Crystal structure of a tyrosine phosphorylated STAT-1 dimer bound to DNA

Structure of an activated Dictyostelium STAT in its DNA-unbound form

Dd-STATa is a STAT protein which transcriptionally regulates cellular differentiation in Dictyostelium discoideum, the only non-metazoan known to employ SH2 domain signaling. The 2.7 A crystal structure of a tyrosine phosphorylated Dd-STATa homodimer reveals a four-domain architecture similar to that of mammalian STATs 1 and 3, but with an inverted orientation for the coiled-coil domain. Dimerization is mediated by SH2 domain:phosphopeptide interactions and by a direct interaction between SH2 domains. The unliganded Dd-STATa dimer adopts a fully extended conformation remarkably different from that of the DNA-bound mammalian STATs, implying a large conformational change upon target site recognition. Buried hydrophilic residues predicted to destabilize the coiled-coil domain suggest how hydrophobic residues may become exposed and mediate nuclear export. Functional and evolutionary implications for metazoan STAT proteins are discussed.

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Identification of the linker-SH2 domain of STAT as the origin of the SH2 domain using two-dimensional structural alignment

The availability of large volumes of genomic sequences presents an unprecedented proteomic challenge to characterize the structure and function of various protein motifs. Primary structural alignment is often unable to accurately identify a given motif due to sequence divergence; however, with the aid of secondary structural prediction for analysis, it becomes feasible to explore protein motifs on a proteome-wide scale. Here we report the use of secondary structural alignment to characterize the Src homology 2 (SH2) domains of both conventional and divergent sequences and divide them into two groups, Src-type and STAT-type. In addition to the basic "alphabetabetabetaalpha" structure (betaBeta), the Src-type SH2 domain contains an extra beta-strand (betaE or betaE-betaF motif). Alternatively, the linker domain-conjugated SH2 domain in STAT contains the alphaB' motif. Combining BLAST data from betaBeta core motif sequences with predicted secondary structural alignment, we have screened for SH2 domains in various eukaryotic model systems including Arabidopsis, Dictyostelium, and Saccharomyces. Two novel genes carrying the linker-SH2 domain of STAT were discovered and subsequently cloned from Arabidopsis. These genes, designated as STAT-type linker-SH2 domain factors (STATL), are found in a wide array of vascular and nonvascular plants, suggesting that the linker-SH2 domain evolved prior to the divergence of plants and animals. Using this approach, we expanded the number of putative SH2 domain-bearing genes in Dictyostelium and comparatively studied the secondary structural profiles of both typical and atypical SH2 domains. Our results indicate that the linker-SH2 domain of the transcription factor STAT is one of the most ancient and fully developed functional domains, serving as a template for the continuing evolution of the SH2 domain essential for phosphotyrosine signal transduction.

Structure of the NCoA-1/SRC-1 PAS-B domain bound to the LXXLL motif of the STAT6 transactivation domain

Signal transducer and activator of transcription 6 (STAT6) regulates transcriptional activation in response to interleukin-4 (IL-4) by direct interaction with coactivators. The CREB-binding protein (p300/CBP) and the nuclear coactivator 1 (NCoA-1), a member of the p160/steroid receptor coactivator family, bind independently to specific regions of the STAT6 transactivation domain and act as coactivators. The interaction between STAT6 and NCoA-1 is mediated by an LXXLL motif in the transactivation domain of STAT6. To define the mechanism of coactivator recognition, we determined the crystal structure of the NCoA-1 PAS-B domain in complex with the STAT6 LXXLL motif. The amphipathic, alpha-helical STAT6 LXXLL motif binds mostly through specific hydrophobic interactions to NCoA-1. A single amino acid of the NCoA-1 PAS-B domain establishes hydrophilic interactions with the STAT6 peptide. STAT6 interacts only with the PAS-B domain of NCoA-1 but not with the homologous regions of NCoA-2 and NCoA-3. The residues involved in binding the STAT6 peptide are strongly conserved between the different NCoA family members. Therefore surface complementarity between the hydrophobic faces of the STAT6 fragment and of the NCoA-1 PAS-B domain almost exclusively defines the binding specificity between the two proteins.

Molecular characterization and IFN signal pathway analysis of Carassius auratus CaSTAT1 identified from the cultured cells in response to virus infection

Type I interferon (IFN) exerts its pleiotropic effects mainly through the JAK-STAT signaling pathway, which is presently best described in mammals. By subtractive suppression hybridization, two fish signaling factors, JAK1 and STAT1, had been identified in the IFN-induced crucian carp Carassius auratus L. blastulae embryonic (CAB) cells after treatment with UV-inactivated grass carp hemorrhagic virus (GCHV). Further, the full-length cDNA of STAT1, termed CaSTAT1, was obtained. It contains 2926 bp and encodes a protein of 718 aa. CaSTAT1 is most similar to rat STAT1 with 59% identity overall and displays all highly conserved domains that the STAT family possesses. Like human STAT1beta, it lacks the C-terminus acting as transcriptional activation domain in mammals. By contrast, only a single transcript was detected in virus-induced CAB cells. Expression analysis showed that CaSTAT1 could be activated by stimulation of CAB cells with poly I:C, active GCHV, UV-inactivated GCHV or CAB IFN, and displayed diverse expression patterns similar to that of mammalian STAT1. Additionally, the expression of an antiviral gene CaMx1 was also induced under the same conditions, and expression difference between CaSTAT1 and CaMx1 was revealed by induction of CAB IFN. These results provide molecular evidence supporting the notion that the fish IFN signaling transduction pathway is similar to that in mammals. Fish IFN exerts its multiple functions, at least antiviral action, through a JAK-STAT pathway.

Novel peptidomimetic inhibitors of signal transducer and activator of transcription 3 dimerization and biological activity

The critical role of signal transducer and activator of transcription 3 (Stat3) in the growth and survival of human tumor cells identifies it as a promising target for cancer drug discovery. We previously identified a Stat3 SH2 domain-binding phosphopeptide, PY*LKTK, and its tripeptide derivatives, PY*L and AY*L (where Y* represents phosphotyrosine), which inhibit Stat3 biochemical activity and biological function. Here, we report novel peptidomimetic compounds based on PY*L (or AY*L) with substitution of the Y-1 residue by benzyl, pyridyl, or pyrazinyl derivatives that are selective and greater than 5-fold more potent in disrupting Stat3 activity in vitro than lead tripeptides. The biological activities of these derivatives mirror that originally observed for peptides. In this context, the representative peptidomimetic ISS 610 with 4-cyanobenzoate substitution inhibits constitutive Stat3 activity in Src-transformed mouse fibroblasts and human breast and lung carcinoma cells. This effect is not evident with the non-phosphorylated counterpart, ISS 610NP, consistent with interaction of peptidomimetics with the SH2 domain of Stat3. Moreover, ISS 610 induces cell growth inhibition and apoptosis of Src-transformed fibroblasts that contain persistently active Stat3. We present the first report of a peptidomimetic approach to design of small-molecule inhibitors of Stat3 that are also among the first examples of disruptors of transcription factor dimerization with the potential for novel cancer therapy.

