Gbetagamma activation site in adenylyl cyclase type II. Adenylyl cyclase type III is inhibited by Gbetagamma

The Gbetagamma complex of heterotrimeric G proteins is the most outstanding example for the divergent regulation of mammalian adenylyl cyclases. The heterodimeric Gbetagamma complex inhibits some isoforms, e.g. ACI, and stimulates the isoforms ACII, -IV, and -VII. Although former studies identified the QEHA region located in the C2 domain of ACII as an important interaction site for Gbetagamma, the determinant of the stimulatory effect of Gbetagamma has not been detected. Here, we identified the C1b domain as the stimulatory region using full-length adenylyl cyclase. The relevant Gbetagamma signal transfer motif in IIC1b was determined as MTRYLESWGAAKPFAHL (amino acids 493-509). Amino acids of this PFAHL motif were absolutely necessary for ACII to be stimulated by Gbetagamma, whereas they were dispensable for Galpha(s) or forskolin stimulation. The PFAHL motif is present in all three adenylyl cyclase isoforms that are activated by Gbetagamma but is absent in other adenylyl cyclase isoforms as well as other known effectors of Gbetagamma. The emerging concept of two contact sites on different molecule halves for effective regulation of adenylyl cyclase is discussed.     

The regulation of type 7 adenylyl cyclase by its C1b region and Escherichia coli peptidylprolyl isomerase, SlyD

Mammalian membrane-bound adenylyl cyclase consists of two highly conserved cytoplasmic domains (C1a and C2a) separated by a less conserved connecting region, C1b, and one of two transmembrane domains, M2. The C1a and C2a domains form a catalytic core that can be stimulated by forskolin and the stimulatory G protein subunit alpha (Galpha(s)). In this study, we analyzed the regulation of type 7 adenylyl cyclase (AC7) by C1b. The C1a, C1b, and C2a domains of AC7 were purified separately. Escherichia coli SlyD protein, a cis-trans peptidylprolyl isomerase (PPIase), copurifies with AC7 C1b (7C1b). SlyD protein can inhibit the Galpha(s)- and/or forskolin-activated activity of both soluble and membrane-bound AC7. Mutant forms of SlyD with reduced PPIase activity are less potent in the inhibition of AC7 activity. Interestingly, different isoforms of mammalian membrane-bound adenylyl cyclase can be either inhibited or stimulated by SlyD protein, raising the possibility that mammalian PPIase may regulate enzymatic activity of mammalian adenylyl cyclase. Purified 7C1b-SlyD complex has a greater inhibitory effect on AC7 activity than SlyD alone. This inhibition by 7C1b is abolished in a 7C1b mutant in which a conserved glutamic acid (amino acid residue 582) is changed to alanine. Inhibition of adenylyl cyclase activity by 7C1b is further confirmed by using 7C1b purified from an E. coli slyD-deficient strain. This inhibitory activity of AC7 is also observed with the 28-mer peptides derived from a region of C1b conserved in AC7 and AC2 but is not observed with a peptide derived from the corresponding region of AC6. This inhibitory activity exhibited by the C1b domain may result from the interaction of 7C1b with 7C1a and 7C2a and may serve to hold AC7 in the basal nonstimulated state.     

Mechanism of Galpha i-mediated inhibition of type V adenylyl cyclase

The topology of mammalian adenylyl cyclase reveals an integral membrane protein composed of an alternating series of membrane and cytoplasmic domains (C1 and C2). The stimulatory G protein, Galpha(s), binds within a cleft in the C2 domain of adenylyl cyclase while Galpha(i) binds within the opposite cleft in the C1 domain. The mechanism of these two regulators also appears to be in opposition. Activation of adenylyl cyclase by Galpha(s) or forskolin results in a 100-fold increase in the apparent affinity of the two domains for one another. We show herein that Galpha(i) reduces C1/C2 domain interaction and thus formation of the adenylyl cyclase catalytic site. Mutants that increase the affinity of C1 for C2 decrease the ability of Galpha(i) to inhibit the enzyme. In addition, Galpha(i) can influence binding of molecules to the catalytic site, which resides at the C1/C2 interface. Adenylyl cyclase can bind substrate analogs in the presence of Galpha(i) but cannot simultaneously bind Galpha(i) and transition state analogs such as 2'd3'-AMP. Galpha(i) also cannot inhibit the membrane-bound enzyme in the presence of manganese, which increases the affinity of adenylyl cyclase for ATP and substrate analogs. Thus homologous G protein alpha-subunits promote bidirectional regulation at the domain interface of the pseudosymmetrical adenylyl cyclase enzyme.     

