Molecular basis for interactions of G protein betagamma subunits with effectors

Custom distinctions in the interaction of G-protein beta subunits with N-type (CaV2.2) versus P/Q-type (CaV2.1) calcium channels

Inhibition of N- (Cav2.2) and P/Q-type (Cav2.1) calcium channels by G-proteins contribute importantly to presynaptic inhibition as well as to the effects of opiates and cannabinoids. Accordingly, elucidating the molecular mechanisms underlying G-protein inhibition of voltage-gated calcium channels has been a major research focus. So far, inhibition is thought to result from the interaction of multiple proposed sites with the Gbetagamma complex (Gbetagamma). Far less is known about the important interaction sites on Gbetagamma itself. Here, we developed a novel electrophysiological paradigm, "compound-state willing-reluctant analysis," to describe Gbetagamma interaction with N- and P/Q-type channels, and to provide a sensitive and efficient screen for changes in modulatory behavior over a broad range of potentials. The analysis confirmed that the apparent (un)binding kinetics of Gbetagamma with N-type are twofold slower than with P/Q-type at the voltage extremes, and emphasized that the kinetic discrepancy increases up to ten-fold in the mid-voltage range. To further investigate apparent differences in modulatory behavior, we screened both channels for the effects of single point alanine mutations within four regions of Gbeta1, at residues known to interact with Galpha. These residues might thereby be expected to interact with channel effectors. Of eight mutations studied, six affected G-protein modulation of both N- and P/Q-type channels to varying degrees, and one had no appreciable effect on either channel. The remaining mutation was remarkable for selective attenuation of effects on P/Q-, but not N-type channels. Surprisingly, this mutation decreased the (un)binding rates without affecting its overall affinity. The latter mutation suggests that the binding surface on Gbetagamma for N- and P/Q-type channels are different. Also, the manner in which this last mutation affected P/Q-type channels suggests that some residues may be important for "steering" or guiding the protein into the binding pocket, whereas others are important for simply binding to the channel.

Keeping G proteins at bay: a complex between G protein-coupled receptor kinase 2 and Gbetagamma

The phosphorylation of heptahelical receptors by heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor kinases (GRKs) is a universal regulatory mechanism that leads to desensitization of G protein signaling and to the activation of alternative signaling pathways. We determined the crystallographic structure of bovine GRK2 in complex with G protein beta1gamma2 subunits. Our results show how the three domains of GRK2-the RGS (regulator of G protein signaling) homology, protein kinase, and pleckstrin homology domains-integrate their respective activities and recruit the enzyme to the cell membrane in an orientation that not only facilitates receptor phosphorylation, but also allows for the simultaneous inhibition of signaling by Galpha and Gbetagamma subunits.

Differential roles of CB1 and CB2 cannabinoid receptors in mast cells

Cannabinoid modulation of immune responses is a pathological consequence of marijuana abuse and a potential outcome of therapeutic application of the drug. Moreover, endogenous cannabinoids are physiological immune regulators. In the present report, we describe alterations in gene transcription that occur after cannabinoid exposure in a mast cell line, RBL2H3. Cannabinoid exposure causes marked changes in the transcript levels for numerous genes, acting both independently of and in concert with immunoreceptor stimulation via Fc epsilon RI. In two mast cell lines, we observed mRNA and protein expression corresponding to both CB1 and CB2 cannabinoid receptor isoforms, contrary to the prevailing view that CB1 is restricted to the CNS. We show that coexpression of the two isoforms is not functionally redundant in mast cells. Analysis of signaling pathways downstream of cannabinoid application reveals that activation of extracellular signal-regulated kinase, AKT, and a selected subset of AKT targets is accomplished by CB2 ligands and nonselective CB1/CB2 agonists in mast cells. CB1 inhibition does not affect AKT or extracellular signal-regulated kinase activation by cannabinoids, indicating that CB2 is the predominant regulatory receptor for these kinases in this cell context. CB1 receptors are, however, functional in these mast cells, since they can contribute to suppression of secretory responses.

Agonist unbinding from receptor dictates the nature of deactivation kinetics of G protein-gated K+ channels

G protein-gated inwardly rectifying K(+) (Kir) channels are found in neurones, atrial myocytes, and endocrine cells and are involved in generating late inhibitory postsynaptic potentials, slowing the heart rate and inhibiting hormone release. They are activated by G protein-coupled receptors (GPCRs) via the inhibitory family of G protein, G(i/o), in a membrane-delimited fashion by the direct binding of Gbetagamma dimers to the channel complex. In this study we are concerned with the kinetics of deactivation of the cloned neuronal G protein-gated K(+) channel, Kir3.1 + 3.2A, after stimulation of a number of GPCRs. Termination of the channel activity on agonist removal is thought to solely depend on the intrinsic hydrolysis rate of the G protein alpha subunit. In this study we present data that illustrate a more complex behavior. We hypothesize that there are two processes that account for channel deactivation: agonist unbinding from the GPCR and GTP hydrolysis by the G protein alpha subunit. With some combinations of agonist/GPCR, the rate of agonist unbinding is slow and rate-limiting, and deactivation kinetics are not modulated by regulators of G protein-signaling proteins. In another group, channel deactivation is generally faster and limited by the hydrolysis rate of the G protein alpha subunit. G protein isoform and interaction with G protein-signaling proteins play a significant role with this group of GPCRs.

Heterotrimer formation, together with isoprenylation, is required for plasma membrane targeting of Gbetagamma

Nascent beta and gamma subunits of heterotrimeric G proteins need to be targeted to the cytoplasmic face of the plasma membrane (PM) in order to transmit signals. We show that beta(1)gamma(2) is poorly targeted to the PM and predominantly localized to endoplasmic reticulum (ER) membranes when expressed in HEK293 cells, but co-expression of a G protein alpha subunit allows strong PM localization of the beta(1)gamma(2). Furthermore, C-terminal isoprenylation of the gamma subunit is necessary but not sufficient for PM localization of beta(1)gamma(2). Isoprenylation of gamma(2) and localization of beta(1)gamma(2) to the ER occurs independently of alpha expression. Efficient PM localization of beta(1)gamma(2) in the absence of co-expressed alpha is observed when a site for palmitoylation, a putative second membrane targeting signal, is introduced into gamma(2). When a mutant of alpha(s) is targeted to mitochondria, beta(1)gamma(2) follows, consistent with an important role for alpha in promoting subcellular localization of betagamma. Furthermore, we directly demonstrate the requirement for alpha by showing that disruption of heterotrimer formation by the introduction of alpha binding mutations into beta(1) impedes PM targeting of beta(1)gamma(2). The results indicate that two membrane targeting signals, lipid modification and alpha binding, make concerted contributions to PM localization of betagamma.

Interaction of Gbeta3s, a splice variant of the G-protein Gbeta3, with Ggamma- and Galpha-proteins

The T-allele of a polymorphism (C825T) in the gene of the G-protein beta3-subunit is associated with a complex phenotype (hypertension, obesity, altered drug responses) and the occurrence of a splice variant termed Gbeta3s which lacks one of the seven WD-domains that compose Gbeta-proteins. Here, we analysed Gbetagamma dimer formation and Galpha activation by Gbeta3s, key functional characteristics of Gbeta-proteins. Cleavage protection assays frequently used to analyse Gbeta1gamma and Gbeta2gamma dimer formation failed for Gbeta3 and Gbeta3s, while in coprecipitation assays, dimerization of Gbeta3 and Gbeta3s with Ggamma5, Ggamma8(c) and Ggamma12 could be demonstrated. Upon expression of Gbeta3s in COS-7 and Sf9 insect cells, binding of GTPgammaS to Galpha-proteins induced by mastoparan-7 and the M(2) muscarinic acetylcholine receptor was facilitated in comparison with cells overexpressing wildtype Gbeta3, as indicated by twofold reduced agonist EC(50) values. Together, these results indicate that Gbeta3s is a biologically active Gbeta-protein that may mediate the enhanced signal transduction observed in cells with the 825T-allele.

