Mapping the Degradable Kinome Provides a Resource for Expedited Degrader Development

Mechanisms and regulation of substrate degradation by the 26S proteasome

The 26S proteasome is involved in degrading and regulating the majority of proteins in eukaryotic cells, which requires a sophisticated balance of specificity and promiscuity. In this Review, we discuss the principles that underly substrate recognition and ATP-dependent degradation by the proteasome. We focus on recent insights into the mechanisms of conventional ubiquitin-dependent and ubiquitin-independent protein turnover, and discuss the plethora of modulators for proteasome function, including substrate-delivering cofactors, ubiquitin ligases and deubiquitinases that enable the targeting of a highly diverse substrate pool. Furthermore, we summarize recent progress in our understanding of substrate processing upstream of the 26S proteasome by the p97 protein unfoldase. The advances in our knowledge of proteasome structure, function and regulation also inform new strategies for specific inhibition or harnessing the degradation capabilities of the proteasome for the treatment of human diseases, for instance, by using proteolysis targeting chimera molecules or molecular glues.

Discovery of DCAF16 Binders for Targeted Protein Degradation

Conventional small-molecule drugs primarily operate by inhibiting protein function, but this approach is limited when proteins lack well-defined ligand-binding pockets. Targeted protein degradation (TPD) offers an alternative approach by harnessing cellular degradation pathways to eliminate specific proteins. Recent studies have expanded the potential of TPD by identifying additional E3 ligases, with DCAF16 emerging as a promising candidate for facilitating protein degradation through both proteolysis-targeting chimera (PROTAC) and molecular glue mechanisms. In this study, we revisited a previously reported compound and discovered that it covalently binds to DCAF16. We further optimized it into a FKBP12-targeting PROTAC, MC-25B. This PROTAC engages DCAF16 at cysteines C177-179, leading to the degradation of nuclear-localized FKBP12. We further demonstrated the versatility of this DCAF16 recruiter by degrading additional endogenous proteins. Compared to the first-generation DCAF16-based PROTAC, which was derived from a fragment electrophile, this DCAF16 recruiter-based PROTAC exhibits improved proteome-wide selectivity.

A Kinetic Scout Approach Accelerates Targeted Protein Degrader Development

Bifunctional molecules such as targeted protein degraders induce proximity to promote gain-of-function pharmacology. These powerful approaches have gained broad traction across academia and the pharmaceutical industry, leading to an intensive focus on strategies that can accelerate their identification and optimization. We and others have previously used chemical proteomics to map degradable target space, and these datasets have been used to develop and train multiparameter models to extend degradability predictions across the proteome. In this study, we now turn our attention to develop generalizable chemistry strategies to accelerate the development of new bifunctional degraders. We implement lysine-targeted reversible-covalent chemistry to rationally tune the binding kinetics at the protein-of-interest across a set of 25 targets. We define an unbiased workflow consisting of global proteomics analysis, IP/MS of ternary complexes and the E-STUB assay, to mechanistically characterize the effects of ligand residence time on targeted protein degradation and formulate hypotheses about the rate-limiting step of degradation for each target. Our key finding is that target residence time is a major determinant of degrader activity, and this can be rapidly and rationally tuned through the synthesis of a minimal number of analogues to accelerate early degrader discovery and optimization.

Unleashing the Power of Covalent Drugs for Protein Degradation

Targeted protein degradation (TPD) has emerged as a significant therapeutic approach for a variety of diseases, including cancer. Advances in TPD techniques, such as molecular glue (MG) and lysosome-dependent strategies, have shown substantial progress since the inception of the first PROTAC in 2001. The PROTAC methodology represents the forefront of TPD technology, with ongoing evaluation in more than 20 clinical trials for the treatment of diverse medical conditions. Two prominent PROTACs, ARV-471 and ARV-110, are currently undergoing phase III and II clinical trials, respectively. Traditional PROTACs are encountering obstacles such as limited binding affinity and a restricted range of E3 ligase ligands for facilitating the protein of interest (POI) degradation. Covalent medicines offer the potential to enhance PROTAC efficacy by enabling the targeting of previously considered "undruggable" shallow binding sites. Strategic alterations allow PROTAC to establish covalent connections with particular target proteins, including Kirsten rat sarcoma viral oncogene homolog (KRAS), Bruton's tyrosine kinase (BTK), epidermal growth factor receptor (EGFR), as well as E3 ligases such as DDB1 and CUL4 associated factor 16 (DCAF16) and Kelch-like ECH-associated protein 1 (Keap1). The concept of covalent degradation has also been utilized in various new forms of degraders, including covalent molecule glue (MG), in-cell click-formed proteolysis targeting chimera (CLIPTAC), HaloPROTAC, lysosome-targeting chimera (LYTAC) and GlueTAC. This review focuses on recent advancements in covalent degraders beyond covalent PROTACs and examines obstacles and future directions pertinent to this field.

Degradome analysis to identify direct protein substrates of small-molecule degraders

Targeted protein degradation (TPD) has emerged as a powerful strategy to selectively eliminate cellular proteins using small-molecule degraders, offering therapeutic promise for targeting proteins that are otherwise undruggable. However, a remaining challenge is to unambiguously identify primary TPD targets that are distinct from secondary downstream effects in the proteome. Here we introduce an approach for selective analysis of protein degradation by mass spectrometry (DegMS) at proteomic scale, which derives its specificity from the exclusion of confounding effects of altered transcription and translation induced by target depletion. We show that the approach efficiently operates at the timescale of TPD (hours) and we demonstrate its utility by analyzing the cyclin K degraders dCeMM2 and dCeMM4, which induce widespread transcriptional downregulation, and the GSPT1 degrader CC-885, an inhibitor of protein translation. Additionally, we apply DegMS to characterize a previously uncharacterized degrader, and identify the zinc-finger protein FIZ1 as a degraded target.

Strategy and Design of In Situ Activated Protein Hydrolysis Targeted Chimeras

Protein hydrolysis targeted chimeras (PROTACs) represent a different therapeutic approach, particularly relevant for overcoming challenges associated with traditional small molecule inhibitors. These challenges include targeting difficult proteins that are often deemed "undruggable" and addressing issues of acquired resistance. PROTACs employ the body's own E3 ubiquitin ligases to induce the degradation of specific proteins of interest (POIs) through the ubiquitin-proteasome pathway. This process is cyclical, allowing for broad applicability, potent protein degradation, and selective targeting. Despite their effectiveness, PROTACs can inadvertently target and degrade nonspecific proteins, potentially resulting in significant side effects and off-target toxicity. To address this concern, researchers have created stimuli-activated PROTACs that enhance targeted protein degradation while minimizing potential harm to healthy cells. These advanced PROTACs aim to improve the precision of degradation in both time and space. This article reviews the strategies for in situ activated PROTACs, highlighting key compounds and research advancements associated with various mechanisms of action. The insights presented here aim to guide further exploration in the field of activated PROTACs.

