Melatonin mt1 and MT2 receptors stimulate c-Jun N-terminal kinase via pertussis toxin-sensitive and -insensitive G proteins

Melatonin is a pineal hormone involved in neuroendocrine processes in mammals. It has been shown that melatonin inhibits the enzymatic activities of adenylyl cyclases and the transcriptional activities of CREB. In this report, we demonstrate that 2-iodomelatonin (2IMT) treatment on COS-7 cells transfected with melatonin receptors (mt1 and MT2) induces c-Jun N-terminal kinase (JNK) activation, which is pertussis toxin (PTX)-sensitive, Ras/Rac-dependent and may involve Src-family protein tyrosine kinases. Moreover, PTX-insensitive Gs, Gz and G16 are capable of linking activated melatonin receptors to the stimulation of JNK. Agonist stimulation on PTX-pretreated COS-7 cells overexpressing mt1 receptor, Galpha(s) and adenylyl cyclase VI led to increased cAMP accumulation. Stimulation of endogenous mt1 receptors in MCF-7 cells was associated with the activation of both JNK and extracellular signal-regulated kinase (ERK). This report demonstrates the stimulatory effect of melatonin receptors on JNK, and provides experimental evidence for a functional coupling between the G(i)-coupled melatonin receptor and Gs, in terms of adenylyl cyclase activation.

G(z alpha) deficient mice: enzyme levels in the autonomic nervous system, neuronal survival and effect of genetic background

Our laboratory has generated a genetically mutant mouse in which the alpha subunit of the heterotrimeric GTP binding protein, G(z) has been made dysfunctional by homologous recombination to determine its in vivo function. These animals show a characteristic failure to thrive phenotype. G(z alpha) is expressed in a variety of nervous system tissues as well as in the adrenal medulla. We therefore examined the autonomic nervous system of the G(z alpha) deficient mouse by measuring the activity of tyrosine hydroxylase and choline acetyltransferase in the superior cervical ganglia, submaxillary gland and the adrenal medulla. Preliminary results using animals of mixed BALB/c and C57BL/6 strains gave inconsistent results. Further experiments demonstrated differences in the activity of tyrosine hydroxylase and choline acetyltransferase between BALB/c and C57BL/6 mouse strains. The analysis of the pure strains showed a reduction in the size and enzyme levels of the adrenal gland and submaxillary glands of the G(z alpha) deficient mouse suggesting a role for adrenal insufficiency and/or nutritional disorders for the failure to thrive phenotype. The survival of sympathetic and sensory neurons was also examined in the G(z alpha) deficient mouse and in the presence of pertussis toxin, sympathetic but not sensory neuronal survival in G(z alpha) deficient mice was significantly attenuated. This suggests that in vivo other pertussis toxin sensitive G proteins may be recruited to compensate for the loss of G(z alpha).

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

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

Inhibition of somatostatin receptor 5-signaling by mammalian regulators of G-protein signaling (RGS) in yeast

Regulators of G-protein signaling (RGSs) are negative regulators of G-protein coupled receptor (GPCR)-mediated signaling that function to limit the lifetime of receptor-activated G(alpha)-proteins. Here we show that four mammalian RGSs differentially inhibit the activation of a FUS1--LacZ reporter gene by the STE2 encoded GPCR in yeast. In order to examine the role of the GPCR in modulating RGS function, we functionally expressed the human somatostatin receptor 5 (SST(5)) in yeast. In the absence of RGSs, FUS1--LacZ activation in response to somatostatin increased in a dose-dependent manner in cells expressing SST(5). In contrast to the results obtained with Ste2p, all RGSs completely inhibited SST(5)-mediated signaling even at concentrations of agonist as high as 10(minus sign5) M. The ability of RGSs to inhibit SST(5) signaling was further assessed in cells expressing modified Gpa1 proteins. Even though SST(5)-mediated FUS1--LacZ activation was 5-fold more efficient with a Gpa1p/G(i3alpha) chimera, response to somatostatin was completely abolished by all four RGSs. Furthermore, we demonstrate that RGS1, RGS2 and RGS5 have reduced ability to inhibit SST(5)-mediated activation of the RGS-resistant Gpa1p(Gly302Ser) mutant suggesting that the ability to interact with the G(alpha)-protein is required for the inhibition of signaling. Taken together, our results indicate that RGSs serve as better GAPs for Gpa1p when activated by SST(5) than when this G-protein is activated by Ste2p.

