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

Novel and recurrent STAT3 mutations in hyper-IgE syndrome patients from different ethnic groups

We performed clinical, immunological and genetic studies of 12 hyper-IgE syndrome (HIES) patients from 4 Hungarian, 2 Lebanese, one Russian, one Polish, and one Swedish families with autosomal dominant (AD) or sporadic forms of the disease to reveal cross-ethnicity of recurrent and novel mutations in the signal transducer and activator of transcription-3 gene (STAT3). Four patients from 3 Hungarian families, and one Russian, and one Swedish patient carried the heterozygous R382W germline mutation at the DNA-binding site of STAT3. The recurrent V637M mutation affecting the SRC homology 2 (SH2) domain was detected in one Lebanese and one Polish family, and the V463del deletion located in the DNA-binding domain was unveiled in another Lebanese family. A novel H332Y mutation affecting the DNA-binding site of STAT3 in three Hungarian patients from a Gypsy family was also found. The segregation of this mutation with HIES, restriction fragment length polymorphism analysis of STAT3 from patients and controls and the negligible production upon IL-6 stimulation of monocyte chemotactic protein-1 by the patient's blood mononuclear cells suggested that the H332Y mutation was disease-causing. These data suggest, that dominant negative mutations of the DNA-binding and SH2 domains of STAT3 cause AD and sporadic cases of HIES in different ethnic groups with R382W as the predominant mutation found in 5 of the 9 families. Functional and genetic data support that the novel H332Y mutation may result in the loss of function of STAT3 and leads to the HIES phenotype.

Inteferons pen the JAK-STAT pathway

Characterization of how interferons (IFNs) mediate their biological response led to identification of the JAK-STAT signaling cascade, where JAKs are receptor-associated kinases and STATs the transcription factors they activate. Today, 4 JAKs and 7 STATs are known to transduce pivotal signals for the over 50 members of the four-helix bundle family of cytokines. This review will provide an overview and historical perspective of the JAK-STAT paradigm.

Overview of protein folds in the immune system

The rapid advancement of X-ray crystallography and nuclear magnetic resonance techniques in recent years has resulted in the solution of macromolecular structures at an unprecedented rate. This review aims at providing a comprehensive description of structures and folds related to the function of the immune system. Focus is placed on immunologically relevant proteins such as immunoreceptors and major histocompatibility complexes. Information is also provided regarding protein structure data banks.

A conserved IFN-alpha receptor tyrosine motif directs the biological response to type I IFNs

Mammalian type I IFNs (IFN-Is) mediate their potent biological activities through an evolutionarily conserved IFN-alpha receptor (IFNAR), consisting of IFNAR1 and IFNAR2. These two chains direct the rapid activation of two founding members of the STAT family of transcription factors, STAT1 and STAT2. To understand how IFN-Is direct the recruitment and activation of STATs, a series of mutant murine IFNAR1 and IFNAR2 receptors were generated and evaluated in IFNAR1 and IFNAR2 knockout cells. These studies reveal that a single conserved IFNAR2 tyrosine, Y(510), plays a critical role in directing the IFN-I-dependent activation of STAT1 and STAT2, both in murine fibroblasts and macrophages. A second IFNAR2 tyrosine, Y(335), plays a more minor role. Likewise, Y(510) > Y(335) play a critical role in the induction of genes and antiviral activity traditionally associated with IFN-Is.

STAT5 activity in pancreatic β-cells

Signal transducer and activator of transcription (STAT)5A and -5B are latent transcription factors activated by cytokines and hormones of the cytokine family. In pancreatic insulin-secreting β-cells, STAT5A and -5B are activated primarily by prolactin and growth hormone stimulation and are important mediators of the potent stimulation of proliferation and insulin production caused by these hormones. STAT5A and -5B are both expressed in β-cells and control the expression of a number of mRNAs implicated in cell replication control, insulin biosynthesis and secretion. In addition to STAT5A and -5B being transcriptional activators, they may also repress gene transcription. By these means, STAT5 proteins increase the levels of anti-apoptotic transcripts in β-cells and repress expression of pro-apoptotic genes. This review focuses on the anti-apoptotic role of STAT5 signaling, providing a mechanism for β-cell resistance to pro-apoptotic cytokines, Type 1 diabetes mellitus and obesity-associated β-cell stress. It is clear from studies of STAT5 signaling in pancreatic β-cells that STAT5 is important for postnatal β-cell compensatory growth (as in pregnancy or obesity) and in the defense against β-cell stress factors.

Tyrosine phosphorylation regulates the partitioning of STAT1 between different dimer conformations

The activation/inactivation cycle of STAT transcription factors entails their transition between different dimer conformations. Unphosphorylated STATs can dimerize in an antiparallel conformation via extended interfaces of the globular N-domains, whereas STAT activation triggers a parallel dimer conformation with mutual phosphortyrosine:SH2 domain interactions, resulting in DNA-binding and nuclear retention. However, despite the crucial role of STAT tyrosine phosphorylation in cytokine signaling, it has not been determined how this modification affects the stability and the conformational flexibility of STAT dimers. Here, we use analytical ultracentrifugation and electrophoretic mobility shift assay (EMSA) to study the association of STAT1 in solution before and after tyrosine phosphorylation. It is revealed that STAT1 formed high-affinity dimers (K(d) of approximately 50 nM) with estimated half-lives of 20-40 min irrespective of the phosphorylation status. Our results demonstrate that parallel and antiparallel conformations of STAT1 were present simultaneously, supported by mutually exclusive interfaces; and the transition between conformations occurred through affinity-driven dissociation/association reactions. Therefore, tyrosine phosphorylation was dispensable for DNA binding, but the phosphorylation enforced preformed SH2 domain-mediated dimers, thus enhancing the DNA-binding activity of STAT1 >200-fold. Moreover, upon STAT1 activation the N-domains adopted an open conformation and engaged in interdimer interactions, as demonstrated by their participation in tetramerization instead of dimerization. Yet, homotypic N-domain interactions are not conserved in the STAT family, because the N-domain dissociation constants of STAT1, STAT3, and STAT4 differed by more than three orders of magnitude. In conclusion, STAT1 constantly oscillated between different dimer conformations, whereby the abundance of the dimerization interfaces was determined by tyrosine phosphorylation.

