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

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.


Introduction
HP1 proteins are important regulators of heterochromatinmediated silencing and chromosome structure in diverse eukaryotes (for recent reviews, see [1,2]). In mammalian cells, the HP1 family is composed of HP1a, HP1b, and HP1c. So far, only HP1b has been inactivated in the mouse, resulting in defective development of neuromuscular junctions and cerebral cortex [3]. Within the nucleus, the three HP1 isoforms all concentrate in foci of dense pericentromeric heterochromatin but are also present in the rest of the nucleoplasm. Consistent with this very general distribution, the mammalian HP1 proteins are detected not only in dense heterochromatic regions but also on active euchromatic genes [4].
On these active genes, HP1 proteins seem to be present both during phases of silencing and transcriptional activity. For example, on the Survivin gene and on an integrated HIV1 LTR, HP1b is detected on the repressed promoter, while HP1c is recruited after transcriptional activation [5,6]. HP1c is however not always associated with active transcription as it participates in the repression of the MMTV LTR and the Sox2 promoter [7,8]. It is noteworthy also that on the HIV1 LTR and on the b-major gene, HP1 proteins are co-localized with the RNA polymerase II (RNAPII), indicating that they are not creating an environment incompatible with recruitment of this polymerase [5,9]. Taken together, these observations suggest that, at least in euchromatin, HP1 proteins are not ''chromatin condensers'' per se, but more likely regulators of enzymatic activities involved in transcription initiation or elongation. HP1 proteins contain two very similar domains known as the chromo-domain (CD) and the chromoshadow-domain (CSD) separated by a less structured hinge region. The CSD is required for dimerization and interaction with many molecular partners that share a PXVXL motif [10]. It is also necessary for the recruitment of HP1 proteins to sites of DNA damage [11]. Concurrently, the CD recognizes and binds histone H3 tails methylated on lysine 9 (K9), an epigenetic mark frequently associated with transcriptional repression [12,13]. In addition, the hinge region of HP1 proteins harbors DNA-and RNA-binding activities and the targeting of these proteins to chromatin likely results from the integration of multiple contacts [14,15].
In contrast to the strong binding to peptides mimicking histone H3 tails methylated on K9, HP1 proteins bind only weakly to reconstituted methylated nucleosomal arrays [16]. Consistent with this, binding of HP1 proteins to purified native chromatin in vitro seems relatively inefficient [15,17]. This binding can be improved by auxiliary factors that may help the recognition of chromatin [16], but it has also been suggested that HP1 can benefit from chromatin opening. Indeed, a more stable incorporation of HP1 proteins occurs in S phase when DNA replication disrupts the histone octamers [17]. Earlier reports also describe the presence in the nucleus of two populations of HP1 proteins with either high or low mobility [18] and it has been proposed that the more stable interaction creates the HP1 population of low mobility [3]. Binding of HP1 proteins may also benefit from ATP-dependent chromatin remodeling as HP1b co-localize with the ACF1-ISWI remodeling complex [19]. In addition, HP1a, but not HP1b and HP1c, interacts with Brg1 and Brm, the mutually exclusive catalytic subunit of the human SWI/SNF (hSWI/SNF) complex, and this interaction favors repression of a reporter construct by a transfected Gal4-HP1a fusion protein ( Figure S1A, S1B, S1C, S1D and [20,21]).
To gain better understanding of HP1 chromatin binding and transcriptional regulation, we have here examined whether these proteins could establish alternative interactions with the histones. This allowed us to identify a contact between the CSD and a region of histone H3 located at the border of the globular domain. This region is also contacted by the hSWI/SNF subunits Brg1 and Brm, and we show that HP1 proteins have a negative effect on hSWI/SNF-mediated chromatin remodeling. Finally, we provide evidence indicating that hSWI/SNF activity is involved in the recruitment of HP1 proteins to chromatin.