A guanylyl cyclase from Paramecium with 22 transmembrane spans. Expression of the catalytic domains and formation of chimeras with the catalytic domains of mammalian adenylyl cyclases

Paramecium has a 280-kDa guanylyl cyclase. The N terminus resembles a P-type ATPase, and the C terminus is a guanylyl cyclase with the membrane topology of canonical mammalian adenylyl cyclases, yet with the cytosolic loops, C1 and C2, inverted compared with the mammalian order. We expressed in Escherichia coli the cytoplasmic domains of the protozoan guanylyl cyclase, independently and linked by a peptide, as soluble proteins. The His(6)-tagged proteins were enriched by affinity chromatography and analyzed by immunoblotting. Guanylyl cyclase activity was reconstituted upon mixing of the recombinant C1a- and C2-positioned domains and in a linked C1a-C2 construct. Adenylyl cyclase activity was minimal. The nucleotide substrate specificity was switched from GTP to ATP upon mutation of the substrate defining amino acids Glu(1681) and Ser(1748) in the C1-positioned domain to the adenylyl cyclase specific amino acids Lys and Asp. Using the C2 domains of mammalian adenylyl cyclases type II or IX and the C2-positioned domain from the Paramecium guanylyl cyclase we reconstituted a soluble, all C2 adenylyl cyclase. All enzymes containing protozoan domains were not affected by Galpha(s)/GTP or forskolin, and P site inhibitors were only slightly effective.     

Gbetagamma that interacts with adenylyl cyclase in opioid tolerance originates from a Gs protein

We previously demonstrated that chronic morphine induces a change in G protein coupling by the mu opioid receptor (MOR) from Gi/o to Gs, concurrent with the instatement of an interaction between Gbetagamma and adenylyl cyclase types II and IV. These two signaling changes confer excitatory effects on the cell in place of the typical inhibition by opioids and are associated with morphine tolerance and dependence. Both signaling changes and these behavioral manifestations of chronic morphine are attenuated by cotreatment with ultra-low-dose naloxone. In the present work, using striatum from chronic morphine-treated rats, we isotyped the Gbeta within Gs and Go heterotrimers that coupled to MOR and compared these to the Gbeta isotype of the Gbetagamma that interacted with adenylyl cyclase II or IV after chronic morphine treatment. Isotyping results show that chronic morphine causes a Gs heterotrimer associated with MOR to release its Gbetagamma to interact with adenylyl cyclase. These data suggest that the switch to Gs coupling by MOR in response to chronic morphine, which is attenuated by ultra-low-dose opioid antagonist cotreatment, leads to a two-pronged stimulation of adenylyl cyclase utilizing both Galpha and Gbetagamma subunits of the Gs protein novel to this receptor.     

Conditional stimulation of type V and VI adenylyl cyclases by G protein betagamma subunits

