Dynamic interactions between 14-3-3 proteins and phosphoproteins regulate diverse cellular processes

CaMKII-mediated phosphorylation of the myosin motor Myo1c is required for insulin-stimulated GLUT4 translocation in adipocytes

The unconventional myosin Myo1c has been implicated in insulin-regulated GLUT4 translocation to the plasma membrane in adipocytes. We show that Myo1c undergoes insulin-dependent phosphorylation at S701. Phosphorylation was accompanied by enhanced 14-3-3 binding and reduced calmodulin binding. Recombinant CaMKII phosphorylated Myo1c in vitro and siRNA knockdown of CaMKIIdelta abolished insulin-dependent Myo1c phosphorylation in vivo. CaMKII activity was increased upon insulin treatment and the CaMKII inhibitors CN21 and KN-62 or the Ca(2+) chelator BAPTA-AM blocked insulin-dependent Myo1c phosphorylation and insulin-stimulated glucose transport in adipocytes. Myo1c ATPase activity was increased after CaMKII phosphorylation in vitro and after insulin stimulation of CHO/IR/IRS-1 cells. Expression of wild-type Myo1c, but not S701A or ATPase dead mutant K111A, rescued the inhibition of GLUT4 translocation by siRNA-mediated Myo1c knockdown. These data suggest that insulin regulates Myo1c function via CaMKII-dependent phosphorylation, and these events play a role in insulin-regulated GLUT4 trafficking in adipocytes likely involving Myo1c motor activity.

The clinicopathological and prognostic impact of 14-3-3 sigma expression on vulvar squamous cell carcinomas

Background

14-3-3 sigma (σ) promotes G2/M cell cycle arrest by sequestering cyclin B1-CDC2 complex in cytoplasm. Down-regulation of 14-3-3σ, which has been demonstrated in various carcinomas, may contribute to malignant transformation. However, the exact role of 14-3-3σ in the pathogenesis of vulvar carcinoma is not fully characterized, and the prognostic impact of 14-3-3σ protein expression is still unknown.

Methods

We investigated the 14-3-3σ expression in a series of 302 vulvar squamous cell carcinomas using immunohistochemistry and its associations with clinicopathological factors and clinical outcome.

Results

In cytoplasm, nucleus and cytoplasm/nucleus of vulvar carcinomas high 14-3-3σ protein expression was found in 72%, 59% and 75% of the carcinomas, respectively, and low levels in 28%, 41% and 25% of the cases, respectively. High level of 14-3-3σ in cytoplasm, nucleus and cytoplasm/nucleus was significantly correlated to large tumor diameter (p = 0.001, p = 0.002 and p = 0.001, respectively) and deep invasion (p = 0.01, p = 0.001 and p = 0.007, respectively). Variations of 14-3-3σ protein expression were not associated to disease-specific survival.

Conclusion

Our results indicate that 14-3-3σ may be involved in the development of a subset of vulvar squamous cell carcinomas by down-regulation of 14-3-3σ protein. Neither cytoplasmic nor nuclear level of 14-3-3σ expression was associated with prognosis.

14-3-3 activation of DNA binding of p53 by enhancing its association into tetramers

Activation of the tumour suppressor p53 on DNA damage involves post-translational modification by phosphorylation and acetylation. Phosphorylation of certain residues is critical for p53 stabilization and plays an important role in DNA-binding activity. The 14-3-3 family of proteins activates the DNA-binding affinity of p53 upon stress by binding to a site in its intrinsically disordered C-terminal domain containing a phosphorylated serine at 378. We have screened various p53 C-terminal phosphorylated peptides for binding to two different isoforms of 14-3-3, epsilon and gamma. We found that phosphorylation at either S366 or T387 caused even tighter binding to 14-3-3. We made by semi-synthesis a tetrameric construct comprised of the tetramerization plus C-terminal domains of p53 that was phosphorylated on S366, S378 and T387. It bound 10 times tighter than did the monomeric counterpart to dimeric 14-3-3. We showed indirectly from binding curves and directly from fluorescence-detection analytical ultracentrifugation that 14-3-3 enhanced the binding of sequence-specific DNA to p53 by causing p53 dimers to form tetramers at lower concentrations. If the in vitro data extrapolate to in vivo, then it is an attractive hypothesis that p53 activity may be subject to control by accessory proteins lowering its tetramer-dimer dissociation constant from its normal value of 120-150 nM.

Interaction with 14-3-3 adaptors regulates the sorting of hMex-3B RNA-binding protein to distinct classes of RNA granules

Stress granules (SG) and processing bodies (PBs) are cytoplasmic ribonucleoprotein particles whose assembly is induced by different stimuli. SG are the site of storage of untranslated transcripts formed in response to environmental stress, whereas PBs are involved in mRNA turnover. We recently characterized a novel family of four human proteins related to the Caenorhabditis elegans Mex-3, a RNA binding protein involved in the establishment of the anterior-posterior embryonic asymmetry and in the maintenance of germline pluripotency. We now report that the adaptor proteins 14-3-3 bind to hMex-3B but not to the three other hMex-3 family members. Serine 462, when phosphorylated, is the major 14-3-3 docking site on hMex-3B, and manipulation of this interaction reveals that 14-3-3 both stabilizes hMex-3B and modulates its ability to bind RNA. Furthermore, the complex formed between hMex-3B and Argonaute proteins is excluded from PBs when the interaction with 14-3-3 is disrupted, whereas the recruitment to SG is not affected. Thus, 14-3-3 exerts combined effects on hMex-3B and acts as a major regulator of the sorting between distinct classes of RNA granules.

The Fat and Warts signaling pathways: new insights into their regulation,mechanism and conservation

A cassette of cytoplasmic Drosophila tumor suppressors, including the kinases Hippo and Warts, has recently been linked to the transmembrane tumor suppressor Fat. These proteins act within interconnected signaling pathways, the principal functions of which are to control the growth and polarity of developing tissues. Recent studies have enhanced our understanding of the basis for signal transduction by Fat and Warts pathways, including the identification of a DNA-binding protein at the end of the pathway, have established the conservation of Fat and Warts signaling from flies to mammals,and have given us new insights into their regulation and biological functions.

Analysis of transient phosphorylation-dependent protein-protein interactions in living mammalian cells using split-TEV

Background

Regulated protein-protein interactions (PPIs) are pivotal molecular switches that are important for the regulation of signaling processes within eukaryotic cells. Cellular signaling is altered in various disease conditions and offers interesting options for pharmacological interventions. Constitutive PPIs are usually mediated by large interaction domains. In contrast, stimulus-regulated PPIs often depend on small post-translational modifications and are thus better suited targets for drug development. However, the detection of modification-dependent PPIs with biochemical methods still remains a labour- and material-intensive task, and many pivotal PPIs that are potentially suited for pharmacological intervention most likely remain to be identified. The availability of methods to easily identify and quantify stimulus-dependent, potentially also transient interaction events, is therefore essential. The assays should be applicable to intact mammalian cells, optimally also to primary cells in culture.

Results

In this study, we adapted the split-TEV system to quantify phosphorylation-dependent and transient PPIs that occur at the membrane and in the cytosol of living mammalian cells. Split-TEV is based on a PPI-induced functional complementation of two inactive TEV protease fragments fused to interaction partners of choice. Genetically encoded transcription-coupled and proteolysis-only TEV reporter systems were used to convert the TEV activity into an easily quantifiable readout. We measured the phosphorylation-dependent interaction between the pro-apoptotic protein Bad and the adapter proteins 14-3-3ε and ζ in NIH-3T3 fibroblasts and in primary cultured neurons. Using split-TEV assays, we show that Bad specifically interacts with 14-3-3 isoforms when phosphorylated by protein kinase Akt-1/PKB at Ser136. We also measured the phosphorylation-dependent Bad/14-3-3 interactions mediated by endogenous and transient Akt-1 activity. We furthermore applied split-TEV assays to measure the phosphorylation-dependent interactions of Neuregulin-1-stimulated ErbB4 receptors with several adapter proteins.

