Showing papers on "Cyclase published in 2005"

Compartmentalization of adenylate cyclase and cAMP signalling.

Abstract: Concepts of cAMP signalling have changed dramatically from the linear cascades of just a few years ago, with the realization that numerous cellular processes affect this motif. These influences include other signalling pathways – most significantly Ca2+, scaffolding proteins (which are themselves variously regulated) to organize the elements of the pathway, and subcellular targeting of components. An obvious implication of this organization is that global measurements of cAMP may trivialize the complexity of the cAMP signals and obscure the regulation of targets. In this presentation, current developments on the targeting and assembly of ACs (adenylate cyclases) and their delivery to selected raft or non-raft domains of the plasma membrane will be discussed, along with the susceptibility of raft-targeted ACs to very discrete modes of increases in the intracellular Ca2+ concentration. Single-cell explorations of cAMP dynamics, as measured with cyclic nucleotide-gated channels, are also described in this paper, particularly as applied to cells in which the composition of AKAP (A-kinase anchoring protein)–PKA (protein kinase A)–PDE (phosphodiesterase) assemblies is probed by RNA interference ablation of defined AKAPs. Abbreviations: AC, adenylate cyclase; AKAP, A-kinase anchoring protein; CCE, capacitative Ca2+ entry; CNG channel, cyclic nucleotide-gated channel; FRET, fluorescence resonance energy transfer; GPCR, G-protein-coupled receptor; HEK-293, cell, human embryonic kidney 293 cell; NHE, Na+/H+ exchanger; PDE, phosphodiesterase; PKA, protein kinase A; TM, transmembrane; TMD, TM domain

Xantha-l encodes a membrane subunit of the aerobic Mg-protoporphyrin IX monomethyl ester cyclase involved in chlorophyll biosynthesis

Abstract: Chlorophyll biosynthesis is a process involving ≈20 different enzymatic steps. Half of these steps are common to the biosynthesis of other tetrapyrroles, such as heme. One of the least understood enzymatic steps is formation of the isocyclic ring, which is a characteristic feature of all (bacterio)chlorophyll molecules. In chloroplasts, formation of the isocyclic ring is an aerobic reaction catalyzed by Mg-protoporphyrin IX monomethyl ester cyclase. An in vitro assay for the aerobic cyclase reaction required membrane-bound and soluble components from the chloroplasts. Extracts from barley (Hordeum vulgare L.) mutants at the Xantha-l and Viridis-k loci showed no cyclase activity. Fractionation of isolated plastids by Percoll gradient centrifugation showed that xantha-l and viridis-k mutants are defective in components associated with chloroplast membranes. The Xantha-l gene, corresponding to Arabidopsis thaliana CHL27, Rubrivivax gelatinosus acsF, Chlamydomonas reinhardtii CRD1, and CTH1 and situated at the short arm of barley chromosome 3 (3H), was cloned, and the mutations in xantha-l35, xantha-l81, and xantha-l82 were characterized. This finding connected biochemical and genetic data because it demonstrated that Xantha-l encodes a membrane-bound cyclase subunit. The evidence suggests that the aerobic cyclase requires at least one soluble and two membrane-bound components.

Origin of asymmetry in adenylyl cyclases: structures of Mycobacterium tuberculosis Rv1900c.

Abstract: Rv1900c, a Mycobacterium tuberculosis adenylyl cyclase, is composed of an N-terminal α/β-hydrolase domain and a C-terminal cyclase homology domain. It has an unusual 7% guanylyl cyclase side-activity. A canonical substrate-defining lysine and a catalytic asparagine indispensable for mammalian adenylyl cyclase activity correspond to N342 and H402 in Rv1900c. Mutagenic analysis indicates that these residues are dispensable for activity of Rv1900c. Structures of the cyclase homology domain, solved to 2.4 A both with and without an ATP analog, form isologous, but asymmetric homodimers. The noncanonical N342 and H402 do not interact with the substrate. Subunits of the unliganded open dimer move substantially upon binding substrate, forming a closed dimer similar to the mammalian cyclase heterodimers, in which one interfacial active site is occupied and the quasi-dyad-related active site is occluded. This asymmetry indicates that both active sites cannot simultaneously be catalytically active. Such a mechanism of half-of-sites-reactivity suggests that mammalian heterodimeric adenylyl cyclases may have evolved from gene duplication of a primitive prokaryote-type cyclase, followed by loss of function in one active site.

