Selective activation of phospholipase C by recombinant G-protein alpha- and beta gamma-subunits

A cytosolic, galphaq- and betagamma-insensitive splice variant of phospholipase C-beta4

Phospholipase C (PLC)-beta4 has been considered to be a mammalian homolog of the NorpA PLC, which is responsible for visual signal transduction in Drosophila. We reported previously the cloning of a cDNA encoding rat phospholipase C-beta4 (PLC-beta4) (Kim, M. J., Bahk, Y. Y., Min, D. S., Lee, S. J., Ryu, S. H., and Suh, P.-G. (1993) Biochem. Biophys. Res. Commun. 194, 706-712). We report now the isolation and characterization of a splice variant (PLC-beta4b). PLC-beta4b is identical to the 130-kDa PLC-beta4 (PLC-beta4a) except that the carboxyl-terminal 162 amino acids of PLC-beta4a are replaced by 10 distinct amino acids. The existence of PLC-beta4b transcripts in the rat brain was demonstrated by reverse transcription-polymerase chain reaction analysis. Immunological analysis using polyclonal antibody specific for PLC-beta4b revealed that this splice variant exists in rat brain cytosol. To investigate functional differences between the two forms of PLC-beta4, transient expression studies in COS-7 cells were conducted. We found that PLC-beta4a was localized mainly in the particulate fraction of the cell, and it could be activated by Galphaq, whereas PLC-beta4b was localized exclusively in the soluble fraction, and it could not be activated by Galphaq. In addition, both PLC-beta4a and PLC-beta4b were not activated by G-protein betagamma-subunits purified from rat brain. These results suggest that PLC-beta4b may be regulated by a mechanism different from that of PLC-beta4a, and therefore it may play a distinct role in PLC-mediated signal transduction.

Reconstitution of bovine A1 adenosine receptors and G proteins in phospholipid vesicles: betagamma-subunit composition influences guanine nucleotide exchange and agonist binding

We have studied the interactions of purified A1 adenosine receptors and G proteins reconstituted into phospholipid vesicles to investigate how the betagamma composition of G protein heterotrimers influences coupling. Recombinant hexahistidine-tagged bovine A1 adenosine receptors were expressed in Sf9 cells and purified to homogeneity by sequential chromatography over heparin-sepharose, xanthine amino congener-agarose, and nickel-nitrilotriacetic acid columns. These receptors were reconstituted with pure recombinant G proteins of defined subunit composition. Receptor-G protein complexes containing alphai2 and beta1gamma2 or beta1gamma3 and stimulated with the agonist, (R)-phenylisopropyladenosine, exchange guanine nucleotide 2-3 times more rapidly than do complexes containing beta1gamma1. This difference is not overcome by increasing the concentration of betagamma subunits. Receptor-G protein complexes containing beta1gamma1 also bind less of the agonist, [125I]-iodoaminobenzyladenosine (125I-ABA), than do complexes containing beta1gamma3. Kinetic experiments show that 125I-ABA dissociates 2-fold more rapidly from receptor-G protein complexes containing beta1gamma1 than from complexes containing the other betagamma subunits. The affinity of the interaction between immobilized Galphai2 subunits and beta1gamma1 or beta1gamma2 measured with an optical biosensor in the absence of receptor is similar. Taken together, these data implicate the gamma-subunit in influencing the interaction between the A1 adenosine receptor and G proteins.

Molecular determinants of selectivity in 5-hydroxytryptamine1B receptor-G protein interactions

The recognition between G protein and cognate receptor plays a key role in specific cellular responses to environmental stimuli. Here we explore specificity in receptor-G protein coupling by taking advantage of the ability of the 5-hydroxytryptamine1B (5-HT1B) receptor to discriminate between G protein heterotrimers containing Galphai1 or Galphat. Gi1 can interact with the 5-HT1B receptor and stabilize a high affinity agonist binding state of this receptor, but Gt cannot. A series of Galphat/Galphai1 chimeric proteins have been generated in Escherichia coli, and their functional integrity has been reported previously (Skiba, N. P., Bae, H., and Hamm, H. E. (1996) J. Biol. Chem. 271, 413-424). We have tested the functional coupling abilities of the Galphat/Galphai1 chimeras to 5-HT1B receptors using high affinity agonist binding and receptor-stimulated guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) binding. In the presence of betagamma subunits, amino acid residues 299-318 of Galphai1 increase agonist binding to the 5-HT1B receptor and receptor stimulation of GTPgammaS binding. Moreover, Galphai1 containing only Galphat amino acid sequences from this region does not show any coupling ability to 5-HT1B receptors. Our studies suggest that the alpha4 helix and alpha4-beta6 loop region of Galphas are an important region for specific recognition between receptors and Gi family members.