Targeting Oncogenic RET Kinase by Simultaneously Inhibiting Kinase Activity and Degrading the Protein

The rearranged-during-transfection (RET) kinase is a validated target for the treatment of RET-altered cancers. Currently approved RET-selective kinase inhibitors, selpercatinib (LOXO-292) and pralsetinib (BLU-667), increase the oncogenic RET protein level upon treatment, which may affect their efficacy. We seek to reduce the oncogenic RET protein level and RET kinase activity simultaneously. Here, we report the development of proteolysis targeting chimera (PROTAC) degraders of oncogenic RET protein. Compound YW-N-7 exhibited dual action of selectively inhibiting and depleting RET protein both in vitro and in vivo. Proteomic analysis indicated that YW-N-7 is highly specific to RET. In cell cultures, reducing RET fusion protein potentiated the activity of LOXO-292. Furthermore, YW-N-7 showed significant activity in inhibiting KIF5B-RET-driven xenograft tumors in animals. This study exemplifies the feasibility of simultaneously inhibiting and degrading oncogenic RET kinase for cancer therapy.

Proteomics: An In-Depth Review on Recent Technical Advances and Their Applications in Biomedicine

Proteins hold pivotal importance since many diseases manifest changes in protein activity. Proteomics techniques provide a comprehensive exploration of protein structure, abundance, and function in biological samples, enabling the holistic characterization of overall changes in organisms. Nowadays, the breadth of emerging methodologies in proteomics is unprecedentedly vast, with constant optimization of technologies in sample processing, data collection, data analysis, and its scope of application is steadily transitioning from the bench to the clinic. Here, we offer an insightful review of the technical developments in proteomics and its applications in biomedicine over the past 5 years. We focus on its profound contributions in profiling disease spectra, discovering new biomarkers, identifying promising drug targets, deciphering alterations in protein conformation, and unearthing protein-protein interactions. Moreover, we summarize the cutting-edge technologies and potential breakthroughs in the proteomics pipeline and provide the principal challenges in proteomics. Based on these, we aspire to broaden the applicability of proteomics and inspire researchers to enhance our understanding of complex biological systems by utilizing such techniques.

Targeting N-Myc in neuroblastoma with selective Aurora kinase A degraders

The N-Myc transcription factor, encoded by MYCN, is a mechanistically validated, yet challenging, target for neuroblastoma (NB) therapy development. In normal neuronal progenitors, N-Myc undergoes rapid degradation, while, in MYCN-amplified NB cells, Aurora kinase A (Aurora-A) binds to and stabilizes N-Myc, resulting in elevated protein levels. Here, we demonstrate that targeted protein degradation of Aurora-A decreases N-Myc levels. A potent Aurora-A degrader, HLB-0532259 (compound 4), was developed from an Aurora-A-binding ligand that engages the Aurora-A/N-Myc complex. HLB-0532259 promotes the degradation of Aurora-A, which elicits concomitant N-Myc degradation, with nanomolar potency and excellent selectivity. HLB-0532259 surpasses the cellular efficacy of established allosteric Aurora-A inhibitors, exhibits favorable pharmacokinetic properties, and elicits tumor reduction in a murine xenograft NB model. This study broadly delineates a strategy for targeting "undruggable" proteins that are reliant on accessory proteins for cellular stabilization.

Chem(Pro)2: the atlas of chemoproteomic probes labelling human proteins

Chemoproteomic probes (CPPs) have been widely considered as powerful molecular biological tools that enable the highly efficient discovery of both binding proteins and modes of action for the studied compounds. They have been successfully used to validate targets and identify binders. The design of CPP has been considered extremely challenging, which asks for the generalization using a large number of probe data. However, none of the existing databases gives such valuable data of CPPs. Herein, a database entitled 'Chem(Pro)2' was therefore developed to systematically describe the atlas of diverse types of CPPs labelling human protein in living cell/lysate. With the booming application of chemoproteomic technique and artificial intelligence in current chemical biology study, Chem(Pro)2 was expected to facilitate the AI-based learning of interacting pattern among molecules for discovering innovative targets and new drugs. Till now, Chem(Pro)2 has been open to all users without any login requirement at: https://idrblab.org/chemprosquare/.

Acquired resistance to tyrosine kinase targeted therapy: mechanism and tackling strategies

Over the past two decades, tyrosine kinase inhibitors (TKIs) have rapidly emerged as pivotal targeted agents, offering promising therapeutic prospects for patients. However, as the cornerstone of targeted therapies, an increasing number of TKIs have been found to develop acquired resistance during treatment, making the challenge of overcoming this resistance a primary focus of current research. This review comprehensively examines the evolution of TKIs from multiple perspectives, with particular emphasis on the mechanisms underlying acquired resistance, innovative drug design strategies, inherent challenges, and future directions.

Exploiting the DCAF16-SPIN4 interaction to identify DCAF16 ligands for PROTAC development

Traditional small molecule drugs often target protein activity directly, but challenges arise when proteins lack suitable functional sites. An alternative approach is targeted protein degradation (TPD), which directs proteins to cellular machinery for proteolytic degradation. Recent studies have identified additional E3 ligases suitable for TPD, expanding the potential of this approach. Among these, DCAF16 has shown promise in facilitating protein degradation through both PROTAC and molecular glue mechanisms. In this study, we developed a homogeneous time resolved fluorescence (HTRF) assay to discover new DCAF16 binders. Using an in-house electrophile library, we identified two diastereomeric compounds, with one engaging DCAF16 at cysteines C177-179 and another reducing its expression. We demonstrated that the compound covalently engaging DCAF16 can be transformed into a PROTAC capable of degrading FKBP12.

