Insulators are multiprotein–DNA complexes that regulate the nuclear architecture. The
Drosophila CP190 protein is a cofactor for the DNA-binding insulator proteins Su(Hw), CTCF, and BEAF-32. The fact that CP190 has been found at genomic sites devoid of either of the known insulator factors has until now been unexplained. We have identified two DNA-binding zinc-finger proteins, Pita, and a new factor named ZIPIC, that interact with CP190 in vivo and in vitro at specific interaction domains. Genomic binding sites for these proteins are clustered with CP190 as well as with CTCF and BEAF-32. Model binding sites for Pita or ZIPIC demonstrate a partial enhancer-blocking activity and protect gene expression from PRE-mediated silencing. The function of the CTCF-bound
MCP insulator sequence requires binding of Pita. These results identify two new insulator proteins and emphasize the unifying function of CP190, which can be recruited by many DNA-binding insulator proteins.Insulators in the
Drosophila and vertebrate genomes have been identified based on their ability to disrupt the communication between an enhancer and a promoter when inserted between them (
Raab and Kamakaka 2010;
Ghirlando et al. 2012;
Herold et al. 2012;
Matzat and Lei 2013;
Chetverina et al. 2014;
Kyrchanova and Georgiev 2014). The growing amount of data show that insulator proteins fulfil an architectural function in mediating inter- and intrachromosomal interactions and in contacting regulatory elements such as promoters or enhancers (
Maksimenko and Georgiev 2014).The best studied
Drosophila insulator proteins, dCTCF (homolog of vertebrate insulator protein CTCF) and Su(Hw) are DNA-binding zinc-finger proteins (
Herold et al. 2012;
Matzat and Lei 2013;
Kyrchanova and Georgiev 2014). Binding sites for dCTCF have been identified in the insulators that separate functional regulatory domains of the
bithorax complex and in many promoter regions (
Moon et al. 2005;
Holohan et al. 2007;
Mohan et al. 2007;
Nègre et al. 2010,
2011;
Ni et al. 2012). The Su(Hw) protein more frequently associates with intergenic sites (
Adryan et al. 2007;
Bushey et al. 2009;
Nègre et al. 2010,
2011;
Soshnev et al. 2012,
2013). As shown in a transgenic assay, dCTCF and Su(Hw) binding sites can support specific distant interactions (
Kyrchanova et al. 2008a,
b), which suggests a key role for these proteins in organizing chromatin architecture.The Su(Hw), dCTCF, and BEAF-32 proteins interact with Centrosomal Protein 190 kD, named CP190 (
Pai et al. 2004;
Gerasimova et al. 2007;
Mohan et al. 2007;
Bartkuhn et al. 2009;
Oliver et al. 2010;
Liang et al. 2014). CP190 (1096 amino acids) contains an N-terminal BTB/POZ domain, an aspartic-acid-rich D-region, four C2H2 zinc-finger motifs, and a C-terminal E-rich domain (
Oliver et al. 2010;
Ahanger et al. 2013). The BTB domain of CP190 forms stable homodimers that may be involved in protein–protein interactions (
Oliver et al. 2010;
Bonchuk et al. 2011). In addition to these motifs, CP190 also contains a centrosomal targeting domain (M) responsible for its localization to centrosomes during mitosis (
Butcher et al. 2004). It has been shown that CP190 is recruited to chromatin via its interaction with the Su(Hw) and dCTCF proteins (
Pai et al. 2004;
Mohan et al. 2007). Inactivation of CP190 affects the activity of the dCTCF-dependent insulator
Fab-8 from the
bithorax complex (
Gerasimova et al. 2007;
Mohan et al. 2007;
Moshkovich et al. 2011) and the
gypsy insulator, which contains 12 binding sites for the Su(Hw) protein (
Pai et al. 2004). Binding of Su(Hw) and CP190 at
gypsy-like sites is mutually dependent, indicating a stabilizing role of CP190 in these cases (
Schwartz et al. 2012).Recent genome-wide ChIP-chip studies provide evidence for an extensive overlap of the CP190 distribution pattern with dCTCF, BEAF-32, and Su(Hw) insulator proteins and the promoters of active genes (
Bartkuhn et al. 2009;
Bushey et al. 2009;
Nègre et al. 2010,
2011;
Schwartz et al. 2012;
Soshnev et al. 2012). Very recently, it has been demonstrated that CP190 bridges DNA-bound insulator factors with promoters (
Liang et al. 2014). These data support the model that CP190 has a global role in the function of insulator proteins. However, there are a number of sites in the
Drosophila genome where CP190 does not colocalize with any known insulator DNA binding protein (IBP), suggesting that there may be some other proteins that recruit CP190 to chromatin (
Schwartz et al. 2012).To identify new factors that associate with CP190, we purified the FLAG-tagged CP190 protein from S2 cells and identified two zinc-finger proteins, CG7928 and Pita, which were shown to interact with CP190 in vivo and in vitro. Genome-wide identification of binding sites for Pita and CG7928 in S2 cells revealed their extensive colocalization with CP190, providing evidence for direct interactions between these proteins, which was supported by binding and in vivo functional assays. Based on these results we termed CG7928 the “
zinc-finger
protein
interacting with
CP190” (ZIPIC).
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