Association of polyomavirus middle tumor antigen with 14-3-3 proteins

Cystatin C deficiency suppresses tumor growth in a breast cancer model through decreased proliferation of tumor cells

Cysteine cathepsins are proteases that, in addition to their important physiological functions, have been associated with multiple pathologies, including cancer. Cystatin C (CstC) is a major endogenous inhibitor that regulates the extracellular activity of cysteine cathepsins. We investigated the role of cystatin C in mammary cancer using CstC knockout mice and a mouse model of breast cancer induced by expression of the polyoma middle T oncoprotein (PyMT) in the mammary epithelium. We showed that the ablation of CstC reduced the rate of mammary tumor growth. Notably, a decrease in the proliferation of CstC knockout PyMT tumor cells was demonstrated ex vivo and in vitro, indicating a role for this protease inhibitor in signaling pathways that control cell proliferation. An increase in phosphorylated p-38 was observed in CstC knockout tumors, suggesting a novel function for cystatin C in cancer development, independent of the TGF-β pathway. Moreover, proteomic analysis of the CstC wild-type and knockout PyMT primary cell secretomes revealed a decrease in the levels of 14-3-3 proteins in the secretome of knock-out cells, suggesting a novel link between cysteine cathepsins, cystatin C and 14-3-3 proteins in tumorigenesis, calling for further investigations.

Chaperone-mediated specificity in Ras and Rap signaling

Ras and Rap proteins are closely related small guanosine triphosphatase (GTPases) that share similar effector-binding domains but operate in a very different signaling networks; Ras has a dominant role in cell proliferation, while Rap mediates cell adhesion. Ras and Rap proteins are regulated by several shared processes such as post-translational modification, phosphorylation, activation by guanine exchange factors and inhibition by GTPase-activating proteins. Sub-cellular localization and trafficking of these proteins to and from the plasma membrane are additional important regulatory features that impact small GTPases function. Despite its importance, the trafficking mechanisms of Ras and Rap proteins are not completely understood. Chaperone proteins play a critical role in trafficking of GTPases and will be the focus of the discussion in this work. We will review several aspects of chaperone biology focusing on specificity toward particular members of the small GTPase family. Understanding this specificity should provide key insights into drug development targeting individual small GTPases.

Dynamic multiprotein assemblies shape the spatial structure of cell signaling

Cell signaling underlies critical cellular decisions. Coordination, efficiency as well as fail-safe mechanisms are key elements. How the cell ensures that these hallmarks are at play are important questions. Cell signaling is often viewed as taking place through discrete and cross-talking pathways; oftentimes these are modularized to emphasize distinct functions. While simple, convenient and clear, such models largely neglect the spatial structure of cell signaling; they also convey inter-modular (or inter-protein) spatial separation that may not exist. Here our thesis is that cell signaling is shaped by a network of multiprotein assemblies. While pre-organized, the assemblies and network are loose and dynamic. They contain transiently-associated multiprotein complexes which are often mediated by scaffolding proteins. They are also typically anchored in the membrane, and their continuum may span the cell. IQGAP1 scaffolding protein which binds proteins including Raf, calmodulin, Mek, Erk, actin, and tens more, with actin shaping B-cell (and likely other) membrane-anchored nanoclusters and allosterically polymerizing in dynamic cytoskeleton formation, and Raf anchoring in the membrane along with Ras, provides a striking example. The multivalent network of dynamic proteins and lipids, with specific interactions forming and breaking, can be viewed as endowing gel-like properties. Collectively, this reasons that efficient, productive and reliable cell signaling takes place primarily through transient, preorganized and cooperative protein-protein interactions spanning the cell rather than stochastic, diffusion-controlled processes.

Role of 14-3-3-beta in the migration and invasion in human malignant glioma cell line U87MG

PURPOSE

To assess the influence of 14-3-3-beta in modulating the migration and invasion of human glioma cells.

METHODS

To profile the genes associated with malignant glioma cell motility, differential display-polymerase chain reaction was performed and the findings were validated by Northern blotting in the U343MG-A, U87MG, and U87MG-10' human glioma cell lines. Antisense 14-3-3-beta cDNA plasmid was transfected into U87MG ('U87-YA-3'). To follow motility changes after transfection, simple scratch test and matrigel assay were performed. Morphological and cytoskeletal changes were documented by light and confocal microscopy. In addition, doubling times of the transfectant and endogenous 14-3-3-beta levels were determined in various glioma cell lines with different motilities.

RESULTS

14-3-3-beta was highly expressed in U87MG cells. U87-YA-3 cells became small and flat, and actin was depolarized. Furthermore, U87-YA-3 cell motility was inhibited markedly versus parental U87MG cells. The doubling times of transfected and parent cells were 32 and 37 hours, respectively. Endogenous 14-3-3-beta expression in the human glioma cell lines was proportional to their migratory and invasive abilities.

CONCLUSION

14-3-3-beta modulates the migration and invasion in U87MG cells, which may be useful in developing therapeutic approaches for the treatment of glioma.

14-3-3ζ as a prognostic marker and therapeutic target for cancer

IMPORTANCE OF THE FIELD

The ubiquitously expressed 14-3-3ζ protein is involved in numerous important cellular pathways involved in cancer. Recent research suggests 14-3-3ζ may play a central role regulating multiple pathways responsible for cancer initiation and progression. This review will provide an overview of 14-3-3 proteins and address the role of 14-3-3ζ overexpression in cancer.

AREAS COVERED IN THIS REVIEW

The review covers the basic role of 14-3-3 in regulation of multiple pathways with a focus on 14-3-3ζ as a clinically relevant biomarker for cancer recurrence.

WHAT THE READER WILL GAIN

14-3-3ζ overexpression has been found in multiple cancers; however, the clinical implications were unclear. Recently, 14-3-3ζ has been identified as a biomarker for poor prognosis and chemoresistance in multiple tumor types, indicating a potential clinical application for using 14-3-3ζ in selecting treatment options and predicting cancer patients' outcome.

TAKE HOME MESSAGE

14-3-3ζ is a potential prognostic marker of cancer recurrence and predictive marker for therapeutic resistance. The overexpression of 14-3-3ζ in multiple cancers suggests that it may be a common target to intervene tumor progression; therefore, more efforts are needed for the development of 14-3-3 inhibitors.

