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

Mechanisms of ATP to cAMP Conversion Catalyzed by the Mammalian Adenylyl Cyclase: A Role of Magnesium Coordination Shells and Proton Wires

We report a mechanism of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) conversion by the mammalian type V adenylyl cyclase revealed in molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) simulations. We characterize a set of computationally derived enzyme-substrate (ES) structures showing an important role of coordination shells of magnesium ions in the solvent accessible active site. In the lowest energy ES conformation, the coordination shell of Mg_A²⁺ does not include the O_δ1 atom of the conserved Asp440 residue. Starting from this conformation, a one-step reaction mechanism is characterized that includes proton transfer from the ribose O^3'H^3' group in ATP to Asp440 via a shuttling water molecule concerted with P^A-O^3A bond cleavage and O^3'-P^A bond formation. The energy profile of this route is consistent with the observed reaction kinetics. The computed energy profiles initiated from higher energy ES complexes are characterized by larger energy expenses to complete the reaction. Consistent with experimental data, we show that the Asp440Ala mutant of the enzyme should exhibit a reduced but retained activity. All considered reaction pathways include proton wires from the O^3'H^3' group via shuttling water molecules.

Molecular basis for GTP recognition by light-activated guanylate cyclase RhGC

Cyclic guanosine 3',5'-monophosphate (cGMP) is an intracellular signalling molecule involved in many sensory and developmental processes. Synthesis of cGMP from GTP is catalysed by guanylate cyclase (GC) in a reaction analogous to cAMP formation by adenylate cyclase (AC). Although detailed structural information is available on the catalytic region of nucleotidyl cyclases (NCs) in various states, these atomic models do not provide a sufficient explanation for the substrate selectivity between GC and AC family members. Detailed structural information on the GC domain in its active conformation is largely missing, and no crystal structure of a GTP-bound wild-type GC domain has been published to date. Here, we describe the crystal structure of the catalytic domain of rhodopsin-GC (RhGC) from Catenaria anguillulae in complex with GTP at 1.7 Å resolution. Our study reveals the organization of a eukaryotic GC domain in its active conformation. We observe that the binding mode of the substrate GTP is similar to that of AC-ATP interaction, although surprisingly not all of the interactions predicted to be responsible for base recognition are present. The structure provides insights into potential mechanisms of substrate discrimination and activity regulation that may be common to all class III purine NCs. DATABASE: Structural data are available in Protein Data Bank database under the accession number 6SIR. ENZYMES: EC4.6.1.2.

From direct to indirect lithium targets: a comprehensive review of omics data

Metal ions are critical to a wide range of biological processes.

Magnesium Uptake by CorA Transporters Is Essential for Growth, Development and Infection in the Rice Blast Fungus Magnaporthe oryzae

Magnaporthe oryzae, the causative organism of rice blast, infects cereal crops and grasses at various stages of plant development. A comprehensive understanding of its metabolism and the implications on pathogenesis is necessary for countering this devastating crop disease. We present the role of the CorA magnesium transporters, MoAlr2 and MoMnr2, in development and pathogenicity of M. oryzae. The MoALR2 and MoMNR2 genes individually complement the Mg²⁺ uptake defects of a S. cerevisiae CorA transporter double mutant. MoALR2 and MoMNR2 respond to extracellular Mg²⁺ and Ca²⁺ levels and their expression is elevated under Mg²⁺ scarce conditions. RNA silencing mediated knockdown of MoALR2 (WT+siALR2, Δmnr2+siALR2 and ALR2+MNR2 simultaneous silencing) drastically alters intracellular cation concentrations and sensitivity to metal ions. MoALR2 silencing is detrimental to vegetative growth and surface hydrophobicity of mycelia, and the transformants display loss of cell wall integrity. MoALR2 is required for conidiogenesis and appressorium development, and is essential for infection. Investigation of knockdown transformants reveal low cAMP levels and altered expression of genes encoding proteins involved in MoMps1 cell wall integrity and cAMP MoPmk1 driven MAP Kinase signaling pathways. In contrast to MoALR2 knockdowns, the MoMNR2 deletion (Δmnr2) shows increased sensitivity to CorA inhibitors as well as altered cation sensitivity, but has limited effect on surface hydrophobicity and severity of plant infection. Interestingly, MoALR2 expression is elevated in Δmnr2. Impairment of development and infectivity of knockdown transformants and altered intracellular cation composition suggest that CorA transporters are essential for Mg²⁺ homeostasis within the cell, and are crucial to maintaining normal gene expression associated with cell structure, signal transduction and surface hydrophobicity in M. oryzae. We suggest that CorA transporters, and especially MoALR2, constitute an attractive target for the development of antifungal agents against this pathogen.

The role of transcriptional regulation in maintaining the availability of mycobacterial adenylate cyclases

Mycobacterium species have a complex cAMP regulatory network indicated by the high number of adenylate cyclases annotated in their genomes. However the need for a high level of redundancy in adenylate cyclase genes remains unknown. We have used semiquantitiative RT-PCR to examine the expression of eight Mycobacterium smegmatis cyclases with orthologs in the human pathogen Mycobacterium tuberculosis, where cAMP has recently been shown to be important for virulence. All eight cyclases were transcribed in all environments tested, and only four demonstrated environmental-mediated changes in transcription. M. smegmatis genes MSMEG_0545 and MSMEG_4279 were upregulated during starvation conditions while MSMEG_0545 and MSMEG_4924 were downregulated in H2O2 and MSMEG_3780 was downregulated in low pH and starvation. Promoter fusion constructs containing M. tuberculosis H37Rv promoters showed consistent regulation compared to their M. smegmatis orthologs. Overall our findings indicate that while low levels of transcriptional regulation occur, regulation at the mRNA level does not play a major role in controlling cellular cyclase availability in a given environment.

