Rescue of functional interactions between the alpha2A-adrenoreceptor and acylation-resistant forms of Gi1alpha by expressing the proteins from chimeric open reading frames

ER/K linked GPCR-G protein fusions systematically modulate second messenger response in cells

FRET and BRET approaches are well established for detecting ligand induced GPCR-G protein interactions in cells. Currently, FRET/BRET assays rely on co-expression of GPCR and G protein, and hence depend on the stoichiometry and expression levels of the donor and acceptor probes. On the other hand, GPCR-G protein fusions have been used extensively to understand the selectivity of GPCR signaling pathways. However, the signaling properties of fusion proteins are not consistent across GPCRs. In this study, we describe and characterize novel sensors based on the Systematic Protein Affinity Strength Modulation (SPASM) technique. Sensors consist of a GPCR and G protein tethered by an ER/K linker flanked by FRET probes. SPASM sensors are tested for the β2-, α1-, and α2- adrenergic receptors, and adenosine type 1 receptor (A₁R), tethered to Gαs-XL, Gαi₂, or Gαq subunits. Agonist stimulation of β2-AR and α2-AR increases FRET signal comparable to co-expressed FRET/BRET sensors. SPASM sensors also retain signaling through the endogenous G protein milieu. Importantly, ER/K linker length systematically tunes the GPCR-G protein interaction, with consequent modulation of second messenger signaling for cognate interactions. SPASM GPCR sensors serve the dual purpose of detecting agonist-induced changes in GPCR-G protein interactions, and linking these changes to downstream signaling.

Cleavage-resistant fusion proteins of the M(2) muscarinic receptor and Gα(i1). Homotropic and heterotropic effects in the binding of ligands

BACKGROUND

G protein-coupled receptors fused to a Gα-subunit are functionally similar to their unfused counterparts. They offer an intriguing view into the nature of the receptor-G protein complex, but their usefulness depends upon the stability of the fusion.

METHODS

Fusion proteins of the M(2) muscarinic receptor and the α-subunit of G(i1) were expressed in CHO and Sf9 cells, extracted in digitonin-cholate, and examined for their binding properties and their electrophoretic mobility on western blots.

RESULTS

Receptor fused to native α(i1) underwent proteolysis near the point of fusion to release a fragment with the mobility of α(i1). The cleavage was prevented by truncation of the α-subunit at position 18. Binding of the agonist oxotremorine-M to the stable fusion protein from Sf9 cells was biphasic, and guanylylimidodiphosphate promoted an apparent interconversion of sites from higher to lower affinity. With receptor from CHO cells, the apparent capacity for N-[(3)H]methylscopolamine was 60% of that for [(3)H]quinuclidinylbenzilate; binding at saturating concentrations of the latter was inhibited in a noncompetitive manner at low concentrations of unlabeled N-methylscopolamine.

CONCLUSIONS

A stable fusion protein of the M(2) receptor and truncated α(i1) resembles the native receptor-G protein complex with respect to the guanyl nucleotide-sensitive binding of agonists and the noncompetitive binding of antagonists.

GENERAL SIGNIFICANCE

Release of the α-subunit is likely to occur with other such fusion proteins, rendering the data ambiguous or misleading. The properties of a chemically stable fusion protein support the notion that signaling proceeds via a stable multimeric complex of receptor and G protein.

Ligand screening system using fusion proteins of G protein-coupled receptors with G protein alpha subunits

G protein-coupled receptors (GPCRs) constitute one of the largest families of genes in the human genome, and are the largest targets for drug development. Although a large number of GPCR genes have recently been identified, ligands have not yet been identified for many of them. Various assay systems have been employed to identify ligands for orphan GPCRs, but there is still no simple and general method to screen for ligands of such GPCRs, particularly of G(i)-coupled receptors. We have examined whether fusion proteins of GPCRs with G protein alpha subunit (Galpha) could be utilized for ligand screening and showed that the fusion proteins provide an effective method for the purpose. This article focuses on the followings: (1) characterization of GPCR genes and GPCRs, (2) identification of ligands for orphan GPCRs, (3) characterization of GPCR-Galpha fusion proteins, and (4) identification of ligands for orphan GPCRs using GPCR-Galpha fusion proteins.

Protean agonism at the dopamine D2 receptor: (S)-3-(3-hydroxyphenyl)-N-propylpiperidine is an agonist for activation of Go1 but an antagonist/inverse agonist for Gi1,Gi2, and Gi3

A range of ligands displayed agonism at the long isoform of the human dopamine D(2) receptor, whether using receptor-G protein fusions or membranes of cells in which pertussis toxin-resistant mutants of individual Galpha(i)-family G proteins could be expressed in an inducible fashion. Varying degrees of efficacy were observed for individual ligands as monitored by their capacity to load [(35)S]GTPgammaS onto each of Galpha(i1),Galpha(i2),Galpha(i3), and Galpha(o1). By contrast, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine was a partial agonist when Galpha(o1) was the target G protein but an antagonist/inverse agonist at Galpha(i1),Galpha(i2), and Galpha(i3). In ligand binding assays, dopamine identified both high- and low-affinity states at each of the dopamine D(2) receptor-G protein fusion proteins, and the high-affinity state was eliminated by guanine nucleotide. (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine bound to an apparent single state of the constructs in which the D(2) receptor was fused to Galpha(i1),Galpha(i2), or Galpha(i3). However, it bound to distinct high- and low-affinity states of the D(2) receptor-Galpha(o1) fusion, with the high-affinity state being eliminated by guanine nucleotide. Likewise, although dopamine identified guanine nucleotide-sensitive high-affinity states of the D(2) receptor when expression of pertussis toxin-resistant forms of each of Galpha(i1), Galpha(i2), Galpha(i3), and Galpha(o1) was induced, (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine identified a high-affinity site only in the presence of Galpha(o1). p-Tyramine displayed a protean ligand profile similar to that of (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine but with lower potency. These results demonstrate (S)-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine to be a protean agonist at the D(2) receptor and may explain in vivo actions of this ligand.

Bring your own G protein

G protein-coupled receptor (GPCR)-Galpha fusion proteins were first characterized more than 10 years ago as a strategy for studying receptor-G protein signaling. A large number of studies have used this approach to characterize receptor coupling to members of the Gs, Gi, and Gq families of Galpha subunits, but this strategy has not been widely used to study Galpha12 and Galpha13. As described in the article by Zhang et al. in this issue of Molecular Pharmacology (p. 1433) characterization of the signaling properties of thromboxane A2 receptor (TPalpha) -Galpha12 and -Galpha13 fusion constructs demonstrates the applicability of this strategy to members of this unique family of Galpha subunits, and how this strategy can be used to resolve otherwise difficult problems of receptor pharmacology associated with these proteins. The general strategy of making receptor-Galpha fusion constructs has wide applicability to a number of research problems, but there are perhaps also "hidden messages" in how different receptor-Galpha subunit fusion pairs behave.

