The structure of the G protein heterotrimer Gi alpha 1 beta 1 gamma 2

The emerging mutational landscape of G proteins and G-protein-coupled receptors in cancer

Aberrant expression and activity of G proteins and G-protein-coupled receptors (GPCRs) are frequently associated with tumorigenesis. Deep sequencing studies show that 4.2% of tumours carry activating mutations in GNAS (encoding Gαs), and that oncogenic activating mutations in genes encoding Gαq family members (GNAQ or GNA11) are present in ~66% and ~6% of melanomas arising in the eye and skin, respectively. Furthermore, nearly 20% of human tumours harbour mutations in GPCRs. Many human cancer-associated viruses also express constitutively active viral GPCRs. These studies indicate that G proteins, GPCRs and their linked signalling circuitry represent novel therapeutic targets for cancer prevention and treatment.

The expanding roles of Gβγ subunits in G protein-coupled receptor signaling and drug action

Gβγ subunits from heterotrimeric G proteins perform a vast array of functions in cells with respect to signaling, often independently as well as in concert with Gα subunits. However, the eponymous term "Gβγ" does not do justice to the fact that 5 Gβ and 12 Gγ isoforms have evolved in mammals to serve much broader roles beyond their canonical roles in cellular signaling. We explore the phylogenetic diversity of Gβγ subunits with a view toward understanding these expanded roles in different cellular organelles. We suggest that the particular content of distinct Gβγ subunits regulates cellular activity, and that the granularity of individual Gβ and Gγ action is only beginning to be understood. Given the therapeutic potential of targeting Gβγ action, this larger view serves as a prelude to more specific development of drugs aimed at individual isoforms.

Molecular targeting of Gα and Gβγ subunits: a potential approach for cancer therapeutics

G-Protein-coupled receptors (GPCRs) signal through G protein α and βγ subunit families to regulate a wide range of physiological and pathophysiological processes. As such, GPCRs are major targets for therapeutic drugs. Downstream targets of GPCRs have also gained interest as a therapeutic approach to complex pathologies involving multiple GPCRs. One such approach involves targeting of the G proteins themselves. Several small molecule Gα and Gβγ modulators have been developed and been tested in various animal models of disease. Here we will discuss the requirements for targeting Gα and Gβγ subunits, the mechanisms of action of currently identified inhibitors, and focus on the potential utility of Gα and Gβγ inhibitors in the treatment of various cancers.

Membrane orientation of Gα(i)β(1)γ(2) and Gβ(1)γ(2) determined via combined vibrational spectroscopic studies

The manner in which the heterotrimeric G protein complexes Gβ1γ2 and Gαiβ1γ2 interact with membranes is likely related to their biological function. We combined complementary measurements from sum frequency generation (SFG) vibrational and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to determine the possible membrane orientations of Gβ1γ2 and the Gαiβ1γ2 heterotrimer more precisely than could be achieved using SFG alone. The most likely orientations of Gβ1γ2 and the Gαiβ1γ2 heterotrimer were both determined to fall within a similar narrow range of twist and tilt angles, suggesting that Gβ1γ2 may bind to Gαi without a significant change in orientation. This "basal" orientation seems to depend primarily on the geranylgeranylated C-terminus of Gγ2 along with basic residues at the N-terminus of Gαi, and suggests that activated G protein-coupled receptors (GPCRs) must reorient G protein heterotrimers at lipid bilayers to catalyze nucleotide exchange. The innovative methodologies developed in this paper can be widely applied to study the membrane orientation of other proteins in situ.

Linking receptor activation to changes in Sw I and II of Gα proteins

G-protein coupled receptors catalyze nucleotide exchange on G proteins, which results in subunit dissociation and effector activation. In the recent β2AR-Gs structure, portions of Switch I and II of Gα are not fully elucidated. We paired fluorescence studies of receptor-Gαi interactions with the β2AR-Gs and other Gi structures to investigate changes in Switch I and II during receptor activation and GTP binding. The β2/β3 loop containing Leu194 of Gαi is located between Switches I and II, in close proximity to IC2 of the receptor and the C-terminus of Gα, thus providing an allosteric connection between these Switches and receptor activation. We compared the environment of residues in myristoylated Gαi proteins in the heterotrimer to that upon receptor activation and subsequent GTP binding. Upon receptor activation, residues in both Switch regions are less solvent-exposed, as compared to the heterotrimer. Upon GTPγS binding, the environment of several residues in Switch I resemble the receptor-bound state, while Switch II residues display effects on their environment which are consistent with their role in GTP binding and Gβγ dissociation. The ability to merge available crystal structures with solution studies is a powerful tool to gain insight into conformational changes associated with receptor-mediated Gi protein activation.

Clonal diversity of recurrently mutated genes in myelodysplastic syndromes

Recent studies suggest that most cases of myelodysplastic syndrome (MDS) are clonally heterogeneous, with a founding clone and multiple subclones. It is not known whether specific gene mutations typically occur in founding clones or subclones. We screened a panel of 94 candidate genes in a cohort of 157 patients with MDS or secondary acute myeloid leukemia (sAML). This included 150 cases with samples obtained at MDS diagnosis and 15 cases with samples obtained at sAML transformation (8 were also analyzed at the MDS stage). We performed whole-genome sequencing (WGS) to define the clonal architecture in eight sAML genomes and identified the range of variant allele frequencies (VAFs) for founding clone mutations. At least one mutation or cytogenetic abnormality was detected in 83% of the 150 MDS patients and 17 genes were significantly mutated (false discovery rate ≤0.05). Individual genes and patient samples displayed a wide range of VAFs for recurrently mutated genes, indicating that no single gene is exclusively mutated in the founding clone. The VAFs of recurrently mutated genes did not fully recapitulate the clonal architecture defined by WGS, suggesting that comprehensive sequencing may be required to accurately assess the clonal status of recurrently mutated genes in MDS.

Restructuring G-protein- coupled receptor activation

G-protein-coupled receptors serve as key signal transduction conduits, linking extracellular inputs with diverse cellular responses. These receptors eluded structural characterization for decades following their identification. A landmark structure of rhodopsin provided a basis for structure-function studies and homology modeling, but advances in receptor biology suffered from a lack of receptor-specific structural insights. The recent explosion in GPCR structures confirms some features predicted by rhodopsin-based models, and more importantly, it reveals unexpected ligand-binding modes and critical aspects of the receptor activation process. The new structures also promise to foster studies testing emerging models for GPCR function such as receptor dimerization and ligand-biased signaling.

