An Activity-Based Oxaziridine Platform for Identifying and Developing Covalent Ligands for Functional Allosteric Methionine Sites: Redox-Dependent Inhibition of Cyclin-Dependent Kinase 4

Activity-based protein profiling (ABPP) is a versatile strategy for enabling identification and characterization of new functional protein sites and discovery of lead compounds for therapeutic development. Yet, the vast majority of ABPP methods applied for covalent drug discovery target highly nucleophilic amino acids such as cysteine or lysine. Here, we report a methionine-directed ABPP platform using Redox-Activated Chemical Tagging (ReACT), which leverages a biomimetic oxidative ligation strategy for selective methionine modification. Application of ReACT to the cancer-driver protein cyclin-dependent kinase 4 (CDK4) as a representative high-value drug target identified three new hyperreactive, ligandable methionine residues, including an allosteric M169 site that is proximal to an activating T172 phosphorylation site. With this information in hand, we designed and synthesized a new methionine-targeting covalent ligand library based on oxaziridine fragments bearing a diverse array of heterocyclic, heteroatom, and stereochemically-rich substituents. ABPP screening of this focused library against a clickable broad-spectrum ReACT probe identified 1oxF11 as a covalent modifier of the CDK4/Cyclin-D1 heterodimer at the M169 site. This compound inhibited CDK4 kinase activity in a dose-dependent manner on purified protein and in live cells. Further biochemical analyses with a phospho-specific CDK4 antibody revealed crosstalk between M169 oxidation and T172 phosphorylation upon 1oxF11 treatment, where M169 oxidation prevented phosphorylation of the activating T172 site on CDK4 and blocked cell cycle progression at the S-phase checkpoint. By identifying a new mechanism for allosteric methionine redox regulation on CDK4 and developing a unique modality for its therapeutic intervention, this work showcases a generalizable platform that provides a starting point for engaging in broader chemoproteomics and protein ligand discovery efforts to find and target previously undruggable methionine sites.