Molecular mimicry of the NF-κB DNA target site by a selected RNA aptamer

During the past two decades, structural and biophysical studies of DNA-protein and RNA-protein complexes have enhanced our understanding of the physico-chemical basis of nucleic acid recognition by proteins. However, it remains unclear what protein surface features are most important for nucleic acid binding and whether the same protein surface could bind specifically to both DNA and RNA. The recently described X-ray crystal structure of the transcription factor NF-kappaB p50 homodimer bound to a high-affinity RNA aptamer allows the direct comparison of NF-kappaB-RNA and NF-kappaB-DNA binding modes. The RNA aptamer, which bears no sequence homology to natural NF-kappaB DNA targets, adopts a structure with similar physico-chemical properties to kappaB DNA and contacts a common nucleic-acid-binding 'consensus surface' on the p50 homodimer.

Sequence, structure and energetic determinants of phosphopeptide selectivity of SH2 domains

Here, we present an approach for the prediction of binding preferences of members of a large protein family for which structural information for a number of family members bound to a substrate is available. The approach involves a number of steps. First, an accurate multiple alignment of sequences of all members of a protein family is constructed on the basis of a multiple structural superposition of family members with known structure. Second, the methods of continuum electrostatics are used to characterize the energetic contribution of each residue in a protein to the binding of its substrate. Residues that make a significant contribution are mapped onto the protein sequence and are used to define a "binding site signature" for the complex being considered. Third, sequences whose structures have not been determined are checked to see if they have binding-site signatures similar to one of the known complexes. Predictions of binding affinity to a given substrate are based on similarities in binding-site signature. An important component of the approach is the introduction of a context-specific substitution matrix suitable for comparison of binding-site residues. The methods are applied to the prediction of phosphopeptide selectivity of SH2 domains. To this end, the energetic roles of all protein residues in 17 different complexes of SH2 domains with their cognate targets are analyzed. The total number of residues that make significant contributions to binding is found to vary from nine to 19 in different complexes. These energetically important residues are found to contribute to binding through a variety of mechanisms, involving both electrostatic and hydrophobic interactions. Binding-site signatures are found to involve residues in different positions in SH2 sequences, some of them as far as 9A away from a bound peptide. Surprisingly, similarities in the signatures of different domains do not correlate with whole-domain sequence identities unless the latter is greater than 50%. An extensive comparison with the optimal binding motifs determined by peptide library experiments, as well as other experimental data indicate that the similarity in binding preferences of different SH2 domains can be deduced on the basis of their binding-site signatures. The analysis provides a rationale for the empirically derived classification of SH2 domains described by Songyang & Cantley, in that proteins in the same group are found to have similar residues at positions important for binding. Confident predictions of binding preference can be made for about 85% of SH2 domain sequences found in SWISSPROT. The approach described in this work is quite general and can, in principle, be used to analyze binding preferences of members of large protein families for which structural information for a number of family members is available. It also offers a strategy for predicting cross-reactivity of compounds designed to bind to a particular target, for example in structure-based drug design.

Dd-STATb, a Dictyostelium STAT protein with a highly aberrant SH2 domain, functions as a regulator of gene expression during growth and early development

Dictyostelium, the only known non-metazoan organism to employ SH2 domain:phosphotyrosine signaling, possesses STATs (signal transducers and activators of transcription) and protein kinases with orthodox SH2 domains. Here, however, we describe a novel Dictyostelium STAT containing a remarkably divergent SH2 domain. Dd-STATb displays a 15 amino acid insertion in its SH2 domain and the conserved and essential arginine residue, which interacts with phosphotyrosine in all other known SH2 domains, is substituted by leucine. Despite these abnormalities, Dd-STATb is biologically functional. It has a subtle role in growth, so that Dd-STATb-null cells are gradually lost from the population when they are co-cultured with parental cells, and microarray analysis identified several genes that are either underexpressed or overexpressed in the Dd-STATb null strain. The best characterised of these,discoidin 1, is a marker of the growth-development transition and it is overexpressed during growth and early development of Dd-STATb null cells. Dimerisation of STAT proteins occurs by mutual SH2 domain:phosphotyrosine interactions and dimerisation triggers STAT nuclear accumulation. Despite its aberrant SH2 domain, the Dd-STATb protein sediments at the size expected for a homodimer and it is constitutively enriched in the nucleus. Moreover, these properties are retained when the predicted site of tyrosine phosphorylation is substituted by phenylalanine. These observations suggest a non-canonical mode of activation of Dd-STATb that does not rely on orthodox SH2 domain:phosphotyrosine interactions.

Identification of a novel conserved motif in the STAT family that is required for tyrosine phosphorylation

The rapid transcriptional activation of cellular genes by either type 1 interferons (IFNalpha/beta) or type 2 interferon (IFNgamma) is responsible for many of the pleiotropic effects of these cytokines, including their antiviral, antigrowth, and immunomodulatory activities. Interferon-stimulated gene expression is mediated by transcription factors termed Stats, which upon being tyrosine-phosphorylated, translocate to the nucleus and bind enhancers of interferon-activated genes. We have recently characterized a new Jurkat cell variant, named H123, where IFNalpha stimulates programmed cell death. H123 clones that are resistant to the apoptotic actions of IFNalpha have been selected. One of these clones (Clone 8) is defective in its responses to IFNalpha with regard to activation of genes that require tyrosine phosphorylation of Stat2. Stimulation of Clone 8 cells with IFNalpha induces normal tyrosine phosphorylation of Stat1 and Stat3. Sequencing of Stat2 RNA reveals a substitution of proline 630 located within the Src homology 2 domain of Stat2 to leucine (P630L). Pro-630 and its adjacent amino acids are conserved in all Stat family members but are absent in other proteins that contain Src homology 2 domains. Expression of Stat2 P630L in cells inhibits IFNalpha-stimulated gene expression. These results not only define a critical motif in Stat2 required for its transcriptional activity, but they also provide evidence that resistance to type one IFNs can be mediated by mutations in Stat2 as well as those previously described for Stat1.

Targeting Stat3 with G-quartet oligodeoxynucleotides in human cancer cells

Stat3 is an oncogene that is activated in many human cancer cells. Genetic approaches that disrupt Stat3 activity result in inhibition of cancer cell growth and enhanced cell apoptosis supporting the development of novel drugs targeting Stat3 for cancer therapy. G-quartet oligodeoxynucleotides (ODNs) were demonstrated to be potent inhibitors of Stat3 DNA binding activity in vitro with the G-quartet ODN, T40214, having an IC(50) of 7 microM. Computer-simulated docking studies indicated that G-quartet ODNs mainly interacted with the SH2 domain of Stat3 and were capable of inserting between the SH2 domains of Stat3 dimers bound to DNA. We demonstrated that the G-rich ODN T40214, which forms a G-quartet structure at intracellular but not extracellular K+ ion concentrations, is delivered efficiently into the cytoplasm and nucleus of cancer cells where it inhibited IL-6-stimulated Stat3 activation and suppressed Stat3-mediated upregulation of bcl-x and mcl-1 gene expression. Thus, G-quartet represents a new class of drug for targeting of Stat3 within cancer cells.

