A conserved H3K14ub-driven H3K9me3 for chromatin compartmentalization.

Compartmentalization of eukaryotic genome into euchromatin and heterochromatin is of critical biological significance1-3. Previous studies have suggested a self-templating pathway involving the reading and writing of histone H3 lysine 9 methylation by SUV39H as the core mechanism for heterochromatin reassembly during cell division1,3. In fission yeast, the mammalian SUV39H homologue Clr4 forms a complex containing ubiquitin ligase Cul4, which catalyses H3K14 mono-ubiquitination (H3K14ub) to promote heterochromatin formation. However, whether heterochromatin reassembly in dividing mammalian cells involves a similar pathway is unknown. Here we identified G2E3 as an H3K14ub-specific, pericentromeric heterochromatin-localized E3 ligase. G2E3-catalysed H3K14ub potentiates histone H3 lysine 9 trimethylation (H3K9me3) by SUV39H and is specifically required for SUV39H compartmentalization and H3K9me3 in pericentromeric heterochromatin. Mechanistically, we found that G2E3 is highly expressed in G2/M phase and associates with mitotic chromosomes in an RNA-dependent manner to catalyse H3K14ub, which is essential for the subsequent sequential recruitment of SUV39H and HP1. The SUV39H chromodomain is a reader of dual H3K9me3 and H3K14ub modifications and SUV39H associates with pericentromeric heterochromatin primarily through its H3K14ub-binding activity. Notably, loss of G2E3 severely impairs pericentromeric heterochromatin organization and results in the aberrant accumulation of SUV39H and H3K9me3 in numerous euchromatin regions and widespread transcriptional repression. Thus, our findings revealed the H3K14ub-dependent SUV39H compartmentalization as a unified mechanism of pericentromeric heterochromatin formation, which is essential for proper euchromatin compartmentalization and transcriptional regulation.