Concanavalin A receptor(s) possibly interacts with at least two kinds of GTP-binding proteins in murine thymocytes

A part of the GTP gamma S-binding activity in murine thymocyte membranes was found to have affinity to a concanavalin A (Con A)-Sepharose column. The material was identified as Gi (inhibitory GTP-binding protein) on the basis of the molecular weight and by islet activating protein-dependent ADP-ribosylation and anti-alpha i (alpha subunit of Gi) immunoblotting. However, when the membranes prepared from Con A-stimulated thymocytes were used, no GTP gamma S-binding activity was detected in the Con A-bound fraction, suggesting that Gi physically and specifically associated with Con A acceptors dissociates upon Con A stimulation. Furthermore, another GTP gamma S-binding protein (25 kDa), which is quite similar to a novel phosphoinositide-specific phospholipase C (PI-PLC)-associated G protein in calf thymocytes (Wang, P., Toyoshima, S., & Osawa, T. (1988) J. Biochem. 103, 137-142), was detected among the Con A-Sepharose-bound proteins with the chemical cross-linking technique. When the 40 kDa and 25 kDa G proteins associated with Con A receptor(s) were isolated and their direct effects on the activity of partially purified PI-PLC as to phosphatidylinositol 4,5-bisphosphate hydrolysis were examined, the 25 kDa G protein was found to enhance the PI-PLC activity more effectively. On the other hand, pretreatment of cells with islet-activating protein completely abolished the inhibitory effect of Con A on the prostaglandin E1 and isoproterenol-induced increases of cellular cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Phosphorylation by cyclic AMP-dependent protein kinase of a human platelet Mr 22,000 GTP-binding protein (smg p21) having the same putative effector domain as the ras gene products

We have purified to near homogeneity a Mr 22,000 GTP-binding protein from human platelet membranes and identified it as the smg-21 gene product (smg p21), having the same putative effector domain as the ras gene products, which we have purified to near homogeneity from bovine brain membranes and characterized. This purified human platelet smg p21 was phosphorylated by cyclic AMP-dependent protein kinase. About one mol of phosphate was maximally incorporated into one mol of the protein. Only serine residue was phosphorylated. Both the guanosine 5'-(3-O-thio)-triphosphate (GTP gamma S)-bound and GDP-bound forms were phosphorylated with the same reaction velocity. The phosphorylation of smg p21 affected neither its GTP gamma S-binding nor GTPase activity. Human platelet smg p21 was not phosphorylated by protein kinase C. A Mr 24,000 GTP-binding protein partially purified from human platelet membranes was not phosphorylated by cyclic AMP-dependent protein kinase or protein kinase C.     

Concanavalin A-induced translocation of part of the GTP-binding activity from the membrane to the cytosol in murine thymocytes

Concanavalin A (Con A) stimulation resulted in the rapid redistribution of part of the GTP-binding activity from the membrane to the cytosol in murine thymocytes. This change in GTP-binding activity was dependent on the Con A concentration. To investigate the relationship between this redistribution and phospholipase C (PLC) activity, the effect of GTP gamma S on the cytosol PLC activity was also examined, and it was found that GTP gamma S enhanced the phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis activity in the cytosol of Con A-stimulated thymocytes more than in that of unstimulated thymocytes. This enhancement by GTP gamma S was also dependent on the Con A concentration. The results suggest that in murine thymocytes, the GTP-binding protein (G-protein) involved in the regulation of PLC activity may be translocated from the membrane to the cytosol upon Con A stimulation. Besides, the dose dependence curve for the change in the GTP gamma S-binding activity was similar to that for inositol phosphates formation in Con A-stimulated thymocytes, suggesting that the translocation of the G-protein is closely related to PLC activation. Furthermore, the effects of cytosol fractions containing the 38-43 and 23-28 kDa GTP-binding subunits of G-proteins on the PIP2 hydrolysis activity of partially purified PLC were examined. The fraction containing the 23-28 kDa subunit evidently enhanced the PLC activity but that containing the 38-43 kDa subunit enhanced the activity to a much lower extent. Moreover, the 23-28 kDa subunit fraction of Con A-stimulated thymocytes was more effective as to enhancement of the PLC activity than that of unstimulated thymocytes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Physical and functional association of cytosolic inositolphospholipid-specific phospholipase C of calf thymocytes with a GTP-binding protein

