Structural and functional relationships of guanosine triphosphate binding proteins

WNT/Frizzled signalling: receptor-ligand selectivity with focus on FZD-G protein signalling and its physiological relevance: IUPHAR Review 3

The wingless/int1 (WNT)/Frizzled (FZD) signalling pathway controls numerous cellular processes such as proliferation, differentiation, cell-fate decisions, migration and plays a crucial role during embryonic development. Nineteen mammalian WNTs can bind to 10 FZDs thereby activating different downstream pathways such as WNT/β-catenin, WNT/planar cell polarity and WNT/Ca(2+) . However, the mechanisms of signalling specification and the involvement of heterotrimeric G proteins are still unclear. Disturbances in the pathways can lead to various diseases ranging from cancer, inflammatory diseases to metabolic and neurological disorders. Due to the presence of seven-transmembrane segments, evidence for coupling between FZDs and G proteins and substantial structural differences in class A, B or C GPCRs, FZDs were grouped separately in the IUPHAR GPCR database as the class FZD within the superfamily of GPCRs. Recently, important progress has been made pointing to a direct activation of G proteins after WNT stimulation. WNT/FZD and G protein coupling remain to be fully explored, although the basic observation supporting the nature of FZDs as GPCRs is compelling. Because the involvement of different (i) WNTs; (ii) FZDs; and (iii) intracellular binding partners could selectively affect signalling specification, in this review we present the current understanding of receptor/ligand selectivity of FZDs and WNTs. We pinpoint what is known about signalling specification and the physiological relevance of these interactions with special emphasis on FZD-G protein interactions.

Molecular basis of cannabinoid CB1 receptor coupling to the G protein heterotrimer Gαiβγ: identification of key CB1 contacts with the C-terminal helix α5 of Gαi

The cannabinoid (CB1) receptor is a member of the rhodopsin-like G protein-coupled receptor superfamily. The human CB1 receptor, which is among the most expressed receptors in the brain, has been implicated in several disease states, including drug addiction, anxiety, depression, obesity, and chronic pain. Different classes of CB1 agonists evoke signaling pathways through the activation of specific subtypes of G proteins. The molecular basis of CB1 receptor coupling to its cognate G protein is unknown. As a first step toward understanding CB1 receptor-mediated G protein signaling, we have constructed a ternary complex structural model of the CB1 receptor and Gi heterotrimer (CB1-Gi), guided by the x-ray structure of β2-adrenergic receptor (β2AR) in complex with Gs (β2AR-Gs), through 824-ns duration molecular dynamics simulations in a fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer environment. We identified a group of residues at the juxtamembrane regions of the intracellular loops 2 and 3 (IC2 and IC3) of the CB1 receptor, including Ile-218(3.54), Tyr-224(IC2), Asp-338(6.30), Arg-340(6.32), Leu-341(6.33), and Thr-344(6.36), as potential key contacts with the extreme C-terminal helix α5 of Gαi. Ala mutations of these residues at the receptor-Gi interface resulted in little G protein coupling activity, consistent with the present model of the CB1-Gi complex, which suggests tight interactions between CB1 and the extreme C-terminal helix α5 of Gαi. The model also suggests that unique conformational changes in the extreme C-terminal helix α5 of Gα play a crucial role in the receptor-mediated G protein activation.

Homogeneous GTP binding assay employing QRET technology

Functional cell signaling assays have become important tools for measuring ligand-induced receptor activation in cell-based biomolecular screening. Guanosine-5'-triphosphate (GTP) is a generic signaling marker responsible for the first intracellular signaling event of the G-protein-coupled receptors (GPCRs). [(35)S]GTPgammaS binding assay is the classical well-established method for measuring agonist-induced G-protein activation requiring a separation of free and bound fractions prior to measurement. Here a novel, separation-free, time-resolved fluorescence GTP binding assay has been developed based on a non-fluorescence resonance energy transfer (FRET) single-label approach and quenching of a nonbound europium-labeled, nonhydrolyzable GTP analog (Eu-GTP). The quenching resonance energy transfer (QRET) method relies on the use of Eu-GTP, providing a time-resolved fluorescent detection as an alternative to the radiolabel [(35)S]GTPgammaS assay. Upon activation of recombinant human alpha(2A)-adrenoceptors (alpha(2A)-AR) expressed in Chinese hamster ovary cells, guanosine-5'-diphosphate is released from the alpha-subunit of Gi-proteins, enabling the subsequent binding of Eu-GTP. Activation of alpha(2A)-AR with 5 different alpha(2)-AR agonists was measured quantitatively using the developed QRET GTP assay and compared to [(35)S]GTPgammaS and heterogeneous Eu-GTP filtration assays. Equal potencies and efficacy rank orders were observed in all 3 assays but with a lower signal-to-background ratio and increased assay variation in the QRET assay compared to the Eu-GTP filtration and the nonhomogeneous [(35)S]GTPgammaS binding assays.

Antifungal protein PAF severely affects the integrity of the plasma membrane of Aspergillus nidulans and induces an apoptosis-like phenotype

The small, basic, and cysteine-rich antifungal protein PAF is abundantly secreted into the supernatant by the beta-lactam producer Penicillium chrysogenum. PAF inhibits the growth of various important plant and zoopathogenic filamentous fungi. Previous studies revealed the active internalization of the antifungal protein and the induction of multifactorial detrimental effects, which finally resulted in morphological changes and growth inhibition in target fungi. In the present study, we offer detailed insights into the mechanism of action of PAF and give evidence for the induction of a programmed cell death-like phenotype. We proved the hyperpolarization of the plasma membrane in PAF-treated Aspergillus nidulans hyphae by using the aminonaphtylethenylpyridinium dye di-8-ANEPPS. The exposure of phosphatidylserine on the surface of A. nidulans protoplasts by Annexin V staining and the detection of DNA strand breaks by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) gave evidence for a PAF-induced apoptotic-like mechanism in A. nidulans. The localization of reactive oxygen species (ROS) by dichlorodihydrofluorescein diacetate and the abnormal cellular ultrastructure analyzed by transmission electron microscopy suggested that ROS-elicited membrane damage and the disintegration of mitochondria played a major role in the cytotoxicity of PAF. Finally, the reduced PAF sensitivity of A. nidulans strain FGSC1053, which carries a dominant-interfering mutation in fadA, supported our assumption that G-protein signaling was involved in PAF-mediated toxicity.

