Search

In addition to the therapeutic applicability of targeted protein degradation (TPD), the modality also harbors unique properties that enable the development of innovative chemical biology tools to interrogate complex biology. TPD offers an all-chemical strategy capable of the potent, durable, selective, reversible, and time-resolved control of the levels of a given target protein in both in vitro and in vivo contexts. These properties are particularly well-suited for enabling the precise perturbation of a given gene to understand its biology, identify dependencies/vulnerabilities in disease contexts, and as a strategy to control gene therapies. To leverage these elegant properties, we developed the AchillesTag (aTAG) degradation system to serve as a tool in target identification and validation efforts. The aTAG degradation system provides a novel degradation tag based on the MTH1 protein paired with three fully validated bifunctional degraders with both in vitro and in vivo applicability. We catalog the development of the aTAG system from selection and validation of the novel MTH1 aTAG, alongside a comprehensive SAR campaign to identify high performing tool degraders. To demonstrate the utility of the aTAG system to dissect a complex biological system, we apply the technology to the control of Chimeric Antigen Receptor (CAR) activity. Using aTAG, we demonstrate the ability to potently and selectively control CAR protein levels, resulting in the exquisite rheostat control of CAR mediated T-cell activity. Furthermore, we showcase the in vivo application of the system via degradation of the aTAG-fused CAR protein in a human xenograft model. The aTAG degradation system provides a complete chemical biology tool to aid foundational target validation efforts that inspire drug discovery campaigns towards therapeutic applicability.

Protein kinases are major drug targets, but the development of highly-selective inhibitors has been challenging due to the similarity of their active sites. The observation of distinct structural states of the fully-conserved Asp-Phe-Gly (DFG) loop has put the concept of conformational selection for the DFG-state at the center of kinase drug discovery. Recently, it was shown that Gleevec selectivity for the Tyr-kinase Abl was instead rooted in conformational changes after drug binding. Here, we investigate whether protein dynamics after binding is a more general paradigm for drug selectivity by characterizing the binding of several approved drugs to the Ser/Thr-kinase Aurora A. Using a combination of biophysical techniques, we propose a universal drug-binding mechanism, that rationalizes selectivity, affinity and long on-target residence time for kinase inhibitors. These new concepts, where protein dynamics in the drug-bound state plays the crucial role, can be applied to inhibitor design of targets outside the kinome., eLife digest Protein kinases are a family of enzymes found in all living organisms. These enzymes help to control many biological processes, including cell division. When particular protein kinases do not work correctly, cells may start to divide uncontrollably, which can lead to cancer. One example is the kinase Aurora A, which is over-active in many common human cancers. As a result, researchers are currently trying to design drugs that reduce the activity of Aurora A in the hope that these could form new anticancer treatments. In general, drugs are designed to be as specific in their action as possible to reduce the risk of harmful side effects to the patient. Designing a drug that affects a single protein kinase, however, is difficult because there are hundreds of different kinases in the body, all with similar structures. Because drugs often work by binding to specific structural features, a drug that targets one protein kinase can often alter the activity of a large number of others too. Gleevec is a successful anti-leukemia drug that specifically works on one target kinase, producing minimal side effects. It was recently discovered that the drug works through a phenomenon called ‘induced fit’. This means that after the drug binds it causes a change in the enzyme’s overall shape that alters the activity of the enzyme. The shape change is complex, and so even small structural differences can change the effect of a particular drug. Do other drugs that target other protein kinases also produce induced fit effects? To find out, Pitsawong, Buosi, Otten, Agafonov et al. studied how three anti-cancer drugs interact with Aurora A: two drugs specifically designed to switch off Aurora A, and Gleevec (which does not target Aurora A). The two drugs that specifically target Aurora A were thought to work by targeting one structural feature of the enzyme. However, the biochemical and biophysical experiments performed by Pitsawong et al. revealed that these drugs instead work through an induced fit effect. By contrast, Gleevec did not trigger an induced fit on Aurora A and so bound less tightly to it. In light of these results, Pitsawong et al. suggest that future efforts to design drugs that target protein kinases should focus on exploiting the induced fit process. This will require more research into the structure of particular kinases.

Protein kinases are major drug targets, but the development of highly-selective inhibitors has been challenging due to the similarity of their active sites. The observation of distinct structural states of the fully-conserved Asp-Phe-Gly (DFG) loop has put the concept of conformational selection for the DFG-state at the center of kinase drug discovery. Recently, it was shown that Gleevec selectivity for the Tyr-kinases Abl was instead rooted in conformational changes after drug binding. Here, we investigate whether protein dynamics after binding is a more general paradigm for drug selectivity by characterizing the binding of several approved drugs to the Ser/Thr-kinase Aurora A. Using a combination of biophysical techniques, we propose a universal drug-binding mechanism, that rationalizes selectivity, affinity and long on-target residence time for kinase inhibitors. These new concepts, where protein dynamics in the drug-bound state plays the crucial role, can be applied to inhibitor design of targets outside the kinome.eLife digestThe Ser/Thr kinase Aurora A is an important target for the development of new anticancer therapies. A longstanding question is how to specifically and effectively inhibit only this kinase in a background of over 550 protein kinases with very similar structures. To this end, understanding the inhibition mechanism of Aurora A by different drugs is essential. Here, we characterize the kinetic mechanism of three distinct kinase drugs, Gleevec (Imatinib), Danusertib (PHA739358) and AT9283 (Pyrazol-4-yl Urea) for Aurora A. We show that inhibitor affinities do not rely exclusively on the recognition of a specific conformation of the Asp-Phe-Gly loop of the kinase. Our quantitative kinetics data put forward an opposing mechanism in which a slow conformational change after drug binding (i.e., induced-fit step) dictates drug affinity.

