Adenylyl cyclase interaction with the D2 dopamine receptor family; differential coupling to Gi, Gz, and Gs

1. The D2-type dopamine receptors are thought to inhibit adenylyl cyclase (AC), via coupling to pertussis toxin (PTX)-sensitive G proteins of the Gi family. We examined whether and to what extent the various D2 receptors (D2S, D2L, D3S, D3L, and D4) couple to the PTX-insensitive G protein Gz, to produce inhibition of AC activity. 2. COS-7 cells were transiently transfected with the individual murine dopamine receptors alone, as well as together with the alpha subunit of Gz. PTX treatment was employed to inactivate endogenous alpha i, and coupling to Gi and Gz was estimated by measuring the inhibition of cAMP accumulation induced by quinpirole, in forskolin-stimulated cells. 3. D2S or D2L receptors can couple to the same extent to Gi and to Gz. The D4 dopamine receptor couples preferably to Gz, resulting in about 60% quinpirole-induced inhibition of cAMP accumulation. The D3S and D3L receptor isoforms couple slightly to Gz and result in 15 and 30% inhibition of cAMP accumulation, respectively. 4. We have demonstrated for the first time that the two D3 receptor isoforms, and not any of the other D2 receptor subtypes, also couple to Gs in both COS-7 and CHO transfected cells, in the presence of PTX. 5. Thus, the differential coupling of the D2 dopamine receptor subtypes to various G proteins may add another aspect to the diversity of dopamine receptor function.

Erf2, a novel gene product that affects the localization and palmitoylation of Ras2 in Saccharomyces cerevisiae

Plasma membrane localization of Ras requires posttranslational addition of farnesyl and palmitoyl lipid moieties to a C-terminal CaaX motif (C is cysteine, a is any aliphatic residue, X is the carboxy terminal residue). To better understand the relationship between posttranslational processing and the subcellular localization of Ras, a yeast genetic screen was undertaken based on the loss of function of a palmitoylation-dependent RAS2 allele. Mutations were identified in an uncharacterized open reading frame (YLR246w) that we have designated ERF2 and a previously described suppressor of hyperactive Ras, SHR5. ERF2 encodes a 41-kDa protein with four predicted transmembrane (TM) segments and a motif consisting of the amino acids Asp-His-His-Cys (DHHC) within a cysteine-rich domain (CRD), called DHHC-CRD. Mutations within the DHHC-CRD abolish Erf2 function. Subcellular fractionation and immunolocalization experiments reveal that Erf2 tagged with a triply iterated hemagglutinin epitope is an integral membrane protein that colocalizes with the yeast endoplasmic reticulum marker Kar2. Strains lacking ERF2 are viable, but they have a synthetic growth defect in the absence of RAS2 and partially suppress the heat shock sensitivity resulting from expression of the hyperactive RAS2(V19) allele. Ras2 proteins expressed in an erf2Delta strain have a reduced level of palmitoylation and are partially mislocalized to the vacuole. Based on these observations, we propose that Erf2 is a component of a previously uncharacterized Ras subcellular localization pathway. Putative members of an Erf2 family of proteins have been uncovered in yeast, plant, worm, insect, and mammalian genome databases, suggesting that Erf2 plays a role in Ras localization in all eucaryotes.

Novel sensing mechanisms and targets for the cAMP-protein kinase A pathway in the yeast Saccharomyces cerevisiae

The cAMP-protein kinase A (PKA) pathway in the yeast Saccharomyces cerevisiae plays a major role in the control of metabolism, stress resistance and proliferation, in particular in connection with the available nutrient conditions. Extensive information has been obtained on the core section of the pathway, i.e. Cdc25, Ras, adenylate cyclase, PKA, and on components interacting directly with this core section, such as the Ira proteins, Cap/Srv2 and the two cAMP phosphodiesterases. Recent work has now started to reveal upstream regulatory components and downstream targets of the pathway. A G-protein-coupled receptor system (Gpr1-Gpa2) acts upstream of adenylate cyclase and is required for glucose activation of cAMP synthesis in concert with a glucose phosphorylation-dependent mechanism. Although a genuine signalling role for the Ras proteins remains unclear, they appear to mediate at least part of the potent stimulation of cAMP synthesis by intracellular acidification. Recently, several new targets of the PKA pathway have been discovered. These include the Msn2 and Msn4 transcription factors mediating part of the induction of STRE-controlled genes by a variety of stress conditions, the Rim15 protein kinase involved in stationary phase induction of a similar set of genes and the Pde1 low-affinity cAMP phosphodiesterase, which specifically controls agonist-induced cAMP signalling. A major issue that remains to be resolved is the precise connection between the cAMP-PKA pathway and other nutrient-regulated components involved in the control of growth and of phenotypic characteristics correlated with growth, such as the Sch9 and Yak1 protein kinases. Cln3 appears to play a crucial role in the connection between the availability of certain nutrients and Cdc28 kinase activity, but it remains to be clarified which nutrient-controlled pathways control Cln3 levels.

