Peptides as probes for G protein signal transduction

Strategies towards Targeting Gαi/s Proteins: Scanning of Protein-Protein Interaction Sites To Overcome Inaccessibility

Heterotrimeric G proteins are classified into four subfamilies and play a key role in signal transduction. They transmit extracellular signals to intracellular effectors subsequent to the activation of G protein-coupled receptors (GPCRs), which are targeted by over 30 % of FDA-approved drugs. However, addressing G proteins as drug targets represents a compelling alternative, for example, when G proteins act independently of the corresponding GPCRs, or in cases of complex multifunctional diseases, when a large number of different GPCRs are involved. In contrast to Gαq, efforts to target Gαi/s by suitable chemical compounds has not been successful so far. Here, a comprehensive analysis was conducted examining the most important interface regions of Gαi/s with its upstream and downstream interaction partners. By assigning the existing compounds and the performed approaches to the respective interfaces, the druggability of the individual interfaces was ranked to provide perspectives for selective targeting of Gαi/s in the future.

Discovery of Pathologic GPCR Aggregation

The family of G-protein-coupled receptors (GPCRs) is one of the most important drug targets. Mechanisms underlying GPCR activation and signaling are therefore of great pharmacologic interest. It was long thought that GPCRs exist and function as monomers. This feature was considered to distinguish GPCRs from other membrane receptors such as receptor tyrosine kinases or cytokine receptors, which signal from dimeric receptor complexes. But during the last two decades it was increasingly recognized that GPCRs can undergo aggregation to form dimers and higher order oligomers, resulting in homomeric and/or heteromeric protein complexes with different stoichiometries. Moreover, this protein complex formation could modify GPCR signaling and function. We contributed to this paradigm shift in GPCR pharmacology by the discovery of the first pathologic GPCR aggregation, which is the protein complex formation between the angiotensin II AT1 receptor and the bradykinin B2 receptor. Increased AT1-B2 heteromerization accounts for the angiotensin II hypersensitivity of pregnant women with preeclampsia hypertension. Since the discovery of AT1-B2, other pathologic GPCR aggregates were found, which contribute to atherosclerosis, neurodegeneration and Alzheimer's disease. As a result of our findings, pathologic GPCR aggregation appears as an independent and disease-specific process, which is increasingly considered as a novel target for pharmacologic intervention.

Structural features of the G-protein/GPCR interactions

BACKGROUND

The details of the functional interaction between G proteins and the G protein coupled receptors (GPCRs) have long been subjected to extensive investigations with structural and functional assays and a large number of computational studies.

SCOPE OF REVIEW

The nature and sites of interaction in the G-protein/GPCR complexes, and the specificities of these interactions selecting coupling partners among the large number of families of GPCRs and G protein forms, are still poorly defined.

MAJOR CONCLUSIONS

Many of the contact sites between the two proteins in specific complexes have been identified, but the three dimensional molecular architecture of a receptor-Gα interface is only known for one pair. Consequently, many fundamental questions regarding this macromolecular assembly and its mechanism remain unanswered.

GENERAL SIGNIFICANCE

In the context of current structural data we review the structural details of the interfaces and recognition sites in complexes of sub-family A GPCRs with cognate G-proteins, with special emphasis on the consequences of activation on GPCR structure, the prevalence of preassembled GPCR/G-protein complexes, the key structural determinants for selective coupling and the possible involvement of GPCR oligomerization in this process.

Signal protein-derived peptides as functional probes and regulators of intracellular signaling

The functionally important regions of signal proteins participating in their specific interaction and responsible for transduction of hormonal signal into cell are rather short in length, having, as a rule, 8 to 20 amino acid residues. Synthetic peptides corresponding to these regions are able to mimic the activated form of full-size signal protein and to trigger signaling cascades in the absence of hormonal stimulus. They modulate protein-protein interaction and influence the activity of signal proteins followed by changes in their regulatory and catalytic sites. The present review is devoted to the achievements and perspectives of the study of signal protein-derived peptides and to their application as selective and effective regulators of hormonal signaling systems in vitro and in vivo. Attention is focused on the structure, biological activity, and molecular mechanisms of action of peptides, derivatives of the receptors, G protein α subunits, and the enzymes generating second messengers.

