An E3 ligase network engages GCN1 to promote the degradation of translation factors on stalled ribosomes.

Ribosomes frequently stall during mRNA translation, resulting in the context-dependent activation of quality control pathways to maintain proteostasis. However, surveillance mechanisms that specifically respond to stalled ribosomes with an occluded A site have not been identified. We discovered that the elongation factor-1α (eEF1A) inhibitor, ternatin-4, triggers the ubiquitination and degradation of eEF1A on stalled ribosomes. Using a chemical genetic approach, we unveiled a signaling network comprising two E3 ligases, RNF14 and RNF25, which are required for eEF1A degradation. Quantitative proteomics revealed the RNF14 and RNF25-dependent ubiquitination of eEF1A and a discrete set of ribosomal proteins. The ribosome collision sensor GCN1 plays an essential role by engaging RNF14, which directly ubiquitinates eEF1A. The site-specific, RNF25-dependent ubiquitination of the ribosomal protein RPS27A/eS31 provides a second essential signaling input. Our findings illuminate a ubiquitin signaling network that monitors the ribosomal A site and promotes the degradation of stalled translation factors, including eEF1A and the termination factor eRF1.

IFITM proteins assist cellular uptake of diverse linked chemotypes.

The search for cell-permeable drugs has conventionally focused on low-molecular weight (MW), nonpolar, rigid chemical structures. However, emerging therapeutic strategies break traditional drug design rules by employing flexibly linked chemical entities composed of more than one ligand. Using complementary genome-scale chemical-genetic approaches we identified an endogenous chemical uptake pathway involving interferon-induced transmembrane proteins (IFITMs) that modulates the cell permeability of a prototypical biopic inhibitor of MTOR (RapaLink-1, MW: 1784 g/mol). We devised additional linked inhibitors targeting BCR-ABL1 (DasatiLink-1, MW: 1518 g/mol) and EIF4A1 (BisRoc-1, MW: 1466 g/mol), uptake of which was facilitated by IFITMs. We also found that IFITMs moderately assisted some proteolysis-targeting chimeras and examined the physicochemical requirements for involvement of this uptake pathway.

Reversible lysine-targeted probes reveal residence time-based kinase selectivity.

The expansion of the target landscape of covalent inhibitors requires the engagement of nucleophiles beyond cysteine. Although the conserved catalytic lysine in protein kinases is an attractive candidate for a covalent approach, selectivity remains an obvious challenge. Moreover, few covalent inhibitors have been shown to engage the kinase catalytic lysine in animals. We hypothesized that reversible, lysine-targeted inhibitors could provide sustained kinase engagement in vivo, with selectivity driven in part by differences in residence time. By strategically linking benzaldehydes to a promiscuous kinase binding scaffold, we developed chemoproteomic probes that reversibly and covalently engage >200 protein kinases in cells and mice. Probe-kinase residence time was dramatically enhanced by a hydroxyl group ortho to the aldehyde. Remarkably, only a few kinases, including Aurora A, showed sustained, quasi-irreversible occupancy in vivo, the structural basis for which was revealed by X-ray crystallography. We anticipate broad application of salicylaldehyde-based probes to proteins that lack a druggable cysteine.

Discovery of Lysine-Targeted eIF4E Inhibitors through Covalent Docking.

Eukaryotic translation initiation factor 4E (eIF4E) binds the m7GTP cap structure at the 5'-end of mRNAs, stimulating the translation of proteins implicated in cancer cell growth and metastasis. eIF4E is a notoriously challenging target, and most of the reported inhibitors are negatively charged guanine analogues with negligible cell permeability. To overcome these challenges, we envisioned a covalent targeting strategy. As there are no cysteines near the eIF4E cap binding site, we developed a covalent docking approach focused on lysine. Taking advantage of a "make-on-demand" virtual library, we used covalent docking to identify arylsulfonyl fluorides that target a noncatalytic lysine (Lys162) in eIF4E. Guided by cocrystal structures, we elaborated arylsulfonyl fluoride 2 to 12, which to our knowledge is the first covalent eIF4E inhibitor with cellular activity. In addition to providing a new tool for acutely inactivating eIF4E in cells, our computational approach may offer a general strategy for developing selective lysine-targeted covalent ligands.

A bifunctional amino acid to study protein-protein interactions.

Protein-protein interactions (PPIs) play crucial roles in regulating essentially all cellular processes. Photo-cross-linking represents a powerful method to study PPIs. To fulfil the requirements for the exploration of different PPIs, there is a continuous demand on the development of novel photo-reactive amino acids with diverse structural properties and functionalities. Reported herein is the development of a bifunctional amino acid termed dzANA, which contains a diazirine, for photo-cross-linking, and a terminal alkyne group, for bioorthogonal tagging. Using known PPIs between histone posttranslational modifications (PTMs) and their binding partners as models, we demonstrate that the dzANA-harbouring peptide-based photoaffinity probes could efficiently and selectively capture the weak and transient PPIs mediated by histone modifications. Our study indicates the potential of dzANA to identify and characterize unknown PPIs.

