Unlocking the potential of chemical probes for methyl-lysine reader proteins

Engineering a methyllysine reader with photoactive amino acid in mammalian cells

Methyllysine sites in proteins are recognized by an array of reader domains that mediate protein-protein interactions for controlling cellular processes. Herein, we engineer a chromodomain, an essential methyllysine reader, to carry 4-azido-l-phenylalanine (AzF) via amber suppressor mutagenesis and demonstrate its potential to bind and crosslink methylated proteins in human cells. We further develop a first-of-its kind chromodomain variant bearing two AzF units with enhanced crosslinking potential suitable for profiling the transient methyllysine interactome.

Site-specific azide-acetyllysine photochemistry on epigenetic readers for interactome profiling

The hydrophobic pocket of the epigenetic reader protein BRD4 has been engineered to carry a photosensitive amino acid to identify novel interacting partners, providing mechanistic insights into BRD4’s function in transcription and beyond.

Chemical modifications on DNA, RNA and histones are recognized by an array of ‘reader’ modules to regulate transcriptional programming and cell fate. However, identification of reader-specific interacting partners in a dynamic cellular environment remains a significant challenge. Herein, we report a chemoproteomic approach termed ‘interaction-based protein profiling’ (IBPP) to characterize novel interacting partners of potentially any reader protein. IBPP harnesses a photosensitive amino acid introduced into the hydrophobic pocket of a reader module to crosslink and enrich transient interacting partners that are inaccessible to traditional methods. Using bromodomain-containing protein 4 (BRD4) as a paradigm, we engineer an ‘aromatic cage’ of the bromodomain to introduce 4-azido-l-phenylalanine (pAzF) without compromising its ability to recognize acetylated lysine residues in histone proteins. We establish the binding efficiency, substrate specificity and crosslinking ability of the engineered ‘reader’ module in biochemical assays. Applying IBPP, we uncovered novel acetylated interacting partners of BRD4, such as transcription factors, expanding on its previously unappreciated role in diverse biological processes. By setting up an azide-acetyllysine photoreaction deep inside the bromodomain aromatic cage as a means to detect protein acetylation, our approach provides a potentially general platform for rapid and unbiased profiling of interacting partners of diverse epigenetic readers whose functions in eukaryotic gene regulation remain convoluted.

Chemical probes for methyl lysine reader domains

The primary intent of a chemical probe is to establish the relationship between a molecular target, usually a protein whose function is modulated by the probe, and the biological consequences of that modulation. In order to fulfill this purpose, a chemical probe must be profiled for selectivity, mechanism of action, and cellular activity, as the cell is the minimal system in which 'biology' can be explored. This review provides a brief overview of progress towards chemical probes for methyl lysine reader domains with a focus on recent progress targeting chromodomains.

Identification of a small-molecule ligand of the epigenetic reader protein Spindlin1 via a versatile screening platform

Epigenetic modifications of histone tails play an essential role in the regulation of eukaryotic transcription. Writer and eraser enzymes establish and maintain the epigenetic code by creating or removing posttranslational marks. Specific binding proteins, called readers, recognize the modifications and mediate epigenetic signalling. Here, we present a versatile assay platform for the investigation of the interaction between methyl lysine readers and their ligands. This can be utilized for the screening of small-molecule inhibitors of such protein-protein interactions and the detailed characterization of the inhibition. Our platform is constructed in a modular way consisting of orthogonal in vitro binding assays for ligand screening and verification of initial hits and biophysical, label-free techniques for further kinetic characterization of confirmed ligands. A stability assay for the investigation of target engagement in a cellular context complements the platform. We applied the complete evaluation chain to the Tudor domain containing protein Spindlin1 and established the in vitro test systems for the double Tudor domain of the histone demethylase JMJD2C. We finally conducted an exploratory screen for inhibitors of the interaction between Spindlin1 and H3K4me3 and identified A366 as the first nanomolar small-molecule ligand of a Tudor domain containing methyl lysine reader.