# RBR E3 ubiquitin ligases
RING-between-RING (RBR) E3 ubiquitin ligases are a family of enzymes that attach ubiquitin to proteins by a two-step mechanism: ubiquitin is first transferred from an E2 conjugating enzyme to a catalytic cysteine in the ligase itself, and only then to the substrate. This E3~ubiquitin thioester (high-energy bond linking ubiquitin to the enzyme's cysteine) intermediate makes RBR ligases mechanistically similar to HECT ligases, even though they bind E2 enzymes through a RING-type domain, hence their description as RING/HECT hybrids.[1](https://www.nature.com/articles/s41467-023-35871-z) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) The family includes parkin, mutated in [Parkinson's disease](https://www.edgepedia.org/parkinsons-disease), and HOIP, the catalytic core of the LUBAC complex that builds linear ubiquitin chains for NF-κB signaling.
| Key fact | Detail |
|---|---|
| Family size | 14 human RBR E3 ligases (earlier reviews counted 12)[1](https://www.nature.com/articles/s41467-023-35871-z) • [3](https://pubmed.ncbi.nlm.nih.gov/24576094/) |
| Domain architecture | RING1, in-between-RING (IBR), and RING2 (Rcat) zinc-binding domains, plus a BRcat domain of the same fold as Rcat but lacking a catalytic cysteine[4](https://preview-www.nature.com/articles/s41467-017-00272-6) • [3](https://pubmed.ncbi.nlm.nih.gov/24576094/) |
| Catalytic mechanism | E2~Ub → E3~Ub thioester on a RING2 cysteine → substrate, a HECT-like two-step transfer[1](https://www.nature.com/articles/s41467-023-35871-z) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) |
| Parkin active site | Cys431, confirmed in cells by loss of mitochondrial degradation on C431 mutation[5](https://pubmed.ncbi.nlm.nih.gov/23770887/) |
| HHARI active site | Cys357, buried at the RING2/Ariadne domain interface in the autoinhibited state[4](https://preview-www.nature.com/articles/s41467-017-00272-6) |
| Chain type | HOIP uniquely generates Met1-linked (linear) ubiquitin chains regulating NF-κB; HOIL-1L forms ester-linked ubiquitin modifications[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) |
| Disease links | Parkin mutations: autosomal recessive juvenile Parkinson's disease, cancer, mycobacterial infection[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) • [5](https://pubmed.ncbi.nlm.nih.gov/23770887/) |
## What RBR ligases are
RBR stands for RING-between-RING: the enzymes carry three tandem zinc-binding domains named RING1, in-between-RING (IBR), and RING2.[4](https://preview-www.nature.com/articles/s41467-017-00272-6) Structural analysis redefined the terminology: the C-terminal RING2 is the Rcat (required-for-catalysis) domain, housing the essential catalytic cysteine, while the central BRcat (benign-catalytic) domain adopts the same fold but lacks a catalytic cysteine.[3](https://pubmed.ncbi.nlm.nih.gov/24576094/) The name reflects history rather than function: neither RING1 nor BRcat behaves like a canonical RING domain, because neither promotes direct transfer from E2 to substrate.
The human family comprises 14 members.[1](https://www.nature.com/articles/s41467-023-35871-z) An earlier review described the group as 12 complex multidomain enzymes,[3](https://pubmed.ncbi.nlm.nih.gov/24576094/) and counts of 12 to 14 appear across the literature; the 2023 figure of 14 is used here. Members are multidomain proteins with regulatory elements outside the catalytic core, including parkin's ubiquitin-like (UBL) domain, HHARI's C-terminal Ariadne domain, and HOIP's N-terminal UBA domain.[1](https://www.nature.com/articles/s41467-023-35871-z)
## The hybrid RING/HECT mechanism
The catalytic cycle has three stages. First, RING1 binds the E2 enzyme charged with ubiquitin (E2~Ub) and holds it in an open conformation, in which ubiquitin is extended rather than folded back against the E2. This is the opposite of canonical RING ligases, which position E2~Ub in a closed conformation primed for direct aminolysis.[4](https://preview-www.nature.com/articles/s41467-017-00272-6) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) Second, in a transthiolation step, the ubiquitin thioester is transferred from the E2's catalytic cysteine to the RBR's catalytic cysteine, forming an E3~Ub intermediate. Third, the RING2-bound ubiquitin is transferred to an acceptor on the substrate, which can vary by ligase and may include a lysine, an N-terminal amino group, or a non-lysine residue.[1](https://www.nature.com/articles/s41467-023-35871-z) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/)