Inactivation of the mouse adenylyl cyclase 3 gene disrupts male fertility and spermatozoon function.

Abstract: Mammalian spermatids and spermatozoa express functional G protein-coupled receptors. However, bicarbonate-regulated soluble adenylyl cyclase (AC), the major AC present in these cells, is not directly coupled to G proteins. To understand how G protein-coupled receptors signal in spermatozoa, we investigated whether a conventional transmembrane cyclase is present and biologically active in these cells. Here, we provide evidence for expression of type 3 AC (AC3) in male germ cells and describe the effects of disruption of the AC3 gene on fertility and function of mouse spermatozoa. As previously reported in rat, AC3 mRNA is expressed in mouse testes and localized, together with soluble AC mRNA, mainly in postmeiotic germ cells. AC3 protein was detected by immunolocalization in round and elongating spermatids in a region corresponding to the developing acrosome and was retained in the mature spermatozoa of the epididymis. Forskolin caused a small increase in cAMP production in mouse spermatozoa, but this incr...

Soluble guanylyl cyclase: the nitric oxide receptor.

Abstract: Soluble guanylyl cyclase is recognized as the most sensitive physiologic receptor for nitric oxide. Binding of nitric oxide to the heme moiety of the cyclase induces its capacity to synthesize the second messenger cGMP. Although the changes in the state of the heme moiety upon exposure of enzyme to NO and its correlation to the stimulation of sGC catalytic activity are well documented, the exact mechanism of such coupling is not understood. Structure-functional studies are required to elucidate this process. In this chapter, we describe the method of expression and purification of recombinant human alpha1/beta1 isoform of sGC in insect cells, which can be a useful tool for such studies. Several approaches that enable characterization of the binding of NO to sGC heme moiety are also described.

Non-specific Lanosterol and Hopanoid Biosynthesis by a Cell-Free System from the Bacterium Methylococcus capsulatus

Abstract: 1. A cell-free system from the bacterium Methylococcus capsulatus was incubated with [12-3H]-squalene; diploptene and diplopterol, normally present in the bacterium, were labelled. 2 The same cell-free system was incubated with (RS)-2,3-epoxy-2,3-dihydro-[3-3H]squalene. Several radioactive 3-hydroxytriterpenes were purifed. Lanosterol, which is normally present in this bacterium, was found labelled as well as 3-epilanosterol. In addition, radioactive 3 alpha-hydroxy and 3 beta-hydroxydiploptene were formed. 3. These data may be explained by the coexistence of two cyclases in M. capsulatus: a squalene/hopane cyclase and a squalene epoxide/lanosterol cyclase. The squalene cyclase exhibits the same lack of substrate specificity as those of Acetobacter pasteurianum and Tetrahymena pyriformis, i.e. in addition to its normal substrate squalene, it can cyclize the two enantiomers of squalene epoxide into 3-hydroxyhopanoids. 4. The presence of a squalene epoxide/lanosterol cyclase activity, which was suspected in view of the unique 3 beta-hydroxy 4 alpha-methyl steroids of M. capsulatus, was demonstrated by the labelling of lanosterol. More surprisingly 3-epilanosterol was also present and labelled. We showed that this does not derive from lanosterol by isomerization via a 3-oxo compound. Therefore the squalene expoxide cyclase of M. capsulatus, like the one of eukaryotes cyclizes the (3S) enantiomer of squalene epoxide into lanosterol. But it is definitely less substrate-specific as it can also cyclize the (3R) enantiomer into 3-epilanosterol.

Guanylyl cyclases across the tree of life.

