p62 / SQSTM1 (Sequestosome-1)

p62 is the prototypical selective autophagy cargo receptor — a 440-amino-acid scaffold that bridges polyubiquitinated cytosolic cargo to the autophagosome membrane via a tripartite functional logic: oligomerization (PB1 domain), ubiquitin recognition (UBA domain), and autophagosome docking (LIR motif binding to lc3 / ATG8 family). Beyond cargo delivery, p62 acts as a signaling hub: it scaffolds aPKC in the NF-κB pathway, activates Nrf2 by sequestering its repressor KEAP1, and senses amino-acid sufficiency to help gate mTORC1 activity. Because p62 is itself an autophagy substrate, its steady-state level serves as a widely-used inverse readout of autophagy flux — but this interpretation requires controlling for transcriptional upregulation via Nrf2, which can elevate p62 mRNA independently of flux changes.

Identity

UniProt: Q13501 (SQSTM1_HUMAN) — Swiss-Prot (manually curated)
NCBI Gene: 8878
HGNC symbol: SQSTM1 (gene); protein commonly called p62
Ensembl: ENSG00000161011
Mouse ortholog: Sqstm1 (one-to-one; mouse p62 functionally interchangeable in most in-vitro assays)
Length: 440 amino acids (canonical isoform) ¹
Molecular weight: ~47 kDa (calculated); commonly referred to as “p62” due to apparent MW on SDS-PAGE

Domain organization

p62 is a modular adaptor organized N-to-C along a linear sequence that allows simultaneous engagement of multiple binding partners ²:

Domain	Residues (approx.)	Function
PB1 (Phox/Bem1p)	3–102	Self-oligomerization; also binds aPKC (scaffold for NF-κB signaling)
ZZ-type zinc finger	123–173	Protein interactions; binds RIP1 kinase (RIPK1); N-degron recognition (ZZA-type)
TRAF6-binding motif	~225–250	Recruits TRAF6 → NF-κB activation
NLS (nuclear)	~256–265	Nuclear import; links to PML nuclear bodies
LIR (LC3-interacting region)	~336–341	Core W-x-x-L motif (W338-D339-E340-L341); binds LC3-I/II, GABARAP family
KIR (KEAP1-interacting)	346–359	Sequesters KEAP1 → liberates Nrf2 for antioxidant transcription; crystal structure shows V348–L355 electron density clearly resolved ³
UBA (ubiquitin-associated)	389–434	Binds K48- and K63-linked polyubiquitin chains; UBA–UBA dimerization competes with ubiquitin binding

Key PTMs:

Ser403 phosphorylation (by CK2, TBK1) — enhances UBA affinity for ubiquitinated cargo; required for efficient aggrephagy ²
Ser349 phosphorylation — promotes KEAP1 binding; Komatsu 2010 shows the D349A mutation strongly impairs KEAP1 interaction (Asp349 stabilizes the KIR type I β-turn via intra-peptide H-bonding), but does not identify the kinase or demonstrate mTORC1-dependent phosphorylation. The mTORC1-Ser349 phosphorylation connection comes from Ichimura 2013 (Mol Cell 56:793–805), not from Komatsu 2010. needs-canonical-id
Lys420 acetylation — modulates ubiquitin-binding and p62-body dynamics

Selective autophagy mechanism

The core cargo-capture and delivery cycle proceeds in four steps:

Oligomerization. The PB1 domain drives head-to-tail self-polymerization, forming filamentous p62 oligomers / condensates. This concentrates the UBA domains and amplifies avidity for polyubiquitinated substrates. PB1-deficient constructs (D69A, R21A point mutants or N-terminal deletions lacking PB1) fail to form cytoplasmic bodies and fail to co-localize with LC3 under starvation; the UBA domain is additionally required for body formation ⁴.
Cargo capture. UBA domain binds K48- and K63-linked polyubiquitin chains on misfolded proteins, aggregates, damaged mitochondria (tagged by PINK1/Parkin during mitophagy), intracellular bacteria (xenophagy), and damaged peroxisomes (pexophagy). Ser403 phosphorylation strengthens affinity. The resulting p62-cargo condensate (a “p62 body”) can grow to micron scale and is highly enriched at quality-control compartments.
Autophagosome docking. The LIR motif (W338-D339-E340-L341) engages the two hydrophobic pockets (HP1, HP2) on the surface of LC3-I, LC3-II, or GABARAP-family proteins presented on the phagophore inner and outer membranes. This interaction was directly demonstrated by Pankiv et al. 2007, who showed that mutation of W338 or L341 abolished the interaction and prevented p62 delivery to autolysosomes ².
Incorporation and degradation. p62 and its cargo are enclosed within the autophagosome, fused with lysosomes, and degraded by lysosomal hydrolases. Because p62 itself is consumed in each round, its steady-state level reflects the balance between synthesis and autophagic degradation.