Ligand-based peptide design and combinatorial peptide libraries to target G protein-coupled receptors

G protein-coupled receptors (GPCRs) are considered to represent the most promising drug targets; it has been repeatedly said that a large fraction of the currently marketed drugs elicit their actions by binding to GPCRs (with cited numbers varying from 30-50%). Closer scrutiny, however, shows that only a modest fraction of (≈60) GPCRs are, in fact, exploited as drug targets, only ≈20 of which are peptide-binding receptors. The vast majority of receptors in the humane genome have not yet been explored as sites of action for drugs. Given the drugability of this receptor class, it appears that opportunities for drug discovery abound. In addition, GPCRs provide for binding sites other than the ligand binding sites (referred to as the "orthosteric site"). These additional sites include (i) binding sites for ligands (referred to as "allosteric ligands") that modulate the affinity and efficacy of orthosteric ligands, (ii) the interaction surface that recruits G proteins and arrestins, (iii) the interaction sites of additional proteins (GIPs, GPCR interacting proteins that regulate G protein signaling or give rise to G protein-independent signals). These sites can also be targeted by peptides. Combinatorial and natural peptide libraries are therefore likely to play a major role in identifying new GPCR ligands at each of these sites. In particular the diverse natural peptide libraries such as the venom peptides from marine cone-snails and plant cyclotides have been established as a rich source of drug leads. High-throughput screening and combinatorial chemistry approaches allow for progressing from these starting points to potential drug candidates. This will be illustrated by focusing on the ligand-based drug design of oxytocin (OT) and vasopressin (AVP) receptor ligands using natural peptide leads as starting points.

Design, synthesis, and preliminary pharmacological evaluation of a set of small molecules that directly activate gi proteins

Heterotrimeric G proteins play a pivotal role in the communication of cells with the environment. G proteins are stimulated by cell surface receptors (GPCR) that catalyze the exchange of GDP, bound to Galpha subunit, with GTP and can per se be the target of drugs. Based on the structure of two nonpeptidic modulators of Gi proteins, a series of new molecules characterized by a long hydrophobic chain and at least two nitrogen atoms protonated at physiological pH was designed. The compounds were tested for their ability to stimulate binding of GTPgammaS to recombinant Gi proteins. Gi activation properties were also evaluated by inhibition of adenylyl cyclase activity in intact lymphocytes. Most compounds were able to stimulate GTPgammaS binding and to inhibit cAMP production at micromolar doses. Among the active compounds, 34 showed good efficacy and was the most potent compound studied, particularly on alpha(o) subtype; its regioisomer, 36, was the most efficacious one. Compound 7 showed also an interesting profile as it showed selectivity toward the alpha(o) subtype, in both efficacy and potency. Some of the compounds synthesized and found to be active may be useful leads to develop more potent and selective Gi protein modulators.

Regulation of receptor-coupling to (multiple) G proteins. A challenge for basic research and drug discovery

G protein-coupled receptors induce intracellular signals via interaction of with cytosolic/peripheral membrane proteins, mainly G proteins. There has been much debate about the mode of interaction between the receptors, G proteins and effectors, their mobility and the ways of determining the specificity of interaction. Additional complexity has been added to system upon the discovery of i) coupling of single receptors to several G proteins and ii) active direction of this by different ligands (stimulus trafficking). These data suggest that the most primary unit in the signal transduction is the receptor complexed with a specific G protein, making the investigation of the mechanism of receptor-G protein selection and interaction even more important. In this review, I will summarize the general knowledge of receptor interaction with G proteins and effectors and the ways of investigating this.

G-protein-directed ligand discovery with peptide combinatorial libraries

Modulators of G-protein signaling have a central role in controlling cell physiology and represent over half of all marketed prescription drugs. G-protein pathways have traditionally been targeted by developing ligands to the extracellular surface of a small subset of the estimated approximately 1000 G-protein-coupled receptors in humans. The intracellular machinery, consisting of the cytosolic receptor surfaces and heterotrimeric G proteins, provides an equivalent diversity of targets that has remained relatively unexplored until now. This review summarizes recent efforts using combinatorial peptide libraries to develop new G-protein signaling modulators targeting intracellular components.

