The Experts below are selected from a list of 326502 Experts worldwide ranked by ideXlab platform
Curt M Horvath - One of the best experts on this subject based on the ideXlab platform.
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A Point Mutation, E95D, in the Mumps Virus V Protein Disengages STAT3 Targeting from STAT1 Targeting
Journal of Virology, 2009Co-Authors: Mamta Puri, W. Paul Duprex, Bertus K. Rima, Ken Lemon, Curt M HorvathAbstract:Mumps virus, like other paramyxoviruses in the Rubulavirus genus, encodes a V protein that can assemble a ubiquitin ligase complex from cellular components, leading to the destruction of cellular signal transducer and activator of transcription (STAT) proteins. While many V proteins target the interferon-activated STAT1 or STAT2 protein, mumps virus V protein is unique in its ability to also target STAT3 for ubiquitin modification and proteasome-mediated degradation. Here we report that a single amino acid substitution in the mumps virus V protein, E95D, results in defective STAT3 targeting while maintaining the ability to target STAT1. Results indicate that the E95D mutation disrupts the ability of the V protein to associate with STAT3. A recombinant mumps virus carrying the E95D mutation in its P and V proteins replicates normally in cultured cells but fails to induce targeting of STAT3. Infection with the recombinant virus results in the differential regulation of a number of cellular genes compared to wild-type mumps virus and increases cell death in infected cells, producing a large-plaque phenotype.
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stat2 is a primary target for measles virus v protein mediated alpha beta interferon signaling inhibition
Journal of Virology, 2008Co-Authors: Aparna Ramachandran, Curt M Horvath, Jean Patrick ParisienAbstract:Measles virus, a member of the Morbillivirus family, infects millions of people each year despite the availability of effective vaccines. The V protein of measles virus is an important virulence factor that can interfere with host innate immunity by inactivating alpha/beta interferon (IFN-α/β) and IFN-γ signaling through protein interactions with signal transducer and activator of transcription proteins STAT1 and STAT2. Here we demonstrate that although STAT1 interference results from protein interactions within a V protein N-terminal region encompassed by amino acids 110 to 130, detection of STAT1 interaction and IFN-γ signaling inhibition requires the presence of cellular STAT2. Cell-specific variability in STAT1 interference was observed to correlate with V protein expression level. A more direct target for measles virus V protein-mediated IFN-α/β evasion is STAT2. Results indicate that the widely conserved C-terminal zinc finger domain of measles virus V protein is both necessary and sufficient to bind STAT2 and disrupt IFN-α/β signal transduction. Mutagenesis and molecular modeling define a contact surface for STAT2 association that includes aspartic acid residue 248 as critical for STAT2 interference and IFN antiviral immune suppression. These findings clearly define the molecular determinants for measles virus IFN evasion and validate specific targets as candidates for therapeutic intervention.
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Enabled interferon signaling evasion in an immune-competent transgenic mouse model of parainfluenza virus 5 infection
Virology, 2007Co-Authors: Thomas Kraus, Lily Garza, Curt M HorvathAbstract:Parainfluenza virus 5 (PIV5 or SV5) infects several mammalian species but is restricted from efficient replication in mice. In humans, PIV5 evades IFN signaling by targeting STAT1 for proteasomal degradation in a STAT2-dependent reaction. In contrast, cell culture experiments have demonstrated that the divergent murine STAT2 protein fails to support STAT1 targeting. Expression of human STAT2 in mouse cells can overcome the species restriction to enable PIV5-induced STAT1 degradation and subsequent IFN antagonism. Here, we describe a transgenic mouse that ubiquitously expresses human STAT2. PIV5 infection induces STAT1 degradation leading to enhanced virus replication and protein expression in the cells from the transgenic mouse but not from the non-transgenic littermates. Importantly, intranasal inoculation with PIV5 results in increased viral load in the lungs of the transgenic mice compared to wild-type littermates. These transgenic mice provide a small animal model to study the role of innate immune evasion in paramyxovirus pathogenesis.
