Targeted Protein Acetylation in Cells Using Heterobifunctional Molecules

Expanding the horizons of targeted protein degradation: A non-small molecule perspective

Targeted protein degradation (TPD) represented by proteolysis targeting chimeras (PROTACs) marks a significant stride in drug discovery. A plethora of innovative technologies inspired by PROTAC have not only revolutionized the landscape of TPD but have the potential to unlock functionalities beyond degradation. Non-small-molecule-based approaches play an irreplaceable role in this field. A wide variety of agents spanning a broad chemical spectrum, including peptides, nucleic acids, antibodies, and even vaccines, which not only prove instrumental in overcoming the constraints of conventional small molecule entities but also provided rapidly renewing paradigms. Herein we summarize the burgeoning non-small molecule technological platforms inspired by PROTACs, including three major trajectories, to provide insights for the design strategies based on novel paradigms.

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.

Protein degraders - from thalidomide to new PROTACs

Recently, the development of protein degraders (protein-degrading compounds) has prominently progressed. There are two remarkable classes of protein degraders: proteolysis-targeting chimeras (PROTACs) and molecular glue degraders (MGDs). Almost 70 years have passed since thalidomide was initially developed as a sedative-hypnotic drug, which is currently recognized as one of the most well-known MGDs. During the last two decades, a myriad of PROTACs and MGDs have been developed, and the molecular mechanism of action (MOA) of thalidomide was basically elucidated, including identifying its molecular target cereblon (CRBN). CRBN forms a Cullin Ring Ligase 4 with Cul4 and DDB1, whose substrate specificity is controlled by its binding ligands. Thalidomide, lenalidomide and pomalidomide, three CRBN-binding MGDs, were clinically approved to treat several intractable diseases (including multiple myeloma). Several other MGDs and CRBN-based PROTACs (ARV-110 and AVR-471) are undergoing clinical trials. In addition, several new related technologies regarding PROTACs and MGDs have also been developed, and achievements of protein degraders impact not only therapeutic fields but also basic biological science. In this article, I introduce the history of protein degraders, from the development of thalidomide to the latest PROTACs and related technologies.

Targeted Protein O-GlcNAcylation Using Bifunctional Small Molecules

Protein O-linked β-N-acetylglucosamine modification (O-GlcNAcylation) plays a crucial role in regulating essential cellular processes. The disruption of the homeostasis of O-GlcNAcylation has been linked to various human diseases, including cancer, diabetes, and neurodegeneration. However, there are limited chemical tools for protein- and site-specific O-GlcNAc modification, rendering the precise study of the O-GlcNAcylation challenging. To address this, we have developed heterobifunctional small molecules, named O-GlcNAcylation TArgeting Chimeras (OGTACs), which enable protein-specific O-GlcNAcylation in living cells. OGTACs promote O-GlcNAcylation of proteins such as BRD4, CK2α, and EZH2 in cellulo by recruiting FKBP12F36V-fused O-GlcNAc transferase (OGT), with temporal, magnitude, and reversible control. Overall, the OGTACs represent a promising approach for inducing protein-specific O-GlcNAcylation, thus enabling functional dissection and offering new directions for O-GlcNAc-targeting therapeutic development.

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.

A chemical magnet: Approaches to guide precise protein localization

Small molecules that chemically induce proximity between two proteins have been widely used to precisely modulate protein levels, stability, and activity. Recently, several studies developed novel strategies that employ heterobifunctional molecules that co-opt shuttling proteins to control the spatial localization of a target protein, unlocking new potential within this domain. Together, these studies lay the groundwork for novel targeted protein relocalization modalities that can rewire the protein circuitry and interactome to influence biological outcomes.

Catalytic Protein Inhibitors

Many of the highest priority targets in a wide range of disease states are difficult-to-drug proteins. The development of reversible small molecule inhibitors for the active sites of these proteins with sufficient affinity and residence time on-target is an enormous challenge. This has engendered interest in strategies to increase the potency of a given protein inhibitor by routes other than further improvement in gross affinity. Amongst these, the development of catalytic protein inhibitors has garnered the most attention and investment, particularly with respect to protein degraders, which catalyze the destruction of the target protein. This article discusses the genesis of the burgeoning field of catalytic inhibitors, the current state of the art, and exciting future directions.

