Divalent cation regulation of adenylyl cyclase. An allosteric site on the catalytic component

Detection of adenylyl cyclase activity using a fluorescent ATP substrate and capillary electrophoresis

A capillary electrophoresis laser-induced fluorescence (CE-LIF) assay was developed for detection of adenylyl cyclase (AC) activity using BODIPY FL ATP (BATP) as substrate. In the assay, BATP was incubated with AC and the resulting mixture of BATP and enzyme product (BODIPY cyclic AMP, BcAMP) separated in 5 min by CE-LIF. Substrate depletion and product accumulation were simultaneously monitored during the course of the reaction. The rate of product formation depended upon the presence of AC activators forskolin or Galpha(s)-GTPgammaS as evidenced by a more rapid BATP turnover to BcAMP compared to basal levels. The CE-LIF assay detected EC50 values for forskolin and Galpha(s)-GTPgammaS of 27 +/- 6 microM and 317 +/- 56 nM, respectively. These EC50 values compared well to those previously reported using [alpha-32P]ATP as substrate. When AC was concurrently activated with 2.5 microM forskolin and 25 nM Galpha(s)-GTPgammaS, the amount of BcAMP formed was 3.4 times higher than the additive amounts of each activator alone indicating a positively cooperative activation by these compounds in agreement with previous assays using radiolabeled substrate. Inhibition of AC activity was also demonstrated using the AC inhibitor 2'-(or-3')-O-(N-methylanthraniloyl) guanosine 5'-triphosphate with an IC50 of 9 +/- 6 nM. The use of a fluorescent substrate combined with CE separation has enabled development of a rapid and robust method for detection of AC activity that is an attractive alternative to the AC assay using radioactive nucleotide and column chromatography. In addition, the assay has potential for high-throughput screening of drugs that act at AC.

Inhibition of adenylyl cyclase activity in brain membrane fractions by arachidonic acid and related unsaturated fatty acids

Pretreatment of mouse brain membranes with arachidonic acid (AA) and related unsaturated fatty acids at 30 degrees C for 10 min decreased basal activity and isoproterenol/guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S)- and forskolin-stimulated activities of adenylyl cyclase to a level less than 5% of control. The presence of the carboxyl group on the fatty acids was essential for the inhibition, because no such inhibition was found with ethyl arachidonate or AA attached to diacylglycerols and phospholipids. The AA-mediated inhibition was observed when the activity was measured in the presence of Mn2+ or forskolin and was insensitive to pertussis toxin or guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS), indicating a mechanism independent of GTP-binding proteins. In addition, the fact that stimulators of the adenylyl cyclase catalytic unit, ATP, GTP gamma S and forskolin, when present during pretreatment, attenuate the inhibitory effect of AA may suggest that the catalytic unit is a target of AA. Bovine serum albumin suppressed the inhibition when present in the mixtures for pretreatment, but could not restore the adenylyl cyclase activity that had been reduced by AA, indicating an irreversible inhibition by AA. The effect of AA was found to be additive to P-site-mediated inhibition. The present study suggests the existence of another mechanism of regulation of adenylyl cyclase by unsaturated fatty acids.

A novel adenylyl cyclase detected in rapidly developing mutants of Dictyostelium

Disruption of either the RDEA or REGA genes leads to rapid development in Dictyostelium. The RDEA gene product displays homology to certain H2-type phosphotransferases, while REGA encodes a cAMP phosphodiesterase with an associated response regulator. It has been proposed that RDEA activates REGA in a multistep phosphorelay. To test this proposal, we examined cAMP accumulation in rdeA and regA null mutants and found that these mutants show a pronounced accumulation of cAMP at the vegetative stage that is not observed in wild-type cells. This accumulation was due to a novel adenylyl cyclase and not to the known Dictyostelium adenylyl cyclases, aggregation stage adenylyl cyclase (ACA) or germination stage adenylyl cyclase (ACG), since it occurred in an acaA/rdeA double mutant and, unlike ACG, was inhibited by high osmolarity. The novel adenylyl cyclase was not regulated by G-proteins and was relatively insensitive to stimulation by Mn2+ ions. Addition of the cAMP phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) permitted detection of the novel adenylyl cyclase activity in lysates of an acaA/acgA double mutant. The fact that disruption of the RDEA gene as well as inhibition of the REGA-phosphodiesterase by IBMX permitted detection of the novel AC activity supports the hypothesis that RDEA activates REGA.