Identification of suitable target/E3 ligase pairs for PROTAC development using a rapamycin-induced proximity assay (RiPA)

The development of proteolysis targeting chimeras (PROTACs), which induce the degradation of target proteins by bringing them into proximity with cellular E3 ubiquitin ligases, has revolutionized drug development. While the human genome encodes more than 600 different E3 ligases, current PROTACs use only a handful of them, drastically limiting their full potential. Furthermore, many PROTAC development campaigns fail because the selected E3 ligase candidates are unable to induce degradation of the particular target of interest. As more and more ligands for novel E3 ligases are discovered, the chemical effort to identify the best E3 ligase for a given target is exploding. Therefore, a genetic system to identify degradation-causing E3 ligases and suitable target/E3 ligase pairs is urgently needed. Here, we used the well-established dimerization of the FKBP12 protein and FRB domain by rapamycin to bring the target protein WDR5 into proximity with candidate E3 ligases. Strikingly, this rapamycin-induced proximity assay (RiPA) revealed that VHL, but not Cereblon, is able to induce WDR5 degradation - a finding previously made by PROTACs, demonstrating its predictive power. By optimizing the steric arrangement of all components and fusing the target protein with a minimal luciferase, RiPA can identify the ideal E3 for any target protein of interest in living cells, significantly reducing and focusing the chemical effort in the early stages of PROTAC development.

PROTAC unleashed: Unveiling the synthetic approaches and potential therapeutic applications

Proteolysis-Targeting Chimeras (PROTACs) are a novel class of bifunctional small molecules that alter protein levels by targeted degradation. This innovative approach uses the ubiquitin-proteasome system to selectively eradicate disease-associated proteins, providing a novel therapeutic strategy for a wide spectrum of diseases. This review delineates detailed synthetic approaches involved in PROTAC building blocks, including the ligand and protein binding parts, linker attached structural components of PROTACs and the actual PROTAC molecules. Furthermore, the recent advancements in PROTAC-mediated degradation of specific oncogenic and other disease-associated proteins, such as those involved in neurodegenerative, antiviral, and autoimmune diseases, were also discussed. Additionally, we described the current landscape of PROTAC clinical trials and highlighted key studies that underscore the translational potential of this emerging therapeutic modality. These findings demonstrate the versatility of PROTACs in modulating the levels of key proteins involved in various severe diseases.

PROTAC-mediated activation, rather than degradation, of a nuclear receptor reveals complex ligand-receptor interaction network

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional molecules containing a ligand for a protein of interest linked to an E3 ubiquitin ligase ligand that induce protein degradation through E3 recruitment to the target protein. Small changes in PROTAC linkers can have drastic consequences, including loss of degradation activity, but the structural mechanisms governing such changes are unclear. To study this phenomenon, we screened PROTACs of diverse targeting modalities and identified dTAG-13 as an activator of the xenobiotic-sensing pregnane X receptor (PXR), which promiscuously binds various ligands. Characterization of dTAG-13 analogs and precursors revealed interplay between the PXR-binding moiety, linker, and E3 ligand that altered PXR activity without inducing degradation. A crystal structure of PXR ligand binding domain bound to a precursor ligand showed ligand-induced binding pocket distortions and a linker-punctured tunnel to the protein exterior at a region incompatible with E3 complex formation, highlighting the effects of linker environment on PROTAC activity.

Harnessing the FBXW7 somatic mutant R465C for targeted protein degradation

Targeted protein degradation (TPD) is a pharmacological strategy that eliminates specific proteins from cells by harnessing cellular proteolytic degradation machinery. In proteasome-dependent TPD, expanding the repertoire of E3 ligases compatible with this approach could enhance the applicability of this strategy across various biological contexts. In this study, we discovered that a somatic mutant of FBXW7, R465C, can be exploited by heterobifunctional compounds for targeted protein degradation. This work demonstrates the potential of utilizing mutant E3 ligases that occur exclusively in diseased cells for TPD applications.

Development of PROTACs using computational approaches

Proteolysis-targeting chimeras (PROTACs) are drugs designed to degrade target proteins via the ubiquitin-proteasome system. With the application of computational biology/chemistry technique in drug design, numerous computer-aided drug design and artificial intelligence (AI)-driven drug design (CADD/AIDD) methods have recently emerged to facilitate the development of PROTAC drugs. We systematically review the role of in silico tools in PROTAC drug design, emphasizing how computational software can model PROTAC action and structure, predict activity, and assist in molecule design. We also discuss current challenges in the rational design of PROTACs from an in silico perspective, such as deviations from small-molecule druggability and the limited availability of training data. We provide an overview of recent discoveries and emerging research in this field, and discuss their potential impact on PROTAC design strategies.

Template-assisted covalent modification underlies activity of covalent molecular glues

Molecular glues are proximity-inducing small molecules that have emerged as an attractive therapeutic approach. However, developing molecular glues remains challenging, requiring innovative mechanistic strategies to stabilize neoprotein interfaces and expedite discovery. Here we unveil a trans-labeling covalent molecular glue mechanism, termed 'template-assisted covalent modification'. We identified a new series of BRD4 molecular glue degraders that recruit CUL4DCAF16 ligase to the second bromodomain of BRD4 (BRD4BD2). Through comprehensive biochemical, structural and mutagenesis analyses, we elucidated how pre-existing structural complementarity between DCAF16 and BRD4BD2 serves as a template to optimally orient the degrader for covalent modification of DCAF16Cys58. This process stabilizes the formation of BRD4-degrader-DCAF16 ternary complex and facilitates BRD4 degradation. Supporting generalizability, we found that a subset of degraders also induces GAK-BRD4BD2 interaction through trans-labeling of GAK. Together, our work establishes 'template-assisted covalent modification' as a mechanism for covalent molecular glues, which opens a new path to proximity-driven pharmacology.

Protacs in cancer therapy: mechanisms, design, clinical trials, and future directions

Cancer develops as a result of changes in both genetic and epigenetic mechanisms, which lead to the activation of oncogenes and the suppression of tumor suppressor genes. Despite advancements in cancer treatments, the primary approach still involves a combination of chemotherapy, radiotherapy, and surgery, typically providing a median survival of approximately five years for patients. Unfortunately, these therapeutic interventions often bring about substantial side effects and toxicities, significantly impacting the overall quality of life for individuals undergoing treatment. Therefore, urgent need of research required which comes up with effective treatment of cancer. This review explores the transformative role of Proteolysis-Targeting Chimeras (PROTACs) in cancer therapy. PROTACs, an innovative drug development strategy, utilize the cell's protein degradation machinery to selectively eliminate disease-causing proteins. The review covers the historical background, mechanism of action, design, and structure of PROTACs, emphasizing their precision in targeting oncogenic proteins. The discussion extends to the challenges, nanotechnology applications, and ongoing clinical trials, showcasing promising results and clinical progress. The review concludes with insights into patents, future directions, and the potential impact of PROTACs in addressing dysregulated protein expression across various diseases. Overall, it provides a concise yet comprehensive overview for researchers, clinicians, and industry professionals involved in developing targeted therapies.