Conclusion

Split-TEV assays are well suited to measure phosphorylation-dependent and transient PPIs that occur specifically at the membrane and in the cytosol of heterologous and primary cultured mammalian cells. Given the high sensitivity of the split-TEV system, all assays were performed in multi-plate formats and could be adapted for higher throughput to screen for pharmacologically active substances.

Prognostic significance of 14-3-3sigma expression in oral squamous cell carcinoma (OSCC)

14-3-3sigma an intracellular phosphoserine binding protein regulates different cellular signalling processes and is involved in cancer development. In this study, we examined the expression of 14-3-3sigma and evaluated its clinical significance in OSCC. Tumour tissue from 95 OSCC patients was analysed for 14-3-3sigma and p53 expression, respectively. The correlation of these proteins with survival and clinical parameters was assessed. 14-3-3sigma high expression was observed in 44.2% of OSCC patients. A significant role of 14-3-3sigma expression on survival was shown by Kaplan-Meier analysis. Median survival time was 4.1years for patients with 14-3-3sigma low tumours, compared with 1.36years for 14-3-3sigma high tumours (P=.0021). Subset analysis in patients receiving adjuvant chemotherapy showed that the overall survival was significantly decreased in 14-3-3sigma high tumours than in 14-3-3sigma low tumours (P=.02). p53 expression was not significant in univariate analyses. In multivariate regression analysis, 14-3-3sigma expression emerged as a significant independent parameter (P=.003). These results provide evidence that 14-3-3sigma expression is involved in OSCC and, in contrast to p53 expression represents a new prognostic marker for OSCC and therapy response. Pending validation targeting 14-3-3sigma might also be a new opportunity to improve therapy.

Novel self-association of the APC molecule affects APC clusters and cell migration

Truncation mutations in the adenomatous polyposis coli (APC) gene are responsible for familial and sporadic colorectal cancer. APC is a multifunctional protein involved in cell migration, proliferation and differentiation. The APC protein forms specific clusters in the cell periphery that correlate with sites of active cell migration. Little is known about the molecular mechanisms that govern these clusters. Here, we identify a novel interaction of an N-terminal region of APC with the extreme C-terminal 300 amino acids of APC and also with itself. The latter interaction is phospho-sensitive and is enhanced by 14-3-3 (YWHA) protein. These interactions modulate the clustering of APC at the ends of membrane protrusions. Overexpressing this domain or inhibiting 14-3-3 proteins disperses APC clusters and leads to decreased cell migration. Moreover, deleting this domain from full-length APC results in less-dynamic clusters compared with wild-type APC. Our data indicate that this newly identified regions in the N-terminal third of APC contributes to the regulation of APC clusters, thus providing a molecular clue for how locally regulated phosphorylation events could mediate the dynamics of APC clusters and contribute to cell migration.

Phosphorylation-dependent interaction of tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein (YWHA) with PADI6 following oocyte maturation in mice

Proteins in the tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein family (YWHA; also known as 14-3-3) are involved in the regulation of many intracellular processes. We have examined the interaction of YWHA with peptidylarginine deiminase type VI (PADI6), an abundant protein in mammalian oocytes, eggs, and early embryos. Peptidylarginine deiminases catalyze the posttranslational modification of peptidylarginine to citrulline. PADI6 is associated with oocyte cytoplasmic sheets, and PADI6-deficient mice are infertile because of disruption of development beyond the two-cell stage. We found that PADI6 undergoes a dramatic developmental change in phosphorylation during oocyte maturation. This change in phosphorylation is linked to an interaction of PADI6 with YWHA in the mature egg. Recombinant glutathione S-transferase YWHA pull-down experiments and transgenic tandem affinity purification with liquid chromatography-mass spectrometry demonstrate a binding interaction between YWHA and PADI6 in mature eggs. YWHA proteins modulate or complement intracellular events involving phosphorylation-dependent switching or protein modification. These results indicate that phosphorylation and/or YWHA binding may serve as a means of intracellular PADI6 regulation.

Implication of ceramide, ceramide 1-phosphate and sphingosine 1-phosphate in tumorigenesis

In the last two decades there has been considerable progress in our understanding of the role of sphingolipids in controlling signal transduction processes, particularly in the mechanisms leading to regulation of cell growth and death. Ceramide is a well-characterized sphingolipid metabolite and second messenger that can be produced by cancer cells in response to a variety of stimuli, including therapeutic drugs, leading to cell cycle arrest and apoptosis. Although this is a promising aspect when thinking of treating cancer, it should be borne in mind that ceramide production may not always be a growth inhibitory or pro-apoptotic signal. In fact, ceramide can be readily converted to sphingosine 1-phosphate (S1P) by the concerted actions of ceramidases and sphingosine kinases, or to ceramide 1-phosphate (C1P) by the action of ceramide kinase. In general, S1P and C1P have opposing effects to ceramide, acting as pro-survival or mitogenic signals in most cell types. This review will address our current understanding of the many roles of ceramide, S1P and C1P in the regulation of cell growth and survival with special emphasis to the emerging role of these molecules and their metabolizing enzymes in controlling tumor progression and metastasis.

Identification of Bombyx mori 14-3-3 orthologs and the interactor Hsp60

The 14-3-3 protein family consists of evolutionarily conserved, acidic 30 kDa proteins composed of seven isoforms named beta, gamma, epsilon, zeta, eta, theta, and sigma in mammalian cells. The dimeric complex of 14-3-3 isoforms, acting as a molecular adaptor, plays a central role in regulation of neuronal function. Since aberrant expression of 14-3-3 is identified in the brains of Alzheimer disease and Parkinson disease, a convenient insect model, if it is available, is highly valuable for studying a pathological role of 14-3-3 in neurodegeneration. Here, we identified the silkworm Bombyx mori 14-3-3 orthologs, zeta and epsilon isoforms highly homologous in amino acid sequences to the human 14-3-3zeta and 14-3-3epsilon. By Western blot, the expression of zeta and epsilon isoforms was identified at substantial levels in the first instar larva, markedly upregulated in the second instar larva, and the highest levels were maintained in the late stage of larva, the pupa, and the adult. Furthermore, by protein overlay and immunoprecipitation, we identified Hsp60 as a 14-3-3-binding partner. The 14-3-3 proteins interacted with the N-terminal fragment of Hsp60. The 14-3-3zeta and epsilon isoforms, along with Hsp60, were expressed widely with overlapping distribution in larval and adult tissues, including brain, fat body, silk gland, Malpighian tube, midgut, ovary, testis, antenna, and pheromone gland. These observations suggest that a molecular adaptor 14-3-3 and a molecular chaperone Hsp60 cooperate to achieve a wide range of cellular functions in B. mori.

Regulation of proline-rich Akt substrate of 40 kDa (PRAS40) function by mammalian target of rapamycin complex 1 (mTORC1)-mediated phosphorylation

The rapamycin-sensitive mammalian target of rapamycin (mTOR) complex 1 (mTORC1) contains mTOR, raptor, mLST8, and PRAS40 (proline-rich Akt substrate of 40 kDa). PRAS40 functions as a negative regulator when bound to mTORC1, and it dissociates from mTORC1 in response to insulin. PRAS40 has been demonstrated to be a substrate of mTORC1, and one phosphorylation site, Ser-183, has been identified. In this study, we used two-dimensional phosphopeptide mapping in conjunction with mutational analysis to show that in addition to Ser-183, mTORC1 also phosphorylates Ser-212 and Ser-221 in PRAS40 when assayed in vitro. Mutation of all three residues to Ala markedly reduces mTORC1-mediated phosphorylation of PRAS40 in vitro. All three sites were confirmed to be phosphorylated in vivo by [(32)P]orthophosphate labeling and peptide mapping. Phosphorylation of Ser-221 and Ser-183 but not Ser-212 is sensitive to rapamycin treatment. Furthermore, we demonstrate that mutation of Ser-221 to Ala reduces the interaction with 14-3-3 to the same extent as mutation of Thr-246, the Akt/protein kinase B-phosphorylated site. We also find that mutation of Ser-221 to Ala increases the inhibitory activity of PRAS40 toward mTORC1. We propose that after mTORC1 kinase activation by upstream regulators, PRAS40 is phosphorylated directly by mTOR, thus contributing to the relief of PRAS40-mediated substrate competition.