Multiple pathways of ERK activation by G protein-coupled receptors

It has only recently been fully realized that G protein-coupled receptors and G proteins play crucial roles in the regulation of cell growth, differentiation and even tumour formation. Naturally occurring mutations in G protein-coupled receptors and in G protein alpha subunits result in uncontrolled cellular proliferation resulting in distinct human diseases. One important mechanism to transduce mitogenic signals from the cell membrane to the cell nucleus is the engagement of the extracellular signal-regulated kinase (ERK)-mitogen-activated protein kinase (MAPK) cascade. A multitude of distinct signal transduction pathways have been deciphered which connect G proteins with the ERK cascade. Both receptor and non-receptor tyrosine kinases play pivotal roles in these signalling pathways. Mitogenic signalling by G protein-coupled receptors can be regarded as a complex interplay between signals emanating from different classes of cell surface receptors which ultimately converge upon a small subset of central signalling proteins in the cell. The characterization of receptor-, G protein- and tyrosine kinase-specific contributions to mitogenic signalling in a particular cell and the identification of proteins serving as a point of convergence in the mitogenic signalling cascade may ultimately allow the design of novel pharmacological approaches to treat diseases involving unrestricted cell proliferation.

Yeast assays for G-protein-coupled receptors

Yeast assays for G-protein-coupled receptors have many attractions due to their simplicity, low cost, and lack of endogenous receptors. Since the first report of functional coupling of the human beta 2 adrenergic receptor to the yeast pheromone-response pathway in 1990, the technology has developed to a point at which more than 30 heterologous GPCRs are now published to couple. Major breakthroughs have come from an understanding of receptor-G protein interactions, alongside advances in knowledge of the structure of heterotrimeric G proteins. Yeast screens have been used to identify ligands both from compound collections and through the autocrine expression of peptide libraries. Yeast genetics has also been applied to a functional analysis of GPCRs and peptide ligands. In this review we describe the historical development of yeast GPCR assay systems and their current applications.

The N terminus of Saccharomyces cerevisiae Sst2p plays an RGS-domain-independent, Mpt5p-dependent role in recovery from pheromone arrest

The Saccharomyces cerevisiae RGS protein Sst2p is involved in desensitization to pheromone and acts as a GTPase-activating protein for the Galpha subunit Gpa1p. Other results indicate that Sst2p acts through Mpt5p and that this action occurs downstream of Fus3p and through Cln3p/Cdc28p. Our results indicate that the interaction of Sst2p with Mpt5p requires the N-terminal MPI (Mpt5p-interacting) domain of Sst2p and is independent of the C-terminal RGS domain. Overexpression of the MPI domain results in an Mpt5p-dependent increase in recovery from pheromone arrest. Overexpression of either intact Sst2p or the MPI domain leads to partial suppression of a gpa1 growth defect, and this suppression is dependent on Mpt5p, indicating that MPI function occurs downstream of Gpa1p and through Mpt5p. Combination of an mpt5 mutation with the GPA1(G302S) mutation, which uncouples Gpa1p from Sst2p, results in pheromone supersensitivity similar to the sst2 mutant, and promotion of recovery by overexpression of Sst2p is dependent on both Mpt5p and the Gpa1p interaction. These results indicate that Sst2p is a bifunctional protein and that the MPI domain acts through Mpt5p independently of the RGS domain. RGS family members from other fungi contain N-terminal domains with sequence similarity to the Sst2p MPI domain, suggesting that MPI function may be conserved.

Plant G proteins: the different faces of GPA1

Biochemical studies suggest that G proteins mediate a variety of signaling processes in plants, yet Arabidopsis has only one gene, GPA1, for a canonical G protein alpha subunit. Recent studies indicate that the GPA1 protein is involved in a number of very different cellular processes.