Targeted protein degradation: current molecular targets, localization, and strategies

Targeted protein degradation (TPD) has revolutionized drug discovery by selectively eliminating specific proteins within and outside the cellular context. Over the past two decades, TPD has expanded its focus beyond well-established targets, exploring diverse proteins beyond cancer-related ones. This evolution extends the potential of TPD to various diseases. Notably, TPD can target proteins at demanding locations, such as the extracellular matrix (ECM) and cellular membranes, presenting both opportunities and challenges for future research. In this review, we comprehensively examine the exciting opportunities in the burgeoning field of TPD, highlighting different targets, their cellular environment, and innovative strategies for modern drug discovery.

Extrapolating Lessons from Targeted Protein Degradation to Other Proximity-Inducing Drugs

Targeted protein degradation (TPD) is an emerging pharmacologic strategy. It relies on small-molecule "degraders" that induce proximity of a component of an E3 ubiquitin ligase complex and a target protein to induce target ubiquitination and subsequent proteasomal degradation. Essentially, degraders thus expand the function of E3 ligases, allowing them to degrade proteins they would not recognize in the absence of the small molecule. Over the past decade, insights gained from identifying, designing, and characterizing various degraders have significantly enhanced our understanding of TPD mechanisms, precipitating in rational degrader discovery strategies. In this Account, I aim to explore how these insights can be extrapolated to anticipate both opportunities and challenges of utilizing the overarching concept of proximity-inducing pharmacology to manipulate other cellular circuits for the dissection of biological mechanisms and for therapeutic purposes.

Discovery and Functional Characterization of a Potent, Selective, and Metabolically Stable PROTAC of the Protein Kinases DYRK1A and DYRK1B

Small-molecule-induced protein degradation has emerged as a promising pharmacological modality for inactivating disease-relevant protein kinases. DYRK1A and DYRK1B are closely related protein kinases that are involved in pathological processes such as neurodegeneration, cancer development, and adaptive immune homeostasis. Herein, we report the development of the first DYRK1 proteolysis targeting chimeras (PROTACs) that combine a new ATP-competitive DYRK1 inhibitor with ligands for the E3 ubiquitin ligase component cereblon (CRBN) to induce ubiquitination and subsequent proteasomal degradation of DYRK1A and DYRK1B. The lead compound (DYR684) promoted fast, efficient, potent, and selective degradation of DYRK1A in cell-based assays. Interestingly, an enzymatically inactive splicing variant of DYRK1B (p65) resisted degradation. Compared to competitive kinase inhibition, targeted degradation of DYRK1 by DYR684 provided improved suppression of downstream signaling. Collectively, our results identify DYRKs as viable targets for PROTAC-mediated degradation and qualify DYR684 as a useful chemical probe for DYRK1A and DYRK1B.

Unveiling the hidden interactome of CRBN molecular glues with chemoproteomics

Targeted protein degradation and induced proximity refer to strategies that leverage the recruitment of proteins to facilitate their modification, regulation or degradation. As prospective design of glues remains challenging, unbiased discovery methods are needed to unveil hidden chemical targets. Here we establish a high throughput affinity purification mass spectrometry workflow in cell lysates for the unbiased identification of molecular glue targets. By mapping the targets of 20 CRBN-binding molecular glues, we identify 298 protein targets and demonstrate the utility of enrichment methods for identifying novel targets overlooked using established methods. We use a computational workflow to estimate target confidence and perform a biochemical screen to identify a lead compound for the new non-ZF target PPIL4. Our study provides a comprehensive inventory of targets chemically recruited to CRBN and delivers a robust and scalable workflow for identifying new drug-induced protein interactions in cell lysates.

Dual-ligand PROTACS mediate superior target protein degradation in vitro and therapeutic efficacy in vivo

Proteolysis targeting chimeras (PROTACs) are revolutionizing the drug development landscape due to their unique ability to selectively degrade disease-associated proteins. Conventional PROTACs are bivalent entities that induce ubiquitination and subsequent proteolysis of a chosen protein of interest (POI) by forming a ternary complex with an E3 ligase. We hypothesized that dual-ligand PROTACs, featuring two copies each of a POI ligand and an E3 ligase ligand, would facilitate the formation of high-avidity, long-lived ternary complexes inside cells, thereby increasing POI degradation potency. To this end, we developed a convergent synthesis route, using l-aspartic acid as a building block for homodimer synthesis, followed by copper-catalyzed azide-alkyne cycloaddition (CuAAC) to conjugate both dimers through a flexible linker. Dual-ligand PROTACs achieved up to a tenfold increase in degradation efficiency and a hundredfold increase in cytotoxicity in vitro across various cancer cell lines compared to their single-ligand counterparts. Furthermore, dual-ligand PROTACs sustain prolonged protein degradation, up to 60 hours after pulsing and washout. In vivo, in a mouse tumor model, the superior therapeutic activity of dual ligand PROTACs was observed.

Discovery of Potent Degraders of the Dengue Virus Envelope Protein

Targeted protein degradation has been widely adopted as a new approach to eliminate both established and previously recalcitrant therapeutic targets. Here, it is reported that the development of small molecule degraders of the envelope (E) protein of dengue virus. Two classes of bivalent E-degraders are developed by linking two previously reported E-binding small molecules, GNF-2, and CVM-2-12-2, to a glutarimide-based recruiter of the CRL4CRBN ligase to effect proteosome-mediated degradation of the E protein. ZXH-2-107 (based on GNF-2) is an E-degrader with ABL inhibitory activity while ZXH-8-004 (based on CVM-2-12-2) is a selective and potent E-degrader. These two compounds provide proof of concept that difficult-to-drug targets such as a viral envelope protein can be effectively eliminated using a bivalent degrader and provide starting points for the future development of a new class of direct-acting antiviral drugs.

