Paraspeckle Protein NONO Regulates Active Chromatin by Allosterically Stimulating NSD1

NSD1 is a key lysine methyltransferase for di-methylation of lysine 36 of histone H3 (H3K36me2) essential for the establishment of active chromatin domains. While the loss of NSD1 catalytic activity halts embryonic development and a gain of that drives oncogenesis in leukemia and glioma, the regulatory mechanisms that control NSD1 activity in these processes remain poorly understood. Here, we uncover that NSD1 requires allosteric activation through the aromatic pocket of its PWWP2 domain. Surprisingly, we identify that NSD1-PWWP2 binds to a non-canonical target, nuclear paraspeckle protein NONO, and this protein-protein interaction allosterically stimulates the catalytic activity of NSD1. Mouse embryonic stem cells (mESC) engineered with mutations in the aromatic pocket of NSD1-PWWP2 cannot differentiate into neural progenitor cells (NPC), and genetic depletion of NONO partially phenocopied this defect at cellular and transcriptional levels, potentially explaining the neurodevelopmental disorder phenotypes in NSD1- or NONO-deficient patients. Our work revealed a novel mechanism driving active chromatin domain formation and has critical implications in the interplay between nuclear paraspeckles and active chromatin, and a vulnerability of NSD1 for therapeutic interventions.