Structural work supports each stage. The structure of fully active human HOIP RBR in its transfer complex with an E2~ubiquitin conjugate showed an elongated E2/E3 architecture with the E2 and E3 catalytic centers ideally aligned for ubiquitin transfer, a geometry that structurally both requires and enables a HECT-like mechanism.[6](https://europepmc.org/article/med/26789245) In that structure, three helix-IBR-fold motifs form ubiquitin-binding regions that engage the activated ubiquitin of the E2~Ub conjugate and, unexpectedly, an additional regulatory ubiquitin molecule.[6](https://europepmc.org/article/med/26789245) Crystal structures of HOIL-1 and RNF216 in RBR/E2~Ub/Ub transthiolation complexes later revealed a conserved transthiolation complex structure that enables efficient E2-to-RBR ubiquitin transfer across the family.[1](https://www.nature.com/articles/s41467-023-35871-z)
RING1's job is not only binding but quality control of the E2. HHARI recruits the transthiolation-specialized E2 UbcH7, inserts a RING1 loop incompatible with closed E2~Ub conformations, and uses UBA-like domain contacts that favor the open state, preventing wasteful discharge of ubiquitin from E2 directly to lysines.[4](https://preview-www.nature.com/articles/s41467-017-00272-6) A ubiquitin-binding site on HHARI's RING2 then captures the E2~Ub and delivers it to the catalytic cysteine; mutations that ablate this binding decrease ligase activity.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) In autoinhibited HHARI, the RING1 and RING2 domains sit about 90 Å apart at opposite ends of the structure, with the UbcH7 and HHARI catalytic cysteines about 54 Å apart, so the E2~Ub intermediate can be recruited even in the autoinhibited conformation.[4](https://preview-www.nature.com/articles/s41467-017-00272-6)
## Regulation and autoinhibition
RBR ligases are tightly repressed and each uses a different release mechanism.
**Parkin** is activated by phosphorylation of its ubiquitin-like (UBL) domain by the kinase PINK1, and by binding of phospho-ubiquitin (ubiquitin phosphorylated on Ser65 by PINK1) to an interface between the RING1 and IBR domains; both events are critical for activation during mitophagy.[1](https://www.nature.com/articles/s41467-023-35871-z)
**HHARI** is kept autoinhibited by its C-terminal Ariadne domain, which buries the catalytic cysteine Cys357 at the RING2/Ariadne interface, and is activated by interaction with NEDDylated cullins or by phosphorylation within the Ariadne domain.[1](https://www.nature.com/articles/s41467-023-35871-z) • [4](https://preview-www.nature.com/articles/s41467-017-00272-6) HHARI and TRIAD1 form the Ariadne family of RBR ligases, defined by this C-terminal auto-inhibitory domain.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/)
**HOIP** is autoinhibited by a UBA domain N-terminal to the RBR module and activated by its cofactors HOIL-1 or Sharpin within the [linear ubiquitin chain assembly complex](https://www.edgepedia.org/linear-ubiquitin-chain-assembly-complex) (LUBAC), and by M1-linked linear di-ubiquitin binding at an allosteric site required for HOIP-mediated NF-κB activation in cells.[1](https://www.nature.com/articles/s41467-023-35871-z) Consistent with this, the active HOIP RBR adopts a conformation markedly different from that of autoinhibited RBRs such as parkin and HHARI.[6](https://europepmc.org/article/med/26789245)
**RNF216** is allosterically activated by K63-linked di-ubiquitin but not by other di-ubiquitin linkages, suggesting a conserved feed-forward mechanism in which a ligase's own chain type activates it.[1](https://www.nature.com/articles/s41467-023-35871-z)
Structural features also maintain the off state at the catalytic site itself. The HOIL-1 RING2 domain contains an unusual Zn2/Cys6 binuclear zinc cluster required for catalytic activity and substrate ubiquitination.[1](https://www.nature.com/articles/s41467-023-35871-z)
## Key members: parkin, HOIP and HHARI
**Parkin** is the most studied RBR ligase. [The 1](https://www.edgepedia.org/the-1).58 Å crystal structure of its R0RBR core was the first atomic-resolution structure of an RBR E3 ligase, and in cells, mutation of Cys431 eliminates parkin-catalyzed degradation of mitochondria, confirming C431 as parkin's cellular active site.[5](https://pubmed.ncbi.nlm.nih.gov/23770887/) Mutations in parkin are linked to autosomal recessive juvenile Parkinson's disease, and also to cancer and mycobacterial infection.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) • [5](https://pubmed.ncbi.nlm.nih.gov/23770887/) One pathogenic mutation illustrates how catalysis fails: substituting Thr415 with Asn substantially decreases ligase activity without altering parkin's structure or solubility, because the equivalent residue in HHARI (Thr341) participates in RING2's ubiquitin-binding site, so the T415N mutation disrupts recruitment of E2~Ub to the catalytic cysteine.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/)
**HOIP** is the catalytic subunit of LUBAC, the complex that assembles Met1-linked linear ubiquitin chains that regulate NF-κB signaling.