In a yeast two-hybrid screen of mouse brain cDNA library, using the N-terminal region of human type V adenylyl cyclase (hACV) as bait, we identified G protein beta2 subunit as an interacting partner. Additional yeast two-hybrid assays showed that the Gbeta(1) subunit also interacts with the N-terminal segments of hACV and human type VI adenylyl cyclase (hACVI). In vitro adenylyl cyclase (AC) activity assays using membranes of Sf9 cells expressing hACV or hACVI showed that Gbetagamma subunits enhance the activity of these enzymes provided either Galpha(s) or forskolin is present. Deletion of residues 77-151, but not 1-76, in the N-terminal region of hACVI obliterated the ability of Gbetagamma subunits to conditionally stimulate the enzyme. Likewise, activities of the recombinant, engineered, soluble forms of ACV and ACVI, which lack the N termini, were not enhanced by Gbetagamma subunits. Transfection of the C terminus of G protein receptor kinase 2 to sequester endogenous Gbetagamma subunits attenuated the ability of isoproterenol to increase cAMP accumulation in COS-7 cells overexpressing hACVI even when G(i) was inactivated by pertussis toxin. Therefore, we conclude that the N termini of human hACV and hACVI are necessary for interactions with, and regulation by, Gbetagamma subunits both in vitro and in intact cells. Moreover, Gbetagamma subunits derived from a source(s) other than G(i) are necessary for the full activation of hACVI by isoproterenol in intact cells.     

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.     

A bifunctional Galphai/Galphas modulatory peptide that attenuates adenylyl cyclase activity

Signaling via G-protein coupled receptors is initiated by receptor-catalyzed nucleotide exchange on Galpha subunits normally bound to GDP and Gbetagamma. Activated Galpha . GTP then regulates effectors such as adenylyl cyclase. Except for Gbetagamma, no known regulators bind the adenylyl cyclase-stimulatory subunit Galphas in its GDP-bound state. We recently described a peptide, KB-752, that binds and enhances the nucleotide exchange rate of the adenylyl cyclase-inhibitory subunit Galpha(i). Herein, we report that KB-752 binds Galpha(s) . GDP yet slows its rate of nucleotide exchange. KB-752 inhibits GTPgammaS-stimulated adenylyl cyclase activity in cell membranes, reflecting its opposing effects on nucleotide exchange by Galpha(i) and Galpha(s).     

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.     

RACK1 regulates specific functions of Gbetagamma

We showed previously that Gbetagamma interacts with Receptor for Activated C Kinase 1 (RACK1), a protein that not only binds activated protein kinase C (PKC) but also serves as an adaptor/scaffold for many signaling pathways. Here we report that RACK1 does not interact with Galpha subunits or heterotrimeric G proteins but binds free Gbetagamma subunits released from activated heterotrimeric G proteins following the activation of their cognate receptors in vivo. The association with Gbetagamma promotes the translocation of RACK1 from the cytosol to the membrane. Moreover, binding of RACK1 to Gbetagamma results in inhibition of Gbetagamma-mediated activation of phospholipase C beta2 and adenylyl cyclase II. However, RACK1 has no effect on other functions of Gbetagamma, such as activation of the mitogen-activated protein kinase signaling pathway or chemotaxis of HEK293 cells via the chemokine receptor CXCR2. Similarly, RACK1 does not affect signal transduction through the Galpha subunits of G(i), G(s), or G(q). Collectively, these findings suggest a role of RACK1 in regulating specific functions of Gbetagamma.     

Involvement of betagamma subunits of G(q/11) in muscarinic M(1) receptor potentiation of corticotropin-releasing hormone-stimulated adenylyl cyclase activity in rat frontal cortex

In the present study, we investigated the involvement of betagamma subunits of G(q/11) in the muscarinic M(1) receptor-induced potentiation of corticotropin-releasing hormone (CRH)-stimulated adenylyl cyclase activity in membranes of rat frontal cortex. Tissue exposure to either one of two betagamma scavengers, the QEHA fragment type II adenylyl cyclase and the GDP-bound form of the alpha subunit of transducin, inhibited the muscarinic M(1) facilitatory effect. Moreover, like acetylcholine (ACh), exogenously added betagamma subunits of transducin potentiated the CRH-stimulated adenylyl cyclase activity, and this effect was not additive with that elicited by ACh. Western blot analysis indicated the expression in frontal cortex of both type II and type IV adenylyl cyclases, two isoforms stimulated by betagamma subunits in synergism with activated G(s). The M(1) receptor-induced enhancement of the adenylyl cyclase response to CRH was counteracted by the G(q/11) antagonist GpAnt-2A but not by GpAnt-2, a preferential G(i/o) antagonist. In addition, the muscarinic facilitatory effect was inhibited by membrane preincubation with antiserum directed against the C terminus of the alpha subunit of G(q/11), whereas the same treatment with antiserum against either G(i1/2) or G(o) was without effect. These data indicate that in membranes of rat frontal cortex, activation of muscarinic M(1) receptors potentiates CRH-stimulated adenylyl cyclase activity through betagamma subunits of G(q/11).     