Synergistic roles for G-protein γ3 and γ7 subtypes in seizure susceptibility as revealed in double knock-out mice

The functions of different G-protein αβγ subunit combinations are traditionally ascribed to their various α components. However, the discovery of similarly diverse γ subtypes raises the possibility that they may also contribute to specificity. To test this possibility, we used a gene targeting approach to determine whether the closely related γ(3) and γ(7) subunits can perform functionally interchangeable roles in mice. In contrast to single knock-out mice that show normal survival, Gng3(-/-)Gng7(-/-) double knock-out mice display a progressive seizure disorder that dramatically reduces their median life span to only 75 days. Biochemical analyses reveal that the severe phenotype is not due to redundant roles for the two γ subunits in the same signaling pathway but rather is attributed to their unique actions in different signaling pathways. The results suggest that the γ(3) subunit is a component of a G(i/o) protein that is required for γ-aminobutyric acid, type B, receptor-regulated neuronal excitability, whereas the γ(7) subunit is a component of a G(olf) protein that is responsible for A(2A) adenosine or D(1) dopamine receptor-induced neuro-protective response. The development of this mouse model offers a novel experimental framework for exploring how signaling pathways integrate to produce normal brain function and how their combined dysfunction leads to spontaneous seizures and premature death. The results underscore the critical role of the γ subunit in this process.

Structural basis for inhibition of mammalian adenylyl cyclase by calcium

Type V and VI mammalian adenylyl cyclases (AC5, AC6) are inhibited by Ca(2+) at both sub- and supramicromolar concentration. This inhibition may provide feedback in situations where cAMP promotes opening of Ca(2+) channels, allowing fine control of cardiac contraction and rhythmicity in cardiac tissue where AC5 and AC6 predominate. Ca(2+) inhibits the soluble AC core composed of the C1 domain of AC5 (VC1) and the C2 domain of AC2 (IIC2). As observed for holo-AC5, inhibition is biphasic, showing "high-affinity" (K(i) = approximately 0.4 microM) and "low-affinity" (K(i) = approximately 100 microM) modes of inhibition. At micromolar concentration, Ca(2+) inhibition is nonexclusive with respect to pyrophosphate (PP(i)), a noncompetitive inhibitor with respect to ATP, but at >100 microM Ca(2+), inhibition appears to be exclusive with respect to PP(i). The 3.0 A resolution structure of Galphas.GTPgammaS/forskolin-activated VC1:IIC2 crystals soaked in the presence of ATPalphaS and 8 microM free Ca(2+) contains a single, loosely coordinated metal ion. ATP soaked into VC1:IIC2 crystals in the presence of 1.5 mM Ca(2+) is not cyclized, and two calcium ions are observed in the 2.9 A resolution structure of the complex. In both of the latter complexes VC1:IIC2 adopts the "open", catalytically inactive conformation characteristic of the apoenzyme, in contrast to the "closed", active conformation seen in the presence of ATP analogues and Mg(2+) or Mn(2+). Structures of the pyrophosphate (PP(i)) complex with 10 mM Mg(2+) (2.8 A) or 2 mM Ca(2+) (2.7 A) also adopt the open conformation, indicating that the closed to open transition occurs after cAMP release. In the latter complexes, Ca(2+) and Mg(2+) bind only to the high-affinity "B" metal site associated with substrate/product stabilization. Ca(2+) thus stabilizes the inactive conformation in both ATP- and PP(i)-bound states.

Regulation of aquaporin 3 expression by magnesium ion

For understanding the actions of magnesium formulations, magnesium oxide and magnesium sulfate as a constituent of antacid, in the gastrointestinal tract, the effect of magnesium ion on the water channel aquaporin 3 (AQP3) known to be permeable mainly to water and glycerol was investigated in Caco-2 cells. The mRNA and protein of aquaporin 3 were detected by real-time RT-PCR and Western blotting, respectively, and found to increase significantly after treatment with magnesium acetate. Inhibitors for signal transducers, MDL-12330A, H-89, U0126, and Ro 31-8220, were shown to repress the increase in expression of the mRNA. A luciferase reporter vector containing bp -1382 to -12 of the 5'-flanking region of the aquaporin 3 gene was constructed for a reporter gene assay. The luciferase activity in transfectants increased on treatment with magnesium acetate. Serial deletion constructs revealed two regions responsible for the magnesium ion-mediated activation, one between bps -404 and -190, and the other between bps -190 and -82. siRNA for the cAMP response element-binding protein (CREB) sequence located between bp -404 and -190 counteracted the magnesium ion-mediated activation of aquaporin 3 transcription. These results suggest that signal transducers, adenylyl cyclase, protein kinase A (PKA), mitogen-activated protein kinase 1/2 (MEK1/2), and mitogen- and stress-activated protein kinase 1 (MSK1), were involved in the signaling pathway for regulating transcription of the aquaporin 3 gene and CREB is one of the transcriptional regulators for aquaporin 3 gene expression mediated by magnesium ion.

Heterodimers of adenylyl cyclases 2 and 5 show enhanced functional responses in the presence of Galpha s

Recent studies have demonstrated that adenylyl cyclase isoforms can form both homo- and heterodimers and that this may be the basic functional unit of these enzymes (see Cooper, D.M.F. and Crossthwaite, A.J. (2006) Trends. Pharmacol. Sci. 8:426-431). Here, we show that adenylyl cyclases 2 and 5 can form a functional heterodimeric complex in HEK293 cells using a combination of BRET, confocal imaging, co-immunoprecipitation and assays of adenylyl cyclase activity. The AC2/5 complex is formed constitutively and is stable in the presence of receptor or forskolin stimulation. The complex formed by AC2/5 is also much more sensitive to the presence of Galpha(s) and forskolin than either of the parent AC isoforms themselves. Finally, we also show that this complex can be detected in native tissues as AC2 and AC5 were localized to the same structures in adult mouse ventricular myocytes and neonatal mouse cardiac fibroblasts and could be co-immunoprecipitated from lysates of mouse heart.