Activation of an alpha2A-adrenoceptor-Galphao1 fusion protein dynamically regulates the palmitoylation status of the G protein but not of the receptor

Post-translational thio-acylation of a fusion protein between the alpha2A-adrenoceptor and the alpha subunit of the G protein G(o1) is both dynamic and regulated by agonist binding. Incorporation of [3H]palmitate into the fusion protein was reduced substantially in the presence of the agonist adrenaline. This was dependent on the concentration of adrenaline and correlated with occupancy of the ligand binding site. Both the receptor and G-protein elements of the fusion construct incorporated [3H]palmitate but this occurred more rapidly for the G-protein element and regulation of acylation by the agonist occurred only for the G protein. The kinetics of de-palmitoylation of the alpha2A-adrenoceptor-Galpha(o1) fusion were accelerated markedly by agonist. Again, this reflected modulation of the G protein but not of the receptor. Agonist-induced regulation of the kinetics of thio-acylation of the G protein was abolished, however, in a mutant unable to bind guanosine 5'-[gamma-[35S]thio]triphosphate ([35S]GTP[S]) in response to adrenaline. Despite the dynamic nature of the post-translational acylation and its regulation by agonist, the ability of adrenaline to activate the G protein, monitored by stimulation of the binding of [35S]GTP[S] to such fusion constructs, was unaffected by the palmitoylation potential of either the receptor or G-protein element.

Comparative analysis of high-affinity ligand binding and G protein coupling of the human CXCR1 chemokine receptor and of a CXCR1-Galpha fusion protein after heterologous production in baculovirus-infected insect cells

In order to perform biochemical and pharmacological characterization of CXCR1, we designed several CXCR1 constructs. All constructs, including a CXCR1-G(i2)alpha fusion protein, were produced in insect cells after infection with recombinant baculovirus. The recombinant receptors exhibited specific high-affinity binding of (125)I-labelled interleukin-8, and Scatchard transformation of the binding data indicated the presence of a population of single homogenous binding sites. Furthermore, the pharmacological profiles for the different CXCR1 constructs produced in the baculovirus-infected insect cells were almost identical to those reported for CXCR1 on human neutrophils. Interestingly, when the CXCR1 constructs were coproduced with G(i2) protein as a result of coinfection with baculoviruses encoding the G(i2)alpha-, the beta- and the gamma- subunits, the B(max) values were significantly increased. Hence, the level of FlagCXCR1Bio, after coproduction with G(i2) protein, was found to be almost 10 times higher than that of the FlagCXCR1Bio alone. However, no differences in the K(i) values were observed of the receptor constructs produced either after single infection or coinfection of insect cells. The addition of guanyl-5'-yl imidodiphosphate resulted in a dramatic reduction of the number of binding sites; however, the K(i) values remained unchanged, indicating coupling of the receptor to the guanine nucleotide-binding protein.

The myristoylated amino terminus of Galpha(i)(1) plays a critical role in the structure and function of Galpha(i)(1) subunits in solution

To determine the role of the myristoylated amino terminus of Galpha in G protein activation, nine individual cysteine mutations along the myristoylated amino terminus of Galpha(i) were expressed in a functionally Cys-less background. Thiol reactive EPR and fluorescent probes were attached to each site as local reporters of mobility and conformational changes upon activation of Galpha(i)GDP by AlF(4)(-), as well as binding to Gbetagamma. EPR and steady state fluorescence anisotropy are consistent with a high degree of immobility for labeled residues in solution all along the amino terminus of myristoylated Galpha(i). This is in contrast to the high mobility of this region in nonmyristoylated Galpha(i) [Medkova, M., et al. (2002) Biochemistry 41, 9962-9972]. Steady state fluorescence measurements revealed pronounced increases in fluorescence upon activation for residues 14-17 and 21 located midway through the 30-amino acid stretch comprising the amino-terminal region. Collectively, the data suggest that myristoylation is an important structural determinant of the amino terminus of Galpha(i) proteins.

Concerted stimulation and deactivation of pertussis toxin-sensitive G proteins by chimeric G protein-coupled receptor-regulator of G protein signaling 4 fusion proteins: analysis of the contribution of palmitoylated cysteine residues to the GAP activity of RGS4

Agonists stimulated high-affinity GTPase activity in membranes of HEK293 cells following coexpression of the alpha 2A-adrenoceptor and a pertussis toxin-resistant mutant of Go1 alpha. Enzyme kinetic analysis of Vmax and Km failed to detect regulation of the effect of agonist by a GTPase activating protein. This did occur, however, when cells were also transfected to express RGS4. Both elements of a fusion protein in which the N-terminus of RGS4 was linked to the C-terminal tail of the alpha 2A-adrenoceptor were functional, as it was able to provide concerted stimulation and deactivation of the G protein. By contrast, the alpha 2A-adrenoceptor-RGS4 fusion protein stimulated but did not enhance deactivation of a form of Go1 alpha that is resistant to the effects of regulator of G protein signaling (RGS) proteins. Employing this model system, mutation of Asn128 but not Asn88 eliminated detectable GTPase activating protein activity of RGS4 against Go1 alpha. Mutation of all three cysteine residues that are sites of post-translational acylation in RGS4 also eliminated GTPase activating protein activity but this was not achieved by less concerted mutation of these sites. These studies demonstrate that a fusion protein between a G protein-coupled receptor and an RGS protein is fully functional in providing both enhanced guanine nucleotide exchange and GTP hydrolysis of a coexpressed G protein. They also provide a direct means to assess, in mammalian cells, the effects of mutation of the RGS protein on function in circumstances in which the spatial relationship and orientation of the RGS to its target G protein is defined and maintained.

Promiscuous coupling at receptor-Galpha fusion proteins. The receptor of one covalent complex interacts with the alpha-subunit of another

Fusion proteins between heptahelical receptors (GPCR) and G protein alpha-subunits show enhanced signaling efficiency in transfected cells. This is believed to be the result of molecular proximity, because the interaction between linked modules of one protein chain, if not constrained by structure, should be strongly favored compared with the same in which partners react as free species. To test this assumption we made a series of fusion proteins (type 1 and 4 opioid receptors with G(o) and beta(2) adrenergic and dopamine 1 receptors with G(sL)) and some mutated analogs carrying different tags and defective GPCR or Galpha subunits. Using cotransfection experiments with readout protocols able to distinguish activation at fused and non-fused alpha-subunits, we found that both the GPCR and the Galpha limb of one fusion protein can freely interact with non-fused proteins and the tethered partners of a neighboring fusion complex. Moreover, a bulky polyanionic inhibitor can suppress with identical potency receptor-Galpha interaction, either when occurring between latched domains of a fused system or separate elements of distinct molecules, indicating that the binding surfaces are equally accessible in both cases. These data demonstrate that there is no entropy drive from the linked condition of fusion proteins and suggest that their signaling may result from the GPCR of one complex interacting with the alpha-subunit of another. Moreover, the enhanced coupling efficiency commonly observed for fusion proteins is not due to the receptor tether, but to the transmembrane helix that anchors Galpha to the membrane.

