PROTEIN COMPLEX REQUIRED FOR OPTIMAL STAT1A-MEDIATED GBP PROMOTER ACTIVATION

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Interferons (IFNs) and other cytokines and growth factors activate the JAK (Janus kinase)/STAT (signal transducer and activator of transcription) pathway. In the IFN signaling, type I IFNs (IFN-a/b) upon binding to their receptor, activate intracellular, receptor-associated, tyrosine kinases JAK1 and Tyk2. These activated JAKs, in turn, phosphorylate specific tyrosine residues on latent cytoplasmic transcription factors which subsequently assemble into a complex called ISGF3 (IFN stimulated gene factor 3). This complex is composed of either STAT1a (91kD) or STAT1b (84kD) and STAT2 (113kD), which together constitute ISGF3-a, and a non-STAT protein called ISGF3-g (48kD) which is a member of the interferon regulatory factor (IRF) family. This complex accumulates in the nucleus, binds to a DNA element, ISRE (IFN-a stimulated response element, a 15 base pair non-dyad symmetrical DNA element), and activates transcription of target genes. ISGF3 is not activated by type II IFN, IFN-g. Instead, IFN-g, upon binding to its receptor and consequent activation of JAK1 and JAK2, induces the phosphorylation of STAT1a (or STAT1b), but not STAT2. The phosphorylated STAT1 then dimerizes and binds to a GAS (IFN-g activated sequence), DNA element that is dyad symmetrical 5¿TTN5AA3¿. Although both STAT1a and STAT1b can bind to GAS, only STAT1a forms functional GAF (IFN-g activated factor) that induces transcription from the GAS elements. Ultimately, one of the most crucial determinants affecting inherent transactivation potential of induced STATs may be a particular nucleoprotein microenvironment. Our objective is to discern patterns of cooperativity between activated STATs and other transcription factors, coactivator, and corepressor complexes within the context of the native promoter sequences. Such interactions are probably necessary to explain the role of STATs in gene activation at different IFN inducible genes. We have approached this problem by comparative study of the complex binding sites in several STAT-responsive genes. While STAT-binding sites exist and are likely required in chromosomal regulatory regions, single GAS elements give very little or no induction on their own in transient transfection assays. Therefore, we started with known native GAS elements and by adding adjacent native nucleotides determined the minimal size sequence that was IFN-responsive when cloned into a reporter gene. Then we determined whether this reporter inducibility correlated with an appearance of a novel band shift using a corresponding oligonucleotide in electrophoretic mobility shift assays (EMSA). We have detected in EMSA constitutive, low-mobility, protein complex that by mutational analysis is shown to be required for optimal STAT-mediated promoter activation. In particular, GBP promoter sequences that contain intact GAS and ISRE that bind STAT1 homodimer and IRF1, respectively, but that cannot bind constitutive low-mobility complex, when cloned in front of the heterologous reporter gene are inactive. Only sequences that bind STAT1, IRF1 and the constitutive low-mobility complex are able to activate a reporter gene upon IFN-g induction. Studies of the DNA affinity and specificity of this complex revealed that its DNA-binding may be affected by mutations within GAS as well as GAS-like site of the GBP promoter, suggesting a possible physical interaction with STATs or occupation of STAT sites that is relieved after appearance of activated STAT1 dimer. DNA affinity of this complex, observed in EMSA, is completely correlated to the transcriptional activation potential of the corresponding reporter constructs in transfection experiments. Currently we are engaged in obtaining larger quantities of partly purified preparation of constitutive low mobility complex which we will further purify in order to identify constituent subunits.