Reduced number and coupling of beta-adrenergic receptors in a modified S49 mouse lymphoma cell line

Long-term culture of S49 wild-type cells in medium containing a high concentration of fetal calf serum leads to a modified (S49m) cell line with a reduced number of beta-adrenergic receptors (R). These S49m cells with a higher rate of proliferation were unable to respond to the beta-adrenergic agonists isoproterenol (ISO) and epinephrine as analysed by measuring adenylate cyclase (ac) activity on purified membranes of these cells. Additionally, no accumulation of cyclic AMP was obtained on S49m intact cells upon stimulation with beta-agonists. Nevertheless, S49m cells were able to respond significantly to the direct activation of the stimulatory guanine nucleotide binding (Gs) protein by aluminium tetrafluoride and sodium fluoride, and to the stimulation of another receptor coupled to the ac system through a Gs protein, by prostaglandin E1 (PGE1). When cloning S49m cells, similar results were obtained upon stimulation with ISO and PGE1 and the cloned cells express the same thy 1.2 and class Id molecules as do S49 cells. The study of S49m cells indicates that they are a beta-adrenergic R-deficient variant distinct from the other variants described for S49 cells.

The effect of GTP on the aluminum fluoride- and forskolin-activated adenylyl cyclase from human embryonic kidney 293 cells

GTP has been shown to inhibit AlF4(-)-stimulated, and to activate forskolin-stimulated adenylyl cyclase activity in the presence of Mg2+ in cell membranes from human embryonic kidney 293 cells. The maximal inhibitory response of AlF4(-)-stimulated adenylyl cyclase activity by GTP was not dependent on the concentration of Mg2+, but was so in the case of forskolin-activated activity at all forskolin concentrations assayed. Mn2+ ions stimulated AlF4(-)- or forskolin-activated adenylyl cyclase activity to a greater extent than Mg2+. The inhibition of AlF4(-)-stimulated cyclase by GTP was still observed with Mn2+, but the activation of forskolin-stimulated cyclase by GTP was not. When assayed together, Mn2+ and Mg2+ showed non-additive behaviours with respect to the amount of cyclic AMP formed after AlF4(-)-stimulation of adenylyl cyclase. The temperature dependence of the activation of adenylyl cyclase by forskolin, AlF4- or under basal conditions was observed to be somehow different in the presence of Mn2+ than in the presence of Mg2+ ions. Cholera toxin treatment produced a markedly increased cyclase activity, specially when assayed with AlF4-. In the case of forskolin-activated adenylyl cyclase, UTP and CTP were unable to reproduce the cyclase activation detected with GTP. However, in the case of AlF4(-)-stimulated adenylyl cyclase, UTP was as good as GTP at inhibiting cyclase activity, and CTP virtually eliminated the activation of the cyclase with AlF4-.

Mutation of a putative ADP-ribosylation motif in the Pasteurella multocida toxin does not affect mitogenic activity

Pasteurella multocida toxin (PMT) is a potent mitogen for Swiss 3T3 fibroblasts and cytotoxic to embryonic bovine lung cells. Site-directed mutagenesis was used to investigate the functional significance of a three amino acid motif in PMT that is present in five other bacterial protein toxins which exhibit ADP-ribosyl transferase activity. Crude lysates of mutant clones were fully cytotoxic for embryonic bovine lung cells. Purified mutant toxin was also as effective at stimulating inositol phosphate turnover and nucleic acid synthesis as wild type toxin. We conclude that this motif has no functional significance in Pasteurella multocida toxin.

Photoaffinity labelling of cardiac membrane GTP-binding proteins in response to insulin

Plasma membranes from rat cardiac ventricular tissue and insulin receptors partially purified by wheat-germ-agglutinin chromatography were subjected to direct photoaffinity labelling with [alpha-32P]GTP in order to elucidate the presence of insulin-receptor-coupled GTP-binding proteins. In plasma membranes three proteins have been identified that exhibit an enhanced photolabelling with the nucleotide in response to insulin. The apparent molecular masses of these proteins were found to be 56, 60 and 74 kDa. Photolabelling of partially purified insulin receptors showed the copurification of the 60-kDa species, whereas the 56-kDa and 74-kDa proteins could not be detected. Furthermore, the 60-kDa G-protein was found to be specifically co-immunoprecipitated with the insulin receptor. Incubation of insulin receptors with insulin increased the labelling of the 60-kDa band to 205 +/- 27% (n = 5) of control. Immuno- and ligand-blotting experiments revealed the additional presence of a 39-kDa G(o)-like protein and two G-proteins with molecular masses of 24 and 26 kDa in the receptor preparation. Under basal conditions the insulin receptor and the 60-kDa G-protein exhibited an apparent inverse distribution between plasma and microsomal membranes with the G-protein being extensively labelled in the microsomal fraction. In conclusion, our data show that, in its native environment, the cardiac insulin receptor couples to at least three GTP-binding proteins. Out of these, a 60-kDa species of microsomal origin, copurifies with the insulin receptor. It is suggested that this G-protein is associated with the insulin receptor and may be involved in insulin receptor signalling in target cells.