N-domain-dependent nonphosphorylated STAT4 dimers required for cytokine-driven activation

The N-terminal protein interaction domain (N-domain) of the signal transducer and activator of transcription-4 (STAT4) is believed to stabilize interactions between two phosphorylated STAT4 dimers to form STAT4 tetramers. Here, we show that nonphosphorylated STAT4 dimers form in vivo before cytokine receptor-driven activation. Mutations in the N-domain dimerization interface abolished assembly of nonphosphorylated STAT4 dimers and prevented STAT4 phosphorylation mediated by cytokine receptors. In addition, N-domain dimerization occurred for other STAT family members but was homotypic in character. This implies a conserved role for N-domain dimerization, which might include influencing interactions with cytokine receptors, favoring homodimer formation or accelerating formation of the phosphorylated STAT dimer.

STAT proteins: from normal control of cellular events to tumorigenesis

Signal transducers and activators of transcription (STAT) proteins comprise a family of transcription factors latent in the cytoplasm that participate in normal cellular events, such as differentiation, proliferation, cell survival, apoptosis, and angiogenesis following cytokine, growth factor, and hormone signaling. STATs are activated by tyrosine phosphorylation, which is normally a transient and tightly regulates process. Nevertheless, several constitutively activated STATs have been observed in a wide number of human cancer cell lines and primary tumors, including blood malignancies and solid neoplasias. STATs can be divided into two groups according to their specific functions. One is made up of STAT2, STAT4, and STAT6, which are activated by a small number of cytokines and play a distinct role in the development of T-cells and in IFNgamma signaling. The other group includes STAT1, STAT3, and STAT5, activated in different tissues by means of a series of ligands and involved in IFN signaling, development of the mammary gland, response to GH, and embriogenesis. This latter group of STATS plays an important role in controlling cell-cycle progression and apoptosis and thus contributes to oncogenesis. Although an increased expression of STAT1 has been observed in many human neoplasias, this molecule can be considered a potential tumor suppressor, since it plays an important role in growth arrest and in promoting apoptosis. On the other hand, STAT3 and 5 are considered as oncogenes, since they bring about the activation of cyclin D1, c-Myc, and bcl-xl expression, and are involved in promoting cell-cycle progression, cellular transformation, and in preventing apoptosis.

Distinct transcriptional activation functions of STAT1alpha and STAT1beta on DNA and chromatin templates

Interferon-induced transcription depends upon tyrosine phosphorylation, subsequent dimerization, and binding to DNA of STAT1. Other factors, including but not necessarily limited to CBP/p300, then bind within the C-terminal 38 amino acid transactivation domain (TAD) to activate transcription. We show that both tyrosine-phosphorylated STAT1alpha (full-length wild-type protein) and STAT1beta (lacking the TAD) stimulate in vitro transcription on a naked DNA template. Furthermore, in a system with purified proteins and naked DNA, STAT1alpha- and STAT1beta-dependent transcription is stimulated by the TRAP/Mediator co-activator complex. Thus STAT1, through some site other than the C-terminal TAD, can interact with TRAP/Mediator or some intermediate protein. Although both STAT1alpha and STAT1beta bind to known STAT sites within in vitro assembled chromatin templates, only STAT1alpha, and not STAT1beta, in cooperation with p300 and acetyl-CoA, stimulated in vitro transcription from chromatin. After interferon-gamma treatment, cells recruit STAT1alpha or -beta to the chromosomal interferon-1 gene, but only STAT1alpha-containing cells recruit p300 and stimulate transcription. We conclude that chromatin remodeling by p300 in vivo makes TRAP/Mediator effective in stimulating transcription.

Interferon-gamma: an overview of signals, mechanisms and functions

Interferon-gamma (IFN-gamma) coordinates a diverse array of cellular programs through transcriptional regulation of immunologically relevant genes. This article reviews the current understanding of IFN-gamma ligand, receptor, signal transduction, and cellular effects with a focus on macrophage responses and to a lesser extent, responses from other cell types that influence macrophage function during infection. The current model for IFN-gamma signal transduction is discussed, as well as signal regulation and factors conferring signal specificity. Cellular effects of IFN-gamma are described, including up-regulation of pathogen recognition, antigen processing and presentation, the antiviral state, inhibition of cellular proliferation and effects on apoptosis, activation of microbicidal effector functions, immunomodulation, and leukocyte trafficking. In addition, integration of signaling and response with other cytokines and pathogen-associated molecular patterns, such as tumor necrosis factor-alpha, interleukin-4, type I IFNs, and lipopolysaccharide are discussed.

Differential roles of C-terminal activation motifs in the establishment of Stat6 transcriptional specificity

Members of the Stat transcription factor family are specifically activated by cytokines, and each Stat mediates its biological effects through the trans-activation of a unique profile of target genes. This specificity is achieved even when Stat proteins mediating opposite transcriptional effects bind to the same palindromic Stat sites in target genes. We show here that the non-conserved sequences of Stat transcription activation domains (TADs) contribute to specificity in promoter activation. Chimeric proteins in which the Stat6 TAD was replaced by that from Stat1alpha or Stat5 exhibited normal interleukin-4-inducible DNA binding activity, but at best modest trans-activation of reporters containing Stat6 binding sites, and a failure to activate the endogenous CD23 promoter in primary B cells. The p160 coactivator nuclear coactivator-1 (Src-1) was specifically recruited by and coactivated Stat6 but not the chimeric Stat6 molecules. Strikingly, transcriptional responses exhibited distinct requirements for the nuclear coactivator-1 interaction motif of the Stat6 C terminus. Together, these findings indicate that the Stat6 TAD contributes to promoter specificity by the differential recruitment of and requirement for a p160-class coactivator.

Functional interaction trap: a strategy for validating the functional consequences of tyrosine phosphorylation of specific substrates in vivo

Protein tyrosine phosphorylation controls diverse signaling pathways, and disregulated tyrosine kinase activity plays a direct role in human diseases such as cancer. Because activated kinases exert their effects by phosphorylating multiple substrate proteins, it is difficult or impossible to assess experimentally the contribution of a particular substrate to a cellular response or activity. To overcome this problem, we have developed a novel approach termed the "functional interaction trap," in which two proteins are induced to interact in a pairwise fashion through an engineered, highly specific binding interface. We show that the functional interaction trap can be used to direct a modified tyrosine kinase to specifically phosphorylate a single substrate of choice in vivo, permitting analysis of the resulting biological output. This strategy provides a powerful tool for validating the functional significance of tyrosine phosphorylation and other post-translational modifications identified by proteomic discovery efforts.