14-3-3 regulates life span by both DAF-16-dependent and -independent mechanisms in Caenorhabditis elegans

Caenorhabditis elegans life span, stress resistance and metabolism are regulated by the Insulin/IGF-1/DAF-2/DAF-16 pathway. DAF-16, a member of FOXO/Forkhead transcription factor family, can be targeted by 14-3-3 proteins to promote stress resistance. We have identified a 14-3-3 C. elegans homolog which promotes life span by both DAF-2-dependent and -independent mechanisms and by an unexpected DAF-16-independent mechanism. Our results demonstrate that C. elegans 14-3-3 proteins modulate stress-responsive genes throughout adulthood. In conclusion, 14-3-3 can be considered as an acute stress-responsive regulator as well as a sustained modulator of the Insulin/IGF-1/DAF-2/DAF-16 regulatory pathway in promoting life expectancy of growing old worms.

Fibroblast growth factor receptor 2 phosphorylation on serine 779 couples to 14-3-3 and regulates cell survival and proliferation

The fibroblast growth factors (FGFs) exert their diverse (or pleiotropic) biological responses through the binding and activation of specific cell surface receptors (FGFRs). While FGFRs are known to initiate intracellular signaling through receptor tyrosine phosphorylation, the precise mechanisms by which the FGFRs regulate pleiotropic biological responses remain unclear. We now identify a new mechanism by which FGFR2 is able to regulate intracellular signaling and cellular responses. We show that FGFR2 is phosphorylated on serine 779 (S779) in response to FGF2. S779, which lies adjacent to the phospholipase Cgamma binding site at Y766, provides a docking site for the 14-3-3 phosphoserine-binding proteins and is essential for the full activation of the phosphatidylinositol 3-kinase and Ras/mitogen-activated protein kinase pathways. Furthermore, S779 signaling is essential for promoting cell survival and proliferation in both Ba/F3 cells and BALB/c 3T3 fibroblasts. This new mode of FGFR2 phosphoserine signaling via the 14-3-3 proteins may provide an increased repertoire of signaling outputs to allow the regulation of pleiotropic biological responses. In this regard, we have identified conserved putative phosphotyrosine/phosphoserine motifs in the cytoplasmic domains of diverse cell surface receptors, suggesting that they may perform important functional roles beyond the FGFRs.

In vivo regulation of Yorkie phosphorylation and localization

Yorkie (Yki), a transcription factor of the Fat and Hippo signaling pathways, is negatively regulated by the Warts kinase. Here, we use Phos-tag gels to characterize Warts-dependent phosphorylation of Yki in vivo, and show that Warts promotes phosphorylation of Yki at multiple sites. We also show that Warts inhibits Yki nuclear localization in vivo, and can promote binding of Yki to 14-3-3 proteins in cultured cells. In vivo assessment of the influence of individual upstream regulators of Warts reveals that some mutants (e.g. fat) have only partial effects on Yki phosphorylation, and weak effects on Yki localization, whereas other genotypes (e.g. ex fat double mutants) have stronger effects on both Yki phosphorylation and localization. We also identify serine 168 as a critical site through which negative regulation of Yki by Warts-mediated phosphorylation occurs, but find that this site is not sufficient to explain effects of Hippo signaling on Yki in vivo. These results identify modulation of subcellular localization as a mechanism of Yki regulation, and establish that this regulation occurs in vivo through multiple sites of Warts-dependent phosphorylation on Yki.

The effect of exercise and insulin on AS160 phosphorylation and 14-3-3 binding capacity in human skeletal muscle

AS160 is an Akt substrate of 160 kDa implicated in the regulation of both insulin- and contraction-mediated GLUT4 translocation and glucose uptake. The effects of aerobic exercise and subsequent insulin stimulation on AS160 phosphorylation and the binding capacity of 14-3-3, a novel protein involved in the dissociation of AS160 from GLUT4 vesicles, in human skeletal muscle are unknown. Hyperinsulinemic-euglycemic clamps were performed on seven men at rest and immediately and 3 h after a single bout of cycling exercise. Skeletal muscle biopsies were taken before and after the clamps. The insulin sensitivity index calculated during the final 30 min of the clamp was 8.0 +/- 0.8, 9.1 +/- 0.5, and 9.2 +/- 0.8 for the rest, postexercise, and 3-h postexercise trials, respectively. AS160 phosphorylation increased immediately after exercise and remained elevated 3 h after exercise. In contrast, the 14-3-3 binding capacity of AS160 and phosphorylation of Akt and AMP-activated protein kinase were only increased immediately after exercise. Insulin increased AS160 phosphorylation and 14-3-3 binding capacity and insulin receptor substrate-1 and Akt phosphorylation, but the response to insulin was not enhanced by prior exercise. In conclusion, the 14-3-3 binding capacity of AS160 is increased immediately after acute exercise in human skeletal muscle, but this is not maintained 3 h after exercise completion despite sustained AS160 phosphorylation. Insulin increases AS160 phosphorylation and 14-3-3 binding capacity, but prior exercise does not appear to enhance the response to insulin.

Regulation of multisite phosphorylation and 14-3-3 binding of AS160 in response to IGF-1, EGF, PMA and AICAR

AS160 (Akt substrate of 160 kDa) mediates insulin-stimulated GLUT4 (glucose transporter 4) translocation, but is widely expressed in insulin-insensitive tissues lacking GLUT4. Having isolated AS160 by 14-3-3-affinity chromatography, we found that binding of AS160 to 14-3-3 isoforms in HEK (human embryonic kidney)-293 cells was induced by IGF-1 (insulin-like growth factor-1), EGF (epidermal growth factor), PMA and, to a lesser extent, AICAR (5-aminoimidazole-4-carboxamide-1-b-D-ribofuranoside). AS160-14-3-3 interactions were stabilized by chemical cross-linking and abolished by dephosphorylation. Eight residues on AS160 (Ser318, Ser341, Thr568, Ser570, Ser588, Thr642, Ser666 and Ser751) were differentially phosphorylated in response to IGF-1, EGF, PMA and AICAR. The binding of 14-3-3 proteins to HA-AS160 (where HA is haemagglutinin) was markedly decreased by mutation of Thr642 and abolished in a Thr642Ala/Ser341Ala double mutant. The AGC (protein kinase A/protein kinase G/protein kinase C-family) kinases RSK1 (p90 ribosomal S6 kinase 1), SGK1 (serum- and glucocorticoid-induced protein kinase 1) and PKB (protein kinase B) displayed distinct signatures of AS160 phosphorylation in vitro: all three kinases phosphorylated Ser318, Ser588 and Thr642; RSK1 also phosphorylated Ser341, Ser751 and to a lesser extent Thr568; and SGK1 phosphorylated Thr568 and Ser751. AMPK (AMP-activated protein kinase) preferentially phosphorylated Ser588, with less phosphorylation of other sites. In cells, the IGF-1-stimulated phosphorylations, and certain EGF-stimulated phosphorylations, were inhibited by PI3K (phosphoinositide 3-kinase) inhibitors, whereas the RSK inhibitor BI-D1870 inhibited the PMA-induced phosphorylations. The expression of LKB1 in HeLa cells and the use of AICAR in HEK-293 cells promoted phosphorylation of Ser588, but only weak Ser341 and Thr642 phosphorylations and binding to 14-3-3s. Paradoxically however, phenformin activated AMPK without promoting AS160 phosphorylation. The IGF-1-induced phosphorylation of the novel phosphorylated Ser666-Pro site was suppressed by AICAR, and by combined mutation of a TOS (mTOR signalling)-like sequence (FEMDI) and rapamycin. Thus, although AS160 is a common target of insulin, IGF-1, EGF, PMA and AICAR, these stimuli induce distinctive patterns of phosphorylation and 14-3-3 binding, mediated by at least four protein kinases.

