Mechanism of inhibition of protein kinase C by 14-3-3 isoforms. 14-3-3 isoforms do not have phospholipase A2 activity

The AGAMOUS-LIKE 16-GENERAL REGULATORY FACTOR 1 module regulates axillary bud outgrowth via catabolism of abscisic acid in cucumber

Lateral branches are important components of shoot architecture and directly affect crop yield and production cost. Although sporadic studies have implicated abscisic acid (ABA) biosynthesis in axillary bud outgrowth, the function of ABA catabolism and its upstream regulators in shoot branching remain elusive. Here, we showed that the MADS-box transcription factor AGAMOUS-LIKE 16 (CsAGL16) is a positive regulator of axillary bud outgrowth in cucumber (Cucumis sativus). Functional disruption of CsAGL16 led to reduced bud outgrowth, whereas overexpression of CsAGL16 resulted in enhanced branching. CsAGL16 directly binds to the promoter of the ABA 8'-hydroxylase gene CsCYP707A4 and promotes its expression. Loss of CsCYP707A4 function inhibited axillary bud outgrowth and increased ABA levels. Elevated expression of CsCYP707A4 or treatment with an ABA biosynthesis inhibitor largely rescued the Csagl16 mutant phenotype. Moreover, cucumber General Regulatory Factor 1 (CsGRF1) interacts with CsAGL16 and antagonizes CsAGL16-mediated CsCYP707A4 activation. Disruption of CsGRF1 resulted in elongated branches and decreased ABA levels in the axillary buds. The Csagl16 Csgrf1 double mutant exhibited a branching phenotype resembling that of the Csagl16 single mutant. Therefore, our data suggest that the CsAGL16-CsGRF1 module regulates axillary bud outgrowth via CsCYP707A4-mediated ABA catabolism in cucumber. Our findings provide a strategy to manipulate ABA levels in axillary buds during crop breeding to produce desirable branching phenotypes.

Autophagy as a new player in the regulation of clock neurons physiology of Drosophila melanogaster

Axonal terminals of the small ventral lateral neurons (sLNvs), the circadian clock neurons of Drosophila, show daily changes in their arborization complexity, with many branches in the morning and their shrinkage during the night. This complex phenomenon is precisely regulated by several mechanisms. In the present study we describe that one of them is autophagy, a self-degradative process, also involved in changes of cell membrane size and shape. Our results showed that autophagosome formation and processing in PDF-expressing neurons (both sLNv and lLNv) are rhythmic and they have different patterns in the cell bodies and terminals. These rhythmic changes in the autophagy activity seem to be important for neuronal plasticity. We found that autophagosome cargos are different during the day and night, and more proteins involved in membrane remodeling are present in autophagosomes in the morning. In addition, we described for the first time that Atg8-positive vesicles are also present outside the sLNv terminals, which suggests that secretory autophagy might be involved in regulating the clock signaling network. Our data indicate that rhythmic autophagy in clock neurons affect the pacemaker function, through remodeling of terminal membrane and secretion of specific proteins from sLNvs.

Pathways to Parkinson's disease: a spotlight on 14-3-3 proteins

14-3-3s represent a family of highly conserved 30 kDa acidic proteins. 14-3-3s recognize and bind specific phospho-sequences on client partners and operate as molecular hubs to regulate their activity, localization, folding, degradation, and protein-protein interactions. 14-3-3s are also associated with the pathogenesis of several diseases, among which Parkinson's disease (PD). 14-3-3s are found within Lewy bodies (LBs) in PD patients, and their neuroprotective effects have been demonstrated in several animal models of PD. Notably, 14-3-3s interact with some of the major proteins known to be involved in the pathogenesis of PD. Here we first provide a detailed overview of the molecular composition and structural features of 14-3-3s, laying significant emphasis on their peculiar target-binding mechanisms. We then briefly describe the implication of 14-3-3s in the central nervous system and focus on their interaction with LRRK2, α-Synuclein, and Parkin, three of the major players in PD onset and progression. We finally discuss how different types of small molecules may interfere with 14-3-3s interactome, thus representing a valid strategy in the future of drug discovery.

Genome-Wide Identification and Characterization of Wheat 14-3-3 Genes Unravels the Role of TaGRF6-A in Salt Stress Tolerance by Binding MYB Transcription Factor

14-3-3 proteins are a large multigenic family of general regulatory factors (GRF) ubiquitously found in eukaryotes and play vital roles in the regulation of plant growth, development, and response to stress stimuli. However, so far, no comprehensive investigation has been performed in the hexaploid wheat. In the present study, A total of 17 potential 14-3-3 gene family members were identified from the Chinese Spring whole-genome sequencing database. The phylogenetic comparison with six 14-3-3 families revealed that the majority of wheat 14-3-3 genes might have evolved as an independent branch and grouped into ε and non-ε group using the phylogenetic comparison. Analysis of gene structure and motif indicated that 14-3-3 protein family members have relatively conserved exon/intron arrangement and motif composition. Physical mapping showed that wheat 14-3-3 genes are mainly distributed on chromosomes 2, 3, 4, and 7. Moreover, most 14-3-3 members in wheat exhibited significantly down-regulated expression in response to alkaline stress. VIGS assay and protein-protein interaction analysis further confirmed that TaGRF6-A positively regulated slat stress tolerance by interacting with a MYB transcription factor, TaMYB64. Taken together, our findings provide fundamental information on the involvement of the wheat 14-3-3 family in salt stress and further investigating their molecular mechanism.

Molecular dynamics simulations and biochemical characterization of Pf14-3-3 and PfCDPK1 interaction towards its role in growth of human malaria parasite

Scaffold proteins play pivotal role as modulators of cellular processes by operating as multipurpose conformation clamps. 14-3-3 proteins are gold-standard scaffold modules that recognize phosphoSer/Thr (pS/pT) containing conserved motifs, and confer conformational changes leading to modulation of functional parameters of their target proteins. Modulation in functional activity of kinases has been attributed to their interaction with 14-3-3 proteins. Herein, we have annotated and characterized PF3D7_0818200 as 14-3-3 isoform I in Plasmodium falciparum 3D7, and its interaction with one of the key kinases of the parasite, Calcium-Dependent Protein Kinase 1 (CDPK1) by performing various analytical biochemistry and biophysical assays. Molecular dynamics simulation studies indicated that CDPK1 polypeptide sequence (61KLGpS64) behaves as canonical Mode I-type (RXXpS/pT) consensus 14-3-3 binding motif, mediating the interaction. The 14-3-3I/CDPK1 interaction was validated in vitro with ELISA and SPR, which confirmed that the interaction is phosphorylation dependent, with binding affinity constant of 670 ± 3.6 nM. The interaction of 14-3-3I with CDPK1 was validated with well characterized optimal 14-3-3 recognition motifs: Mode I-type ARSHpSYPA and Mode II-type RLYHpSLPA, by simulation studies and ITC. This interaction was found to marginally enhance CDPK1 functional activity. Furthermore, interaction antagonizing peptidomimetics showed growth inhibitory impact on the parasite indicating crucial physiological role of 14-3-3/CDPK1 interaction. Overall, this study characterizes 14-3-3I as a scaffold protein in the malaria parasite and unveils CDPK1 as its previously unidentified target. This sets a precedent for the rational design of 14-3-3 based PPI inhibitors by utilizing 14-3-3 recognition motif peptides, as a potential antimalarial strategy.