Results
The chromoshadow-domain interacts with the globular domain of histone H3 We investigated whether HP1 proteins could bind histone H3 independently of the well-characterized association of the CD with methylated K9. To this end, we tested the binding of HP1a and HP1c to either purified or recombinant B10-epitope-tagged histones immobilized on nitrocellulose membrane. As expected, the HP1 proteins bound strongly to purified histone H3 but not to histone H4 ( Figure 1A, lanes 3 and 4). Interestingly, we also observed weaker but significant binding to full-length recombinant histone H3 produced in E. coli and therefore not methylated on K9 ( Figure 1A, lane 1). This binding was not observed on the tail region alone ( Figure 1A, lane 2). This is in accordance with earlier studies showing interaction of HP1 proteins with the globular domain of recombinant histone H3 [17,22]. In GST pull down assays, we also observed weaker, but persisting histone H3 binding after mutation of the CD at position V22, abolishing interaction of HP1a with the methylated histone H3 tail ( Figure 1B, lane 2). This again suggested the presence of additional contact points between HP1a and histone H3.
The structure of the CSD is very similar to that of the CD ( Figure 1C), prompting us to probe for an interaction with the histones via this domain. To this end, we further mutated HP1a V22M at position I126 inside the CSD. This position is equivalent to I25 in the CD, an amino acid that, when mutated, prevents the domain from interacting with histone H3 [22]. This position was chosen because V22 has no equivalent in the CSD. The double mutant no longer interacted with H3, indicating that in both the CD and the CSD, the first b strand is involved in histone interaction ( Figure 1B, lane 3). Mutation of the CSD at I126 also affected the repressing activity of HP1a. This was visualized by cotransfecting in MCF7 cells an MMTV/Gal4 reporter construct and expression plasmids for Gal4-HP1a fusions where HP1a was either WT or with an I126F mutation ( Figure 1D and 1E).
We next mapped the region of histone H3 involved in the interaction with the HP1a CSD. A non-modified histone H3 truncation mutant spanning from aa 1 to 76 produced in E.coli was sufficient to interact with HP1a, while a shorter construct containing only the H3 tail region (aa 1-47) failed to do so. The interaction was disrupted by deletion of the CSD, confirming its implication in the contact with H3 ( Figure 1F and 1G, 3 top panels). We noted also that all mutations affecting HP1 dimerization abolished the CSD-H3 interaction, while this interaction resisted incubation with a DNA intercalating agent (data not shown). This series of experiments also showed that HP1c and Drosophila dHP1a had binding activities similar to that of HP1a ( Figure 1G, bottom panels).
We finally identified aa 35 to 66 as the minimal region binding both HP1a and HP1c ( Figure 1H, lane 2 and 4). We termed this region the Shadock for ''chromoShadow docking''. This region contains a PXXVXL motif resembling the PXVXL motif frequently found in proteins interacting with the CSD of HP1 proteins [10]. Mutation of the valine in this sequence (V46) abolished binding to HP1a but not HP1c ( Figure 1H, lane 5). Consistent with this, the Shadock could be further shortened to aa 44 to 66 without disrupting binding of HP1c ( Figure 1H, lane 3). These observations show that the two proteins have overlapping but not identical binding sites.
Brg1, Brm, and HP1 contact overlapping regions in the globular domain of histone H3 The Shadock region is located at the entry/exit site of the nucleosome but is partially hidden inside the nucleosomal barrel Author Summary HP1 proteins are transcriptional regulators frequently associated with gene silencing, a phenomenon involving masking of promoter DNA by dense chromatin. Owing to their chromo-domain, these proteins can read and bind an epigenetic mark that on many non-expressed genes is present on histone H3 at the surface of the nucleosome (the fundamental packing unit of chromatin). However, the binding to this mark does not explain the repressing activity of HP1 proteins. Here, we show that these proteins can establish a second contact with histone H3, independently of the epigenetic mark. This second contact site is located inside the nucleosome, in a position likely to be inaccessible. Interestingly, this site is also contacted by a subunit of the SWI/SNF complex and this contact is required for the ATP-dependent chromatin remodeling catalyzed by SWI/SNF. We provide evidence suggesting that HP1 proteins use the SWI/SNF chromatin remodeling to gain access to the contact site inside the nucleosome and to prevent further remodeling by competing with SWI/SNF for binding at this position. These observations lead us to suggest that HP1 proteins function as gatekeepers on promoters, detecting and stopping unwanted exposure of internal nucleosomal sites.
