Genetic selection of mammalian adenylyl cyclases insensitive to stimulation by Gsalpha

Gα(i/o)-coupled receptor-mediated sensitization of adenylyl cyclase: 40 years later

Heterologous sensitization of adenylyl cyclase (also referred to as superactivation, sensitization, or supersensitization of adenylyl cyclase) is a cellular adaptive response first described 40 years ago in the laboratory of Dr. Marshall Nirenberg. This apparently paradoxical cellular response occurs following persistent activation of Gαi/o-coupled receptors and causes marked enhancement in the activity of adenylyl cyclases, thereby increasing cAMP production. Since our last review in 2005, significant progress in the field has led to a better understanding of the relevance of, and the cellular biochemical processes that occur during the development and expression of heterologous sensitization. In this review we will discuss the recent advancements in the field and the mechanistic hypotheses on heterologous sensitization.

Dopamine D(2) Receptor-Mediated Heterologous Sensitization of AC5 Requires Signalosome Assembly

Chronic dopamine receptor activation is implicated in several central nervous system disorders. Although acute activation of Gα_i-coupled D₂ dopamine receptors inhibits adenylyl cyclase, persistent activation enhances adenylyl cyclase activity, a phenomenon called heterologous sensitization. Previous work revealed a requirement for Gα_s in D₂-induced heterologous sensitization of AC5. To elucidate the mechanism of Gα_s dependency, we expressed Gα_s mutants in Gα_s-deficient Gnas^E2−/E2− cells. Neither Gα_s-palmitoylation nor Gα_s-Gβγ interactions were required for sensitization of AC5. Moreover, we found that coexpressing βARKct-CD8 or Sar1(H79G) blocked heterologous sensitization. These studies are consistent with a role for Gα_s-AC5 interactions in sensitization however, Gβγ appears to have an indirect role in heterologous sensitization of AC5, possibly by promoting proper signalosome assembly.

Autoinhibitory regulation of soluble adenylyl cyclase

Soluble adenylyl cyclase is an evolutionarily conserved bicarbonate sensor that plays a crucial role in cAMP dependent processes that occur during mammalian fertilization. sAC protein is expressed at the highest levels in male germ cells, and is found to occur as one of two known isoforms: a truncated protein (sAC(t)) that consists almost exclusively of the two conserved catalytic domains (C1 and C2), and a full-length form (sAC(fl)) that contains an additional noncatalytic C-terminal region. Several studies suggested sAC(t) was more active than sAC(fl). We now demonstrate that the specific activity of sAC(t) is at least 10-fold higher than the specific activity of sAC(fl). Using deletion analysis and a novel genetic screen to identify activating mutations, we uncovered an autoinhibitory region just C-terminal to the C2 domain. Kinetic analysis of purified recombinant sAC revealed this autoinhibitory domain functions to lower the enzyme's V(max) without altering its affinity for substrate or regulation by any of the known modulators of sAC activity. Our results identify an additional regulatory mechanism specific to the sAC(fl) isoform.

D2 dopamine receptor-induced sensitization of adenylyl cyclase type 1 is G alpha(s) independent

Acute activation of D2 dopamine receptors inhibits adenylyl cyclase (EC 4.6.1.1), whereas persistent activation of these inhibitory receptors results in a compensatory increase in cyclic AMP accumulation. This sensitization of adenylyl cyclase is thought to involve enhanced Galpha(s)-adenylyl cyclase interactions; however, the absolute requirement of Galpha(s) has not been determined. The present study used a Galpha(s)-deficient cell line to examine directly the role of Galpha(s) in D2 dopamine receptor-induced sensitization of recombinant adenylyl cyclase type 1 (AC1) and 5 (AC5). In acute experiments, quinpirole activation of the D2 dopamine receptor inhibited AC1 and AC5 activity, indicating that the acute regulatory properties of AC1 and AC5 were retained in the absence of Galpha(s). Subsequent experiments revealed that short-term (2 h) activation of the D2 dopamine receptor resulted in significantly enhanced forskolin-stimulated AC1 activity in the absence of Galpha(s), whereas sensitization of forskolin-stimulated AC5 activity appeared to require Galpha(s). The Galpha(s)-independent sensitization of AC1 was explored further using AC1-selective activation protocols (A23187 and CCE) following short- and long-term agonist treatment. These studies revealed that persistent activation of D2 dopamine receptors sensitized AC1 activity to Ca2+ stimulation in cells devoid of endogenous Galpha(s) and demonstrate directly that sensitization of AC1 is Galpha(s)-independent.