Intrinsic disorder associated with 14-3-3 proteins and their partners

Protein-protein interactions (PPIs) mediate a variety of cellular processes and form complex networks, where connectivity is achieved owing to the "hub" proteins whose interaction with multiple protein partners is facilitated by the intrinsically disordered protein regions (IDPRs) and posttranslational modifications (PTMs). Universal regulatory proteins of the eukaryotic 14-3-3 family nicely exemplify these concepts and are the focus of this chapter. The extremely wide interactome of 14-3-3 proteins is characterized by high levels of intrinsic disorder (ID) enabling protein phosphorylation and consequent specific binding to the well-structured 14-3-3 dimers, one of the first phosphoserine/phosphothreonine binding modules discovered. However, high ID enrichment also challenges structural studies, thereby limiting the progress in the development of small molecule modulators of the key 14-3-3 PPIs of increased medical importance. Besides the well-known structural flexibility of their variable C-terminal tails, recent studies revealed the strong and conserved ID propensity hidden in the N-terminal segment of 14-3-3 proteins (~40 residues), normally forming the α-helical dimerization region, that may have a potential role for the dimer/monomer dynamics and recently reported moonlighting chaperone-like activity of these proteins. We review the role of ID in the 14-3-3 structure, their interactome, and also in selected 14-3-3 complexes. In addition, we discuss approaches that, in the future, may help minimize the disproportion between the large amount of known 14-3-3 partners and the small number of 14-3-3 complexes characterized with atomic precision, to unleash the whole potential of 14-3-3 PPIs as drug targets.

Pharmacoproteomics Profile in Response to Acamprosate Treatment of an Alcoholism Animal Model

Acamprosate is an FDA-approved medication for the treatment of alcoholism that is unfortunately only effective in certain patients. Although acamprosate is known to stabilize the hyper-glutamatergic state in alcoholism, pharmacological mechanisms of action in brain tissue remains unknown. To investigate the mechanism of acamprosate efficacy, the authors employ a pharmacoproteomics approach using an animal model of alcoholism, type 1 equilibrative nucleoside transporter (ENT1) null mice. The results demonstrate that acamprosate treatment significantly decreased both ethanol drinking and preference in ENT1 null mice compared to that of wild-type mice. Then, to elucidate acamprosate efficacy mechanism in ENT1 null mice, the authors utilize label-free quantification proteomics comparing both genotype and acamprosate treatment effects in the nucleus accumbens (NAc). A total of 1040 protein expression changes are identified in the NAc among 3634 total proteins detected. The proteomics and Western blot result demonstrate that acamprosate treatment decreased EAAT expression implicating stabilization of the hyper-glutamatergic condition in ENT1 null mice. Pathway analysis suggests that acamprosate treatment in ENT1 null mice seems to rescue glutamate toxicity through restoring of RTN4 and NF-κB medicated neuroimmune signaling compared to wild-type mice. Overall, pharmacoproteomics approaches suggest that neuroimmune restoration is a potential efficacy mechanism in the acamprosate treatment of certain sub-populations of alcohol dependent subjects.

Hidden disorder propensity of the N-terminal segment of universal adapter protein 14-3-3 is manifested in its monomeric form: Novel insights into protein dimerization and multifunctionality

The multiplicity of functions of 14-3-3 proteins, integrated into many cellular interactions and signaling networks, is primarily based upon their dimeric α-helical structure that is capable of binding phosphorylated protein partners as well as displaying a "moonlighting" chaperone-like activity. The structure and functions of 14-3-3 proteins are regulated in different ways, including Ser58 phosphorylation in the interface, which shifts equilibrium towards the formation of protein monomers whose role is poorly understood. While modification of Ser58 induced only partial dissociation, the engineered triple mutation of human 14-3-3ζ located in the first α-helix deeply monomerized the protein, allowing for a structural analysis of the monomeric form. Dimer-incapable 14-3-3 proteins retained binding capacity and specificity towards some phosphopartners, and also demonstrated increased chaperone-like activity on various substrates. Here, we found a substantial propensity of the N-terminal segment (~40 residues) of 14-3-3 proteins to intrinsic disorder, showing remarkable conservation across different isoforms and organisms. We hypothesized that this intrinsic disorder propensity, hidden in the α-helical 14-3-3 dimer, can be manifested upon its dissociation and interrogated novel monomeric 14-3-3ζ carrying both monomerizing and S58E mutations (14-3-3ζmS58E). CD spectroscopy showed that, at physiological temperatures, this protein has ~10-15% reduced helicity relative to the wild type protein, corresponding to roughly 40 residues. Along with the known flexibility of C-terminus, SAXS-based modeling of the 14-3-3ζmS58E structure strongly suggested pliability of its N-terminus. The unraveled disorder propensity of the N-terminal tails of 14-3-3 proteins provides new clues for better understanding of the molecular mechanisms of dimerization and multifunctionality of these universal adapter proteins.

14-3-3 Proteins are Regulators of Autophagy

14-3-3 proteins are implicated in the regulation of proteins involved in a variety of signaling pathways. 14-3-3-dependent protein regulation occurs through phosphorylation-dependent binding that results, in many cases, in the release of survival signals in cells. Autophagy is a cell digestion process that contributes to overcoming nutrient deprivation and is initiated under stress conditions. However, whether autophagy is a cell survival or cell death mechanism remains under discussion and may depend on context. Nevertheless, autophagy is a cellular process that determines cell fate and is tightly regulated by different signaling pathways, some of which, for example MAPK, PI3K and mTOR, are tightly regulated by 14-3-3 proteins. It is therefore important to understand the role of 14-3-3 protein in modulating the autophagic process. Within this context, direct binding of 14-3-3 to mTOR regulatory proteins, such as TSC2 and PRAS40, connects 14-3-3 with autophagy regulatory processes. In addition, 14-3-3 binding to human vacuolar protein sorting 34 (hVps34), a class III phosphatidylinositol-3-kinase (PI3KC3), indicates the involvement of 14-3-3 proteins in regulating autophagosome formation. hVps34 is involved in vesicle trafficking processes such as autophagy, and its activation is needed for initiation of autophagy. Chromatography and overlay techniques suggest that hVps34 directly interacts with 14-3-3 proteins under physiological conditions, thereby maintaining hVps34 in an inactive state. In contrast, nutrient starvation promotes dissociation of the 14-3-3–hVps34 complex, thereby enhancing hVps34 lipid kinase activity. Thus, 14-3-3 proteins are regulators of autophagy through regulating key components of the autophagic machinery. This review summarizes the role of 14-3-3 protein in the control of target proteins involved in regulating the master switches of autophagy.

Direct interaction between scaffolding proteins RACK1 and 14-3-3ζ regulates brain-derived neurotrophic factor (BDNF) transcription

RACK1 is a scaffolding protein that spatially and temporally regulates numerous signaling cascades. We previously found that activation of the cAMP signaling pathway induces the translocation of RACK1 to the nucleus. We further showed that nuclear RACK1 is required to promote the transcription of the brain-derived neurotrophic factor (BDNF). Here, we set out to elucidate the mechanism underlying cAMP-dependent RACK1 nuclear translocation and BDNF transcription. We identified the scaffolding protein 14-3-3ζ as a direct binding partner of RACK1. Moreover, we found that 14-3-3ζ was necessary for the cAMP-dependent translocation of RACK1 to the nucleus. We further observed that the disruption of RACK1/14-3-3ζ interaction with a peptide derived from the RACK1/14-3-3ζ binding site or shRNA-mediated 14-3-3ζ knockdown inhibited cAMP induction of BDNF transcription. Together, these data reveal that the function of nuclear RACK1 is mediated through its interaction with 14-3-3ζ. As RACK1 and 14-3-3ζ are two multifunctional scaffolding proteins that coordinate a wide variety of signaling events, their interaction is likely to regulate other essential cellular functions.

Properties of the monomeric form of human 14-3-3ζ protein and its interaction with tau and HspB6

Dimers formed by seven isoforms of the human 14-3-3 protein participate in multiple cellular processes. The dimeric form has been extensively characterized; however, little is known about the structure and properties of the monomeric form of 14-3-3. The monomeric form is involved in the assembly of homo- and heterodimers, which could partially dissociate back into monomers in response to phosphorylation at Ser58. To obtain monomeric forms of human 14-3-3ζ, we produced four protein constructs with different combinations of mutated (M) or wild-type (W) segments E(5), (12)LAE(14), and (82)YREKIE(87). Under a wide range of expression conditions in Escherichia coli, the MMM and WMM mutants were insoluble, whereas WMW and MMW mutants were soluble, highly expressed, and purified to homogeneity. WMW and MMW mutants remained monomeric over a wide range of concentrations while retaining the α-helical structure characteristic of wild-type 14-3-3. However, WMW and MMW mutants were highly susceptible to proteolysis and had much lower thermal stabilities than the wild-type protein. Using WMW and MMW mutants, we show that the monomeric form interacts with the tau protein and with the HspB6 protein, in both cases forming complexes with a 1:1 stoichiometry, in contrast to the 2:1 and/or 2:2 complexes formed by wild-type 14-3-3. Significantly, this interaction requires phosphorylation of tau protein and HspB6. Because of minimal changes in structure, MMW and especially WMW mutant proteins are promising candidates for analyzing the effect of monomerization on the physiologically important properties of 14-3-3ζ.

