The GppNHp-activated adenylyl cyclase complex from turkey erythrocyte membranes can be isolated with its beta gamma subunits

G protein-coupled receptor-effector macromolecular membrane assemblies (GEMMAs)

G protein-coupled receptors (GPCRs) are the largest group of receptors involved in cellular signaling across the plasma membrane and a major class of drug targets. The canonical model for GPCR signaling involves three components - the GPCR, a heterotrimeric G protein and a proximal plasma membrane effector - that have been generally thought to be freely mobile molecules able to interact by 'collision coupling'. Here, we synthesize evidence that supports the existence of GPCR-effector macromolecular membrane assemblies (GEMMAs) comprised of specific GPCRs, G proteins, plasma membrane effector molecules and other associated transmembrane proteins that are pre-assembled prior to receptor activation by agonists, which then leads to subsequent rearrangement of the GEMMA components. The GEMMA concept offers an alternative and complementary model to the canonical collision-coupling model, allowing more efficient interactions between specific signaling components, as well as the integration of the concept of GPCR oligomerization as well as GPCR interactions with orphan receptors, truncated GPCRs and other membrane-localized GPCR-associated proteins. Collision-coupling and pre-assembled mechanisms are not exclusive and likely both operate in the cell, providing a spectrum of signaling modalities which explains the differential properties of a multitude of GPCRs in their different cellular environments. Here, we explore the unique pharmacological characteristics of individual GEMMAs, which could provide new opportunities to therapeutically modulate GPCR signaling.

N terminus of type 5 adenylyl cyclase scaffolds Gs heterotrimer

According to accepted doctrine, agonist-bound G protein-coupled receptors catalyze the exchange of GDP for GTP and facilitate the dissociation of Galpha and Gbetagamma, which in turn regulate their respective effectors. More recently, the existence of preformed signaling complexes, which may include receptors, heterotrimeric G proteins, and/or effectors, is gaining acceptance. We show herein the existence of a preformed complex of inactive heterotrimer (Galpha(s) x betagamma) and the effector type 5 adenylyl cyclase (AC5), localized by the N terminus of AC5. GST fusions of AC5 N terminus (5NT) bind to purified G protein subunits (GDP-Galpha(s) and Gbetagamma) with apparent affinities of 270 +/- 21 and 190 +/- 7 nM, respectively. GDP-bound Galpha(s) and Gbetagamma did not compete, but rather facilitated their interaction with 5NT, consistent with the isolation of a ternary complex (5NT, Galpha(s), and Gbetagamma) by gel filtration. The AC5/Gbetagamma interaction was also demonstrated by immunoprecipitation and fluorescence resonance energy transfer (FRET) and the binding site of heterotrimer Galpha(s) x betagamma mapped to amino acids 60 to 129 of 5NT. Deletion of this region in full-length AC5 resulted in significant reduction of FRET between Gbetagamma and AC. 5NT also interacts with the catalytic core of AC, mainly via the C1 domain, to enhance Galpha(s)--and forskolin-stimulated activity of C1/C2 domains. The N terminus also serves to constrain Galpha(i)-mediated inhibition of AC5, which is relieved in the presence of Gbetagamma. These results reveal that 5NT plays a key regulatory role by interacting with the catalytic core and scaffolding inactive heterotrimeric G proteins, forming a preassembled complex that is potentially braced for GPCR activation.

Membrane signalling complexes: implications for development of functionally selective ligands modulating heptahelical receptor signalling

Technological development has considerably changed the way in which we evaluate drug efficacy and has led to a conceptual revolution in pharmacological theory. In particular, molecular resolution assays have revealed that heptahelical receptors may adopt multiple active conformations with unique signalling properties. It is therefore becoming widely accepted that ligand ability to stabilize receptor conformations with distinct signalling profiles may allow to direct the stimulus generated by an activated receptor towards a specific signalling pathway. This capacity to induce only a subset of the ensemble of responses regulated by a given receptor has been termed "functional selectivity" (or "stimulus trafficking"), and provides the bases for a highly specific regulation of receptor signalling. Concomitant with these observations, heptahelical receptors have been shown to associate with G proteins and effectors to form multimeric arrays. These complexes are constitutively formed during protein synthesis and are targeted to the cell surface as integral signalling units. Herein we summarize evidence supporting the existence of such constitutive signalling arrays and analyze the possibility that they may constitute viable targets for developing ligands with "functional selectivity".