Signal transducer and activator of transcription 5A/B in prostate and breast cancers

Protein kinase signaling pathways, such as Janus kinase 2-Signal transducer and activator of transcription 5A/B (JAK2-STAT5A/B), are of significant interest in the search for new therapeutic strategies in both breast and prostate cancers. In prostate cancer, the components of the JAK2-STAT5A/B signaling pathway provide molecular targets for small-molecule inhibition of survival and growth signals of the cells. At the same time, new evidence suggests that the STAT5A/B signaling pathway is involved in the transition of organ-confined prostate cancer to hormone-refractory disease. This implies that the active JAK2-STAT5A/B signaling pathway potentially provides the means for pharmacological intervention of clinical prostate cancer progression. In addition, active STAT5A/B may serve as a prognostic marker for identification of those primary prostate cancers that are likely to progress to aggressive disease. In breast cancer, the role of STAT5A/B is more complex. STAT5A/B may have a dual role in the regulation of malignant mammary epithelium. Data accumulated from mouse models of breast cancer suggest that in early stages of breast cancer STAT5A/B may promote malignant transformation and enhance growth of the tumor. This is in contrast to established breast cancer, where STAT5A/B may mediate the critical cues for maintaining the differentiation of mammary epithelium. In addition, present data suggest that activation of STAT5A/B in breast cancer predicts favorable clinical outcome. The dual nature of STAT5A/B action in breast cancer makes the therapeutic use of STAT5 A/B more complex.

Overview of protein structural and functional folds

This overview provides an illustrated, comprehensive survey of some commonly observed protein‐fold families and structural motifs, chosen for their functional significance. It opens with descriptions and definitions of the various elements of protein structure and associated terminology. Following is an introduction into web‐based structural bioinformatics that includes surveys of interactive web servers for protein fold or domain annotation, protein‐structure databases, protein‐structure‐classification databases, structural alignments of proteins, and molecular graphics programs available for personal computers. The rest of the overview describes selected families of protein folds in terms of their secondary, tertiary, and quaternary structural arrangements, including ribbon‐diagram examples, tables of representative structures with references, and brief explanations pointing out their respective biological and functional significance.

Transcription factor signal transducer and activator of transcription 5 promotes growth of human prostate cancer cells in vivo

PURPOSE

Signal transducer and activator of transcription 5a/b (Stat5a/b) is the key mediator of prolactin effects in prostate cancer cells via activation of Janus-activated kinase 2. Prolactin is a locally produced growth factor in human prostate cancer. Prolactin protein expression and constitutive activation of Stat5a/b are associated with high histologic grade of clinical prostate cancer. Moreover, activation of Stat5a/b in primary prostate cancer predicts early disease recurrence. Here, we inhibited Stat5a/b by several different methodologic approaches. Our goal was to establish a proof of principle that Stat5a/b is critical for prostate cancer cell viability in vitro and for prostate tumor growth in vivo.

EXPERIMENTAL DESIGN

We inhibited Stat5a/b protein expression by antisense oligonucleotides or RNA interference and transcriptional activity of Stat5a/b by adenoviral expression of a dominant-negative mutant of Stat5a/b in prostate cancer cells in culture. Moreover, Stat5a/b activity was suppressed in human prostate cancer xenograft tumors in nude mice. Stat5a/b regulation of Bcl-X(L) and cyclin D1 protein levels was shown by antisense suppression of Stat5a/b protein expression followed by Western blotting.

RESULTS AND CONCLUSIONS

We show here that inhibition of Stat5a/b by antisense oligonucleotides, RNA interference, or adenoviral expression of dominant-negative Stat5a/b effectively kills prostate cancer cells. Moreover, we show that Stat5a/b is critical for human prostate cancer xenograft growth in nude mice. The effects of Stat5a/b on the viability of prostate cancer cells involve Stat5a/b regulation of Bcl-X(L) and cyclin D1 protein levels but not the expression or activation of Stat3. This work establishes Stat5a/b as a therapeutic target protein for prostate cancer. Pharmacologic inhibition of Stat5a/b in prostate cancer can be achieved by small-molecule inhibitors of transactivation, dimerization, or DNA binding of Stat5a/b.

Roles of unphosphorylated STATs in signaling

The seven members of the signal transducer and activator of transcription (STAT) family of transcription factors are activated in response to many different cytokines and growth factors by phosphorylation of specific tyrosine residues. The STAT1 and STAT3 genes are specific targets of activated STATs 1 and 3, respectively, resulting in large increases in the levels of these unphosphorylated STATs (U-STATs) in response to the interferons (STAT1) or ligands that active gp130, such as IL-6 (STAT3). U-STATs drive gene expression by novel mechanisms distinct from those used by phosphorylated STAT (P-STAT) dimers. In this review, we discuss the roles of U-STATs in transcription and regulation of gene expression.

Crystal structure of unphosphorylated STAT3 core fragment

Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcriptional factors that play an important role in cytokine and growth factor signaling. Here we report a 3.05 A-resolution crystal structure of an unphosphorylated STAT3 core fragment. The overall monomeric structure is very similar to that of the phosphorylated STAT3 core fragment. However, the dimer interface observed in the unphosphorylated STAT1 core fragment structure is absent in the STAT3 structure. Solution studies further demonstrate that the core fragment of STAT3 is primarily monomeric. Mutations corresponding to those in STAT1, which lead to disruption of the core fragment interface and prolonged tyrosine phosphorylation, show little or no effect on the tyrosine phosphorylation kinetics of STAT3. These results highlight the structural and biochemical differences between STAT3 and STAT1, and suggest different regulation mechanisms of these two proteins.

Choose your partners: dimerization in eukaryotic transcription factors

In many eukaryotic transcription factor gene families, proteins require a physical interaction with an identical molecule or with another molecule within the same family to form a functional dimer and bind DNA. Depending on the choice of partner and the cellular context, each dimer triggers a sequence of regulatory events that lead to a particular cellular fate, for example, proliferation or differentiation. Recent syntheses of genomic and functional data reveal that partner choice is not random; instead, dimerization specificities, which are strongly linked to the evolution of the protein family, apply. Our focus is on understanding these interaction specificities, their functional consequences and how they evolved. This knowledge is essential for understanding gene regulation and designing a new generation of drugs.

Structural basis for the binding of high affinity phosphopeptides to Stat3

Signal transducer and activator of transcription 3 (Stat3) is constitutively active in a number of cancers where it participates in aberrant transcription of prosurvival, cell cycling, and angiogenesis genes. Since Stat3 initiates its signaling activity through binding of its SH2 domain to phosphotyrosine residues on cell surface receptors, inhibitors targeting this region of the protein are potential chemotherapeutic agents. To date, no NMR or X-ray crystallographic structures of high-affinity phosphopeptides complexed with the Stat3 SH2 domain are available to aid in the development of peptidomimetic antagonists. Examination of the crystal structures of several STAT proteins and the complex of Stat1 with Ac-pTyr-Asp-Lys-Pro-His-NH(2) led to a hypothesis that the specificity determinant for Stat3, glutamine at position pY+3 in pTyr-Xxx-Xxx-Gln sequences, resides in a unique pocket on the protein surface at the juncture of the third strand of the central beta-sheet and a unique, STAT specific alpha-helix. Docking of Ac-pTyr-Leu-Pro-Gln-NHBn to the SH2 domain of Stat3 using molecular modeling showed that the Gln binds tightly in this pocket and participates in a network of hydrogen bonds. Novel interactions between the peptide main chain and the protein were also discovered. Phosphopeptide structure-affinity studies using unnatural amino acids and glutamine derivatives provide evidence for the peptide-protein interactions revealed by the model and lend support to the binding hypothesis.