14-3-3 proteins and regulation of cytoskeleton

The proteins of the 14-3-3 family are universal adapters participating in multiple processes running in the cell. We describe the structure, isoform composition, and distribution of 14-3-3 proteins in different tissues. Different elements of 14-3-3 structure important for dimer formation and recognition of protein targets are analyzed in detail. Special attention is paid to analysis of posttranslational modifications playing important roles in regulation of 14-3-3 function. The data of the literature concerning participation of 14-3-3 in regulation of intercellular contacts and different elements of cytoskeleton formed by microfilaments are analyzed. We also describe participation of 14-3-3 in regulation of small G-proteins and protein kinases important for proper functioning of cytoskeleton. The data on the interaction of 14-3-3 with different components of microtubules are presented, and the probable role of 14-3-3 in developing of certain neurodegenerative diseases is discussed. The data of the literature concerning the role of 14-3-3 in formation and normal functioning of intermediate filaments are also reviewed. It is concluded that due to its adapter properties 14-3-3 plays an important role in cytoskeleton regulation. The cytoskeletal proteins that are abundant in the cell might compete with the other protein targets of 14-3-3 and therefore can indirectly regulate many intracellular processes that are dependent on 14-3-3.

Post-translational modification of 14-3-3 isoforms and regulation of cellular function

14-3-3 is now well established as a family of dimeric proteins that can modulate interaction between proteins involved in a wide range of functions. In many cases, these proteins show a distinct preference for a particular isoform(s) of 14-3-3 and in many cases a specific repertoire of dimer formation influences the particular proteins that 14-3-3 interact. Well over 200 proteins have been shown to interact with 14-3-3. The purpose of this review is to give an overview of the recently identified post-translational modifications of 14-3-3 isoforms and how this regulates function, interaction, specificity of dimerisation between isoforms and cellular location of target proteins. The association between 14-3-3 and its targets usually involves phosphorylation of the interacting protein which has been the subject of many reviews and discussion of this is included in other reviews in this series. However, it is now realised that in some cases the phosphorylation and a number of other, novel covalent modifications of 14-3-3 isoforms may modulate interaction and dimerisation of 14-3-3. Since this aspect is now emerging to be of major importance in the mechanism of regulation by 14-3-3 isoforms and has not been the focus of previous reviews, this will be detailed here.

Analysis of dynamic tyrosine phosphoproteome in LFA-1 triggered migrating T-cells

The ordered, directional migration of T-lymphocytes is a key process during immune surveillance and response. This requires cell adhesion to the high endothelial venules or to the extracellular matrix by a series of surface receptor/ligand interactions involving adhesion molecules of the integrin family including lymphocyte function associated molecule-1 (LFA-1) and intercellular adhesion molecules (ICAMs). Reversible protein phosphorylation is emerging as a key player in the regulation of biological functions with tyrosine phosphorylation playing a crucial role in signal transduction. Thus, the study of this type of post-translational modification at the proteomic level has great biological significance. In this work, phospho-enriched cell lysates from LFA-1-triggered migrating human T-cells were subjected to immunoaffinity purification of tyrosine phosphorylated proteins, mass spectrometric, and bioinformatic analysis. In addition to the identification of several well-documented proteins, the analysis suggested involvement of a number of new and novel proteins in LFA-1 induced T-cell migration. This dataset expands the list of the signaling components of the LFA-1 induced phosphotyrosine protein complexes in migrating T-cells that will be extremely useful in the study of their specific roles within LFA-1 associated signaling pathways. Identification of proteins previously not reported in the context of LFA-1 stimulated signal transduction might provide new insights into understanding the LFA-1 signaling networks and aid in the search for new potential therapeutic targets.

Protein tyrosine phosphatase interacting protein 51--a jack-of-all-trades protein

Protein tyrosine phosphatase interacting protein 51 (PTPIP51) interacts both in vitro and in vivo with PTP1B, a protein tyrosine phosphatase involved in cellular regulation. PTPIP51 is known to be expressed in many different types of tissues. It is involved in cellular processes such as proliferation, differentiation and apoptosis. Nevertheless, the exact cellular function of PTPIP51 is still unknown. The present review summarizes our current knowledge of the PTPIP51 gene and its mRNA and protein structure.

Identification of a functional splice variant of 14-3-3E1 in rainbow trout

The 14-3-3 protein family is a family of regulatory proteins involved in diverse cellular processes. The presence of 14-3-3 isoforms and the diversity of cellular processes regulated by 14-3-3 isoforms suggest functional specificity of the isoforms. In this study, we report the identification and characterization of a new isoform of the rainbow trout 14-3-3E1 gene generated by alternative splicing. The new isoform contains an insertion of 48 nucleotides (from intron 5) in the coding region of 14-3-3E1 which results in the introduction of a premature stop codon between exon 5 and exon 6. Thus, the alternatively spliced form of 14-3-3E1 (14-3-3E1DeltaC17) lacks 17 amino acid residues at the C terminus encoded by the last exon (exon 6). Reverse-transcription polymerase chain reaction analysis revealed that the wild-type 14-3-3E1 (14-3-3E1wt) is ubiquitously expressed, while 14-3-3E1DeltaC17 shows tissue-specific as well as stage-specific expression during ovarian development and early embryogenesis. Analysis by yeast two-hybrid system demonstrated that 14-3-3E1Delta17 interacts with a number of proteins including ATP synthase, ankyrin repeat domain 13b, cytochrome c subunit VIa, cytochrome c subunit VIb, 60S ribosomal protein L34, solute carrier family 17 member 6 (SLC17A6), troponin I, and an unknown protein. Although all of these proteins except for SLC17A6 also interact with 14-3-3E1wt, 14-3-3E1Delta17 appears to have higher binding affinity with these proteins than 14-3-3E1wt. These findings suggest that alternative splicing affects the function and tissue-specific expression of 14-3-3E1.

Role of 14-3-3 protein and oxidative stress in diabetic cardiomyopathy

Cardiovascular disease is a leading cause of death worldwide. Diabetes mellitus is a well-known and important risk factor for cardiovascular diseases. The occurrence of diabetic cardiomyopathy is independent of hypertension, coronary artery disease, or any other known cardiac diseases. There is growing evidence that excess generation of highly reactive free radicals, largely due to hyperglycemia, causes oxidative stress, which further exacerbates the development and progression of diabetes and its complications. Diabetic cardiomyopathy is characterized by morphologic and structural changes in the myocardium and coronary vasculature mediated by the activation of various signaling pathways. Myocardial apoptosis, hypertrophy and fibrosis are the most frequently proposed mechanisms to explain cardiac changes in diabetic cardiomyopathy. Mammalian 14-3-3 proteins are dimeric phosphoserine-binding proteins that participate in signal transduction and regulate several aspects of cellular biochemistry. 14-3-3 protein regulates diabetic cardiomyopathy via multiple signaling pathways. This review focuses on emerging evidence suggesting that 14-3-3 protein plays a key role in the pathogenesis of the cardiovascular complications of diabetes, which underlie the development and progression of diabetic cardiomyopathy.

