GAIP, a protein that specifically interacts with the trimeric G protein G alpha i3, is a member of a protein family with a highly conserved core domain

Membrane-associated GAIP is a phosphoprotein and can be phosphorylated by clathrin-coated vesicles

GAIP (G alpha interacting protein) is a member of the RGS (regulators of G protein signaling) family and accelerates the turnover of GTP bound to Galphai, Galphaq, and Galpha13. There are two pools of GAIP-a soluble and a membrane-anchored pool. The membrane-anchored pool is found on clathrin-coated vesicles (CCVs) and pits in rat liver and AtT-20 pituitary cells. By treatment of a GAIP-enriched rat liver fraction with alkaline phosphatase, we found that membrane-bound GAIP is phosphorylated. By immunoprecipitation carried out on [(32)P]orthophosphate-labeled AtT-20 pituitary cells stably expressing GAIP, (32)P-labeling was associated exclusively with the membrane pool of GAIP. Phosphoamino acid analysis revealed that phosphorylation of GAIP occurred largely on serine residues. Recombinant GAIP could be phosphorylated at its N terminus with purified casein kinase 2 (CK2). It could also be phosphorylated by isolated CCVs in vitro. Phosphorylation was Mn(2+)-dependent, using both purified CK2 and CCVs. Ser-24 was identified as one of the phosphorylation sites. Our results establish that GAIP is phosphorylated and that only the membrane pool is phosphorylated, suggesting that GAIP can be regulated by phosphorylation events taking place at the level of clathrin-coated pits and vesicles.

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.

Down-regulation of beta-catenin by the colorectal tumor suppressor APC requires association with Axin and beta-catenin

The tumor suppressor adenomatous polyposis coli (APC) is mutated in familial adenomatous polyposis and in sporadic colorectal tumors. APC forms a complex with beta-catenin, Axin, and glycogen synthase kinase-3beta and induces the degradation of beta-catenin. In the present study, we examined whether APC association with Axin is required for degradation of beta-catenin. We found that a fragment of APC that induces beta-catenin degradation was rendered inactive by disruption of its Axin-binding sites. Also, overexpression of an Axin fragment spanning the regulator of the G-protein signaling domain inhibited APC-mediated beta-catenin degradation. An APC fragment with mutated beta-catenin-binding sites but intact Axin-binding sites also failed to induce degradation of beta-catenin. These results suggest that APC requires interaction with Axin and beta-catenin to down-regulate beta-catenin.

Natriuretic peptides inhibit G protein activation. Mediation through cross-talk between cyclic GMP-dependent protein kinase and regulators of G protein-signaling proteins

Atrial natriuretic peptide (ANP) inhibits the proliferation of many cells, in part through interfering with signal transduction enacted by G protein-coupled growth factor receptors. Signaling interactions between ANP and the G protein-coupled growth factor receptor ligand, endothelin-3 (ET-3), regulate astrocyte proliferation at a very proximal but undefined point. Here, we find that ANP inhibits the ability of ET-3 to activate Galpha(q) and Galpha(i) in these cells. ANP stimulated the translocation of endogenous regulators of G protein-signaling (RGS) proteins 3 and 4 from the cytosol to the cell membrane, and enhanced their association with Galpha(q) and Galpha(i). ANP effects were significantly blocked by HS-142-1, an inhibitor of guanylate cyclase activation, or by ET-3. KT5823, an inhibitor of cyclic GMP-dependent protein kinase (PKG) reversed the RGS translocation induced by ANP; conversely, expression of an active catalytic subunit of PKG-I, or 8-bromo-cyclic GMP stimulated RGS translocation. ANP caused the phosphorylation of both RGS proteins in a PKG-dependent fashion, and the expressed PKG (in the absence of ANP) also stimulated RGS phosphorylation. A novel cross-talk between PKG and RGS proteins is stimulated by ANP and leads to the increased translocation and association of RGS proteins with Galpha. The rapid inactivation of G proteins provides a mechanism by which ANP inhibits downstream signaling to the cell proliferation program.

Inverse agonism at heptahelical receptors: concept, experimental approach and therapeutic potential

Inverse agonists (negative antagonists) are ligands that stabilize the inactive conformation (R) of receptors according to the two-state receptor model. The active conformation (R*) of heptahelical receptors, i.e. G protein-coupled receptors, has high affinity for G proteins. According to ternary complex models of receptor activation, the R*G complex is in equilibrium with R + G, with spontaneous activity in the absence of agonist. Inverse agonists, having a higher affinity for R, shift R*G towards R + G, decreasing the spontaneous activity of receptors. Agonists have the opposite effect, with a higher affinity for R*. Neutral antagonists have the same affinity for R and R* and compete for both agonists and inverse agonists. Inverse agonists have been recently proposed for a variety of heptahelical receptors. Methods to detect inverse agonists among antagonists are based on the determination of ligand affinity at R and R* with binding experiments, and on the modulation of G protein activity (GTP binding and hydrolysis) or of effector activity. Receptor inverse agonists, but also G protein antagonists and GTPase inhibitors, decrease spontaneous G protein activity corresponding to R*G. Receptor agonists, G protein agonists and GTPase inhibitors increase effector basal activity, but receptor inverse agonists decrease it. The therapeutic potential of inverse agonists is proposed in human diseases ascribed to constitutively active mutant receptors and may be extended to diseases related to wild-type receptor over-expression leading to the increase of R*. Some of the therapeutic effects of presently used receptor antagonists may be related to their inverse agonist properties. Inverse agonists lead to receptor upregulation, offering new approaches to tolerance and dependence to drugs.

Regulator of G protein signaling 1 (RGS1) markedly impairs Gi alpha signaling responses of B lymphocytes

Regulator of G protein signaling (RGS) proteins modulate signaling through pathways that use heterotrimeric G proteins as transducing elements. RGS1 is expressed at high levels in certain B cell lines and can be induced in normal B cells by treatment with TNF-alpha. To determine the signaling pathways that RGS1 may regulate, we examined the specificity of RGS1 for various G alpha subunits and assessed its effect on chemokine signaling. G protein binding and GTPase assays revealed that RGS1 is a Gi alpha and Gq alpha GTPase-activating protein and a potential G12 alpha effector antagonist. Functional studies demonstrated that RGS1 impairs platelet activating factor-mediated increases in intracellular Ca+2, stromal-derived factor-1-induced cell migration, and the induction of downstream signaling by a constitutively active form of G12 alpha. Furthermore, germinal center B lymphocytes, which are refractory to stromal-derived factor-1-triggered migration, express high levels of RGS1. These results indicate that RGS proteins can profoundly effect the directed migration of lymphoid cells.

Calnuc, an EF-hand Ca(2+) binding protein, specifically interacts with the C-terminal alpha5-helix of G(alpha)i3

Calnuc (nucleobindin) was previously shown to be present both in the cytosol and in the Golgi and to be the major Golgi Ca(2+) binding protein. In this study we verified the existence of the cytosolic pool of calnuc and investigated its interaction with G(alpha)i3. Cytosolic calnuc was released by mild digitonin permeabilization. In pulse-chase experiments, the two pools of calnuc had different mobilities, suggesting different posttranslational modifications. That calnuc interacts with G(alpha)i3 in vivo was verified by the finding that G(alpha)i3 could be crosslinked intracellularly to calnuc and co-immunoprecipitated from NIH 3T3 cells stably overexpressing either activated (Q204L) or inactivated (G203A) G(alpha)i3. Binding was Ca(2+) and Mg(2+)-dependent. Calnuc and G(alpha)i3-GFP codistributed primarily in the Golgi region. By yeast two-hybrid analysis, the binding site on G(alpha)i3 for calnuc was mapped to the C-terminal region because removal of the last 12 amino acids (but not 11) abolished the interaction. Peptide competition indicated that calnuc, with its coiled-coil domain constituted by the two EF-hands, binds to G(alpha)i3's C-terminal alpha5-helix. These results demonstrate that calnuc may play an important role in G protein- and Ca(2+)-regulated signal transduction events.

