| Protein Name | HUWE1 (MULE/ARF-BP1) |
| Gene | HUWE1 |
| UniProt | Q7Z6Z7 |
| Molecular Weight | ~482 kDa |
| Length | 4,374 amino acids |
| Subcellular Localization | Cytoplasm, nucleus, mitochondria-associated |
| Protein Family | HECT-domain E3 ubiquitin ligases |
HUWE1 is one of the largest human proteins and a HECT-domain E3 ubiquitin ligase that catalyzes polyubiquitination of a broad substrate repertoire controlling apoptosis, DNA repair, mitochondrial dynamics, and protein quality control. In neurons, HUWE1 functions as a master proteostasis regulator, ubiquitinating misfolded proteins for proteasomal degradation, controlling mitochondrial fusion-fission balance through MFN2 ubiquitination, and modulating apoptosis through MCL-1 and p53 regulation. Loss-of-function mutations cause X-linked intellectual disability, and HUWE1 dysfunction contributes to neurodegeneration[1].
HUWE1 contains multiple functional domains across its massive polypeptide chain. The N-terminal region contains ARM/HEAT repeats for substrate recognition and protein-protein interactions. The UBA (ubiquitin-associated) domain recognizes ubiquitinated substrates. The WWE domain mediates protein-protein interactions involved in ubiquitin signaling. The BH3 (BCL-2 homology 3) domain directly binds MCL-1 for targeted ubiquitination. The C-terminal HECT (Homologous to E6-AP C-Terminus) domain contains the catalytic cysteine that accepts ubiquitin from E2 conjugating enzymes and transfers it to substrates[2].
The HECT domain forms a bilobed structure: the N-lobe binds the E2 enzyme (primarily UbcH5/UBE2D family), while the C-lobe contains the catalytic cysteine (Cys4341) that forms a thioester intermediate with ubiquitin before transfer to the substrate lysine.
HUWE1 ubiquitinates misfolded and aggregation-prone proteins, generating K48-linked polyubiquitin chains that target them for proteasomal degradation. This activity prevents protein aggregation in neurons, which are post-mitotic and cannot dilute toxic aggregates through cell division[3].
HUWE1 ubiquitinates MFN2 (mitofusin-2), promoting mitochondrial fission under stress conditions. This is coordinated with PINK1/Parkin mitophagy: HUWE1-mediated MFN2 degradation on damaged mitochondria prevents their re-fusion with the healthy network, facilitating selective removal by mitophagy[4].
Through ubiquitination and degradation of the anti-apoptotic protein MCL-1, HUWE1 lowers the apoptotic threshold. HUWE1 also degrades p53 (as ARF-BP1), and the balance between these activities determines neuronal survival versus death decisions under stress[1:1].
HUWE1 dysfunction in Alzheimer's and Parkinson's disease leads to impaired clearance of misfolded tau and alpha-synuclein, mitochondrial network fragmentation from dysregulated MFN2 ubiquitination, and aberrant apoptosis from MCL-1/p53 imbalance[5].
Mutations in HUWE1 cause Turner type, Juberg-Marsidi, and Brooks syndromes, confirming HUWE1's essential role in brain development[6].
Approaches include enhancing HUWE1 catalytic activity to boost aggregate clearance, substrate-specific modulation of HUWE1-mediated ubiquitination, and targeting the MCL-1/p53 axis to optimize neuronal survival thresholds.
Zhong Q et al. Mule/ARF-BP1, a BH3-only E3 ubiquitin ligase, catalyzes the polyubiquitination of Mcl-1 and regulates apoptosis. Cell. 2005. ↩︎ ↩︎
Chen D et al. ARF-BP1/Mule is a critical mediator of the ARF tumor suppressor. Cell. 2005. ↩︎
Zhao X et al. The HECT-domain ubiquitin ligase Huwe1 controls neural differentiation and proliferation by destabilizing the N-Myc oncoprotein. Nature Cell Biology. 2008. ↩︎
Leboucher GP et al. Stress-induced phosphorylation and proteasomal degradation of mitofusin 2 facilitates mitochondrial fragmentation and apoptosis. Molecular Cell. 2012. ↩︎
Gong Y et al. Ubiquitin ligase HUWE1 regulates neuronal proteostasis. Acta Neuropathologica Communications. 2021. ↩︎
Friez MJ et al. HUWE1 mutations in Juberg-Marsidi and Brooks syndromes: the results of an X-chromosome exome sequencing study. BMJ Open. 2016. ↩︎