Assembly of a Ca2+-dependent BK channel signaling complex by binding to beta2 adrenergic receptor

Large-conductance voltage and Ca2+-activated potassium channels (BKCa) play a critical role in modulating contractile tone of smooth muscle, and neuronal processes. In most mammalian tissues, activation of beta-adrenergic receptors and protein kinase A (PKAc) increases BKCa channel activity, contributing to sympathetic nervous system/hormonal regulation of membrane excitability. Here we report the requirement of an association of the beta2-adrenergic receptor (beta2AR) with the pore forming alpha subunit of BKCa and an A-kinase-anchoring protein (AKAP79/150) for beta2 agonist regulation. beta2AR can simultaneously interact with both BKCa and L-type Ca2+ channels (Cav1.2) in vivo, which enables the assembly of a unique, highly localized signal transduction complex to mediate Ca2+- and phosphorylation-dependent modulation of BKCa current. Our findings reveal a novel function for G protein-coupled receptors as a scaffold to couple two families of ion channels into a physical and functional signaling complex to modulate beta-adrenergic regulation of membrane excitability.

G protein-coupled receptors form stable complexes with inwardly rectifying potassium channels and adenylyl cyclase

A large number of studies have demonstrated co-purification or co-immunoprecipitation of receptors with G proteins. We have begun to look for the presence of effector molecules in these receptor complexes. Co-expression of different channel and receptor permutations in COS-7 and HEK 293 cells in combination with co-immunoprecipitation experiments established that the dopamine D(2) and D(4), and beta(2)-adrenergic receptors (beta(2)-AR) form stable complexes with Kir3 channels. The D(4)/Kir3 and D(2) receptor/Kir3 interaction does not occur when the channel and receptor are expressed separately and mixed prior to immunoprecipitation, indicating that the interaction is not an artifact of the experimental protocol and reflects a biosynthetic event. The observed complexes are stable in that they are not disrupted by receptor activation or modulation of G protein alpha subunit function. However, using a peptide that binds Gbetagamma (betaARKct), we show that Gbetagamma is critical for dopamine receptor-Kir3 complex formation, but not for maintenance of the complex. We also provide evidence that Kir3 channels and another effector, adenylyl cyclase, are stably associated with the beta(2)-adrenergic receptor and can be co-immunoprecipitated by anti-receptor antibodies. Using bioluminescence resonance energy transfer, we have shown that in living cells under physiological conditions, beta(2)AR interacts directly with Kir3.1/3.4 and Kir3.1/3.2c heterotetramers as well as with adenylyl cyclase. All of these interactions are stable in the presence of receptor agonists, suggesting that these signaling complexes persist during signal transduction. In addition, we provide evidence that the receptor-effector complexes are also found in vivo. The observation that several G protein-coupled receptors form stable complexes with their effectors suggests that this arrangement might be a general feature of G protein-coupled signal transduction.

A beta2 adrenergic receptor signaling complex assembled with the Ca2+ channel Cav1.2

The existence of a large number of receptors coupled to heterotrimeric guanine nucleotide binding proteins (G proteins) raises the question of how a particular receptor selectively regulates specific targets. We provide insight into this question by identifying a prototypical macromolecular signaling complex. The beta(2) adrenergic receptor was found to be directly associated with one of its ultimate effectors, the class C L-type calcium channel Ca(v)1.2. This complex also contained a G protein, an adenylyl cyclase, cyclic adenosine monophosphate-dependent protein kinase, and the counterbalancing phosphatase PP2A. Our electrophysiological recordings from hippocampal neurons demonstrate highly localized signal transduction from the receptor to the channel. The assembly of this signaling complex provides a mechanism that ensures specific and rapid signaling by a G protein-coupled receptor.

