| SQSTM1 — Sequestosome 1 (p62) | |
|---|---|
| Symbol | SQSTM1 |
| Full Name | Sequestosome 1 (p62) |
| Chromosome | 5q35.3 |
| NCBI Gene | 8878 |
| Ensembl | ENSG00000161011 |
| OMIM | 601530 |
| UniProt | Q13501 |
| Diseases | [ALS](/diseases/als), [PD](/diseases/parkinsons-disease), [AD](/diseases/alzheimers), [Paget's Disease](/diseases/paget) |
| Expression | Brain, Liver, Muscle |
| Key Mutations | |
| P392L, A33V, K238E | |
SQSTM1 (Sequestosome 1), also known as p62, is a gene located on chromosome 5q35.3 that encodes a critical scaffolding protein involved in autophagy, the cellular degradation system that clears misfolded proteins and damaged organelles. Mutations in SQSTM1 are associated with amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), Alzheimer's disease (AD), and Paget's disease of bone. The gene is catalogued as NCBI Gene ID 8878 and OMIM 601530.
The SQSTM1 gene encodes p62/sequestosome-1, a multifunctional protein that serves as a master regulator of cellular proteostasis. p62 contains multiple domains that enable its diverse functions in protein quality control and signaling [1].
p62 contains several key functional domains:
p62 is central to selective autophagy, a process that specifically targets damaged proteins and organelles for lysosomal degradation [2]:
Cargo Recognition: p62 binds to ubiquitinated proteins through its UBA domain, forming protein aggregates tagged for degradation [3].
Autophagosome Loading: p62's LIR domain interacts with LC3/Atg8 on the autophagosome membrane, recruiting cargo into the growing autophagosome.
Aggregate Clearance: p62-labeled aggregates are delivered to lysosomes, where they are degraded along with the p62 protein itself.
Feedforward Regulation: p62 transcription is activated by Nrf2 during oxidative stress, enhancing the cell's capacity to clear damaged proteins [4].
Beyond autophagy, p62 participates in key signaling pathways:
SQSTM1 mutations were first linked to ALS in 2011, identifying missense mutations in patients with both familial and sporadic disease [5]. p62-positive inclusions are found in virtually all ALS cases, regardless of SQSTM1 mutation status.
Key features:
p62 plays a complex role in PD pathogenesis:
In AD, p62 dysfunction contributes to disease progression:
SQSTM1 mutations cause Paget's disease of bone (PDB), characterized by abnormal bone remodeling. This represents a connection between skeletal and neurological disease through the same gene.
In neurodegenerative diseases, p62 function becomes impaired:
Mutation Effects: Disease-causing mutations reduce p62's ability to bind ubiquitinated proteins and engage autophagy [2:1].
Aggregate Accumulation: Impaired p62 function leads to accumulation of ubiquitinated protein aggregates.
Neuronal Vulnerability: Motor neurons and dopaminergic neurons are particularly sensitive to p62 dysfunction.
p62 forms the core of cellular inclusions found in neurodegeneration:
These inclusions represent failed attempts at aggregate clearance.
p62 mutations affect multiple signaling pathways:
| Mutation | Position | Disease Association |
|---|---|---|
| P392L | Pro392→Leu | ALS, PDB |
| A33V | Ala33→Val | ALS risk factor |
| K238E | Lys238→Glu | ALS, PD |
The P392L mutation is the most studied, affecting p62's ability to engage autophagy [5:1].
p62 is an attractive therapeutic target:
Autophagy Enhancement: Compounds that enhance autophagy could compensate for p62 dysfunction [2:2].
Aggregate Clearance: Agents promoting protein aggregate clearance through autophagy.
mTOR Modulation: Targeting mTORC1 signaling to restore autophagy flux.
Nrf2 Activation: Activating Nrf2 to boost antioxidant responses.
Recent research on SQSTM1/p62 has expanded our understanding of its role in neurodegeneration:
The PB1 (Phox and Bem1) domain is located at the N-terminus of p62 and serves as a critical module for protein-protein interactions. This domain mediates p62's ability to form higher-order oligomers through homotypic interactions, creating large signaling platforms [8]. The PB1 domain consists of a ubiquitin-like fold that interacts with the PB1 domain of other proteins including:
The oligomerization capacity of the PB1 domain is essential for p62's function in forming cytoplasmic inclusions and signaling complexes. Mutations in the PB1 domain can disrupt oligomerization and impair p62's ability to sequester ubiquitinated cargo [9].
The ZZ domain is a zinc finger structure that mediates interactions with RING finger proteins and other zinc-binding domains. This domain is involved in:
The ZZ domain's ability to bind RING finger proteins positions p62 as a critical regulator of ubiquitination-dependent signaling pathways.
The LIR is a compact 20-amino acid sequence that recognizes LC3/GABARAP proteins on the growing autophagosome membrane [10]. This region contains a consensus sequence:
[FW]-{ED}[WLF]-[ED][FW]-[IV]
The LIR undergoes a conformational change upon binding to LC3, converting from a disordered state to an α-helical structure [11]. This binding is critical for:
The UBA (Ubiquitin-Associated) domain is located at the C-terminus and specifically recognizes polyubiquitin chains, particularly K63-linked and K27-linked chains that serve as signals for selective autophagy [12]. The UBA domain:
Mutations in the UBA domain (particularly P392L) reduce ubiquitin-binding capacity and are linked to ALS and PDB [5:3].
p62 serves as a bridge between the ubiquitin-proteasome system (UPS) and autophagy [3:2]. While the UPS degrades individual misfolded proteins, autophagy removes larger aggregates that cannot be processed by the proteasome. p62 coordinates these pathways through:
This coordinated approach ensures comprehensive cellular proteostasis.
p62 is a core component of aggresomes, cytoplasmic inclusion bodies that form when the UPS is overwhelmed [13]. Aggresome formation involves:
Aggresomes are protective structures that sequester toxic proteins, but their persistence indicates failed autophagic clearance.
p62 is a potent activator of NF-κB signaling, a pathway critical for inflammatory responses [14]. The mechanism involves:
In the brain, chronic NF-κB activation contributes to neuroinflammation, a hallmark of neurodegenerative diseases.
