Expression and analysis of Gs alpha mutants with decreased ability to activate adenylylcyclase

In-depth analysis of Gαs protein activity by probing different fluorescently labeled guanine nucleotides

G proteins are interacting partners of G protein-coupled receptors (GPCRs) in eukaryotic cells. Upon G protein activation, the ability of the Gα subunit to exchange GDP for GTP determines the intracellular signal transduction. Although various studies have successfully shown that both Gαs and Gαi have an opposite effect on the intracellular cAMP production, with the latter being commonly described as "more active", the functional analysis of Gαs is a comparably more complicated matter. Additionally, the thorough investigation of the ubiquitously expressed variants of Gαs, Gαs(short) and Gαs(long), is still pending. Since the previous experimental evaluation of the activity and function of the Gαs isoforms is not consistent, the focus was laid on structural investigations to understand the GTPase activity. Herein, we examined recombinant human Gαs by applying an established methodological setup developed for Gαi characterization. The ability for GTP binding was evaluated with fluorescence and fluorescence anisotropy assays, whereas the intrinsic hydrolytic activity of the isoforms was determined by a GTPase assay. Among different nucleotide probes, BODIPY FL GTPγS exhibited the highest binding affinity towards the Gαs subunit. This work provides a deeper understanding of the Gαs subunit and provides novel information concerning the differences between the two protein variants.

Structures of Ric-8B in complex with Gα protein folding clients reveal isoform specificity mechanisms

Mammalian Ric-8 proteins act as chaperones to regulate the cellular abundance of heterotrimeric G protein α subunits. The Ric-8A isoform chaperones Gαi/o, Gα12/13, and Gαq/11 subunits, while Ric-8B acts on Gαs/olf subunits. Here, we determined cryoelectron microscopy (cryo-EM) structures of Ric-8B in complex with Gαs and Gαolf, revealing isoform differences in the relative positioning and contacts between the C-terminal α5 helix of Gα within the concave pocket formed by Ric-8 α-helical repeat elements. Despite the overall architectural similarity with our earlier structures of Ric-8A complexed to Gαq and Gαi1, Ric-8B distinctly accommodates an extended loop found only in Gαs/olf proteins. The structures, along with results from Ric-8 protein thermal stability assays and cell-based Gαolf folding assays, support a requirement for the Gα C-terminal region for binding specificity, and highlight that multiple structural elements impart specificity for Ric-8/G protein binding.

Gain-of-function screen of α-transducin identifies an essential phenylalanine residue necessary for full effector activation

Two regions on the α subunits of heterotrimeric GTP-binding proteins (G-proteins), the Switch II/α2 helix (which changes conformation upon GDP-GTP exchange) and the α3 helix, have been shown to contain the binding sites for their effector proteins. However, how the binding of Gα subunits to their effector proteins is translated into the stimulation of effector activity is still poorly understood. Here, we took advantage of a reconstituted rhodopsin-coupled phototransduction system to address this question and identified a distinct surface and an essential residue on the α subunit of the G-protein transducin (α_T) that is necessary to fully activate its effector enzyme, the cGMP phosphodiesterase (PDE). We started with a chimeric G-protein α subunit (α_T*) comprising residues mainly from α_T and a short stretch of residues from the G_i1 α subunit (α_i1), which only weakly stimulates PDE activity. We then reinstated the α_T residues by systematically replacing the corresponding α_i1 residues within α_T* with the aim of fully restoring PDE stimulatory activity. These experiments revealed that the αG/α4 loop and a phenylalanine residue at position 283 are essential for conferring the α_T* subunit with full PDE stimulatory capability. We further demonstrated that this same region and amino acid within the α subunit of the G_s protein (α_s) are necessary for full adenylyl cyclase activation. These findings highlight the importance of the αG/α4 loop and of an essential phenylalanine residue within this region on Gα subunits α_T and α_s as being pivotal for their selective and optimal stimulation of effector activity.

A positive genotype-phenotype correlation in a large cohort of patients with Pseudohypoparathyroidism Type Ia and Pseudo-pseudohypoparathyroidism and 33 newly identified mutations in the GNAS gene

Maternally inherited inactivating GNAS mutations are the most common cause of parathyroid hormone (PTH) resistance and Albright hereditary osteodystrophy (AHO) leading to pseudohypoparathyroidism type Ia (PHPIa) due to Gsα deficiency. Paternally inherited inactivating mutations lead to isolated AHO signs characterizing pseudo-pseudohypoparathyroidism (PPHP). Mutations are distributed throughout the Gsα coding exons of GNAS and there is a lack of genotype–phenotype correlation. In this study, we sequenced exon 1–13 of GNAS in a large cohort of PHPIa- and PPHP patients and identified 58 different mutations in 88 patients and 27 relatives. Thirty-three mutations including 15 missense mutations were newly discovered. Furthermore, we found three hot spots: a known hotspot (p.D190MfsX14), a second at codon 166 (p.R166C), and a third at the exon 5 acceptor splice site (c.435 + 1G>A), found in 15, 5, and 4 unrelated patients, respectively. Comparing the clinical features to the molecular genetic data, a significantly higher occurrence of subcutaneous calcifications in patients harboring truncating versus missense mutations was demonstrated. Thus, in the largest cohort of PHPIa patients described to date, we extend the spectrum of known GNAS mutations and hot spots and demonstrate for the first time a correlation between the genetic defects and the expression of a clinical AHO-feature.

