Polycomb Repressive Complex 2 and H3K27me3 Cooperate with H3K9 Methylation To Maintain Heterochromatin Protein 1α at Chromatin

Methylation of histone H3 on lysine 9 or 27 is crucial for heterochromatin formation. Previously considered hallmarks of, respectively, constitutive and facultative heterochromatin, recent evidence has accumulated in favor of coexistence of these two marks and their cooperation in gene silencing maintenance. H3K9me2/3 ensures anchorage at chromatin of heterochromatin protein 1 alpha (HP1 alpha), a main component of heterochromatin. HP1 alpha chromoshadow domain, involved in dimerization and interaction with partners, has additional but still unclear roles in HP1 alpha recruitment to chromatin. Because of previously suggested links between polycomb repressive complex 2 (PRC2), which catalyzes H3K27 methylation, and HP1 alpha, we tested whether PRC2 may regulate HP1 alpha abundance at chromatin. We found that the EZH2 and SUZ12 subunits of PRC2 are required for HP1 alpha stability, as knockdown of either protein led to HP1 alpha degradation. Similar results were obtained upon overexpression of H3K27me2/3 demethylases. We further showed that binding of HP1 alpha/beta/gamma to H3K9me3 peptides is greatly increased in the presence of H3K27me3, and this is dependent on PRC2. These data fit with recent proteomic studies identifying PRC2 as an indirect H3K9me3 binder in mouse tissues and suggest the existence of a cooperative mechanism of HP1 alpha anchorage at chromatin involving H3 methylation on both K9 and K27 residues.