Abstract: This review explores the origins, diversity and functions of guanylyl cyclases in cellular organisms. In eukaryotes both cGMP and cAMP are produced by the conserved class III cyclase domains, while prokaryotes use five more unrelated catalysts for cyclic nucleotide synthesis. The class III domain is found embedded in proteins with a large variety of membrane topologies and other functional domains, but the vertebrate guanylyl cyclases take only two forms, the receptor guanylyl cyclases with single transmembrane domain and the soluble enzymes with heme binding domain. The invertebrates additionally show a soluble guanylyl cyclase that cannot bind heme, while the more basal metazoans may lack the heme binding enzymes altogether. Fungi, the closest relatives of the metazoans, completely lack guanylyl cylases, but they appear again in the Dictyostelids, the next relative in line. Remarkably, the two Dictyostelid guanylyl cyclases have little in common with the vertebrate enzymes. There is a soluble guanylyl cyclase, which shows greatest sequence and structural similarity to the vertebrate soluble adenylyl cyclase, and a membrane-bound form with the same configuration as the dodecahelical adenylyl cyclases of vertebrates. There is a difference, the pseudosymmetric C1 and C2 catalytic domains have swapped position in the Dictyostelium enzyme. Unlike the vertebrate guanylyl cyclases, the Dictyostelium enzymes are activated by heterotrimeric G-proteins. Swapped C1 and C2 domains are also found in the structurally similar guanylyl cyclases of ciliates and apicomplexans, but these enzymes additionally harbour an amino-terminal ATPase module with ten transmembrane domains. G-protein regulation could not be demonstrated for these enzymes. Higher plants lack class III cyclase domains, but an unexplored wealth of guanylyl cyclases is present in the green alga Chlamydomonas. Progenitors of all structural variants of the eukaryote guanylyl cyclases are found among the prokaryote adenylyl cyclases. This and the close similarity of many guanylyl cyclases to adenylyl cyclases suggests a paraphyletic origin for the eukaryote enzymes with multiple events of conversion of substrate specificity.

Bordetella pertussis Adenylate Cyclase Toxin Modulates Innate and Adaptive Immune Responses: Distinct Roles for Acylation and Enzymatic Activity in Immunomodulation and Cell Death

Abstract: Adenylate cyclase toxin (CyaA) of Bordetella pertussis belongs to the repeat in toxin family of pore-forming toxins, which require posttranslational acylation to lyse eukaryotic cells. CyaA modulates dendritic cell (DC) and macrophage function upon stimulation with LPS. In this study, we examined the roles of acylation and enzymatic activity in the immunomodulatory and lytic effects of CyaA. The adenylate cyclase activity of CyaA was necessary for its modulatory effects on murine innate immune cells. In contrast, acylation was not essential for the immunomodulatory function of CyaA, but was required for maximal caspase-3 activation and cytotoxic activity. The wild-type acylated toxin (A-CyaA) and nonacylated CyaA (NA-CyaA), but not CyaA with an inactive adenylate cyclase domain (iAC-CyaA), enhanced TLR-ligand-induced IL-10 and inhibited IL-12, TNF-α, and CCL3 production by macrophages and DC. In addition, both A-CyaA and NA-CyaA, but not iAC-CyaA, enhanced surface expression of CD80 and decreased CpG-stimulated CD40 and ICAM-1 expression on immature DC. Furthermore, both A-CyaA and NA-CyaA promoted the induction of murine IgG1 Abs, Th2, and regulatory T cells against coadministered Ags in vivo, whereas iAC-CyaA had more limited adjuvant activity. In contrast, A-CyaA and iAC-CyaA induced caspase-3 activation and cell death in macrophages, but these effects were considerably reduced or absent with NA-CyaA. Our findings demonstrate that the enzymatic activity plays a critical role in the immunomodulatory effects of CyaA, whereas acylation facilitates the induction of apoptosis and cell lysis, and as such, NA-CyaA has considerable potential as a nontoxic therapeutic molecule with potent anti-inflammatory properties.

Acylation of lysine 860 allows tight binding and cytotoxicity of Bordetella adenylate cyclase on CD11b-expressing cells.