Ric-8B is a GTP-dependent G protein alphas guanine nucleotide exchange factor

ric-8 (resistance to inhibitors of cholinesterase 8) genes have positive roles in variegated G protein signaling pathways, including Gα(q) and Gα(s) regulation of neurotransmission, Gα(i)-dependent mitotic spindle positioning during (asymmetric) cell division, and Gα(olf)-dependent odorant receptor signaling. Mammalian Ric-8 activities are partitioned between two genes, ric-8A and ric-8B. Ric-8A is a guanine nucleotide exchange factor (GEF) for Gα(i)/α(q)/α(12/13) subunits. Ric-8B potentiated G(s) signaling presumably as a Gα(s)-class GEF activator, but no demonstration has shown Ric-8B GEF activity. Here, two Ric-8B isoforms were purified and found to be Gα subunit GDP release factor/GEFs. In HeLa cells, full-length Ric-8B (Ric-8BFL) bound endogenously expressed Gα(s) and lesser amounts of Gα(q) and Gα(13). Ric-8BFL stimulated guanosine 5'-3-O-(thio)triphosphate (GTPγS) binding to these subunits and Gα(olf), whereas the Ric-8BΔ9 isoform stimulated Gα(s short) GTPγS binding only. Michaelis-Menten experiments showed that Ric-8BFL elevated the V(max) of Gα(s) steady state GTP hydrolysis and the apparent K(m) values of GTP binding to Gα(s) from ∼385 nm to an estimated value of ∼42 μM. Directionality of the Ric-8BFL-catalyzed Gα(s) exchange reaction was GTP-dependent. At sub-K(m) GTP, Ric-BFL was inhibitory to exchange despite being a rapid GDP release accelerator. Ric-8BFL binds nucleotide-free Gα(s) tightly, and near-K(m) GTP levels were required to dissociate the Ric-8B·Gα nucleotide-free intermediate to release free Ric-8B and Gα-GTP. Ric-8BFL-catalyzed nucleotide exchange probably proceeds in the forward direction to produce Gα-GTP in cells.

Structural basis for the specific inhibition of heterotrimeric Gq protein by a small molecule

Heterotrimeric GTP-binding proteins (G proteins) transmit extracellular stimuli perceived by G protein-coupled receptors (GPCRs) to intracellular signaling cascades. Hundreds of GPCRs exist in humans and are the targets of a large percentage of the pharmaceutical drugs used today. Because G proteins are regulated by GPCRs, small molecules that directly modulate G proteins have the potential to become therapeutic agents. However, strategies to develop modulators have been hampered by a lack of structural knowledge of targeting sites for specific modulator binding. Here we present the mechanism of action of the cyclic depsipeptide YM-254890, which is a recently discovered Gq-selective inhibitor. YM-254890 specifically inhibits the GDP/GTP exchange reaction of alpha subunit of Gq protein (Galphaq) by inhibiting the GDP release from Galphaq. X-ray crystal structure analysis of the Galphaqbetagamma-YM-254890 complex shows that YM-254890 binds the hydrophobic cleft between two interdomain linkers connecting the GTPase and helical domains of the Galphaq. The binding stabilizes an inactive GDP-bound form through direct interactions with switch I and impairs the linker flexibility. Our studies provide a novel targeting site for the development of small molecules that selectively inhibit each Galpha subunit and an insight into the molecular mechanism of G protein activation.

Ric-8B stabilizes the alpha subunit of stimulatory G protein by inhibiting its ubiquitination

The alpha subunit of stimulatory G protein (G alpha(s)) activates adenylyl cyclase, which catalyzes cAMP production, and regulates many physiological aspects, such as cardiac regulation and endocrine systems. Ric-8B (resistance to inhibitors of cholinesterase 8B) has been identified as the G alpha(s)-binding protein; however, its role in G(s) signaling remains obscure. In this study, we present evidence that Ric-8B specifically and positively regulates G(s) signaling by stabilizing the G alpha(s) protein. An in vitro biochemical study suggested that Ric-8B does not possess guanine nucleotide exchange factor activity. However, knockdown of Ric-8B attenuated beta-adrenergic agonist-induced cAMP accumulation, indicating that Ric-8B positively regulates G(s) signaling. Interestingly, overexpression and knockdown of Ric-8B resulted in an increase and a decrease in the G alpha(s) protein, respectively, without affecting the G alpha(s) mRNA level. We found that the G alpha(s) protein is ubiquitinated and that this ubiquitination is inhibited by Ric-8B. This Ric-8B-mediated inhibition of G alpha(s) ubiquitination requires interaction between Ric-8B and G alpha(s) because Ric-8B splicing variants, which are defective for G alpha(s) binding, failed to inhibit the ubiquitination. Taken together, these results suggest that Ric-8B plays a critical and specific role in the control of G alpha(s) protein levels by modulating G alpha(s) ubiquitination and positively regulates G(s) signaling.

Receptor-mediated activation of heterotrimeric G-proteins: current structural insights

G-protein-coupled receptors (GPCRs) serve as catalytic activators of heterotrimeric G-proteins (Galphabetagamma) by exchanging GTP for the bound GDP on the Galpha subunit. This guanine nucleotide exchange factor activity of GPCRs is the initial step in the G-protein cycle and determines the onset of various intracellular signaling pathways that govern critical physiological responses to extracellular cues. Although the structural basis for many steps in the G-protein nucleotide cycle have been made clear over the past decade, the precise mechanism for receptor-mediated G-protein activation remains incompletely defined. Given that these receptors have historically represented a set of rich drug targets, a more complete understanding of their mechanism of action should provide further avenues for drug discovery. Several models have been proposed to explain the communication between activated GPCRs and Galphabetagamma leading to the structural changes required for guanine nucleotide exchange. This review is focused on the structural biology of G-protein signal transduction with an emphasis on the current hypotheses regarding Galphabetagamma activation. We highlight several recent results shedding new light on the structural changes in Galpha that may underlie GDP release.