Autophagy flux readout — critical caveat

Increased p62 protein accumulation is widely interpreted as evidence of impaired autophagy flux. This interpretation is valid only when p62 transcription is held constant. Nrf2 activation — which can occur downstream of oxidative stress, KEAP1 sequestration by p62 itself (a positive feedback loop), or exogenous Nrf2 inducers — transcriptionally upregulates SQSTM1 mRNA, increasing p62 protein independently of flux ³. In aging settings where both oxidative stress and autophagy decline concurrently, p62 changes must be interpreted alongside mRNA measurements or flow-based flux assays (e.g., chloroquine-chase for LC3-II turnover).

Selective autophagy variants

p62 participates as a primary or secondary receptor across multiple selective autophagy contexts:

Cargo type	Process	Co-receptors / notes
Ubiquitinated aggregates	aggrephagy	p62 is the principal receptor; NBR1 acts redundantly
Damaged/depolarized mitochondria	mitophagy	Parkin ubiquitinates OMM proteins → p62 recruited; OPTN/NDP52 also act in parallel
Intracellular bacteria	xenophagy	p62 + NDP52 + OPTN; bactericidal urgency context
Damaged peroxisomes	pexophagy	PEX5/PEX14 route distinct from p62; p62 provides backup
Protein bodies / stress granules	stress-granule autophagy	p62 condensates co-dissolve with cargo

Signaling roles (beyond autophagy)

p62 functions as a multivalent scaffold that can activate or modulate several signaling axes independently of its cargo-receptor role:

NF-κB activation. Via its PB1 domain, p62 associates with aPKC (atypical PKC: PKCζ / PKCι) and adaptor proteins. The ZZ domain binds RIPK1 and recruits TRAF6, an E3 ubiquitin ligase that K63-polyubiquitinates itself and NEMO/IKKγ, ultimately triggering IKK complex activation and NF-κB nuclear translocation ⁵. In aging contexts, p62 scaffolding at NF-κB may contribute to low-level chronic inflammation (see chronic-inflammation).

Nrf2/KEAP1 pathway. The KIR motif (residues 346–359; V348–L355 resolved by crystal structure ³) binds KEAP1 with high affinity; Asp349 stabilizes the KIR type I β-turn, and Ser349 phosphorylation (attributed to mTORC1 by Ichimura 2013, not Komatsu 2010) further promotes KEAP1 binding. KEAP1 normally functions as an adaptor that targets Nrf2 for CUL3-based ubiquitination and proteasomal degradation. p62 sequesters KEAP1 away from Nrf2, preventing Nrf2 degradation and allowing it to translocate to the nucleus and activate the antioxidant response element (ARE) gene program — including SQSTM1 itself and NQO1, HMOX1, GCLC ³. This p62–KEAP1–Nrf2 axis creates a positive-feedback amplifier for stress resistance; it is also exploited by cancer cells to sustain antioxidant defenses.

mTORC1 amino-acid sensing. p62 has been reported to interact with mTORC1 regulatory components at the lysosomal surface in an amino-acid-dependent manner, contributing to mTORC1 activation via the RAGULATOR–RAG GTPase axis. The exact molecular details remain debated. no-mechanism

Model: p62 accumulation as a readout of autophagy deficiency

The canonical demonstration that autophagy deficiency causes p62 accumulation comes from conditional Atg7 knockout studies. Komatsu et al. 2007 crossed Sqstm1-null mice with Atg7 conditional knockouts (neuronal CaMKII-Cre driver, per training knowledge — local PDF corrupted, re-verification needed) and showed that the ubiquitin-positive inclusion bodies that accumulate in Atg7-KO neurons contain p62 as a required structural component: Sqstm1/Atg7 double-knockout neurons formed few inclusions ⁶. This established p62 as the organizer of the ubiquitin inclusion body phenotype — not merely a passive passenger. needs-replication

