Phosphorylation and activation of the DNA binding activity of purified stat1 by the Janus protein-tyrosine kinases and the epidermal growth factor receptor

The activation of Janus protein-tyrosine kinases (Jaks) and the subsequent phosphorylation and activation of latent signal transducers and activators of transcription (Stats) are common elements in signal transduction through the cytokine receptor superfamily. To assess the role and specificity of Jaks in Stat activation, we have utilized baculovirus expression systems to produce Stat1 and the Jaks. Co-expression of Stat1 with Tyk2, Jak1, or Jak2 resulted in the specific tyrosine phosphorylation of Stat1 at Tyr⁷⁰¹, the residue phosphorylated in mammalian cells stimulated with interferon γ. Alternatively, Stat1, purified to apparent homogeneity from insect cell extracts, was phosphorylated at Tyr⁷⁰¹ in Jak immune complex kinase reactions. Phosphorylation of purified Stat1 was necessary and sufficient for the acquisition of DNA binding activity. The specificity in both systems was indicated by the inability of a Jak2 catalytically inactive mutant (Jak2- Glu⁸⁸²) or the Tec protein-tyrosine kinase to phosphorylate Stat1. However, immune complex-purified epidermal growth factor receptor was capable of phosphorylating purified Stat1 at Tyr⁷⁰¹ and activating its DNA binding activity in in vitro reactions.