Direct stimulation of Bruton's tyrosine kinase by G(q)-protein alpha-subunit

Heterotrimeric guanine-nucleotide-binding regulatory proteins (G proteins) transduce signals from a wide variety of cell-surface receptors to generate physiological responses. Protein-tyrosine kinases are another group of critical cellular signal transducers and their malfunction often leads to cancer. Although activation of G-protein-coupled receptors can elicit rapid stimulation of cellular protein-tyrosine phosphorylation, the mechanism used by G proteins to activate protein-tyrosine kinases is unclear. Here we show that the purified alpha-subunit of the G(q) class of G proteins (G[alpha]q) directly stimulates the activity of a purified non-receptor kinase, Bruton's tyrosine kinase (Btk), whereas purified alpha-subunits from G(il), G(O) or G(z) proteins do not. G(alpha)q can also activate Btk in vivo. Furthermore, in Btk-deficient cells, stimulation of another kinase, a p38 MAP kinase, by Gq-coupled receptors is blocked. Our results demonstrate that certain protein-tyrosine kinases can be direct effectors of G proteins.

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.

Activation of the calcium-permeable cation channel CD20 by alpha subunits of the Gi protein

When the calcium-permeable cation channel CD20 is expressed in Balb/c 3T3 cells, it is activated by insulin-like growth factor-I (IGF-I) via the IGF-I receptor (Kanzaki, M., Nie, L., Shibata, H., and Kojima, I. (1997) J. Biol. Chem. 272, 4964-4969). The present study was conducted to investigate the role of G proteins in the regulation of the CD20 channel. In the excised patch clamp mode, activation of the CD20 channel by IGF-I required GTP, Mg2+, and ATP in the bath solution, and removal of either GTP or ATP attenuated the activation. Non-hydrolyzable ATP could substitute for ATP, and guanyl-5'-yl thiophosphate blocked the activation of the channel by IGF-I. The CD20 channel was also activated by guanosine 5'-3-O-(thio)triphosphate, and ATP was not required for the activation. Addition of a preparation of Gi/Go holoprotein purified from bovine brain activated the CD20, and the beta-adrenergic receptor kinase peptide did not affect the number of channel openings induced by the G protein. The CD20 channel was stimulated by the GTP-bound form of recombinant Gi2 alpha subunit purified from Sf9 cells. The Gi3 alpha subunit was less effective, and the Gi1 alpha subunit had no effect. Purified recombinant beta1gamma2 subunits did not affect the activity of the channel. Finally, IGF-I-induced activation of CD20 was inhibited by an antibody against Gi2 alpha subunit. These findings indicate that the CD20 channel expressed in Balb/c 3T3 cells is activated by the IGF-I receptor via the alpha subunits of heterotrimeric G proteins.

Alpha 2A-adrenergic receptor stimulated calcium release is transduced by Gi-associated G(beta gamma)-mediated activation of phospholipase C

Proposed mechanisms by which alpha 2-adrenergic receptors (alpha 2AR) regulate intracellular calcium ([Ca2+]i) include stimulation and inhibition of cell surface calcium channels, stimulation of calcium release via receptor coupling to Gq with subsequent activation of phospholipase C and release of IP3, or stimulation of calcium release via coupling to Gi in an IP3-independent manner. These potential mechanisms were explored in cells that expressed alpha(2A)AR endogenously (HEL cells), permanently transfected CHO cells, and transiently transfected COS-7 cells. Each cell type displayed agonist (UK14304)-dependent increases in [Ca2+]i that were blocked by yohimbine, ablated by pertussis toxin, and largely unaffected by chelation of extracellular calcium. Furthermore, calcium release was associated with IP3 accumulation and was blocked by an inhibitor of phospholipase C (PLC). When expressed in CHO cells, a mutated alpha(2A)AR which has the amino and carboxyl termini of the third intracellular loop substituted with beta 2AR sequence poorly coupled to Gi in adenylyl cyclase assays, and likewise displayed virtually no coupling to increased [Ca2+]i. These results all point toward a Gi- versus a Gq-mediated coupling pathway triggering release of intracellular calcium stores. The possibility that G(beta gamma) subunits released from alpha(2A)AR-Gi coupling is the mechanism of PLC activation was explored in COS-7 cells by coexpressing alpha(2A)AR with the G(beta gamma) inhibitors transducin or a carboxy-terminal portion of the beta AR kinase. Both beta gamma inhibitors markedly inhibited alpha(2A)AR modulation of [Ca2+]i while not affecting thromboxane A2 receptor mediated stimulation of [Ca2+]i via Gq coupling. Thus, alpha(2A)AR couple to calcium release via Gi-associated G(beta gamma) subunits. This coupling is present in multiple cell types and should be considered a major signal transduction pathway of this receptor.