Principles of interleukin (IL)-6-type cytokine signalling and its regulation

The IL (interleukin)-6-type cytokines IL-6, IL-11, LIF (leukaemia inhibitory factor), OSM (oncostatin M), ciliary neurotrophic factor, cardiotrophin-1 and cardiotrophin-like cytokine are an important family of mediators involved in the regulation of the acute-phase response to injury and infection. Besides their functions in inflammation and the immune response, these cytokines play also a crucial role in haematopoiesis, liver and neuronal regeneration, embryonal development and fertility. Dysregulation of IL-6-type cytokine signalling contributes to the onset and maintenance of several diseases, such as rheumatoid arthritis, inflammatory bowel disease, osteoporosis, multiple sclerosis and various types of cancer (e.g. multiple myeloma and prostate cancer). IL-6-type cytokines exert their action via the signal transducers gp (glycoprotein) 130, LIF receptor and OSM receptor leading to the activation of the JAK/STAT (Janus kinase/signal transducer and activator of transcription) and MAPK (mitogen-activated protein kinase) cascades. This review focuses on recent progress in the understanding of the molecular mechanisms of IL-6-type cytokine signal transduction. Emphasis is put on the termination and modulation of the JAK/STAT signalling pathway mediated by tyrosine phosphatases, the SOCS (suppressor of cytokine signalling) feedback inhibitors and PIAS (protein inhibitor of activated STAT) proteins. Also the cross-talk between the JAK/STAT pathway with other signalling cascades is discussed.

STATs dimerize in the absence of phosphorylation

Upon activation by tyrosine kinases, members of the STAT family of transcription factors form stable dimers that are able to rapidly translocate to the nucleus and bind DNA. Although crystal structures of activated, near full-length, Stat1 and Stat3 illustrate how STATs bind to DNA, they provide little insight into the dynamic regulation of STAT activity. To explore the unique structural changes Stat1 and Stat3 undergo when they become activated, full-length inactive recombinant proteins were prepared. To our surprise, even though these proteins are unable to bind DNA, our studies demonstrate that they exist as stable homodimers. Similarly, the Stat1 and Stat3 found in the cytoplasm of unstimulated cells also exhibit a dimeric structure. These observations indicate that Stat1 and Stat3 exist as stable homodimers prior to activation.

STAT4 requires the N-terminal domain for efficient phosphorylation

STAT4 (signal transducer and activator of transcription-4) mediates biological effects in response to interleukin-12 (IL-12). STAT4 has multiple domains that have distinct functions in signaling and gene activation. To characterize the role of the STAT4 N-terminal domain in mediating STAT4 biological function, we have generated STAT4-deficient transgenic mice that express human full-length STAT4 or an N-terminal deletion mutant (Delta N-STAT4) lacking the N-terminal 51 amino acids. Whereas full-length STAT4 rescued IL-12 responsiveness, T lymphocytes expressing the STAT4 N-terminal mutant failed to proliferate in response to IL-12 and had limited Th1 cell development as evidenced by minimal interferon-gamma production. Deletion of the N-terminal domain resulted in failure of STAT4 to be phosphorylated following IL-12 stimulation despite similar phosphorylation of JAK2 and TYK2 in full-length STAT4 and Delta N-STAT4 transgenic T cells. We demonstrate that the lack of phosphorylation in cultured cells is due to reduced efficiency of phosphorylation of Delta N-STAT4 by Janus kinases. These data support a new model wherein the N-terminal domain is required to mediate the phosphorylation of STAT4 in addition to the previously documented role in gene transactivation.

DNA binding controls inactivation and nuclear accumulation of the transcription factor Stat1

Cytokine-dependent gene transcription greatly depends on the tyrosine phosphorylation ("activation") of Stat proteins at the cell membrane. This rapidly leads to their accumulation in the nucleus by an unknown mechanism. We performed microinjections of recombinant Stat1 protein to show that nuclear accumulation of phosphorylated Stat1 can occur without cytokine stimulation of cells. Microinjection of Stat1 antibody and treatment of cells with kinase or phosphatase inhibitors revealed that nuclear accumulation is a highly dynamic process sustained by Stat1 nucleocytoplasmic cycling and continuous kinase activity. By characterizing nuclear accumulation mutants, it is demonstrated that nuclear import and nuclear retention are two separate steps leading up to nuclear accumulation, with nonspecific DNA binding of activated Stat1 being sufficient for nuclear retention. Critical for nuclear buildup of Stat1 and the subsequent nuclear export is the point of time of tyrosine dephosphorylation, because our data indicate that activated Stat1 is incapable of leaving the nucleus and requires dephosphorylation to do so. It is demonstrated that the inactivation of Stat1 is controlled by its exchange reaction with DNA, whereby DNA binding protects Stat1 from dephosphorylation in a sequence-specific manner. Thus, during nuclear accumulation, a surprisingly simple mechanism integrates central aspects of cytokine-dependent gene regulation, for example, receptor monitoring, promoter occupancy, and transcription factor inactivation.

Principles of interleukin (IL)-6-type cytokine signalling and its regulation

The IL (interleukin)-6-type cytokines IL-6, IL-11, LIF (leukaemia inhibitory factor), OSM (oncostatin M), ciliary neurotrophic factor, cardiotrophin-1 and cardiotrophin-like cytokine are an important family of mediators involved in the regulation of the acute-phase response to injury and infection. Besides their functions in inflammation and the immune response, these cytokines play also a crucial role in haematopoiesis, liver and neuronal regeneration, embryonal development and fertility. Dysregulation of IL-6-type cytokine signalling contributes to the onset and maintenance of several diseases, such as rheumatoid arthritis, inflammatory bowel disease, osteoporosis, multiple sclerosis and various types of cancer (e.g. multiple myeloma and prostate cancer). IL-6-type cytokines exert their action via the signal transducers gp (glycoprotein) 130, LIF receptor and OSM receptor leading to the activation of the JAK/STAT (Janus kinase/signal transducer and activator of transcription) and MAPK (mitogen-activated protein kinase) cascades. This review focuses on recent progress in the understanding of the molecular mechanisms of IL-6-type cytokine signal transduction. Emphasis is put on the termination and modulation of the JAK/STAT signalling pathway mediated by tyrosine phosphatases, the SOCS (suppressor of cytokine signalling) feedback inhibitors and PIAS (protein inhibitor of activated STAT) proteins. Also the cross-talk between the JAK/STAT pathway with other signalling cascades is discussed.

DNA binding activity of cytoplasmic phosphorylated Stat6 is masked by an interaction with a detergent-sensitive factor

Signal transducer and activator of transcription (Stat) 6 is vital to interleukin (IL)-4 and IL-13 responses and the generation of Th2 immunity. We investigated the cellular location of phosphorylated Stat6 and Stat6 DNA binding activity in A201.1 murine B cells and primary splenocytes. Phosphorylated Stat6 was present in cytoplasmic and nuclear extracts from IL-4-treated cells. Confocal microscopy confirmed the presence of phosphorylated Stat6 in the cytoplasm of IL-4-treated cells. In contrast, Stat6 DNA binding activity was present in nuclear extracts, but not in cytoplasmic extracts. Thus, cytoplasmic extracts from IL-4-stimulated cells were devoid of Stat6 DNA binding activity despite the presence of phosphorylated Stat6. Addition of cytoplasmic extracts to nuclear extracts did not inhibit Stat6 DNA binding present in the nuclear extracts. Detergent treatment restored Stat6 DNA binding activity in cytoplasmic extracts of IL-4-stimulated cells. Thus, DNA binding activity of cytoplasmic phosphorylated Stat6 is masked by a factor dissociable by detergent treatment.

