Ubiquitin

Overview

Ubiquitin
Linkage	Function
K48	Proteasomal degradation
K63	Endocytosis, signaling, [autophagy](/entities/autophagy)
K11	Cell cycle regulation
K27	DNA damage response
K33	Mitochondrial quality control
K29	Lysosomal degradation
K6	Mitochondrial dynamics
M1 (linear)	[NF-κB](/entities/nf-kb) signaling
Associated Diseases	ALS, ALZHEIMER, ALZHEIMER DISEASE, ALZHEIMER'S DISEASE, AMYOTROPHIC LATERAL SCLEROSIS
KG Connections	2864 edges

Ubiquitin is a highly conserved 76-amino acid protein that covalently attaches to target proteins to mark them for various cellular fates.^{1Overview of the ubiquitin-proteasome system2014 · Current Protocols in Protein Science · PMID 25092124Open reference2Impairment of the ubiquitin-proteasome system in Alzheimer's diseases and other tauopathies2016 · Ageing Research Reviews · PMID 26732485Open reference} 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

1. Polyubiquitin chain recognition

2. Substrate unfolding

3. Translocation into 20S core

4. Proteolytic cleavage (β1, β2, β5)

5. 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 tangles^{3Tau-driven ubiquitination and neurodegeneration2017 · Acta Neuropathologica Communications · DOI 10.1186/s40478-017-0463-3Open reference}

• Impaired proteasome function

• p62 accumulation in plaques

• Failed aggregate clearance

• UPS 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 ubiquitination

• Proteasome impairment

• Autophagy dysfunction

Huntington’s Disease

• Mutant 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 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

• Protein Aggregation in Neurodegeneration

• Autophagy Mechanisms

• PINK1-Parkin Pathway

• Tau Protein

• Alpha-Synuclein

External Links

• UniProt: ubiquitin

• PubMed: ubiquitin