Harmony of Protein Tags and Chimeric Molecules Empowers Targeted Protein Ubiquitination and Beyond

Post-translational modifications (PTMs) are crucial mechanisms that underlie the intricacies of biological systems and disease mechanisms. This review focuses on the latest advancements in the design of heterobifunctional small molecules that hijack PTM machineries for target-specific modifications in living systems. A key innovation in this field is the development of proteolysis-targeting chimeras (PROTACs), which promote the ubiquitination of target proteins for proteasomal degradation. The past decade has seen several adaptations of the PROTAC concept to facilitate targeted (de)phosphorylation and acetylation. Protein fusion tags have been particularly vital in these proof-of-concept studies, aiding in the investigation of the functional roles of post-translationally modified proteins linked to diseases. This overview delves into protein-tagging strategies that enable the targeted modulation of ubiquitination, phosphorylation, and acetylation, emphasizing the synergies and challenges of integrating heterobifunctional molecules with protein tags in PTM research. Despite significant progress, many PTMs remain to be explored, and protein tag-assisted PTM-inducing chimeras will continue to play an important role in understanding the fundamental roles of protein PTMs and in exploring the therapeutic potential of manipulating protein modifications, particularly for targets not yet addressed by existing drugs.

Linking ATP and allosteric sites to achieve superadditive binding with bivalent EGFR kinase inhibitors

Bivalent molecules consisting of groups connected through bridging linkers often exhibit strong target binding and unique biological effects. However, developing bivalent inhibitors with the desired activity is challenging due to the dual motif architecture of these molecules and the variability that can be introduced through differing linker structures and geometries. We report a set of alternatively linked bivalent EGFR inhibitors that simultaneously occupy the ATP substrate and allosteric pockets. Crystal structures show that initial and redesigned linkers bridging a trisubstituted imidazole ATP-site inhibitor and dibenzodiazepinone allosteric-site inhibitor proved successful in spanning these sites. The re-engineered linker yielded a compound that exhibited significantly higher potency (~60 pM) against the drug-resistant EGFR L858R/T790M and L858R/T790M/C797S, which was superadditive as compared with the parent molecules. The enhanced potency is attributed to factors stemming from the linker connection to the allosteric-site group and informs strategies to engineer linkers in bivalent agent design.

TAC-tics for Leveraging Proximity Biology in Drug Discovery

Chemically induced proximity (CIP) refers to co-opting naturally occurring biological pathways using synthetic molecules to recruit neosubstrates that are not normally encountered or to enhance the affinity of naturally occurring interactions. Leveraging proximity biology through CIPs has become a rapidly evolving field and has garnered considerable interest in basic research and drug discovery. PROteolysis TArgeting Chimera (PROTAC) is a well-established CIP modality that induces the proximity between a target protein and an E3 ubiquitin ligase, causing target protein degradation via the ubiquitin-proteasome system. Inspired by PROTACs, several other induced proximity modalities have emerged to modulate both proteins and RNA over recent years. In this review, we summarize the critical advances and opportunities in the field, focusing on protein degraders, RNA degraders and non-degrader modalities such as post-translational modification (PTM) and protein-protein interaction (PPI) modulators. We envision that these emerging proximity-based drug modalities will be valuable resources for both biological research and therapeutic discovery in the future.

