Interaction of gdt1 and protein kinase A (PKA) in the growth-differentiation-transition in Dictyostelium

The gdt1 gene is a negative regulator of the growth-differentiation-transition (GDT) in Dictyostelium. gdt1- cells express the GDT marker discoidin earlier and at higher levels and prematurely enter the differentiation pathway. Protein kinase A is a positive regulator of the GDT and is required for multicellular development. Disruption of the PKA catalytic subunit or overexpression of a constitutively active mutant of the regulatory subunit results in cells which do not form multicellular aggregates and which show strongly reduced levels of discoidin. We have created PKA-/gdt1- double mutants and show that these display high levels of discoidin expression but no aggregation, suggesting that gdt1 may be a downstream target of PKA in a branched signaling cascade initiating differentiation. Data obtained with the PKA inhibitor H89 support these result: in wild type cells H89 inhibits discoidin expression while in gdt1- mutants there is no obvious effect. However, since PKA-/gdt1- cells display less discoidin expression than the single gdt1 mutant, we propose that PKA and gdt1 are in two parallel interacting pathways. To get insight into the mechanism how PKA may block gdt1, we have tested two putative PKA phosphorylation sites in the protein and found that one of them is efficiently phosphorylated by PKA in vitro. A model for the interplay between PKA and gdt1 during the growth-differentiation-transition is discussed.     

Interaction of gdt1 and protein kinase A (PKA) in the growth-differentiation-transition in Dictyostelium

Abstract The gdt1 gene is a negative regulator of the growth-differentiation-transition (GDT) in Dictyostelium . gdt1 ⁻ cells express the GDT marker discoidin earlier and at higher levels and prematurely enter the differentiation pathway. Protein kinase A is a positive regulator of the GDT and is required for multicellular development. Disruption of the PKA catalytic subunit or overexpression of a constitutively active mutant of the regulatory subunit results in cells which do not form multicellular aggregates and which show strongly reduced levels of discoidin. We have created PKA ⁻/gdt1⁻ double mutants and show that these display high levels of discoidin expression but no aggregation, suggesting that gdt1 may be a downstream target of PKA in a branched signalling cascade initiating differentiation. Data obtained with the PKA inhibitor H89 support these result: in wild type cells H89 inhibits discoidin expression while in gdt1 ⁻ mutants there is no obvious effect. However, since PKA⁻/gdt1⁻ cells display less discoidin expression than the single gdt1 mutant, we propose that PKA and gdt1 are in two parallel interacting pathways.
To get insight into the mechanism how PKA may block gdt1, we have tested two putative PKA phosphorylation sites in the protein and found that one of them is efficiently phosphorylated by PKA in vitro. A model for the interplay between PKA and gdt1 during the growth-differentiation-transition is discussed.     

Galpha3 and protein kinase A represent cross-talking pathways for gene expression in Dictyostelium discoideum

Heterotrimeric G proteins and protein kinase A (PKA) are regulators of development in Dictyostelium discoideum. It has been reported that disruption of the Dictyostelium Galpha3 gene (galpha3-) blocks development and expression of several early development genes, characteristics that are reminiscent of mutants lacking the catalytic subunit of PKA (pkac-). The hypothesis that Galpha3 and PKA signaling pathways may interact to control developmental gene expression was tested by comparing the regulation of seven genes expressed early in development in the wild-type and in galpha3- and pkac- mutants, and comparing PKA activity in the wild-type and in a galpha3- mutant. The expression patterns of six genes were affected similarly by the Galpha3 and PKA mutations, while the expression of only one gene, the cAMP receptor 1 (cAR1), differed between the mutants. PKA activity, measured by phosphorylation of the PKA-specific substrate Kemptide, was higher in galpha3- cells than in wild-type cells, suggesting that Galpha3 normally exerts an inhibitory effect on PKA activity. Although some early development genes appear to require both Galpha3 and PKA for expression, the differing response of cAR1 expression and the inhibitory effect of Galpha3 on PKA activity suggest that Galpha3 and PKA are members of interacting pathways controlling gene expression early in development.     