The critical protein interactions and structures that elicit growth deregulation in cancer and viral replication

One of the greatest challenges in biomedicine is to define the critical targets and network interactions that are subverted to elicit growth deregulation in human cells. Understanding and developing rational treatments for cancer requires a definition of the key molecular targets and how they interact to elicit the complex growth deregulation phenotype. Viral proteins provide discerning and powerful probes to understand both how cells work and how they can be manipulated using a minimal number of components. The small DNA viruses have evolved to target inherent weaknesses in cellular protein interaction networks to hijack the cellular DNA and protein replication machinery. In the battle to escape the inevitability of senescence and programmed cell death, cancers have converged on similar mechanisms, through the acquisition and selection of somatic mutations that drive unchecked cellular replication in tumors. Understanding the dynamic mechanisms through which a minimal number of viral proteins promote host cells to undergo unscheduled and pathological replication is a powerful strategy to identify critical targets that are also disrupted in cancer. Viruses can therefore be used as tools to probe the system-wide protein-protein interactions and structures that drive growth deregulation in human cells. Ultimately this can provide a path for developing system context-dependent therapeutics. This review will describe ongoing experimental approaches using viruses to study pathways deregulated in cancer, with a particular focus on viral cellular protein-protein interactions and structures.

Lessons in signaling and tumorigenesis from polyomavirus middle T antigen

The small DNA tumor viruses have provided a very long-lived source of insights into many aspects of the life cycle of eukaryotic cells. In recent years, the emphasis has been on cancer-related signaling. Here we review murine polyomavirus middle T antigen, its mechanisms, and its downstream pathways of transformation. We concentrate on the MMTV-PyMT transgenic mouse, one of the most studied models of breast cancer, which permits the examination of in situ tumor progression from hyperplasia to metastasis.

Lessons from polyoma middle T antigen on signaling and transformation: A DNA tumor virus contribution to the war on cancer

Middle T antigen (MT) is the principal oncogene of murine polyomavirus. Its study has led to the discovery of the roles of tyrosine kinase and phosphoinositide 3-kinase (PI3K) signaling in mammalian growth control and transformation. MT is necessary for viral transformation in tissue culture cells and tumorigenesis in animals. When expressed alone as a transgene, MT causes tumors in a wide variety of tissues. It has no known catalytic activity, but rather acts by assembling cellular signal transduction molecules. Protein phosphatase 2A, protein tyrosine kinases of the src family, PI3K, phospholipase Cgamma1 as well as the Shc/Grb2 adaptors are all assembled on MT. Their activation sets off a series of signaling cascades. Analyses of virus mutants as well as transgenic animals have demonstrated that the effects of a given signal depend not only tissue type, but on the genetic background of the host animal. There remain many opportunities as we seek a full molecular understanding of MT and apply some of its lessons to human cancer.

Laser capture microdissection and protein microarray analysis of human non-small cell lung cancer: differential epidermal growth factor receptor (EGPR) phosphorylation events associated with mutated EGFR compared with wild type

Little is known about lung carcinoma epidermal growth factor (EGF) kinase pathway signaling within the context of the tissue microenvironment. We quantitatively profiled the phosphorylation and abundance of signal pathway proteins relevant to the EGF receptor within laser capture microdissected untreated, human non-small cell lung cancer (NSCLC) (n = 25) of known epidermal growth factor receptor (EGFR) tyrosine kinase domain mutation status. We measured six phosphorylation sites on EGFR to evaluate whether EGFR mutation status in vivo was associated with the coordinated phosphorylation of specific multiple phosphorylation sites on the EGFR and downstream proteins. Reverse phase protein array quantitation of NSCLC revealed simultaneous increased phosphorylation of EGFR residues Tyr-1148 (p < 0.044) and Tyr-1068 (p < 0.026) and decreased phosphorylation of EGFR Tyr-1045 (p < 0.002), HER2 Tyr-1248 (p < 0.015), IRS-1 Ser-612 (p < 0.001), and SMAD Ser-465/467 (p < 0.011) across all classes of mutated EGFR patient samples compared with wild type. To explore which subset of correlations was influenced by ligand induction versus an intrinsic phenotype of the EGFR mutants, we profiled the time course of 115 cellular signal proteins for EGF ligand-stimulated (three dosages) NSCLC mutant and wild type cultured cell lines. EGFR mutant cell lines (H1975 L858R) displayed a pattern of EGFR Tyr-1045 and HER2 Tyr-1248 phosphorylation similar to that found in tissue. Persistence of phosphorylation for AKT Ser-473 following ligand stimulation was found for the mutant. These data suggest that a higher proportion of the EGFR mutant carcinoma cells may exhibit activation of the phosphatidylinositol 3-kinase/protein kinase B (AKT)/mammalian target of rapamycin (MTOR) pathway through Tyr-1148 and Tyr-1068 and suppression of IRS-1 Ser-612, altered heterodimerization with ERBB2, reduced response to transforming growth factor beta suppression, and reduced ubiquitination/degradation of the EGFR through EGFR Tyr-1045, thus providing a survival advantage. This is the first comparison of multiple, site-specific phosphoproteins with the EGFR tyrosine kinase domain mutation status in vivo.

A RNA interference screen identifies the protein phosphatase 2A subunit PR55gamma as a stress-sensitive inhibitor of c-SRC

Protein Phosphatase type 2A (PP2A) represents a family of holoenzyme complexes with diverse biological activities. Specific holoenzyme complexes are thought to be deregulated during oncogenic transformation and oncogene-induced signaling. Since most studies on the role of this phosphatase family have relied on the use of generic PP2A inhibitors, the contribution of individual PP2A holoenzyme complexes in PP2A-controlled signaling pathways is largely unclear. To gain insight into this, we have constructed a set of shRNA vectors targeting the individual PP2A regulatory subunits for suppression by RNA interference. Here, we identify PR55γ and PR55δ as inhibitors of c-Jun NH₂-terminal kinase (JNK) activation by UV irradiation. We show that PR55γ binds c-SRC and modulates the phosphorylation of serine 12 of c-SRC, a residue we demonstrate to be required for JNK activation by c-SRC. We also find that the physical interaction between PR55γ and c-SRC is sensitive to UV irradiation. Our data reveal a novel mechanism of c-SRC regulation whereby in response to stress c-SRC activity is regulated, at least in part, through loss of the interaction with its inhibitor, PR55γ.