Partial rescue of functional interactions of a nonpalmitoylated mutant of the G-protein G alpha s by fusion to the beta-adrenergic receptor

Most heterotrimeric G-protein alpha subunits are posttranslationally modified by palmitoylation, a reversible process that is dynamically regulated. We analyzed the effects of Galpha(s) palmitoylation for its intracellular distribution and ability to couple to the beta-adrenergic receptor (betaAR) and stimulate adenylyl cyclase. Subcellular fractionation and immunofluorescence microscopy of stably transfected cyc(-) cells, which lack endogenous Galpha(s), showed that wild-type Galpha(s) was predominantly localized at the plasma membrane, but the mutant C3A-Galpha(s), which does not incorporate [(3)H]palmitate, was mostly associated with intracellular membranes. In agreement with this mislocalization, C3A-Galpha(s) showed neither isoproterenol- or GTPgammaS-stimulated adenylyl cyclase activation nor GTPgammaS-sensitive high-affinity agonist binding, all of which were present in the wild-type Galpha(s) expressing cells. Fusion of C3A-Galpha(s) with the betaAR [betaAR-(C3A)Galpha(s)] partially rescued its ability to induce high-affinity agonist binding and to stimulate adenylyl cyclase activity after isoproterenol or GTPgammaS treatment. In comparison to results with the WT-Galpha(s) and betaAR (betaAR-Galpha(s)) fusion protein, the betaAR-(C3A)Galpha(s) fusion protein was about half as efficient at coupling to the receptor and effector. Chemical depalmitoylation by hydroxylamine of membranes expressing betaAR-Galpha(s) reduced the high-affinity agonist binding and adenylyl cyclase activation to a similar degree as that observed in betaAR-(C3A)Galpha(s) expressing membranes. Altogether, these findings indicate that palmitoylation ensured proper localization of Galpha(s) and facilitated bimolecular interactions of Galpha(s) with the betaAR and adenylyl cyclase.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

A novel strategy to engineer functional fluorescent inhibitory G-protein alpha subunits

Signaling studies in living cells would be greatly facilitated by the development of functional fluorescently tagged G-protein alpha subunits. We have designed G(i/o)alpha subunits fused to the cyan fluorescent protein and assayed their function by studying the following two signal transduction pathways: the regulation of G-protein-gated inwardly rectifying K(+) channels (Kir3.0 family) and adenylate cyclase. Palmitoylation and myristoylation consensus sites were removed from G(i/o) alpha subunits (G(i1)alpha, G(i2)alpha, G(i3)alpha, and G(oA)alpha) and a mutation introduced at Cys(-4) rendering the subunit resistant to pertussis toxin. This construct was fused in-frame with cyan fluorescent protein containing a short peptide motif from GAP43 that directs palmitoylation and thus membrane targeting. Western blotting confirmed G(i/o)alpha protein expression. Confocal microscopy and biochemical fractionation studies revealed membrane localization. Each mutant G(i/o) alpha subunit significantly reduced basal current density when transiently expressed in a stable cell line expressing Kir3.1 and Kir3.2A, consistent with the sequestration of the Gbetagamma dimer by the mutant Galpha subunit. Moreover, each subunit was able to support A1-mediated and D2S-mediated channel activation when transiently expressed in pertussis toxin-treated cells. Overexpression of tagged G(i3)alpha and G(oA)alpha alpha subunits reduced receptor-mediated and forskolin-induced cAMP mobilization.

Agonists activate Gi1 alpha or Gi2 alpha fused to the human mu opioid receptor differently

As preferential coupling of opioid receptor to various inhibitory Galpha subunits is still under debate, we have investigated the selectivity of the human mu opioid receptor fused to a pertussis toxin insensitive C351I Gi1 alpha or C352I Gi2 alpha in stably transfected HEK 293 cells. Overall agonist binding affinities were increased for both fusion constructs when compared to the wild type receptor. [35 S]GTPgammaS binding was performed on pertussis toxin treated cells to monitor coupling efficiency of the fusion constructs. Upon agonist addition hMOR-C351I Gi1 a exhibited an activation profile similar to the non-fused receptor while hMOR-C352I Gi2 alpha was poorly activated. Interestingly no correlation could be drawn between agonist binding affinity and efficacy. Upon agonist addition, forskolin-stimulated cAMP production, as measured using a reporter gene assay, was inhibited by signals transduced via the fused Gi1 alpha and Gi2 alpha mainly. In contrast both fusion constructs were able to initiate ERK-MAPK phosphorylation via coupling to endogenous G proteins only. In conclusion our data indicate that hMOR couples more efficiently to Gi1 alpha than Gi2 alpha and that the coupling efficacy is clearly agonist-dependent.

Enhanced detection of receptor constitutive activity in the presence of regulators of G protein signaling: applications to the detection and analysis of inverse agonists and low-efficacy partial agonists

Fusion proteins between the human 5-hydroxytryptamine (5-HT)(1A) receptor and either wild type or certain pertussis toxin-resistant forms of G(o1)alpha and G(i1)alpha display constitutive GTPase activity that can be inhibited by the inverse agonist spiperone. Addition of recombinant regulator of G protein signaling (RGS) 1 or RGS16 to membranes expressing these fusion proteins resulted in elevation of this constitutive GTPase activity without significantly altering the binding affinity of antagonist/inverse agonist ligands. For a 5-HT(1A) receptor-(Cys(351)Ile)G(o1)alpha fusion protein the increase in basal GTPase activity was greater than 4-fold. Enzyme kinetic analysis demonstrated that the effect of RGS1 was as a GTPase-activating protein for the fusion construct. In the presence of the RGS proteins, both agonists and inverse agonists produced much more robust regulation of high-affinity GTPase activity than in their absence. This allowed detection of the partial agonist nature of WAY100635, which has been described previously as a neutral antagonist at the 5-HT(1A) receptor. Of a range of ligands studied, only haloperidol functioned as a neutral ligand in the presence of RGS1. These studies show that addition of a recombinant RGS protein provides a simple and novel means to elevate the fraction of basal membrane GTPase activity contributed by the constitutive activity of a receptor. By so doing, it also greatly enhances the ability to detect and analyze the effects of inverse agonists and to discriminate between neutral ligands and those with low levels of positive intrinsic efficacy.

Reciprocal mutations of highly conserved residues in transmembrane helices 2 and 7 of the alpha(2A)-adrenoceptor restore agonist activation of G(i1)alpha

Fusion proteins were constructed between the alpha(2A)-adrenoceptor and the alpha-subunit of the G-protein G(i1). Mutation of the highly conserved Asp(79) in transmembrane (TM) helix 2 of the receptor to Asn reduced the capacity of agonists to activate G(i1)alpha by 95% without altering [3H]antagonist or agonist ligand-binding affinity. A reciprocal mutation in TM helix 7 (Asn(422)Asp) was without effect on signalling effectiveness. Combination of these two mutations overcame the effect of the Asp(79)Asp mutation. By examining alterations in this helix 2-helix 7 microdomain, we further demonstrate the utility of receptor-G-protein fusion proteins to quantitate mutational effects on receptor-G-protein interactions and information transfer.