Helix 8 Leu in the CB1 cannabinoid receptor contributes to selective signal transduction mechanisms

The intracellular C-terminal helix 8 (H8) of the CB(1) cannabinoid receptor deviates from the highly conserved NPXXY(X)(5,6)F G-protein-coupled receptor motif, possessing a Leu instead of a Phe. We compared the signal transduction capabilities of CB(1) with those of an L7.60F mutation and an L7.60I mutation that mimics the CB(2) sequence. The two mutant receptors differed from wild type (WT) in their ability to regulate G-proteins in the [(35)S]guanosine 5'-3-O-(thio)triphosphate binding assay. The L7.60F receptor exhibited attenuated stimulation by agonists WIN-55,212-2 and CP-55,940 but not HU-210, whereas the L7.60I receptor exhibited impaired stimulation by all agonists tested as well as by the inverse agonist rimonabant. The mutants internalized more rapidly than WT receptors but could equally sequester G-proteins from the somatostatin receptor. Both the time course and maximal N-type Ca(2+) current inhibition by WIN-55,212-2 were reduced in the mutants. Reconstitution experiments with pertussis toxin-insensitive G-proteins revealed loss of coupling to Galpha(i3) but not Galpha(0A) in the L7.60I mutant, whereas the reduction in the time course for the L7.60F mutant was governed by Galpha(i3). Furthermore, Galpha(i3) but not Galpha(0A) enhanced basal facilitation ratio, suggesting that Galpha(i3) is responsible for CB(1) tonic activity. Co-immunoprecipitation studies revealed that both mutant receptors were associated with Galpha(i1) or Galpha(i2) but not with Galpha(i3). Molecular dynamics simulations of WT CB(1) receptor and each mutant in a 1-palmitoyl-2-oleoylphosphatidylcholine bilayer suggested that the packing of H8 is different in each. The hydrogen bonding patterns along the helix backbones of each H8 also are different, as are the geometries of the elbow region of H8 (R7.56(400)-K7.58(402)). This study demonstrates that the evolutionary modification to NPXXY(X)(5,6)L contributes to maximal activity of the CB(1) receptor and provides a molecular basis for the differential coupling observed with chemically different agonists.     

Genome integrity is regulated by the Caenorhabditis elegans Rad51D homolog rfs-1

Multiple mechanisms ensure genome maintenance through DNA damage repair, suppression of transposition, and telomere length regulation. The mortal germline (Mrt) mutants in Caenorhabditis elegans are defective in maintaining genome integrity, resulting in a progressive loss of fertility over many generations. Here I show that the high incidence of males (him)-15 locus, defined by the deficiency eDf25, is allelic to rfs-1, the sole rad-51 paralog group member in C. elegans. The rfs-1/eDf25 mutant displays a Mrt phenotype and mutant animals manifest features of chromosome fusions prior to the onset of sterility. Unlike other Mrt genes, rfs-1 manifests fluctuations in telomere lengths and functions independently of telomerase. These data suggest that rfs-1 is a novel regulator of genome stability.     

Galphaq-coupled receptor internalization specifically induced by Galphaq signaling. Regulation by EBP50

In the present report, we investigated the effect of ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) expression on the agonist-induced internalization of the thromboxane A(2) beta receptor (TPbeta receptor). Interestingly, we found that EBP50 almost completely blocked TPbeta receptor internalization, which could not be reversed by overexpression of G protein-coupled receptor (GPCR) kinases and arrestins. Because we recently demonstrated that EBP50 can bind to and inhibit Galpha(q), we next studied whether Galpha(q) signaling could induce TPbeta receptor internalization, addressing the long standing question about the relationship between GPCR signaling and their internalization. Expression of a constitutively active Galpha(q) mutant (Galpha(q)-R183C) resulted in a robust internalization of the TPbeta receptor, which was unaffected by expression of dominant negative mutants of arrestin-2 and -3, but inhibited by expression of EBP50 or dynamin-K44A, a dominant negative mutant of dynamin. Phospholipase Cbeta and protein kinase C did not appear to significantly contribute to internalization of the TPbeta receptor, suggesting that Galpha(q) induces receptor internalization through a phospholipase Cbeta- and protein kinase C-independent pathway. Surprisingly, there appears to be specificity in Galpha protein-mediated GPCR internalization. Galpha(q)-R183C also induced the internalization of CXCR4 (Galpha(q)-coupled), whereas it failed to do so for the beta(2)-adrenergic receptor (Galpha(s)-coupled). Moreover, Galpha(s)-R201C, a constitutively active form of Galpha(s), had no effect on internalization of the TPbeta, CXCR4, and beta(2)-adrenergic receptors. Thus, we showed that Galpha protein signaling can lead to internalization of GPCRs, with specificity in both the Galpha proteins and GPCRs that are involved. Furthermore, a new function has been described for EBP50 in its capacity to inhibit receptor endocytosis.     

