Ibf1 and Ibf2 are novel CP190-interacting proteins required for insulator function

摘要

Insulators are DNA-protein complexes that play a central role in chromatin organization and regulation of gene expression. In Drosophila different proteins, dCTCF, Su(Hw), and BEAF bind to specific subsets of insulators most of them having in common CP190. It has been shown that there are a number of CP190-binding sites that are not shared with any other known insulator protein, suggesting that other proteins could cooperate with CP190 to regulate insulator activity. Here we report on the identification of two previously uncharacterized proteins as CP190-interacting proteins, that we have named Ibf1 and Ibf2. These proteins localize at insulator bodies and associate with chromatin at CP190-binding sites throughout the genome. We also show that Ibf1 and Ibf2 are DNA-binding proteins that form hetero-oligomers that mediate CP190 binding to chromatin. Moreover, Ibf1 and Ibf2 are necessary for insulator activity in enhancer-blocking assays and Ibf2 null mutation cause a homeotic phenotype. Taken together our data reveal a novel pathway of CP190 recruitment to chromatin that is required for insulator activity. Synopsis CP190 is a common component of various Drosophila insulator complexes, but its role at genomic binding sites not shared with other known insulator factors is unclear. This study now identifies two new proteins that interact and cooperate with CP190 in insulator function. Characterization of CP190 interactors identifies Ibf1 and Ibf2 as novel insulator binding proteins. Ibf1 and Ibf2 are necessary for enhancer-blocking function of Drosophila insulators. Chromatin binding of the Ibf1/Ibf2 complex involves recognition of a novel DNA motif. Recruitment of the CP190 insulator protein to a subset of its chromatin binding sites depends on Ibf1/Ibf2. Two new factors join the ranks of fly insulator proteins that cooperate with CP190, helping to understand CP190 action at genomic binding sites not shared with other known insulator proteins. © 2014 The Authors.