Some RING-type E3 ubiquitin transferase (EC
2.3.2.27) are not able to bind a substrate protein directly. Instead, they form complexes with a cullin scaffold protein and a substrate recognition module, which are known as CRL (Cullin-RING-Ligase) complexes. The cullin protein needs to be activated by the ubiquitin-like protein NEDD8 in a process known as neddylation. Like ubiquitin, the NEDD8 protein ends with two glycine residues. The E1 NEDD8-activating enzyme activates NEDD8 in an ATP-dependent reaction by forming a high-energy thioester intermediate between NEDD8 and one of its cysteine residues. The activated NEDD8 is subsequently transferred to a cysteine residue of EC
2.3.2.34, E2 NEDD8-conjugating enzyme, and is eventually conjugated to a lysine residue of specific substrates in the presence of the appropriate E3 transferase (EC
2.3.2.32, cullin-RING-type E3 NEDD8 transferase).
