RSC Chemical Biology

Targeted protein degradation (TPD) is a rapidly advancing therapeutic strategy that selectively eliminates disease-associated proteins by co-opting the cells protein degradation machinery. Covalent modification of proteins with ubiquitin is a critical event in TPD, yet the analytical tools for quantifying the ubiquitination kinetics have been limited. Here, we present a real-time, high-throughput fluorescent assay utilizing purified, FRET-active E2-Ub conjugates to monitor ubiquitin transfer. This assay is highly versatile, requiring no engineering of the target protein or ligase, thereby accelerating assay development and minimizing the risk of artifacts. The single-step, single-turnover nature of the monitored reaction enables rigorous and quantitative analysis of ubiquitination kinetics. We show that this assay can be used to measure key degrader characteristics such as degrader affinity for the target protein, degrader affinity for the ligase, affinity of ternary complex assembly, and catalytic efficiency of the ternary complex. The high sensitivity and accuracy of this comprehensive, single-assay approach to ternary complex characterization will empower the discovery and optimization of heterobifunctional degraders and molecular glues.

Proteolysis-targeting chimeras (PROTACs) co-op the ubiquitin system for targeted protein degradation, creating opportunities to interrogate cellular functions of proteins through "chemical knockdown". However, matched pairs of protein degraders and inhibitors, that possess high specificity and chemical complementarity, for individual components of the ubiquitin system have remained scarce. This includes reagents to modulate activity and abundance of deubiquitinases (DUBs), which critically regulate ubiquitin-mediated signaling. Here, using an integrated chemical biology approach, we explored the cellular function of the DUB USP7 as a case study comparing inhibition and degradation of this DUB in melanoma and pancreatic cancer cells. Through the synthesis of a degrader library, we identified potent USP7 PROTACs for each cancer type, established BRET-based ternary complex formation and quantified degradation efficiency. USP7 degraders and their cognate inhibitor were subsequently employed to characterize treatment-induced phenotypic alterations. Proteomic and cellular analyses revealed that highly specific degradation of USP7 modulated both shared and distinct protein sets across cancer cell types, without impacting cell growth. Notably, cellular responses to USP7 degradation differed markedly from those to USP7 inhibition. Moreover, our data uncovered broad proteomic and metabolic changes induced by prolonged USP7 inhibitor treatment. Collectively, our work provides a chemical toolbox of comprehensively characterized reagents to distinguish on-target phenotypes which will aid the understanding of the role of USP7 in malignant diseases. More broadly, our data emphasize the importance of increased specificity via PROTAC-mediated degradation and the potential of this modality to distinguish catalytic from non-catalytic as well as cell-line specific functions of DUBs.

The development of selective inhibitors of Deubiquitylase enzymes (DUBs) is difficult due to a high level of homology in the active sites of the {approx} 100 such enzymes in the human proteome. A potential way to achieve this in a more facile manner would be to develop PROTACs or molecular glues that engage the target DUB in a less conserved region outside of the catalytic domain. However, this raises the concern that auto-deubiquitylation would make DUBs poor substrates for this modality. Here we describe a chemical genetics system to evaluate this issue. We find that some DUBs are readily degradable via the Ubiquitin-proteasome pathway and some are not. Of the latter category, some resist turnover through auto-deubiquitylation and some are simply poor proteasome substrates.

Sm Endonuclease (SmEn) is a promiscuous, highly active nuclease widely used in protein purification, 2D protein gels, and gene and cell therapy. We aimed to recombinantly and economically produce this reagent using E. coli. Despite widespread application of E. coli for recombinant production of proteins, cytoplasmic expression of this protein resulted in no activity accumulation. We therefore investigated translocation of SmEn to the periplasm of E. coli by evaluating several signal sequences, E. coli host cells, and incubation conditions. For rapid feedback, we developed a crude lysate-based nuclease activity assay that enabled convenient screening and identified suitable conditions for active SmEn accumulation. Signal sequence selection was most influential with additional benefit gained by slowing synthesis either using the transcriptionally weakened strain, C43 (DE3) or by reducing incubation temperature. While our study provides valuable insights for optimizing a nuclease translocation and reducing production costs, more research is needed to explore the influence of mRNA secondary structure at the translation initiation region on protein expression and translocation. Overall, our rapid screening assay facilitated the development of an effective production process for a protein with potential cytoplasmic toxicity as well as the need of disulfide bond formation.