Excess of Gbetae over Gqalphae in vivo prevents dark, spontaneous activity of Drosophila photoreceptors

Drosophila melanogaster photoreceptor cells are capable of detecting single photons. This utmost sensitivity is critically dependent on the maintenance of an exceedingly low, dark, spontaneous activity of photoreceptor cells. However, the underlying mechanisms of this hallmark of phototransduction are not fully understood. An analysis of the Drosophila visual heterotrimeric (alphabetagamma) Gq protein revealed that wild-type Drosophila flies have about a twofold excess of Gbeta over Galpha subunits of the visual Gq protein. Studies of Gbetae mutants in which the excess of Gbeta was genetically eliminated showed dramatic dark, spontaneous activity of the photoreceptor cells, whereas concurrent genetic reduction of the Galpha subunit, which restored the excess of Gbeta, abolished this effect. These results indicate that an excess of Gbeta over Galpha is a strategy used in vivo for the suppression of spontaneous activity, thereby yielding a high signal to noise ratio, which is characteristic of the photoreceptor light response. This mechanism could be relevant to the regulation of G protein signaling in general.

Modulation of EGF receptor-mediated vulva development by the heterotrimeric G-protein Galphaq and excitable cells in C. elegans

The extent to which excitable cells and behavior modulate animal development has not been examined in detail. Here, we demonstrate the existence of a novel pathway for promoting vulval fates in C. elegans that involves activation of the heterotrimeric Galphaq protein, EGL-30. EGL-30 acts with muscle-expressed EGL-19 L-type voltage-gated calcium channels to promote vulva development, and acts downstream or parallel to LET-60 (RAS). This pathway is not essential for vulval induction on standard Petri plates, but can be stimulated by expression of activated EGL-30 in neurons, or by an EGL-30-dependent change in behavior that occurs in a liquid environment. Our results indicate that excitable cells and animal behavior can provide modulatory inputs into the effects of growth factor signaling on cell fates, and suggest that communication between these cell populations is important for normal development to occur under certain environmental conditions.

Isoform-specific regulation of adenylyl cyclase: a potential target in future pharmacotherapy

Adenylyl cyclase (AC) is a target enzyme of multiple G-protein-coupled receptors (GPCRs). In the past decade, the cloning, structure and biochemical properties of nine AC isoforms were reported, and each isoform of AC shows distinct patterns of tissue distribution and biochemical/pharmacological properties. In addition to the conventional regulators of this enzyme, such as calmodulin (CaM) or PKC, novel regulators, for example, caveolin, have been identified. Most importantly, these regulators work on AC in an isoform dependent manner. Recent studies have demonstrated that certain classic AC inhibitors, i.e., P-site inhibitors, show an isoform-dependent inhibition of AC. The side chain modifications of forskolin, a diterpene extract from Coleus forskolii, markedly enhance its isoform selectivity. When taken together, these findings suggest that it is feasible to develop new pharmacotherapeutic agents that target AC isoforms to regulate various neurohormonal signals in a highly tissue-/organ-specific manner.

Endocrine manifestations of stimulatory G protein alpha-subunit mutations and the role of genomic imprinting

The heterotrimeric G protein G(s) couples hormone receptors (as well as other receptors) to the effector enzyme adenylyl cyclase and is therefore required for hormone-stimulated intracellular cAMP generation. Receptors activate G(s) by promoting exchange of GTP for GDP on the G(s) alpha-subunit (G(s)alpha) while an intrinsic GTPase activity of G(s)alpha that hydrolyzes bound GTP to GDP leads to deactivation. Mutations of specific G(s)alpha residues (Arg(201) or Gln(227)) that are critical for the GTPase reaction lead to constitutive activation of G(s)-coupled signaling pathways, and such somatic mutations are found in endocrine tumors, fibrous dysplasia of bone, and the McCune-Albright syndrome. Conversely, heterozygous loss-of-function mutations may lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, brachydactyly, sc ossifications, and mental deficits. Similar mutations are also associated with progressive osseous heteroplasia. Interestingly, paternal transmission of GNAS1 mutations leads to the AHO phenotype alone (pseudopseudohypoparathyroidism), while maternal transmission leads to AHO plus resistance to several hormones (e.g., PTH, TSH) that activate G(s) in their target tissues (pseudohypoparathyroidism type IA). Studies in G(s)alpha knockout mice demonstrate that G(s)alpha is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues (e.g., renal proximal tubule, the major site of renal PTH action), while being biallelically expressed in most other tissues. Disrupting mutations in the maternal allele lead to loss of G(s)alpha expression in proximal tubules and therefore loss of PTH action in the kidney, while mutations in the paternal allele have little effect on G(s)alpha expression or PTH action. G(s)alpha has recently been shown to be also imprinted in human pituitary glands. The G(s)alpha gene GNAS1 (as well as its murine ortholog Gnas) has at least four alternative promoters and first exons, leading to the production of alternative gene products including G(s)alpha, XLalphas (a novel G(s)alpha isoform that is expressed only from the paternal allele), and NESP55 (a chromogranin-like protein that is expressed only from the maternal allele). A fourth alternative promoter and first exon (exon 1A) located approximately 2.5 kb upstream of the G(s)alpha promoter is normally methylated on the maternal allele and transcriptionally active on the paternal allele. In patients with isolated renal resistance to PTH (pseudohypoparathyroidism type IB), the exon 1A promoter region has a paternal-specific imprinting pattern on both alleles (unmethylated, transcriptionally active), suggesting that this region is critical for the tissue-specific imprinting of G(s)alpha. The GNAS1 imprinting defect in pseudohypoparathyroidism type IB is predicted to decrease G(s)alpha expression in renal proximal tubules. Studies in G(s)alpha knockout mice also demonstrate that this gene is critical in the regulation of lipid and glucose metabolism.

