Ubiquitin is a highly conserved 76-amino acid protein that covalently attaches to target proteins to mark them for various cellular fates. 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
- 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 - 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
| Linkage |
Function |
| K48 |
Proteasomal degradation |
| K63 |
Endocytosis, signaling, autophagy |
| K11 |
Cell cycle regulation |
| K27 |
DNA damage response |
| K33 |
Mitochondrial quality control |
| K29 |
Lysosomal degradation |
| K6 |
Mitochondrial dynamics |
| M1 (linear) |
NF-κB signaling |
- Mixed linkage chains
- Branched ubiquitin chains
- More complex regulatory functions
- 20S core particle (α-rings + β-rings)
- 19S regulatory caps (6 ATPases)
- Recognizes poly-K48 chains
- Degrades ubiquitinated proteins
- Polyubiquitin chain recognition
- Substrate unfolding
- Translocation into 20S core
- Proteolytic cleavage (β1, β2, β5)
- Peptide release
- Selective autophagy receptors (p62, OPTN, NDP52)
- Recognize poly-K63 and K27 chains
- Link cargo to autophagosomes
- LC3 interaction domain (LIR)
- Clearance of protein aggregates
- p62/SQSTM1 as key receptor
- Ubiquitin chain specificity
- Impaired in neurodegeneration
- Ubiquitinated tau tangles
- Impaired proteasome function
- p62 accumulation in plaques
- Failed aggregate clearance
- UPS dysfunction precedes symptoms
- LRRK2 mutations affect ubiquitination
- Parkin E3 ligase dysfunction
- PINK1-Parkin mitophagy impairment
- Ubiquitinated Lewy bodies
- G2019S LRRK2 alters autophagy
- Ubiquitinated inclusions
- Mutations in UBQLN2 (ubiquilin 2)
- TDP-43 ubiquitination
- Proteasome impairment
- Autophagy dysfunction
- Mutant huntingtin aggregates
- Impaired UPS function
- Ubiquitinated inclusion bodies
- Autophagy compensation
- Therapeutic targeting potential
- Tau and TDP-43 pathology
- Ubiquitin chain abnormalities
- OPTN mutations
- p62 accumulation
- Misfolded proteins in ER
- Retrotranslocation
- Ubiquitination by E3 ligases
- Proteasomal degradation
- PINK1-Parkin mitophagy
- Ubiquitin chains on mitochondria
- Autophagy receptor recruitment
- Mitochondrial dynamics
- Proteasomal degradation in nucleus
- Chromatin-associated degradation
- Histone ubiquitination
- Natural compounds (EGCG, curcumin)
- Proteasome activators
- Overcoming inhibition
- mTOR inhibitors (rapamycin)
- Trehalose
- Exercise
- Caloric restriction
- Small molecule modulators
- Gene therapy approaches
- Specific ligase targeting
- USP14 inhibition
- OTUB1 enhancement
- Proteasome-associated DUBs
- Ubiquitinated proteins in CSF
- p62 levels
- Serum ubiquitin
- Proteasome activity assays
- Autophagic flux
- Ubiquitin chain analysis