-!- RBR-type E3 ubiquitin transferases have two RING fingers separated by an internal motif (IBR, for In Between RING). -!- The enzyme interacts with the CRL (Cullin-RING ubiquitin Ligase) complexes formed by certain RING-type E3 ubiquitin transferase (see EC 2.3.2.27), which include a neddylated cullin scaffold protein and a substrate recognition module. -!- The RING1 domain binds an EC 2.3.2.23, and transfers the ubiquitin that is bound to it to an internal cysteine residue in the RING2 domain, followed by the transfer of the ubiquitin from RING2 to the substrate. -!- Once the substrate has been ubiquitinated by the RBR-type ligase, it can be ubiqutylated further using ubiquitin carried directly on E2 enzymes, in a reaction catalyzed by EC 2.3.2.27. -!- Activity of the RBR-type enzyme is dependent on neddylation of the cullin protein in the CRL complex. -!- Cf. EC 2.3.2.26, EC 2.3.2.27, and EC 2.3.2.32.