Role of receptors in Bacillus thuringiensis crystal toxin activity

Bacillus thuringiensis produces crystalline protein inclusions with insecticidal or nematocidal properties. These crystal (Cry) proteins determine a particular strain's toxicity profile. Transgenic crops expressing one or more recombinant Cry toxins have become agriculturally important. Individual Cry toxins are usually toxic to only a few species within an order, and receptors on midgut epithelial cells have been shown to be critical determinants of Cry specificity. The best characterized of these receptors have been identified for lepidopterans, and two major receptor classes have emerged: the aminopeptidase N (APN) receptors and the cadherin-like receptors. Currently, 38 different APNs have been reported for 12 different lepidopterans. Each APN belongs to one of five groups that have unique structural features and Cry-binding properties. While 17 different APNs have been reported to bind to Cry toxins, only 2 have been shown to mediate toxin susceptibly in vivo. In contrast, several cadherin-like proteins bind to Cry toxins and confer toxin susceptibility in vitro, and disruption of the cadherin gene has been associated with toxin resistance. Nonetheless, only a small subset of the lepidopteran-specific Cry toxins has been shown to interact with cadherin-like proteins. This review analyzes the interactions between Cry toxins and their receptors, focusing on the identification and validation of receptors, the molecular basis for receptor recognition, the role of the receptor in resistant insects, and proposed models to explain the sequence of events at the cell surface by which receptor binding leads to cell death.

A mechanism of cell death involving an adenylyl cyclase/PKA signaling pathway is induced by the Cry1Ab toxin of Bacillus thuringiensis

Many pathogenic organisms and their toxins target host cell receptors, the consequence of which is altered signaling events that lead to aberrant activity or cell death. A significant body of literature describes various molecular and cellular aspects of toxins associated with bacterial invasion, colonization, and host cell disruption. However, there is little information on the molecular and cellular mechanisms associated with the insecticidal action of Bacillus thuringiensis (Bt) Cry toxins. Recently, we reported that the Cry1Ab toxin produced by Bt kills insect cells by activating a Mg(2+)-dependent cytotoxic event upon binding of the toxin to its receptor BT-R(1). Here we show that binding of Cry toxin to BT-R(1) provokes cell death by activating a previously undescribed signaling pathway involving stimulation of G protein (G(alphas)) and adenylyl cyclase, increased cAMP levels, and activation of protein kinase A. Induction of the adenylyl cyclase/protein kinase A pathway is manifested by sequential cytological changes that include membrane blebbing, appearance of ghost nuclei, cell swelling, and lysis. The discovery of a toxin-induced cell death pathway specifically linked to BT-R(1) in insect cells should provide insights into how insects evolve resistance to Bt and into the development of new, safer insecticides.

Structures, mechanism, regulation and evolution of class III nucleotidyl cyclases

Cyclic 3',5'-guanylyl and adenylyl nucleotides function as second messengers in eukaryotic signal transduction pathways and as sensory transducers in prokaryotes. The nucleotidyl cyclases (NCs) that catalyze the synthesis of these molecules comprise several evolutionarily distinct groups, of which class III is the largest. The domain structures of prokaryotic and eukaryotic class III NCs are diverse, including a variety of regulatory and transmembrane modules. Yet all members of this family contain one or two catalytic domains, characterized by an evolutionarily ancient topological motif (betaalphaalphabetabetaalphabeta) that is preserved in several other enzymes that catalyze the nucleophilic attack of a 3'-hydroxyl upon a 5' nucleotide phosphate. Two dyad-related catalytic domains compose one catalytic unit, with the catalytic sites formed at the domain interface. The catalytic domains of mononucleotidyl cyclases (MNCs) and diguanylate cyclases (DGCs) are called cyclase homology domains (CHDs) and GGDEF domains, respectively. Prokaryotic NCs usually contain only one catalytic domain and are catalytically active as intermolecular homodimers. The different modes of dimerization in class III NCs probably evolved concurrently with their mode of binding substrate. The catalytic mechanism of GGDEF domain homodimers is not completely understood, but they are expected to have a single active site with each subunit contributing equivalent determinants to bind one GTP molecule or half a c-diGMP molecule. CHD dimers have two potential dyad-related active sites, with both CHDs contributing determinants to each site. Homodimeric class III MNCs have two equivalent catalytic sites, although such enzymes may show half-of-sites reactivity. Eukaryotic class III MNCs often contain two divergent CHDs, with only one catalytically competent site. All CHDs appear to use a common catalytic mechanism, which requires the participation of two magnesium or manganese ions for binding polyphosphate groups and nucleophile activation. In contrast, mechanisms for purine recognition and specificity are more diverse. Class III NCs are subject to regulation by small molecule effectors, endogenous domains, or exogenous protein partners. Many of these regulators act by altering the interface of the catalytic domains and therefore the integrity of the catalytic site(s). This review focuses on both conserved and divergent mechanisms of class III NC function and regulation.

Nitric oxide-dependent allosteric inhibitory role of a second nucleotide binding site in soluble guanylyl cyclase

The mechanism of desensitization of the nitric oxide (NO) receptor (alpha1.beta1 isoform of soluble guanylyl cyclase, sGC) is not known. Models of the structure of alpha1.beta1, based on the x-ray crystal structure of adenylyl cyclase (AC) suggest the existence of a nucleotide-like binding site, in addition to the putative catalytic site. We have previously reported that mutating residues that coordinate Mg(2+)GTP (substrate) binding in alpha1.beta1 into those present in AC fully reverts GC activity to AC activity. The wild-type form of alpha1.beta1 (GC-wt) and the mutant form (AC-mut, alpha1R592Q.beta1E473K,C541D) were purified, and their sensitivities to various nucleotides were assessed. In using the AC-mut as well as other mutants that coordinate purine binding, we were able to distinguish allosteric inhibitory effects of guanine nucleotides from competitively inhibitory effects on catalytic activity. Here we report that several nucleotide analogs drastically alter sGC and AC-mut activity by acting at a second nucleotide site, likely pseudosymmetric to the catalytic site. In particular, Mg(2+)GTP gamma S and Mg(2+)ATP gamma S inhibited cyclase activity through a mixed, non-competitive mechanism that was only observable under NO stimulation and not under basal conditions. The non-competitive pattern of inhibition was not present in mutants carrying the substitution beta1D477A, the pseudosymmetric equivalent to alpha1D529 (located in the substrate-binding site and involved in substrate binding and catalysis), or with the double mutations alpha1E525K,C594D, the pseudosymmetric equivalent to beta1E473K,C541D. Taken together these data suggest that occupation of the second site by nucleotides may underlie part of the mechanism of desensitization of sGC.