Abstract: The Bordetella adenylate cyclase toxin-hemolysin (CyaA, ACT, or AC-Hly) forms cation-selective membrane channels and delivers into the cytosol of target cells an adenylate cyclase domain (AC) that catalyzes uncontrolled conversion of cellular ATP to cAMP. Both toxin activities were previously shown to depend on post-translational activation of proCyaA to CyaA by covalent palmitoylation of the internal Lys983 residue (K983). CyaA, however, harbors a second RTX acylation site at residue Lys860 (K860), and the role of K860 acylation in toxin activity is unclear. We produced in E. coli the CyaA-K860R and CyaA-K983R toxin variants having the Lys860 and Lys983 acylation sites individually ablated by arginine substitutions. When examined for capacity to form membrane channels and to penetrate sheep erythrocytes, the CyaA-K860R acylated on Lys983 was about 1 order of magnitude more active than CyaA-K983R acylated on Lys860, although, in comparison to intact CyaA, both monoacylated constructs exhibited markedly reduced activities in erythrocytes. Channels formed in lipid bilayers by CyaA-K983R were importantly less selective for cations than channels formed by CyaA-K860R, intact CyaA, or proCyaA, showing that, independent of its acylation status, the Lys983 residue may play a role in toxin structures that determine the distribution of charged residues at the entry or inside of the CyaA channel. While necessary for activity on erythrocytes, acylation of Lys983 was also sufficient for the full activity of CyaA on CD11b+ J774A.1 monocytes. In turn, acylation of Lys860 alone did not permit toxin activity on erythrocytes, while it fully supported the high-affinity binding of CyaA-K983R to the toxin receptor CD11b/CD18 and conferred on CyaA-K983R a reduced but substantial capacity to penetrate and kill the CD11b+ cells. This is the first evidence that acylation of Lys860 may play a role in the biological activity of CyaA, even if redundant to the acylation of Lys983.

Non-specific Biosynthesis of Hopane Triterpenes by a Cell-Free System from Acetobacter pasteurianum

Abstract: 1. A cell-free system from the bacterium Acetobacter pasteurianum was incubated with [12-3H]squalene; diploptene and diplopterol, hopanoids normally present in the bacterium, were labelled. Their radioactivity was confirmed by purification using thin-layer chromatography, synthesis of derivatives and recrystallization to constant specific activity. This demonstrates the direct cyclization of squalene into diploptene and diplopterol, catalysed by a squalene cyclase activity in A. pasteurianum. 2. The same cell-free system transformed (RS)-2,3-epoxy-2,3-dihydro-[12,13-3H]squalene into labelled 3 alpha-hydroxyhop-22(29)-ene, 3 beta-hydroxyhop-22(29)-ene, hopane-3 alpha,22-diol and hopane-3 beta,22-diol. Their radioactivity was similarly confirmed. This bacterial homogenate is thus capable of cyclizing an unnatural substrate, 2,3-epoxy-squalene, into 3-hydroxyhopanoids normally absent in the bacterium. 3. The 3 alpha-hydroxy and 3 beta-hydroxyhopanoids could have been enzymatically interconverted via the 3-oxo compound. Synthetic racemic (RS)-2,3-epoxy-2,3-dihydro-[3-3H]squalene was incubated and gave rise to 3-3H-labelled 3 alpha and 3 beta-hydroxyhopanoids. This excludes an isomerization via a 3-oxo compound which would give unlabelled 3-hydroxyhopanoids. 4. In conclusion, the cyclase of A. pasteurianum accepts the replacement of the normal substrate, squalene, by the corresponding epoxide. Furthermore it is not selective in the stereochemistry of the epoxide and cyclizes both enantiomers, contrary to the epoxysqualene cyclase of eukaryotes.

