NPM1 mediates genome-nucleolus interactions and the establishment of their repressive chromatin states

Abstract

Repressive chromatin domains are often located at the nuclear lamina (NL) or nucleolus. Although nucleolar associated domains (NADs) have been recently mapped, the mechanisms of NAD association with nucleoli and the functional significance of their localization remain unclear. Here, we show that NAD association with nucleoli is mediated by nucleophosmin (NPM1), a factor located within the granular component, the outer layer of the nucleolus. NPM1 binds NADs, interacts with the histone lysine methyltransferase G9a (EHMT2), and is required for establishing H3K9me2 at NADs. Loss of NPM1 or expression of NPM1 mutant lacking the DNA binding domain (NPM1{Delta}DBD) caused NAD dissociation from nucleoli and H3K9me2 reduction specifically at NADs. G9a is dispensable for NAD contacts with nucleoli and interacts with NPM1{Delta}DBD, indicating that NADs acquire G9a-mediated H3K9me2 only after associating with NPM1 at nucleoli. The results provide mechanistic insights into how genomic domains associate with nucleoli and acquire their repressive chromatin state. Additionally, our findings suggest that the nucleolus not only serves as a scaffold for positioning repressive chromatin domains but also plays a direct role in establishing these chromatin states.