Synthetic Gene Circuits for Regulation of Next-Generation Cell-Based Therapeutics

Arming human cells with synthetic gene circuits enables to expand their capacity to execute superior sensing and response actions, offering tremendous potential for innovative cellular therapeutics. This can be achieved by assembling components from an ever-expanding molecular toolkit, incorporating switches based on transcriptional, translational, or post-translational control mechanisms. This review provides examples from the three classes of switches, and discusses their advantages and limitations to regulate the activity of therapeutic cells in vivo. Genetic switches designed to recognize internal disease-associated signals often encode intricate actuation programs that orchestrate a reduction in the sensed signal, establishing a closed-loop architecture. Conversely, switches engineered to detect external molecular or physical cues operate in an open-loop fashion, switching on or off upon signal exposure. The integration of such synthetic gene circuits into the next generation of chimeric antigen receptor T-cells is already enabling precise calibration of immune responses in terms of magnitude and timing, thereby improving the potency and safety of therapeutic cells. Furthermore, pre-clinical engineered cells targeting other chronic diseases are gathering increasing attention, and this review discusses the path forward for achieving clinical success. With synthetic biology at the forefront, cellular therapeutics holds great promise for groundbreaking treatments.

DNA-encoded library-enabled discovery of proximity-inducing small molecules

Small molecules that induce protein-protein associations represent powerful tools to modulate cell circuitry. We sought to develop a platform for the direct discovery of compounds able to induce association of any two preselected proteins, using the E3 ligase von Hippel-Lindau (VHL) and bromodomains as test systems. Leveraging the screening power of DNA-encoded libraries (DELs), we synthesized ~1 million DNA-encoded compounds that possess a VHL-targeting ligand, a variety of connectors and a diversity element generated by split-and-pool combinatorial chemistry. By screening our DEL against bromodomains in the presence and absence of VHL, we could identify VHL-bound molecules that simultaneously bind bromodomains. For highly barcode-enriched library members, ternary complex formation leading to bromodomain degradation was confirmed in cells. Furthermore, a ternary complex crystal structure was obtained for our most enriched library member with BRD4BD1 and a VHL complex. Our work provides a foundation for adapting DEL screening to the discovery of proximity-inducing small molecules.

Small-molecule tools for YEATS domain proteins

Chromatin reader domains are protein folds that bind to post-translational modifications of histones and other chromatin-associated proteins. Compared to other families of reader domains, the discovery that YEATS domains bind to acylated lysines is relatively recent. Four human proteins harbor a YEATS domain, and each is present in protein complexes that regulate chromatin and transcription (ENL, AF9, YEATS2, and YEATS4). Without chemical tools to enable temporally resolved perturbations, it is often unclear how reader domains contribute to protein function. Here, we will discuss recent progress in developing small-molecule tools for YEATS domains and highlight their usefulness for making biological discoveries.

Targeted protein modification as a paradigm shift in drug discovery

Targeted Protein Modification (TPM) is an umbrella term encompassing numerous tools and approaches that use bifunctional agents to induce a desired modification over the POI. The most well-known TPM mechanism is PROTAC-directed protein ubiquitination. PROTAC-based targeted degradation offers several advantages over conventional small-molecule inhibitors, has shifted the drug discovery paradigm, and is acquiring increasing interest as over ten PROTACs have entered clinical trials in the past few years. Targeting the protein of interest for proteasomal degradation by PROTACS was the pioneer of various toolboxes for selective protein degradation. Nowadays, the ever-increasing number of tools and strategies for modulating and modifying the POI has expanded far beyond protein degradation, which phosphorylation and de-phosphorylation of the protein of interest, targeted acetylation, and selective modification of protein O-GlcNAcylation are among them. These novel strategies have opened new avenues for achieving more precise outcomes while remaining feasible and minimizing side effects. This field, however, is still in its infancy and has a long way to precede widespread use and translation into clinical practice. Herein, we investigate the pros and cons of these novel strategies by exploring the latest advancements in this field. Ultimately, we briefly discuss the emerging potential applications of these innovations in cancer therapy, neurodegeneration, viral infections, and autoimmune and inflammatory diseases.

Linking ATP and allosteric sites to achieve superadditive binding with bivalent EGFR kinase inhibitors

Bivalent molecules consisting of groups connected through bridging linkers often exhibit strong target binding and unique biological effects. However, developing bivalent inhibitors with the desired activity is challenging due to the dual motif architecture of these molecules and the variability that can be introduced through differing linker structures and geometries. We report a set of alternatively linked bivalent EGFR inhibitors that simultaneously occupy the ATP substrate and allosteric pockets. Crystal structures show that initial and redesigned linkers bridging a trisubstituted imidazole ATP-site inhibitor and dibenzodiazepinone allosteric-site inhibitor proved successful in spanning these sites. The reengineered linker yielded a compound that exhibited significantly higher potency (~60 pM) against the drug-resistant EGFR L858R/T790M and L858R/T790M/C797S, which was superadditive as compared with the parent molecules. The enhanced potency is attributed to factors stemming from the linker connection to the allosteric-site group and informs strategies to engineer linkers in bivalent agent design.