Identification of a nonmammalian Golf subtype: functional role in olfactory signaling of airborne odorants in Xenopus laevis

Attempts to identify the Galpha subtypes in the two compartments of the olfactory system from Xenopus, which are supposed to be specialized for detecting aquatic and volatile odorous compounds, revealed that a Galpha(o1) subtype is characteristic for the "water nose," the lateral diverticulum, whereas a novel Galpha(s) subtype predominates in the "air nose," the medial diverticulum. The newly identified Galpha(s)-type is more closely related to Galpha(olf) of rat and human than to the known Galpha(s)-isoform of Xenopus; it is therefore considered the first identified nonmammalian Galpha(olf) subtype. Sequence comparison of Galpha(olf) from amphibia and mammals revealed a particular conservation within the alpha-helical domains, which are supposed to control the GDP/GTP-exchange rate. The selective expression of different Galpha subtypes in the two anatomically separated and functionally specialized nasal compartments parallels the expression of distinct classes of olfactory receptors. Moreover, biochemical analysis revealed that stimulation with appropriate odorous compounds elicits the formation of inositol trisphosphate in the lateral diverticulum. In contrast, cyclic adenosine monophosphate signals were induced in the medial diverticulum, and this response appears to be mediated by the novel Galpha(olf) subtype. The data indicate that olfactory sensory neurons in each of the nasal cavities are equipped not only with defined sets of receptor types but also with a distinct molecular machinery for the chemo-electrical transduction process.

Isolation of a novel gene from Schizosaccharomyces pombe: stm1+ encoding a seven-transmembrane loop protein that may couple with the heterotrimeric Galpha 2 protein, Gpa2

A putative seven transmembrane protein gene, stm1(+), which is required for proper recognition of nitrogen starvation signals, was isolated as a multicopy suppressor of a ras1 synthetic lethal mutant in Schizosaccharomyces pombe. Under nitrogen-deficient conditions, transcription of the stm1 gene was induced; deletion of stm1 was associated with early entry into G(1) arrest. Under nutritionally sufficient conditions, overexpression of Stm1 inhibited vegetative cell growth, resulted in decreased intracellular cAMP levels, increased the expression of the meiosis-specific genes ste11, mei2, and mam2, and facilitated sexual development in homothallic cells. However inhibition of vegetative cell growth and reduction of cAMP levels were not observed in a deletion mutant of the heterotrimeric G protein Galpha2 gene, gpa2, that is responsible for regulating intracellular cAMP levels, a key factor in determining the sexual development in S. pombe. Stm1 protein was shown to interact with Gpa2 through its C-terminal transmembrane domains 5-7. Mutation at Lys(199) in the C-terminal domain (stm1(K199A)) abolished the Stm1 overexpression effect on lowering cAMP levels. Induction of ste11, a meiosis-specific gene transcription factor, by Stm1 overexpression was enhanced in gpa2-deleted cells but was absent in a deletion mutant of sty1, a key protein kinase that links mitotic control with environmental signals and induces stress-responsive genes. Moreover, deletion of both stm1 and ras1 caused delayed entry into G(1) arrest in S. pombe when the cells were grown in a nitrogen-deficient medium. Thus we consider that the stm1 gene can function through Gpa2-dependent and/or -independent pathways and may play a role in providing the prerequisite state for entering the pheromone-dependent differentiation cycle in which heterotrimeric Galpha1 protein, Gpa1, and Ras1 play major roles. Stm1 could function as a sentinel molecule sensing the nutritional state of the cells, stopping the proliferative cell cycle, and preparing the cell to enter meiosis under nutritionally deficient conditions.

Effects of subchronic lithium chloride treatment on G-protein subunits (Golf, Ggamma7) and adenylyl cyclase expressed specifically in the rat striatum

Lithium salt has been widely used as a treatment for mania, but the mechanism of its effect remains unknown. Previously, by studying c-fos expression, we showed that the striatum was a possible target region for the antimanic effects of lithium salt. The present study focused on the effect of subchronic lithium chloride treatment on G-proteins (Golf, Ggamma7) and adenylyl cyclase type V, which are expressed specifically in the rat striatum. Subchronic lithium chloride treatment significantly increased the level of Golf protein, a stimulant alpha-subunit of G-protein, by 53.5% (P<0.01), but the levels of Ggamma7 and adenylyl cyclase type V did not change. This increased level of Golf protein was found after 2 weeks of lithium chloride treatment, but not after 1 week, and the level returned to the basal level 1 week after withdrawal of lithium chloride. This result suggests that the level of Golf protein increases to compensate for the suppression of the adenylyl cyclase system by lithium, and that this increase may account for the "rebound" phenomenon, which is the relapse observed after abrupt discontinuation of lithium salt treatment.

