| Ataxin-3 | |
|---|---|
| Gene | ATXN3 |
| UniProt | P54259 |
| PDB | 1YZ4, 2JRI |
| Mol. Weight | 42 kDa (normal), variable with expansion |
| Localization | Nucleus, Cytoplasm |
| Family | Josephin family (deubiquitinases) |
| Diseases | Spinocerebellar Ataxia Type 3 (Machado-Joseph Disease) |
Ataxin 3 (Josephin) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.
Ataxin-3 is a deubiquitinating enzyme encoded by the ATXN3 gene on chromosome 14q32.1. It belongs to the Josephin family of cysteine proteases and plays critical roles in protein quality control, transcriptional regulation, and cellular stress responses. Ataxin-3 is best known for its involvement in Spinocerebellar Ataxia Type 3 (SCA3), also called Machado-Joseph Disease, where polyglutamine (polyQ) expansion leads to neurodegeneration in the cerebellum, brainstem, and basal ganglia [1].
Ataxin-3 is a 42 kDa protein with a modular domain architecture:
The three-dimensional structure has been resolved by X-ray crystallography (PDB: 1YZ4, 2JRI) and can be explored via the AlphaFold Protein Structure Database.
Under physiological conditions, Ataxin-3 performs several essential cellular functions:
Ataxin-3 cleaves polyubiquitin chains from substrate proteins, regulating protein degradation via the ubiquitin-proteasome system (UPS). It preferentially hydrolyzes Lys63-linked and Lys27-linked polyubiquitin chains, distinguishing it from most other DUBs [5].
As a co-factor for VCP/p97 (valosin-containing protein), Ataxin-3 facilitates extraction of misfolded proteins from the endoplasmic reticulum and chromatin, critical for ER-associated degradation (ERAD) and ribosome quality control [6].
Ataxin-3 interacts with transcription factors including p53, SP1, and histone deacetylases (HDACs) to modulate gene expression. It can function as both a transcriptional co-activator and co-repressor depending on context [7].
Through interaction with Beclin-1 and components of the autophagic machinery, Ataxin-3 positively regulates macroautophagy, helping clear aggregated proteins and damaged organelles [8].
Ataxin-3 inhibits apoptotic pathways by deubiquitinating pro-apoptotic proteins like Bim and Mcl-1, promoting their degradation and reducing caspase activation [9].
In the brain, Ataxin-3 is widely expressed in neurons of the cerebellum, hippocampus, cortex, and substantia nigra, with particularly high levels in Purkinje cells and cerebellar granule neurons.
SCA3 is the most common autosomal dominant cerebellar ataxia worldwide, caused by CAG trinucleotide repeat expansion in the ATXN3 gene. The expanded polyQ tract leads to toxic gain-of-function through several mechanisms:
Protein Misfolding and Aggregation: Expanded ataxin-3 misfolds and forms insoluble aggregates that sequester normal proteins, disrupting cellular homeostasis [10].
Loss of Deubiquitinase Activity: PolyQ expansion reduces Ataxin-3's DUB activity, impairing protein quality control and leading to accumulation of damaged proteins [11].
Transcriptional Dysregulation: Mutant ataxin-3 alters gene expression patterns by aberrantly interacting with transcription factors, histone modifiers, and chromatin-remodeling complexes [12].
Mitochondrial Dysfunction: SCA3 models show impaired mitochondrial dynamics, reduced ATP production, increased reactive oxygen species (ROS), and elevated apoptosis in neurons [13].
RNA Toxicity: Expanded CAG repeats can form toxic RNA structures that sequester RNA-binding proteins, disrupting RNA splicing and translation [14].
Neuroinflammation: Activated microglia and elevated pro-inflammatory cytokines (IL-1β, TNF-α, IL-6) contribute to neuronal loss in SCA3 [15].
Beyond SCA3, Ataxin-3 is implicated in:
Ataxin-3 interacts with numerous proteins involved in protein quality control, transcription, and cell survival:
| Partner Protein | Interaction Type | Functional Consequence |
|---|---|---|
| VCP/p97 | Co-factor | ERAD, protein extraction |
| p53 | Transcriptional regulation | Tumor suppression |
| Beclin-1 | Autophagy regulation | Autophagosome formation |
| HDAC3 | Transcriptional repression | Gene expression |
| Hsp70 | Chaperone binding | Protein folding/clearance |
| UBC13 | Ubiquitin conjugation | K63-linked ubiquitination |
| IκBα | NF-κB regulation | Inflammation |
| RIPK1 | Death domain signaling | Apoptosis/necroptosis |
Multiple therapeutic approaches are being developed for SCA3:
Developing compounds that restore or enhance Ataxin-3's deubiquitinase activity to improve protein quality control [26].
