Structural basis for inactivation of PRC2 by G-quadruplex RNA.

The histone methyltransferase PRC2 (Polycomb Repressive Complex 2) silences genes via successively attaching three methyl groups to lysine 27 of histone H3. PRC2 associates with numerous pre-mRNA and lncRNA transcripts with a binding preference for G-quadruplex RNA. Here, we present a 3.3Ã…-resolution cryo-EM structure of PRC2 bound to a G-quadruplex RNA. Notably, RNA mediates the dimerization of PRC2 by binding both protomers and inducing a protein interface comprised of two copies of the catalytic subunit EZH2, which limits nucleosome DNA interaction and occludes H3 tail accessibility to the active site. Our results reveal an unexpected mechanism for RNA-mediated inactivation of a chromatin-modifying enzyme. Furthermore, the flexible loop of EZH2 that helps stabilize RNA binding also facilitates the handoff between RNA and DNA, an activity implicated in PRC2 regulation by RNA. One-Sentence Summary: Cryo-EM structure of RNA-bound PRC2 dimer elucidates an unexpected mechanism of PRC2 inhibition by RNA.