The yeast trimeric guanine nucleotide-binding protein alpha subunit, Gpa2p, controls the meiosis-specific kinase Ime2p activity in response to nutrients

Saccharomyces cerevisiae Gpa2p, the alpha subunit of a heterotrimeric guanine nucleotide-binding protein (G protein), is involved in the regulation of vegetative growth and pseudohyphal development. Here we report that Gpa2p also controls sporulation by interacting with the regulatory domain of Ime2p (Sme1p), a protein kinase essential for entrance of meiosis and sporulation. Protein-protein interactions between Gpa2p and Ime2p depend on the GTP-bound state of Gpa2p and correlate with down-regulation of Ime2p kinase activity in vitro. Overexpression of Ime2p inhibits pseudohyphal development and enables diploid cells to sporulate even in the presence of glucose or nitrogen. In contrast, overexpression of Gpa2p in cells simultaneously overproducing Ime2p results in a drastic reduction of sporulation efficiency, demonstrating an inhibitory effect of Gpa2p on Ime2p function. Furthermore, deletion of GPA2 accelerates sporulation on low-nitrogen medium. These observations are consistent with the following model. In glucose-containing medium, diploid cells do not sporulate because Ime2p is inactive or expressed at low levels. Upon starvation, expression of Gpa2p and Ime2p is induced but sporulation is prevented as long as nitrogen is present in the medium. The negative control of Ime2p kinase activity is exerted at least in part through the activated form of Gpa2p and is released as soon as nutrients are exhausted. This model attributes a switch function to Gpa2p in the meiosis-pseudohyphal growth decision.

Differential expression of G proteins in the mouse olfactory system

Transmembrane signaling events at the dendrites and axons of olfactory receptor neurons mediate distinct functions. Whereas odorant recognition and chemosensory transduction occur at the dendritic membranes of olfactory neurons, signal propagation, axon sorting and target innervation are functions of their axons. The roles of G proteins in transmembrane signaling at the dendrites have been studied extensively, but axonal G proteins have not been investigated in detail. We used immunohistochemistry to visualize expression of alpha subunits of G(o) and G(i2) in the mouse olfactory system. G(o) is expressed ubiquitously on axons of olfactory receptor neurons throughout the olfactory neuroepithelium and in virtually all glomeruli in the main olfactory bulb. In contrast, expression of G(i2) is restricted to a sub-population of olfactory neurons, along the dorsal septum and the dorsal recess of the nasal cavity, which projects primarily to medial regions of the olfactory bulb, with the exception of glomeruli adjacent to the pathway of the vomeronasal nerve. In contrast to the overlapping expression patterns of G(o) and G(i2) in the main olfactory system, neurons expressing G(o) and those expressing G(i2) in the accessory olfactory bulb are more clearly separated, in agreement with previous studies. Vomeronasal axons terminating in glomeruli in the rostral region of the accessory olfactory bulb express G(i2), whereas those projecting to the caudal region express G(o). Characterization of the expression patterns of G(i2) and G(o) in the olfactory projection is essential for future studies aimed at relating transmembrane signaling events to signal propagation, axon sorting and target innervation.

Cyclic AMP-dependent protein kinase regulates pseudohyphal differentiation in Saccharomyces cerevisiae