Signal transduction by a nondissociable heterotrimeric yeast G protein

Many signal transduction pathways involve heterotrimeric G proteins. The accepted model for activation of heterotrimeric G proteins states that the protein dissociates to the free G(alpha) (GTP)-bound subunit and free G(betagamma) dimer. On GTP hydrolysis, G(alpha) (GDP) then reassociates with G(betagamma) [Gilman, A. G. (1987) Annu. Rev. Biochem. 56, 615-649]. We reexamined this hypothesis, by using the mating G protein of the yeast Saccharomyces cerevisiae encoded by the genes GPA1, STE4, and STE18. In the absence of mating pheromone, the G(alpha) (Gpa1) subunit represses the mating pathway. On activation by binding of pheromone to a serpentine receptor, the G(betagamma) (Ste4, Ste18) dimer transmits the signal to a mitogen-activated protein kinase cascade, leading to gene activation, arrest in the G(1) stage of the cell cycle, production of shmoos (mating projections), and cell fusion. We found that a Ste4-Gpa1 fusion protein transmitted the pheromone signal and activated the mating pathway as effectively as when Ste4 (G(beta)) and Gpa1 (G(alpha)) were coexpressed as separate proteins. Hence, dissociation of this G protein is not required for its activation. Rather, a conformational change in the heterotrimeric complex is likely to be involved in signal transduction.

Signal transduction by a nondissociable heterotrimeric yeast G protein

Many signal transduction pathways involve heterotrimeric G proteins. The accepted model for activation of heterotrimeric G proteins states that the protein dissociates to the free G(alpha) (GTP)-bound subunit and free G(betagamma) dimer. On GTP hydrolysis, G(alpha) (GDP) then reassociates with G(betagamma) [Gilman, A. G. (1987) Annu. Rev. Biochem. 56, 615-649]. We reexamined this hypothesis, by using the mating G protein of the yeast Saccharomyces cerevisiae encoded by the genes GPA1, STE4, and STE18. In the absence of mating pheromone, the G(alpha) (Gpa1) subunit represses the mating pathway. On activation by binding of pheromone to a serpentine receptor, the G(betagamma) (Ste4, Ste18) dimer transmits the signal to a mitogen-activated protein kinase cascade, leading to gene activation, arrest in the G(1) stage of the cell cycle, production of shmoos (mating projections), and cell fusion. We found that a Ste4-Gpa1 fusion protein transmitted the pheromone signal and activated the mating pathway as effectively as when Ste4 (G(beta)) and Gpa1 (G(alpha)) were coexpressed as separate proteins. Hence, dissociation of this G protein is not required for its activation. Rather, a conformational change in the heterotrimeric complex is likely to be involved in signal transduction.

The complex regulation of receptor-coupled G-proteins

Heterotrimeric G-proteins are associated with the cytoplasmic surface of the cell membrane as oligomeric structures. The oligomeric structures were deduced from a variety of studies including target (irradiation) analysis, hydrodynamic evaluation of detergent extracted material, and cross-linking of G-proteins in their membrane environment. From the functional mass determined by target analysis, it was estimated that one receptor (for glucagon) is associated with 8-10 units of Gs, the heterotrimeric G-protein that stimulates adenylyl cyclase. It is proposed that the receptor associates with each monomer of the chain via weak and strong binding forces that are dictated according to whether either GTP or GDP is bound to the alpha-subunits (weak forces) or, due to the hormone-induced release of the nucleotides during the exchange reaction, these subunits become transiently devoid of nucleotides (strong forces). The hormone-induced changes in type and degree of nucleotide binding allow for movement of the receptor along the oligomeric chain and filling of the nucleotide binding sites with the activating nucleotide, GTP. In this manner, the receptor catalytically activates Gs. It is suggested that the dynamic instability of the oligomeric chain produced by the asymmetric distribution of GTP and GDP along the chain results in release of a GTP-monomer from one end and association of a GDP-monomer at the opposite end. Adenylyl cyclase associates with the released GTP-monomer inducing a transient state of the coupled proteins. In a Mg-dependent fashion, hydrolysis of GTP occurs resulting in re-organization of the coupled proteins such that alpha and beta gamma interact with distinct domains of the cyclase molecule. The final state of the coupled process determines the degree of cyclase activity. Release of Pi from its binding site restores association of alpha and beta gamma to the GDP-bound form of the heterotrimer. The latter associates with the oligomeric structure of G-proteins to complete the cycle of events in the overall process of hormonal activation of the system.

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.