MAINCullin-RING Ligases (CRLs) are modular, multi-subunit complexes that constitute a major class of ubiquitin E3 ligases 1,2 . CRLs coordinate the ubiquitination of substrates as either a signal for degradation via the 26S proteasome, or to alter the function of the target protein 2,3 . The CRL2 E3 ligase consists of a Cullin-2 (CUL2) scaffold in association with a catalytic RING-box protein (RBX1), with the substrate adaptors Elongin B (ELOB) and C (ELOC) at its N-terminal 4 . When associated with the von Hippel-Lindau (VHL) tumour suppressor substrate receptor, the CRL2 complex is the primary regulator of the Hypoxia Inducible Factor 1-α (HIF-1α) transcription factor 5,6 .
Mutations in the interface between VHL, ELOB and ELOC can deactivate CRL2leading to an accumulation of HIF-1α, which can in turn drive tumorigenesis through the over-activation of oncogenes 7 . Moreover, CRL2 has recently been identified as a potential target for small molecular inhibitors and PROTACs -a new class of cancer drugs that promote degradation of tumorigenic gene products 8-10 . These fascinating systems have been described in detail by a number of excellent reviews 1,2,3 .Activation of CRL2, in common with other members of the CRL family, involves a cascade of E1, E2 and E3 enzymes, which conjugate the ubiquitin-like protein NEDD8 (N8) to residue K689 on the Cullin-2 scaffold 11 . In its activated state, CRL2~N8 (the ~ stylization denotes a covalent interaction) recruits the ubiquitin-conjugated E2 enzyme via the RING domain of RBX1 12 . Ubiquitination now takes place, covalently adding ubiquitin to the substrate molecule docked at the CRL2 N-terminal. The activity of CRL2 is negatively regulated by the 331 kDa Constitutive Photomorphogenesis 9 Signalosome (CSN) complex, frequently referred to as the COP9 signalosome complex [13][14][15] . The CSN was originally identified as consisting of eight subunits