Sensitization of adenylate cyclase by Galpha i/o-coupled receptors

Activation of receptors coupled to inhibitory G proteins (Galpha i/o) has opposing consequences for cyclic AMP accumulation and the activity of cyclic AMP-dependent protein kinase, depending on the duration of stimulation. Acute activation inhibits the activity of adenylate cyclase, thereby attenuating cyclic AMP accumulation; in contrast, persistent activation of Galpha i/o-coupled receptors produces a paradoxical enhancement of adenylate cyclase activity, thus increasing cyclic AMP accumulation when the action of the inhibitory receptor is terminated. This heterologous sensitization of cyclic AMP signaling, also called superactivation or supersensitization, likely represents a cellular adaptive response, a mechanism by which the cell compensates for chronic inhibitory input. Recent advances in our knowledge of G protein-mediated signaling, regulation of adenylate cyclase, and other cellular signaling mechanisms have extensively increased our insight into the mechanisms and significance of this phenomenon. In particular, recent evidence points to the Galpha(s)-adenylate cyclase interface as a locus for the expression of the sensitized adenylate cyclase response, and to isoform-specific phosphorylation of adenylate cyclase as one mechanism that can produce sensitization. Galpha i/o-coupled receptor-induced heterologous sensitization may contribute to enhanced Galpha(s)-coupled receptor signaling following neurotransmitter elevations induced by the administration of drugs of abuse and during other types of neuronal function or dysfunction. This review will focus on recent advances in our understanding of signaling pathways that are involved in sensitization and describe the potential role of sensitization in neuronal function.

Modeling of Galpha(s) and Galpha(i) regulation of human type V and VI adenylyl cyclase

We examined the kinetics of Galpha(s) and Galpha(i) regulation of human type V and type VI adenylyl cyclase (AC V and AC VI) in order to better model interactions between AC and its regulators. Activation of AC VI by Galpha(s) displayed classical Michaelis-Menten kinetics, whereas AC V activation by Galpha(s) was cooperative with a Hill coefficient of 1.4. The basal activity of human AC V, but not that of AC VI, was inhibited by Galpha(i). Both enzymes showed greater inhibition by Galpha(i) at low Galpha(s) concentrations; however, human AC V was activated by Galpha(i) at high Galpha(s) concentrations. Neither regulator had an effect on the K(m) for Mg-ATP. Mutations made within the Galpha(s) binding pocket of AC V (N1090D) and VI (F1078S) displayed 6- and 14-fold greater EC(50) values for Galpha(s) activation but had no effect on Galpha(i) inhibition of basal activity or K(m) for Mg-ATP. Galpha(s)-stimulated AC VI-F1078S was not significantly inhibited by Galpha(i), despite normal inhibition by Galpha(i) upon forskolin stimulation. Mechanistic models for Galpha(s) and Galpha(i) regulation of AC V and VI were derived to describe these results. Our models are consistent with previous studies, predicting a decrease in affinity of Galpha(i) in the presence of Galpha(s). For AC VI, Galpha(s) is required for inhibition but not binding by Galpha(i). For AC V, binding of two molecules of Galpha(s) and Galpha(i) to an AC dimer are required to fully describe the data. These models highlight the differences between AC V and VI and the complex interactions with two important regulators.

Using molecular tools to dissect the role of Galphas in sensitization of AC1

Short-term activation of Galpha(i/o)-coupled receptors inhibits adenylyl cyclase, whereas persistent activation of Galpha(i/o)-coupled receptors results in a compensatory sensitization of adenylyl cyclase activity after subsequent activation by Galpha(s) or forskolin. Several indirect observations have suggested the involvement of increased Galpha(s)-adenylyl cyclase interactions in the expression of sensitization; however, evidence supporting a direct role for Galpha(s) has not been well established. In the present report, we used two genetic approaches to further examine the role of Galpha(s) in heterologous sensitization of Ca(2+)-sensitive type 1 adenylyl cyclase (AC1). In the first approach, we constructed Galpha(s)-insensitive mutants of AC1 (F293L and Y973S) that retained sensitivity to Ca2+ and forskolin activation. Persistent (2 h) activation of the D2 dopamine receptor resulted in a significant augmentation of basal or Ca(2+)- and forskolin-stimulated AC1 activity; however, sensitization of Galpha(s)-insensitive mutants of AC1 was markedly reduced compared with wild-type AC1. In the second strategy, we examined the requirement of an intact receptor-Galpha(s) signaling pathway for the expression of sensitization using dominant-negative Galpha(s) mutants (alpha3beta5 G226A/A366S or alpha3beta5 G226A/E268A/A366S) to disrupt D1 dopamine receptor activation of recombinant AC1. D1 dopamine receptor-Galpha(s) signaling was attenuated in the presence of alpha3beta5 G226A/A366S or alpha3beta5 G226A/E268A/A366S, but D2 agonist-induced sensitization of Ca(2+)-stimulated AC1 activity was not altered. Together, the present findings directly support the hypothesis that the expression of sensitization of AC1 involves Galpha(s)-adenylyl cyclase interactions.

