Expression regulation by Yaf2 in mouse embryonic stem cells

The Polycomb group (PcG) proteins are the key epigenetic regulators with well-established roles in stem cell maintenance. To date, biochemical analyses have identified noncanonical PRC1(nc-PRC1) complexes characterized by the presence of Rybp/Yaf2 and the lack of CBX proteins. The biological significance of these nc-PRC1s and the potential mechanisms by which they mediate gene repression are largely unknown. Here we show that disruption of Yaf2 results in compromised proliferation and abnormal differentiation in mouse embryonic stem cells (mESCs). Deletion analysis identifies the region encompassing amino acid residues 102–150 required for the assembly of Yaf2 into nc-PRC1 complexes. Furthermore, we found that substitution of serine 166 by alanine (S166A) caused a loss of Yaf2-phosphorylation signal in mESCs. Loss of such Yaf2-phosphorylation compromises Ring1B-mediated H2A monoubiquitination and in turn lose its ability to repress target gene expression. We therefore propose that Yaf2 phosphorylation enhances its ability to maintain the pluripotent state in mESCs.