Profiling Cellular Morphological Changes Induced by Dual-Targeting PROTACs of Aurora Kinase and RNA-Binding Protein YTHDF2

Proteolysis targeting chimeras (PROTACs) are new chemical modalities that degrade proteins of interest, including established kinase targets and emerging RNA-binding proteins (RBPs). Whereas diverse sets of biochemical, biophysical and cellular assays are available for the evaluation and optimizations of PROTACs in understanding the involved ubiquitin-proteasome-mediated degradation mechanism and the structure-degradation relationship, a phenotypic method profiling the cellular morphological changes is rarely used. In this study, first, we reported the only examples of PROTACs degrading the mRNA-binding protein YTHDF2 via screening of multikinase PROTACs. Second, we reported the profiling of cellular morphological changes of the dual kinase- and RBP-targeting PROTACs using the unbiased cell painting assay (CPA). The CPA analysis revealed the high biosimilarity with the established aurora kinase cluster and annotated aurora kinase inhibitors, which reflected the association between YTHDF2 and the aurora kinase signaling network. Broadly, the results demonstrated that the cell painting assay can be a straightforward and powerful approach to evaluate PROTACs. Complementary to the existing biochemical, biophysical and cellular assays, CPA provided a new perspective in characterizing PROTACs at the cellular morphology.

Discovery of electrophilic degraders that exploit SNAr chemistry

Targeted covalent inhibition (TCI) and targeted protein degradation (TPD) have proven effective in pharmacologically addressing formerly 'undruggable' targets. Integration of both methodologies has resulted in the development of electrophilic degraders where recruitment of a suitable E3 ubiquitin ligase is achieved through formation of a covalent bond with a cysteine nucleophile. Expanding the scope of electrophilic degraders requires the development of electrophiles with tempered reactivity that enable selective ligase recruitment and reduce cross-reactivity with other cellular nucleophiles. In this study, we report the use of chemical moieties that enable nucleophilic aromatic substitution (SNAr) reactions in the rational design of electrophilic protein degraders. Appending an SNAr covalent warhead to several preexisting small molecule inhibitors transformed them into degraders, obviating the need for a defined E3 ligase recruiter. The SNAr covalent warhead is versatile; it can recruit various E3 ligases, including DDB1 and CUL4 associated factor 11 (DCAF11), DDB1 and CUL4 associated factor 16 (DCAF16), and possibly others. The incorporation of an SNAr covalent warhead into the BRD4 inhibitor led to the discovery of degraders with low picomolar degradation potency. Furthermore, we demonstrate the broad applicability of this approach through rational functional switching from kinase inhibitors into potent degraders.

A Kinetic Scout Approach Accelerates Targeted Protein Degrader Development

Bifunctional molecules such as targeted protein degraders induce proximity to promote gain-of-function pharmacology. These powerful approaches have gained broad traction across academia and the pharmaceutical industry, leading to an intensive focus on strategies that can accelerate their identification and optimization. We and others have previously used chemical proteomics to map degradable target space, and these datasets have been used to develop and train multiparameter models to extend degradability predictions across the proteome. In this study, we now turn our attention to develop generalizable chemistry strategies to accelerate the development of new bifunctional degraders. We implement lysine-targeted reversible-covalent chemistry to rationally tune the binding kinetics at the protein-of-interest across a set of 25 targets. We define an unbiased workflow consisting of global proteomics analysis, IP/MS of ternary complexes and the E-STUB assay, to mechanistically characterize the effects of ligand residence time on targeted protein degradation and formulate hypotheses about the rate-limiting step of degradation for each target. Our key finding is that target residence time is a major determinant of degrader activity, and this can be rapidly and rationally tuned through the synthesis of a minimal number of analogues to accelerate early degrader discovery and optimization efforts.

Targeted Degradation of Protein Kinase A via a Stapled Peptide PROTAC

Proteolysis-targeting chimeras (PROTACs) are bifunctional molecules that bind and recruit an E3 ubiquitin ligase to a targeted protein of interest, often through the utilization of a small molecule inhibitor. To expand the possible range of kinase targets that can be degraded by PROTACs, we sought to develop a PROTAC utilizing a hydrocarbon-stapled peptide as the targeting agent to bind the surface of a target protein of interest. In this study, we describe the development of a proteolysis-targeting chimera, dubbed Stapled Inhibitor Peptide - PROTAC or StIP-TAC, linking a hydrocarbon-stapled peptide with an E3 ligase ligand for targeted degradation of Protein Kinase A (PKA). This StIP-TAC molecule stimulated E3-mediated protein degradation of PKA, and this effect could be reversed by the addition of the proteasomal inhibitor MG-132. Further, StIP-TAC treatment led to a significant reduction in PKA substrate phosphorylation. Since many protein targets of interest lack structural features that make them amenable to small molecule targeting, development of StIP-TACs may broaden the potential range of protein targets using a PROTAC-mediated proteasomal degradation approach.

Targeted protein degradation using chimeric human E2 ubiquitin-conjugating enzymes

Proteins can be targeted for degradation by engineering biomolecules that direct them to the eukaryotic ubiquitination machinery. For instance, the fusion of an E3 ubiquitin ligase to a suitable target binding domain creates a 'biological Proteolysis-Targeting Chimera' (bioPROTAC). Here we employ an analogous approach where the target protein is recruited directly to a human E2 ubiquitin-conjugating enzyme via an attached target binding domain. Through rational design and screening we develop E2 bioPROTACs that induce the degradation of the human intracellular proteins SHP2 and KRAS. Using global proteomics, we characterise the target-specific and wider effects of E2 vs. VHL-based fusions. Taking SHP2 as a model target, we also employ a route to bioPROTAC discovery based on protein display libraries, yielding a degrader with comparatively weak affinity capable of suppressing SHP2-mediated signalling.

New therapies on the horizon: Targeted protein degradation in neuroscience

This minireview explores the burgeoning field of targeted protein degradation (TPD) and its promising applications in neuroscience and clinical development. TPD offers innovative strategies for modulating protein levels, presenting a paradigm shift in small-molecule drug discovery and therapeutic interventions. Importantly, small-molecule protein degraders specifically target and remove pathogenic proteins from central nervous system cells without the drug delivery challenges of genomic and antibody-based modalities. Here, we review recent advancements in TPD technologies, highlight proteolysis targeting chimera (PROTAC) protein degrader molecules with proximity-induced degradation event-driven and iterative pharmacology, provide applications in neuroscience research, and discuss the high potential for translation of TPD into clinical settings.