A splice variant of the G protein beta 3-subunit implicated in disease states does not modulate ion channels

A single-nucleotide polymorphism (C825T) in the GNB3 gene produces an alternative splice variant of the heterotrimeric G protein beta3 subunit (Gbeta3). Translation of the alternatively spliced mRNA results in a protein product, Gbeta3-s, in which 41 amino acids are deleted from Gbeta3. Interestingly, previous studies indicate that the C825T allele occurs with a high frequency in patients with certain vascular disorders. However, little information is available regarding the functional role Gbeta3-s might play in ion channel modulation. To examine this aspect, Gbeta3 or Gbeta3-s, along with either Ggamma2 or Ggamma5, were expressed in rat sympathetic neurons by nuclear microinjection of vector encoding the desired protein. In contrast to Gbeta3, expression of Gbeta3-s did not modulate N-type Ca(2+) or G protein-gated inwardly rectifying K(+) channels. In addition, Gbeta3-s did not appear to complex with a pertussis toxin-insensitive mutant of Galpha(i2) or couple to natively expressed alpha(2)-adrenergic receptors. Finally, fluorescence resonance energy transfer (FRET) measurements indicated that enhanced yellow fluorescent protein (EYFP)-labeled Gbeta3-s does not form a Gbetagamma heterodimer when coexpressed with enhanced cyan fluorescent protein (ECFP)-labeled Ggamma2. Therefore, when expressed in sympathetic neurons, Gbeta3-s appears to lack biological activity--hence pathological conditions in patients carrying the homozygous C825T allele may result from a functional knockout of Gbeta3.

Rhodopsin controls a conformational switch on the transducin gamma subunit

Rhodopsin, a prototypical G protein-coupled receptor, catalyzes the activation of a heterotrimeric G protein, transducin, to initiate a visual signaling cascade in photoreceptor cells. The betagamma subunit complex, especially the C-terminal domain of the transducin gamma subunit, Gtgamma(60-71)farnesyl, plays a pivotal role in allosteric regulation of nucleotide exchange on the transducin alpha subunit by light-activated rhodopsin. We report that this domain is unstructured in the presence of an inactive receptor but forms an amphipathic helix upon rhodopsin activation. A K65E/E66K charge reversal mutant of the gamma subunit has diminished interactions with the receptor and fails to adopt the helical conformation. The identification of this conformational switch provides a mechanism for active GPCR utilization of the betagamma complex in signal transfer to G proteins.

Identification and characterization of G beta 3s2, a novel splice variant of the G-protein beta 3 subunit

The T-allele of a polymorphism (C825T) in the gene for the G-protein beta 3 subunit (GNB3) is associated with cardiovascular and metabolic disorders, distinct cellular features and altered drug responses. The molecular mechanisms that give rise to this complex phenotype have been linked to the occurrence of G beta 3s, a splice variant of GNB3. G beta 3s is predominantly expressed in cells with the 825T-allele. In the present study we describe the identification and characterization of an additional G beta 3 splice variant referred to as G beta 3s2. Its mRNA is expressed in heart, blood cells and tumour tissue, and its expression is also tightly associated with the GNB3 825T-allele. G beta 3s2 is generated by alternative splicing using non-canonical splice sites. G beta subunits belong to the family of propeller proteins and consist of seven regular propeller blades. Transcripts for G beta 3s2 are lacking 129 bp of the coding sequence of the wild-type G beta 3 protein. Thus the predicted structure consists of only six propeller blades, which resembles the structure of G beta 3s. Co-immunoprecipitation analyses indicated that G beta 3s2 dimerizes with different G gamma subunits, e.g. G gamma 5, G gamma 8(C) and G gamma 12. In Sf9 insect cells, expression of G beta 3s2 together with G gamma 12 enhances receptor-stimulated activation of G alpha(i2). Expression of G beta 3s2 in mammalian cells activated the mitogen-activated protein kinase cascade. Together, these results suggest that G beta 3s2 is a biologically active G beta variant which may play a role in the manifestation of the complex phenotype associated with the 825T-allele.

Activation of G-protein Galpha subunits by receptors through Galpha-Gbeta and Galpha-Ggamma interactions

Activation of the Galpha subunit of heterotrimeric GTP-binding proteins by transmembrane receptors requires the propagation of structural signals from the receptor-binding site to the nucleotide-binding site at the opposite side of the protein. In a previous model, it was suggested that the Gbeta-Ggamma dimer is tilted away from Galpha by a lever-arm motion of the Galpha N-terminal helix. Here, we propose that the motion occurs in the opposite direction, close-packing the Galpha-Gbeta interface and creating a novel interface between the helical domain of Galpha and the N terminus of Ggamma, which determines the specificity of activation.

The betagamma subunit of heterotrimeric G proteins interacts with RACK1 and two other WD repeat proteins

A yeast two-hybrid approach was used to discern possible new effectors for the betagamma subunit of heterotrimeric G proteins. Three of the clones isolated are structurally similar to Gbeta, each exhibiting the WD40 repeat motif. Two of these proteins, the receptor for activated C kinase 1 (RACK1) and the dynein intermediate chain, co-immunoprecipitate with Gbetagamma using an anti-Gbeta antibody. The third protein, AAH20044, has no known function; however, sequence analysis indicates that it is a WD40 repeat protein. Further investigation with RACK1 shows that it not only interacts with Gbeta(1)gamma(1) but also unexpectedly with the transducin heterotrimer Galpha(t)beta(1)gamma(1). Galpha(t) alone does not interact, but it must contribute to the interaction because the apparent EC(50) value of RACK1 for Galpha(t)beta(1)gamma(1) is 3-fold greater than that for Gbeta(1)gamma(1) (0.1 versus 0.3 microm). RACK1 is a scaffold that interacts with several proteins, among which are activated betaIIPKC and dynamin-1 (1). betaIIPKC and dynamin-1 compete with Gbeta(1)gamma(1) and Galpha(t)beta(1)gamma(1) for interaction with RACK1. These findings have several implications: 1) that WD40 repeat proteins may interact with each other; 2) that Gbetagamma interacts differently with RACK1 than with its other known effectors; and/or 3) that the G protein-RACK1 complex may constitute a signaling scaffold important for intracellular responses.

Role of dynamic interactions in effective signal transfer for Gbeta stimulation of phospholipase C-beta 2

Heterotrimeric G protein subunits regulate their effectors by protein-protein interactions. The regions involved in these direct interactions have either signal transfer or general binding functions (Buck, E., Li, J., Chen, Y., Weng, G., Scarlata, S., and Iyengar, R. (1999) Science 283, 1332-1335). Although key determinants of signal transfer regions for G protein subunits have been identified, the mechanisms of signal transfer are not fully understood. We have used a combinatorial peptide approach to analyze one Gbeta region, Gbeta86-105, involved in signal transfer to the effector phospholipase C (PLC)-beta2 to gain a more mechanistic understanding of Gbeta/PLC-beta2 signaling. Binding and functional studies with the combinatorial peptides on interaction with and stimulation/inhibition of phospholipase Cbeta2 indicate that binding affinity can be resolved from EC(50) for functional effects, such that peptides that have wild type binding affinities have 15- to 20-fold lower EC(50) values. Although more potent, these peptides display a much lower extent of maximal stimulation. These peptides synergize with Gbetagamma or peptides encoding the second Gbeta42-54 signal transfer region in maximally stimulating phospholipase C-beta2. Other combinatorial peptides from the Gbeta86-105 region that bind to PLC-beta2 by themselves submaximally stimulate and extensively inhibit Gbetagamma stimulation of PLC-beta2. The intrinsic stimulation function can be attributed to Arg-96 and Ser-97, the synergy function to Trp-99, and the binding affinity to Thr-87, Val-90, Pro-94, Arg-96, Ser-97, and Val-100. These results indicate that, even within signal transfer regions, residues involved in binding can be resolved from those involved in signal transfer and that signal transfer is likely to be achieved through dynamic rather than steady-state interactions.