Engineering a V(2) vasopressin receptor agonist- and regulator of G-protein-signaling-sensitive G protein

It is extremely difficult to detect guanine nucleotide exchange or hydrolysis stimulated by receptors which couple to G(s)alpha. Furthermore, G(s)alpha is largely resistant to the GTPase-activating properties of RGS proteins. Coexpression of the vasopressin V(2) receptor with a series of chimeric G protein alpha subunits in which the C-terminal 6-12 amino acids of G(i1)alpha were replaced with the equivalent sequence of G(s)alpha allowed robust vasopressin-stimulated [(35)S]GTPgammaS binding. Vasopressin did not stimulate the GTPase activity of fusion proteins between the V(2) receptor and either G(s)alpha or G(i1)alpha. However, it produced a concentration-dependent stimulation of V(max) for a V(2) receptor-G(i1)alpha/Gs6alpha fusion protein. This construct bound [(3)H]vasopressin with high affinity and this was competed by other ligands with rank order anticipated for the V(2) receptor. RGS1 enhanced vasopressin stimulation of V(2) receptor-G(i1)alpha/G(s)6alpha in a concentration-dependent manner. RGS-GAIP was substantially less potent. Enzyme kinetic analysis demonstrated that RGS1 increased both V(max) of the GTPase activity and the observed K(m) for GTP, consistent with RGS1 accelerating the rate of GTP hydrolysis of the chimeric G protein, whereas the agonist vasopressin accelerates guanine nucleotide exchange. This approach provides a sensitive assay for V(2) receptor agonist ligands and may be amenable to many other G(s)alpha-coupled receptors.

Coordinated agonist regulation of receptor and G protein palmitoylation and functional rescue of palmitoylation-deficient mutants of the G protein G11alpha following fusion to the alpha1b-adrenoreceptor: palmitoylation of G11alpha is not required for interaction with beta*gamma complex

Transfection of either the alpha(1b)-adrenoreceptor or Galpha(11) into a fibroblast cell line derived from a Galpha(q)/Galpha(11) double knockout mouse failed to produce elevation of intracellular [Ca(2+)] upon the addition of agonist. Co-expression of these two polypeptides, however, produced a significant stimulation. Co-transfection of the alpha(1b)-adrenoreceptor with the palmitoylation-resistant C9S,C10S Galpha(11) also failed to produce a signal, and much reduced and kinetically delayed signals were obtained using either C9S Galpha(11) or C10S Galpha(11). Expression of a fusion protein between the alpha(1b)-adrenoreceptor and Galpha(11) allowed [Ca(2+)](i) elevation, and this was also true for a fusion protein between the alpha(1b)-adrenoreceptor and C9S,C10S Galpha(11), since this strategy ensures proximity of the two polypeptides at the cell membrane. For both fusion proteins, co-expression of transducin alpha, as a beta.gamma-sequestering agent, fully attenuated the Ca(2+) signal. Both of these fusion proteins and one in which an acylation-resistant form of the receptor was linked to wild type Galpha(11) were also targets for agonist-regulated [(3)H]palmitoylation and bound [(35)S]guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) in an agonist concentration-dependent manner. The potency of agonist to stimulate [(35)S]GTPgammaS binding was unaffected by the palmitoylation potential of either receptor or G protein. These studies provide clear evidence for coordinated, agonist-mediated regulation of the post-translational acylation of both a receptor and partner G protein and demonstrate the capacity of such fusions to bind and then release beta.gamma complex upon agonist stimulation whether or not the G protein can be palmitoylated. They also demonstrate that Ca(2+) signaling in EF88 cells by such fusion proteins is mediated via release of the G protein beta.gamma complex.

Differential capacities of the RGS1, RGS16 and RGS-GAIP regulators of G protein signaling to enhance alpha2A-adrenoreceptor agonist-stimulated GTPase activity of G(o1)alpha

Recombinant RGS1, RGS16 and RGS-GAIP, but not RGS2, were able to substantially further stimulate the maximal GTPase activity of G(o1)alpha promoted by agonists at the alpha2A-adrenoreceptor in a concentration-dependent manner. Kinetic analysis of the regulation of an alpha2A-adrenoreceptor-G(o1)alpha fusion protein by all three RGS proteins revealed that they had similar affinities for the receptor-G protein fusion. However, their maximal effects on GTP hydrolysis varied over threefold with RGS16 > RGS1 > RGS-GAIP. Both RGS1 and RGS16 reduced the potency of the alpha2A-adrenoreceptor agonist adrenaline by some 10-fold. A lower potency shift was observed for the partial agonist UK14304 and the effect was absent for the weak partial agonist oxymetazoline. Each of these RGS proteins altered the intrinsic activity of both UK14304 and oxymetazoline relative to adrenaline. Such results require the RGS interaction with G(o1)alpha to alter the conformation of the alpha2A-adrenoreceptor and are thus consistent with models invoking direct interactions between RGS proteins and receptors. These studies demonstrate that RGS1, RGS16 and RGS-GAIP show a high degree of selectivity to regulate alpha2A-adrenoreceptor-activated G(o1)alpha rather than G(i1)alpha, G(i2)alpha or G(i3)alpha and different capacities to inactivate this G protein.

Two active molecular phenotypes of the tachykinin NK1 receptor revealed by G-protein fusions and mutagenesis

The NK1 neurokinin receptor presents two non-ideal binding phenomena, two-component binding curves for all agonists and significant differences between agonist affinity determined by homologous versus heterologous competition binding. Whole cell binding with fusion proteins constructed between either Galpha(s) or Galpha(q) and the NK1 receptor with a truncated tail, which secured non-promiscuous G-protein interaction, demonstrated monocomponent agonist binding closely corresponding to either of the two affinity states found in the wild-type receptor. High affinity binding of both substance P and neurokinin A was observed in the tail-truncated Galpha(s) fusion construct, whereas the lower affinity component was displayed by the tail-truncated Galpha(q) fusion. The elusive difference between the affinity determined in heterologous versus homologous binding assays for substance P and especially for neurokinin A was eliminated in the G-protein fusions. An NK1 receptor mutant with a single substitution at the extracellular end of TM-III-(F111S), which totally uncoupled the receptor from Galpha(s) signaling, showed binding properties that were monocomponent and otherwise very similar to those observed in the tail-truncated Galpha(q) fusion construct. Thus, the heterogenous pharmacological phenotype displayed by the NK1 receptor is a reflection of the occurrence of two active conformations or molecular phenotypes representing complexes with the Galpha(s) and Galpha(q) species, respectively. We propose that these molecular forms do not interchange readily, conceivably because of the occurrence of microdomains or "signal-transductosomes" within the cell membrane.