Dissociation of membrane-anchored heterotrimeric G-protein induced by G(α) subunit binding to GTP

Heterotrimeric G-proteins' activation on the intracellular side of the cell membrane is initiated by stimulation of the G-Protein Coupled Receptors (GPCRs) extra-cellular part. This two-step activation mechanism includes (1) an exchange between GDP and GTP molecules in the G(α) subunit and (2) a dissociation of the whole G(αβγ) complex into two membrane-anchored blocks, namely the isolated G(α) and G(βγ) subunits. Although X-ray data are available for both inactive G(αβγ):GDP and active G(α):GTP complexes, intermediate steps involved in the molecular mechanism of the dissociation have not yet been addressed at the molecular level. In this study, we first built a membrane-anchored intermediate G(iαβγ):GTP complex. This model was then equilibrated by molecular dynamics simulations before the Targeted Molecular Dynamics (TMD) technique was used to force the G(α) subunit to evolve from its inactive (GDP-bound) to its active (GTP-bound) conformations, as described by available X-ray data. The TMD constraint was applied only to the G(α) subunit so that the resulting global rearrangements acting on the whole G(αβγ):GTP heterotrimer could be analyzed. We showed how these mainly local conformational changes of G(α) could initiate large domain:domain motions of the whole complex, the G(βγ) behaving as an almost quasi-rigid block. This separation of the two G(α):GTP and G(βγ) subunits required the loss of several interactions at the G(α):G(βγ) interface that were reported. This study provided an atomistic view of the crucial intermediate step of the G-proteins activation, e.g., the dissociation, that could hardly be elucidated by the experiment.

Crystal structure of the human PRMT5:MEP50 complex

Protein arginine methyltransferases (PRMTs) play important roles in several cellular processes, including signaling, gene regulation, and transport of proteins and nucleic acids, to impact growth, differentiation, proliferation, and development. PRMT5 symmetrically di-methylates the two-terminal ω-guanidino nitrogens of arginine residues on substrate proteins. PRMT5 acts as part of a multimeric complex in concert with a variety of partner proteins that regulate its function and specificity. A core component of these complexes is the WD40 protein MEP50/WDR77/p44, which mediates interactions with binding partners and substrates. We have determined the crystal structure of human PRMT5 in complex with MEP50 (methylosome protein 50), bound to an S-adenosylmethionine analog and a peptide substrate derived from histone H4. The structure of the surprising hetero-octameric complex reveals the close interaction between the seven-bladed β-propeller MEP50 and the N-terminal domain of PRMT5, and delineates the structural elements of substrate recognition.

Regulation of Ca(V)2 calcium channels by G protein coupled receptors

Voltage gated calcium channels (Ca²⁺ channels) are key mediators of depolarization induced calcium influx into excitable cells, and thereby play pivotal roles in a wide array of physiological responses. This review focuses on the inhibition of Ca(V)2 (N- and P/Q-type) Ca²⁺-channels by G protein coupled receptors (GPCRs), which exerts important autocrine/paracrine control over synaptic transmission and neuroendocrine secretion. Voltage-dependent inhibition is the most widespread mechanism, and involves direct binding of the G protein βγ dimer (Gβγ) to the α1 subunit of Ca(V)2 channels. GPCRs can also recruit several other distinct mechanisms including phosphorylation, lipid signaling pathways, and channel trafficking that result in voltage-independent inhibition. Current knowledge of Gβγ-mediated inhibition is reviewed, including the molecular interactions involved, determinants of voltage-dependence, and crosstalk with other cell signaling pathways. A summary of recent developments in understanding the voltage-independent mechanisms prominent in sympathetic and sensory neurons is also included. This article is part of a Special Issue entitled: Calcium channels.

Identifying the hotspots on the top faces of WD40-repeat proteins from their primary sequences by β-bulges and DHSW tetrads

The analysis of 36 available crystal structures of WD40 repeat proteins reveals widespread existence of a beta-bulge formed at the beginning of strand a and the end of strand b, termed as WD_b–a bulge: among a total of 259 WD40 blades, there are 243 such β-bulges. The R₁ positions in these WD_b–a bulges have fair distributions of Arg, His, Ile, Leu, Lys, Met, Phe, Trp, Tyr and Val residues. These residues protrude on the top face of the WD40 proteins and can serve as hotspots for protein-protein interactions. An analysis of 29 protein complexes formed by 17 WD proteins reveals that these R₁ residues, along with two other residues (R₁-2 and D-1), are indeed widely involved in protein-protein interactions. Interestingly, these WD_b–a bulges can be easily identified by the 4-amino acid sequences of (V, L, I), R₁, R₂, (V, L, I), along with some other significant amino acids. Thus, the hotspots of WD40 proteins on the top face can be readily predicted based on the primary sequences of the proteins. The literature-reported mutagenesis studies for Met30, MDV1, Tup11, COP1 and SPA1, which crystal structures are not available, can be readily understood based on the feature-based method. Applying the method, the twelve potential hotspots on the top face of Tup11 from S. japonicas have been identified. Our ITC measurements confirm seven of them, Tyr382, Arg284, Tyr426, Tyr508, Leu559, Lys575 and Ile601, are essential for recognizing Fep1. The ITC measurements further convinced that the feature-based method provides accurate prediction of hotspots on the top face.

GDP release preferentially occurs on the phosphate side in heterotrimeric G-proteins

After extra-cellular stimulation of G-Protein Coupled Receptors (GPCRs), GDP/GTP exchange appears as the key, rate limiting step of the intracellular activation cycle of heterotrimeric G-proteins. Despite the availability of a large number of X-ray structures, the mechanism of GDP release out of heterotrimeric G-proteins still remains unknown at the molecular level. Starting from the available X-ray structure, extensive unconstrained/constrained molecular dynamics simulations were performed on the complete membrane-anchored Gi heterotrimer complexed to GDP, for a total simulation time overcoming 500 ns. By combining Targeted Molecular Dynamics (TMD) and free energy profiles reconstruction by umbrella sampling, our data suggest that the release of GDP was much more favored on its phosphate side. Interestingly, upon the forced extraction of GDP on this side, the whole protein encountered large, collective motions in perfect agreement with those we described previously including a domain to domain motion between the two ras-like and helical sub-domains of G_α.

Despite the availability of many structural and biochemical data, the activation of G-proteins remains to be understood at the molecular level. We used a computation tool to decipher the first limiting step of this activation: GDP release. Combining different methods of analysis, we propose that the GDP exit occurs on its phosphate side. This study helped to rationalize some experimental observations from the literature and opens many perspectives concerning the study of G-proteins activation and their putative inhibition.