Molecular biological approaches to unravel adenylyl cyclase signaling and function

Signal transduction through the cell membrane requires the participation of one or more plasma membrane proteins. For many transmembrane signaling events adenylyl cyclases (ACs) are the final effector enzymes which integrate and interpret divergent signals from different pathways. The enzymatic activity of adenylyl cyclases is stimulated or inhibited in response to the activation of a large number of receptors in virtually all cells of the human body. To date, ten different mammalian isoforms of adenylyl cyclase (AC) have been cloned and characterized. Each isoform has its own distinct tissue distribution and regulatory properties, providing possibilities for different cells to respond diversely to similar stimuli. The product of the enzymatic reaction catalyzed by ACs, cyclic AMP (cAMP) has been shown to play a crucial role for a variety of fundamental physiological cell functions ranging from cell growth and differentiation, to transcriptional regulation and apoptosis. In the past, investigations into the regulatory mechanisms of ACs were limited by difficulties associated with their purification and the availability of the proteins in any significant amount. Moreover, nearly every cell expresses several AC isoforms. Therefore, it was difficult to perform biochemical characterization of the different AC isoforms and nearly impossible to assess the physiological roles of the individual isoforms in intact cells, tissue or organisms. Recently, however, different molecular biological approaches have permitted several breakthroughs in the study of ACs. Recombinant technologies have allowed biochemical analysis of adenylyl cyclases in-vitro and the development of transgenic animals as well as knock-out mice have yielded new insights in the physiological role of some AC isoforms. In this review, we will focus mainly on the most novel approaches and concepts, which have delineated the mechanisms regulating AC and unravelled novel functions for this enzyme.

Modulation of rat cardiac sodium channel by the stimulatory G protein alpha subunit

1. Modulation of cardiac sodium currents (INa) by the G protein stimulatory alpha subunit (Gsalpha) was studied using patch-clamp techniques on freshly dissociated rat ventricular myocytes. 2. Whole-cell recordings showed that stimulation of beta-adrenergic receptors with 10 microM isoprenaline (isoproterenol, ISO) enhanced INa by 68.4 +/- 9.6 % (mean +/- s.e.m.; n = 7, P < 0.05 vs. baseline). With the addition of 22 microgram ml-1 protein kinase A inhibitor (PKI) to the pipette solution, 10 microM ISO enhanced INa by 30.5 +/- 7.0 % (n = 7, P < 0.05 vs. baseline). With the pipette solution containing both PKI and 20 microgram ml-1 anti-Gsalpha IgG or 20 microgram ml-1 anti-Gsalpha IgG alone, 10 microM ISO produced no change in INa. 3. The effect of Gsalpha on INa was not due to changes in the steady-state activation or inactivation curves, the time course of current decay, the development of inactivation, or the recovery from inactivation. 4. Whole-cell INa was increased by 45.2 +/- 5.3% (n = 13, P < 0.05 vs. control) with pipette solution containing 1 microM Gsalpha27-42 peptide (amino acids 27-42 of rat brain Gsalpha) without altering the properties of Na+ channel kinetics. Furthermore, application of 1 nM Gsalpha27-42 to Na+ channels in inside-out macropatches increased the ensemble-averaged INa by 32.5 +/- 6.8 % (n = 8, P < 0.05 vs. baseline). The increase in INa was reversible upon Gsalpha27-42 peptide washout. Single channel experiments showed that the Gsalpha27-42 peptide did not alter the Na+ single channel current amplitude, the mean open time or the mean closed time, but increased the number of functional channels (N) in the patch. 5. Application of selected short amino acid segments (Gsalpha27-36, Gsalpha33-42 and Gsalpha30-39) of the 16 amino acid Gsalpha peptide (Gsalpha27-42 peptide) showed that only the C-terminal segment of this peptide (Gsalpha33-42) significantly increased INa in a dose-dependent fashion. These results show that cardiac INa is regulated by Gsalpha via a mechanism independent of PKA that results in an increase in the number of functional Na+ channels. In addition, a 10 residue domain (amino acids 33-42) near the N-terminus of Gsalpha is important in modulating cardiac Na+ channels.

G protein regulation of adenylate cyclase

Adenylate cyclase integrates positive and negative signals that act through G protein-coupled cell-surface receptors with other extracellular stimuli to finely regulate levels of cAMP within the cell. Recently, the structures of the cyclase catalytic core complexed with the plant diterpene forskolin, and a cyclase-forskolin complex bound to an activated form of the stimulatory G protein subunit Gs alpha have been solved by X-ray crystallography. These structures provide a wealth of detail about how different signals could converge at the core cyclase domains to regulate catalysis. In this article, William Simonds reviews recent advances in the molecular and structural biology of this key regulatory enzyme, which provide new insight into its ability to integrate multiple signals in diverse cellular contexts.

Identification of effector residues on photoreceptor G protein, transducin

Transducin is a photoreceptor-specific heterotrimeric GTP-binding protein that plays a key role in the vertebrate visual transduction cascade. Here, using scanning site-directed mutagenesis of the chimeric Galphat/Galphai1 alpha-subunit (Galphat/i), we identified Galphat residues critical for interaction with the effector enzyme, rod cGMP phosphodiesterase (PDE). Our evidence suggests that residue Ile208 in the switch II region directly interacts with the effector in the active GTP-bound conformation of Galphat. Residues Arg201, Arg204, and Trp207 are essential for the conformation-dependent Galphat/effector interaction either via direct contacts with the inhibitory PDE gamma-subunit or by forming an effector-competent conformation through the communication network between switch II and the switch III/alpha3-helix domain of Galphat. Residues His244 and Asn247 in the alpha3 helix of Galphat are responsible for the conformation-independent effector-specific interaction. Insertion of these residues rendered the Galphat/i chimera with the ability to bind PDE gamma-subunit and stimulate PDE activity approaching that of native Galphat. Comparative analysis of the interactions of Galphat/i mutants with PDE and RGS16 revealed two adjacent but distinct interfaces on transducin. This indicates a possibility for a functional trimeric complex, RGS/Galpha/effector, that may play a central role in turn-off mechanisms of G protein signaling systems, particularly in phototransduction.