RNA Editing of the Human Serotonin 5-HT2CReceptor Disrupts Transactivation of the Small G-Protein RhoA

The human serotonin 5-HT2C receptor undergoes adenosineto-inosine RNA editing at five positions, generating multiple receptor isoforms with altered G-protein coupling properties. In the current study, we demonstrate that RNA editing regulates the pattern of intracellular signaling. The non-edited human 5-HT2C receptor isoform INI activates phospholipase D via the G13 heterotrimer G-protein. We present evidence that transactivation of the small G-protein RhoA is required for phospholipase D activation. In contrast, neither transactivation of RhoA nor phospholipase D activation was detected in cells expressing the fully edited VGV isoform. The ability to activate phospholipase C is also reduced in VGV-expressing cells, but not to the extent found for the phospholipase D signal. We conclude that RNA editing represents a novel mechanism for regulating 5-HT2C receptor signaling to pathways linked to actin cytoskeletal organization and regulated exocytosis.

Identification of the G protein-activating sequence of the single-transmembrane natriuretic peptide receptor C (NPR-C)

Rat natriuretic peptide clearance receptor (NPR-C) contains four sequences capable of inhibiting adenylyl cyclase. We have undertaken mutational and deletion studies on the intracellular domain of rat NPR-C to determine which of these sequences is functionally relevant. Nine mutant receptors were constructed by deletion of 11 or 28 COOH-terminal residues or by site-directed mutagenesis of basic residues in a 17-amino acid sequence, R(469)RNHQEESNIGKHRELR(485), corresponding to the main active peptide. Substitution of arginine residues (R(469)R(470)) flanking the NH(2) terminus abolished G(i1) and G(i2) and PLC-beta activities and inhibition of adenylyl cyclase. Substitution of one or two basic residues (H(481) and/or R(482) or R(485)) in the COOH-terminal motif (H(481)RELR(485)) greatly decreased or abolished G protein and PLC-beta activities and inhibition of adenylyl cyclase. This implies that sequences NH(2)-terminal to the motif or COOH-terminal to R(470) could not sustain receptor activity in situ, although they exhibited activity when used as synthetic peptides. Deletion of the 11 COOH-terminal residues (E(486) to A(496)) suggested an autoinhibitory function for this sequence. We conclude that the 17-amino acid sequence (R(469) to R(485)) in the middle region of the intracellular domain of NPR-C is both necessary and sufficient for activation of G proteins and effector enzymes.

Phospholipase D activation by endogenous 5-hydroxytryptamine 2C receptors is mediated by Galpha13 and pertussis toxin-insensitive Gbetagamma subunits

Phospholipase D activation was measured in primary cultures of rat choroid plexus epithelial cells, which endogenously express the 5-hydroxytryptamine (5-HT) 2C receptor, as well as a heterologous cell line expressing the cloned receptor. In both systems, serotonin stimulation of the 5-HT(2C) receptor activates phospholipase D in addition to phospholipase C, the traditional effector. Specific inhibitors and membrane permeable blocking peptides were used to determine which heterotrimeric G-proteins were involved. Results suggest that both alpha and free betagamma subunits from G(13) heterotrimers are responsible for phospholipase D activation.

Involvement of both G(q/11) and G(s) proteins in gonadotropin-releasing hormone receptor-mediated signaling in L beta T2 cells

The hypothalamic hormone gonadotropin-releasing hormone (GnRH) stimulates the synthesis and release of the pituitary gonadotropins. GnRH acts through a plasma membrane receptor that is a member of the G protein-coupled receptor (GPCR) family. These receptors interact with heterotrimeric G proteins to initiate downstream signaling. In this study, we have investigated which G proteins are involved in GnRH receptor-mediated signaling in L beta T2 pituitary gonadotrope cells. We have shown previously that GnRH activates ERK and induces the c-fos and LH beta genes in these cells. Signaling via the G(i) subfamily of G proteins was excluded, as neither ERK activation nor c-Fos and LH beta induction was impaired by treatment with pertussis toxin or a cell-permeable peptide that sequesters G beta gamma-subunits. GnRH signaling was partially mimicked by adenoviral expression of a constitutively active mutant of G alpha(q) (Q209L) and was blocked by a cell-permeable peptide that uncouples G alpha(q) from GPCRs. Furthermore, chronic activation of G alpha(q) signaling induced a state of GnRH resistance. A cell-permeable peptide that uncouples G alpha(s) from receptors was also able to inhibit ERK, c-Fos, and LH beta, indicating that both G(q/11) and G(s) proteins are involved in signaling. Consistent with this, GnRH caused GTP loading on G(s) and G(q/11) and increased intracellular cAMP. Artificial elevation of cAMP with forskolin activated ERK and caused a partial induction of c-Fos. Finally, treatment of G alpha(q) (Q209L)-infected cells with forskolin enhanced the induction of c-Fos showing that the two pathways are independent and additive. Taken together, these results indicate that the GnRH receptor activates both G(q) and G(s) signaling to regulate gene expression in L beta T2 cells.