Basic fibroblast growth factor utilizes both types of component subunits of Gs for dual signaling in human adipocytes. Stimulation of adenylyl cyclase via Galph(s) and inhibition of NADPH oxidase by Gbeta gamma(s)

Basic fibroblast growth factor (bFGF), a ligand of receptor protein-tyrosine kinases, promoted the dissociation of G(s) and had antagonistic stimulatory and inhibitory effects on adenylyl cyclase and NADPH oxidase in human fat cell plasma membranes. The bFGF-induced activation of adenylyl cyclase was blocked by COOH-terminal anti-Galpha(s), indicating that it was mediated by Galpha(s). The inhibitory action of bFGF was mimicked by exogenously supplied Gbetagamma-subunits and was reversed by anti-Gbeta(1/2), or betaARK-CT, a COOH-terminal beta-adrenergic receptor kinase fragment that specifically binds free Gbetagamma, indicating that it was transduced by Gbetagamma complexes. The bFGF-induced inhibition of NADPH-dependent H(2)O(2) generation was also reversed by peptide 100-119, an inhibitor of G(s) activation by ligand-occupied beta-adrenergic receptors, indicating that the Gbetagamma complexes mediating the inhibitory action of the growth factor are derived from G(s). The findings suggest a direct, non-kinase-dependent, coupling of bFGF receptor(s) to G(s) and provide the first example of a ligand of receptor protein-tyrosine kinases that is capable of utilizing both types of component subunits of a single heterotrimeric G protein for dual signaling in a single cell type.     

Mutagenesis in the switch IV of the helical domain of the human Gsalpha reduces its GDP/GTP exchange rate

The Galpha subunits of heterotrimeric G proteins are constituted by a conserved GTPase "Ras-like" domain (RasD) and by a unique alpha-helical domain (HD). Upon GTP binding, four regions, called switch I, II, III, and IV, have been identified as undergoing structural changes. Switch I, II, and III are located in RasD and switch IV in HD. All Galpha known functions, such as GTPase activity and receptor, effector, and Gbetagamma interaction sites have been found to be localized in RasD, but little is known about the role of HD and its switch IV region. Through the construction of chimeras between human and Xenopus Gsalpha we have previously identified a HD region, encompassing helices alphaA, alphaB, and alphaC, that was responsible for the observed functional differences in their capacity to activate adenylyl cyclase (Antonelli et al. [1994]: FEBS Lett 340:249-254). Since switch IV is located within this region and contains most of the nonconservative amino acid differences between both Gsalpha proteins, in the present work we constructed two human Gsalpha mutant proteins in which we have changed four and five switch IV residues for the ones present in the Xenopus protein. Mutants M15 (hGsalphaalphaS133N, M135P, P138K, P143S) and M17 (hGsalphaalphaS133N, M135P, V137Y, P138K, P143S) were expressed in Escherichia coli, purified, and characterized by their ability to bind GTPgammaS, dissociate GDP, hydrolyze GTP, and activate adenylyl cyclase. A decreased rate of GDP release, GTPgammaS binding, and GTP hydrolysis was observed for both mutants, M17 having considerably slower kinetics than M15 for all functions tested. Reconstituted adenylyl cyclase activity with both mutants showed normal activation in the presence of AlF(4)(-), but a decreased activation with GTPgammaS, which is consistent with the lower GDP dissociating rate they displayed. These data provide new evidence on the role that HD is playing in modulating the GDP/GTP exchange of the Gsalpha subunit.     

