HP1Hs{alpha} Promotes Nucleosome Associations that Drive Chromatin Condensation [Gene Regulation]

January 10th, 2014 by Azzaz, A. M., Vitalini, M. W., Thomas, A. S., Price, J., Blacketer, M. J., Cryderman, D. E., Zirbel, L. N., Woodcock, C. L., Elcock, A. H., Wallrath, L. L., Shogren-Knaak, M. A.

HP1Hsα-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 HP1Hsα-nucleosome interactions contribute to the structure of heterochromatin. We show that HP1Hsα preferentially binds histone H3K9Me3-containing nucleosomal arrays in favor of non-methylated nucleosomal arrays and that non-specific DNA interactions and pre-existing chromatin compaction promote binding. The chromo and chromo shadow domains of HP1Hsα play an essential role in HP1Hsα-nucleosome interactions, while the hinge region appears to have a less significant role. Electron microscopy of HP1Hsα-associated nucleosomal arrays showed that HP1Hsα caused nucleosome associations within an array, facilitating chromatin condensation. Differential sedimentation of HP1Hsα associated nucleosomal arrays showed that HP1Hsα 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 HP1Hsα-nucleosome interactions cause chromatin condensation, a process that regulates many chromosome events.