Dimension	Status	Notes
Pathway conserved in humans?	yes	LIR/UBA/PB1 domains and their functions are conserved
Phenotype conserved in humans?	yes	p62+ inclusions are hallmarks of ALS, FTD, PD, Paget disease — all human diseases
Replicated in humans?	partial	Genetic KO data is mouse; human disease mutations in SQSTM1 provide supporting evidence

Knockout and disease phenotypes

Sqstm1 global knockout (mouse): Mice lacking p62 are viable and fertile but develop late-onset obesity, insulin resistance, and type 2 diabetes-like metabolic phenotypes ⁷. This is thought to reflect disruption of p62-mediated aPKC–NF-κB scaffolding in metabolic tissues, rather than autophagic cargo-receptor function per se. The absence of immediate autophagy catastrophe demonstrates that other cargo receptors (NBR1, OPTN, NDP52) provide functional redundancy for bulk ubiquitinated-cargo clearance.

Conditional Atg7-KO (neuronal): Loss of atg7 in post-mitotic neurons causes progressive accumulation of p62- and ubiquitin-positive cytoplasmic inclusion bodies, behavioral abnormalities, and neurodegeneration without amyloid — demonstrating that autophagy is the primary clearance route for aggregation-prone proteins in neurons, with p62 marking the failure points ⁶. ⚠️ Local PDF for this reference is corrupted; the Cre driver identity (CaMKII-Cre vs. nestin-Cre) and quantitative details require re-verification.

Human disease mutations (SQSTM1):

Paget disease of bone (PDB3): Multiple loss-of-function and dominant-negative mutations in the UBA domain (e.g., P392L) impair ubiquitin binding, causing osteoclast dysregulation. OMIM #167250.
Frontotemporal dementia / ALS (FTDALS3): ALS-associated mutations affect p62 oligomerization and stress granule dynamics. p62+ cytoplasmic inclusions are a defining neuropathological feature of TDP-43 proteinopathy in both ALS and FTD.
Neurodegeneration with ataxia, dystonia, gaze palsy (NADGP): Recently characterized via biallelic LoF SQSTM1 mutations.
Distal myopathy with rimmed vacuoles (DMRV): Rimmed vacuoles contain p62 and ubiquitin, consistent with autophagic clearance failure.

Aging-specific context

Decline of p62 in aged tissues. Multiple studies report reduced p62 protein in aged rodent brain and muscle, consistent with a general decline in autophagy flux with age. This is interpreted as a vicious cycle: as mTOR becomes more active and upstream autophagy-initiation signals (AMPK, ULK1) weaken in aged tissues, p62-dependent cargo clearance decreases, misfolded proteins accumulate, and proteotoxicity grows. However, quantitative human data for age-dependent p62 decline is sparse, and the confounding of Nrf2-driven transcription complicates interpretation. needs-human-replication needs-replication

Pro-longevity effects of p62 upregulation. In Drosophila, ubiquitous induction of Ref(2)P (the p62 ortholog) from day 30 (midlife) onward significantly extends lifespan and healthspan in females only (lifespan extension in male flies was not observed) ⁸. The mechanism requires both autophagy (Atg1 RNAi suppresses the benefit) and mitochondrial fission (Drp1^K38A dominant-negative abrogates the lifespan benefit); mitophagy is required for the longevity effect. The paper also shows improved proteostasis (reduced ubiquitinated aggregates in flight muscle). The “reduced oxidative damage” framing is not directly stated in the paper — the mechanism is mitochondrial fission → mitophagy → improved mitochondrial function. needs-human-replication

Dimension	Status	Notes
Pathway conserved in humans?	yes	LIR-mediated selective autophagy operates in human cells
Phenotype conserved in humans?	partial	p62-OE lifespan experiments are in fly only; human SQSTM1 gain-of-function not studied
Replicated in humans?	no	No human interventional data targeting p62 levels specifically

SQSTM1 mutations in ALS/FTD and aggregation diseases. p62 positive inclusions are among the most reliable neuropathological markers of TDP-43 and FUS proteinopathies (ALS/FTD), synucleinopathies (Parkinson’s), and tauopathies. Whether p62 accumulation is pathogenic (directly toxic, sequestering important cargo away from degradation) or protective (concentrating misfolded proteins for bulk clearance) remains contested. contradictory-evidence

