Evaluation of acyllysine isostere interactions with the aromatic pocket of the AF9 YEATS domain

Negative Cooperativity in the UHRF1 TTD-PHD Dual Domain Masks the Contributions of Cation-π Interactions Between Trimethyllysine and the TTD Aromatic Cage

UHRF1 is a promising epigenetic target in oncology, but inhibitor development has proven challenging due to the interplay between its tandem Tudor domain (TTD) and plant homeodomain (PHD). The TTD binds trimethyllysine (Kme3) at position 9 while the PHD binds Arg at position 2 on histone 3. Herein, we report how the PHD influences TTD recognition of the histone 3 tail containing Kme3 (H3K9me3) versus its neutral isostere, tert-butyl norleucine (tBuNle). Our findings show that the dual domain binds both peptides equally, supporting tBuNle's potential for inhibitor development. However, unexpectedly, the binding mechanism of H3K9me3 differs between the single and dual domains. In the TTD alone, Kme3 is bound in the aromatic cage via electrostatically tunable cation-π interactions, but in the dual domain, Kme3 binding is independent of electrostatics in the aromatic cage-an unprecedented observation. Computational studies suggest cation-π interactions should contribute in both cases. The contrasting experimental and computational results point to an unusual example of negative chelate cooperativity: interactions between the histone and PHD mask the mechanism of TTD recognition of K9me3. This work underscores the complexity of histone post-translational modification (PTM) readout in multi-domain proteins and demonstrates the first example of a masked cation-π interaction.

Epigenetics-targeted drugs: current paradigms and future challenges

Epigenetics governs a chromatin state regulatory system through five key mechanisms: DNA modification, histone modification, RNA modification, chromatin remodeling, and non-coding RNA regulation. These mechanisms and their associated enzymes convey genetic information independently of DNA base sequences, playing essential roles in organismal development and homeostasis. Conversely, disruptions in epigenetic landscapes critically influence the pathogenesis of various human diseases. This understanding has laid a robust theoretical groundwork for developing drugs that target epigenetics-modifying enzymes in pathological conditions. Over the past two decades, a growing array of small molecule drugs targeting epigenetic enzymes such as DNA methyltransferase, histone deacetylase, isocitrate dehydrogenase, and enhancer of zeste homolog 2, have been thoroughly investigated and implemented as therapeutic options, particularly in oncology. Additionally, numerous epigenetics-targeted drugs are undergoing clinical trials, offering promising prospects for clinical benefits. This review delineates the roles of epigenetics in physiological and pathological contexts and underscores pioneering studies on the discovery and clinical implementation of epigenetics-targeted drugs. These include inhibitors, agonists, degraders, and multitarget agents, aiming to identify practical challenges and promising avenues for future research. Ultimately, this review aims to deepen the understanding of epigenetics-oriented therapeutic strategies and their further application in clinical settings.

WDR5 Binding to Histone Serotonylation Is Driven by an Edge-Face Aromatic Interaction with Unexpected Electrostatic Effects

Histone serotonylation has emerged as a key post-translational modification. WDR5 preferentially binds to serotonylated histone 3 (H3), and this binding event has been associated with tumorigenesis. Herein, we utilize genetic code expansion, structure-activity relationship studies, and computation to study an edge-face aromatic interaction between WDR5 Phe149 and serotonin on H3 that is key to this protein-protein interaction. We find experimentally that this edge-face aromatic interaction is unaffected by modulating the electrostatics of the face component but is weakened by electron-withdrawing substituents on the edge component. Overall, these results elucidate that this interaction is governed by van der Waals forces as well as electrostatics of the edge ring, a result that clarifies discrepancies among previous theoretical models and model system studies of this interaction type. This is the first evaluation of the driving force of an edge-face aromatic interaction at a protein-protein interface and provides a key benchmark for the nature of these understudied interactions that are abundant in the proteome.