A soluble inositolphospholipid-specific phospholipase C (PI-phospholipase C) has been purified 5,800-fold from the cytosolic fraction of calf thymocytes. The purification was achieved by sequential column chromatographies on DEAE-Sepharose CL-6B, heparin-Sepharose CL-6B, Sephacryl S-300, Mono S, and Superose 12, followed by column chromatography on Sephadex G-100 in the presence of 1% sodium cholate. The enzyme thus purified was found to be homogeneous on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The molecular weight of the enzyme was estimated to be 68 kDa by SDS-PAGE. The enzyme is specific for inositol phospholipids. Phosphatidylinositol and phosphatidylinositol 4,5-bisphosphate (PIP2) were hydrolyzed, but phosphatidylcholine and phosphatidylethanolamine were not affected by the enzyme. GTP gamma S-binding activity was detected in the enzyme fractions after all the purification steps, but not in the final enzyme preparation. The PI-phospholipase C and GTP gamma S-binding activities in the partially purified enzyme preparation could be separated by the column chromatography on Sephadex G-100 only in the presence of 1% sodium cholate. Thus, the soluble PI-phospholipase C has affinity to a GTP-binding protein. SDS-PAGE of the GTP-binding fractions eluted from the Sephadex G-100 column gave three visible bands of 54, 41, and 27 kDa polypeptide was specifically ADP-ribosylated by pertussis toxin. Furthermore, it was found that GTP and GTP gamma S (10 microM and 1 mM) could enhance the PIP2 hydrolysis activity of the partially purified enzyme in the presence of 3 mM EGTA, but the purified enzyme after separation from the GTP-binding activity was not affected by GTP and GTP gamma S. The soluble PI-phospholipase C of calf thymocytes may be not only physically but also functionally associated with a GTP-binding protein.     

Purification and partial amino acid sequences of a phospholipase C-associated GTP-binding protein from calf thymocytes

We recently identified a phosphoinositide-specific phospholipase C (PI-PLC)-stimulating GTP-binding protein (G protein) in calf thymocyte cytosol (Wang, P., Toyoshima, S., & Osawa, T. (1987) J. Biochem. 102, 1275-1287; and (1988) 103, 137-142). In this study we completely purified a G protein whose properties are quite similar to the G protein mentioned above from the calf thymocyte membrane and determined partial amino acid sequences of it. The purification was achieved by first treating the membrane with GTP gamma S, followed by sequential column chromatographies on DEAE-Sepharose CL-6B, Sephacryl S-200, Mono Q, and Mono S. The G protein was purified in a GTP gamma S-binding form and assayed as to the radioactivity of the [35S]GTP gamma S-bound PI-PLC-associated G protein standard obtained from calf thymocyte cytosol. The purified G protein could stimulate the activity of a partially purified PI-PLC for phosphatidylinositol 4,5-bisphosphate hydrolysis. From approximately 5.6 g of membrane protein we obtained about 5 micrograms of a purified sample. The purified G protein showed a molecular weight of 21 kDa on SDS-PAGE and one of 25 kDa on gel filtration. The partial amino acid sequences were determined by treating the purified sample with lysylendopeptidase, purifying the resultant peptide fragments on a HPLC-reverse phase column and then sequencing the peptide fragments with a sequencer. Comparison of the obtained sequences with those of known lower molecular weight GTP-binding proteins suggested that, although structurally similar to rho gene products, this is a novel G protein.     