Regulation of angiotensin II-induced G protein signaling by phosducin-like protein

Phosducin-like protein (PhLP) is a broadly expressed member of the phosducin (Pd) family of G protein betagamma subunit (Gbetagamma)-binding proteins. Though PhLP has been shown to bind Gbetagamma in vitro, little is known about its physiological function. In the present study, the effect of PhLP on angiotensin II (Ang II) signaling was measured in Chinese hamster ovary cells expressing the type 1 Ang II receptor and various amounts of PhLP. Up to 3.6-fold overexpression of PhLP had no effect on Ang II-stimulated inositol trisphosphate (IP(3)) formation, whereas further increases caused an abrupt decrease in IP(3) production with half-maximal inhibition occurring at 6-fold PhLP overexpression. This threshold level for inhibition corresponds to the cellular concentration of cytosolic chaperonin complex, a recently described binding partner that preferentially binds PhLP over Gbetagamma. Results of pertussis toxin sensitivity, GTPgammaS binding, and immunoprecipitation experiments suggest that PhLP inhibits phospholipase Cbeta activation by dual mechanisms: (i) steric blockage of Gbetagamma activation of PLCbeta and (ii) interference with Gbetagamma-dependent cycling of G(q)alpha by the receptor. These results suggest that G protein signaling may be regulated through controlling the cellular concentration of free PhLP by inducing its expression or by regulating its binding to the chaperonin.

Thrombin receptors activate G(o) proteins in endothelial cells to regulate intracellular calcium and cell shape changes

Thrombin receptors couple to G(i/o), G(q), and G(12/13) proteins to regulate a variety of signal transduction pathways that underlie the physiological role of endothelial cells in wound healing or inflammation. Whereas the involvement of G(i), G(q), G(12), or G(13) proteins in thrombin signaling has been investigated extensively, the role of G(o) proteins has largely been ignored. To determine whether G(o) proteins could contribute to thrombin-mediated signaling in endothelial cells, we have developed minigenes that encode an 11-amino acid C-terminal peptide of G(o1) proteins. Previously, we have shown that use of the C-terminal minigenes can specifically block receptor activation of G protein families (). In this study, we demonstrate that G(o) proteins are present in human microvascular endothelial cells (HMECs). Moreover, we show that thrombin receptors can stimulate [(35)S]guanosine-5'-O-(3-thio)triphosphate binding to G(o) proteins when co-expressed in Sf9 membranes. The potential coupling of thrombin receptors to G(o) proteins was substantiated by transfection of the G(o1) minigene into HMECs, which led to a blockade of thrombin-stimulated release of [Ca(2+)](i) from intracellular stores. Transfection of the beta-adrenergic kinase C terminus blocked the [Ca(2+)](i) response to the same extent as with G(o1) minigene peptide, suggesting that this G(o)-mediated [Ca(2+)](i) transient was caused by Gbetagamma stimulation of PLCbeta. Transfection of a G(i1/2) minigene had no effect on thrombin-stimulated [Ca(2+)](i) signaling in HMEC, suggesting that Gbetagamma derived from G(o) but not G(i) could activate PLCbeta. The involvement of G(o) proteins on events downstream from calcium signaling was further evidenced by investigating the effect of G(o1) minigenes on thrombin-stimulated stress fiber formation and endothelial barrier permeability. Both of these effects were sensitive to pertussis toxin treatment and could be blocked by transfection of G(o1) minigenes but not G(i1/2) minigenes. We conclude that the G(o) proteins play a role in thrombin signaling distinct from G(i1/2) proteins, which are mediated through their Gbetagamma subunits and involve coupling to calcium signaling and cytoskeletal rearrangements.

Astrocyte mGlu(2/3)-mediated cAMP potentiation is calcium sensitive: studies in murine neuronal and astrocyte cultures

Signal transduction mechanisms of group II metabotropic glutamate receptors (mGlu(2/3)) remains a matter of some controversy, therefore we sought to gain new insights into its regulation by studying cAMP production in cultured neurons and astrocytes, and by examining inter-relationships of mGlu(2/3)-induced signalling with cellular calcium and various signalling cascades. mGlu(2/3) agonists 2R,4R-4-aminopyrrolidine-2,4-dicarboxylic acid (2R,4R-APDC) and (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268) inhibited 10 microM forskolin-stimulated production of cAMP in murine cortical neurons, striatal neurons and forebrain astrocytes in the absence of extracellular Ca(2+). These agonists potentiated cAMP production in the presence of 1.8 mM Ca(2+) in astrocytes only. This potentiation was dependent on the extracellular Ca(2+) concentration (0.001-10 mM) and inhibited by the mGlu(2/3) antagonist LY341495 (1 microM), adenosine deaminase (1 U/ml) and the adenosine A(2A) receptor antagonist ZM241385 (1 microM). Pre-incubation with the phospholipase C (PLC) inhibitor U73122 (10 microM), L-type Ca(2+)-channel blockers nifedipine (1 microM) and nimodipine (1 microM), the calmodulin kinase II (CaMKII) inhibitor KN-62 (10 microM) or pertussis toxin (100 ng/ml) inhibited this potentiation. In the absence of 1.8 mM Ca(2+), thapsigargin (1 microM) facilitated the potentiation of cAMP production. Measurement of the Ca(2+)-binding dye Fluo-3/AM showed that, compared to Ca(2+)-free conditions, thapsigargin and 1.8 mM Ca(2+) elevated [Ca(2+)](i) in astrocytes; the latter effect being prevented by L-type Ca(2+)-channel blockers. Potentiation of cAMP production was also demonstrated when astrocytes were stimulated with the beta-adrenoceptor agonist isoprenaline (10 microM) in the presence of 1.8 mM Ca(2+), but not with the adenosine agonist NECA (10 microM) or the group I mGlu receptor agonist DHPG (100 microM). BaCl(2) (1.8 mM) in place of Ca(2+) did not facilitate forskolin-stimulated mGlu(2/3)-potentiation of cAMP. In short, this study in astrocytes demonstrates that under physiological Ca(2+) and adenylate cyclase stimulation an elevation of cAMP production is achieved that is mediated by PLC/IP(3)- and CaMKII-dependent pathways and results in the release of endogenous adenosine which acts at G(s) protein-coupled A(2A) receptors. These findings provide new insights into mGlu(2/3) signalling in astrocytes versus neurons, and which could determine the functional phenotypy of astrocytes under physiological and pathological conditions.

Role of the G protein gamma subunit in beta gamma complex modulation of phospholipase Cbeta function

The G protein betagamma complex regulates a wide range of effectors, including the phospholipase C isozymes (PLCbetas). Different domains on the beta subunit are known to contact phospholipase Cbeta and affect its regulation. In contrast, the role of the gamma subunit in Gbetagamma modulation of PLCbeta function is not known. Results here show that the gamma subunit C-terminal domain is involved in mediating Gbetagamma interactions with phospholipase Cbeta. Mutations were introduced to alter the position of the post-translational prenyl modification at the C terminus of the gamma subunit with reference to the beta subunit. These mutants were appropriately post-translationally modified with the geranylgeranyl moiety. A deletion that shortened the C-terminal domain, insertions that extended this domain, and a point mutation, F59A, that disrupted the interaction of this domain with the beta subunit were all affected in their ability to activate PLCbeta to varying degrees. All mutants, however, interacted equally effectively with the G(o)alpha subunit. The results indicate that the G protein gamma subunit plays a direct role in the modulation of effector function by the betagamma complex.

