The Polycomb repressive complexes PRC1 and PRC2 are key mediators of heritable gene silencing in multicellular organisms. Here we characterise AEBP2, a known PRC2 cofactor which, in vitro, has been shown to stimulate PRC2 activity. We show that AEBP2 localises specifically to PRC2 target loci, including the inactive X chromosome. Proteomic analysis confirms that AEBP2 associates exclusively with PRC2 complexes. However, analysis of embryos homozygous for a targeted mutation of Aebp2 unexpectedly revealed a Trithorax phenotype, normally linked to antagonism of Polycomb function. Consistent with this we observe elevated levels of PRC2 mediated histone H3K27 methylation at target loci in Aebp2 mutant embryonic stem cells. We further demonstrate that mutant ES cells assemble atypical hybrid PRC2 sub-complexes, potentially accounting for enhancement of Polycomb activity, and suggesting that AEBP2 normally plays a role in defining the mutually exclusive composition of PRC2 sub-complexes.
The Company of Biologists