Copy number gain and oncogenic activity of YWHAZ/14-3-3zeta in head and neck squamous cell carcinoma

Gene amplification, a common mechanism for oncogene activation in cancers, has been used in the discovery of novel oncogenes. Low-level copy number gains are frequently observed in head and neck squamous cell carcinomas (HNSCCs) where numerous amplification events and potential oncogenes have already been reported. Recently, we applied restriction landmark genome scanning to study gene amplifications in HNSCC and located novel and uncharacterized regions in primary tumor samples. Gain on chromosome 8q22.3, the location of YWHAZ (14-3-3zeta), is found in 30-40% HNSCC cases. Data obtained from fluorescence in situ hybridization and immunohistochemistry on HNSCC tissue microarrays confirmed frequent low-level YWHAZ copy number gain and protein overexpression. YWHAZ mRNA was frequently upregulated in patients' tumor tissues. Furthermore, YWHAZ RNAi significantly suppressed the growth rate of HNSCC cell lines, and overexpression of YWHAZ in HaCaT immortalized human skin keratinocytes promotes overgrowth, as well as morphological changes. Reduced YWHAZ levels increased the G1/G0-phase proportion, decreased the S-phase proportion and the rate of DNA synthesis. Based on this evidence, we suggest that YWHAZ is a candidate proto-oncogene and deserves further investigation into its role in HNSCC carcinogenesis.

Binding of 14-3-3 proteins to a single stranded oligodeoxynucleotide aptamer

A synthetic library of ca. 10(13) single stranded oligodeoxynucleotides, each comprising a randomized 40mer sequence and homogeneous 10mer flanking regions, was screened for binding to recombinant human 14-3-3gamma. A single aptamer, which showed similar affinities (K(D) approximately 10(-8)M) for six isoforms of the protein, has been shown to bind to undenatured 14-3-3 protein in the cerebral spinal fluid of scrapie infected sheep.

Expression of PTH1R constructs in LLC-PK1 cells: protein nuclear targeting is mediated by the PTH1R NLS

This study demonstrates that the PTH1R NLS can target a fusion protein to the nucleus, and that this is blocked by sequences downstream of the NLS. GFP fused to the NLS showed a significant increase in nuclear targeting compared to GFP alone or GFP fused to a peptide of the same length. In previous studies, we demonstrated that the type I PTH/PTHrP receptor (PTH1R) localizes to the nucleus of cells within rat liver, kidney, uterus, ovary and gut. Similarly, nuclear localization of the PTH1R was observed in the cultured osteoblast-like cells MC3T3-E1, UMR106, ROS 17/2.8 and SaOS-2. We have identified a putative bipartite nuclear localization signal (NLS), from residues 471-488 in the protein sequence of the PTH1R. In this study, several PTH1R constructs were made in the Enhanced Green Fluorescent Protein (EGFP) expression vector (Clontech), transiently transfected into LLC-PK1 Clone 46 cells, and the resultant fusion protein expression followed by fluorescence microscopy. This particular clone of LLC-PK1 shows no biochemical response in vitro to parathyroid hormone. Constructs included the entire PTH1R sequence (PTH1R-GFP), the putative NLS fused to the C-terminus of GFP (GFP-NLS) or the NLS through to the C-terminus of the PTH1R fused to GFP (GFP-NLSCT). Deconvolution fluorescence microscopy of cells transfected with PTH1R-GFP showed abundant fluorescent signal throughout the cells with distinctly fluorescing plasma membranes. These cells also exhibited an increase in cAMP production in response to (0-10(-8) M) hPTH(1-34), with an increase in cAMP from 11 fmol/mug of protein to 101 fmol/microg. In contrast, cells transfected with the GFP-NLS construct showed significant nuclear sequestration of fluorescence as compared to GFP alone, GFP-NLSCT, or a short amino acid sequence fused to GFP (GFP-FFVAIYCFCNGEVQAEI). These results indicate that the NLS at residues 471-488 of the mature rat PTH1R is functional and plays a role in targeting the PTH1R the nucleus, also the addition of GFP to the C-terminus of the PTH1R still allows cAMP generation which will be useful for further studies.

Inactivation of 14-3-3 Protein Exacerbates Cardiac Hypertrophy and Fibrosis through Enhanced Expression of Protein Kinase C .BETA.2 in Experimental Diabetes

Diabetic cardiomyopathy is associated with cardiac hypertrophy and fibrosis. Activation of protein kinase C (PKC) has been implicated in the diabetes-induced cardiovascular complications. PKCbeta2 isoform is preferentially found to be activated in the diabetic myocardium. However, the role of PKCbeta2 in diabetic cardiomyopathy is not clear. 14-3-3 family members are dimeric phosphoserine-binding proteins that regulate signal transduction, apoptotic and checkpoint control pathways, and have been shown to bind with PKC isozymes and negatively regulate their enzymatic activities. The present study tests whether 14-3-3 protein regulates cardiac hypertrophy and fibrosis in streptozotocin (STZ)-induced diabetic mice, using transgenic mice with cardiac specific over-expression of dominant negative (DN) 14-3-3 protein. In addition, we examined the relationship between 14-3-3 protein and PKCbeta2 in the diabetic myocardium. Cardiac myocyte diameter, content of cardiac fibrosis, left ventricular tissue expressions of atrial natriuretic peptide, transforming growth factor beta1, collagen III and PKCbeta2 were significantly elevated 28 and 56 d after STZ injection in transgenic DN-14-3-3 mice, when compared to their non-transgenic counterparts. These results clearly demonstrate that the functional inactivation of 14-3-3 protein in DN-14-3-3 mice exacerbates diabetes-induced cardiac hypertrophy and fibrosis. The exacerbations of cardiac hypertrophy and fibrosis were significantly and positively correlated with the enhanced expression of PKCbeta2 in DN-14-3-3 mice. Our results indicate for the first time that 14-3-3 protein negatively regulates cardiac hypertrophy and fibrosis, possibly through controlling the expression of PKCbeta2 in the diabetic myocardium.

Dominant negative 14-3-3 promotes cardiomyocyte apoptosis in early stage of type I diabetes mellitus through activation of JNK

14-3-3 family members are dimeric, phosphoserine binding proteins that regulate signal transduction, apoptotic, and checkpoint control pathways. Recently, cardiomyocyte apoptosis has been characterized in type I diabetes mellitus. In order to study the molecular mechanism underlying diabetes-induced cardiomyocyte apoptosis, we examined the role of 14-3-3 protein and MAPK pathways in transgenic mice with cardiac specific expression of dominant negative 14-3-3eta (DN-14-3-3). p38 MAPK was highly activated 1, 28, and 56 days after diabetes induction by streptozotocin, whereas peak JNK activation was found on day 3 and decreased afterwards. In contrast, ERK1/2 were not activated in diabetic myocardium. Cardiomyocyte apoptosis was peaked on day 3 and decreased on 7, 28, and 56 days. p38 MAPK and JNK activation as well as cardiomyocyte apoptosis were greatly increased in DN-14-3-3 mice relative to non-transgenic mice. Moreover, we found a significant correlation between JNK activation and apoptosis in diabetic myocardium. These results indicate for the first time that 14-3-3 protein plays a critical anti-apoptotic role in diabetic myocardium by inhibiting the JNK pathway.

Activity-dependent expression of acyl-coenzyme a-binding protein in retinal muller glial cells evoked by optokinetic stimulation

Long-term horizontal optokinetic stimulation (HOKS) decreases the gain of the horizontal optokinetic reflex and evokes the second phase of optokinetic afternystagmus (OKAN-II). We investigated the possible molecular constituents of this adaptation. We used a differential display reverse transcriptase-PCR screen for mRNAs isolated from retinas of rabbits that received HOKS. In each rabbit, we compared mRNAs from the retina stimulated in the posterior-->anterior (preferred) direction with mRNAs from the retina stimulated in the anterior-->posterior (null) direction. Acyl-CoA-binding protein (ACBP) mRNA was one of four mRNAs selected by this screen, the proteins of which interact with GABA receptors. HOKS in the preferred direction increased ACBP mRNA transcription and ACBP protein expression. ACBP was localized to Muller glial cells by hybridization histochemistry and by immunohistochemistry. ACBP interacts with the alpha1-subunit of the GABA(A) receptor, as determined by a yeast two-hybrid technique. This interaction was confirmed by coimmunoprecipitation of ACBP and the alpha1-subunit of the GABA(A) receptor using an antibody to GABA(A)alpha1. The interaction was also confirmed by a "pull-down" assay in which histidine-tagged ACBP was used to pull down the GABA(A)alpha1. ACBP does not cross the blood-brain barrier. However, smaller truncated proteolytic fragments of ACBP do, increasing the excitability of central cortical neurons. Muller cells may secrete ACBP in the inner plexiform layer, thereby decreasing the sensitivity of GABA(A) receptors expressed on the surface of ganglion cell dendrites. Because retinal directional sensitivity is linked to GABAergic transmission, HOKS-induced expression of ACBP could provide a molecular basis for adaptation to HOKS and for the genesis of OKAN-II.

