Allosteric Activation Dictates PRC2 Activity Independent of Its Recruitment to Chromatin

PRC2 is a therapeutic target for several types of cancers currently undergoing clinical trials. Its activity is regulated by a positive feedback loop whereby its terminal enzymatic product, H3K27me3, is specifically recognized and bound by an aromatic cage present in its EED subunit. The ensuing allosteric activation of the complex stimulates H3K27me3 deposition on chromatin. Here we report a stepwise feedback mechanism entailing key residues within distinctive interfacing motifs of EZH2 or EED that are found to be mutated in cancers and/or Weaver syndrome. PRC2 harboring these EZH2 or EED mutants manifested little activity in vivo but, unexpectedly, exhibited similar chromatin association as wild-type PRC2, indicating an uncoupling of PRC2 activity and recruitment. With genetic and chemical tools, we demonstrated that targeting allosteric activation overrode the gain-of-function effect of EZH2^Y646X oncogenic mutations. These results revealed critical implications for the regulation and biology of PRC2 and a vulnerability in tackling PRC2-addicted cancers. Lee and Yu et al. dissect the stepwise mechanism of PRC2 activation involving the EZH2-SRM and EED anchorage regions. Mutations in these regions diminish PRC2 activity in vivo and override the hyperactivity of oncogenic EZH2 mutations but do not affect PRC2 association with chromatin. α-Helical mimetics against SRM abrogate PRC2 activation.