Phospholipase C beta and membrane action of calcitriol and estradiol

We have shown that estrogens and calcitriol, the hormonally active form of vitamin D, increase the concentration of intracellular calcium ([Ca2+]i) within 5 s by mobilizing calcium from the endoplasmic reticulum and the formation of inositol 1,4, 5-trisphosphate and diacylglycerol. Because the activation of effectors as phospholipase C (PLC) coupled to G-proteins is the early event in the signal transduction pathway leading to the inositol 1,4,5-trisphosphate formation and to [Ca2+]i increase, we described different PLC isoforms (beta1, beta2, gamma1, and gamma2, but not beta4) in female rat osteoblasts using Western immunoblotting. The data showed that phospholipase C beta was involved in the mobilization of Ca2+ from the endoplasmic reticulum of Fura-2-loaded confluent osteoblasts by calcitriol and 17beta estradiol, and PLC gamma was ineffective. The data also showed that only a PLC beta1 linked to a Pertussis toxin-insensitive G-protein and a PLC beta2 coupled to a Pertussis toxin-sensitive G-protein are involved in the effects of calcitriol and 17beta estradiol on the mobilization of Ca2+ from intracellular Ca2+ stores. In conclusion, these results may be an important step toward understanding membrane effects of these steroids and may be an additional argument in favor of membrane receptors to steroid hormones.

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.

Selective activation of effector pathways by brain-specific G protein beta5

While multiple G protein beta and gamma subunit isoforms have been identified, the implications of this potential diversity of betagamma heterodimers for signaling through betagamma-regulated effector pathways remains unclear. Furthermore the molecular mechanism(s) by which the betagamma complex modulates diverse mammalian effector molecules is unknown. Effector signaling by the structurally distinct brain-specific beta5 subunit was assessed by transient cotransfection with gamma2 in COS cells and compared with beta1. Transfection of either beta1 or beta5 with gamma2 stimulated the activity of cotransfected phospholipase C-beta2 (PLC-beta2), as previously reported. In contrast, cotransfection of beta1 but not beta5 with gamma2 stimulated the mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK) pathways even though the expression of beta5 in COS cells was evident by immunoblotting. The G protein beta5 expressed in transfected COS cells was properly folded as its pattern of stable C-terminal proteolytic fragments was identical to that of native brain beta5. The inability of beta5 to activate the MAPK and JNK pathways was not overcome by cotransfection with three additional Ggamma isoforms. These results suggest it is the Gbeta subunit which determines the pattern of downstream signaling by the betagamma complex and imply that the structural features of the betagamma complex mediating effector regulation may differ among effectors.

Membrane binding of phospholipases C-beta 1 and C-beta 2 is independent of phosphatidylinositol 4,5-bisphosphate and the alpha and beta gamma subunits of G proteins

We have measured the membrane binding affinities of purified phosphatidylinositol-specific phospholipases C-beta 1 and C-beta 2 to membranes of varying lipid composition using fluorescence methods. Our studies show that these proteins bind with affinities of 10(-5)-10(-4) M, with a small dependence on lipid type. Binding was relatively insensitive to the presence of phosphatidylinositol-specific phospholipases C-beta s' major physiological substrate, phosphatidylinositiol 4,5-bisphosphate, as well as the presence of Ca2+, which is required for activity. The presence of purified GTP gamma S-activated alpha 11 subunits of heterotrimeric guanine nucleotide binding proteins (G proteins) did not alter the membrane binding affinity of phosphatidylinositol-specific phospholipases C-beta 1, even though alpha 11 is a potent activator of this protein. Similarly, the presence of purified beta gamma subunits of G proteins did not alter the membrane association of phosphatidylinositol-specific phospholipases C-beta 2 even though these subunits strongly activate this isoform. These results argue against a recruitment model for PLC-beta activation by G proteins, negatively charged lipids, Ca2+, or substrate, and suggest that activation occurs through association of the membrane-bound species.

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.

Dopamine D1A receptor regulation of phospholipase C isoform

In LTK- cells stably transfected with rat D1A receptor cDNA, fenoldopam, a D1 agonist, increased phosphatidylinositol 4, 5-bisphosphate hydrolysis in a time-dependent manner. In the cytosol, phospholipase C (PLC) activity increased (50 +/- 7%) in 30 s, returned to basal level at 4 h, and decreased below basal values by 24 h; in the membrane, PLC activity also increased (36 +/- 13%) in 30 s, returned to basal level at 10 min, and decreased below basal value at 4 and 24 h. Fenoldopam also increased PLC-gamma protein in a time-dependent manner. The latter was blocked by the D1 antagonist SKF83742 and by a D1A antisense oligodeoxynucleotide, indicating involvement of the D1A receptor. The fenoldopam-induced increase in PLC-gamma and activity was mediated by protein kinase A (PKA) since it was blocked by the PKA antagonist Rp-8-CTP-adenosine cyclic 3':5'-monophosphorothioate (Rp-8-CTP-cAMP-S) and mimicked by direct stimulation of adenylyl cyclase with forskolin or by a PKA agonist, Sp-cAMP-S. Protein kinase C (PKC) was also involved, since the fenoldopam-induced increase in PLC-gamma protein was blocked by two different PKC inhibitors, calphostin C and chelerythrine; calphostin C also blocked the fenoldopam-induced increase in PLC activity. In addition, forskolin and a PKA agonist, Sp-8-CTP-cAMP-S, increased PKC activity, and direct stimulation of PKC with phorbol 12-myristate 13-acetate increased PLC-gamma protein and activity, effects that were blocked by calphostin C. We suggest that the D1A-mediated stimulation of PLC occurs as a result of PKA activation. PKA then stimulates PLC-gamma in cytosol and membrane via activation of PKC.