G protein specificity: traffic direction required

This review focuses on the coupling specificity of the Galpha and Gbetagamma subunits of pertussis toxin (PTX)-sensitive G(i/o) proteins that mediate diverse signaling pathways, including regulation of ion channels and other effectors. Several lines of evidence indicate that specific combinations of G protein alpha, beta and gamma subunits are required for different receptors or receptor-effector networks, and that a higher degree of specificity for Galpha and Gbetagamma is observed in intact systems than reported in vitro. The structural determinants of receptor-G protein specificity remain incompletely understood, and involve receptor-G protein interaction domains, and perhaps other scaffolding processes. By identifying G protein specificity for individual receptor signaling pathways, ligands targeted to disrupt individual pathways of a given receptor could be developed.

Dancing with multiple partners

Transmembrane proteins, such as G protein-coupled receptors (GPCRs) and integrins, activate intracellular signaling pathways through interactions with downstream binding partners. Woodside discusses two examples in which GPCRs and integrins interact in a noncompeting manner with more than one partner. The specific GPCR described is the thrombin receptor, in experiments where G protein peptides selectively block signaling through a particular G protein that does not appear to inhibit coupling of the receptor to other G proteins. The second system described is the alphaIIbbeta3 integrin and its activation of the nonreceptor tyrosine kinase Syk. Syk appeared capable of interacting with both the integrin and intracellular domains of immune response receptors, because binding of Syk to the integrin was not inhibited by peptides based on the Syk binding site in immune response receptors. Thus, multiple, noncompeting binding partners add to the complexity of signal transduction outputs from a single receptor complex.

Gbeta residues that do not interact with Galpha underlie agonist-independent activity of K+ channels

Gbetagamma subunits interact directly and activate G protein-gated Inwardly Rectifying K(+) (GIRK) channels. Little is known about the identity of functionally important interactions between Gbetagamma and GIRK channels. We tested the effects of all mammalian Gbeta subunits on channel activity and showed that whereas Gbeta1-4 subunits activate heteromeric GIRK channels independently of receptor activation, Gbeta5 does not. Gbeta1 and Gbeta5 both bind the N and C termini of the GIRK1 and GIRK4 channel subunits. Chimeric analysis between the Gbeta1 and Gbeta5 proteins revealed a 90-amino acid stretch that spans blades two and three of the seven-propeller structure and is required for channel activation. Within this region, eight non-conserved amino acids were critical for the activity of Gbeta1, as mutation of each residue to its counterpart in Gbeta5 significantly reduced the ability of Gbeta1 to stimulate channel activity. In particular, mutation of residues Ser-67 and Thr-128 to the corresponding Gbeta5 residues completely abolished Gbeta1 stimulation of GIRK channel activity. Mapping these functionally important residues on the three-dimensional structure of Gbeta1 shows that Ser-67, Ser-98, and Thr-128 are the only surface accessible residues. Galpha(i)1 interacts with Ser-98 but not with Ser-67 and Thr-128 in the heterotrimeric Galphabetagamma structure. Further characterization of the three mutant proteins showed that they fold properly and interact with Ggamma2. Of the three identified functionally important residues, the Ser-67 and Thr-128 Gbeta mutants significantly inhibited basal currents of a channel point mutant that displays Gbetagamma-mediated basal but not agonist-induced currents. Our findings indicate that the presence of Gbeta residues that do not interact with Galpha are involved in Gbetagamma interactions in the absence of agonist stimulation.

Cloning, tissue distribution, and functional expression of the human G protein beta 4-subunit

Heterotrimeric G proteins (Galphabetagamma) play an essential role in coupling membrane receptors to effector proteins such as ion channels and enzymes. Among the five mammalian Gbeta-subunits cloned, the human G protein beta4 has not been described. The purpose of the present study was to functionally characterize the newly identified human Gbeta4 subunit. The Gbeta4 open reading frame (ORF) was amplified utilizing PCR from brain cDNA. Amplification primers were generated following 5' rapid amplification of cDNA ends (5'-RACE) from an expressed sequence tag (EST) containing the predicted 3' end of the protein. Multiple tissue cDNA panel analysis showed that Gbeta4 mRNA was strongly expressed in lung and placenta, whereas it is weakly expressed in brain and heart. Heterologous overexpression of Gbeta4gamma2 or Gbeta4gamma4 in rat sympathetic neurons resulted in tonic modulation of N-type voltage-gated Ca(2+) and G protein-gated inwardly rectifying K(+) currents. Furthermore, coexpression of Gbeta4gamma2 and Galpha(oA) resulted in heterotrimer formation. These results show that the newly cloned Gbeta subunit shares several properties with other human Gbeta family members.

Opposing changes of trimeric G protein levels during ontogenetic development of rat brain

Developmental changes in the distribution of guanine nucleotide-binding regulatory proteins (G proteins) were investigated in the rat brain during postnatal development. Using a standard or high-resolution urea-SDS-PAGE and specific polyclonal antipeptide antibodies oriented against G(i)alpha1/G(i)alpha2, G(i)alpha3, G(s)alpha, G(o)alpha1/G(o)alpha2, G(q)alpha/G(11)alpha and Gbeta subunit, all these proteins were determined by quantitative immunoblotting in homogenates prepared from cortex, thalamus, hippocampus and pituitary of 1-, 7-, 12-, 18-, 25- and 90-day-old animals. The levels of the majority of G protein alpha subunits, namely G(i)alpha1, G(i)alpha2, G(i)alpha3, G(o)alpha1, G(o)alpha2, G(q)alpha, G(11)alpha and Gbeta, were high already at birth. Whereas the short variant of G(s)alpha, G(s)alphaS, rose sharply in all tested brain regions between postnatal day (PD) 1 and 90, the long variant of G(s)alpha, G(s)alphaL, was unchanged in cortex and thalamus and slightly increased in hippocampus. An increase was observed also in expression of G(i)alpha1/G(i)alpha2 and G(o)alpha1 protein, while G(o)alpha2 remained constant. Minority protein G(o)alpha* dramatically increased in cortex and thalamus, was unchanged in hippocampus and not detectable in pituitary. By contrast, the highest levels of G(i)alpha3 and G(q)alpha/G(11)alpha were detected as early as at PD 1. During the next 90 days, the immunological signal of G(i)alpha3 almost disappeared and G(q)alpha/G(11)alpha continuously declined to the levels corresponding to 50% of the levels determined at birth. Expression of Gbeta subunit was basically unchanged during postnatal development. Our present analysis indicates that G(s)alpha, G(i)alpha/G(o)alpha and G(q)alpha/G(11)alpha proteins are differently expressed in the course of brain development. Differential expression of the individual alpha subunits of trimeric G proteins during postnatal development suggests their different roles in maturation of the brain tissue.