( Figure 2A). Interestingly, this region also includes the H3 aN helix previously shown to play an important role in nucleosome mobility [23] and mutations in this region were recently shown to affect chromatin remodeling by yeast SWI/SNF [24]. Besides, we found that histone H3 was co-immunoprecipitated with the hSWI/SNF catalytic subunit Brg1 in an in vitro assay ( Figure 2C). We therefore investigated whether Brg1 would target this region during remodeling. In these experiments, we used a truncation mutant of the Brg1 protein centered on the ATPase domain (DBrg1-1, Figure 2B). This mutant, sufficiently short to be produced in E. coli, shows remodeling activity similar to full length Brg1 [25]. When expressed as a GST fusion, DBrg1-1 had affinity for histone H3 but not for other histones bound to nitrocellulose membrane ( Figure 2D, lane 5). GST pull down assays further showed that DBrg1-1 bound to both purified and recombinant histone H3 ( Figure 2E, lanes 2 and 3). Additional Brg1 deletion mutants showed that regions Cterminal of the helicase domain could mediate the interaction ( Figure 2F). These regions were previously reported as essential for in vitro remodeling [25]. Binding properties of Brg1 to histone H3 could essentially be recapitulated with Brm, the alternative catalytic subunit of the hSWI/SNF complex ( Figure S1E, S1F, S1G, S1H). H3 deletion mutants further revealed that the interaction of Brg1 with the histone was dependent on the region from aa 35 to 66 also involved in interaction with the HP1 proteins ( Figure 2G, lane 2). Best binding was however achieved when this region was extended by 10 aa (aa 25 to 66, Figure 2G, lane 1).
To determine whether this interaction was important for chromatin remodeling, we used Restriction Enzyme Accessibility (REA) assays [26]. Nucleosomal arrays were assembled by the use of DNA templates consisting of two sets of five 5S nucleosome positioning sequences that flank DNA sufficient to assemble two nucleosomes, one of which overlaps a unique HhaI site. Accessibility of this site is increased in the presence of full length Brg1 and ATP, reflecting chromatin remodeling (in Figure 2H, lane 1 compare top and bottom panels). In these assays, we challenged the remodeling by Brg1 with H3 deletion mutants, reasoning that these polypeptides could interfere with the binding of Brg1 with its normal nucleosomal substrate. We observed a good correlation between the ability of the H3 mutants to inhibit the remodeling reaction and their ability to bind Brg1 (see the effect of 200nM and 400nM competing protein in Figure 2H and 2I, and compare with binding in Figure 2G). These observations show that the contact between Brg1 and H3 is important for remodeling.

HP1a and HP1c repress chromatin remodeling by hSWI/ SNF in vitro
The binding of both HP1 and Brg1/Brm proteins to the Shadock domain of histone H3 suggested a competition between these two sets of transcriptional regulators. To investigate this possibility, we tested the effect of HP1a on chromatin remodeling by the hSWI/SNF complex in REA assays. Addition of recombinant flag-tagged HP1a (F-HP1a) prevented the hSWI/ SNF-dependent increase of HhaI accessibility without modifying the intrinsic accessibility of the restriction site ( Figure 3A). Quantification showed that 50% inhibition of site accessibility was reached when nucleosomes, F-HP1a and hSWI/SNF were present at a molar ratio of approx. 1:70:10. In a similar assay F-HP1a also repressed remodeling by Brg1 and the truncated DBrg1 ( Figure S2B).
We next analyzed the effect of F-HP1a on remodeling of a mononucleosome substrate. The mononucleosome was assembled on a 202 bp template containing a unique PstI site only 5% accessible in the absence of hSWI/SNF complex and ATP. As observed with the polynucleosomal template, F-HP1a inhibited remodeling of this substrate ( Figure 3B). Repression was moderately less efficient as 50% inhibition was reached at a  nucleosomes:F-HP1a:hSWI/SNF ratio of 1:100:10. Flag-tagged F-HP1c also repressed hSWI/SNF remodeling with an efficiency similar to that of F-HP1a ( Figure 3C). Measuring the kinetics of the repression revealed however that HP1c was significantly slower than HP1a ( Figure S2C).