Thrombospondin stimulates focal adhesion disassembly through Gi- and phosphoinositide 3-kinase-dependent ERK activation

The matricellular protein thrombospondin (TSP) stimulates stress fiber and focal adhesion disassembly through a sequence (hep I) in its heparin-binding domain. TSP/hep I signals focal adhesion disassembly by binding cell surface calreticulin (CRT) and activating phosphoinositide 3-kinase (PI3K). However, other components of this signaling pathway have not been identified. We now show that TSP induces focal adhesion disassembly through activation of pertussis toxin (PTX)-sensitive G proteins and ERK phosphorylation. PTX pretreatment inhibits TSP/hep I-mediated focal adhesion disassembly as well as PI3K activation. In addition, membrane-permeable Galpha(i2)- and Gbetagamma-blocking peptides inhibit hep I-mediated focal adhesion disassembly. Hep I stimulates a transient increase in ERK activation, which is abrogated by both PTX and PI3K inhibitors. Inhibiting ERK activation with MEK inhibitors blocks hep I-mediated focal adhesion disassembly, indicating that ERK activation is required for cytoskeletal reorganization. G protein signals and ERK phosphorylation are induced by TSP binding to cell surface CRT, because CRT null mouse embryonic fibroblasts (MEF) fail to stimulate ERK phosphorylation in response to TSP/hep I treatment. These data show that G(i) protein and ERK, in concert with PI3K, are stimulated by TSP.CRT interactions at the cell surface to induce de-adhesive changes in the cytoskeleton.

Activation and inhibition of G proteins by lipoamines

We have previously shown that alkyl-substituted amino acid derivatives directly activate G(i/o) proteins. N-Dodecyl-N(alpha),N(epsilon)-(bis-l-lysinyl)-l-lysine amide (FUB132) is a new representative of this class of compounds with increased efficacy. Here, we characterized the molecular mechanism of action of this class of compounds. FUB132 and its predecessor FUB86 were selective receptomimetics for G(i/o) because they stimulated the guanine nucleotide exchange reaction of purified G(i/o) as documented by an increased rate of GDP release, GTP gamma S binding, and GTP hydrolysis. In contrast to the receptomimetic peptide mastoparan, stimulation of G proteins by lipoamines required the presence of neither G beta gamma-dimers nor lipids. On the contrary, G beta gamma-dimers suppressed the stimulatory effect of FUB132. The stimulation of G(i/o) by lipoamines and by mastoparan was not additive. A peptide derived from the C terminus of G alpha(o3), but not a corresponding G alpha(q)-derived peptide, quenched the FUB132-induced activation of G alpha(o). In membranes prepared from human embryonic kidney 293 cells that stably expressed the G(i/o)-coupled human A(1)-adenosine receptor, lipoamines impeded high-affinity agonist binding. In contrast, antagonist binding was not affected. We conclude that alkyl-substituted amines target a site, most likely at the C terminus of G alpha(i/o)-subunits, that is also contacted by receptors. However, because G beta gamma-dimers blunt rather than enhance their efficacy, their mechanism of action differs fundamentally from that of a receptor. Thus, despite their receptomimetic effect in vitro, alkyl-substituted amines and related polyamines are poor direct G protein activators in vivo. In the presence of G beta gamma, they rather antagonize G protein-coupled receptor signaling.

