SQSTM1/p62

SQSTM1/p62

Introduction

Sqstm1 P62 is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

SQSTM1 (Sequestosome 1), also known as p62, is a multifunctional scaffold and signaling adaptor protein that serves as a central hub connecting [protein-aggregation](/mechanisms/protein-aggregation), selective [autophagy](/mechanisms/autophagy-lysosome-neurodegeneration)mechanisms/autophagy), the [ubiquitin-proteasome-system](/mechanisms/ubiquitin-proteasome-system), and cellular stress response pathways [@deng2020]. As a selective [autophagy](/entities/autophagy) receptor, p62 recognizes ubiquitinated cargo — including misfolded proteins, damaged [mitochondrial-dynamics](/entities/mitochondrial-dynamics), and intracellular pathogens — and targets them for lysosomal degradation via interaction with LC3/GABARAP on autophagic membranes. [@ref2019]

SQSTM1/p62

Introduction

Sqstm1 P62 is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

SQSTM1 (Sequestosome 1), also known as p62, is a multifunctional scaffold and signaling adaptor protein that serves as a central hub connecting [protein-aggregation](/mechanisms/protein-aggregation), selective [autophagy](/mechanisms/autophagy-lysosome-neurodegeneration)mechanisms/autophagy), the [ubiquitin-proteasome-system](/mechanisms/ubiquitin-proteasome-system), and cellular stress response pathways [@deng2020]. As a selective [autophagy](/entities/autophagy) receptor, p62 recognizes ubiquitinated cargo — including misfolded proteins, damaged [mitochondrial-dynamics](/entities/mitochondrial-dynamics), and intracellular pathogens — and targets them for lysosomal degradation via interaction with LC3/GABARAP on autophagic membranes. [@ref2019]

Mutations in SQSTM1 are causally linked to Paget disease of bone and have been identified as genetic risk factors for [als](/diseases/amyotrophic-lateral-sclerosis) and [ftd](/diseases/frontotemporal-dementia) [@ref2019]. Beyond its role as a cargo receptor, p62 is a critical signaling node that integrates the [nf-kb](/entities/nf-kb) inflammatory pathway, the Keap1-NRF2 antioxidant response, and mTORC1 nutrient sensing, making it a key determinant of neuronal survival and vulnerability. [@snchezmartn2018]

Protein Structure and Functional Domains

p62 is a 440-amino-acid protein organized into multiple functional domains, each mediating distinct protein-protein interactions [@snchezmartn2018]: [@ciuffa2020]

PB1 Domain (Phox and Bem1p, residues 21–103)

The N-terminal PB1 domain mediates: [@liu2022]

• Self-oligomerization: p62 polymerizes into helical filaments through head-to-tail PB1-PB1 interactions, forming punctate structures known as p62 bodies [@ciuffa2020]
• Interaction with NBR1: Hetero-oligomerization with the autophagy receptor NBR1 enhances cargo recognition
• Interaction with MEKK3: Mediates activation of the MEK-ERK and [nf-kb](/entities/nf-kb) signaling pathways
• Interaction with PKCζ/λ: Regulates atypical protein kinase C signaling

ZZ-type Zinc Finger Domain (residues 128–163)

• Binds RIP1 (RIPK1) to activate [nf-kb](/entities/nf-kb) signaling
• Coordinates zinc ions for structural stability
• Functions as an N-recognin that binds N-terminal arginylated substrates for selective autophagy

TRAF6-Binding Domain (TB, residues 225–250)

• Binds TRAF6 (TNF Receptor-Associated Factor 6) to activate [nf-kb](/entities/nf-kb)
• Required for mTORC1 activation on lysosomal membranes via TRAF6-mediated K63-linked ubiquitination

LIR Motif (LC3-Interacting Region, residues 335–341)

The LIR motif (consensus: W/Y-X-X-L/I/V) directly binds to Atg8-family proteins (LC3A/B/C, GABARAP, GABARAPL1/2) on autophagosomal membranes: [@haack2016]

• Bridges ubiquitinated cargo to the autophagy machinery
• Essential for selective [autophagy](/mechanisms/autophagy-lysosome-neurodegeneration)mechanisms/autophagy) of protein aggregates (aggrephagy), damaged mitochondria ([mitophagy), and pathogens (xenophagy)

KIR Motif (Keap1-Interacting Region, residues 349–352)

• Binds Keap1 (Kelch-like ECH-associated protein 1) through a STGE motif
• Competes with NRF2 for Keap1 binding, leading to NRF2 stabilization
• Phosphorylation of Ser349 (human) enhances KIR-Keap1 affinity, activating the NRF2 antioxidant response

UBA Domain (Ubiquitin-Associated, residues 386–434)

The C-terminal UBA domain: [@berkamp2021]

• Recognizes K48-linked and K63-linked polyubiquitin chains on misfolded proteins and other cargo
• Essential for targeting ubiquitinated aggregates to autophagosomes
• UBA domain ubiquitination regulates p62 activity and aggregate clearance

Signaling Functions

Selective Autophagy Receptor

p62 is the prototypical selective autophagy receptor, mediating multiple forms of selective autophagy [@pankiv2007].