Mutations uncover a role for two magnesium ions in the catalytic mechanism of adenylyl cyclase

The recent determination of the crystal structure of adenylyl cyclase has elucidated many structural features that determine the regulatory properties of the enzyme. In addition, the characterization of adenylyl cyclase by mutagenic techniques and the identification of the binding site for P-site inhibitors have led to modeling studies that describe the ATP-binding site. Despite these advances, the catalytic mechanism of adenylyl cyclase remains uncertain, especially with respect to the role that magnesium ions may play in this process. We have identified four mutant mammalian adenylyl cyclases defective in their metal dependence, allowing us to further characterize the function of metal ions in the catalytic mechanism of this enzyme. The wild-type adenylyl cyclase shows a biphasic Mg2+ dose-response curve in which the high-affinity component displays cooperativity (Hill coefficient of 1.4). Two mutations (C441R and Y442H) reduce the affinity of the adenylyl cyclase for Mg2+ dramatically without affecting the binding of MgATP, suggesting that there is a metal requirement in addition to the ATP-bound Mg2+. The results of this study thus demonstrate multiple metal requirements of adenylyl cyclase and support the existence of a Mg2+ ion essential for catalysis and distinct from the ATP-bound ion. We propose that adenylyl cyclase employs a catalytic mechanism analogous to that of DNA polymerase, in which two key magnesium ions facilitate the nucleophilic attack of the 3'-hydroxyl group and the subsequent elimination of pyrophosphate.

Catalytic mechanism of the adenylyl and guanylyl cyclases: modeling and mutational analysis

The adenylyl and guanylyl cyclases catalyze the formation of 3', 5'-cyclic adenosine or guanosine monophosphate from the corresponding nucleoside 5'-triphosphate. The guanylyl cyclases, the mammalian adenylyl cyclases, and their microbial homologues function as pairs of homologous catalytic domains. The crystal structure of the rat type II adenylyl cyclase C2 catalytic domain was used to model by homology a mammalian adenylyl cyclase C1-C2 domain pair, a homodimeric adenylyl cyclase of Dictyostelium discoideum, a heterodimeric soluble guanylyl cyclase, and a homodimeric membrane guanylyl cyclase. Mg2+ATP or Mg2+GTP were docked into the active sites based on known stereochemical constraints on their conformation. The models are consistent with the activities of seven active-site mutants. Asp-310 and Glu-432 of type I adenylyl cyclase coordinate a Mg2+ ion. The D310S and D310A mutants have 10-fold reduced Vmax and altered [Mg2+] dependence. The NTP purine moieties bind in mostly hydrophobic pockets. Specificity is conferred by a Lys and an Asp in adenylyl cyclase, and a Glu, an Arg, and a Cys in guanylyl cyclase. The models predict that an Asp from one domain is a general base in the reaction, and that the transition state is stabilized by a conserved Asn-Arg pair on the other domain.

Adenylyl cyclase 6 is selectively regulated by protein kinase A phosphorylation in a region involved in Galphas stimulation

Receptors activate adenylyl cyclases through the Galphas subunit. Previous studies from our laboratory have shown in certain cell types that express adenylyl cyclase 6 (AC6), heterologous desensitization included reduction of the capability of adenylyl cyclases to be stimulated by Galphas. Here we further analyze protein kinase A (PKA) effects on adenylyl cyclases. PKA treatment of recombinant AC6 in insect cell membranes results in a selective loss of stimulation by high (>10 nM) concentrations of Galphas. Similar treatment of AC1 or AC2 did not affect Galphas stimulation. Conversion of Ser-674 in AC6 to an Ala blocks PKA phosphorylation and PKA-mediated loss of Galphas stimulation. A peptide encoding the region 660-682 of AC6 blocks stimulation of AC6 and AC2 by high concentrations of Galphas. Substitution of Ser-674 to Asp in the peptide renders the peptide ineffective, indicating that the region 660-682 of AC6 is involved in regulation of signal transfer from Galphas. This region contains a conserved motif present in most adenylyl cyclases; however, the PKA phosphorylation site is unique to members of the AC6 family. These observations suggest a mechanism of how isoform selective regulatory diversity can be obtained within conserved regions involved in signal communication.