Glia development in the embryonic CNS of Drosophila

Glial cells are pivotal players during the development and function of complex nervous systems. In Drosophila, recent genetic analyses have revealed several genes that control differentiation and function of CNS glial cells and their interactions with neurons can be studied in detail at the CNS midline, where it is essential for the correct establishment of the commissural axon pattern.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

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.

An exploration of the sequence of a 2.9-Mb region of the genome of Drosophila melanogaster: the Adh region

A contiguous sequence of nearly 3 Mb from the genome of Drosophila melanogaster has been sequenced from a series of overlapping P1 and BAC clones. This region covers 69 chromosome polytene bands on chromosome arm 2L, including the genetically well-characterized "Adh region." A computational analysis of the sequence predicts 218 protein-coding genes, 11 tRNAs, and 17 transposable element sequences. At least 38 of the protein-coding genes are arranged in clusters of from 2 to 6 closely related genes, suggesting extensive tandem duplication. The gene density is one protein-coding gene every 13 kb; the transposable element density is one element every 171 kb. Of 73 genes in this region identified by genetic analysis, 49 have been located on the sequence; P-element insertions have been mapped to 43 genes. Ninety-five (44%) of the known and predicted genes match a Drosophila EST, and 144 (66%) have clear similarities to proteins in other organisms. Genes known to have mutant phenotypes are more likely to be represented in cDNA libraries, and far more likely to have products similar to proteins of other organisms, than are genes with no known mutant phenotype. Over 650 chromosome aberration breakpoints map to this chromosome region, and their nonrandom distribution on the genetic map reflects variation in gene spacing on the DNA. This is the first large-scale analysis of the genome of D. melanogaster at the sequence level. In addition to the direct results obtained, this analysis has allowed us to develop and test methods that will be needed to interpret the complete sequence of the genome of this species. Before beginning a Hunt, it is wise to ask someone what you are looking for before you begin looking for it. Milne 1926

Solution structure of human GAIP (Galpha interacting protein): a regulator of G protein signaling

The solution structure of the human protein GAIP (Galpha interacting protein), a regulator of G protein signaling, has been determined by NMR techniques. Dipolar couplings of the oriented protein in two different liquid crystal media have been used in the structure calculation. The solution structure of GAIP is compared to the crystal structure of an homologous protein from rat (RGS4) complexed to the alpha-subunit of a G protein. Some of RGS4 residues involved in the Galpha-RGS binding interface have similar orientations in GAIP (free form), indicating that upon binding these residues do not suffer conformational rearrangements, and therefore, their role does not seem to be restricted to Galpha interaction but also to RGS folding and stability. We suggest that other structural differences between the two proteins may be related to the process of binding as well as to a distinct efficiency in their respective GTPase activating function.

Antagonism between G(o)alpha and G(q)alpha in Caenorhabditis elegans: the RGS protein EAT-16 is necessary for G(o)alpha signaling and regulates G(q)alpha activity

To elucidate the cellular role of the heterotrimeric G protein G(o), we have taken a molecular genetic approach in Caenorhabditis elegans. We screened for suppressors of activated GOA-1 (G(o)alpha) that do not simply decrease its expression and found mutations in only two genes, sag-1 and eat-16. Animals defective in either gene display a hyperactive phenotype similar to that of goa-1 loss-of-function mutants. Double-mutant analysis indicates that both sag-1 and eat-16 act downstream of, or parallel to, G(o)alpha and negatively regulate EGL-30 (G(q)alpha) signaling. eat-16 encodes a regulator of G protein signaling (RGS) most similar to the mammalian RGS7 and RGS9 proteins and can inhibit endogenous mammalian G(q)/G(11) in COS-7 cells. Animals defective in both sag-1 and eat-16 are inviable, but reducing function in egl-30 restores viability, indicating that the lethality of the eat-16; sag-1 double mutant is due to excessive G(q)alpha activity. Analysis of these mutations indicates that the G(o) and G(q) pathways function antagonistically in C. elegans, and that G(o)alpha negatively regulates the G(q) pathway, possibly via EAT-16 or SAG-1. We propose that a major cellular role of G(o) is to antagonize signaling by G(q).

Amino-terminal cysteine residues of RGS16 are required for palmitoylation and modulation of Gi- and Gq-mediated signaling

RGS proteins (Regulators of G protein Signaling) are a recently discovered family of proteins that accelerate the GTPase activity of heterotrimeric G protein alpha subunits of the i, q, and 12 classes. The proteins share a homologous core domain but have divergent amino-terminal sequences that are the site of palmitoylation for RGS-GAIP and RGS4. We investigated the function of palmitoylation for RGS16, which shares conserved amino-terminal cysteines with RGS4 and RGS5. Mutation of cysteine residues at residues 2 and 12 blocked the incorporation of [3H]palmitate into RGS16 in metabolic labeling studies of transfected cells or into purified RGS proteins in a cell-free palmitoylation assay. The purified RGS16 proteins with the cysteine mutations were still able to act as GTPase-activating protein for Gialpha. Inhibition or a decrease in palmitoylation did not significantly change the amount of protein that was membrane-associated. However, palmitoylation-defective RGS16 mutants demonstrated impaired ability to inhibit both Gi- and Gq-linked signaling pathways when expressed in HEK293T cells. These findings suggest that the amino-terminal region of RGS16 may affect the affinity of these proteins for Galpha subunits in vivo or that palmitoylation localizes the RGS protein in close proximity to Galpha subunits on cellular membranes.

Structural basis for the selectivity of the RGS protein, GAIP, for Galphai family members. Identification of a single amino acid determinant for selective interaction of Galphai subunits with GAIP

GAIP is a regulator of G protein signaling (RGS) that accelerates the rate of GTP hydrolysis by some G protein alpha subunits. In the present studies, we have examined the structural basis for the ability of GAIP to discriminate among members of the Galphai family. Galphai1, Galphai3, and Galphao interacted strongly with GAIP, whereas Galphai2 interacted weakly and Galphas did not interact at all. A chimeric G protein composed of a Galphai2 N terminus and a Galphai1 C terminus interacted as strongly with GAIP as native Galphai1, whereas a chimeric N-terminal Galphai1 with a Galphai2 C terminus did not interact. These results suggest that the determinants responsible for GAIP selectivity between these two Galphais reside within the C-terminal GTPase domain of the G protein. To further localize residues contributing to G protein-GAIP selectivity, a panel of 15 site-directed Galphai1 and Galphai2 mutants were assayed. Of the Galphai1 mutants tested, only that containing a mutation at aspartate 229 located at the N terminus of Switch 3 did not interact with GAIP. Furthermore, the only Galphai2 variant that interacted strongly with GAIP contained a replacement of the corresponding Galphai2 Switch 3 residue (Ala230) with aspartate. To determine whether GAIP showed functional preferences for Galpha subunits that correlate with the binding data, the ability of GAIP to enhance the GTPase activity of purified alpha subunits was tested. GAIP catalyzed a 3-5-fold increase in the rate of GTP hydrolysis by Galphai1 and Galphai2(A230D) but no increase in the rate of Galphai2 and less than a 2-fold increase in the rate of Galphai1(D229A) under the same conditions. Thus, GAIP was able to discriminate between Galphai1 and Galphai2 in both binding and functional assays, and in both cases residue 229/230 played a critical role in selective recognition.

