Thiirane linkers directed histone H2A diubiquitination suggests plasticity in 53BP1 recognition

Efficient Chemical Synthesis of Multi-Monoubiquitylated and Diubiquitylated Histones by the α-Halogen Ketone-Mediated Strategy

The chemical synthesis of homogeneously ubiquitylated histones is a powerful approach to decipher histone ubiquitylation-dependent epigenetic regulation. Among the various methods, α-halogen ketone-mediated conjugation chemistry has recently been an attractive strategy to generate single-monoubiquitylated histones for biochemical and structural studies. Herein, we report the use of this strategy to prepare not only dual- and even triple-monoubiquitylated histones but also diubiquitin-modified histones. We were surprised to find that the synthetic efficiencies of multi-monoubiquitylated histones were comparable to those of single-monoubiquitylated ones, suggesting that this strategy is highly tolerant to the number of ubiquitin monomers installed onto histones. The facile generation of a series of single-, dual-, and triple-monoubiquitylated H3 proteins enabled us to evaluate the influence of ubiquitylation patterns on the binding of DNA methyltransferase 1 (DNMT1) to nucleosomes. Our study highlights the potential of site-specific conjugation chemistry to generate chemically defined histones for epigenetic studies.

The expedient, CAET-assisted synthesis of dual-monoubiquitinated histone H3 enables evaluation of its interaction with DNMT1

Site-selective conjugation chemistry has proven effective to synthesize homogenously ubiquitinated histones. Recently, a powerful strategy using 2-((2-chloroethyl) amino) ethane-1-thiol (CAET) as a bifunctional handle was developed to generate chemically stable ubiquitin chains without racemization and homodimerization. Herein, we extend this strategy to the expedient synthesis of ubiquitinated histones, exemplifying its utility to not only synthesize single-monoubiquitinated histones, but dual-monoubiquitinated histones as well. The synthetic histones enabled us to evaluate the binding of DNMT1 to ubiquitinated nucleosomes and map the hotspots of this interaction. Our work highlights the potential of modern chemical protein synthesis to synthesize ubiquitinated histones for epigenetic studies.

Synthetic modifications of histones and their functional evaluation

Post-transrational modifications (PTMs) of histones play a key role in epigenetic regulation. Unraveling the roles of each epigenetic mark can provide new insights into their biological mechanisms. On the other hand, it is generally difficult to prepare homogeneously-modified histones/nucleosomes to investigate their specific functions. Therefore, synthetic approaches to acquire precisely mimicked histones/nucleosomes are in great demand, and further development of this research field is anticipated. In this review, synthetic strategies to modify histones/nucleosomes, including cysteine modifications, transformations of dehydroalanine residues and lysine acylation using a catalyst system, are cited. In addition, the functional evaluation of synthetically modified histones/nucleosomes is described.

Total chemical synthesis of the phosphorylated p62 UBA domain reveals that Ser407Pi but not Ser403Pi enhances ubiquitin binding

As an autophagic adaptor, p62 specifically targets ubiquitinated proteins to an autophagosome for lysosomal degradation through a critical ubiquitin-associated (UBA) domain. Recent research studies reported that the Ser403 and Ser407 sites on the UBA domain were modified by phosphorylation, increasing the binding affinity between p62 and ubiquitin (Ub). However, the exact role of each phosphorylation site in the regulation of the UBA domain and Ub binding remains unclear. In this text, we applied total chemical synthesis to prepare four types of phosphorylated UBAs, among which the bisphosphorylated UBA was successfully synthesized via the pseudo-dipeptide unit and auxiliary-mediated hydrazide-based native chemical ligation (NCL). Isothermal titration calorimetry (ITC) assays showed that the phosphorylation at S407 enhanced the binding affinity between UBA and Ub, while that at S403 did not. It was suggested that phosphorylation at S407 might be important for promoting the interplay between the UBA domain and Ub, whereas phosphorylation at S403 was not directly involved in this interaction.

The Bre1/Rad6 machinery: writing the central histone ubiquitin mark on H2B and beyond

Mono-ubiquitination on H2B (H2Bub1) is an evolutionarily conserved histone post-translational modification implicated in various important physiological processes including DNA replication, transcription activation, and DNA damage repair. The Bre1/Rad6 ubiquitination machinery is currently considered to be the sole writer of H2Bub1, but the mechanistic basis by which it operates is unclear. Recently, the RING-type E3 ligase Bre1 was proposed to associate with the E2 enzyme Rad6 through a novel interaction between Bre1 RBD (Rad6 binding domain) and Rad6; and the RING domain of Bre1 that is responsible for the nucleosomal acidic patch binding also interacts with Rad6 to stimulate its catalytic activity. Recent discoveries have yielded evidence for the phenomenon of liquid-liquid phase separation in the context of H2Bub1, and its regulation by other histone post-translational modifications. This review summarizes current knowledge about Bre1/Rad6-mediated H2B ubiquitination, including the physiological functions and the molecular basis for writing and regulation of this central histone ubiquitin mark. Possible models for the Bre1/Rad6 machinery bound to nucleosomes bearing different modifications in the writing step are also disclosed.