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Identification of the Nuclear Export Signal and STAT-Binding Domains of the Nipah Virus V Protein Reveals Mechanisms Underlying Interferon Evasion
Journal of Virology, 2004Co-Authors: Jason J. Rodriguez, Cristian D. Cruz, Curt M HorvathAbstract:The V proteins of Nipah virus and Hendra virus have been demonstrated to bind to cellular STAT1 and STAT2 proteins to form high-molecular-weight complexes that inhibit interferon (IFN)-induced antiviral transcription by preventing STAT nuclear accumulation. Analysis of the Nipah virus V protein has revealed a region between amino acids 174 and 192 that functions as a CRM1-dependent nuclear export signal (NES). This peptide is sufficient to complement an export-defective human immunodeficiency virus Rev protein, and deletion and substitution mutagenesis revealed that this peptide is necessary for both V protein shuttling and cytoplasmic retention of STAT1 and STAT2 proteins. However, the NES is not required for V-dependent IFN signaling inhibition. IFN signaling is blocked primarily by interaction between Nipah virus V residues 100 to 160 and STAT1 residues 509 to 712. Interaction with STAT2 requires a larger Nipah virus V segment between amino acids 100 and 300, but deletion of residues 230 to 237 greatly reduced STAT2 coprecipitation. Further, V protein interactions with cellular STAT1 is a prerequisite for STAT2 binding, and sequential immunoprecipitations demonstrate that V, STAT1, and STAT2 can form a tripartite complex. These findings characterize essential regions for Henipavirus V proteins that represent potential targets for therapeutic intervention.
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hendra virus v protein inhibits interferon signaling by preventing STAT1 and stat2 nuclear accumulation
Journal of Virology, 2003Co-Authors: Jason J. Rodriguez, Linfa Wang, Curt M HorvathAbstract:The V protein of the recently emerged paramyxovirus, Nipah virus, has been shown to inhibit interferon (IFN) signal transduction through cytoplasmic sequestration of cellular STAT1 and STAT2 in high-molecular-weight complexes. Here we demonstrate that the closely related Hendra virus V protein also inhibits cellular responses to IFN through binding and cytoplasmic sequestration of both STAT1 and STAT2, but not STAT3. These findings demonstrate a V protein-mediated IFN signal evasion mechanism that is a general property of the known Henipavirus species.
Chienkuo Lee - One of the best experts on this subject based on the ideXlab platform.
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stat3 cooperates with phospholipid scramblase 2 to suppress type i interferon response
Frontiers in Immunology, 2018Co-Authors: Minghsun Tsai, Chienkuo LeeAbstract:Type I interferon (IFN-I) is a pluripotent cytokine that modulates innate and adaptive immunity. We have previously shown that STAT3 suppresses IFN-I response in a manner dependent on its N-terminal domain (NTD), but independent of its DNA-binding and transactivation ability. Using the yeast two-hybrid system, we have identified phospholipid scramblase 2 (PLSCR2) as a STAT3 NTD-binding partner and a suppressor of IFN-I response. Overexpression of PLSCR2 attenuates ISRE-driven reporter activity, which is further aggravated by co-expression of STAT3. Moreover, PLSCR2 deficiency enhances IFN-I-induced gene expression and antiviral activity without affecting the activation or nuclear translocation of STAT1 and STAT2 or the assembly of ISGF3 complex. Instead, PLSCR2 impedes promoter occupancy by ISGF3, an effect further intensified by the presence of STAT3. Moreover, palmitoylation of PLSCR2 is required for its binding to STAT3 and for this suppressive activity. In addition to STAT3, PLSCR2 also interacts with STAT2, which facilitates the suppressive effect on ISGF3-mediated transcriptional activity. Together, these results define the role of a novel STAT3-PLSCR2 axis in fine-tuning IFN-I response.
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Data_Sheet_1_STAT3 Cooperates With Phospholipid Scramblase 2 to Suppress Type I Interferon Response.PDF
2018Co-Authors: Minghsun Tsai, Chienkuo LeeAbstract:Type I interferon (IFN-I) is a pluripotent cytokine that modulates innate and adaptive immunity. We have previously shown that STAT3 suppresses IFN-I response in a manner dependent on its N-terminal domain (NTD), but independent of its DNA-binding and transactivation ability. Using the yeast two-hybrid system, we have identified phospholipid scramblase 2 (PLSCR2) as a STAT3 NTD-binding partner and a suppressor of IFN-I response. Overexpression of PLSCR2 attenuates ISRE-driven reporter activity, which is further aggravated by co-expression of STAT3. Moreover, PLSCR2 deficiency enhances IFN-I-induced gene expression and antiviral activity without affecting the activation or nuclear translocation of STAT1 and STAT2 or the assembly of ISGF3 complex. Instead, PLSCR2 impedes promoter occupancy by ISGF3, an effect further intensified by the presence of STAT3. Moreover, palmitoylation of PLSCR2 is required for its binding to STAT3 and for this suppressive activity. In addition to STAT3, PLSCR2 also interacts with STAT2, which facilitates the suppressive effect on ISGF3-mediated transcriptional activity. Together, these results define the role of a novel STAT3–PLSCR2 axis in fine-tuning IFN-I response.
