Interaction of HP1 and Brg1/Brm with the Globular Domain of Histone H3 Is Required for HP1-Mediated Repression

Abstract

The heterochromatin-enriched HP1 proteins play a critical role in regulation of transcription. These proteins contain two related domains known as the chromo- and the chromoshadow-domain. The chromo-domain binds histone H3 tails methylated on lysine 9. However, in vivo and in vitro experiments have shown that the affinity of HP1 proteins to native methylated chromatin is relatively poor and that the opening of chromatin occurring during DNA replication facilitates their binding to nucleosomes. These observations prompted us to investigate whether HP1 proteins have additional histone binding activities, envisioning also affinity for regions potentially occluded by the nucleosome structure. We find that the chromoshadow-domain interacts with histone H3 in a region located partially inside the nucleosomal barrel at the entry/exit point of the nucleosome. Interestingly, this region is also contacted by the catalytic subunits of the human SWI/SNF complex. In vitro, efficient SWI/SNF remodeling requires this contact and is inhibited in the presence of HP1 proteins. The antagonism between SWI/SNF and HP1 proteins is also observed in vivo on a series of interferon-regulated genes. Finally, we show that SWI/SNF activity favors loading of HP1 proteins to chromatin both in vivo and in vitro. Altogether, our data suggest that HP1 chromoshadow-domains can benefit from the opening of nucleosomal structures to bind chromatin and that HP1 proteins use this property to detect and arrest unwanted chromatin remodeling.

Methylation of Lysine 9 in Histone H3 Directs Alternative Modes of Highly Dynamic Interaction of Heterochromatin Protein hHP1β with the Nucleosome

“…Previous studies reported that the CSD of hHPI␣ and hHPI␥ bind a portion of the globular domain of H3, named the "Shadock" region (16). However, because those results came from biochemical assays with isolated histones, it is possible that the interaction does not occur in the integral nucleosome assembly, where the "Shadock" region is partially buried (44).…”

“…Interactions between hHP1s with the core region of H3 (12,16,17), of hHP1␣ with H2A.Z (18), of hHP1␣ with linker histone H1 (12,19,20), and of human or Xenopus HP1␣ with DNA (19,21,22) or RNA (23) have been reported. In addition, it has been questioned whether DmHP1 makes stable contacts with chromatin in the absence of accessory factors (24).…”

Basic concepts of epigenetics

“…Silencing and transcription patterns are altered by environmental cues and chromodomain, MeCP2, methylated CpGs, MBD1/SETDB1/ CAF-1 and human Swi/Snf ATPases, via their chromoshadow PXVXL motif. 25,[146][147][148] HP1 proteins are involved in nucleosomal constriction and counteract expansive chromatin remodeling complexes such as Brg1 and Brm. 148 Moreover, HP1 can also interact with BAHD1, which interacts directly with MBD1 and HDAC5, allowing for organized heterochromatin that lacks acetylated H4 and is enriched in H3K27me3.…”

“…25,[146][147][148] HP1 proteins are involved in nucleosomal constriction and counteract expansive chromatin remodeling complexes such as Brg1 and Brm. 148 Moreover, HP1 can also interact with BAHD1, which interacts directly with MBD1 and HDAC5, allowing for organized heterochromatin that lacks acetylated H4 and is enriched in H3K27me3. 149 The histone mark H3K9me3 seems to be of utmost importance in the stability of constrictive HP1, which, if lost by phosphorylation of H3S10, can lead to nucleosomal expansion and formation of euchromatin.…”

Beyond the histone tale: HP1α deregulation in breast cancer epigenetics

“…37,46,47 This H3 region is abbreviated Shadock for "chromoShadow docking" and contains a variant of the PxVxL motif, PGTVAL, required for HP1 binding and besides also capable forming interaction with BRG1. 46 Efficient SWI/SNF remodeling requires this H3 contact and is inhibited in the presence of HP1 proteins. SWI/ SNF ATPase activity facilitates the HP1 binding for functional detection and arrest of chromatin remodeling.…”