Role of the prenyl group on the G protein gamma subunit in coupling trimeric G proteins to A1 adenosine receptors

The coupling of receptors to heterotrimeric G proteins is determined by interactions between the receptor and the G protein alpha subunits and by the composition of the betagamma dimers. To determine the role of the gamma subunit prenyl modification in this interaction, the CaaX motifs in the gamma1 and gamma2 subunits were altered to direct modification with different prenyl groups, recombinant betagamma dimers expressed in the baculovirus/Sf9 insect cell system, and the dimers purified. The activity of the betagamma dimers was compared in two assays: formation of the high affinity agonist binding conformation of the A1 adenosine receptor and receptor-catalyzed exchange of GDP for GTP on the alpha subunit. The beta1gamma1 dimer (modified with farnesyl) was significantly less effective than beta1gamma2 (modified with geranylgeranyl) in either assay. The beta1gamma1-S74L dimer (modified with geranylgeranyl) was nearly as effective as beta1gamma2 in either assay. The beta1gamma2-L71S dimer (modified with farnesyl) was significantly less active than beta1gamma2. Using 125I-labeled betagamma subunits, it was determined that native and altered betagamma dimers reconstituted equally well into Sf9 membranes containing A1 adenosine receptors. These data suggest that the prenyl group on the gamma subunit is an important determinant of the interaction between receptors and G protein gamma subunits.

G protein gamma subunits with altered prenylation sequences are properly modified when expressed in Sf9 cells

The gamma subunits of heterotrimeric G proteins undergo post-translational prenylation and carboxylmethylation after formation of the betagamma dimer, modifications that are essential for alpha-betagamma, betagamma-receptor, and betagamma-effector interactions. We have determined the specific prenyl group present on the beta1gamma1, beta1gamma2, and beta1gamma3 dimers purified from baculovirus-infected Sf9 cells by specific binding to G protein alpha subunits immobilized on agarose. These recombinant dimers undergo the same post-translational modifications determined for gamma1 and gamma2 isolated from mammalian tissues. Furthermore, infection of Sf9 cells with a recombinant baculovirus encoding an alteration of the gamma1 CaaX sequence (gamma1-S74L) resulted in geranylgeranylation of the resulting gamma1 subunit, and alteration of the gamma2 CaaX sequence to CAIS (gamma2-L71S) resulted in farnesylation. Both of these altered gamma subunits were able to associate stably with beta1, and the resulting betagamma dimer bound tightly to alpha-agarose and eluted specifically with aluminum fluoride. These results indicate that Sf9 insect cells properly process the CaaX motif in G protein gamma subunits and are a useful model system to study the role of prenylation in the protein-protein interactions in which the betagamma subunits participate.

Role for GTP in glucose-induced phospholipase C activation in pancreatic islets

We have previously demonstrated a permissive role for GTP in insulin secretion; in the current studies, we examined the effect of GTP on phospholipase C (PLC) activation to explore one possible mechanism for that observation. In rat islets preexposed to the GTP synthesis inhibitors mycophenolic acid (MPA) or mizoribine (MZ), PLC activation induced by 16.7 mM glucose (or by 20 mM alpha-ketoisocaproic acid) was inhibited 63% without altering the labeling of phosphoinositide substrates. Provision of guanine, which normalizes islet GTP content and insulin release, prevented the inhibition of PLC by MPA. Glucose-induced phosphoinositide hydrolysis was blocked by removal of extracellular Ca2+ or by diazoxide. PLC induced directly by Ca2+ influx (i.e., 40 mM K+) was reduced 42% in MPA-pretreated islets but without inhibition of the concomitant insulin release. These data indicate that glucose-induced PLC activation largely reflects Ca2+ entry and demonstrate (for the first time in intact cells) that adequate GTP is necessary for glucose (and Ca(2+)-)-induced PLC activation but not for maximal Ca(2+)-induced exocytosis.