On WD40 proteins: propelling our knowledge of transcriptional control?

A direct effect of post-translational modifications (PTMs) on nucleosomes is the formation of a dynamic platform able to assemble the transcriptional machinery and to recruit chromatin modifiers. The histone code hypothesis suggests that histone PTMs can act as binding sites for chromatin readers and effector proteins, such as the bromodomains, that selectively interact with acetylated lysines, or the "Royal family" and the PHD finger domains, which are able to recognize methylated arginines and lysines. In this review we will discuss recent data describing the function of WD40 proteins as a new class of histone readers, with particular emphasis on the ones able to recognize methylated arginine and lysine residues. We will discuss how WDR5, a classical seven-bladed WD40 propeller, is able to bind with similar affinities both the catalytic subunit of the Trithorax-like complexes, and the histone H3 tail either unmodified or symmetrically dimethylated on arginine 2 (H3R2me2s). Furthermore, we will speculate on how these mutually exclusive interactions of WDR5 may play a role in mediating different degrees of H3K4 methylations at both promoters and distal regulatory sites. Finally, we will summarize recent literature elucidating how other WD40 proteins such as NURF55, EED and LRWD1 recognize methylated histone tails, highlighting similarities and differences among them.

Nucleotide exchange factors: Kinetic analyses and the rationale for studying kinetics of GEFs

Exchange factors are enzymes that catalyze the exchange of GTP for GDP on guanine nucleotide binding proteins. Progress in understanding the molecular basis of action and the cellular functions of these enzymes has largely come from structural determinations (e.g., crystal structures) and studying effects on cells when expression levels of the exchange factors are perturbed or mutated exchange factors are expressed. Proportionally little effort has been expended on studying the kinetics of exchange; however, reaction rates are central to understanding enzymes. Here, we discuss the importance of kinetic analysis of exchange factors for guanine nucleotide binding proteins, with a focus on ADP-ribosylation factor (Arf) and heterotrimeric G proteins, for providing unique insights into molecular mechanisms and regulation as well as how kinetic analyses are used to complement other approaches.

Role of naofen in apoptosis of hepatocytes induced by lipopolysaccharide through mitochondrial signaling in rats

AIM

  Lipopolysaccharide (LPS) causes apoptosis of hepatocytes, which is probably mediated by inflammatory substances released from Kupffer cells (KCs). Recently, we have reported that naofen, a newly found intracellular WD40-repeat protein, has a role in inducing the apoptosis in HEK293 cells. Hence, the present study was undertaken to investigate a role of naofen in the LPS-induced apoptosis of rat hepatocytes.

METHODS

  Rats were treated with i.v. injections of LPS, and livers were extirpated to evaluate expression of naofen and apoptosis. In in vitro experiments, hepatocytes and KCs were separately isolated from rat livers. The incubation medium for KCs treated with LPS (KC-CM) was used for hepatocyte culture.

RESULTS

  Intravenous injections of LPS enhanced the expression of naofen in the livers. Livers showed terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive staining, and elevated caspase-3 activity. In isolated KCs or hepatocytes, LPS hardly affected naofen expression and caspase-3 activity, whereas incubation of hepatocytes with KC-CM enhanced both naofen expression and caspase-3 activation. Transfection of hepatocyte with naofen siRNA prevented such effects of KC-CM, and clearly eliminated KC-CM-induced reduction of Bcl-2 and Bcl-xL. In contrast, overexpression of naofen in hepatocytes downregulated Bcl-2 and Bcl-xL, released cytochrome c from mitochondria, and activated caspase-3.

CONCLUSION

  These results indicate that LPS may induce the hepatic apoptosis in association with enhanced naofen expression, and that naofen may mediate the activation of caspase-3 through downregulating the Bcl-2 and Bcl-xL expression, and releasing cytochrome c from mitochondria to cytoplasm.

Direct inhibition of the cold-activated TRPM8 ion channel by Gαq

Activation of the TRPM8 ion channel in sensory nerve endings produces a sensation of pleasant coolness. Here we show that inflammatory mediators such as bradykinin and histamine inhibit TRPM8 in intact sensory nerves, but do not do so via conventional signalling pathways. The G-protein subunit Ga_q instead binds to TRPM8 and when activated by a Gq-coupled receptor directly inhibits ion channel activity. Deletion of Ga_q largely abolished inhibition of TRPM8, and inhibition was rescued by a Ga_q chimera whose ability to activate downstream signalling pathways was completely ablated. Activated Ga_q protein, but not Gβγ, potently inhibits TRPM8 in excised patches. We conclude that Ga_q pre-forms a complex with TRPM8 and inhibits activation of TRPM8, following activation of G-protein coupled receptors, by a direct action. This signalling mechanism may underlie the abnormal cold sensation caused by inflammation.

Probing heterotrimeric G protein activation: applications to biased ligands

Cell surface G protein-coupled receptors (GPCRs) drive numerous signaling pathways involved in the regulation of a broad range of physiologic processes. Today, they represent the largest target for modern drugs development with potential application in all clinical fields. Recently, the concept of "ligand-directed trafficking" has led to a conceptual revolution in pharmacological theory, thus opening new avenues for drug discovery. Accordingly, GPCRs do not function as simple on-off switch but rather as filters capable of selecting the activation of specific signals and thus generating texture responses to ligands, a phenomenon often referred to as ligand-biased signaling. Also, one challenging task today remains optimization of pharmacological assays with increased sensitivity so to better appreciate the inherent texture of ligands. However, considering that a single receptor has pleiotropic signaling properties and that each signal can crosstalk at different levels, biased activity remains thus difficult to evaluate. One strategy to overcome these limitations would be examining the initial steps following receptor activation. Even, if some G protein independent functions have been recently described, heterotrimeric G protein activation remains a general hallmark for all GPCRs families and the first cellular event subsequent to agonist binding to the receptor. Herein, we review the different methodologies classically used or recently developed to monitor G protein activation and discussed them in the context of G protein biased-ligands.

Signal activation and inactivation by the Gα helical domain: a long-neglected partner in G protein signaling

Heterotrimeric guanine nucleotide-binding proteins (G proteins) are positioned at the top of many signal transduction pathways. The G protein α subunit is composed of two domains, one that resembles Ras and another that is composed entirely of α helices. Historically most attention has focused on the Ras-like domain, but emerging evidence reveals that the helical domain is an active participant in G protein signaling.