Regulation of protein phosphorylation in Dictyostelium discoideum

We have examined the phosphorylation of the cyclic adenosine 3':5' monophosphate (cAMP) cell surface chemotactic receptor and a 36 kDa membrane-associated protein (p36) in Dictyostelium discoideum. The activity of CAR-kinase, the enzyme responsible for the phosphorylation of the cAMP receptor, was studied in plasma membrane preparations. It was found that, as in intact cells, the receptor was rapidly phosphorylated in membranes incubated with [gamma 32P] adenosine triphosphate (ATP) but only in the presence of cAMP. This phosphorylation was not observed in membranes prepared from cells which did not display significant cAMP binding activity. cAMP could induce receptor phosphorylation at low concentrations, while cyclic guanosine 3':5' monophosphate (cGMP) could elicit receptor phosphorylation only at high concentrations. Neither ConA, Ca2+, or guanine nucleotides had an effect on CAR-kinase. It was also observed that 2-deoxy cAMP but not dibutyryl cAMP induced receptor phosphorylation. The data suggest that the ligand occupied form of the cAMP receptor is required for CAR-kinase activity. Although the receptor is rapidly dephosphorylated in vivo, we were unable to observe its dephosphorylation in vitro. In contrast, p36 was rapidly dephosphorylated. Also, unlike the cAMP receptor, the phosphorylation of p36 was found to be regulated by the addition of guanine nucleotides. Guanosine diphosphate (GDP) enhanced the phosphorylation while guanosine triphosphate (GTP) decreased the radiolabeling of p36 indicating that GTP can compete with ATP for the nucleotide triphosphate binding site of p36 kinase. Thus was verified using radiolabeled GTP as the phosphate donor. Competition experiments with GTP gamma S, ATP, GTP, CTP, and uridine triphosphate (UTP) indicated that the phosphate donor site of p36 kinase is relatively non-specific.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in phosphatidylinositol phospholipase C isoenzymes and in their association with GTP gamma S-binding activity in guinea pig uterine smooth muscle during pregnancy.

The nature distribution and associated GTP gamma S binding activity of phosphatidylinositol phospholipase C (PI-PLC) has been studied in non-pregnant and pregnant guinea pig uterine smooth muscle. Cytosolic fractions partially purified by Q-Sepharose and heparin-Agarose chromatography show two isoenzyme forms, one with an apparent molecular weight of 58 kD that crossreacts with PI-PLC alpha and a has Km for phosphatidylinositol of 292 +/- 72.6 microM, designated alpha, and a form that has an apparent molecular weight of 86 kD and a substrate Km of 54 +/- 20 microM designated delta. Approximately 80% of the total PI-PLC activity was recovered in the cytosolic fraction and this increased 8-10 fold for both isoenzymes from the non-pregnant to the late pregnant uterus and the proportion of the alpha isoenzyme increased from approximately 40% to 55% of the total. PI-PLC alpha but not delta activity had GTP gamma S binding activity associated with it after Q-Sepharose or heparin-Agarose chromatography. This associated activity accounted for 2% of the total GTP gamma S-binding activity in the non-pregnant uterus and 31% of that in the near-term uterus. On separation of the PI-PLCa-GTP gamma S-binding complex by gel filtration on Sephacryl S200 gave two peaks one of 118 kD accounting for two-thirds of all the binding and two-thirds of the enzyme activity and a 58 kD peak. The 118 kD peak could not be separated by treatment with 0.5% cholate, but in this form enzyme activity was protected from detergent inactivation found with the 58 kD form. In sodium dodecyl sulphate polyacrylamide-gel electrophoresis PI-PLC alpha was released from the 118 kD complex and showed an apparent molecular weight of 61.5 kD. All the activity in the residual membrane fraction could be released by washing with buffer followed by, 2 M KCl and then 2 M KCl plus 0.5% cholate. This released isoenzyme forms that appeared identical to those in the cytosolic fraction and with GTP gamma S-binding activity associated with PI-PLC alpha. It is concluded that in the near term guinea pig uterus there is a dramatic increase in the capacity for inositol polyphosphate production. Moreover the dramatic increase in GTP gamma S-binding activity associated with PI-PLC alpha implies large changes in the extent and possibly nature of the putative G-protein activation of this pathway.(ABSTRACT TRUNCATED AT 400 WORDS)     