• Aggrephagy: Clearance of ubiquitinated protein aggregates — p62 oligomerizes via PB1 domain to concentrate and compact aggregates, then delivers them to autophagosomes via LIR-LC3 interaction
• [mitophagy](/mechanisms/mitophagy): Recognition of ubiquitinated outer mitochondrial membrane proteins downstream of [pink1](/proteins/pink1-protein)/[prkn](/genes/prkn) signaling
• Xenophagy: Targeting of ubiquitin-coated intracellular bacteria
• Lipophagy: Selective degradation of lipid droplets
• Ferritinophagy: NCOA4-mediated ferritin degradation, linking p62 to iron metabolism and [ferroptosis](/mechanisms/ferroptosis)

Keap1-NRF2 Antioxidant Pathway

p62 activates the NRF2 antioxidant defense system through a non-canonical mechanism [@snchezmartn2018]: [@komatsu2010]

This pathway is disrupted in multiple [neurodegenerative diseases, where impaired autophagy leads to p62 accumulation, chronic NRF2 activation, and eventual exhaustion of antioxidant defenses. [@turco2019]

mTORC1 Nutrient Sensing

p62 promotes mTORC1 activation on lysosomal membranes: [@rubino2012]

• Amino acid stimulation activates MEKK3 via the PB1 domain, leading to p38δ-mediated phosphorylation of p62
• Phosphorylated p62 recruits TRAF6, which K63-ubiquitinates [mtor-neurodegeneration](/mechanisms/mtor-neurodegeneration) to promote its activation
• This creates a regulatory circuit: mTORC1 activity suppresses autophagy (including p62 degradation), while p62 promotes mTORC1 activity

NF-κB Inflammatory Signaling

p62 modulates [nf-kb](/entities/nf-kb) signaling at multiple levels: [@zatloukal2002]

• Scaffolds TRAF6 and RIP1 to facilitate [nf-kb](/entities/nf-kb) activation downstream of TNF-α and IL-1β receptors
• Delivers polyubiquitinated TRAF6 to the autophagy pathway for degradation, providing negative feedback
• Links [neuroinflammation](/mechanisms/neuroinflammation) to protein quality control

p62 Bodies and Phase Separation

p62 forms cellular punctate structures known as p62 bodies, which have properties of liquid-liquid phase-separated condensates [@ciuffa2020]:

• Formation: PB1 domain-mediated polymerization drives p62 into filamentous assemblies that condense with ubiquitinated cargo
• Dynamics: p62 bodies exhibit liquid-like properties including fusion, fission, and internal mixing
• Function: Concentrates cargo and autophagy machinery, increasing efficiency of selective autophagy
• Pathological transition: In disease states, p62 bodies can transition from liquid-like to solid aggregate states, becoming resistant to autophagic clearance
• Relationship to [liquid-liquid-phase-separation](/mechanisms/liquid-liquid-phase-separation): p62 condensation intersects with other phase-separated structures including [stress-granules](/mechanisms/stress-granules) and [tdp-43](/proteins/tdp-43) aggregates

Role in Neurodegenerative Diseases

ALS and FTD

SQSTM1 mutations are established genetic risk factors for both [als](/diseases/amyotrophic-lateral-sclerosis) and [ftd](/diseases/frontotemporal-dementia), supporting the concept of an ALS-FTD disease continuum [@ref2019]:

• Causative mutations: Over 25 SQSTM1 variants have been identified in ALS/FTD patients, predominantly in the UBA domain (e.g., P392L, A390X) and LIR motif
• Pathogenic mechanisms:
• UBA domain mutations impair ubiquitin binding, reducing clearance of ubiquitinated protein aggregates
• LIR motif mutations disrupt LC3 interaction, blocking autophagic delivery
• ALS-FTLD-linked mutations disrupt selective autophagy and the NFE2L2/NRF2 antioxidant stress pathway
• D329G/D329H variants cannot be proteolyzed by caspase-8, impairing mTORC1 activation
• [tdp-43](/proteins/tdp-43) pathology: p62-positive, ubiquitin-positive inclusions containing [tdp-43](/proteins/tdp-43) are a hallmark of ALS/FTD. p62 colocalizes with [tdp-43](/proteins/tdp-43) aggregates and may either facilitate their clearance (functional p62) or contribute to inclusion formation (dysfunctional p62)
• [c9orf72](/genes/c9orf72): [c9orf72](/genes/c9orf72) repeat expansions produce dipeptide repeat proteins that form p62-positive aggregates