ABCD of the phosphodiesterase family: interaction and differential activity in COPD

Phosphodiesterases (PDEs) are important enzymes that hydrolyze the cyclic nucleotides adenosine 3'5'-cyclic monophosphate (cAMP) and guanosine 3'5'-cyclic monophosphate (cGMP) to their inactive 5' monophosphates. They are highly conserved across species and as well as their role in signal termination, they also have a vital role in intra-cellular localization of cyclic nucleotide signaling and integration of the cyclic nucleotide pathways with other signaling pathways. Because of their pivotal role in intracellular signaling, they are now of considerable interest as therapeutic targets in a wide variety diseases, including COPD where PDE inhibitors may have bronchodilator, anti-inflammatory and pulmonary vasodilator actions. This review examines the diversity and cellular localization of the isoforms of PDE, the known and speculative relevance of this to the treatment of COPD, and the range of PDE inhibitors in development together with a discussion of their possible role in treating COPD.

Phosducin interacts with the G-protein βγ-dimer of ciliate protozoanBlepharisma japonicumupon illumination

Immunological techniques and high-resolution FRET analysis were employed to investigate the in vivo colocalization and interaction of phosducin(Pdc) with the βγ-subunits of G-protein (Gβγ) in the ciliate Blepharisma japonicum. Immunological techniques revealed that illumination of cells resulted in a decrease in phosphorylation levels of Pdc and its colocalization with Gβγ. The observed light-induced Pdc dephosphorylation was also accompanied by significant enhancement of Gβγ binding by this molecule. Possible formation of the Pdc–Gβγ complex in cells exposed to light was corroborated by FRET between these proteins. Treatment of cells with okadaic acid, an inhibitor of phosphatase activity, entirely prevented Pdc dephosphorylation by light, colocalization of this phosphoprotein with Gβγ and generation of the Pdc–Gβγ complex. Cell fractionation and immunoblotting revealed that in cells exposed to light, the formation of Pdc–Gβγ complex and its translocation into the cytoplasm occur simultaneously with a change in the gel migration of Gβ. Moreover, a 33 kDa immunoanalog of 14-3-3 protein was identified and we showed that this protein is bound by phosphorylated Pdc in a cell adapted to darkness. The results of this study provide additional detailed characterization of the functional properties of the ciliate Pdc. The likely functional role of Pdc in Blepharisma is discussed.

Cyclic nucleotide-dependent protein kinases inhibit binding of 14-3-3 to the GTPase-activating protein Rap1GAP2 in platelets

GTPase-activating proteins are required to terminate signaling by Rap1, a small guanine nucleotide-binding protein that controls integrin activity and cell adhesion. Recently, we identified Rap1GAP2, a GTPase-activating protein of Rap1 in platelets. Here we show that 14-3-3 proteins interact with phosphorylated serine 9 at the N terminus of Rap1GAP2. Platelet activation by ADP and thrombin enhances serine 9 phosphorylation and increases 14-3-3 binding to endogenous Rap1GAP2. Conversely, inhibition of platelets by endothelium-derived factors nitric oxide and prostacyclin disrupts 14-3-3 binding. These effects are mediated by cGMP- and cAMP-dependent protein kinases that phosphorylate Rap1GAP2 at serine 7, adjacent to the 14-3-3 binding site. 14-3-3 binding does not change the GTPase-activating function of Rap1GAP2 in vitro. However, 14-3-3 binding attenuates Rap1GAP2 mediated inhibition of cell adhesion. Our findings define a novel crossover point of activatory and inhibitory signaling pathways in platelets.

Aberrant overexpression of an epithelial marker, 14-3-3sigma, in a subset of hematological malignancies

Background

14-3-3σ is a p53-mediated cell-cycle inhibitor in epithelial cells. The expression of 14-3-3σ is frequently altered in cancers of epithelial origin associated with altered DNA methylation. Since its involvement in a non-epithelial tumor is unknown, we examined 14-3-3σ expression in patients with haematological malignancies.

Methods

We analyzed 41 hematopoietic cell lines and 129 patients with a variety of hematological malignancies for 14-3-3σ expression with real-time RT-PCR. We also examined protein levels by Western blot analysis and DNA methylation status of the 14-3-3σ gene by methylation-specific PCR analysis of bisulfite-treated DNA. In addition, mutations of p53 gene were identified by RT-PCR-SSCP analysis and the expression levels of 14-3-3σ were compared with those of other cell-cycle inhibitor genes, CDKN2A and ARF.

Results

The expression levels of 14-3-3σ mRNA in almost all cell lines were low and comparable to those in normal hematopoietic cells except for 2 B-cell lines. On the contrary, 14-3-3σ mRNA was aberrantly overexpressed frequently in mature lymphoid malignancies (30 of 93, 32.3%) and rarely in acute leukemia (3 of 35, 8.6%). 14-3-3σ protein was readily detectable and roughly reflected the mRNA level. In contrast to epithelial tumors, methylation status of the 14-3-3σ gene was not associated with expression in hematological malignancies. Mutations of p53 were identified in 12 patients and associated with lower expression of 14-3-3σ. The expression levels of 14-3-3σ, CDKN2A and ARF were not correlated with but rather reciprocal to one another, suggesting that simultaneous overexpression of any two of them is incompatible with tumor growth.

Conclusion

14-3-3σ, an epithelial cell marker, was overexpressed significantly in a subset of mature lymphoid malignancies. This is the first report of aberrant 14-3-3σ expression in non-epithelial tumors in vivo. Since the significance of 14-3-3σ overexpression is unknown even in epithelial tumors such as pancreatic cancers, further analysis of regulation and function of the 14-3-3σ gene in non-epithelial as well as epithelial tumors is warranted.

Delta-catenin-induced dendritic morphogenesis. An essential role of p190RhoGEF interaction through Akt1-mediated phosphorylation

Delta-catenin was first identified through its interaction with Presenilin-1 and has been implicated in the regulation of dendrogenesis and cognitive function. However, the molecular mechanisms by which delta-catenin promotes dendritic morphogenesis were unclear. In this study, we demonstrated delta-catenin interaction with p190RhoGEF, and the importance of Akt1-mediated phosphorylation at Thr-454 residue of delta-catenin in this interaction. We have also found that delta-catenin overexpression decreased the binding between p190RhoGEF and RhoA, and significantly lowered the levels of GTP-RhoA but not those of GTP-Rac1 and -Cdc42. Delta-catenin T454A, a defective form in p190RhoGEF binding, did not decrease the binding between p190RhoGEF and RhoA. Delta-catenin T454A also did not lower GTP-RhoA levels and failed to induce dendrite-like process formation in NIH 3T3 fibroblasts. Furthermore, delta-catenin T454A significantly reduced the length and number of mature mushroom shaped spines in primary hippocampal neurons. These results highlight signaling events in the regulation of delta-catenin-induced dendrogenesis and spine morphogenesis.

