RGS8 accelerates G-protein-mediated modulation of K+ currents

Unique isoform of Galpha -interacting protein (RGS-GAIP) selectively discriminates between two Go-mediated pathways that inhibit Ca2+ channels

Regulators of G-protein signaling (RGS) proteins constitute a large family of GTPase-activating proteins for heterotrimeric G proteins. More than 20 RGS genes have been identified in mammals. One of these, the Galpha-interacting protein (GAIP), preferentially interacts with members of the G(i)/G(o) subfamily of G proteins in mammalian cells, but its selectivity among members of this subfamily in vitro is limited. Here we report the cloning and functional characterization of a unique cDNA isoform of GAIP, derived from embryonic chicken dorsal root ganglion neurons. Chick GAIP is composed of 199 amino acids, organized into a conserved RGS domain (85% identical to human GAIP), and a unique, short N terminus (only 41% identical, 50% homologous to known mammalian orthologues). Consistent with this unique primary structure, chick GAIP has physiological properties that distinguish it from mammalian GAIPs. We have explored the selectivity of chick GAIP in electrophysiological assays of two G(o)-mediated forms of Ca(2+) channel inhibition produced by gamma-aminobutyric acid in chick dorsal root ganglion neurons, voltage-independent inhibition (mediated by G(o)alpha) and voltage-dependent inhibition (mediated by G(o)betagamma). Dialyzing recombinant chick GAIP in these cells selectively reduced voltage-independent inhibition without affecting voltage-dependent inhibition. Mammalian GAIP, tested under identical conditions in previous studies, demonstrated no selectivity between these two inhibitory processes; thus, our results suggest that the functional specificity of chick GAIP is likely to be determined by its unique N terminus.

Interaction of 14-3-3 protein with regulator of G protein signaling 7 is dynamically regulated by tumor necrosis factor-alpha

Regulators of G protein signaling (RGS) constitute a family of proteins with a conserved RGS domain of approximately 120 amino acids that accelerate the intrinsic GTP hydrolysis of activated Galpha(i) and Galpha(q) subunits. The phosphorylation-dependent interaction of 14-3-3 proteins with a subset of RGS proteins inhibits their GTPase-accelerating activity in vitro. The inhibitory interaction between 14-3-3 and RGS7 requires phosphorylation of serine 434 of RGS7. We now show that phosphorylation of serine 434 is dynamically regulated by TNF-alpha. Cellular stimulation by TNF-alpha transiently decreased the phosphorylation of serine 434 of RGS7, abrogating the inhibitory interaction with 14-3-3. We examined the effect of 14-3-3 on RGS-mediated deactivation kinetics of G protein-coupled inwardly rectifying K(+) channels (GIRKs) in Xenopus oocytes. 14-3-3 inhibited the function of wild-type RGS7, but not that of either RSG7(P436R) or RGS4, two proteins that do not bind 14-3-3. Our findings are the first evidence that extracellular signals can modulate the activity of RGS proteins by regulating their interaction with 14-3-3.

Graded contribution of the Gbeta gamma binding domains to GIRK channel activation

G protein coupled inwardly rectifying K(+) channels (GIRK/Kir3.x) are mainly activated by a direct interaction with Gbetagamma subunits, released upon the activation of inhibitory neurotransmitter receptors. Although Gbetagamma binding domains on all four subunits have been found, the relative contribution of each of these binding sites to channel gating has not yet been defined. It is also not known whether GIRK channels open once all Gbetagamma sites are occupied, or whether gating is a graded process. We used a tandem tetrameric approach to enable the selective elimination of specific Gbetagamma binding domains in the tetrameric context. Here, we show that tandem tetramers are fully operational. Tetramers with only one wild-type channel subunit showed receptor-independent high constitutive activity. The presence of two or three wild-type subunits reconstituted receptor activation gradually. Furthermore, a tetramer with no GIRK1 Gbetagamma binding domain displayed slower kinetics of activation. The slowdown in activation was found to be independent of regulator of G protein signaling or receptor coupling, but this slowdown could be reversed once only one Gbetagamma binding domain of GIRK1 was added. These results suggest that partial activation can occur under low Gbetagamma occupancy and that full activation can be accomplished by the interaction with three Gbetagamma binding subunits.

Alternative splicing of RGS8 gene determines inhibitory function of receptor type-specific Gq signaling

The regulators of G protein signaling (RGS) proteins modulate heterotrimeric G protein signaling. RGS8 is a brain-specific RGS protein of 180 aa. Here we identified a short isoform of RGS8, RGS8S, that arises by alternative splicing. RGS8S cDNA encodes a N terminus of 7 aa instead of amino acids 1-9 of RGS8 and 10-180 of RGS8. The subcellular distribution of RGS8 and RGS8S did not differ significantly in transfected cells. RGS8S accelerated, not as efficiently as RGS8, the turning on and off of Gi/o-mediated modulation of G protein-gated inwardly rectifying K(+) channels in Xenopus oocytes. We next examined the effects of RGS8 and RGS8S on Gq-mediated signaling. RGS8 decreased the amplitude of the response upon activation of m1 muscarinic or substance P receptors, but did not remarkably inhibit signaling from m3 muscarinic receptors. In contrast, RGS8S showed much less inhibition of the response of either of these Gq-coupled receptors. By quantitative analysis of the inhibitory effect and the protein expression level, we confirmed that the difference of inhibitory effect is caused by both the qualitative difference between RGS8 and RGS8S and the quantitative difference of the protein expression level. We also confirmed that the receptor-type specificity of inhibition is not caused by the difference of the expression level of the receptors. In summary, we showed that 9 aa in the N terminus of RGS8 contribute to the function to inhibit Gq-coupled signaling in a receptor type-specific manner and that the regulatory function of RGS8S is especially diminished on Gq-coupled responses.

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

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

Regulators of G-protein signalling as new central nervous system drug targets

G-protein-coupled receptors (GPCRs) are major targets for drug discovery. The regulator of G-protein signalling (RGS)-protein family has important roles in GPCR signal transduction. RGS proteins contain a conserved RGS-box, which is often accompanied by other signalling regulatory elements. RGS proteins accelerate the deactivation of G proteins to reduce GPCR signalling; however, some also have an effector function and transmit signals. Combining GPCR agonists with RGS inhibitors should potentiate responses, and could markedly increase the agonist's regional specificity. The diversity of RGS proteins with highly localized and dynamically regulated distributions in brain makes them attractive targets for pharmacotherapy of central nervous system disorders.

Activation and deactivation kinetics of alpha 2A- and alpha 2C-adrenergic receptor-activated G protein-activated inwardly rectifying K+ channel currents

Although G protein-coupled receptor-mediated signaling is one of the best studied biological events, little is known about the kinetics of these processes in intact cells. Experiments with neurons from alpha(2A)-adrenergic receptor knockout mice suggested that the alpha(2A)-receptor subtype inhibits neurotransmitter release with higher speed and at higher action potential frequencies than the alpha(2C)-adrenergic receptor. Here we investigated whether these functional differences between presynaptic alpha(2)-adrenergic receptor subtypes are the result of distinct signal transduction kinetics of these two receptors and their coupling to G proteins. alpha(2A)- and alpha(2C)-receptors were stably expressed in HEK293 cells at moderate ( approximately 2 pmol/mg) or high (17-24 pmol/mg) levels. Activation of G protein-activated inwardly rectifying K(+) (GIRK) channels was similar in extent and kinetics for alpha(2A)- and alpha(2C)-receptors at both expression levels. However, the two receptors differed significantly in their deactivation kinetics after removal of the agonist norepinephrine. alpha(2C)-Receptor-activated GIRK currents returned much more slowly to base line than did alpha(2A)-stimulated currents. This observation correlated with a higher affinity of norepinephrine at the murine alpha(2C)- than at the alpha(2A)-receptor subtype and may explain why alpha(2C)-adrenergic receptors are especially suited to control sympathetic neurotransmission at low action potential frequencies in contrast to the alpha(2A)-receptor subtype.