Protein Phosphatase type 2A (PP2A) represent a family of holoenzyme complexes involved in wide range of activities such as growth, differentiation, and cell death. The PP2A holoenzyme complex is made up of a catalytic, a structural, and one of various “B” subunits. These “B” subunits are thought to provide the substrate specificity required for PP2A activity. Previous work on PP2A has mostly been derived by inhibiting the catalytic subunit through chemical inhibition, as such inhibiting all of the pathways associated with PP2A. To identify individual “B” subunits involved in specific cellular processes we have generated a “B” subunit gene knockdown library, which allows us to inhibit each of the known “B” subunits individually. One of the many pathways regulated by PP2A is the c-Jun NH₂-terminal kinase (JNK) kinase pathway, which, depending on stimulus, can affect either cell survival or cell proliferation. Here we report that the “B” subunit PR55γ acts as a negative regulator of JNK activity and cell death. We show that PR55γ influences JNK activity by inhibiting one of its upstream regulators, the proto-oncogene c-SRC, through dephosphorylation at one of the key residues on c-SRC, a site we show to be critical for c-SRC activation following cell stress. Overall our work describes the novel function of a specific PP2A subunit involved in cell survival and identifies a novel mechanism of c-SRC regulation.

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

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

KEYWORDS

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

Latent protein LANA2 from Kaposi's sarcoma-associated herpesvirus interacts with 14-3-3 proteins and inhibits FOXO3a transcription factor

The Kaposi's sarcoma-associated herpesvirus latent protein LANA2 has been suggested to have an important role in the transforming activity of the virus based on its capacity to inhibit p53 and PKR-dependent apoptosis as well as the interferon-dependent response. Here, we describe a novel interaction between LANA2 and both the phosphoserine/phosphothreonine-binding 14-3-3 proteins and the transcription factor FOXO3a. In addition, our results indicate that LANA2 inhibits the transcriptional activity of FOXO3a and blocks the G2/M arrest induced by 14-3-3 protein overexpression. These results suggest a novel mechanism by which LANA2 may promote tumorigenesis.

A novel function of 14-3-3 protein: 14-3-3zeta is a heat-shock-related molecular chaperone that dissolves thermal-aggregated proteins

The 14-3-3 proteins are highly conserved molecules that function as intracellular adaptors in a variety of biological processes, such as signal transduction, cell cycle control, and apoptosis. Here, we show that a 14-3-3 protein is a heat-shock protein (Hsp) that protects cells against physiological stress as its new cellular function. We have observed that, in Drosophila cells, the 14-3-3zeta is up-regulated under heat stress conditions, a process mediated by a heat shock transcription factor. As the biological action linked to heat stress, 14-3-3zeta interacted with apocytochrome c, a mitochondrial precursor protein of cytochrome c, in heat-treated cells, and the suppression of 14-3-3zeta expression by RNA interference resulted in the formation of significant amounts of aggregated apocytochrome c in the cytosol. The aggregated apocytochrome c was converted to a soluble form by the addition of 14-3-3zeta protein and ATP in vitro. 14-3-3zeta also resolubilized heat-aggregated citrate synthase and facilitated its reactivation in cooperation with Hsp70/Hsp40 in vitro. Our observations provide the first direct evidence that a 14-3-3 protein functions as a stress-induced molecular chaperone that dissolves and renaturalizes thermal-aggregated proteins.

14-3-3 isoforms are induced by aldosterone and participate in its regulation of epithelial sodium channels

Aldosterone increases sodium absorption across renal collecting duct cells primarily by increasing the apical membrane expression of ENaC, the sodium entry channel. Nedd4-2, a ubiquitin-protein isopeptide ligase, tags ENaC with ubiquitin for internalization and degradation, but when it is phosphorylated by the aldosterone-induced kinase, SGK1, Nedd4-2 is inhibited and apical ENaC density and sodium absorption increase. We evaluated the hypothesis that 14-3-3 proteins participate in the aldosterone-mediated regulation of ENaC by associating with phosphorylated Nedd4-2. Mouse cortical collecting duct (mCCD) epithelia cultured on filters expressed several 14-3-3 isoforms; this study focused on an isoform whose expression was induced 3-fold by aldosterone, 14-3-3beta. In polarized mCCD epithelia, aldosterone elicited significant, time-dependent increases in the expression of alpha-ENaC, SGK1, phospho-Nedd4-2, and 14-3-3beta without altering total Nedd4-2. Aldosterone decreased the interaction of alpha-ENaC with Nedd4-2, and with similar kinetics increased the association of 14-3-3beta with phospho-Nedd4-2. Short interfering RNA-induced knockdown of 14-3-3beta blunted the aldosterone-induced increase in alpha-ENaC expression, returned alpha-ENaC-Nedd4-2 binding toward prealdosterone levels, and blocked the aldosterone-stimulated increase in transepithelial sodium transport. Incubation of cell extracts with a selective phospho-Nedd4-2 antibody blocked the aldosterone-induced association of 14-3-3beta with Nedd4-2, implicating SGK1 phosphorylation at Ser-328 as the primary site of 14-3-3beta binding. Our studies show that aldosterone increases the expression of 14-3-3beta, which interacts with phospho-Nedd4-2 to block its interaction with ENaC, thus enhancing sodium absorption by increasing apical membrane ENaC density.

14-3-3 proteins: a historic overview

This chapter includes a historic overview of 14-3-3 proteins with an emphasis on the differences between potentially cancer-relevant isoforms on the genomic, protein and functional level. The focus will therefore be on mammalian 14-3-3s although many important developments in the field have involved Drosophila 14-3-3 proteins for example and the cross-fertilisation from parallel studies on plant 14-3-3 should not be underestimated. In the major part of this review I will attempt to focus on some novel data and aspects of 14-3-3 structure and function, in particular regulation of 14-3-3 isoforms by oncogene-related protein kinase phosphorylation and aspects of 14-3-3 research with which newcomers to the field may be less familiar.

Structural determinants of 14-3-3 binding specificities and regulation of subcellular localization of 14-3-3-ligand complexes: a comparison of the X-ray crystal structures of all human 14-3-3 isoforms

14-3-3 proteins are a ubiquitous class of regulatory proteins found in all eukaryotic cells and were the first class of molecules to be recognized as discrete phosphoserine/threonine binding modules. 14-3-3 proteins bind a large number of different substrates to regulate a wide array of cellular signaling events including cell cycle progression and DNA damage responses, programmed cell death, cytoskeletal dynamics, transcriptional control of gene expression, as well as processes directly related to cancer progression. In this review, the structural basis of phosphorylation-dependent binding of 14-3-3 to peptide and protein ligands is discussed along with mechanisms that govern how 14-3-3 regulates the function of its bound ligands. The X-ray crystal structures of all human 14-3-3 proteins bound to peptides have now been solved. Here, we use structural comparisons between isoforms as a framework for discussion of ligand binding by 14-3-3 as well as the mechanisms through which post-translational modification of the different isoforms alters their function.