Regulators of G-protein signaling (RGS) 1 and 16 are induced in response to bacterial lipopolysaccharide and stimulate c-fos promoter expression

Regulators of G-protein signaling (RGS) are negative regulators of G-protein-coupled receptor (GPCR) signaling. Sepsis is a pathophysiological condition that is induced primarily in response to bacterial infection and is associated with decreased responsiveness to a number of vasoactive GPCR agonists. Using a degenerate RT-PCR screen, we report that RGS1 and RGS16 were amplified from the heart and aorta of septic animals. By Northern blot analysis, RGS1 and RGS16 were upregulated, with their respective levels increasing 6- and 50-fold in septic hearts. Using a yeast-based bioassay, both RGS1 and RGS16 were found to be equipotent in their ability to attenuate GPCR signaling. These results suggest that both RGS1 and RGS16 contribute to the sepsis-mediated decrease in GPCR signaling. Elevated levels of some RGSs may also lead to an increase in Gbetagamma-activated signaling pathways in the absence of GPCR agonists. Using a c-fos luciferase reporter gene that is responsive to Gbetagamma-activated signaling pathways, we observed a respective 8- and 7-fold increase in the basal luciferase in serum-deprived transfected mammalian cells overexpressing RGS1 or RGS16. This suggests that RGSs play a role in promoting the sepsis-mediated increases in the activation of intracellular signal transduction pathways.

Clathrin-coated vesicles bearing GAIP possess GTPase-activating protein activity in vitro

Galpha-interacting protein (GAIP) is a member of the RGS (regulators of G protein signaling) family, which serve as GAPs (GTPase-activating proteins) for Galpha subunits. Previously, we demonstrated that GAIP is localized on clathrin-coated vesicles (CCVs). Here, we tested whether GAIP-enriched vesicles could accelerate the GTPase activity of Galphai proteins. A rat liver fraction containing vesicular carriers (CV2) was enriched (4.5x) for GAIP by quantitative immunoblotting, and GAIP was detected on some of the vesicles in the CV2 fraction by immunoelectron microscopy. When liver fractions were added to recombinant Galphai3 and tested for GAP activity, only the CV2 fraction contained GAP activity. Increasing amounts of CV2 increased the activity, whereas immunodepletion of the CV2 fraction with an antibody against the C terminus of GAIP decreased GAP activity. CCV fractions were prepared from rat liver by using a protocol that maintains the clathrin coats. GAIP was enriched in these fractions and was detected on CCVs by immunogold labeling. Addition of increasing amounts of CCV to recombinant Galphai3 protein increased the GTPase activity. We conclude that CCVs possess GAP activity for Galphai3 and that membrane-associated GAIP is capable of interacting with Galphai3. The reconstitution of the interaction between a heterotrimeric G protein and GAIP on CCVs provides biochemical evidence for a model whereby the G protein and its GAP are compartmentalized on different membranes and come into contact at the time of vesicle fusion. Alternatively, they may be located on the same membrane and segregate at the time of vesicle budding.

Regulators of G protein signaling proteins as determinants of the rate of desensitization of presynaptic calcium channels

Norepinephrine inhibits omega-conotoxin GVIA-sensitive presynaptic Ca2+ channels in chick dorsal root ganglion neurons through two pathways, one mediated by Go and the other by Gi. These pathways desensitize at different rates. We have found that recombinant Galpha interacting protein (GAIP) and regulators of G protein signaling (RGS)4 selectively accelerate the rate of desensitization of Go- and Gi-mediated pathways, respectively. Blockade of endogenous RGS proteins using antibodies raised against Galpha interacting protein and RGS4 slows the rate of desensitization of these pathways in a selective manner. These results demonstrate that different RGS proteins may interact with Gi and Go selectively, giving rise to distinct time courses of transmitter-mediated effects.

A PDZ protein regulates the distribution of the transmembrane semaphorin, M-SemF

M-SemF is a membrane-associated, neurally enriched member of the semaphorin family of axon guidance signals. We considered whether the cytoplasmic domain of M-SemF might possess a signaling function and/or might control the distribution of M-SemF on the cell surface. We identify a PDZ-containing neural protein as an M-SemF cytoplasmic domain-associated protein (SEMCAP-1). SEMCAP-2 is a closely related nonneuronal protein. SEMCAP-1 has recently also been identified as GIPC, by virtue of its interaction with the RGS protein GAIP in vitro (De Vries, L., Lou, X., Zhao, G., Zheng, B., and Farquhar, M. G. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 12340-12345). Expression studies support the notion that SEMCAP-1(GIPC) interacts with M-SemF, but not GAIP, in brain. Lung SEMCAP-2 and SEMCAP-1(GIPC) are potential partners for both GAIP and M-SemF. The protein interaction requires the single PDZ domain of SEMCAP-1(GIPC) and the carboxyl-terminal four residues of M-SemF, ESSV. While SEMCAP-1(GIPC) also interacts with SemC, it does not interact with other proteins containing a class I PDZ binding motif, nor does M-SemF interact with other class I PDZ proteins. Co-expression of SEMCAP-1(GIPC) induces the redistribution of dispersed M-SemF into detergent-resistant aggregates in HEK293 cells. Thus, SEMCAP-1(GIPC) appears to regulate the subcellular distribution of M-SemF in brain, and SEMCAPs could link M-SemF to G protein signal transduction pathways.

Interaction of heterotrimeric G protein Galphao with Purkinje cell protein-2. Evidence for a novel nucleotide exchange factor

The heterotrimeric G protein Galphao is ubiquitously expressed throughout the central nervous system, but many of its functions remain to be defined. To search for novel proteins that interact with Galphao, a mouse brain library was screened using the yeast two-hybrid interaction system. Pcp2 (Purkinje cell protein-2) was identified as a partner for Galphao in this system. Pcp2 is expressed in cerebellar Purkinje cells and retinal bipolar neurons, two locations where Galphao is also expressed. Pcp2 was first identified as a candidate gene to explain Purkinje cell degeneration in pcd mice (Nordquist, D. T., Kozak, C. A., and Orr, H. T. (1988) J. Neurosci. 8, 4780-4789), but its function remains unknown as Pcp2 knockout mice are normal (Mohn, A. R., Feddersen, R. M., Nguyen, M. S., and Koller, B. H. (1997) Mol. Cell. Neurosci. 9, 63-76). Galphao and Pcp2 binding was confirmed in vitro using glutathione S-transferase-Pcp2 fusion proteins and in vitro translated [35S]methionine-labeled Galphao. In addition, when Galphao and Pcp2 were cotransfected into COS cells, Galphao was detected in immunoprecipitates of Pcp2. To determine whether Pcp2 could modulate Galphao function, kinetic constants kcat and koff of bovine brain Galphao were determined in the presence and absence of Pcp2. Pcp2 stimulates GDP release from Galphao more than 5-fold without affecting kcat. These findings define a novel nucleotide exchange function for Pcp2 and suggest that the interaction between Pcp2 and Galphao is important to Purkinje cell function.