ADP-ribosylation is a chemically versatile modification of proteins, RNA and DNA that regulates various important signaling pathways, many of which are implicated in human diseases. Despite being discovered 60 years ago in the form of poly-ADP-ribosylation - the most studied form of this modification - investigating ADP-ribosylation at the molecular level has historically been challenging. By applying serine ADP-ribosylation-based antibody engineering technology, we have developed the first site-specific, as well as sensitive, mono-ADP-ribosylation modular antibodies. Here, we extend the scope of this technology to poly-ADP-ribosylation. By combining serine poly-ADP-ribosylated peptides as antigens, PARP1 serine mono- and poly-ADP-ribosylation for validation, phage display and the SpyTag protein ligation systems, we developed modular antibodies that are highly specific for poly-ADP-ribosylation. SpyTag-based coupling of horseradish peroxidase at specific positions, distant from the antigen-binding region of the antibody, yields a format that simplifies immunoblotting while dramatically enhancing poly-ADP-ribosylation detection sensitivity. Additionally, the creation of synthetic immunoglobulin formats - mouse, rabbit and human - enables straightforward co-detection of mono- and poly-ADP-ribosylation in cells by immunofluorescence. Our new tools improve and simplify the detection of poly-ADP-ribosylation, particularly in immunoblotting, enabling specific investigations of this key cellular signal in the context of other distinct forms of ADP-ribosylation.

Targeted protein degradation (TPD) with Proteolysis Targeting Chimeras (PROTACs), heterobifunctional compounds consisting of protein targeting ligands linked to recruiters of E3 ubiquitin ligases, has arisen as a powerful therapeutic modality to induce the proximity of target proteins with E3 ligases to ubiquitinate and degrade specific proteins in cells. Thus far, PROTACs have primarily exploited the recruitment of E3 ubiquitin ligases or their substrate adapter proteins but have not exploited the recruitment of more core components of the ubiquitin-proteasome system (UPS). In this study, we used covalent chemoproteomic approaches to discover a covalent recruiter against the E2 ubiquitin conjugating enzyme UBE2D--EN67--that targets an allosteric cysteine, C111, without affecting the enzymatic activity of the protein. We demonstrated that this UBE2D recruiter could be used in heterobifunctional degraders to degrade neo-substrate targets in a UBE2D-dependent manner, including BRD4 and the androgen receptor. Overall, our data highlight the potential for the recruitment of core components of the UPS machinery, such as E2 ubiquitin conjugating enzymes, for TPD, and underscore the utility of covalent chemoproteomic strategies for identifying novel recruiters for additional components of the UPS.

Peptide-based biopesticides represent a promising strategy for sustainable disease control in agriculture. Synthetic antifungal peptides incorporating the {gamma}-core motif of plant defensins offer multiple modes of action (MoA) and potential as biofungicides. We investigated a synthetic variant of the olive defensin OefDef1.1 for antifungal activity, structure-function relationships, and MoA against Botrytis cinerea, the necrotrophic pathogen causing gray mold. A disulfide-bridged peptide, GMAOe1C_V1*, derived from OefDef1.1 (G32-Y53) and modified with hydrophobic amino acid substitutions, inhibited B. cinerea growth in vitro and reduced lesion formation in detached leaves. Foliar application of GMAOe1C_V1* suppressed disease symptoms in pepper plants. Mechanistically, GMAOe1C_V1* rapidly permeabilized fungal plasma membranes and accumulated in vacuoles, triggering vacuolar expansion and cell death. It also inhibited protein synthesis in vitro and in vivo, suggesting a role as a translation inhibitor. Alanine scanning mutagenesis of the non-disulfide bridged variant identified the 7RHSKH11 motif as essential for antifungal activity. Circular dichroism revealed an unstructured conformation with minimal secondary structure. Transcriptomic analysis of GMAOe1C_V1* treated B. cinerea germlings showed downregulation of genes involved in mitochondrial function and amino acid biosynthesis. These findings demonstrate the potential of an olive defensin-derived peptide as a bio-inspired antifungal agent with multifaceted MoA, supporting its development for crop protection.

