Unlike IFNα - activated ISGF3 complex in which tyrosine - phosphorylated STAT proteins are required, IRF - 9 could successfully interact with either
wt STAT2 or mutant STAT2 - Y690F in the absence of IFNα, although the interaction between IRF - 9 and
wt STAT2 could be obviously enhanced by IFNα via tyrosine phosphorylation of STAT2 (Fig. 2A).