14-3-3 proteins as potential oncogenes

14-3-3 proteins are a family of highly conserved cellular proteins that play key roles in the regulation of central physiological pathways. More than 200 14-3-3 target proteins have been identified, including proteins involved in mitogenic and cell survival signaling, cell cycle control and apoptotic cell death. Importantly, the involvement of 14-3-3 proteins in the regulation of various oncogenes and tumor suppressor genes points to a potential role in human cancer. The present review summarizes current findings implicating a 14-3-3 role in cancer while discussing potential mechanisms and points of action of 14-3-3 during cancer development and progression.

14-3-3 proteins: a number of functions for a numbered protein

Many signal transduction events are orchestrated by specific interactions of proteins mediated through discrete phosphopeptide-binding motifs. Although several phosphospecific-binding domains are now known, 14-3-3s were the first proteins recognized to specifically bind a discrete phosphoserine or phosphothreonine motif. The 14-3-3 proteins are a family of ubiquitously expressed, exclusively eukaryotic proteins with an astonishingly large number of binding partners. Consequently, 14-3-3s modulate an enormous and diverse group of cellular processes. The effects of 14-3-3 proteins on their targets can be broadly defined using three categories: (i) conformational change; (ii) physical occlusion of sequence-specific or structural protein features; and (iii) scaffolding. This review will describe the current state of knowledge on 14-3-3 proteins, highlighting several important advances, and will attempt to provide a framework by which 14-3-3 functions can be understood.

Protein and mRNA characterization in high and low metastasis adenoid cystic carcinoma cell lines

Metastasis and invasion, the important characteristics of malignant tumors, are closely associated with a series of changes in the expression of genes and proteins. In this study, we compare mRNA and protein expression in high and low metastasis adenoid cystic carcinoma cell lines by mRNA suppression subtractive hybridization and two-dimensional electrophoresis combined with peptide mass fingerprint analysis. 34 differentially expressed genes were obtained using suppression subtractive hybridization experiments including 6 highly expressed gene sequences in the high metastasis cell line, and 28 in the low metastasis cell line. RNA dot blot hybridization further confirmed the results after excluding false positives. For protein analysis, ten significantly different protein spots were detected using two-dimensional gel electrophoresis technique combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (MALDI- TOF-MS). The results then compare with the SWISS PROT database. These results suggest that high tumor metastasis of adenoid cystic carcinoma is associated with multiple genes whose function include angiogenesis, protein synthesis, signal transduction, modulation of cell cycle, molecular chaperones, and immune co-stimulating molecule. Moreover, the results of the phenotypic function-related expression mapping analysis at the mRNA and protein level revealed obvious complementarities, providing important clues for further study of the molecular mechanism of metastasis, metastasis control and possible targets for cancer gene therapy.

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

14-3-3 proteins exert an extraordinarily widespread influence on cellular processes in all eukaryotes. They operate by binding to specific phosphorylated sites on diverse target proteins, thereby forcing conformational changes or influencing interactions between their targets and other molecules. In these ways, 14-3-3s 'finish the job' when phosphorylation alone lacks the power to drive changes in the activities of intracellular proteins. By interacting dynamically with phosphorylated proteins, 14-3-3s often trigger events that promote cell survival--in situations from preventing metabolic imbalances caused by sudden darkness in leaves to mammalian cell-survival responses to growth factors. Recent work linking specific 14-3-3 isoforms to genetic disorders and cancers, and the cellular effects of 14-3-3 agonists and antagonists, indicate that the cellular complement of 14-3-3 proteins may integrate the specificity and strength of signalling through to different cellular responses.

14-3-3 Binds to and Mediates Phosphorylation of Microtubule-associated Tau Protein by Ser9-phosphorylated Glycogen Synthase Kinase 3β in the Brain

In mammalian brain, tau, glycogen synthase kinase 3beta (GSK3beta), and 14-3-3, a phosphoserine-binding protein, are parts of a multiprotein tau phosphorylation complex. Within the complex, 14-3-3 simultaneously binds to tau and GSK3beta (Agarwal-Mawal, A., Qureshi, H. Y., Cafferty, P. W., Yuan, Z., Han, D., Lin, R., and Paudel, H. K. (2003) J. Biol. Chem. 278, 12722-12728). The molecular mechanism by which 14-3-3 connects GSK3beta to tau within the complex is not clear. In this study, we find that GSK3beta within the tau phosphorylation complex is phosphorylated on Ser(9). From extracts of rat brain and rat primary cultured neurons, Ser(9)-phosphorylated GSK3beta precipitates with glutathione-agarose beads coated with glutathione S-transferase-14-3-3. Similarly, from rat brain extract, Ser(9)-phosphorylated GSK3beta co-immunoprecipitates with tau. In vitro, 14-3-3 binds to GSK3beta only when the kinase is phosphorylated on Ser(9). In transfected HEK-293 cells, 14-3-3 binds to Ser(9)-phosphorylated GSK3beta and does not bind to GSK3beta (S9A). Tau, on the other hand, binds to both GSK3beta (WT) and GSK3beta (S9A). Moreover, 14-3-3 enhances the binding of tau with Ser(9)-phosphorylated GSK3beta by approximately 3-fold but not with GSK3beta (S9A). Similarly, 14-3-3 stimulates phosphorylation of tau by Ser(9)-phosphorylated GSK3beta but not by GSK3beta (S9A). In transfected HEK-293 cells, Ser(9) phosphorylation suppresses GSK3beta-catalyzed tau phosphorylation in the absence of 14-3-3. In the presence of 14-3-3, however, Ser(9)-phosphorylated GSK3beta remains active and phosphorylates tau. Our data indicate that within the tau phosphorylation complex, 14-3-3 connects Ser(9)-phosphorylated GSK3beta to tau and Ser(9)-phosphorylated GSK3beta phosphorylates tau.

Suppression of AP-1 constitutive activity interferes with polyomavirus MT antigen transformation ability

Polyomavirus (Py) encodes a potent oncogene, the middle T antigen (MT), that induces cell transformation by binding to and activating several cytoplasmic proteins which take part in transduction of growth factors-induced mitogenic signal to the nucleus. We have previously reported that the AP-1 transcriptional complex is a target for MT during cell transformation although, its activation was not sufficient for establishment of the transformed phenotype. Here we show that expression of a dominant-negative cJun mutant in MT transformed cell lines inhibits its transformation ability, indicating that constitutive AP-1 activity is necessary for cell transformation mediated by MT. Evidences also suggest that proliferation of MT transformed cells in low serum concentrations and their ability to form colonies in agarose are controlled by distinct mechanisms.

