Targeted protein degradation is a novel pharmacology established by drugs that recruit target proteins to E3 ubiquitin ligases. Based on the structure of the degrader and the target, different E3 interfaces are critically involved, thus forming defined "functional hotspots". Understanding disruptive mutations in functional hotspots informs on the architecture of the assembly, and highlights residues susceptible to acquire resistance phenotypes. Here, we employ haploid genetics to show that hotspot mutations cluster in substrate receptors of hijacked ligases, where mutation type and frequency correlate with gene essentiality. Intersection with deep mutational scanning revealed hotspots that are conserved or specific for chemically distinct degraders and targets. Biophysical and structural validation suggests that hotspot mutations frequently converge on altered ternary complex assembly. Moreover, we validated hotspots mutated in patients that relapse from degrader treatment. In sum, we present a fast and widely accessible methodology to characterize small-molecule degraders and associated resistance mechanisms.

Targeted protein degradation (TPD) is a new pharmacology based on small-molecule degraders that induce proximity between a protein of interest (POI) and an E3 ubiquitin ligase. Of the approximately 600 E3s encoded in the human genome, only around 2% can be co-opted with degraders. This underrepresentation is caused by a paucity of discovery approaches to identify degraders for defined E3s. This hampers a rational expansion of the druggable proteome and stymies critical advancements in the field, such as tissue- and cell-specific degradation. Here, we focus on dynamic NEDD8 conjugation, a post-translational, regulatory circuit that controls the activity of 250 cullin RING E3 ligases (CRLs). Leveraging this regulatory layer enabled us to develop a scalable assay to identify compounds that alter the interactome of an E3 of interest by tracing their abundance after pharmacologically induced auto-degradation. Initial validation studies are performed for CRBN and VHL, but proteomics studies indicate broad applicability for many CRLs. Among amenable ligases, we select CRLDCAF15 for a proof-of-concept screen, leading to the identification of a novel DCAF15-dependent molecular glue degrader inducing the degradation of RBM23 and RBM39. Together, this strategy empowers the scalable identification of degraders specific to a ligase of interest.

Targeted protein degradation is a new pharmacologic paradigm established by drugs that recruit target proteins to E3 ubiquitin ligases via a ternary ligase-degrader-target complex. Based on the structure of the degrader and the neosubstrate, different E3 ligase interfaces are critically involved in this process, thus forming defined functional hotspots. Understanding disruptive mutations in functional hotspots informs on the architecture of the underlying assembly, and highlights residues prone to cause drug resistance. Until now, their identification was driven by structural methods with limited scalability. Here, we employ haploid genetics to show that hotspot mutations cluster in the substrate receptors of the hijacked ligases and find that type and frequency of mutations are shaped by the essentiality of the harnessed ligase. Intersection with deep mutational scanning data revealed hotspots that are either conserved, or specific for chemically distinct degraders or recruited neosubstrates. Biophysical and structural validation suggest that hotspot mutations frequently converge on altered ternary complex assembly. Moreover, we identified and validated hotspots mutated in patients that relapse from degrader treatment. In sum, we present a fast and experimentally widely accessible methodology that empowers the characterization of small-molecule degraders and informs on associated resistance mechanisms.