Profile of RGS expression in single rat atrial myocytes

'Regulators of G protein signaling' (RGS proteins) are members of a large family of GTPase-activating proteins that are differentially expressed in various cell types and accelerate the termination of heterotrimeric G protein signaling. To identify RGS proteins that may affect autonomic regulation of atrial excitability, we screened the expression of nineteen RGS genes (RGS subfamilies A, B, C, and D) in single spontaneously beating rat atrial myocytes maintained in primary culture. Expression profiling by single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that seven distinct RGS genes are endogenously expressed in atrial myocytes which were also identified in poly(A)(+) mRNA from rat atria (RGS2, RGS3, RGS4, RGS6, RGS10, GAIP, and RGSZ2). Other RGS transcripts were detected in atrial poly(A)(+) mRNA but not single atrial myocytes (RGS5, RGS12, RGS16, and RGS18), and therefore are likely to originate from non-myocyte sources in atrial tissue. The single-cell RT-PCR experiments also led to the identification of putative splice variants for RGS6 and GAIP. Immunocytochemistry using RGS-specific antibodies confirmed the presence of selected RGS proteins in the cultured atrial myocytes. These results demonstrate a rich diversity of RGS expression in atrial myocytes whose specific role in G-protein signaling is yet to be determined. The identification of endogenous RGS proteins in atrial myocytes will facilitate targeted suppression and/or deletion studies to determine how each RGS protein may affect atrial excitability and its short-term and long-term regulation by G-protein signaling events.

Ca(2+) elevation evoked by membrane depolarization regulates G protein cycle via RGS proteins in the heart

Regulators of G protein signaling (RGS), which act as GTPase activators, are a family of cytosolic proteins emerging rapidly as an important means of controlling G protein-mediated cell signals. The importance of RGS action has been verified in vitro for various kinds of cell function. Their in situ modes of action in intact cells are, however, poorly understood. Here we show that an increase in intracellular Ca(2+) evoked by membrane depolarization controls the RGS action on G protein activation of muscarinic K(+) (K(G)) channel in the heart. Acetylcholine-induced K(G) current exhibits a slow time-dependent increase during hyperpolarizing voltage steps, referred to as "relaxation." This reflects the relief from the decrease in available K(G) channel number induced by cell depolarization. This phenomenon is abolished when an increase in intracellular Ca(2+) is prevented. It is also abolished when a calmodulin inhibitor or a mutant RGS4 is applied that can bind to calmodulin but that does not accelerate GTPase activity. Therefore, an increase in intracellular Ca(2+) and the resultant formation of Ca(2+)/calmodulin facilitate GTPase activity of RGS and thus decrease the available channel number on depolarization. These results indicate a novel and probably general pathway that Ca(2+)-dependent signaling regulates the G protein cycle via RGS proteins.

Bridging with GAPs: receptor communication through RGS proteins

The regulators of heterotrimeric guanosine triphosphate (GTP)-binding protein (G protein) signaling (RGS proteins) were named for their ability to act as GTP-activating proteins (GAPs) for G proteins and, thus, limit the signal generated by G protein-coupled receptors (GPCRs). In addition to this characteristic biochemical trait, RGS proteins constitute a large family of structurally diverse proteins with variable sequence motifs that permit additional specific interactions. RGS proteins may also serve as a bridge from GPCRs to receptor tyrosine kinases or transmembrane channels, allowing signals from GPCRs to regulate signaling through other types of receptors, and vice versa.

RGS8 protein is distributed in dendrites and cell body of cerebellar Purkinje cell

RGS8 was originally identified as an RGS protein specifically expressed in neuronally differentiated P19 cells. We generated a polyclonal antibody specific to rat RGS8 using a synthetic peptide. When nonneural cells (DDT1MF2, CHO, and NIH3T3) transfected with rat RGS8 cDNA were immuno-stained with this antibody, the RGS8 protein was mainly detected in the nuclei. Since RGS8 mRNA was exclusively expressed in Purkinje cells of the cerebellum in the rat brain, we further examined the cellular distribution of the RGS8 protein in Purkinje cells using cultured cerebellar cells and tissue sections of the cerebellum. The RGS8 protein was excluded from the nuclei and distributed in the cell body and dendrites, but not in the axons of Purkinje cells. These results demonstrate the presence of a mechanism controlling the distribution of RGS8 protein in cerebellar Purkinje cells.

Differential regulation of G protein-gated inwardly rectifying K(+) channel kinetics by distinct domains of RGS8

1. The contribution of endogenous regulators of G protein signalling (RGS) proteins to G protein modulated inwardly rectifying K(+) channel (GIRK) activation/deactivation was examined by expressing mutants of Galpha(oA) insensitive to both pertussis toxin (PTX) and RGS proteins in rat sympathetic neurons. 2. GIRK channel modulation was reconstituted in PTX-treated rat sympathetic neurons following heterologous expression of G protein subunits. Under these conditions, noradrenaline-evoked GIRK channel currents displayed: (1) a prominent lag phase preceding activation, (2) retarded activation and deactivation kinetics, and (3) a lack of acute desensitization. 3. Unexpectedly, heterologous expression of RGS8 in neurons expressing PTX-i-RGS-insensitive Galpha(oA) shortened the lag phase and restored rapid activation, but retarded the deactivation phase further. These effects were found to arise from the N-terminus, but not the core domain, of RGS8 thus suggesting actions on channel modulation independently of GTPase acceleration. 4. These findings indicate that different domains of RGS8 make distinct contributions to the temporal regulation of GIRK channels. The RGS8 core domain accelerates termination of the G-protein cycle presumably by increasing Galpha GTPase activity. In contrast, the N-terminal domain of RGS8 appears to promote entry into the G protein cycle, possibly by enhancing coupling of receptors to the G protein heterotrimer. Together, these opposing effects should allow for an increase in temporal fidelity without a dramatic decrease in signal strength.

Phosphorylation and nuclear translocation of a regulator of G protein signaling (RGS10)

Heterotrimeric G proteins are involved in the transduction of hormonal and sensory signals across plasma membranes of eukaryotic cells. Hence, they are a critical point of control for a variety of agents that modulate cellular function. Activation of these proteins is dependent on GTP binding to their alpha (Galpha) subunits. Regulators of G protein signaling (RGS) bind specifically to activated Galpha proteins, potentiating the intrinsic GTPase activity of the Galpha proteins and thus expediting the termination of Galpha signaling. Although there are several points in most G protein controlled signaling pathways that are affected by reversible covalent modification, little evidence has been shown addressing whether or not the functions of RGS proteins are themselves regulated by such modifications. We report in this study the acute functional regulation of RGS10 thru the specific and inducible phosphorylation of RGS10 protein at serine 168 by cAMP-dependent kinase A. This phosphorylation nullifies the RGS10 activity at the plasma membrane, which controls the G protein-dependent activation of the inwardly rectifying potassium channel. Surprisingly, the phosphorylation-mediated attenuation of RGS10 activity was not manifested in an alteration of its ability to accelerate GTPase activity of Galpha. Rather, the phosphorylation event correlates with translocation of RGS10 from the plasma membrane and cytosol into the nucleus.

