Identification of Two Non-Peptidergic Small Molecule Inhibitors of CBX2 Binding to K27 Trimethylated Oligonucleosomes.

The dysregulation of the PRC1/2 complex plays a key role in lineage plasticity in prostate cancer and may be required to maintain neuroendocrine phenotype. [1] CBX2, a key component of the canonical PRC1 complex, is an epigenetic reader, recognizing trimethylated lysine on histone 3 (H3K27me3) [2] and is overexpressed in metastatic neuroendocrine prostate cancer. [3,4] We implemented a screening strategy using nucleosome substrates to identify inhibitors of CBX2 binding to chromatin. Construct design and phosphorylation state of CBX2 were critical for successful implementation and execution of an HTS library screen. A rigorous screening funnel including counter and selectivity assays allowed us to quickly focus on true positive hit matter. Two distinct non-peptide-like chemotypes were identified and confirmed in orthogonal biochemical and biophysical assays demonstrating disruption of CBX2 binding to nucleosomes and direct binding to purified CBX2, respectively.

Post-translational insertion of boron in proteins to probe and modulate function.

Boron is absent in proteins, yet is a micronutrient. It possesses unique bonding that could expand biological function including modes of Lewis acidity not available to typical elements of life. Here we show that post-translational Cß-Bγ bond formation provides mild, direct, site-selective access to the minimally sized residue boronoalanine (Bal) in proteins. Precise anchoring of boron within complex biomolecular systems allows dative bond-mediated, site-dependent protein Lewis acid-base-pairing (LABP) by Bal. Dynamic protein-LABP creates tunable inter- and intramolecular ligand-host interactions, while reactive protein-LABP reveals reactively accessible sites through migratory boron-to-oxygen Cß-Oγ covalent bond formation. These modes of dative bonding can also generate de novo function, such as control of thermo- and proteolytic stability in a target protein, or observation of transient structural features via chemical exchange. These results indicate that controlled insertion of boron facilitates stability modulation, structure determination, de novo binding activities and redox-responsive 'mutation'.

Novel irreversible covalent BTK inhibitors discovered using DNA-encoded chemistry.

Libraries of DNA-Encoded small molecules created using combinatorial chemistry and synthetic oligonucleotides are being applied to drug discovery projects across the pharmaceutical industry. The majority of reported projects describe the discovery of reversible, i.e. non-covalent, target modulators. We synthesized multiple DNA-encoded chemical libraries terminated in electrophiles and then used them to discover covalent irreversible inhibitors and report the successful discovery of acrylamide- and epoxide-terminated Bruton's Tyrosine Kinase (BTK) inhibitors. We also demonstrate their selectivity, potency and covalent cysteine engagement using a range of techniques including X-ray crystallography, thermal transition shift assay, reporter displacement assay and intact protein complex mass spectrometry. The epoxide BTK inhibitors described here are the first ever reported to utilize this electrophile for this target.

Histone chaperone exploits intrinsic disorder to switch acetylation specificity.

Histones, the principal protein components of chromatin, contain long disordered sequences, which are extensively post-translationally modified. Although histone chaperones are known to control both the activity and specificity of histone-modifying enzymes, the mechanisms promoting modification of highly disordered substrates, such as lysine-acetylation within the N-terminal tail of histone H3, are not understood. Here, to understand how histone chaperones Asf1 and Vps75 together promote H3 K9-acetylation, we establish the solution structural model of the acetyltransferase Rtt109 in complex with Asf1 and Vps75 and the histone dimer H3:H4. We show that Vps75 promotes K9-acetylation by engaging the H3 N-terminal tail in fuzzy electrostatic interactions with its disordered C-terminal domain, thereby confining the H3 tail to a wide central cavity faced by the Rtt109 active site. These fuzzy interactions between disordered domains achieve localization of lysine residues in the H3 tail to the catalytic site with minimal loss of entropy, and may represent a common mechanism of enzymatic reactions involving highly disordered substrates.

Genetic Incorporation of Olefin Cross-Metathesis Reaction Tags for Protein Modification.

