Proc. Natl. Acad. Sci. U. S. A. 1997; PubMed Scopus Google Scholar). from a with a as as and were shown to have the amount of PLC-β2 with S.B. Lee X. Bae Y.S. Rhee S.G. 1996; PubMed Google Scholar).