Updated project metadata. PROTACs induce the degradation of target proteins by hijacking an E3 ligase. However, it is usually difficult to directly identify the E3 ligase recruited by a new E3 ligase ligand in a PROTAC using the co-immunoprecipitation method because the formation of the complex is transient and very dynamic. TurboID assay is a powerful proximity labeling method that can sensitively detect weak and transient protein-protein interactions by biotinylating proteins that interact with a bait protein fused with an engineered biotin ligase. Here, we adapted TurboID technology to identify and characterize the specific E3 ligase(s) recruited by 955, a potent piperlongumine (PL)-SNS-032 conjugated CDK9 PROTAC, to mediate CDK9 degradation.