The ins and outs of STAT1 nuclear transport

There is an inherent elegance in being in the right place at the right time. The STAT1 transcription factor possesses regulatory signals that ensure its distribution to the right cellular location at the right time. Latent STAT1 resides primarily in the cytoplasm, and there it responds to hormone signaling through tyrosine phosphorylation by Janus kinases or growth factor receptors. After phosphorylation, STAT1 dimerizes, and this conformational change reveals a nuclear import signal that is recognized by a specific nuclear import carrier. In the nucleus, the STAT1 dimer dissociates from the import carrier and binds to specific DNA target sites in the promoters of regulated genes. STAT1 is subsequently dephosphorylated in the nucleus by a constitutively active tyrosine phosphatase, leading to its dissociation from DNA. A nuclear export signal of STAT1 appears to be masked when dimers are bound to DNA, but it becomes accessible to the CRM1 export carrier after dissociation from DNA. CRM1 binds STAT1 and transports the transcription factor back to the cytoplasm. Studies show that the regulatory trafficking signals that guide the nuclear import and export of STAT1 reside within its DNA binding domain. The location of these signals indicates that their function has coevolved with the ability of STAT1 to bind DNA and regulate gene expression. The nuclear import and subsequent recycling of STAT1 to the cytoplasm are integral to its function as a signal transducer and activator of transcription.

SUMO modification of STAT1 and its role in PIAS-mediated inhibition of gene activation

The PIAS (protein inhibitors of activated STAT) family of proteins were first discovered as inhibitors of activated signal transducers and activators of transcription (STATs). More recently these proteins have been shown to function as E3 ligases that promote the SUMO modification of a number of transcription regulators. We have investigated the relationship between the effects of PIAS proteins on STAT1 transcriptional activity and the ability of the PIAS proteins to function as SUMO E3 ligases. We demonstrate that STAT1 is a substrate for SUMO modification and that PIASx-alpha, but not PIAS1, functions as an E3 ligase to promote STAT1 modification. In addition, we have mapped the major site for SUMO modification on STAT1 to lysine 703. This lysine residue is in close proximity to the regulatory tyrosine residue at position 701, whose phosphorylation mediates STAT1 activation in response to cytokine signaling. Mutation of lysine 703 to arginine abolishes SUMO modification of STAT1 both in vitro and in vivo. However, this mutation does not affect the activation of STAT1 or the ability of either PIAS1 or PIASx-alpha to function as an inhibitor of STAT1-mediated transcription activation. Our findings demonstrate that inhibition of STAT1 by PIAS proteins does not require SUMO modification of STAT1 itself. SUMO modification of STAT1 may nonetheless be functionally important given the close proximity between the SUMO modification site and tyrosine 701.

A novel sequence in the coiled-coil domain of Stat3 essential for its nuclear translocation

Stat3 is activated by cytokines and growth factors via specific tyrosine phosphorylation, dimerization, and nuclear translocation. However, the mechanism involved in its nuclear translocation is unclear. In this study, by systematic deletion and site-directed mutagenesis we identified Arg-214/215 in the alpha-helix 2 region of the coiled-coil domain of Stat3 as a novel sequence element essential for its nuclear translocation, stimulated by epidermal growth factor as well as by interleukin-6. Furthermore, we identified Arg-414/417 in the DNA binding domain as also required for the nuclear localization of Stat3. This sequence element corresponds to Lys-410/413 of Stat1, a reported sequence for Stat1 nuclear translocation. On the other hand, Leu-411 of Stat3, corresponding to Leu-407 of Stat1, a necessary residue for Stat1 nuclear transport, is not essential for Stat3 nuclear import. The mutant of Arg-214/215 or Arg-414/417 was shown to be tyrosyl-phosphorylated normally but failed to enter the nucleus in response to epidermal growth factor or interleukin-6. The defect, however, can be rescued by the wild-type Stat3 but cannot be compensated by these two mutants. Mutations on Arg-414/417, but not Arg-214/215, destroy the DNA binding activity of Stat3. Our data for the first time identified a sequence element located in the coiled-coil domain that is involved in the ligand-induced nuclear translocation of Stat3. This novel sequence together with a conserved sequence element in the DNA binding domain coordinates to mediate the nuclear translocation of Stat3.

Leptin-induced transactivation of NPY gene promoter mediated by JAK1, JAK2 and STAT3 in the neural cell lines

Neuropeptide Y (NPY) plays an important role in the central and sympathetic regulation of food intake and blood pressure. Although the NPY gene expression is regulated by a number of agents such as leptin, the mechanism responsible for leptin-induced regulation of the transcription of the NPY gene remains to be explored. In this study, the NPY gene promoter was transactivated by leptin in N18TG2, NG108-15 and PC12 cells which expressed the functional leptin receptor. The long isoform of leptin receptor (OB-Rb) could induce the transactivation, but the C-terminal truncated form (OB-Ra) could not. When dominant negative type of STAT3, JAK1 or JAK2 and was co-expressed, the leptin-induced transactivation was suppressed almost completely. The leptin-response element which confers NPY gene transactivation by leptin was determined in the 221-bp region of rat NPY gene promoter (-553/-335), where two STAT3-binding site-like elements (TCCAGTA) exist. These results indicated that activation of JAK1, JAK2 and STAT3 is necessary for leptin-induced transactivation of NPY gene through the leptin-response element in these neural cells.

Signal transduction pathways regulated by prolactin and Src result in different conformations of activated Stat5b

Stat5 is activated by a broad spectrum of cytokines, as well as non-receptor tyrosine kinases, such as Src. In this study, the DNA binding properties of the two closely related Stat5 proteins, Stat5a and Stat5b, induced either by prolactin (Prl) or by Src were analyzed by electrophoretic mobility shift assays using several different Stat5 binding sites. Src-induced Stat5b-DNA binding complexes consistently displayed a slightly faster mobility than those induced by Prl, as well as differences in their ability to be supershifted by anti-Stat5 antibodies. IP-Westerns performed using specific antibodies directed at the N and C termini of Stat5b suggested that depending on the activating stimulus, Stat5b exhibited different conformations, which influenced antibody accessibility at its C terminus. These conformational differences may in part be due to differential effects of Prl and Src on Stat5b tyrosine phosphorylation, since Src induced several additional sites of tyrosine phosphorylation of Stat5b at residues other than Tyr-699, including Tyr-724 and Tyr-679. The latter Tyr-679 is conserved in all mammalian Stat5bs, but is not present in Stat5a. A Stat 5bY679F mutant induced by Src kinase exhibited an altered pattern of nuclear localization as compared with wild-type Stat5b. Furthermore, this mutation inhibited v-Src-induced cyclin D1-luciferase reporter activity in transient transfection assays performed in Stat5a/b-deficient MEFs, suggesting that Tyr-679 phosphorylation may play a role in v-Src induced proliferation. Thus, depending on the signal transduction pathway responsible for activation, different conformations of activated Stat5 may result in selective biological responses.