Autographa californica nucleopolyhedrovirus infection of Spodoptera frugiperda cells: a global analysis of host gene regulation during infection, using a differential display approach

Autographa californica nucleopolyhedrovirus (AcMNPV), the type member of the virus family Baculoviridae, infects pest insects and has been the subject of many studies for its development as a biopesticide. It is also the virus upon which most of the commercial baculovirus protein expression systems are based. AcMNPV infection of cultured host Spodoptera frugiperda (Sf9) cells can induce a number of alterations of host cell properties including altering the cellular cytoskeleton, an arrest of the cell cycle in G(2)/M, and the global shutoff of host protein translation. Additionally, several cellular transcripts have been shown to be down-regulated following AcMNPV infection. In this study, we take a differential display approach to address whether a global down-regulation of Sf9 host transcripts occurs at late times of infection. Additionally, we also use this approach to search for host mRNAs which are up-regulated at early times of infection, and may be important for facilitating baculovirus infection. From these experiments we can confirm a global down-regulation of Sf9 mRNA levels at late times of infection. We also found that up-regulation of individual host gene RNA levels at early times of infection did not occur frequently. One host transcript which was found to be transiently up-regulated as a result of AcMNPV infection was an Sf9 Hsc70 gene. Hsc70 proteins have been shown to play a vital role in the life-cycle of other large DNA viruses, which suggests that this protein is also important for baculovirus infection.

14-3-3 connects glycogen synthase kinase-3 beta to tau within a brain microtubule-associated tau phosphorylation complex

In a recent study, we reported that in bovine brain extract, glycogen synthase kinase-3beta and tau are parts of an approximately 400-500 kDa microtubule-associated tau phosphorylation complex (Sun, W., Qureshi, H. Y., Cafferty, P. W., Sobue, K., Agarwal-Mawal, A., Neufield, K. D., and Paudel, H. K. (2002) J. Biol. Chem. 277, 11933-11940). In this study, we find that when purified brain microtubules are subjected to Superose 12 gel filtration column chromatography, the dimeric scaffold protein 14-3-3 zeta co-elutes with the tau phosphorylation complex components tau and GSK3 beta. From gel filtration fractions containing the tau phosphorylation complex, 14-3-3 zeta, GSK3 beta, and tau co-immunoprecipitate with each other. From extracts of bovine brain, COS-7 cells, and HEK-293 cells transfected with GSK3 beta, 14-3-3 zeta co-precipitates with GSK3 beta, indicating that GSK3 beta binds to 14-3-3 zeta. From HEK-293 cells transfected with tau, GSK3 beta, and 14-3-3 zeta in different combinations, tau co-immunoprecipitates with GSK3 beta only in the presence of 14-3-3 zeta. In vitro, approximately 10-fold more tau binds to GSK3 beta in the presence of than in the absence of 14-3-3 zeta. In transfected HEK-293 cells, 14-3-3 zeta stimulates GSK3 beta-catalyzed tau phosphorylation in a dose-dependent manner. These data indicate that in brain, the 14-3-3 zeta dimer simultaneously binds and bridges tau and GSK3 beta and stimulates GSK3 beta-catalyzed tau phosphorylation.

Polyoma virus middle T antigen and its role in identifying cancer-related molecules

Most cancer researchers take for granted some of the basic concepts about the molecular changes that underlie tumorigenesis. These include the principles that tyrosine kinases and the phosphorylation of phosphatidylinositol by phosphatidylinositol 3-kinases are important in the signalling pathways that control proliferation and apoptosis, and hence cancer formation. However, how many know that a small DNA mouse virus was crucial in establishing both of these tenets?

Forward transport. 14-3-3 binding overcomes retention in endoplasmic reticulum by dibasic signals

Proteins with dibasic retention motifs are subject to retrograde transport to endoplasmic reticulum (ER) by COPI-coated vesicles. As forward transport requires escape from ER retention, general release mechanisms have been expected. Here, KCNK3 potassium channels are shown to bear two cytoplasmic trafficking motifs: an N-terminal dibasic site that binds beta-COP to hold channels in ER and a C-terminal "release" site that binds the ubiquitous intracellular regulator 14-3-3beta on a nonclassical motif in a phosphorylation-dependent fashion to suppress beta-COP binding and allow forward transport. The strategy appears to be common. The major histocompatibility antigen class II-associated invariant chain Iip35 exhibits dibasic retention, carries a release motif, and shows mutually exclusive binding of beta-COP and 14-3-3beta on adjacent N-terminal sites. Other retained proteins are demonstrated to carry functional 14-3-3beta release motifs.

The human cruciform-binding protein, CBP, is involved in DNA replication and associates in vivo with mammalian replication origins

We previously identified and purified from human (HeLa) cells a 66-kDa cruciform-binding protein, CBP, with binding specificity for cruciform DNA regardless of its sequence. DNA cruciforms have been implicated in the regulation of initiation of DNA replication. CBP is a member of the 14-3-3 family of proteins, which are conserved regulatory molecules expressed in all eukaryotes. Here, the in vivo association of CBP/14-3-3 with mammalian origins of DNA replication was analyzed by studying its association with the monkey replication origins ors8 and ors12, as assayed by a chromatin immunoprecipitation assay and quantitative PCR analysis. The association of the 14-3-3beta, -epsilon, -gamma, and -zeta isoforms with these origins was found to be approximately 9-fold higher, compared with other portions of the genome, in logarithmically growing cells. In addition, the association of these isoforms with ors8 and ors12 was also analyzed as a function of the cell cycle. Higher binding of 14-3-3beta, -epsilon, -gamma, and -zeta isoforms with ors8 and ors12 was found at the G(1)/S border, by comparison with other stages of the cell cycle. The CBP/14-3-3 cruciform binding activity was also found to be maximal at the G(1)/S boundary. The involvement of 14-3-3 in mammalian DNA replication was analyzed by studying the effect of anti-14-3-3beta, -epsilon, -gamma, and -zeta antibodies in the in vitro replication of p186, a plasmid containing the minimal replication origin of ors8. Anti-14-3-3epsilon, -gamma, and -zeta antibodies alone or in combination inhibited p186 replication by approximately 50-80%, while anti-14-3-3beta antibodies had a lesser effect ( approximately 25-50%). All of the antibodies tested were also able to interfere with CBP binding to cruciform DNA. The results indicate that CBP/14-3-3 is an origin-binding protein, acting at the initiation step of DNA replication by binding to cruciform-containing molecules, and dissociates after origin firing.