Interaction between the RGS domain of RGS4 with G protein alpha subunits mediates the voltage-dependent relaxation of the G protein-gated potassium channel

1. In native cardiac myocytes, there is a time dependence to the G protein-gated inwardly rectifying K(+) (K(G)) channel current during voltage steps that accelerates as the concentration of acetylcholine is increased. This phenomenon has been called 'relaxation' and is not reproduced in the reconstituted Kir3.1/Kir3.4 channel in Xenopus oocytes. We have shown that RGS4, a regulator of G protein signalling, restores relaxation to the reconstituted Kir3.1/Kir3.4 channel. In this study, we examined the mechanism of this phenomenon by expressing various combinations of membrane receptors, G proteins, Kir3.0 subunits and mutants of RGS4 in Xenopus oocytes. 2. RGS4 restored relaxation to K(G) channels activated by the pertussis toxin (PTX)-sensitive G protein-coupled m(2)-muscarinic receptor but not to those activated by the G(s) protein-coupled beta(2)-adrenergic receptor. 3. RGS4 induced relaxation not only in heteromeric K(G) channels composed of Kir3.1 and Kir3.4 but also in homomeric assemblies of either an active mutant of Kir3.1 (Kir3.1/F137S) or an isoform of Kir3.2 (Kir3.2d). 4. Truncation mutants of RGS4 showed that the RGS domain itself was essential to reproduce the effect of wild-type RGS4 on the K(G) channel. 5. The mutation of residues in the RGS domain which interact with the alpha subunit of the G protein (G(alpha)) impaired the effect of RGS4. 6. This study therefore shows that interaction between the RGS domain and PTX-sensitive G(alpha) subunits mediates the effect of RGS4 on the agonist concentration-dependent relaxation of K(G) channels.

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

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

Molecular biological approaches to unravel adenylyl cyclase signaling and function

Signal transduction through the cell membrane requires the participation of one or more plasma membrane proteins. For many transmembrane signaling events adenylyl cyclases (ACs) are the final effector enzymes which integrate and interpret divergent signals from different pathways. The enzymatic activity of adenylyl cyclases is stimulated or inhibited in response to the activation of a large number of receptors in virtually all cells of the human body. To date, ten different mammalian isoforms of adenylyl cyclase (AC) have been cloned and characterized. Each isoform has its own distinct tissue distribution and regulatory properties, providing possibilities for different cells to respond diversely to similar stimuli. The product of the enzymatic reaction catalyzed by ACs, cyclic AMP (cAMP) has been shown to play a crucial role for a variety of fundamental physiological cell functions ranging from cell growth and differentiation, to transcriptional regulation and apoptosis. In the past, investigations into the regulatory mechanisms of ACs were limited by difficulties associated with their purification and the availability of the proteins in any significant amount. Moreover, nearly every cell expresses several AC isoforms. Therefore, it was difficult to perform biochemical characterization of the different AC isoforms and nearly impossible to assess the physiological roles of the individual isoforms in intact cells, tissue or organisms. Recently, however, different molecular biological approaches have permitted several breakthroughs in the study of ACs. Recombinant technologies have allowed biochemical analysis of adenylyl cyclases in-vitro and the development of transgenic animals as well as knock-out mice have yielded new insights in the physiological role of some AC isoforms. In this review, we will focus mainly on the most novel approaches and concepts, which have delineated the mechanisms regulating AC and unravelled novel functions for this enzyme.

Mechanisms governing subcellular localization and function of human RGS2

RGS proteins negatively regulate heterotrimeric G proteins at the plasma membrane. RGS2-GFP localizes to the nucleus, plasma membrane, and cytoplasm of HEK293 cells. Expression of activated G(q) increased RGS2 association with the plasma membrane and decreased accumulation in the nucleus, suggesting that signal-induced redistribution may regulate RGS2 function. Thus, we identified and characterized a conserved N-terminal domain in RGS2 that is necessary and sufficient for plasma membrane localization. Mutational and biophysical analyses indicated that this domain is an amphipathic alpha-helix that binds vesicles containing acidic phospholipids. However, the plasma membrane targeting function of the amphipathic helical domain did not appear to be essential for RGS2 to attenuate signaling by activated G(q). Nevertheless, truncation mutants indicated that the N terminus is essential, potentially serving as a scaffold that binds receptors, signaling proteins, or nuclear components. Indeed, the RGS2 N terminus directs nuclear accumulation of GFP. Although RGS2 possesses a nuclear targeting motif, it lacks a nuclear import signal and enters the nucleus by passive diffusion. Nuclear accumulation of RGS2 does not limit its ability to attenuate G(q) signaling, because excluding RGS2 from the nucleus was without effect. RGS2 may nonetheless regulate signaling or other processes in the nucleus.

Cloning and characterization of 13 novel transcripts and the human RGS8 gene from the 1q25 region encompassing the hereditary prostate cancer (HPC1) locus

The aim of this study was to develop a saturated transcript map of the region encompassing the HPC1 locus to identify the susceptibility genes involved in hereditary prostate cancer (OMIM 176807) and hyperparathyroidism-jaw tumor syndrome (OMIM 145001). We previously reported the generation of a 6-Mb BAC/PAC contig of the candidate region and employed various strategies, such as database searching, exon-trapping, direct cDNA hybridization, and sample sequencing of BACs, to identify all potential transcripts. These efforts led to the identification and precise localization on the BAC contig of 59 transcripts representing 22 known genes and 37 potential transcripts represented by ESTs and exon traps. Here we report the detailed characterization of these ESTs into full-length transcript sequences, their expression pattern in various tissues, their genomic organization, and their homology to known genes. We have also identified an Alu insertion polymorphism in the intron of one of the transcripts. Overall, data on 13 novel transcripts and the human RGS8 gene (homologue of the rat RGS8 gene) are presented in this paper. Ten of the 13 novel transcripts are expressed in prostate tissue and represent positional candidates for HPC1.

Control of the cardiac muscarinic K+ channel by beta-arrestin 2

Control of the cardiac muscarinic K(+) current (i(K,ACh)) by beta-arrestin 2 has been studied. In Chinese hamster ovary cells transfected with m2 muscarinic receptor, muscarinic K(+) channel, receptor kinase (GRK2), and beta-arrestin 2, desensitization of i(K,ACh) during a 3-min application of 10 micrometer ACh was significantly increased as compared with that in cells transfected with receptor, channel, and GRK2 only (fade in current increased from 45 to 78%). The effect of beta-arrestin 2 was lost if cells were not co-transfected with GRK2. Resensitization (recovery from desensitization) of i(K,ACh) in cells transfected with beta-arrestin 2 was significantly slowed (time constant increased from 34 to 232 s). Activation and deactivation of i(K,ACh) on application and wash-off of ACh in cells transfected with beta-arrestin 2 were significantly slowed from 0.9 to 3.1 s (time to half peak i(K,ACh)) and from 6.2 to 13.8 s (time to half-deactivation), respectively. In cells transfected with a constitutively active beta-arrestin 2 mutant, desensitization occurred in the absence of agonist (peak current significantly decreased from 0.4 +/- 0.05 to 0.1 +/- 0.01 nA). We conclude that beta-arrestin 2 has the potential to play a major role in desensitization and other aspects of the functioning of the muscarinic K(+) channel.