Characterization of phosphopeptide motifs specific for the Src homology 2 domains of signal transducer and activator of transcription 1 (STAT1) and STAT3

Signal transducers and activators of transcription (STAT) 1 and STAT3 are activated by overlapping but distinct sets of cytokines. STATs are recruited to the different cytokine receptors through their Src homology (SH) 2 domains that make highly specific interactions with phosphotyrosine-docking sites on the receptors. We used a degenerate phosphopeptide library synthesized on 35-microm TentaGel beads and fluorescence-activated bead sorting to determine the sequence specificity of the peptide-binding sites of the SH2 domains of STAT1 and STAT3. The large bead library allowed not only peptide sequencing of pools of beads but also of single beads. The method was validated through surface plasmon resonance measurements of the affinities of different peptides to the STAT SH2 domains. Furthermore, when selected peptides were attached to a truncated erythropoietin receptor and stably expressed in DA3 cells, activation of STAT1 or STAT3 could be achieved by stimulation with erythropoietin. The combined analysis of pool sequencing, the individual peptide sequences, and plasmon resonance measurements allowed the definition of SH2 domain binding motifs. STAT1 preferentially binds peptides with the motif phosphotyrosine-(aspartic acid/glutamic acid)-(proline/arginine)-(arginine/proline/glutamine), whereby a negatively charged amino acid at +1 excludes a proline at +2 and vice versa. STAT3 preferentially binds peptides with the motif phosphotyrosine-(basic or hydrophobic)-(proline or basic)-glutamine. For both STAT1 and STAT3, specific high affinity phosphopeptides were identified that can be used for the design of inhibitory molecules.

Molecular recognition by SH2 domains

In this chapter, we have described the biophysical investigations which have dissected the mechanisms of SH2 domain function. Due to nearly a decade and a half of investigation on SH2 domains, much about their binding mechanism has been characterized. SH2 domains have been found to have a positively charged binding cavity, largely conserved between different SH2 domains, which coordinates binding of the pTyr in the target. The ionic interactions between this pocket and the pTyr, in particular, between Arg beta B5 and the phosphate, provide the majority of the binding energy stabilizing SH2 domain-target interactions. The specificity in SH2 domain-target interactions emanates most often from the interactions between the residues C-terminal to the pTyr in the target and the specificity determining residues in the C-terminal half of the SH2 domain. However, the interactions in the specificity determining region of SH2 domains are weak, and hence single SH2 domains show only a modest level of specificity for tyrosine phosphorylated targets. Greater specificity in SH2 domain-containing protein-tyrosine phosphorylated target interactions can be achieved by placing SH2 domains in tandem (as is often found) or possibly through specific localization of SH2 domain-containing proteins within the cell. Although a relatively good understanding of how SH2 domains function in isolation has been obtained, the ways in which SH2 domain binding is coupled to allosteric transmission of signals in larger SH2 domain-containing proteins are still not clear. Hence, the future should bring further investigations of the mechanisms by which SH2 domain ligation alters the enzymatic activity and cellular localization of SH2 domain-containing proteins.

Signal transducer and activator of transcription proteins in leukemias

Signal transducer and activator of transcription (STAT) proteins are a 7-member family of cytoplasmic transcription factors that contribute to signal transduction by cytokines, hormones, and growth factors. STAT proteins control fundamental cellular processes, including survival, proliferation, and differentiation. Given the critical roles of STAT proteins, it was hypothesized that inappropriate or aberrant activation of STATs might contribute to cellular transformation and, in particular, leukemogenesis. Constitutive activation of mutated STAT3 has in fact been demonstrated to result in transformation. STAT activation has been extensively studied in leukemias, and mechanisms of STAT activation and the potential role of STAT signaling in leukemogenesis are the focus of this review. A better understanding of mechanisms of dysregulation of STAT signaling pathways may serve as a basis for designing novel therapeutic strategies that target these pathways in leukemia cells.

IL-13 receptors and signaling pathways: an evolving web

IL-13 is an immunoregulatory cytokine secreted predominantly by activated T(H)2 cells. Over the past several years, it has become evident that IL-13 is a key mediator in the pathogenesis of allergic inflammation. IL-13 shares many functional properties with IL-4, stemming from the fact that they share a common receptor subunit, the alpha subunit of the IL-4 receptor (IL-4Ralpha). Characterization of IL-13-deficient mice, IL-4-deficient mice, and IL-4 receptor alpha-deficient (IL-4Ralpha(-/-)) mice have demonstrated nonredundant roles for IL-13. IL-13 mediates its effects by interacting with a complex receptor system comprised of IL-4Ralpha and two IL-13 binding proteins, IL-13Ralpha1 and IL-13Ralpha2. IL-13 receptors are expressed on human B cells, basophils, eosinophils, mast cells, endothelial cells, fibroblasts, monocytes, macrophages, respiratory epithelial cells, and smooth muscle cells. However, functional IL-13 receptors have not been demonstrated on human or mouse T cells. Thus unlike IL-4, IL-13 does not appear to be important in the initial differentiation of CD4 T cells into T(H)2-type cells but rather appears to be important in the effector phase of allergic inflammation. This is further supported by many in vivo observations, including that administration of IL-13 resulted in allergic inflammation, tissue-specific overexpression of IL-13 in the lungs of transgenic mice resulted in airway inflammation and mucus hypersecretion, IL-13 blockade abolished allergic inflammation independently of IL-4, and IL-13 appears to be more important than IL-4 in mucus hypersecretion. Given the importance of IL-13 as an effector molecule, regulation at the level of its receptors might be an important mechanism of modulating IL-13 responses and thus propagation of the allergic response. Accordingly, IL-13 is an attractive, novel therapeutic target for pharmacologic intervention in allergic disorders. This review will summarize the current understanding of the IL-13 receptors and signaling pathways, emphasizing recent observations.

GRIM-19, a death-regulatory gene product, suppresses Stat3 activity via functional interaction

Signal transducer and activator of transcription 3 (Stat3) is a latent cytoplasmic transcription factor that can be activated by cytokines and growth factors. Stat3 plays important roles in cell growth, anti-apoptosis and cell transformation, and is constitutively active in various cancers. We examined its potential regulators by yeast two-hybrid screening. GRIM-19, a gene product related to interferon-beta- and retinoic acid-induced cancer cell death, was identified and demonstrated to interact with Stat3 in various cell types. The interaction is specific for Stat3, but not for Stat1 and Stat5a. The interaction regions in both proteins were mapped, and the cellular localization of the interaction was examined. GRIM-19 itself co-localizes with mitochondrial markers, and forms aggregates at the perinulear region with co-expressed Stat3, which inhibits Stat3 nuclear translocation stimulated by epidermal growth factor (EGF). GRIM-19 represses Stat3 transcriptional activity and its target gene expression, and also suppresses cell growth in Src-transformed cells and a Stat3-expressing cell line. Our data suggest that GRIM-19 is a novel negative regulator of Stat3.