Pathway membership and key interactors

autophagy — central cargo receptor; p62 level is a primary flux readout
mitophagy — recruited to Parkin-ubiquitinated OMM proteins; acts alongside OPTN, NDP52
ubiquitin-proteasome-system — crossroads: p62 is also present at aggresomes formed when the proteasome is overwhelmed
nf-kb — PB1-aPKC scaffold and TRAF6 recruiter; also a transcriptional target of NF-κB (NF-κB response element in SQSTM1 promoter — feedforward)
lc3 — direct LIR-mediated interaction; the p62–LC3 axis is the canonical cargo-to-autophagosome bridge
atg7 — upstream E1-like enzyme required for LC3 lipidation; Atg7-KO is the primary genetic tool for p62 accumulation phenotypes
ulk1 — upstream kinase that phosphorylates p62 (and initiates phagophore nucleation); AMPK-dependent ULK1 activity is the proximal trigger for p62-body resolution under nutrient stress
mtor — mTORC1 inhibits ULK1 → reduces p62 flux; Ser349 phosphorylation (which promotes KEAP1 binding) has been attributed to mTORC1 by Ichimura 2013 (Mol Cell 56:793–805), not to Komatsu 2010
pink1, parkin — in mitophagy, Parkin-generated K63-pUb chains on VDAC1/BNIP3L serve as p62 UBA docking sites
tfeb — parallel lysosomal biogenesis transcription factor; p62 and TFEB are both mTORC1 substrates and both induced by autophagy

Pharmacological relevance

There is currently no clinically approved agent that specifically targets p62. Indirect modulation:

Rapamycin / rapalogs — mTORC1 inhibition activates ULK1, increasing p62 flux and reducing steady-state p62 levels
AMPK activators (metformin, AICAR) — upstream of ULK1; promote p62-body resolution
Proteasome inhibitors (bortezomib, carfilzomib) — cause p62 and ubiquitin accumulation at aggresomes (convergent pathway)
Nrf2 activators (sulforaphane, bardoxolone) — increase p62 mRNA transcriptionally via the ARE, confounding flux interpretation

Limitations and knowledge gaps

p62 as flux readout requires mRNA controls. Transcriptional upregulation via Nrf2 or NF-κB can raise p62 protein even when flux is unchanged or improved. Many published experiments report only protein without mRNA or LC3-II flux controls. needs-replication
Age-dependent p62 decline in humans is poorly quantified. Rodent data exist; matched human tissue studies with proper controls for transcription are scarce. needs-human-replication
p62-OE longevity in mammals untested. The Drosophila p62-OE lifespan extension has not been replicated in a mammalian system. needs-human-replication
mTORC1 interaction mechanism at lysosome. p62’s role in the RAGULATOR-RAG axis is debated; the exact binding interfaces and physiological significance under normal nutrient conditions are not fully resolved. no-mechanism
Pathogenic vs. protective role of p62 inclusions. In ALS/FTD and synucleinopathies, whether p62 bodies are cytoprotective condensates or toxic structures remains an open and contested question. contradictory-evidence
Park 1995 DOI mismatch. The DOI 10.1074/jbc.270.27.16243 — identified as the original sequestosome cloning paper — resolves in the archive to a Photosystem I paper (Synechocystis PCC 6803). This is a known BUG-2 archive DOI mismatch. The original sequestosome-naming paper has not been independently confirmed to a verified DOI in this pass; excluded from citation. needs-canonical-id
Komatsu 2007 Cell PDF corrupted. The local PDF at is a Cell Press recruitment media kit, not the paper. Claims attributed to Komatsu 2007 (Cre driver identity, double-KO phenotype quantification) remain unverified from source. A re-download or PMC author manuscript is needed.
Ser349 phosphorylation kinase misattributed in prior draft. The claim that “mTORC1 phosphorylates Ser349 to enhance KEAP1 binding” was incorrectly attributed to Komatsu 2010. Komatsu 2010 only shows Asp349 structural importance (D349A impairs binding); the mTORC1–Ser349 link is from Ichimura 2013 (Mol Cell 56:793–805). Corrected in body text above.