Modulation of adenyl cyclase activity in the gills of Tapes philippinarum

Adenyl cyclase (AC) plays a pivotal role in cell signaling. The AC system of bivalves has received little attention so far, and our study has been addressed to the characterization of AC properties in the gills of T. philippinarum. The enzyme showed a Km value of 0.77 mM for ATP in the presence of 5 mM Mg2+; in the absence of agonists, it was poorly affected by GTP, while it was stimulated by GTPgammaS and GppNHp up to 14-fold and 4-fold, respectively. Similarly to other invertebrates, the enzyme activity was scarcely stimulated by forskolin. The receptor agonist serotonin (5-HT) significantly stimulated the AC activity, and the pharmacological profile of the 5-HT receptor/s was as follows: (+)butaclamol > dihydroergocryptine > methysergide > prazosin > yohimbine. The AC activity was assessed in vitro in the presence of tributyltin chloride and HgCl2, which reduced the AC activity only at the highest dose tested (10-100 microM). Our data indicate the presence of a membrane-bound AC in gill membranes of T. philippinarum, coupled to Gs proteins and to a specific class of 5-HT receptors. Such receptors show a pharmacological profile slightly different from that reported for 5-HT invertebrate receptors cloned so far.

Zinc inhibition of adenylyl cyclase correlates with conformational changes in the enzyme

We have previously demonstrated that Zn(2+) inhibits hormone and forskolin stimulation of cAMP synthesis in intact N18TG2 cells, corresponding plasma membranes, and of recombinant adenylyl cyclase isoforms. If, however, the enzyme is pre-activated by hormone or forskolin, Zn(2+) inhibition is attenuated [J. Biol. Chem. 277 (2002) 11859]. We have extended our analyses of this inhibition to investigations of soluble adenylyl cyclase, composed of the CI and CII domains of the full-length protein. The properties of Zn(2+) inhibition of the soluble enzyme parallel that of the full-length protein, including the fact that inhibition is not competitive with Mg(2+). By monitoring intrinsic and extrinsic fluorescence, we demonstrate changes in enzyme conformers in response to the addition of varied effectors. The data suggest a possible mechanism by which Zn(2+) inhibits adenylyl cyclase activity.

Structural basis for the inhibition of mammalian membrane adenylyl cyclase by 2 '(3')-O-(N-Methylanthraniloyl)-guanosine 5 '-triphosphate

Membrane-bound mammalian adenylyl cyclase (mAC) catalyzes the synthesis of intracellular cyclic AMP from ATP and is activated by stimulatory G protein alpha subunits (Galpha(s)) and by forskolin (FSK). mACs are inhibited with high potency by 2 '(3')-O-(N-methylanthraniloyl) (MANT)-substituted nucleotides. In this study, the crystal structures of the complex between Galpha(s).GTPgammaS and the catalytic C1 and C2 domains from type V and type II mAC (VC1.IIC2), bound to FSK and either MANT-GTP.Mg(2+) or MANT-GTP.Mn(2+) have been determined. MANT-GTP coordinates two metal ions and occupies the same position in the catalytic site as P-site inhibitors and substrate analogs. However, the orientation of the guanine ring is reversed relative to that of the adenine ring. The MANT fluorophore resides in a hydrophobic pocket at the interface between the VC1 and IIC2 domains and prevents mAC from undergoing the "open" to "closed" domain rearrangement. The K(i) of MANT-GTP for inhibition of VC1.IIC2 is lower in the presence of mAC activators and lower in the presence of Mn(2+) compared with Mg(2+), indicating that the inhibitor binds more tightly to the catalytically most active form of the enzyme. Fluorescence resonance energy transfer-stimulated emission from the MANT fluorophore upon excitation of Trp-1020 in the MANT-binding pocket of IIC2 is also stronger in the presence of FSK. Mutational analysis of two non-conserved amino acids in the MANT-binding pocket suggests that residues outside of the binding site influence isoform selectivity toward MANT-GTP.

Analysis of nitric oxide-sensitive guanylyl cyclase in human platelets before and after aggregation

Nitric oxide (NO) inhibits cell adhesion to vascular endothelium and platelet aggregation through activation of soluble guanylyl cyclase (sGC) and a consequent increase in cGMP. The aim of the present study was to analyze NO-sensitive sGC in human platelets before and after aggregation. NO-sensitive sGC activity was tested in the cytosol and membrane fractions of native human platelets and ADP-induced platelet aggregates in the presence of 3 mM Mn2+ as cofactor. After ADP-induced platelet aggregation there was a significant increase of sGC activity in membranes. Western blot analysis showed a partial translocation of the enzyme to the plasma membrane. These findings support recent data that sGC is associated with cellular membranes in various tissues and cell types and that this membrane association is influenced by the activation state in human platelets (Nat Cell Biol 2002; 4: 307-11). Using 3 mM Mg2+ instead of Mn2+ as cofactor, a sharp decrease of sGC activity was apparent in the cytosol of aggregated platelets. Kinetic analysis of the cytosolic enzyme and concentration-response curves for free Mg2+ showed that platelet aggregation changes binding of free Mg2+ but not binding of the substrate complex Mg.GTP. This effect was specific for free Mg2+ and was not seen for Mn2+. In addition, changes in free Mg2+ concentration in a physiological range markedly influenced NO-stimulated sGC activity. This provides a possible explanation for the increased platelet aggregability in patients with low intraplatelet Mg2+ levels.