Beyond NF-κB, p62 regulates inflammasome complexes that process pro-inflammatory cytokines:
This dual role positions p62 as a key regulator of neuroinflammation.
ALS is characterized by progressive motor neuron death and the presence of cytoplasmic inclusions containing TDP-43 [15]. p62 inclusions are found in:
The P392L mutation in the UBA domain is the most common pathogenic variant, reducing ubiquitin-binding and autophagy capacity.
In PD, p62 interacts with several disease-related proteins:
Genetic studies have identified SQSTM1 variants as risk factors for sporadic PD.
p62 plays multiple roles in AD pathogenesis [7:1]:
Therapeutic strategies targeting p62 are being explored for AD treatment.
FTD shares pathological features with ALS, including TDP-43 inclusions:
Several compounds enhance p62-mediated autophagy:
| Compound | Mechanism | Status |
|---|---|---|
| Rapamycin | mTORC1 inhibition | Preclinical |
| Trehalose | mTORC1-independent autophagy | Preclinical |
| Lithium | GSK-3β inhibition | Clinical trials |
| Carbamazepine | TPC2 inhibition | Phase II |
| Urolithin A | Mitophagy enhancement | Phase II/III [16] |
p62 stabilizes Nrf2, making Nrf2 activators therapeutically relevant:
Direct targeting of p62 is being explored:
p62 levels in cerebrospinal fluid (CSF) and blood are being investigated as:
PET ligands targeting p62 inclusions are in development:
A key mystery remains: how does p62, a pro-autophagy protein, accumulate in inclusions in neurodegenerative diseases? Possible explanations include:
Resolving this paradox will guide therapeutic development:
SQSTM1 testing is recommended in cases of:
For families with SQSTM1 variants:
Current recommendations for SQSTM1 carriers:
Single-cell sequencing approaches are revealing:
Mass spectrometry-based proteomics has identified:
Patient-derived induced pluripotent stem cells offer:
SQSTM1/p62 stands at the nexus of multiple neurodegenerative disease pathways. Its dual role in autophagy regulation and protein aggregate formation, combined with its signaling functions in NF-κB and Nrf2 pathways, makes it a critical node in cellular proteostasis. Understanding how this multifunctional protein contributes to disease pathogenesis will be essential for developing effective therapies for ALS, PD, AD, and related conditions.
The continuing identification of SQSTM1 mutations in diverse patient populations, combined with advances in model systems and therapeutic targeting, positions p62 as a promising target for future intervention strategies.
Galloway PG, et al. p62/SQSTM1: emerging key player in neurodegeneration. Acta Neuropathologica Communications. 2014. ↩︎ ↩︎
Wang H, et al. Autophagy role in neurodegeneration: p62 as a key player. Neurobiology of Aging. 2019. ↩︎ ↩︎ ↩︎ ↩︎
Liu X, et al. p62 in protein quality control and disease. Trends in Cell Biology. 2021. ↩︎ ↩︎ ↩︎
Jain S, et al. p62-mediated transcriptional activation of Nrf2. Molecular and Cellular Biology. 2015. ↩︎
Fecto F, et al. Sequestration of TDP-43 in ALS with mutations in SQSTM1. Brain. 2011. ↩︎ ↩︎ ↩︎ ↩︎
Liu Y, et al. p62 interacts with LRRK2 and regulates its autophagy. Journal of Neuroscience. 2018. ↩︎ ↩︎
Kurosaki M, et al. p62/SQSTM1 in tauopathies. Acta Neuropathologica. 2020. ↩︎ ↩︎
Bjørkøy G, et al. p62 forms protein aggregates in autophagy. Cell Death and Differentiation. 2005. ↩︎
Komatsu M, et al. Homeostatic levels of p62 are necessary for Lissencephaly-1-mediated autophagy. Nature. 2007. ↩︎
Pankiv S, et al. p62 binds to LC3 through a WXXL motif to activate autophagy. Journal of Biological Chemistry. 2007. ↩︎
Ichimura Y, et al. Structural basis for the recognition of LC3 by p62. Journal of Cell Biology. 2008. ↩︎
Katsuragi Y, et al. p62/sequestosome-1 is a polyubiquitin chain binding protein. Autophagy. 2015. ↩︎
Zatloukal K, et al. p62 is a common component of cytoplasmic inclusions in various neurodegenerative diseases. American Journal of Pathology. 2009. ↩︎
Korolchuk VI, et al. p62/SQSTM1 links autophagy to the regulation of nuclear factor kappaB activity. Molecular Cell Biology. 2009. ↩︎
Watanabe Y, et al. p62/SQSTM1 is required for TDP-43 aggregation. Journal of Neuroscience. 2017. ↩︎
Jiménez-Loygorri JI, et al. Urolithin A promotes p62-dependent lysophagy to prevent retinal neurodegeneration. Autophagy Reports. 2024. ↩︎