A comparative analysis of the time course of cardiac Ca2+ current response to rapid applications of beta-adrenergic and dihydropyridine agonists

A fast perfusion system was used to analyze the kinetics of the response of L-type calcium current (ICa) to rapid exposures to beta-adrenergic or dihydropyridine agonists in whole-cell patch-clamped frog ventricular myocytes. The perfusion system was based on the lateral motion of an array of plastic capillary tubes from which solutions flowed at a velocity of approximately 5 cm/s. Movement from one capillary to the adjacent one occurred in < 20 ms and complete exchange of extracellular solution was achieved in < 50 ms as demonstrated by the block of ICa by fastflow application of Cd during a depolarizing pulse. Fastflow applications of increasing concentrations of isoprenaline (Iso) led to a dose-dependent stimulation of ICa at [Iso] > 1 nM. The response of ICa to Iso always started after a delay of several seconds. The delay duration decreased as [Iso] increased, and was typically approximately 3 s at 10 microM Iso. The rising phase of ICa increase was monophasic and independent of [Iso] > 100 nM. For short applications of Iso (8.8 s), half maximal and maximal stimulation of ICa occurred approximately 20 s and approximately 40 s after the beginning of Iso application, respectively. When Iso was applied during a depolarizing pulse (with Ba as the charge carrier), IBa never increased during that pulse. The kinetics of the ICa response to Iso were not affected by varying the voltage clamp protocols or the ionic composition of intracellular and extracellular solutions. In comparison with the effects of Iso, the stimulatory effect of the dihydropyridine agonist (-)Bay K 8644 on ICa was approximately 15 times faster: delay, half-time to maximal and time to maximal responses were 15 times shorter with (-)Bay K 8644 than with Iso. It is concluded that frog ventricular myocytes respond slowly to a quick application of beta-adrenergic agonists.

Four-state models and regulation of contraction of smooth muscle. I. Physical considerations, stability, and solutions

Cyclic four-state models are frequently used in biology to represent a variety of molecular behaviors. A common experimental strategy to test such models is to follow the behavior of the real system after some of the rate constants are changed in a stepwise manner. We analyze the mathematical behavior of a simple example of such a model applicable to the regulation of contraction of smooth muscle, but our results apply in general to any linear, cyclic four-state model. We discuss detailed balance and requirements for linearity. We find that the only way to have sustained oscillations is for the rate constants of the model to be themselves oscillatory. We state conditions for decaying oscillations and find that in models that do not follow strictly first-order kinetics and do not satisfy detailed balance, these conditions can hold. We show analytically that the response of any state to step changes in the rate constants is the sum of three weighted exponentials plus a constant term, the steady-state value. We provide explicit expressions for the time dependence of all state variables. We discuss a simple way to use these results to obtain numerical solutions in cases where the rate constants change in an arbitrary way.

Plasma-membrane-independent pool of the alpha subunit of the stimulatory guanine-nucleotide-binding regulatory protein in a low-density-membrane fraction of S49 lymphoma cells

We report that compartmentalisation of the stimulatory guanine-nucleotide-binding regulatory protein (Gs) exists in S49 lymphoma cells. In addition to the previously reported cytosolic form of the alpha subunit of Gs (Gs alpha) [Ransnäs, L. A., Svoboda P., Jasper, J. R. & Insel, P. A. (1989) Proc. Natl Acad. Sci. USA 86, 7900-7903], three membrane-bound forms of Gs alpha were identified through rate-zonal centrifugation in sucrose density gradients, Gs alpha-specific anti-peptide serum and an adenylate cyclase complementation assay. The sedimentation profile of the first pool of Gs alpha in the high-density portion of the gradient (1.13-1.16 g/cm3) is identical with that of beta-adrenergic-receptor binding, Na/K-ATPase and adenylate cyclase activity, and may therefore be identified as plasma-membrane fragments. The second pool, which was recovered in the middle portion of the gradient (1.09-1.11 g/cm3), contains a much lower total amount of Gs alpha and correlates with the endoplasmic reticulum (microsomal) enzyme markers, NADPH-cytochrome-c reductase and glucose-6-phosphatase. The identity of the third pool of Gs alpha located at the top of the gradient (1.06-1.08 g/cm3), is unknown. The Golgi apparatus marker, UDPgalactose:N-acetylglucosamine glycosyltransferase, was partially recovered in this area; however, this enzyme was also present in the high-density portion of the gradient. Complete absence of specific adenylate cyclase and Na/K-ATPase activity indicates that this low-density (light) membrane form of Gs alpha is distinct from any plasma-membrane fragments. Furthermore, sedimentation at 100,000 x g proves its particulate (membrane) character. The light membrane form of Gs alpha subunit is functionally active in an adenylate cyclase complementation assay using cyc- membranes devoid of Gs alpha. Overall, our data indicates that a substantial portion of Gs alpha is localized in membrane pools other than plasma membrane.

ADP-ribosylation of small GTP-binding proteins by Bacillus cereus

A human pathogenic strain of Bacillus cereus produces an exoenzyme which selectively ADP-ribosylates 20-25 kDa GTP-binding proteins in platelet membranes. Pre-ADP-ribosylation of rho proteins of human platelet membranes with Clostridium botulinum exoenzyme C3 or Clostridium limosum exoenzyme inhibits subsequent ADP-ribosylation by the exoenzyme from B. cereus indicating similar substrate specificity of the transferases. The ADP-ribosyltransferase from B. cereus reveals no immunological cross-reactivity with C. botulinum C3 and C. limosum exoenzyme.

Regulation of adenylyl cyclase from Paramecium by an intrinsic potassium conductance

Hyperpolarization of the cell membrane of Paramecium stimulates adenosine 3',5'-monophosphate (cAMP) formation. Manipulations of the K+ resting conductance of the ciliate by adaptation in different buffers affected excitability of the cAMP generating system. Blockade of K+ channels inhibited hyperpolarization-stimulated cAMP formation. A mutant of Paramecium that is unable to control its K+ resting conductance had a defect in cAMP formation. Purified adenylyl cyclase, when incorporated into an artificial lipid bilayer membrane, revealed properties of a voltage-independent K+ channel. This indicates that the adenylyl cyclase of Paramecium has a secondary function as carrier of the K+ resting conductance. A hyperpolarization-activated K+ efflux appears to directly regulate adenylyl cyclase activity in vivo.