The role of G protein methylation in the function of a geranylgeranylated beta gamma isoform

The gamma subunit of heterotrimeric G proteins is isoprenylated and methylated on its carboxyl terminal cysteine residue. While retinal transducin is farnesylated, all other gamma subunits are modified by geranylgeranylation. An immobilized form of pig liver esterase (iPLE) is able to hydrolyze the methyl ester of a geranylgeranylated beta gamma isoform (beta 1 gamma 2). Since methylation is the only reversible reaction in the isoprenylation pathway, it could be a site of regulation of G protein activity. With both the methylated and demethylated beta 1 gamma 2 now available, the role of methylation for a geranylgeranylated heterotrimeric G protein may be addressed. Here, it is reported that methylation has no effect on the ability of beta gamma to interact with an alpha subunit, as probed by ADP-ribosylation studies with pertussis toxin, and has a small effect (less than 2-fold) on the ability of geranylgeranylated beta gamma to activate phosphatidylinositol-specific phospholipase C (PIPLC) and phosphoinositide 3 kinase (PI3K). In binding studies, demethylation only slightly decreased the ability of beta 1 gamma 2 to adhere to azolectin vesicles. Therefore, methylation of heterotrimeric G proteins appears to have only a minor effect in signal transduction processes which can be correlated to a decrease in hydrophobicity of the beta gamma subunit.

Pertussis toxin-sensitive activation of phospholipase C by the C5a and fMet-Leu-Phe receptors

Signal transduction pathways that mediate C5a and fMet-Leu-Phe (fMLP)-induced pertussis toxin (PTx)-sensitive activation of phospholipase C (PLC) have been investigated using a cotransfection assay system in COS-7 cells. The abilities of the receptors for C5a and fMLP to activate PLC beta2 and PLC beta3 through the Gbetagamma subunits of endogenous Gi proteins in COS-7 cells were tested because both PLC beta2 and PLC beta3 were shown to be activated by the betagamma subunits of G proteins in in vitro reconstitution assays. Neither of the receptors can activate endogenous PLC beta3 or recombinant PLC beta3 in transfected COS-7 cells. However, both receptors can clearly activate PLC beta2 in a PTx-sensitive manner, suggesting that the receptors may interact with endogenous PTx-sensitive G proteins and activate PLC beta2 probably through the Gbetagamma subunits. These findings were further corroborated by the results that PLC beta3 could only be slightly activated by Gbeta1gamma1 or Gbeta1gamma5 in the cotransfection assay, whereas the Gbetagamma subunits strongly activated PLC beta2 under the same conditions. PLC beta3 can be activated by Galphaq, Galpha11, and Galpha16 in the cotransfection assay. In addition, the Ggamma2 and Ggamma3 mutants with substitution of the C-terminal Cys residue by a Ser residue, which can inhibit wild type Gbetagamma-mediated activation of PLC beta2, were able to inhibit C5a or fMLP-mediated activation of PLC beta2. These Ggamma mutants, however, showed little effect on m1-muscarinic receptor-mediated PLC activation, which is mediated by the Gq class of G proteins. These results all confirm that the Gbetagamma subunits are involved in PLC beta2 activation by the two chemoattractant receptors and suggest that in COS-7 cells activation of PLC beta3 by Gbetagamma may not be the primary pathway for the receptors.

Metabotropic glutamate receptor subtype 1A activates adenylate cyclase when expressed in baby hamster kidney cells

1. The coupling of subtype mGluR1a of the metabotropic glutamate receptor family to phosphoinositide hydrolysis, cAMP-formation and arachidonic acid release was characterized when the receptor was expressed in baby hamster kidney (BHK) cells. 2. When measuring cAMP-formation in BHK cells expressing mGluR1a, the rank order of agonist potency as well as the efficacy of mGluR1a antagonists was comparable to what has been observed when measuring mGluR1a-mediated phosphoinositide (PI)-hydrolysis. 3. However, while the presence of extracellular calcium increased the efficacy of quisqualate to stimulate phosphoinositide hydrolysis no significant effects were observed when measuring quisqualate-induced cAMP-formation. 4. Pretreatment of mGluR1a-expressing cells with pertussis toxin increased quisqualate-induced cAMP-formation in contrast to the observed partial inhibition of PI-hydrolysis by pertussis toxin. Cholera toxin increased cAMP-formation in BHK cells but showed no effects on PI-hydrolysis. 5. While quisqualate also stimulated [3H]-arachidonic acid release from BHK cells expressing mGluR1a this effect may be secondary to activation of phospholipase C. 6. These data further suggest that mGluR1a is coupled to PI-hydrolysis as well as cAMP-formation via different G-proteins which can be discriminated by their sensitivity to pertussis toxin.

