EPOP and MTF2 Activate PRC2 Activity through DNA-sequence specificity

The Polycomb repressive complex 2 (PRC2) establishes transcriptionally silent chromatin by catalyzing histone H3 lysine 27 trimethylation (H3K27me3). Although cofactors such as MTF2 and JARID2 are known to direct PRC2 to nucleation sites and stimulate its catalytic activity, the role of the PRC2-associated factor EPOP has remained unclear and even controversial. Here, we combine biochemical reconstitution and a cellular EED-rescue system to define EPOP’s contribution to initial PRC2 recruitment and de novo H3K27me3 deposition. We show that EPOP, like MTF2, directly stimulates PRC2 histone methyltransferase activity in vitro. In vivo, EPOP is dispensable for initial PRC2 recruitment but promotes de novo H3K27me3 deposition in cooperation with MTF2 and JARID2. Mechanistically, electrophoretic mobility shift assays and reconstituted chromatin substrates reveal that EPOP and MTF2 enhance PRC2–nucleosome binding in a sequence- and chromatin-dependent manner: EPOP preferentially potentiates activity on GA-rich dinucleosomes, whereas MTF2 favors GCN-rich substrates. Together, these findings position EPOP as a positive regulator of PRC2 function during the establishment of H3K27me3 domains, and reveal a paradigm in which distinct cofactors confer DNA-sequence-specific control of de novo H3K27me3 deposition.