Olefin cross-metathesis (CM) is a viable reaction for the modification of alkene-containing proteins. Although allyl sulfide or selenide side-chain motifs in proteins can critically enhance the rate of CM reactions, no efficient method for their site-selective genetic incorporation into proteins has been reported to date. Here, through the systematic evaluation of olefin-bearing unnatural amino acids for their metabolic incorporation, we have discovered S-allylhomocysteine (Ahc) as a genetically encodable Met analogue that is not only processed by translational cellular machinery but also a privileged CM substrate residue in proteins. In this way, Ahc was used for efficient Met codon reassignment in a Met-auxotrophic strain of E. coli (B834 (DE3)) as well as metabolic labeling of protein in human cells and was reactive toward CM in several representative proteins. This expands the use of CM in the toolkit for "tag-and-modify" functionalization of proteins.

Structural characterization of the Asf1-Rtt109 interaction and its role in histone acetylation.

Acetylation of histone H3 at lysine-56 by the histone acetyltransferase Rtt109 in lower eukaryotes is important for maintaining genomic integrity and is required for C. albicans pathogenicity. Rtt109 is activated by association with two different histone chaperones, Vps75 and Asf1, through an unknown mechanism. Here, we reveal that the Rtt109 C-terminus interacts directly with Asf1 and elucidate the structural basis of this interaction. In addition, we find that the H3 N-terminus can interact via the same interface on Asf1, leading to a competition between the two interaction partners. This, together with the recruitment and position of the substrate, provides an explanation of the role of the Rtt109 C-terminus in Asf1-dependent Rtt109 activation.

Posttranslational mutagenesis: A chemical strategy for exploring protein side-chain diversity.

Posttranslational modification of proteins expands their structural and functional capabilities beyond those directly specified by the genetic code. However, the vast diversity of chemically plausible (including unnatural but functionally relevant) side chains is not readily accessible. We describe C (sp3)-C (sp3) bond-forming reactions on proteins under biocompatible conditions, which exploit unusual carbon free-radical chemistry, and use them to form Cß-Cγ bonds with altered side chains. We demonstrate how these transformations enable a wide diversity of natural, unnatural, posttranslationally modified (methylated, glycosylated, phosphorylated, hydroxylated), and labeled (fluorinated, isotopically labeled) side chains to be added to a common, readily accessible dehydroalanine precursor in a range of representative protein types and scaffolds. This approach, outside of the rigid constraints of the ribosome and enzymatic processing, may be modified more generally for access to diverse proteins.

Synthetic Nucleosomes Reveal that GlcNAcylation Modulates Direct Interaction with the FACT Complex.

Transcriptional regulation can be established by various post-translational modifications (PTMs) on histone proteins in the nucleosome and by nucleobase modifications on chromosomal DNA. Functional consequences of histone O-GlcNAcylation (O-GlcNAc=O-linked ß-N-acetylglucosamine) are largely unexplored. Herein, we generate homogeneously GlcNAcylated histones and nucleosomes by chemical post-translational modification. Mass-spectrometry-based quantitative interaction proteomics reveals a direct interaction between GlcNAcylated nucleosomes and the "facilitates chromatin transcription" (FACT) complex. Preferential binding of FACT to GlcNAcylated nucleosomes may point towards O-GlcNAcylation as one of the triggers for FACT-driven transcriptional control.

Optimization of protein samples for NMR using thermal shift assays.

Maintaining a stable fold for recombinant proteins is challenging, especially when working with highly purified and concentrated samples at temperatures >20 °C. Therefore, it is worthwhile to screen for different buffer components that can stabilize protein samples. Thermal shift assays or ThermoFluor(®) provide a high-throughput screening method to assess the thermal stability of a sample under several conditions simultaneously. Here, we describe a thermal shift assay that is designed to optimize conditions for nuclear magnetic resonance studies, which typically require stable samples at high concentration and ambient (or higher) temperature. We demonstrate that for two challenging proteins, the multicomponent screen helped to identify ingredients that increased protein stability, leading to clear improvements in the quality of the spectra. Thermal shift assays provide an economic and time-efficient method to find optimal conditions for NMR structural studies.

The histone chaperone sNASP binds a conserved peptide motif within the globular core of histone H3 through its TPR repeats.