Functional expression and FRET analysis of green fluorescent proteins fused to G-protein subunits in rat sympathetic neurons

1. cDNA constructs coding for a yellow-emitting green fluorescent protein (GFP) mutant fused to the N-terminus of the G-protein subunit beta 1 (YFP-beta 1) and a cyan-emitting GFP mutant fused to the N-terminus of the G-protein subunit gamma 2 (CFP-gamma 2) were heterologously expressed in rat superior cervical ganglion (SCG) neurons following intranuclear injection of the tagged subunits. The ability of the tagged subunits to modulate effectors, form a heterotrimer and couple to receptors was characterized using the whole-cell patch-clamp technique. Fluorescent resonance energy transfer (FRET) was also measured to determine the protein-protein interaction between the two fusion proteins. 2. Similar to co-expression of untagged beta 1/gamma 2, co-expression of YFP-beta 1/gamma 2, beta 1/CFP-gamma 2, or YFP-beta 1/CFP-gamma 2 resulted in a significant increase in basal N-type Ca(2+) channel facilitation when compared to uninjected neurons. Furthermore, the noradrenaline (NA)-mediated inhibition of Ca(2+) channels was significantly attenuated. 3. Co-expression of YFP-beta 1/CFP-gamma 2 with G-protein-gated inwardly rectifying K(+) channels (GIRK1 and GIRK4) resulted in tonic GIRK currents that were blocked by Ba(2+). 4. The ability of the tagged subunits to form heterotrimers was tested by co-injecting either tagged or untagged G beta 1 and G gamma 2 with excess G alpha(oA) cDNA. Under these conditions, the NA-mediated Ca(2+) current inhibition was significantly decreased when compared to uninjected neurons. 5. Coupling to the alpha 2-adrenergic receptor was reconstituted in neurons expressing pertussis toxin (PTX)-insensitive G alpha(oA) and either tagged or untagged G beta 1 gamma 2 subunits. Application of NA to PTX-treated cells resulted in a voltage-dependent inhibition of N-type Ca(2+) currents. 6. FRET measurements in the SCG revealed an in vivo interaction between YFP-beta 1 and CFP-gamma 2. Co-expression of untagged beta 1 significantly decreased the interaction between the two fusion proteins. 7. In summary, the attachment of GFP mutants to the N-terminus of G beta 1 or G gamma 2 does not qualitatively impair their ability to form a heterotrimer, modulate effectors (N-type Ca(2+) and GIRK channels), or couple to receptors.

The role of electrostatic interactions in the regulation of the membrane association of G protein beta gamma heterodimers

In this paper we report calculations of electrostatic interactions between the transducin (G(t)) betagamma heterodimer (G(t)betagamma) and phospholipid membranes. Although membrane association of G(t)betagamma is due primarily to the hydrophobic penetration into the membrane interior of a farnesyl chain attached to the gamma subunit, structural studies have revealed that there is a prominent patch of basic residues on the surface of the beta subunit surrounding the site of farnesylation that is exposed upon dissociation from the G(t)alpha subunit. Moreover, phosducin, which produces dissociation of G(t)betagamma from membranes, interacts directly with G(t)betagamma and introduces a cluster of acidic residues into this region. The calculations, which are based on the finite difference Poisson-Boltzmann method, account for a number of experimental observations and suggest that charged residues play a role in mediating protein-membrane interactions. Specifically, the calculations predict the following. 1) Favorable electrostatic interactions enhance the membrane partitioning due to the farnesyl group by an order of magnitude although G(t)betagamma has a large net negative charge (-12). 2) This electrostatic attraction positions G(t)betagamma so that residues implicated in mediating the interaction of G(t)betagamma with its membrane-bound effectors are close to the membrane surface. 3) The binding of phosducin to G(t)betagamma diminishes the membrane partitioning of G(t)betagamma by an order of magnitude. 4) Lowering the ionic strength of the solution converts the electrostatic attraction into a repulsion. Sequence analysis and homology model building suggest that our conclusions may be generalized to other Gbetagamma and phosducin isoforms as well.

Chemical modification of transducin with iodoacetic acid: transducin-alpha carboxymethylated at Cys(347) allows transducin binding to Light-activated rhodopsin but prevents its release in the presence of GTP

Modification of transducin (T) with iodoacetic acid (IAA) inhibited its light-dependent guanine nucleotide-binding activity. Approximately 1 mol of [(3)H]IAA was incorporated per mole of T. Cys(347), located on the alpha-subunit of T (T(alpha)), was identified as the major labeled residue in the [(3)H]IAA-modified holoenzyme. In contrast, Cys(135) and Cys(347) were modified with [(3)H]IAA in the isolated T(alpha). IAA-modified T was able to bind tightly to photoexcited rhodopsin (R*), but GTP did not promote the dissociation of the complex between alkylated T and R*. In addition, R* protected against the inhibition of T by IAA. A comparable inactivation of T and analogous interactions between T and R* were observed when 2-nitro 5-thiocyanobenzoic acid (NTCBA) was used as the modifying reagent (J. O. Ortiz and J. Bubis, 2001, Effects of differential sulfhydryl group-specific labeling on the rhodopsin and guanine nucleotide binding activities of transducin, Arch. Biochem. Biophys. 387, 233-242). However, while carboxymethylated T was capable of liberating GDP in the presence of R*, NTCBA-modified T was unable to release the guanine nucleotide diphosphate upon incubation with the photoactivated receptor. Thus, IAA-labeling stabilized a T:R* complex intermediate carrying the empty nucleotide pocket conformation of T. On the other hand, NTCBA-modified T seemed to be "locked" in the GDP-bound state of T, even in the presence of R*.

Beta-adrenergic signaling in the heart: dual coupling of the beta2-adrenergic receptor to G(s) and G(i) proteins

Beta-adrenergic receptor (AR) subtypes are archetypical members of the G protein-coupled receptor (GPCR) superfamily. Whereas both beta1AR and beta2AR stimulate the classic G(s)-adenylyl cyclase-3',5'-adenosine monophosphate (cAMP)-protein kinase A (PKA) signaling cascade, beta2AR couples to both G(s) and G(i) proteins, activating bifurcated signaling pathways. In the heart, dual coupling of the beta2AR to G(s) and G(i) results in compartmentalization of the G(s)-stimulated cAMP signal, thus selectively affecting plasma membrane effectors (such as L-type Ca(2+) channels) and bypassing cytoplasmic target proteins (such as phospholamban and myofilament contractile proteins). More important, the beta2AR-to-G(i) branch delivers a powerful cell survival signal that counters apoptosis induced by the concurrent G(s)-mediated signal or by a wide range of assaulting factors. This survival pathway sequentially involves G(i), G(beta)(gamma), phosphoinositide 3-kinase, and Akt. Furthermore, cardiac-specific transgenic overexpression of betaAR subtypes in mice results in distinctly different phenotypes in terms of the likelihood of cardiac hypertrophy and heart failure. These findings indicate that stimulation of the two betaAR subtypes activates overlapping, but different, sets of signal transduction mechanisms, and fulfills distinct or even opposing physiological and pathophysiological roles. Because of these differences, selective activation of cardiac beta2AR may provide catecholamine-dependent inotropic support without cardiotoxic consequences, which might have beneficial effects in the failing heart.

Ggamma in dictyostelium: its role in localization of gbetagamma to the membrane is required for chemotaxis in shallow gradients

G-protein-mediated signal transduction pathways play an essential role in the developmental program of the simple eukaryotic organism Dictyostelium discoideum. Database searches have yielded 11 Galpha-subunits, a single Gbeta-subunit, but no Ggamma-subunits. We report here the purification, cDNA isolation, and functional analysis of a Ggamma-subunit. Like Gbeta, the Ggamma appears to be unique and hybridization studies show that Ggamma and Gbeta are expressed in parallel during development. Species-wide sequence comparisons of Ggamma-subunits and gamma-like domains of RGS proteins reveal short stretches of highly conserved residues as well as the common CXXL motif at the COOH-terminal of Ggammas that target Gbetagammas to plasma membrane. Overexpression of a CSVL-deleted Ggamma (GgammaDelta) in wild-type cells shifts Gbetagamma to the cytosol and selectively impairs certain G-protein-mediated signal transduction pathways. These cells are able to respond to increments in the stimulus, but are unable to sense chemoattractant gradients. They neither move directionally nor recruit PH-domains to their leading edge. Thus, a full complement of membrane-tethered Gbetagamma is required for sensing shallow gradients, but is not essential for responses to increments in extracellular stimuli.