Analysis of the Gs/mitogen-activated protein kinase pathway in mutant S49 cells

Heterotrimeric G protein-coupled receptors can activate the mitogen-activated protein kinase (MAPK) cascade. Recent studies using pharmacological inhibitors or dominant-negative mutants of signaling molecules have advanced our understanding of the pathways from G protein-coupled receptors to MAPK. However, molecular genetic analysis of these pathways is inadequate in mammalian cells. Here, using the well characterized Gsalpha- and protein kinase A-deficient S49 mouse lymphoma cells, we provide the molecular genetic evidence that Gsalpha is responsible for transducing the beta-adrenergic receptor signal to MAPK in a protein kinase A-dependent pathway involving Rap1 and Raf (but not Ras) molecules.

Genetic selection of mammalian adenylyl cyclases insensitive to stimulation by Gsalpha

We describe the development of a genetic system allowing for the isolation of mutant mammalian adenylyl cyclases defective in their responses to G protein subunits, thus allowing for the identification of structural elements within the cyclase that are responsible for the recognition of these regulators. Expression of mammalian type V adenylyl cyclase in a cyclase-deleted yeast strain can conditionally complement the lethal phenotype of this strain. Type V adenylyl cyclase-expressing yeast grow only when the cyclase is activated by coexpression of Gsalpha or addition of forskolin to the medium; however, growth arrest is observed in the presence of both activators or under basal conditions. Utilizing this genetic system, we have isolated 25 adenylyl cyclase mutants defective in their response to Gsalpha. Sequence analysis and biochemical characterization of these mutants have identified residues in both cytoplasmic domains of the cyclase that are involved in the specific binding of and regulation by Gsalpha.

Alpha helix content of G protein alpha subunit is decreased upon activation by receptor mimetics

To elucidate the mechanism whereby liganded receptor molecules enhance nucleotide exchange of GTP-binding regulatory proteins (G proteins), changes in the secondary structure of the recombinant Gi1 alpha subunit (Gi1alpha) upon binding with receptor mimetics, compound 48/80 and mastoparan, were analyzed by circular dichroism spectroscopy. Compound 48/80 enhanced the initial rate of GTPgammaS binding to soluble Gi1alpha 2.6-fold with an EC50 of 30 microg/ml. With the same EC50, the mimetic decreased the magnitude of ellipticity, which is ascribed to a reduction in alpha helix content of the Gi1alpha by 7%. Likewise, mastoparan also enhanced the rate of GTPgammaS binding by 3.0-fold and decreased the magnitude of ellipticity of Gi1alpha similar to compound 48/80. In corresponding experiments using a K349P-Gi1alpha, a Gi1alpha counterpart of the unc mutant in Gsalpha in which Pro was substituted for Lys349, enhancement of the GTPgammaS binding rate by both activators was quite small. In addition, compound 48/80 showed a negligible effect on the circular dichroism spectrum of the mutant. On the other hand, a proteolytic fragment of Gi1alpha lacking the N-terminal 29 residues was activated and showed decreased ellipticity upon interaction with the compound, as did the wild-type Gi1alpha. Taken together, our results strongly suggest that the activator-induced unwinding of the alpha helix of the G protein alpha subunit is mechanically coupled to the enhanced release of bound GDP from the alpha subunit.

Crystal structure of the catalytic domains of adenylyl cyclase in a complex with Gsalpha.GTPgammaS

The crystal structure of a soluble, catalytically active form of adenylyl cyclase in a complex with its stimulatory heterotrimeric G protein alpha subunit (Gsalpha) and forskolin was determined to a resolution of 2.3 angstroms. When P-site inhibitors were soaked into native crystals of the complex, the active site of adenylyl cyclase was located and structural elements important for substrate recognition and catalysis were identified. On the basis of these and other structures, a molecular mechanism is proposed for the activation of adenylyl cyclase by Gsalpha.

Crystal structure of the adenylyl cyclase activator Gsalpha

The crystal structure of Gsalpha, the heterotrimeric G protein alpha subunit that stimulates adenylyl cyclase, was determined at 2.5 A in a complex with guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). Gsalpha is the prototypic member of a family of GTP-binding proteins that regulate the activities of effectors in a hormone-dependent manner. Comparison of the structure of Gsalpha.GTPgammaS with that of Gialpha.GTPgammaS suggests that their effector specificity is primarily dictated by the shape of the binding surface formed by the switch II helix and the alpha3-beta5 loop, despite the high sequence homology of these elements. In contrast, sequence divergence explains the inability of regulators of G protein signaling to stimulate the GTPase activity of Gsalpha. The betagamma binding surface of Gsalpha is largely conserved in sequence and structure to that of Gialpha, whereas differences in the surface formed by the carboxyl-terminal helix and the alpha4-beta6 loop may mediate receptor specificity.

Identification of common and distinct residues involved in the interaction of alphai2 and alphas with adenylyl cyclase

The G protein alpha subunits, alphas and alphai2, have stimulatory and inhibitory effects, respectively, on a common effector protein, adenylyl cyclase. These effects require a GTP-dependent conformational change that involves three alpha subunit regions (Switches I-III). alphas residues in three adjacent loops, including Switch II, specify activation of adenylyl cyclase. The adenylyl cyclase-specifying region of alphai2 is located within a 78-residue segment that includes two of these loops but none of the conformational switch regions. We have used an alanine-scanning mutagenesis approach within Switches I-III and the 78-residue segment of alphai2 to identify residues required for inhibition of adenylyl cyclase. We found a cluster of conserved residues in Switch II in which substitutions cause major losses in the abilities of both alphai2 and alphas to modulate adenylyl cyclase activity but do not affect alpha subunit expression or the GTP-induced conformational change. We also found two regions within the 78-residue segment of alphai2 in which substitutions reduce the ability of alphai2 to inhibit adenylyl cyclase, one of which corresponds to an effector-activating region of alphas. Thus, both alphai2 and alphas interact with adenylyl cyclase using: 1) conserved Switch II residues that communicate the conformational state of the alpha subunit and 2) divergent residues that specify particular effectors and the nature of their modulation.