Mutations in the STAT5A gene are associated with embryonic survival and milk composition in cattle

The objective of this study was to investigate the association of the signal transducer and activator of transcription 5A (STAT5A) gene with fertilization rate, embryonic survival, and milk production and composition in cattle. The STAT proteins are transcription factors that are specifically activated to regulate gene transcription when cells encounter cytokines and growth factors. The STAT5A gene is a member of the interferon-tau (IFN-tau) and placental lactogen (PL) signaling pathway, which is involved in both milk production and initiation of pregnancy. Using the DNA-pooling sequencing approach, a total of 12 single nucleotide polymorphisms (SNP) were identified, 1 exonic and 11 intronic. For the study of association of these SNP with embryonic survival, 1,551 embryos were produced in vitro from 160 cows and 3 sires. Significant associations with embryonic survival were found for 7, 5, and 2 SNP for embryos produced from sires 1, 2, and 3 respectively. The association of fertilization rate with STAT5A polymorphisms was evaluated in more than 2,300 oocytes. Significant associations were found for 6, 2, and 2 SNP for sires 1, 2, and 3 respectively. For sire 1, 5 SNP showed significant associations with both embryonic survival and fertilization rate compared with 1 SNP for sires 2 and 3. To determine if embryonic losses had occurred before the blastocyst stage, 145 of the surviving embryos were harvested at d 7 of development and genotyped for the single exonic SNP12195. A significant segregation distortion was observed between oocytes produced from 2 sires carrying the same genotype. Thus, it is most likely that STAT5A is associated with 2 mechanisms of embryo death. One is a prefertilization mechanism involving sperm factors that cause low fertilization rate. The second is a postfertilization mechanism that causes incompatibility between the male pronucleus and the oocyte, which in turn leads to death of the embryo before the blastocyst stage. Association testing of SNP12195 (exon 8) and SNP14217 (intron 9) with milk composition revealed that allele G of SNP12195 was associated with a decrease in both protein and fat percentages. However, SNP14217 in intron 9 showed no significant association with milk production or health traits. The G allele of SNP12195 was also associated with low embryonic survival, making this SNP an attractive candidate for progeny testing programs in dairy cattle.

Rationally designed inhibitors identify STAT3 N-domain as a promising anticancer drug target

Activation of the signal transducer and activator of transcription 3 (STAT3) is frequently detected in many cancer types. Activated STAT3 may participate in oncogenesis by stimulating cell proliferation and resisting apoptosis, as well as promoting tumor angiogenesis, invasion, and migration. Many STAT3-dependent cellular responses are mediated through interactions with other proteins, and the amino-terminal domain (N-domain) of STAT3 was proposed to be responsible for this. Our NMR studies revealed that synthetic analogs of the STAT4 second alpha-helix bind to the N-domain and perturb its structure. Structural data available for the STAT4 N-domain was used for the rational design of STAT3 helix 2 analogs with enhanced biological activity. Cell-permeable derivatives of the STAT3 second helix were found to directly and specifically bind to STAT3 but not STAT1 as determined by FRET analysis in cells expressing GFP-STAT3 and GFP-STAT1. Furthermore, they potently induced apoptotic death in breast cancer cells but not normal breast cells or STAT3-deficient fibroblasts. The inhibitors caused significant changes in the mitochondrial potential of cancer cells, leading to cell death. These compounds not only are promising drug candidates but also offer a convenient tool for studying the mechanisms of action of STAT transcription factors and have facilitated our understanding of the crucial role of the N-domain in STAT3 function.

Computational study on mechanism of G-quartet oligonucleotide T40214 selectively targeting Stat3

The mounting evidences have shown that signal transducer and activator of transcription 3 (Stat3) is a critical target for cancer therapy. Recently, we developed a G-quartet oligonucleotide T40214 as a novel and potent Stat3 inhibitor. T40214 specifically inhibited DNA-binding activity of Stat3 and significantly suppressed the growth of many tumor xenografts in nude mice. To determine the mechanism of GQ-ODNs selectively targeting Stat3, we established a 3D model of complex T40214/p-Stat3 dimer based on experimental evidences. The binding site of T40214 within Stat3 dimer was determined by statistical docking analysis. The results indicated that T40214 strongly interacted within the region from residue E638 through E652 of Stat3 dimer. The binding model refined by Hex docking disclosed that T40214 binds to SH2 domain of Stat3 and forms H-bonds with residues Q643, Q644, N646, and N647, which are critical for the binding interaction. The 3D models also suggested that T40214 inhibits Stat3 activity through disrupting the binding interaction between Stat3 dimer and DNA duplex for transcription. Our computational studies provided a platform for future structure-based drug design of novel Stat3 inhibitors.

Structure of the SOCS4-ElonginB/C complex reveals a distinct SOCS box interface and the molecular basis for SOCS-dependent EGFR degradation

Tyrosine kinase signaling is tightly controlled by negative feedback inhibitors including suppressors of cytokine signaling (SOCS). SOCS assemble as SH2 domain substrate recognition modules in ElonginB/C-cullin ubiquitin ligases. In accordance, SOCS4 reduces STAT3 signaling from EGFR through increased receptor degradation. Variable C-termini in SOCS4-SOCS7 exclude these family members from a SOCS2-type domain arrangement in which a strictly conserved C terminus determines domain packing. The structure of the SOCS4-ElonginC-ElonginB complex reveals a distinct SOCS structural class. The N-terminal ESS helix functionally replaces the CIS/SOCS1-SOCS3 family C terminus in a distinct SH2-SOCS box interface that facilitates further interdomain packing between the extended N- and C-terminal regions characteristic for this subfamily. Using peptide arrays and calorimetry the STAT3 site in EGFR (pY(1092)) was identified as a high affinity SOCS4 substrate (K(D) = 0.5 microM) revealing a mechanism for EGFR degradation. SOCS4 also bound JAK2 and KIT with low micromolar affinity, whereas SOCS2 was specific for GH-receptor.