Receptor-mediated phosphorylation of spermatozoan proteins

These studies are the first to report egg peptide-mediated stimulation of protein phosphorylation in spermatozoa. Speract (Gly-Phe-Asp-Leu-Asn-Gly-Gly-Gly-Val-Gly) or resact (Cys-Val-Thr-Gly-Ala-Pro-Gly-Cys-Val-Gly-Gly-Gly-Arg-Leu-NH2) stimulated the incorporation of 32P into various proteins of isolated spermatozoan membranes in the presence, but not absence, of GTP. The Mr of three of the phosphorylated proteins were 52,000, 75,000, and 100,000. GTP gamma S (guanosine 5'-O-(3-thiotriphosphate] but not GDP beta S (guanosine 5'-O-(2-thiodiphosphate] or GMP-PNP (guanylyl imidodiphosphate) also supported the peptide-mediated stimulation of protein phosphorylation. The peptides markedly stimulated guanylate cyclase activity, and GTP gamma S or GTP but not GMP-PNP served as effective substrates for the enzyme. The accumulation of cyclic AMP was not stimulated by the peptides. Subsequently, it was shown that added cyclic GMP or cyclic AMP increased 32P incorporation into the same membrane proteins as those observed in the presence of peptide and GTP. The amount of cyclic GMP (up to 3 microM) formed by membranes in the presence of peptide and 100 microM GTP equated with the amount of added cyclic GMP required to increase the 32P content of a Mr 75,000 protein selected for further study. 32P-Peptide maps of the Mr 75,000 protein indicated that the same domains were phosphorylated in response to cyclic nucleotides or to egg peptide and GTP. Intact cells were subsequently incubated with 32P to determine if the radiolabeled proteins observed in isolated membranes also would be obtained in intact cells. The 32P contents of proteins of Mr 52,000, 75,000, and 100,000 were significantly increased by the addition of resact. Peptide maps confirmed that the increased 32P incorporation obtained in a Mr 75,000 protein of isolated membranes occurred on the same protein domains as the 32P found on the Mr 75,000 protein of intact cells. These results suggest that a GTP or GTP gamma S requirement for peptide-mediated protein phosphorylation in spermatozoan membranes is mainly due to the enhanced formation of cyclic GMP, and it therefore is likely that peptide-induced elevations of cyclic nucleotide concentrations in spermatozoa are responsible for the specific increases in 32P associated with at least three sperm proteins, all apparently localized on the plasma membrane.     

Phosphorylation of smg p21, a ras p21-like GTP-binding protein, by cyclic AMP-dependent protein kinase in a cell-free system and in response to prostaglandin E1 in intact human platelets

We have separated multiple small Mr GTP-binding proteins (G proteins) from bovine brain membranes by several column chromatographies and purified to near homogeneity four of them, including a novel Mr 24,000 G protein (smg p25A), a novel Mr 22,000 G protein (smg p21), the rho protein (rho p20), and the c-Ki-ras protein (c-Ki-ras p21). Among these small Mr G proteins, only smg p21 is phosphorylated stoichiometrically by cAMP-dependent protein kinase (protein kinase A), and c-Ki-ras p21 is phosphorylated to a small extent by protein kinase A in a cell-free system. None of smg p25A, rho p20, and other partially purified small Mr G proteins is phosphorylated by protein kinase A. Neither smg p21 nor other small Mr G proteins are phosphorylated by protein kinase C. About 1 mol of phosphate is maximally incorporated into 1 mol of smg p21 by protein kinase A. Only serine residue(s) are phosphorylated. The guanosine 5'-3-O-(thio) triphosphate (GTP gamma S)-bound and GDP-bound forms of smg p21 are phosphorylated with the same reaction velocity. The phosphorylation of smg p21 affects neither its GTP gamma S-binding nor GTPase activity. smg p21 is found in human platelets, and this human platelet smg p21 is also phosphorylated by protein kinase A at the same site(s) as bovine brain smg p21 in a cell-free system. When intact human platelets are stimulated by prostaglandin E1 known to elevate the cAMP level, four proteins with apparent Mr values of 240,000, 50,000, 24,000, and 22,000 are phosphorylated. These four proteins are also phosphorylated by the action of dibutyryl cAMP but not by the action of thrombin, Ca2+ ionophore A23187, or 12-O-tetradecanoylphorbol-13-acetate. Among the four proteins, the Mr 22,000 protein is identified as smg p21. The site(s) of phosphorylation of smg p21 by protein kinase A in a cell-free system are identical to that phosphorylated in response to prostaglandin E1 in intact platelets. These results indicate that among many small Mr G proteins, smg p21 is selectively phosphorylated by protein kinase A and that this G protein is also phosphorylated by this protein kinase in response to prostaglandin E1 in intact human platelets.     