Differential inhibition of adenylyl cyclase isoforms and soluble guanylyl cyclase by purine and pyrimidine nucleotides

Mammals express nine membranous adenylyl cyclase isoforms (ACs 1-9), a structurally related soluble guanylyl cyclase (sGC) and a soluble AC (sAC). Moreover, Bacillus anthracis and Bacillus pertussis produce the AC toxins, edema factor (EF), and adenylyl cyclase toxin (ACT), respectively. 2'(3')-O-(N-methylanthraniloyl)-guanosine 5'-[gamma-thio]triphosphate is a potent competitive inhibitor of AC in S49 lymphoma cell membranes. These data prompted us to study systematically the effects of 24 nucleotides on AC in S49 and Sf9 insect cell membranes, ACs 1, 2, 5, and 6, expressed in Sf9 membranes and purified catalytic subunits of membranous ACs (C1 of AC5 and C2 of AC2), sAC, sGC, EF, and ACT in the presence of MnCl(2). N-Methylanthraniloyl (MANT)-GTP inhibited C1.C2 with a K(i) of 4.2 nm. Phe-889 and Ile-940 of C2 mediate hydrophobic interactions with the MANT group. MANT-inosine 5'-[gamma-thio]triphosphate potently inhibited C1.C2 and ACs 1, 5, and 6 but exhibited only low affinity for sGC, EF, ACT, and G-proteins. Inosine 5'-[gamma-thio]triphosphate and uridine 5'-[gamma-thio]triphosphate were mixed G-protein activators and AC inhibitors. AC5 was up to 15-fold more sensitive to inhibitors than AC2. EF and ACT exhibited unique inhibitor profiles. At sAC, 2',5'-dideoxyadenosine 3'-triphosphate was the most potent compound (IC(50), 690 nm). Several MANT-adenine and MANT-guanine nucleotides inhibited sGC with K(i) values in the 200-400 nm range. UTP and ATP exhibited similar affinities for sGC as GTP and were mixed sGC substrates and inhibitors. The exchange of MnCl(2) against MgCl(2) reduced inhibitor potencies at ACs and sGC 1.5-250-fold, depending on the nucleotide and cyclase studied. The omission of the NTP-regenerating system from cyclase reactions strongly reduced the potencies of MANT-ADP, indicative for phosphorylation to MANT-ATP by pyruvate kinase. Collectively, AC isoforms and sGC are differentially inhibited by purine and pyrimidine nucleotides.

Regulation and organization of adenylyl cyclases and cAMP

Adenylyl cyclases are a critically important family of multiply regulated signalling molecules. Their susceptibility to many modes of regulation allows them to integrate the activities of a variety of signalling pathways. However, this property brings with it the problem of imparting specificity and discrimination. Recent studies are revealing the range of strategies utilized by the cyclases to solve this problem. Microdomains are a consequence of these solutions, in which cAMP dynamics may differ from the broad cytosol. Currently evolving methodologies are beginning to reveal cAMP fluctuations in these various compartments.

Kinetic properties of "soluble" adenylyl cyclase. Synergism between calcium and bicarbonate

"Soluble" adenylyl cyclase (sAC) is a widely expressed source of cAMP in mammalian cells that is evolutionarily, structurally, and biochemically distinct from the G protein-responsive transmembrane adenylyl cyclases. In contrast to transmembrane adenylyl cyclases, sAC is insensitive to heterotrimeric G protein regulation and forskolin stimulation and is uniquely modulated by bicarbonate ions. Here we present the first report detailing kinetic analysis and biochemical properties of purified recombinant sAC. We confirm that bicarbonate regulation is conserved among mammalian sAC orthologs and demonstrate that bicarbonate stimulation is consistent with an increase in the V(max) of the enzyme with little effect on the apparent K(m) for substrate, ATP-Mg(2+). Bicarbonate can further increase sAC activity by relieving substrate inhibition. We also identify calcium as a direct modulator of sAC activity. In contrast to bicarbonate, calcium stimulates sAC activity by decreasing its apparent K(m) for ATP-Mg(2+). Because of their different mechanisms, calcium and bicarbonate synergistically activate sAC; therefore, small changes of either calcium or bicarbonate will lead to significant changes in cellular cAMP levels.

Loss of G protein gamma 7 alters behavior and reduces striatal alpha(olf) level and cAMP production

The G protein beta gamma-dimer is required for receptor interaction and effector regulation. However, previous approaches have not identified the physiologic roles of individual subtypes in these processes. We used a gene knockout approach to demonstrate a unique role for the G protein gamma(7)-subunit in mice. Notably, deletion of Gng7 caused behavioral changes that were associated with reductions in the alpha(olf)-subunit content and adenylyl cyclase activity of the striatum. These data demonstrate that an individual gamma-subunit contributes to the specificity of a given signaling pathway and controls the formation or stability of a particular G protein heterotrimer.

A critical interplay between Ca2+ inhibition and activation by Mg2+ of AC5 revealed by mutants and chimeric constructs

Adenylyl cyclase type 5 (AC5) is sensitive to both high and low affinity inhibition by Ca(2+). This property provides a sensitive feedback mechanism of the Ca(2+) entry that is potentiated by cAMP in sources where AC5 is commonly expressed (e.g. myocardium). Remarkably little is known about the molecular mechanism whereby Ca(2+) inhibits AC5. Because previous studies had showed that Ca(2+) antagonized the activation of adenylyl cyclase brought about by Mg(2+), we have now evaluated the Mg(2+)-binding domain in the catalytic site as the potential site of the interaction, using a number of mutations of AC5 with impaired Mg(2+) activation. Mg(2+) activation exerted contrasting effects on the high and low affinity Ca(2+) inhibition. In both wild type and mutants, activation by Mg(2+) decreased the absolute amount of high affinity inhibition without affecting the K(i) value, whereas the K(i) value for low affinity inhibition was decreased. These effects were directly proportional to the sensitivity of the mutants to Mg(2+). Parallel changes were noted in the efficacies of Ca(2+), Sr(2+), and Ba(2+) in the mutant species, suggesting a simple mutation in a shared domain. Strikingly, forskolin, which activates by a mechanism different from Mg(2+), did not modify inhibition by Ca(2+). Deletion of the N terminus and the C1b domain of AC5 and a chimera formed with AC2 confirmed that the catalytic domain alone was responsible for high affinity inhibition. We therefore conclude that both low and high affinity inhibition by Ca(2+) are exerted on different conformations of the Mg(2+)-binding sites in the catalytic domain of AC5.