Impaired response to interferon-alpha/beta and lethal viral disease in human STAT1 deficiency

The receptors for interferon-alpha/beta (IFN-alpha/beta) and IFN-gamma activate components of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway, leading to the formation of at least two transcription factor complexes. STAT1 interacts with STAT2 and p48/IRF-9 to form the transcription factor IFN-stimulated gene factor 3 (ISGF3). STAT1 dimers form gamma-activated factor (GAF). ISGF3 is induced mainly by IFN-alpha/beta, and GAF by IFN-gamma, although both factors can be activated by both types of IFN. Individuals with mutations in either chain of the IFN-gamma receptor (IFN-gammaR) are susceptible to infection with mycobacteria. A heterozygous STAT1 mutation that impairs GAF but not ISGF3 activation has been found in other individuals with mycobacterial disease. No individuals with deleterious mutations in the IFN-alpha/beta signaling pathway have been described. We report here two unrelated infants homozygous with respect to mutated STAT1 alleles. Neither IFN-alpha/beta nor IFN-gamma activated STAT1-containing transcription factors. Like individuals with IFN-gammaR deficiency, both infants suffered from mycobacterial disease, but unlike individuals with IFN-gammaR deficiency, both died of viral disease. Viral multiplication was not inhibited by recombinant IFN-alpha/beta in cell lines from the two individuals. Inherited impairment of the STAT1-dependent response to human IFN-alpha/beta thus results in susceptibility to viral disease.

A three-dimensional model of Suppressor Of Cytokine Signalling 1 (SOCS-1)

Suppressor Of Cytokine Signalling 1 (SOCS-1) is one of the proteins responsible for the negative regulation of the JAK-STAT pathway triggered by many cytokines. This important inhibition involves complex formation between SOCS-1 and JAK2, which requires particular structural domains (KIR, ESS and SH2) on SOCS-1. A three-dimensional theoretical model of SOCS-1 is presented here. The model was generated by the application of different modelling techniques, including threading, structure-based modelling, surface analysis and protein docking. The structure accounts for the interactions between SOCS-1 and two other key proteins in the JAK-STAT pathway, namely JAK2 and Elongin BC. The proposed model for the interaction between SOCS-1 and JAK2 suggests that the SOCS-1 suppress the kinase activity of JAK2 by obstructing the catalytic groove of the tyrosine kinase. Subsequent interaction of the JAK-SOCS complex with Elongin BC was also modelled. A sequence and structural comparison between the SH2 domain of SOCS-1 and the SH2 domains of other proteins highlights key residues that could be responsible for SOCS-1 specificity. Currently available mutational data are evaluated. The results are consistent with the experimental data and they provide deeper insights into the inhibitory function of SOCS-1 at a molecular level.

Structural modeling of ataxin-3 reveals distant homology to adaptins

Spinocerebellar ataxia type 3 (SCA3) is a polyglutamine disorder caused by a CAG repeat expansion in the coding region of a gene encoding ataxin-3, a protein of yet unknown function. Based on a comprehensive computational analysis, we propose a structural model and structure-based functions for ataxin-3. Our predictive strategy comprises the compilation of multiple sequence and structure alignments of carefully selected proteins related to ataxin-3. These alignments are consistent with additional information on sequence motifs, secondary structure, and domain architectures. The application of complementary methods revealed the homology of ataxin-3 to ENTH and VHS domain proteins involved in membrane trafficking and regulatory adaptor functions. We modeled the structure of ataxin-3 using the adaptin AP180 as a template and assessed the reliability of the model by comparison with known sequence and structural features. We could further infer potential functions of ataxin-3 in agreement with known experimental data. Our database searches also identified an as yet uncharacterized family of proteins, which we named josephins because of their pronounced homology to the Josephin domain of ataxin-3.

A reinterpretation of the dimerization interface of the N-terminal domains of STATs

The crystal structures of the N-terminal domain (N-domain) and the core region of the STAT family of transcription factors have been determined previously. STATs can form cooperative higher order structures (tetramers or higher oligomers) while bound to DNA. The crystal packing in the STAT4 N-domain crystal structure, determined at 1.5 A resolution, suggests two alternate organizations of the N-domain dimer. We now present the results of site directed mutagenesis of residues predicted to be involved at each dimer interface. Our results indicate that the dimer interface suggested earlier as being physiologically relevant is, in fact, unlikely to be so. Given the alternative model for the N-domain dimer, the ability of the N-domain to mediate interactions of two STAT dimers on DNA remains unchanged.

Protein kinase C delta associates with the interleukin-6 receptor subunit glycoprotein (gp) 130 via Stat3 and enhances Stat3-gp130 interaction

The transcriptional regulation of Stat proteins is controlled through their C-terminal domains, which harbor both a tyrosine phosphorylation site, required for dimerization and subsequent nuclear translocation, and a serine phosphorylation site, required for maximum transcriptional activity. Previously, we reported that protein kinase Cdelta (PKCdelta) phosphorylates and interacts with Stat3 in an interleukin (IL)-6-dependent manner. In this study, we further characterized this interaction, and investigated the potential role of such an interaction. We show here that the catalytic domain of PKCdelta interacts with the Src homology 2 domain and part of the adjacent C-terminal transactivation domain of Stat3. This interaction, which does not seem to involve a classical phosphotyrosine SH2-mediated binding, however, significantly enhances the interaction of Stat3 and the IL-6 receptor subunit glycoprotein (gp) 130, which is the initial step for Stat3 activation by IL-6. Expression of a dominant negative PKCdelta or depletion of the endogenous PKCdelta by phorbol 12-myristate 3-acetate treatment abrogates the association of Stat3 with gp130. At the same time, PKCdelta is recruited to gp130 via association with Stat3, which may facilitate its phosphorylation on the gp130 receptor. Finally, we identified Thr-890, a putative PKC phosphorylation site on gp130, to be critical for the effect of PKCdelta. Our data indicate that PKCdelta plays important regulatory roles in IL-6 signaling.

Structure of the sporulation-specific transcription factor Ndt80 bound to DNA

Progression through the middle phase of sporulation in Saccharomyces cerevisiae is promoted by the successful completion of recombination at the end of prophase I. Completion of meiotic recombination allows the activation of the sporulation-specific transcription factor Ndt80, which binds to a specific DNA sequence, the middle sporulation element (MSE), and activates approximately 150 genes to enable progression through meiosis. Here, we isolate the DNA-binding domain of Ndt80 and determine its crystal structure both free and in complex with an MSE-containing DNA. The structure reveals that Ndt80 is a member of the Ig-fold family of transcription factors. The structure of the DNA-bound form, refined at 1.4 A, reveals an unexpected mode of recognition of 5'-pyrimidine- guanine-3' dinucleotide steps by arginine residues that simultaneously recognize the 3'-guanine base through hydrogen bond interactions and the 5'-pyrimidine through stacking/van der Waals interactions. Analysis of the DNA-binding affinities of MSE mutants demonstrates the central importance of these interactions, and of the AT-rich portion of the MSE. Functional similarities between Ndt80 and the Caenorhabditis elegans p53 homolog suggest an evolutionary link between Ndt80 and the p53 family.