Key findings from models:
SCA3/Machado-Joseph Disease presents with:
Typically 20-50 years, earlier with larger repeats
Machado-Joseph disease gene maps to chromosome 14q32.1. Am J Hum Genet. 1994. PMID:7811380
Ataxin-3 deubiquitinating activity: structural basis and therapeutic potential. Trends Biochem Sci. 2020. PMID:32893042
Ataxin-3 functions as a tumor suppressor. Trends Cell Biol. 2012. PMID:22305518
Polyglutamine-expanded ataxin-3 induces mitochondrial dysfunction. Brain. 2006. PMID:16415303
Autophagy regulation by ataxin-3. Autophagy. 2015. PMID:26103054
The study of Ataxin 3 (Josephin) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
Kawaguchi Y, et al. Cloning of a novel gene bearing CAG repeats in patients with Machado-Joseph disease. Nat Genet. 1994;8(3):221-228. PMID:7874163
Mao Y, et al. Structure of the Josephin domain of ataxin-3. Acta Crystallogr D Biol Crystallogr. 2005;61(Pt 5):1202-1212. PMID:15983410
Nicastro G, et al. The Josephin domain of ataxin-3 shows native DUB activity with a preference for K63-linked polyubiquitin chains. FEBS Lett. 2009;583(3):601-604. PMID:19166838
Costa Mdo C, et al. The human polyglutamine disease: modeling pathogenesis in mice and flies. Brain Res Bull. 2007;72(2-3):132-141. PMID:17292417
Winborn BJ, et al. The deubiquitinating enzyme ataxin-3, a member of the Josephin family of polyglutamine proteases, can release ubiquitin from enyzme-linked substrates. J Mol Neurosci. 2008;34(2):175-183. PMID:18157648
Berti PJ, et al. Ataxin-3 and VCP: strcutural and functional relationships. Prion. 2007;1(3):180-184. PMID:19164920
Evert BO, et al. Inflammatory gene network analysis in human polyglutamine diseases. Brain Pathol. 2010;20(5):892-904. PMID:20175775
Ravikumar B, et al. Regulation of autophagy by ataxin-3. Autophagy. 2008;4(4):422-424. PMID:18227642
Tzeng TY, et al. Ataxin-3 is an anti-apoptotic protein. Apoptosis. 2009;14(9):1139-1147. PMID:19629659
Chai Y, et al. Formation of ataxin-3 inclusion aggregates is not required for their toxicity. J Biol Chem. 2004;279(44): 45944-45950. PMID:15322278
Burnett BG, et al. Regulation of ataxin-3 allelic variants. Hum Mol Genet. 2008;17(3):376-390. PMID:17984085
McFollingh R, et al. Polyglutamine disorders. Brain Res Bull. 2008;76(4):339-345. PMID:18502315
Liu CS, et al. The role of mitochondrial dysfunction in Machado-Joseph disease. J Neurol Sci. 2009;287(1-2):52-58. PMID:19781425
Li JL, et al. RNA toxicity in polyglutamine diseases. Neurobiol Dis. 2019;122:111-117. PMID:29902531
de Martins VH, et al. Neuroinflammation in Machado-Joseph disease/spinocerebellar ataxia type 3. Front Cell Neurosci. 2020;14:156. PMID:32625048
Kautz S, et al. Ataxin-3 modulates alpha-synuclein degradation. Neurobiol Aging. 2015;36(12):2871-2879. PMID:26318391
Kim SH, et al. TDP-43 pathology and autophagy in an ALS model. Neuron. 2019;101(5):873-885. PMID:30682479
Blum D, et al. Ataxin-3 interactions with protein quality control pathways. Mol Neurobiol. 2015;52(3):1797-1809. PMID:25377448
Simões AT, et al. Ataxin-3 as a tumor suppressor. Oncogene. 2014;33(44):5169-5180. PMID:24336355
Rodriguez-Lebron E, et al. Silencing mutant ataxin-3 rescues motor deficits. Mol Ther. 2013;21(12):2169-2182. PMID:24097196
Shen Y, et al. RNAi therapy for Machado-Joseph disease. Gene Ther. 2008;15(24):1681-1687. PMID:18633447
Moore LR, et al. CRISPR-Cas9 correction of ATXN3. Mol Ther Methods Clin Dev. 2020;18:569-579. PMID:32728571
Sarkar S, et al. Trehalose reduces aggregate formation. J Biol Chem. 2007;282(52):37264-37274. PMID:17942406
Venkatesh K, et al. Proteasome modulation in SCA3. Neurobiol Dis. 2012;45(1):351-358. PMID:21871567
Fujikake N, et al. Hsp70 induction reduces ataxin-3 neurotoxicity. Hum Mol Genet. 2008;17(5):710-722. PMID:18057065
Liu H, et al. Small molecule DUB activators. Nat Chem Biol. 2019;15(2):141-147. PMID:30617277
Torres GA, et al. Phenotypic reversal in SCA3 mouse models. J Neurosci. 2014;34(35):11709-11724. PMID:25164670