Structure-guided engineering of type I-F CASTs for targeted gene insertion in human cells

Conventional genome editing tools rely on DNA double-strand breaks (DSBs) and host recombination proteins to achieve large insertions, resulting in a heterogeneous mixture of undesirable editing outcomes. We recently leveraged a type I-F CRISPR-associated transposase (CAST) from the Pseudoalteromonas Tn7016 transposon (PseCAST) for DSB-free, RNA-guided DNA integration in human cells, taking advantage of its programmability and large payload capacity. PseCAST is the only characterized CAST system that has achieved human genomic DNA insertions, but multiple lines of evidence suggest that DNA binding may be a critical bottleneck that limits high-efficiency activity. Here we report structural determinants of target DNA recognition by the PseCAST QCascade complex using single-particle cryogenic electron microscopy (cryoEM), which revealed novel subtype-specific interactions and RNA-DNA heteroduplex features. By combining our structural data with target DNA library screens and rationally engineered protein mutations, we uncovered CAST variants that exhibit increased integration efficiency and modified PAM stringency. Structure predictions of key interfaces in the transpososome holoenzyme also revealed opportunities for the design of hybrid CASTs, which we leveraged to build chimeric systems that combine high-activity DNA binding and DNA integration modules. Collectively, our work provides unique structural insights into type I-F CAST systems while showcasing multiple diverse strategies to investigate and engineer new RNA-guided transposase architectures for human genome editing applications.

Targeted protein degradation: from mechanisms to clinic

Targeted protein degradation refers to the use of small molecules to induce the selective degradation of proteins. In its most common form, this degradation is achieved through ligand-mediated neo-interactions between ubiquitin E3 ligases - the principal waste disposal machines of a cell - and the protein targets of interest, resulting in ubiquitylation and subsequent proteasomal degradation. Notable advances have been made in biological and mechanistic understanding of serendipitously discovered degraders. This improved understanding and novel chemistry has not only provided clinical proof of concept for targeted protein degradation but has also led to rapid growth of the field, with dozens of investigational drugs in active clinical trials. Two distinct classes of protein degradation therapeutics are being widely explored: bifunctional PROTACs and molecular glue degraders, both of which have their unique advantages and challenges. Here, we review the current landscape of targeted protein degradation approaches and how they have parallels in biological processes. We also outline the ongoing clinical exploration of novel degraders and provide some perspectives on the directions the field might take.

PROTAC-Mediated HDAC7 Protein Degradation Unveils Its Deacetylase-Independent Proinflammatory Function in Macrophages

Class IIa histone deacetylases (Class IIa HDACs) play critical roles in regulating essential cellular metabolism and inflammatory pathways. However, dissecting the specific roles of each class IIa HDAC isoform is hindered by the pan-inhibitory effect of current inhibitors and a lack of tools to probe their functions beyond epigenetic regulation. In this study, a novel PROTAC-based compound B4 is developed, which selectively targets and degrades HDAC7, resulting in the effective attenuation of a specific set of proinflammatory cytokines in both lipopolysaccharide (LPS)-stimulated macrophages and a mouse model. By employing B4 as a molecular probe, evidence is found for a previously explored role of HDAC7 that surpasses its deacetylase function, suggesting broader implications in inflammatory processes. Mechanistic investigations reveal the critical involvement of HDAC7 in the Toll-like receptor 4 (TLR4) signaling pathway by directly interacting with the TNF receptor-associated factor 6 and TGFβ-activated kinase 1 (TRAF6-TAK1) complex, thereby initiating the activation of the downstream mitogen-activated protein kinase/nuclear factor-κB (MAPK/NF-κB) signaling cascade and subsequent gene transcription. This study expands the insight into HDAC7's role within intricate inflammatory networks and highlights its therapeutic potential as a novel target for anti-inflammatory treatments.

Ubiquitin-specific proximity labeling for the identification of E3 ligase substrates

Protein ubiquitylation controls diverse processes within eukaryotic cells, including protein degradation, and is often dysregulated in disease. Moreover, small-molecule degraders that redirect ubiquitylation activities toward disease targets are an emerging and promising therapeutic class. Over 600 E3 ubiquitin ligases are expressed in humans, but their substrates remain largely elusive, necessitating the development of new methods for their discovery. Here we report the development of E3-substrate tagging by ubiquitin biotinylation (E-STUB), a ubiquitin-specific proximity labeling method that biotinylates ubiquitylated substrates in proximity to an E3 ligase of interest. E-STUB accurately identifies the direct ubiquitylated targets of protein degraders, including collateral targets and ubiquitylation events that do not lead to substrate degradation. It also detects known substrates of E3 ligase CRBN and VHL with high specificity. With the ability to elucidate proximal ubiquitylation events, E-STUB may facilitate the development of proximity-inducing therapeutics and act as a generalizable method for E3-substrate mapping.

Discovery of a first-in-class protein degrader for the c-ros oncogene 1 (ROS1)

The c-ros oncogene 1 (ROS1), an oncogenic driver, is known to induce non-small cell lung cancer (NSCLC) when overactivated, particularly through the formation of fusion proteins. Traditional targeted therapies focus on inhibiting ROS1 activity with ROS 1 inhibitors to manage cancer progression. However, a new strategy involving the design of protein degraders offers a more potent approach by completely degrading ROS1 fusion oncoproteins, thereby effectively blocking their kinase activity and enhancing anti-tumour potential. Utilizing PROteolysis-TArgeting Chimera (PROTAC) technology and informed by molecular docking and rational design, we report the first ROS1-specific PROTAC, SIAIS039. This degrader effectively targets multiple ROS1 fusion oncoproteins (CD74-ROS1, SDC4-ROS1 and SLC34A2-ROS1) in engineered Ba/F3 cells and HCC78 cells, demonstrating anti-tumour effects against ROS1 fusion-driven cancer cells. It suppresses cell proliferation, induces cell cycle arrest, and apoptosis, and inhibits clonogenicity. The anti-tumour efficacy of SIAIS039 surpasses two approved drugs, crizotinib and entrectinib, and matches that of the top inhibitors, including lorlatinib and taletrectinib. Mechanistic studies confirm that the degradation induced by 039 requires the participation of ROS1 ligands and E3 ubiquitin ligases, and involves the proteasome and ubiquitination. In addition, 039 exhibited excellent oral bioavailability in a mouse xenograft model, highlighting its potential for clinical application. In conclusion, our study presents a promising and novel therapeutic strategy for ROS1 fusion-positive NSCLC by targeting ROS1 fusion oncoproteins for degradation, laying the foundation for the development of further PROTAC and offering hope for patients with ROS1 fusion-positive NSCLC.