Absence of complementation in non-allelic mutants of Dictyostelium discoideum with defects in the transition from growth to development

In Dictyostelium discoideum, growth and development are mutually exclusive and the transition between the two phases of the life cycle is regulated by the environment. This regulation is disturbed in HBW3, a chemically induced mutant with an unknown molecular defect. The mutant develops rapidly and expresses developmental markers during growth. Here we show that HBW3 fails to complement another mutant which has a similar phenotype: a targeted knock-out of the gdt1 gene. We further show that both mutations are recessive and that both are located on chromosome III, suggesting that the two mutations might be allelic. Molecular analysis, however, demonstrates that the gdt1 gene is not mutated in HBW3. Thus, although a wild-type copy of each gene is present in diploid cell lines, the defects due to the recessive mutations synergize to produce a detectable phenotype. The phenotypic similarities and differences between the two mutants are discussed.     

The barley amo1 locus is tightly linked to the starch synthase IIIa gene and negatively regulates expression of granule-bound starch synthetic genes

In this study of barley starch synthesis, the interaction between mutations at the sex6 locus and the amo1 locus has been characterized. Four barley genotypes, the wild type, sex6, amo1, and the amo1sex6 double mutant, were generated by backcrossing the sex6 mutation present in Himalaya292 into the amo1 'high amylose Glacier'. The wild type, amo1, and sex6 genotypes gave starch phenotypes consistent with previous studies. However, the amo1sex6 double mutant yielded an unexpected phenotype, a significant increase in starch content relative to the sex6 phenotype. Amylose content (as a percentage of starch) was not increased above the level observed for the sex6 mutation alone; however, on a per seed basis, grain from lines containing the amo1 mutation (amo1 mutants and amo1sex6 double mutants) synthesize significantly more amylose than the wild-type lines and sex6 mutants. The level of granule-bound starch synthase I (GBSSI) protein in starch granules is increased in lines containing the amo1 mutation (amo1 and amo1sex6). In the amo1 genotype, starch synthase I (SSI), SSIIa, starch branching enzyme IIa (SBEIIa), and SBEIIb also markedly increased in the starch granules. Genetic mapping studies indicate that the ssIIIa gene is tightly linked to the amo1 locus, and the SSIIIa protein from the amo1 mutant has a leucine to arginine residue substitution in a conserved domain. Zymogram analysis indicates that the amo1 phenotype is not a consequence of total loss of enzymatic activity although it remains possible that the amo1 phenotype is underpinned by a more subtle change. It is therefore proposed that amo1 may be a negative regulator of other genes of starch synthesis.     

Truncated products of the vestigial proliferation gene induce apoptosis.

The vestigial (vg) gene in D. melanogaster, whose mutant phenotype is characterized by wing atrophy, encodes a novel nuclear protein involved in cell proliferation. The original vg mutant (vgBG) displays massive apoptosis in the wing imaginal disc. Here we tested the hypothesis that the vg mutant phenotype could be due: (i) to lack of cell proliferation in null mutants due to the absence of the Vg product and, (ii) to apoptosis in vgBG and other mutants due to the presence of a major Vg truncated product. In agreement with our hypothesis no cell death was observed in null vg mutants, and the anticell death baculovirus P35 product is unable to rescue the mutant phenotype caused by absence of the Vg product. In addition, expression of the antiproliferative gene dacapo, the homolog of p21, induces a mutant wing phenotype without inducing cell death. In contrast the wing phenotype of the original vg mutant could be reproduced by the ectopic expression of the reaper cell death gene when expressed by vg regulatory sequences. In agreement with the hypothesis, the classic vg mutant spontaneously displays an increase in reaper expression in the wing disc and its phenotype can be partially rescued by the P35 product. Finally, we showed that ectopic expression of a truncated Vg product is able on its own to induce ectopic cell death and reaper expression. Our results shed new light on the function of the vg gene, in particular, they suggest that the normal and truncated products affect vg target genes in different ways.     

