Background: UHRF1 is an important epigenetic regulator connecting DNA methylation and histone methylations. Results: PHD-H3 interaction is independent of the TTD, whereas TTD-H3K9me3 interaction the PHD. Conclusion: Both TTD and PHD are essential for specific recognition of H3K9me3 by human UHRF1. Significance: This work reveals how UHRF1 recognizes H3K9me3, which is important for its cellular localization and DNA methylation.

DNMT1 is an important epigenetic regulator that plays a key role in the maintenance of DNA methylation. Here we determined the crystal structure of DNMT1 in complex with USP7 at 2.9 Å resolution. The interaction between the two proteins is primarily mediated by an acidic pocket in USP7 and Lysine residues within DNMT1's KG linker. This intermolecular interaction is required for USP7-mediated stabilization of DNMT1. Acetylation of the KG linker Lysine residues impair DNMT1–USP7 interaction and promote the degradation of DNMT1. Treatment with HDAC inhibitors results in an increase in acetylated DNMT1 and decreased total DNMT1 protein. This negative correlation is observed in differentiated neuronal cells and pancreatic cancer cells. Our studies reveal that USP7-mediated stabilization of DNMT1 is regulated by acetylation and provide a structural basis for the design of inhibitors, targeting the DNMT1–USP7 interaction surface for therapeutic applications.

UHRF1 is an important epigenetic regulator for maintenance DNA methylation. UHRF1 recognizes hemi-methylated DNA (hm-DNA) and trimethylation of histone H3K9 (H3K9me3), but the regulatory mechanism remains unknown. Here we show that UHRF1 adopts a closed conformation, in which a C-terminal region (Spacer) binds to the tandem Tudor domain (TTD) and inhibits H3K9me3 recognition, whereas the SET-and-RING-associated (SRA) domain binds to the plant homeodomain (PHD) and inhibits H3R2 recognition. Hm-DNA impairs the intramolecular interactions and promotes H3K9me3 recognition by TTD–PHD. The Spacer also facilitates UHRF1–DNMT1 interaction and enhances hm-DNA-binding affinity of the SRA. When TTD–PHD binds to H3K9me3, SRA-Spacer may exist in a dynamic equilibrium: either recognizes hm-DNA or recruits DNMT1 to chromatin. Our study reveals the mechanism for regulation of H3K9me3 and hm-DNA recognition by URHF1.

We report on the role of 5f-orbital participation in the unexpected inversion of the s-bond metathesis reactivity trend of triamidoamine thorium(IV) and uranium(IV) alkyls. Reaction of KCH 2 Ph with [U(Tren TIPS)(I)] [2a, Tren TIPS ¼ N(CH 2 CH 2 NSiPr i 3) 3 3À ] gave the cyclometallate [U {N(CH 2 CH 2 NSiPr i 3) 2 (CH 2 CH 2 NSiPr i 2 C[H]MeCH 2)}] (3a) with the intermediate benzyl complex not observable. In contrast, when [Th(Tren TIPS)(I)] (2b) was treated with KCH 2 Ph, [Th(Tren TIPS)(CH 2 Ph)] (4) was isolated; which is notable as Tren N-silylalkyl metal alkyls tend to spontaneously cyclometallate. Thermolysis of 4 results in the extrusion of toluene and formation of the cyclometallate [Th {N(CH 2 CH 2 NSiPr i 3) 2 (CH 2 CH 2 NSiPr i 2 C[H]MeCH 2)}] (3b). This reactivity is the reverse of what would be predicted. Since the bonding of thorium is mainly electrostatic it would be predicted to undergo facile cyclometallation, whereas the more covalent uranium system might be expected to form an isolable benzyl intermediate. The thermolysis of 4 follows well-defined first order kinetics with an activation energy of 22.3 AE 0.1 kcal mol À1 , and Eyring analyses yields DH ‡ ¼ 21.7 AE 3.6 kcal mol À1 and DS ‡ ¼ À10.5 AE 3.1 cal K À1 mol À1 , which is consistent with a s-bond metathesis reaction. Computational examination of the reaction profile shows that the inversion of the reactivity trend can be attributed to the greater f-orbital participation of the bonding for uranium facilitating the s-bond metathesis transition state whereas for thorium the transition state is more ionic resulting in an isolable benzyl complex. The activation barriers are computed to be 19.0 and 22.2 kcal mol À1 for the uranium and thorium cases, respectively, and the latter agrees excellently with the experimental value. Reductive decomposition of "[U(Tren TIPS)(CH 2 Ph)]" to [U(Tren TIPS)] and bibenzyl followed by cyclometallation to give 3a with elimination of dihydrogen was found to be endergonic by 4 kcal mol À1 which rules out a redox-based cyclometallation route for uranium.