Development of an Orally Bioavailable LCK PROTAC Degrader as a Potential Therapeutic Approach to T-Cell Acute Lymphoblastic Leukemia

Despite significant advances over recent years, the treatment of T cell acute lymphoblastic leukemia (T-ALL) remains challenging. We have recently shown that a subset of T-ALL cases exhibited constitutive activation of the lymphocyte-specific protein tyrosine kinase (LCK) and were consequently responsive to treatments with LCK inhibitors and degraders such as dasatinib and dasatinib-based PROTACs. Here we report the design, synthesis and in vitro/vivo evaluation of SJ45566, a potent and orally bioavailable LCK PROTAC.

Targeted Protein Degradation: Current and Emerging Approaches for E3 Ligase Deconvolution

Targeted protein degradation (TPD), including the use of proteolysis-targeting chimeras (PROTACs) and molecular glue degraders (MGDs) to degrade proteins, is an emerging strategy to develop novel therapies for cancer and beyond. PROTACs or MGDs function by inducing the proximity between an E3 ligase and a protein of interest (POI), leading to ubiquitination and consequent proteasomal degradation of the POI. Notably, one major issue in TPD is the lack of ligandable E3 ligases, as current studies predominantly use CUL4CRBN and CUL2VHL. The TPD community is seeking to expand the landscape of ligandable E3 ligases, but most discoveries rely on phenotypic screens or serendipity, necessitating systematic target deconvolution. Here, we examine and discuss both current and emerging E3 ligase deconvolution approaches for degraders discovered from phenotypic screens or monovalent glue chemistry campaigns, highlighting future prospects for identifying more ligandable E3 ligases.

Quantitative Measurement of Rate of Targeted Protein Degradation

Targeted protein degradation (TPD) is a therapeutic approach that leverages the cell's natural machinery to degrade targets instead of inhibiting them. This is accomplished by using mono- or bifunctional small molecules designed to induce the proximity of target proteins and E3 ubiquitin ligases, leading to ubiquitination and subsequent proteasome-dependent degradation of the target. One of the most significant attributes of the TPD approach is its proposed catalytic mechanism of action, which permits substoichiometric exposure to achieve the desired pharmacological effects. However, apart from one in vitro study, studies supporting the catalytic mechanism of degraders are largely inferred based on potency. A more comprehensive understanding of the degrader catalytic mechanism of action can help aspects of compound development. To address this knowledge gap, we developed a workflow for the quantitative measurement of the catalytic rate of degraders in cells. Comparing a selective and promiscuous BTK degrader, we demonstrate that both compounds function as efficient catalysts of BTK degradation, with the promiscuous degrader exhibiting faster rates due to its ability to induce more favorable ternary complexes. By leveraging computational modeling, we show that the catalytic rate is highly dynamic as the target is depleted from cells. Further investigation of the promiscuous kinase degrader revealed that the catalytic rate is a better predictor of optimal degrader activity toward a specific target compared to degradation magnitude alone. In summary, we present a versatile method for mapping the catalytic activity of any degrader for TPD in cells.

Development of PROTACS degrading KRAS and SOS1

The Kirsten rat sarcoma virus-son of sevenless 1 (KRAS-SOS1) axis drives tumor growth preferentially in pancreatic, colon, and lung cancer. Now, KRAS G12C mutated tumors can be successfully treated with inhibitors that covalently block the cysteine of the switch II binding pocket of KRAS. However, the range of other KRAS mutations is not amenable to treatment and the G12C-directed agents Sotorasib and Adragrasib show a response rate of only approximately 40%, lasting for a mean period of 8 months. One approach to increase the efficacy of inhibitors is their inclusion into proteolysis-targeting chimeras (PROTACs), which degrade the proteins of interest and exhibit much higher antitumor activity through multiple cycles of activity. Accordingly, PROTACs have been developed based on KRAS- or SOS1-directed inhibitors coupled to either von Hippel-Lindau (VHL) or Cereblon (CRBN) ligands that invoke the proteasomal degradation. Several of these PROTACs show increased activity in vitro and in vivo compared to their cognate inhibitors but their toxicity in normal tissues is not clear. The CRBN PROTACs containing thalidomide derivatives cannot be tested in experimental animals. Resistance to such PROTACS arises through downregulation or inactivation of CRBN or factors of the functional VHL E3 ubiquitin ligase. Although highly active KRAS and SOS1 PROTACs have been formulated their clinical application remains difficult.

The impact of E3 ligase choice on PROTAC effectiveness in protein kinase degradation

Proteolysis targeting chimera (PROTACs) provide a novel therapeutic approach that is revolutionizing drug discovery. The success of PROTACs largely depends on the combination of their three fragments: E3 ligase ligand, linker and protein of interest (POI)-targeting ligand. We summarize the pivotal significance of the precise combination of the E3 ligase ligand with the POI-recruiting warhead, which is crucial for the successful execution of cellular processes and achieving the desired outcomes. Therefore, the key to our selection was the use of at least two ligands recruiting two different ligases. This approach enables a direct comparison of the impacts of the specific ligases on target degradation.

Application of AlphaFold models in evaluating ligandable cysteines across E3 ligases

Proteolysis Targeting Chimeras (PROTACs) are an emerging therapeutic modality and chemical biology tools for Targeted Protein Degradation (TPD). PROTACs contain a ligand targeting the protein of interest, a ligand recruiting an E3 ligase and a linker connecting these two ligands. There are over 600 E3 ligases known so far, but only a handful have been exploited for TPD applications. A key reason for this is the scarcity of ligands binding various E3 ligases and the paucity of structural data available, which complicates ligand design across the family. In this study, we aim to progress PROTAC discovery by proposing a shortlist of E3 ligases that can be prioritized for covalent targeting by performing systematic structural ligandability analysis on a chemoproteomic dataset of potentially reactive cysteines across hundreds of E3 ligases. One of the goals of this study is to apply AlphaFold (AF) models for ligandability evaluations, as for a vast majority of these ligases an experimental structure is not available in the protein data bank (PDB). Using a combination of pocket features, AF model quality and additional aspects, we propose a shortlist of E3 ligases and corresponding cysteines that can be prioritized to potentially discover covalent ligands and expand the PROTAC toolbox.