Lithium administration modulates platelet Gi in humans

Platelet G proteins were assessed in 7 normal volunteers before and after 14 days of lithium administration at therapeutic plasma levels. Cholera and pertussis toxin catalyzed ADP-ribosylation of platelet membrane proteins were measured by SDS-PAGE. Immunoblotting with specific antibodies was used to measure platelet membrane alpha i content. There was a statistically significant 37% increase in pertussis toxin mediated ADP-ribosylation of a 40,000 Mr protein in platelet membranes after lithium administration, but cholera toxin mediated ADP-ribosylation of a 45,000 Mr protein and alpha i immunoblotting were unchanged by lithium. Increased pertussis toxin stimulated ADP-ribosylation in the absence of changes in alpha i content could be explained by a shift in platelet Gi in favor of its undissociated, inactive form. This would be consistent with increased platelet adenylyl cyclase activity found in these same subjects after lithium.

Clostridium botulinum C3 ADP-ribosyltransferase

C3 and C3-like ADP-ribosyltransferases modify the low-molecular-mass GTP-binding proteins Rho and Rac. ADP-ribosylation occurs in asparagine-41, which is located in the putative effector region of these highly conserved regulatory proteins. First studies indicate that the Rho proteins are somehow involved in the regulation of cytoskeletal proteins, e.g., microfilament proteins. Although the precise mechanism of the interaction of the C3 substrate with cytoskeletal elements is unclear, it appears that the ADP-ribosylation by C3 renders the GTP-binding protein biologically inactive. Thus C3 and/or C3-like ADP-ribosyltransferases may be useful instruments with which to study the physiological functions of its eukaryotic substrates. Moreover, those studies may help to elucidate whether these exoenzymes are of pathophysiological and pathogenetic relevance in diseases caused by clostridia producing these agents.

Regulation of IMP dehydrogenase gene expression by its end products, guanine nucleotides

To study the regulation of IMP dehydrogenase (IMPDH), the rate-limiting enzyme of guanine nucleotide biosynthesis, we examined the effects of nucleosides, nucleotides, nucleotide analogs, or the IMPDH inhibitor mycophenolic acid (MPA) on the steady-state levels of IMPDH mRNA. The results indicated that IMPDH gene expression is regulated inversely by the intracellular level of guanine ribonucleotides. We have shown that treatment with guanosine increased the level of cellular guanine ribonucleotides and subsequently reduced IMPDH steady-state mRNA levels in a time- and dose-dependent manner. Conversely, MPA treatment diminished the level of guanine ribonucleotides and increased IMPDH mRNA levels. Both of these effects on the steady-state level of IMPDH mRNA could be negated by cotreatment with guanosine and MPA. The down regulation of IMPDH gene expression by guanosine or its up regulation by MPA was not due to major changes in transcriptional initiation and elongation or mRNA stability in the cytoplasm but rather was due to alterations in the levels of the IMPDH mRNA in the nucleus. These results suggest that IMPDH gene expression is regulated by a posttranscriptional, nuclear event in response to fluctuations in the intracellular level of guanine ribonucleotides.

A novel approach to detect toxin-catalyzed ADP-ribosylation in intact cells: its use to study the action of Pasteurella multocida toxin

Certain microbial toxins are ADP-ribosyltransferases, acting on specific substrate proteins. Although these toxins have been of great utility in studies of cellular regulatory processes, a simple procedure to directly study toxin-catalyzed ADP-ribosylation in intact cells has not been described. Our approach was to use [2-3H]adenine to metabolically label the cellular NAD+ pool. Labeled proteins were then denatured with SDS, resolved by PAGE, and detected by flurography. In this manner, we show that pertussis toxin, after a dose-dependent lag period, [3H]-labeled a 40-kD protein intact cells. Furthermore, incubation of the gel with trichloroacetic acid at 95 degrees C before fluorography caused the release of label from bands other than the pertussis toxin substrate, thus, allowing its selective visualization. The modification of the 40-kD protein was ascribed to ADP-ribosylation of a cysteine residue on the basis of inhibition of labeling by nicotinamide and the release of [3H]ADP-ribose from the labeled protein by mercuric acetate. Cholera toxin catalyzed the [3H]-labeling of a 46-kD protein in the [2-3H]adenine-labeled cells. Pretreatment of the cells with pertussis toxin before the labeling of NAD+ with [2-3H]adenine blocked [2-3H]ADP-ribosylation catalyzed by pertussis toxin, but not that by cholera toxin. Thus, labeling with [2-3H]adenine permits the study of toxin-catalyzed ADP-ribosylation in intact cells. Pasteurella multocida toxin has recently been described as a novel and potent mitogen for Swiss 3T3 cell and acts to stimulate the phospholipase C-mediated hydrolysis of polyphosphoinositides. The basis of the action of the toxin is not known. Using the methodology described here, P. multocida toxin was not found to act by ADP-ribosylation.

Purification and characterization of a GTP-binding protein serving as pertussis toxin substrate in starfish oocytes

In response to a meiosis-inducing hormone, 1-methyladenine (1-MA), starfish oocytes undergo reinitiation of meiosis with germinal vesicle breakdown. The 1-MA-initiated signal is, however, inhibited by prior microinjection of pertussis toxin into the oocytes (Shilling, F., Chiba, K., Hoshi, M., Kishimoto, T., and Jaffe, L.A. (1989) Dev. Biol. 133, 605-608), suggesting that a pertussis-toxin-sensitive guanine-nucleotide-binding protein (G protein) is involved in the 1-MA-induced signal transduction. Based on these findings, we purified a G protein serving as the substrate of pertussis toxin from the plasma membranes of starfish oocytes. The purified G protein had an alpha beta gamma-trimeric structure consisting of 39-kDa alpha, 37-kDa beta, and 8-kDa gamma subunits. The 39-kDa alpha subunit contained a site for ADP-ribosylation catalyzed by pertussis toxin. The alpha subunit was also recognized by antibodies specific for a common GTP-binding site of many mammalian alpha subunits or a carboxy-terminal ADP-ribosylation site of mammalian inhibitory G-alpha. An antibody raised against mammalian 36-/35-kDa beta subunits strongly reacted with the 37-kDa beta subunit of starfish G protein. The purified starfish G protein had a GTP-binding activity with a high affinity and displayed a low GTPase activity. The activity of the G protein serving as the substrate for pertussis-toxin-catalyzed ADP-ribosylation was inhibited by its association with a non-hydrolyzable GTP analogue. Thus, the starfish G protein appeared to be similar to mammalian G proteins at least in terms of its structure and properties of nucleotide binding and the pertussis toxin substrate. A possible role of the starfish G protein is also discussed in the signal transduction between 1-MA receptors and reinitiation of meiosis with germinal vesicle breakdown.

