The RBR family of E3 ligases has a distinctive organisation of RING fingers called 'RING-betweenRING-RING' (RBR), characterised by two RING fingers with a cysteine-rich region called the 'in-between-RING' domain that separates them (1, 2, 3). The domains in the RBR E3 ligases have been further named as RING1-BRcat-Rcat according to their structure and function: the C-terminal Rcat (required-for-catalysis) domain is essential for catalytic activity, whereas the central BRcat (benign-catalytic) domain adopts the same fold as the Rcat, but lacks a catalytic cysteine residue and ubiquitination activity (4). These ligases have a unique mechanism of elongating ubiquitin chains that distinguishes them from other E3 ligases.

1. http://www.ebi.ac.uk/interpro/entry/IPR031127

2. Marín et al., 2004. Parkin and relatives: the RBR family of ubiquitin ligases. Physiol Genomics., 17(3):253-63. PMID: 15152079

3. Marín, 2009. RBR ubiquitin ligases: Diversification and streamlining in animal lineages. J Mol Evol., 69(1):54-64. PMID: 19526189

4. Spratt et al., 2014. RBR E3 ubiquitin ligases: new structures, new insights, new questions. Biochem J., 458(3):421-37. PMID: 24576094