DISTINCT STAT STRUCTURE PROMOTES INTERACTION OF STAT2 WITH THE P48 SUBUNIT OF THE INTERFERON-ALPHA-STIMULATED TRANSCRIPTION FACTOR ISGF3

Cells express a variety of STAT (signal transducer and activator of tr anscription) transcription factors that are structurally homologous an d yet function specifically in response to particular cytokines, The f unctions of the individual STATs are dependent on distinct protein-pro tein interactions. STAT1 and STATE are activated by tyrosine phosphory lation in response to type I interferons-alpha/beta (IFN-alpha/beta) a nd subsequently farm a multimeric transcription factor designated the IFN-alpha-stimulated gene factor 3 (ISGF3). ISGF3 is a unique STAT com plex because it also contains a Iron-STAT molecule, p48, which is a cr itical DNA-binding component. We provide evidence that STATE specifica lly interacts with p48 in vivo before and after IFN-alpha stimulation, The specificity of ISGF3 formation is therefore a result of the disti nct nature of the STATE molecule, Coimmunoprecipitation assays demonst rate p48 association with STAT2 but not STAT1. Hybrid STAT2.STAT1 mole cules were used to identify a region of STATE which specifically assoc iates with p48. The region of STATE interaction spans an amino-termina l region of two predicted coiled coils, The studies demonstrate the in vivo existence of a STAT2.p48 complex and a distinct STAT2.STAT1 comp lex after IFN-alpha stimulation. Data suggest that distinct bipartite complexes STAT2.p48 and STAT2.STAT1 translocate to the nucleus and ass ociate on the DNA target site as ISGF3.