LEDGF activation of PKC gamma and gap junction disassembly in lens epithelial cells

Lens epithelium-derived growth factor (LEDGF) has been shown to enhance survival of lens epithelial cells (LECs) against stress. The objectives of these studies are to determine how LEDGF controls PKC gamma activity in normal LECs: how this control of PKC gamma regulates the phosphorylation of Connexin 43, the inhibition of gap junction activity, and the prevention of assembly of gap junctions in LECs. A rabbit LEC line, N/N1003A, was grown in the absence or presence of LEDGF. PKC gamma protein was translocated from the cytosolic fractions to the membrane fractions upon addition of LEDGF at 10 ng ml(-1). In whole cell extracts of N/N1003A cells, co-immunoprecipitation assays showed a protein-protein interaction between PKC gamma and Connexin 43. In the presence of LEDGF the activation of PKC gamma enhanced the phosphorylation of Connexin 43 by four-fold compared to the absence of LEDGF. The addition of LEDGF for 30 min resulted in a 65% decrease in gap junction Connexin 43 at the cell surface and a 70% decrease in gap junction activity. These results suggest that the activation of PKC gamma by LEDGF plays a major role in gap junction assembly/disassembly, which may enhance survival of LECs against osmolarity-stress induced by high sugar concentration.

14-3-3 proteins; bringing new definitions to scaffolding

The 14-3-3 proteins are a part of an emerging family of proteins and protein domains that bind to serine/threonine-phosphorylated residues in a context specific manner, analogous to the Src homology 2 (SH2) and phospho-tyrosine binding (PTB) domains. 14-3-3 proteins bind and regulate key proteins involved in various physiological processes such as intracellular signaling (e.g. Raf, MLK, MEKK, PI-3 kinase, IRS-1), cell cycling (e.g. Cdc25, Wee1, CDK2, centrosome), apoptosis (e.g. BAD, ASK-1) and transcription regulation (e.g. FKHRL1, DAF-16, p53, TAZ, TLX-2, histone deacetylase). In contrast to SH2 and PTB domains, which serve mainly to mediate protein-protein interactions, 14-3-3 proteins in many cases alter the function of the target protein, thus allowing them to serve as direct regulators of their targets. This review focuses on the various mechanisms employed by the 14-3-3 proteins in the regulation of their diverse targets, the structural basis for 14-3-3-target protein interaction with emphasis on the role of 14-3-3 dimerization in target protein binding and regulation and provides an insight on 14-3-3 regulation itself.

The expanding superfamily of phospholipase A(2) enzymes: classification and characterization

The phospholipase A(2) (PLA(2)) superfamily consists of a broad range of enzymes defined by their ability to catalyze the hydrolysis of the middle (sn-2) ester bond of substrate phospholipids. The hydrolysis products of this reaction, free fatty acid and lysophospholipid, have many important downstream roles, and are derived from the activity of a diverse and growing superfamily of PLA(2) enzymes. This review updates the classification of the various PLA(2)'s now described in the literature. Four criteria have been employed to classify these proteins into one of the 11 Groups (I-XI) of PLA(2)'s. First, the enzyme must catalyze the hydrolysis of the sn-2 ester bond of a natural phospholipid substrate, such as long fatty acid chain phospholipids, platelet activating factor, or short fatty acid chain oxidized phospholipids. Second, the complete amino acid sequence of the mature protein must be known. Third, each PLA(2) Group should include all of those enzymes that have readily identifiable sequence homology. If more than one homologous PLA(2) gene exists within a species, then each paralog should be assigned a Subgroup letter, as in the case of Groups IVA, IVB, and IVC PLA(2). Homologs from different species should be classified within the same Subgroup wherever such assignments are possible as is the case with zebra fish and human Group IVA PLA(2) orthologs. The current classification scheme does allow for historical exceptions of the highly homologous Groups I, II, V, and X PLA(2)'s. Fourth, catalytically active splice variants of the same gene are classified as the same Group and Subgroup, but distinguished using Arabic numbers, such as for Group VIA-1 PLA(2) and VIA-2 PLA(2)'s. These four criteria have led to the expansion or realignment of Groups VI, VII and VIII, as well as the addition of Group XI PLA(2) from plants.

14-3-3 proteins and growth control

The 14-3-3 proteins constitute a family that is highly conserved in a wide range of organisms, including higher eukaryotes, invertebrates and plants. Variants of 14-3-3 proteins assembled in homo- and heterodimers were found to interact with diverse cellular proteins. Until recently, the biological role of 14-3-3 members was still poorly understood. However, the results of an increasing number of studies on their structure and function are converging to define 14-3-3 proteins as a novel type of adaptor that modulates interactions between components involved in signal transduction pathway and in cell cycle control.

Protein kinase C activation by acidic proteins including 14-3-3

14-3-3 proteins may function as adapter or scaffold proteins in signal transduction pathways. We reported previously that several 14-3-3 isotypes bind to protein kinase C (PKC)-zeta and facilitate coupling of PKC-zeta to Raf-1 [van der Hoeven, van der Wal, Ruurs, van Dijk and van Blitterswijk (2000) Biochem. J. 345, 297-306], an event that boosts the mitogen-activated protein kinase (ERK) pathway in Rat-1 fibroblasts. The present work investigated whether bound 14-3-3 would affect PKC-zeta activity. Using recombinant 14-3-3 proteins and purified PKC-zeta in a convenient, newly developed in vitro kinase assay, we found that 14-3-3 proteins stimulated PKC-zeta activity in a dose-dependent fashion up to approx. 2.5-fold. Activation of PKC-zeta by 14-3-3 isotypes was unrelated to their mutual affinity, estimated by co-immunoprecipitation from COS cell lysates. Accordingly, PKC-zeta with a defective (point-mutated) 14-3-3-binding site, showed the same 14-3-3-stimulated activity as wild-type PKC-zeta. As 14-13-3 proteins are acidic, we tested several other acidic proteins, which turned out to stimulate PKC-zeta activity in a similar fashion, whereas neutral or basic proteins did not. These effects were not restricted to the atypical PKC-zeta, but were also found for classical PKC. Together, the results suggest that the stimulation of PKC activity by 14-3-3 proteins is non-specific and solely due to the acidic nature of these proteins, quite similar to that known for acidic lipids.

Increased levels of 14-3-3 gamma and epsilon proteins in brain of patients with Alzheimer's disease and Down syndrome

The 14-3-3 family consists of homo- and heterodimeric proteins representing a novel type of "adaptor proteins" modulating the interaction between components of signal transduction pathways. 14-3-3 isoforms interact with phosphoserine motifs on many proteins as kinases, phosphatases, apoptosis related proteins etc. Performing protein mapping by 2D electrophoresis in human brain we identified two isoforms, 14-3-3 gamma and epsilon and decided to determine these two multifunctional proteins in several brain regions of aged patients with Alzheimer's disease (AD) and Down Syndrome (DS) with AD neuropathology in comparison with control brains. 14-3-3 gamma and 14-3-3 epsilon proteins were increased in several brain regions of AD and DS patients. These changes may contribute to the complex pathomechanisms of AD and AD in DS, evolving inevitably from the fourth decade of life. Deranged 14-3-3 isoforms gamma and epsilon may reflect impaired signaling and/or apoptosis in the brain as several kinases (protein kinase C, Ras, mitogen-activated kinase MEK) involved in signaling and apoptotic factors as bcl-2-related proteins BAD and BAG-1 are binding to 14-3-3 motifs.