Regulation of IMP dehydrogenase gene expression by its end products, guanine nucleotides

To study the regulation of IMP dehydrogenase (IMPDH), the rate-limiting enzyme of guanine nucleotide biosynthesis, we examined the effects of nucleosides, nucleotides, nucleotide analogs, or the IMPDH inhibitor mycophenolic acid (MPA) on the steady-state levels of IMPDH mRNA. The results indicated that IMPDH gene expression is regulated inversely by the intracellular level of guanine ribonucleotides. We have shown that treatment with guanosine increased the level of cellular guanine ribonucleotides and subsequently reduced IMPDH steady-state mRNA levels in a time- and dose-dependent manner. Conversely, MPA treatment diminished the level of guanine ribonucleotides and increased IMPDH mRNA levels. Both of these effects on the steady-state level of IMPDH mRNA could be negated by cotreatment with guanosine and MPA. The down regulation of IMPDH gene expression by guanosine or its up regulation by MPA was not due to major changes in transcriptional initiation and elongation or mRNA stability in the cytoplasm but rather was due to alterations in the levels of the IMPDH mRNA in the nucleus. These results suggest that IMPDH gene expression is regulated by a posttranscriptional, nuclear event in response to fluctuations in the intracellular level of guanine ribonucleotides.

Beta-adrenoceptor distribution in murine lymphoid cell lines

beta-Adrenergic receptors (R) on several tumor lymphoid cell lines were characterized both directly by beta radioligand binding of 125iodo-cyanopindolol (125I-CYP) to intact cells and membranes, and functionally by assessing hormone-dependent changes in cyclic 3',5' adenosine monophosphate (cAMP) levels on intact cells and measuring adenylate cyclase (a.c.) activity on membranes. Only two lymphoid cell types, BW 5147 (a T cell derived lymphoma cell line) and TIB 221 (a B cell derived line) displayed significant amounts of beta-adrenergic R by 125I-CYP specific binding. Despite this, no stimulation of the a.c. activity was found in the presence of beta-adrenergic agonists in these cells in comparison with native lymphocytes or cells of the well-known S49 cell line used as a positive control. beta-Adrenoceptor specific uncoupling was confirmed by aluminum tetrafluoride (AlFl4) direct activation of the a.c. system in the beta R-bearing cell membranes and by an increase in cAMP production induced by PGE1, another hormone that activates the a.c. Structural characterization of beta-adrenoceptors by photoaffinity-labeling demonstrates that uncoupling was not due to a structural alteration of the beta-adrenergic R expressed in these lymphoma cell lines, as these R gave similar results as native or S49 cells. It can be concluded that functional beta-adrenoceptors are absent in these lymphoma cells. The possible implication of alternative transmission pathways and original neuroendocrine control in tumor lymphoid cells is discussed.

Interaction of mastoparan with the low molecular mass GTP-binding proteins rho/rac

Mastoparan, which has been shown to active G proteins, inhibits the ADP-ribosylation of 20 kDa human platelet membrane proteins catalyzed by Clostridium botulinum exoenzyme C3 half-maximally and maximally (90%) at 20 and 100 microM concentrations, respectively. Inhibition of ADP-ribosylation was enhanced by GTP-gamma S. Mastoparan increased GTP hydrolysis by porcine brain rho protein and stimulated GTP binding in a concentration dependent manner. The data suggest that mastoparan not only interacts with heterotrimeric G proteins but also with low molecular mass GTP-binding proteins of the rho/rac family.

Chemical and functional analysis of components of adenylyl cyclase from human platelets treated with phorbolesters

Human platelets, prelabeled with [32P]phosphate were treated with tetradecanoylphorbol acetate (TPA) for 5 min at 37 degrees C. Phosphorylation of the components of adenylyl cyclase was determined in membranes using specific antibodies against G-proteins and the catalytic moiety. Less than 0.01 mol of [32P]phosphate/mol could be detected in immunoprecipitates using antibodies against sequences within the alpha-subunit of the GTP binding protein Gi. TPA, however, caused the incorporation of 0.67-1.1 mol of [32P]phosphate per mol of catalyst while 0.13-0.2 mol were found in the absence of TPA. Lack of modification of the alpha-subunit of Gi was also indicated by the results of reconstitution experiments with purified Gi alpha from bovine brain: adenylyl cyclase in membranes from untreated platelets was significantly more inhibited by added G1 alpha, than that from TPA treated cells. While beta, gamma-subunits were like-wise inhibitory no difference dependent on platelet-pretreatment could be observed.

Structural heterogeneity of membrane receptors and GTP-binding proteins and its functional consequences for signal transduction

Recent information obtained, mainly by recombinant cDNA technology, on structural heterogeneity of hormone and transmitter receptors, of GTP-binding proteins (G-proteins) and, especially, of G-protein-linked receptors is reviewed and the implications of structural heterogeneity for diversity of hormone and transmitter actions is discussed. For the future, three-dimensional structural analysis of membrane proteins participating in signal transmission and transduction pathways is needed in order to understand the molecular basis of allosteric regulatory mechanisms governing the interactions between these proteins including hysteretic properties and cell-cybernetic aspects.

