14-3-3 Proteins directly regulate Ca2+/calmodulin-dependent protein kinase kinase α through phosphorylation-dependent multisite binding

Look for the Scaffold: Multifaceted Regulation of Enzyme Activity by 14-3-3 Proteins

Enzyme activity is regulated by several mechanisms, including phosphorylation. Phosphorylation is a key signal transduction process in all eukaryotic cells and is thus crucial for virtually all cellular processes. In addition to its direct effect on protein structure, phosphorylation also affects protein-protein interactions, such as binding to scaffolding 14-3-3 proteins, which selectively recognize phosphorylated motifs. These interactions then modulate the catalytic activity, cellular localisation and interactions of phosphorylated enzymes through different mechanisms. The aim of this mini-review is to highlight several examples of 14-3-3 protein-dependent mechanisms of enzyme regulation previously studied in our laboratory over the past decade. More specifically, we address here the regulation of the human enzymes ubiquitin ligase Nedd4-2, procaspase-2, calcium-calmodulin dependent kinases CaMKK1/2, and death-associated protein kinase 2 (DAPK2) and yeast neutral trehalase Nth1.

KN-93 promotes HDAC4 nucleus translocation to promote fatty acid oxidation in myocardial infarction

Myocardial infarction (MI) is a potentially fatal disease that causes a significant number of deaths worldwide. The strategy of increasing fatty acid oxidation in myocytes is considered a therapeutic avenue to accelerate metabolism to meet energy demands. We conducted the study aiming to investigate the effect of KN-93, which induces histone deacetylase (HDAC)4 shuttling to the nucleus, on fatty acid oxidation and the expression of related genes. A mouse model of myocardial infarction was induced by isoprenaline administration. Heart damage was assessed by the detection of cardiac injury markers. The level of fatty acid oxidation level was evaluated by testing the expression of related genes. Both immunofluorescence and immunoblotting in the cytosol or nucleus were utilized to observe the distribution of HDAC4. The interaction between HDAC4 and specificity protein (SP)1 was confirmed by co-immunoprecipitation. The acetylation level of SP1 was tested after KN-93 treatment and HDAC4 inhibitor. Oxygen consumption rate and immunoblotting experiments were used to determine whether the effect of KN-93 on increasing fatty acid oxidation is through HDAC4 and SP1. Administration of KN-93 significantly reduced cardiac injury in myocardial infarction and promoted fatty acid oxidation both in vitro and in vivo. KN-93 was shown to mediate nuclear translocation of HDAC4. HDAC4 was found to interact with SP1 and reduce SP1 acetylation. HDAC4 or SP1 inhibitors attenuated the effect of KN-93 on fatty acid oxidation. In conclusion, KN-93 promotes HDAC4 translocation to the nucleus, thereby potentially enhancing fatty acid oxidation by SP1.

14-3-3 protein inhibits CaMKK1 by blocking the kinase active site with its last two C-terminal helices

Ca2+ /CaM-dependent protein kinase kinases 1 and 2 (CaMKK1 and CaMKK2) phosphorylate and enhance the catalytic activity of downstream kinases CaMKI, CaMKIV, and protein kinase B. Accordingly, CaMKK1 and CaMKK2 regulate key physiological and pathological processes, such as tumorigenesis, neuronal morphogenesis, synaptic plasticity, transcription factor activation, and cellular energy homeostasis, and promote cell survival. Both CaMKKs are partly inhibited by phosphorylation, which in turn triggers adaptor and scaffolding protein 14-3-3 binding. However, 14-3-3 binding only significantly affects CaMKK1 function. CaMKK2 activity remains almost unchanged after complex formation for reasons still unclear. Here, we aim at structurally characterizing CaMKK1:14-3-3 and CaMKK2:14-3-3 complexes by SAXS, H/D exchange coupled to MS, and fluorescence spectroscopy. The results revealed that complex formation suppresses the interaction of both phosphorylated CaMKKs with Ca2+ /CaM and affects the structure of their kinase domains and autoinhibitory segments. But these effects are much stronger on CaMKK1 than on CaMKK2 because the CaMKK1:14-3-3γ complex has a more compact and rigid structure in which the active site of the kinase domain directly interacts with the last two C-terminal helices of the 14-3-3γ protein, thereby inhibiting CaMKK1. In contrast, the CaMKK2:14-3-3 complex has a looser and more flexible structure, so 14-3-3 binding only negligibly affects the catalytic activity of CaMKK2. Therefore, Ca2+ /CaM binding suppression and the interaction of the kinase active site of CaMKK1 with the last two C-terminal helices of 14-3-3γ protein provide the structural basis for 14-3-3-mediated CaMKK1 inhibition.