Domain analysis of human transmembrane guanylyl cyclase receptors: implications for regulation

Abstract: In the human genome, sequence analysis indicates there are five functional transmembrane guanylyl cyclases, enzymes that synthesize the intracellular second messenger, cGMP. Two, GC-A and GC-B or NPR-A and NPR-B, are widely distributed receptors for atrial natriuretic peptide, brain natriuretic peptide and C-type natriuretic peptide, more commonly known as ANP, BNP and CNP, respectively. One cyclase, GC-C or StaR, is predominantly found in the intestinal epithelium and is the receptor for guanylin and uroguanylin, as well as for the bacterial pathogen, heat-stable enterotoxin (Sta). The remaining two cyclases, GC-E and GC-F or RetGC-1 and RetGC-2, are expressed in the retina and regulate the dark cycle of phototransduction. Unlike the other family members, GC-E and GC-F have no known extracellular ligands. Instead, they are activated under low calcium conditions by guanylyl cyclase activating proteins called GCAPs. All five members consist of an extracellular ligand binding domain, single transmembrane spanning domain, and intracellular kinase homology, dimerization and guanylyl cyclase catalytic domains. In the first part of this review, the tissue expression, ligands and "knockout" phenotypes of each receptor are summarized and individual domains are compared. In the second part, regulation by ATP, calcium, protein kinase C and phosphorylation is discussed.

Production of calcium-mobilizing metabolites by a novel member of the ADP-ribosyl cyclase family expressed in Schistosoma mansoni.

Abstract: ADP-ribosyl cyclases are structurally conserved enzymes that are best known for catalyzing the production of the calcium-mobilizing metabolite, cyclic adenosine diphosphate ribose (cADPR), from nicotinamide adenine dinucleotide (NAD + ). However, these enzymes also produce adenosine diphosphate ribose (ADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP + ), both of which have been shown to modulate calcium mobilization in vitro. We have now characterized a new member of the cyclase family from Schistosoma mansoni, a member of the Platyhelminthes phylum. We show that the novel NAD(P) + catabolizing enzyme (NACE) expressed by schistosomes is structurally most closely related to the cyclases cloned from Aplysia but also shows significant homology with the mammalian cyclases, CD38 and CD157. NACE expression is developmentally regulated in schistosomes, and the GPI-anchored protein is localized to the outer tegument of the adult schistosome. Importantly, NACE, like all members of the cyclase family, is a multifunctional enzyme and catalyzes NAD + glycohydrolase and base-exchange reactions to produce ADPR and NAADP + . However, despite being competent to generate a cyclic product from NGD + , a nonphysiologic surrogate substrate, NACE is so far the only enzyme in the cyclase family that is unable to produce significant amounts of cADPR (<0.02% of reaction products) using NAD + as the substrate. This suggests that the other calcium-mobilizing metabolites produced by NACE may be more important for calcium signaling in schistosomes. Alternatively, the function of NACE may be to catabolize extracellular NAD + to prevent its use by host enzymes that utilize this source of NAD + to

'Swell dialysis' demonstrates that adenylate cyclase in Trypanosoma brucei is regulated by calcium ions

Abstract: A new technique, 'swell dialysis', is described that allows the study of cellular enzymes in situ if they are located in the cytoplasm or on the cytoplasmic face of any membrane. The technique is potentially applicable to any population of individual eukaryotic cells that lack a cell wall. In this study swell dialysis has been applied to an examination of the regulation of adenylate cyclase in blood-stream forms of Trypanosoma brucei (427-12/ICI-060). Adenylate cyclase in this protozoon is shown to be controlled by Ca2+. There is a 16-fold stimulation of the cyclase at 100 microM Ca2+ over basal activity without added Ca2+. The ability of Ca2+ to stimulate adenylate cyclase is lost upon rupture of the cell, which is reminiscent of the loss of control of the adenylate cyclase in Escherichia coli by sugars of the phosphotransferase system when cell breakage occurs. The physiological function of the Ca2+ activation of adenylate cyclase in T. brucei has not been established but a possible role in the change of surface coat in bloodstream forms should be considered.

