Critical Role of the UBL Domain in Stimulating the E3 Ubiquitin Ligase Activity of UHRF1 toward Chromatin

摘要

class="head no_bottom_margin" id="sec1title">IntroductionDNA and histone modifications regulate chromatin function and mediate processes such as transcription, DNA repair, and DNA replication. Maintaining chromatin modifications after DNA replication is essential for chromatin homeostasis, especially for silenced regions of the genome, such as repetitive elements. The maintenance methyltransferase DNMT1 ensures that cytosine methylation at CpG dinucleotides is propagated to daughter cells. To achieve this, DNMT1 is recruited to newly replicated hemi-methylated DNA in order to copy CpG methylation marks to the newly synthesized DNA strand. The E3 ubiquitin ligase UHRF1 (ubiquitin-like containing PHD and RING finger domains protein 1) was found to play a critical role in this targeting process (, ). UHRF1 is a multi-domain epigenetic regulator (A) that reads histone H3 Lys-9 di- and tri-methylation (H3K9me2/3) marks in the context of an otherwise unmodified N terminus through its linked TTD-PHD (tandem-tudor domain and plant homeodomain) module (, ) in addition to binding hemi-methylated CpGs via its SRA (SET and RING-associated) domain (, ). The recruitment of DNMT1 to newly replicated chromatin critically depends on the ubiquitylation of Lys-14, Lys-18, and/or Lys-23 in histone H3 that is catalyzed by the E3 ubiquitin ligase activity of UHRF1 harbored in its C-terminal RING domain (). The tandem mono-ubiquitin marks on histone H3 are recognized by DNMT1 via a ubiquitin interaction motif in its RFTS (replication foci targeting sequence) domain (). In contrast to the targeting of DNMT1 to ubiquitylated histone H3, which has been elucidated in structural detail (), the enzymatic mechanism by which UHRF1 deposits ubiquitin onto the histone is not completely understood.The UBL Domain of UHRF1 Contacts Other Regions within the Protein(A) Schematic of full-length mouse UHRF1 (Np95) indicating domain architecture and function. Red lines indicate known intra-molecular interactions.(B) Crosslinking with mass spectrometry (XL-MS) profile of UHRF1. Crosslinks involving the UBL domain are shown in purple and other intra-protein crosslinks in red. Individual domains within UHRF1 are annotated.(C) Proposed model of UHRF1 folding.