A mechanistic basis for luciferase bioluminescence provides a glimpse into its evolutionary role for organism survival, as it provides a blueprint to engineer luciferase enzymes for advanced technological applications. Gaussia Luciferase is among the brightest natural luciferases, but (1) the evolutionary development of its luminescence behavior remains unclear, (2) recent fundamental studies utilized E. Coli expression systems instead of eukaryotic expression systems, and (3) notable mutants have been discovered but not integrated into comprehensive mechanistic analysis. We describe new mechanistic observations from GLuc by addressing these gaps. We monitored fluorescent coelenerazine-to-coelenteramide conversion to study turnover kinetics of mammalian-derived GLuc; this assay characterized the positive cooperativity kinetics of GLuc. The non-luminescent mutants, R76A and R147A, still turn over the substrate with high efficiency, each demonstrating sustained positive cooperativity. Through mass spectrometry, mutational analysis, and analytical liquid chromatography, we demonstrate that GLuc undergoes methionine oxidation during substrate turnover, and that this impacts the flash-type luminescence of the luciferase; we did not observe indications of covalent attachment with the substrate, product, or their intermediates. Chromatography on luciferases derived from ancestral sequence reconstruction highlighted that the extent of methionine-induced surface changes was greater for earlier ancestral luciferases. Ancestral sequence reconstruction also revealed that earlier ancestral copepod luciferases produced less light when compared to GLuc.

Diatoms are photosynthetic microalgae that fix a significant fraction of the worlds carbon. Because of their photosynthetic efficiency and high-lipid content, diatoms are priority candidates for biofuel production. Here, we report that sporulating Bacillus thuringiensis when in co-culture with a marine diatom Phaeodactylum tricornutum significantly increases the diatom cell count. Bioassay-guided purification led to the identification of two diketopiperazines (DKPs) that both stimulate P. tricornutum growth and increase its lipid content. RNA-seq analysis revealed upregulation of a small set of P. tricornutum genes involved in iron starvation response and nutrient recycling when DKP was added to the diatom culture. This work demonstrates that two DKPs produced by a bacterium could positively impact P. tricornutum growth and lipid content, offering new approaches to enhance P. tricornutum-based biofuel production. As increasing numbers of DKPs are isolated from marine microbes, the work gives potential clues to bacterially produced growth factors for marine microalgae. One sentence summaryTwo diketopiperazines (DKPs) produced by sporulating bacterium Bacillus thuringiensis stimulate diatom P. tricornutum growth and increase diatom lipid content.

Lanthipeptides are ribosomally synthesized and post-translationally modified peptides characterized by the presence of thioether crosslinks. Class II lanthipeptide synthetases are bifunctional enzymes responsible for the multistep chemical modification of these natural products. ProcM is a class II lanthipeptide synthetase known for its remarkable substrate tolerance and ability to install diverse (methyl)lanthionine rings with high accuracy. Previous studies suggested that the final ring pattern of the lanthipeptide product may be determined by the substrate sequence rather than by ProcM, and that ProcM operates by a kinetically controlled mechanism, wherein the ring pattern is dictated by the relative rates of the individual cyclization reactions. This study utilizes kinetic assays to determine if rates of isolated modifications can predict the final ring pattern present in prochlorosins. Changes in the core substrate sequence resulted in changes to the reaction rates of ring formation as well as a change in the order of modifications. Additionally, individual chemical reaction rates were significantly impacted by the presence of other modifications on the peptide. These findings indicate that the rates of isolated modifications are capable of predicting the final ring pattern but are not necessarily a good predictor of the order of modification in WT ProcA3.3 and its variants.

