Nature Chemical Biology, 2019, 15, 737–746
Ligand-dependent protein degradation has emerged as a compelling strategy to pharmacologically control the protein content of cells. So far, however, only a limited number of E3 ligases have been found to support this process. Here, we use a chemical proteomic strategy that leverages broadly reactive, cysteine-directed electrophilic fragments coupled to selective ligands for intracellular proteins (for example, SLF for FKBP12, JQ1 for BRD4) to screen for heterobifunctional degrader compounds (or proteolysis targeting chimeras, PROTACs) that operate by covalent adduction of E3 ligases. This approach identified DCAF16—a poorly characterized substrate recognition component of CUL4-DDB1 E3 ubiquitin ligases—as a target of electrophilic PROTACs that promote the nuclear-restricted degradation of proteins. We find that only a modest fraction (~10–40%) of DCAF16 needs to be modified to support protein degradation, pointing to the potential for electrophilic PROTACs to induce neosubstrate degradation without substantially perturbing the function of the participating E3 ligase.
Linking of fragments in neighboring binding sites is one of the optimization strategies in fragment-based drug discovery, where additive or even more substantial bioactivity improvements can be realized. However, such efforts present a considerable challenge when one fragment binds covalently to the target protein, as small modifications can influence the correct positioning of the covalent warhead toward the targeted nucleophilic residue. Here, we present a case study of fragment linking that yielded single-digit micromolar, covalent inhibitors of the SARS-CoV-2 main protease, starting from fragments that were inactive in the biochemical assay. Using structural information from a recent, high-throughput crystallographic fragment screen, we show that the success of fragment linking in the design of targeted covalent inhibitors is heavily impacted by several factors, including the warhead type, the labeling chemistry, and even subtle changes in the designed linker. Notably, we observe that induced fit effects might override the original fragment orientations in the linked molecule, highlighting the need for reliable structure verification, especially in consecutive rounds of fragment elaboration.
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DOI Ansgar Oberheide, Maxime van den Oetelaar, Jakob Scheele, Jan Borggräfe, Semmy Engelen, Michael Sattler, Christian Ottmann, ...
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