Divergent regulation of KCNQ1/E1 by targeted recruitment of protein kinase A to distinct sites on the channel complex

The slow delayed rectifier potassium current, IKs, conducted through pore-forming Q1 and auxiliary E1 ion channel complexes is important for human cardiac action potential repolarization. During exercise or fright, IKs is up-regulated by protein kinase A (PKA)-mediated Q1 phosphorylation to maintain heart rhythm and optimum cardiac performance. Sympathetic up-regulation of IKs requires recruitment of PKA holoenzyme (two regulatory - RI or RII - and two catalytic Cα subunits) to Q1 C-terminus by an A kinase anchoring protein (AKAP9). Mutations in Q1 or AKAP9 that abolish their functional interaction result in long QT syndrome type 1 and 11, respectively, which increases the risk of sudden cardiac death during exercise. Here, we investigated the utility of a targeted protein phosphorylation (TPP) approach to reconstitute PKA regulation of IKs in the absence of AKAP9. Targeted recruitment of endogenous Cα to E1-YFP using a GFP/YFP nanobody (nano) fused to RIIα enabled acute cAMP-mediated enhancement of IKs, reconstituting physiological regulation of the channel complex. By contrast, nano-mediated tethering of RIIα or Cα to Q1-YFP constitutively inhibited IKs by retaining the channel intracellularly in the endoplasmic reticulum and Golgi. Proteomic analysis revealed that distinct phosphorylation sites are modified by Cα targeted to Q1-YFP compared to free Cα. Thus, functional outcomes of synthetically recruited PKA on IKs regulation is critically dependent on the site of recruitment within the channel complex. The results reveal insights into divergent regulation of IKs by phosphorylation across different spatial and time scales, and suggest a TPP approach to develop new drugs to prevent exercise-induced sudden cardiac death.

The rise of degrader drugs

The cancer genomics revolution has served up a plethora of promising and challenging targets for the drug discovery community. The field of targeted protein degradation (TPD) uses small molecules to reprogram the protein homeostasis system to destroy desired target proteins. In the last decade, remarkable progress has enabled the rational development of degraders for a large number of target proteins, with over 20 molecules targeting more than 12 proteins entering clinical development. While TPD has been fully credentialed by the prior development of immunomodulatory drug (IMiD) class for the treatment of multiple myeloma, the field is poised for a "Gleevec moment" in which robust clinical efficacy of a rationally developed novel degrader against a preselected target is firmly established. Here, we endeavor to provide a high-level evaluation of exciting developments in the field and comment on steps that may realize the full potential of this new therapeutic modality.

Proximity-inducing modalities: the past, present, and future

Living systems use proximity to regulate biochemical processes. Inspired by this phenomenon, bifunctional modalities that induce proximity have been developed to redirect cellular processes. An emerging example of this class is molecules that induce ubiquitin-dependent proteasomal degradation of a protein of interest, and their initial development sparked a flurry of discovery for other bifunctional modalities. Recent advances in this area include modalities that can change protein phosphorylation, glycosylation, and acetylation states, modulate gene expression, and recruit components of the immune system. In this review, we highlight bifunctional modalities that perform functions other than degradation and have great potential to revolutionize disease treatment, while also serving as important tools in basic research to explore new aspects of biology.