Expression levels of RGS7 and RGS4 proteins determine the mode of regulation of the G protein-activated K(+) channel and control regulation of RGS7 by G beta 5

Regulators of G protein signaling RGS4 and RGS7 accelerate the kinetics of K(+) channels (GIRKs) in the Xenopus oocyte system. Here, via quantitative analysis of RGS expression, we reveal biphasic effects of RGSs on GIRK regulation. At low concentrations, RGS4 inhibited basal GIRK activity, but stimulated it at high concentrations. RGS7, which is associated with the G protein subunit G beta 5, is regulated by G beta 5 by two distinct mechanisms. First, G beta 5 augments RGS7 activity, and second, it increases its expression. These dual effects resolve previous controversies regarding RGS4 and RGS7 function and indicate that they modulate signaling by mechanisms supplementary to their GTPase-activating protein activity.

GTPase-activating proteins for heterotrimeric G proteins: regulators of G protein signaling (RGS) and RGS-like proteins

GTPase-activating proteins (GAPs) regulate heterotrimeric G proteins by increasing the rates at which their subunits hydrolyze bound GTP and thus return to the inactive state. G protein GAPs act allosterically on G subunits, in contrast to GAPs for the Ras-like monomeric GTP-binding proteins. Although they do not contribute directly to the chemistry of GTP hydrolysis, G protein GAPs can accelerate hydrolysis >2000-fold. G protein GAPs include both effector proteins (phospholipase C-¿, p115RhoGEF) and a growing family of regulators of G protein signaling (RGS proteins) that are found throughout the animal and fungal kingdoms. GAP activity can sharpen the termination of a signal upon removal of stimulus, attenuate a signal either as a feedback inhibitor or in response to a second input, promote regulatory association of other proteins, or redirect signaling within a G protein signaling network. GAPs are regulated by various controls of their cellular concentrations, by complex interactions with G¿ or with G¿5 through an endogenous G-like domain, and by interaction with multiple other proteins.

Muscarinic stimulation of alpha1E Ca channels is selectively blocked by the effector antagonist function of RGS2 and phospholipase C-beta1

Neuronal alpha1E Ca channel subunits are widely expressed in mammalian brain, where they are thought to form R-type Ca channels. Recent studies have demonstrated that R-type channels contribute to neurosecretion and dendritic Ca influx, but little is known concerning their modulation. Here we show that alpha1E channels are strongly stimulated, and only weakly inhibited, through M1 muscarinic acetylcholine receptors. Both forms of channel modulation are mediated by pertussis toxin-insensitive G-proteins. Channel stimulation is blocked by regulator of G-protein signaling 2 (RGS2) or the C-terminal region of phospholipase C-beta1 (PLCbeta1ct), which have been previously shown to function as GTPase-activating proteins for Galphaq. In contrast, RGS2 and PLCbeta1ct do not block inhibition of alpha1E through M1 receptors. Inhibition is prevented, however, by the C-terminal region of beta-adrenergic receptor kinase 1, which sequesters Gbetagamma dimers. Thus, stimulation of alpha1E is mediated by a pertussis toxin-insensitive Galpha subunit (e.g., Galphaq), whereas inhibition is mediated by Gbetagamma. The ability of RGS2 and PLCbeta1ct to selectively block stimulation indicates these proteins functioned primarily as effector antagonists. In support of this interpretation, RGS2 prevented stimulation of alpha1E with non-hydrolyzable guanosine 5'-0-(3-thiotriphosphate). We also report strong muscarinic stimulation of rbE-II, a variant alpha1E Ca channel that is insensitive to voltage-dependent inhibition. Our results predict that Galphaq-coupled receptors predominantly stimulate native R-type Ca channels. Receptor-mediated enhancement of R-type Ca currents may have important consequences for neurosecretion, dendritic excitability, gene expression, or other neuronal functions.

Regulator of G protein signaling 8 (RGS8) requires its NH2 terminus for subcellular localization and acute desensitization of G protein-gated K+ channels

Functional roles of the NH(2)-terminal region of RGS (regulators of G protein signaling) 8 in G protein signaling were studied. The deletion of the NH(2)-terminal region of RGS8 (DeltaNRGS8) resulted in a partial loss of the inhibitory function in pheromone response of yeasts, although Galpha binding was not affected. To examine roles in subcellular distribution, we coexpressed two fusion proteins of RGS8-RFP and DeltaNRGS8-GFP in DDT1MF2 cells. RGS8-RFP was highly concentrated in nuclei of unstimulated cells. Coexpression of constitutively active Galpha(o) resulted in translocation of RGS8 protein to the plasma membrane. In contrast, DeltaNRGS8-GFP was distributed diffusely through the cytoplasm in the presence or absence of active Galpha(o). When coexpressed with G protein-gated inwardly rectifying K(+) channels, DeltaNRGS8 accelerated both turning on and off similar to RGS8. Acute desensitization of G protein-gated inwardly rectifying K(+) current observed in the presence of RGS8, however, was not induced by DeltaNRGS8. Thus, we, for the first time, showed that the NH(2) terminus of RGS8 contributes to the subcellular localization and to the desensitization of the G protein-coupled response.

Molecular cloning and characterization of a novel regulator of G-protein signaling from mouse hematopoietic stem cells

A novel regulator of G-protein signaling (RGS) has been isolated from a highly purified population of mouse long-term hematopoietic stem cells, and designated RGS18. It has 234 amino acids consisting of a central RGS box and short divergent NH(2) and COOH termini. The calculated molecular weight of RGS18 is 27,610 and the isoelectric point is 8.63. Mouse RGS18 is expressed from a single gene and shows tissue specific distribution. It is most highly expressed in bone marrow followed by fetal liver, spleen, and then lung. In bone marrow, RGS18 level is highest in long-term and short-term hematopoietic stem cells, and is decreased as they differentiate into more committed multiple progenitors. The human RGS18 ortholog has a tissue-specific expression pattern similar to that of mouse RGS18. Purified RGS18 interacts with the alpha subunit of both G(i) and G(q) subfamilies. The results of in vitro GTPase single-turnover assays using Galpha(i) indicated that RGS18 accelerates the intrinsic GTPase activity of Galpha(i). Transient overexpression of RGS18 attenuated inositol phosphates production via angiotensin receptor and transcriptional activation through cAMP-responsive element via M1 muscarinic receptor. This suggests RGS18 can act on G(q)-mediated signaling pathways in vivo.

Erk1/2-dependent phosphorylation of Galpha-interacting protein stimulates its GTPase accelerating activity and autophagy in human colon cancer cells

Galpha-interacting protein (GAIP) is a regulator of G protein signaling (RGS) that accelerates the rate of GTP hydrolysis by the alpha-subunit of the trimeric G(i3) protein. Both proteins are part of a signaling pathway that controls lysosomal-autophagic catabolism in human colon cancer HT-29 cells. Here we show that GAIP is phosphorylated by an extracellular signal-regulated (Erk1/2) MAP kinase-dependent pathway sensitive to amino acids, MEK1/2 (PD098059), and protein kinase C (GF109203X) inhibitors. An in vitro phosphorylation assay demonstrates that Erk2-dependent phosphorylation of GAIP stimulates its GTPase-activating protein activity toward the Galpha(i3) protein (k = 0.187 +/- 0.001 s(-)(1), EC(50) = 1.12 +/- 0.10 microm) when compared with unphosphorylated recombinant GAIP (k = 0.145 +/- 0.003 s(-)(1), EC(50) = 3.16 +/- 0. 12 microm) or to GAIP phosphorylated by other Ser/Thr protein kinases (protein kinase C, casein kinase II). This stimulation and the phosphorylation of GAIP by Erk2 were abrogated when serine at position 151 in the RGS domain was substituted by an alanine residue using site-directed mutagenesis. Furthermore, the lysosomal-autophagic pathway was not stimulated in S151A-GAIP mutant-expressing cells when compared with wild-type GAIP-expressing cells. These results demonstrate that the GTPase-activating protein activity of GAIP is stimulated by Erk2 phosphorylation. They also suggested that Erk1/2 and GAIP are engaged in the signaling control of a major catabolic pathway in intestinal derived cells.