Molecular Mechanisms Underlying Ca2+/Calmodulin-Dependent Protein Kinase Kinase Signal Transduction

Ca²⁺/calmodulin-dependent protein kinase kinase (CaMKK) is the activating kinase for multiple downstream kinases, including CaM-kinase I (CaMKI), CaM-kinase IV (CaMKIV), protein kinase B (PKB/Akt), and 5′AMP-kinase (AMPK), through the phosphorylation of their activation-loop Thr residues in response to increasing the intracellular Ca²⁺ concentration, as CaMKK itself is a Ca²⁺/CaM-dependent enzyme. The CaMKK-mediated kinase cascade plays important roles in a number of Ca²⁺-dependent pathways, such as neuronal morphogenesis and plasticity, transcriptional activation, autophagy, and metabolic regulation, as well as in pathophysiological pathways, including cancer progression, metabolic syndrome, and mental disorders. This review focuses on the molecular mechanism underlying CaMKK-mediated signal transduction in normal and pathophysiological conditions. We summarize the current knowledge of the structural, functional, and physiological properties of the regulatory kinase, CaMKK, and the development and application of its pharmacological inhibitors.

Structural insights into the functional roles of 14-3-3 proteins

Signal transduction cascades efficiently transmit chemical and/or physical signals from the extracellular environment to intracellular compartments, thereby eliciting an appropriate cellular response. Most often, these signaling processes are mediated by specific protein-protein interactions involving hundreds of different receptors, enzymes, transcription factors, and signaling, adaptor and scaffolding proteins. Among them, 14-3-3 proteins are a family of highly conserved scaffolding molecules expressed in all eukaryotes, where they modulate the function of other proteins, primarily in a phosphorylation-dependent manner. Through these binding interactions, 14-3-3 proteins participate in key cellular processes, such as cell-cycle control, apoptosis, signal transduction, energy metabolism, and protein trafficking. To date, several hundreds of 14-3-3 binding partners have been identified, including protein kinases, phosphatases, receptors and transcription factors, which have been implicated in the onset of various diseases. As such, 14-3-3 proteins are promising targets for pharmaceutical interventions. However, despite intensive research into their protein-protein interactions, our understanding of the molecular mechanisms whereby 14-3-3 proteins regulate the functions of their binding partners remains insufficient. This review article provides an overview of the current state of the art of the molecular mechanisms whereby 14-3-3 proteins regulate their binding partners, focusing on recent structural studies of 14-3-3 protein complexes.