Bombesin activation of phospholipase C beta 1 in rat acinar pancreatic cells involves the pertussis toxin-sensitive G alpha i3 protein

Bombesin stimulation of inositol 1,4,5-trisphosphate (Ins P3) formation in rat sonicated pancreatic acinar cells was inhibited by an antibody directed against the pertussis toxin (PTX)-sensitive GTP-binding G alpha i3 protein but not by an anti-G alpha q-11 antibody. After solubilization and gel filtration, [125I-Tyr4]bombesin binding sites were recovered in a peak of protein of 67 approximately 90 kDa with a maximal enrichment corresponding to a molecular mass of 83-kDa. Results obtained from the non-hydrolysable GTP analog guanosine-5'-[gamma-thio]triphosphate (GTP gamma S) binding, PTX-stimulated ADP-ribosylation and immunoblotting showed that the 83-kDa fraction contained the G alpha i3 protein but not the G alpha q-11 protein. Furthermore, GTP gamma S increased the bombesin binding dissociation constant (KD) from 0.32 to 0.60 nM, while the anti-G alpha i3 antibody decreased the maximal binding capacity (Bmax) from 50 to 25 fmol/mg protein without affecting the KD. Mixing solubilized bombesin binding sites with a phospholipase C (PLC) preparation from rat pancreas reconstituted a bombesin-stimulated PLC activity which was markedly inhibited by the anti-G alpha i3 antibody but unaffected by the anti-G alpha q-11 antibody. In addition, this stimulation was inhibited by an anti-PLC beta 1 antibody. This result supports the involvement of the PLC beta 1 isoform in bombesin receptor activation.

A 50 KDa protein modulates guanine nucleotide binding of transglutaminase II

Regulation of cellular response is an important mechanism for controlling cellular functions. The transmembrane signaling of the hormone receptors is regulated by GTP-binding proteins (GTPases) and their associated proteins. Our previous studies demonstrated that the bifunctional GTP-binding protein, G alpha h (transglutaminase II), consistently copurified with an approximately 50 kDa protein (G Beta h) which is dissociated from G alpha h upon activation with GTP gamma S or AlF4-. Present immunological and biochemical studies on the regulation of the GTPase cycle of G alpha h, which involves the alpha 1-adrenoceptor and 50 KDa G beta h, reveal that the 50 kDa protein is indeed a G alpha h-associated protein and down regulates functions of G alpha h. Thus, polyclonal antibody against G Beta h coimmunoprecipitates GDP-bound G alpha h but not the GDP-AlF4--bound form. The GTP gamma S binding and GTPase activity of G alpha h are inhibited in a G beta h concentration dependent manner. Supporting this notion, G beta h accelerated GTP gamma S release from G alpha h and changes the affinity of G alpha h from GTP to GDP. Moreover, the ternary complex preparation exhibits TGase activity that is inhibited in the presence of the alpha 1-agonist and GTP. The GTP gamma S binding by the ternary complex, consisting of the alpha 1-agonist, the receptor, and Gh, is also inhibited by G beta h. The inhibition of GTP gamma S binding with the ternary complex requires a > or = 2.7-fold higher concentration of G beta h than the G alpha h alone, indicating that the receptor enhances the affinity of G alpha h for GTP. In addition, G beta h copurifies with an alpha 1-agonist, adrenoceptor, and G alpha h ternary complex, showing that the complex is a heterotetramer. Our data also suggest that G beta h does not directly interact with alpha 1-adrenoceptor. These findings clearly demonstrate that G alpha h associates with a novel protein which modulates the affinity of G alpha h for guanine nucleotides and that the GDP-bound Gh is the ground state for the counterpart activator, the alpha 1-adrenoceptor, in this signaling system.

Phosphoinositide-specific phospholipase C and mitogenic signaling

The importance of PLC activation in cell proliferation is evident from the fact that the hydrolysis of PtdIns(4,5)P2 is one of the early events that follow the interaction of many growth factors and mitogens with their respective receptors. However, the importance of PLC activation is not restricted to proliferation; it is one of the most common transmembrane signaling events elicited by receptors that regulate many other cellular processes, including differentiation, metabolism, secretion, contraction, and sensory perception. It is also clear that cell proliferation signaling does not always require PLC, as indicated by the fact that growth factors such as insulin and CSF-1 do not appear to elicit the hydrolysis of PtdIns(4,5)P2, even though the intracellular domains of their receptors carry a PTK domain and the receptors show topologies very similar to those of the PLC-activating growth factors PDGF, EGF, and FGF. The growth factor-dependent activation of PLC is initiated by the formation of a complex between the receptor PTK and PLC-gamma; the formation of this complex is mediated by a specific interaction between a tyrosine phosphate residue on the intracellular domain of PTK and the SH2 domain of PLC-gamma. The receptor PTK subsequently phosphorylates PLC-gamma, of which two distinct isozymes, PLC-gamma 1 and PLC-gamma 2, have been identified. Proliferation of T cells and B cells in response to the aggregation of their respective cell surface receptors is also accompanied by the activation of PLC-gamma isozymes at an early stage. Unlike growth factor receptors, the T cell and B cell receptors lack intrinsic PTK activity but associate with several non-receptor PTKs of the Src and Syk families. Although the specific kinases are not known, one or more of these enzymes phosphorylate and activate PLC-gamma 1 and PLC-gamma 2. Transduction of growth signals by G protein-coupled receptors such as those for thrombin or bombesin also requires PtdIns(4,5)P2 hydrolysis, which, in this instance, is mediated by PLC-beta isozymes. The PLC-beta subfamily consists of four distinct members: PLC-beta 1, PLC-beta 2, PLC-beta 3, and PLC-beta 4. Agonist interaction with specific G protein-coupled receptors causes the dissociation of Gq proteins into G alpha and G beta gamma subunits and the exchange of GDP bound to G alpha for GTP. The resulting GTP-bound G alpha subunit then activates PLC-beta isoforms by binding to the carboxyl-terminal region of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