We finally used the REA assay to test several HA-tagged HP1a constructs ( Figure 3D). Consistent with a role for the CSD in the repression, truncation of the carboxyl terminal region abolished the repressing effect of HP1a on remodeling by hSWI/SNF. In contrast, mutants defective in either histone H3 K9me binding (HP1a V22M), DNA/RNA binding (DDNA), or interaction with Brm/Brg1 (HP1a dBrg1) were not affected in their ability to repress hSWI/SNF chromatin remodeling ( Figure 3E). In addition, we observed HP1a-mediated repression on a nucleosomal array reconstituted with non-modified histones produced in E. coli (data not shown), confirming that binding of the CD to H3 K9me was not required for inhibition of hSWI/SNF activity.

Antagonistic regulation of interferon-inducible genes by hSWI/SNF and HP1 proteins
We next set up to identify genes where the antagonism between Brg1/Brm and HP1 proteins could be visualized in vivo. Our attention was brought to interferon-regulated genes that are wellcharacterized hSWI/SNF targets and that, like the viral HIV1 LTR, are highly and rapidly inducible in response to outside stimuli [27][28][29]. To confirm the effect of hSWI/SNF on the transcription of these genes, we knocked down Brm in HeLa cells with two different siRNAs ( Figure 4A, lanes 1-3). Brm was here preferred over Brg1 as Brm is degraded during each mitosis and is therefore very efficiently depleted with siRNAs [30]. This depletion resulted in repression of the interferon-inducible genes we tested, including IFIT1, IFIT3, OASL, and OAS1, with the exception of IFIT5 ( Figure 4B). In contrast, these genes, again with the exception of IFIT5, were activated upon knock-down of either HP1a or HP1b ( Figure 4A, lanes 4-7, and Figure 4C). These data show that several interferon-regulated genes rely on hSWI/SNF for their activation and on HP1a and HP1b for their repression.
Knock-down of HP1c did not significantly affect the expression of the interferon-inducible genes in the absence of stimulation ( Figure 4A, lanes 8-9 and 4C). However, we noted a moderate but reproducible decrease of the interferon-inducibility of the genes after depletion of HP1c, again with the exception of IFIT5 ( Figure 4D). These data are consistent with earlier reports showing an implication of HP1c in efficient expression of some genes [5,6].
Finally, we note that the activation of the interferon-inducible genes was observed only on 4 out of 5 tested genes, with HP1a and HP1b, but not with HP1c or GAPDH siRNAs. In addition, our siRNAs were designed to minimize the interferon response [31]. We therefore ruled out a possible non-specific stimulation of the interferon pathway by the double-stranded siRNAs.

Recruitment of HP1c to chromatin is dependent on Brm/ Brg1
Earlier studies have shown that HP1 proteins bind poorly to chromatin under physiological salt conditions while they associate tightly with destabilized nucleosomes from cells in S-phase [15,17]. This would be consistent with nucleosomal structures preventing HP1 proteins to access either the DNA or the Shadock region inside the nucleosome barrel. It also suggests a possible effect of chromatin remodeling on the loading of HP1 proteins to chromatin.