Myocardial beta-adrenergic adenylate cyclase complex in a canine model of chagasic cardiomyopathy

Infection of beagles with an opossum-derived strain of Trypanosoma cruzi (Tc-O) results in features of early and chronic chagasic cardiomyopathy, that is, increases in PR interval, atrioventricular block, and frequent ventricular premature contractions, ventricular tachycardia, and decreased left ventricular ejection fraction. These signs are not observed in animals infected with a canine strain of T. cruzi (Tc-D). To understand the biochemical basis for these early cardiac effects, we examined the beta-adrenergic adenylate cyclase complex in myocardial membranes prepared from animals infected with either of the two strains. In animals infected with Tc-O (symptomatic), the maximum velocity (Vmax) decreased and concentration of agonist resulting in 50% of Vmax (Kact) increased for isoproterenol-dependent adenylate cyclase activity; in animals infected with Tc-D (asymptomatic), Vmax and Kact for isoproterenol were unchanged from control, uninfected animals. beta-Receptor density decreased by 20% in symptomatic animals with no change in affinity, whereas no differences were observed between uninfected and infected asymptomatic animals. A complex pattern of changes was apparent in the guanine nucleotide binding protein, Gs, in the setting of infection. Alterations in cholera toxin-dependent ADP-ribosylation patterns as well as immunochemical detection with anti-G alpha s antisera suggested a change in the biochemical nature of the Gs species and not necessarily a physical loss of this protein. Reconstitution of adenylate cyclase activity in cyc- membranes demonstrated a decrease in hormone-sensitive Gs activity in membranes prepared from symptomatic animals without a change in activity demonstrable in the presence of Gpp(NH)p. Collectively, the results suggest that the depression in beta-adrenergic adenylate cyclase activity associated with symptomatic infection of beagles with T. cruzi occurs primarily as a result of changes in the Gs protein complex, most likely resulting in an uncoupling of the beta-adrenergic receptor from the Gs protein.

Somatostatin stimulates Ca(2+)-activated K+ channels through protein dephosphorylation

The neuropeptide somatostatin inhibits secretion from electrically excitable cells in the pituitary, pancreas, gut and brain. In mammalian pituitary tumour cells somatostatin inhibits secretion through two distinct pertussis toxin-sensitive mechanisms. One involves inhibition of adenylyl cyclase, the other an unidentified cyclic AMP-independent mechanism that reduces Ca2+ influx by increasing membrane conductance to potassium. Here we demonstrate that the predominant electrophysiological effect of somatostatin on metabolically intact pituitary tumour cells is a large, sustained increase in the activity of the large-conductance Ca(2+)- and voltage-activated K+ channels (BK). This action of somatostatin does not involve direct effects of Ca2+, cAMP or G proteins on the channels. Our results indicate instead that somatostatin stimulates BK channel activity through protein dephosphorylation.

Diversity of G proteins in signal transduction

The heterotrimeric guanine nucleotide-binding proteins (G proteins) act as switches that regulate information processing circuits connecting cell surface receptors to a variety of effectors. The G proteins are present in all eukaryotic cells, and they control metabolic, humoral, neural, and developmental functions. More than a hundred different kinds of receptors and many different effectors have been described. The G proteins that coordinate receptor-effector activity are derived from a large gene family. At present, the family is known to contain at least sixteen different genes that encode the alpha subunit of the heterotrimer, four that encode beta subunits, and multiple genes encoding gamma subunits. Specific transient interactions between these components generate the pathways that modulate cellular responses to complex chemical signals.

ADP-ribosylation and de-ADP-ribosylation of the rho protein by Clostridium botulinum exoenzyme C3. Regulation by EDTA, guanine nucleotides and pH

Pretreatment of rho protein purified from pig brain cytosol with EDTA (3 mM) for 10 min at 30 degrees C inhibited its ADP-ribosylation by Clostridium botulinum C3 ADP-ribosyltransferase by more than 90%. The EDTA effect was not caused by alteration of C3. GDP or GDP beta S present during the pretreatment period completely prevented the decrease in ADP-ribosylation with half-maximal and maximal effects at 3 and 300 microM, respectively. GTP or GTP gamma S were less efficacious in preventing the decrease in ADP-ribosylation, but were more potent (half-maximal and maximal effects at 0.1 and 3 microM, respectively). [32P]ADP-ribose incorporated in pig brain rho by C3 was de-ADP-ribosylated by the enzyme in the presence of nicotinamide and at low pH. Concomitantly, [32P]NAD was formed. The pH optima for ADP-ribosylation and de-ADP-ribosylation were pH 7.5 and 5.5, respectively. De-ADP-ribosylation was most efficient with nicotinamide, less effective with 3-acetylpyridine and not observed with 3-aminopyridine, 4-aminopyridine, 4-acetylpyridine and isonicotinic acid. As observed for the ADP-ribosylation, the de-ADP-ribosylation by C3 was maximal with the GDP-bound form of rho and blocked after EDTA treatment.

Regulation of Ca2+ current in frog ventricular cardiomyocytes by 5'-guanylylimidodiphosphate and acetylcholine

1. Calcium currents (ICa) were measured in frog ventricular myocytes using the whole-cell patch clamp technique and a perfused pipette. The effect of internal perfusion with the hydrolysis-resistant GTP analogue, GppNHp (5'guanylylimidodiphosphate), on basal ICa and ICa stimulated with forskolin or isoprenaline was examined to gain insight into the role of G proteins in ICa regulation. 2. Without added guanine nucleotides, isoprenaline stimulated ICa approximately 14-fold with an EC50 of 0.09 microM. Forskolin stimulated ICa approximately 10-fold with an EC50 of 0.30 microM. 3. Internal 30 microM-GppNHp produced an approximately 80% decrease in ICa elevated by 0.3 microM-isoprenaline or 3 microM-forskolin. The inhibition of isoprenaline stimulation was due to a decrease in the maximal stimulation from approximately 14-fold to approximately 14-fold without a significant change in the EC50. In contrast, the reduction in forskolin stimulation was due to a 22-fold increase in the EC50 to 11.4 microM, with little change in maximal stimulation. 4. The inhibition of stimulated ICa by GppNHp is likely to be mediated by a G protein, because the effects of GppNHp are irreversible, and are blocked by excess GTP. ICa is affected similarly by GppNHp and by ACh. This suggests that GppNHp activates the same G protein that is normally activated by ACh, but activation by GppNHp occurs in the absence of agonist occupation of the muscarinic receptor. 5. The increase in the EC50 for forskolin produced by internal GppNHp was reversed by exposure to isoprenaline, which itself did not affect ICa amplitude. On average, exposure to isoprenaline in the presence of GppNHp caused an irreversible 81-fold decrease in the EC50 for forskolin to 0.14 microM. Stimulation of ICa by forskolin after internal GppNHp and exposure to isoprenaline was completely blocked by the protein kinase A inhibitor PKI(5-22). 6. These effects do not involve the phospholipase C system, because they are not mimicked by phorbol esters or internal inositol 1,4,5-trisphosphate (IP3) and are not blocked by bromophenacyl bromide or neomycin. 7. Direct effects of G proteins on ICa were not evident, because internal perfusion with PKI(5-22) completely inhibited isoprenaline- or forskolin-stimulated increases in ICa, and neither ACh nor internal GppNHp (30-500 microM) affected basal ICa or ICa elevated by internally perfused cyclic AMP. 8. These results suggest that the predominant site of action of the inhibitory G protein activated by either GppNHp or ACh is adenylyl cyclase. Furthermore, the internally perfused frog cardiomyocytes may provide a useful approach for probing the detailed interactions of G proteins, forskolin, and adenylyl cyclase in an intact cell.

