Fig. 7. Molecular mechanism underlying dynamic heterochromatin.

Proposed model for dynamic heterochromatin maintenance in mESC: a KAP1/TRIM28 is recruited by DNA-binding KRAB-ZFPs to interstitial repeats, such as IAP ERVs. KAP1/TRIM28 orchestrates heterochromatin formation and maintenance, recruiting amongst others histone methyltransferase SETDB1, histone H3.3 chaperone DAXX and chromatin remodeler Smarcad1. Through SETDB1-mediated histone H3K9me3 methylation, HP1 is recruited and presumably contributes to heterochromatin formation by compaction/phase separation. b Smarcad1 ATP-dependent remodeling activity leads to a disassembly of nucleosomes and permanent eviction of existing histones. The primary reason for histone eviction is unknown but may facilitate access of KRAB-ZFPs to the underlying DNA sequence to amplify KAP1 recruitment. c In wildtype cells DNA is only accessible very transiently since nucleosomes are immediately reformed using H3.3, not canonical histone H3.1/2. The deposited H3.3 may be newly synthesized, thus does not carry H3K9me3 methylation. Hence, SETDB1 needs to reestablish H3K9me3 on the dynamic nucleosome substrates. In the absence of H3.3, nucleosome reassembly is impaired, leaving DNA accessible. In the absence of Smarcad1, nucleosomes are not evicted, alleviating the requirement for H3.3 as a substrate for new nucleosomes and suspending nucleosome dynamics at heterochromatin.