IQGAP1 and RNA Splicing in the Context of Head and Neck via Phosphoproteomics

IQGAP1 (IQ motif-containing GTPase-activating protein 1) scaffolds several signaling pathways in mammalian cells that are implicated in carcinogenesis, including the RAS and PI3K pathways that involve multiple protein kinases. IQGAP1 has been shown to promote head and neck squamous cell carcinoma (HNSCC); however, the underlying mechanism(s) remains unclear. Here, we report a mass spectrometry-based analysis identifying differences in phosphorylation of cellular proteins in vivo and in vitro in the presence or absence of IQGAP1. By comparing the esophageal phosphoproteome profiles between Iqgap1+/+ and Iqgap1-/- mice, we identified RNA splicing as one of the most altered cellular processes. Serine/arginine-rich splicing factor 6 (SRSF6) was the protein with the most downregulated levels of phosphorylation in Iqgap1-/- tissue. We confirmed that the absence of IQGAP1 reduced SRSF6 phosphorylation both in vivo and in vitro. We then expanded our analysis to human normal oral keratinocytes. Again, we found factors involved in RNA splicing to be highly altered in the phosphoproteome profile upon genetic disruption of IQGAP1. Both the Clinical Proteomic Tumor Analysis Consortium (CPTAC) and the Cancer Genome Atlas (TCGA) data sets indicate that phosphorylation of splicing-related proteins is important in HNSCC prognosis. The Biological General Repository for Interaction Datasets (BioGRID) repository also suggested multiple interactions between IQGAP1 and splicing-related proteins. Based on these collective observations, we propose that IQGAP1 regulates the phosphorylation of splicing proteins, which potentially affects their splicing activities and, therefore, contributes to HNSCC. Raw data are available from the MassIVE database with identifier MSV000087770.

Protein kinase A negatively regulates VEGF-induced AMPK activation by phosphorylating CaMKK2 at serine 495

Activation of AMP-activated protein kinase (AMPK) in endothelial cells by vascular endothelial growth factor (VEGF) via the Ca2+/calmodulin-dependent protein kinase kinase 2 (CaMKK2) represents a pro-angiogenic pathway, whose regulation and function is incompletely understood. This study investigates whether the VEGF/AMPK pathway is regulated by cAMP-mediated signalling. We show that cAMP elevation in endothelial cells by forskolin, an activator of the adenylate cyclase, and/or 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of phosphodiesterases, triggers protein kinase A (PKA)-mediated phosphorylation of CaMKK2 (serine residues S495, S511) and AMPK (S487). Phosphorylation of CaMKK2 by PKA led to an inhibition of its activity as measured in CaMKK2 immunoprecipitates of forskolin/IBMX-treated cells. This inhibition was linked to phosphorylation of S495, since it was not seen in cells expressing a non-phosphorylatable CaMKK2 S495C mutant. Phosphorylation of S511 alone in these cells was not able to inhibit CaMKK2 activity. Moreover, phosphorylation of AMPK at S487 was not sufficient to inhibit VEGF-induced AMPK activation in cells, in which PKA-mediated CaMKK2 inhibition was prevented by expression of the CaMKK2 S495C mutant. cAMP elevation in endothelial cells reduced basal and VEGF-induced acetyl-CoA carboxylase (ACC) phosphorylation at S79 even if AMPK was not inhibited. Together, this study reveals a novel regulatory mechanism of VEGF-induced AMPK activation by cAMP/PKA, which may explain, in part, inhibitory effects of PKA on angiogenic sprouting and play a role in balancing pro- and anti-angiogenic mechanisms in order to ensure functional angiogenesis.

The 14-3-3 Proteins as Important Allosteric Regulators of Protein Kinases

Phosphorylation by kinases governs many key cellular and extracellular processes, such as transcription, cell cycle progression, differentiation, secretion and apoptosis. Unsurprisingly, tight and precise kinase regulation is a prerequisite for normal cell functioning, whereas kinase dysregulation often leads to disease. Moreover, the functions of many kinases are regulated through protein–protein interactions, which in turn are mediated by phosphorylated motifs and often involve associations with the scaffolding and chaperon protein 14-3-3. Therefore, the aim of this review article is to provide an overview of the state of the art on 14-3-3-mediated kinase regulation, focusing on the most recent mechanistic insights into these important protein–protein interactions and discussing in detail both their structural aspects and functional consequences.