Functional characterization of the adenylyl cyclase gene sgs-1 by analysis of a mutational spectrum in Caenorhabditis elegans

The sgs-1 (suppressor of activated Galpha(s)) gene encodes one of the four adenylyl cyclases in the nematode C. elegans and is most similar to mammalian adenylyl cyclase type IX. We isolated a complete loss-of-function mutation in sgs-1 and found it to result in animals with retarded development that arrest in variable larval stages. sgs-1 mutant animals exhibit lethargic movement and pharyngeal pumping and (while not reaching adulthood) have a mean life span that is > 50% extended compared to wild type. An extensive set of reduction-of-function mutations in sgs-1 was isolated in a screen for suppressors of a neuronal degeneration phenotype induced by the expression of a constitutively active version of the heterotrimeric Galpha(s) subunit of C. elegans. Although most of these mutations change conserved residues within the catalytic domains of sgs-1, mutations in the less-conserved transmembrane domains are also found. The sgs-1 reduction-of-function mutants are viable and have reduced locomotion rates, but do not show defects in pharyngeal pumping or life span.

Expression, purification, and assay of cytosolic (catalytic) domains of membrane-bound mammalian adenylyl cyclases

The identification and isolation of the soluble catalytic domains of adenylyl cyclase have provided investigators with useful reagents for the study of these enzymes. They have permitted detailed mechanistic investigation of the actions of forskolin, Gs alpha, and the inhibitory G protein, Gi alpha. They have served as critical reagents for the development of plausible models of the catalytic mechanism of the enzyme. They have enabled X-ray crystallographic analysis of adenylyl cyclase; this technique was not approachable with the small quantities of the membrane-bound enzyme available previously. The information obtained by using the soluble domains of adenylyl cyclase has provided templates for description of the behavior of many forms of purine nucleotide cyclases from a variety of species. We now appreciate both adenylyl cyclases and guanylyl cyclases as dimeric enzymes with a 2-fold symmetrical domain arrangement (or pseudosymmetrical in the case of heterodimerization). The active sites are located at the interface between the two domains, both of which contribute binding surfaces.

Genetic selection of regulatory mutants of mammalian adenylyl cyclases

Initial steps in the identification of the Gs alpha-binding site present in mammalian adenylyl cyclases can be achieved with the use of the yeast genetic system described. It must be stressed that this system serves as a means to identify mutants that are candidates; biochemical analysis of these mutants is a next and necessary step in the confirmation of these phenotypes. The system described can be readily adapted for the isolation of additional classes of mammalian adenylyl cyclase mutants including mutants with altered regulation toward forskolin, catalytic abnormalities, or enhanced sensitivities toward activators. In addition, this system can be employed for the isolation of constitutively active adenylyl cyclase mutants, or by coexpressing other adenylyl cyclase isoforms and their known regulators, mutations in the binding sites for these molecules can be elucidated.

Zinc inhibition of cAMP signaling

Zn(2+) is required as either a catalytic or structural component for a large number of enzymes and thus contributes to a variety of important biological processes. We report here that low micromolar concentrations of Zn(2+) inhibited hormone- or forskolin-stimulated cAMP production in N18TG2 neuroblastoma cells. Similarly, low concentrations inhibited hormone- and forskolin-stimulated adenylyl cyclase (AC) activity in membrane preparations and did so primarily by altering the V(max) of the enzyme. Zn(2+) also inhibited recombinant isoforms, indicating that this reflects a direct interaction with the enzyme. The IC(50) for Zn(2+) inhibition was approximately 1-2 microm with a Hill coefficient of 1.33. The dose-response curve for Zn(2+) inhibition was identical for AC1, AC5, and AC6 as well as for the C441R mutant of AC5 whose defect appears to be in one of the catalytic metal binding sites. However, AC2 displayed a distinct dose-response curve. These data in combination with the findings that Zn(2+) inhibition was not competitive with Mg(2+) or Mg(2+)/ATP suggest that the inhibitory Zn(2+) binding site is distinct from the metal binding sites involved in catalysis. The prestimulated enzyme was found to be less susceptible to Zn(2+) inhibition, suggesting that the ability of Zn(2+) to inhibit AC could be significantly influenced by the coincidence timing of the input signals to the enzyme.

Regulation of adenylyl cyclases by a region outside the minimally functional cytoplasmic domains

The highly conserved topological structure of G protein-activated adenylyl cyclases seems unnecessary because the soluble cytoplasmic domains retain regulatory and catalytic properties. Yet, we previously isolated a constitutively active mutant of the Dictyostelium discoideum adenylyl cyclase harboring a single point mutation in the region linking the cytoplasmic and membrane domains (Leu-394). We show here that multiple amino acid substitutions at Leu-394 also display constitutive activity. The constitutive activity of these mutants is not dependent on G proteins or cytosolic regulators, although some of the mutants can be activated to higher levels than wild type. Combining a constitutive mutation such as L394T with K482N, a point mutation that renders the enzyme insensitive to regulators, restores an enzyme with wild type properties of low basal activity and the capacity to be activated by G proteins. Thus regions located outside the cytoplasmic loops of adenylyl cyclases are not only important in the acquisition of an activated conformation, they also have impact on other regions within the catalytic core of the enzyme.

The gametocyte-activating factor xanthurenic acid stimulates an increase in membrane-associated guanylyl cyclase activity in the human malaria parasite Plasmodium falciparum

Sex is an obligate step in the life cycle of the malaria parasite and occurs in the midgut of the mosquito vector. With both Plasmodium falciparum and Plasmodium berghei, the tryptophan metabolite xanthurenic acid induces the release of motile male gametes from red blood cells (exflagellation), a prerequisite for fertilization. The addition of cGMP or phosphodiesterase inhibitors to cultures of mature gametocytes has also been shown to stimulate exflagellation. Here, we demonstrate that there is a guanylyl cyclase activity associated with mature P. falciparum gametocyte membrane preparations, which is dependent on the presence of Mg(2+)/Mn(2+) but is inhibited by Ca(2+). Significantly, this activity is increased on addition of xanthurenic acid. In contrast, a xanthurenic acid precursor (3-hydroxykynurenine), which is not an inducer of exflagellation, does not induce this guanylyl cyclase activity. These results therefore suggest that xanthurenic acid-induced exflagellation may be mediated by activation of the parasite cGMP signalling pathway.