In response to nitrogen starvation, diploid cells of the yeast Saccharomyces cerevisiae differentiate to a filamentous growth form known as pseudohyphal differentiation. Filamentous growth is regulated by elements of the pheromone mitogen-activated protein (MAP) kinase cascade and a second signaling cascade involving the receptor Gpr1, the Galpha protein Gpa2, Ras2, and cyclic AMP (cAMP). We show here that the Gpr1-Gpa2-cAMP pathway signals via the cAMP-dependent protein kinase, protein kinase A (PKA), to regulate pseudohyphal differentiation. Activation of PKA by mutation of the regulatory subunit Bcy1 enhances filamentous growth. Mutation and overexpression of the PKA catalytic subunits reveal that the Tpk2 catalytic subunit activates filamentous growth, whereas the Tpk1 and Tpk3 catalytic subunits inhibit filamentous growth. The PKA pathway regulates unipolar budding and agar invasion, whereas the MAP kinase cascade regulates cell elongation and invasion. Epistasis analysis supports a model in which PKA functions downstream of the Gpr1 receptor and the Gpa2 and Ras2 G proteins. Activation of filamentous growth by PKA does not require the transcription factors Ste12 and Tec1 of the MAP kinase cascade, Phd1, or the PKA targets Msn2 and Msn4. PKA signals pseudohyphal growth, in part, by regulating Flo8-dependent expression of the cell surface flocculin Flo11. In summary, the cAMP-dependent protein kinase plays an intimate positive and negative role in regulating filamentous growth, and these findings may provide insight into the roles of PKA in mating, morphogenesis, and virulence in other yeasts and pathogenic fungi.

Gbetagamma and palmitate target newly synthesized Galphaz to the plasma membrane

The subcellular location of a signaling protein determines its ability to transmit messages accurately and efficiently. Three different lipid modifications tether heterotrimeric G proteins to membranes: alpha subunits are myristoylated and/or palmitoylated, and gamma subunits are prenylated. In a previous study, we examined the role of lipid modifications in maintaining the membrane attachment of a G protein alpha subunit, alphaz, which is myristoylated and palmitoylated (Morales, J., Fishburn, C. S., Wilson, P. T., and Bourne, H. R. (1998) Mol. Biol. Cell 9, 1-14). Now we extend this analysis by characterizing the mechanisms that target newly synthesized alphaz to the plasma membrane (PM) and analyze the role of lipid modifications in this process. In comparison with newly synthesized alphas, which is palmitoylated but not myristoylated, alphaz moves more rapidly to the membrane fraction following synthesis in the cytosol. Newly synthesized alphaz associates randomly with cellular membranes, but with time accumulates at the PM. Palmitoylated alphaz is present only in PM-enriched fractions, whereas a nonpalmitoylated mutant of alphaz (alphazC3A) associates less stably with the PM than does wild-type alphaz. Expression of a C-terminal fragment of the beta-adrenoreceptor kinase, which sequesters free betagamma, impairs association of both alphaz and alphazC3A with the PM, suggesting that the alpha subunit must bind betagamma in order to localize at the PM. Based on these findings, we propose a model in which, following synthesis on soluble ribosomes, myristoylated alphaz associates randomly and reversibly with membranes; upon association with the PM, alphaz binds betagamma, which promotes its palmitoylation, thus securing it in the proper place for transmitting the hormonal signal.

Golfalpha is expressed in primary sensory neurons outside of the olfactory neuroepithelium

Golfalpha is the alpha chain of a trimolecular stimulatory G protein originally described as the G protein responsible for signal transduction in odourant recognition within neurons of the olfactory neuroepithelium. While applying the technique of mRNA differential display to herpes simplex virus infected tissue, a partial cDNA clone corresponding to the mouse homologue of Golfalpha was isolated from sensory dorsal root ganglia. Levels of this transcript were reduced following viral infection and this reduction was enhanced in CD8(+) depleted mice. The presence of this G protein within sensory ganglia was confirmed with Northern blotting and PCR and in situ hybridization studies localised Golfalpha expression exclusively to neurons within this tissue.

The yeast pheromone-responsive G alpha protein stimulates recovery from chronic pheromone treatment by two mechanisms that are activated at distinct levels of stimulus

The pheromone response of Saccharomyces cerevisiae is mediated by a receptor-coupled heterotrimeric G protein. The beta gamma subunit of the G protein stimulates a PAK/MAP kinase cascade that leads to cellular changes preparatory to mating, while the pheromone-responsive G alpha protein, Gpa1, antagonizes the G beta gamma-induced signal. In its inactive conformation, Gpa1 sequesters G beta gamma and tethers it to the receptor. In its active conformation, Gpa1 stimulates adaptive mechanisms that downregulate the mating signal, but which are independent of alpha-beta gamma binding. To elucidate these potentially novel signaling functions of G alpha in yeast, epistasis analyses were performed using N388D, a hyperadaptive mutant form of Gpa1, and null alleles of various loci that have been implicated in adaptation. The results of these experiments indicate the existence of signaling thresholds that affect the yeast mating reaction. At low pheromone concentration, the Regulator of G Protein Signaling (RGS) homologue and putative guanosine triphosphatase (GTPase) activating protein, Sst2, appears to stimulate sequestration of G beta gamma by Gpa1. Throughout the range of pheromone concentrations sufficient to cause cell cycle arrest, Gpa1 stimulates adaptive mechanisms that are partially dependent on Msg5 and Mpt5. Gpa1-mediated adaptation appears to be independent of Afr1, Akr1, and the carboxy-terminus of the pheromone receptor.

