Cryo-EM structure of the human COP1-DET1 ubiquitin ligase complex

Ubiquitin modification regulates fundamental cellular processes by controlling protein stability and function. The ubiquitin ligase COP1, conserved from plants to humans, targets developmental transcription factors. COP1 can act on its own or be incorporated into CULLIN4–RING ligase (CRL4) complexes via the adaptor DET1. Despite its importance, the architecture and mechanisms of COP1- and DET1-containing assemblies remain incompletely defined. Here, we present cryo-EM structures of human COP1 bound to DDB1–DDA1–DET1 (DDD) and Ube2e2, capturing an inactive, stacked assembly. Co-expression with COP1 substrates such as c-Jun or ETS2 disrupts this arrangement and drives a conformational transition into a distinct dimeric state that permits substrate access. Structural modeling delineates the spatial organization of the COP1 WD40 domains responsible for substrate recruitment. DET1 functions as a scaffold, linking COP1 and Ube2e2 to initiate ubiquitin transfer to substrates, while DDB1 recruits the CULLIN4–RBX1 complex to enable Ube2d3-mediated ubiquitin chain elongation. Together, these findings reveal dynamic switching between structural states of the CRL4DET1–COP1 E3 ligase and a substrate-triggered activation mechanism, providing a framework for understanding substrate-specific regulation and E3 ligase assembly dynamics.