CSN- and CAND1-dependent remodelling of the budding yeast SCF complex

Cullin–RING ligases (CRLs) are ubiquitin E3 enzymes with variable substrate-adaptor and -receptor subunits. All CRLs are activated by modification of the cullin subunit with the ubiquitin-like protein Nedd8 (neddylation). The protein CAND1 (Cullin-associated-Nedd8-dissociated-1) also promotes CRL activity, even though it only interacts with inactive ligase complexes. The molecular mechanism underlying this behaviour remains largely unclear. Here, we find that yeast SCF (Skp1–Cdc53–F-box) Cullin–RING complexes are remodelled in a CAND1-dependent manner, when cells are switched from growth in fermentable to non-fermentable carbon sources. Mechanistically, CAND1 promotes substrate adaptor release following SCF deneddylation by the COP9 signalosome (CSN). CSN- or CAND1-mutant cells fail to release substrate adaptors. This delays the formation of new complexes during SCF reactivation and results in substrate degradation defects. Our results shed light on how CAND1 regulates CRL activity and demonstrate that the cullin neddylation–deneddylation cycle is not only required to activate CRLs, but also to regulate substrate specificity through dynamic substrate adaptor exchange. CAND1 promotes the activity of Cullin–RING ubiquitin ligases, but binds exclusively to inactive unneddylated forms of the enzyme. By identifying a simple means to reversibly activate this complex in budding yeast, Zemla et al . resolve this paradox and show that CAND1 acts as an exchange factor for substrate adaptors.