The G protein beta gamma subunit transduces the muscarinic receptor signal for Ca2+ release in Xenopus oocytes

At least 30 G protein-linked receptors stimulate phosphatidylinositol 4,5-bisphosphate phosphodiesterase (phospholipase C beta, PLC beta) through G protein subunits to release intracellular calcium from the endoplasmic reticulum (Clapham, D. E. (1995) Cell 80, 259-268). Although both G alpha and G beta gamma G protein subunits have been shown to activate purified PLC beta in vitro, G alpha q has been presumed to mediate the pertussis toxin-insensitive response in vivo. In this study, we show that G beta gamma plays a dominant role in muscarinic-mediated activation of PLC beta by employing the Xenopus oocyte expression system. Antisense nucleotides and antibodies to G alpha q/11 blocked the m3-mediated signal transduction by inhibiting interaction of the muscarinic receptor with the G protein. Agents that specifically bound free G beta gamma subunits (G alpha-GDP and a beta-adrenergic receptor kinase fragment) inhibited acetylcholine-induced signal transduction to PLC beta, and injection of G beta gamma subunits into oocytes directly induced release of intracellular Ca2+. We conclude that receptor coupling specificity of the G alpha q/G beta gamma heterotrimer is determined by G alpha q; G beta gamma is the predominant signaling molecule activating oocyte PLC beta.

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.

Synergy between the inositol phosphate responses to transfected human adenosine A1-receptors and constitutive P2-purinoceptors in CHO-K1 cells

1. The effect of adenosine A1-receptor and P2-purinoceptor agonists on [3H]-inositol phosphate accumulation has been investigated in CHO-K1 cells transfected with the human adenosine A1-receptor. 2. Adenosine receptor agonists stimulated [3H]-inositol phosphate accumulation in CHO-K1 cells with a rank potency order of N6-cyclopentyladenosine (CPA) > 5'-N-ethylcarboxamidoadenosine (NECA) > 2-chloroadenosine > N6-2-(4-aminophenyl) ethyladenosine (APNEA). The responses to both CPA and APNEA were antagonized by the A1 selective antagonist, 1,3-dipropylcyclopentylxanthine (DPCPX) yielding KD values of 1.2 nM and 4.3 nM respectively. 3. ATP, UTP and ATP gamma S were also able to stimulate [3H]-inositol phosphate accumulation in these cells with EC50 values of 1.9 microM, 1.3 microM and 5.0 microM respectively. 2-Methyl-thio-ATP was a weak agonist of this response (EC50 > 100 microM). 4. The [3H]-inositol phosphate response to CPA was completely attenuated by pertussis toxin treatment (24 h; 100 ng ml-1). In contrast, the responses to ATP, UTP and ATP gamma S were only reduced by circa 30% in pertussis toxin-treated cells. 5. The simultaneous addition of CPA and either ATP, UTP or ATP gamma S produced a large augmentation of [3H]-inositol phospholipid hydrolysis. This was due to an increase in the maximal response and was significantly greater than the predicted additive response for activation of these two receptor systems. The synergy was not observed in pertussis toxin-treated cells. 6. No synergy was observed between the [3H]-inositol phosphate responses to histamine and ATP in CHO-K1 cells transfected with the bovine histamine H1-receptor. In these cells the response to histamine was completely resistant to inhibition by pertussis toxin treatment. 7. This study provides a clear demonstration of a synergy between pertussis toxin-sensitive and insensitive receptor systems in a model cell system which is an ideal host for transfected cDNA sequences. This model system should provide a unique opportunity to unravel the mechanisms underlying this example of receptor cross-talk involving phospholipase C.