Exploitation of Proximity-Mediated Effects in Drug Discovery: An Update of Recent Research Highlights in Perturbing Pathogenic Proteins and Correlated Issues

The utilization of proximity-mediated effects to perturb pathogenic proteins of interest (POIs) has emerged as a powerful strategic alternative to conventional drug design approaches based on target occupancy. Over the past three years, the burgeoning field of targeted protein degradation (TPD) has witnessed the expansion of degradable POIs to membrane-associated, extracellular, proteasome-resistant, and even microbial proteins. Beyond TPD, researchers have achieved the proximity-mediated targeted protein stabilization, the recruitment of intracellular immunophilins to disturb undruggable targets, and the nonphysiological post-translational modifications of POIs. All of these strides provide new avenues for innovative drug discovery aimed at battling human malignancies and other major diseases. This perspective presents recent research highlights and discusses correlated issues in developing therapeutic modalities that exploit proximity-mediated effects to modulate pathogenic proteins, thereby guiding future academic and industrial efforts in this field.

Proximity-Based Modalities for Biology and Medicine

Molecular proximity orchestrates biological function, and blocking existing proximities is an established therapeutic strategy. By contrast, strengthening or creating neoproximity with chemistry enables modulation of biological processes with high selectivity and has the potential to substantially expand the target space. A plethora of proximity-based modalities to target proteins via diverse approaches have recently emerged, opening opportunities for biopharmaceutical innovation. This Outlook outlines the diverse mechanisms and molecules based on induced proximity, including protein degraders, blockers, and stabilizers, inducers of protein post-translational modifications, and agents for cell therapy, and discusses opportunities and challenges that the field must address to mature and unlock translation in biology and medicine.

Acetylation Targeting Chimera Enables Acetylation of the Tumor Suppressor p53

With advances in chemically induced proximity technologies, heterobifunctional modalities such as proteolysis targeting chimeras (PROTACs) have been successfully advanced to clinics for treating cancer. However, pharmacologic activation of tumor-suppressor proteins for cancer treatment remains a major challenge. Here, we present a novel Acetylation Targeting Chimera (AceTAC) strategy to acetylate the p53 tumor suppressor protein. We discovered and characterized the first p53Y220C AceTAC, MS78, which recruits histone acetyltransferase p300/CBP to acetylate the p53Y220C mutant. MS78 effectively acetylated p53Y220C lysine 382 (K382) in a concentration-, time-, and p300-dependent manner and suppressed proliferation and clonogenicity of cancer cells harboring the p53Y220C mutation with little toxicity in cancer cells with wild-type p53. RNA-seq studies revealed novel p53Y220C-dependent upregulation of TRAIL apoptotic genes and downregulation of DNA damage response pathways upon acetylation induced by MS78. Altogether, the AceTAC strategy could provide a generalizable platform for targeting proteins, such as tumor suppressors, via acetylation.

Targeted protein posttranslational modifications by chemically induced proximity for cancer therapy

Post-translational modifications (PTMs) regulate all aspects of protein function. Therefore, upstream regulators of PTMs, such as kinases, acetyltransferases, or methyltransferases, are potential therapeutic targets for human diseases, including cancer. To date, multiple inhibitors and/or agonists of these PTM upstream regulators are in clinical use, while others are still in development. However, these upstream regulators control not only the PTMs of disease-related target proteins but also other disease-irrelevant substrate proteins. Thus, nontargeted perturbing activities may introduce unwanted off-target toxicity issues that limit the use of these drugs in successful clinical applications. Therefore, alternative drugs that solely regulate a specific PTM of the disease-relevant protein target may provide a more precise effect in treating disease with relatively low side effects. To this end, chemically induced proximity has recently emerged as a powerful research tool, and several chemical inducers of proximity (CIPs) have been used to target and regulate protein ubiquitination, phosphorylation, acetylation, and glycosylation. These CIPs have a high potential to be translated into clinical drugs and several examples such as PROTACs and MGDs are now in clinical trials. Hence, more CIPs need to be developed to cover all types of PTMs, such as methylation and palmitoylation, thus providing a full spectrum of tools to regulate protein PTM in basic research and also in clinical application for effective cancer treatment.

