The PHD domain of Np95 (mUHRF1) is involved in large-scale reorganization of pericentromeric heterochromatin.

Heterochromatic chromosomal regions undergo large-scale reorganization and progressively aggregate, forming chromocenters. These are dynamic structures that rapidly adapt to various stimuli that influence gene expression patterns, cell cycle progression, and differentiation. Np95-ICBP90 (m- and h-UHRF1) is a histone-binding protein expressed only in proliferating cells. During pericentromeric heterochromatin (PH) replication, Np95 specifically relocalizes to chromocenters where it highly concentrates in the replication factories that correspond to less compacted DNA. Np95 recruits HDAC and DNMT1 to PH and depletion of Np95 impairs PH replication. Here we show that Np95 causes large-scale modifications of chromocenters independently from the H3:K9 and H4:K20 trimethylation pathways, from the expression levels of HP1, from DNA methylation and from the cell cycle. The PHD domain is essential to induce this effect. The PHD domain is also required in vitro to increase access of a restriction enzyme to DNA packaged into nucleosomal arrays. We propose that the PHD domain of Np95-ICBP90 contributes to the opening and/or stabilization of dense chromocenter structures to support the recruitment of modifying enzymes, like HDAC and DNMT1, required for the replication and formation of PH.

Np95 is implicated in pericentromeric heterochromatin replication and in major satellite silencing.

Heterochromatin plays an important role in transcriptional repression, for the correct segregation of chromosomes and in the maintenance of genome stability. Pericentric heterochromatin (PH) replication and formation have been proposed to occur in the pericentric heterochromatin duplication body (pHDB). A central question is how the underacetylated state of heterochromatic histone H4 tail is established and controlled, because it is a key event during PH replication and is essential to maintain the compacted and silenced state of these regions. Np95 is a cell cycle regulated and is a nuclear histone-binding protein that also recruits HDAC-1 to target promoters. It is essential for S phase and for embryonic formation and is implicated in chromosome stability. Here we show that Np95 is part of the pHDB, and its functional ablation causes a strong reduction in PH replication. Depletion of Np95 also causes a hyperacetylation of lysines 8, 12, and 16 of heterochromatin histone H4 and an increase of pericentromeric major satellite transcription, whose RNAs are key players for heterochromatin formation. We propose that Np95 is a new relevant protein involved in heterochromatin replication and formation.