Defining the specificity space of the human SRC homology 2 domain

Src homology 2 (SH2) domains are the largest family of interaction modules encoded by the human genome to recognize tyrosine-phosphorylated sequences and thereby play pivotal roles in transducing and controlling cellular signals emanating from protein-tyrosine kinases. Different SH2 domains select for distinct phosphopeptides, and the function of a given SH2 domain is often dictated by the specific motifs that it recognizes. Therefore, deciphering the phosphotyrosyl peptide motif recognized by an SH2 domain is the key to understanding its cellular function. Here we cloned all 120 SH2 domains identified in the human genome and determined the phosphotyrosyl peptide binding properties of 76 SH2 domains by screening an oriented peptide array library. Of these 76, we defined the selectivity for 43 SH2 domains and refined the binding motifs for another 33 SH2 domains. We identified a number of novel binding motifs, which are exemplified by the BRDG1 SH2 domain that selects specifically for a bulky, hydrophobic residue at P + 4 relative to the Tyr(P) residue. Based on the oriented peptide array library data, we developed scoring matrix-assisted ligand identification (or SMALI), a Web-based program for predicting binding partners for SH2-containing proteins. When applied to SH2D1A/SAP (SLAM-associated protein), a protein whose mutation or deletion underlies the X-linked lymphoproliferative syndrome, SMALI not only recapitulated known interactions but also identified a number of novel interacting proteins for this disease-associated protein. SMALI also identified a number of potential interactors for BRDG1, a protein whose function is largely unknown. Peptide in-solution binding analysis demonstrated that a SMALI score correlates well with the binding energy of a peptide to a given SH2 domain. The definition of the specificity space of the human SH2 domain provides both the necessary molecular basis and a platform for future exploration of the functions for SH2-containing proteins in cells.

Historical developments in the research of interferon receptors

Interferons (IFNs) were discovered 50 years ago independently by Isaacs and Lindemann and by Nagata and Kojima. When it was later realized that IFNs are active at very low concentrations, research began to determine how their powerful effects were generated from such a small initial signal. It has since been established that interferons, as well as all other cytokines, employ cell surface receptors to translate their presence in the serum to a potent cellular response to a viral infection. These receptor complexes are composed of multiple distinct glycosylated transmembrane polypeptides, a number of protein tyrosine kinases, and interact transiently with a large variety of other proteins including transcription factors, phosphatases, signaling repressors, and adaptor proteins coupling the receptor to alternative signaling pathways. Three major receptor complexes exist that are exclusive to each of three major classes of interferon. Even though the effects of each major class of interferon vary physiologically, each receptor complex interacts with its ligand in similar ways and activates similar signaling cascades. In this mini-review, we take a historical perspective at the major events in the characterization of interferon receptors, discussing interesting results that still need to be explained.

STAT dynamics

The ability of transcription factors to gain entrance to the nucleus is critical to their role in gene expression. Signal transducers and activators of transcription (STATs) are latent DNA binding factors activated by specific tyrosine phosphorylation. There are seven mammalian STAT genes encoding proteins that display constitutive nuclear localization and/or conditional nuclear localization. This review will focus on STAT1 and STAT2 that are activated in response to interferon and exhibit conditional nuclear localization. The dynamic redistribution of STAT1 and STAT2 between the cytoplasm and the nucleus is coordinate with their gain of ability to bind DNA.

A systematic comparative and structural analysis of protein phosphorylation sites based on the mtcPTM database

mtcPTM is a new database of phosphorylated protein sequences and atomic models. Analysis of the phosphosites in mtcPTM showed that phosphorylation sites are found in a highly heterogeneous range of structural and sequence contexts.

mtcPTM is an online repository of human and mouse phosphosites in which data are hierarchically organized to preserve biologically relevant experimental information, thus allowing straightforward comparisons of phosphorylation patterns found under different conditions. The database also contains the largest available collection of atomic models of phosphorylatable proteins. Detailed analysis of this structural dataset reveals that phosphorylation sites are found in a heterogeneous range of structural and sequence contexts. mtcPTM is available on the web .

Constitutive nuclear import of latent and activated STAT5a by its coiled coil domain

Signal transducer and activator of transcription 5a (STAT5a) is a critical transcription factor for a number of physiological processes including hematopoiesis and mammary gland development. Cytokines such as growth hormone, prolactin, erythropoietin, and interleukin-2 stimulate the activation of STAT5a by tyrosine phosphorylation. Tyrosine phosphorylation confers a conformational change and the ability to bind specific target DNA. To execute its function as a signaling molecule and transcription factor, accurate cellular localization of STAT5a is essential. This study explores the nuclear trafficking of STAT5a both before phosphorylation and after tyrosine phosphorylation. With the use of live cell imaging we demonstrate the continuous shuttling of STAT5a in and out of the nucleus. Evaluation of a series of mutations and deletions identifies a region within the coiled coil domain of STAT5a that is critical for nuclear import of both unphosphorylated and tyrosine-phosphorylated forms. The mechanism that regulates transport of STAT5a through nuclear pore complexes into the nucleus is therefore independent of tyrosine phosphorylation. However, after tyrosine phosphorylation, STAT5a accumulates in the nucleus because of its retention by DNA binding. These findings should provide a foundation for further studies that involve targeting the activity of STAT5a.

A genetically encoded metabolically stable analogue of phosphotyrosine in Escherichia coli

p-Carboxymethyl- l-phenylalanine (pCMF), a phosphotyrosine (pTyr) mimetic that is resistant to protein tyrosine phosphatase hydrolysis, was cotranslationally incorporated into proteins in Escherichia coli using an orthogonal amber suppressor tRNA/aminoacyl-tRNA synthetase (aaRS) pair. The pCMF-specific aaRS was identified from a large library of Methanococcus jannaschii tyrosyl-tRNA synthetase active-site mutants by a combination of positive and negative genetic selections. When pCMF was substituted for Tyr701 in human signal transducer and activator of transcription-1 (STAT1), a constitutively active mutant was obtained that dimerizes and binds a DNA oligonucleotide duplex that contains the M67 site recognized by Tyr701-phosphorylated STAT1. Genetic incorporation of pCMF into proteins should provide a new tool for the preparation of stable analogues of a wide array of phosphoproteins involved in signal transduction pathways, as well as the development of peptide-based, cellularly expressed inhibitors of pTyr binding proteins.

Phosphorylation of IRF8 in a pre-associated complex with Spi-1/PU.1 and non-phosphorylated Stat1 is critical for LPS induction of the IL1B gene

Rapid induction of transcription is known to be mediated by factors which bind DNA following post-translational modification. We report here that non-tyrosine phosphorylated (NTP)-Stat1 is involved in a cooperative interaction with Spi-1/PU.1 and IRF8 to form a pre-associated, poised complex for IL1B gene induction. A double point mutation at a putative STAT binding site, which overlaps this composite Spi-1 x IRF8 site located in the LPS and IL-1 response element (LILRE), inhibited human IL1B LPS-dependent reporter activity to about 10 percent of the control wild type vector. Chromatin immunoprecipitation revealed stimulation-independent constitutive binding of IRF8, Spi-1 and NTP-Stat1 at the LILRE, while binding of C/EBP beta was activated at an adjacent C/EBP beta site after LPS stimulation. In contrast to Stat1, IRF8 was tyrosine phosphorylated following LPS treatment. Supporting the involvement of NTP-Stat1, LPS-induced IL1B reporter activity in monocytes was enhanced by ectopic expression of NTP-Stat1 Y701F. In contrast, co-expression of a Y211F IRF8 mutein functioned as a dominant-negative inhibitor of LPS-induced IL1B reporter activity. In vitro DNA binding using extracts from LPS-treated monocytes confirmed that the LILRE enhancer constitutively binds a trimolecular complex containing IRF8, Spi-1 and NTP-Stat1. Binding studies using in vitro-expressed proteins revealed that NTP-Stat1 enhanced the binding of Spi-1 and IRF8 to the LILRE. Co-expression of TRAF6, an LPS surrogate, with Spi-1 and IRF8 enhanced IL1B reporter activity in HEK293R cells, which was dramatically reduced when Y211F IRF8 was co-expressed. These results suggest that the rapid transcriptional induction of an important inflammatory gene is dependent upon constitutive cooperative binding of a Spi-1 x IRF8 x NTP-Stat1 complex to the LILRE, which primes the gene for immediate induction following IRF8 phosphorylation. Phosphorylation of chromatin pre-associated factors like IRF8 may be an important strategy for the rapid transcriptional activation of genes involved in innate immunity.