Targeted Biomolecule Regulation Platform: A Split-and-Mix PROTAC Approach

The development of bifunction al molecules, which can enable targeted RNA degradation, targeted protein acetylation, or targeted protein degradation, remains a time-consuming process that requires tedious optimization. We propose a split-and-mix nanoplatform that serves as a self-adjustable platform capable of facile screening, programmable ligand ratios, self-optimized biomolecule spatial recognition, and multifunctional applications. Herein, we demonstrate the potential of our proposed nanoplatform by showcasing proteolysis-targeting chimeras (PROTACs), namely, split-and-mix PROTAC (SM-PROTAC). We highlight the scope of our platform through the targeted disruption of intracellular therapeutic targets involving ERα, CDK4/6, AR, MEK1/2, BRD2/4, BCR-ABL, etc. These studies confirm the effectiveness and universality of the SM-PROTAC platform for proximity-induced applications. This platform is programmable, with significant potential applications to biomolecule regulation, including the fields of epigenetics, gene editing, and biomolecule modification regulation.

Strategies for Conditional Regulation of Proteins

Design of the next-generation of therapeutics, biosensors, and molecular tools for basic research requires that we bring protein activity under control. Each protein has unique properties, and therefore, it is critical to tailor the current techniques to develop new regulatory methods and regulate new proteins of interest (POIs). This perspective gives an overview of the widely used stimuli and synthetic and natural methods for conditional regulation of proteins.

A heterobifunctional molecule system for targeted protein acetylation in cells

Protein acetylation is a vital biological process that regulates myriad cellular events. Despite its profound effects on protein function, there are limited research tools to dynamically and selectively regulate protein acetylation. To address this, we developed an acetylation tagging system, called AceTAG, to target proteins for chemically induced acetylation directly in live cells. AceTAG uses heterobifunctional molecules composed of a ligand for the lysine acetyltransferase p300/CBP and a FKBP12^F36V ligand. Target proteins are genetically tagged with FKBP12^F36V and brought in proximity with p300/CBP by AceTAG molecules to subsequently undergo protein-specific acetylation. Targeted acetylation of proteins in cells using AceTAG is selective, rapid, and can be modulated in a dose-dependent fashion, enabling controlled investigations of acetylated protein targets directly in cells. In this protocol, we focus on (1) generation of AceTAG constructs and cell lines, (2) in vitro characterization of AceTAG mediated ternary complex formation and cellular target engagement studies; and (3) in situ characterization of AceTAG induced acetylation of targeted proteins by immunoblotting and quantitative proteomics. The robust procedures described herein should enable the use of AceTAG to explore the roles of acetylation for a variety of protein targets.

Advancing Targeted Protein Degradation via Multiomics Profiling and Artificial Intelligence

Only around 20% of the human proteome is considered to be druggable with small-molecule antagonists. This leaves some of the most compelling therapeutic targets outside the reach of ligand discovery. The concept of targeted protein degradation (TPD) promises to overcome some of these limitations. In brief, TPD is dependent on small molecules that induce the proximity between a protein of interest (POI) and an E3 ubiquitin ligase, causing ubiquitination and degradation of the POI. In this perspective, we want to reflect on current challenges in the field, and discuss how advances in multiomics profiling, artificial intelligence, and machine learning (AI/ML) will be vital in overcoming them. The presented roadmap is discussed in the context of small-molecule degraders but is equally applicable for other emerging proximity-inducing modalities.

Emerging TACnology: Heterobifunctional Small Molecule Inducers of Targeted Posttranslational Protein Modifications

Posttranslational modifications (PTMs) play an important role in cell signaling and they are often deregulated in disease. This review addresses recent advances in the development of heterobifunctional small molecules that enable targeting or hijacking PTMs. This emerging field is spearheaded by proteolysis-targeting chimeras (PROTACs), that induce ubiquitination of their targets and, thus, tag them for degradation by the proteasome. Within the last decade, several improvements have been made to enhance spatiotemporal control of PROTAC-induced degradation as well as cell permeability. Inspired by the success story of PROTACs, additional concepts based on chimeric small molecules have emerged such as phosphatase-recruiting chimeras (PhoRCs). Herein, an overview of strategies causing (de-)phosphorylation, deubiquitination as well as acetylation is provided, and the opportunities and challenges of heterobifunctional molecules for drug discovery are highlighted. Although significant progress has been achieved, a plethora of PTMs have not yet been covered and PTM-inducing chimeras will be helpful tools for chemical biology and could even find application in pharmacotherapy.