New aspects of the glucose activation of the H(+)-ATPase in the yeast Saccharomyces cerevisiae

The glucose-induced activation of plasma membrane ATPase from Saccharomyces cerevisiae was first described by Serrano in 1983. Many aspects of this signal transduction pathway are still obscure. In this paper, evidence is presented for the involvement of Snf3p as the glucose sensor related to this activation process. It is shown that, in addition to glucose detection by Snf3p, sugar transport is also necessary for activation of the ATPase. The participation of the G protein, Gpa2p, in transducing the internal signal (phosphorylated sugars) is also demonstrated. Moreover, the involvement of protein kinase C in the regulation of ATPase activity is confirmed. Finally, a model pathway is presented for sensing and transmission of the glucose activation signal of the yeast H(+)-ATPase.

C18orf2, a novel, highly conserved intronless gene within intron 5 of the GNAL gene on chromosome 18p11

We have characterized a novel intronless human gene (C18orf2) which is embedded in intron 5 of the G-protein gene (GNAL) on chromosome 18p11. This gene codes for a 199 amino acid polypeptide with a predicted molecular weight of 22.1 kDa. It is highly homologous to a number of predicted developmental proteins in organisms ranging from yeasts to Drosophila. C18orf2 mRNA was found to be expressed in various tissues.

The olfactory G protein G(alphaolf) possesses a lower GDP-affinity and deactivates more rapidly than G(salphashort): consequences for receptor-coupling and adenylyl cyclase activation

The olfactory G protein G(alphaolf) differs from the short splice variant of G(salpha) (G(salphaS)) in 80 amino acids, but little is known about biochemical differences between G(alphaolf) and G(salphaS). We addressed this question by analyzing fusion proteins of the beta2-adrenoceptor (beta2AR) and G(alphaolf) and G(salphaS), respectively, using Sf9 insect cells as expression system. The fusion ensured defined receptor/G protein stoichiometry and efficient coupling. High-affinity agonist binding studies showed that G(alphaolf) possesses a lower GDP-affinity than G(salphaS) As a result, the agonist-free beta2AR and the beta2AR occupied by partial agonists were more efficient at promoting GDP-dissociation from G(alphaolf) than from G(salphaS) a assessed by guanosine 5'-O-(3-thiotriphosphate) binding, adenylyl cyclase (AC) activity and GTP hydrolysis. Basal AC activity in the absence of GTP was almost sixfold lower in membranes expressing beta2AR-G(alphaolf) than in membranes expressing beta2AR-G(salphaS) at similar levels, reflecting the lower abundance of G(alphaolf-GDP) relative to G(salphaS-GDP). The maximum agonist-stimulated AC activity with beta2AR-G(salphaS) was more than twofold higher than with beta2AR-G(alphaolf), but the relative agonist-stimulation of AC with beta2AR-G(alphaolf) was much greater than with beta2AR-G(salphaS). The difference in maximum AC activity can be explained by more rapid deactivation of G(alphaolf-GTP) by GTP hydrolysis and GTP dissociation relative to G(salphaS-GTP). Taken together, there are biochemical differences between G(alphaolf) and G(salphaS), supporting different roles of these G proteins in vivo.

Overexpression of the heterotrimeric G-protein alpha-subunit enhances phytochrome-mediated inhibition of hypocotyl elongation in Arabidopsis

Plant heterotrimeric G-proteins have been implicated in a number of signaling processes. However, most of these studies are based on biochemical or pharmacological approaches. To examine the role of heterotrimeric G-proteins in plant development, we generated transgenic Arabidopsis expressing the Galpha subunit of the heterotrimeric G-protein under the control of a glucocorticoid-inducible promoter. With the conditional overexpression of either the wild type or a constitutively active version of Arabidopsis Galpha, transgenic seedlings exhibited a hypersensitive response to light. This enhanced light sensitivity was more exaggerated in a relatively lower intensity of light and was observed in white light as well as far-red, red, and blue light conditions. The enhanced responses in far-red and red light required functional phytochrome A and phytochrome B, respectively. Furthermore, the response to far-red light depended on functional FHY1 but not on FIN219 and FHY3. This dependence on FHY1 indicates that the Arabidopsis Galpha protein may act only on a discrete branch of the phytochrome A signaling pathway. Thus, our results support the involvement of a heterotrimeric G-protein in the light regulation of Arabidopsis seedling development.