Recruitment of RXR by homotetrameric RARalpha fusion proteins is essential for transformation

While formation of higher-order oncogenic transcriptional complexes is critical for RARalpha fusion proteins in acute promyelocytic leukemia, the essential components and their roles in mediating transformation are still largely unknown. To this end, the present study demonstrates that homodimerization is not sufficient for RARalpha fusion-mediated transformation, which requires higher-order homotetramerization. Surprisingly, intrinsic homo-oligomeric DNA binding by the fusion proteins is also dispensable. Importantly, higher-order RXR/RARalpha fusion hetero-oligomeric complexes that aberrantly recruit transcriptional corepressors to downstream targets are essential for transformation. Intervention of RXR-dependent pathways by panRXR-agonists or RXRalpha shRNAs suppresses RARalpha fusion-mediated transformation. Taken together, these results define the oncogenic threshold for self-association and reveal the pathological significance of higher-order RARalpha fusion/RXR hetero-oligomeric complexes and their potential value as a therapeutic target.

Survival pathways in hypertrophy and heart failure: the gp130-STAT3 axis

Circulating levels of interleukin (IL)-6 and related cytokines are elevated in patients with congestive heart failure and after myocardial infarction. Serum IL-6 concentrations are related to decreasing functional status of these patients and provide important prognostic information.Moreover, in the failing human heart, multiple components of the IL-6- glycoprotein (gp)130 receptor system are impaired, implicating an important role of this system in cardiac pathophysiology.Experimental studies have shown that the common receptor subunit of IL-6 cytokines is phosphorylated in response to pressure overload and myocardial infarction and that it subsequently activates at least three different downstream signaling pathways, the signal transducers and activators of transcription 1 and 3 (STAT1/3), the Src-homology tyrosine phosphatase 2 (SHP2)-Ras-ERK, and the PI3K-Akt system. Gp130 receptor mediated signaling promotes cardiomyocyte survival, induces hypertrophy, modulates cardiac extracellular matrix and cardiac function. In this regard, the gp130 receptor system and its main downstream mediator STAT3 play a key role in cardioprotection. This review summarizes the current knowledge of IL-6 cytokines, gp130 receptor and STAT3 signaling in the heart exposed to physiological (aging, pregnancy) and pathophysiological stress (ischemia, pressure overload, inflammation and cardiotoxic agents) with a special focus on the potential role of individual IL-6 cytokines.

Defects in growth hormone receptor signaling

Severe growth failure and insulin-like growth factor (IGF) deficiency were first reported 40 years ago in patients who ultimately proved to have mutations in the gene encoding the growth hormone receptor (GHR). So far, over 250 similar patients, encompassing more than 60 different mutations of GHR, have been reported. The GHR is a member of the cytokine receptor superfamily and has been shown to signal, at least in part, through the Janus-family tyrosine kinase-signal transducer and activator of transcription (JAK-STAT) pathway. Six patients, from five distinct families, have been reported to have phenotypes similar to that of patients with GHR defects but with wild-type receptors and homozygosity for five different mutations of the STAT5b gene. These patients define a new cause of GH insensitivity and primary IGF deficiency and confirm the crucial role of STAT5b in GH-mediated IGF-I gene transcription.

Isoform selective inhibition of STAT1 or STAT3 homo-dimerization via peptidomimetic probes: Structural recognition of STAT SH2 domains

The identification of constitutively activated STAT (Signal Transducers and Activators of Transcription) proteins in aberrant cell signaling pathways has led to investigations targeting the selective disruption of specific STAT isoforms directly associated with oncogenisis. We have identified, through the design of a library of peptidomimetic inhibitors, agents that selectively disrupt STAT1 or STAT3 homo-dimerization at low micromolar concentrations. ISS840 has 20-fold higher inhibition of STAT1 homo-dimerization (IC(50) value of 31 microM) relative to STAT3 (IC(50) value of 560 microM).

Growth hormone insensitivity and severe short stature in siblings: a novel mutation at the exon 13-intron 13 junction of the STAT5b gene

BACKGROUND/AIMS

Growth hormone insensitivity (GHI) is characterized by severe short stature, high serum growth hormone (GH), low serum IGF-I and IGFBP-3 levels and is classically associated with genetic defects of the GH receptor (GHR). Recently, mutations of the STAT5b gene have been identified and shown to be associated with GHI and severe IGF deficiency. We investigated 2 sisters from a consanguineous family from Kuwait, with clinical and biochemical features of GHI, in whom no molecular defects in the GHR were identified.

METHODS

Serum and DNA were analyzed.

RESULTS

In addition to GHI, siblings 2 and 1 presented with, respectively, a diagnosis of juvenile idiopathic arthritis and recurrent pulmonary infections. Molecular analysis of the STAT5b gene revealed a novel homozygous deletion of a G at the junction of exon 13-intron 13. The parents, who are of normal height, were heterozygous for the mutation.

CONCLUSIONS

This is the first STAT5b defect to be identified in siblings, further supporting the autosomal recessive mode of transmission of STAT5b deficiency. The results affirm that defective STAT5b is an etiology for IGF deficiency and the GHI phenotype, and emphasize the importance of considering this diagnosis in patients with IGF deficiency, especially when associated with diverse immunological problems.

Multiple functions of the IKK-related kinase IKKepsilon in interferon-mediated antiviral immunity

IKKepsilon is an IKK (inhibitor of nuclear factor kappaBkinase)-related kinase implicated in virus induction of interferon-beta (IFNbeta). We report that, although mice lacking IKKepsilon produce normal amounts of IFNbeta, they are hypersusceptible to viral infection because of a defect in the IFN signaling pathway. Specifically, a subset of type I IFN-stimulated genes are not activated in the absence of IKKepsilon because the interferon-stimulated gene factor 3 complex (ISGF3) does not bind to promoter elements of the affected genes. We demonstrate that IKKepsilon is activated by IFNbeta and that IKKepsilon directly phosphorylates signal transducer and activator of transcription 1 (STAT1), a component of ISGF3. We conclude that IKKepsilon plays a critical role in the IFN-inducible antiviral transcriptional response.