Analytical pharmacology of G protein-coupled receptors by stoichiometric expression of the receptor and G(alpha) protein subunits

The description of a new family of recombinant proteins, which are constructed by the covalent fusion of the cDNA encoding a G protein-coupled receptor with that of a G(alpha) protein subunit, has recently been introduced as an original strategy to explore receptor pharmacology under defined experimental conditions. As such, a controlled 1:1 stoichiometry of receptor and G(alpha) protein expression can be achieved, as well as a forced spatial proximity to each other. Fusion proteins have been revealed as active at the receptor ligand binding level and functional at the G(alpha) protein and effector level. Insights on analytical pharmacological data are discussed for wild-type and mutant receptors interacting with a given G(alpha) protein subunit and different subtypes of either wild-type or mutant G(alpha) proteins activated by a single receptor subtype. A possible alteration of the receptor:G(alpha) protein selectivity may occur due either to the spatial proximity of both protein partners or to a constraint receptor state unable to accommodate to different G(alpha) protein states. Coactivation of endogenous G(alpha) proteins in host cells expressing a fusion protein has also been observed, but depends mainly on the coupling efficiency of the receptor and G(alpha) protein engaged in the fusion process. The ligand's apparent intrinsic activity has been shown to be either enhanced, attenuated, or unmodified when the functional responses of a fusion protein are compared to the coexpression of both fusion protein partners.

Coupling efficacy and selectivity of the human mu-opioid receptor expressed as receptor-Galpha fusion proteins in Escherichia coli

Two constructs encoding the human micro-opioid receptor (hMOR) fused at its C terminus to either one of two Galpha subunits, Galpha(o1) (hMOR-Galpha(o1)) and Galpha(i2) (hMOR-Galpha(i2)), were expressed in Escherichia coli at levels suitable for pharmacological studies (0.4-0.5 pmol/mg). Receptors fused to Galpha(o1) or to Galpha(i2) maintained high-affinity binding of the antagonist diprenorphine. Affinities of the micro-selective agonists morphine, [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), and endomorphins as well as their potencies and intrinsic activities in stimulating guanosine 5'-O-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding were assessed in the presence of added purified Gbetagamma subunits. Both fusion proteins displayed high-affinity agonist binding and agonist-stimulated [(35)S]GTPgammaS binding. In the presence of Gbetagamma dimers, the affinities of DAMGO and endomorphin-1 and -2 were higher at hMOR-Galpha(i2) than at hMOR-Galpha(o1), whereas morphine displayed similar affinities at the two chimeras. Potencies of the four agonists in stimulating [(35)S]GTPgammaS binding at hMOR-Galpha(o1) were similar, whereas at hMOR-Galpha(i2), endomorphin-1 and morphine were more potent than DAMGO and endomorphin-2. The intrinsic activities of the four agonists at the two fusion constructs were similar. The results confirm hMOR coupling to Galpha(o1) and Galpha(i2) and support the hypothesis of the existence of multiple receptor conformational states, depending on the nature of the G protein to which it is coupled.

The regulator of G protein signaling RGS4 selectively enhances alpha 2A-adreoreceptor stimulation of the GTPase activity of Go1alpha and Gi2alpha

Agonist-stimulated high affinity GTPase activity of fusion proteins between the alpha(2A)-adrenoreceptor and the alpha subunits of forms of the G proteins G(i1), G(i2), G(i3), and G(o1), modified to render them insensitive to the action of pertussis toxin, was measured following transient expression in COS-7 cells. Addition of a recombinant regulator of G protein signaling protein, RGS4, did not significantly affect basal GTPase activity nor agonist stimulation of the fusion proteins containing Galpha(i1) and Galpha(i3) but markedly enhanced agonist-stimulation of the proteins containing Galpha(i2) and Galpha(o1.) The effect of RGS4 on the alpha(2A)-adrenoreceptor-Galpha(o1) fusion protein was concentration-dependent with EC(50) of 30 +/- 3 nm and the potency of the receptor agonist UK14304 was reduced 3-fold by 100 nm RGS4. Equivalent reconstitution with Asn(88)-Ser RGS4 failed to enhance agonist function on the alpha(2A)-adrenoreceptor-Galpha(o1) or alpha(2A)-adrenoreceptor-Galpha(i2) fusion proteins. Enzyme kinetic analysis of the GTPase activity of the alpha(2A)-adrenoreceptor-Galpha(o1) and alpha(2A)-adrenoreceptor-Galpha(i2) fusion proteins demonstrated that RGS4 both substantially increased GTPase V(max) and significantly increased K(m) of the fusion proteins for GTP. The increase in K(m) for GTP was dependent upon RGS4 amount and is consistent with previously proposed mechanisms of RGS function. Agonist-stimulated GTPase turnover number in the presence of 100 nm RGS4 was substantially higher for alpha(2A)-adrenoreceptor-Galpha(o1) than for alpha(2A)-adrenoreceptor-Galpha(i2). These studies demonstrate that although RGS4 has been described as a generic stimulator of the GTPase activity of G(i)-family G proteins, selectivity of this interaction and quantitative variation in its function can be monitored in the presence of receptor activation of the G proteins.

Edg2 receptor functionality: gialpha1 coexpression and fusion protein studies

Recombinant receptor cell lines are widely used in G-protein-coupled receptor selectivity studies. To unequivocally interpret the results of such studies, it is essential that the host cell line does not endogenously express the receptor of interest and in addition is unresponsive to the receptor's natural ligand. Here we describe an approach to overcome such difficulties associated with orphan receptors or, as in the present case, receptors whose endogenous ligand ubiquitously affects mammalian cells. The functional heterologous assay system described is for the hEdg2 receptor, which uses lysophosphatidic acid as its endogenous ligand. Once activated, this receptor mediates its effects via multiple secondary messenger pathways, including a Gi-coupled pathway. We have transiently expressed a pertussis toxin-insensitive hEdg2 receptor-ratGialpha1 fusion protein into human embryonic kidney cells and have monitored the ability of compounds to stimulate [(35)S]GTPgammaS binding in membranes prepared from these cells after pretreatment with toxin. Because the assay conditions used favor Gi-mediated responses and because endogenous Gialpha subunits are rendered inactive, the response measured is, by definition, fusion protein-mediated. Consequently, we have developed an assay that monitors definitively Edg2 receptor-mediated responses in a mammalian cell line. A limited structure activity relationship study suggests that the lysophospholipid carbon chain has a role in receptor activation and in addition indicates that certain modifications to the phosphate group are tolerated.

Coupling of the muscarinic m2 receptor to G protein-activated K(+) channels via Galpha(z) and a receptor-Galpha(z) fusion protein. Fusion between the receptor and Galpha(z) eliminates catalytic (collision) coupling

G protein-activated K(+) channel (GIRK), which is activated by the G(betagamma) subunit of heterotrimeric G proteins, and muscarinic m2 receptor (m2R) were coexpressed in Xenopus oocytes. Acetylcholine evoked a K(+) current, I(ACh), via the endogenous pertussis toxin (PTX)-sensitive G(i/o) proteins. Activation of I(ACh) was accelerated by increasing the expression of m2R, suggesting a collision coupling mechanism in which one receptor catalytically activates several G proteins. Coexpression of the alpha subunit of the PTX-insensitive G protein G(z), Galpha(z), induced a slowly activating PTX-insensitive I(ACh), whose activation kinetics were also compatible with the collision coupling mechanism. When GIRK was coexpressed with an m2R x Galpha(z) fusion protein (tandem), in which the C terminus of m2R was tethered to the N terminus of Galpha(z), part of I(ACh) was still eliminated by PTX. Thus, the m2R of the tandem activates the tethered Galpha(z) but also the nontethered G(i/o) proteins. After PTX treatment, the speed of activation of the m2R x Galpha(z)-mediated response did not depend on the expression level of m2R x Galpha(z) and was faster than when m2R and Galpha(z) were coexpressed as separate proteins. These results demonstrate that fusing the receptor and the Galpha strengthens their coupling, support the collision-coupling mechanism between m2R and the G proteins, and suggest a noncatalytic (stoichiometric) coupling between the G protein and GIRK in this model system.