Reconstitution of mitogen-activated protein kinase phosphorylation cascades in bacteria. Efficient synthesis of active protein kinases

Mitogen-activated protein (MAP) kinase pathways include a three-kinase cascade terminating in a MAP kinase family member. The middle kinase in the cascade is a MAP/extracellular signal-regulated kinase (ERK) kinase or MEK family member and is highly specific for its MAP kinase target. The first kinase in the cascade, a MEK kinase (MEKK), is characterized by its ability to activate one or more MEK family members. A two-plasmid bacterial expression system was employed to express active forms of the following MEK and MAP kinase family members: ERK1, ERK2, alpha-SAPK, and p38 and their upstream activators, MEK1, -2, -3, and -4. In each kinase module, the upstream activator, a constitutively active mutant of MEK1 or MEKK1, was expressed from a low copy plasmid, while one or two downstream effector kinases were expressed from a high copy plasmid with different antibiotic resistance genes and origins of replication. Consistent with their high activity, ERK1 and ERK2 were doubly phosphorylated on Tyr and Thr, were recognized by an antibody specific to the doubly phosphorylated forms, and were inactivated by either phosphoprotein phosphatase 2A or phosphotyrosine phosphatase type 1. Likewise, activated p38 and alpha-stress-activated protein kinase could also be inactivated by either phosphatase, and alpha-stress-activated protein kinase was recognized by an antibody specific to the doubly phosphorylated forms. These three purified, active MAP kinases have specific activities in the range of 0.6-2.3 micromol/min/mg. Coexpression of protein kinases with their substrates in bacteria is of great value in the preparation of numerous phosphoproteins, heretofore not possible in procaryotic expression systems.

Does subunit dissociation necessarily accompany the activation of all heterotrimeric G proteins?

Heterotrimeric (alpha beta gamma) G proteins mediate a variety of signal transduction events in virtually every cell of every eukaryotic organism. The predominant hypothesis is that dissociation of the alpha-subunit from the G beta gamma-subunit complex necessarily accompanies the activation of these proteins, and that the alpha-subunit is primarily responsible for regulating the response of effector molecules. However, there is increasing evidence that both the alpha-subunit and the beta gamma-subunit complex function in regulating effector activity. Furthermore, data for some G proteins suggest that they function as activated heterotrimers rather than as dissociated subunits.

G protein mechanisms: insights from structural analysis

This review is concerned with the structures and mechanisms of a superfamily of regulatory GTP hydrolases (G proteins). G proteins include Ras and its close homologs, translation elongation factors, and heterotrimeric G proteins. These proteins share a common structural core, exemplified by that of p21ras (Ras), and significant sequence identity, suggesting a common evolutionary origin. Three-dimensional structures of members of the G protein superfamily are considered in light of other biochemical findings about the function of these proteins. Relationships among G protein structures are discussed, and factors contributing to their low intrinsic rate of GTP hydrolysis are considered. Comparison of GTP- and GDP-bound conformations of G proteins reveals how specific contacts between the gamma-phosphate of GTP and the switch II region stabilize potential effector-binding sites and how GTP hydrolysis results in collapse (or reordering) of these surfaces. A GTPase-activating protein probably binds to and stabilizes the conformation of its cognate G protein that recognizes the transition state for hydrolysis, and may insert a catalytic residue into the G protein active site. Inhibitors of nucleotide release, such as the beta gamma subunit of a heterotrimeric G protein, bind selectively to and stabilize the GDP-bound state. Release factors, such as the translation elongation factor, Ts, also recognize the switch regions and destabilize the Mg(2+)-binding site, thereby promoting GDP release. G protein-coupled receptors are expected to operate by a somewhat different mechanism, given that the GDP-bound form of many G protein alpha subunits does not contain bound Mg2+.

Mapping of effector binding sites of transducin alpha-subunit using G alpha t/G alpha i1 chimeras

The G protein transducin has been an often-used model for biochemical, structural, and mechanistic studies of G protein function. Experimental studies have been limited, however, by the inability to express quantities of mutants in heterologous systems with ease. In this study we have made a series of G alpha t/G alpha i1 chimeras differing at as few as 11 positions from native G alpha t. Ten chimeras are properly folded, contain GDP, can assume an A1F4(-)-induced activated conformation, and interact with beta gamma t and light-activated rhodopsin. They differ dramatically in their affinity for GDP, from Gi-like (initial rates 225 mumol/mol s) to Gt-like (initial rates 4.9 mumol/mol s). We have used these chimeras to define contact sites on G alpha t with the effector enzyme cGMP phosphodiesterase. G alpha t GTP but not G alpha t GDP activates it by removing the phosphodiesterase (PDE) gamma inhibitory subunit. In solution, G alpha t GTP interacts with PDE gamma (Kd 12 nM), while G alpha t GDP binds PDE gamma more weakly (Kd 0.88 microM). The interaction of G alpha i GDP with PDE gamma is undetectable, but G alpha i GDP-A1F4- interacts weakly with PDE gamma (Kd 2.4 microM). Using defined G alpha t/G alpha i chimeras, we have individuated the regions on G alpha t most important for interaction with PDE gamma in the basal and activated states. The G alpha t sequence encompassing alpha helix 3 and the alpha 3/beta 5 loop contributes most binding energy to interaction with PDE gamma. Another composite P gamma interaction site is the conserved switch, through which the GTP-bound G alpha t as well as G alpha i1 interact with P gamma. Competition studies between PDE gamma and truncated regions of PDE gamma provide evidence for the point-to-point interactions between the two proteins. The amino-terminal 1-45 segment containing the central polycationic region binds to G alpha t's alpha 3 helix and alpha 3/beta 5 loop, while the COOH-terminal region of P gamma, 63-87, binds in concert to the conserved switch regions. The first interaction provides specific interaction with both the GDP- and GTP-liganded G alpha t, while the second one is conserved between G alpha t and G alpha i1 and dependent on the activated conformation.

