The role of the histone H2A ubiquitinase Sce in Polycomb repression

Abstract

Polycomb group (PcG) proteins exist in multiprotein complexes that modify chromatin to repress transcription. Drosophila PcG proteins Sex combs extra (Sce; dRing) and Posterior sex combs (Psc) are core subunits of PRC1-type complexes. The Sce:Psc module acts as an E3 ligase for monoubiquitylation of histone H2A, an activity thought to be crucial for repression by PRC1-type complexes. Here, we created an Sce knockout allele and show that depletion of Sce results in loss of H2A monoubiquitylation in developing Drosophila. Genome-wide profiling identified a set of target genes co-bound by Sce and all other PRC1 subunits. Analyses in mutants lacking individual PRC1 subunits reveals that these target genes comprise two distinct classes. Class I genes are misexpressed in mutants lacking any of the PRC1 subunits. Class II genes are only misexpressed in animals lacking the Psc-Su(z)2 and Polyhomeotic (Ph) subunits but remain stably repressed in the absence of the Sce and Polycomb (Pc) subunits. Repression of class II target genes therefore does not require Sce and H2A monoubiquitylation but might rely on the ability of Psc-Su(z)2 and Ph to inhibit nucleosome remodeling or to compact chromatin. Similarly, Sce does not provide tumor suppressor activity in larval tissues under conditions in which Psc-Su(z)2, Ph and Pc show such activity. Sce and H2A monoubiquitylation are therefore only crucial for repression of a subset of genes and processes regulated by PRC1-type complexes. Sce synergizes with the Polycomb repressive deubiquitinase (PR-DUB) complex to repress transcription at class I genes, suggesting that H2A monoubiquitylation must be appropriately balanced for their transcriptional repression.

The emerging role of Polycomb repressors in the response to DNA damage

“…Deletion of the RING1A and RING1B mouse homologues in mouse embryonic stem cells can release stalled Pol II and is accompanied by the derepression of PcG target genes (Stock et al, 2007). Two main mechanisms by which PcG complexes accomplish Pol II stalling have been described: chromatin compaction (Eskeland et al, 2010;Francis et al, 2004) and regulation of histone H2A monoubiquitylation (Gutiérrez et al, 2012;Scheuermann et al, 2010;Stock et al, 2007). However, the relative contribution of these mechanisms to PcG-mediated gene silencing remains to be investigated (Simon and Kingston, 2009).…”

BMI1–RING1B is an autoinhibited RING E3 ubiquitin ligase

“…The importance of H2A K119ub in PRC1-enforced gene repression has been studied in several systems [11][12][13] (reviewed in refs 14,15). Introduction of a catalytically inactive RING1B mutant into Ring1A À / À Ring1B conditional knockout embryonic stem (ES) cells 12 established that H2A ubiquitination is dispensable for chromatin compaction but essential for maintaining repression of target genes and for maintaining ES cells in a dedifferentiated state.…”

“…Introduction of a catalytically inactive RING1B mutant into Ring1A À / À Ring1B conditional knockout embryonic stem (ES) cells 12 established that H2A ubiquitination is dispensable for chromatin compaction but essential for maintaining repression of target genes and for maintaining ES cells in a dedifferentiated state. Although some details of PRC1 function are different in Drosophila 15 , it appears that H2A ubiquitination is again critical for a subset of PRC1 function 13 . In accord with the importance of H2A ubiquitination, the process is highly regulated: a number of H2A K119 deubiquitinases have been identified and assigned critical functions (recently reviewed in ref.…”

From Flies to Mice: The Emerging Role of Non-Canonical PRC1 Members in Mammalian Development

“…In the next decades, a plethora of additional PcG genes were identified such as Sex combs extra (Sce) [37,38], cramped (crm) [39], Scm related gene containing four mbt domains (Sfmbt) [40] and Suppressor of zeste 12 (Su(z)12) [41], and proved that they all play a role in the transmission of the silent state of homeotic genes. Sixty years after the isolation of the first Pc allele [1], De Ayala et al published a classical Polycomb screen in Drosophila and described calypso, a non-redundant Drosophila gene showing dominant PcG type homeotic transformations [37].…”

“…Some Ring finger proteins were shown to be able to interact with ubiquitin conjugating enzymes [71,72]. SCE and its human homologs RING1/RING1A and RNF2/RING1B are catalytically active ubiquitin ligases [38,73], which were found to be the core catalytic subunits of all mammalian Polycomb Repressive Complex 1s (PRC1s).…”