Insights into ligand pharmacology using receptor-G-protein fusion proteins

Production of chimeric DNAs in which the 5' end of G-protein alpha-subunits are linked directly to the 3' tail of a G-protein-coupled receptor has recently offered an unusual strategy to explore the detailed pharmacology of receptor-G-protein interactions. Expression of such fusion proteins ensures a 1:1 stoichiometry of receptor and G-protein expression and their proximity to each other. The capacity of such fusion proteins to be regarded as agonist-activated GTPases that allow simple enzyme kinetics to be applied to issues of ligand efficacy will be considered. In addition, the effects of point mutations, in both receptors and G proteins, on ligand function are particularly amenable to the types of robust quantitative analyses that can be produced using such fusion proteins.

An Asp79Asn mutation of the alpha2A-adrenoceptor interferes equally with agonist activation of individual Gialpha-family G protein subtypes

The quantitative effects of an Asp79Asn mutation in the porcine alpha2A-adrenoceptor on adrenaline-mediated stimulation of the alpha subunit of individual members of the Gi family of G proteins were assessed by measuring GTP turnover number for fusion proteins between the wild type or mutated receptor and pertussis toxin-resistant forms of each of Gi1, Gi2 and Gi3. In each case the receptor mutation limited activation of the G protein to 8-14% of that produced by the wild type receptor. Previous demonstration that in a single cell this mutation selectively interferes with alpha2A-adrenoceptor regulation of distinct effector end points transduced by Gi family members must therefore reflect differential requirements for amplification or the cellular location of individual, co-expressed, G proteins.

Functional analysis of a human A(1) adenosine receptor/green fluorescent protein/G(i1)alpha fusion protein following stable expression in CHO cells

Fusion proteins between the human A(1) adenosine receptor and the pertussis toxin resistant (Cys351Gly) mutant of the G-protein alpha subunit G(i1)alpha (A1/Gi), and between the human A(1) adenosine receptor, the Aequorea victoria green fluorescent protein (GFP) and Cys351Gly G(i1)alpha (A1/GFP/Gi), were expressed in CHO cells. The agonist NECA caused a stimulation of [(35)S]GTPgammaS binding at both fusion proteins with similar concentration dependence as at the native receptor. However in the presence of pertussis toxin NECA stimulation of [(35)S]GTPgammaS binding was only seen at the A1/GFP/Gi fusion protein. The regulation of the adenylyl cyclase and MAP kinase effector systems by both fusion proteins was attenuated following pertussis toxin treatment. These studies demonstrate for the first time the characterisation of a fusion protein between a G-protein coupled receptor, GFP and a G-protein alpha subunit.

Kinetics of ternary complex formation with fusion proteins composed of the A(1)-adenosine receptor and G protein alpha-subunits

High affinity agonist binding to G protein-coupled receptors depends on the formation of a ternary complex between agonist, receptor, and G protein. This process is too slow to be accounted for by a simple diffusion-controlled mechanism. We have tested if the interaction between activated receptor and G protein is rate-limiting by fusing the coding sequence of the human A(1)-adenosine receptor to that of Galpha(i-1) (A(1)/Galpha(i-1)) and of Galpha(o) (A(1)/Galpha(o)). Fusion proteins of the expected molecular mass were detected following transfection of HEK293 cells. Ternary complex formation was monitored by determining the kinetics for binding of the high affinity agonist (-)-N(6)-3[(125)I](iodo-4-hydroxyphenylisopropyl)adenosine; these were similar in the wild-type receptor and the fusion proteins over the temperature range of 10 to 30 degrees C. Agonist dissociation may be limited by the stability of the ternary complex. This assumption was tested by creating fusion proteins in which the Cys(351) of Galpha(i-1) was replaced with glycine (A(1)/Galpha(i-1)C351G) or isoleucine (A(1)/Galpha(i-1)C351I) to lower the affinity of the receptor for the G protein. In these mutated fusion proteins, the dissociation rate of the ternary complex was accelerated; in contrast, the rate of the forward reaction was not affected. We therefore conclude that (i) receptor activation per se rather than its interaction with the G protein is rate-limiting in ternary complex formation; (ii) the stability of the ternary complex is determined by the dissociation rate of the G protein. These features provide for a kinetic proofreading mechanism that sustains the fidelity of receptor-G protein coupling.

GPCR-Galpha fusion proteins: molecular analysis of receptor-G-protein coupling

The efficiency of interactions between G-protein-coupled receptors (GPCRs) and heterotrimeric guanine nucleotide-binding proteins (G proteins) is greatly influenced by the absolute and relative densities of these proteins in the plasma membrane. The study of these interactions has been facilitated by the use of GPCR-Galpha fusion proteins, which are formed by the fusion of GPCR to Galpha. These fusion proteins ensure a defined 1:1 stoichiometry of GPCR to Galpha and force the physical proximity of the signalling partners. Thus, fusion of GPCR to Galpha enhances coupling efficiency can be used to study aspects of receptor-G-protein coupling that could not otherwise be examined by co-expressing GPCRs and G proteins as separate proteins. The results of studies that have made use of GPCR-Galpha fusion proteins will be discussed in this article, along with the strengths and limitations of this approach.

Fatty acylation of proteins: new insights into membrane targeting of myristoylated and palmitoylated proteins

Covalent attachment of myristate and/or palmitate occurs on a wide variety of viral and cellular proteins. This review will highlight the latest advances in our understanding of the enzymology of N-myristoylation and palmitoylation as well as the functional consequences of fatty acylation of key signaling proteins. The role of myristate and palmitate in promoting membrane binding as well as specific membrane targeting will be reviewed, with emphasis on the Src family of tyrosine protein kinases and alpha subunits of heterotrimeric G proteins. The use of myristoyl switches and regulated depalmitoylation as mechanisms for achieving reversible membrane binding and regulated signaling will also be explored.

