Structural complementarity offsets low receptor affinity in E7820-mediated RBM39 degradation by CRL4DCAF15

The investigational drugs E7820, indisulam and tasisulam (aryl-sulfonamides) promote the degradation of the splicing factor RBM39 in a proteasome and CRL4DCAF15 ubiquitin ligase-dependent mechanism, however the molecular details of this activity remain elusive. Here we present the cryo-EM structure of DDB1-DCAF15-DDA1 bound to RBM39 and E7820 at 4.4 Å resolution, together with crystal structures of engineered subcomplexes. We show that DCAF15 adopts a novel fold stabilized by DDA1, and that extensive protein-protein contacts between the ligase and substrate mitigate the low affinity interaction between aryl-sulfonamides and DCAF15. Our data demonstrates how aryl-sulfonamides neo-functionalize a shallow, non-conserved pocket on DCAF15 to selectively bind and degrade RBM39 and RBM23 without the requirement for a high affinity ligand, which has broad implications for the de novo discovery of molecular glue degraders.