Effects of mutations in the WD40 domain of α-COP on its interaction with the COPI coatomer in Saccharomyces cerevisiae

Replacement of glycine 227 in the fifth WD40 motif of α-COP/Ret1p/Soo1p by charged or aromatic amino acids is responsible for the temperature-dependent osmo-sensitivity of Saccharomyces cerevisiae, while truncations of WD40 motifs exerted a reduction in cell growth rate and impairment in assembly of cell-wall associated proteins such as enolase and Gas1p. Yeast two-hybrid analysis revealed that the ret1-1/soo1-1 mutation of α-COP abolished the interaction with β- and ɛ-COP, respectively, and that the interaction between α-COP and β-COP relied on the WD40 domain of α-COP. Furthermore, although the WD40 domain is dispensable for interaction of α-COP with ɛ-COP, structural alterations in the WD40 domain could impair the interaction.

Differences in intradomain and interdomain motion confer distinct activation properties to structurally similar Gα proteins

Proteins with similar crystal structures can have dissimilar rates of substrate binding and catalysis. Here we used molecular dynamics simulations and biochemical analysis to determine the role of intradomain and interdomain motions in conferring distinct activation rates to two Gα proteins, Gα(i1) and GPA1. Despite high structural similarity, GPA1 can activate itself without a receptor, whereas Gα(i1) cannot. We found that motions in these proteins vary greatly in type and frequency. Whereas motion is greatest in the Ras domain of Gα(i1), it is greatest in helices αA and αB from the helical domain of GPA1. Using protein chimeras, we show that helix αA from GPA1 is sufficient to confer rapid activation to Gα(i1). Gα(i1) has less intradomain motion than GPA1 and instead displays interdomain displacement resembling that observed in a receptor-heterotrimer crystal complex. Thus, structurally similar proteins can have distinct atomic motions that confer distinct activation mechanisms.

A constitutively active Gα subunit provides insights into the mechanism of G protein activation

The activation of Gα subunits of heterotrimeric G proteins by G protein-coupled receptors (GPCRs) is a critical event underlying a variety of biological responses. Understanding how G proteins are activated will require structural and biochemical analyses of GPCRs complexed to their G protein partners, together with structure-function studies of Gα mutants that shed light on the different steps in the activation pathway. Previously, we reported that the substitution of a glycine for a proline at position 56 within the linker region connecting the helical and GTP-binding domains of a Gα chimera, designated αT*, yields a more readily exchangeable state for guanine nucleotides. Here we show that GDP-GTP exchange on αT*(G56P), in the presence of the light-activated GPCR, rhodopsin (R*), is less sensitive to the β1γ1 subunit complex than to wild-type αT*. We determined the X-ray crystal structure for the αT*(G56P) mutant and found that the G56P substitution leads to concerted changes that are transmitted to the conformationally sensitive switch regions, the α4-β6 loop, and the β6 strand. The α4-β6 loop has been proposed to be a GPCR contact site that signals to the TCAT motif and weakens the binding of the guanine ring of GDP, whereas the switch regions are the contact sites for the β1γ1 complex. Collectively, these biochemical and structural data lead us to suggest that αT*(G56P) may be adopting a conformation that is normally induced within Gα subunits by the combined actions of a GPCR and a Gβγ subunit complex during the G protein activation event.

The C-terminus of the G protein α subunit controls the affinity of nucleotides

The C-terminus of the G protein α subunit has a well-known role in determining the selective coupling with the cognate G protein-coupled receptor (GPCR). In fact, rhodopsin, a prototypical GPCR, exhibits active state [metarhodopsin II (MII)] stabilization by interacting with G protein [extra formation of MII (eMII)], and the extent of stabilization is affected by the C-terminal sequence of Gα. Here we examine the relationship between the amount of eMII and the activation efficiency of Gi mutants whose Giα forms have different lengths of the C-terminal sequence of Goα. The results show that both the activation efficiencies of Gi and the amounts of eMII were affected by mutations; however, there was no correlation between them. This finding suggested that the C-terminal region of Gα not only stabilizes MII (active state) but also affects the nucleotide-binding site of Gα. Therefore, we measured the activation efficiency of these mutants by MII at several concentrations of GDP and GTP and calculated the rate constants of GDP release, GDP uptake, and GTP uptake. These rate constants of the Gi mutants were substantially different from those of the wild type, indicating that the replacement of the amino acid residues in the C-terminus alters the affinity of nucleotides. The rate constants of GDP uptake and GTP uptake showed a strong correlation, suggesting that the C-terminus of Giα controls the accessibility of the nucleotide-binding site. Therefore, our results strongly suggest that there is a long-range interlink between the C-terminus of Giα and its nucleotide-binding site.

A P-loop mutation in Gα subunits prevents transition to the active state: implications for G-protein signaling in fungal pathogenesis

Heterotrimeric G-proteins are molecular switches integral to a panoply of different physiological responses that many organisms make to environmental cues. The switch from inactive to active Gαβγ heterotrimer relies on nucleotide cycling by the Gα subunit: exchange of GTP for GDP activates Gα, whereas its intrinsic enzymatic activity catalyzes GTP hydrolysis to GDP and inorganic phosphate, thereby reverting Gα to its inactive state. In several genetic studies of filamentous fungi, such as the rice blast fungus Magnaporthe oryzae, a G42R mutation in the phosphate-binding loop of Gα subunits is assumed to be GTPase-deficient and thus constitutively active. Here, we demonstrate that Gα(G42R) mutants are not GTPase deficient, but rather incapable of achieving the activated conformation. Two crystal structure models suggest that Arg-42 prevents a typical switch region conformational change upon Gα_i1(G42R) binding to GDP·AlF₄⁻ or GTP, but rotameric flexibility at this locus allows for unperturbed GTP hydrolysis. Gα(G42R) mutants do not engage the active state-selective peptide KB-1753 nor RGS domains with high affinity, but instead favor interaction with Gβγ and GoLoco motifs in any nucleotide state. The corresponding Gα_q(G48R) mutant is not constitutively active in cells and responds poorly to aluminum tetrafluoride activation. Comparative analyses of M. oryzae strains harboring either G42R or GTPase-deficient Q/L mutations in the Gα subunits MagA or MagB illustrate functional differences in environmental cue processing and intracellular signaling outcomes between these two Gα mutants, thus demonstrating the in vivo functional divergence of G42R and activating G-protein mutants.