A Saccharomyces cerevisiae G-protein coupled receptor, Gpr1, is specifically required for glucose activation of the cAMP pathway during the transition to growth on glucose

In the yeast Saccharomyces cerevisiae the accumulation of cAMP is controlled by an elaborate pathway. Only two triggers of the Ras adenylate cyclase pathway are known. Intracellular acidification induces a Ras-mediated long-lasting cAMP increase. Addition of glucose to cells grown on a non-fermentable carbon source or to stationary-phase cells triggers a transient burst in the intracellular cAMP level. This glucose-induced cAMP signal is dependent on the G alpha-protein Gpa2. We show that the G-protein coupled receptor (GPCR) Gpr1 interacts with Gpa2 and is required for stimulation of cAMP synthesis by glucose. Gpr1 displays sequence homology to GPCRs of higher organisms. The absence of Gpr1 is rescued by the constitutively activated Gpa2Val-132 allele. In addition, we isolated a mutant allele of GPR1, named fil2, in a screen for mutants deficient in glucose-induced loss of heat resistance, which is consistent with its lack of glucose-induced cAMP activation. Apparently, Gpr1 together with Gpa2 constitute a glucose-sensing system for activation of the cAMP pathway. Deletion of Gpr1 and/or Gpa2 affected cAPK-controlled features (levels of trehalose, glycogen, heat resistance, expression of STRE-controlled genes and ribosomal protein genes) specifically during the transition to growth on glucose. Hence, an alternative glucose-sensing system must signal glucose availability for the Sch9-dependent pathway during growth on glucose. This appears to be the first example of a GPCR system activated by a nutrient in eukaryotic cells. Hence, a subfamily of GPCRs might be involved in nutrient sensing.

Supersensitization of the adenylyl cyclase system in Chinese hamster ovary cells co-expressing cloned opioid receptors and Gz, a PTX-insensitive G protein

Chronic and/or sustained stimulation of opioid receptors has been shown to lead to an increase in activity of the adenylyl cyclase (AC) system via pertussis toxin (PTX)-sensitive Gi/o, family G protein. In the present study, we examined whether supersensitization of the AC system is induced via a PTX-insensitive G protein, Gz, activation of which leads to the inhibition of AC activity. In Chinese hamster ovary (CHO) cells expressing either mu- or kappa-opioid receptors, acute treatment with morphine or U69,593, but not naloxone or norbinaltorphimine, respectively, suppressed forskolin-induced cyclic AMP (cAMP) accumulation, while sustained (4 h) treatment with the opioid agonists induced cAMP overshoot above the naive level (supersensitization of the AC system). Both effects were completely blocked by pretreatment with PTX. In CHO cells co-expressing mu- or kappa-opioid receptors and alpha(z), inhibitory effects of cAMP accumulation by acute treatment with the opioid agonists and supersensitization of the AC system by sustained treatment with them were induced despite pretreatment with PTX. These data suggest that supersensitization of the AC system is induced by sustained opioid agonist treatment not only via Gi/o but also via Gz.

Olfactory receptors, Golf alpha and adenylyl cyclase mRNA expressions in the rat heart during ontogenic development

Golf alpha (a G heterotrimeric protein which shares a high homology with Gs alpha) expression was studied in the rat heart before birth and until weaning. Since Golf alpha in the neuro-olfactory epithelium is coupled to olfactory receptors and type III adenylyl cyclase, we looked for the presence of such molecules in the heart. Golf alpha mRNA was detected in the rat heart, highest levels being found in 21-day old fetuses until 3 days post partum. The protein amounts measured by Western blots paralleled the Golf alpha mRNA levels. Immunohistochemical studies revealed the presence of Golf alpha in atrial and ventricular cardiomyocytes. OL1 and latrophilin mRNAs, G protein-coupled olfactory receptors, were expressed at early postnatal stages. Adenylyl cyclase mRNAs for type II, type III, type V and type VI were expressed before birth and until weaning. Elements for an unexpected signaling pathway involving odorant receptors like OL1 and latrophilin, Golf alpha and type III adenylyl cyclase were expressed in rat heart, and appeared developmentally regulated.