Eukaryotic chromatin is a complex yet dynamic structure, which is regulated in part by the assembly and disassembly of nucleosomes. Key to this process is a group of proteins termed histone chaperones that guide the thermodynamic assembly of nucleosomes by interacting with soluble histones. Here we investigate the interaction between the histone chaperone sNASP and its histone H3 substrate. We find that sNASP binds with nanomolar affinity to a conserved heptapeptide motif in the globular domain of H3, close to the C-terminus. Through functional analysis of sNASP homologues we identified point mutations in surface residues within the TPR domain of sNASP that disrupt H3 peptide interaction, but do not completely disrupt binding to full length H3 in cells, suggesting that sNASP interacts with H3 through additional contacts. Furthermore, chemical shift perturbations from(1)H-(15)N HSQC experiments show that H3 peptide binding maps to the helical groove formed by the stacked TPR motifs of sNASP. Our findings reveal a new mode of interaction between a TPR repeat domain and an evolutionarily conserved peptide motif found in canonical H3 and in all histone H3 variants, including CenpA and have implications for the mechanism of histone chaperoning within the cell.

Generation of a synthetic GlcNAcylated nucleosome reveals regulation of stability by H2A-Thr101 GlcNAcylation.

O-GlcNAcylation is a newly discovered histone modification implicated in transcriptional regulation, but no structural information on the physical effect of GlcNAcylation on chromatin exists. Here, we generate synthetic, pure GlcNAcylated histones and nucleosomes and reveal that GlcNAcylation can modulate structure through direct destabilization of H2A/H2B dimers in the nucleosome, thus promoting an 'open' chromatin state. The results suggest that a plausible molecular basis for one role of histone O-GlcNAcylation in epigenetic regulation is to lower the barrier for RNA polymerase passage and hence increase transcription.

Designing logical codon reassignment - Expanding the chemistry in biology.

Over the last decade, the ability to genetically encode unnatural amino acids (UAAs) has evolved rapidly. The programmed incorporation of UAAs into recombinant proteins relies on the reassignment or suppression of canonical codons with an amino-acyl tRNA synthetase/tRNA (aaRS/tRNA) pair, selective for the UAA of choice. In order to achieve selective incorporation, the aaRS should be selective for the designed tRNA and UAA over the endogenous amino acids and tRNAs. Enhanced selectivity has been achieved by transferring an aaRS/tRNA pair from another kingdom to the organism of interest, and subsequent aaRS evolution to acquire enhanced selectivity for the desired UAA. Today, over 150 non-canonical amino acids have been incorporated using such methods. This enables the introduction of a large variety of structures into proteins, in organisms ranging from prokaryote, yeast and mammalian cells lines to whole animals, enabling the study of protein function at a level that could not previously be achieved. While most research to date has focused on the suppression of 'non-sense' codons, recent developments are beginning to open up the possibility of quadruplet codon decoding and the more selective reassignment of sense codons, offering a potentially powerful tool for incorporating multiple amino acids. Here, we aim to provide a focused review of methods for UAA incorporation with an emphasis in particular on the different tRNA synthetase/tRNA pairs exploited or developed, focusing upon the different UAA structures that have been incorporated and the logic behind the design and future creation of such systems. Our hope is that this will help rationalize the design of systems for incorporation of unexplored unnatural amino acids, as well as novel applications for those already known.

Structural insights into how 5-hydroxymethylation influences transcription factor binding.

Transcription factor binding and high resolution crystallographic studies (1.3 Å) of Dickerson-Drew duplexes with cytosine, methylcytosine and hydroxymethylcytosine bases provide evidence that C-5 cytosine modifications could regulate transcription by context dependent effects on DNA transcription factor interactions.

DNA modification under mild conditions by Suzuki-Miyaura cross-coupling for the generation of functional probes.

Quick and clean: A method for Pd-catalyzed Suzuki-Miyaura cross-coupling to iododeoxyuridine (IdU) in DNA is described. Key to the reactivity is the choice of the ligand and the buffer. A covalent [Pd]-DNA intermediate was isolated and characterized. Photocrosslinking probes were generated to trap proteins that bind to epigenetic DNA modifications.

Rapid cross-metathesis for reversible protein modifications via chemical access to Se-allyl-selenocysteine in proteins.