Heterotrimeric G-proteins function as molecular switches to convey cellular signals. When a G-protein coupled receptor encounters its ligand at the cellular membrane, it catalyzes guanine nucleotide exchange on the Gα subunit, resulting in a shift from an inactive to an active conformation. G-protein signaling pathways are conserved from mammals to plants and fungi, including the rice blast fungus Magnaporthe oryzae. A mutation in the Gα subunit (G42R), previously thought to eliminate its GTPase activity, leading to constitutive activation, has been utilized to investigate roles of heterotrimeric G-protein signaling pathways in multiple species of filamentous fungi. Here, we demonstrate through structural, biochemical, and cellular approaches that G42R mutants are neither GTPase deficient nor constitutively active, but rather are unable to transition to the activated conformation. A direct comparison of M. oryzae fungal strains harboring either G42R or truly constitutively activating mutations in two Gα subunits, MagA and MagB, revealed markedly different phenotypes. Our results suggest that activation of MagB is critical for pathogenic development of M. oryzae in response to hydrophobic surfaces, such as plant leaves. Furthermore, the lack of constitutive activity by Gα(G42R) mutants prompts a re-evaluation of its use in previous genetic experiments in multiple fungal species.

Myristoylation exerts direct and allosteric effects on Gα conformation and dynamics in solution

Coupling of heterotrimeric G proteins to activated G protein-coupled receptors results in nucleotide exchange on the Gα subunit, which in turn decreases its affinity for both Gβγ and activated receptors. N-Terminal myristoylation of Gα subunits aids in membrane localization of inactive G proteins. Despite the presence of the covalently attached myristoyl group, Gα proteins are highly soluble after GTP binding. This study investigated factors facilitating the solubility of the activated, myristoylated protein. In doing so, we also identified myristoylation-dependent differences in regions of Gα known to play important roles in interactions with receptors, effectors, and nucleotide binding. Amide hydrogen-deuterium exchange and site-directed fluorescence of activated proteins revealed a solvent-protected amino terminus that was enhanced by myristoylation. Furthermore, fluorescence quenching confirmed that the myristoylated amino terminus is in proximity to the Switch II region in the activated protein. Myristoylation also stabilized the interaction between the guanine ring and the base of the α5 helix that contacts the bound nucleotide. The allosteric effects of myristoylation on protein structure, function, and localization indicate that the myristoylated amino terminus of Gα(i) functions as a myristoyl switch, with implications for myristoylation in the stabilization of nucleotide binding and in the spatial regulation of G protein signaling.

The p101 subunit of PI3Kγ restores activation by Gβ mutants deficient in stimulating p110γ

G-protein-regulated PI3Kγ (phosphoinositide 3-kinase γ) plays a crucial role in inflammatory and allergic processes. PI3Kγ, a dimeric protein formed by the non-catalytic p101 and catalytic p110γ subunits, is stimulated by receptor-released Gβγ complexes. We have demonstrated previously that Gβγ stimulates both monomeric p110γ and dimeric p110γ/p101 lipid kinase activity in vitro. In order to identify the Gβ residues responsible for the Gβγ–PI3Kγ interaction, we examined Gβ1 mutants for their ability to stimulate lipid and protein kinase activities and to recruit PI3Kγ to lipid vesicles. Our findings revealed different interaction profiles of Gβ residues interacting with p110γ or p110γ/p101. Moreover, p101 was able to rescue the stimulatory activity of Gβ1 mutants incapable of modulating monomeric p110γ. In addition to the known adaptor function of p101, in the present paper we show a novel regulatory role of p101 in the activation of PI3Kγ.

Chaperone-mediated assembly of G protein complexes

G protein signaling depends on the ability of the individual subunits of the G protein heterotrimer to assemble into functional complexes. Formation of the G protein βγ (Gβγ) dimer is particularly challenging because it is an obligate dimer in which the individual subunits are unstable on their own. Recent studies have revealed an intricate chaperone system that brings the Gβ and Gγ subunits together. This system includes the cytosolic chaperonin containing TCP-1 (CCT) and its co-chaperone phosducin-like protein 1 (PhLP1). CCT assists Gβ in achieving its β-propeller structure, while PhLP1 releases Gβ from CCT and facilitates its interaction with Gγ. Once Gβγ is formed, PhLP1 remains bound until it is displaced by the Gα subunit and the G protein heterotrimer is brought together. Another obligate dimer is the complex between the G protein β(5) subunit and a regulator of G protein signaling protein (Gβ(5)-RGS). Gβ(5)-RGS also requires CCT for Gβ(5) folding, but PhLP1 plays a different role. It stabilizes the interaction between Gβ(5) and CCT, perhaps to increase folding efficiency. After Gβ(5) folding PhLP1 must subsequently release, allowing the RGS protein to bind and form the Gβ(5)-RGS dimer directly on CCT. Gβ(5)-RGS is then freed from CCT to interact with its membrane anchoring protein and form a stable complex that turns off the G protein signal by catalyzing GTP hydrolysis on Gα.

Allosteric mechanisms of G protein-Coupled Receptor signaling: a structural perspective

G protein-Coupled Receptors (GPCRs) use a complex series of intramolecular conformational changes to couple agonist binding to the binding and activation of cognate heterotrimeric G protein (Gαβγ). The mechanisms underlying this long-range activation have been identified using a variety of biochemical and structural approaches and have primarily used visual signal transduction via the GPCR rhodopsin and cognate heterotrimeric G protein transducin (G(t)) as a model system. In this chapter, we review the methods that have revealed allosteric signaling through rhodopsin and transducin. These methods can be applied to a variety of GPCR-mediated signaling pathways.

Preferential assembly of G-αβγ complexes directed by the γ subunits

Assembly of the G-αβγ heterotrimer is required for receptor signaling. Although much has been learned about the assembly process itself, the identities of the G-αβγ combinations that actually exist in physiological setting are largely unknown. Moreover, there is uncertainty regarding whether the individual subunits associate by a random process, or combine by a regulated process to form quasi-stable G-αβγ complexes. In this chapter, we will focus on emerging genetic -evidence that supports the latter model. Specifically, we will discuss how use of gene targeted mice has revealed preferential assembly of the striatal-specific Gα(olf)β(2)γ(7) complex occurs by a sequential process that is directed by the γ(7) subunit. The existence of specific G-αβγ complexes responsible for transducing the signals from different receptors may have profound implications by providing a possible explanation for biased agonism.