We have engineered a mutant of Dictyostelium discoideum ( Dicty ) myosin II that contains the same fast-reacting “SH1” thiol as in muscle myosin, spin-labeled it, and performed electron paramagnetic resonance (EPR) to compare the structure of the force-generating region of the two myosins. Dicty myosin serves as a model system for muscle myosin because of greater ease of mutagenesis, expression, and crystallization. The catalytic domains of these myosins have nearly identical crystal structures in the apo state, but there are significant differences in ATPase kinetics, and there are no crystal structures of skeletal muscle myosin with bound nucleotides, so another structural technique is needed. Previous EPR studies, with a spin label attached to SH1 in muscle myosin, have resolved the key structural states of this region. Therefore, we have performed identical experiments on both myosins spin-labeled at equivalent sites. Spectra were identical for the two myosins in the apo and ADP-bound states. With bound ADP and phosphate analogs, ( i ) both proteins exhibit two resolved structural states (prepowerstroke, postpowerstroke) in a single biochemical state (defined by the bound nucleotide), and ( ii ) these structural states are essentially identical in the two myosins but ( iii ) are occupied to different extents as a function of the biochemical state. We conclude that ( i ) myosin structural and biochemical states do not have a one-to-one correspondence, and ( ii ) Dicty myosin can serve as a good analog for structural studies of muscle myosin only if differences in the coupling between biochemical and structural states are taken into account.

SummaryMolecular recognition plays a central role in biology, and protein dynamics has been acknowledged to be important in this process. However, it is highly debated whether conformational changes happen before ligand binding to produce a binding-competent state (conformational selection) or are caused in response to ligand binding (induced fit). Proposals for both mechanisms in protein/protein recognition have been primarily based on structural arguments. However, the distinction between them is a question of the probabilities of going via these two opposing pathways. Here, we present a direct demonstration of exclusive conformational selection in protein/protein recognition by measuring the flux for rhodopsin kinase binding to its regulator recoverin, an important molecular recognition in the vision system. Using nuclear magnetic resonance (NMR) spectroscopy, stopped-flow kinetics, and isothermal titration calorimetry, we show that recoverin populates a minor conformation in solution that exposes a hydrophobic binding pocket responsible for binding rhodopsin kinase. Protein dynamics in free recoverin limits the overall rate of binding.

Catalytic function of many enzymes is comprised of a number of microscopic steps including enzyme structural rearrangements and the chemical steps, in which bonds of the substrate(s) are broken/made to synthesize the product(s). Separation of these processes is a challenging task that hinders our understanding of enzyme catalysis. We have used nuclear magnetic resonance (NMR) and rapid-quenching techniques to independently study these processes in adenylate kinase (ADK) and to characterize their energetic contribution at atomic resolution. Adenylate kinase is an important enzyme that catalyzes a reversible reaction: 2ADP ↔ ATP +AMP, and is involved in maintaining energy homeostasis in a cell. Two binding sites of ADK and three different ligands result in a variety of biochemical states (determined by the bound ligands). Each of these states has different dynamic properties making the overall structural dynamics of ADK complex. We have tackled this problem by engineering a loss-of-function ADK mutant (with greatly reduced rate of turnover), which allowed us to “trap” the enzyme in well-defined biochemical states. Our results lead to a detailed energy profile of ADK, providing insights into the molecular mechanism of its functioning. This work reveals fundamental principles of enzyme catalysis and highlights the role of protein's intrinsic dynamics.

Cancer is a serious illness and the second leading cause of death in the US. It is caused by misregulation of the signaling cascades and uncontrolled cell proliferation. Protein kinases are the key players in the regulatory machinery, and in the last 20 years they have become an increasingly important drug targets. The main requirement for such drugs is selectivity: ideally, only one kinase should be affected by an individual drug. The prominent success story is an FDA-approved chronic myeloid leukemia therapeutic Gleevec. Gleevec is a highly specific inhibitor of the Abl kinase and has very little potency towards a closely related Src subfamily of kinases. This specificity is particularly intriguing considering that the drug-binding pockets look nearly identical in these kinases. We combined NMR and pre-steady-state kinetic experiments to monitor Gleevec binding in real time and with residue-specific precision. Our data establish a novel model that fully and quantitatively explains the observed 3000-fold difference in Gleevec's affinities to Abl and Src, solving a long-standing paradox in the field. These results reveal the general principles of kinase-drug interactions and highlight the molecular mechanisms behind nanomolar affinities found in clinically relevant therapeutics.