-!- Some RING-type E3 ubiquitin transferase (EC 2.3.2.27) are not able to bind a substrate protein directly. -!- Instead, they form a complex with a cullin scaffold protein and a substrate recognition module, which is named CRL for Cullin-RING- Ligase. -!- The cullin protein needs to be activated by the ubiquitin-like protein NEDD8 in a process known as neddylation. -!- The transfer of NEDD8 from a NEDD8-specific E2 enzyme onto the cullin protein is a secondary function of the RING-type E3 ubiquitin transferase in the CRL complex. -!- The process requires auxiliary factors that belong to the DCN1 (defective in cullin neddylation 1) family.

-!- RING E3 ubiquitin transferases serve as mediators bringing the ubiquitin-charged E2 ubiquitin-conjugating enzyme (EC 2.3.2.23) and an acceptor protein together to enable the direct transfer of ubiquitin through the formation of an isopeptide bond between the C-terminal glycine residue of ubiquitin and the epsilon-amino group of an L-lysine residue of the acceptor protein. -!- Unlike EC 2.3.2.26 the RING-E3 domain does not form a catalytic thioester intermediate with ubiquitin. -!- Many members of the RING-type E3 ubiquitin transferase family are not able to bind a substrate directly, and form a complex with a cullin scaffold protein and a substrate recognition module (the complexes are named CRL for Cullin-RING-Ligase). -!- In these complexes, the RING-type E3 ubiquitin transferase provides an additional function, mediating the transfer of a NEDD8 protein from a dedicated E2 carrier to the cullin protein (see EC 2.3.2.32). -!- Cf. EC 2.3.2.31.