Targeting fusion oncoproteins in childhood cancers: challenges and future opportunities for developing therapeutics

Fusion oncoproteins are associated with childhood cancers and have proven challenging to target, aside from those that include kinases. As part of its efforts for targeting childhood cancers, the National Cancer Institute recently conducted a series on Novel Chemical Approaches for Targeting Fusion Oncoproteins. Key learnings on leading platforms and technologies that can be used to advance the development of molecular therapeutics that target fusion oncoproteins in childhood cancers are described. Recent breakthroughs in medicinal chemistry and chemical biology provide new ground and creative strategies to exploit for the development of targeted agents for improving outcomes against these recalcitrant cancers.

The cyclimids: Degron-inspired cereblon binders for targeted protein degradation

Cereblon (CRBN) is an E3 ligase substrate adapter widely exploited for targeted protein degradation (TPD) strategies. However, achieving efficient and selective target degradation is a preeminent challenge with ligands that engage CRBN. Here, we report that the cyclimids, ligands derived from the C-terminal cyclic imide degrons of CRBN, exhibit distinct modes of interaction with CRBN and offer a facile approach for developing potent and selective bifunctional degraders. Quantitative TR-FRET-based characterization of 60 cyclimid degraders in binary and ternary complexes across different substrates revealed that ternary complex binding affinities correlated strongly with cellular degradation efficiency. Our studies establish the unique properties of the cyclimids as versatile warheads in TPD and a systematic biochemical approach for quantifying ternary complex formation to predict their cellular degradation activity, which together will accelerate the development of ligands that engage CRBN.

Discovery of Potent Degraders of the Dengue Virus Envelope Protein

Targeted protein degradation has been widely adopted as a new approach to eliminate both established and previously recalcitrant therapeutic targets. Here we report the development of small molecule degraders of the envelope (E) protein of dengue virus. We developed two classes of bivalent E-degraders, linking two previously reported E-binding small molecules, GNF-2 and CVM-2-12-2, to a glutarimide-based recruiter of the CRL4CRBN ligase to effect proteosome-mediated degradation of the E protein. ZXH-2-107 (based on GNF-2) is an E degrader with ABL inhibition while ZXH-8-004 (based on CVM-2-12-2) is a selective and potent E-degrader. These two compounds provide proof-of-concept that difficult-to-drug targets such as a viral envelope protein can be effectively eliminated using a bivalent degrader and provide starting points for the future development of a new class antiviral drugs.

Chemical proteomics reveals the target landscape of 1,000 kinase inhibitors

Medicinal chemistry has discovered thousands of potent protein and lipid kinase inhibitors. These may be developed into therapeutic drugs or chemical probes to study kinase biology. Because of polypharmacology, a large part of the human kinome currently lacks selective chemical probes. To discover such probes, we profiled 1,183 compounds from drug discovery projects in lysates of cancer cell lines using Kinobeads. The resulting 500,000 compound-target interactions are available in ProteomicsDB and we exemplify how this molecular resource may be used. For instance, the data revealed several hundred reasonably selective compounds for 72 kinases. Cellular assays validated GSK986310C as a candidate SYK (spleen tyrosine kinase) probe and X-ray crystallography uncovered the structural basis for the observed selectivity of the CK2 inhibitor GW869516X. Compounds targeting PKN3 were discovered and phosphoproteomics identified substrates that indicate target engagement in cells. We anticipate that this molecular resource will aid research in drug discovery and chemical biology.

Altered Metabolism and Inflammation Driven by Post-translational Modifications in Intervertebral Disc Degeneration

Intervertebral disc degeneration (IVDD) is a prevalent cause of low back pain and a leading contributor to disability. IVDD progression involves pathological shifts marked by low-grade inflammation, extracellular matrix remodeling, and metabolic disruptions characterized by heightened glycolytic pathways, mitochondrial dysfunction, and cellular senescence. Extensive posttranslational modifications of proteins within nucleus pulposus cells and chondrocytes play crucial roles in reshaping the intervertebral disc phenotype and orchestrating metabolism and inflammation in diverse contexts. This review focuses on the pivotal roles of phosphorylation, ubiquitination, acetylation, glycosylation, methylation, and lactylation in IVDD pathogenesis. It integrates the latest insights into various posttranslational modification-mediated metabolic and inflammatory signaling networks, laying the groundwork for targeted proteomics and metabolomics for IVDD treatment. The discussion also highlights unexplored territories, emphasizing the need for future research, particularly in understanding the role of lactylation in intervertebral disc health, an area currently shrouded in mystery.

Degron tagging for rapid protein degradation in mice

Degron tagging allows proteins of interest to be rapidly degraded, in a reversible and tuneable manner, in response to a chemical stimulus. This provides numerous opportunities for understanding disease mechanisms, modelling therapeutic interventions and constructing synthetic gene networks. In recent years, many laboratories have applied degron tagging successfully in cultured mammalian cells, spurred by rapid advances in the fields of genome editing and targeted protein degradation. In this At a Glance article, we focus on recent efforts to apply degron tagging in mouse models, discussing the distinct set of challenges and opportunities posed by the in vivo environment.

Proteome-scale discovery of protein degradation and stabilization effectors

Targeted protein degradation and stabilization are promising therapeutic modalities because of their potency, versatility and their potential to expand the druggable target space1,2. However, only a few of the hundreds of E3 ligases and deubiquitinases in the human proteome have been harnessed for this purpose, which substantially limits the potential of the approach. Moreover, there may be other protein classes that could be exploited for protein stabilization or degradation3-5, but there are currently no methods that can identify such effector proteins in a scalable and unbiased manner. Here we established a synthetic proteome-scale platform to functionally identify human proteins that can promote the degradation or stabilization of a target protein in a proximity-dependent manner. Our results reveal that the human proteome contains a large cache of effectors of protein stability. The approach further enabled us to comprehensively compare the activities of human E3 ligases and deubiquitinases, identify and characterize non-canonical protein degraders and stabilizers and establish that effectors have vastly different activities against diverse targets. Notably, the top degraders were more potent against multiple therapeutically relevant targets than the currently used E3 ligases cereblon and VHL. Our study provides a functional catalogue of stability effectors for targeted protein degradation and stabilization and highlights the potential of induced proximity screens for the discovery of new proximity-dependent protein modulators.