loco encodes an RGS protein required for Drosophila glial differentiation

In Drosophila, glial cell development depends on the gene glial cells missing (gcm). gcm activates the expression of other transcription factors such as pointed and repo, which control subsequent glial differentiation. In order to better understand glial cell differentiation, we have screened for genes whose expression in glial cells depends on the activity of pointed. Using an enhancer trap approach, we have identified loco as such a gene. loco is expressed in most lateral CNS glial cells throughout development. Embryos lacking loco function have an normal overall morphology, but fail to hatch. Ultrastructural analysis of homozygous mutant loco embryos reveals a severe glial cell differentiation defect. Mutant glial cells fail to properly ensheath longitudinal axon tracts and do not form the normal glial-glial cell contacts, resulting in a disruption of the blood-brain barrier. Hypomorphic loco alleles were isolated following an EMS mutagenesis. Rare escapers eclose which show impaired locomotor capabilities. loco encodes the first two known Drosophila members of the family of Regulators of G-protein signalling (RGS) proteins, known to interact with the alpha subunits of G-proteins. loco specifically interacts with the Drosophila alphai-subunit. Strikingly, the interaction is not confined to the RGS domain. This interaction and the coexpression of LOCO and Galphai suggests a function of G-protein signalling for glial cell development.

RGS7 and RGS8 differentially accelerate G protein-mediated modulation of K+ currents

The recently discovered family of RGS (regulators of G protein signaling) proteins acts as GTPase activating proteins which bind to alpha subunits of heterotrimeric G proteins. We previously showed that a brain-specific RGS, RGS8 speeds up the activation and deactivation kinetics of the G protein-coupled inward rectifier K+ channel (GIRK) upon receptor stimulation (Saitoh, O., Kubo, Y., Miyatani, Y., Asano, T., and Nakata, H. (1997) Nature 390, 525-529). Here we report the isolation of a full-length rat cDNA of another brain-specific RGS, RGS7. In situ hybridization study revealed that RGS7 mRNA is predominantly expressed in Golgi cells within granule cell layer of cerebellar cortex. We observed that RGS7 recombinant protein binds preferentially to Galphao, Galphai3, and Galphaz. When co-expressed with GIRK1/2 in Xenopus oocytes, RGS7 and RGS8 differentially accelerate G protein-mediated modulation of GIRK. RGS7 clearly accelerated activation of GIRK current similarly with RGS8 but the acceleration effect of deactivation was significantly weaker than that of RGS8. These acceleration properties of RGS proteins may play important roles in the rapid regulation of neuronal excitability and the cellular responses to short-lived stimulations.

Modulation of renal tubular cell function by RGS3

The recently discovered family of regulators of G protein signaling (RGS) accelerates the intrinsic GTPase activity of certain Galpha subunits, thereby terminating G protein signaling. Particularly high mRNA levels of one family member, RGS3, are found in the adult kidney. To establish the temporal and spatial renal expression pattern of RGS3, a polyclonal antiserum was raised against the COOH terminus of RGS3. Staining of mouse renal tissue at different gestational stages revealed high levels of RGS3 within the developing and mature tubular epithelial cells. We tested whether RGS3 can modulate tubular migration, an important aspect of tubular development, in response to G protein-mediated signaling. Several mouse intermedullary collecting duct (mIMCD-3) cell lines were generated that expressed RGS3 under the control of an inducible promoter. Lysophosphatidic acid (LPA) is a potent chemoattractant that mediates its effects through heterotrimeric G proteins. We found that induction of RGS3 significantly reduced LPA-mediated cell migration in RGS3-expressing mIMCD-3 clones, whereas chemotaxis induced by hepatocyte growth factor remained unaffected by RGS3. Our findings suggest that RGS3 modulates tubular functions during renal development and in the adult kidney.

RGS proteins determine signaling specificity of Gq-coupled receptors

Regulators of G protein signaling (RGS) proteins accelerate GTP hydrolysis by Galpha subunits, thereby attenuating signaling. RGS4 is a GTPase-activating protein for Gi and Gq class alpha subunits. In the present study, we used knockouts of Gq class genes in mice to evaluate the potency and selectivity of RGS4 in modulating Ca2+ signaling transduced by different Gq-coupled receptors. RGS4 inhibited phospholipase C activity and Ca2+ signaling in a receptor-selective manner in both permeabilized cells and cells dialyzed with RGS4 through a patch pipette. Receptor-dependent inhibition of Ca2+ signaling by RGS4 was observed in acini prepared from the rat and mouse pancreas. The response of mouse pancreatic acini to carbachol was about 4- and 33-fold more sensitive to RGS4 than that of bombesin and cholecystokinin (CCK), respectively. RGS1 and RGS16 were also potent inhibitors of Gq-dependent Ca2+ signaling and acted in a receptor-selective manner. RGS1 showed approximately 1000-fold higher potency in inhibiting carbachol than CCK-dependent signaling. RGS16 was as effective as RGS1 in inhibiting carbachol-dependent signaling but only partially inhibited the response to CCK. By contrast, RGS2 inhibited the response to carbachol and CCK with equal potency. The same pattern of receptor-selective inhibition by RGS4 was observed in acinar cells from wild type and several single and double Gq class knockout mice. Thus, these receptors appear to couple Gq class alpha subunit isotypes equally. Difference in receptor selectivity of RGS proteins action indicates that regulatory specificity is conferred by interaction of RGS proteins with receptor complexes.

GAIP, a Galphai-3-binding protein, is associated with Golgi-derived vesicles and protein trafficking

Proteins of the regulators of G protein signaling (RGS) family bind to Galpha subunits to downregulate their signaling in a variety of systems. Galpha-interacting protein (GAIP) is a mammalian RGS protein that shows high affinity for the activated state of Galphai-3, a protein known to regulate post-Golgi trafficking of secreted proteins in kidney epithelial cells. This study aimed to localize GAIP in epithelial cells and to investigate its potential role in the regulation of membrane trafficking. LLC-PK1 cells were stably transfected with a c-myc-tagged GAIP cDNA. In the transfected and untransfected cells, GAIP was found in the cytosol and on cell membranes. Immunogold labeling showed that membrane-bound GAIP was localized on budding vesicles around Golgi stacks. When an in vitro assay was used to generate vesicles from isolated rat liver and Madin-Darby canine kidney cell Golgi membranes, GAIP was found to be concentrated in fractions of newly budded Golgi vesicles. Finally, the constitutive trafficking and secretion of sulfated proteoglycans was measured in cell lines overexpressing GAIP. We show evidence for GAIP regulation of secretory trafficking before the level of the trans-Golgi network but not in post-Golgi secretion. The location and functional effects of GAIP overlap only partially with those of Galphai-3 and suggest multiple roles for GAIP in epithelial cells.

Subcellular localization of the Galphai3 protein and G alpha interacting protein, two proteins involved in the control of macroautophagy in human colon cancer HT-29 cells

Autophagic sequestration is controlled by the Galphai3 protein in human colon cancer HT-29 cells. Immunofluorescence and subcellular fractionation studies showed that the Galphai3 protein is preferentially associated with Golgi membranes but co-localization was also observed with the endoplasmic reticulum (ER) membrane. The Galphai2 protein, which is not involved in the control of autophagic sequestration, is associated with the plasma membrane. Transfection of chimaeric Galphai proteins (Galphai3/2, Galphai2/3) containing the N- and C-terminal parts of the relevant Galphai demonstrated that the C-terminal part of the Galphai3 protein, by governing its membrane localization [de Almeida, Holtzman, Peters, Ercolani, Ausiello and Stow (1994) J. Cell Sci. 107, 507-515], is important in the control of macroautophagic sequestration. G alpha interacting protein (GAIP),which stimulates the GTPase activity of the Galphai3 protein and favours macroautophagic sequestration in HT-29 cells,was shown, by immunofluorescence studies using confocal microscopy, to be confined to the cytoplasm. The cytoplasmic distribution of GAIP only partially overlaps with that of the Galphai3 protein. However, the presence of the two proteins on Golgi and ER membranes was confirmed by subcellular fractionation. These results point to the importance of the cytoplasmic localization of the Galphai3 protein and GAIP in controlling autophagic sequestration in HT-29 cells.

