Polycomb-group complex 1 acts as an E3 ubiquitin ligase for Geminin to sustain hematopoietic stem cell activity

Abstract

Polycomb-group (PcG) genes encode multimeric nuclear protein complexes, PcG complex 1 and 2. PcG complex 2 was proved to induce transcription repression and to further methylate histone H3 at lysine-27 (H3K27). Subsequently PcG complex 1 is recruited through recognition of methylated H3K27 and maintains the transcription silencing by mediating monoubiquitination of histone H2A at lysine-119. Genetic evidence demonstrated a crucial role for PcG complex 1 in stem cells, and Bmi1, a member of PcG complex 1, was shown to sustain adult stem cells through direct repression of the INK4a locus encoding cyclin-dependent kinase inhibitor, p16CKI, and p19ARF. The molecular functions of PcG complex 1, however, remain insufficiently understood. In our study, deficiency of Rae28, a member of PcG complex 1, was found to impair ubiquitin-proteasome-mediated degradation of Geminin, an inhibitor of DNA replication licensing factor Cdt1, and to increase protein stability. The resultant accumulation of Geminin, based on evidence from retroviral transduction experiments, presumably eliminated hematopoietic stem cell activity in Rae28-deficient mice. Rae28 mediates recruiting Scmh1, which provides PcG complex 1 an interaction domain for Geminin. Moreover, PcG complex 1 acts as the E3 ubiquitin ligase for Geminin, as we demonstrated in vivo as well as in vitro by using purified recombinant PcG complex 1 reconstituted in insect cells. Our findings suggest that PcG complex 1 supports the activity of hematopoietic stem cells, in which high-level Geminin expression induces quiescence securing genome stability, by enhancing cycling capability and hematopoietic activity through direct regulation of Geminin.Rae28 ͉ Scmh1 ͉ HSCs ͉ ubiquitination ͉ DNA replication licensing

RNF2/Ring1b negatively regulates p53 expression in selective cancer cell types to promote tumor development

“…Previous studies revealed that Rnf2 acts as an E3 ubiquitin ligase (21,23,28). Various E3 ligases regulate p53 activity through direct degradation, changes in subcellular localization, or the alteration of transcription activity (3,10,12).…”

“…In addition to its monoubiquitination activity, the PRC1 complex also has poly-ubiquitination activity. For example, PRC1 poly-ubiquitinates Geminin, a DNA replication inhibitor, to sustain adult hematopoietic stem cell activity (23). Protein structure analysis revealed that within the PRC1 complex, Bmi1 and RNF2 heterodimerize via their N-terminal RING domains to form an active E3 ubiquitin ligase (24)(25)(26).…”

Id1 enhances RING1b E3 ubiquitin ligase activity through the Mel-18/Bmi-1 polycomb group complex

“…Our results also confirmed that Id1 overexpression had no effect on tri-methylation of histone H3 lysine 27. We then investigated the expression of geminin, a target of RING1b E3 ubiquitin ligase, (Ohtsubo et al, 2008) and found that overexpression of Id1 in MCF7 cells downregulated geminin at the protein level but not the mRNA level. Similarly, Id1 knockdown led to a reduction of histone H2A ubiquitination and upregulation of geminin expression in both MCF7 and MDA-MB-231 cell lines (Figure 4a, left and right panels).…”

“…It is known that geminin is not detected in the G1 phase and increases during progression into the S phase (McGarry and Kirschner, 1998). Geminin was supposed to have a crucial role in supporting stem cells by regulating cellular proliferation, differentiation and further genome stability (Ohtsubo et al, 2008). Another study regarded geminin as a critical regulator for maintaining an undifferentiated state through the regulation of a chromatinremodeling complex (Seo et al, 2005).…”

“…In particular, Bmi-1 is very critical for RING1b E3 ligase activity-mediated gene silencing, because knockdown of Bmi-1 led to loss of histone H2A ubiquitination and upregulation of its target, HoxC5 genes (Wei et al, 2006). The PRC1 complex has also been shown to function as an E3 ubiquitin ligase for degradation of target protein, such as geminin, which prevented DNA replication and maintained the undifferentiated cell state (Saxena and Dutta, 2005;Seo et al, 2005;Ohtsubo et al, 2008). Moreover, a recent study has shown that RING1b protein also could be selfubiquitinated and this is crucial for its H2A ubiquitination activity (Zaaroor-Regev et al, 2010).…”