Single-domain antibodies, known as nanobodies (Nbs), are widely used in structural biology, therapeutics, and as molecular probes in biology and biotechnology. Nbs towards soluble proteins are routinely developed via alpaca immunization or directed evolution in yeast cell-surface display. However, for membrane proteins, the targets are generally detergent-solubilized, and there remains a need for Nb development methods against membrane proteins in a native-like membrane environment. To address this need, we present a protocol for Nb selection via extraction of membrane proteins into amphiphilic polymers such as styrene-maleic acid to produce purified membrane proteins in stable liponanoparticles. Proof of generality is demonstrated by applying the pipeline to four membrane-resident enzymes of differing fold, oligomerization state, and membrane topology (reentrant membrane helix, transmembrane, membrane-associated). Following screening for optimal stabilization into liponanoparticles, Nbs were selected against four target proteins from glycoconjugate biosynthesis pathways. The selected Nbs showed high affinity and selectivity towards their target proteins with KD apparent values ranging from 15 nM to 200 nM, depending on the Nb-protein conjugate. In accordance with their tight binding, various Nb-protein complexes were found to be stable to size-exclusion chromatography purification. The Nbs were also amenable to sortase-mediated ligation, enabling their conversion into molecular probes for the target membrane protein. The ability to select for such high-affinity Nb against membrane proteins in SMALP will facilitate their widespread application in cell biology and biomedical applications.

Increasing evidence suggests the protein arginine methyltransferase PRMT5 as a contributor to tumorigenesis in various cancer types and several inhibitors have entered clinical trials. Robust assays to determine cellular target engagement and selectivity are an important asset for the optimisation of inhibitors and the design of relevant in vivo studies. Here we report a suite of chemical biology assays enabling quantitative assessment of PRMT5 inhibitor in-cell target engagement and global selectivity profiling using a representative set of inhibitors. With the help of a bespoke cellular probe, we assess inhibitor target occupancy in cells in relation to biochemical and functional cellular assays. Investigating the influence of SAM, the natural cofactor of PRMT5, our results support the hypothesis that SAM positively contributes to the engagement of substrate-competitive inhibitors via a PRMT5:SAM:inhibitor ternary complex. Extensive proteomic profiling studies by drug affinity chromatography and thermal profiling further indicate high specificity of the clinical PRMT5 inhibitor GSK3326595 (pemrametostat). Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=65 SRC="FIGDIR/small/477145v2_ufig1.gif" ALT="Figure 1"> View larger version (16K): org.highwire.dtl.DTLVardef@11e448org.highwire.dtl.DTLVardef@a27194org.highwire.dtl.DTLVardef@cec270org.highwire.dtl.DTLVardef@3f76aa_HPS_FORMAT_FIGEXP M_FIG C_FIG

The escalating incidence of antimicrobial resistance in Staphylococcus aureus, particularly the methicillin-resistant strain (MRSA), necessitates the development of novel therapeutic strategies. Cationic lipidated oligomers (CLOs) have emerged as promising membrane-active antimicrobial agents; however, their mechanisms of action remain insufficiently understood. In this study, untargeted metabolomics was employed to systematically profile the temporal metabolic perturbations induced by two structurally distinct CLOs, C12-o-DMEN-10 and C12-o-BEDA-10, in MRSA across four defined time points (0.25, 0.5, 1, and 3 hours). These CLOs previously demonstrated differential antibacterial activity, as evidenced by dose-dependent propidium iodide (PI) uptake and growth inhibition assays. Metabolomic analysis revealed pronounced and sustained disruptions in bacterial membrane lipid metabolism, including significant depletion of phosphatidylglycerols ([&ge;] -2.5 log2FC, p < 0.05), alongside elevated levels of phosphatidylethanolamines, lysophospholipids, and fatty acid-derived metabolites indicative of membrane destabilization and lipid remodelling. Although both CLOs affected overlapping metabolic pathways, they differed in the extent and temporal dynamics of their effects. These findings provide mechanistic insights into CLO-mediated antibacterial activity and highlight the value of metabolomics in elucidating both direct and downstream cellular responses, which extend beyond the scope of conventional membrane integrity assays, such as PI fluorescence.