Dimeric tubulin-stimulated adenylyl cyclase activity is augmented after long-term amitriptyline treatment

We have investigated altered association of tubulin dimers interacting with G proteins and modulating adenylyl cyclase (AC) as a result of long-term amitriptyline (AMT) treatment. Gpp(NH)p-stimulated, but not basal or manganese-stimulated, AC activity was significantly augmented in cortex membranes prepared from rats chronically treated with AMT. The enhancement of AC activity by Gpp(NH)p-liganded tubulin (tubulin-Gpp(NH)p) was significantly higher in chronically AMT-treated rats than in control rats. Moreover, in cortex membranes from controls, tubulin-Gpp(NH)p prepared from chronically AMT-treated rats was more effective to activate AC activity than tubulin-Gpp(NH)p from controls. Immunoblotting and photoaffinity guanine nucleotide binding procedures showed no significant differences in the amount and the function of G proteins between controls and AMT-treated groups. It is suggested that long-term AMT treatment causes alteration in the functional interaction between tubulin and G protein, and this modification may participate in enhanced coupling of Gs to the catalytic subunit of AC induced by the chronic antidepressant treatment.

Distinct characteristics of the basal activities of adenylyl cyclases 2 and 6

Regulation of basal activities of adenylyl cyclase (AC) 2 and 6, expressed in Sf9 cells by infection with recombinant baculovirus, was studied. An antipeptide antibody that recognizes AC2 and AC6 with equal sensitivity was used to establish that equivalent levels were expressed. Basal activities of AC2 and AC6 were compared at varying concentrations of Mg2+ or Mn2+ ions; AC2 had 15- and 10-fold greater activity than AC6, respectively. At 20 mM Mg2+, the Km values for ATP were 88 and 39 microM for AC2 and AC6, respectively, whereas their Vmax values were 281 and 11 pmol/mg protein.min. With 100 microM forskolin and either Mg2+ or Mn2+, the difference in activities between AC2 and AC6 was reduced to approximately 2-fold. Forskolin stimulated AC6 greater than 40-fold at 0.5-2 mM Mg2+, whereas AC2 was stimulated 4-6-fold. At 20 mM Mg2+, AC2 was stimulated 2-fold by forskolin, whereas AC6 was stimulated 18-fold. With Mg2+ alone, activities of AC2 and AC6 were not saturable up to 20 mM and yielded curvilinear Hofstee transformations. With forskolin, activities of both AC2 and AC6 were saturable by 10 mM Mg2+ and yielded linear Hofstee transformations. These data indicate that there are substantial differences in the basal enzymatic activities of adenylyl cyclase isoforms, due to differential regulation by Mg2+ ions rather than intrinsic catalytic capabilities. Thus the presence and relative abundance of adenylyl cyclase subtypes could greatly affect the resting cellular cAMP levels with consequent effects on important biological functions, such as differentiation and proliferation.

Actions of lithium on the cyclic AMP signalling system in various regions of the brain--possible relations to its psychotropic actions. A study on the adenylate cyclase in rat cerebral cortex, corpus striatum and hippocampus