Interaction site of GTP binding Gh (transglutaminase II) with phospholipase C

The GTP binding G alpha h (transglutaminase II) mediates the alpha 1B-adrenoreceptor signal to a 69-kDa phospholipase C (PLC). Thus, G alpha h possesses both GTPase and transglutaminase activities with a signal transfer role. The recognition sites of this unique GTP binding protein for either the receptor or the effector are completely unknown. A site on human heart G alpha h (hhG alpha h) has been identified that interacts with and stimulates PLC. Expressed mutants of hhG alpha h with deleted C-terminal regions lost the response to (-)-epinephrine and GTP and failed to coimmunoprecipitate PLC by the specific Gh7 alpha antibody. The interaction regions were further defined by studies with synthetic peptides of hhG alpha h and a chimera in which residues Val665-Lys672 of hhG alpha h were substituted with Ile707-Ser714 residues of human coagulation factor XIIIa. Thus, eight amino acid residues near the C terminus of hhG alpha h are critical for recognition and stimulation of PLC.

Gq alpha protein function in vivo: genetic dissection of its role in photoreceptor cell physiology

Heterotrimeric G proteins mediate a variety of signaling processes by coupling seven-transmembrane receptors to intracellular effector molecules. The Drosophila phototransduction cascade is a G protein-coupled signaling cascade that utilizes a phospholipase C (PLC beta) effector. PLC beta has been shown to be activated by Gq alpha in reconstituted systems. To determine whether a Gq-like protein couples rhodopsin to PLC, and to study its function, we isolated a mutant defective in a photoreceptor-specific Gq protein, DGq. We now demonstrate that Gq is essential for the activation of the phototransduction cascade in vivo. We also generated transgenic flies expressing DGq under an inducible promoter and show that it is possible to manipulate the sensitivity of a photoreceptor cell by controlled expression of DGq. Characterization of quantum bumps in mutants expressing less that 1% of the levels of DGq revealed that the rhodopsin-G protein interaction does not determine the gain of the single photon responses. Together, these results provide significant insight into the role of Gq in regulating the output of a photoreceptor cell.

Alteration of prostaglandin E receptors in advanced breast tumour cell lines

We studied the direct effects of PGE2, often produced at high levels by mammary tumours, on three human breast cancer cell lines diversely advanced in malignancy regarding differentiation and tumorigenicity in nude mice. We evaluated PGE2 effect on cell growth, PGE2 receptor level and functionality. Our results show that PGE2 induces cAMP accumulation and inhibits the growth of the most differentiated breast cancer cells. We also demonstrate that loss and probably dysfunction of PGE2 receptors is related to an advanced tumorigenic phenotype of the cells. Thus, it seems that during progression of breast cancer, the cell growth escapes from control by PGE2. Nevertheless, it is possible to control the growth of advanced breast cancer cells in vitro by direct induction of intracellular cAMP accumulation.

Differential regulation of G protein alpha-subunit GTPase activity by peptides derived from the third cytoplasmic loop of the alpha 2-adrenergic receptor

The effect of peptides homologous to segments of a G protein-coupled receptor on the GTPase activity of recombinant Go alpha (rGo alpha) and Gs alpha (rGs alpha) has been tested. These peptides contain overlapping sequences spanning from amino acid 212 of the putative fifth transmembrane domain to amino acid 229 of the third cytoplasmic loop of the alpha 2 adrenergic receptor. Interestingly, two peptides (comprising residues 212-227 and 214-227) strongly inhibit the basal GTPase activity of both rGo alpha and rGs alpha. Instead, a C-terminally extended peptide (residues 216-229) stimulates rGo alpha but slightly inhibits rGs alpha. Circular dichroism spectroscopy of the peptides reveals that an a helical structure is more easily inducible in the inhibitory ones. These findings constitute an example of peptides representing cytoplasmic receptor sequences that differentially modulate the GTPase activity of recombinant G protein alpha-subunits.

Mapping regions of G alpha q interacting with PLC beta 1 using multiple overlapping synthetic peptides

The heterotrimeric G-protein alpha-chain G alpha q plays a critical role mediating receptor-linked activation of the beta isoforms of PLC which hydrolyse membrane inositol-containing phospholipids to generate the second messengers inositol 1,4,5-trisphosphate and diacylglycerol. Despite knowledge of the three-dimensional structure of two G-protein alpha-chains (G alpha t and G alpha i1) as well as high regional amino acid conservation between members of the G-protein alpha-chain family, the precise molecular domains of G alpha q mediating activation of PLC beta 1 are unknown. To map sites responsible for effector interaction we employed 188 peptides each of 15 residues and corresponding to overlapping regions of the complete G alpha q sequence. These were tested for their ability to inhibit G alpha q-dependent activation of recombinant PLC beta 1 using an in vitro reconstitution assay. Peptides from two regions of G alpha q mediated up to 100% inhibition of GTP gamma S-stimulated PLC beta 1 activity, and representative peptides from each of these regions were half-maximally effective at 69.3 +/- 27.4 microM (n = 4) (G alpha q: 251-265) and 110.0 +/- 41.9 microM (n = 4) (G alpha q: 306-319). G alpha q regions described by inhibitory peptides are conserved selectively in other G-protein alpha-chains linked to PLC beta 1 activation (G alpha 11, G alpha 14) and correspond spatially to sites of effector interaction identified in G alpha s by scanning mutagenesis and in transducin using site-specific antibodies and peptides. Computer transducin using site-specific antibodies and peptides. Computer homology modelling of G alpha q based on the crystal structure of transducin indicates that regions interacting with PLC beta 1 form two parallel alpha-helices lying at the surface of the G alpha q structure. These observations provide the first description of two regions within G alpha q critically important for activating PLC beta 1, and moreover, indicate that effector binding domains identified in transducin and G alpha s are also conserved spatially in G alpha q.