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stat2 irf9 directs a prolonged isgf3 like transcriptional response and antiviral activity in the absence of STAT1
Biochemical Journal, 2015Co-Authors: Katarzyna Blaszczyk, Adam Olejnik, Hanna Nowicka, Lilla Ozgyin, Yiling Chen, Stefan Chmielewski, Kaja Kostyrko, Joanna Wesoly, Balint L Balint, Chienkuo LeeAbstract:Evidence is accumulating for the existence of a signal transducer and activator of transcription 2 (STAT2)/interferon regulatory factor 9 (IRF9)-dependent, STAT1-independent interferon alpha (IFNα) signalling pathway. However, no detailed insight exists into the genome-wide transcriptional regulation and the biological implications of STAT2/IRF9-dependent IFNα signalling as compared with interferon-stimulated gene factor 3 (ISGF3). In STAT1-defeicient U3C cells stably overexpressing human STAT2 (hST2-U3C) and STAT1-deficient murine embryonic fibroblast cells stably overexpressing mouse STAT2 (mST2-MS1KO) we observed that the IFNα-induced expression of 2′-5′-oligoadenylate synthase 2 (OAS2) and interferon-induced protein with tetratricopeptide repeats 1 (Ifit1) correlated with the kinetics of STAT2 phosphorylation, and the presence of a STAT2/IRF9 complex requiring STAT2 phosphorylation and the STAT2 transactivation domain. Subsequent microarray analysis of IFNα-treated wild-type (WT) and STAT1 KO cells overexpressing STAT2 extended our observations and identified ∼120 known antiviral ISRE-containing interferon-stimulated genes (ISGs) commonly up-regulated by STAT2/IRF9 and ISGF3. The STAT2/IRF9-directed expression profile of these IFN-stimulated genes (ISGs) was prolonged as compared with the early and transient response mediated by ISGF3. In addition, we identified a group of ‘STAT2/IRF9-specific’ ISGs, whose response to IFNα was ISGF3-independent. Finally, STAT2/IRF9 was able to trigger an antiviral response upon encephalomyocarditis virus (EMCV) and vesicular stomatitis Indiana virus (VSV). Our results further prove that IFNα-activated STAT2/IRF9 induces a prolonged ISGF3-like transcriptome and generates an antiviral response in the absence of STAT1. Moreover, the existence of ‘STAT2/IRF9-specific’ target genes predicts a novel role of STAT2 in IFNα signalling.
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STAT1 and stat2 but not stat3 arbitrate contradictory growth signals elicited by alpha beta interferon in t lymphocytes
Molecular and Cellular Biology, 2005Co-Authors: Ramon Gimeno, Christian Schindler, Chienkuo Lee, David E LevyAbstract:Alpha/beta interferon (IFN-α/β) triggers antiviral and antiproliferative responses in target cells through modulation of gene expression. The JAK-STAT pathway is the major mediator of these biological effects through the activation of the transcription factors STAT1 and STAT2, and gene ablation studies have demonstrated that both STAT1 and STAT2 are required for most antiviral responses induced by IFN-α/β. However, additional signaling pathways are also activated by IFN. Here, we show that these additional pathways provoke a proliferative response in activated T lymphocytes. While activation of IFN-stimulated gene factor 3 produces a dominant inhibitory signal capable of overriding the mitogenic response, absence of either STAT1 or STAT2 leads to a proliferative response to IFN. Growth stimulation by IFN-α/β is independent of other STAT proteins, particularly of STAT3, since T lymphocytes from STAT1-STAT3 double-knockout mice are growth stimulated by IFN-α/β treatment. IFN-α/β can cooperate with numerous T-cell mitogens, including interleukin-2 (IL-2), IL-4, IL-7, and IL-12, and can contribute to the rapid restoration of the thymus following glucocorticoid-mediated ablation. These results underscore the complexity of the cellular response to IFN and suggest that the ultimate outcome of IFN action results from a balance between growth-inhibitory and -stimulatory effects.
Jean-christophe Renauld - One of the best experts on this subject based on the ideXlab platform.