Stabilization of Protein-Protein Interactions between CaMKK2 and 14-3-3 by Fusicoccins

Ca²⁺/calmodulin-dependent protein kinase kinase 2 (CaMKK2) regulates several key physiological and pathophysiological processes, and its dysregulation has been implicated in obesity, diabetes, and cancer. CaMKK2 is inhibited through phosphorylation in a process involving binding to the scaffolding 14-3-3 protein, which maintains CaMKK2 in the phosphorylation-mediated inhibited state. The previously reported structure of the N-terminal CaMKK2 14-3-3-binding motif bound to 14-3-3 suggested that the interaction between 14-3-3 and CaMKK2 could be stabilized by small-molecule compounds. Thus, we investigated the stabilization of interactions between CaMKK2 and 14-3-3γ by Fusicoccin A and other fusicoccanes-diterpene glycosides that bind at the interface between the 14-3-3 ligand binding groove and the 14-3-3 binding motif of the client protein. Our data reveal that two of five tested fusicoccanes considerably increase the binding of phosphopeptide representing the 14-3-3 binding motif of CaMKK2 to 14-3-3γ. Crystal structures of two ternary complexes suggest that the steric contacts between the C-terminal part of the CaMKK2 14-3-3 binding motif and the adjacent fusicoccane molecule are responsible for differences in stabilization potency between the study compounds. Moreover, our data also show that fusicoccanes enhance the binding affinity of phosphorylated full-length CaMKK2 to 14-3-3γ, which in turn slows down CaMKK2 dephosphorylation, thus keeping this protein in its phosphorylation-mediated inhibited state. Therefore, targeting the fusicoccin binding cavity of 14-3-3 by small-molecule compounds may offer an alternative strategy to suppress CaMKK2 activity by stabilizing its phosphorylation-mediated inhibited state.

CaMKK2 is inactivated by cAMP-PKA signaling and 14-3-3 adaptor proteins

The calcium-calmodulin-dependent protein kinase kinase-2 (CaMKK2) is a key regulator of cellular and whole-body energy metabolism. It is known to be activated by increases in intracellular Ca²⁺, but the mechanisms by which it is inactivated are less clear. CaMKK2 inhibition protects against prostate cancer, hepatocellular carcinoma, and metabolic derangements induced by a high-fat diet; therefore, elucidating the intracellular mechanisms that inactivate CaMKK2 has important therapeutic implications. Here we show that stimulation of cAMP-dependent protein kinase A (PKA) signaling in cells inactivates CaMKK2 by phosphorylation of three conserved serine residues. PKA-dependent phosphorylation of Ser⁴⁹⁵ directly impairs calcium-calmodulin activation, whereas phosphorylation of Ser¹⁰⁰ and Ser⁵¹¹ mediate recruitment of 14-3-3 adaptor proteins that hold CaMKK2 in the inactivated state by preventing dephosphorylation of phospho-Ser⁴⁹⁵ We also report the crystal structure of 14-3-3ζ bound to a synthetic diphosphorylated peptide that reveals how the canonical (Ser⁵¹¹) and noncanonical (Ser¹⁰⁰) 14-3-3 consensus sites on CaMKK2 cooperate to bind 14-3-3 proteins. Our findings provide detailed molecular insights into how cAMP-PKA signaling inactivates CaMKK2 and reveals a pathway to inhibit CaMKK2 with potential for treating human diseases.

Proteomic characterization of early lung response to breast cancer metastasis in mice

INTRODUCTION

The tumor-promoting rearrangement of the lungs facilitates the process of cancer cell survival in a foreign microenvironment and enables their protection against immune defense. The study aimed to define the fingerprint of the early rearrangement of the lungs via the proteomic profiling of the lung tissue in the experimental model of tumor metastasis in a murine 4T1 mammary adenocarcinoma.

MATERIALS AND METHODS

The studies were performed on 7-8-week-old BALB/c female mice. Viable 4T1 cancer cells were orthotopically inoculated into the right mammary fat pad. The experiment was performed in the early phase of the tumor metastasis one and two weeks after cancer cell inoculation. The comparative analysis of protein profiles was carried out with the aid of the two-dimensional difference in gel electrophoresis (2D-DIGE). Proteins, of which expression differed significantly, were identified using nano-liquid chromatography coupled to a high-resolution mass spectrometry (nanoLC/hybrid ion trap- Orbitrap XL Discovery).