To investigate whether hSWI/SNF activity could influence HP1 recruitment, we carried out chromatin immuno-precipitations (ChIP) on the IFIT3 promoter before and after knock down of Brm with siRNAs. To minimize the impact of the histone H3K9 methylation repression mark on recruitment of HP1 proteins, we followed the promoter during transcriptional activation and we concentrated our study on HP1c that is not associated with repression of basal IFIT3 transcriptional activity ( Figure 4C). The experiments were carried out with a HeLa-derived cell line stably expressing moderate levels of epitope-tagged HP1c, thus allowing us to detect the protein with both anti-HP1c and anti-HA tag antibodies ( Figure 5A). Upon stimulation with interferon a, recruitment of both HP1c and Brm increased ( Figure 5B, siGAPDH). This is consistent with an implication of HP1c in efficient expression of the IFIT3 gene as observed in Figure 4D. The recruitment of HP1c was essentially abolished upon depletion of Brm ( Figure 5B, siBrm). This decreased recruitment was not due to the silencing of the IFIT3 promoter, as Brm depletion reduces the transcriptional activity of the gene only approx. 2-fold ( Figure 4D). ChIP-reChIP experiments further showed that Brm and HP1c were present on the same chromatin fragments, suggesting that their recruitment is interdependent. This corecruitment could be visualized with anti-Brm followed by anti-HA ChIP-reChIP as well as the inverse combination ( Figure 5C).
We next investigated the timing of the recruitment of HP1c and Brg1 on an integrated HIV1 LTR ( Figure 5D). As mentioned in the introduction, transcriptional activation of this promoter results in eviction of HP1ß and increased recruitment of HP1c [5]. ChIP analysis showed that recruitment of RNAPII, Brg1 and HP1c peaks 60 min. after induction with PMA. Subsequently, low levels of Brg1 recruitment were restored while levels of RNAPII and HP1c remained high. These observations are therefore compatible with transient recruitment of Brg1 helping the loading of HP1c onto sites internal to the nucleosome. They also suggest that recruitment of HP1c may limit the duration of hSWI/SNFmediated chromatin remodeling during transcriptional activation.

Brg1-mediated remodeling facilitates binding of HP1 proteins to nucleosomes
To investigate in vitro whether chromatin opening could facilitate binding of HP1 proteins to nucleosomes, we finally used a recombinant nucleosomal array associated with streptavidin beads ( Figure 6A and 6B). Consistent with earlier studies, this array assembled at relatively low ionic strength was poorly bound by Drosophila dHP1a ( Figure 6C, lane 2 and [16,32]). The Drosophila protein was here preferred because it could be more efficiently purified than its human counterparts and showed limited direct binding to Brg1 ( Figure S3). The binding of this protein was significantly increased in the presence of full length recombinant purified Brg1 and ATP ( Figure 6C, lanes 3 and 4). The effect was inhibited in the presence of cS-ATP, showing that it was dependent on the remodeling activity of Brg1 ( Figure 6C, lane 5). Some binding was also observed in the presence of high levels of Brg1 and non-hydrolysable cATP ( Figure 6C, lane 6), possibly explained either by a contamination of our preparation of Brg1 with ATP or by the interaction of dHP1a with Brg1.

Discussion
We show here that, in addition to the contact of the CD with methylated H3K9, HP1 proteins use the CSD to associate with histone H3 at the level of the entry/exit point of the nucleosome barrel. This region on histone H3 that we termed Shadock is also contacted by Brg1 and Brm, the catalytic subunits of the hSWI/ SNF complex, and chromatin remodeling can favor loading of HP1 proteins onto chromatin. We provide also evidence suggesting that this loading ultimately decreases the efficiency of hSWI/SNF remodeling.
FRAP experiments have shown that the CSD domain is required for the binding of HP1 proteins to native chromatin in vivo, with actually a stronger contribution than the CD when observation are made in euchromatic regions [33]. In addition, deletion of the CSD abolishes HP1-mediated transcriptional repression in transfection assays [34]. In the same type of experiment, we show that a point mutation in the CSD disrupting the interaction with the H3 Shadock region (I126F) is sufficient to interfere with the repressing activity. Furthermore, our REA assays show that the CSD is required for inhibition of chromatin remodeling by hSWI/SNF and that this remodeling complex contacts the same region on H3 as the HP1 proteins. Taken together, these observations strongly suggest that HP1 chromatin binding and repression activities are largely mediated by the histone binding activity of the CSD.
