The Stat1 binding motif of the interferon-gamma receptor is sufficient to mediate Stat5 activation and its repression by SOCS3

Signal transduction via the interferon-gamma (IFN-gamma) receptor requires the tyrosine phosphorylation of signal transducers and activators of transc ription (Stats). Whereas tyrosine phosphorylation of Stat1 occurs in all ce lls, activation of Stat5 by IFN-gamma is cell type-restricted. Here we inve stigated the mechanism of Stat5 activation by the IFN-gamma receptor. In tr ansfection assays both Stat5 isoforms, Stat5a and Stat5b, were phosphorylat ed on tyrosine in response to IFN-gamma. Stat5 activation required the pres ence of tyrosine 420 (Tyr-420) in the murine IFNGR1 receptor chain, which a lso serves as the Stat1 binding site. Moreover, a peptide including Tyr-440 , the Stat1 binding site of the human IFNGR1 chain, conferred the ability u pon a synthetic receptor to activate Stat5. Suppressor of cytokine signalin g 3 (SOCS3) inhibited the activation of Stat5 by the IFN-gamma receptor, an d the Tyr-440-containing peptide stretch was sufficient for repression. SOC S3 expression had little effect on the activity of Jak kinases not associat ed with cytokine receptors. In IFN-gamma -treated, Stat1-deficient fibrobla sts Stat5 was inefficient in inducing transcription of a Stat-dependent rep orter gene, suggesting it does not per se make a major contribution to the expression of IFN-gamma -responsive genes.