Regulators of G protein signaling attenuate the G protein-mediated inhibition of N-type Ca channels

Regulators of G protein signaling (RGS) proteins bind to the alpha subunits of certain heterotrimeric G proteins and greatly enhance their rate of GTP hydrolysis, thereby determining the time course of interactions among Galpha, Gbetagamma, and their effectors. Voltage-gated N-type Ca channels mediate neurosecretion, and these Ca channels are powerfully inhibited by G proteins. To determine whether RGS proteins could influence Ca channel function, we recorded the activity of N-type Ca channels coexpressed in human embryonic kidney (HEK293) cells with G protein-coupled muscarinic (m2) receptors and various RGS proteins. Coexpression of full-length RGS3T, RGS3, or RGS8 significantly attenuated the magnitude of receptor-mediated Ca channel inhibition. In control cells expressing alpha1B, alpha2, and beta3 Ca channel subunits and m2 receptors, carbachol (1 microM) inhibited whole-cell currents by approximately 80% compared with only approximately 55% inhibition in cells also expressing exogenous RGS protein. A similar effect was produced by expression of the conserved core domain of RGS8. The attenuation of Ca current inhibition resulted primarily from a shift in the steady state dose-response relationship to higher agonist concentrations, with the EC50 for carbachol inhibition being approximately 18 nM in control cells vs. approximately 150 nM in RGS-expressing cells. The kinetics of Ca channel inhibition were also modified by RGS. Thus, in cells expressing RGS3T, the decay of prepulse facilitation was slower, and recovery of Ca channels from inhibition after agonist removal was faster than in control cells. The effects of RGS proteins on Ca channel modulation can be explained by their ability to act as GTPase-accelerating proteins for some Galpha subunits. These results suggest that RGS proteins may play important roles in shaping the magnitude and kinetics of physiological events, such as neurosecretion, that involve G protein-modulated Ca channels.

RGS3 inhibits G protein-mediated signaling via translocation to the membrane and binding to Galpha11

In the present study, we investigated the function and the mechanism of action of RGS3, a member of a family of proteins called regulators of G protein signaling (RGS). Polyclonal antibodies against RGS3 were produced and characterized. An 80-kDa protein was identified as RGS3 by immunoprecipitation and immunoblotting with anti-RGS3 antibodies in a human mesangial cell line (HMC) stably transfected with RGS3 cDNA. Coimmunoprecipitation experiments in RGS3-overexpressing cell lysates revealed that RGS3 bound to aluminum fluoride-activated Galpha11 and to a lesser extent to Galphai3 and that this binding was mediated by the RGS domain of RGS3. A role of RGS3 in postreceptor signaling was demonstrated by decreased calcium responses and mitogen-activated protein (MAP) kinase activity induced by endothelin-1 in HMC stably overexpressing RGS3. Moreover, depletion of endogenous RGS3 by transfection of antisense RGS3 cDNA in NIH 3T3 cells resulted in enhanced MAP kinase activation induced by endothelin-1. The study of intracellular distribution of RGS3 indicated its unique cytosolic localization. Activation of G proteins by AlF4-, NaF, or endothelin-1 resulted in redistribution of RGS3 from cytosol to the plasma membrane as determined by Western blotting of the cytosolic and particulate fractions with RGS3 antiserum as well as by immunofluorescence microscopy. Agonist-induced translocation of RGS3 occurred by a dual mechanism involving both C-terminal (RGS domain) and N-terminal regions of RGS3. Thus, coexpression of RGS3 with a constitutively active mutant of Galpha11 (Galpha11-QL) resulted in the binding of RGS3, but not of its N-terminal fragment, to the membrane fraction and in its interaction with Galpha11-QL in vitro without any stimuli. However, both full-length RGS3 and its N-terminal domain translocated to the plasma membrane upon stimulation of intact cells with endothelin-1 as assayed by immunofluorescence microscopy. The effect of endothelin-1 was also mimicked by calcium ionophore A23187, suggesting the importance of Ca2+ in the mechanism of redistribution of RGS3. These data indicate that RGS3 inhibits G protein-coupled receptor signaling by a complex mechanism involving its translocation to the membrane in addition to its established function as a GTPase-activating protein.

dRGS7 encodes a Drosophila homolog of EGL-10 and vertebrate RGS7

We identified a Drosophila gene encoding a homolog of the regulator of G-protein signaling (RGS) protein family. This gene (dRGS7) is expressed in neurons of the embryo and adult fly and is predicted to encode a 428-amino acid protein with >55% overall amino acid sequence identity with the vertebrate protein RGS7 and the C. elegans EGL-10. The dRGS7 protein is 50% conserved in the C-terminal RGS domain with RGS7 and EGL-10 but, remarkably, displays much greater conservation with the N-terminal regions of these proteins. This finding implies a conserved function for these homologs from divergent species involving domains outside the RGS domain. The dRGS7 protein also has a domain of similarity with Dishevelled and pleckstrin, raising the possibility that these proteins interact with common signaling components.

Role of regulator of G protein signaling in desensitization of the glucose-dependent insulinotropic peptide receptor

The glucose-dependent insulinotropic peptide receptor (GIP-R) is a member of the G protein-coupled receptors. Recent studies have indicated that elevated serum GIP concentrations in type II diabetic patients might induce desensitization of the GIP-R, and this mechanism could contribute to impaired insulin secretion. The cellular and molecular mechanisms governing GIP desensitization are unknown. Here, we report the results of studies on a new family of proteins known as regulators of G protein signaling (RGS) that have been shown to mediate the desensitization process of other receptors. GIP-R and RGS1, -2, -3, and -4 complementary DNAs were cotransfected into human embryonic kidney cells (L293). GIP-stimulated cAMP generation in control cells and in those coexpressing RGS1, -3, and -4 displayed a dose-dependent increase 10 min after GIP treatment. In contrast, RGS2 expression inhibited the GIP-induced cAMP response by 50%, a response similar to that of cells desensitized by preincubation with 10(-7) M GIP. In betaTC3 cells, preincubation of GIP attenuated GIP-induced insulin release by 45% at 15 min and by 55% at 30 min. Expression of RGS2 in the betaTC3 cells significantly decreased GIP-stimulated insulin secretion, whereas glucose-induced insulin release was not affected. RGS2 messenger RNA was identified by Northern blot analysis to be expressed endogenously in betaTC3 and L293 cells, and its level was significantly induced by GIP treatment in betaTC3 cells. Moreover, RGS2 bound Gs alpha protein in an in vitro system, suggesting that RGS2 attenuated the Gs-adenylate cyclase signaling pathway. These results suggest a potential role for RGS2 in modulating GIP-mediated insulin secretion in pancreatic islet cells.

