Role of Histone H3 Lysine 27 Methylation in Polycomb-Group Silencing

TLDR

Polycomb group proteins maintain HOX gene silencing, and although histone methylation is known to support long‑term repression, a direct link between PcG activity and histone methylation had not been demonstrated. The authors sought to purify and characterize the human EED‑EZH2 complex, the homolog of the Drosophila ESC‑E(Z) complex. The EED‑EZH2 complex was purified and characterized as the human equivalent of the Drosophila ESC‑E(Z) complex. The purified complex specifically methylates histone H3 at lysine 27, and this methylation co‑localizes with E(Z) binding at a Polycomb response element, promotes Polycomb recruitment, and correlates with gene repression, thereby linking histone methylation to PcG‑mediated silencing.

Abstract

Polycomb group (PcG) proteins play important roles in maintaining the silent state of HOX genes. Recent studies have implicated histone methylation in long-term gene silencing. However, a connection between PcG-mediated gene silencing and histone methylation has not been established. Here we report the purification and characterization of an EED-EZH2 complex, the human counterpart of the Drosophila ESC-E(Z) complex. We demonstrate that the complex specifically methylates nucleosomal histone H3 at lysine 27 (H3-K27). Using chromatin immunoprecipitation assays, we show that H3-K27 methylation colocalizes with, and is dependent on, E(Z) binding at an Ultrabithorax (Ubx) Polycomb response element (PRE), and that this methylation correlates with Ubx repression. Methylation on H3-K27 facilitates binding of Polycomb (PC), a component of the PRC1 complex, to histone H3 amino-terminal tail. Thus, these studies establish a link between histone methylation and PcG-mediated gene silencing.