Overexpression of the heterotrimeric G-protein alpha-subunit enhances phytochrome-mediated inhibition of hypocotyl elongation in Arabidopsis

Plant heterotrimeric G-proteins have been implicated in a number of signaling processes. However, most of these studies are based on biochemical or pharmacological approaches. To examine the role of heterotrimeric G-proteins in plant development, we generated transgenic Arabidopsis expressing the Galpha subunit of the heterotrimeric G-protein under the control of a glucocorticoid-inducible promoter. With the conditional overexpression of either the wild type or a constitutively active version of Arabidopsis Galpha, transgenic seedlings exhibited a hypersensitive response to light. This enhanced light sensitivity was more exaggerated in a relatively lower intensity of light and was observed in white light as well as far-red, red, and blue light conditions. The enhanced responses in far-red and red light required functional phytochrome A and phytochrome B, respectively. Furthermore, the response to far-red light depended on functional FHY1 but not on FIN219 and FHY3. This dependence on FHY1 indicates that the Arabidopsis Galpha protein may act only on a discrete branch of the phytochrome A signaling pathway. Thus, our results support the involvement of a heterotrimeric G-protein in the light regulation of Arabidopsis seedling development.

At acidic pH, the GPA2-cAMP pathway is necessary to counteract the ORD1-mediated repression of the hypoxic SRP1/TIR1 yeast gene

The hypoxic SRP1/TIR1 gene encodes a stress-response cell wall mannoprotein and this gene is downregulated at acidic pH. The stress-responsive HOG pathway is necessary to maintain hypoxic TIR1 expression, but only at acidic pH. However, unlike known HOG pathway-dependent genes, TIR1 is under positive cAMP control and this effect is mediated by GPA2 but not by RAS2. Genetic analysis showed that ord1 mutation was epistatic to the gpa2 mutation, thereby indicating that Gpa2p is needed to counteract the Ord1 factor, which is involved in the repression of hypoxic TIR1 expression, while the HOG pathway appears to be independent from Ord1 repression. In addition, an increased ORD1 gene expression was observed in the Deltagpa2 mutant cells, meaning that GPA2 maintains a low basal level of ORD1 transcripts. Thus, cAMP allows partial relief of the TIR1 repression exerted by Ord1p. However, this is contradicted at acidic pH by the HOG pathway requirement because Hog1p is activated under stress conditions when the cAMP cellular content is low. The opposite effects of the GPA2-cAMP and HOG pathways are likely to explain the diminished hypoxic expression of TIR1 at acidic pH.

G protein regulation of ion channels and abscisic acid signaling in Arabidopsis guard cells

The phytohormone abscisic acid (ABA) promotes plant water conservation by decreasing the apertures of stomatal pores in the epidermis through which water loss occurs. We found that Arabidopsis thaliana plants harboring transferred DNA insertional mutations in the sole prototypical heterotrimeric GTP-binding (G) protein alpha subunit gene, GPA1, lack both ABA inhibition of guard cell inward K(+) channels and pH-independent ABA activation of anion channels. Stomatal opening in gpa1 plants is insensitive to inhibition by ABA, and the rate of water loss from gpa1 mutants is greater than that from wild-type plants. Manipulation of G protein status in guard cells may provide a mechanism for controlling plant water balance.

Modulation of cell proliferation by heterotrimeric G protein in Arabidopsis

The alpha subunit of a prototypical heterotrimeric GTP-binding protein (G protein), which is encoded by a single gene (GPA1) in Arabidopsis, is a modulator of plant cell proliferation. gpa1 null mutants have reduced cell division in aerial tissues throughout development. Inducible overexpression of GPA1 in Arabidopsis confers inducible ectopic cell division. GPA1 overexpression in synchronized BY-2 cells causes premature advance of the nuclear cycle and the premature appearance of a division wall. Results from loss of function and ectopic expression and activation of GPA1 indicate that this subunit is a positive modulator of cell division in plants.

Biochemical analysis of yeast G(alpha) mutants that enhance adaptation to pheromone

The mating-specific heterotrimeric G(alpha) protein of Saccharomyces cerevisiae, Gpa1, negatively regulates activation of the pheromone response pathway both by sequestering G(beta)gamma and by triggering an adaptive response through an as yet unknown mechanism. Previous genetic studies identified mutant alleles of GPA1 that downregulate the pheromone response independently of the pheromone receptor (GPA1E364K), or through a receptor-dependent mechanism (GPA1N388D). To further our understanding of the mechanism of action of these mutant alleles, their corresponding proteins were purified and subjected to biochemical analysis. The receptor-dependent activity of Gpa1N388D was further analyzed using yeast strains expressing constitutively active receptor (Ste2) mutants, and C-terminal truncation mutant forms of Gpa1. A combination of G(alpha) affinity chromatography, GTP binding/hydrolysis studies, and genetic analysis allowed us to assign a distinct mechanism of action to each of these mutant proteins.