RNA editing of the human serotonin 5-HT2C receptor alters receptor-mediated activation of G13 protein

The recent completion of the human genome predicted the presence of only 30,000 genes, stressing the importance of mechanisms that increase molecular diversity at the post-transcriptional level. One such post-transcriptional event is RNA editing, which generates multiple protein isoforms from a single gene, often with profound functional consequences. The human serotonin 5-HT(2C) receptor undergoes RNA editing that creates multiple receptor isoforms. One consequence of RNA editing of cell surface receptors may be to alter the pattern of activation of heterotrimeric G-proteins and thereby shift preferred intracellular signaling pathways. We examined the ability of the nonedited 5-HT(2C) receptor isoform (INI) and two extensively edited isoforms, VSV and VGV, to interact with various G-protein alpha subunits. Two functional assays were utilized: the cell-based functional assay, Receptor Selection/Amplification Technology(TM), in which the pharmacological consequences of co-expression of 5HT(2C) receptor isoforms with G-protein alpha subunits in fibroblasts were studied, and 5HT(2C) receptor-mediated rearrangements of the actin cytoskeleton in stable cell lines. These studies revealed that the nonedited 5-HT(2C) receptor functionally couples to G(q) and G(13). In contrast, coupling to G(13) was not detected for the extensively edited 5-HT(2C) receptors. Thus, RNA editing represents a novel mechanism for regulating the pattern of activation of heterotrimeric G-proteins, molecular switches that control an enormous variety of biological processes.

Dissecting G protein-coupled receptor signaling pathways with membrane-permeable blocking peptides. Endogenous 5-HT(2C) receptors in choroid plexus epithelial cells

To determine the intracellular signaling mechanism of the 5-HT(2C) receptor endogenously expressed in choroid plexus epithelial cells, we implemented a strategy of targeted disruption of protein-protein interactions. This strategy entails the delivery of conjugated membrane-permeable peptides that disrupt domain interaction at specific steps in the signaling cascade. As proof of concept, two peptides targeted against receptor-G protein interaction domains were examined. Only G(q)CT, which targets the receptor-G(q) protein interacting domain, disrupted 5-HT(2C) receptor-mediated phosphatidylinositide hydrolysis. G(s)CT, targeting the receptor-G(s) protein, disrupted beta2 adrenergic receptor-mediated activation of cAMP but not 5-HT(2C) receptor-mediated phosphatidylinositide hydrolysis. The peptide MPS-PLCbeta1M, mimicking the domain of phospholipase Cbeta1 (PLCbeta1) interacting with active Galpha(q), also blocked 5-HT(2C) receptor activation. In contrast, peptides PLCbeta2M and Phos that bind to and sequester free Gbetagamma subunits were ineffective at blocking 5-HT(2C) receptor-mediated phosphoinositol turnover. However, both peptides disrupted Gbetagamma-mediated alpha(2A) adrenergic receptor activation of mitogen-activated protein kinase. These results provide the first direct demonstration that active Galpha(q) subunits mediate endogenous 5-HT(2C) receptor activation of PLCbeta and that Gbetagamma subunits released from Galpha(q) heterotrimeric proteins are not involved. Comparable results were obtained with metabotropic glutamate receptor 5 expressed in astrocytes. Thus, conjugated, membrane-permeable peptides are effective tools for the dissection of intracellular signals.

Role of third intracellular loop of galanin receptor type 1 in signal transduction

To determine the domains essential for G-protein coupling of the human galanin receptor type 1 (GalR1), we have used both GalR1 mutants and synthetic receptor-derived peptides in(125)I-galanin and [(35)S]-GTPgammaS binding studies. Replacement of potential phosphorylation sites by Leu in the third intracellular loop (IC3) of GalR1 did not affect K(D)values for the receptor. Peptides derived form the IC3 loop, and especially the N-terminal part of it were able to increase the rate of [(35)S]-GTPgammaS binding to the trimeric Gialpha1beta1gamma2, but not to Gsalphabeta1gamma2, whereas the peptides corresponding to the IC1 and IC2 loops had no such effect. IC3 loop peptides also inhibited the binding of(125)I-galanin to GalR1 in membranes from Rin m5F cells. Our results suggest that the IC3 loop of GalR1, especially its N-terminal part, defines the coupling of the receptor to the Gialpha1beta1gamma2 protein and consequently, to the signal transduction cascade.

