HP1 binding creates a local barrier against transcription activation and persists during chromatin decondensation

Mouse pericentric repeats form transcriptionally silent and compacted domains known as chromocenters. These prototypic heterochromatin compartments are marked by heterochromatin protein 1 (HP1). However, its contributions to chromocenter structure and function remain debated. We investigated the role of HP1 by recruiting the activators VP16, p65, and VPR to mouse fibroblast chromocenters and analyzed its silencing activity with a transcription reporter. Upon chromocenter decondensation and transcription activation, interactions of HP1 with chromatin and H3K9 trimethylation remained stable, suggesting stoichiometric binding rather than higher-order assembly. HP1-mediated repression required promoter-proximal binding and effectively suppressed VP16-triggered activation but not the stronger activation by VPR. These observations are explained by a 1D lattice binding model, which conceptualizes chromocenters as arrays of repeat units that can independently switch between silenced and activated states. Our findings provide a quantitative framework that explains how chromocenter organization responds to transcriptional activation while maintaining local heterochromatin features. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=189 HEIGHT=200 SRC="FIGDIR/small/627308v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@83816corg.highwire.dtl.DTLVardef@1a603c6org.highwire.dtl.DTLVardef@41d8b4org.highwire.dtl.DTLVardef@fa4d0d_HPS_FORMAT_FIGEXP M_FIG C_FIG HIGHLIGHTSO_LIHP1 represses transcription at mouse chromocenters and an ectopic reporter when competing with transcriptional activators VP16, p65 and VPR C_LIO_LIHP1 repression requires promoter-proximal binding and effectively counteracts the weak activator VP16 C_LIO_LIThe strong activator VPR overcomes HP1-mediated repression while HP1 remains bound to chromatin C_LIO_LIHP1 binding and H3K9me3 persist during chromocenter decondensation and transcriptional activation C_LIO_LIA 1D lattice binding model explains how independent repeat units transition between silenced and activated states without requiring phase separation C_LI