Sequence analysis of the potent mitogenic toxin of Pasteurella multocida

Pasteurella multocida toxin is a potent mitogen for cultured Swiss 3T3 cells where it causes an accumulation of inositol phosphates and activation of protein kinase C. The gene sequence described here coded for a 146 kDa protein. The ORF was preceded by a ribosome binding site and followed by a stem loop. There was no evidence for a signal sequence. The gene had a low G + C base ratio which differs from the rest of the Pasteurella genome. There was no significant homology with other known proteins, although a motif found in certain bacterial toxins which are ADP-ribosyl transferases is present. A recombinant expressing only part of the PMT gene was not mitogenic.

Morphological alterations of Xenopus oocytes induced by valine-14 p21rho depend on isoprenylation and are inhibited by Clostridium botulinum C3 ADP-ribosyltransferase

Microinjection of the constitutively active recombinant Val-14 p21rho A into Xenopus oocytes induced dramatic morphological changes with redistribution of pigments from the animal pole resulting in spotted oocytes. The effects induced by Val-14 p21rho A were regulated by progesterone in a dose-dependent manner whereas prior ADP-ribosylation of the rho protein blocked its activity. About 30 min after microinjection, p21 rho was associated with the plasma membrane. The membrane association of p21rho and its biological activity were inhibited by lovastatin, an inhibitor of the 3-hydroxy-3-methylglutaryl coenzyme A reductase. The findings suggest that membrane attachment and biological activity of p21rho depend on isoprenylation of the GTP-binding protein.

ADP-ribosylation of actin isoforms by Clostridium botulinum C2 toxin and Clostridium perfringens iota toxin

The substrate specificities of the actin-ADP-ribosylating toxins, Clostridium botulinum C2 toxin and Clostridium perfringens iota toxin were studied by using five different preparations of actin isoforms: alpha-skeletal muscle actin, alpha-cardiac muscle actin, gizzard gamma-smooth muscle actin, spleen beta- and gamma-cytoplasmic actin, and aortic smooth muscle actin containing alpha- and gamma-smooth muscle actin isoforms. C. perfringens iota toxin ADP-ribosylated all actin isoforms tested, whereas C. botulinum C2 toxin did not modify alpha-skeletal muscle actin or alpha-cardiac muscle actin. Spleen beta/gamma-cytoplasmic actin and gizzard gamma-smooth muscle actin were substrates of C. botulinum C2 toxin. In the aortic smooth muscle actin preparation, gamma-smooth muscle actin but not alpha-smooth muscle actin was ADP-ribosylated by C. botulinum C2 toxin. The data indicate that, in contrast to C. perfringens iota toxin, C. botulinum C2 toxin ADP-ribosylates only beta/gamma-cytoplasmic and gamma-smooth muscle actin and suggest that the N-terminal region of actin isoforms define the substrate specificity for ADP-ribosylation by C. botulinum C2 toxin.

Purification and characterization of five different alpha subunits of guanine-nucleotide-binding proteins in bovine brain membranes. Their physiological properties concerning the activities of adenylate cyclase and atrial muscarinic K+ channels

We have purified five different alpha subunits of guanine-nucleotide-binding proteins (G proteins) from bovine brain membranes as active forms bound to guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]). All the purified alpha subunits were interacted with beta gamma subunits and served as a substrate for pertussin-catalyzed ADP-ribosylation. Based on the findings of immunoblot analyses using specific antibodies raised against various alpha subunits of G proteins, three of them were identified as alpha i-1, alpha i-2 and alpha i-3, and the other two were classified into alpha o type. One of the alpha o-type proteins was the most abundant in the brain membranes (termed alpha o), and the other (alpha o2) appeared to differ from alpha o in its proteolytic digestion data. The physiological properties of these purified GTP[gamma S]-bound alpha subunits towards adenylate cyclase and atrial muscarinic K+ channels were studied. The nucleotide-bound forms of alpha i-1, alpha i-2, alpha i-3 and alpha o2 inhibited the adenylate cyclase activity of S49 cyc- membranes which had been reconstituted with GTP[gamma S]-treated Gs; this inhibition appeared to be mainly competitive with the activated Gs, alpha i-1 having the most potent inhibitory activity among them. GTP[gamma S]-bound alpha o, however, could not inhibit the Gs-stimulated activity at all. On the other hand, all the GTP[gamma S]-bound alpha subunits activated atrial muscarinic K+ channels, accompanied by a lag time, at picomolar concentrations. The beta gamma subunits resolved from G proteins also activated the K+ channels without a lag time at nanomolar concentration. The maximum activation by the beta gamma subunits appeared to be more potent than that by any of the alpha subunits. These results suggest that alpha and beta gamma subunits might activate the K+ channels by mechanisms different from each other.

