DNA methylation, catalyzed by DNA methyltransferases (DNMTs), is an epigenetic mark that needs to be faithfully replicated during mitosis in order to maintain cell phenotype during successive cell divisions. This epigenetic mark is located on the 5′-carbon of the cytosine mainly within cytosine–phosphate–guanine (CpG) dinucleotides. DNA methylation is asymmetrically positioned on both DNA strands, temporarily generating a hemi-methylated state after DNA replication. Hemi-methylation is a particular status of DNA that is recognized by ubiquitin-like containing plant homeodomain (PHD) and really interesting new gene (RING) finger domains 1 (UHRF1) through its SET- (Su(var)3-9, Enhancer-of-zeste and Trithorax) and RING-associated (SRA) domain. This interaction is considered to be involved in the recruitment of DNMT1 to chromatin in order to methylate the adequate cytosine on the newly synthetized DNA strand. The UHRF1/DNMT1 tandem plays a pivotal role in the inheritance of DNA methylation patterns, but the fine-tuning mechanism remains a mystery. Indeed, because DNMT1 experiences difficulties in finding the cytosine to be methylated, it requires the help of a guide, i.e., of UHRF1, which exhibits higher affinity for hemi-methylated DNA vs. non-methylated DNA. Two models of the UHRF1/DNMT1 dialogue were suggested to explain how DNMT1 is recruited to chromatin: (i) an indirect communication via histone H3 ubiquitination, and (ii) a direct interaction of UHRF1 with DNMT1. In the present review, these two models are discussed, and we try to show that they are compatible with each other.

The role of the mitotic phosphorylation of the amino (NH ₂ ) terminus of Centromere Protein A (CENP-A), the histone variant epigenetic centromeric marker, remains elusive. Here, we show that the NH ₂ terminus of human CENP-A is essential for mitotic progression and that localization of CENP-C, another key centromeric protein, requires only phosphorylation of the CENP-A NH ₂ terminus, and is independent of the CENP-A NH ₂ terminus length and amino acid sequence. Mitotic CENP-A nucleosomal complexes contain CENP-C and phosphobinding 14-3-3 proteins. In contrast, mitotic nucleosomal complexes carrying nonphosphorylatable CENP-A–S7A contained only low levels of CENP-C and no detectable 14-3-3 proteins. Direct interactions between the phosphorylated form of CENP-A and 14-3-3 proteins as well as between 14-3-3 proteins and CENP-C were demonstrated. Taken together, our results reveal that 14-3-3 proteins could act as specific mitotic “bridges,” linking phosphorylated CENP-A and CENP-C, which are necessary for the platform function of CENP-A centromeric chromatin in the assembly and maintenance of active kinetochores.

MeCP2 binds hydroxymethylated CA repeats: Despites of decades of research on the Rett syndrome protein MeCP2, its function remains unclear. Ibrahim et al. show that MeCP2 is a hydroxymethylated cytosine-adenosine (CA) repeat-binding protein that modulates chromatin architecture at a distance from the transcription start site (see the Perspective by Zhou and Zoghbi). MeCP2 accumulates and spreads around modified CA repeats and competes for nucleosome occupancy. Loss of MeCP2 results in a widespread increase in nucleosome density inside lamina-associated domains and transcriptional dysregulation of genes enriched in CA repeats. These results shed light on the underlying molecular mechanism of Rett syndrome, a severe disease associated with mutations in MeCP2. Science , abd5581, this issue p. eabd5581 ; see also abj5027, p. 1390