Modulation of the Ca(2+)-activated Cl(-) channel by 14-3-3epsilon

We have previously reported an association of 14-3-3epsilon isoform with calmodulin. Using the voltage-clamp technique, the present study investigated the potential role of 14-3-3 in modulating the Ca(2+)-activated Cl(-) channel (CaCC) endogenously expressed in Xenopus oocytes. Injection of 14-3-3epsilon antisense oligodeoxynucleotides resulted in potentiation of the ionomycin-induced Cl(-) current, while 14-3-3 peptide and calmodulin inhibitor, W13, suppressed the antisense-potentiated current. The data suggest that 14-3-3epsilon plays an inhibitory role in modulating the CaCC by interacting with the calmodulin-dependent pathway. The potential role of 14-3-3epsilon in other tissues and its therapeutic potential for cystic fibrosis are discussed.

14-3-3 proteins block apoptosis and differentially regulate MAPK cascades

14-3-3 family members are dimeric phosphoserine-binding proteins that participate in signal transduction and checkpoint control pathways. In this work, dominant-negative mutant forms of 14-3-3 were used to disrupt 14-3-3 function in cultured cells and in transgenic animals. Transfection of cultured fibroblasts with the R56A and R60A double mutant form of 14-3-3zeta (DN-14-3-3zeta) inhibited serum-stimulated ERK MAPK activation, but increased the basal activation of JNK1 and p38 MAPK. Fibroblasts transfected with DN-14-3-3zeta exhibited markedly increased apoptosis in response to UVC irradiation that was blocked by pre-treatment with a p38 MAPK inhibitor, SB202190. Targeted expression of DN-14-3-3eta to murine postnatal cardiac tissue increased the basal activation of JNK1 and p38 MAPK, and affected the ability of mice to compensate for pressure overload, which resulted in increased mortality, dilated cardiomyopathy and massive cardiomyocyte apoptosis. These results demonstrate that a primary function of mammalian 14-3-3 proteins is to inhibit apoptosis.

14-3-3Gamma interacts with and is phosphorylated by multiple protein kinase C isoforms in PDGF-stimulated human vascular smooth muscle cells

It has recently been demonstrated that some members of the 14-3-3 protein family play an important role in signal transduction leading to cellular proliferation. We have previously shown that expression of 14-3-3gamma is induced by growth factors in human vascular smooth muscle cells (VSMC). In this study, we cloned the human homolog of 14-3-3gamma and observed many potential phosphorylation sites, suggesting the potential for post-translational modification. In VSMC treated with platelet-derived growth factor (PDGF), 14-3-3gamma protein was expressed and phosphorylated in an activation-dependent manner. Platelet-derived growth factor-induced phosphorylation could be inhibited by phosphokinase C (PKC) inhibitory compounds, and 14-3-3gamma could be phosphorylated in the absence of PDGF by compounds that activate PKC. We also demonstrated interaction between 14-3-3gamma and several PKC isoforms (alpha, beta, gamma, theta, and delta), implicating these PKC family isoforms as the kinases responsible for PDGF-induced 14-3-3gamma phosphorylation. We found that 14-3-3gamma interacted with the signal transduction protein Raf-1, suggesting that 14-3-3gamma provides a link between this protein and PKC. Thus, 14-3-3gamma may represent a signal transduction protein that is regulated transcriptionally and post-transcriptionally by growth factors.

In vivo and in vitro association of 14-3-3 epsilon isoform with calmodulin: implication for signal transduction and cell proliferation

Using a yeast two-hybrid screen, human 14-3-3 epsilon protein was found to interact with human calmodulin. In vitro binding assay between human 14-3-3 epsilon protein/peptide and calmodulin was demonstrated by native gel electrophoresis, and the interaction was shown to be calcium dependent. Our results, along with the association of the 14-3-3 epsilon protein with other signaling proteins, suggest that the 14-3-3 protein could provide a link between signal transduction and cell proliferation.

14-3-3 proteins in neuronal development and function

The 14-3-3 proteins are small, cytosolic, evolutionarily conserved proteins expressed abundantly in the nervous system. Although they were discovered more than 30 yr ago, their function in the nervous system has remained enigmatic. Several recent studies have helped to clarify their biological function. Crystallographic investigations have revealed that 14-3-3 proteins exist as dimers and that they contain a specific region for binding to other proteins. The interacting proteins, in turn, contain a 14-3-3 binding motif; proteins that interact with 14-3-3 dimers include PKC and Raf, protein kinases with critical roles in neuronal signaling. These proteins are capable of activating Raf in vitro, and this role has been verified by in vivo studies in Drosophila. Most interestingly, mutations in the Drosophila 14-3-3 genes disrupt neuronal differentiation, synaptic plasticity, and behavioral plasticity, establishing a role for these proteins in the development and function of the nervous system.

Exoenzyme S from P. aeruginosa ADP ribosylates rab4 and inhibits transferrin recycling in SLO-permeabilized reticulocytes

ADP-ribosylation of rab proteins by exoenzyme S (Exo S) of P. aeruginosa was studied using reticulocytes. 14-3-3 protein, the eukaryotic cofactor that is obligatory for Exo S activity, was found in association with reticulocyte endocytic vesicles and exosomes, vesicles previously shown to be enriched with rab4. Incubation of purified endocytic vesicles with Exo S triggered rab4 ADP-ribosylation. Transferrin recycling in SLO-permeabilized reticulocytes was highly impaired when Exo S was added to the cells, suggesting that ADP-ribosylation affected rab4 function. Moreover, in vitro ADP-ribosylation of different rab proteins was studied using the cofactor activity extracted from reticulocytes.

Human Signaling Protein 14-3-3ζ Interacts With Platelet Glycoprotein Ib Subunits Ibα and Ibβ

The initiation of primary hemostasis is mediated by interaction of the platelet glycoprotein Ib (GPIb) surface receptor and its arterial subendothelial von Willebrand factor (vWF) ligand. The intracellular signaling immediately following GPIb receptor occupancy connecting the adhesive event to platelet activation and aggregation has not been well characterized. The 14-3-3 proteins are a 27- to 30-kD ubiquitous protein family with diverse biologic roles, including functional modulation of several prominent signaling proteins. We used the yeast two-hybrid system and confocal microscopy to characterize the recently described interaction between GPIb and platelet 14-3-3ζ, and provide evidence for the potential signaling role of this protein. Two-hybrid interactions suggest that platelet 14-3-3ζ associates with the cytoplasmic domain of GPIb subunits Ibα and Ibβ in transformed yeast cells. The 14-3-3 interaction with GPIbβ may be partly mediated through the latter's phosphorylated serine 166 residue as its mutagenesis results in 20% to 40% reduced interaction. There was 51% to 59% reduced interaction between GPIb and three 14-3-3ζ deletion mutants compared with full-length 14-3-3ζ, suggesting that either theN-terminal dimerization or membrane-binding domains or more than one noncontiguous 14-3-3ζ element may be required for optimal GPIb interaction. Confocal studies of platelets and a megakaryocyte cell line provided additional evidence for interaction of 14-3-3ζ with GPIbα and GPIbβ. We also found that, similar to the signaling mediators phosphatidylinositol 3-kinase and Src, platelet cytoskeletal 14-3-3ζ content is increased following vWF and ristocetin stimulation. We suggest that platelet 14-3-3ζ interacts with GPIbα and Ibβ, that this interaction may be partly mediated through phosphoserine recognition, and that 14-3-3ζ cytoskeletal translocation may serve as a GPIb post–receptor occupancy signaling event.