Class IIa histone deacetylases: regulating the regulators

In the last decade, the identification of enzymes that regulate acetylation of histones and nonhistone proteins has revealed the key role of dynamic acetylation and deacetylation in various cellular processes. Mammalian histone deacetylases (HDACs), which catalyse the removal of acetyl groups from lysine residues, are grouped into three classes, on the basis of similarity to yeast counterparts. An abundance of experimental evidence has established class IIa HDACs as crucial transcriptional regulators of various developmental and differentiation processes. In the past 5 years, a tremendous effort has been dedicated to characterizing the regulation of these enzymes. In this review, we summarize the latest discoveries in the field and discuss the molecular and structural determinants of class IIa HDACs regulation. Finally, we emphasize that comprehension of the mechanisms underlying class IIa HDAC functions is essential for potential therapeutic applications.

Gill-specific transcriptional regulation of Na+/K+ -ATPase alpha-subunit in the euryhaline shore crab Pachygrapsus marmoratus: sequence variants and promoter structure

The sodium pump (Na+/K+ -ATPase) has been implicated in osmoregulatory ion transport in many aquatic animals. In the euryhaline hyper-hypoosmoregulating shore crab Pachygrapsus marmoratus, induction of Na+/K+ -ATPase alpha-subunit mRNA varies between gills in response to osmotic stress. Following transfer of crabs from normal seawater (36 per thousand salinity) to diluted seawater (10 per thousand), a condition in which gills exhibit net ion uptake, alpha-subunit mRNA expression is upregulated in all tested gills, albeit with differing time courses. By contrast, following transfer from seawater to hypertonic (45 per thousand) seawater, a condition in which the animal is excreting ions, alpha-subunit mRNA is induced primarily in gill no. 7 (nine in total), suggesting that this gill may be associated specifically with ion excretion in P. marmoratus. Full-length sequencing of alpha-subunit cDNA revealed the existence of two isoforms differing only in the inclusion of an 81-nucleotide segment within the N-terminal open reading frame of the long (D) form in comparison to the short (C) form. The 81-nucleotide segment encodes a 14-3-3 protein binding site that may facilitate movement of the alpha-subunit protein between intracellular compartments and the plasma membrane. mRNA expression of the two forms followed similar patterns upon salinity transfer. Genomic DNA sequencing of the putative promoter region of the alpha-subunit gene demonstrated a spectrum of predicted transcription factor binding sites that are likely associated with the complex expression pattern observed among gills following osmotic stress.

Transdifferentiation of peripheral blood mononuclear cells into epithelial-like cells

Bone marrow-derived stem cells have the potential to transdifferentiate into unexpected peripheral cells. We hypothesize that circulating bone marrow-derived stem cells might have the capacity to transdifferentiate into epithelial-like cells and release matrix metalloproteinase-1-modulating factors such as 14-3-3varsigma for dermal fibroblasts. We have characterized a subset of peripheral blood mononuclear cells (PBMCs) that develops an epithelial-like profile. Our findings show that these cells develop epithelial-like morphology and express 14-3-3varsigma and keratin-5, -8 as early as day 7 and day 21, respectively. When compared with control, conditioned media collected from PBMCs in advanced epithelial-like differentiation (cultures on days 28, 35, and 42) increased the matrix metalloproteinase-1 expression in dermal fibroblasts (P </= 0.01). The depletion of 14-3-3varsigma from these conditioned media by immunoprecipitation reduced the effect by 39.5% (P value, 0.05). Therefore, the releasable 14-3-3varsigma from PBMC-derived epithelial-like cells is involved in this process. Our findings provide new insights into the PBMC transdifferentiation to generate epithelial-like cells and subsequently release of 14-3-3varsigma that will disclose new therapeutic alternatives for different dermal clinical settings.

14-3-3-dependent inhibition of the deubiquitinating activity of UBPY and its cancellation in the M phase

The deubiquitinating enzyme UBPY, also known as USP8, regulates cargo sorting and membrane traffic at early endosomes. Here we demonstrate the regulatory mechanism of the UBPY catalytic activity. We identified 14-3-3 epsilon, gamma, and zeta as UBPY-binding proteins using co-immunoprecipitation followed by mass spectrometric analysis. The 14-3-3 binding of UBPY was inhibited by mutating the consensus 14-3-3-binding motif RSYS(680)SP, by phosphatase treatment, and by competition with the Ser(680)-phosphorylated RSYS(680)SP peptide. Metabolic labeling with [(32)P]orthophosphate and immunoblotting using antibody against the phosphorylated 14-3-3-binding motif showed that Ser(680) is a major phosphorylation site in UBPY. These results indicated that 14-3-3s bind to the region surrounding Ser(680) in a phosphorylation-dependent manner. The mutation at Ser(680) led to enhanced ubiquitin isopeptidase activity of UBPY toward poly-ubiquitin chains and a cellular substrate, epidermal growth factor receptor, in vitro and in vivo. Moreover, addition of 14-3-3epsilon inhibited the UBPY activity in vitro. Finally, UBPY was dephosphorylated at Ser(680) and dissociated from 14-3-3s in the M phase, resulting in enhanced activity of UBPY during cell division. We conclude that UBPY is catalytically inhibited in a phosphorylation-dependent manner by 14-3-3s during the interphase, and this regulation is cancelled in the M phase.

Hippocampal signaling protein levels are different in early and late metestrus in the rat

Early and late metestrus in the rat differ by progesterone levels. As it is known that progesterone shows a potential negative effect on cognitive performances and can counteract the estradiol-induced neural effects, we intended to study signaling proteins in the hippocampus, a structure representing a main brain area of cognitive function. Female OFA Sprague-Dawley rats were used in the studies and estrous phases were determined using vaginal smears. Hippocampal tissue was taken, proteins extracted, run on two-dimensional gel electrophoresis and proteins were identified by mass spectrometry methods (MALDI-TOF-TOF and nano-LC-ESI-MS/MS). Individual signaling protein levels quantified by specific software were shown to vary between the two phases, including NG,NG-dimethylarginine dimethylaminohydrolase 1 for nitric oxide signaling, guanine nucleotide-binding proteins, septin 6, septin 11, G-septin alpha, and 14-3-3 protein gamma. Results from this study indicate that early and late metestrus show differences in signaling pathways, that may help to design further investigations at the protein level and may assist to interpret literature on protein expression and brain protein levels in female rats. Moreover, signaling differences in hippocampus are challenging cognitive studies during these two metestrus phases probably revealing cognitive differences between early and late metestrus.

14-3-3 Expression in Denervated Hippocampus after Entorhinal Cortex Lesion Assessed by Culture-Derived Isotope Tags in Quantitative Proteomics

Activation of astrocytes accompanies many brain pathologies. Reactive astrocytes have a beneficial role in acute neurotrauma but later on might inhibit regeneration. 2D-gel electrophoresis and mass spectrometry were applied to study the proteome difference in denervated hippocampus in wildtype mice and mice lacking intermediate filament proteins glial fibrillary acidic protein (GFAP) and vimentin (GFAP-/-Vim-/-) that show attenuated reactive gliosis and enhanced posttraumatic regeneration. Proteomic data and immunohistochemical analyses showed upregulation of the adapter protein 14-3-3 four days postlesion and suggested that 14-3-3 upregulation after injury is triggered by reactive gliosis. Culture-derived isotope tags (CDIT) and mass spectrometry demonstrated that 14-3-3 epsilon was the major isoform upregulated in denervated hippocampus and that its upregulation was attenuated in GFAP-/-Vim-/- mice and thus most likely connected to reactive gliosis.

Proteomic screen in the simple metazoan Hydra identifies 14-3-3 binding proteins implicated in cellular metabolism, cytoskeletal organisation and Ca2+ signalling

Background

14-3-3 proteins have been implicated in many signalling mechanisms due to their interaction with Ser/Thr phosphorylated target proteins. They are evolutionarily well conserved in eukaryotic organisms from single celled protozoans and unicellular algae to plants and humans. A diverse array of target proteins has been found in higher plants and in human cell lines including proteins involved in cellular metabolism, apoptosis, cytoskeletal organisation, secretion and Ca²⁺ signalling.