ChIP and ChIP reChIP assays show that Brm is required for the loading of HP1c to the IFIT3 promoter during transcriptional activation. We note that on this promoter, we have another example of the switch from HP1a/HP1ß to HP1c during transcriptional activation, also observed on the HIV1 LTR and on the Survivin promoter [5,6]. This switch could suggest that, in vivo, histone H3K9 methylation is the determining factor for the recruitment of HP1a and HP1ß, while recruitment of HP1c would be more dependent on chromatin opening by hSWI/SNF or the RNAPII. The role of HP1 proteins on active promoters is still enigmatic. The transient recruitment of Brg1 to the HIV1 promoter in sync with HP1c recruitment shown in Figure 5D suggests that HP1 proteins could be involved in limiting remodeling on activated promoters. However, HP1 proteins may also have a role further downstream in connection with mRNA maturation [35].
The presence of HP1a on the IFIT1 and IFIT3 promoters and thereby the direct regulation of these genes by the HP1 protein was established by ChIP experiments (data not shown). However, the ChIP approach did not allow us to estimate the impact of hSWI/SNF remodeling on the recruitment of HP1a, as depletion of Brm leads to repression of the IFIT1 and IFIT3 promoters. Under these conditions, it was not possible to part between enhanced recruitment due to increased histone H3K9 methylation and decreased recruitment associated with reduced hSWI/SNF activity. However, our in vitro nucleosome-array binding assay suggests that HP1 proteins other than HP1c can benefit from chromatin remodeling to bind nucleosomes. It must here be noted that this assay does not allow us to discriminate between histone and DNA binding. Nevertheless, repression of hSWI/SNF remodeling does not appear to rely on DNA binding as an HP1a mutant no longer binding DNA still represses. In addition, HP1c does not bind to DNA in our hands and prefers nucleosomes (data not shown). Yet, it is efficient in repressing hSWI/SNF remodeling. Finally, we note that methylation of histone H3 on K9 is not required for repression of hSWI/SNF remodeling in vitro, further suggesting that the contact of the CSD with the Shadock, and not other contacts, is important for the repressing activity of HP1 proteins.
The interaction of HP1a CSD with histone H3 required HP1 dimerization and was disrupted by the mutation of V46 in the PXVXL-like sequence in the Shadock. It is therefore possible that this interaction can be structurally compared to that of other molecular partners of CSDs such as CAF1 or TIF1 proteins [36]. Interestingly, the rH3(35-66)V46A and rH3(44-66) mutants bind HP1c but not HP1a, while H3 constructs with a full Shadock region bind both HP1 proteins. These observations show that HP1a and HP1c have neighboring but distinct sites of interaction on the histone. While this manuscript was in revision, it was shown that phosphorylation of histone H3 on tyrosine 41 by JAK2 compromises the binding of the chromoshadow-domain of HP1a to an H3 peptide spanning from aa 31 to 56 [37]. This modification is outside the HP1c binding site and it therefore seems possible that HP1a and HP1c are differentially regulated by post-translational modifications in the H3 Shadock region.
The region of histone H3 contacted by the HP1 proteins also associates with the hSWI/SNF subunits Brg1 and Brm. This is compatible with earlier studies in yeast showing that residues K56 and L61 are involved in SWI/SNF recruitment [38,39]. In yeast, it was suggested that the H3 aN helix is targeted and remodeled by the SWI/SNF complex [40]. More recently, mutagenesis within this region was shown to affect the efficiency of yeast SWI/SNF remodeling in vitro [24]. Our competition experiments with polypeptides mimicking the H3 aN helix further suggest that interaction of Brg1 with this region is essential for the chromatin remodeling activity of the hSWI/SNF complex. Consistent with this, the sequences downstream of the helicase domain of Brg1/ Brm that mediate the interaction with the H3 Shadock domain have earlier been reported as essential for efficient remodeling in vitro [25]. We therefore speculate that the competing binding of Brg1/Brm and HP1 proteins to a same region of histone H3 during unwanted remodeling can at least in part explain the inhibiting effect of HP1 on chromatin opening by hSWI/SNF.
