Polycomb-like proteins link the PRC2 complex to CpG islands.

Abstract

The Polycomb repressive complex 2 (PRC2) mainly mediates transcriptional repression1,2 and plays essential roles in various biological processes including the maintenance of cell identity and proper differentiation. Polycomb-like proteins (PCLs), including PHF1, MTF2 and PHF19, are PRC2 associated factors that form sub-complexes with PRC2 core components3, and have been proposed to modulate PRC2’s enzymatic activity or its recruitment to specific genomic loci4–13. Mammalian PRC2 binding sites are enriched in CG content, which correlate with CpG islands that display a low level of DNA methylation14. However, the mechanism of PRC2 recruitment to CpG islands is not fully understood. In this study, we solved the crystal structures of the N-terminal domains of PHF1 and MTF2 with bound CpG-containing DNAs in the presence of H3K36me3-containing histone peptides. We found that the extended homologous (EH) regions of both proteins fold into a winged-helix structure, which specifically binds to the unmethylated CpG motif but in a manner completely different from the canonical winged-helix motif-DNA recognition. We further showed that the PCL EH domains are required for efficient recruitment of PRC2 to CpG island-containing promoters in mouse embryonic cells. Our research provides the first direct evidence demonstrating that PCLs are critical for PRC2 recruitment to CpG islands, thereby further clarifying their roles in transcriptional regulation in vivo.