Single amino acid substitutions and deletions that alter the G protein coupling properties of the V2 vasopressin receptor identified in yeast by receptor random mutagenesis

To facilitate structure-function relationship studies of the V2 vasopressin receptor, a prototypical G(s)-coupled receptor, we generated V2 receptor-expressing yeast strains (Saccharomyces cerevisiae) that required arginine vasopressin-dependent receptor/G protein coupling for cell growth. V2 receptors heterologously expressed in yeast were unable to productively interact with the endogenous yeast G protein alpha subunit, Gpa1p, or a mutant Gpa1p subunit containing the C-terminal G alpha(q) sequence (Gq5). In contrast, the V2 receptor efficiently coupled to a Gpa1p/G alpha(s) hybrid subunit containing the C-terminal G alpha(s) sequence (Gs5), indicating that the V2 receptor retained proper G protein coupling selectivity in yeast. To gain insight into the molecular basis underlying the selectivity of V2 receptor/G protein interactions, we used receptor saturation random mutagenesis to generate a yeast library expressing mutant V2 receptors containing mutations within the second intracellular loop. A subsequent yeast genetic screen of about 30,000 mutant receptors yielded four mutant receptors that, in contrast to the wild-type receptor, showed substantial coupling to Gq5. Functional analysis of these mutant receptors, followed by more detailed site-directed mutagenesis studies, indicated that single amino acid substitutions at position Met(145) in the central portion of the second intracellular loop of the V2 receptor had pronounced effects on receptor/G protein coupling selectivity. We also observed that deletion of single amino acids N-terminal of Met(145) led to misfolded receptor proteins, whereas single amino acid deletions C-terminal of Met(145) had no effect on V2 receptor function. These findings highlight the usefulness of combining receptor random mutagenesis and yeast expression technology to study mechanisms governing receptor/G protein coupling selectivity and receptor folding.

Cyclic AMP-dependent protein kinase controls virulence of the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that infects the human central nervous system. This pathogen elaborates two specialized virulence factors: the antioxidant melanin and an antiphagocytic immunosuppressive polysaccharide capsule. A signaling cascade controlling mating and virulence was identified. The PKA1 gene encoding the major cyclic AMP (cAMP)-dependent protein kinase catalytic subunit was identified and disrupted. pka1 mutant strains were sterile, failed to produce melanin or capsule, and were avirulent. The PKR1 gene encoding the protein kinase A (PKA) regulatory subunit was also identified and disrupted. pkr1 mutant strains overproduced capsule and were hypervirulent in animal models of cryptococcosis. pkr1 pka1 double mutant strains exhibited phenotypes similar to that of pka1 mutants, providing epistasis evidence that the Pka1 catalytic subunit functions downstream of the Pkr1 regulatory subunit. The PKA pathway was also shown to function downstream of the Galpha protein Gpa1 and to regulate cAMP production by feedback inhibition. These findings define a Galpha protein-cAMP-PKA signaling pathway regulating differentiation and virulence of a human fungal pathogen.

The role of hexose transport and phosphorylation in cAMP signalling in the yeast Saccharomyces cerevisiae

Glucose-induced cAMP signalling in Saccharomyces cerevisiae requires extracellular glucose detection via the Gpr1-Gpa2 G-protein coupled receptor system and intracellular glucose-sensing that depends on glucose uptake and phosphorylation. The glucose uptake requirement can be fulfilled by any glucose carrier including the Gal2 permease or by intracellular hydrolysis of maltose. Hence, the glucose carriers do not seem to play a regulatory role in cAMP signalling. Also the glucose carrier homologues, Snf3 and Rgt2, are not required for glucose-induced cAMP synthesis. Although no further metabolism beyond glucose phosphorylation is required, neither Glu6P nor ATP appears to act as metabolic trigger for cAMP signalling. This indicates that a regulatory function may be associated with the hexose kinases. Consistently, intracellular acidification, another known trigger of cAMP synthesis, can bypass the glucose uptake requirement but not the absence of a functional hexose kinase. This may indicate that intracellular acidification can boost a downstream effect that amplifies the residual signal transmitted via the hexose kinases when glucose uptake is too low.