Results

We found that the simple metazoan Hydra has four 14-3-3 isoforms. In order to investigate whether the diversity of 14-3-3 target proteins is also conserved over the whole animal kingdom we isolated 14-3-3 binding proteins from Hydra vulgaris using a 14-3-3-affinity column. We identified 23 proteins that covered most of the above-mentioned groups. We also isolated several novel 14-3-3 binding proteins and the Hydra specific secreted fascin-domain-containing protein PPOD. In addition, we demonstrated that one of the 14-3-3 isoforms, 14-3-3 HyA, interacts with one Hydra-Bcl-2 like protein in vitro.

Conclusion

Our results indicate that 14-3-3 proteins have been ubiquitous signalling components since the start of metazoan evolution. We also discuss the possibility that they are involved in the regulation of cell numbers in response to food supply in Hydra.

The role of stratifin in fibroblast-keratinocyte interaction

Stratifin is a member of 14-3-3 protein family, a highly conserved group of proteins constituted by seven isoforms. They are involved in numerous crucial intracellular functions such as cell cycle and apoptosis, regulation of signal transduction pathways, cellular trafficking, cell proliferation and differentiation, cell survival, and protein folding and processing, among others. At epidermal level, stratifin (also called 14-3-3 sigma) has been described as molecule with relevant functions. For instance, this isoform is a marker associated with keratinocyte differentiation. In this maturation process, the presence of dominant negative molecules of p53 induces a "stemness condition" of keratinocyte precursor cells and suppression of stratifin expression. In addition, the recently described keratinocyte-releasable form of stratifin is involved in dermal fibroblast MMP-1 over-expression through c-Fos and c-Jun activity. This effect is mediated, at least in part, by p38 mitogen-activated protein kinase (MAPK). Other MMP family members such as stromelysin-1 (MMP-3), stromelysin-2 (MMP-10), neutrophil collagenase (MMP-8), and membrane-type MMP-24 (MT5-MMP) are also up-regulated by stratifin. Within fibroproliferative disorder of skin, hypertrophic scar and keloids exhibit a high content of collagen, proteoglycans, and fibronectin. Thus, the MMP profile induced by stratifin is an interesting starting point to establish new therapeutic tools to control the process of wound healing. In this review, we will focus on site of synthesis and mode of action of stratifin in skin and wound healing.

Nuclear localization of endogenous RGK proteins and modulation of cell shape remodeling by regulated nuclear transport

The members of the RGK small GTP-binding protein family, Kir/Gem, Rad, Rem and Rem2, are multifunctional proteins that regulate voltage-gated calcium channel activity and cell shape remodeling. Calmodulin (CaM) or CaM 14-3-3 are regulators of RGK functions and their association defines the subcellular localization of RGK proteins. Abolition of CaM association results in the accumulation of RGK proteins in the nucleus, whereas 14-3-3 binding maintains them in the cytoplasm. Kir/Gem possesses nuclear localization signals (NLS) that mediate nuclear accumulation through an importin alpha5-dependent pathway (see Mahalakshmi RN, Nagashima K, Ng MY, Inagaki N, Hunziker W, Béguin P. Nuclear transport of Kir/Gem requires specific signals and importin alpha5 and is regulated by Calmodulin and predicted service phosphorylations. Traffic 2007; doi: 10.1111/j.1600-0854.2007.00598.x). Because the extent of nuclear localization depends on the RGK protein and the cell type, the mechanism and regulation of nuclear transport may differ. Here, we extend our analysis to the other RGK members and show that Rem also binds importin alpha5, whereas Rad associates with importins alpha3, alpha5 and beta through three conserved NLS. Predicted phosphorylation of a serine residue within the bipartite NLS affects, as observed for Kir/Gem, nuclear accumulation of Rem, but not that of Rad or Rem2. We also identify an additional regulatory phosphorylation for all RGK proteins that prevents binding of 14-3-3 and thereby interferes with their cytosolic relocalization by 14-3-3. Functionally, nuclear localization of RGK proteins contributes to the suppression of RGK-mediated cell shape remodeling. Importantly, we show that endogenous RGK proteins are localized predominantly in the nucleus of individual cells of the brain cortex 'in situ' as well as in primary hippocampal cells, indicating that transport between the nucleus and their site of action in the cytoplasm (i.e., cytoskeleton, endoplasmic reticulum or plasma membrane) is of physiological relevance for the regulation of RGK protein function.

PRAS40 is a target for mammalian target of rapamycin complex 1 and is required for signaling downstream of this complex

Signaling through the mammalian target of rapamycin complex 1 (mTORC1) is positively regulated by amino acids and insulin. PRAS40 associates with mTORC1 (which contains raptor) but not mTORC2. PRAS40 interacts with raptor, and this requires an intact TOR-signaling (TOS) motif in PRAS40. Like TOS motif-containing proteins such as eIF4E-binding protein 1 (4E-BP1), PRAS40 is a substrate for phosphorylation by mTORC1. Consistent with this, starvation of cells of amino acids or treatment with rapamycin alters the phosphorylation of PRAS40. PRAS40 binds 14-3-3 proteins, and this requires both amino acids and insulin. Binding of PRAS40 to 14-3-3 proteins is inhibited by TSC1/2 (negative regulators of mTORC1) and stimulated by Rheb in a rapamycin-sensitive manner. This confirms that PRAS40 is a target for regulation by mTORC1. Small interfering RNA-mediated knockdown of PRAS40 impairs both the amino acid- and insulin-stimulated phosphorylation of 4E-BP1 and the phosphorylation of S6. However, this has no effect on the phosphorylation of Akt or TSC2 (an Akt substrate). These data place PRAS40 downstream of mTORC1 but upstream of its effectors, such as S6K1 and 4E-BP1.

The PGE2-induced inhibition of the PLD activation pathway stimulated by fMLP in human neutrophils is mediated by PKA at the PI3-Kgamma level

Prostaglandin E2 (PGE2), an eicosanoid that modulates inflammation, inhibits several chemoattractant-elicited functions in neutrophils such as chemotaxis, production of superoxide anions, adhesion, secretion of cytotoxic enzymes and synthesis of leukotriene B4. We previously reported that PGE2 inhibits the fMLP signaling pathway that leads to PLD activation through suppression of PI3-Kgamma activity and the decreased recruitment to membranes of PLD activation factors, PKC, Rho and Arf-GTPases. This effect is mediated via the EP2 receptors known to raise cAMP in cells. The inhibition of most fMLP-induced functional responses by PGE2 via EP2 receptors is mediated by PKA, except the chemotactic response. We have investigated the role of PKA in the EP2-mediated inhibition of the PLD activation pathway. H-89, a selective PKA pharmacological inhibitor suppressed the inhibitory effects of PGE2 at all stages of the PLD pathway activated by fMLP, i.e. PLD activity, translocation to membranes of PKCalpha, Rho and Arf-GTPases, calcium influx, tyrosine phosphorylation of proteins and finally translocation of p110gamma catalytic subunit of PI3-K to membranes. However, neither PLD nor PI3-Kgamma was substrate of PKA. These data provide evidence that PGE2-stimulated PKA activity regulates the PLD pathway stimulated by fMLP at the level of PI3-Kgamma and that the inhibition of PI3-Kgamma activation by PKA is a complex mechanism that remains to be completely elucidated.

Proteomic analysis of in vivo 14-3-3 interactions in the yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae produces two 14-3-3 proteins, Bmh1 and Bmh2, whose exact functions have remained unclear. Here, we performed a comprehensive proteomic analysis using multistep immunoaffinity purification and mass spectrometry and identified 271 yeast proteins that specifically bind to Bmh1 and -2 in a phosphorylation-dependent manner. The identified proteins have diverse biochemical functions and cellular roles, including cell signaling, metabolism, and cell cycle regulation. Importantly, there are a number of protein subsets that are involved in the regulation of yeast physiology through a variety of cell signaling pathways, including stress-induced transcription, cell division, and chitin synthesis at the cell wall. In fact, we found that a yeast mutant deficient in Bmh1 and -2 had defects in signal-dependent response of the MAPK (Hog1 and Mpk1) cascade and exhibited an abnormal accumulation of chitin at the bud neck. We propose that Bmh1 and -2 are common regulators of many cell signaling modules and pathways mediated by protein phosphorylation and regulate a variety of biological events by coordinately controlling the identified multiplex phosphoprotein components.