It has been estimated that in most industrialized countries 1 person out of every 1000 in the population is undergoing lithium treatment to stabilize their episodic mood disturbances due to manic-depressive illness. Lithium may stabilize mood swings by altering the action of certain neurotransmitters at the synaptic level in the brain. Recent research suggests that lithium alters neurotransmission by affecting neurotransmitter-coupled second messenger systems. A major second messenger system is the adenylate cyclase, which generates intracellular cAMP from ATP. The adenylate cyclases (type I-IV) are regulated by stimulatory and inhibitory receptors, which either stimulate or inhibit the adenylate cyclase activity. The stimulatory and inhibitory neurotransmitter-receptor signals are transferred to the catalytic unit of the adenylate cyclase by Gs and Gi, respectively. The activated receptor induces GTP stimulation of the heterotrimeric G protein, leading to a dissociation of the protein into the active alpha*GTP and the beta gamma complex. The former stimulates the catalytic unit of adenylate cyclase. The stimulation is terminated by a GTPase located on the alpha subunit that converts GTP to inactive GDP. At present, G proteins are known to play a central role in coupling receptors to effector proteins. In addition to extracellular regulation due to neurotransmitters, some adenylate cyclases (type I, III) are regulated by CaM as a consequence of enhanced intracellular concentrations of free Ca2+. The Ca(2+)-dependent stimulation of adenylate cyclase by CaM is assumed to occur by a direct effect on the catalytic unit. The catalytic units sensitive to Ca(2+)-CaM are also subjected to regulation by stimulatory and inhibitory neurotransmitter stimuli. Magnesium is essential for adenylate cyclase activity, since MgATP2- is the enzyme substrate. Furthermore, one Mg2+ site located on the G protein regulates both the receptor agonist affinity and the dissociation of the G protein during the activation cycle. A second Mg2+ site on the catalytic unit is responsible for Mg2+ regulation of the catalytic activity. The present work aimed at investigating the mechanisms by which lithium in vitro and after chronic treatment (ex vivo) affects adenylate cyclase activities in various regions of the rat brain. Lithium in vitro and ex vivo inhibited the selective stimulation of adenylate cyclase by Ca(2+)-CaM in the cerebral cortex. Furthermore, lithium in vitro interacted directly with the catalytic unit of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium inhibition of cardiac adenylyl cyclase. Evidence for two distinct sites of inhibition

Increasing the free calcium concentration from 10(-8) M to 10(-4) M inhibited cardiac sarcolemmal adenylyl cyclase activated by the addition of 5 X 10(-4) M forskolin or 1 X 10(-4) M GTP or Gpp(NH)p. The calcium inhibition curve in the presence of all three activators was shallow and best fit by a two site model of high affinity (less than 1.0 microM) and low affinity (greater than 0.1 mM). Gpp(NH)p appeared to decrease the sensitivity of adenylyl cyclase to inhibition by calcium at the high affinity site. Similar inhibition constants were obtained with each of the activators. Calmodulin content of native freeze-thaw vesicles was 76.2 +/- 14.2 ng/mg. Treatment of the vesicles with 1 mM EGTA to remove calmodulin significantly reduced calmodulin content to 19.7 +/- 1.35 ng/mg. This treatment had no significant effect on the calcium inhibition profile. Increasing free calcium to 3 X 10(-6) M was shown to have no effect on the EC50 estimated for either Gpp(NH)p or forskolin but did slightly increase the EC50 estimated for Mg2+ in the presence of maximal concentrations of either activator. Nevertheless, maximally stimulating concentrations of Mg2+ were unable to overcome calcium inhibition. Pretreatment of sarcolemmal membranes with pertussis toxin was shown to have no significant effect on calcium inhibition of adenylyl cyclase. The results suggest that the overall inhibitory action of calcium was most likely calmodulin independent and involved a direct interaction with the catalytic subunit at two distinct sites of high and low affinity. At the low affinity site calcium most likely competes with Mg2+ for an allosteric divalent cation binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Forskolin inhibition of cyclic AMP generation in J774 macrophages

Forskolin inhibited cyclic AMP generation in J774 macrophage cells in response to isoproterenol. Forskolin, 10 nM-0.1 mM, also inhibited the adenylate cyclase activity of membrane preparations. The basal activity and the isoproterenol-, cholera toxin-, fluoride- or GppNHp-stimulated activities were maximally depressed by 10 microM forskolin (30-70% inhibition, EC50 = 0.3-0.5 microM). This effect was achieved similarly in membranes from pertussis toxin-treated cells. Forskolin required guanine nucleotides for inhibition. In the absence of GTP the decrease in basal activity was reversed into stimulation (EC50 = 10 microM forskolin). Reversal of inhibition into activation also followed the addition of 1 mM MnCl2 (EC50 = 10 microM forskolin). 1,9-Dideoxyforskolin was ineffective to alter adenylate cyclase activity. In contrast, a water-soluble derivative of forskolin was as active as forskolin to regulate activity. The results suggest that forskolin may interact with adenylate cyclase to cause either activation or inhibition depending on the degree of activation of Ns and on its interaction with the catalyst.

