Distinct roles of the NH2- and COOH-terminal domains of the protein inhibitor of activated signal transducer and activator of transcription (STAT) 1 (PIAS1) in cytokine-induced PIAS1-Stat1 interaction

sTATs are activated by tyrosine phosphorylation on cytokine stimulation. A tyrosine-phosphorytated STAT forms a functional dimer through reciprocal Sr c: homology 2 domain (SH2)-phosphotyrosyl peptide interactions. IFN treatme nt induces the association of PIAS1 and stat1, which results in the inhibit ion of Stat1-mediated gene activation. The molecular basis of the cytokine- dependent PIAS1-Stat1 interaction has not been understood. We report here t hat a region near the COOH terminus of PIAS1 (amino acids 392-541) directly interacts with the NH2-terminal domain of Stat1 (amino acids 1-191). A mut ant PIAS1 lacking the Stat1-interacting domain tailed to inhibit Stat1-medi ated gene activation. By using a modified yeast two-hybrid assay, we demons trated that PIAS1 specifically interacts with the Stat1 dimer, but not tyro sine-phosphorylated or -unphosphorylated Stat1 monomer, In addition, wherea s the NH2-terminal region of PIAS1 does not interact with Stat1, it serves as a modulatory domain by preventing the interaction of the COOH-terminal d omain of PIAS1 with the Stat1 monomer. Thus, the cytokine-induced PIAS1-Sta t1 interaction is mediated through the specific recognition of the dimeric form of stat1 by PIAS1.