Small molecular weight GTP-binding proteins in human erythrocyte ghosts

GTP-binding proteins (G proteins) were extracted from human erythrocyte ghosts by sodium cholate and purified by gel filtration on an Ultrogel AcA-44 column followed by hydroxyapatite column chromatography. At least two peaks of G proteins were separated by hydroxyapatite column chromatography. The second peak contained G proteins recognized by the antibodies against the respective alpha subunits of Gs and Gi, and the ras protein, while the G protein of the first peak was not recognized by any of these antibodies. The G protein of the first peak was purified further by Mono Q HR5/5 column chromatography. The purified G protein showed a molecular weight of about 22 kDa on SDS-polyacrylamide gel electrophoresis. This G protein (22K G) specifically bound guanosine 5'-(3-O-thio) triphosphate (GTP gamma S), GTP and GDP with a Kd value for GTP gamma S of about 50 nM. GTP gamma S-binding to 22K G was inhibited by pretreatment with N-ethylmaleimide. The G proteins recognized by the antibodies against the alpha subunit of Go and the ADP-ribosylation factor for Gs, designated as ARF, were not detected in human erythrocyte ghosts. These results indicate that there are at least two species of small molecular weight G proteins in human erythrocyte ghosts: one is the ras protein and the other is a novel G protein of 22K G.     

Purification and characterization of c-Ki-ras p21 from bovine brain crude membranes

In the present studies, we attempted to purify the native molecular forms of the c-ras proteins (c-ras p21s) from bovine brain crude membranes and separated at least three GTP-binding proteins (G proteins) cross-reactive with the antibody recognizing all of Ha-, Ki-, and N-ras p21s. Among them, one G protein with a Mr of about 21,000 was highly purified and characterized. The Mr 21,000 G protein bound maximally about 0.6 mol of [35S]guanosine 5'-(3-O-thio)triphosphate (GTP gamma S)/mol of protein with a Kd value of about 30 nM. [35S]GTP gamma S-binding to Mr 21,000 G protein was inhibited by GTP and GDP, but not by other nucleotides such as ATP, UTP, and CTP. [35S]GTP gamma S-binding to Mr 21,000 G protein was inhibited by pretreatment with N-ethylmaleimide. Mr 21,000 G protein hydrolyzed GTP to liberate Pi with a turnover number of about 0.01 min-1. Mr 21,000 G protein was not copurified with the beta gamma subunits of the G proteins regulatory for adenylate cyclase. Mr 21,000 G protein was not recognized by the antibody against the ADP-ribosylation factor for Gs. The peptide map of Mr 21,000 G protein was different from those of the G proteins with Mr values of 25,000 and 20,000, designated as smg p25A and rho p20, respectively, which we have recently purified from bovine brain crude membranes. The partial amino acid sequence of Mr 21,000 G protein was identical with that of human c-Ki-ras 2B p21. These results indicate that Mr 21,000 G protein is bovine brain c-Ki-ras 2B p21 and that c-Ki-ras 2B p21 is present in bovine brain membranes.     

Inhibition by cyclic AMP of guanine nucleotide-induced activation of phosphoinositide-specific phospholipase C in human platelets

Phosphoinositide-specific phospholipase C (PLC) activity of human platelet membranes was activated by the nonhydrolyzable guanine nucleotide GTP gamma S. This activation did not occur in either membranes prepared from dibutyryl cyclic AMP-pretreated platelets (A-membranes) or those prepared from untreated cells and subsequently incubated with cyclic AMP (cAMP) (B-membranes). This cAMP-mediated inhibition was abolished in the presence of inhibitors of cAMP-dependent protein kinase (A-kinase), suggesting that the inhibition was due to phosphorylation of (a) protein component(s). No significant differences were observed in the basal PLC activity and the extent of pertussis toxin-catalyzed ADP-ribosylation among control membranes and the two types of phosphorylated membranes (A- and B-membranes). GTP-binding activities of Gs, Gi and GTP-binding proteins of lower molecular masses were not altered by the phosphorylation of the membranes. These findings suggest that a GTP-binding protein is involved in the GTP gamma S-mediated activation of PLC and that cAMP (plus A-kinase) inhibits this activation by phosphorylating a membrane protein (probably a 240-kDa protein), rather than the GTP-binding protein or PLC itself. It is likely that this phosphorylation uncouples the GTP-binding protein from PLC.     