The commonality of protein interaction networks determined in neurodegenerative disorders (NDDs)

MOTIVATION

Neurodegenerative disorders (NDDs) are progressive and fatal disorders, which are commonly characterized by the intracellular or extracellular presence of abnormal protein aggregates. The identification and verification of proteins interacting with causative gene products are effective ways to understand their physiological and pathological functions. The objective of this research is to better understand common molecular pathogenic mechanisms in NDDs by employing protein-protein interaction networks, the domain characteristics commonly identified in NDDs and correlation among NDDs based on domain information.

RESULTS

By reviewing published literatures in PubMed, we created pathway maps in Kyoto Encyclopedia of Genes and Genomes (KEGG) for the protein-protein interactions in six NDDs: Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), dentatorubral-pallidoluysian atrophy (DRPLA) and prion disease (PRION). We also collected data on 201 interacting proteins and 13 compounds with 282 interactions from the literature. We found 19 proteins common to these six NDDs. These common proteins were mainly involved in the apoptosis and MAPK signaling pathways. We expanded the interaction network by adding protein interaction data from the Human Protein Reference Database and gene expression data from the Human Gene Expression Index Database. We then carried out domain analysis on the extended network and found the characteristic domains, such as 14-3-3 protein, phosphotyrosine interaction domain and caspase domain, for the common proteins. Moreover, we found a relatively high correlation between AD, PD, HD and PRION, but not ALS or DRPLA, in terms of the protein domain distributions.

AVAILABILITY

http://www.genome.jp/kegg/pathway/hsa/hsa01510.html (KEGG pathway maps for NDDs).

The proline-rich Akt substrate of 40 kDa (PRAS40) is a physiological substrate of mammalian target of rapamycin complex 1

The proline-rich Akt substrate of 40 kilodaltons (PRAS40) was identified as a raptor-binding protein that is phosphorylated directly by mammalian target of rapamycin (mTOR) complex 1 (mTORC1) but not mTORC2 in vitro, predominantly at PRAS40 (Ser(183)). The binding of S6K1 and 4E-BP1 to raptor requires a TOR signaling (TOS) motif, which contains an essential Phe followed by four alternating acidic and small hydrophobic amino acids. PRAS40 binding to raptor was severely inhibited by mutation of PRAS40 (Phe(129) to Ala). Immediately carboxyl-terminal to Phe(129) are two small hydrophobic amino acid followed by two acidic residues. PRAS40 binding to raptor was also abolished by mutation of the major mTORC1 phosphorylation site, Ser(183), to Asp. PRAS40 (Ser(183)) was phosphorylated in intact cells; this phosphorylation was inhibited by rapamycin, by 2-deoxyglucose, and by overexpression of the tuberous sclerosis complex heterodimer. PRAS40 (Ser(183)) phosphorylation was also inhibited reversibly by withdrawal of all or of only the branched chain amino acids; this inhibition was reversed by overexpression of the Rheb GTPase. Overexpressed PRAS40 suppressed the phosphorylation of S6K1 and 4E-BP1 at their rapamycin-sensitive phosphorylation sites, and reciprocally, overexpression of S6K1 or 4E-BP1 suppressed phosphorylation of PRAS40 (Ser(183)) and its binding to raptor. RNA interference-induced depletion of PRAS40 enhanced the amino acid-stimulated phosphorylation of both S6K1 and 4E-BP1. These results establish PRAS40 as a physiological mTORC1 substrate that contains a variant TOS motif. Moreover, they indicate that the ability of raptor to bind endogenous substrates is limiting for the activity of mTORC1 in vivo and is therefore a potential locus of regulation.

Nuclear Transport of Kir/Gem Requires Specific Signals and Importin α5 and Is Regulated by Calmodulin and Predicted Serine Phosphorylations

Kir/Gem, together with Rad, Rem and Rem2, is a member of the RGK small GTP-binding protein family. These multifunctional proteins regulate voltage-gated calcium channel (VGCC) activity and cell-shape remodeling. Calmodulin and 14-3-3 binding modulate the functions of RGK proteins. Intriguingly, abolishing the binding of calmodulin or calmodulin and 14-3-3 results in nuclear accumulation of RGK proteins. Under certain conditions, the Ca(v)beta3-subunit of VGCCs can be translocated into the nucleus along with the RGK proteins, resulting in channel inactivation. The mechanism by which nuclear localization of RGK proteins is accomplished and regulated, however, is unknown. Here, we identify specific nuclear localization signals (NLS) in Kir/Gem that are both required and sufficient for nuclear transport. Importin alpha5 binds to Kir/Gem, and its depletion using RNA interference impairs nuclear translocation of this RGK protein. Calmodulin and predicted phosphorylations on serine residues within or in the vicinity of a C-terminal bipartite NLS regulate nuclear translocation by interfering with the association between importinalpha5 and Kir/Gem. These predicted phosphorylations, however, do not affect Kir/Gem-mediated calcium channel downregulation but rather, as shown in the accompanying paper (Mahalakshmi RN, Ng MY, Guo K, Qi Z, Hunziker W, Béguin P. Nuclear localization of endogenous RGK proteins and modulation of cell shape remodeling by regulated nuclear transport. Traffic 2007; doi:10.1111/j.1600-0854.2007.00599.x), interfere with cell-shape remodeling.

Papillomavirus binding factor (PBF)-mediated inhibition of cell growth is regulated by 14-3-3beta

The cellular factor, papillomavirus (PV)-binding factor (PBF)/Huntington's disease binding protein 2 (HDBP2), was identified by its ability to bind regulatory sequences of certain papillomavirus types as well as the Huntington's disease gene. PBF is thought to be a novel nuclear-shuttling transcription factor with unknown function. To further characterize PBF, we identified 14-3-3beta as an interaction partner. We demonstrated that PBF binds to 14-3-3beta using two motifs. Akt-kinase and an unidentified kinase that are activated by the PI3K-signaling pathway were able to phosphorylate these motifs, allowing PBF to associate with 14-3-3beta. This interaction may contribute to the control of the subcellular localization of PBF, which migrated into the nucleus in the absence of growth factors. Over-expression of PBF resulted in the inhibition of cell growth, which was enhanced using a 14-3-3 binding-deficient PBF mutant. Thus, our experiments characterized PBF as a new cellular factor mediating the effects of PI3K/Akt signaling and 14-3-3 on cell growth.

Transglutaminase 2 kinase activity facilitates protein kinase A-induced phosphorylation of retinoblastoma protein

Transglutaminase 2 (TG2, tissue transglutaminase) is a multifunctional protein involved in cross-linking a variety of proteins, including retinoblastoma protein (Rb). Here we show that Rb is also a substrate for the recently identified serine/threonine kinase activity of TG2 and that TG2 phosphorylates Rb at the critically important Ser780 residue. Furthermore, phosphorylation of Rb by TG2 destabilizes the Rb.E2F1 complex. TG2 phosphorylation of Rb was abrogated by high Ca2+ concentrations, whereas TG2 transamidating activity was inhibited by ATP. TG2 was itself phosphorylated by protein kinase A (PKA). Phosphorylation of TG2 by PKA attenuated its transamidating activity and enhanced its kinase activity. Activation of PKA in mouse embryonic fibroblasts (MEF) with dibutyryl-cAMP enhanced phosphorylation of both TG2 and Rb by a process that was inhibited by the PKA inhibitor H89. Treatment with dibutyryl-cAMP enhanced Rb phosphorylation in MEFtg2+/+ cells but not in MEFtg2-/- cells. These data indicate that Rb is a substrate for TG2 kinase activity and suggest that phosphorylation of Rb, which results from activation of PKA in fibroblasts, is indirect and requires TG2 kinase activity.