An antibody directed against residues 100-119 within the alpha-helical domain of Galpha(s) defines a novel contact site for beta-adrenergic receptors

A polyclonal antiserum that recognizes residues 100-119 within the alpha-helical domain of Galpha(s) (K-20) caused a dissociation of G(s) into its component subunits and activated a cholera toxin-sensitive high affinity GTPase. Consistently, the antibody mimicked the stimulatory effects of the beta-adrenergic agonist, isoproterenol, on adenylyl cyclase, which is mediated by Galpha(s), and its inhibitory action on NADPH-dependent H(2)O(2) generation, a Gbetagamma-mediated response. A peptide corresponding to the target sequence of K-20 not only neutralized the receptor-mimetic effects of the antibody but inhibited the whole spectrum of isoproterenol action as well, including its antagonistic effects on adenylyl cyclase and NADPH-dependent H(2)O(2) generation. By contrast, COOH-terminal anti-Galpha(s) selectively inhibited the stimulatory effect of isoproterenol on cAMP formation without affecting its inhibitory effect on NADPH-dependent H(2)O(2) generation. The data are consistent with the concept that beta-adrenergic receptors interact with multiple sites on Galpha(s) each playing a distinct role, and strongly suggest that antibody K-20 defines a novel contact site for beta-adrenergic receptors that localizes to the alpha-helical domain and is essential for eliciting the complete spectrum of beta-adrenergic responses.

G protein antagonists

Heterotrimeric G proteins couple membrane-bound heptahelical receptors to their cellular effector systems (ion channels or enzymes generating a second messenger). In current pharmacotherapy, the input to G protein-regulated signalling is typically manipulated by targeting the receptor with appropriate agonists or antagonists and, to a lesser extent, by altering second messenger levels, most notably by inhibiting phosphodiesterases that hydrolyse cyclic nucleotides. When stimulated, G proteins undergo a cycle of activation and deactivation in which the alpha-subunits and the betagamma-dimers sequentially expose binding sites for their reaction partners (receptors, guanine nucleotides and effectors, as well as regulatory proteins). These domains can be blocked by inhibitors and this produces effects that cannot be achieved by receptor antagonists. Here, the structural and mechanistic information on G protein antagonists is summarized and an outline of the arguments supporting the hypothesis that G proteins per se are also potential drug targets is provided.

Chimaeric G alpha proteins: their potential use in drug discovery

Approaches that allow ligand occupancy of a wide range of G protein-coupled receptors to be converted into robust assays amenable to relatively high-throughput analysis are ideal for screening for novel ligands at this class of receptor. Many attempts have been made to design universal ligand-screening systems such that any GPCR can be screened using a common assay end-point. Manipulation of the G protein within the assay system offers the possibility of achieving this. To better understand the domains involved in the interactions between G protein-coupled receptors, G proteins and effector polypeptides and the fine details of these contacts, a wide range of chimaeric G protein alpha subunits have been produced. Graeme Milligan and Stephen Rees discuss the information generated by such studies and the ways in which such chimaeric G proteins can be integrated into assay systems for drug discovery.

Receptor-mediated and receptor-independent activation of G-proteins in rat brain membranes

High-affinity GTPase activity intrinsic to G-proteins, which serves as an index of G-protein activation elicited through agonist-stimulated receptors as well as by receptor-independent direct G-protein activators like mastoparan, was measured in rat brain membranes. Receptor-mediated high-affinity GTPase activity was detectable preferentially for the Gi subfamily associated with adenylyl cyclase inhibition mediated by group II metabotropic glutamate, pirenzepine-insensitive muscarinic acetylcholine, GABA(B), adenosine A1, dopamine D2-like (striatum), and serotonin 5-HT1A (hippocampus) receptors. The pharmacological characteristics of such receptor-mediated high-affinity GTPase activities were presented. Mastoparan, a tetradecapeptide from wasp venom which has been shown to directly activate Gi and Go, inhibited low-affinity GTP hydrolyzing activity, probably due to its activating effect on nucleoside diphosphokinase (NDPK). When NDPK activity was inhibited completely by UDP, mastoparan stimulated high-affinity GTPase activity in a concentration-dependent manner. There are many compounds other than mastoparan with apparently diverse structural properties which have been shown to directly activate G-proteins. The relevance and possible participation of receptor-independent mode of G-protein activation for some neuropeptides were discussed.