Footnotes

UniProt Q13501 (SQSTM1_HUMAN), Swiss-Prot entry, accessed 2026-05-04 · 440 aa, manually curated · canonical human isoform ↩
pankiv-2007-p62-lc3-lir · doi:10.1074/jbc.M702824200 · in-vitro (HeLa, HEK293) · model: co-immunoprecipitation, fluorescence microscopy · 4,465 citations (100th percentile) · Pankiv S et al., J Biol Chem 2007 · defines W338-x-x-L341 LIR consensus; Ser403 phosphorylation enhances flux; key paper establishing p62 as direct LC3-binding cargo receptor · ⚠️ PDF download failed (no PMC candidate URLs; publisher paywalled); claims derived from training knowledge. no-fulltext-access ↩ ↩² ↩³
komatsu-2010-p62-nrf2-keap1 · doi:10.1038/ncb2021 · in-vivo (mouse, Mx1-Cre × Atg7^flox/flox hepatocyte-specific KO, poly(I)·poly(C) induced) + in-vitro (primary hepatocytes, HEK293T) · 2,347 citations (100th percentile) · Komatsu M et al., Nat Cell Biol 12:213–223, 2010 · PDF verified · KIR = residues 346–359 (V348–L355 electron density clearly resolved in crystal structure); p62 competes with Nrf2-ETGE for KEAP1 DC domain binding at 1:1 stoichiometry (K_d = 5.4±0.3 × 10^5 M^−1 by ITC); p62 accumulation sequesters KEAP1 → Nrf2 activation → SQSTM1 transcriptional upregulation (positive feedback); Asp349 critical for type I β-turn of KIR (D349A markedly impairs KEAP1 binding); Ser349 phosphorylation not attributed to mTORC1 in this paper ↩ ↩² ↩³ ↩⁴
bjorkoy-2005-p62-selective-autophagy · doi:10.1083/jcb.200507002 · in-vitro (HeLa, S-GFP-p62 HeLa, NIH3T3, SHSY-5Y) · model: huntingtin aggregates in cell culture · 3,207 citations (100th percentile) · Bjørkøy G et al., J Cell Biol 2005 · shows p62 bodies degraded by autophagy; both PB1 (oligomerization) and UBA (ubiquitin-binding) domains required for cytoplasmic body formation; p62 forms a shell surrounding huntingtin aggregates; protective against huntingtin-induced cell death ↩
doi:10.1042/ebc20170035 · review · Essays Biochem 2017 · 710 citations (100th percentile) · Lamark T, Johansen T · comprehensive review of p62/SQSTM1 selective autophagy paradigm; NF-κB signaling scaffold mechanism; KIR/KEAP1/Nrf2 axis · ⚠️ not_oa — PDF unavailable; cannot verify claims attributed to this review. no-fulltext-access ↩
komatsu-2007-p62-atg7-inclusions · doi:10.1016/j.cell.2007.10.035 · in-vivo (mouse, Sqstm1-/- × CaMKII-Cre Atg7fl/fl) · model: neuronal autophagy deficiency · 2,094 citations (100th percentile) · Komatsu M et al., Cell 2007 · demonstrates p62 is required to form ubiquitin+ inclusion bodies in Atg7-KO neurons; double-KO neurons fail to form inclusions; establishes p62 as organizing scaffold for autophagy-deficiency phenotype · ⚠️ LOCAL PDF CORRUPTED (downloaded a Cell Press media kit); claims derived from training knowledge, not PDF read. Re-verification needed. needs-replication ↩ ↩²
bjorkoy-lamark-2006-p62-review · doi:10.4161/auto.2.2.2405 · review · Autophagy 2006 · 322 citations · Bjørkøy G, Lamark T, Johansen T · overview of p62 as missing link between aggregates and autophagy; Sqstm1-/- metabolic phenotype description · ⚠️ PDF download failed (bronze OA, no candidate URLs); claims not verified against source. no-fulltext-access ↩
aparicio-2019-ref2p-drosophila-lifespan · doi:10.1016/j.celrep.2019.06.070 · in-vivo (Drosophila melanogaster, female) · n > 147 (log-rank) · model: daGS>UAS-dp62 midlife induction (day 30 onward), ubiquitous · Aparicio R, Rana A, Walker DW, Cell Rep 28:1029–1040, 2019 · lifespan extension in females only (males: not significant); requires Atg1-dependent autophagy and Drp1-dependent mitochondrial fission; proteostasis improvement in flight muscle; not yet replicated in mammals needs-human-replication ↩

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