# EPOP and MTF2 activate PRC2 activity through DNA-sequence specificity

## TL;DR

This study shows how EPOP and MTF2 help PRC2 silence genes by targeting specific DNA sequences in chromatin.

## Contribution

EPOP is identified as a new PRC2 cofactor that enhances H3K27me3 deposition through DNA sequence specificity.

## Key findings

- EPOP and MTF2 form distinct PRC2 subcomplexes with different DNA-binding preferences.
- EPOP promotes PRC2 histone methyltransferase activity in a GCN-rich DNA-dependent manner.
- EPOP and MTF2 work with JARID2 to establish H3K27me3 domains at specific chromatin sites.

## Abstract

Polycomb repressive complex 2 (PRC2) silences gene expression by depositing H3K27me3 and is essential for development and disease regulation. How PRC2 activity is initiated at defined genomic sites remains incompletely understood. We identify EPOP as a PRC2-associated factor that promotes histone methyltransferase activity and facilitates PRC2 binding to chromatin. EPOP and MTF2 define mutually exclusive PRC2 subcomplexes that exhibit distinct binding preferences for chromatin substrates, driven in part by the underlying DNA sequences within linker regions. These findings reveal a mechanism by which PRC2 activity is spatially regulated in the genome and provide insight into subcomplex-specific control of gene silencing.

Polycomb Repressive Complex 2 (PRC2) facilitates the formation of facultative heterochromatin, instrumental to tissue specific gene expression. PRC2 catalyzes trimethylation of lysine 27 of histone H3 (H3K27me3), which is targeted for chromatin compaction by PRC1. Importantly, PRC2-associated cofactors regulate its distinct activities, as in the case of MTF2 and JARID2 that direct PRC2 to specific chromatin nucleation sites based on preferred DNA-binding motifs. Here, we investigated EPOP whose role in regulating PRC2 was not well-defined. We find that both EPOP and MTF2 stimulate PRC2 histone methyltransferase (HMT) activity in vitro. Unlike MTF2, EPOP is ineffectual in PRC2 chromatin recruitment as evidenced by an EED-rescue system in vivo but promotes H3K27me3 deposition de novo in cooperation with MTF2 and JARID2. Binding assays using reconstituted dinucleosome substrates revealed that similar to MTF2, EPOP promotes PRC2 chromatin-binding activity in a distinct DNA-sequence-dependent manner (GCN-rich and GA-rich, respectively). Thus, EPOP and MTF2 in conjunction with JARID2 foster PRC2-mediated HMT activity at chromatin sites comprising cofactor-preferred DNA-binding sequences during the formation of H3K27me3-chromatin domains.