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

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

G protein coupled receptor signaled apoptosis is associated with activation of a cation insensitive acidic endonuclease and intracellular acidification

Apoptosis associated oligonucleosomal fragmentation of DNA can result from the activation of endonucleases that exhibit different pH optima and are either sensitive or insensitive to divalent cations. DNA fragmentation due to activation of cation sensitive endonucleases occurs in the absence of a change in intracellular pH whereas intracellular acidification is a feature of apoptosis characterized by activation of cation insensitive acidic endonuclease. We have reported earlier that somatostatin (SST) induced DNA fragmentation and apoptosis is signaled in a receptor subtype selective manner uniquely via human somatostatin receptor subtype 3 (hSSTR3). In the present study we investigated the pH dependence and cation sensitivity of endonuclease induced in hSSTR3 expressing CHO-K1 cells by the SST agonist octreotide (OCT) and its effect on intracellular pH. We show that OCT induced apoptosis is associated with selective stimulation of a divalent cation insensitive acidic endonuclease. The intracellular pH of of cells undergoing OCT induced apoptosis was 0.9 pH units lower than that of control cells. The effect of OCT on endonuclease and pH was inhibited by orthovanadate as well as by pretreatment with pertussis toxin, suggesting that hSSTR3 initiated cytotoxic signaling is protein tyrosine phosphatase mediated and is G protein dependent. These findings suggest that intracellular acidification and activation of acidic endonuclease mediate wild type p53 associated apoptosis signaled by hormones acting via G protein coupled receptors.

A betagamma dimer derived from G13 transduces the angiotensin AT1 receptor signal to stimulation of Ca2+ channels in rat portal vein myocytes

A G protein composed of alpha13, beta1, and gamma3 subunits selectively couples the angiotensin AT1A receptors to increase cytoplasmic Ca2+ concentration ([Ca2+]i) in rat portal vein myocytes (Macrez-Leprêtre, N., Kalkbrenner, F., Morel, J. L., Schultz, G., and Mironneau, J. (1997) J. Biol. Chem. 272, 10095-10102). We show here that Gbetagamma transduces the signal leading to stimulation of L-type Ca2+ channels. Intracellular dialysis through the patch pipette of a carboxyl-terminal anti-betacom antibody and a peptide corresponding to the Gbetagamma binding region of the beta-adrenergic receptor kinase 1 inhibited the stimulation of Ca2+ channels and the increase in [Ca2+]i evoked by angiotensin II. The Gbetagamma binding peptide did not prevent the dissociation of the heterotrimeric G protein into its subunits, as it did not block activation of phospholipase C-beta by Galphaq in response to stimulation of alpha1-adrenoreceptors. Transient overexpression of the beta-adrenergic receptor kinase 1 fragment and of Galpha subunits also inhibited the angiotensin II-induced increase in [Ca2+]i. Both anti-alpha13 antibody and carboxyl-terminal alpha13 peptide abrogated the angiotensin II-induced stimulation of Ca2+ channels. We conclude that activation of angiotensin AT1 receptors requires all three alpha, beta, and gamma subunits of G13 for receptor-G protein interaction, whereas the transduction of the signal to L-type Ca2+ channels is mediated by Gbetagamma.

An evaluation of strategies available for the identification of GTP-binding proteins required in intracellular signalling pathways

Strategies which can be used to elucidate the nature of a GTP-binding regulatory protein (G-protein) involved in an intracellular pathway of interest in the complex environment of the cell are described and evaluated. A desirable strategy is considered to be one in which the first stage indicates a requirement for one or more G-proteins, provides information on whether a monomeric, trimeric or other type of G-protein is involved, and gives some idea of the G-protein sub-class. In the second stage the specific G-protein involved is identified. Approaches available for investigations in the first stage include the use of analogues of GTP and GDP, AlF4-, inhibitors of G-protein isoprenylation, bacterial toxins which covalently modify G-proteins, and the introduction of a purified GDP dissociation inhibitor, GDP exchange and/or GTP-ase activating protein. Identification of the specific G-protein in the second stage can be achieved using anti G-protein antibodies, G-protein-or receptor-derived peptides, antisense G-protein RNA and over-expressed, constitutively-active or dominant-negative G-protein mutants. The correct interpretation of results obtained with GTP and GDP analogues and AlF4- in the first stage is complex and often difficult, and requires a thorough understanding of the functions and mechanisms of activation of G-proteins. Nevertheless, it is important to reach the correct conclusion at this stage since considerable time and expense are usually required for investigations in the second stage.