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role of the interleukin il 28 receptor tyrosine residues for antiviral and antiproliferative activity of il 29 interferon λ1 similarities with type i interferon signaling
Journal of Biological Chemistry, 2004Co-Authors: Laure Dumoutier, Amel Tounsi, Thomas Michiels, Caroline Sommereyns, Sergei V Kotenko, Jean-christophe RenauldAbstract:Interferon (IFN)-lambda1, -lambda2, and -lambda3 are the latest members of the class II cytokine family and were shown to have antiviral activity. Their receptor is composed of two chains, interleukin-28R/likely interleukin or cytokine or receptor 2 (IL-28R/LICR2) and IL-10Rbeta, and mediates the tyrosine phosphorylation of STAT1, STAT2, STAT3, and STAT5. Here, we show that activation of this receptor by IFN-lambda1 can also inhibit cell proliferation and induce STAT4 phosphorylation, further extending functional similarities with type I IFNs. We used IL-28R/LICR2-mutated receptors to identify the tyrosines required for STAT activation, as well as antiproliferative and antiviral activities. We found that IFN-lambda1-induced STAT2 tyrosine phosphorylation is mediated through tyrosines 343 and 517 of the receptor, which showed some similarities with tyrosines from type I IFN receptors involved in STAT2 activation. These two tyrosines were also responsible for antiviral and antiproliferative activities of IFN-lambda1. By contrast, STAT4 phosphorylation ( and to some extent STAT3 activation) was independent from IL-28R/LICR2 tyrosine residues. Taken together, these observations extend the functional similarities between IFN-lambdas and type I IFNs and shed some new light on the mechanisms of activation of STAT2 and STAT4 by these cytokines.
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role of the interleukin il 28 receptor tyrosine residues for antiviral and antiproliferative activity of il 29 interferon lambda 1 similarities with type i interferon signaling
Journal of Biological Chemistry, 2004Co-Authors: Laure Dumoutier, Amel Tounsi, Thomas Michiels, Caroline Sommereyns, Sergei V Kotenko, Jean-christophe RenauldAbstract:Interferon (IFN)-lambda1, -lambda2, and -lambda3 are the latest members of the class II cytokine family and were shown to have antiviral activity. Their receptor is composed of two chains, interleukin-28R/likely interleukin or cytokine or receptor 2 (IL-28R/LICR2) and IL-10Rbeta, and mediates the tyrosine phosphorylation of STAT1, STAT2, STAT3, and STAT5. Here, we show that activation of this receptor by IFN-lambda1 can also inhibit cell proliferation and induce STAT4 phosphorylation, further extending functional similarities with type I IFNs. We used IL-28R/LICR2-mutated receptors to identify the tyrosines required for STAT activation, as well as antiproliferative and antiviral activities. We found that IFN-lambda1-induced STAT2 tyrosine phosphorylation is mediated through tyrosines 343 and 517 of the receptor, which showed some similarities with tyrosines from type I IFN receptors involved in STAT2 activation. These two tyrosines were also responsible for antiviral and antiproliferative activities of IFN-lambda1. By contrast, STAT4 phosphorylation ( and to some extent STAT3 activation) was independent from IL-28R/LICR2 tyrosine residues. Taken together, these observations extend the functional similarities between IFN-lambdas and type I IFNs and shed some new light on the mechanisms of activation of STAT2 and STAT4 by these cytokines.
Eleanor N. Fish - One of the best experts on this subject based on the ideXlab platform.
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The interferon-inducible Stat2:STAT1 heterodimer preferentially binds in vitro to a consensus element found in the promoters of a subset of interferon-stimulated genes.
Journal of Interferon and Cytokine Research, 2001Co-Authors: Julien Ghislain, Melody Nguyen, Thomas Wong, Eleanor N. FishAbstract:Regulated expression of type I interferon (IFN)-stimulated genes (ISG) requires the binding of the signal transducer and activator of transcription (Stat) complexes to enhancer elements located in the ISG promoters. These enhancer elements include the IFN-stimulated response element (ISRE) and the palindromic IFN-γ activation site (GAS) element. Regulated expression of ISRE containing ISG depends on IFN-stimulated gene factor 3 (ISGF3), a heterodimer involving STAT1 and Stat2 in association with a DNA-binding adapter protein, p48/IFN regulatory factor-9 (IRF-9). Several GAS binding Stat complexes involving STAT1, Stat3, Stat4, and Stat5 have been described, but their contribution to GAS-dependent ISG expression remains to be established. We and others previously identified an IFN-α-inducible Stat2:1 heterodimer that exhibits binding to the GAS element of the IRF-1 gene. These previous studies raise the possibility that Stat2:1 may participate in the transcriptional activation of the subset of ISG containi...