Synthesis and assembly of G protein βγ dimers: comparison of in vitro and in vivo studies

The heterotrimeric GTP-binding proteins (G proteins) are the canonical cellular machinery used with the approximately 700 G protein-coupled receptors (GPCRs) in the human genome to transduce extracellular signals across the plasma membrane. The synthesis of the constituent G protein subunits, and their assembly into Gβγ dimers and G protein heterotrimers, determines the signaling repertoire for G-protein/GPCR signaling in cells. These synthesis/assembly -processes are intimately related to two other overlapping events in the intricate pathway leading to formation of G protein signaling complexes, posttranslational modification and intracellular trafficking of G proteins. The assembly of the Gβγ dimer is a complex process involving multiple accessory proteins and organelles. The mechanisms involved are becoming increasingly appreciated, but are still incompletely understood. In vitro and in vivo (cellular) studies provide different perspectives of these processes, and a comparison of them can provide insight into both our current level of understanding and directions to be taken in future investigations.

Structure-function relationships of the G domain, a canonical switch motif

GTP-binding (G) proteins constitute a class of P-loop (phosphate-binding loop) proteins that work as molecular switches between the GDP-bound OFF and the GTP-bound ON state. The common principle is the 160-180-residue G domain with an α,β topology that is responsible for nucleotide-dependent conformational changes and drives many biological functions. Although the G domain uses a universally conserved switching mechanism, its structure, function, and GTPase reaction are modified for many different pathways and processes.

RACK1, A multifaceted scaffolding protein: Structure and function

The Receptor for Activated C Kinase 1 (RACK1) is a member of the tryptophan-aspartate repeat (WD-repeat) family of proteins and shares significant homology to the β subunit of G-proteins (Gβ). RACK1 adopts a seven-bladed β-propeller structure which facilitates protein binding. RACK1 has a significant role to play in shuttling proteins around the cell, anchoring proteins at particular locations and in stabilising protein activity. It interacts with the ribosomal machinery, with several cell surface receptors and with proteins in the nucleus. As a result, RACK1 is a key mediator of various pathways and contributes to numerous aspects of cellular function. Here, we discuss RACK1 gene and structure and its role in specific signaling pathways, and address how posttranslational modifications facilitate subcellular location and translocation of RACK1. This review condenses several recent studies suggesting a role for RACK1 in physiological processes such as development, cell migration, central nervous system (CN) function and circadian rhythm as well as reviewing the role of RACK1 in disease.

Epigenetic repressor-like genes are differentially regulated during grapevine (Vitis vinifera L.) development

Grapevine sexual reproduction involves a seasonal separation between inflorescence primordia (flowering induction) and flower development. We hypothesized that a repression mechanism implicating epigenetic changes could play a role in the seasonal separation of these two developmental processes in grapevine. Therefore, the expression of five grapevine genes with homology to the Arabidopsis epigenetic repressor genes FERTILIZATION INDEPENDENT ENDOSPERM (FIE), EMBRYONIC FLOWER 2 (EMF2), CURLY LEAF (CLF), MULTICOPY SUPPRESSOR OF IRA 1 (MSI1) and SWINGER (SWN) was analyzed during the development of buds and vegetative and reproductive organs. During bud development, the putative grapevine epigenetic repressor genes VvCLF, VvEMF2, VvMSI1, VvSWN and VvFIE are mainly expressed in latent buds at the flowering induction period, but also detected during bud burst and inflorescence/flower development. The overlapping expression patterns of grapevine PcG-like genes in buds suggest that chromatin remodeling mechanisms could be operating during grapevine bud development for controlling processes such as seasonal flowering, dormancy and bud burst. Furthermore, the expression of grapevine PcG-like genes was also detected in fruits and vegetative organs, suggesting that epigenetic changes could be at the basis of the regulation of various proliferation-differentiation cell transitions that occur during grapevine development.

Understanding molecular recognition by G protein βγ subunits on the path to pharmacological targeting

Heterotrimeric G proteins, composed of Gα and Gβγ subunits, transduce extracellular signals via G-protein-coupled receptors to modulate many important intracellular responses. The Gβγ subunits hold a central position in this signaling system and have been implicated in multiple aspects of physiology and the pathophysiology of disease. The Gβ subunit belongs to a large family of WD40 repeat proteins with a circular β-bladed propeller structure. This structure allows Gβγ to interact with a broad range of proteins to play diverse roles. How Gβγ interacts with and regulates such a wide variety of partners yet maintains specificity is an interesting problem in protein-protein molecular recognition in signal transduction, where signal transfer by proteins is often driven by modular conserved recognition motifs. Evidence has accumulated that one mechanism for Gβγ multitarget recognition is through an intrinsically flexible protein surface or "hot spot" that accommodates multiple modes of binding. Because each target has a unique recognition mode for Gβγ subunits, it suggests that these interactions could be selectively manipulated with small molecules, which could have significant therapeutic potential.

Central and C-terminal domains of heterotrimeric G protein gamma subunits differentially influence the signaling necessary for primordial germ cell migration

Heterotrimeric G protein signaling is involved in many pathways essential to development including those controlling cell migration, proliferation, differentiation and apoptosis. One key developmental event known to rely on proper heterotrimeric G protein signaling is primordial germ cell (PGC) migration. We previously developed an in vivo PGC migration assay that identified differences in the signaling capacity of G protein gamma subunits. In this study we developed Gγ subunit chimeras to determine the regions of Gγ isoforms that are responsible for these differences. The central section of the Gγ subunit was found to be necessary for the ability of a Gγ subunit to mediate signaling involved in PGC migration. Residues found in the carboxy-terminal segment of Gγ transducin (gngt1) were found to be responsible for the ability of this subunit to disrupt PGC migration. The type of prenylation did not affect the ability of a Gγ subunit to reverse prenylation-deficient-Gγ-induced PGC migration defects. However, a version of gng2, engineered to be farnesylated instead of geranylgeranylated, still lacks the ability to reverse PGC migration defects known to result from treatment of zebrafish with geranylgeranyl transferase inhibitors (GGTI), supporting the notion that Gγ subunits are one of several protein targets that need to be geranylgeranylated to orchestrate the proper long-range migration of PGCs.