Direct activation of fission yeast adenylate cyclase by the Gpa2 Gα of the glucose signaling pathway

Abstract: G protein-mediated signaling is implicated in yeast and fungal cAMP pathways. By two-hybrid screens and pull-down experiments, we show that the fission yeast Gpa2 Gα binds an N-terminal domain of adenylate cyclase, comprising a moderately conserved sequence within a region otherwise poorly related to other fungal adenylate cyclases. Overexpressing this domain in yeast perturbs cAMP signaling, which is restored by Gpa2 coexpression. Mutations affecting this domain, over 1,100 residues from the catalytic domain, alter glucose-triggered cAMP signaling. This is evidence for direct activation of adenylate cyclase by a fungal G protein and suggests a distinct activation mechanism from that of mammals.

Schizosaccharomyces pombe adenylate cyclase suppressor mutations suggest a role for cAMP phosphodiesterase regulation in feedback control of glucose/cAMP signaling

Abstract: Mutations affecting the Schizosaccharomyces pombe cAMP phosphodiesterase (PDE) gene cgs2+ were identified in a screen for suppressors of mutant alleles of the adenylate cyclase gene (git2+/cyr1+), which encode catalytically active forms of the enzyme that cannot be stimulated by extracellular glucose signaling. These mutations suppress both the git2− mutant alleles used in the suppressor selection and mutations in git1+, git3+, git5+, git7+, git10+, and git11+, which are all required for adenylate cyclase activation. Notably, these cgs2 mutant alleles fail to suppress mutations in gpa2+, which encodes the Gα subunit of a heterotrimeric G protein required for adenylate cyclase activation, although the previously identified cgs2-2 allele does suppress loss of gpa2+. Further analysis of the cgs2-s1 allele reveals a synthetic interaction with the gpa2R176H-activated allele, with respect to derepression of fbp1-lacZ transcription in glucose-starved cells. In addition, direct measurements of cAMP levels show that cgs2-s1 cells maintain normal basal cAMP levels, but are severely defective in feedback regulation upon glucose detection. These results suggest that PDE activity in S. pombe may be coordinately regulated with adenylate cyclase activity as part of the feedback regulation mechanism to limit the cAMP response to glucose detection.

Functional Desensitisation of β‐Adrenergic Receptors of Avian Erythrocytes by Catecholamines and Adenosine 3′,5′‐Phosphate

Abstract: Prolonged exposure to beta-adrenergic agonists of pigeon erythrocytes causes a reversible loss (70%) of catecholamine-stimulated adenylate cyclase activity without reduction in the number of beta-adrenergic receptors. In addition a less pronounced decrease in non-stimulated and NaF-stimulated adenylate cyclase activity (15-22%) is observed, appearing at different agonist concentrations and at a different rate. Dibutyryladenosine 3',5'-phosphate and the phosphodiesterase inhibitor methylisobutylxanthine partially mimick the action of the beta-adrenergic agonist, thus pointing to a possible role of adenosine 3',5'-phosphate in establishing desensitization. When adenylate cyclase from desensitized cells is stimulated with 5'-guanylyl-imidodiphosphate in the presence or absence of catecholamines the lag period preceding the attainment of maximal activity is extended. Likewise the rate of reversal by GTP or GTP of persistent activation of adenylate cyclase is slowed down. This is therefore interpreted to mean that the loss in hormonal stimulation on treatment of pigeon red blood cells with beta-adrenergic agonists is due to a delayed exchange of GDP against GTP on the regulatory GTP-binding protein. Furthermore, we conclude that events causing the refractory state in avian erythrocytes should occur at a site distal to the beta-adrenergic receptor.

Long-Term Morphine Treatment Enhances Proteasome-Dependent Degradation of Gβ in Human Neuroblastoma SH-SY5Y Cells: Correlation with Onset of Adenylate Cyclase Sensitization