Cellular and subcellular expression of Golf/Gs and Gq/G11 alpha-subunits in rat pancreatic endocrine cells

We studied the cellular and subcellular localization of Galpha-subunits in pancreas by immunocytochemistry. Golfalpha and G11alpha were specifically localized in islet insulin B-cells and glucagon A-cells, respectively. Gsalpha and Gqalpha labeling was more abundant in B-cells. The presence of Golfalpha in B-cells was confirmed by in situ hybridization. In B-cells, Golfalpha and Gsalpha were found in the Golgi apparatus, plasma membrane (PM) and, remarkably, in mature and immature insulin secretory granules, mainly at the periphery of the insulin grains. Gqalpha was detected on the rough endoplasmic reticulum (RER) near the Golgi apparatus. In A-cells, the Galpha-subunits were mostly within the glucagon granules: G11alpha gave the strongest signal, Gsalpha less strong, Gq was scarce, and Golf was practically absent. Gqalpha and Gsalpha immunoreactivity was detected in acinar cells, although it was much weaker than that in islet cells. The cell-dependent distribution of the Galpha-subunits indicates that the stimulatory pathways for pancreatic function differ in acinar and in islet B- and A-cells. Furthermore, the G-protein subunits in islet cell secretory granules might be functional and participate in granule trafficking and hormone secretion.

Activation of human D3 dopamine receptor inhibits P/Q-type calcium channels and secretory activity in AtT-20 cells

The D3 dopamine receptor is postulated to play an important role in the regulation of neurotransmitter secretion at both pre- and postsynaptic terminals. However, this hypothesis and the underlying mechanisms remain untested because of the lack of D3-selective ligands, paucity of appropriate model secretory systems, and the weak and inconsistent coupling of D3 receptors to classical signal transduction pathways. The absence of ligands that selectively discriminate between D3 and D2 receptors in vivo precludes the study of D3 receptor function in the brain and necessitates the use of heterologous expression systems. In this report we demonstrate that activation of the human D3 dopamine receptor expressed in the AtT-20 neuroendocrine cell line causes robust inhibition of P/Q-type calcium channels via pertussis toxin-sensitive G-proteins. In addition, using the vesicle trafficking dye FM1-43, we show that D3 receptor activation significantly inhibits spontaneous secretory activity in these cells. Our results not only support the hypothesis that the D3 receptor can regulate secretory activity but also provide insight into the underlying signaling mechanisms. We propose a functional model in which the D3 receptor tightly regulates neurotransmitter release at a synapse by only allowing the propagation of spikes above a certain frequency or burst-duration threshold.

Galphaolf identification by RT-PCR in purified normal pancreatic B cells and in islets from rat models of non-insulin-dependent diabetes

Recent reports using immunohistochemistry have shown that Galphaolf which shares 88% homology with Galphas was expressed in pancreatic islets. To test the specificity of the expression of this G protein isotype in rat islet cells, B and non-B cells were separated by flow cytometry. The expression of Galphaolf and adenylyl cyclases (AC) of types II, III, V, and VI was evaluated by reverse-transcriptase polymerase chain reaction (RT-PCR). Since alterations in the expression of AC III were recently reported in the GK rat (a model of non-insulin-dependent diabetes mellitus, NIDDM), we also have analyzed the mRNA expression of Galphaolf and AC isoforms in pancreatic islets from GK rats and from adult rats neonatally treated by streptozotocin (nSTZ rats), another model of NIDDM. Southern blots of amplicons generated with specific primers of Galphaolf revealed the presence of a 540-bp band only in B cells. AC of types II, III, V, and VI were expressed both in B and non-B cells. However, AC III mRNA was clearly more abundant in non-B than in B cells. Moreover, in B cells the expression of AC VI was higher than that of AC V, whereas equal expressions of AC V and AC VI were found in non-B cells. In GK rat islets, the mRNA expressions of Galphaolf, AC II, and AC III were clearly increased and no change in AC V and AC VI was found. In nSTZ rat islets, Galphaolf expression was barely detectable, but AC II and AC III mRNA levels were higher than those observed in controls. In conclusion, Galphaolf mRNA appeared specifically expressed in islet B cells and was increased in GK islets. The steady-state mRNA levels of AC II and AC III were clearly increased in the islets of the two rat models of NIDDM. Thus, alterations in the expression of G protein isotypes and AC isoforms could contribute to the diabetic phenotype.