RESULTS

Palpable primary tumors were noted in the 2^nd week after cancer cell inoculation. The investigated period preceded the formation of numerous macrometastases in the lungs, however the metastasis-promoting changes were visible very early. Primary tumor-induced inflammation developed in the lungs as early as after the 1^st week and progressed during the 2^nd week, accompanied by increased concentration of 2-OH-E⁺, an oxidative stress marker, and imbalance in nitric oxide metabolites, pointing to endothelium dysfunction. The early proteomic changes in the lungs in the 1^st week after 4T1 cell inoculation resulted in the reorganization of lung tissue structure [actin, cytoplasmic 1 (Actb), tubulin beta chain (Tubb5), lamin-B1 (Lmnb1), serine protease inhibitor A3K (Serpina3k)] and activation of defense mechanisms [selenium-binding protein 1 (Selenbp1), endoplasmin (Hsp90b1), stress 70 protein, mitochondrial (Hspa9), heat shock protein HSP 90-beta (Hsp90ab1)], but also modifications in metabolic pathways [glucose-6-phosphate 1-dehydrogenase X (G6pdx), ATP synthase subunit beta, mitochondrial (Atp5b), L-lactate dehydrogenase B chain (Ldhb)]. Further development of the solid tumor after the 2^nd week following cancer cell inoculation, secretion of prolific tumor-derived factors as well as the presence of the increasing number of circulating cancer cells and extravasation processes further impose reorganization of the lung tissue [Actb, vimentin (Vim), clathrin light chain A (Clta)], altering additional metabolic pathways [annexin A5 (Anxa5), Rho GDP-dissociation inhibitor 2 (Arhgdib), complement 1 Q subcomponent-binding protein, mitochondrial (C1qbp), 14-3-3 protein zeta/delta (Ywhaz), peroxiredoxin-6 (Prdx6), chitinase-like protein 4 (Chi3l4), reticulocalbin-1 (Rcn1), EF-hand domain-containing protein D2 (Efhd2), calumenin (Calu)]. Interestingly, many of differentially expressed proteins were involved in calcium homeostasis (Rcn1, Efhd2, Calu, Actb, Vim, Lmnb1, Clta, Tubb5, Serpina3k, Hsp90b1, Hsp90ab1, Hspa9. G6pdx, Atp5b, Anxa5, Arhgdib, Ywhaz).

CONCLUSION

The analysis enabled revealing the importance of calcium signaling during the early phase of metastasis development, early cytoskeleton and extracellular matrix reorganization, activation of defense mechanisms and metabolic adaptations. It seems that the tissue response is an interplay between pro- and anti-metastatic mechanisms accompanied by inflammation, oxidative stress and dysfunction of the barrier endothelial cells.

AMP-activated protein kinase-mediated feedback phosphorylation controls the Ca2+/calmodulin (CaM) dependence of Ca2+/CaM-dependent protein kinase kinase β

The Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ)/5'-AMP-activated protein kinase (AMPK) phosphorylation cascade affects various Ca²⁺-dependent metabolic pathways and cancer growth. Unlike recombinant CaMKKβ that exhibits higher basal activity (autonomous activity), activation of the CaMKKβ/AMPK signaling pathway requires increased intracellular Ca²⁺ concentrations. Moreover, the Ca²⁺/CaM dependence of CaMKKβ appears to arise from multiple phosphorylation events, including autophosphorylation and activities furnished by other protein kinases. However, the effects of proximal downstream kinases on CaMKKβ activity have not yet been evaluated. Here, we demonstrate feedback phosphorylation of CaMKKβ at multiple residues by CaMKKβ-activated AMPK in addition to autophosphorylation in vitro, leading to reduced autonomous, but not Ca²⁺/CaM-activated, CaMKKβ activity. MS analysis and site-directed mutagenesis of AMPK phosphorylation sites in CaMKKβ indicated that Thr¹⁴⁴ phosphorylation by activated AMPK converts CaMKKβ into a Ca²⁺/CaM-dependent enzyme as shown by completely Ca²⁺/CaM-dependent CaMKK activity of a phosphomimetic T144E CaMKKβ mutant. CaMKKβ mutant analysis indicated that the C-terminal domain (residues 471-587), including the autoinhibitory region, plays an important role in stabilizing an inactive conformation in a Thr¹⁴⁴ phosphorylation-dependent manner. Furthermore, immunoblot analysis with anti-phospho-Thr¹⁴⁴ antibody revealed phosphorylation of Thr¹⁴⁴ in CaMKKβ in transfected COS-7 cells that was further enhanced by exogenous expression of AMPKα. These results indicate that AMPK-mediated feedback phosphorylation of CaMKKβ regulates the CaMKKβ/AMPK signaling cascade and may be physiologically important for intracellular maintenance of Ca²⁺-dependent AMPK activation by CaMKKβ.

Systematic Synergy of Glucose and GLP-1 to Stimulate Insulin Secretion Revealed by Quantitative Phosphoproteomics

GLP-1 synergizes with glucose in regulating pancreatic β-cell function, including facilitating β-cell survival and insulin secretion. Though it has been widely accepted that phosphorylation is extremely important in regulating β-cell functions, our knowledge to the global mechanism is still limited. Here we performed a quantitative phosphoproteomics study to systematically present the synergistic regulation of INS-1E cell phosphoproteome mediated by glucose and GLP-1. We generated the largest pancreatic β-cell phosphoproteome by identifying 25,327 accurately localized phosphorylation sites on 5,389 proteins. Our results discovered several novel kinases regulated by glucose, GLP-1 or their synergism, and some of these kinases might act as downstream molecules of GLP-1 mediated PKA signaling cascade. A few phosphosites were regulated by both GLP-1 and glucose alone, and these target proteins were highly related to their biological function on pancreatic β-cells. Finally, we found glucose and GLP-1 executed their synergistic effect at multiple levels, especially at pathway level. Both GLP-1 and glucose participated in regulating every single step of the secretion pathway, and systematically synergized their effects in inducing insulin secretion.

A 14-3-3 Family Protein from Wild Soybean (Glycine Soja) Regulates ABA Sensitivity in Arabidopsis

It is widely accepted that the 14-3-3 family proteins are key regulators of multiple stress signal transduction cascades. By conducting genome-wide analysis, researchers have identified the soybean 14-3-3 family proteins; however, until now, there is still no direct genetic evidence showing the involvement of soybean 14-3-3s in ABA responses. Hence, in this study, based on the latest Glycine max genome on Phytozome v10.3, we initially analyzed the evolutionary relationship, genome organization, gene structure and duplication, and three-dimensional structure of soybean 14-3-3 family proteins systematically. Our results suggested that soybean 14-3-3 family was highly evolutionary conserved and possessed segmental duplication in evolution. Then, based on our previous functional characterization of a Glycine soja 14-3-3 protein GsGF14o in drought stress responses, we further investigated the expression characteristics of GsGF14o in detail, and demonstrated its positive roles in ABA sensitivity. Quantitative real-time PCR analyses in Glycine soja seedlings and GUS activity assays in PGsGF14O:GUS transgenic Arabidopsis showed that GsGF14o expression was moderately and rapidly induced by ABA treatment. As expected, GsGF14o overexpression in Arabidopsis augmented the ABA inhibition of seed germination and seedling growth, promoted the ABA induced stomata closure, and up-regulated the expression levels of ABA induced genes. Moreover, through yeast two hybrid analyses, we further demonstrated that GsGF14o physically interacted with the AREB/ABF transcription factors in yeast cells. Taken together, results presented in this study strongly suggested that GsGF14o played an important role in regulation of ABA sensitivity in Arabidopsis.