GTP-dependent binding of Gi, G(o) and Gs to the gamma-subunit of the effector of Gt

The gamma-subunit of the cGMP-phosphodiesterase (PDE gamma) of retinal rods forms a tight complex with the activated alpha-subunit of transducin (Gt alpha GTP gamma S). We observe that while PDE gamma is not the physiological effector of other G alpha subtypes, it can still detectably interact with them. This interaction is strong with Gi1 alpha and Gi3 alpha (Kd approximately 10 nM) and weaker with Go alpha and Gs alpha (Kd approximately 1 microM). For all these G alpha subtypes, similar intrinsic fluorescence changes are observed upon PDE gamma binding. Moreover, similar relative decreases in affinity are obtained when the GDP forms of Gi1 alpha, Gi3 alpha or Gt alpha are used in lieu of the GTP forms. This points to a conserved GTP-dependent effector-interaction domain.

Separate GTP binding and GTPase activating domains of a G alpha subunit

Most members of the guanosine triphosphatase (GTPase) superfamily hydrolyze guanosine triphosphate (GTP) quite slowly unless stimulated by a GTPase activating protein or GAP. The alpha subunits (G alpha) of the heterotrimeric G proteins hydrolyze GTP much more rapidly and contain an approximately 120-residue insert not found in other GTPases. Interactions between a G alpha insert domain and a G alpha GTP-binding core domain, both expressed as recombinant proteins, show that the insert acts biochemically as a GAP. The results suggest a general mechanism for GAP-dependent hydrolysis of GTP by other GTPases.

Cell-specific physical and functional coupling of human 5-HT1A receptors to inhibitory G protein alpha-subunits and lack of coupling to Gs alpha

We have studied the physical and functional linkages of heterologously expressed human 5-HT1A receptors to G protein alpha-subunits in HeLa and CHO-K1 cells. HeLa cells expressed immunoreactivity to G(i) proteins with an apparent rank order of G(i) alpha 3 (approximately 1 pmol/mg of protein) >> G(i) alpha 1 (approximately 0.1 pmol/mg) >> G(i) alpha 2 (< 0.02 pmol/mg), whereas CHO-K1 cells expressed immunoreactivity to G(i) alpha 2 (approximately 5 pmol/mg) >> G(i) alpha 3 (approximately 0.7 pmol/mg), but not to G(i) alpha 1. Both cell lines expressed large and small forms of Gs alpha, but neither expressed detectable G(o) alpha. Agonist-promotable physical coupling of the 5-HT1A receptor to G proteins was examined with high-affinity agonist binding and with co-immunoprecipitation using rabbit anti-receptor IgG fractions. Agonist treatment induced coupling of the 5-HT1A receptors to G proteins with an apparent rank order of G(i) alpha 3 > G(i) alpha 1, G(i) alpha 2 in HeLa cells and G(i) alpha 3 > G(i) alpha 2 in CHO-K1 cells. Agonist-promotable functional coupling of the 5-HT1A receptors to inhibition of adenylylcyclase was measured in membranes derived from HeLa and CHO-K1 cells expressing approximately 2.5-3 pmol of receptors/mg of protein by preincubation with antisera raised against the carboxyl termini of the G(i) protein alpha-subunits. A noteworthy difference between the two cell types was that antisera against the predominant G protein (G(i) alpha 2) were substantially more efficacious than G(i) alpha 3 antisera at blocking functional coupling to adenylylcyclase inhibition in CHO-K1 cells, whereas in HeLa cells, antisera against nonpredominant G proteins (G(i) alpha 1/G(i) alpha 2) were equally as effective as those against the predominant G protein (G(i) alpha 3). No physical or functional coupling of the 5-HT1A receptor to Gs alpha isoforms was detected in either cell line. These findings suggest that the 5-HT1A receptor can physically couple to multiple distinct G(i) proteins in mammalian cell membranes and that functional coupling to adenylylcyclase inhibition may be mediated by G(i) alpha 1, G(i) alpha 2, and G(i) alpha 3. One factor influencing the relative importance of those G proteins for 5-HT1A receptor-inhibited adenylylcyclase activity appears to be their-relative levels of expression.(ABSTRACT TRUNCATED AT 250 WORDS)

G protein mutations in human disease

The heterotrimeric G proteins couple cell-surface receptors for extracellular signals to intracellular effectors that generate second messengers. Abnormal G protein signalling, resulting from posttranslational modifications by bacterial toxins, altered gene expression, or gene mutations, may lead to diverse biological consequences. Mutations within G protein alpha subunit genes that lead to either constitutive activation or loss of function have been identified. Such G protein mutations play a role in the pathogenesis of several human diseases, including sporadic endocrine tumors, McCune-Albright syndrome, and Albright hereditary osteodystrophy.

Why there is no gene for alcoholism

The search for and recognition of biological and genetic markers of alcoholism are discussed in the context of a heuristic model of human alcoholism as a complex, multilocus, heterogeneous disorder. Implications of this model for the interpretation of results from both linkage and association studies are presented.