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the proximal tyrosines of the cytoplasmic domain of the β chain of the type i interferon receptor are essential for signal transducer and activator of transcription stat 2 activation evidence that two stat2 sites are required to reach a threshold of
Journal of Biological Chemistry, 1999Co-Authors: Owen W Nadeau, David E Levy, Paul Domanski, Anna Usacheva, Shahab Uddin, Leonidas C Platanias, Paula M Pitha, Regina Raz, Beata Majchrzak, Eleanor N. FishAbstract:The precise role of the different subunits (alpha/IFNAR1 and betaL/IFNAR2) of the type I interferon receptor (IFN-R) in the activation of signal transducer and activator of transcription (Stat) 1, Stat2, and Stat3 has not yet been established. In this report we demonstrate that there are functionally redundant phosphotyrosine-dependent and -independent binding sites for Stat2 in the alpha and beta subunits of the type I IFN-R. Expression of a type I IFN-R containing only the constitutive Stat2 site or the proximal tyrosines of betaL, but not the docking site on the alpha chain (Tyr466 and Tyr481), supported low levels of Stat2 activation. However, the presence of only one intact Stat2 site did not lead to induction of interferon-stimulated gene factor 3 (ISGF3) or an antiviral state. Normal levels of Stat2 tyrosine phosphorylation, induction of ISGF3, and an antiviral effect always required the proximal tyrosines of betaL and at least one of the other Stat2 sites (Tyralpha466, 481 or betaL404-462). These data suggest that a threshold of Stat2 tyrosine phosphorylation is required for complete activation of ISGF3. Interestingly, a receptor in which all tyrosines were mutated to phenylalanine shows normal Stat3 phosphorylation and low levels of activation of STAT1.
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Application of Genomic DNA Affinity Chromatography Identifies Multiple Interferon-α-regulated Stat2 Complexes
Journal of Biological Chemistry, 1996Co-Authors: Julien J. Ghislain, Eleanor N. FishAbstract:Abstract Interferon-α (IFN-α)-induced signal transduction is mediated by the phosphorylation-activation of the signal transducer and activator of transcription (STAT) proteins STAT1, Stat2, and Stat3. Previous studies have shown that these activated STATs dimerize to form four distinct STAT complexes which translocate to the nucleus and activates transcription by binding to specific promoter elements. The interferon-stimulated gene factor-3 (ISGF3) consists of Stat2 and STAT1 heterodimers in association with a DNA-binding protein, p48, that binds to the interferon stimulated response element. Homo- and heterodimers of STAT1 and Stat3 bind to the palindromic interferon response element (pIRE). In this report we demonstrate the utility of a biochemical procedure that we have developed, based on genomic DNA affinity chromatography, for the identification of IFN-α-induced STAT complexes. Using this approach, we identified ISGF3-independent Stat2-containing STAT complexes. Results from the analysis of Stat2 complexes in the electrophoretic mobility shift assay were consistent with genomic DNA affinity chromatography results and identified a Stat2:1 complex that binds with low affinity to the pIRE of the interferon regulatory factor-1 gene. Immunoprecipitation studies of Stat2 revealed an IFN-α dependent co-precipitation of both STAT1 and Stat3. Taken together, our results suggest that IFN-α activates, in addition to ISGF3, other Stat2-containing STAT complexes, one of which binds to an element related to the interferon regulatory factor-1 pIRE.
George R. Stark - One of the best experts on this subject based on the ideXlab platform.
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alternative activation of STAT1 and stat3 in response to interferon γ
Journal of Biological Chemistry, 2004Co-Authors: Yulan Qing, George R. StarkAbstract:Interferon-γ (IFNγ) is a pluripotent cytokine whose major biological effects are mediated through a pathway in which STAT1 is the predominant and essential transcription factor. STAT3 can also be activated weakly by IFNγ, but the mechanism of activation and function of STAT3 as a part of the interferon response are not known. Here we show that STAT3 activation is much stronger and more prolonged in STAT1-null mouse embryo fibroblasts than in wild-type cells. In response to IFNγ, SRC-family kinases are required to activate STAT3 (but not STAT1) through tyrosine phosphorylation, whereas the receptor-bound kinases JAK1 and JAK2 are required to activate both STATs. Tyrosine 419 of the IFNγ receptor subunit 1 (IFNGR1) is required to activate both STATs, suggesting that STAT1 and STAT3 compete with each other for the same receptor phosphotyrosine motif. Activated STAT3 can replace STAT1 in STAT1-null cells to drive the transcription of certain genes, for example, socs-3 and c/ebpδ, which have γ-activated sequence motifs in their promoters. Work from Ian Kerr's laboratory reveals that the gp130-linked interleukin-6 receptor, which usually activates STAT3 predominantly, activates STAT1 efficiently when STAT3 is absent (Costa-Pereira, A. P., Tininini, S., Strobl, B., Alonzi, T., Schlaak, J. F., Is'harc, H., Gesualdo, I., Newman, S. J., Kerr, I. M., and Poli, V. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 8043–8047). Because STAT1 and STAT3 have opposing biological effects (STAT3 is an oncogene, and STAT1 is a tumor suppressor), the reciprocal activation of these two transcription factors in response to IFNγ or interleukin-6 suggests that their relative abundance, which may vary substantially in different normal cell types, under different conditions or in tumors is likely to have a major impact on how cells behave in response to different cytokines.
