Structural and mechanistic insights into the CAND1-mediated SCF substrate receptor exchange

Modular SCF (SKP1-CUL1-Fbox) ubiquitin E3 ligases orchestrate multiple cellular pathways in eukaryotes. Their variable SKP1-Fbox substrate receptor (SR) modules enable regulated substrate recruitment and subsequent proteasomal degradation. CAND proteins are essential for the efficient and timely exchange of SRs. To gain structural understanding of the underlying molecular mechanism, we reconstituted a human CAND1-driven exchange reaction of substrate-bound SCF alongside its co-E3 ligase DCNL1 and visualized it by cryo-EM. We describe high-resolution structural intermediates, including a ternary CAND1-SCF complex, as well as conformational and compositional intermediates representing SR- or CAND1-dissociation. We describe in molecular detail how CAND1-induced conformational changes in CUL1/RBX1 provide an optimized DCNL1-binding site and reveal an unexpected dual role for DCNL1 in CAND1-SCF dynamics. Moreover, a partially dissociated CAND1-SCF conformation accommodates cullin neddylation, leading to CAND1 displacement. Our structural findings, together with functional biochemical assays, help formulate a detailed model for CAND-SCF regulation. [Display omitted] •Structure of a transient CAND1-CUL1/RBX1 and substrate-bound receptor complex•DCNL1-stabilized CAND1-CUL1/RBX1 complex is recalcitrant to SR association•Structural snapshots of CAND1 dissociation, induced by substrate receptor•Partial dissociation of CAND1 enables CUL1 neddylation and its full ejection Cullin-RING ligases, through their modular architecture, enable the timely regulation of diverse cellular signaling processes. Shaaban et al. reveal how CAND1, aided by DCNL1, facilitates this process by coordinating the assembly of cullin-RING ligase complexes.