AGS3 and signal integration by Galpha(s)- and Galpha(i)-coupled receptors: AGS3 blocks the sensitization of adenylyl cyclase following prolonged stimulation of a Galpha(i)-coupled receptor by influencing processing of Galpha(i)

AGS3-LONG and AGS3-SHORT contain G-protein regulatory motifs that interact with and stabilize the GDP-bound conformation of Galpha(i) > Galpha(o). AGS3 and related proteins may influence signal strength or duration as well as the adaptation of the signaling system associated with sustained stimulation. To address these issues, we determined the effect of AGS3 on the integration of stimulatory (Galpha(s)-mediated vasoactive intestinal peptide receptor) and inhibitory (Galpha(i)-mediated alpha(2)-adrenergic receptor (alpha(2)-AR)) signals to adenylyl cyclase in Chinese hamster ovary cells. AGS3-SHORT and AGS3-LONG did not alter the VIP-induced increase in cAMP or the inhibitory effect of alpha(2)-AR activation. System adaptation was addressed by determining the influence of AGS3 on the sensitization of adenylyl cyclase that occurs following prolonged activation of a Galpha(i)-coupled receptor. Incubation of cells with the alpha(2)-AR agonist UK14304 (1 microm) for 18 h resulted in a approximately 1.8-fold increase in the vasoactive intestinal peptide-induced activation of adenylyl cyclase, and this was associated with a decrease in membrane-associated Galpha(i3). Both effects were blocked by AGS3-SHORT. AGS3-SHORT also decreased the rate of Galpha(i3) decay. A mutant AGS3-SHORT incapable of binding G-protein was inactive. These data suggest that AGS3 and perhaps other G-protein regulatory motif-containing proteins increase the stability of Galpha(i) in the membrane, which influences the adaptation of the cell to prolonged activation of Galpha(i)-coupled receptors.     

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.     

Structural model of a complex between the heterotrimeric G protein, Gsalpha, and tubulin

A number of studies have demonstrated interplay between the cytoskeleton and G protein signaling. Many of these studies have determined a specific interaction between tubulin, the building block of microtubules, and G proteins. The alpha subunits of some heterotrimeric G proteins, including Gsalpha, have been shown to interact strongly with tubulin. Binding of Galpha to tubulin results in increased dynamicity of microtubules due to activation of GTPase of tubulin. Tubulin also activates Gsalpha via a direct transfer of GTP between these molecules. Structural insight into the interaction between tubulin and Gsalpha was required, and was determined, in this report, through biochemical and molecular docking techniques. Solid phase peptide arrays suggested that a portion of the amino terminus, alpha2-beta4 (the region between switch II and switch III) and alpha3-beta5 (just distal to the switch III region) domains of Gsalpha are important for interaction with tubulin. Molecular docking studies revealed the best-fit models based on the biochemical data, showing an interface between the two molecules that includes the adenylyl cyclase/Gbetagamma interaction regions of Gsalpha and the exchangeable nucleotide-binding site of tubulin. These structural models explain the ability of tubulin to facilitate GTP exchange on Galpha and the ability of Galpha to activate tubulin GTPase.     

Activation of phospholipase C-epsilon by heterotrimeric G protein betagamma-subunits

