InterPro domain: IPR031127

This 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 ]. 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 [ 3 ]. These ligases have a unique mechanism of elongating ubiquitin chains that distinguishes them from other E3 ligases [ 4 , 5 ].

On the basis of sequence conservation within the RBR segment, RBR proteins are assigned to 15 subfamilies (A-I, P, S, T, U, X, Z). There are two RBR proteins in the yeast Saccharomyces cerevisiae, six in Drosophila melanogaster, 10 in Caenorhabditis elegans, about 40 in Arabidopsis thaliana, around 23 in the zebrafish (Danio rerio), and about 15 in humans [ 6 ].

1. Parkin and relatives: the RBR family of ubiquitin ligases. Physiol. Genomics 17, 253-63
2. RBR ubiquitin ligases: Diversification and streamlining in animal lineages. J. Mol. Evol. 69, 54-64
3. RBR E3 ubiquitin ligases: new structures, new insights, new questions. Biochem. J. 458, 421-37
4. UBCH7 reactivity profile reveals parkin and HHARI to be RING/HECT hybrids. Nature 474, 105-8
5. RBR E3-ligases at work. EMBO Rep. 15, 142-54
6. The ring between ring fingers (RBR) protein family. Genome Biol. 8, 209