Surface plasmon resonance biosensor technology (SPR) is ideally suited for fragment-based lead discovery. However, generally suitable experimental procedures or detailed protocols are lacking, especially for structurally or physico-chemically challenging targets or when tool compounds are lacking. Success depends on accounting for the features of both the target and the chemical library, purposely designing screening experiments for identification and validation of hits with desired specificity and mode-of-action, and availability of orthogonal methods capable of confirming fragment hits. By adopting a multiplexed strategy, the range of targets and libraries amenable to an SPR biosensor-based approach for identifying hits is considerably expanded. We here illustrate innovative strategies using five challenging targets and variants thereof. Two libraries of 90 and 1056 fragments were screened using two different flow-based SPR biosensor systems, allowing different experimental approaches. Practical considerations and procedures accounting for the characteristics of the proteins and libraries, and that increase robustness, sensitivity, throughput and versatility are highlighted.

We describe the synthesis of C-5 indole-tagged pyrimidine and C-8 indole-tagged purine nucleoside phosphoramidites and their incorporation into double-stranded DNA 15 base pairs in length. Of the 23 sequence modifications tested, two induced the DNA duplex to adopt a Z-like left-handed conformation under physiological salt conditions, bypassing the specific sequences typically required for a left-handed Z-DNA structure. The impact of these modifications varied with the linker type: flexible propyl linkers exhibited distinct effects compared to rigid propargyl linkers. Notably, modifications positioned directly on or near a restriction site emphasized the pivotal role of linker rigidity in controlling DNA conformation. Specifically, the conformational change induced by the flexible linker impacted nuclease and restriction endonuclease cleavage, reducing sequence specificity. In contrast, the rigid linker suppressed this effect. Furthermore, our findings indicate that nucleic acid duplexes modified with indole-linked nucleotides using a flexible propyl linker have a pronounced tendency to form BZ or Z-like regions in longer DNA sequences. A higher density of modifications may even induce a full Z-like conformation throughout the duplex. These modified nucleotides hold potential for the development of novel antisense therapeutics and introducing valuable tools for in vitro screening of small molecules targeting distorted B-DNA, BZ-DNA, and Z-DNA structures.

Targeted protein degradation (TPD) represents a potent chemical biology paradigm that leverages the cellular degradation machinery to pharmacologically eliminate specific proteins of interest. Although multiple E3 ligases have been discovered to facilitate TPD, there exists a compelling requirement to diversify the pool of E3 ligases available for such applications. This expansion will broaden the scope of potential protein targets, accommodating those with varying subcellular localizations and expression patterns. In this study, we describe a CRISPR-based transcriptional activation screen focused on human E3 ligases, with the goal of identifying E3 ligases that can facilitate heterobifunctional compound-mediated target degradation. This approach allows us to address the limitations associated with investigating candidate degrader molecules in specific cell lines that either lack or have low levels of the desired E3 ligases. Through this approach, we identified a candidate proteolysis-targeting chimera (PROTAC), 22-SLF, that induces the degradation of FKBP12 when the FBXO22 gene transcription is activated. 22-SLF induced the degradation of endogenous FKBP12 in a FBXO22-dependent manner across multiple cancer cell lines. Subsequent mechanistic investigations revealed that 22-SLF interacts with C227 and/or C228 in FBXO22 to achieve the target degradation. Finally, we demonstrated the versatility of FBXO22-based PROTACs by effectively degrading another endogenous protein BRD4. This study uncovers FBXO22 as an E3 ligase capable of supporting ligand-induced protein degradation through electrophilic PROTACs. The platform we have developed can readily be applied to elucidate protein degradation pathways by identifying E3 ligases that facilitate either small molecule-induced or endogenous protein degradation.

While protein ubiquitination is an extensively studied post-translational modification, many aspects of this process remain unclear. Ubiquitin conjugation involves the action of three different types of enzymes working in concert to install ubiquitin onto substrate proteins. Despite efforts, an in vitro mid/high-throughput screen to quickly determine which enzymes work together to build ubiquitin chains and directly analyze the type(s) of chains formed does not exist. In this study, we developed a new multiplexed mass spectrometry-based E1-E2-E3 assay that enables the analysis of whether E2/E3 pairs work together to form ubiquitin chains and concomitantly reports on the nature of the formed ubiquitin chain type(s). The assay employs synthetic modified neutron-encoded monoUb substrates with a distinct molecular weight, enabling the simultaneous analysis of these substrates. Overall, various E2-E3 pairs were screened for their ability to build Ub chains, which furnished a three-dimensional overview of linkage selectivity over time and enzyme concentration. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=110 SRC="FIGDIR/small/655449v1_ufig1.gif" ALT="Figure 1"> View larger version (22K): org.highwire.dtl.DTLVardef@130203aorg.highwire.dtl.DTLVardef@93c9adorg.highwire.dtl.DTLVardef@9d9bedorg.highwire.dtl.DTLVardef@167ea0c_HPS_FORMAT_FIGEXP M_FIG C_FIG

