Polycomb group and SCF ubiquitin ligases are found in a novel BCOR complex that is recruited to BCL6 targets

Department of Genetics, Cell Biology and Development, 6-160 Jackson Hall, 321 Church Street SE, Minneapolis, MN 55455, USA.
Molecular and Cellular Biology (Impact Factor: 4.78). 10/2006; 26(18):6880-9. DOI: 10.1128/MCB.00630-06
Source: PubMed

ABSTRACT The corepressor BCOR potentiates transcriptional repression by the proto-oncoprotein BCL6 and suppresses the transcriptional activity of a common mixed-lineage leukemia fusion partner, AF9. Mutations in human BCOR cause male lethal, X-linked oculofaciocardiodental syndrome. We identified a BCOR complex containing Polycomb group (PcG) and Skp-Cullin-F-box subcomplexes. The PcG proteins include RING1, RYBP, NSPC1, a Posterior Sex Combs homolog, and RNF2, an E3 ligase for the mono-ubiquitylation of H2A. BCOR complex components and mono-ubiquitylated H2A localize to BCL6 targets, indicating that the BCOR complex employs PcG proteins to expand the repertoire of enzymatic activities that can be recruited by BCL6. This also suggests that BCL6 can target PcG proteins to DNA. In addition, the BCOR complex contains components of a second ubiquitin E3 ligase, namely, SKP1 and FBXL10 (JHDM1B). We show that BCOR coimmunoprecipitates isoforms of FBXL10 which contain a JmjC domain that recently has been determined to have histone H3K36 demethylase activity. The recruitment of two distinct classes of E3 ubiquitin ligases and a histone demethylase by BCOR suggests that BCOR uses a unique combination of epigenetic modifications to direct gene silencing.

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    • "In addition, the mammalian Kdm2 homolog Fbxl10 binds to CpG islands through its CXXC domain and recruits the PRC1 proteins RING1B and Nspc1 to DNA in embryonic stem cells (Wu et al., 2013). Moreover, several transcription factors have been involved in the recruitment of a subset of PRC1s (Gearhart et al., 2006; Trojer et al., 2011). Therefore, the use of DNAbinding proteins to tether PRC1s to target genes may be a conserved mechanism. "
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    ABSTRACT: From mammals to plants, the Polycomb Group (PcG) machinery plays a crucial role in maintaining the repression of genes that are not required in a specific differentiation status. However, the mechanism by which PcG machinery mediates gene repression is still largely unknown in plants. Compared to animals, few PcG proteins have been identified in plants, not only because just some of these proteins are clearly conserved to their animal counterparts, but also because some PcG functions are carried out by plant-specific proteins, most of them as yet uncharacterized. For a long time, the apparent lack of Polycomb Repressive Complex (PRC)1 components in plants was interpreted according to the idea that plants, as sessile organisms, do not need a long-term repression as they must be able to respond rapidly to environmental signals; however, some PRC1 components have been recently identified, indicating that this may not be the case. Furthermore, new data regarding the recruitment of PcG complexes and maintenance of PcG repression in plants have revealed important differences to what has been reported so far. This review highlights recent progress in plant PcG function, focusing on the role of the putative PRC1 components.
    Molecular Plant 10/2013; 7(3). DOI:10.1093/mp/sst150 · 6.34 Impact Factor
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    • "BCoR is a large protein of 1721 amino acids with three ankyrin repeats but otherwise little homology to known protein sequences. It interacts with HDACs 1, 3, 4 and 5 [38] and engages in macromolecular complexes for epigenetic modifications to direct gene silencing [39]. BCoR has no DNA binding domain but is thought to function in association with transcription factors like BCL-6, AF9 or Sp1 [40,41]. "
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    ABSTRACT: Nuclear inclusions of aggregated proteins have primarily been characterized for molecules with aberrant poly-glutamine repeats and for mutated or structurally altered proteins. They were termed "nuclear aggresomes" and misfolding was shown to promote association with molecular chaperones and proteasomes. Here, we report that two components of a transcriptional repressor complex (BCL-6 and BCoR) of wildtype amino acid sequence can independently or jointly induce the formation of nuclear aggregates when overexpressed. The observation that the majority of cells rapidly downregulate BCL-6/BCoR levels, supports the notion that expression of these proteins is under tight control. The inclusions occur when BCL-6/BCoR expression exceeds 150-fold of endogenous levels. They preferentially develop in the nucleus by a gradual increase in aggregate size to form large, spheroid structures which are not associated with heat shock proteins or marked by ubiquitin. In contrast, we find the close association of BCL-6/BCoR inclusions with PML bodies and a reduction in aggregation upon the concomitant overexpression of histone deacetylases or heat shock protein 70. In summary, our data offer a perspective on nuclear aggregates distinct from classical "nuclear aggresomes": Large complexes of spheroid structure can evolve in the nucleus without being marked by the cellular machinery for protein refolding and degradation. However, nuclear proteostasis can be restored by balancing the levels of chaperones.
    PLoS ONE 10/2013; 8(10):e76845. DOI:10.1371/journal.pone.0076845 · 3.23 Impact Factor
    • "We went on to investigate the role of Bcor in Gata6 repression. Bcor is a co-repressor that interacts with the transcription factor Bcl6, and was of particular interest because it has been shown to be associated with a non-canonical PRC1 complex containing Ring1b, Nspc1 (Pcgf1 - Mouse Genome Informatics), Rybp and Kdm2b (Gearhart et al., 2006; Sánchez et al., 2007; Lagarou et al., 2008; Gao et al., 2012; Tavares et al., 2012). To validate further the role of Bcor in Gata6 repression, we carried out an independent knockdown experiment using a second hairpin to deplete Bcor in the reporter cell line (Fig. 4A). "
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    ABSTRACT: The use of whole-genome pooled shRNA libraries in loss-of-function screening in tissue culture models provides an effective means to identify novel factors acting in pathways of interest. Embryonic stem cells (ESCs) offer a unique opportunity to study processes involved in stem cell pluripotency and differentiation. Here, we report a genome-wide shRNA screen in ESCs to identify novel components involved in repression of the Gata6 locus, using a cell viability-based screen, which offers the benefits of stable shRNA integration and a robust and simple protocol for hit identification. Candidate factors identified were enriched for transcription factors and included known Polycomb proteins and other chromatin-modifying factors. We identified the protein Bcor, which is known to associate in complexes with the Polycomb protein Ring1B, and verified its importance in Gata6 repression in ESCs. Potential further applications of such a screening strategy could allow the identification of factors important for regulation of gene expression and pluripotency.
    Development 10/2013; 140(19):4110-5. DOI:10.1242/dev.094615 · 6.46 Impact Factor
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