Substrate engagement by F-box proteins promotes NEDD8 modification of cullins, which is necessary for the activation of cullin-RING E3 ubiquitin ligases (CRLs). However, the mechanism by which substrate recruitment triggers cullin neddylation remains unclear. Here, we identify DCNL1 (defective in cullin neddylation 1-like 1) as a component of CRL2 called ECV (elongins BC/CUL2/VHL) and show that molecular suppression of DCNL1 attenuates CUL2 neddylation. DCNL1 via its DAD patch binds to CUL2 but is also able to bind VHL independent of CUL2 and the DAD patch. The engagement of the substrate hypoxia-inducible factor 1␣ (HIF1␣) to the substrate receptor VHL increases DCNL1 binding to VHL as well as to CUL2. Notably, an engineered mutant form of HIF1␣ that associates with CUL2, but not DCNL1, fails to trigger CUL2 neddylation and retains ECV in an inactive state. These findings support a model in which substrate engagement prompts DCNL1 recruitment that facilitates the initiation of CUL2 neddylation and define DCNL1 as a "substrate sensor switch" for ECV activation.
Cullin-RING ligases (CRLs), the largest family of E3 ubiquitin ligases, are multiprotein complexes involved in protein degradation (1, 2). They are assembled on a cullin scaffold (CUL1, CUL2, CUL3, CUL4A, CUL4B, and CUL5) and contain a RING finger protein (RBX1 or RBX2) at the cullin C-terminal domain (CTD) as well as a substrate receptor, which generally associates with the cullin N-terminal domain (NTD) via adaptor proteins (2). CUL3-based CRLs are the exception in which the Broad complex, Tramtrack, and Bric-a-Brac (BTB)-domain containing protein mediates both substrate specificity and binding to CUL3 without the need of adaptor proteins (3).The process of attaching ubiquitin onto a CRL substrate requires three enzymes. Using ATP, an E1 activating enzyme forms a thioester bond between its catalytic cysteine residue and the C-terminal carboxyl group of ubiquitin (4). The activated ubiquitin is then transferred onto the catalytic cysteine of an E2 conjugating enzyme (4). CRL through RBX1/2 helps transfer ubiquitin from E2 onto a lysine residue on the bound substrate (2). Neddylation, the process of conjugating the ubiquitin-like protein NEDD8 onto a substrate, follows an analogous cascade (5, 6). Cullins are modified by NEDD8 (7). Structural studies involving the CTD of CUL5 (CUL5 CTD )-RBX1 and NEDD8ϳCUL5 CTD -RBX1 have demonstrated that in an unneddylated state, RBX1 has limited movement due to inhibitory cullin subdomains, thus restraining its ability to recruit ubiquitin-charged E2 (8). Neddylation causes a conformational change in cullin structure that releases RBX1 from a confined configuration to enhance the binding of ubiquitin-charged E2 and thus CRL ubiquitylation capability (8, 9).The molecular events involved in the process of neddylation of cullins are not as straightforward as the resulting effects. For CUL1 neddylation to take place, rotation of the RING domain of RBX1 is needed in order to bring the catalytic Cys111 of UBC12 (NEDD8 E2 enz...