DYRK1A binds to an evolutionarily conserved WD40-repeat protein WDR68 and induces its nuclear translocation

DYRK1A is encoded in the Down's syndrome critical region on human chromosome 21, and plays an important role in the functional and developmental regulation of many types of cells, including neuronal cells. Here we have identified WDR68, an evolutionarily conserved protein with WD40-repeat domains, as a cellular binding partner of DYRK1A. WDR68 was originally identified in petunia as AN11 that controls the pigmentation of flowers by stimulating the transcription of anthocyanin biosynthetic genes. Experiments with RNA interference showed that WDR68 was indispensable for the optimal proliferation and survival of mammalian cultured cell, and WDR68 depletion induced cell apoptosis. DYRK1A and DYRK1B, but not DYRK2, DYRK3, or DYRK4, bound to endogenous and expressed WDR68. The N-terminal domain, but not the catalytic kinase domain or the C-terminal domain of DYRK1A, was responsible for the WDR68 binding. Deletions in the N-terminal or C-terminal region outside of the central WD40-repeats of WDR68 abolished its binding to DYRK1A, suggesting that WD40 repeats are not sufficient for the association with DYRK1A. Immunofluorescent staining revealed that WDR68 was distributed throughout the cell. Importantly, nuclear accumulation of WDR68 was observed upon co-expression of the wild type and a kinase-dead mutant of DYRK1A. Taken together, these results suggest that DYRK1A binds specifically to WDR68 in cells, and that the binding, but not the phosphorylation event, induces the nuclear translocation of WDR68.

Conformational changes in the G protein Gs induced by the β2 adrenergic receptor

G protein-coupled receptors represent the largest family of membrane receptors that instigate signalling through nucleotide exchange on heterotrimeric G proteins. Nucleotide exchange, or more precisely, GDP dissociation from the G protein α-subunit, is the key step towards G protein activation and initiation of downstream signalling cascades. Despite a wealth of biochemical and biophysical studies on inactive and active conformations of several heterotrimeric G proteins, the molecular underpinnings of G protein activation remain elusive. To characterize this mechanism, we applied peptide amide hydrogen-deuterium exchange mass spectrometry to probe changes in the structure of the heterotrimeric bovine G protein, Gs (the stimulatory G protein for adenylyl cyclase) on formation of a complex with agonist-bound human β(2) adrenergic receptor (β(2)AR). Here we report structural links between the receptor-binding surface and the nucleotide-binding pocket of Gs that undergo higher levels of hydrogen-deuterium exchange than would be predicted from the crystal structure of the β(2)AR-Gs complex. Together with X-ray crystallographic and electron microscopic data of the β(2)AR-Gs complex (from refs 2, 3), we provide a rationale for a mechanism of nucleotide exchange, whereby the receptor perturbs the structure of the amino-terminal region of the α-subunit of Gs and consequently alters the 'P-loop' that binds the β-phosphate in GDP. As with the Ras family of small-molecular-weight G proteins, P-loop stabilization and β-phosphate coordination are key determinants of GDP (and GTP) binding affinity.

Structural flexibility of the G alpha s alpha-helical domain in the beta2-adrenoceptor Gs complex

The active-state complex between an agonist-bound receptor and a guanine nucleotide-free G protein represents the fundamental signaling assembly for the majority of hormone and neurotransmitter signaling. We applied single-particle electron microscopy (EM) analysis to examine the architecture of agonist-occupied β(2)-adrenoceptor (β(2)AR) in complex with the heterotrimeric G protein Gs (Gαsβγ). EM 2D averages and 3D reconstructions of the detergent-solubilized complex reveal an overall architecture that is in very good agreement with the crystal structure of the active-state ternary complex. Strikingly however, the α-helical domain of Gαs appears highly flexible in the absence of nucleotide. In contrast, the presence of the pyrophosphate mimic foscarnet (phosphonoformate), and also the presence of GDP, favor the stabilization of the α-helical domain on the Ras-like domain of Gαs. Molecular modeling of the α-helical domain in the 3D EM maps suggests that in its stabilized form it assumes a conformation reminiscent to the one observed in the crystal structure of Gαs-GTPγS. These data argue that the α-helical domain undergoes a nucleotide-dependent transition from a flexible to a conformationally stabilized state.

The many blades of the β-propeller proteins: conserved but versatile

The β-propeller is a highly symmetrical structure with 4-10 repeats of a four-stranded antiparallel β-sheet motif. Although β-propeller proteins with different blade numbers all adopt disc-like shapes, they are involved in a diverse set of functions, and defects in this family of proteins have been associated with human diseases. However, it has remained ambiguous how variations in blade number could alter the function of β-propellers. In addition to the regularly arranged β-propeller topology, a recently discovered β-pinwheel propeller has been found. Here, we review the structural and functional diversity of β-propeller proteins, including β-pinwheels, as well as recent advances in the typical and atypical propeller structures.

Regulators of G-protein signaling and their Gα substrates: promises and challenges in their use as drug discovery targets

Because G-protein coupled receptors (GPCRs) continue to represent excellent targets for the discovery and development of small-molecule therapeutics, it is posited that additional protein components of the signal transduction pathways emanating from activated GPCRs themselves are attractive as drug discovery targets. This review considers the drug discovery potential of two such components: members of the "regulators of G-protein signaling" (RGS protein) superfamily, as well as their substrates, the heterotrimeric G-protein α subunits. Highlighted are recent advances, stemming from mouse knockout studies and the use of "RGS-insensitivity" and fast-hydrolysis mutations to Gα, in our understanding of how RGS proteins selectively act in (patho)physiologic conditions controlled by GPCR signaling and how they act on the nucleotide cycling of heterotrimeric G-proteins in shaping the kinetics and sensitivity of GPCR signaling. Progress is documented regarding recent activities along the path to devising screening assays and chemical probes for the RGS protein target, not only in pursuits of inhibitors of RGS domain-mediated acceleration of Gα GTP hydrolysis but also to embrace the potential of finding allosteric activators of this RGS protein action. The review concludes in considering the Gα subunit itself as a drug target, as brought to focus by recent reports of activating mutations to GNAQ and GNA11 in ocular (uveal) melanoma. We consider the likelihood of several strategies for antagonizing the function of these oncogene alleles and their gene products, including the use of RGS proteins with Gα(q) selectivity.

Silver spoons and other personal reflections

The intent is to tell a story-hopefully one that is at various times serious, light-hearted, or provocative-that describes my life in biomedical science, especially focusing on the 50 years from 1961 (as a college senior) to the present.