Mode of action of lithium on the catalytic unit of adenylate cyclase from rat brain

The action of lithium on calcium-calmodulin-activated and forskolin-activated catalytic unit of adenylate cyclase has been studied. The catalytic unit was solubilized from rat brain and separated from the guanine nucleotide binding protein by gel filtration. Calcium-calmodulin-stimulated and forskolin-stimulated catalytic unit activities were inhibited in the presence of 2 mM and 1 mM of lithium, respectively. No inhibitory effect was observed on the basal activity. The inhibitory effect of lithium on the stimulated activities was antagonized by magnesium. Lithium did not influence the interaction of the enzyme substrate (ATP) with the catalytic unit. The present results indicate that lithium interacts directly with the catalytic unit of the adenylate cyclase system. In the neuron, lithium might interfere with a divalent cation site on the catalytic unit.

Regulation of Dictyostelium discoideum adenylate cyclase by manganese and adenosine analogs

Adenylate cyclase is the critical enzyme in the chemotactic signal relay mechanism of the slime mold amoeba, Dictyostelium discoideum. However, few studies examining the regulation of this enzyme have been performed in vitro due to the instability of enzyme activity in crude lysates. For studies presented in this communication, a membrane preparation has been isolated that exhibits a high specific activity adenylate cyclase that is stable during storage at -70 degrees C and under assay conditions at 27 degrees C. The enzyme was activated by micromolar concentrations of MnCl2. GTP and its non-hydrolyzable analog, guanosine 5'-(beta, gamma-imino)triphosphate, inhibited the enzyme non-competitively in the presence of either Mg2+ or Mn2+. However, this inhibition was more pronounced in the presence of Mn2+. Since guanylate cyclase activity in the D. discoideum membranes was less than 10% of the adenylate cyclase activity, there could not be a significant contribution by guanylate cyclase toward the production of cyclic AMP. Experiments indicate that D. discoideum adenylate cyclase was also regulated by adenosine analogs. The enzyme was inhibited by 2',5'-dideoxyadenosine and 2'-deoxyadenosine and inhibition was augmented by the presence of Mn2+. However, the inhibition was not entirely consistent with that which would be expected for the P-site of eukaryotic systems because some purine-modified adenosine analogs also inhibited the enzyme. Guanine nucleotides had no effect on the inhibition by either purine-modified or ribose-modified adenosine analogs. The binding of cyclic AMP to its receptor on the D. discoideum membranes was not affected by either MnCl2 or adenosine analogs.

Effects of lithium on calmodulin-stimulated adenylate cyclase activity in cortical membranes from rat brain

The influence of lithium on calmodulin-stimulated adenylate cyclase activity has been studied in vitro and after chronic treatment. Chronic lithium treatment decreased calcium-calmodulin-stimulated adenylate cyclase activity in rat cortical membranes, while no effect was observed on GTP-stimulated activity. Lithium in vitro inhibited adenylate cyclase activity stimulated by isoprenaline, GTP or calcium-calmodulin. Calcium-calmodulin-stimulated activity was more sensitive to lithium (2 mM) than isoprenaline- and GTP-stimulated activities (5 mM) and activities by these agents combined. Lithium had no effect on the unstimulated enzyme activity. The inhibitory effect of lithium in vitro on calcium-calmodulin-stimulated adenylate cyclase activity was antagonized by magnesium. The inhibition induced by lithium in vitro on the GTP-stimulated adenylate cyclase activity was increased by substituting manganese for magnesium in the assay media. Furthermore, the manganese-stimulated activity was also reduced by lithium. The latter effect was not observed in calmodulin-depleted membranes, but the inhibitory effect of lithium could be restored by addition of exogenous calmodulin. The present results suggest that lithium might influence the interaction of calmodulin with the enzyme and/or interfere with the divalent cation site(s) on the adenylate cyclase system.