Examining the efficiency of receptor/G-protein coupling with a cleavable beta2-adrenoceptor-gsalpha fusion protein

Reconstitution of high-affinity agonist binding at the beta2-adrenoceptor (beta2AR) expressed in Sf9 insect cells requires a large excess of the stimulatory G-protein of adenylyl cyclase, Gsalpha, relative to receptor [R. Seifert, T. W. Lee, V. T. Lam & B. K. Kobilka, (1998) Eur. J. Biochem. 255, 369-382]. In a fusion protein of the beta2AR and Gsalpha (beta2AR-Gsalpha), which has only a 1 : 1 stoichiometry of receptor and G-protein, high-affinity agonist binding and agonist-stimulated GTP hydrolysis, guanosine 5'-O-(3-thiotriphosphate) (GTP[S]) binding and adenylyl cyclase (AC) activation are more efficient than in the nonfused coexpression system. In order to analyze the stability of the receptor/G-protein interaction, we constructed a fusion protein with a thrombin-cleavage site between beta2AR and Gsalpha (beta2AR-TS-Gsalpha). beta2AR-TS-Gsalpha efficiently reconstituted high-affinity agonist binding, agonist-stimulated GTP hydrolysis, GTP[S] binding and AC activation. Thrombin cleaves approximately 70% of beta2AR-TS-Gsalpha molecules in Sf9 membranes. Thrombin cleavage did not impair high-affinity agonist binding and GTP[S] binding but strongly reduced ligand-regulated GTPase activity and AC activity. We conclude that fusion of the beta2AR to Gsalpha promotes tight physical association of the two partners and that this association remains stable for a single activation/deactivation cycle even after cleavage of the link between the receptor and G-protein. Dilution of Gsalpha in the membrane and release of activated Gsalpha into the cytosol can both prevent cleaved beta2AR-TS-Gsalpha from undergoing multiple activation/deactivation cycles.

Comparative analysis of the efficacy of A1 adenosine receptor activation of Gi/o alpha G proteins following coexpression of receptor and G protein and expression of A1 adenosine receptor-Gi/o alpha fusion proteins

HEK293T cells were transiently transfected to express either the human A1 adenosine receptor together with pertussis toxin-resistant cysteine-to-glycine forms of the alpha subunits of Gi1 (C351G), Gi2 (C352G), and Gi3 (C351G) and wild-type Go1alpha or fusion proteins comprising the A1 adenosine receptor and these Gi/o G proteins to compare A1 adenosine receptor agonist-mediated activation of these Gi family G proteins upon coexpression of individual Gi/o G proteins and receptor versus expression as receptor-G protein fusion proteins. Addition of the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) to membranes of pertussis toxin-treated cells resulted in a concentration-dependent stimulation of [35S]GTPgammaS binding with comparable amounts of NECA required to produce half-maximal stimulation following transfection of A1 adenosine receptor and Gi/o G proteins either as fusion proteins or as separate polypeptides. However, the magnitude of agonist-mediated activation of GTPgammaS binding was greatly enhanced by expressing the A1 adenosine receptor and Gi family G proteins from chimaeric open reading frames. This observation was consistent following the study of more than 40 agonists. No preferential activation of any G protein was observed with more than 40 A1 receptor agonists following cotransfection of receptor with G protein or transfection of receptor-G protein fusion proteins. These studies demonstrate the utility of using fusion proteins to study receptor-G protein interaction, show that the A1 adenosine receptor couples equally well to the Gi/o G proteins Gi1alpha, G i2alpha, Gi3alpha, and Go1alpha, and demonstrate that for a range of agonists there is no selectivity for activation of any particular A1 adenosine receptor-Gi/o G protein combination.

Coronary arteriolar dilation to acidosis: role of ATP-sensitive potassium channels and pertussis toxin-sensitive G proteins

BACKGROUND

We previously demonstrated that coronary arteriolar dilation in response to acidosis is mediated by the opening of ATP-sensitive potassium (KATP) channels. However, the signal transduction involved in the KATP-channel activation during acidosis has not been elucidated. A recent study in cardiac myocytes implied that pertussis toxin (PTX)-sensitive G proteins may be involved in the signal transduction for KATP-channel activation. However, it remains unclear whether this transduction process also occurs in the vascular tissue and, in particular, whether it exerts functional dilation in response to acidosis.

METHODS AND RESULTS

To examine the signaling pathway for acidosis-induced dilation, porcine coronary arterioles were isolated, cannulated, and pressurized for in vitro study. The GTPase activity in reconstituted G proteins was examined at different levels of pH. Extravascular acidosis (pH 7.3 to 7.0) produced a graded dilation of coronary arterioles. This dilation was not affected by removal of endothelium but was significantly attenuated after inhibition of KATP channels and G proteins by glibenclamide and PTX, respectively. Glibenclamide and PTX attenuated the acidosis-induced arteriolar dilation to the same extent, and combined administration of both inhibitors did not further inhibit the vasodilation. These results indicated that both inhibitors act on the same vasodilatory pathway. Furthermore, vasodilation of coronary arterioles to the KATP-channel opener pinacidil and to the endothelium-independent vasodilator sodium nitroprusside was not affected by PTX. Because PTX inhibited acidosis-induced vasodilation without inhibiting KATP-channel function, it is suggested that PTX inhibits the vasodilatory pathway upstream from KATP channels. GTPase activity in reconstituted G proteins was significantly enhanced by a reduction in pH, indicating that G proteins were directly activated by acidosis.

CONCLUSIONS

On the basis of these findings, we conclude that acidosis-induced coronary arteriolar dilation is mediated by the opening of smooth muscle KATP channels through the activation of PTX-sensitive G proteins.

Modulation of relative intrinsic activity of agonists at the alpha-2A adrenoceptor by mutation of residue 351 of G protein gi1alpha

Compared with epinephrine, the relative intrinsic activity of a series of partial agonists to activate fusion proteins between the porcine alpha-2A adrenoceptor and the alpha-subunit of Gi1 was reduced after a single-point mutation (Cys351Gly) in the G protein. Although UK14304 was close to a full agonist at the fusion construct containing wild-type (Cys351)Gi1alpha, it was a partial agonist at that containing Gly351Gi1alpha. Moreover, although clonidine functioned as a good partial agonist to activate the fusion protein containing Cys351Gi1alpha, it was essentially an antagonist at the Gly351Gi1alpha-containing fusion protein. By contrast, incorporation of Ile351Gi1alpha into the fusion protein resulted in all partial agonists displaying higher intrinsic activity relative to epinephrine to activate this fusion protein than the one containing the wild-type G protein sequence. This is the first demonstration that the relative intrinsic activity of a series of agonists can be modified by a point mutation in a G protein rather than a receptor and indicates that the nature of a key contact site between a G protein and a receptor can selectively regulate partial agonist function. We provide a model for this based on the hydrophobicity of a key receptor-G protein alpha-subunit interaction interface.

Signalling functions of protein palmitoylation

Covalent lipid modifications anchor numerous signalling proteins to the cytoplasmic face of the plasma membrane. These modifications mediate protein-membrane and protein-protein interactions and are often essential for function. Protein palmitoylation, due to its reversible nature, may be particularly important for modulating protein function during cycles of activation and deactivation. Despite intense investigation, the exact functions of protein palmitoylation are not well understood. However, it is clear that palmitoylation can affect a protein's affinity for membranes, subcellular localization, and interactions with other proteins. In this review, recent advances in understanding the functions and mechanisms of protein palmitoylation are discussed, with particular emphasis on how this lipid affects the biochemistry and cell biology of signalling proteins.

