Application of genomic DNA affinity chromatography identifies multiple interferon-alpha-regulated Stat2 complexes

Interferon-alpha (IFN-alpha)-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-alpha-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-alpha dependent co-precipitation of both Stat1 and Stat3. Taken together, our results suggest that IFN-alpha 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.