Genetic and physical interaction of Ssp1 CaMKK and Rad24 14-3-3 during low pH and osmotic stress in fission yeast

The Ssp1 calmodulin kinase kinase (CaMKK) is necessary for stress-induced re-organization of the actin cytoskeleton and initiation of growth at the new cell end following division in Schizosaccharomyces pombe. In addition, it regulates AMP-activated kinase and functions in low glucose tolerance. ssp1⁻ cells undergo mitotic delay at elevated temperatures and G₂ arrest in the presence of additional stressors. Following hyperosmotic stress, Ssp1-GFP forms transient foci which accumulate at the cell membrane and form a band around the cell circumference, but not co-localizing with actin patches. Hyperosmolarity-induced localization to the cell membrane occurs concomitantly with a reduction of its interaction with the 14-3-3 protein Rad24, but not Rad25 which remains bound to Ssp1. The loss of rad24 in ssp1⁻ cells reduces the severity of hyperosmotic stress response and relieves mitotic delay. Conversely, overexpression of rad24 exacerbates stress response and concomitant cell elongation. rad24⁻ does not impair stress-induced localization of Ssp1 to the cell membrane, however this response is almost completely absent in cells overexpressing rad24.

A Glycine soja 14-3-3 protein GsGF14o participates in stomatal and root hair development and drought tolerance in Arabidopsis thaliana

It is well established that 14-3-3 proteins are key regulators of multiple stress signal transduction cascades. However, the biological functions of soybean 14-3-3 proteins, especially in plant drought response, are not yet known. In this study, we characterized a Glycine soja 14-3-3 gene, GsGF14o, which is involved in plant development and drought response. GsGF14o expression was greatly induced by drought stress, as evidenced by the quantitative real-time PCR and β-glucuronidase (GUS) activity analysis. GsGF14o overexpression in Arabidopsis thaliana resulted in decreased drought tolerance during seed germination and seedling growth. Furthermore, silencing of AtGF14µ, the most homologous 14-3-3 gene of GsGF14o, led to enhanced drought tolerance at both the seed germination and seedling stage. Unexpectedly, GsGF14o transgenic lines showed reduced water loss and transpiration rates compared with wild-type plants, which was demonstrated to be the consequence of the decreased stomatal size. At the same time, the smaller stomata due to GsGF14o overexpression led to a relatively slow net photosynthesis rate, which led to a growth penalty under drought stress. We further demonstrated that GsGF14o overexpression caused deficits in root hair formation and development, and thereby reduced the water intake capacity of the transgenic root system. In addition, GsGF14o overexpression down-regulated the transcript levels of drought-responsive marker genes. Finally, we also investigated the tissue-specific accumulation of GsGF14o by using a GUS activity assay. Collectively, the results presented here confirm that GsGF14o plays a dual role in drought stress responses through its involvement in the regulation of stomatal size and root hair development.

The many faces of calmodulin in cell proliferation, programmed cell death, autophagy, and cancer

Calmodulin (CaM) is a ubiquitous Ca(2+) receptor protein mediating a large number of signaling processes in all eukaryotic cells. CaM plays a central role in regulating a myriad of cellular functions via interaction with multiple target proteins. This review focuses on the action of CaM and CaM-dependent signaling systems in the control of vertebrate cell proliferation, programmed cell death and autophagy. The significance of CaM and interconnected CaM-regulated systems for the physiology of cancer cells including tumor stem cells, and processes required for tumor progression such as growth, tumor-associated angiogenesis and metastasis are highlighted. Furthermore, the potential targeting of CaM-dependent signaling processes for therapeutic use is discussed.