Real time visualization of agonist-mediated redistribution and internalization of a green fluorescent protein-tagged form of the thyrotropin-releasing hormone receptor

The long isoform of the rat thyrotropin-releasing hormone receptor (TRHR) was modified by the addition of a vesicular stomatitis virus (VSV) epitope tag and green fluorescent protein (GFP). VSV-TRHR-GFP bound TRH with affinity similar to that of the unmodified receptor and stimulated [3H]inositol phosphate production. A clone stably expressing VSV-TRHR-GFP at some 120,000 copies/cell was selected to visualize this receptor during cellular exposure to TRH. Internalization was detected within 3-5 min after treatment with 1 x 10(-7) M TRH, with dramatic reductions in plasma membrane localization achieved within 10-15 min. The TRHR antagonist/inverse agonist chlordiazepoxide competitively inhibited internalization. Hyperosmotic sucrose inhibited internalization of VSV-TRHR-GFP, measured both by intact cell [3H]TRH binding studies and by confocal microscopy. Now TRH caused a redistribution of VSV-TRHR-GFP to highly punctate but plasma membrane-delineated foci. Pretreatment with the microtubule-disrupting agent nocodazole allowed internalization of the VSV-TRHR-GFP construct but only into vesicles that remained in close apposition to the plasma membrane. Covisualization of VSV-TRHR-GFP and Texas Red transferrin initially indicated entirely separate localizations. After exposure to TRH substantial amounts of VSV-TRHR-GFP were present in vesicles overlapping those containing Texas Red transferrin. Such results demonstrate the G protein-coupling capacity and provide real time visualization of the processes of internalization of a TRH-receptor-GFP construct in response to agonist.

The G-protein betagamma complex

The vast majority of signalling pathways in mammalian cells are mediated by heterotrimeric (alpha betagamma) G proteins. Reviewed here is regulation of signal transduction by the betagamma complex at different protein interfaces: subunit-subunit, receptor-G protein and G protein-effector. The role of diverse beta and gamma subunit types in achieving specificity in signalling and potentially unidentified functions for these subunits also are discussed.

Quantitative analysis of a cysteine351glycine mutation in the G protein Gi1alpha: effect on alpha2A-adrenoceptor-Gi1alpha fusion protein activation

Fusion proteins were constructed between the porcine alpha2A-adrenoceptor and either wild-type (Cys351) or a pertussis toxin-resistant (Gly351) form of the G protein Gi1alpha. Addition of adrenaline to membranes expressing the fusion proteins resulted in concentration-dependent stimulation of their high affinity GTPase activity. The alpha2A-adrenoceptor-wild type Gi1alpha fusion protein produced substantially higher maximal stimulation of GTPase activity in response to adrenaline than that containing Gly351 Gi1alpha. Treatment of the fusion proteins as agonist-regulated enzymes allowed measurement of Vmax and turnover number for adrenaline-stimulation of the GTPase activity of each fusion construct. The turnover number of the alpha2A-adrenoceptor-Cys351 Gly Gi1alpha fusion protein was only 44'S, of that for the alpha2A-adrenoceptor-wild type Gi1alpha fusion protein. These data provide the first direct quantitative evaluation of the effects of a mutation of a G protein on the capacity of an agonist-occupied receptor to activate the mutant.

Agonist occupation of an alpha2A-adrenoreceptor-Gi1alpha fusion protein results in activation of both receptor-linked and endogenous Gi proteins. Comparisons of their contributions to GTPase activity and signal transduction and analysis of receptor-G protein activation stoichiometry

A fusion protein between a pertussis toxin-resistant (C351G) mutant of the alpha subunit of the G protein Gi1 and the porcine alpha2A-adrenoreceptor was stably expressed in Rat 1 fibroblasts. Agonists caused stimulation of high affinity GTPase activity, which was partially prevented by pertussis toxin treatment, demonstrating that the toxin-resistant component of the GTPase activity was derived from the receptor-fused G protein and the remainder from endogenous Gialpha. Half-maximal stimulation of the GTPase activity of endogenous Gi was achieved with lower concentrations of agonist. Although the Km for GTP of the fusion protein-linked Gi was lower than for the endogenous G protein, Vmax measurements demonstrated that adrenaline activated some 5 mol of endogenous Gi/mol of fusion protein-linked Gi. The isolated alpha2A-adrenoreceptor could activate Gs; however, the fusion protein did not. Compared with adrenaline, the efficacy of a range of partial agonists to stimulate endogenous Gialpha was greater than for the fusion protein-constrained C351G Gi1alpha. alpha2A-Adrenoreceptor agonists could stimulate both p44 mitogen-activated protein kinase and p70 S6 kinase and inhibit forskolin-amplified adenylyl cyclase activity in untreated alpha2A-adrenoreceptor-C351G Gi1alpha fusion protein-expressing cells, but these signals were abolished following pertussis toxin treatment. These results demonstrate conclusively, and for the first time, that agonist occupancy of a receptor-G protein fusion protein can result in activation of G proteins other than that physically linked to the receptor. This was selective between G protein classes. Analysis of the contributions of fusion protein-linked and endogenous G proteins to agonist-stimulated GTPase activity provided a direct and original measure of receptor-G protein activation stoichiometry.

New aspects of G-protein-coupled receptor signalling and regulation

Research on the structure, regulation and signalling properties of the family of seven-transmembrane-helix, heterotrimeric guanine nucleotide-binding protein (G-protein)-coupled receptors (GPCRs) continues at a frantic pace. This reflects their central role in transmission of hormone- and neurotransmitter-encoded information across the plasma membrane of cells. The location of the ligand-binding sites on the extracellular face of the membrane has made them obvious targets for therapeutic intervention in a wide range of conditions resulting from endocrine imbalance. Furthermore, based on the identification of many novel GPCR sequences emerging from expressed sequence tags (ESTs) and other DNA sequencing programmes, it has become clear that the GPCR family is likely to be considerably larger than appreciated in even the recent past. Although neither the natural ligands nor synthetic pharmaceuticals have yet been identified for these so-called ;orphan' GPCRs, they offer the potential for a plethora of new therapeutic targets. Within a short review, it is impossible to cover all the current developments in this field and the topics selected represent a personal view of recent highlights of areas that provide both novel and general insights into the function and regulation of GPCRs.

Measurement of agonist efficacy using an alpha2A-adrenoceptor-Gi1alpha fusion protein

A fusion protein was constructed between the porcine alpha2A-adrenoceptor and a pertussis toxin-insensitive (Cys351Gly) form of the alpha subunit of the G protein Gi1. Addition of agonist ligands to membranes of COS-7 cells transiently transfected to express this construct, and treated with pertussis toxin prior to cell harvest, resulted in stimulation of both high affinity GTPase activity and enhanced binding of [35S]GTPgammaS. By considering the fusion protein as an agonist-activated enzyme and measuring Vmax of GTP hydrolysis a range of agonist ligands displayed varying efficacy in their capacity to activate the receptor-associated G protein with adrenaline = noradrenaline = alpha-methylnoradrenaline > UK14304 > BHT933 > or = xylazine = clonidine. A similar rank order was observed following independent co-expression of the alpha2A-adrenoceptor and Cys351Gly-Gi1alpha. These data demonstrate the utility and applicability of using a receptor-G protein fusion protein approach, in which the stoichiometry of receptor and G protein is fixed at 1:1, to measure and further understand the nature of agonist efficacy.