How do 14-3-3 proteins work?-- Gatekeeper phosphorylation and the molecular anvil hypothesis

14-3-3 proteins were the first signaling molecules to be identified as discrete phosphoserine/threonine binding modules. This family of proteins, which includes seven isotypes in human cells and up to 15 in plants, plays critical roles in cell signaling events that control progress through the cell cycle, transcriptional alterations in response to environmental cues, and programmed cell death. Despite over 30 years of research, distinct roles for most isotypes remain unknown. Though 14-3-3 proteins perform different functions for different ligands, general mechanisms of 14-3-3 action include changes in activity of bound ligands, altered association of bound ligands with other cellular components, and changes in intracellular localization of 14-3-3-bound cargo. We present a speculative model where binding of 14-3-3 to multiple sites on some ligands results in global ligand conformational changes that mediate their biological effects. For these multi-site ligands, one binding site is likely to function as a 'gatekeeper' whose phosphorylation is necessary for 14-3-3 binding but may not always be sufficient for full biological activity. If correct, then 14-3-3 may prove to be a bona fide phosphodependent signaling chaperone.

Cell transformation by the middle T-antigen of polyoma virus

The polyoma virus region expressed early in the lytic cycle encodes three proteins, or T-antigens, that together cause the infected cell to enter the cell cycle and so provide a suitable cellular environment for replication of the viral genome. Under some circumstances infection does not kill the cell, but the T-antigens are still produced, resulting in the cell becoming transformed and tumorigenic. Most of this transforming action is exerted by the middle T-antigen, which has the ability to convert established cell lines to an oncogenic state. Middle T is a membrane bound polypeptide that interacts with a number of the proteins used by tyrosine kinase associated receptors to stimulate mitogenesis, so MT can be considered as a permanently active analogue of a receptor. Through a defined series of interactions, MT assembles a large multi-protein complex at the cell membrane, consisting of MT, the core dimer of protein phosphatase 2A, an src-family tyrosine kinase, and via phosphotyrosines, ShcA, phosphatidylinositol (3') kinase, and phospholipase Cgamma-1. Tyrosine phosphorylation stimulates PI3K and PLCgamma-1 enzymatic activity, and on ShcA creates binding sites for Grb2 with its associated Sos1 and Gab1. This activates p21(ras), and hence, the MAP kinase cascade. Consequently, MT can be used as a model for studying cell transformation and growth factor receptor signalling pathways.

ShcA and Grb2 mediate polyoma middle T antigen-induced endothelial transformation and Gab1 tyrosine phosphorylation

Middle T antigen (PymT) is the principal transforming component of polyomavirus, and rapidly induces hemangiomas in neonatal mice. PymT, a membrane-associated scaffold, recruits and activates Src family tyrosine kinases, and, once tyrosine phosphorylated, binds proteins with PTB and SH2 domains such as ShcA, phosphatidylinositol 3-kinase (PI3K) and phospholipase Cgamma-1 (PLCgamma-1). To explore the pathways required for endothelial transformation in vivo, we introduced PymT mutant forms into mice. PymT variants unable to bind PI3K and PLCgamma-1 directly induced hemangiomas similarly to wild type, but a mutant unable to bind ShcA was transformation compromised. Requirement for a ShcA PTB domain- binding site was suppressed by replacing this motif in PymT with YXN sequences, which bind the Grb2 SH2 domain upon phosphorylation. Surprisingly, PymT recruitment of ShcA and Grb2 correlated with PI3K activation. PymT mimics activated receptor tyrosine kinases by forming a ShcA-Grb2-Gab1 complex, thus inducing Gab1 tyrosine phosphorylation, which itself is associated with PI3K. Therefore, PymT activation of ShcA-Grb2 signaling is critical for endothelial transformation, and PymT can stimulate Grb2 signaling to both the MAP kinase and PI3K pathways.

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.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Mammalian 14-3-3beta associates with the Chlamydia trachomatis inclusion membrane via its interaction with IncG

Chlamydiae replicate intracellularly within a vacuole that is modified early in infection to become fusogenic with a subset of exocytic vesicles. We have recently identified four chlamydial inclusion membrane proteins, IncD-G, whose expression is detected within the first 2 h after internalization. To gain a better understanding of how these Inc proteins function, a yeast two-hybrid screen was employed to identify interacting host proteins. One protein, 14-3-3beta, was identified that interacted specifically with IncG. The interaction between 14-3-3beta and IncG was confirmed in infected HeLa cells by indirect immunofluorescence microscopy and interaction with a GFP-14-3-3beta fusion protein. 14-3-3 proteins are phosphoserine-binding proteins. Immunoprecipitation studies with [32P]-orthophosphate-labelled cells demonstrated that IncG is phosphorylated in both chlamydia-infected HeLa cells and in yeast cells expressing IncG. Site-directed mutagenesis of predicted 14-3-3 phosphorylation sites demonstrated that IncG binds to 14-3-3beta via a conserved 14-3-3-binding motif (RS164RS166F). Finally, indirect immunofluorescence demonstrated that 14-3-3beta interacts with Chlamydia trachomatis inclusions but not C. psittaci or C. pneumoniae inclusions. 14-3-3beta is the first eukaryotic protein found to interact with the chlamydial inclusion; however, its unique role in C. trachomatis pathogenesis remains to be determined.

Expression of major capsid protein VP-1 in the absence of viral particles in thymomas induced by murine polyomavirus

Thymomas induced by polyomavirus strain PTA in mice are known to express the major capsid protein VP-1. Since the expression of a late structural protein such as VP-1 is considered a sign of virus replication, the present work attempted to clarify the implication of the presence of this protein in tumor cells. Electron microscopy of tumors showed a striking absence of viral particles in the vast majority of the cells. However, immunoelectron microscopy of the same samples demonstrated intranuclear VP-1 in most cells despite the absence of viral particles. Very little infectious virus was recovered from tumors. A change in the electrophoretic mobility of VP-1 from thymomas was detected compared with VP-1 from productively infected cells. The data presented in this work prove that the expression of VP-1 in polyomavirus-induced tumors is not synonymous with the presence of infectious virus, suggesting a possible defect in viral encapsidation.