[1](https://www.nature.com/articles/s41467-023-35871-z) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) Its activity depends on the cofactors HOIL-1 or Sharpin and on allosteric activation by its own linear-chain product.[1](https://www.nature.com/articles/s41467-023-35871-z)
**HHARI (ARIH1)** couples RBR chemistry to the cullin-RING ligase system: it is activated by NEDDylated cullins, which relieves Ariadne-domain autoinhibition.[1](https://www.nature.com/articles/s41467-023-35871-z)
## Chain-type output and linkage specificity
HOIP is the E3 responsible for generating Met1-linked linear ubiquitin chains, while HOIL-1L is a LUBAC subunit with distinct catalytic activity that forms ester-linked ubiquitin modifications.[8](https://www.nature.com/articles/s41556-020-0517-9) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) RNF216, by contrast, is K63-linkage-specific and is allosterically activated by K63 di-ubiquitin, a feed-forward arrangement in which the product linkage promotes further catalysis.[1](https://www.nature.com/articles/s41467-023-35871-z) Across the family, linkage choice appears tied to allosteric ubiquitin binding: distinct ubiquitin linkages can activate distinct RBR ligases.[1](https://www.nature.com/articles/s41467-023-35871-z)
## How RBR ligases compare with HECT and RING ligases
The three E3 classes differ in how ubiquitin travels from E2 to substrate. Canonical RING E3s bind E2~Ub and promote a closed conformation that allows ubiquitin to pass directly to substrate, with no covalent intermediate on the E3. HECT E3s and RBR E3s both form an E3~Ub thioester intermediate; RBRs achieve this despite containing an E2-binding RING domain.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) RBRs therefore combine RING-like E2 recruitment through RING1 with HECT-like two-step transfer through the Rcat cysteine, which is why they are called RING-HECT hybrids.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC5675740/)
E2 choice is part of the mechanism. UbcH7 residue Lys96 is a key E2/E3 specificity determinant for HHARI; introducing this residue into UbcH5b substantially increases that E2's activity with HHARI.[4](https://preview-www.nature.com/articles/s41467-017-00272-6) The evidence available here is qualitative on speed and processivity: no quantitative transfer rates or processivity comparisons between RBR, HECT and RING ligases are covered by these sources.
## Disease roles and therapeutic targeting
Parkin mutations cause autosomal recessive juvenile Parkinson's disease and are also linked to cancer and mycobacterial infection.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) • [5](https://pubmed.ncbi.nlm.nih.gov/23770887/) Pathogenic mutations can break catalysis at different points, as the T415N example shows, or at the active site itself, where C431 is required for parkin-driven mitochondrial degradation in cells.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/) • [5](https://pubmed.ncbi.nlm.nih.gov/23770887/)
On the HOIP side, the allosteric linear di-ubiquitin binding site in HOIP is required for HOIP-mediated NF-κB activation in cells, making LUBAC central to inflammatory signaling.[1](https://www.nature.com/articles/s41467-023-35871-z) The evidence record contains no data on drugs against HOIP/LUBAC or parkin activators, or on clinical status since 2023, so no therapeutic-development claims can be made here.
## References
1. The unifying catalytic mechanism of the RING-between-RING E3 ubiquitin ligase family. Nature Communications. https://www.nature.com/articles/s41467-023-35871-z
2. Molecular insights into RBR E3 ligase ubiquitin transfer mechanisms. https://pmc.ncbi.nlm.nih.gov/articles/PMC4967960/
3. RBR E3 ubiquitin ligases: new structures, new insights, new questions. https://pubmed.ncbi.nlm.nih.gov/24576094/
4. Structural insights into the mechanism and E2 specificity of the RBR E3 ubiquitin ligase HHARI. Nature Communications. https://preview-www.nature.com/articles/s41467-017-00272-6
5. Structure and function of Parkin E3 ubiquitin ligase reveals aspects of RING and HECT ligases. https://pubmed.ncbi.nlm.nih.gov/23770887/
6. Structure of a HOIP/E2~ubiquitin complex reveals RBR E3 ligase mechanism and regulation. Nature. https://europepmc.org/article/med/26789245
7. RING-Between-RING E3 ligases: Emerging themes amid the variations. https://pmc.ncbi.nlm.nih.gov/articles/PMC5675740/
8. S41556 020 0517 9 (nature.com). https://www.nature.com/articles/s41556-020-0517-9
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Enzyme classes and activities › Ubiquitination and protein-modification enzymes › Ubiquitin conjugation machinery › RBR E3 ligases*
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