Abstract: The initial aim of this study was to identify protein changes associated with long-term morphine treatment in a recombinant human neuroblastoma SH-SY5Y clone (sc2) stably overexpressing the human mu-opioid (MOP) receptor. In MOP receptor-overexpressing sc2 cells, short-term morphine exposure was found to be much more potent and efficacious in inhibiting forskolin-elicited production of cAMP, and long-term morphine exposure was shown to induce a substantially higher degree of opiate dependence, as reflected by adenylate cyclase sensitization, than it did in wild-type neuroblastoma cells. Differential proteomic analysis of detergent-resistant membrane rafts isolated from untreated and chronically morphine-treated sc2 cells revealed long-term morphine exposure to have reliably induced a 30 to 40% decrease in the abundance of five proteins, subsequently identified by mass spectrometry as G protein subunits alphai(2), alphai(3), beta(1), and beta(2), and prohibitin. Quantitative Western blot analyses of whole-cell extracts showed that long-term morphine treatment-induced down-regulation of Gbeta but not of the other proteins is highly correlated (r(2) = 0.96) with sensitization of adenylate cyclase. Down-regulation of Gbeta and adenylate cyclase sensitization elicited by long-term morphine treatment were suppressed in the presence of carbobenzoxy-l-leucyl-l-leucyl-l-norvalinal (MG-115) or lactacystin. Thus, sustained activation of the MOP receptor by morphine in sc2 cells seems to promote proteasomal degradation of Gbeta to sensitize adenylate cyclase. Together, our data suggest that the long-term administration of opiates may elicit dependence by altering the neuronal balance of heterotrimeric G proteins and adenylate cyclases, with the ubiquitin-proteasome pathway playing a pivotal role.

Calmodulin stimulation of adenylate cyclase of intestinal epithelium.

Abstract: The present work analyzes the calmodulin stimulation of rat intestinal epithelial adenylate cyclase. It is shown that: (1) activation of intestinal adenylate cyclase by calmodulin occurs in the presence of 1 – 30 μM free Ca2+; (2) in the 0.5 – 10 mM MgCl2 concentration range, the pattern of calmodulin action is not modified by Mg2+; calmodulin acts by increasing the maximal velocity of the intestinal adenylate cyclase without altering its affinity for ATP and Mg2+; (3) the calmodulin effect does not require the presence of guanine nucleotide and is even additive to that induced by GTP or its synthetic non-hydrolyzable analog guanosine 5′-[β,γ-imido]triphosphate; (4) calmodulin and sodium fluoride exert an additive effect on the enzyme activity; (5) calmodulin and prostaglandins E1 or E2 display additive stimulatory actions, whatever the dose of prostaglandins tested; calmodulin, therefore, does not modify the affinity of intestinal adenylate cyclase for its hormonal effectors; (6) trifluopeazine in the 25–200 μM range of concentrations (K0.5= 85 μM) suppresses the adenylate cyclase activity which has been stimulated by 5 μM calmodulin; (7) calmodulin is active in the 0.1 – 10 μM concentration range, half-maximal stimulation is induced by 0.9 μM protein, i.e. a dose of calmodulin that is much below the calmodulin content of the intestinal epithelial cell; (8) at low (0.5 mM) and high (5 mM) Mg2- concentration, a sixfold and threefold maximal increase of basal activities are observed, respectively. Our data indicate that calmodulin stimulates intestinal adenylate cyclase by a mechanism that is different from the hormonal activation and suggest the presence of a specific regulatory site for calmodulin on the enzyme.

Cerebellar astrocytes co-express several ADP receptors. Presence of functional P2Y13-like receptors

Abstract: Astrocytes exhibit a form of excitability based on variations of intracellular Ca2+ concentration in response to various stimuli, including ADP, ATP, UTP and dinucleotides. Here, we investigate the presence of the recently cloned ADP-sensitive receptors, P2Y12 and P2Y13 subtypes, which are negatively coupled to adenylate cyclase, in cerebellar astrocytes. We checked the effect of specific agonists, 2-methylthioadenosine diphosphate (2MeSADP) and ADP, on adenylate cyclase stimulation induced by isoproterenol. Both agonists significantly reduced the cAMP accumulation induced by isoproterenol. The inhibitory effect was concentration-dependent with IC50 values of 46 ± 13 and 23 ± 14 nM for 2MeSADP and ADP, respectively. The experiments were carried out in the presence of MRS-2179, a specific antagonist of P2Y1 receptor, to avoid any contribution of this receptor. Using fura-2 microfluorimetry we also proved that astrocytes responded to 2MeSADP stimulations with calcium responses in the absence and also in the presence of MRS-2179. Both effects, inhibition of adenylate cyclase and intracellular calcium mobilization, were not modified by 2MeSAMP, an antagonist of P2Y12 receptor, suggesting that were mediated by P2Y13-like receptors.