Regulation of chemotactic and proadhesive responses to chemoattractant receptors by RGS (regulator of G-protein signaling) family members

Serpentine Galphai-linked receptors support rapid adhesion and directed migration of leukocytes and other cell types. The intracellular mechanisms mediating and regulating chemoattractant-directed adhesion and locomotion are only now beginning to be explored. RGS (for regulator of G-protein signaling) proteins are a recently described family that regulate Galphai-stimulated pathways by acting as GTPase-activating proteins. Little is known about the GTPase activity of the Galphai proteins involved in adhesion and chemotaxis, or the significance of their regulation to these responses. Using transiently transfected lymphoid cells as a model system, we show that expression of RGS1, RGS3, and RGS4 inhibits chemoattractant-induced migration. In contrast, RGS2, a regulator of Galphaq activity, had no effect on cell migration to any chemoattractant. RGS1, RGS3, and RGS4 also reduced rapid chemoattractant-triggered adhesion, although the proadhesive response appears quantitatively less sensitive to RGS action than chemotaxis. The results suggest that the duration of the Galphai signal may be a particularly important parameter in the chemotactic responses of leukocytes, and demonstrate the potential for RGS family members to regulate cellular adhesive and migratory behaviors.

GIPC, a PDZ domain containing protein, interacts specifically with the C terminus of RGS-GAIP

We have identified a mammalian protein called GIPC (for GAIP interacting protein, C terminus), which has a central PDZ domain and a C-terminal acyl carrier protein (ACP) domain. The PDZ domain of GIPC specifically interacts with RGS-GAIP, a GTPase-activating protein (GAP) for Galphai subunits recently localized on clathrin-coated vesicles. Analysis of deletion mutants indicated that the PDZ domain of GIPC specifically interacts with the C terminus of GAIP (11 amino acids) in the yeast two-hybrid system and glutathione S-transferase (GST)-GIPC pull-down assays, but GIPC does not interact with other members of the RGS (regulators of G protein signaling) family tested. This finding is in keeping with the fact that the C terminus of GAIP is unique and possesses a modified C-terminal PDZ-binding motif (SEA). By immunoblotting of membrane fractions prepared from HeLa cells, we found that there are two pools of GIPC-a soluble or cytosolic pool (70%) and a membrane-associated pool (30%). By immunofluorescence, endogenous and GFP-tagged GIPC show both a diffuse and punctate cytoplasmic distribution in HeLa cells reflecting, respectively, the existence of soluble and membrane-associated pools. By immunoelectron microscopy the membrane pool of GIPC is associated with clusters of vesicles located near the plasma membrane. These data provide direct evidence that the C terminus of a RGS protein is involved in interactions specific for a given RGS protein and implicates GAIP in regulation of additional functions besides its GAP activity. The location of GIPC together with its binding to GAIP suggest that GAIP and GIPC may be components of a G protein-coupled signaling complex involved in the regulation of vesicular trafficking. The presence of an ACP domain suggests a putative function for GIPC in the acylation of vesicle-bound proteins.

RGSZ1, a Gz-selective RGS protein in brain. Structure, membrane association, regulation by Galphaz phosphorylation, and relationship to a Gz gtpase-activating protein subfamily

We cloned the cDNA for human RGSZ1, the major Gz-selective GTPase-activating protein (GAP) in brain (Wang, J., Tu, Y., Woodson, J., Song, X., and Ross, E. M. (1997) J. Biol. Chem. 272, 5732-5740) and a member of the RGS family of G protein GAPs. Its sequence is 83% identical to RET-RGS1 (except its N-terminal extension) and 56% identical to GAIP. Purified, recombinant RGSZ1, RET-RGS1, and GAIP each accelerated the hydrolysis of Galphaz-GTP over 400-fold with Km values of approximately 2 nM. RGSZ1 was 100-fold selective for Galphaz over Galphai, unusually specific among RGS proteins. Other enzymological properties of RGSZ1, brain Gz GAP, and RET-RGS1 were identical; GAIP differed only in Mg2+ dependence and in its slightly lower selectivity for Galphaz. RGSZ1, RET-RGS1, and GAIP thus define a subfamily of Gz GAPs within the RGS proteins. RGSZ1 has no obvious membrane-spanning region but is tightly membrane-bound in brain. Its regulatory activity in membranes depends on stable bilayer association. When co-reconstituted into phospholipid vesicles with Gz and m2 muscarinic receptors, RGSZ1 increased agonist-stimulated GTPase >15-fold with EC50 <12 nM, but RGSZ1 added to the vesicle suspension was <0.1% as active. RGSZ1, RET-RGS1, and GAIP share a cysteine string sequence, perhaps targeting them to secretory vesicles and allowing them to participate in the proposed control of secretion by Gz. Phosphorylation of Galphaz by protein kinase C inhibited the GAP activity of RGSZ1 and other RGS proteins, providing a mechanism for potentiation of Gz signaling by protein kinase C.

Vesicle budding on Golgi membranes: regulation by G proteins and myosin motors

One of the main functions of the Golgi complex is to generate transport vesicles for the post-Golgi trafficking of proteins in secretory pathways. Many different populations of vesicles are distinguished by unique sets of structural and regulatory proteins which participate in vesicle budding and fusion. Monomeric and heterotrimeric G proteins regulate vesicle budding and secretory traffic into and out of the Golgi complex. An inventory of G protein alpha subunits associated with Golgi membranes highlights their diverse involvement and potential for coupling Golgi trafficking, through various signal transduction pathways, to cell growth or other more specialized cell functions. Cytoskeletal proteins are now also known to associate specifically with the Golgi complex and Golgi-derived vesicles. Amongst these, conventional and unconventional myosins are recruited to vesicle membranes. Several roles in vesicle budding and vesicle trafficking can be proposed for these actin-based motors.

Expression of GTPase-deficient Gialpha2 results in translocation of cytoplasmic RGS4 to the plasma membrane

The members of a recently identified protein family termed regulators of G-protein signaling (RGS) act as GTPase-activating proteins for certain Galpha subunits in vitro, but their physiological effects in cells are uncertain in the face of similar biochemical activity and overlapping patterns of tissue expression. Consistent with its activity in in vitro GTPase-activating protein assays, RGS4 interacts efficiently with endogenous proteins of the Gi and Gq subclasses of Galpha subunits but not with G12alpha or Gsalpha. Unlike other RGS proteins such as RGS9, RGS-GAIP, and Sst2p, which have been reported to be largely membrane-associated, a majority of cellular RGS4 is found as a soluble protein in the cytoplasm. However, the expression of a GTPase-deficient Gialpha subunit (Gialpha2-Q204L) resulted in the translocation of both wild type RGS4 and a non-Gialpha-binding mutant (L159F) to the plasma membrane. These data suggest that RGS4 may be recruited to the plasma membrane indirectly by G-protein activation and that multiple RGS proteins within a given cell might be differentially localized to determine a physiologic response to a G-protein-linked stimulus.

The mammalian calcium-binding protein, nucleobindin (CALNUC), is a Golgi resident protein

We have identified CALNUC, an EF-hand, Ca2+-binding protein, as a Golgi resident protein. CALNUC corresponds to a previously identified EF-hand/calcium-binding protein known as nucleobindin. CALNUC interacts with Galphai3 subunits in the yeast two-hybrid system and in GST-CALNUC pull-down assays. Analysis of deletion mutants demonstrated that the EF-hand and intervening acidic regions are the site of CALNUC's interaction with Galphai3. CALNUC is found in both cytosolic and membrane fractions. The membrane pool is tightly associated with the luminal surface of Golgi membranes. CALNUC is widely expressed, as it is detected by immunofluorescence in the Golgi region of all tissues and cell lines examined. By immunoelectron microscopy, CALNUC is localized to cis-Golgi cisternae and the cis-Golgi network (CGN). CALNUC is the major Ca2+-binding protein detected by 45Ca2+-binding assay on Golgi fractions. The properties of CALNUC and its high homology to calreticulin suggest that it may play a key role in calcium homeostasis in the CGN and cis-Golgi cisternae.