Receptor inhibition of pheromone signaling is mediated by the Ste4p Gbeta subunit

The pheromone response pathway of the yeast Saccharomyces cerevisiae is initiated in MATa cells by binding of alpha-factor to the alpha-factor receptor. MATa cells in which the a-factor receptor is inappropriately expressed exhibit reduced pheromone signaling, a phenomenon termed receptor inhibition. In cells undergoing receptor inhibition, activation of the signaling pathway occurs normally at early time points but decreases after prolonged exposure to pheromone. Mutations that suppress the effects of receptor inhibition were obtained in the STE4 gene, which encodes the beta-subunit of the G protein that transmits the pheromone response signal. These mutations mapped to the N terminus and second WD repeat of Ste4p in regions that are not part of its Galpha binding surface. A STE4 allele containing several of these mutations, called STE4(SD13), reversed the signaling defect seen at late times in cells undergoing receptor inhibition but had no effect on the basal activity of the pathway. Moreover, the signaling properties of STE4(SD13) were indistinguishable from those of STE4 in wild-type MATa and MATalpha cells. These results demonstrate that the effect of the STE4(SD13) allele is specific to the receptor inhibition function of STE4. STE4(SD13) suppressed the signaling defect conferred by receptor inhibition in a MATa strain containing a deletion of GPA1, the G protein alpha-subunit gene; however, STE4(SD13) had no effect in a MATalpha strain containing a GPA1 deletion. Suppression of receptor inhibition by STE4(SD13) in a MATa strain containing a GPA1 deletion was unaffected by deletion of STE2, the alpha-factor receptor gene. The results presented here are consistent with a model in which an a-specific gene product other than Ste2p detects the presence of the a-factor receptor and blocks signaling by inhibiting the function of Ste4p.

Mapping of a yeast G protein betagamma signaling interaction

The mating pathway of Saccharomyces cerevisiae is widely used as a model system for G protein-coupled receptor-mediated signal transduction. Following receptor activation by the binding of mating pheromones, G protein betagamma subunits transmit the signal to a MAP kinase cascade, which involves interaction of Gbeta (Ste4p) with the MAP kinase scaffold protein Ste5p. Here, we identify residues in Ste4p required for the interaction with Ste5p. These residues define a new signaling interface close to the Ste20p binding site within the Gbetagamma coiled-coil. Ste4p mutants defective in the Ste5p interaction interact efficiently with Gpa1p (Galpha) and Ste18p (Ggamma) but cannot function in signal transduction because cells expressing these mutants are sterile. Ste4 L65S is temperature-sensitive for its interaction with Ste5p, and also for signaling. We have identified a Ste5p mutant (L196A) that displays a synthetic interaction defect with Ste4 L65S, providing strong evidence that Ste4p and Ste5p interact directly in vivo through an interface that involves hydrophobic residues. The correlation between disruption of the Ste4p-Ste5p interaction and sterility confirms the importance of this interaction in signal transduction. Identification of the Gbetagamma coiled-coil in Ste5p binding may set a precedent for Gbetagamma-effector interactions in more complex organisms.

Regulators of pseudohyphal differentiation in Saccharomyces cerevisiae identified through multicopy suppressor analysis in ammonium permease mutant strains

Nitrogen-starved diploid cells of the yeast Saccharomyces cerevisiae differentiate into a filamentous, pseudohyphal growth form. Recognition of nitrogen starvation is mediated, at least in part, by the ammonium permease Mep2p and the Galpha subunit Gpa2p. Genetic activation of the pheromone-responsive MAP kinase cascade, which is also required for filamentous growth, only weakly suppresses the filamentation defect of Deltamep2/Deltamep2 and Deltagpa2/Deltagpa2 strain. Surprisingly, deletion of Mep1p, an ammonium permease not previously thought to regulate differentiation, significantly enhances the potency of MAP kinase activation, such that the STE11-4 allele induces filamentation to near wild-type levels in Deltamep1/Deltamep1 Deltamep2/Deltamep2 and Deltamep1/Deltamep1 Deltagpa2/Deltagpa2 strains. To identify additional regulatory components, we isolated high-copy suppressors of the filamentation defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant. Multicopy expression of TEC1, PHD1, PHD2 (MSS10/MSN1/FUP4), MSN5, CDC6, MSS11, MGA1, SKN7, DOT6, HMS1, HMS2, or MEP2 each restored filamentation in a Deltamep1/Deltamep1 Deltamep2/Deltamep2 strain. Overexpression of SRK1 (SSD1), URE2, DAL80, MEP1, or MEP3 suppressed only the growth defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant strain. Characterization of these genes through deletion analysis and epistasis underscores the complexity of this developmental pathway and suggests that stress conditions other than nitrogen deprivation may also promote filamentous growth.