Rabbit intestine contains a protein that inhibits the dissociation of GDP from and the subsequent binding of GTP to rhoB p20, a ras p21-like GTP-binding protein

A novel regulatory protein for rhoB p20, a ras p21-like GTP-binding protein (G protein), was partially purified from the cytosol fraction of rabbit intestine. This protein, designated as rhoB p20 GDP dissociation inhibitor (GDI), inhibited the dissociation of GDP from rhoB p20. rhoB p20 GDI also inhibited the binding of guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) to the GDP-bound form of rhoB p20 but not of that to the guanine nucleotide-free form. GDI did not affect the GTPase activity of rhoB p20 and by itself showed no GTP gamma S-binding activity. GDI was inactive for other ras p21/ras p21-like G proteins including c-Ha-ras p21, smg p21 and smg p25A. The Mr value of GDI was estimated to be about 27,000 from the S value. These results indicate that rabbit intestine contains a novel regulatory protein that inhibits the dissociation of GDP from and thereby the subsequent binding of GTP to rhoB p20.     

Effect of cyclic AMP and cyclic GMP on the autophosphorylation of elongation factor 1 from wheat embryos

Extensively purified EF-1H (EF-1 alpha beta beta' gamma) from wheat embryos had a protein kinase activity and phosphorylated EF-1 beta which is one of the two EF-Ts-like factors (EF-1 beta and EF-1 beta'). In this reaction ATP and GTP were equally effective as phosphate donors, and threonine residue was phosphorylated. At 10(-7)M, 3', 5' cyclic GMP stimulated both the phosphorylation and phe-tRNA binding reactions, whereas 3', 5' cyclic AMP inhibited both reactions. These findings indicate that phosphorylation of EF-1H may play an important role in the translational control of protein biosynthesis at the elongation step.     

Detection of G Proteins in Purified Bovine Brain Myelin

Following a previous report on detection of muscarinic receptors in myelin with the implied presence of G proteins, we now demonstrate by more direct means the presence of such proteins and their quantification. Using [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTP gamma S) as the binding ligand, purified myelin from bovine brain was found to contain approximately half the binding activity of whole white matter (138 +/- 9 vs. 271 +/- 18 pmol/mg of protein). Scatchard analysis of saturation binding data revealed two slopes, a result suggesting at least two binding populations. This binding was inhibited by GTP and its analog but not by 5'-adenylylimidodiphosphate [App(NH)p], GMP, or UTP. Following sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) of myelin proteins and blotting on nitrocellulose, [alpha-32P]GTP bound to three bands in the 21-27-kDa range in a manner inhibited by GTP and GTP gamma S but not App(NH)p. ADP-ribosylation of myelin with [32P]NAD+ and cholera toxin labeled a protein of 43 kDa, whereas reaction with pertussis toxin labeled two components of 40 kDa. Cholate extract of myelin subjected to chromatography on a column of phenyl-Sepharose gave at least three major peaks of [35S]GTP gamma S binding activity. SDS-PAGE and immunoblot analyses of peak I indicated the presence of Go alpha, Gi alpha, and Gs alpha. Further fractionation of peak II by diethyl-aminoethyl-Sephacel chromatography gave one [35S]GTP gamma S binding peak with the low-molecular-mass (21-27 kDa) proteins and a second showing two major protein bands of 36 and 40 kDa on SDS-PAGE.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that a GTP binding protein regulates phosphorylation of the CD3 antigen in human T lymphocytes

The role of guanine nucleotide binding regulatory proteins (G proteins) in the regulation of phosphorylation of the gamma subunit of the CD3 antigen has been examined. CD3 gamma chain phosphorylation in isolated T cell microsomes was stimulated by the G protein activator guanosine 5'-0 thiotriphosphate (GTP gamma S), but cyclic adenosine monophosphate and guanosine 5'-diphosphate were ineffective at inducing gamma chain phosphorylation. The effect of GTP gamma S was rapid and transient; a half maximal effect was observed with 50 microM of the nucleotide. gamma polypeptide phosphorylated in vitro in GTP gamma S stimulated microsomes incorporated phosphate on Serines 123 and 126. These data are consistent with the involvement of a G protein in the signalling mechanisms that regulate the phosphorylation of the CD3 gamma chain.     

