Affinity of Stat2 for the subunits of the interferon alpha receptor

The interferon alpha receptor is composed of two subunits: IFNaR1 and IFNaR2. Interferon alpha binding to the receptor induces phosphorylation of tyrosine 466 on IFNaR1, which in turn binds the SH2 domain of the latent transcription factor Stat2 to initiate signaling. Stat2 also binds to IFNaR2 in a constitutive, phosphorylation-independent manner. To explore the function of the Stat2-IFNaR2 interaction and its possible relationship to the SH2-dependent docking of Stat2 to phosphorylated IFNaR1, the affinity of Stat2 for each of the receptor subunits was determined. Recombinant proteins corresponding to the cytoplasmic domains of the receptor subunits and the central core region of Stat2 were partially purified and used in affinity precipitation experiments to demonstrate that Stat2 binds more avidly to IFNaR2 than to phosphorylated IFNaR1. Surface plasmon resonance based biosensor analysis confirmed this finding; Stat2 bound IFNaR2 (K-d = 45 nM) approximately 6-fold stronger than it bound tyrosine 466-phosphorylated IFNaR1 (Kd = 245 nM). Affinity precipitation experiments involving all three proteins (Stat2, phosphorylated IFNaR1, and IFNaR2) indicated that the Stat2-receptor interactions are independent of one another. The relevance of these data to possible models of interferon alpha signal transduction is discussed.