Endogenous regulators of G protein signaling proteins regulate presynaptic inhibition at rat hippocampal synapses

Presynaptic inhibition mediated by G protein-coupled receptors (GPCRs) can develop and decay in a few seconds. This time course is too rapid to be accounted for by the intrinsic GTPase activity of Galpha subunits alone. Here, we test the hypothesis that endogenous regulators of G protein signaling (RGS proteins) are required for rapid, brief presynaptic inhibition. Endogenous G protein alpha subunits were uncoupled from GPCRs by treating cultures with pertussis toxin (PTX). Adenoviral expression of mutant PTX-insensitive (PTX-i) Galpha(i1-3) or Galpha(o) subunits rescued adenosine-induced presynaptic inhibition in cultured hippocampal neurons. Expression of double mutant Galpha(i1) or Galpha(o) subunits that were both PTX-insensitive and unable to bind RGS proteins (PTX/RGS-i) also rescued presynaptic inhibition. Presynaptic inhibition mediated by PTX/RGS-i subunits decayed much more slowly after agonist removal than that mediated by PTX-i subunits or native G proteins. The onset of presynaptic inhibition mediated by PTX/RGS-i Galpha(o) was also slower than that mediated by PTX-i Galpha(o). In contrast, the onset of presynaptic inhibition mediated by PTX/RGS-i Galpha(i1) was similar to that mediated by PTX-i Galpha(i1). These results suggest that endogenous RGS proteins regulate the time course of G protein signaling in mammalian central nervous system presynaptic terminals.

RGS proteins: lessons from the RGS9 subfamily

RGS proteins enhance the time resolution of G protein signaling cascades by accelerating GTP hydrolysis of G alpha subunits of heterotrimeric G proteins. RGS9-1, a photoreceptor-specific RGS protein, is the first vertebrate member of this sizeable family whose physiological function in a well-defined G protein pathway has been identified. It is essential for normal subsecond recovery kinetics of the light responses in retinal photoreceptors. Understanding this role allows RGS9-1 to serve as a useful model for understanding how specificity and regulation of RGS function are achieved. In addition to the catalytic RGS domain, shared among all members of this family, RGS9-1 contains several other domains, which are also found in a closely related subset of RGS proteins, the RGS9 subfamily. One of these domains, the G gamma-like (GGL) domain, has been identified as the attachment site for G beta 5 proteins, which act as obligate subunits for this subfamily. Results from RGS9-1 and other subfamily members suggest that specificity is achieved by cell type-specific transcription, RNA processing, and G beta 5-dependent protein stabilization. In addition, membrane localization via specific targeting domains likely plays an important role.

G protein modulation of recombinant P/Q-type calcium channels by regulators of G protein signalling proteins

1. Fast synaptic transmission is triggered by the activation of presynaptic Ca2+ channels which can be inhibited by Gbetagamma subunits via G protein-coupled receptors (GPCR). Regulators of G protein signalling (RGS) proteins are GTPase-accelerating proteins (GAPs), which are responsible for >100-fold increases in the GTPase activity of G proteins and might be involved in the regulation of presynaptic Ca2+ channels. In this study we investigated the effects of RGS2 on G protein modulation of recombinant P/Q-type channels expressed in a human embryonic kidney (HEK293) cell line using whole-cell recordings. 2. RGS2 markedly accelerates transmitter-mediated inhibition and recovery from inhibition of Ba2+ currents (IBa) through P/Q-type channels heterologously expressed with the muscarinic acetylcholine receptor M2 (mAChR M2). 3. Both RGS2 and RGS4 modulate the prepulse facilitation properties of P/Q-type Ca2+ channels. G protein reinhibition is accelerated, while release from inhibition is slowed. These kinetics depend on the availability of G protein alpha and betagamma subunits which is altered by RGS proteins. 4. RGS proteins unmask the Ca2+ channel beta subunit modulation of Ca2+ channel G protein inhibition. In the presence of RGS2, P/Q-type channels containing the beta2a and beta3 subunits reveal significantly altered kinetics of G protein modulation and increased facilitation compared to Ca2+ channels coexpressed with the beta1b or beta4 subunit.

G-protein mediated gating of inward-rectifier K+ channels

G-protein regulated inward-rectifier potassium channels (GIRK) are part of a superfamily of inward-rectifier K+ channels which includes seven family members. To date four GIRK subunits, designated GIRK1-4 (also designated Kir3.1-4), have been identified in mammals, and GIRK5 has been found in Xenopus oocytes. GIRK channels exist in vivo both as homotetramers and heterotetramers. In contrast to the other mammalian GIRK family members, GIRK1 can not form functional channels by itself and has to assemble with GIRK2, 3 or 4. As the name implies, GIRK channels are modulated by G-proteins; they are also modulated by phosphatidylinositol 4,5-bisphosphate, intracellular sodium, ethanol and mechanical stretch. Recently a family of GTPase activating proteins known as regulators of G-protein signaling were shown to be the missing link for the fast deactivation kinetics of GIRK channels in native cells, which contrast with the slow kinetics observed in heterologously expressed channels. GIRK1, 2 and 3 are highly abundant in brain, while GIRK4 has limited distribution. Here, GIRK1/2 seems to be the predominant heterotetramer. In general, neuronal GIRK channels are involved in the regulation of the excitability of neurons and may contribute to the resting potential. Interestingly, only the GIRK1 and 4 subunits are distributed in the atrial and sinoatrial node cells of the heart and are involved in the regulation of cardiac rate. Our main objective of this review is to assess the current understanding of the G-protein modulation of GIRK channels and their physiological importance in mammals.

Regulators of G protein signaling: a bestiary of modular protein binding domains

Members of the newly discovered regulator of G protein signaling (RGS) families of proteins have a common RGS domain. This RGS domain is necessary for conferring upon RGS proteins the capacity to regulate negatively a variety of Galpha protein subunits. However, RGS proteins are more than simply negative regulators of signaling. RGS proteins can function as effector antagonists, and recent evidence suggests that RGS proteins can have positive effects on signaling as well. Many RGS proteins possess additional C- and N-terminal modular protein-binding domains and motifs. The presence of these additional modules within the RGS proteins provides for multiple novel regulatory interactions performed by these molecules. These regions are involved in conferring regulatory selectivity to specific Galpha-coupled signaling pathways, enhancing the efficacy of the RGS domain, and the translocation or targeting of RGS proteins to intracellular membranes. In other instances, these domains are involved in cross-talk between different Galpha-coupled signaling pathways and, in some cases, likely serve to integrate small GTPases with these G protein signaling pathways. This review discusses these C- and N-terminal domains and their roles in the biology of the brain-enriched RGS proteins. Methods that can be used to investigate the function of these domains are also discussed.