Analysis of the life cycle of stat6. Continuous cycling of STAT6 is required for IL-4 signaling

Signal transducer and activator of transcription (Stat)6 is a transcription factor important for the development of Th2 cells and regulation of gene expression by IL-4 and IL-13. It is known that Stat6 is rapidly activated in response to IL-4; however, the fate of activated Stat6 is less clear. We examined the fate of activated Stat6 and found that during continuous exposure to IL-4, Stat6 activity was sustained for 72 h and that the maintenance of a constant level of activated Stat6 did not require new protein synthesis. In contrast, when cells were pulsed with IL-4 and then incubated in the absence of IL-4, the half-life of Stat6 phosphorylation and DNA binding activity was less than 1 h. Stat6 did not accumulate in the nucleus, and protein degradation did not play a major role in the disappearance of activated Stat6. Inhibition of kinase activity by staurosporine or the JAK inhibitor, AG490, revealed that maintenance of Stat6 activation in the continuous presence of IL-4 required ongoing phosphorylation of latent cytoplasmic Stat6 molecules. Cells treated with an inhibitor of nuclear export, leptomycin B, were unable to maintain Stat6 activation. Thus, the maintenance of Stat6 activation requires a constant cycle of activation, deactivation, nuclear export, and reactivation.

Chromatin-remodelling factor BRG1 selectively activates a subset of interferon-α-inducible genes

Brahma-related gene 1 (BRG1 ) is a key component of the ATP-dependent chromatin-remodelling SWI2-SNF2 complex and has been implicated in regulating gene expression, cell-cycle control and tumorigenesis. Here we report that BRG1 interacts with signal transducer and activator of transcription 2 (STAT2) - a transcription factor that regulates gene expression mediated by interferon-alpha (IFN-alpha). BRG1 enhances the IFN-alpha-induced expression of 9-27 and IFI27 but not that of four other target genes tested, showing that the activation of different target genes by STAT2 may involve alternative chromatin modifiers. Our results also suggest that the recruitment and activation of BRG1 may require other cis-acting and trans-acting elements in addition to STAT2. Our study links the SWI2-SNF2 complex to the regulation of cytokine-induced gene expression and may identify a molecular mechanism of BRG1-mediated gene activation and tumorigenesis.

Negative regulation of STAT92E by an N-terminally truncated STAT protein derived from an alternative promoter site

Previously unrecognized mRNAs originating from a dual promoter at the stat92E locus are described. One of these encodes a truncated protein, DeltaNSTAT92E, that lacks the N-terminal 133 amino acids. Antibodies detect both the full-length and truncated molecules early in embryogenesis (1-5 h), and mRNA detection by specific RT-PCR reactions accords with the protein distribution. Given that the N termini of mammalian STATs are known to have positive functions in transcriptional activation, we explored the role of DeltaNSTAT92E early in embryogenesis. By increasing the DeltaNSTAT92E-to-STAT92E ratio in overexpression and RNAi experiments, we observe phenotypes compatible with suppression of wild-type STAT92E activity. We therefore conclude that the short form of STAT92E is a naturally occurring dominant-negative product that can be added to the growing list of negative regulators of STAT activity.

Stats: transcriptional control and biological impact

Extracellular proteins bound to cell-surface receptors can change nuclear gene expression patterns in minutes, with far-reaching consequences for development, cell growth and homeostasis. The signal transducer and activator of transcription (STAT) proteins are among the most well studied of the latent cytoplasmic signal-dependent transcription-factor pathways. In addition to several roles in normal cell decisions, dysregulation of STAT function contributes to human disease, making the study of these proteins an important topic of current research.

Arginine/lysine-rich nuclear localization signals mediate interactions between dimeric STATs and importin alpha 5

Interferon stimulation results in tyrosine phosphorylation, dimerization, and nuclear import of STATs (signal transducers and activators of transcription). Proteins to be targeted into the nucleus usually contain nuclear localization signals (NLSs), which interact with importin alpha. Importin alpha binds to importin beta, which docks the protein complex to nuclear pores, and the complex translocates into the nucleus. Here we show that baculovirus-produced and -activated STAT1 homodimers and STAT1-STAT2 heterodimers directly interacted with importin alpha 5 (NPI-1). This interaction was very stable and was dependent on lysines 410 and 413 of STAT1. Only STAT dimers that had two intact NLS elements, one in each monomer, were able to bind to importin alpha 5. STAT-importin alpha 5 complexes apparently consisted of two STAT and two importin alpha molecules. STAT NLS-dependent colocalization of importin alpha 5 with STAT1 or STAT2 was seen in the nucleus of transfected cells. gamma-Activated sequence DNA elements efficiently inhibited STAT binding to importin alpha 5 suggesting that the DNA and importin alpha binding sites are close to each other in STAT dimers. Our results demonstrate that specific NLSs in STATs mediate direct interactions of STAT dimers with importin alpha, which activates the nuclear import process.

Chk2 activation and phosphorylation-dependent oligomerization

The tumor suppressor gene CHK2 encodes a versatile effector serine/threonine kinase involved in responses to DNA damage. Chk2 has an amino-terminal SQ/TQ cluster domain (SCD), followed by a forkhead-associated (FHA) domain and a carboxyl-terminal kinase catalytic domain. Mutations in the SCD or FHA domain impair Chk2 checkpoint function. We show here that autophosphorylation of Chk2 produced in a cell-free system requires trans phosphorylation by a wortmannin-sensitive kinase, probably ATM or ATR. Both SQ/TQ sites and non-SQ/TQ sites within the Chk2 SCD can be phosphorylated by active Chk2. Amino acid substitutions in the SCD and the FHA domain impair auto- and trans-kinase activities of Chk2. Chk2 forms oligomers that minimally require the FHA domain of one Chk2 molecule and the SCD within another Chk2 molecule. Chk2 oligomerization in vivo increases after DNA damage, and when damage is induced by gamma irradiation, this increase requires ATM. Chk2 oligomerization is phosphorylation dependent and can occur in the absence of other eukaryotic proteins. Chk2 can cross-phosphorylate another Chk2 molecule in an oligomeric complex. Induced oligomerization of a Chk2 chimera in vivo concomitant with limited DNA damage augments Chk2 kinase activity. These results suggest that Chk2 oligomerization regulates Chk2 activation, signal amplification, and transduction in DNA damage checkpoint pathways.

Interdomain interaction of Stat3 regulates its Src homology 2 domain-mediated receptor binding activity

Activation of Stat proteins by cytokines is initiated by their Src homology 2 (SH2) domain-mediated association with the cytokine receptors. Previously, we identified an essential role of the coiled-coil domain of Stat3 in binding of the receptor peptides derived from the interleukin-6 receptor subunit, gp130. In this study, we further investigated the molecular basis of this regulation. We found that the C-terminal domain of Stat3 negatively regulates its receptor binding activity only in the absence of the first alpha-helix of the coiled-coil domain, which leads to a hypothesis of intramolecular interaction. Physical interactions between the coiled-coil domain and the C-terminal domain, as well as the SH2 domain, were indeed detected. Furthermore, a sub-region of the C-terminal domain (amino acids 720-740), which is also involved in the interaction with the coiled-coil domain, was demonstrated to be critical for the regulation of the receptor binding. Correspondingly, phosphorylation on Ser-727 within this region inhibits this interaction. In agreement with the peptide binding results, both the coiled-coil domain and the C-terminal sub-region are necessary for the functional recruitment of Stat3 to the cellular gp130 in response to interleukin-6, suggesting that the interdomain interaction is a prerequisite for the SH2-mediated receptor binding in interleukin-6 signaling.