In vitro binding assays suggest that Brg1 has an affinity for the globular domain of H3 that is higher than that of HP1a ( Figure  S4). In a mechanism based on competition of Brg1 and HP1 proteins for binding to overlapping sites on histone H3, this difference in affinity may explain that a 7-fold excess of HP1a over Brg1 is required to obtain 50% inhibition of hSWI/SNF remodeling in the REA assays. It must finally be noted that the activity of the hSWI/SNF complex is also inhibited by the Polycomb Group (PcG) class II complex in vitro [26,41]. This complex, involved in gene silencing, includes a CD protein that, like HP1 proteins, binds methylated histone H3 tails with a preference for methylation on K27 rather than K9 [42,43]. However, Polycomb proteins contain no CSDs and it seems that PRC1 relies essentially on a structuring effect on the nucleosomal template, repressing remodeling by creating more condensed chromatin [44].
hSWI/SNF and HP1 proteins have many common target promoters including E2F1, human thymidine kinase, c-Myc, Sox2, Cyclin E, and the MMTV and HIV1 LTRs [5,7,[45][46][47][48][49][50][51]. Likewise, we show here that several interferon-inducible genes that require hSWI/SNF for their activation are under the negative control of HP1a and HP1b. We note however that not all hSWI/SNF target genes we tested were affected by knock-down of HP1 proteins, including for example DraL and SPARC. These genes were both expressed at relatively high levels in the cells we used and may therefore not have any repressive structure on their promoter (data not shown). All considered, we suggest that on promoters where SWI/SNF functions as a repressor, the opening of the chromatin may, as previously suggested, be the event initiating HP1 stable recruitment [8,20]. Where SWI/SNF functions as an activator, HP1 proteins recruited by methylated histone H3 tails may instead function as sensors of unwanted SWI/SNF activity, binding nucleosome domains uncovered by the remodeling, and thereby block the reaction (see proposed model in Figure 7). Finally, after the activation, additional HP1 proteins may be involved in controlling excessive remodeling activity. In that sense, HP1 proteins could very generally function as gatekeepers using the exposure of domains internal to the nucleosome to detect and restrict chromatin opening.

Materials and Methods
Transient transfection assays MCF7 cells were transfected by calcium phosphate precipitation as previously described [52]. When indicated, 10 26 M dexamethasone (dex) was added to the medium. 40 hours post-transfection, luciferase assays were performed using the Promega luciferase kit, following the instructions of the manufacturer.

Restriction enzyme accessibility (REA) assays
Flag-epitope tagged human HP1a and Brg1 were expressed in SF9 cells using a bac-to-bac expression system (Gibco) and purified on M2 anti-flag beads (Sigma). hSWI/SNF was purified from a Flag-tagged Ini1 HeLa cell line as described previously [53]. HAtagged HP1a constructs were expressed in E.coli and purified using their additional 6xHis affinity tag. HA-HP1aDDNA contains a deletion from aa 87 to 111. HA-HP1aDBrg1 was previously described [20]. The polynucleosome template was assembled by gradient salt dialysis with HeLa core histones and a 5S arrays fragment [53,54]. The mononucleosome template was assembled on a 202 bp TPT containing DNA fragment by salt dialysis [55] and incubated at 37uC for 3 hours before use. REA assays were performed as previously described [26].

In vitro interaction experiments
GST-HP1 fusions and GST-DBrg1 were constructed in pGEX3X and pET41 plasmids respectively. HA-tagged HP1 and B10-tagged histone H3 were constructed in pET-28 and pET15b plasmids respectively that also providing a 6xHis affinity tag used for purification. Pull-down experiments were performed in ELB buffer (50mM HEPES pH 7, 250mM NaCl, 1mM EDTA, 0.1% NP40, 1xComplete protease inhibitor cocktail from Roche). Bound proteins were eluted in 100mM Tris pH 8, 20mM glutathione, resolved by SDS-PAGE and detected by western blotting using the B10 monoclonal mouse anti-estrogen receptor a antibodies (Euromedex ERB10-As) or anti-histone H3 rabbit polyclonal (Abcam ab1791). Overlay assays and nucleosome array binding assays were previously described [14,16].