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Functional subdomains of STAT2 required for preassociation with the alpha interferon receptor and for signaling.
Molecular and cellular biology, 1997Co-Authors: Stewart Leung, Ian M. Kerr, George R. StarkAbstract:Two members of the STAT signal transducer and activator of transcription family, STAT1 and STAT2, are rapidly phosphorylated on tyrosine in response to alpha interferon (IFN-alpha). Previous work showed that in the mutant human cell line U6A, which lacks STAT2 and is completely defective in IFN-alpha signaling, the phosphorylation of STAT1 is very weak, revealing that activation of STAT1 depends on STAT2. We now find that STAT2 binds to the cytoplasmic domain of the IFNAR2c (also known as IFNAR2-2) subunit of the IFN-alpha receptor in extracts of untreated cells. STAT1 also binds but only when STAT2 is present. The activities of chimeric STAT2-STAT1 proteins were assayed in U6A cells to define regions required for IFN-alpha signaling. Previous work showed that a point mutation in the Src homology 2 (SH2) domain prevents STAT2 from binding to phosphotyrosine 466 of the IFNAR1 subunit of the activated receptor. However, we now find that the entire SH2 domain of STAT2 can be replaced by that of STAT1 without loss of function, revealing that other regions of STAT2 are required for its specific interaction with the receptor. A chimeric protein, in which the N-terminal third of STAT2 has replaced the corresponding region of STAT1, did preassociate with the IFNAR2c subunit of the receptor, became phosphorylated when IFN-alpha was added, and supported the phosphorylation of endogenous STAT1. These results are consistent with a model in which STAT2 and STAT1 are prebound to the IFNAR2c subunit of the resting receptor. Upon activation, the IFNAR1 subunit is phosphorylated on Tyr-466, allowing the SH2 domain of STAT2 to bind to it; this is followed by the sequential phosphorylation of STAT2 and STAT1.
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formation of STAT1 stat2 heterodimers and their role in the activation of irf 1 gene transcription by interferon α
Journal of Biological Chemistry, 1996Co-Authors: Stewart Leung, Sajjad A Qureshi, James E Darnell, George R. StarkAbstract:Abstract An upstream inverted repeat (IR) element mediates transcriptional activation of the interferon response factor-1 gene (IRF-1) by interferon (IFN)-α and IFN-. IFN-α and IFN- fail to induce IRF-1 in cells that lack signal transducer and activator of transcription 1 (STAT1), and STAT1 homodimers bind to IR elements in extracts of IFN-α-treated cells. We now report that STAT2 also plays an important role in the IFN-α-mediated transcriptional activation of the IRF-1 gene. A new factor, most likely a STAT1-STAT2 heterodimer, was detected with an IR probe in extracts of IFN-α-treated cells. STAT1 and STAT2 are already known to combine with p48, a DNA-binding protein, to form IFN-stimulated gene factor 3 (ISGF3), which binds to IFN-stimulated response elements (ISREs) distinct from the IR of the IRF-1 gene. In extracts of U2A cells, which lack p48, STAT1-STAT2 heterodimers were still formed, indicating that they do not contain p48. We manipulated the intracellular levels of STAT1-STAT2 heterodimers and STAT1 homodimers to examine their roles in the induction of IRF-1 by IFN-α. Although both dimers can induce IRF-1 transcription, the heterodimers are more potent and thus may be the major activators in vivo. Deletion analysis reveals that the C-terminal domain of STAT2 is important for transcriptional activation mediated by both STAT1-STAT2 heterodimers and ISGF3.