Role of cAMP-dependent protein kinase during growth and early development of Dictyostelium discoideum

cAMP-dependent protein kinase (PKA) is an essential regulator of gene expression and cell differentiation during multicellular development of Dictyostelium discoideum. Here we show that PKA activity also regulates gene expression during the growth phase and at the transition from growth to development. Overexpression of PKA leads to overexpression of the discoidinIgamma promoter, while expression of the discoidinIgamma promoter is reduced when PKA activity is reduced, either by expression of a dominant negative mutant of the regulatory subunit or by disruption of the gene for the catalytic subunit (PKA-C). The discoidin phenotype of PKA-C null cells is cell autonomous. In particular, normal secretion of discoidin-inducing factors was demonstrated. In addition, PKA-C null cells are able to respond to media conditioned by PSF and CMF. We conclude that PKA is a major activator of discoidin expression. However, it is not required for production or transduction of the inducing extracellular signals. Therefore, PKA-dependent and PKA-independent pathways regulate the expression of the discoidin genes.     

BON1 interacts with the protein kinases BIR1 and BAK1 in modulation of temperature-dependent plant growth and cell death in Arabidopsis

The Arabidopsis copine gene BON1 encodes a calcium-dependent phospholipid-binding protein involved in plant growth homeostasis and disease resistance. However, the biochemical and molecular mechanisms by which BON1 modulates plant growth and defense responses are not well understood. Here, we show that BON1 interacts physically with the leucine-rich-repeat receptor-like kinases BIR1 (BAK1-interacting receptor-like kinase 1) and pathogen-associated molecular pattern (PAMP) receptor regulator BAK1 in vitro and in vivo. Additionally, bon1 and bir1 mutants exhibit synergistic interaction. While a bir1 null mutant has similar growth and cell-death defects compared with bon1, a bir1 bon1 double mutant displays more severe phenotypes than does the single mutants. The bon1-1 and bir1-1 phenotypes are partially suppressed by overexpression of BIR1 and BON1, respectively. Furthermore, the bir1 phenotype is attenuated by a loss-of-function mutation in the resistance (R) gene SNC1 (Suppressor of npr1-1, constitutive 1), which mediates defense responses in bon1. Intriguingly, BON1 and BIR1 can be phosphorylated by BAK1 in vitro. Our findings suggest that BIR1 functions as a negative regulator of plant resistance and that BON1 and BIR1 might modulate both PAMP- and R protein-triggered immune responses.     

Arabidopsis extra-large G proteins (XLGs) regulate root morphogenesis

As in mammalian systems, heterotrimeric G proteins, composed of alpha, beta and gamma subunits, are present in plants and are involved in the regulation of development and cell signaling. Besides the sole prototypical G protein alpha subunit gene, GPA1, the Arabidopsis thaliana genome has three extra-large GTP-binding protein (XLG)-encoding genes: XLG1 (At2g23460), XLG2 (At4g34390) and XLG3 (At1g31930). The C-termini of the XLGs are Galpha domains that are homologous to GPA1, whereas their N-termini each contain a cysteine-rich region and a putative nuclear localization signal (NLS). GFP fusions with each XLG confirmed nuclear localization. All three XLG genes are expressed in essentially all plant organs, with strong expression in vascular tissues, primary root meristems and lateral root primordia. Analysis of single, double and triple T-DNA insertional mutants of the XLG genes revealed redundancy in XLG function. Dark-grown xlg1-1 xlg2-1 xlg3-1 triple mutant plants showed markedly increased primary root length compared with wild-type plants. This phenotype was not observed in dark-grown xlg single mutants, and was suppressed upon complementation of the xlg triple mutant with each XLG. Root cell sizes of the xlg triple mutant and root morphology were highly similar to those of wild-type roots, suggesting that XLGs may regulate cell proliferation. Dark-grown roots of the xlg triple mutants also showed altered sensitivity to sugars, ABA hyposensitivity and ethylene hypersensitivity, whereas seed germination in xlg triple mutants was hypersensitive to osmotic stress and ABA. As plant-specific proteins, regulatory mechanisms of XLGs may differ from those of conventional Galphas.     