GTP binding proteins: a key role in cellular communication

One of the major steps in the understanding of the hormonal and sensory transduction mechanisms in eukaryotic cells has been the discovery of a family of GTP binding proteins which couple receptors to specific cellular effectors. The absolute requirement of GTP for hormonal stimulation of adenylate cyclase was the initial observation which led to the purification of the protein involved: Gs. Gs couples stimulatory receptors to adenylate cyclase. It is a heterotrimer composed of an alpha chain (45 or 52 kDa), a beta chain (35-36 kDa) and a gamma chain (8 kDa). Several other G proteins of known functions have been purified: Gi, which couples inhibitory receptors to adenylate cyclase, and transducin which couples photoexcited rhodopsin to cyclic GMP phosphodiesterase. Some G proteins of uncertain function have also been purified: Go, a G protein mainly localized in nervous tissues and Gp, a G protein isolated from placenta and platelets. All these G proteins have a common design. Like Gs they all consist of 3 chains: alpha, beta and gamma. The beta chains are nearly identical, whereas the gamma chains are more variable. The alpha chains are different, but share common domains (especially at the level of the GTP binding site). These domains of homologies are also similar to those of other GTP binding proteins, such as the product of the ras gene (p21) and the initiation or elongation factors. alpha Chains are also ADP ribosylated by bacterial toxins. Gs and transducin are targets for cholera toxin, whereas Gi, Go and transducin are targets for pertussis toxin.(ABSTRACT TRUNCATED AT 250 WORDS)     

G protein-effector coupling: binding of rod phosphodiesterase inhibitory subunit to transducin

The cyclic GMP phosphodiesterase of retinal rods is composed of three distinct polypeptides: alpha (90 kDa), beta (86 kDa), and gamma (10 kDa). In this multimeric form, the enzyme is inhibited. Its activity is stimulated by the interaction with the GTP-bound form of the T alpha subunit of transducin and reversed upon the recombination of the inhibitory gamma subunit with the catalytic alpha beta subunit. We show here by a novel coimmunoprecipitation technique that the gamma subunit, but not the alpha beta subunit, forms a 1:1 complex with T alpha. The binding of gamma to T alpha is nucleotide-dependent and is facilitated by GTP gamma S or Gpp(NH)p. This study provides convincing evidence that the T alpha-GTP subunit of transducin stimulates phosphodiesterase activity by binding to gamma and physically carrying it away from alpha beta.     

Stimulation by GTP-binding proteins (Gi, Go) of partially purified phospholipase C activity from human platelet membranes

A phospholipase C exhibiting preferential hydrolytic activity for polyphosphoinositides was partially purified from the deoxycholate extract of human platelet membranes by Q-Sepharose and Heparin-Sepharose column chromatographies. The activity of this purified phospholipase C free of the GTP gamma S-binding activity was stimulated at a similar level by addition of purified rat brain Gi or Go. These results suggest that GTP-binding proteins may interact directly with a solubilized membrane phospholipase C to stimulate its activity.     

Cyclic AMP-Dependent Protein Phosphorylation in Chemosensory Neurons: Identification of Cyclic Nucleotide-Regulated Phosphoproteins in Olfactory Cilia

Chemosensory dendritic membranes (olfactory cilia) contain protein kinase activity that is stimulated by cyclic AMP and more efficiently by the nonhydrolyzable GTP analog guanosine-5'-O-(3-thio)triphosphate (GTP gamma S). In control nonsensory (respiratory) cilia, the cyclic AMP-dependent protein kinase is practically GTP gamma S-insensitive. GTP gamma S activation of the olfactory enzyme appears to be mediated by a stimulatory GTP-binding protein (G-protein) and adenylate cyclase previously shown to be enriched in the sensory membranes. Protein kinase C activity cannot be detected in the chemosensory cilia preparation under the conditions tested. Incubation of olfactory cilia with [gamma-32P]ATP leads to the incorporation of [32P]phosphate into many polypeptides, four of which undergo covalent modification in a cyclic nucleotide-dependent manner. The phosphorylation of one polypeptide, pp24, is strongly and specifically enhanced by cyclic AMP at concentrations lower than 1 microM. This phosphoprotein is not present in respiratory cilia, but is seen also in membranes prepared from olfactory neuroepithelium after cilia removal. Cyclic AMP-dependent protein kinase and phosphoprotein pp24 may be candidate components of the molecular machinery that transduces odor signals.