Cross-metathesis (CM) has recently emerged as a viable strategy for protein modification. Here, efficient protein CM has been demonstrated through biomimetic chemical access to Se-allyl-selenocysteine (Seac), a metathesis-reactive amino acid substrate, via dehydroalanine. On-protein reaction kinetics reveal a rapid reaction with rate constants of Seac-mediated-CM comparable or superior to off-protein rates of many current bioconjugations. This use of Se-relayed Seac CM on proteins has now enabled reactions with substrates (allyl GlcNAc, N-allyl acetamide) that were previously not possible for the corresponding sulfur analogue. This CM strategy was applied to histone proteins to install a mimic of acetylated lysine (KAc, an epigenetic marker). The resulting synthetic H3 was successfully recognized by antibody that binds natural H3-K9Ac. Moreover, Cope-type selenoxide elimination allowed this putative marker (and function) to be chemically expunged, regenerating an H3 that can be rewritten to complete a chemically enabled "write (CM)-erase (ox)-rewrite (CM)" cycle.

Stereodefined trisubstituted enolates as a unique entry to all-carbon quaternary stereogenic centers in acyclic systems.

This protocol describes a new approach for the preparation of stereodefined trisubstituted chiral enolate species, avoiding conventional asymmetric enolization of carbonyl compounds. This protocol was developed as a single-flask synthetic sequence and therefore does not require isolation or purification of intermediate compounds. The sequence starts from a regioselective carbocupration reaction of readily accessible chiral ynamides; this is followed by oxidation of the generated vinylcuprate with a commonly available oxidizing reagent (tert-butyl hydroperoxide) in order to generate an enolate that completely retains its configuration. This synthetic protocol has been applied to the preparation of aldol and Mannich-type adducts. The procedure reported here requires a simple reaction setup commonly available in all synthetic laboratories and takes ∼6 h for completion and 2 h for isolation and purification. Final products are valuable diastereomerically and enantiomerically enriched building blocks for organic synthesis containing all-carbon quaternary stereocenters in acyclic systems.

Forming all-carbon quaternary stereogenic centres in acyclic systems from alkynes.

The formation of all-carbon quaternary stereocentres in acyclic systems is one of the most difficult contemporary challenges in modern synthetic organic chemistry. Particularly challenging is the preparation of all-carbon quaternary stereocentres in aldol adducts; this difficulty is problematic because the aldol reaction represents one of the most valuable chemical transformations in organic synthesis. The main problem that limits the formation of these stereocentres is the absence of an efficient method of preparing stereodefined trisubstituted enolates in acyclic systems. Here we describe a different approach that involves the formation of two new stereogenic centres--including the all-carbon quaternary one--via a combined carbometalation-oxidation reaction of an organocuprate to give a stereodefined trisubstituted enolate. We use this method to generate a series of aldol and Mannich products from ynamides with excellent diastereomeric and enantiomeric ratios and moderate yields.

Chemical discrimination between dC and 5MedC via their hydroxylamine adducts.

The presence of the methylated nucleobase (5Me)dC in CpG islands is a key factor that determines gene silencing. False methylation patterns are responsible for deteriorated cellular development and are a hallmark of many cancers. Today genes can be sequenced for the content of (5Me)dC only with the help of the bisulfite reagent, which is based exclusively on chemical reactivity differences established by the additional methyl group. Despite intensive optimization of the bisulfite protocol, the method still has specificity problems. Most importantly ∼95% of the DNA analyte is degraded during the analysis procedure. We discovered that the reagent O-allylhydroxylamine is able to discriminate between dC and (5Me)dC. The reagent, in contrast to bisulfite, does not exploit reactivity differences but gives directly different reaction products. The reagent forms a stable mutagenic adduct with dC, which can exist in two states (E versus Z). In case of dC the allylhydroxylamine adduct switches into the E-isomeric form, which generates dC to dT transition mutations that can easily be detected by established methods. Significantly, the (5Me)dC-adduct adopts exclusively the Z-isomeric form, which causes the polymerase to stop. O-allylhydroxylamine does allow differentiation between dC and (5Me)dC with high accuracy, leading towards a novel and mild chemistry for methylation analysis.