Regulation of GIRK channel deactivation by Galpha(q) and Galpha(i/o) pathways

G protein regulated inward rectifying potassium channels (GIRKs) are activated by G protein coupled receptors (GPCRs) via the G protein betagamma subunits. However, little is known about the effects of different GPCRs on the deactivation kinetics of transmitter-mediated GIRK currents. In the present study we investigated the influence of different GPCRs in the presence and absence of RGS proteins on the deactivation kinetics of GIRK channels by coexpressing the recombinant protein subunits in Xenopus oocytes. The stimulation of both G(i/o)- and G(q)-coupled pathways accelerated GIRK deactivation. GIRK currents deactivated faster upon stimulation of G(i/o)- and G(q)-coupled pathways by P(2)Y(2) receptors (P(2)Y(2)Rs) than upon activation of the G(i/o)-coupled pathway alone via muscarinic acetylcholine receptor M2 (M(2) mAChRs). This acceleration was found to be dependent on phospholipase C (PLC) and protein kinase C (PKC) activities and intracellular calcium. With the assumption that RGS2 has a higher affinity for Galpha(q) than Galpha(i/o), we demonstrated that the deactivation kinetics of GIRK channels can be differentially regulated by the relative amount of RGS proteins. These data indicate that transmitter-mediated deactivation of GIRK currents is modulated by crosstalk between G(i/o)- and G(q)-coupled pathways.

The regulator of G protein signaling family

Regulator of G protein signaling (RGS) proteins are responsible for the rapid turnoff of G protein-coupled receptor signaling pathways. The major mechanism whereby RGS proteins negatively regulate G proteins is via the GTPase activating protein activity of their RGS domain. Structural and mutational analyses have characterized the RGS/G alpha interaction in detail, explaining the molecular mechanisms of the GTPase activating protein activity of RGS proteins. More than 20 RGS proteins have been isolated, and there are indications that specific RGS proteins regulate specific G protein-coupled receptor pathways. This specificity is probably created by a combination of cell type-specific expression, tissue distribution, intracellular localization, posttranslational modifications, and domains other than the RGS domain that link them to other signaling pathways. In this review we discuss what has been learned so far about the role of RGS proteins in regulating G protein-coupled receptor signaling and point out areas that may be fruitful for future research.

Regulators of G-protein signaling in receptor complexes

G protein signaling pathways regulate heart development and adult cardiac function. G protein activity is controlled by the interplay between receptor-catalyzed activation and the inhibitory regulators of G protein signaling (RGS) proteins. Most RGS proteins are GTPase accelerating proteins (GAPs) for Gi and Gq class G protein alpha subunits, and thereby terminate signaling. However, RGS proteins also provide scaffolding properties to help assemble or maintain signaling complexes. Thus, RGS proteins are kinetic regulators that may sharpen both signal activation and termination. The five subfamilies of mammalian RGS proteins contain a characteristic RGS domain and distinct flanking domains that convey lipid and/or protein interactions within receptor complexes. The RGS domain provides GAP activity and additional interactions with the receptor complex. Distantly related RGS-like (RGL) proteins provide other regulatory and effector functions in G protein signaling pathways. RGS and RGL proteins provide exciting new therapeutic targets to combat cardiovascular diseases.

A regulator of G protein signalling (RGS) protein confers agonist-dependent relaxation gating to a G protein-gated K+ channel

1. The effects of RGS4 on the voltage-dependent relaxation of G protein-gated K+ (KG) channels were examined by heterologous expression in Xenopus oocytes. 2. While the relaxation kinetics was unaffected by the acetylcholine concentration ([ACh]) in the absence of RGS4, it became dependent on [ACh] when RGS4 was co-expressed. 3. Kinetic analyses indicated that RGS4 confers to the KG channel a voltage-independent inhibitory gating mechanism, which was attenuated by ACh in a concentration-dependent fashion. 4. In vitro biochemical studies showed that RGS4 could bind to the protein complex containing KG channel subunits. 5. Since the native cardiac KG channel exhibited similar agonist-dependent relaxation kinetics to that mediated by RGS4, it is suggested that KG channel gating is a novel physiological target of RGS protein-mediated regulation.

Endogenous regulator of G-protein signaling proteins modify N-type calcium channel modulation in rat sympathetic neurons

Experiments using heterologous overexpression indicate that regulator of G-protein signaling (RGS) proteins play important roles in Gbetagamma-mediated ion channel modulation. However, the roles subserved by endogenous RGS proteins have not been extensively examined because tools for functionally inhibiting natively expressed RGS proteins are lacking. To address this void, we used a strategy in which Galpha(oA) was rendered insensitive to pertussis toxin (PTX) and RGS proteins by site-directed mutagenesis. Either PTX-insensitive (PTX-i) or both PTX- and RGS-insensitive (PTX/RGS-i) mutants of Galpha(oA) were expressed along with Gbeta(1) and Ggamma(2) subunits in rat sympathetic neurons. After overnight treatment with PTX to suppress natively expressed Galpha subunits, voltage-dependent Ca(2+) current inhibition by norepinephrine (NE) (10 microm) was reconstituted in neurons expressing either PTX-i or PTX/RGS-i Galpha(oA). When compared with neurons expressing PTX-i Galpha(oA), the steady-state concentration-response relationships for NE-induced Ca(2+) current inhibition were shifted to lower concentrations in neurons expressing PTX/RGS-i Galpha(oA). In addition to an increase in agonist potency, the expression of PTX/RGS-i Galpha(oA) dramatically retarded the current recovery after agonist removal. Interestingly, the alteration in current recovery was accompanied by a slowing in the onset of current inhibition. Together, our data suggest that endogenous RGS proteins contribute to membrane-delimited Ca(2+) channel modulation by regulating agonist potency and kinetics of G-protein-mediated signaling in neuronal cells.

Molecular cloning and characterization of Xenopus RGS5

We identified six genes that encode putative RGS proteins (XRGSI-VI) in developing Xenopus embryos using PCR amplification with degenerate primers corresponding to the conserved region (RGS domain) of known RGS proteins. RT-PCR analysis revealed that mRNAs of these XRGSs are differentially expressed during embryogenesis. At stage 1, only XRGSII mRNA was detected. On the other hand, expression of XRGSVI mRNA increased apparently at stage 14 and expression of three of other XRGS (III, IV, V) elevated between stage 25 and 40. To further characterize XRGS proteins expressed in Xenopus embryos, we isolated a cDNA clone for XRGSIII. Based on determined nucleotide sequence, XRGSIII was considered as a Xenopus homologue of mammalian RGS5 (XRGS5). Genetic analysis using the pheromone response halo assay showed that expression of XRGS5 inhibits yeast response to alpha-factor, suggesting that XRGS5 negatively regulates the G-protein-mediated signaling pathway in developing Xenopus embryos.

Heterotrimeric G-proteins of a filamentous fungus regulate cell wall composition and susceptibility to a plant PR-5 protein

Membrane permeabilizing plant defensive proteins first encounter the fungal cell wall that can harbor specific components that facilitate or prevent access to the plasma membrane. However, signal transduction pathways controlling cell wall composition in filamentous fungi are largely unknown. We report here that the deposition of cell wall constituents that block the action of osmotin (PR-5), an antifungal plant defense protein, against Aspergillus nidulans requires the activity of a heterotrimeric G-protein mediated signaling pathway. The guanidine nucleotide GDPbetaS, that locks G-proteins in a GDP-bound inactive form, inhibits osmotin-induced conidial lysis. A dominant interfering mutation in FadA, the alpha-subunit of a heterotrimeric G-protein, confers resistance to osmotin. A deletion mutation in SfaD, the beta-subunit of a heterotrimeric G-protein also increases osmotin resistance. Aspergillus nidulans strains bearing these mutations also have increased tolerance to SDS, reduced cell wall porosity and increased chitin content in the cell wall.

