Molecular characterization of Celtix-1, a bromodomain protein interacting with the transcription factor interferon regulatory factor 2

Abstract

Transcriptional control at the G1/S‐phase transition of the cell cycle requires functional interactions of multimeric promoter regulatory complexes that contain DNA binding proteins, transcriptional cofactors, and/or chromatin modifying enzymes. Transcriptional regulation of the human histone H4/n gene (FO108) is mediated by Interferon Regulatory Factor‐2 (IRF‐2), as well as other histone gene promoter factors. To identify proteins that interact with cell‐cycle regulatory factors, we performed yeast two‐hybrid analysis with IRF‐2 and identified a novel human protein termed Celtix‐1 which binds to IRF‐2. Celtix‐1 contains several phylogenetically conserved domains, including a bromodomain, which is found in a number of transcriptional cofactors. Using a panel of IRF‐2 deletion mutants in yeast two‐hybrid assays, we established that Celtix‐1 contacts the C‐terminus of IRF‐2. Celtix‐1 directly interacts with IRF‐2 based on binding studies with glutathione S‐transferase (GST)/IRF‐2 fusion proteins, and immunofluorescence studies suggest that Celtix‐1 and IRF‐2 associate in situ. Celtix‐1 is distributed throughout the nucleus in a heterodisperse pattern. A subset of Celtix‐1 colocalizes with the hyperacetylated forms of histones H3 and H4, as well as with the hyperphosphorylated, transcriptionally active form of RNA polymerase II. We conclude that the bromodomain protein Celtix‐1 is a novel IRF‐2 interacting protein that associates with transcriptionally active chromatin in situ. J. Cell. Physiol. 185:269–279, 2000.