ProxyBind: a Compendium of Binding Sites for Proximity-Induced Pharmacology

Proximity-induced pharmacology (ProxPharm) is a novel paradigm in drug discovery where a small molecule brings two proteins in close proximity to elicit a signal, generally from one protein onto another. The potential of ProxPharm compounds as a new therapeutic modality is firmly established by proteolysis targeting chimeras (PROTACs) that bring an E3 ubiquitin ligase in proximity to a target protein to induce ubiquitination and subsequent degradation of the target. The concept can be expanded to induce other post-translational modifications via the recruitment of different types of protein-modifying enzymes. To survey the human proteome for opportunities in proximity pharmacology, we systematically mapped non-catalytic drug binding pockets on the structure of protein-modifying enzymes available from the Protein Databank. In addition to binding sites exploited by previously reported ProxPharm compounds, we identified putative ligandable non-catalytic pockets in 236 kinases, 45 phosphatases, 37 deubiquitinases, 14 methyltransferases, 11 acetyltransferases, 13 glycosyltransferases, 4 deacetylases, 7 demethylases and 2 glycosidases, including cavities occupied by chemical matter that may serve as starting points for future ProxPharm compounds. This systematic survey confirms that proximity pharmacology is a versatile modality with largely unexplored and promising potential and reveals novel opportunities to pharmacologically rewire molecular circuitries. All data is available from the ProxyBind database at https://polymorph.sgc.utoronto.ca/proxybind/index.php.

Methods to characterize and discover molecular degraders in cells

Cells use many post-translational modifications (PTMs) to tailor proteins and transduce cellular signals. Recent years have witnessed the rapid growth of small molecule and enzymatic strategies to purposely manipulate one particular PTM, ubiquitination, on desired target proteins in cells. These approaches typically act by induced proximity between an E3 ligase and a target protein resulting in ubiquitination and degradation of the substrate in cells. In this review, we cover recent approaches to study molecular degraders and discover their induced substrates in vitro and in live cells. Methods that have been adapted and applied to the development of molecular degraders are described, including global proteomics, affinity-purification, chemical proteomics and enzymatic strategies. Extension of these strategies to edit additional PTMs in cells is also discussed. This review is intended to assist researchers who are interested in editing PTMs with new modalities to select suitable method(s) and guide their studies.

Protein–Protein Interaction Prediction for Targeted Protein Degradation

Protein–protein interactions (PPIs) play a fundamental role in various biological functions; thus, detecting PPI sites is essential for understanding diseases and developing new drugs. PPI prediction is of particular relevance for the development of drugs employing targeted protein degradation, as their efficacy relies on the formation of a stable ternary complex involving two proteins. However, experimental methods to detect PPI sites are both costly and time-intensive. In recent years, machine learning-based methods have been developed as screening tools. While they are computationally more efficient than traditional docking methods and thus allow rapid execution, these tools have so far primarily been based on sequence information, and they are therefore limited in their ability to address spatial requirements. In addition, they have to date not been applied to targeted protein degradation. Here, we present a new deep learning architecture based on the concept of graph representation learning that can predict interaction sites and interactions of proteins based on their surface representations. We demonstrate that our model reaches state-of-the-art performance using AUROC scores on the established MaSIF dataset. We furthermore introduce a new dataset with more diverse protein interactions and show that our model generalizes well to this new data. These generalization capabilities allow our model to predict the PPIs relevant for targeted protein degradation, which we show by demonstrating the high accuracy of our model for PPI prediction on the available ternary complex data. Our results suggest that PPI prediction models can be a valuable tool for screening protein pairs while developing new drugs for targeted protein degradation.