Modular design of Gbeta as the basis for reversible specificity in effector stimulation

The G protein Gbetagamma subunit complex stimulates effectors by direct interactions utilizing extensive Gbeta regions over the surface of its propeller structure that faces the Galpha subunit. Our previous experiments have shown the resolved functions of signal transfer and general binding for Gbeta regions involved in stimulation of the effector phospholipase C-beta2, PLC-beta2, within the region Gbeta-(86-135), which comprises three beta strands arranged in a structurally contiguous fashion (Buck, E., Li, J., Chen, Y., Weng, G., Sacarlata, S., and Iyengar, R. (1999) Science 283, 1332-1335). This raises an important question as to why mutagenesis studies indicate that an extensive set of sites all over the Gbeta propeller structure and outside the 86-135 region are involved in Gbeta regulation of PLC-beta2. Using peptides to define functions of these Gbeta regions, we find that Gbeta signaling to PLC-beta2 relies on a collection of modular signal transfer and general binding units, each with lower apparent affinity relative to Gbetagamma-PLC interactions. Gbeta-(42-54) functions as a signal transfer region, Gbeta-(228-249) and Gbeta-(321-340) function in general binding, and Gbeta-(64-84) and Gbeta-(300-313) seem to play a structural role rather than a direct contact with the effector. A substitution within the Gbeta-(42-54) signal transfer region that increases the K(act) of this peptide for PLC-beta2 is accompanied by an increase in the observed maximal extent of signal transfer. We conclude that the lower K(act) for individual signal transfer regions may result in a decrease in the maximal effect of signal transfer. The spatial resolution of the signal transfer and general binding regions over a wide surface of Gbeta allow geometrical constraints to achieve specificity even with relatively low affinity interactions.

Binding of G alpha(o) N terminus is responsible for the voltage-resistant inhibition of alpha(1A) (P/Q-type, Ca(v)2.1) Ca(2+) channels

G-protein-mediated inhibition of presynaptic voltage-dependent Ca(2+) channels is comprised of voltage-dependent and -resistant components. The former is caused by a direct interaction of Ca(2+) channel alpha(1) subunits with G beta gamma, whereas the latter has not been characterized well. Here, we show that the N terminus of G alpha(o) is critical for the interaction with the C terminus of the alpha(1A) channel subunit, and that the binding induces the voltage-resistant inhibition. An alpha(1A) C-terminal peptide, an antiserum raised against G alpha(o) N terminus, and a G alpha(o) N-terminal peptide all attenuated the voltage-resistant inhibition of alpha(1A) currents. Furthermore, the N terminus of G alpha(o) bound to the C terminus of alpha(1A) in vitro, which was prevented either by the alpha(1A) channel C-terminal or G alpha(o) N-terminal peptide. Although the C-terminal domain of the alpha(1B) channel showed similar ability in the binding with G alpha(o) N terminus, the above mentioned treatments were ineffective in the alpha(1B) channel current. These findings demonstrate that the voltage-resistant inhibition of the P/Q-type, alpha(1A) channel is caused by the interaction between the C-terminal domain of Ca(2+) channel alpha(1A) subunit and the N-terminal region of G alpha(o).

RGS12 and RGS14 GoLoco motifs are G alpha(i) interaction sites with guanine nucleotide dissociation inhibitor Activity

The regulators of G-protein signaling (RGS) proteins accelerate the intrinsic guanosine triphosphatase activity of heterotrimeric G-protein alpha subunits and are thus recognized as key modulators of G-protein-coupled receptor signaling. RGS12 and RGS14 contain not only the hallmark RGS box responsible for GTPase-accelerating activity but also a single G alpha(i/o)-Loco (GoLoco) motif predicted to represent a second G alpha interaction site. Here, we describe functional characterization of the GoLoco motif regions of RGS12 and RGS14. Both regions interact exclusively with G alpha(i1), G alpha(i2), and G alpha(i3) in their GDP-bound forms. In GTP gamma S binding assays, both regions exhibit guanine nucleotide dissociation inhibitor (GDI) activity, inhibiting the rate of exchange of GDP for GTP by G alpha(i1). Both regions also stabilize G alpha(i1) in its GDP-bound form, inhibiting the increase in intrinsic tryptophan fluorescence stimulated by AlF(4)(-). Our results indicate that both RGS12 and RGS14 harbor two distinctly different G alpha interaction sites: a previously recognized N-terminal RGS box possessing G alpha(i/o) GAP activity and a C-terminal GoLoco region exhibiting G alpha(i) GDI activity. The presence of two, independent G alpha interaction sites suggests that RGS12 and RGS14 participate in a complex coordination of G-protein signaling beyond simple G alpha GAP activity.

Identification of a surface on the beta-propeller protein RACK1 that interacts with the cAMP-specific phosphodiesterase PDE4D5

A strategy of mutagenesis followed by yeast two-hybrid assay was used to determine the sites on the WD-repeat protein Receptor for Activated C Kinase 1 (RACK1) necessary for it to interact with the cAMP-specific phosphodiesterase isoform PDE4D5. Analysis of deletion mutations demonstrated that WD-repeats 5-7, inclusively, of RACK1 contained the major site for interaction with PDE4D5. A reverse two-hybrid screen focusing on WD-repeats 5-7 of RACK1 isolated 11 single amino acid mutations from within this region that blocked the interaction. The ability of these mutations to block the interaction was confirmed by "pull-down" assays using bacterially expressed glutathione-S-transferase (GST)-RACK1 and mammalian cell-expressed PDE4D5. A model of RACK1 structure, based on the structural similarity of RACK1 to other beta-propeller WD-repeat proteins, indicated that the majority of the amino acids identified by mutagenesis are clustered in a discrete surface of RACK1. We propose that this surface of RACK1 is the major site for its interaction with the unique amino-terminal region of PDE4D5.

Parallel regulation of mitogen-activated protein kinase kinase 3 (MKK3) and MKK6 in Gq-signaling cascade

Heterotrimeric G protein G(q) stimulates the activity of p38 mitogen-activated protein kinase (MAPK) in mammalian cells. To investigate the signaling mechanism whereby alpha and betagamma subunits of G(q) activate p38 MAPK, we introduced kinase-deficient mutants of mitogen-activated protein kinase kinase 3 (MKK3), MKK4, and MKK6 into human embryonal kidney 293 cells. The activation of p38 MAPK by Galpha(q) and Gbetagamma was blocked by kinase-deficient MKK3 and MKK6 but not by kinase-deficient MKK4. In addition, Galpha(q) and Gbetagamma stimulated MKK3 and MKK6 activities. The MKK3 and MKK6 activations by Galpha(q), but not by Gbetagamma, were dependent on phospholipase C and c-Src. Galpha(q) stimulated MKK3 in a Rac- and Cdc42-dependent manner and MKK6 in a Rho-dependent manner. On the other hand, Gbetagamma activated MKK3 in a Rac- and Cdc42-dependent manner and MKK6 in a Rho-, Rac-, and Cdc42-dependent manner. Gbetagamma-induced MKK3 and MKK6 activations were dependent on a tyrosine kinase other than c-Src. These results suggest that Galpha(q) and Gbetagamma stimulate the activity of p38 MAPK by regulating MKK3 and MKK6 through parallel signaling pathways.

Modulation of the G protein regulator phosducin by Ca2+/calmodulin-dependent protein kinase II phosphorylation and 14-3-3 protein binding

Phototransduction is a canonical G protein-mediated cascade of retinal photoreceptor cells that transforms photons into neural responses. Phosducin (Pd) is a Gbetagamma-binding protein that is highly expressed in photoreceptors. Pd is phosphorylated in dark-adapted retina and is dephosphorylated in response to light. Dephosphorylated Pd binds Gbetagamma with high affinity and inhibits the interaction of Gbetagamma with Galpha or other effectors, whereas phosphorylated Pd does not. These results have led to the hypothesis that Pd down-regulates the light response. Consequently, it is important to understand the mechanisms of regulation of Pd phosphorylation. We have previously shown that phosphorylation of Pd by cAMP-dependent protein kinase moderately inhibits its association with Gbetagamma. In this study, we report that Pd was rapidly phosphorylated by Ca(2+)/calmodulin-dependent kinase II, resulting in 100-fold greater inhibition of Gbetagamma binding than cAMP-dependent protein kinase phosphorylation. Furthermore, Pd phosphorylation by Ca(2+)/calmodulin-dependent kinase II at Ser-54 and Ser-73 led to binding of the phosphoserine-binding protein 14-3-3. Importantly, in vivo decreases in Ca(2+) concentration blocked the interaction of Pd with 14-3-3, indicating that Ca(2+) controls the phosphorylation state of Ser-54 and Ser-73 in vivo. These results are consistent with a role for Pd in Ca(2+)-dependent light adaptation processes in photoreceptor cells and also suggest other possible physiological functions.