Binding of purified 14-3-3 zeta signaling protein to discrete amino acid sequences within the cytoplasmic domain of the platelet membrane glycoprotein Ib-IX-V complex

The glycoprotein (GP) Ib-IX-V complex constitutively expressed on the platelet plasma membrane mediates initial adhesion of circulating platelets to vessel wall matrix at high shear, and shear-induced platelet aggregation. In both cases, this involves binding of GP Ib-IX-V to the adhesive glycoprotein, von Willebrand Factor (vWF). vWF binding to GP Ib-IX-V rapidly induces platelet activation, leading to cytoskeletal rearrangement, shape change, and secretion that enables alphaIIbbeta3 integrin (GP IIb-IIIa)-dependent platelet aggregation. All these events are critical in (patho)physiological thrombus formation. The recent discovery that the signaling protein, 14-3-3 zeta, copurifies with the GP Ib-IX complex (minus GP V) [Du, X., Harris, S. J., Tetaz, T. J., Ginsberg, M. H., & Berndt, M. C. (1994) J. Biol. Chem. 269, 18287-18290] indicated a potential mechanism for vWF-dependent signaling. The aim of the present study was to identify discrete amino acid sequences that bind 14-3-3 zeta within the cytoplasmic domain of the receptor. As an initial screening assay, overlapping synthetic peptides based on the cytoplasmic domains of GP Ibalpha (100 residues), GP Ibbeta (34 residues), GP IX (5 residues), and GP V (16 residues) were immobilized and assessed for the ability to bind purified 14-3-3 zeta. The C-terminal sequence GHSL of GP Ibalpha was identified as one 14-3-3 zeta interactive sequence, consistent with previous results [Du, X., Fox, J. E., & Pei, S. (1996) J. Biol. Chem. 271, 7362-7367]. Binding of 125I-labeled 14-3-3 zeta to GHSL-containing peptides was inhibitable by unlabeled 14-3-3 zeta and by anti-14-3-3 zeta IgG. Ala-walking through the GHSL sequence suggested all residues were necessary for optimal binding. In addition, 14-3-3 zeta bound with lower affinity to a peptide based on the central region of the GP Ibalpha cytoplasmic domain (Arg-557-Gly-575), whereas peptide sequences within the cytoplasmic domains of GP Ibbeta (Arg-160-Arg-175) and GP V (Lys-529-Gly-544) bound 14-3-3 zeta with comparable affinity to the GHSL-containing peptide. Soluble GHSL-containing peptides, GP Ibbeta- and GP V-based peptides semidissociated 14-3-3 zeta from GP Ib-IX-V or GP Ib-IX in platelet extracts as analyzed by immunoprecipitation, suggesting these sequences, at least partially, mediate the GP Ib-IX-V-14-3-3 zeta interaction in cells. Further, phosphorylation of the GP Ibbeta peptide at a site corresponding to a protein kinase A phosphorylation site (Ser-166) enhanced the affinity of 14-3-3 zeta binding by approximately 8-fold, suggesting phosphorylation as a potential mechanism for regulating 14-3-3 zeta association with the GP Ib-IX-V complex.

Effects of peroxisome proliferators and 12-O-tetradecanoyl phorbol-13-acetate on intercellular communication and connexin43 in two hamster fibroblast systems

Effects of 12-O-tetradecanoyl phorbol 13-acetate (TPA) and the hepatic peroxisome proliferators (HPPs) clofibrate, methyl clofenapate (MCP), di(2-ethylhexyl)phthalate (DEHP) and mono(2-ethylhexyl)phthalate (MEHP) were studied in 2 gap junctional intercellular communication (GJIC) systems, metabolic cooperation in V79 cells and microinjection/dye transfer in Syrian hamster embryo (SHE) cells and V79 cells. TPA inhibited GJIC in both systems but was considerably more potent in V79 cells. SHE cells showed a rapid and transient inhibition of GJIC after exposure to HPPs, with maximal inhibition occurring at 5-15 min. The transient inhibition could be caused by metabolization of the compounds. Clofibrate and MEHP produced strong inhibition of metabolic cooperation in V79 cells at high concentrations, while the effect of MCP and DEHP was lower. However, DEHP, MEHP and clofibrate strongly inhibited dye transfer in V79 cells after a 30 min exposure. Clofibrate also showed a dose- and time-dependent effect on dye transfer in V79 cells. The phosphorylation status of the gap junction protein connexin43 (Cx43) changed minimally in SHE cells after exposure to TPA or HPPs. Cx43 from V79 cells was strongly affected by TPA, but not by HPPs. Immunofluorescence of Cx43 disappeared in both cell types when they were exposed to TPA and MEHP, but not to the other HPPs. Thus, there is no direct correlation between the inhibition of GJIC and changes in the phosphorylation status of Cx43 or the appearance of Cx43 in immunofluorescence experiments. The discrepancies may partly be explained by binding of accessory proteins to Cx43. We point out sequences that may be involved in such binding.

14-3-3 inhibits the Dictyostelium myosin II heavy-chain-specific protein kinase C activity by a direct interaction: identification of the 14-3-3 binding domain

Myosin II heavy chain (MHC) specific protein kinase C (MHC-PKC), isolated from Dictyostelium discoideum, regulates myosin II assembly and localization in response to the chemoattractant cyclic AMP. Immunoprecipitation of MHC-PKC revealed that it resides as a complex with several proteins. We show herein that one of these proteins is a homologue of the 14-3-3 protein (Dd14-3-3). This protein has recently been implicated in the regulation of intracellular signaling pathways via its interaction with several signaling proteins, such as PKC and Raf-1 kinase. We demonstrate that the mammalian 14-3-3 zeta isoform inhibits the MHC-PKC activity in vitro and that this inhibition is carried out by a direct interaction between the two proteins. Furthermore, we found that the cytosolic MHC-PKC, which is inactive, formed a complex with Dd14-3-3 in the cytosol in a cyclic AMP-dependent manner, whereas the membrane-bound active MHC-PKC was not found in a complex with Dd14-3-3. This suggests that Dd14-3-3 inhibits the MHC-PKC in vivo. We further show that MHC-PKC binds Dd14-3-3 as well as 14-3-3 zeta through its C1 domain, and the interaction between these two proteins does not involve a peptide containing phosphoserine as was found for Raf-1 kinase. Our experiments thus show an in vivo function for a member of the 14-3-3 family and demonstrate that MHC-PKC interacts directly with Dd14-3-3 and 14-3-3 zeta through its C1 domain both in vitro and in vivo, resulting in the inhibition of the kinase.

Over-expression of plant 14-3-3 proteins in tobacco: enhancement of the plasmalemma K+ conductance of mesophyll cells

Two cDNA clones encoding 14-3-3 homologous proteins were isolated from Vicia faba. Deduced amino acid sequences share different degrees of homology with other plant 14-3-3 proteins. Both clones, under the control of the CaMV 35S promoter, were transformed into tobacco plants. Immunoblotting showed three different forms of ca. 31, 34, and 37 kDa, indicating a covalent modification of the expressed 14-3-3 proteins. These forms were mainly present in the microsomal fraction. Patch-clamp studies of mesophyll protoplasts of the transformants revealed a strongly enhanced K+ conductance compared to the wild type. This indicates the involvement of 14-3-3 proteins in ion channel regulation, presumably by modulating kinase activities or binding the channel.

Molecular cloning and tissue-specific expression of the mouse homologue of the rat brain 14-3-3 theta protein: characterization of its cellular and developmental pattern of expression in the male germ line

The highly conserved 14-3-3 family of proteins, originally reported as brain-specific and then found in various somatic cells and oocytes, interacts with several important signal transduction kinases so that actually the 14-3-3 protein are considered as modulators of multiple signal transduction pathways. Here we show that a 14-3-3 protein is also expressed in the male germ cells, thus extending the protein cellular distribution to a cell line never reported to express 14-3-3 proteins. Screening of a mouse spermatogenic cells lambda gt11 cDNA library with affinity-purified polyclonal antibodies to the tyrosine kinase SP42 allowed the isolation of several positive clones. Sequencing of a positive cDNA clone revealed a 735-nucleotide open reading frame encoding a protein of 245 amino acids (27,778 Da). The predicted protein was found to be identical to the most recently discovered 14-3-3 isoform, the theta subtype from a rat brain. Here we demonstrate that 14-3-3 theta mRNA is highly expressed in testis and brain only. Western immunoblot analyses confirm the Northern blot data. Developmental Northern and Western blot analyses are consistent with an expression and translation of the 14-3-3 theta gene throughout spermatogenesis. However, analysis of RNA from purified populations of spermatogenic cells at different developmental stages and immunohistochemistry on adult testis sections reveal that within the testis the 14-3-3 theta gene products are most abundant in meiotic prophase spermatocytes, and, above all, in differentiating spermatids. Both testicular and epididymal spermatozoa are negative. The present study is the first report on the presence and molecular characterization of the 14-3-3 theta gene product in the male germ line. Our observations suggest that this specific member of the 14-3-3 protein family could play distinct modulatory roles in the complex development of the mammalian male germ cell lineage.