Forskolin: unique diterpene activator of adenylate cyclase in pregnant and nonpregnant guinea pig myometrial membranes

In guinea pig myometrium, beta-adrenergic receptors are functionally coupled to adenylate cyclase. beta-Adrenergic receptor agonists in the presence of guanosine triphosphate stimulate adenylate cyclase activity, thus increasing 3'5'-cyclic adenosine monophosphate synthesis and promoting myometrial relaxation. In pregnant animals close to term (65 days), beta-adrenergic receptor density as well as basal and (-)isoproterenol-dependent (in the presence of guanosine triphosphate) adenylate cyclase activity is significantly higher than that in nonpregnant animals or those in early pregnancy. Since this system appears to be made up of at least three components (beta-adrenergic receptor, guanosine triphosphate-binding protein, and a catalytic component), these observations on total adenylate cyclase activity may reflect alterations in one or more of these components. To answer the question whether the catalytic unit of this system can be directly assayed and whether its activity is influenced by pregnancy, we have performed in vitro experiments to measure the enzymatic activity of the catalytic component of the beta-adrenergic receptor-adenylate cyclase complex in guinea pig myometrial membranes. We have used two compounds that stimulate the catalytic component: forskolin and manganese chloride. Forskolin, regardless of the presence or absence of guanosine triphosphate, is the most potent stimulator of adenylate cyclase activity in myometrial membranes from nonpregnant and pregnant animals; manganese chloride is a less potent activator. The degree of adenylate cyclase stimulation by forskolin tends to be higher in uteri from pregnant (greater than or equal to 0.5 gestation) than from nonpregnant or postpartum animals. It was concluded: that adenylate cyclase stimulation by forskolin does not depend on the presence of beta-adrenergic receptor agonists or guanosine triphosphate and that with advancing gestation there might be a qualitative or quantitative change with regard to the interaction between forskolin and the presumed catalytic component of the beta-adrenergic receptor-adenylate cyclase complex.

Manganese and manganese-ATP interactions with bovine sperm adenylate cyclase

The adenylate cyclase of mammalian spermatozoa shares some of the properties of the isolated catalytic component from somatic cell adenylate cyclases. One of these properties is the large apparent stimulation by Mn2+. We have used the direct linear plot according to Eisenthal and Cornish-Bowden to explore whether this apparent stimulation is due to direct stimulation by Mn2+ or due to complexation of free ATP, a postulated inhibitor of cyclase activity. We have observed the activity of the particulate adenylate cyclase from bovine caudal epididymal spermatozoa as a function of calculated equilibrium values for the concentrations of Mn2+, free ATP, and the enzyme's substrate, the manganese-ATP complex. Direct linear plots for activity and substrate concentration over the apparent inhibitory concentration range of free ATP give the pattern expected for a hyperbolic substrate response. By contrast, direct linear plots in which Mn2+ concentration varies over its apparent stimulatory range show that as Mn2+ concentration increases, activities are higher than would be predicted for a hyperbolic substrate response. We conclude that for particulate bovine sperm adenylate cyclase, free ATP is not strongly inhibitory, and Mn2+ is a positive effector, reaching half-maximal stimulation at 0.2 mM. The unique nature of the sperm adenylate cyclase and its possible regulation by Mn2+ under physiological conditions is discussed.

Ethanol's effects on cortical adenylate cyclase activity

The effects of ethanol on beta-adrenergic receptor-coupled adenylate cyclase (AC) of mouse cerebral cortex were examined. The addition of ethanol (20-500 mM) to incubation mixtures containing cortical membranes demonstrated that ethanol could increase AC activity and potentiate the stimulatory effects of guanylyl-imidodiphosphate [Gpp(NH)p] on AC activity. Ethanol increased the rate of activation of AC by guanine nucleotides and concomitantly decreased the EC50 for magnesium required to achieve maximal stimulation of cortical AC. The EC50 values for Gpp(NH)p and isoproterenol stimulation of AC activity were also altered by ethanol. Ethanol was capable of stimulating AC extracted by use of digitonin. The AC activity in the digitonin extract was no longer sensitive to the addition of Gpp(NH)p or NaF, but was still stimulated by ethanol. We propose multiple sites of action for ethanol in stimulating cortical AC activity. These sites include actions at the beta-adrenergic receptor, at the G/F coupling proteins, and at the catalytic unit of cortical AC. Comparison of ethanol's actions on cortical beta receptor coupled AC activity with prior reported actions of ethanol on striatal dopamine (DA)-sensitive AC indicated differential sensitivities of these two AC systems to ethanol. These differences may be determined by specific coupling characteristics of the striatal and cortical AC systems or by differences in the plasma membranes in which striatal and cortical AC systems are located.