GABA spillover activates postsynaptic GABA(B) receptors to control rhythmic hippocampal activity

In the hippocampus, interneurons provide synaptic inhibition via the transmitter GABA, which can activate GABA(A) and GABA(B) receptors (GABA(A)Rs and GABA(B)Rs). Generally, however, GABA released by a single interneuron activates only GABA(A)Rs on its targets, despite the abundance of GABA(B)RS. Here, I show that during hippocampal rhythmic activity, simultaneous release of GABA from several interneurons activates postsynaptic GABA(B)Rs and that block of GABA(B)Rs increases oscillation frequency. Furthermore, if GABA uptake is inhibited, even GABA released by a single interneuron is enough to activate GABA(B)Rs. This occurs also on cells not directly contacted by that interneuron, indicating that GABA has to overcome uptake and exit the synaptic cleft to reach GABA(B)RS. Thus, activation of extrasynaptic GABA(B)Rs by pooling of GABA is an important mechanism regulating hippocampal network activity.

Competition between internal AlF(4)(-) and receptor-mediated stimulation of dorsal raphe neuron G-proteins coupled to calcium current inhibition

Intracellular aluminum fluoride (AlF(4)(-)), placed in a patch pipette, activated a G-protein, resulting in a "tonic" inhibition of the Ca(2+) current of isolated serotonergic neurons of the rat dorsal raphe nucleus. Serotonin (5-HT) also inhibits the Ca(2+) current of these cells. After external bath application and quick removal of 5-HT to an AlF(4)(-) containing cell, there was a reversal or transient disinhibition (TD) of the inhibitory effect of AlF(4)(-) on Ca(2+) current. A short predepolarization of the membrane potential to +70 mV, a condition that is known to reverse G-protein-mediated inhibition, reversed the inhibitory effect of AlF(4)(-) on Ca(2+) current and brought the Ca(2+) current to the same level as that seen at the peak of the TD current. With AlF(4)(-) in the pipette, the TD phenomenon could be eliminated by lowering pipette MgATP, or by totally chelating pipette Al(3+). In the presence of AlF(4)(-), but with either lowered MgATP or extreme efforts to eliminate pipette Al(3+), the rate of recovery from 5-HT on wash was slowed, a condition opposite to that where a TD occurred. The putative complex of AlF(4)(-)-bound G-protein (Galpha.GDP. AlF(4)(-)) appeared to free G-betagamma-subunits, mimicking the effect on Ca(2+) channels of the G.GTP complex. The ON-rate of the inhibition of Ca(2+) current, after a depolarizing pulse, by betagamma-subunits released by AlF(4)(-) in the pipette was significantly slower than that of the agonist-activated G-protein. The OFF-rate of the AlF(4)(-)-mediated inhibition in response to a depolarizing pulse, a measure of the affinity of the free G-betagamma-subunit for the Ca(2+) channel, was slightly slower than that of the agonist stimulated G-protein. In summary, AlF(4)(-) modified the OFF-rate kinetics of G-protein activation by agonists, but had little effect on the kinetics of the interaction of the betagamma-subunit with Ca(2+) channels. Agonist application temporarily reversed the effects of AlF(4)(-), making it a complementary tool to GTP-gamma-S for the study of G-protein interactions.

Co-expression of Gbeta5 enhances the function of two Ggamma subunit-like domain-containing regulators of G protein signaling proteins

Regulators of G protein signaling (RGS) stimulate the GTPase activity of G protein Galpha subunits and probably play additional roles. Some RGS proteins contain a Ggamma subunit-like (GGL) domain, which mediates a specific interaction with Gbeta5. The role of such interactions in RGS function is unclear. RGS proteins can accelerate the kinetics of coupling of G protein-coupled receptors to G-protein-gated inwardly rectifying K(+) (GIRK) channels. Therefore, we coupled m2-muscarinic acetylcholine receptors to GIRK channels in Xenopus oocytes to evaluate the effect of Gbeta5 on RGS function. Co-expression of either RGS7 or RGS9 modestly accelerated GIRK channel kinetics. When Gbeta5 was co-expressed with either RGS7 or RGS9, the acceleration of GIRK channel kinetics was strongly increased over that produced by RGS7 or RGS9 alone. RGS function was not enhanced by co-expression of Gbeta1, and co-expression of Gbeta5 alone had no effect on GIRK channel kinetics. Gbeta5 did not modulate the function either of RGS4, an RGS protein that lacks a GGL domain, or of a functional RGS7 construct in which the GGL domain was omitted. Enhancement of RGS7 function by Gbeta5 was not a consequence of an increase in the amount of plasma membrane or cytosolic RGS7 protein.

Altered regulation of potassium and calcium channels by GABA(B) and adenosine receptors in hippocampal neurons from mice lacking Galpha(o)

To examine the role of G(o) in modulation of ion channels by neurotransmitter receptors, we characterized modulation of ionic currents in hippocampal CA3 neurons from mice lacking both isoforms of Galpha(o). In CA3 neurons from Galpha(o)(-/-) mice, 2-chloro-adenosine and the GABA(B)-receptor agonist baclofen activated inwardly rectifying K(+) currents and inhibited voltage-dependent Ca(2+) currents just as effectively as in Galpha(o)(+/+) littermates. However, the kinetics of transmitter action were dramatically altered in Galpha(o)(-/-) mice in that recovery on washout of agonist was much slower. For example, recovery from 2-chloro-adenosine inhibition of calcium current was more than fourfold slower in neurons from Galpha(o)(-/-) mice [time constant of 12.0 +/- 0.8 (SE) s] than in neurons from Galpha(o)(+/+) mice (time constant of 2.6 +/- 0.2 s). Recovery from baclofen effects was affected similarly. In neurons from control mice, effects of both baclofen and 2-chloro-adenosine on Ca(2+) currents and K(+) currents were abolished by brief exposure to external N-ethyl-maleimide (NEM). In neurons lacking Galpha(o), some inhibition of Ca(2+) currents by baclofen remained after NEM treatment, whereas baclofen activation of K(+) currents and both effects of 2-chloro-adenosine were abolished. These results show that modulation of Ca(2+) and K(+) currents by G protein-coupled receptors in hippocampal neurons does not have an absolute requirement for Galpha(o). However, modulation is changed in the absence of Galpha(o) in having much slower recovery kinetics. A likely possibility is that the very abundant Galpha(o) is normally used but, when absent, can readily be replaced by G proteins with different properties.

The dual modulation of GIRK1/GIRK2 channels by opioid receptor ligands

It is well known that activation of the cloned kappa-opioid receptor by nanomolar concentrations of U50488H (trans-(+/-)-3, 4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl-benzeneacetamide) , a selective kappa-opioid receptor agonist, leads to the opening of GIRK1 channels. In this study, we demonstrate that the cloned kappa-opioid receptor functionally couples to GIRK1/GIRK2 channels (G-protein-coupled inwardly rectifying K(+) channels), mimicking the probable heteromultimeric state of neuronal GIRK channels. We also show that micromolar concentrations of U50488H reduce GIRK1/GIRK2 current through direct GIRK1/GIRK2 channel block in a voltage-independent manner (IC(50)=70.28+/-3.68 microM). Similarly, it was found that propoxyphene, methadone, and naloxone also can block GIRK1/GIRK2 current. In contrast, elevated concentrations of morphine (up to 1 mM) did not cause channel block. The related inwardly rectifying K(+) channel, IRK1, was not affected by elevated concentrations of these drugs. We conclude that nanomolar concentrations of opioid receptor ligands activate GIRK1/GIRK2 channels through a receptor-mediated pathway, while micromolar concentrations of some opioid receptor ligands inhibit GIRK1/GIRK2 channels by direct channel block.