14-3-3 protein family members have a regulatory role in retinoic acid-mediated induction of cytokeratins in F9 cells

We have found that the expression of five 14-3-3 protein isoforms is induced during the retinoic acid (RA)-mediated differentiation of mouse embryonal carcinoma F9 cells. The induced expression of the 14-3-3 proteins is presumed to have a role in enhancing the mitogen-activated protein kinase (MAPK) activity during RA-mediated F9 cell differentiation, because using genetically engineered budding yeast we showed that these isoforms enhanced the signaling in the MAPK cascade mainly through the interaction with Raf-1. Then we assessed the role of increased MAPK activity in F9 cell differentiation by interfering with signaling in the MAPK cascade in F9 cells. The exogenous expression of dominant-negative MEK1 efficiently abrogated RA-mediated induction of the cytokeratins EndoA and EndoC in the F9 cells. These results suggest that the 14-3-3 proteins play a role in the efficient induction of the cytokeratins during F9 cell differentiation through their signal enhancing activity in the MAPK cascade.

14-3-3 proteins: structure, function, and regulation

The 14-3-3 proteins are a family of conserved regulatory molecules expressed in all eukaryotic cells. A striking feature of the 14-3-3 proteins is their ability to bind a multitude of functionally diverse signaling proteins, including kinases, phosphatases, and transmembrane receptors. This plethora of interacting proteins allows 14-3-3 to play important roles in a wide range of vital regulatory processes, such as mitogenic signal transduction, apoptotic cell death, and cell cycle control. In this review, we examine the structural basis for 14-3-3-ligand interactions, proposed functions of 14-3-3 in various signaling pathways, and emerging views of mechanisms that regulate 14-3-3 actions.

14-3-3zeta is an effector of tau protein phosphorylation

Neurofibrillary tangles associated with Alzheimer's disease are composed mainly of paired helical filaments that are formed by the aggregation of abnormally phosphorylated microtubule-associated protein tau. 14-3-3, a highly conserved protein family that exists as seven isoforms and regulates diverse cellular processes is present in neurofibrillary tangles (Layfield, R., Fergusson, J., Aitken, A., Lowe, J., Landon, M., Mayer, R. J. (1996) Neurosci. Lett. 209, 57-60). The role of 14-3-3 in Alzheimer's disease pathogenesis is not known. In this study, we found that the 14-3-3zeta isoform is associated with tau in brain extract and profoundly stimulates cAMP-dependent protein kinase catalyzed in vitro phosphorylation on Ser(262)/Ser(356) located within the microtubule-binding region of tau. 14-3-3zeta binds to both phosphorylated and nonphosphorylated tau, and the binding site is located within the microtubule-binding region of tau. From brain extract, 14-3-3zeta co-purifies with microtubules, and tubulin blocks 14-3-3zeta-tau binding. Among four 14-3-3 isoforms tested, beta and zeta but not gamma and epsilon associate with tau. Our data suggest that 14-3-3zeta is a tau protein effector and may be involved in the abnormal tau phosphorylation occurring during Alzheimer's disease ontogeny.

Association of the cyclin-dependent kinases and 14-3-3 sigma negatively regulates cell cycle progression

14-3-3 sigma, implicated in cell cycle arrest by p53, was cloned by expression cloning through cyclin-dependent kinase 2 (CDK2) association. 14-3-3 sigma shares cyclin-CDK2 binding motifs with different cell cycle regulators, including p107, p130, p21(CIP1), p27(KIP1), and p57(KIP2), and is associated with cyclin.CDK complexes in vitro and in vivo. Overexpression of 14-3-3 sigma obstructs cell cycle entry by inhibiting cyclin-CDK activity in many breast cancer cell lines. Overexpression of 14-3-3 sigma can also inhibit cell proliferation and prevent anchorage-independent growth of these cell lines. These findings define 14-3-3 sigma as a negative regulator of the cell cycle progression and suggest that it has an important function in preventing breast tumor cell growth.

Human endothelial cells expressing polyoma middle T induce tumors

The middle T oncogene of murine polyomavirus (PymT) rapidly transforms and immortalizes murine embryonic endothelial cells (EC), leading to the formation of vascular tumors in newborn mice, by recruitment of host, non-transformed EC. These tumors are reminiscent of human vascular tumors like cavernous hemangioma, Kaposi's sarcoma or those characterizing Kasabach-Merrit syndrome. Here we investigate the in vitro and in vivo behavior of human primary umbilical cord vein EC expressing PymT. While PymT has been unable to transform human fibroblasts in earlier experiments or controls done here, mT expressing EC (PymT-EC) derived by infection with pLX-PymT retrovirus induce hemangiomas in nu/nu mice. These tumors contain not only human cells but also recruited mouse EC as shown by the presence of human and murine CD31 positive EC. In vitro analysis shows that PymT-EC retain endothelial specific markers like CD31, Von Willebrand factor, and VE-cadherin, and reach the confluence without signs of overgrowth. They are also responsive to vascular endothelial growth factor-A. However, their proliferation rate is increased. The balance between urokinase-type plasminogen activator and plasminogen activator inhibitor-1 is modified; RNA and catalytic activity for the former are elevated while PAI-1 RNA is reduced. In contrast with murine model, where the PymT EC cells become immortal, the effects induced by PymT in human EC are transient. After 12-15 passages, human PymT EC stop proliferating, assume a senescent phenotype, and lose the ability to induce hemangiomas. At the same time both the amount of middle T protein and the level of activation of pp60c-src lower.

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.

Virocrine transformation: the intersection between viral transforming proteins and cellular signal transduction pathways

This review describes a mechanism of viral transformation involving activation of cellular signaling pathways. We focus on four viral oncoproteins: the E5 protein of bovine papillomavirus, which activates the platelet-derived growth factor beta receptor; gp55 of spleen focus forming virus, which activates the erythropoietin receptor; polyoma virus middle T antigen, which resembles an activated receptor tyrosine kinase; and LMP-1 of Epstein-Barr virus, which mimics an activated tumor necrosis factor receptor. These examples indicate that diverse viruses induce cell transformation by activating cellular signal transduction pathways. Study of this mechanism of viral transformation will provide new insights into viral tumorigenesis and cellular signal transduction.

Hepatitis C virus core protein interacts with 14-3-3 protein and activates the kinase Raf-1

Persistent hepatitis C virus (HCV) infection is a major cause of chronic liver dysfunction in humans and is epidemiologically closely associated with the development of human hepatocellular carcinoma. Among HCV components, core protein has been reported to be implicated in cell growth regulation both in vitro and in vivo, although mechanisms explaining those effects are still unclear. In the present study, we identified that members of the 14-3-3 protein family associate with HCV core protein. 14-3-3 protein bound to HCV core protein in a phosphoserine-dependent manner. Introduction of HCV core protein caused a substantial increase in Raf-1 kinase activity in HepG2 cells and in a yeast genetic assay. Furthermore, the HCV core-14-3-3 interaction was essential for Raf-1 kinase activation by HCV core protein. These results suggest that HCV core protein may represent a novel type of Raf-1 kinase-activating protein through its interaction with 14-3-3 protein and may contribute to hepatocyte growth regulation.