Brahma-related gene 1-dependent STAT3 recruitment at IL-6-inducible genes

IL-6 is an immunoregulatory cytokine with multiple functions in hemopoiesis, proliferation, and tumorigenesis. IL-6 triggers phosphorylation, dimerization, and nuclear translocation of STAT3, which binds to target promoters and activates transcription. Brahma-related gene 1 (BRG1), the enzymatic engine of the yeast-mating type-switching and sucrose-nonfermenting chromatin-remodeling complex, is essential for recruitment of STAT1 or STAT1/STAT2-containing complexes to IFN targets. We hypothesized that BRG1 might also be required for STAT3 recruitment. In this study, we show that induction of a subset of human IL-6-responsive genes is BRG1 dependent. BRG1 is constitutively present at these targets and is required for STAT3 recruitment, downstream histone modifications, and IL-6-induced chromatin remodeling. IL-6-induced recruitment of STAT3 to the IFN regulatory factor 1 promoter and subsequent mRNA synthesis is BRG1 dependent, even though IFN-gamma-mediated STAT1 recruitment to this locus is BRG1 independent. BRG1 also increased basal expression of IFN-induced transmembrane protein 3 and IFN-gamma-induced protein 16, and the basal chromatin accessibility at the promoter of IFN regulatory factor 1. The effect on basal expression was STAT3 independent, as revealed by small interfering RNA knockdown. Together with prior observations, these data reveal that BRG1 has a broad role in mediating STAT accessibility at multiple cytokine-responsive promoters and exposes promoter specific differences in both the effect of BRG1 on basal chromatin accessibility and on access of different STAT proteins to the same target.

STAT1 as a key modulator of cell death

Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcription factors that mediate various biological responses, including cell proliferation, survival, apoptosis, and differentiation. Among the members of the STAT family, accumulating evidence now indicates an important role for STAT1 in various forms of cell death. Depending upon stimuli or cell types, STAT1 can modulate a broad spectrum of cell death, comprising both apoptotic and non-apoptotic pathways. STAT1-dependent regulation of cell death is largely dependent on a transcriptional mechanism such as the activation of death-promoting genes. However, non-transcriptional mechanisms such as STAT1 interaction with TRADD, p53, or HDAC have been implicated in the regulation of cell death by STAT1. Furthermore, STAT1 itself is also subject to complex forms of regulation such as post-translational protein modification, which can critically affect STAT1 signaling and STAT1-dependent cell death. Given the reports showing that dysregulation of STAT1 signaling is associated with various pathological conditions, including the development of cancer, a better understanding of the mechanism underlying STAT1 regulation of cell death may lead to successful strategies for targeting STAT1 in such pathological settings.

Signal transducer and activator of transcription signals in allergic disease

Signal transducer and activator of transcription (STAT) proteins are a group of transcription factors that transmit signals from the extracellular milieu of cells to the nucleus. They are crucial for the signaling of many cytokines that are mediators of allergic inflammation and impact various cell types critical to allergy including epithelial cells, mast cells, lymphocytes, dendritic cells, and eosinophils. Dysregulation of STAT signaling has been implicated in allergic disease, highlighting the importance of these ubiquitous molecules in allergic inflammation and the potential of these pathways as a target for therapeutic intervention. This review will summarize the current understanding of the roles of STAT signaling in allergic disease and the potential of targeting STATs for the treatment of allergic disorders, emphasizing recent observations.

Dephosphorylation of phosphotyrosine on STAT1 dimers requires extensive spatial reorientation of the monomers facilitated by the N-terminal domain

We report experiments that infer a radical reorientation of tyrosine-phosphorylated parallel STAT1 dimers to an antiparallel form. Such a change in structure allows easy access to a phosphatase. With differentially epitope-tagged molecules, we show that the two monomers of a dimer remain together during dephosphorylation although they most likely undergo spatial reorientation. Extensive single amino acid mutagenesis within crystallographically established domains, manipulation of amino acids in an unstructured tether that connects the N-terminal domain (ND) to the core of the protein, and the demonstration that overexpressed ND can facilitate dephosphorylation of a core molecule lacking an ND all support this model: When the tyrosine-phosphorylated STAT1 disengages from DNA, the ND dimerizes and somehow assists in freeing the reciprocal pY-SH2 binding between the monomers of the dimer while ND ND dimerization persists. The core of the monomers rotate allowing reciprocal association of the coiled:coil and DNA-binding domains to present pY at the two ends of an antiparallel dimer for ready dephosphorylation.

Novel STAT1 alleles in otherwise healthy patients with mycobacterial disease

The transcription factor signal transducer and activator of transcription-1 (STAT1) plays a key role in immunity against mycobacterial and viral infections. Here, we characterize three human STAT1 germline alleles from otherwise healthy patients with mycobacterial disease. The previously reported L706S, like the novel Q463H and E320Q alleles, are intrinsically deleterious for both interferon gamma (IFNG)–induced gamma-activating factor–mediated immunity and interferon alpha (IFNA)–induced interferon-stimulated genes factor 3–mediated immunity, as shown in STAT1-deficient cells transfected with the corresponding alleles. Their phenotypic effects are however mediated by different molecular mechanisms, L706S affecting STAT1 phosphorylation and Q463H and E320Q affecting STAT1 DNA-binding activity. Heterozygous patients display specifically impaired IFNG-induced gamma-activating factor–mediated immunity, resulting in susceptibility to mycobacteria. Indeed, IFNA-induced interferon-stimulated genes factor 3–mediated immunity is not affected, and these patients are not particularly susceptible to viral disease, unlike patients homozygous for other, equally deleterious STAT1 mutations recessive for both phenotypes. The three STAT1 alleles are therefore dominant for IFNG-mediated antimycobacterial immunity but recessive for IFNA-mediated antiviral immunity at the cellular and clinical levels. These STAT1 alleles define two forms of dominant STAT1 deficiency, depending on whether the mutations impair STAT1 phosphorylation or DNA binding.

Mendelian susceptibility to mycobacterial disease is a rare syndrome. It is defined by the occurrence of severe disease caused by low virulence mycobacteria in otherwise healthy individuals, in whom antiviral immune response is not affected. Eleven known genetic defects, affecting five genes, have been involved in this type of deficient response to infection, involving immune-mediator molecules IL12 and interferon gamma: IL12B, IL12RB1, IFNGR1, IFNGR2, and STAT1. The signal transducer and activator of transcription-1 (STAT1) amino acid change L706S was previously shown to cause disease by impairing STAT1 phosphorylation. Here, we report two new STAT1 mutations that impair STAT1 DNA-binding activity. We show, by functional analysis of the three STAT1 mutant alleles, that they are intrinsically deleterious for both interferon gamma–induced antimycobacterial immunity, which is mediated through gamma-activated factor and for interferon alpha–induced antiviral immunity, which is mediated through interferon-stimulated genes factor 3. Interestingly, the three alleles are dominant for interferon gamma–induced gamma-activated factor–mediated antimycobacterial immunity, but recessive for interferon alpha–induced interferon-stimulated genes factor 3–mediated antiviral immunity at the cellular and clinical levels. These two new STAT1 alleles, which affect the binding of STAT1 to DNA, define distinct novel genetic causes of Mendelian susceptibility to mycobacterial disease and provide further insight into the molecular mechanism of disease.