Adenylate cyclase in the Drosophila memory mutant rutabaga displays an altered Ca2+ sensitivity

Adenylate cyclase in washed, crude membrane fractions prepared from the Drosophila conditioning mutant, rutabaga, displays an altered responsiveness to Ca2+. The results are of interest since the modulation of adenylate cyclase activity by Ca2+ has recently been suggested to play a role in molecular events that underlie memory formation.

Forskolin regulation of liver membrane adenylyl cyclase

The effects of forskolin on rat liver plasma membrane adenylyl cyclase were studied. The diterpene stimulated the Vmax of the enzyme system with apparent Km values of 3-5 microM. Stimulations were marked both in the absence (20-fold over control) as well as in the presence of various stimulators such as GTP, GuoPP[NH]P, NaF alone or in combination with glucagon. Except with GTP, where stimulations of activities by forskolin and the nucleotide were synergistic (more than additive), stimulations of combinations of GuoPP[NH]P, NaF or glucagon with forskolin were additive. Forskolin did not alter significantly the apparent Km values of the enzyme for MgATP or MnATP or the apparent Ka values (concentrations giving stimulations that are 50% of maximum) for Mg or Mn ions, GTP, GuoPP[NH]P or NaF. Forskolin caused a decrease in the concentration of glucagon required for half-maximal stimulation from 5 microM to 1.5 microM. Except for this effect on the Ka for the glucagon, the only kinetic parameter altered was the Vmax under all conditions tested. Although proteolysis stimulated liver membrane adenylyl cyclase under control conditions, it did not enhance forskolin-stimulated activities. More extensive proteolysis, which resulted in decreased activities in the absence of forskolin, also resulted in reduced forskolin-stimulated activities. 'Uncoupling' of the guanine-nucleotide-binding regulatory component, that mediates guanine nucleotide stimulation by addition of 30 mM MnCl2, did not result in 'uncoupling' of forskolin stimulation. The data indicate that the diterpene forskolin stimulates adenylyl cyclase activity by a novel mechanism that differs from that by which NaF or guanylyl nucleotides affect this membrane-bound system and that the diterpene should be a useful tool with which to explore as yet unrecognized modes of regulation of cyclic AMP production.

Inhibition of NS-protein-stimulated human-platelet adenylate cyclase by epinephrine and stable GTP analogs

The influence of purified, preactivated NS protein, mediating adenylate cyclase stimulation by hormones and guanine nucleotides, was studied on adenylate cyclase activity in membranes of human platelets. The preactivated coupling protein caused a large increase in platelet enzyme activity. The activation occurred after a short lag phase and exhibited saturation. NS protein increased the V of the platelet adenylate cyclase without change in the enzyme's affinity for the substrate, MgATP, whereas the apparent affinity for Mg2+ was increased by more than one order of magnitude. The NS-protein-activated human platelet adenylate cyclase was inhibited by the hormone, epinephrine, and by the stable GTP analogs, guanosine 5'-[beta, gamma-imido]triphosphate and guanosine 5'-[gamma-thio]triphosphate. The stable GTP analog or hormone-induced adenylate cyclase inhibition was not competitive with regard to the concentration of NS protein. Inhibition of NS-protein-stimulated platelet adenylate cyclase caused by stable GTP analogs appeared to be persistent, was amplified by epinephrine and was accompanied by a large decrease in the enzyme's apparent affinity for Mg2+. The data suggest that the hormone-sensitive adenylate cyclase is regulated by two guanine-nucleotide-binding sites, NS and Ni, mediating enzyme stimulation and inhibition, respectively, by hormones and guanine nucleotides, and that the two coupling components interact in a non-competitive manner with the adenylate cyclase, exhibiting high and low affinity for Mg2+ when affected by NS and Ni, respectively.