Analysis of the roles of 14-3-3 in the platelet glycoprotein Ib-IX-mediated activation of integrin alpha(IIb)beta(3) using a reconstituted mammalian cell expression model

We have reconstituted the platelet glycoprotein (GP) Ib-IX-mediated activation of the integrin alpha(IIb)beta(3) in a recombinant DNA expression model, and show that 14-3-3 is important in GPIb-IX signaling. CHO cells expressing alpha(IIb)beta(3) adhere poorly to vWF. Cells expressing GPIb-IX adhere to vWF in the presence of botrocetin but spread poorly. Cells coexpressing integrin alpha(IIb)beta(3) and GPIb-IX adhere and spread on vWF, which is inhibited by RGDS peptides and antibodies against alpha(IIb)beta(3). vWF binding to GPIb-IX also activates soluble fibrinogen binding to alpha(IIb)beta(3) indicating that GPIb-IX mediates a cellular signal leading to alpha(IIb)beta(3) activation. Deletion of the 14-3-3-binding site in GPIbalpha inhibited GPIb-IX-mediated fibrinogen binding to alpha(IIb)beta(3) and cell spreading on vWF. Thus, 14-3-3 binding to GPIb-IX is important in GPIb-IX signaling. Expression of a dominant negative 14-3-3 mutant inhibited cell spreading on vWF, suggesting an important role for 14-3-3. Deleting both the 14-3-3 and filamin-binding sites of GPIbalpha induced an endogenous integrin-dependent cell spreading on vWF without requiring alpha(IIb)beta(3), but inhibited vWF-induced fibrinogen binding to alpha(IIb)beta(3). Thus, while different activation mechanisms may be responsible for vWF interaction with different integrins, GPIb-IX-mediated activation of alpha(IIb)beta(3) requires 14-3-3 interaction with GPIbalpha.

The cytoplasmic domain of the platelet glycoprotein Ibalpha is phosphorylated at serine 609

The alpha chain of the platelet von Willebrand factor receptor, glycoprotein (GP) Ib, is not known to be phosphorylated. Here, we report that the cytoplasmic domain of GPIbalpha is phosphorylated at Ser(609); this was detected by immunoblotting with an anti-phosphopeptide antibody, anti-pS609, that specifically recognizes the GPIbalpha C-terminal sequence S(606)GHSL(610) only when Ser(609) is phosphorylated. Immunoabsorption with anti-pS609 removed almost all of the GPIbalpha from platelet lysates, indicating a high proportion of GPIbalpha phosphorylation. Anti-pS609 inhibited GPIb-IX binding to the intracellular signaling molecule, 14-3-3zeta. Dephosphorylation of GPIb-IX with potato acid phosphatase inhibited anti-pS609 binding and also 14-3-3zeta binding. A synthetic phosphopeptide corresponding to the GPIbalpha C-terminal sequence (SIRYSGHpSL), but not a nonphosphorylated identical peptide, abolished GPIb-IX binding to 14-3-3zeta. Thus, phosphorylation at Ser(609) of GPIbalpha is important for 14-3-3zeta binding to GPIb-IX. In certain regions of spreading platelets, particularly at the periphery, there was a reduction in GPIbalpha staining by anti-pS609 as observed under a confocal microscope, indicating that a subpopulation of GPIbalpha molecules in these regions is dephosphorylated. These data suggest that phosphorylation and dephosphorylation at Ser(609) of GPIbalpha regulates GPIb-IX interaction with 14-3-3 and may play important roles in the process of platelet adhesion and spreading.

Catalytically inactive protein phosphatase 2A can bind to polyomavirus middle tumor antigen and support complex formation with pp60(c-src)

Interaction between the heterodimeric form of protein phosphatase 2A (PP2A) and polyomavirus middle T antigen (MT) is required for the subsequent assembly of a transformation-competent MT complex. To investigate the role of PP2A catalytic activity in MT complex formation, we undertook a mutational analysis of the PP2A 36-kDa catalytic C subunit. Several residues likely to be involved in the dephosphorylation mechanism were identified and mutated. The resultant catalytically inactive C subunit mutants were then analyzed for their ability to associate with a cellular (B subunit) or a viral (MT) B-type subunit. Strikingly, while all of the inactive mutants were severely impaired in their interaction with B subunit, most of these mutants formed complexes with polyomavirus MT. These findings indicate a potential role for these catalytically important residues in complex formation with cellular B subunit, but not in complex formation with MT. Transformation-competent MT is known to associate with, and modulate the activity of, several cellular proteins, including pp60(c-src) family kinases. To determine whether association of MT with an active PP2A A-C heterodimer is necessary for subsequent association with pp60(c-src), catalytically inactive C subunits were examined for their ability to form complexes containing pp60(c-src) in MT-expressing cells. Two catalytically inactive C subunit mutants that efficiently formed complexes with MT also formed complexes that included an active pp60(c-src) kinase, demonstrating that PP2A activity is not essential in cis in MT complexes for subsequent pp60(c-src) association.

Phosphorylation-dependent association of the Ras-related GTP-binding protein Rem with 14-3-3 proteins

Rem belongs to a subfamily of Ras-related GTPases that includes Rad, Gem, and Kir. These proteins are unique among the Ras superfamily since their expression is under transcriptional regulation and they contain distinct amino and carboxyl termini. To gain insight into the cellular function of Rem, we have undertaken an expression screen using a mouse embryo cDNA library to identify Rem-interacting proteins and find that Rem interacts with a series of 14-3-3 isoforms (epsilon, eta, theta, and zeta). Immunoprecipitation studies demonstrate an interaction that is independent of the nucleotide state of Rem. Rem is phosphorylated in vivo, and binding of Rem to 14-3-3zeta is abolished by pretreating Rem with protein phosphatase 1. Thus, the association of Rem and 14-3-3zeta is phosphorylation-dependent. Examination of the interaction between 14-3-3zeta and various Rem deletion mutants mapped a critical binding site to the C-terminus of Rem. Finally, we demonstrate the interaction of Rad but not the newly identified Rem2 protein with 14-3-3 proteins. These results suggest that 14-3-3 may allow the recruitment of distinct proteins that participate in Rem-mediated signal transduction pathways.