Nucleosome compaction facilitates HP1 gamma binding to methylated H3K9

摘要

The alpha, beta and gamma isoforms of mammalian heterochromatin protein 1 (HP1) selectively bind to methylated lysine 9 of histone H3 via their chromodomains. Although the phenotypes of HP1-knockout mice are distinct for each isoform, the molecular mechanisms underlying HP1 isoform-specific function remain elusive. In the present study, we found that in contrast to HP1 alpha, HP1 gamma could not bind tri-methylated H3 lysine 9 in a reconstituted tetra-nucleosomes when the nucleosomes were in an uncompacted state. The hinge region connecting HP1's chromodomain and chromoshadow domain contributed to the distinct recognition of the nucleosomes by HP1 alpha and HP1 gamma. HP1 gamma, but not HP1 alpha, was strongly enhanced in selective binding to tri-methylated lysine 9 in histone H3 by the addition of Mg2+ or linker histone H1, which are known to induce compaction of nucleosomes. We propose that this novel property of HP1 gamma recognition of lysine 9 in the histone H3 tail in different nucleosome structures plays a role in reading the histone code.