Eukaryotic chromatin is organized into distinct domains that are important for the regulation of gene activity. For example, genes within heterochromatic domains are usually transcriptionally silent. The heterochromatin protein-1 (HP1) family members are crucial for the formation of heterochromatin and bind modified nucleosomes in which Lys9 of histone H3 is trimethylated (H3K9me3). This interaction is important for recruiting HP1 to the chromosomes. During mitosis, a majority of HP1 is released from the chromatin, but the way in which HP1-chromatin association is regulated in a cell cycle–dependent manner had not been characterized. Two independent studies by Fischle et al. and Hirota et al. (Nature, advance online publication 12 October 2005, 10.1038/nature04219 and 10.1038/nature042254) now address this question.

Fischle et al. show that the overall level of H3K9me3 is not altered during mitosis, suggesting that other factors cause HP1 to dissociate from the chromatin. One possible factor is the mitotic phosphorylation of H3 Ser10 (H3S10ph). Both Fischle et al. and Hirota et al. use antibodies specific for the dual modifications (H3K9me3S10ph) to show that H3K9me3S10ph is predominantly found in mitotic chromatin (bottom row). Notably, the occurrence of the dual H3 modifications coincides with the release of HP1 from chromatin (top row).

It is known that H3 Ser10 is phosphorylated during mitosis by the chromosomal passenger complex (CPC) that contains the Aurora B kinase. Inhibition of Aurora B kinase by hesperadin (both studies), by RNA interference (both studies) or by depletion of CPC (Fischle et al.) produced mitotic chromatin that contained H3K9me3 without Ser10 phosphorylaton. In these cases, HP1 failed to dissociate from chromatin, confirming the role of H3S10ph in regulating HP1-chromatin association. Using in vitro binding assays with H3 peptides, both studies show that phosphorylation of Ser10 is necessary and sufficient to induce HP1 release from mitotic chromatin. These results indicate that transient phosphorylation of H3 Ser10 during mitosis functions like an on/off 'switch', regulating the association of HP1 with chromatin. These studies support a previously proposed 'switch hypothesis' as a mechanism for regulating protein-protein interactions.