The STAT3 NH₂-terminal domain stabilizes enhanceosome assembly by interacting with the p300 bromodomain

Signal transducer and activator of transcription 3 (STAT3) is a latent transcription factor mainly activated by the interleukin-6 cytokine family. Previous studies have shown that activated STAT3 recruits p300, a coactivator whose intrinsic histone acetyltransferase activity is essential for transcription. Here we investigated the function of the STAT3 NH ₂-terminal domain and how its interaction with p300 regulates STAT3 signal transduction. In STAT3^-/- mouse embryonic fibroblasts, a stably expressed NH₂ terminus-deficient STAT3 mutant (STAT3-ΔN) was unable to efficiently induce either STAT3-mediated reporter activity or endogenous mRNA expression. Chromatin immunoprecipitation assays were performed to determine whether the NH₂-terminal domain regulates p300 recruitment or stabilizes enhanceosome assembly. Despite equivalent levels of STAT3 binding, cells expressing the STAT3-ΔN mutant were unable to recruit p300 and RNA polymerase II to the native socs3 promoter as efficiently as those expressing STAT3-full length. We previously reported that the STAT3 NH ₂-terminal domain is acetylated by p300 at Lys-49 and Lys-87. By introducing K49R/K87R mutations, here we found that the acetylation status of the STAT3 NH₂-terminal domain regulates its interaction with p300. In addition, the STAT3 NH₂-terminal binding site maps to the p300 bromodomain, a region spanning from amino acids 995 to 1255. Finally a p300 mutant lacking the bromodomain (p300-ΔB) exhibited a weaker binding to STAT3, and the enhanceosome formation on the socs3 promoter was inhibited when p300-ΔB was overexpressed. Taken together, our data suggest that the STAT3 NH₂-terminal domain plays an important role in the interleukin-6 signaling pathway by interacting with the p300 bromodomain, thereby stabilizing enhanceosome assembly.