PLC-epsilon was identified recently as a phosphoinositide-hydrolyzing phospholipase C (PLC) containing catalytic domains (X, Y, and C2) common to all PLC isozymes as well as unique CDC25- and Ras-associating domains. Novel regulation of this PLC isozyme by the Ras oncoprotein and alpha-subunits (Galpha(12)) of heterotrimeric G proteins was illustrated. Sequence analyses of PLC-epsilon revealed previously unrecognized PH and EF-hand domains in the amino terminus. The known interaction of Gbetagamma subunits with the PH domains of other proteins led us to examine the capacity of Gbetagamma to activate PLC-epsilon. Co-expression of Gbeta(1)gamma(2) with PLC-epsilon in COS-7 cells resulted in marked stimulation of phospholipase C activity. Gbeta(2) and Gbeta(4) in combination with Ggamma(1), Ggamma(2), Ggamma(3), or Ggamma(13) also activated PLC-epsilon to levels similar to those observed with Gbeta(1)-containing dimers of these Ggamma-subunits. Gbeta(3) in combination with the same Ggamma-subunits was less active, and Gbeta(5)-containing dimers were essentially inactive. Gbetagamma-promoted activation of PLC-epsilon was blocked by cotransfection with either of two Gbetagamma-interacting proteins, Galpha(i1) or the carboxyl terminus of G protein receptor kinase 2. Pharmacological inhibition of PI3-kinase-gamma had no effect on Gbeta(1)gamma(2)-promoted activation of PLC-epsilon. Similarly, activation of Ras in the action of Gbetagamma is unlikely, because a mutation in the second RA domain of PLC-epsilon that blocks Ras activation of PLC failed to alter the stimulatory activity of Gbeta(1)gamma(2). Taken together, these results reveal the presence of additional functional domains in PLC-epsilon and add a new level of complexity in the regulation of this novel enzyme by heterotrimeric G proteins.     

Adenylyl cyclase regulates signal onset via the inhibitory GTP-binding protein, Gi

Adenylyl cyclase, the enzyme that converts ATP to cAMP, is regulated by its stimulatory and inhibitory GTP-binding proteins, G(s) and G(i), respectively. Recently, we demonstrated that besides catalyzing the synthesis of cAMP, type V adenylyl cyclase (ACV) can act as a GTPase-activating protein for Galpha(s) and also enhance the ability of activated receptors to stimulate GTP-GDP exchange on heterotrimeric G(s) (Scholich, K., Mullenix, J. B., Wittpoth, C., Poppleton, H. M., Pierre, S. C., Lindorfer, M. A., Garrison, J. C., and Patel, T. B. (1999) Science 283, 1328-1331). This latter action of ACV would facilitate the rapid onset of signaling via G(s). Because the C1 region of ACV interacts with the inhibitory GTP-binding protein Galpha(i), we investigated whether the receptor-mediated activation of heterotrimeric G(i) was also regulated by ACV and its subdomains. Our data show that ACV and its C1 domain increased the ability of a muscarinic receptor mimetic peptide (MIII-4) to enhance activation of heterotrimeric G(i) such that the amount of peptide required to stimulate G(i) in steady-state GTPase activity assays was 3-4 orders of magnitude less than without the C1 domain. Additionally, the MIII-4-mediated binding of guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) to G(i) was also markedly increased in the presence of ACV or its C1 domain. In contrast, the C2 domain of ACV was not able to alter either the GTPase activity or the GTPgammaS binding to G(i) in the presence of MIII-4. Furthermore, in adenylyl cyclase assays employing S49 cyc(-) cell membranes, the C1 (but not the C2) domain of ACV enhanced the ability of peptide MIII-4 as well as endogenous somatostatin receptors to activate endogenous G(i) and to inhibit adenylyl cyclase activity. These data demonstrate that adenylyl cyclase and its C1 domain facilitate receptor-mediated activation of G(i).     

Identification of new Gβγ interaction sites in adenylyl cyclase 2

The role of Gβγ in adenylyl cyclase (AC) signaling is complicated due to its role as a conditional activator (AC2, AC4 and AC7) and an inhibitor (AC1, AC3 and AC8). AC2 is stimulated by Gα_s and if Gβγ is present the stimulation is synergistic. The precise mechanism of this synergistic activation is still not known. In order to further elucidate the role of Gβγ in AC2 activation by Gα_s, peptides derived from the C1 domains of AC2 were synthesized and the ability of the various peptides to regulate AC2 function was tested. Our results identify two new Gβγ-binding sites in the AC2 C1 domain, AC2 C1a 339-360 and AC2 C1b 578-602 that are involved with stimulation of AC2 by Gβγ. These two regions are different from the previously described QEHA motif in the C2 domain of AC2. Further, the recently discovered PFAHL motif was confirmed to bind and to be involved with stimulation of AC2 by Gβγ. These functional studies indicate that multiple regions of AC2 are involved in the interaction with Gβγ.     

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.