Beyond Proteolysis-Targeting Chimeric Molecules: Designing Heterobifunctional Molecules Based on Functional Effectors

In recent years, with the successful development of proteolysis-targeting chimeric molecules (PROTACs), the potential of heterobifunctional molecules to contribute to reenvisioning drug design, especially small-molecule drugs, has been increasingly recognized. Inspired by PROTACs, diverse heterobifunctional molecules have been reported to simultaneously bind two or more molecules and bring them into proximity to interaction, such as ribonuclease-recruiting, autophagy-recruiting, lysosome-recruiting, kinase-recruiting, phosphatase-recruiting, glycosyltransferase-recruiting, and acetyltransferase-recruiting chimeras. On the basis of the heterobifunctional principle, more opportunities for advancing drug design by linking potential effectors to a protein of interest (POI) have emerged. Herein, we introduce heterobifunctional molecules other than PROTACs, summarize the limitations of existing molecules, list the main challenges, and propose perspectives for future research directions, providing insight into alternative design strategies based on substrate-proximity-based targeting.

Chasing molecular glue degraders: screening approaches

Protein-protein interactions (PPIs) govern all biological processes. Some small molecules modulate PPIs through induced protein proximity. In particular, molecular glue degraders are monovalent compounds that orchestrate interactions between a target protein and an E3 ubiquitin ligase, prompting the proteasomal degradation of the former. This and other pharmacological strategies of targeted protein degradation (e.g. proteolysis-targeting chimeras - PROTACs) overcome some limitations of traditional occupancy-based therapeutics. Here, we provide an overview of the "molecular glue" concept, with a special focus on natural and synthetic inducers of proximity to E3s. We then briefly highlight the serendipitous discoveries of some clinical and preclinical molecular glue degraders, and discuss the first examples of intentional discoveries. Specifically, we outline the different screening strategies reported in this rapidly evolving arena and our thoughts on future perspectives. By mastering the ability to influence PPIs, molecular glue degraders can induce the degradation of unligandable proteins, thus providing an exciting path forward to broaden the targetable proteome.

Kinase-targeting small-molecule inhibitors and emerging bifunctional molecules

Kinases are among the most successful drug targets. To date, 72 small-molecule kinase inhibitors (SMKIs) have been approved by the US FDA, together with ~500 SMKIs in clinical trials. Although the topic has been heavily reviewed in recent years, an overview that focused on the currently approved SMKIs in combination with the emerging kinase-targeting bifunctional molecules is absent. Herein, we first provide an updated overview of the approved SMKIs, with an emphasis on their binding modes, classified in groups of type I and II ATP-competitive inhibitors, type III and IV allosteric inhibitors, and covalent inhibitors. We then highlight the novel chemical modalities in kinase targeting by using different types of proximity-inducing bifunctional molecules for kinase degradation and modifications.

Chemical Biology Tools for Protein Lysine Acylation

Lysine acylation plays pivotal roles in cell physiology, including DNA transcription and repair, signal transduction, immune defense, metabolism, and many other key cellular processes. Molecular mechanisms of dysregulated lysine acylation are closely involved in the pathophysiological progress of many human diseases, most notably cancers. In recent years, chemical biology tools have become instrumental in studying the function of post-translational modifications (PTMs), identifying new "writers", "erasers" and "readers", and in targeted therapies. Here, we describe key developments in chemical biology approaches that have advanced the study of lysine acylation and its regulatory proteins (2016-2021). We further discuss the discovery of ligands (inhibitors and PROTACs) that are capable of targeting regulators of lysine acylation. Next, we discuss some current challenges of these chemical biology probes and suggest how chemists and biologists can utilize chemical probes with more discriminating capacity. Finally, we suggest some critical considerations in future studies of PTMs from our perspective.

Molecular probes for cellular imaging of post-translational proteoforms

Specific post-translational modification (PTM) states of a protein affect its property and function; understanding their dynamics in cells would provide deep insight into diverse signaling pathways and biological processes. However, it is not trivial to visualize post-translational modifications in a protein- and site-specific manner, especially in a living-cell context. Herein, we review recent advances in the development of molecular imaging tools to detect diverse classes of post-translational proteoforms in individual cells, and their applications in studying precise roles of PTMs in regulating the function of cellular proteins.