Identification of a discrete region of the G protein gamma subunit conferring selectivity in beta gamma complex formation

The identification of multiple G protein beta and gamma subunit subtypes suggests a potential diversity of beta gamma heterodimers, which may contribute to the specificity of signal transduction between receptors and effectors. The assembly of beta and gamma subtypes is selective. For example, gamma 1 can assemble with beta 1 but not with beta 2, whereas gamma 2 assembles with both beta isoforms. To identify the structural features of the beta and gamma subunits governing selectivity in heterodimer assembly, a series of nonisoprenylated chimeras of gamma 1 and gamma 2 was constructed, and their interaction with beta 1 and beta 2 was assessed by their ability to direct beta expression to the cytosol in cotransfected COS cells. All of the gamma 1/gamma 2 chimeras were capable of interacting with beta 1 as judged by the cotransfection assay. Chimeras containing gamma 2 sequence near the middle of the molecule between two conserved sequence motifs were capable of interacting as well with beta 2. Among 12 divergent residues in this region, it was found that replacement of three consecutive amino acids in gamma 1, Glu-Glu-Phe (residues 38-40), with the three corresponding amino acids of gamma 2, Ala-Asp-Leu (residues 35-37), conferred the ability to assemble with beta 2. The reciprocal chimera containing Glu-Glu-Phe in the context of gamma 2 failed to assemble with beta 2. The last residue of this triplet is occupied by a leucine in all known mammalian gamma subunits except gamma 1 and appears to be a key determinant of the ability of a gamma subunit to assemble with beta 2. This locus maps to a region of predicted alpha-helical structure in the gamma subunit, likely to represent a point of physical contact with the beta subunit.

A novel GTP-binding protein gamma-subunit, G gamma 8, is expressed during neurogenesis in the olfactory and vomeronasal neuroepithelia

A novel heterotrimeric G-protein gamma-subunit has been cloned, and its function has been confirmed by expression and purification. This gamma-subunit is only detected in the olfactory epithelium, the vomeronasal epithelium and, to a lesser extent, the olfactory bulb. It is absent from all other tissues studied including the nasal respiratory epithelium. During development, expression of G gamma 8 in the olfactory epithelium parallels neurogenesis, peaking shortly after birth and declining in the adult. In situ hybridization studies localize expression of this novel gamma-subunit to the sensory neurons; hybridization is strongest in the region of the epithelium that contains immature neurons. Unlike proteins that are expressed only in mature olfactory neurons (e.g. olfactory marker protein or Golf alpha), expression of G gamma 8 in the olfactory epithelium is relatively unaffected by olfactory bulbectomy. In the vomeronasal epithelium expression of G gamma 8 is also highest in the developing neurons. Taken together, these findings are consistent with a very specific role for G gamma 8 in the development and turnover of olfactory and vomeronasal neurons.

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.

Transmitter-mediated inhibition of N-type calcium channels in sensory neurons involves multiple GTP-binding proteins and subunits

The modulation of voltage-activated Ca2+ channels by neurotransmitters and peptides is very likely a primary means of regulating Ca(2+)-dependent physiological functions such as neurosecretion, muscle contraction, and membrane excitability. In neurons, N-type Ca2+ channels (defined as omega-conotoxin GVIA-sensitive) are one prominent target for transmitter-mediated inhibition. This inhibition is widely thought to result from a shift in the voltage independence of channel gating. Recently, however, voltage-independent inhibition has also been described for N channels. As embryonic chick dorsal root ganglion neurons express both of these biophysically distinct modulatory pathways, we have utilized these cells to test the hypothesis that the voltage-dependent and -independent actions of transmitters are mediated by separate biochemical pathways. We have confirmed this hypothesis by demonstrating that the two modulatory mechanisms activated by a single transmitter involve not only different classes of G protein but also different G protein subunits.

Regulation of phospholipase C-beta isozymes by G-proteins

Phospholipase C (PLC) isozymes are known to be regulated, in part, by heterotrimeric GTP-binding protein (G-protein) subunits, including Galpha subunits of the G(q) family and Gbetagamma subunits. New data show that PLC can also be regulated by a high molecular weight G-protein that doubles as a cellular transglutaminase. Furthermore, a soluble phosphatidylinositol transfer protein (PITP) has been implicated in sustaining the activity of PLC by delivering substrate to the plasma membrane. Such diverse regulatory mechanisms imply that the PLC isozymes are precisely controlled and have specific roles in generating second messengers in response to various external stimuli.

Cloned adenosine A3 receptors: pharmacological properties, species differences and receptor functions

In this review, Joel Linden summarizes what is known about a new and intriguing member of the adenosine receptor family, the A3 receptor. This receptor exhibits unusually large differences in structure, tissue distribution and pharmacological properties between species. Rat A3 receptors are resistant to blockade by xanthine antagonists, but human and sheep A3 receptors can be potently blocked by certain xanthines, notably acidic 8-phenylxanthines. One function of the receptor is to facilitate degranulation of mast cells, and a role for mast cells and A3 receptors in mediating myocardial preconditioning has been proposed. Therefore, selective antagonists of A3 receptors have potential for the treatment of allergic, inflammatory and possibly ischaemic disorders.