Axin, an inhibitor of the Wnt signalling pathway, interacts with beta-catenin, GSK-3beta and APC and reduces the beta-catenin level

BACKGROUND

The Wnt/Wingless signalling pathway plays an important role in both embryonic development and tumorigenesis. Beta-catenin and Axin are positive and negative effectors of the Wnt signalling pathway, respectively.

RESULTS

We found that Axin interacts with beta-catenin and glycogen synthase kinase-3beta (GSK-3beta). Furthermore, the regulation of the G-protein signalling (RGS) domain of Axin is associated with the colorectal tumour suppressor adenomatous polyposis coli (APC). Overexpression of Axin in the human colorectal cancer cell line SW480 induced a drastic reduction in the level of -catenin. Interaction with beta-catenin and GSK-3beta was required for the Axin-mediated beta-catenin reduction.

CONCLUSION

Axin interacts with beta-catenin, GSK-3beta and APC, and negatively regulates the Wnt signalling pathway, presumably by regulating the level of beta-catenin.

A point mutation in Galphao and Galphai1 blocks interaction with regulator of G protein signaling proteins

Regulator of G protein-signaling (RGS) proteins accelerate GTP hydrolysis by Galpha subunits and are thought to be responsible for rapid deactivation of enzymes and ion channels controlled by G proteins. We wanted to identify and characterize Gi-family alpha subunits that were insensitive to RGS action. Based on a glycine to serine mutation in the yeast Galpha subunit Gpa1(sst) that prevents deactivation by Sst2 (DiBello, P. R., Garrison, T. R., Apanovitch, D. M., Hoffman, G., Shuey, D. J., Mason, K., Cockett, M. I., and Dohlman, H. G. (1998) J. Biol. Chem. 273, 5780-5784), site-directed mutagenesis of alphao and alphai1 was done. G184S alphao and G183S alphai1 show kinetics of GDP release and GTP hydrolysis similar to wild type. In contrast, GTP hydrolysis by the G --> S mutant proteins is not stimulated by RGS4 or by a truncated RGS7. Quantitative flow cytometry binding studies show IC50 values of 30 and 96 nM, respectively, for aluminum fluoride-activated wild type alphao and alphai1 to compete with fluorescein isothiocyanate-alphao binding to glutathione S-transferase-RGS4. The G --> S mutant proteins showed a greater than 30-100-fold lower affinity for RGS4. Thus, we have defined the mechanism of a point mutation in alphao and alphai1 that prevents RGS binding and GTPase activating activity. These mutant subunits should be useful in biochemical or expression studies to evaluate the role of endogenous RGS proteins in Gi function.

Plasma membrane localization is required for RGS4 function in Saccharomyces cerevisiae

RGS4, a mammalian GTPase activating protein for G protein alpha subunits, was identified by its ability to inhibit the pheromone response pathway in Saccharomyces cerevisiae. To define regions of RGS4 necessary for its function in vivo, we assayed mutants for activity in this system. Deletion of the N-terminal 33 aa of RGS4 (Delta1-33) yielded a nonfunctional protein and loss of plasma membrane localization. These functions were restored by addition of a C-terminal membrane-targeting sequence to RGS4 (Delta1-33). Thus, plasma membrane localization is tightly coupled with the ability of RGS4 to inhibit signaling. Fusion of the N-terminal 33 aa of RGS4 to green fluorescent protein was sufficient to localize an otherwise soluble protein to the plasma membrane, defining this N-terminal region as a plasma membrane anchorage domain. RGS4 is palmitoylated, with Cys-2 and Cys-12 the likely sites of palmitoylation. Surprisingly, mutation of the cysteine residues within the N-terminal domain of RGS4 did not affect plasma membrane localization in yeast or the ability to inhibit signaling. Features of the N-terminal domain other than palmitoylation are responsible for the plasma membrane association of RGS4 and its ability to inhibit pheromone response in yeast.

RGS-GAIP, a GTPase-activating protein for Galphai heterotrimeric G proteins, is located on clathrin-coated vesicles

RGS-GAIP (Galpha-interacting protein) is a member of the RGS (regulator of G protein signaling) family of proteins that functions to down-regulate Galphai/Galphaq-linked signaling. GAIP is a GAP or guanosine triphosphatase-activating protein that was initially discovered by virtue of its ability to bind to the heterotrimeric G protein Galphai3, which is found on both the plasma membrane (PM) and Golgi membranes. Previously, we demonstrated that, in contrast to most other GAPs, GAIP is membrane anchored and palmitoylated. In this work we used cell fractionation and immunocytochemistry to determine with what particular membranes GAIP is associated. In pituitary cells we found that GAIP fractionated with intracellular membranes, not the PM; by immunogold labeling GAIP was found on clathrin-coated buds or vesicles (CCVs) in the Golgi region. In rat liver GAIP was concentrated in vesicular carrier fractions; it was not found in either Golgi- or PM-enriched fractions. By immunogold labeling it was detected on clathrin-coated pits or CCVs located near the sinusoidal PM. These results suggest that GAIP may be associated with both TGN-derived and PM-derived CCVs. GAIP represents the first GAP found on CCVs or any other intracellular membranes. The presence of GAIP on CCVs suggests a model whereby a GAP is separated in space from its target G protein with the two coming into contact at the time of vesicle fusion.

G-protein signaling: fine-tuning signaling kinetics

Mammalian 'regulators of G protein signaling' (RGS proteins) help shut off G-protein-mediated signaling by GTPase activation. But new evidence shows that RGS proteins can also speed up the activation of signaling. The combined effect is a change in signaling kinetics without a decrease in signaling intensity.

Signal transduction in the Caenorhabditis elegans nervous system

Caenorhabditis elegans interacts with its environment by sensing chemicals, touch, and temperature; genetic analysis of each of these responses has led to the identification of candidate signaling molecules within sensory neurons. A molecular model for touch sensation has emerged from studies of the mechanosensory response; the receptors and signal transduction mechanisms in olfactory neurons are being elucidated; and an unusual neuroendocrine role for a TGF-beta-related peptide in chemosensory neurons has been discovered. Presynaptic and postsynaptic components of neuronal synapses have been identified in behavioral and pharmacological mutant screens. Mutations have been found in multiple classes of nicotinic acetylcholine receptor genes, excitatory and inhibitory glutamate receptor genes, and candidate gap junction genes, allowing their function to be studied in vivo. Different G-protein signaling pathways have characteristic effects on behavior, neuronal degeneration, and embryonic development.