Regulator of G-protein signaling 3 (RGS3) inhibits Gbeta1gamma 2-induced inositol phosphate production, mitogen-activated protein kinase activation, and Akt activation

Regulator of G-protein signaling 3 (RGS3) enhances the intrinsic rate at which Galpha(i) and Galpha(q) hydrolyze GTP to GDP, thereby limiting the duration in which GTP-Galpha(i) and GTP-Galpha(q) can activate effectors. Since GDP-Galpha subunits rapidly combine with free Gbetagamma subunits to reform inactive heterotrimeric G-proteins, RGS3 and other RGS proteins may also reduce the amount of Gbetagamma subunits available for effector interactions. Although RGS6, RGS7, and RGS11 bind Gbeta(5) in the absence of a Ggamma subunit, RGS proteins are not known to directly influence Gbetagamma signaling. Here we show that RGS3 binds Gbeta(1)gamma(2) subunits and limits their ability to trigger the production of inositol phosphates and the activation of Akt and mitogen-activated protein kinase. Co-expression of RGS3 with Gbeta(1)gamma(2) inhibits Gbeta(1)gamma(2)-induced inositol phosphate production and Akt activation in COS-7 cells and mitogen-activated protein kinase activation in HEK 293 cells. The inhibition of Gbeta(1)gamma(2) signaling does not require an intact RGS domain but depends upon two regions in RGS3 located between acids 313 and 390 and between 391 and 458. Several other RGS proteins do not affect Gbeta(1)gamma(2) signaling in these assays. Consistent with the in vivo results, RGS3 inhibits Gbetagamma-mediated activation of phospholipase Cbeta in vitro. Thus, RGS3 may limit Gbetagamma signaling not only by virtue of its GTPase-activating protein activity for Galpha subunits, but also by directly interfering with the activation of effectors.

G Protein beta subunit types differentially interact with a muscarinic receptor but not adenylyl cyclase type II or phospholipase C-beta 2/3

In comparison with the alpha subunit of G proteins, the role of the beta subunit in signaling is less well understood. During the regulation of effectors by the betagamma complex, it is known that the beta subunit contacts effectors directly, whereas the role of the beta subunit is undefined in receptor-G protein interaction. Among the five G protein beta subunits known, the beta(4) subunit type is the least studied. We compared the ability of betagamma complexes containing beta(4) and the well characterized beta(1) to stimulate three different effectors: phospholipase C-beta2, phospholipase C-beta3, and adenylyl cyclase type II. beta(4)gamma(2) and beta(1)gamma(2) activated all three of these effectors with equal efficacy. However, nucleotide exchange in a G protein constituting alpha(o)beta(4)gamma(2) was stimulated significantly more by the M2 muscarinic receptor compared with alpha(o)beta(1)gamma(2). Because alpha(o) forms heterotrimers with beta(4)gamma(2) and beta(1)gamma(2) equally well, these results show that the beta subunit type plays a direct role in the receptor activation of a G protein.

Effects of differential sulfhydryl group-specific labeling on the rhodopsin and guanine nucleotide binding activities of transducin

The role of transducin sulfhydryl groups was examined by chemical modification with four different reagents: 4-acetamido-4'-maleimidyl-stilbene-2, 2' disulfonic acid (AMDA); 4-vinyl pyridine (VP); 2-nitro-5-thiocyano benzoic acid (NTCBA); and 2, 5-dimethoxystilbene-4'-maleimide (DM). All these compounds rapidly inhibited the [3H]GMPpNp-binding activity of transducin stimulated by photoexcited rhodopsin (R*). Sedimentation experiments showed that the labeling of transducin with AMDA or VP hindered its binding to R* while NTCBA-modified transducin was capable of interacting with the photoreceptor protein. In contrast, DM-labeled transducin precipitated even in the absence of R*. Photoactivated rhodopsin was capable of protecting against the observed AMDA and NTCBA inhibition in transducin function, but not against the inactivation caused by VP or DM. These results suggest the existence of different functional cysteines on transducin that are located in the proximity of the interaction site with the photoreceptor protein, near the guanine nucleotide binding site, or in regions involved in the structural changes taking place upon protein activation. With the use of these reagents, transducin appears to be "frozen" in various conformational stages of its cycle, providing conditions for studying two of the initial steps of the visual process: the light-dependent binding of transducin to rhodopsin and the transducin guanine nucleotide exchange reaction.

Ggamma-like (GGL) domains: new frontiers in G-protein signaling and beta-propeller scaffolding

The standard model of signal transduction from G-protein-coupled receptors (GPCRs) involves guanine nucleotide cycling by a heterotrimeric G-protein assembly composed of Galpha, Gbeta, and Ggamma subunits. The WD-repeat beta-propeller protein Gbeta and the alpha-helical, isoprenylated polypeptide Ggamma are considered obligate dimerization partners; moreover, conventional Gbetagamma heterodimers are considered essential to the functional coupling of Galpha subunits to receptors. However, our recent discovery of a Gbeta5 binding site (the Ggamma-like or "GGL" domain) within several regulators of G-protein signaling (RGS) proteins revealed the potential for functional GPCR/Galpha coupling in the absence of a conventional Ggamma subunit. In addition, we posit that the interaction between Gbeta5 isoforms and the GGL domains of RGS proteins represents a general mode of binding between beta-propeller proteins and their partners, extending beyond the realm of G-protein-linked signal transduction.

Mutant G protein alpha subunit activated by Gbeta gamma: a model for receptor activation?

How receptors catalyze exchange of GTP for GDP bound to the Galpha subunit of trimeric G proteins is not known. One proposal is that the receptor uses the G protein's betagamma heterodimer as a lever, tilting it to pull open the guanine nucleotide binding pocket of Galpha. To test this possibility, we designed a mutant Galpha that would bind to betagamma in the tilted conformation. To do so, we excised a helical turn (four residues) from the N-terminal region of alpha(s), the alpha subunit of G(S), the stimulatory regulator of adenylyl cyclase. In the presence, but not in the absence, of transiently expressed beta(1) and gamma(2), this mutant (alpha(s)Delta), markedly stimulated cAMP accumulation. This effect depended on the ability of the coexpressed beta protein to interact normally with the lip of the nucleotide binding pocket of alpha(s)Delta. We substituted alanine for an aspartate in beta(1) that binds to a lysine (K206) in the lip of the alpha subunit's nucleotide binding pocket. Coexpressed with alpha(s)Delta and gamma(2), this mutant, beta(1)-D228A, elevated cAMP much less than did beta(1)-wild type; it did bind to alpha(s)Delta normally, however, as indicated by its unimpaired ability to target alpha(s)Delta to the plasma membrane. We conclude that betagamma can activate alpha(s) and that this effect probably involves both a tilt of betagamma relative to alpha(s) and interaction of beta with the lip of the nucleotide binding pocket. We speculate that receptors use a similar mechanism to activate trimeric G proteins.