G protein alpha subunit G alpha z couples neurotransmitter receptors to ion channels in sympathetic neurons

The functional roles subserved by G(alpha)z, a G protein alpha subunit found predominantly in neuronal tissues, have remained largely undefined. Here, we report that G(alpha)z coupled neurotransmitter receptors to N-type Ca2+ channels when transiently overexpressed in rat sympathetic neurons. The G(alpha)z-mediated inhibition was voltage dependent and PTX insensitive. Recovery from G(alpha)z-mediated inhibition was extremely slow but accelerated by coexpression with RGS proteins. G(alpha)z selectively interacted with a subset of receptors that ordinarily couple to N-type Ca2+ channels via PTX-sensitive Go/i proteins. In addition, G(alpha)z rescued the activation of heterologously expressed GIRK channels in PTX-treated neurons. These results suggest that G(alpha)z is capable of coupling receptors to ion channels and might underlie PTX-insensitive ion channel modulation observed in neurons under physiological and pathological conditions.

N-terminal domain of Gpa1 (G protein alpha) subunit) is sufficient for plasma membrane targeting in yeast Saccharomyces cerevisiae

G proteins play a central role in transmitting signals from cell surface receptors to effector proteins inside the cell. Signaling can only occur, however, if all these protein components are properly assembled and localized at the plasma membrane. Past studies have shown that certain segments within the N-terminal region of the G protein alpha subunit are necessary for membrane attachment. Here we identify a region within the yeast G alpha (Gpa1) that is sufficient for membrane attachment, as well as for specific targeting to the plasma membrane. Initially, we constructed chimeric proteins that replace the N terminus of mammalian Gsalpha with the corresponding sequence from Gpa1. Gsalpha is inefficiently targeted to the yeast plasma membrane and therefore cannot fully complement the loss of Gpa1. Gpa1-Gsaplha chimeras were assayed for proper membrane localization by functional complementation of a gpa1Delta;) mutant, and by sucrose density gradient fractionation of cell membranes. Most of the chimeras tested, including one with only the N-terminal 7 amino acids from Gpa1, exhibited normal membrane targeting and complementing activity. We also fused various lengths of N-terminal Gpa1 sequence to glutathione-S-transferase (GST), a heterologous protein normally expressed in the cytoplasm. The first 67- 36- or 9-amino acids of Gpa1 were all sufficient to direct GST specifically to the plasma membrane in yeast. This analysis defines the extreme N terminus of Gpa1 as the primary determinant of proper membrane targeting, and represents an essential step towards isolating and identifying G protein-targeting proteins within the plasma membrane.

Crosstalk between cAMP and pheromone signalling pathways in Ustilago maydis

In the phytopathogenic basidiomycete Ustilago maydis mating and dikaryon formation are controlled by a pheromone/receptor system and the multiallelic b locus. Recently, a gene encoding a G protein alpha subunit, gpa3, was isolated and has subsequently been implicated in pheromone signal transduction. Mutants deleted for gpa3 are sterile and nonpathogenic, and exhibit a morphology that is similar to that of mutants with defects in the adenylate cyclase gene uac1. We have found that the sterility and mutant morphology of gpa3 deletion strains can be rescued by exogenous cAMP. In these mutants and in the corresponding wild-type strains, exogenous cAMP stimulates pheromone gene expression to a level comparable to that seen in the pheromone-stimulated state. In addition, we demonstrate that uac1 is epistatic to gpa3. We conclude that Gpa3 controls the cAMP signalling pathway in U. maydis and discuss how this pathway feeds into the pheromone response.

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.