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.

Sensitization of adenylate cyclase: a general mechanism of neuroadaptation to persistent activation of Galpha(i/o)-coupled receptors?

Acute activation of Galphas-coupled receptors stimulates cyclic AMP accumulation leading to the activation of downstream signaling cascades. These Galphas-mediated events can be countered by acute activation of inhibitory G proteins (Galpha(i/o)), which inhibit the activity of adenylate cyclase, thereby attenuating cyclic AMP accumulation. Furthermore, an additional, less direct mechanism for Galpha(i/o) proteins modulation of cyclic AMP signaling also has been described. Persistent activation of several Galpha(i/o)-coupled receptors has been shown to result in a subsequent paradoxical enhancement of adenylate cyclase activity in response to drug-stimulated cyclic AMP accumulation. This sensitization of adenylate cyclase likely represents a cellular adaptive response following prolonged activation of inhibitory receptors. Recent advances in our knowledge of G protein signaling, adenylate cyclase regulation, and other cellular signaling mechanisms have extensively increased our insight into this phenomenon. It is now thought that sensitization occurs as part of a compensatory mechanism by which the cell adapts to chronic inhibitory input. Such a mechanism may be involved in modulating Galphas-coupled receptor signaling following neurotransmitter elevations that occur in psychiatric disease states or following the administration of many drugs of abuse. This review will focus on recent advances in the understanding of molecular signaling pathways that are involved in sensitization and describe the potential role of sensitization in neuronal cell function.

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.

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.

The signal transfer regions of G alpha(s)

The crystal structure of soluble functional fragments of adenylyl cyclase complexed with G alpha(s) and forskolin, shows three regions of G alpha(s) in direct contact with adenylyl cyclase. The functions of these three regions are not known. We tested synthetic peptides encoding these regions of G alpha(s) on the activities of full-length adenylyl cyclases 2 and 6. A peptide encoding the Switch II region (amino acids 222-247) stimulated both adenylyl cyclases 2- to 3-fold. Forskolin synergized the stimulation. Addition of peptides in the presence of activated G alpha(s) partially inhibited G alpha(s) stimulation. Corresponding Switch II region peptides from G alpha(q) and G alpha(i) did not stimulate adenylyl cyclase. A peptide encoding the Switch I region (amino acids 199-216) also stimulated AC2 and AC6. The stimulatory effects of the two peptides at saturating concentrations were non-additive. A peptide encoding the third contact region (amino acids 268-286) located in the alpha 3-beta 5 region, inhibits basal, forskolin, and G alpha(s)-stimulated enzymatic activities. Since this region in G alpha(s) interacts with both the central cytoplasmic loop and C-terminal tail of adenylyl cyclases this peptide may be involved in blocking interactions between these two domains. These functional data in conjunction with the available structural information suggest that G alpha(s) activation of adenylyl cyclase is a complex event where the alpha 3-beta 5 loop of G alpha(s) may bring together the central cytoplasmic loop and C-terminal tail of adenylyl cyclase thus allowing the Switch I and Switch II regions to function as signal transfer regions to activate adenylyl cyclase.

Dopamine D2 receptor-induced heterologous sensitization of adenylyl cyclase requires Galphas: characterization of Galphas-insensitive mutants of adenylyl cyclase V

