Human Heterochromatin Protein 1α Promotes Nucleosome Associations That Drive Chromatin Condensation

Background: Heterochromatin is enriched for di- and tri-methylated lysine 9 of histone H3 (H3K9Me2/3) and heterochromatin protein 1 (HP1(Hs .)). Results: The association of HP1(Hs) with H3K9Me3-containing nucleosome arrays facilitated array compaction and cross-array interactions. Conclusion: HP1(Hs) association caused intra- and inter-array associations, leading to chromatin condensation and looping. Significance: An understanding of HP1(Hs)-nucleosome interactions provides insights on the structure and functions of heterochromatin. HP1(Hs)-containing heterochromatin is located near centric regions of chromosomes and regulates DNA-mediated processes such as DNA repair and transcription. The higher-order structure of heterochromatin contributes to this regulation, yet the structure of heterochromatin is not well understood. We took a multidisciplinary approach to determine how HP1(Hs)-nucleosome interactions contribute to the structure of heterochromatin. We show that HP1(Hs) preferentially binds histone H3K9Me3-containing nucleosomal arrays in favor of non-methylated nucleosomal arrays and that nonspecific DNA interactions and pre-existing chromatin compaction promote binding. The chromo and chromo shadow domains of HP1(Hs) play an essential role in HP1(Hs)-nucleosome interactions, whereas the hinge region appears to have a less significant role. Electron microscopy of HP1(Hs)-associated nucleosomal arrays showed that HP1(Hs) caused nucleosome associations within an array, facilitating chromatin condensation. Differential sedimentation of HP1(Hs)-associated nucleosomal arrays showed that HP1(Hs) promotes interactions between arrays. These strand-to-strand interactions are supported by in vivo studies where tethering the Drosophila homologue HP1a to specific sites promotes interactions with distant chromosomal sites. Our findings demonstrate that HP1(Hs)-nucleosome interactions cause chromatin condensation, a process that regulates many chromosome events.