Alzheimer's Disease

p62 dysfunction contributes to AD pathology:

• Accumulates in neurofibrillary tangles with hyperphosphorylated tau]
• Co-localizes with [amyloid-beta](/proteins/amyloid-beta) plaques
• Reduced p62 expression correlates with impaired [autophagy](/mechanisms/autophagy-lysosome-neurodegeneration)mechanisms/autophagy) in AD brains
• p62-mediated NRF2 activation provides initial neuroprotection but becomes overwhelmed as disease progresses
• Polymorphisms in SQSTM1 may modulate AD risk

Parkinson's Disease

p62 intersects with key PD pathways:

• Functions as a [mitophagy](/mechanisms/mitophagy) receptor downstream of [pink1](/proteins/pink1-protein)/[prkn](/genes/prkn), delivering ubiquitinated mitochondria to autophagosomes
• Accumulates in Lewy bodies with [alpha-synuclein](/proteins/alpha-synuclein)
• p62-mediated aggrephagy targets α-synuclein aggregates for degradation
• [lrrk2](/proteins/lrrk2-protein) kinase activity regulates p62 phosphorylation and autophagic flux

Huntington's Disease

• p62 is sequestered into mutant [huntingtin](/proteins/huntingtin) aggregates in HD
• Sequestration reduces p62 availability for normal autophagic functions
• p62 overexpression enhances clearance of mutant [huntingtin](/proteins/huntingtin) in cellular models

Childhood-Onset Neurodegeneration

Biallelic loss-of-function mutations in SQSTM1 cause a severe childhood-onset neurodegenerative syndrome characterized by progressive ataxia, dystonia, supranuclear gaze palsy, and cognitive decline, demonstrating that complete p62 loss is incompatible with normal neuronal function [@haack2016].

p62 as a Diagnostic Marker

p62 immunostaining is used diagnostically in neuropathology:

• p62-positive inclusions: Present in ALS, FTD-TDP, and [c9orf72](/genes/c9orf72)-associated disease
• Neurofibrillary tangles: p62 co-stains with tau] tangles in AD and other tauopathies
• Lewy bodies: p62 co-stains with α-synuclein in PD and [lewy-body-dementia](/diseases/lewy-body-dementia)
• Polyglucosan bodies: p62-positive in [lafora-disease](/diseases/lafora-disease) and other glycogen storage disorders

Therapeutic Implications

Autophagy Enhancement

Strategies to enhance p62-mediated selective autophagy include:

• Rapamycin and rapalogs: Inhibit mTORC1 to induce autophagy, though this also reduces p62 expression
• Trehalose: [mtor-neurodegeneration](/mechanisms/mtor-neurodegeneration)-independent autophagy inducer that enhances p62-mediated aggregate clearance
• Spermidine: Natural polyamine that induces autophagy and enhances p62-dependent cargo recognition

p62-Targeted Approaches

• [autophagy](/mechanisms/autophagy-lysosome-neurodegeneration)mechanisms/autophagy)-targeting chimeras (AUTACs): Bifunctional molecules linking a target-binding moiety to a p62-recruiting tag, directing specific proteins for autophagic degradation
• p62 stabilizers: Small molecules that enhance p62 oligomerization and cargo recognition
• NRF2 activators: Dimethyl fumarate and other NRF2 activators act partly through the p62-Keap1 axis

Gene Therapy

• SQSTM1 gene delivery to restore p62 function in [neurons](/entities/neurons) with loss-of-function mutations
• Strategies to enhance p62 expression in aging [neurons](/entities/neurons) where levels decline

Background

The study of Sqstm1 P62 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.

External Links

• [PubMed](https://pubmed.ncbi.nlm.nih.gov/) - Biomedical literature
• [Alzheimer's Disease Neuroimaging Initiative](https://adni.loni.usc.edu/) - Research data
• [Allen Brain Atlas](https://brain-map.org/) - Brain gene expression data

Brain Atlas Resources

• Allen Human Brain Atlas: [SQSTM1/p62 expression search](https://human.brain-map.org/microarray/search/show?search_term=SQSTM1%2Fp62)
• Allen Mouse Brain Atlas: [SQSTM1/p62 search](https://mouse.brain-map.org/search/index.html?query=SQSTM1%2Fp62)
• Allen Cell Type Atlas: [Transcriptomic cell type reference](https://portal.brain-map.org/atlases-and-data/rnaseq)
• BrainSpan Developmental Transcriptome: [SQSTM1/p62 developmental expression](https://www.brainspan.org/rnaseq/search/index.html?search_term=SQSTM1%2Fp62)

See Also

• [alpha-synuclein](/proteins/alpha-synuclein)
• [Huntingtin ([HTT](/proteins/huntingtin))[/proteins/[huntingtin](/proteins/huntingtin)

References