Whereas acute stimulation of Galphai/o-coupled receptors inhibits the activity of adenylyl cyclase, a delayed consequence of persistent activation of the receptors is heterologous sensitization, an enhanced responsiveness of adenylyl cyclase to activators such as forskolin or agonists of Galphas-coupled receptors. Galphas-insensitive mutants of adenylyl cyclase type V were used to test the hypothesis that heterologous sensitization requires Galphas-dependent activation of adenylyl cyclase. When adenylyl cyclase was stably expressed in human embryonic kidney (HEK) 293 cells with the D2L dopamine receptor, basal, forskolin-stimulated, and isoproterenol-stimulated cyclic AMP accumulation were all enhanced by 2-h pretreatment with the D2 receptor agonist quinpirole. Transient expression of wild-type adenylyl cyclase and three Galphas-insensitive mutants (F379L, R1021Q, and F1093S) in HEK293 cells stably expressing the D2L receptor demonstrated that all three mutants had little or no responsiveness to beta-adrenergic receptor-mediated activation of Galphas but that the mutants retained sensitivity to forskolin and to D2L receptor-mediated inhibition. Transiently expressed adenylyl cyclase V was robustly sensitized by 2-h pretreatment with quinpirole. In contrast, the Galphas-insensitive mutants displayed no sensitization of forskolin-stimulated cyclic AMP accumulation, indicating that responsiveness to Galphas is required for the expression of heterologous sensitization.

An isoform-specific interaction of the membrane anchors affects mammalian adenylyl cyclase type V activity

The nine membrane-bound mammalian adenylyl cyclases (ACs) contain two highly diverged membrane anchors, M1 and M2, with six transmembrane spans each and two conserved cytosolic domains which coalesce into a pseudoheterodimeric catalytic unit. Previously, the catalytic segments, bacterially expressed as soluble proteins, were characterized extensively whereas the function of the membrane domains remained unexplored. Using the catalytic C1 and C2 domains of AC type V we employed the membrane anchors from type V and VII ACs for construction of enzymes with duplicated, inverted, fully swapped and chimeric membrane anchors. Further, in the M1 membrane domain individual transmembrane spans were removed or exchanged between type V and VII ACs. The constructs were expressed in HEK293 cells, the expression levels and membrane localization was assessed by Western blotting. Cell-free basal, forskolin-, GTP gamma S-and G(s alpha)/GTP gamma S-stimulated AC activities were determined. The results demonstrate that enzymatic activities were only maintained when the M1 and M2 membrane domains were derived from either AC V or VII. Constructs with chimeric membrane domains, i.e. M1 from type V and M2 from type VII AC or vice versa, were essentially inactive although the expression levels and membrane localization appeared to be normal. The data indicate a functionally important interaction of the membrane domains of ACs in that they seem to interact in a pair-like, isoform delimited manner. This interaction directly impinges on the formation of the catalytic interface. We propose that protein-protein interactions of the AC membrane domains may constitute another, yet unexplored level of AC regulation.

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.

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.

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 adenylyl and guanylyl cyclase superfamily

New structures solved in 1997 revealed that the adenylyl cyclase core consists of a pair of catalytic domains arranged in a wreath. Homologous catalytic domains are arranged in diverse adenylyl and guanylyl cyclases as symmetric homodimers or pseudosymmetric heterodimers. The kinship of the adenylyl and guanylyl cyclases has been confirmed by the structure-based interconversion of their nucleotide specificities. Catalysis is activated when two metal-binding aspartate residues on one domain are juxtaposed with a key aspargine-arginine pair on the other. Allosteric activators of mammalian adenylyl cyclase, forskolin and the stimulatory G protein alpha subunit, promote the catalytically optimal juxtaposition of the two domains.

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.

Identification of a Gialpha binding site on type V adenylyl cyclase

The stimulatory G protein alpha subunit Gsalpha binds within a cleft in adenylyl cyclase formed by the alpha1-alpha2 and alpha3-beta4 loops of the C2 domain. The pseudosymmetry of the C1 and C2 domains of adenylyl cyclase suggests that the homologous inhibitory alpha subunit Gialpha could bind to the analogous cleft within C1. We demonstrate that myristoylated guanosine 5'-3-O-(thio)triphosphate-Gialpha1 forms a stable complex with the C1 (but not the C2) domain of type V adenylyl cyclase. Mutagenesis of the membrane-bound enzyme identified residues whose alteration either increased or substantially decreased the IC50 for inhibition by Gialpha1. These mutations suggest binding of Gialpha within the cleft formed by the alpha2 and alpha3 helices of C1, analogous to the Gsalpha binding site in C2. Adenylyl cyclase activity reconstituted by mixture of the C1 and C2 domains of type V adenylyl cyclase was also inhibited by Gialpha. The C1b domain of the type V enzyme contributed to affinity for Gialpha, but the source of C2 had little effect. Mutations in this soluble system faithfully reflected the phenotypes observed with the membrane-bound enzyme. The pseudosymmetrical structure of adenylyl cyclase permits bidirectional regulation of activity by homologous G protein alpha subunits.

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.