The Polycomb Group Protein L3mbtl2 Assembles an Atypical PRC1-Family Complex that Is Essential in Pluripotent Stem Cells and Early Development

Abstract

SUMMARY L3mbtl2 has been implicated in transcriptional repression and chromatin compaction but its biological function has not been defined. Here we show that disruption of L3mbtl2 results in embryonic lethality with failure of gastrulation. This correlates with compromised proliferation and abnormal differentiation of L3mbtl2−/− embryonic stem (ES) cells. L3mbtl2 regulates genes by recruiting a Polycomb Repressive Complex1 (PRC1)-related complex, resembling the previously described E2F6-complex, and including G9A, Hdac1, and Ring1b. Presence of L3mbtl2 at target genes is associated with H3K9-dimethylation, low histone-acetylation, and H2AK119-ubiquitination, but the latter is neither dependent on L3mbtl2 nor sufficient for repression. Genome wide studies revealed that the L3mbtl2-dependent complex predominantly regulates genes not bound by canonical PRC1 and PRC2. However, some developmental regulators are repressed by the combined activity of all three complexes. Together, we have uncovered a highly selective, essential role for an atypical PRC1-family complex in ES cells and early development.

BMI1–RING1B is an autoinhibited RING E3 ubiquitin ligase

“…For example, non-canonical PRC1 ligases could have an expanded list of client substrates and if so, it might explain why they have not maintained optimal binding to the nucleosome surface. The requirement for non-canonical PRC1 complexes in ES cell proliferation 62 , retroviral repression 63 , and regulation of metabolic genes and cell cycle progression 25 may be consistent with a need for greater flexibility (faster intrinsic rate, lower affinity for substrate) compared with the action of canonical PRC1 during development (where precision may be of paramount importance). The recent rapid progress in this area suggests that important elements of PCGF-RING1 E3 ligase function and regulation may yet be discovered.…”

Characterization of non-canonical Polycomb Repressive Complex 1 subunits during early mouse embryogenesis

“…5,30 Likewise, the NuRD complex associates with L3MBTL2 in ES cells. 26 In sum, our work contributes to the molecular characterization of the embryonic chromatin after fertilization, and places non-canonical PRC1 activity as a potential important player to consider when understanding the molecular definition of epigenetic reprogramming in vivo.…”

“…13 A further subdivision into functionally distinct families is provided by the presence of one of the 6 PCGF homologues. 13 The high heterogeneity of PRC1 complexes is thought to allow a combinatorial assembly of multiple subunits to integrate additional biochemical activities including readers for other histone modifications-such as HP1s or the Malignant Brain Tumor (MBT) family members 26 -as well as histone modifiers-such as HDACs. 11 In the mouse embryo, PcG proteins support 'facultative' heterochromatin establishment in the male pronucleus 3,25,28 , and a mutation in K27 within H3.3, the main histone variant incorporated in the male pronucleus, results in developmental arrest and heterochromatin defects.…”

Proteomic profile of maternal-aged blastocoel fluid suggests a novel role for ubiquitin system in blastocyst quality

“…Accumulated evidence in the literature has currently demonstrated that the monoubiquitylation of histone H2A at lysine 119 is crucially associated with transcriptionally repressed chromatin, as a result of the repression activity of Polycomb group proteins [42][43][44][45][46]. The latter proteins are crucial developmental regulators that maintain transcriptional repression of hundreds of genes involved in development from early embryogenesis through birth to adulthood [37,[47][48][49][50], as well as in signaling or cancer development using chromatin-based epigenetic mechanisms [51,52]. In particular, the Polycomb group RING finger protein 1 (PCGF1) acts as key cell growth regulator that promotes cell cycle progression and proliferation by transcriptional repression of the CDK cyclin inhibitor p21Waf1/Cip1 [53].…”