Potential regulatory roles for G protein-coupled receptor kinases and beta-arrestins in gonadotropin-releasing hormone receptor signaling

GnRH stimulates gonadotropin secretion, which desensitizes unless the releasing hormone is secreted or administered in a pulsatile fashion. The mechanism of desensitization is unknown, but as the GnRH receptor is G protein coupled, it might involve G protein-coupled receptor kinases (GRKs). Such kinases phosphorylate the intracellular regions of seven-transmembrane receptors, permitting beta-arrestin to bind, which prevents the receptor from activating G proteins. Here, we tested the effect of GRKs and beta-arrestins on GnRH-induced inositol trisphosphate (IP3) production in COS cells transfected with the GnRH receptor complementary DNA. GRK2, -3, and -6 overexpression inhibited IP3 production by 50-75% during the 30 sec of GnRH treatment. Coexpression of GRK2 and beta-arrestin-2 suppressed GnRH-induced IP3 production more than that of either alone. Immunocytochemical staining of rat anterior pituitary revealed that all cells expressed GRK2, -3, and -6; all cells also expressed the beta-arrestins. Western blots on cytosolic extracts of rat pituitaries revealed the presence of GRK2/3 and beta-arrestin-1 and -2. The expression of GRKs and beta-arrestins by gonadotropes and their inhibition of GnRH-stimulated IP3 production in COS-1 cells expressing the GnRH receptor suggest a potential regulatory role for the GRK/beta arrestin paradigm in GnRH receptor signaling.

Mutational analysis of the Asn residue essential for RGS protein binding to G-proteins

Members of the RGS family serve as GTPase-activating proteins (GAPs) for heterotrimeric G-proteins and negatively regulate signaling via G-protein-coupled receptors. The recently resolved crystal structure of RGS4 bound to Gialpha1 suggests two potential mechanisms for the GAP activity of RGS proteins as follows: stabilization of the Gialpha1 switch regions by RGS4 and the catalytic action of RGS4 residue Asn128. To elucidate a role of the Asn residue for RGS GAP function, we have investigated effects of the synthetic peptide corresponding to the Galpha binding domain of human retinal RGS (hRGSr) containing the key Asn at position 131, and we have carried out mutational analysis of Asn131. Synthetic peptide hRGSr-(123-140) retained its ability to bind the AlF4--complexed transducin alpha-subunit, Gtalpha.AlF4-, but failed to elicit stimulation of Gtalpha GTPase activity. Wild-type hRGSr stimulated Gtalpha GTPase activity by approximately 10-fold with an EC50 value of 100 nM. Mutant hRGSr proteins with substitutions of Asn131 by Ser and Gln had a significantly reduced affinity for Gtalpha but were capable of substantial stimulation of Gtalpha GTPase activity, 80 and 60% of Vmax, respectively. Mutants hRGSr-Leu131, hRGSr-Ala131, and hRGSr-Asp131 were able to accelerate Gtalpha GTPase activity only at very high concentrations (>10 microM) which appears to correlate with a further decrease of their affinity for transducin. Two mutants, hRGSr-His131 and hRGSr-Delta131, had no detectable binding to transducin. Mutational analysis of Asn131 suggests that the stabilization of the G-protein switch regions rather than catalytic action of the Asn residue is a key component for the RGS GAP action.

Selective uncoupling of RGS action by a single point mutation in the G protein alpha-subunit

Heterotrimeric G proteins function as molecular relays, shuttling between cell surface receptors and intracellular effectors that propagate a signal. G protein signaling is governed by the rates of GTP binding (catalyzed by the receptor) and GTP hydrolysis. RGS proteins (regulators of G protein signaling) were identified as potent negative regulators of G protein signaling pathways in simple eukaryotes and are now known to act as GTPase-activating proteins (GAPs) for G protein alpha-subunits in vitro. It is not known, however, if Galpha GAP activity is responsible for the regulatory action of RGS proteins in vivo. We describe here a Galpha mutant in yeast (gpa1(sst)) that phenotypically mimics the loss of its cognate RGS protein (SST2). The gpa1(sst) mutant is resistant to an activated allele of SST2 in vivo and is unresponsive to RGS GAP activity in vitro. The analogous mutation in a mammalian Gqalpha is also resistant to RGS action in transfected cells. These mutants demonstrate that RGS proteins act through Galpha and that RGS-GAP activity is responsible for their desensitizing activity in cells. The Galphasst mutant will be useful for uncoupling RGS-mediated regulation from other modes of signal regulation in whole cells and animals.

Asexual sporulation in Aspergillus nidulans

The formation of mitotically derived spores, called conidia, is a common reproductive mode in filamentous fungi, particularly among the large fungal class Ascomycetes. Asexual sporulation strategies are nearly as varied as fungal species; however, the formation of conidiophores, specialized multicellular reproductive structures, by the filamentous fungus Aspergillus nidulans has emerged as the leading model for understanding the mechanisms that control fungal sporulation. Initiation of A. nidulans conidiophore formation can occur either as a programmed event in the life cycle in response to intrinsic signals or to environmental stresses such as nutrient deprivation. In either case, a development-specific set of transcription factors is activated and these control the expression of each other as well as genes required for conidiophore morphogenesis. Recent progress has identified many of the earliest-acting genes needed for initiating conidiophore development and shown that there are at least two antagonistic signaling pathways that control this process. One pathway is modulated by a heterotrimeric G protein that when activated stimulates growth and represses both asexual and sexual sporulation as well as production of the toxic secondary metabolite, sterigmatocystin. The second pathway apparently requires an extracellular signal to induce sporulation-specific events and to direct the inactivation of the first pathway, removing developmental repression. A working model is presented in which the regulatory interactions between these two pathways during the fungal life cycle determine whether cells grow or develop.

Substitution of transducin ser202 by asp abolishes G-protein/RGS interaction

Known RGS proteins stimulate GTPase activity of Gi and Gq family members, but do not interact with Gsalpha and G12alpha. To determine the role of specific Galpha residues for RGS protein recognition, six RGS contact residues of chimeric transducin alpha-subunit (Gtalpha) corresponding to the residues that differ between Gialpha and Gsalpha have been replaced by Gsalpha residues. The ability of human retinal RGS (hRGSr) to bind mutant Gtalpha subunits and accelerate their GTPase activity was investigated. Substitutions Thr178 --> Ser, Ile181 --> Phe, and Lys205 --> Arg of Gtalpha did not alter its interaction with hRGSr. The Lys176 --> Leu mutant had the same affinity for hRGSr as Gtalpha, but the maximal GTPase stimulation by hRGSr was reduced by approximately 2.5-fold. The substitution His209 --> Gln led to a 3-fold decrease in the affinity of hRGSr for the Gtalpha mutant without significantly affecting the maximal GTPase enhancement. The Ser202 --> Asp mutation abolished Gtalpha recognition by hRGSr. A counteracting replacement of Glu129 by Ala in hRGSr did not restore the interaction of hRGSr with the Gtalpha Ser202 --> Asp mutant. Our data suggest that the Ser residue at position 202 of Gtalpha is critical for the specificity of RGS proteins toward Gi and Gq families of G-proteins. Consequently, the corresponding residue, Asp229 of Gsalpha, is likely responsible for the inability of RGS proteins to interact with Gsalpha.

The core domain of RGS16 retains G-protein binding and GAP activity in vitro, but is not functional in vivo

The regulators of G-protein signaling (RGS) family members contain a conserved region, the RGS domain, and are GTPase-activating proteins for many members of G-protein alpha-subunits. We report here that the core domain of RGS16 is sufficient for in vitro biochemical functions as assayed by its G-protein binding affinity and its ability to stimulate GTP hydrolysis by G alpha(o) protein. RGS16 also requires, in addition to the RGS domain, the divergent N-terminus for its biological function in the attenuation of pheromone signaling in yeast, whereas its C-terminus region is dispensable. Together with other evidence, these data support the notion that RGS proteins interact with other cellular factors and may serve to link specific G-proteins to different downstream effectors in G-protein-mediated signaling pathways.