Antibody drug conjugates have become one of the most actively developed classes of drugs in recent years. Their great potential comes from combining the strengths of large and small molecule therapeutics: the exquisite specificity of antibodies and the highly potent nature of cytotoxic compounds. More recently, the approach of engineering antibody drug conjugate scaffolds to achieve highly controlled drug to antibody ratios has focused on substituting or inserting cysteines to facilitate site-specific conjugation. Herein, we characterise an antibody scaffold engineered with an inserted cysteine that formed an unexpected disulfide bridge. A combination of mass spectrometry and biophysical techniques have been used to understand how the additional disulfide bridge forms, interconverts and changes the stability and structural dynamics of the antibody. Insight is gained into the local and global destabilisation associated with the engineering and subsequent disulfide bonded variant that will inform future engineering strategies.

The direct-to-biology approach enables rapid, high-throughput evaluation of large compound libraries in biological assays, eliminating costly and time-consuming purification steps. This strategy has been widely used in the development of proteolysis-targeting chimeras (PROTACs) to facilitate rapid linker optimization and the identification of active degraders from thousands of crude candidate compounds. Similarly, some other direct-to-biology biophysical assays have been utilized for the optimization of small-molecule ligands. However, a direct-to-biology strategy for evaluating cellular target engagement has not yet been demonstrated. Here, we extend this approach to the cellular thermal shift assay (CETSA). By systematically comparing crude reaction mixtures of reported DCAF11 covalent ligands with their corresponding purified analogues, we demonstrate that unpurified compounds can be directly evaluated in CETSA to assess cellular target engagement.

Deubiquitinases (DUBs) are proteases that hydrolyze isopeptide bonds linking ubiquitin to protein substrates, which can lead to reduced substrate degradation through the ubiquitin proteasome system. Deregulation of DUB activity has been implicated in many disease states, including cancer, neurodegeneration and inflammation, making them potentially attractive targets for therapeutic intervention. The >100 known DUB enzymes have been classified primarily by their conserved active sites, but we are still building our understanding of their substrate profiles, localization and regulation of DUB activity in diverse contexts. Ubiquitin-derived covalent activity-based probes (ABPs) are the premier tool for DUB activity profiling, but their large recognition element impedes cellular permeability and presents an unmet need for small molecule ABPs which account for local DUB concentration, protein interactions, complexes, and organelle compartmentalization in intact cells or organisms. Here, through comprehensive warhead profiling we identify cyanopyrrolidine (CNPy) probe IMP-2373 (12), a small molecule pan-DUB ABP to monitor DUB activity in physiologically relevant live cell systems. Through chemical proteomics and targeted assays we demonstrate that IMP-2373 quantitatively engages more than 35 DUBs in live cells across a range of non-toxic concentrations, and in diverse cell lines and disease models, and we demonstrate its application to quantification of changes in intracellular DUB activity during MYC deregulation in a model of B cell lymphoma. IMP-2373 thus offers a complementary tool to ubiquitin ABPs to monitor dynamic DUB activity in the context of disease-relevant phenotypes. SYNOPSIS TOC Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=112 SRC="FIGDIR/small/509970v1_ufig1.gif" ALT="Figure 1"> View larger version (28K): org.highwire.dtl.DTLVardef@13285acorg.highwire.dtl.DTLVardef@1e602b1org.highwire.dtl.DTLVardef@1baeff6org.highwire.dtl.DTLVardef@1e00eb9_HPS_FORMAT_FIGEXP M_FIG C_FIG