Isolation of a G protein that is modified by learning and reduces potassium currents in Hermissenda

In Hermissenda crassicornis conditioned to associate light and rotation, type B photoreceptor neurons exhibit pairing-specific decreases in the potassium currents IA and IK-Ca, which account for many of the behavioral changes elicited by associative conditioning. To determine which proteins are involved in storage of this memory, high-performance liquid chromatography was used to examine proteins from Hermissenda eyes. Conditioning-specific changes in four phosphoproteins were observed 24 hours after conditioning. One of these proteins, cp20, was purified to apparent homogeneity and found to be a G protein. When injected back into Hermissenda type B cells, cp20 reduced IK and IK-Ca in a manner indistinguishable from the reduction caused by conditioning, suggesting that this protein may play a crucial role in memory acquisition or retention.

Isolation of a G protein that is modified by learning and reduces potassium currents in Hermissenda

In Hermissenda crassicornis conditioned to associate light and rotation, type B photoreceptor neurons exhibit pairing-specific decreases in the potassium currents IA and IK-Ca, which account for many of the behavioral changes elicited by associative conditioning. To determine which proteins are involved in storage of this memory, high-performance liquid chromatography was used to examine proteins from Hermissenda eyes. Conditioning-specific changes in four phosphoproteins were observed 24 hours after conditioning. One of these proteins, cp20, was purified to apparent homogeneity and found to be a G protein. When injected back into Hermissenda type B cells, cp20 reduced IK and IK-Ca in a manner indistinguishable from the reduction caused by conditioning, suggesting that this protein may play a crucial role in memory acquisition or retention.

Cell differentiation and altered IMP dehydrogenase expression induced in human T-lymphoblastoid leukemia cells by mycophenolic acid and tiazofurin

The IMP dehydrogenase inhibitors mycophenolic acid (MPA) and tiazofurin (TZ) induce a time- and dose-dependent inhibition of cell growth, as well as differentiation in T-lymphoid CEM-2 leukemia cells. The differentiated cells have acquired a suppressor/cytotoxic T-lymphocyte phenotype characterized by reactivity with maturation-specific monoclonal antibodies. Coadministration of guanosine and hypoxanthine reduces the growth inhibition and diminishes the induction of differentiation by either MPA or TZ. No such reduction was observed for differentiation induced by phorbol 12-myristate 13-acetate (PMA), another inducer of a suppressor/cytotoxic phenotype in CEM-2 cells. During the first 2 days of treatment with MPA or TZ, a pattern of stable IMPDH mRNA levels and increased amounts of cellular enzyme was observed, perhaps, because of compensation for the inhibitor-mediated decrease in cellular IMPDH activity or a MPA- or TZ-mediated decrease in proteolysis of IMPDH. PMA treatment decreased the levels of IMPDH mRNA, protein, and activity. In addition, treatment of CEM-2 cells with either IMPDH inhibitors or PMA caused different alterations of the ribonucleotide pools. The lack of a consistent pattern of IMPDH expression in CEM-2 cells treated with IMPDH inhibitors or PMA indicates that no general association exists between the induction of cell differentiation and the expression of IMPDH. Nevertheless, our results indicating that IMPDH inhibitors can induce differentiation in CEM-2 cells suggest that this treatment may provide a useful approach to circumvent the differentiation block in some tumor cells.

Inhibition of cytochalasin D-stimulated G-actin ATPase by ADP-ribosylation with Clostridium perfringens iota toxin

Clostridium perfringens iota toxin belongs to a novel family of actin-ADP-ribosylating toxins. The effects of ADP-ribosylation of skeletal muscle actin by Clostridium perfringens iota toxin on cytochalasin D-stimulated actin ATPase activity was studied. Cytochalasin D stimulated actin-catalysed ATP hydrolysis maximally by about 30-fold. ADP-ribosylation of actin completely inhibited cytochalasin D-stimulated ATP hydrolysis. Inhibition of ATPase activity occurred at actin concentrations below the critical concentration (0.1 microM), at low concentrations of Mg2+ (50 microM) and even in the actin-DNAase I complex, indicating that ADP-ribosylation of actin blocks the ATPase activity of monomeric actin and that the inhibitory effect is not due to inhibition of the polymerization of actin.

Pasteurella multocida toxin: potent mitogen for cultured fibroblasts

Native Pasteurella multocida toxin (PMT) is shown to be an extremely potent mitogen for Swiss 3T3 fibroblasts. Half-maximal stimulation of DNA synthesis was obtained at concentrations of 1 and 2 pM for recombinant PMT (rPMT) and PMT, respectively. The degree of rPMT-induced DNA synthesis was comparable to that elicited by 10% fetal bovine serum and, moreover, was observed in the complete absence of other factors. Cell proliferation was also enhanced by rPMT. The toxin was also a potent mitogen for BALB/c and NIH 3T3 cells, 3T6 cells, and tertiary mouse embryo or human fibroblasts. The mitogenic activity of rPMT was heat-labile. A polyclonal antiserum to PMT inhibited DNA synthesis when added early, but not late, during treatment of the Swiss 3T3 cells with rPMT. A similar time-dependent action of methylamine was also observed. Furthermore, transient exposure of the cells to rPMT at 37 degrees C, but not at 4 degrees C, resulted in a stimulation of DNA synthesis. Thus, toxin action may require cell entry and processing via an acidic compartment. The toxin, at mitogenic concentrations, caused a large increase in the production of inositol phosphates. In contrast, rPMT did not increase the intracellular concentration of cyclic AMP in Swiss 3T3 cells. The basis of rPMT action may afford a unique insight into molecular signaling events involved in the control of cell proliferation.

Botulinum ADP-ribosyltransferase C3: a new tool to study low molecular weight GTP-binding proteins

It is well known that certain bacterial toxins, e.g. cholera and pertussis toxins, ADP-ribosylate eukaryotic regulatory proteins. They have become invaluable tools in the study of G protein-linked receptors. Less well appreciated is the fact that certain strains of Clostridium botulinum types C and D produce an ADP-ribosyltransferase, termed C3. This enzyme is structurally and functionally distinct from botulinum neurotoxins C1 and D. Its substrate is the 21 kDa GTP-binding protein rho. Klaus Aktories and Alan Hall explain why C3 is now an important tool in analysing the regulatory function of the ras-related protein rho.