The G protein beta subunit is a determinant in the coupling of Gs to the beta 1-adrenergic and A2a adenosine receptors

The signaling specificity of five purified G protein betagamma dimers, beta(1)gamma(2), beta(2)gamma(2), beta(3)gamma(2), beta(4)gamma(2), and beta(5)gamma(2), was explored by reconstituting them with G(s) alpha and receptors or effectors in the adenylyl cyclase cascade. The ability of the five betagamma dimers to support receptor-alpha-betagamma interactions was examined using membranes expressing the beta(1)-adrenergic or A2a adenosine receptors. These receptors discriminated among the defined heterotrimers based solely on the beta isoform. The beta(4)gamma(2) dimer demonstrated the highest coupling efficiency to either receptor. The beta(5)gamma(2) dimer coupled poorly to each receptor, with EC(50) values 40-200-fold higher than those observed with beta(4)gamma(2). Strikingly, whereas the EC(50) of the beta(1)gamma(2) dimer at the beta(1)-adrenergic receptor was similar to beta(4)gamma(2), its EC(50) was 20-fold higher at the A2a adenosine receptor. Inhibition of adenylyl cyclase type I (AC1) and stimulation of type II (AC2) by the betagamma dimers were measured. betagamma dimers containing Gbeta(1-4) were able to stimulate AC2 similarly, and beta(5)gamma(2) was much less potent. beta(1)gamma(2), beta(2)gamma(2), and beta(4)gamma(2) inhibited AC1 equally; beta(3)gamma(2) was 10-fold less effective, and beta(5)gamma(2) had no effect. These data argue that the beta isoform in the betagamma dimer can determine the specificity of signaling at both receptors and effectors.

Insulin and insulin-like growth factor I receptors utilize different G protein signaling components

We examined the role of heterotrimeric G protein signaling components in insulin and insulin-like growth factor I (IGF-I) action. In HIRcB cells and in 3T3L1 adipocytes, treatment with the Galpha(i) inhibitor (pertussis toxin) or microinjection of the Gbetagamma inhibitor (glutathione S-transferase-betaARK) inhibited IGF-I and lysophosphatidic acid-stimulated mitogenesis but had no effect on epidermal growth factor (EGF) or insulin action. In basal state, Galpha(i) and Gbeta were associated with the IGF-I receptor (IGF-IR), and after ligand stimulation the association of IGF-IR with Galpha(i) increased concomitantly with a decrease in Gbeta association. No association of Galpha(i) was found with either the insulin or EGF receptor. Microinjection of anti-beta-arrestin-1 antibody specifically inhibited IGF-I mitogenic action but had no effect on EGF or insulin action. beta-Arrestin-1 was associated with the receptors for IGF-I, insulin, and EGF in a ligand-dependent manner. We demonstrated that Galpha(i), betagamma subunits, and beta-arrestin-1 all play a critical role in IGF-I mitogenic signaling. In contrast, neither metabolic, such as GLUT4 translocation, nor mitogenic signaling by insulin is dependent on these protein components. These results suggest that insulin receptors and IGF-IRs can function as G protein-coupled receptors and engage different G protein partners for downstream signaling.

G protein betagamma subunit-mediated presynaptic inhibition: regulation of exocytotic fusion downstream of Ca2+ entry

The nervous system can modulate neurotransmitter release by neurotransmitter activation of heterotrimeric GTP-binding protein (G protein)-coupled receptors. We found that microinjection of G protein betagamma subunits (Gbetagamma) mimics serotonin's inhibitory effect on neurotransmission. Release of free Gbetagamma was critical for this effect because a Gbetagamma scavenger blocked serotonin's effect. Gbetagamma had no effect on fast, action potential-evoked intracellular Ca2+ release that triggered neurotransmission. Inhibition of neurotransmitter release by serotonin was still seen after blockade of all classical Gbetagamma effector pathways. Thus, Gbetagamma blocked neurotransmitter release downstream of Ca2+ entry and may directly target the exocytotic fusion machinery at the presynaptic terminal.

Characterization of a phospholipase C beta 2-binding site near the amino-terminal coiled-coil of G protein beta gamma subunits

In previous work (Sankaran, B., Osterhout, J., Wu, D., and Smrcka, A. V. (1998) J. Biol. Chem. 273, 7148-7154), we showed that overlapping peptides, N20K (Asn(564)-Lys(583)) and E20K (Glu(574)-Lys(593)), from the catalytic domain of phospholipase C (PLC) beta2 block Gbetagamma-dependent activation of PLC beta2. The peptides could also be directly cross-linked to betagamma subunits with a heterobifunctional cross-linker succinimidyl 4-[N-maleimidomethyl]-cyclohexane-1-carboxylate. Cross-linking of peptides to Gbeta(1) was inhibited by PLC beta2 but not by alpha(i1)(GDP), indicating that the peptide-binding site on beta(1) represents a binding site for PLC beta2 that does not overlap with the alpha(i1)-binding site. Here we identify the site of peptide cross-linking and thereby define a site for PLC beta2 interaction with beta subunits. Each of the 14 cysteine residues in beta(1) were altered to alanine. The ability of the PLC beta2-derived peptide to cross-link to each betagamma mutant was then analyzed to identify the reactive sulfhydryl moiety on the beta subunit required for the cross-linking reaction. We find that C25A was the only mutation that significantly affected peptide cross-linking. This indicates that the peptide is specifically binding to a region near cysteine 25 of beta(1) which is located in the amino-terminal coiled-coil region of beta(1) and identifies a PLC-binding site distinct from the alpha subunit interaction site.

Membrane raft microdomains in chemokine receptor function

Cell chemotaxis requires the acquisition and maintenance of both spatial and functional asymmetry between initially equivalent cell parts. In leukocytes one becomes the leading edge and the other, the rear edge or uropod. The acquisition of this cell polarity is controlled by an array of chemoattractants, including those of the chemokine family. We propose that chemokine receptor activation in highly organized lipid raft domains is a major determinant for the correct localization of the signaling pathways leading to the cell asymmetries required for migration. The lateral organization imposed by membrane raft microdomains is discussed in the context of other chemokine receptor activities, such as its role as a human immunodeficiency virus (HIV) coreceptor.

Partial agonist clonidine mediates alpha(2)-AR subtypes specific regulation of cAMP accumulation in adenylyl cyclase II transfected DDT1-MF2 cells

alpha2-Adrenergic receptor (alpha(2)-AR) activation in the pregnant rat myometrium at midterm potentiates beta(2)-AR stimulation of adenylyl cyclase (AC) via Gbetagamma regulation of the type II isoform of adenylyl cyclase. However, at term, alpha(2)-AR activation inhibits beta(2)-AR stimulation of AC. This phenomenon is associated with changes in alpha(2)-AR subtype expression (midterm alpha(2A/D)-AR >> alpha(2B)-AR; term alpha(2B) >or =alpha(2A/D)-AR), without any change in ACII mRNA, suggesting that alpha(2A/D)- and alpha(2B)-AR differentially regulate beta(2)-cAMP production. To address this issue, we have stably expressed the same density of alpha(2A/D)- or alpha(2B)-AR with AC II in DDT1-MF2 cells. Clonidine (partial agonist) increased beta(2)-AR-stimulated cAMP production in alpha(2A/D)-AR-ACII transfectants but inhibited it in alpha(2B)-AR-ACII transfectants. In contrast, epinephrine (full agonist) enhanced beta(2)-stimulated ACII in both alpha(2A)- and alpha(2B)-ACII clonal cell lines. 4-Azidoanilido-[alpha-(32)P]GTP-labeling of activated G proteins indicated that, in alpha(2B)-AR transfectants, clonidine activated only Gi(2), whereas epinephrine, the full agonist, effectively coupled to Gi(2) and Gi(3). Thus, partial and full agonists selectively activate G proteins that lead to drug specific effects on effectors. Moreover, these data indicate that Gi(3) activation is required for potentiation of beta(2)-AR stimulation of AC by alpha(2A/D) and alpha(2B)-AR in DDT1-MF2 cells. This may reflect an issue of the amount of Gbetagamma released upon receptor activation and/or betagamma composition of Gi(3) versus Gi(2).