Interactions within the coiled-coil domain of RetGC-1 guanylyl cyclase are optimized for regulation rather than for high affinity

RetGC-1, a member of the membrane guanylyl cyclase family of proteins, is regulated in photoreceptor cells by a Ca(2+)-binding protein known as GCAP-1. Proper regulation of RetGC-1 is essential in photoreceptor cells for normal light adaptation and recovery to the dark state. In this study we show that cGMP synthesis by RetGC-1 requires dimerization, because critical functions in the catalytic site must be provided by each of the two polypeptide chains of the dimer. We also show that an intact alpha-helical coiled-coil structure is required to provide dimerization strength for the catalytic domain of RetGC-1. However, the dimerization strength of this domain must be precisely optimized for proper regulation by GCAP-1. We found that Arg(838) within the dimerization domain establishes the Ca(2+) sensitivity of RetGC-1 by determining the strength of the coiled-coil interaction. Arg(838) substitutions dominantly enhance cGMP synthesis even at the highest Ca(2+) concentrations that occur in normal dark-adapted photoreceptor cells. Molecular dynamics simulations suggest that Arg(838) substitutions disrupt a small network of salt bridges to allow an abnormal extension of coiled-coil structure. Substitutions at Arg(838) were first identified by linkage to the retinal degenerative disease, autosomal dominant cone rod dystrophy (adCORD). Consistent with the characteristics of this disease, the Arg(838)-substituted RetGC-1 mutants exhibit a dominant biochemical phenotype. We propose that accelerated cGMP synthesis in humans with adCORD is the primary cause of cone-rod degeneration.

Calcium ion downregulates soluble guanylyl cyclase activity: evidence for a two-metal ion catalytic mechanism

The inhibition of soluble guanylyl cyclase by Ca2+ has been kinetically characterized and the results support a two-metal-ion catalytic mechanism for formation of cGMP. Ca2+ reversibly inhibits both the basal and NO-stimulated forms of bovine lung soluble guanylyl cyclase. Inhibition is independent of the activator identity and concentration, revealing that Ca2+ interacts with a site independent of the heme regulatory site. Inhibition by Ca2+ is competitive with respect to Mg2+ in excess of substrate, with Kis values of 29 +/- 4 and 6.6 +/- 0.6 microM for the basal and activated states, respectively. Ca2+ inhibits noncompetitively with respect to the substrate MgGTP in both activity states. The qualitatively similar inhibition pattern and quantitatively different Ki values between the basal and NO-stimulated states suggest that the Ca2+ binding site undergoes some structural modification upon activation of the enzyme. The competitive nature of Ca2+ inhibition with respect to excess Mg2+ is consistent with a two-metal-ion mechanism for cyclization.

Isolation and characterization of constitutively active mutants of mammalian adenylyl cyclase

A genetic screen in Saccharomyces cerevisiae identified mutations in mammalian adenylyl cyclase that activate the enzyme in the absence of G(s)alpha. Thirteen of these mutant proteins were characterized biochemically in an assay system that depends on a mixture of the two cytosolic domains (C(1) and C(2)) of mammalian adenylyl cyclases. Three mutations, I1010M, K1014N, and P1015Q located in the beta4-beta5 loop of the C(2) domain of type II adenylyl cyclase, increase enzymatic activity in the absence of activators. The K1014N mutation displays both increased maximal activity and apparent affinity for the C(1) domain of type V adenylyl cyclase in the absence of activators of the enzyme. The increased affinity of the mutant C(2) domain of adenylyl cyclase for the wild type C(1) domain was exploited to isolate a complex containing VC(1), IIC(2), and G(s)alpha-guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) in the absence of forskolin and a complex of VC(1), IIC(2), forskolin, and P-site inhibitor in the absence of G(s)alpha-GTPgammaS. The isolation of these complexes should facilitate solution of crystal structures of low activity states of adenylyl cyclase and thus determination of the mechanism of activation of the enzyme by forskolin and G(s)alpha.

Guanylyl cyclase activity associated with putative bifunctional integral membrane proteins in Plasmodium falciparum

We report here that guanylyl cyclase activity is associated with two large integral membrane proteins (PfGCalpha and PfGCbeta) in the human malaria parasite Plasmodium falciparum. Unusually, the proteins appear to be bifunctional; their amino-terminal regions have strong similarity with P-type ATPases, and the sequence and structure of the carboxyl-terminal regions conform to that of G protein-dependent adenylyl cyclases, with two sets of six transmembrane sequences, each followed by a catalytic domain (C1 and C2). However, amino acids that are enzymatically important and present in the C2 domain of mammalian adenylyl cyclases are located in the C1 domain of the P. falciparum proteins and vice versa. In addition, certain key residues in these domains are more characteristic of guanylyl cyclases. Consistent with this, guanylyl cyclase activity was obtained following expression of the catalytic domains of PfGCbeta in Escherichia coli. In P. falciparum, expression of both genes was detectable in the sexual but not the asexual blood stages of the life cycle, and PfGCalpha was localized to the parasite/parasitophorous vacuole membrane region of gametocytes. The profound structural differences identified between mammalian and parasite guanylyl cyclases suggest that aspects of this signaling pathway may be mechanistically distinct.

Inhibition by calcium of mammalian adenylyl cyclases

Ca(2+) regulates mammalian adenylyl cyclases in a type-specific manner. Stimulatory regulation is moderately well understood. By contrast, even the concentration range over which Ca(2+) inhibits adenylyl cyclases AC5 and AC6 is not unambiguously defined; even less so is the mechanism of inhibition. In the present study, we compared the regulation of Ca(2+)-stimulable and Ca(2+)-inhibitable adenylyl cyclases expressed in Sf9 cells with tissues that predominantly express these activities in the mouse brain. Soluble forms of AC5 containing either intact or truncated major cytosolic domains were also examined. All adenylyl cyclases, except AC2 and the soluble forms of AC5, displayed biphasic Ca(2+) responses, suggesting the presence of two Ca(2+) sites of high ( approximately 0.2 microM) and low affinity ( approximately 0.1 mM). With a high affinity, Ca(2+) (i) stimulated AC1 and cerebellar adenylyl cyclases, (ii) inhibited AC6 and striatal adenylyl cyclase, and (iii) was without effect on AC2. With a low affinity, Ca(2+) inhibited all adenylyl cyclases, including AC1, AC2, AC6, and both soluble forms of AC5. The mechanism of both high and low affinity inhibition was revealed to be competition for a stimulatory Mg(2+) site(s). A remarkable selectivity for Ca(2+) was displayed by the high affinity site, with a K(i) value of approximately 0.2 microM, in the face of a 5000-fold excess of Mg(2+). The present results show that high and low affinity inhibition by Ca(2+) can be clearly distinguished and that the inhibition occurs type-specifically in discrete adenylyl cyclases. Distinction between these sites is essential, or quite spurious inferences may be drawn on the nature or location of high affinity binding sites in the Ca(2+)-inhibitable adenylyl cyclases.