G protein heterotrimer Galpha13beta1gamma3 couples the angiotensin AT1A receptor to increases in cytoplasmic Ca2+ in rat portal vein myocytes

The subunit composition of angiotensin AT1 receptor-activated G protein was identified by using antisense oligonucleotide injection into the nucleus of rat portal vein myocytes. In these cells, we have previously shown that increases in the cytoplasmic calcium concentration ([Ca2+]i) induced by activation of angiotensin AT1 receptors were dependent on extracellular Ca2+ entry by L-type Ca2+ channels and subsequent Ca2+-induced Ca2+ release from the intracellular stores. The angiotensin AT1 receptor-activated increases in [Ca2+]i were selectively inhibited by injection of antisense oligonucleotides directed against the mRNAs coding for the alpha13, beta1, and gamma3 subunits. A correlating reduction in Galpha13, Gbeta1, and Ggamma3 protein expression was confirmed by immunocytochemistry. In addition, anti-alpha13 antibody and synthetic peptide corresponding to the carboxyl terminus of the Galpha13 subunit inhibited, in a concentration-dependent manner, the angiotensin AT1 receptor-mediated Ca2+ response. Reverse transcription-polymerase chain reaction analysis showed that only the angiotensin AT1A receptor was expressed in rat portal vein smooth muscle. Furthermore, injection of anti-AT1A oligonucleotides selectively inhibited the angiotensin II-induced increase in [Ca2+]i. We conclude that the receptor-activated signal leading to increases in [Ca2+]i is transduced by the heterotrimeric G13 protein composed of alpha13/beta1/gamma3 subunits and that the carboxyl terminus of the Galpha13 subunit interacts with the angiotensin AT1A receptor.

G protein mechanisms: insights from structural analysis

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

Functional and structural complexity of signal transduction via G-protein-coupled receptors

A prerequisite for the maintenance of homeostasis in a living organism is fine-tuned communication between different cells. The majority of extracellular signaling molecules, such as hormones and neurotransmitters, interact with a three-protein transmembrane signaling system consisting of a receptor, a G protein, and an effector. These single components interact sequentially and reversibly. Considering that hundreds of G-protein-coupled receptors interact with a limited repertoire of G proteins, the question of coupling specificity is worth considering. G-protein-mediated signal transduction is a complex signaling network with diverging and converging transduction steps at each coupling interface. The recent realization that classical signaling pathways are intimately intertwined with growth-factor-signaling cascades adds another level of complexity. Elaborate studies have significantly enhanced our knowledge of the functional anatomy of G-protein-coupled receptors, and the concept has emerged that receptor function can be modulated with high specificity by coexpressed receptor fragments. These results may have significant clinical impact in the future.

Agonist-specific conformational changes in the yeast alpha-factor pheromone receptor

The yeast alpha-factor pheromone receptor is a member of the G-protein-coupled receptor family. Limited trypsin digestion of yeast membranes was used to investigate ligand-induced conformational changes in this receptor. The agonist, alpha-factor, accelerated cleavage in the third intracellular loop, whereas the antagonist, desTrp1,Ala3-alpha-factor, reduced the cleavage rate. Thus, the enhanced accessibility of the third intracellular loop is specific to the agonist. alpha-Factor inhibited cleavage weakly at a second site near the cytoplasmic terminus of the seventh transmembrane helix, whereas the antagonist showed a stronger inhibition of cleavage at this site and at another site in the C-terminal domain of the receptor. The alpha-factor-induced conformational changes appeared to be inherent properties of the receptor, as they were retained in G-protein-deficient mutants. Moreover, a mutant receptor (ste2-L236H) that affects the third loop and is defective for G-protein coupling retained the ability to undergo the agonist-induced conformational changes. These results are consistent with a model in which G-protein activation is limited by the availability of specific contacts between the G protein and the third intracellular loop of the receptor. The antagonist appears to promote a distinct conformational state that differs from either the unoccupied or the agonist-occupied state.