Characterization of phylogenetically distant members of the adenylate cyclase family from mycobacteria: Rv1647 from Mycobacterium tuberculosis and its orthologue ML1399 from M. leprae.

Abstract: Analysis of the genome sequence of Mycobacterium tuberculosis H37Rv has identified 16 genes that are similar to the mammalian adenylate and guanylate cyclases. Rv1647 was predicted to be an active adenylate cyclase but its position in a phylogenetically distant branch from the other enzymes characterized so far from M. tuberculosis makes it an interestingly divergent nucleotide cyclase to study. In agreement with its divergence at the sequence level from other nucleotide cyclases, the cloning, expression and purification of Rv1647 revealed differences in its biochemical properties from the previously characterized Rv1625c adenylate cyclase. Adenylate cyclase activity of Rv1647 was activated by detergents but was resistant to high concentrations of salt. Mutations of substrate-specifying residues to those present in guanylate cyclases failed to convert the enzyme into a guanylate cyclase, and did not alter its oligomeric status. Orthologues of Rv1647 could be found in M. leprae, M. avium and M. smegmatis. The orthologue from M. leprae (ML1399) was cloned, and the protein was expressed, purified and shown biochemically to be an adenylate cyclase, thus representing the first adenylate cyclase to be described from M. leprae. Importantly, Western-blot analysis of subcellular fractions from M. tuberculosis and M. leprae revealed that the Rv1647 and ML1399 gene products respectively were expressed in these bacteria. Additionally, M. tuberculosis was also found to express the Rv1625c adenylate cyclase, suggesting that multiple adenylate cyclase proteins may be expressed simultaneously in this organism. These results suggest that class III cyclase-like gene products probably have an important role to play in the physiology and perhaps the pathology of these medically important bacteria.

Histidine residue at position 234 of oxidosqualene-lanosterol cyclase from saccharomyces cerevisiae simultaneously influences cyclization, rearrangement, and deprotonation reactions.

Abstract: Oxidosqualene cyclases catalyze the biotransformation of acyclic (3S)-2,3-oxidosqualene (OS) to a variety of polycyclic sterols and triterpenoids, generating over 100 distinct triterpenoid skeletons with the formula C30H50O. [1–3,4 and references therein] Product specificity is species-dependent and precisely controlled by the prefolded substrate conformation as well as by interactions between the carbocationic intermediate for deprotonation and the functional groups of catalytic amino acid residues of the enzyme. The transformation mechanisms of this single class of enzymes can vary widely. For example, the triterpenes lanosterol, cycloartenol, and parkeol are formed from a preorganized chair–boat–chair substrate conformation of OS, and cationic cyclization to the protosteryl cation is followed by skeletal rearrangements until the final deprotonation step. Formation of the pentacyclic b-amyrin and lupeol proceed similarly except that OS is in the chair–chair–chair conformation (this results in stereochemical differences in the products relative to the chair–boat–chair substrate conformation), and the cationic cyclization to the dammarenyl cation is followed by annulation of a fifth ring. Various strategies have been used to probe the complex cyclization/rearrangement reaction mechanism, both for the purpose of understanding these complex enzymes and also to engineer cyclases to generate new product profiles. For example, site-directed mutagenesis was used to identify the residues responsible for the product specificity of b-amyrin synthase (PNY) and lupeol synthase (OEW). Two residues of PNY from Panax ginseng, Trp259 and Tyr261, were found to play important roles in the reaction mechanism to direct b-amyrin and/or lupeol formation. We and others independently identified several critical residues from oxidosqualene-lanosterol cyclase (ERG7) from Saccharomyces cerevisiae and oxidosqualenecycloartenol synthase (CAS) from Arabidopsis thaliana, and demonstrated their roles in facilitating tetracyclic formation and/or stabilizing the lanosteryl cation for deprotonation, as