RGSZ1, a Gz-selective regulator of G protein signaling whose action is sensitive to the phosphorylation state of Gzalpha

Regulators of G protein signaling (RGS) are a family of proteins that attenuate the activity of the trimeric G proteins. RGS proteins act as GTPase-activating proteins (GAPs) for the alpha subunits of several trimeric G proteins, much like the GAPs that regulate the activity of monomeric G proteins such as Ras. RGS proteins have been cloned from many eukaryotes, and those whose biochemical activity has been characterized regulate the members of the Gi family of G proteins; some forms can also act on Gq proteins. In an ongoing effort to elucidate the role of Gzalpha in cell signaling, the yeast two-hybrid system was employed to identify proteins that could interact with a mutationally activated form of Gzalpha. A novel RGS, termed RGSZ1, was identified that is most closely related to two existing RGS proteins termed RetRGS1 and GAIP. Northern blot analysis revealed that expression of RGSZ1 was limited to brain, and expression was particularly high in the caudate nucleus. Biochemical characterization of recombinant RGSZ1 protein revealed that RGSZ1 was indeed a GAP and, most significantly, showed a marked preference for Gzalpha over other members of the Gialpha family. Phosphorylation of Gzalpha by protein kinase C, an event known to occur in cells and that was previously shown to influence alpha-betagamma interactions of Gz, rendered the G protein much less susceptible to RGSZ1 action.

Support for a chromosome 18p locus conferring susceptibility to functional psychoses in families with schizophrenia, by association and linkage analysis

The action of antipsychotic drugs on dopamine receptors suggests that dopaminergic signal transmission may play a role in the development of schizophrenia. We tested eight candidate genes (coding for dopamine receptors, the dopamine transporter, and G-proteins) in 59 families from Germany and Israel, for association. A P value of .00055 (.0044 when corrected for the no. of markers tested) was obtained for the intronic CA-repeat marker G-olfalpha on chromosome 18p. The value decreased to .000088 (.0007) when nine sibs with recurrent unipolar depressive disorder were included. Linkage analysis using SSLP markers densely spaced around G-olfalpha yielded a maximum two-point LOD score of 3.1 for a marker 0.5 cM distal to G-olfalpha. Multipoint analysis under the assumption of heterogeneity supported this linkage-whether the affected pheotype was defined narrowly or broadly-as did nonparametric linkage (NPL). In 12 families with exclusively maternal transmission of the disease, the NPL value also supported linkage to this marker. In order to test for association/linkage disequilibrium in the presence of linkage, the sample was restricted to independent offspring. When this sample was combined with 65 additional simplex families (each of them comprising one schizophrenic offspring and his or her parents), the 124-bp allele of G-olfalpha was transmitted 47 times and was not transmitted 21 times (P=.009). These results suggest the existence, on chromosome 18p, of a potential susceptibility locus for functional psychoses.

Influence of Gz and Gi2 transducer proteins in the affinity of opioid agonists to mu receptors

The affinity displayed by different opioids to mu receptors (ORs) was determined in mouse brain membranes incubated with antibodies directed to Galpha subunits of the guanine nucleotide-binding proteins Gi2 and Gz. Assays were conducted with 10 pm 125I-Tyr27-beta-endorphin in the presence of 300 nm N, N-diallyl-Tyr-(alpha-aminoisobutyric acid)2-Phe-Leu-OH (ICI-174 864), which prevented the binding of the iodinated neuropeptide to delta-ORs. Gpp(NH)p or the preincubation of mouse brain membranes with IgGs to Gi2alpha or Gzalpha subunits, promoted reductions in the affinity exhibited by the labelled probe. The potencies of beta-endorphin, [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO) and [D-Pen2,5]enkephalin (DPDPE) were reduced after impairing the coupling of mu-ORs to Gi2 or Gz proteins. Morphine showed a loss of affinity towards the mu-OR after preincubation of membranes with IgGs to Gzalpha subunits. However, it retained its potency after treatment with the anti-Gi2alpha IgGs. Conversely, [D-Ala2, D-Leu5]enkephalin (DADLE) and [D-Ser2, Leu5] enkephalin-Thr6 (DSLET) showed decreased affinity to mu-ORs after treatment with anti-Gi2alpha IgGs, with no noticeable change following the use of IgGs to Gzalpha subunits. The affinity exhibited by the opioid antagonists naloxone, naltrexone, naloxonazine and [Cys2,Tyr3,Orn5, Pen7 amide]somatostatin analogue (CTOP) remained unchanged after either treatment. Therefore, the affinity exhibited by opioid agonists of mu-ORs, but not antagonists, depends on the nature of the G-protein coupled to these receptors.