A ternary intermediate of CAND1-SCF (I) reveals a three-way interface between the tip of CAND1pincer, CUL1CR1, and SKP1. CUL1 can simultaneously accommodate CAND1pincerand SKP1 because SKP1clash-loopis disordered, and SKP1helix2is partially displaced to avoid a direct clash. The co-E3 DCNL1 preferentially associates with CAND1-bound CUL1/RBX1 but disordered SR (II). DCNL1 binding, in turn, further induces conformational changes in CAND1 and CUL1NTD, which hinder SR binding and ultimately result in a pool of stable, inactive DCNL1-occupied CAND1-CUL1/RBX1 complexes (III). Future work is needed to fully understand the conformational transitions between states (I) and (II). Although RBX1RINGis in an orientation incompatible with CUL1 neddylation in (III), a plausible albeit speculative sequence of events that can lead to an activated SCF (IV) is proposed. Alternatively, the transient CAND1-SCF complex (I) can decay into a partially dissociated CAND1 intermediate (V) with CAND1C-terminal-armdetached from CUL1NTD, whereas the SR-substrate complex remains stably bound. RBX1RINGin this state features significant conformational freedom, greatly increasing the likelihood of neddylation and thus leading to CAND1 dissociation and SCF activation (IV).