Overexpression of phosphorylated STAT-1alpha in the labial salivary glands of patients with Sjögren's syndrome

OBJECTIVE

To clarify the molecular mechanisms of Sjögren's syndrome (SS), we analyzed the functional role of the STAT-1 gene, one of the interferon-gamma (IFNgamma)-inducible genes, in labial salivary glands (LSGs) from SS patients.

METHODS

The expression of STAT-1 messenger RNA (mRNA) was examined by real-time polymerase chain reaction (PCR) analysis, and the phosphorylation of STAT-1 protein (Tyr(701) and Ser(727) pSTAT-1) was investigated by Western blot and immunohistochemical analyses. The expression of IFNgamma-inducible 10-kd protein (IP-10), IFN regulatory factor 1 (IRF-1), and Fas was also examined by real-time PCR and immunohistochemical analyses.

RESULTS

STAT-1alpha and STAT-1beta mRNA were highly expressed in LSGs from SS patients. The level of STAT-1alpha protein in SS LSGs was higher than that in 3 control LSGs, whereas STAT-1beta protein was not clearly detected by Western blot analysis. Moreover, Tyr(701) and Ser(727) pSTAT-1alpha proteins were specifically detected in SS LSGs. Immunohistochemical analysis showed localization of Tyr(701) pSTAT-1 in infiltrating lymphocytes and the adjacent ductal epithelium from SS patients. Ser(727) pSTAT-1 was localized only in the ductal epithelium of SS LSGs. The STAT-1-inducible genes IP-10 and IRF-1 and the Fas genes were highly expressed in SS LSGs and were colocalized with Ser(727) pSTAT-1-positive, but not Tyr(701) pSTAT-1-positive, cells.

CONCLUSION

We found evidence of the up-regulation of STAT-1alpha mRNA and protein in LSGs from SS patients, as well as the presence of pSTAT-1alpha in ductal epithelium from SS patients. Our findings suggest that STAT-1alpha, especially Ser(727) pSTAT-1, may function as a key molecule in the pathogenesis of SS.

Regulation of cytokine signaling pathways by PIAS proteins

Cytokines activate multiple signal transduction pathways to regulate gene expression. STATs and NF-kB are two important families of transcription factors activated by cytokines. Abnormal regulation of STAT and NF-kB activities has been associated with human diseases. The protein inhibitor of activated STAT (PIAS) protein family has been proposed to interact with over 60 proteins, many of which are transcription factors involved in the immune system. PIAS proteins regulate transcription through several mechanisms, including blocking the DNA-binding activity of transcription factors, recruiting transcriptional co-repressors and promoting protein sumoylation. This article is to review the role of PIAS proteins in the regulation of STAT and NF-kB signaling pathways.

White spot syndrome virus annexes a shrimp STAT to enhance expression of the immediate-early gene ie1

Although the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway is part of the antiviral response in arthropods such as Drosophila, here we show that white spot syndrome virus (WSSV) uses a shrimp STAT as a transcription factor to enhance viral gene expression in host cells. In a series of deletion and mutation assays using the WSSV immediate-early gene ie1 promoter, which is active in shrimp cells and also in insect Sf9 cells, an element containing a STAT binding motif was shown to be important for the overall level of WSSV ie1 promoter activity. In the Sf9 insect cell line, a specific protein-DNA complex was detected by using electrophoresis mobility shift assays (EMSA) with the 32P-labeled STAT binding motif of the WSSV ie1 promoter as the probe. When recombinant Penaeus monodon STAT (rPmSTAT) was overexpressed in Sf9 cells, EMSA with specific antibodies confirmed that the STAT was responsible for the formation of the specific protein-DNA complex. Another EMSA showed that in WSSV-infected P. monodon, levels of activated PmSTAT were higher than in WSSV-free P. monodon. A transactivation assay of the WSSV ie1 promoter demonstrated that increasing the level of rPmSTAT led to dose-dependent increases in ie1 promoter activity. These results show that STAT directly transactivates WSSV ie1 gene expression and contributes to its high promoter activity. We conclude that WSSV successfully annexes a putative shrimp defense mechanism, which it uses to enhance the expression of viral immediate-early genes.

Jak2 and Tyk2 are necessary for lineage-specific differentiation, but not for the maintenance of self-renewal of mouse embryonic stem cells

As the LIF-induced Jak1/STAT3 pathway has been reported to play a crucial role in self-renewal of mESCs, we sought to determine if Jak2, which is also expressed in mESCs, might also be involved in the pathway. By employing an RNAi strategy, we established both Jak2 and Jak2/Tyk2 knockdown mESC clones. Both Jak2 and Jak2/Tyk2 knockdown clones maintained the undifferentiated state as wild-type controls, even in a very low concentration of LIF. However, we observed not only faster onset of differentiation but also differential expression of tissue-specific lineage genes for ectodermal and mesodermal, but not endodermal origins from embryoid bodies generated from both types of knockdown clones compared to the wild-type. Furthermore, the reduced level of Jak2 caused differentiation of mESCs in the presence of LIF when the Wnt pathway was activated by LiCl treatment. Taken together, we demonstrated that Jak2 and Tyk2 are not involved in LIF-induced STAT3 pathway for self-renewal of mESCs, but play a role in early lineage decision of mESCs to various differentiated cell types.

Hepatitis C virus core protein blocks interferon signaling by interaction with the STAT1 SH2 domain

Emerging data have indicated that hepatitis C virus (HCV) subverts the host antiviral response to ensure its persistence. We previously demonstrated that HCV protein expression suppresses type I interferon (IFN) signaling by leading to the reduction of phosphorylated STAT1 (P-STAT1). We also demonstrated that HCV core protein directly bound to STAT1. However, the detailed mechanisms by which HCV core protein impacts IFN signaling components have not been fully clarified. In this report, we show that the STAT1 interaction domain resides in the N-terminal portion of HCV core (amino acids [aa] 1 to 23). This domain is also required to produce P-STAT1 reduction and inhibit IFN signaling transduction. Conversely, the C-terminal region of STAT1, specifically the SH2 domain (aa 577 to 684), is required for the interaction of HCV core with STAT1. The STAT1 SH2 domain is critical for STAT1 hetero- or homodimerization. We propose a model by which the binding of HCV core to STAT1 results in decreased P-STAT, blocked STAT1 heterodimerization to STAT2, and, therefore, reduced IFN-stimulated gene factor-3 binding to DNA and disrupted IFN-stimulated gene transcription.