Phosphoproteomic identification of targets of the Arabidopsis sucrose nonfermenting-like kinase SnRK2.8 reveals a connection to metabolic processes

SnRK2.8 is a member of the sucrose nonfermenting-related kinase family that is down-regulated when plants are deprived of nutrients and growth is reduced. When this kinase is over expressed in Arabidopsis, the plants grow larger. To understand how this kinase modulates growth, we identified some of the proteins that are phosphorylated by this kinase. A new phosphoproteomic method was used in which total protein from plants overexpressing the kinase was compared with total protein from plants in which the kinase was inactivated. Protein profiles were compared on two-dimensional gels following staining by a dye that recognizes phosphorylated amino acids. Candidate target proteins were confirmed with in vitro phosphorylation assays, using the kinase and target proteins that were purified from Escherichia coli. Seven target proteins were confirmed as being phosphorylated by SnRK2.8. Certain targets, such as 14-3-3 proteins, regulate as yet unidentified proteins, whereas other targets, such as glyoxalase I and ribose 5-phosphate isomerase, detoxify byproducts from glycolysis and catalyze one of the final steps in carbon fixation, respectively. Also, adenosine kinase and 60S ribosomal protein were confirmed as targets of SnRK2.8. Using mass spectrometry, we identified phosphorylated residues in the SnRK2.8, the 14-3-3kappa, and the 14-3-3chi. These data show that the expression of SnRK2.8 is correlated with plant growth, which may in part be due to the phosphorylation of enzymes involved in metabolic processes.

Post-transcriptional control of the Saccharomyces cerevisiae proteome by 14-3-3 proteins

14-3-3 proteins form a family of conserved eukaryotic proteins binding to over 200 different proteins involved in nearly all cellular processes. The yeast Saccharomyces cerevisiae has two genes encoding 14-3-3 proteins, BMH1 and BMH2. As 14-3-3 proteins are essential in most S. cerevisiae strains, we constructed a bmh mutant with suboptimal 14-3-3 protein activity. Here, we report the effect of these bmh mutations on the proteome as determined by two-dimensional gel electrophoresis and mass spectrometry. We identified 26 proteins of which the levels increased by more than 2.0-fold and 51 proteins of which the levels decreased by more than 2.0-fold in the bmh mutant compared with those of the wild-type strain. For only 9 of these proteins, a more than 2.0-fold alteration was found at the transcriptional level. The levels of many proteins involved in gluconeogenesis, including Fba1, Eno1, Eno2, Tpi1, Pck1, Mdh2, Tdh2, Tdh3, and Gpm1, increased in the mutant, whereas the levels of several proteins involved in amino acid biosynthesis and translation and heat shock proteins were lower. Our studies indicate that 14-3-3 proteins control the S. cerevisiae proteome at the post-transcriptional level, in agreement with the binding of 14-3-3 proteins to proteins involved in protein synthesis and degradation. In addition, our studies suggest a key role in the regulation of carbohydrate metabolism by 14-3-3 proteins.

KEYWORDS

14-3-3 proteins * Saccharomyces cerevisiae * proteome * gluconeogenesis * BMH1 * BMH2.

Functional Specificity of PMCA Isoforms?

In mammals, four different genes encode four PMCA isoforms. PMCA1 and PMCA4 are expressed ubiquitously. PMCA2 and PMCA3 are expressed prevalently in the central nervous systems. More than 30 variants are generated by mechanisms of alternative splicing. The physiological meaning of the existence of such elevated number of isoforms is not clear, but it would be plausible to relate it to the cell-specific demands of Ca2+ homeostasis. To characterize functional specificity of PMCA variants we have investigated two aspects: the effects of the overexpression of the different PMCA variants on cellular Ca2+ handling and the existence of possible isoform-specific interactions with partner proteins using a yeast two-hybrid technique. The four basic PMCA isoforms were coexpressed in CHO cells together with the Ca2+-sensitive recombinant photoprotein aequorin. The effects of their overexpression on Ca2+ homeostasis were monitored in the living cells. They had revealed that the ubiquitous isoforms 1 and 4 are less effective in reducing the Ca2+ peaks generated by cell stimulation as compared to the neuron-specific isoforms 2 and 3. To establish whether these differences were related to different and new physiological regulators of the pump, the 90 N-terminal residues of PMCA2 and PMCA4 have been used as baits for the search of molecular partners. Screening of a human brain cDNA library with the PMCA4 bait specified the epsilon-isoform of protein 14-3-3, whereas no 14-3-3 epsilon clone was obtained with the PMCA2 bait. Overexpression of PMCA4/14-3-3 epsilon (but not of PMCA2/14-3-3 epsilon) in HeLa cells together with targeted aequorins showed that the ability of the cells to export Ca2+ was impaired. Thus, the interaction with 14-3-3 epsilon inhibited PMCA4 but not PMCA2. The role of PMCA2 has been further characterized by Ca2+ measurements in cells overexpressing different splicing variants. The results indicated that the combination of alternative splicing at two different sites in the pump structure was responsible for different functional characteristics of the pumps.

Structure of a 14-3-3 coordinated hexamer of the plant plasma membrane H+ -ATPase by combining X-ray crystallography and electron cryomicroscopy

Regulatory 14-3-3 proteins activate the plant plasma membrane H(+)-ATPase by binding to its C-terminal autoinhibitory domain. This interaction requires phosphorylation of a C-terminal, mode III, recognition motif as well as an adjacent span of approximately 50 amino acids. Here we report the X-ray crystal structure of 14-3-3 in complex with the entire binding motif, revealing a previously unidentified mode of interaction. A 14-3-3 dimer simultaneously binds two H(+)-ATPase peptides, each of which forms a loop within the typical 14-3-3 binding groove and therefore exits from the center of the dimer. Several H(+)-ATPase mutants support this structure determination. Accordingly, 14-3-3 binding could result in H(+)-ATPase oligomerization. Indeed, by using single-particle electron cryomicroscopy, the 3D reconstruction of the purified H(+)-ATPase/14-3-3 complex demonstrates a hexameric arrangement. Fitting of 14-3-3 and H(+)-ATPase atomic structures into the 3D reconstruction map suggests the spatial arrangement of the holocomplex.

Topoisomerase II binds importin alpha isoforms and exportin/CRM1 but does not shuttle between the nucleus and cytoplasm in proliferating cells

Resistance to anticancer drugs that target DNA topoisomerase II (topo II) isoforms alpha and/or beta is associated with decreased nuclear and increased cytoplasmic topo IIalpha. Earlier studies have confirmed that functional nuclear localization and export signal sequences (NLS and NES) are present in both isoforms. In this study, we show that topo II alpha and beta bind and are imported into the nucleus by importin alpha1, alpha3, and alpha5 in conjunction with importin beta. Topo IIalpha also binds exportin/CRM1 in vitro. However, wild-type topo IIalpha has only been observed in the cytoplasm of cells that are entering plateau phase growth. This suggests that topo IIalpha may shuttle between the nucleus and the cytoplasm with the equilibrium towards the nucleus in proliferating cells but towards the cytoplasm in plateau phase cells. The CRM1 inhibitor Leptomycin B increases the nuclear localization of GFP-tagged topo IIalpha with a mutant NLS, suggesting that its export is being inhibited. However, homokaryon shuttling experiments indicate that fluorescence-tagged wild-type topo II alpha and beta proteins do not shuttle in proliferating Cos-1 or HeLa cells. We conclude that topo II alpha and beta nuclear export is inhibited in proliferating cells so that these proteins do not shuttle.