The Stat1 Binding Motif of the Interferon-γ Receptor Is Sufficient to Mediate Stat5 Activation and Its Repression by SOCS3

Abstract

Signal transduction via the interferon-γ (IFN-γ) receptor requires the tyrosine phosphorylation of signal transducers and activators of transcription (Stats). Whereas tyrosine phosphorylation of Stat1 occurs in all cells, activation of Stat5 by IFN-γ is cell type-restricted. Here we investigated the mechanism of Stat5 activation by the IFN-γ receptor. In transfection assays both Stat5 isoforms, Stat5a and Stat5b, were phosphorylated on tyrosine in response to IFN-γ. Stat5 activation required the presence of tyrosine 420 (Tyr-420) in the murine IFNGR1 receptor chain, which also serves as the Stat1 binding site. Moreover, a peptide including Tyr-440, the Stat1 binding site of the human IFNGR1 chain, conferred the ability upon a synthetic receptor to activate Stat5. Suppressor of cytokine signaling 3 (SOCS3) inhibited the activation of Stat5 by the IFN-γ receptor, and the Tyr-440-containing peptide stretch was sufficient for repression. SOCS3 expression had little effect on the activity of Jak kinases not associated with cytokine receptors. In IFN-γ-treated, Stat1-deficient fibroblasts Stat5 was inefficient in inducing transcription of a Stat-dependent reporter gene, suggesting it does not per se make a major contribution to the expression of IFN-γ-responsive genes.