Ubiquitin
Overview <table class="infobox infobox-protein">
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Ubiquitin
Overview <table class="infobox infobox-protein">
Ubiquitin is a highly conserved 76-amino acid protein that covalently attaches to target proteins to mark them for various cellular fates.[@overview2014][@impairment2016] This post-translational modification, called ubiquitination or ubiquitylation, is a fundamental regulatory mechanism controlling protein degradation, signaling, trafficking, and localization. Dysregulation of ubiquitin biology is central to many neurodegenerative diseases.
Structure and Biochemistry
Ubiquitin Molecule
Molecular weight: 8.5 kDa
Seven lysine residues (K6, K11, K27, K29, K33, K48, K63)
N-terminal methionine (M1)
C-terminal glycine (G76) for conjugation
E1-E2-E3 Enzymatic Cascade E1 - Activating enzymes:
~2 enzymes in humans
ATP-dependent ubiquitin activation
Forms thioester bond with ubiquitin C-terminus
E2 - Conjugating enzymes:
~40 enzymes in humans
Receive ubiquitin from E1
Determine chain linkage type
E3 - Ligase enzymes:
>600 enzymes in humans
Substrate recognition
Transfer ubiquitin to target proteins
Ubiquitin Chain Types
Homotypic Chains (Same Linkage)
Heterotypic Chains
Mixed linkage chains
Branched ubiquitin chains
More complex regulatory functions
The Ubiquitin-Proteasome System (UPS)
26S Proteasome Structure
20S core particle (α-rings + β-rings)
19S regulatory caps (6 ATPases)
Recognizes poly-K48 chains
Degrades ubiquitinated proteins
Degradation Process
Polyubiquitin chain recognition
Substrate unfolding
Translocation into 20S core
Proteolytic cleavage (β1, β2, β5)
Peptide release
Autophagy-Lysosome Pathway
Ubiquitin in Autophagy
Selective autophagy receptors (p62, OPTN, NDP52)
Recognize poly-K63 and K27 chains
Link cargo to autophagosomes
LC3 interaction domain (LIR)
Aggrephagy
Clearance of protein aggregates
p62/SQSTM1 as key receptor
Ubiquitin chain specificity
Impaired in neurodegeneration
Disease Relevance
Alzheimer's Disease
Ubiquitinated [tau](/proteins/tau) tangles[@taudriven2017]
Impaired proteasome function
p62 accumulation in plaques
Failed aggregate clearance
[UPS](/mechanisms/ubiquitin-proteasome-system) dysfunction precedes symptoms
Parkinson's Disease
LRRK2 mutations affect ubiquitination
Parkin E3 ligase dysfunction
PINK1-Parkin mitophagy impairment
Ubiquitinated Lewy bodies
G2019S LRRK2 alters autophagy
Amyotrophic Lateral Sclerosis
Ubiquitinated inclusions
Mutations in UBQLN2 (ubiquilin 2)
[TDP-43](/mechanisms/tdp-43-proteinopathy) ubiquitination
Proteasome impairment
Autophagy dysfunction
Huntington's Disease
Mutant [huntingtin](/proteins/huntingtin) aggregates
Impaired UPS function
Ubiquitinated inclusion bodies
Autophagy compensation
Therapeutic targeting potential
Frontotemporal Dementia
Tau and TDP-43 pathology
Ubiquitin chain abnormalities
OPTN mutations
p62 accumulation
Quality Control Mechanisms
ER-Associated Degradation (ERAD)
Misfolded proteins in ER
Retrotranslocation
Ubiquitination by E3 ligases
Proteasomal degradation
Mitochondrial Quality Control
PINK1-Parkin mitophagy
Ubiquitin chains on mitochondria
Autophagy receptor recruitment
Mitochondrial dynamics
Nuclear Quality Control
Proteasomal degradation in nucleus
Chromatin-associated degradation
Histone ubiquitination
Therapeutic Approaches
Proteasome Enhancement
Natural compounds (EGCG, curcumin)
Proteasome activators
Overcoming inhibition
Autophagy Induction
[mTOR](/mechanisms/mtor-signaling-pathway) inhibitors (rapamycin)
Trehalose
Exercise
Caloric restriction
E3 Ligase Modulation
Small molecule modulators
Gene therapy approaches
Specific ligase targeting
Deubiquitinating Enzymes (DUBs)
USP14 inhibition
OTUB1 enhancement
Proteasome-associated DUBs
Biomarkers
Protein Aggregation Markers
Ubiquitinated proteins in CSF
p62 levels
Serum ubiquitin
Activity Measures
Proteasome activity assays
Autophagic flux
Ubiquitin chain analysis
See Also
[Ubiquitin-Proteasome System](/mechanisms/ubiquitin-proteasome-system)
[Protein Aggregation in Neurodegeneration](/mechanisms/protein-aggregation)
[Autophagy Mechanisms](/mechanisms/autophagy)
[PINK1-Parkin Pathway](/mechanisms/pink1-parkin-pathway)
[Tau Protein](/proteins/tau)
[Alpha-Synuclein](/proteins/alpha-synuclein)
External Links
[UniProt: ubiquitin](https://www.uniprot.org/)
[PubMed: ubiquitin](https://pubmed.ncbi.nlm.nih.gov/?term=ubiquitin+neurodegeneration)
References
[Unknown, Overview of the ubiquitin-proteasome system (2014)](https://pubmed.ncbi.nlm.nih.gov/25092124/)
[Unknown, Impairment of the ubiquitin-proteasome system in Alzheimer's diseases and other tauopathies (2016)](https://pubmed.ncbi.nlm.nih.gov/26732485/)
[Unknown, Tau-driven ubiquitination and neurodegeneration (2017)](https://doi.org/10.1186/s40478-017-0463-3)
Pathway Diagram The following diagram shows key molecular relationships for Ubiquitin based on knowledge graph edges:
Mermaid diagram (expand to render)
Pathway Diagram The following diagram shows the key molecular relationships involving Ubiquitin discovered through SciDEX knowledge graph analysis:
Mermaid diagram (expand to render)
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