The thick aleurone1 mutant defines a negative regulation of maize aleurone cell fate that functions downstream of defective kernel1

The maize (Zea mays) aleurone layer occupies the single outermost layer of the endosperm. The defective kernel1 (dek1) gene is a central regulator required for aleurone cell fate specification. dek1 mutants have pleiotropic phenotypes including lack of aleurone cells, aborted embryos, carotenoid deficiency, and a soft, floury endosperm deficient in zeins. Here we describe the thick aleurone1 (thk1) mutant that defines a novel negative function in the regulation of aleurone differentiation. Mutants possess multiple layers of aleurone cells as well as aborted embryos. Clonal sectors of thk1 mutant tissue in otherwise normal endosperm showed localized expression of the phenotype with sharp boundaries, indicating a localized cellular function for the gene. Sectors in leaves showed expanded epidermal cell morphology but the mutant epidermis generally remained in a single cell layer. Double mutant analysis indicated that the thk1 mutant is epistatic to dek1 for several aspects of the pleiotropic dek1 phenotype. dek1 mutant endosperm that was mosaic for thk1 mutant sectors showed localized patches of multilayered aleurone. Localized sectors were surrounded by halos of carotenoid pigments and double mutant kernels had restored zein profiles. In sum, loss of thk1 function restored the ability of dek1 mutant endosperm to accumulate carotenoids and zeins and to differentiate aleurone. Therefore the thk1 mutation defines a negative regulator that functions downstream of dek1 in the signaling system that controls aleurone specification and other aspects of endosperm development. The thk1 mutation was found to be caused by a deletion of approximately 2 megabases.     

G(i)-dependent activation of c-Jun N-terminal kinase in human embryonal kidney 293 cells

Heterotrimeric G proteins stimulate the activities of two stress-activated protein kinases, c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase in mammalian cells. In this study, we examined whether alpha subunits of G(i) family activate JNK using transient expression system in human embryonal kidney 293 cells. Constitutively activated mutants of Galpha(i1), Galpha(i2), and Galpha(i3) increased JNK activity. In contrast, constitutively activated Galpha(o) and Galpha(z) mutants did not stimulate JNK activity. To examine the mechanism of JNK activation by Galpha(i), kinase-deficient mutants of mitogen-activated protein kinase kinase 4 (MKK4) and 7 (MKK7), which are known to be JNK activators, were transfected into the cells. However, Galpha(i)-induced JNK activation was not blocked effectively by kinase-deficient MKK4 and MKK7. In addition, activated Galpha(i) mutant failed to stimulate MKK4 and MKK7 activities. Furthermore, JNK activation by Galpha(i) was inhibited by dominant-negative Rho and Cdc42 and tyrosine kinase inhibitors, but not dominant-negative Rac and phosphatidylinositol 3-kinase inhibitors. These results indicate that Galpha(i) regulates JNK activity dependent on small GTPases Rho and Cdc42 and on tyrosine kinase but not on MKK4 and MKK7.     

Polarity exchange at the interface of regulators of G protein signaling with G protein alpha-subunits

RGS proteins are GTPase-activating proteins (GAPs) for G protein alpha-subunits. This GAP activity is mediated by the interaction of conserved residues on regulator of G protein signaling (RGS) proteins and Galpha-subunits. We mutated the important contact sites Glu-89, Asn-90, and Asn-130 in RGS16 to lysine, aspartate, and alanine, respectively. The interaction of RGS16 and its mutants with Galpha(t) and Galpha(i1) was studied. The GAP activities of RGS16N90D and RGS16N130A were strongly attenuated. RGS16E89K increased GTP hydrolysis of Galpha(i1) by a similar extent, but with an about 100-fold reduced affinity compared with non-mutated RGS16. As Glu-89 in RGS16 is interacting with Lys-210 in Galpha(i1), this lysine was changed to glutamate for compensation. Galpha(i1)K210E was insensitive to RGS16 but interacted with RGS16E89K. In rat uterine smooth muscle cells, wild type RGS16 abolished G(i)-mediated alpha(2)-adrenoreceptor signaling, whereas RGS16E89K was without effect. Both Galpha(i1) and Galpha(i1)K210E mimicked the effect of alpha(2)-adrenoreceptor stimulation. Galpha(i1)K210E was sensitive to RGS16E89K and 10-fold more potent than Galpha(i1). Analogous mutants of Galpha(q) (Galpha(q)K215E) and RGS4 (RGS4E87K) were created and studied in COS-7 cells. The activity of wild type Galpha(q) was counteracted by wild type RGS4 but not by RGS4E87K. The activity of Galpha(q)K215E was inhibited by RGS4E87K, whereas non-mutated RGS4 was ineffective. We conclude that mutation of a conserved lysine residue to glutamate in Galpha(i) and Galpha(q) family members renders these proteins insensitive to wild type RGS proteins. Nevertheless, they are sensitive to glutamate to lysine mutants of RGS proteins. Such mutant pairs will be helpful tools in analyzing Galpha-RGS specificities in living cells.