C. elegans STAT: evolution of a regulatory switch

STAT transcription factors have been implicated in many biological processes, particularly host immune defense and development. Here we characterize a STAT orthologue from the nematode, C. elegans. We show that this protein, termed STA-1, is structurally and functionally related to other vertebrate and invertebrate STAT proteins, recognizing a cis DNA element conserved through phylogeny. Unexpectedly, STA-1 lacks the conserved amino-terminal oligomerization domain found in vertebrate and other invertebrate STAT proteins, a feature also lacking in orthologues from a distantly related nematode species and from the slime mold, Dictyostelium discoideum. This absence suggests that a primordial STAT protein lacked this domain, which was accreted later in evolution to provide further regulatory control of STAT signaling. Derivation of null mutants demonstrated that STA-1 is not required for nematode viability, despite its widespread expression in multiple tissues of the worm. However, mutant STA-1 proteins that lack functional coiled-coil and DNA binding domains could still be activated and accumulated in the nucleus, suggesting that DNA binding is not a necessary prerequisite for nuclear retention of activated STAT proteins. Our results shed new light on the evolution and function of the STAT signaling pathway and on the structural requirements for STAT activation.

Structure, function, and regulation of STAT proteins

The Signal Transducer and Activator of Transcription (STAT) family of proteins was first discovered in the 1990's as key proteins in cytokine signaling. Since then, the field has greatly advanced in the past 15 years, providing significant insight into the structure, function, and regulation of STATs. STATs are latent cytoplasmic transcription factors consisting of seven mammalian members. They are Tyr phosphorylated upon activation, a post-translational modification critical for dimerization, nuclear import, DNA binding, and transcriptional activation. In recent years, unphosphorylated STATs have also been observed to dimerize and drive transcription, albeit by yet an obscure mechanism. In addition, the function of cytoplasmic STATs is beginning to emerge. Here, we describe the structure, function, and regulation of both unphosphorylated and phosphorylated STATs. STAT isoforms from alternative splicing or proteolytic processing, and post-translational modifications affecting STAT activities are also discussed.

The amino-terminal domain of human signal transducers and activators of transcription 1: overexpression, purification and characterization

The dual functional signal transducers and activators of transcription (STAT) proteins are latent cytoplasmic transcription factors that play crucial roles in host defense. Animals that lack these proteins are highly susceptible to microbial and viral infections and chemically induced primary tumours. We have over expressed the amino-terminal domain of human STAT1 (hSTAT1) in Escherichia coli and purified it by affinity chromatography and gel filtration chromatography. The entire process has been monitored by gel electrophoresis. The pure protein has been characterized by mass spectrometry and 2-dimensional nuclear magnetic resonance (2D-NMR) spectroscopy. Our results indicate that the N-terminus of hSTAT1 exists as a dimer in solution.

The human and mouse complement of SH2 domain proteins-establishing the boundaries of phosphotyrosine signaling

SH2 domains are interaction modules uniquely dedicated to the recognition of phosphotyrosine sites and are embedded in proteins that couple protein-tyrosine kinases to intracellular signaling pathways. Here, we report a comprehensive bioinformatics, structural, and functional view of the human and mouse complement of SH2 domain proteins. This information delimits the set of SH2-containing effectors available for PTK signaling and will facilitate the systems-level analysis of pTyr-dependent protein-protein interactions and PTK-mediated signal transduction. The domain-based architecture of SH2-containing proteins is of more general relevance for understanding the large family of protein interaction domains and the modular organization of the majority of human proteins.

IL-6 signaling via the STAT3/SOCS3 pathway: functional analysis of the conserved STAT3 N-domain

The conserved N-domain of the STAT proteins has been implicated in several activities crucial to cytokine signaling including receptor recruitment and STAT activation, cooperative DNA binding and STAT-dependent gene expression. We evaluated the role of the STAT3 N-domain in the IL-6 signal transduction pathway leading to Socs3 gene expression, an essential mechanism that controls the quality and magnitude of IL-6-dependent transcriptional responses. Based on the model for STAT N-domain function in cooperative gene expression and the presence of tandem STAT binding motifs in the murine Socs3 promoter, we anticipated that stabilizing interactions between adjacent STAT3 dimers via N-domain sequences might be essential for Socs3 gene expression. This was underscored by the tight conservation in the location and sequence of the tandem STAT binding sites between the murine and human Socs3 promoters. Using reconstitution into Stat3-/- mouse embryonic fibroblasts (Stat3-/- MEFs), we find that a STAT3 N-domain deletion mutant (Delta 133STAT3) is activated by tyrosine phosphorylation in response to IL-6 and then undergoes dephosphorylation with kinetics similar to full-length STAT3. These results highlight important differences compared to other STATs where the N-domain has been shown to mediate activation (STAT4) or dephosphorylation (STAT1). STAT3 binds predominantly to a single STAT consensus site in the Socs3 promoter, despite the presence of an adjacent STAT motif. Significantly, Delta 133STAT3 stimulates expression of the endogenous Socs3 gene in Stat3-/- MEFs upon IL-6 treatment with an activity similar to reconstituted STAT3, demonstrating that the N-domain is dispensable for Socs3 gene expression. We propose that the Socs3 gene in its chromosomal context is activated by the IL-6/STAT3 pathway independent of STAT3 N-domain sequences.

Regulation of Stat3 nuclear import by importin α5 and importin α7 via two different functional sequence elements

Regulated import of STAT proteins into the nucleus through the nuclear pores is a vital event. We previously identified Arg214/215 in the coiled-coil domain and Arg414/417 in the DNA binding domain involved in the ligand-induced nuclear translocation of Stat3. In this study, we investigated the mechanism for Stat3 nuclear transport. We report here that among five ubiquitously expressed human importin alphas, importin alpha5 and alpha7, but not importin alpha1, alpha3, and alpha4, bind to Stat3 upon cytokine stimulation. Similar results were observed for Stat1, but not for Stat5a and 5b, which were unable to interact with any of the importin alphas. The C-terminus of importin alpha5 is necessary but not sufficient for Stat3 binding. Truncation mutant of Stat3 (aa1-320) that contains Arg214/215 exhibits specific binding to importin alpha5, and an exclusive nuclear localization. Point mutations of Arg214/215 in this mutant destroy importin alpha5 binding and its nuclear localization. In contrast, the truncation mutant (aa320-770) including Arg414/417 fails to interact with importin alpha5 and is localized in the cytoplasm. However, both sequence elements are necessary for the full-length Stat3's interaction with importin alpha5. These results suggest that Arg214/215 is likely the binding site for importin alpha5, whereas Arg414/417 may not be involved in the direct binding, but necessary for maintaining the proper conformation of Stat3 dimer for importin binding. A model for Stat3 nuclear translocation is proposed based on these data.