Structure and sites of phosphorylation of 14-3-3 protein: role in coordinating signal transduction pathways

The 14-3-3 family are homo- and heterodimeric proteins whose biological role has been unclear for some time, although they are now gaining acceptance as a novel type of 'adaptor' protein that modulates interactions between components of signal transduction pathways, rather than by direct activation or inhibition. It is becoming apparent that phosphorylation of the binding partner and possibly also the 14-3-3 proteins may regulate these interactions. 14-3-3 isoforms interact with a novel phosphoserine (Sp) motif on many proteins, RSX1,2SpXP. The two isoforms that interact with Raf-1 are phosphorylated in vivo on Ser185 in a consensus sequence motif for proline-directed kinases. The crystal structure of 14-3-3 indicates that this phosphorylation could regulate interaction of 14-3-3 with its target proteins. We have now identified a number of additional phosphorylation sites on distinct mammalian and yeast isoforms.

Raf-1 kinase and exoenzyme S interact with 14-3-3zeta through a common site involving lysine 49

14-3-3 proteins are a family of conserved dimeric molecules that bind to a range of cellular proteins involved in signal transduction and oncogenesis. Our solution of the crystal structure of 14-3-3zeta revealed a conserved amphipathic groove that may allow the association of 14-3-3 with diverse ligands (Liu, D., Bienkowska, J., Petosa, C., Collier, R. J., Fu, H., and Liddington, R. (1995) Nature 376, 191-194). Here, the contributions of three positively charged residues (Lys-49, Arg-56, and Arg-60) that lie in this Raf-binding groove were investigated. Two of the charge-reversal mutations greatly (K49E) or partially (R56E) decreased the interaction of 14-3-3zeta with Raf-1 kinase, whereas R60E showed only subtle effects on the binding. Interestingly, these mutations exhibited similar effects on the functional interaction of 14-3-3zeta with another target protein, exoenzyme S (ExoS), an ADP-ribosyltransferase from Pseudomonas aeruginosa. The EC50 values of 14-3-3zeta required for ExoS activation increased by approximately 110-, 5-, and 2-fold for the K49E, R56E, and R60E mutants, respectively. The drastic reduction of 14-3-3zeta/ligand affinity by the K49E mutation is due to a local electrostatic effect, rather than the result of a gross structural alteration, as evidenced by partial proteolysis and circular dichroism analysis. This work identifies the first point mutation (K49E) that dramatically disrupts 14-3-3zeta/ligand interactions. The parallel effects of this single point mutation on both Raf-1 binding and ExoS activation strongly suggest that diverse associated proteins share a common structural binding determinant on 14-3-3zeta.

Inhibition of cytosolic phospholipase A2 by annexin V in differentiated permeabilized HL-60 cells. Evidence of crucial importance of domain I type II Ca2+-binding site in the mechanism of inhibition

Annexin V belongs to a family of proteins that interact with phospholipids in a Ca2+-dependent manner. This protein has been demonstrated to have anti-phospholipase A2 activity. However, this effect has never yet been reported with the 85-kDa cytosolic PLA2 (cPLA2). We studied, in a model of differentiated and streptolysin O-permeabilized HL-60 cells, the effect of annexin V on cPLA2 activity after stimulation by calcium, GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)), formyl-Met-Leu-Phe, or phorbol 12-myristate 13-acetate. Both recombinant and human placental purified annexin V inhibit cPLA2 activity whatever the stimulus used. The decrease of arachidonic acid release is of 40 and 50%, respectively, at [Ca2+] of 3 and 10 microM. The mechanism of inhibition was also analyzed. cPLA2 requires calcium and protein kinase C (PKC) or mitogen-activated protein kinase phosphorylation for its activation. As annexin V was shown to be an endogenous inhibitor of PKC, PKC-stimulated cPLA2 activity was analyzed. Using GF109203x, a specific PKC inhibitor, we demonstrated that this pathway is of minor importance in our model. cPLA2 inhibition by annexin V is not linked to PKC inhibition. To test the hypothesis of phospholipid depletion, mutants of annexin V were constructed using mutagenesis directed to Ca2+ site. We demonstrate that the Ca2+ site located in domain I is necessary for the inhibitory effect of annexin V on cPLA2 activity. The site in domain IV is also involved but with less efficiency. In contrast, mutations in site II and III do not modify this effect. Moreover, annexin V mutated on all sites does not inhibit cPLA2. Thus, we propose a predominant role of module (I/IV) in the biological action of annexin V, which, in physiological conditions, may control cPLA2 activity by depletion of the phospholipid substrate.

Learning-induced alterations in hippocampal PKC-immunoreactivity: a review and hypothesis of its functional significance

1. To localize protein kinase C (PKC) in the hippocampus, PKC activity measures, mRNA in situ hybridization, and [3H]phorbol ester binding techniques were used until in the 1980s antibodies became available for in situ immunocytochemistry. In the late 1980s, PKC-isoform-specific antibodies were first used to map hippocampal PKC at the cellular and subcellular level. The mammalian hippocampus contains all four Ca(2+)-dependent PKC isoforms, but the (sub)cellular localization is both isoform- and species-specific. 2. Hippocampally-dependent spatial and associative learning in rat, mice and rabbit induce an increase in PKC immunoreactivity (ir) in hippocampal principal cells studied 24 hours after the animals had learned the task. Among the four Ca(2+)-dependent PKC subtypes, this increase is selective for the gamma-isoform. The presence of the gamma-isoform in dendritic spines (the most likely site for synaptic plasticity and information storage), in contrast to PKC alpha, beta 1, and beta 2, may underlie the isoform-selectivity. 3. Compared to fully trained animals, subjects halfway training showed intermediate levels of increased PKC gamma-ir. Poor learners that were not able to learn the task showed considerably less enhanced PKC gamma-ir as compared to good learners. 4. Associative learning induced a decrease in astroglial PKC beta 2 and gamma-ir in those regions where a simultaneous increase in neuronal PKC gamma-ir was observed. This decrease most likely reflects PKC down-regulation, enabling the astrocytes to maintain their K+ buffering capacity necessary to support neuronal activity such as accompanying learning and memory. 5. Western blot analyses revealed that the increase in PKC gamma-ir was not due to an increase in total amount of PKC gamma, translocation, or the proteolytic generation of the fragment PKM. The increase in PKC gamma-ir must therefore reflect a learning-induced conformational change in the PKC gamma molecule that results in the exposure of the antigenic site(s). 6. Although a large number of hippocampal pyramidal cells display learning-induced enhancement of PKC gamma-ir at the 24 hours post-training time point, this does not indicate, however, that all synapses in these neurons are used, or that the maximal PKC signal transduction capacity per call has been reached. 7. The enhanced PKC gamma-ir may reflect a form of activated PKC, since PKC stimulation by phorbol esters (both in hippocampal slices and mildly aldehyde fixed sections) mimicked the increase in PKC gamma-ir similar as seen after learning. 8. The most likely transmitter systems which may have induced the altered PKC gamma-ir are acetylcholine and glutamate. Their contribution and interaction at the cellular level are depicted in a schematic circuit terminating on a CA1 pyramidal cell (Fig. 4). 9. Several functional roles for PKC gamma in learning and memory are discussed, and a hypothetical model is proposed based on an endogeneous PKC inhibitor protein that may explain altered antibody-binding to PKC gamma after learning (Fig. 6). 10. The immunocytochemical approach can contribute significantly to the ongoing attempts to decipher part of the cellular and biochemical mechanism of learning and memory. The development of ever more specific and better characterized antibodies reactive with different sites of proteins like PKC gamma will offer the necessary tools for further immunocytochemical research to help unravel complex brain functions.