Photo-lysine captures proteins that bind lysine post-translational modifications.

Post-translational modifications (PTMs) have key roles in regulating protein-protein interactions in living cells. However, it remains a challenge to identify these PTM-mediated interactions. Here we develop a new lysine-based photo-reactive amino acid, termed photo-lysine. We demonstrate that photo-lysine, which is readily incorporated into proteins by native mammalian translation machinery, can be used to capture and identify proteins that recognize lysine PTMs, including 'readers' and 'erasers' of histone modifications.

Developing diazirine-based chemical probes to identify histone modification 'readers' and 'erasers'.

Post translational modifications (PTMs, e.g., phosphorylation, acetylation and methylation) of histone play important roles in regulating many fundamental cellular processes such as gene transcription, DNA replication and damage repair. While 'writer' and 'eraser' enzymes modify histones by catalyzing the addition and removal of histone PTMs, 'reader' proteins recognize these modified histones and 'translate' the PTMs by executing distinct cellular programs. Therefore, identification of the regulating enzymes and binding partners of histone PTMs is essential for understanding their regulatory mechanisms and cellular functions. Here we report the development of diazirine-based photoaffinity probes for identification of 'readers' and 'erasers' of histone PTMs. When compared with previously described benzophenone-based photoaffinity probes, the present probes demonstrate significantly improved photo-cross-linking rates, yields and specificities for capturing proteins that recognize a trimethylation mark on histone H3 lysine 4 (H3K4Me3). Furthermore, we show that the diazirine-based probes can also be used to identify enzymes that catalyse the removal of histone lysine acetylation and malonylation. This study provides new chemical tools for examining PTM-mediated protein-protein interactions and broadens the scope of our photo-cross-linking strategy from finding histone PTM 'readers' to identifying dynamic and transient interactions between PTMs and their 'erasers'.

Integrative chemical biology approaches for identification and characterization of "erasers" for fatty-acid-acylated lysine residues within proteins.

Acylation of proteins with fatty acids is important for the regulation of membrane association, trafficking, subcellular localization, and activity of many cellular proteins. While significant progress has been made in our understanding of the two major forms of protein acylation with fatty acids, N-myristoylation and S-palmitoylation, studies of the acylation of lysine residues, within proteins, with fatty acids have lagged behind. Demonstrated here is the use of integrative chemical biology approaches to examine human sirtuins as de-fatty-acid acylases in vitro and in cells. Photo-crosslinking chemistry is used to investigate enzymes which recognize fatty-acid acylated lysine. Human Sirt2 was identified as a robust lysine de-fatty-acid acylase in vitro. The results also show that Sirt2 can regulate the acylation of lysine residues, of proteins, with fatty acids within cells.

Identification of 'erasers' for lysine crotonylated histone marks using a chemical proteomics approach.

Posttranslational modifications (PTMs) play a crucial role in a wide range of biological processes. Lysine crotonylation (Kcr) is a newly discovered histone PTM that is enriched at active gene promoters and potential enhancers in mammalian cell genomes. However, the cellular enzymes that regulate the addition and removal of Kcr are unknown, which has hindered further investigation of its cellular functions. Here we used a chemical proteomics approach to comprehensively profile 'eraser' enzymes that recognize a lysine-4 crotonylated histone H3 (H3K4Cr) mark. We found that Sirt1, Sirt2, and Sirt3 can catalyze the hydrolysis of lysine crotonylated histone peptides and proteins. More importantly, Sirt3 functions as a decrotonylase to regulate histone Kcr dynamics and gene transcription in living cells. This discovery not only opens opportunities for examining the physiological significance of histone Kcr, but also helps to unravel the unknown cellular mechanisms controlled by Sirt3, that have previously been considered solely as a deacetylase.

Boron-trihalide-promoted regioselective ring-opening reactions of gem-difluorocyclopropyl ketones.

Boron trihalide-promoted ring-opening reactions of gem-difluorocyclopropyl ketones to give the corresponding ß-trifluoromethyl ketones and ß-halodifluoromethyl ketones were described. It was found that boron trihalides act as both Lewis acids and nucleophiles and the proximal bond prefers to cleave in this transformation.

A novel reaction of gem-difluorocyclopropyl ketones with nitriles leading to 2-fluoropyrroles.

The CF3SO3H-promoted ring opening of gem-difluorocyclopropyl ketones prefers to undergo proximal bond cleavage and the subsequent cyclization with nitriles occurred smoothly to give 2-fluoropyrroles.