G protein selectivity is a determinant of RGS2 function

RGS (regulator of G protein signaling) proteins are GTPase-activating proteins that attenuate signaling by heterotrimeric G proteins. Whether the biological functions of RGS proteins are governed by quantitative differences in GTPase-activating protein activity toward various classes of Galpha subunits and how G protein selectivity is achieved by differences in RGS protein structure are largely unknown. Here we provide evidence indicating that the function of RGS2 is determined in part by differences in potency toward G(q) versus G(i) family members. RGS2 was 5-fold more potent than RGS4 as an inhibitor of G(q)-stimulated phosphoinositide hydrolysis in vivo. In contrast, RGS4 was 8-fold more potent than RGS2 as an inhibitor of G(i)-mediated signaling. RGS2 mutants were identified that display increased potency toward G(i) family members without affecting potency toward G(q). These mutations and the structure of RGS4-G(i)alpha(1) complexes suggest that RGS2-G(i)alpha interaction is unfavorable in part because of the geometry of the switch I binding pocket of RGS2 and a potential interaction between the alpha8-alpha9 loop of RGS2 and alphaA of G(i) class alpha subunits. The results suggest that the function of RGS2 relative to other RGS family members is governed in part by quantitative differences in activity toward different classes of Galpha subunits.

G protein-dependent Ca2+ signaling complexes in polarized cells

Polarized cells signal in a polarized manner. This is exemplified in the patterns of [Ca2+]i waves and [Ca2+]i oscillations evoked by stimulation of G protein-coupled receptors in these cells. Organization of Ca(2+)-signaling complexes in cellular microdomains, with the aid of scaffolding proteins, is likely to have a major role in shaping G protein-coupled [Ca2+]i signal pathways. In epithelial cells, these domains coincide with sites of [Ca2+]i-wave initiation and local [Ca2+]i oscillations. Cellular microdomains enriched with Ca(2+)-signaling proteins have been found in several cell types. Microdomains organize communication between Ca(2+)-signaling proteins in the plasma membrane and internal Ca2+ stores in the endoplasmic reticulum through the interaction between the IP3 receptors in the endoplasmic reticulum and Ca(2+)-influx channels in the plasma membrane. Ca2+ signaling appears to be controlled within the receptor complex by the regulators of G protein-signaling (RGS) proteins. Three domains in RGS4 and related RGS proteins contribute important regulatory features. The RGS domain accelerates GTP hydrolysis on the G alpha subunit to uncouple receptor stimulation from IP3 production; the C-terminus may mediate interaction with accessory proteins in the complex; and the N-terminus acts in a receptor-selective manner to confer regulatory specificity. Hence, RGS proteins have both catalytic and scaffolding function in Ca2+ signaling. Organization of Ca(2+)-signaling proteins into complexes within microdomains is likely to play a prominent role in the localized control of [Ca2+]i and in [Ca2+]i oscillations.

G Protein Regulation of Inwardly Rectifying K(+) Channels

Inwardly rectifying K(+) (Kir) channels respond to receptor-stimulated signaling cascades that involve G proteins and other cytosolic messengers. Channel activity is controlled both by direct coupling of G protein subunits and by phosphorylation via protein serine/threonine and tyrosine kinases. The coincidence of both forms of Kir channel signaling may give rise to complex cellular responses.

RGS1 is expressed in monocytes and acts as a GTPase-activating protein for G-protein-coupled chemoattractant receptors

The leukocyte response to chemoattractants is transduced by the interaction of transmembrane receptors with GTP-binding regulatory proteins (G-proteins). RGS1 is a member of a protein family constituting a newly appreciated and large group of proteins that act as deactivators of G-protein signaling pathways by accelerating the GTPase activity of G-protein alpha subunits. We demonstrate here that RGS1 is expressed in human monocytes; by immunofluorescence and subcellular fractionation RGS1 was localized to the plasma membrane. By using a mixture of RGS1 and plasma membranes, we were able to demonstrate GAP activity of RGS1 on receptor-activated G-proteins; RGS1 did not affect ligand-stimulated GDP-GTP exchange. We found that RGS1 desensitizes a variety of chemotactic receptors including receptors for N-formyl-methionyl-leucyl-phenylalanine, leukotriene B4, and C5a. Interaction of RGS proteins and ligand-induced G-protein signaling can be demonstrated by determining GTPase activity using purified RGS proteins and plasma membranes.

Emerging roles for RGS proteins in cell signalling

Regulators of G-protein signalling (RGS proteins) are a family of highly diverse, multifunctional signalling proteins that share a conserved 120 amino acid domain (RGS domain). RGS domains bind directly to activated Galpha subunits and act as GTPase-activating proteins (GAPs) to attenuate and/or modulate hormone and neurotransmitter receptor-initiated signalling by both Galpha-GTP and Gbetagamma. Apart from this structural domain, which is shared by all known RGS proteins, these proteins differ widely in their overall size and amino acid identity and possess a remarkable variety of structural domains and motifs. These biochemical features impart signalling functions and/or enable RGS proteins to interact with a growing list of unexpected protein-binding partners with diverse cellular roles. New appreciation for the broader cellular functions of RGS proteins challenges established models of G-protein signalling and serves to identify these proteins as central participants in receptor signalling and cell physiology.

Rapid GTP binding and hydrolysis by G(q) promoted by receptor and GTPase-activating proteins

Receptor-promoted GTP binding and GTPase-activating protein (GAP)-promoted GTP hydrolysis determine the onset and termination of G protein signaling; they coordinately control signal amplitude. The mechanisms whereby cells independently regulate signal kinetics and signal amplitude are therefore central to understanding G protein function. We have used quench-flow kinetic methods to measure the rates of the individual reactions of the agonist-stimulated GTPase cycle for G(q) during steady-state signaling. G(q) and m1 muscarinic cholinergic receptor were co-reconstituted into proteoliposomes with one of two GAPs: phospholipase C (PLC)-beta1, the major G(q)-regulated effector protein, and RGS4, a GAP commonly thought to be an inhibitor of G(q) signaling. In this system, the rate constant for GAP-stimulated hydrolysis of Galpha(q)-bound GTP at 30 degrees C was 9-12 s(-1) for PLC-beta1 and 22-27 s(-1) for RGS4. These rates are 1,000- to 2,000-fold faster than in the absence of a GAP and far faster than measured previously. G(q) can thus hydrolyze bound GTP with deactivation half-times of 25-75 ms at 30 degrees C, commensurate with physiological rates of signal termination. GDP/GTP exchange, which reactivates G(q), was the principal rate-limiting step for the GTPase cycle and was also faster than previously thought. At physiological concentrations of GTP, exchange was limited by the rate of dissociation of GDP from the receptor-G(q) complex, with a maximal rate of 1.8 s(-1) at 30 degrees C. Comparison of activation and deactivation rates help explain how GDP/GTP exchange balance rapid GTP hydrolysis to maintain steady-state signal amplitude.

Inwardly rectifying potassium channels

Inwardly rectifying potassium (Kir) channels regulate the resting membrane potential of the cell and thereby modulate the electrical activity of cardiac and neuronal cells, insulin secretion and epithelial K(+) transport. Considerable progress in understanding the molecular structure of Kir channels and the way in which they are regulated by extracellular and intracellular modulators has been made during the past year.