Crystal structure of the β2 adrenergic receptor-Gs protein complex

G protein-coupled receptors (GPCRs) are responsible for the majority of cellular responses to hormones and neurotransmitters as well as the senses of sight, olfaction and taste. The paradigm of GPCR signaling is the activation of a heterotrimeric GTP binding protein (G protein) by an agonist-occupied receptor. The β₂ adrenergic receptor (β₂AR) activation of Gs, the stimulatory G protein for adenylyl cyclase, has long been a model system for GPCR signaling. Here we present the crystal structure of the active state ternary complex composed of agonist-occupied monomeric β₂AR and nucleotide-free Gs heterotrimer. The principal interactions between the β₂AR and Gs involve the amino and carboxyl terminal α-helices of Gs, with conformational changes propagating to the nucleotide-binding pocket. The largest conformational changes in the β₂AR include a 14 Å outward movement at the cytoplasmic end of transmembrane segment 6 (TM6) and an alpha helical extension of the cytoplasmic end of TM5. The most surprising observation is a major displacement of the alpha helical domain of Gαs relative to the ras-like GTPase domain. This crystal structure represents the first high-resolution view of transmembrane signaling by a GPCR.

A concerted mechanism for opening the GDP binding pocket and release of the nucleotide in hetero-trimeric G-proteins

G-protein hetero-trimers play a fundamental role in cell function. Their dynamic behavior at the atomic level remains to be understood. We have studied the Gi hetero-trimer through a combination of molecular dynamics simulations and normal mode analyses. We showed that these big proteins could undergo large-amplitude conformational changes, without any energy penalty and with an intrinsic dynamics centered on their GDP binding pocket. Among the computed collective motions, one of the modes (mode 17) was particularly able to significantly open both the base and the phosphate sides of the GDP binding pocket. This mode describing mainly a motion between the Ras-like and the helical domains of G(α) was in close agreement with some available X-ray data and with many other biochemical/biophysical observations including the kink of helix α5. The use of a new protocol, which allows extraction of the GDP ligand along the computed normal modes, supported that the exit of GDP was largely coupled to an opening motion along mode 17. We propose for the first time a "concerted mechanism" model in which the opening of the GDP pocket and the kink of the α5 helix occur concomitantly and favor GDP release from G(αβγ) complexes. This model is discussed in the context of the G-protein-coupled receptor/G-protein interaction close to the cell membrane.

Conformational flexibility and binding interactions of the G protein βγ heterodimer

Previous NMR experiments on unbound G protein βγ heterodimer suggested that particular residues in the binding interface are mobile on the nanosecond timescale. In this work we performed nanosecond-timescale molecular dynamics simulations to investigate conformational changes and dynamics of Gβγ in the presence of several binding partners: a high-affinity peptide (SIGK), phosducin, and the GDP-bound α subunit. In these simulations, the high mobility of GβW99 was reduced by SIGK, and it appeared that a tyrosine might stabilize GβW99 by hydrophobic or aromatic stacking interactions in addition to hydrogen bonds. Simulations of the phosducin-Gβγ complex showed that the mobility of GβW99 was restricted, consistent with inferences from NMR. However, large-scale conformational changes of Gβγ due to binding, which were hypothesized in the NMR study, were not observed in the simulations, most likely due to their short (nanosecond) duration. A pocket consisting of hydrophobic amino acids on Gα appears to restrict GβW99 mobility in the crystal structure of the Gαβγ? heterotrimer. The simulation trajectories are consistent with this idea. However, local conformational changes of residues GβW63, GβW211, GβW297, GβW332, and GβW339 were detected during the MD simulations. As expected, the magnitude of atomic fluctuations observed in simulations was greater for α than for the βγ subunits, suggesting that α has greater flexibility. These observations support the notion that to maintain the high mobility of GβW99 observed by solution NMR requires that the Gβ-α interface must open up on time scale longer than can be observed in nanosecond scale simulations.

Real-time visualization of heterotrimeric G protein Gq activation in living cells

Background

Gq is a heterotrimeric G protein that plays an important role in numerous physiological processes. To delineate the molecular mechanisms and kinetics of signalling through this protein, its activation should be measurable in single living cells. Recently, fluorescence resonance energy transfer (FRET) sensors have been developed for this purpose.

Results

In this paper, we describe the development of an improved FRET-based Gq activity sensor that consists of a yellow fluorescent protein (YFP)-tagged Gγ2 subunit and a Gαq subunit with an inserted monomeric Turquoise (mTurquoise), the best cyan fluorescent protein variant currently available. This sensor enabled us to determine, for the first time, the k_on (2/s) of Gq activation. In addition, we found that the guanine nucleotide exchange factor p63RhoGEF has a profound effect on the number of Gq proteins that become active upon stimulation of endogenous histamine H1 receptors. The sensor was also used to measure ligand-independent activation of the histamine H1 receptor (H1R) upon addition of a hypotonic stimulus.

Conclusions

Our observations reveal that the application of a truncated mTurquoise as donor and a YFP-tagged Gγ2 as acceptor in FRET-based Gq activity sensors substantially improves their dynamic range. This optimization enables the real-time single cell quantification of Gq signalling dynamics, the influence of accessory proteins and allows future drug screening applications by virtue of its sensitivity.

The extreme C-terminal region of Gαs differentially couples to the luteinizing hormone and beta2-adrenergic receptors

The mechanisms of G protein coupling to G protein-coupled receptors (GPCR) share general characteristics but may exhibit specific interactions unique for each GPCR/G protein partnership. The extreme C terminus (CT) of G protein α-subunits has been shown to be important for association with GPCR. Hypothesizing that the extreme CT of Gα(s) is an essential component of the molecular landscape of the GPCR, human LH receptor (LHR), and β(2)-adrenergic receptor (β(2)-AR), a model cell system was created for the expression and manipulation of Gα(s) subunits in LHR(+) s49 ck cells that lack endogenous Gα(s). On the basis of studies involving truncations, mutations, and chain extensions of Gα(s), the CT was found to be necessary for LHR and β(2)-AR signaling. Some general similarities were found for the responses of the two receptors, but significant differences were also noted. Computational modeling was performed with a combination of comparative modeling, molecular dynamics simulations, and rigid body docking. The resulting models, focused on the Gα(s) CT, are supported by the experimental observations and are characterized by the interaction of the four extreme CT amino acid residues of Gα(s) with residues in LHR and β(2)-AR helix 3, (including R of the DRY motif), helix 6, and intracellular loop 2. This portion of Gα(s) recognizes the same regions of the two GPCR, although with differences in the details of selected interactions. The predicted longer cytosolic extensions of helices 5 and 6 of β(2)-AR are expected to contribute significantly to differences in Gα(s) recognition by the two receptors.