Contribution of Electrostatic CH3-π Interactions to Recognition of Histone Asymmetric Dimethylarginine by the SPIN1 Triple Tudor Domain

Methylation of arginine (Arg) residues on histones creates a new binding epitope, enabling recognition by aromatic cage binding pockets in Tudor domains; these protein-protein interactions (PPIs) govern gene expression. Despite their biological importance, the molecular details of methylated Arg recognition are poorly understood. While the desolvation, hydrogen bonding, and guanidinium stacking of methylated Arg have been explored in model systems and proposed to contribute to binding, direct interactions between the methyl groups and the aromatic residues in the binding pocket have not previously been investigated. Herein, we mechanistically study the CH3-π interactions between the SPIN1 triple Tudor domain and histone asymmetric dimethylarginine. We find that these CH3-π interactions are electrostatically tunable, exhibiting cation-π character, albeit attenuated relative to cation-π interactions with quaternary ammonium ions, offering key insight into how methylation of Arg alters its binding epitope to enable new PPIs.

Trimethyllysine Reader Proteins Exhibit Widespread Charge-Agnostic Binding via Different Mechanisms to Cationic and Neutral Ligands

In the last 40 years, cation-π interactions have become part of the lexicon of noncovalent forces that drive protein binding. Indeed, tetraalkylammoniums are universally bound by aromatic cages in proteins, suggesting that cation-π interactions are a privileged mechanism for binding these ligands. A prominent example is the recognition of histone trimethyllysine (Kme3) by the conserved aromatic cage of reader proteins, dictating gene expression. However, two proteins have recently been suggested as possible exceptions to the conventional understanding of tetraalkylammonium recognition. To broadly interrogate the role of cation-π interactions in protein binding interactions, we report the first large-scale comparative evaluation of reader proteins for a neutral Kme3 isostere, experimental and computational mechanistic studies, and structural analysis. We find unexpected widespread binding of readers to a neutral isostere with the first examples of readers that bind the neutral isostere more tightly than Kme3. We find that no single factor dictates the charge selectivity, demonstrating the challenge of predicting such interactions. Further, readers that bind both cationic and neutral ligands differ in mechanism: binding Kme3 via cation-π interactions and the neutral isostere through the hydrophobic effect in the same aromatic cage. This discovery explains apparently contradictory results in previous studies, challenges traditional understanding of molecular recognition of tetraalkylammoniums by aromatic cages in myriad protein-ligand interactions, and establishes a new framework for selective inhibitor design by exploiting differences in charge dependence.

Reading and erasing of histone crotonyllysine mimics by the AF9 YEATS domain and SIRT2 deacylase

Lysine acylations on histones and their recognition by chromatin-binding reader domains and removal by histone deacylases function as an important mechanism for eukaryotic gene regulation. Histone lysine crotonylation (Kcr) is an epigenetic mark associated with active transcription, and its installation and removal are dynamically regulated by cellular epigenetic enzymes. Here, we report binding studies and enzyme assays with histone H3K9 peptides bearing simplest Kcr analogs with varying hydrocarbon chain length, bulkiness, rigidity and polarity. We demonstrate that the AF9 YEATS domain displays selectivity for binding of different acylation modifications on histone H3K9 peptides and exhibits preference for bulkier cinnamoylated lysine over crotonylated lysine and its mimics. SIRT2 shows deacylase activity against most of acylated H3K9 peptides bearing different crotonyllysine mimics, however, it displays a poor ability for the removal of cinnamoyl and trifluorocrotonyl groups. These results demonstrate different substrate selectivities of epigenetic proteins acting on crotonyllysine and pave the way for rational design and development of AF9 YEATS and SIRT2 inhibitors for treatment of human diseases, including cancer.

Molecular Recognition of Methacryllysine and Crotonyllysine by the AF9 YEATS Domain

Histone lysine methacrylation and crotonylation are epigenetic marks that play important roles in human gene regulation. Here, we explore the molecular recognition of histone H3 peptides possessing methacryllysine and crotonyllysine at positions 18 and 9 (H3K18 and H3K9) by the AF9 YEATS domain. Our binding studies demonstrate that the AF9 YEATS domain displays a higher binding affinity for histones possessing crotonyllysine than the isomeric methacryllysine, indicating that AF9 YEATS distinguishes between the two regioisomers. Molecular dynamics simulations reveal that the crotonyllysine/methacryllysine-mediated desolvation of the AF9 YEATS domain provides an important contribution to the recognition of both epigenetic marks. These results provide important knowledge for the development of AF9 YEATS inhibitors, an area of biomedical interest.