14-3-3 proteins sequester a pool of soluble TRIM32 ubiquitin ligase to repress autoubiquitylation and cytoplasmic body formation

Deregulated expression of tripartite motif-containing protein 32 (TRIM32, an E3 ubiquitin-protein ligase) contributes to various diseases. Here we report, using quantitative proteomics and biochemistry, that 14-3-3 proteins bind to phosphorylated TRIM32 and prevent TRIM32 autoubiquitylation and the formation of TRIM32-containing cytoplasmic bodies, which are potential autoregulatory mechanisms that can reduce the concentration of soluble free TRIM32. The 14-3-3-TRIM32 interaction is dependent on protein-kinase-A-catalyzed phosphorylation of TRIM32 at Ser651. We found that the inhibitory effect of 14-3-3 is, in part, a consequence of disrupting the propensity of TRIM32 to undergo higher-order self-association without affecting its dimerization. Consequently, dimerized TRIM32 bound to 14-3-3 was sequestered in a distinct cytoplasmic pool away from the microtubule network, whereas a TRIM32 mutant that cannot bind 14-3-3 underwent multimerization and was unavailable to facilitate cell growth. Our results reveal a novel connection between ubiquitylation and phosphorylation pathways, which could modulate a variety of cell events by stimulating the formation of the 14-3-3-TRIM32 signaling complex.

Ca2+/Calmodulin-dependent protein kinase kinase beta is regulated by multisite phosphorylation

Ca(2+)/calmodulin-dependent protein kinase kinase β (CaMKKβ) is a serine/threonine-directed kinase that is activated following increases in intracellular Ca(2+). CaMKKβ activates Ca(2+)/calmodulin-dependent protein kinase I, Ca(2+)/calmodulin-dependent protein kinase IV, and the AMP-dependent protein kinase in a number of physiological pathways, including learning and memory formation, neuronal differentiation, and regulation of energy balance. Here, we report the novel regulation of CaMKKβ activity by multisite phosphorylation. We identify three phosphorylation sites in the N terminus of CaMKKβ, which regulate its Ca(2+)/calmodulin-independent autonomous activity. We then identify the kinases responsible for these phosphorylations as cyclin-dependent kinase 5 (CDK5) and glycogen synthase kinase 3 (GSK3). In addition to regulation of autonomous activity, we find that phosphorylation of CaMKKβ regulates its half-life. We find that cellular levels of CaMKKβ correlate with CDK5 activity and are regulated developmentally in neurons. Finally, we demonstrate that appropriate phosphorylation of CaMKKβ is critical for its role in neurite development. These results reveal a novel regulatory mechanism for CaMKKβ-dependent signaling cascades.

Schizosaccharomyces pombe cell division cycle under limited glucose requires Ssp1 kinase, the putative CaMKK, and Sds23, a PP2A-related phosphatase inhibitor

Calcium/calmodulin-dependent protein kinase (CaMK) is required for diverse cellular functions, and similar kinases exist in fungi. Although mammalian CaMK kinase (CaMKK) activates CaMK and also evolutionarily-conserved AMP-activated protein kinase (AMPK), CaMKK is yet to be established in yeast. We here report that the fission yeast Schizosaccharomyces pombe Ssp1 kinase, which controls G2/M transition and response to stress, is the putative CaMKK. Ssp1 has a CaM binding domain (CBD) and associates with 14-3-3 proteins as mammalian CaMKK does. Temperature-sensitive ssp1 mutants isolated are defective in the tolerance to limited glucose, and this tolerance requires the conserved stretch present between the kinase domain and CBD. Sds23, multi-copy suppressor for mutants defective in type 1 phosphatase and APC/cyclosome, also suppresses the ssp1 phenotype, and is required for the tolerance to limited glucose. We demonstrate that Sds23 binds to type 2A protein phosphatases (PP2A) and PP2A-related phosphatase Ppe1, and that Sds23 inhibits Ppe1 phosphatase activity. Ssp1 and Ppe1 thus seem to antagonize in utilizing limited glucose. We also show that Ppk9 and Ssp2 are the catalytic subunits of AMPK and AMPK-related kinases, respectively, which bind to common beta-(Amk2) and gamma-(Cbs2) subunits.