Retroviral DNA integration

DNA integration is a unique enzymatic process shared by all retroviruses and retrotransposons. During integration, double-stranded linear viral DNA is inserted into the host genome in a process catalyzed by the virus-encoded integrase (IN). The mechanism involves a series of nucleophilic attacks, the first of which removes the terminal 2 bases from the 3' ends of the long terminal repeats and of the second which inserts the viral DNA into the host genome. IN specifically recognizes the DNA sequences at the termini of the viral DNA, juxtaposing both ends in an enzyme complex that inserts the viral DNA into a single site in a concerted manner. Small duplications of the host DNA, characteristic of the viral IN, are found at the sites of insertion. At least two host proteins, HMG-I(Y) and BAF, have been shown to increase the efficiency of the integration reaction.

Trypanosoma cruzi adenylyl cyclase is encoded by a complex multigene family

The parasitic protozoan Trypanosoma cruzi undergoes several differentiation events during its life cycle. Some of these transitions are thought to involve activation of adenylyl cyclase via the binding of peptide ligands to the cell surface. Here we describe the characterisation of the adenylyl cyclase gene family of T. cruzi. Two complete genes and one pseudogene have been sequenced. The protein products appear to have a large extracellular domain, a single transmembrane helix and a cytosolic catalytic domain. The adenylyl cyclase genes are present on at least six chromosomes and are scattered rather than clustered. They form a large polymorphic family in which the extracellular domain is particularly variable. An Escherichia coli adenylyl cyclase mutant could be complemented by expression of the catalytic domain of the T. cruzi enzyme. The recombinant protein had adenylyl cyclase activity in vitro, which was enhanced by increasing concentrations of divalent cations (Mn2+ > Mg2+). This constitutively active recombinant protein will be a useful tool for dissecting the catalytic mechanism of adenylyl cyclase.

Activating mutation of adenylyl cyclase reverses its inhibition by G proteins

We have implemented a yeast genetic selection developed previously by our laboratory to identify mutant mammalian type V adenylyl cyclases insensitive to inhibition by G(ialpha.) One mutation isolated was localized to the first cytoplasmic domain at a Phe residue (position 400), which is conserved in all nine isoforms of membrane-bound mammalian adenylyl cyclase. Biochemical characterization of the F400Y mutant revealed a dramatic conversion of the G(ialpha) response from inhibitory to stimulatory. This mutation results in additional activating effects. The mutant exhibits an enhanced sensitivity toward activation by either G(salpha) or forskolin. Synergism between G(salpha) and forskolin is not observed for the F400Y mutant, presumably because the mutant already is in the sensitized state. Additionally, an enhancement of the basal unstimulated activity was observed. This mutation, which is the first demonstration of an activating point in a mammalian adenylyl cyclase, mimics a sensitized conformation of the wild-type enzyme that underlies the synergism between stimulatory inputs, and additionally, removes the inhibitory regulatory input provided by G(ialpha). Because sensitizing adenylyl cyclase toward its stimulators can have profound biological implications, this raises the possibility that naturally occurring mutations resembling those at the Phe400 residue may be associated with human disease states.

Two-metal-Ion catalysis in adenylyl cyclase

Adenylyl cyclase (AC) converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate, a ubiquitous second messenger that regulates many cellular functions. Recent structural studies have revealed much about the structure and function of mammalian AC but have not fully defined its active site or catalytic mechanism. Four crystal structures were determined of the catalytic domains of AC in complex with two different ATP analogs and various divalent metal ions. These structures provide a model for the enzyme-substrate complex and conclusively demonstrate that two metal ions bind in the active site. The similarity of the active site of AC to those of DNA polymerases suggests that the enzymes catalyze phosphoryl transfer by the same two-metal-ion mechanism and likely have evolved from a common ancestor.

The 5-HT1A receptor and its ligands: structure and function

An overview is presented on progress made in research on 5-HT1A receptors and their ligands since their discovery in 1983. Molecular biology has offered new tools, for example cloned 5-HT1A receptors, their mutants and chimeras to study structure and function. Many compounds, belonging to different chemical classes, display high affinity and selectivity for 5-HT1A receptors. The majority of these compounds are agonists or partial agonists, full antagonists are still scarce. Agonists and partial agonists are active in various animal models of anxiety and depression. Partial receptor agonists have been proven to be effective in general anxiety disorder and depression in man. Potential therapeutic applications for 5-HT1A receptor antagonists are evaluated, for example, in cognition disorders.

From protein sequence to function

As genome sequences and protein structures are deciphered, we wish to predict their corresponding functions. Many functions cannot be told from from the sequence, however, although there has been progress in this quest for an impossible Grail. Furthermore, a structure and its corresponding sequence become most interesting when one knows the function. Inductive reasoning, based on the integration of biological and sequence knowledge, should enable sequence and functional data to be combined in a productive way.

The interactions of adenylate cyclases with P-site inhibitors

Recent kinetic, binding and crystallographic studies using P-site inhibitors of mammalian adenylate bases provide new insights into the catalytic mechanism of these highly regulated enzymes. Here, Carmen Dessauer and colleagues discuss the conformational states of adenylate cyclase, the structural determinants of inhibitor binding and the potential uses of these inhibitors as pharmacological agents.

The structure, catalytic mechanism and regulation of adenylyl cyclase

The recent structure determinations of the mammalian effector enzyme adenylyl cyclase reveal the structure of its catalytic core, provide new insights into its catalytic mechanism and suggest how diverse signaling molecules regulate its activity.