The proximal tyrosines of the cytoplasmic domain of the beta 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 interferon alpha-induced Stat2 tyrosine phosphorylation that allows normal formation of interferon-stimulated gene factor 3

The precise role of the different subunits (alpha/IFNAR1 and beta(L)/IFNAR2 ) of the type I interferon receptor (IFN-R) in the activation of signal tra nsducer and activator of transcription (Stat) 1, Stat2, and Stat3 has not y et been established. In this report we demonstrate that there are functiona lly redundant phosphotyrosine-dependent and -independent binding sites for Stat2 in the alpha and beta subunits of the type I IFN-R, Expression of a t ype I IFN-R containing only the constitutive Stat2 site or the proximal tyr osines of beta(L), but not the docking site on the a chain (Tyr(466) and Ty r(481)), supported low levels of Stat2 activation. However, the presence of only one intact Stat2 site did not lead to induction of interferon-stimula ted gene factor 3 (ISGF3) or an antiviral state. Normal levels of Stat2 tyr osine phosphorylation, induction of ISGF3, and an antiviral effect always r equired the proximal tyrosines of beta(L) and at least one of the other Sta t2 sites (Tyr(alpha 466,481) or beta(L404-462)). These data suggest that a threshold of Stat2 tyrosine phosphorylation is required for complete activa tion of ISGF3. Interestingly, a receptor in which all tyrosines were mutate d to phenylalanine shows normal Stat3 phosphorylation and low levels of act ivation of Stat1.