lc3

LC3 / MAP1LC3B

The canonical mammalian autophagosome membrane marker. LC3B (gene MAP1LC3B) is a ubiquitin-like modifier that becomes covalently lipidated and inserted into the autophagosomal membrane — enabling biochemical quantification of autophagy flux, cargo recruitment via LIR-motif cargo receptors, and selective autophagy targeting. It is the most widely used single readout of autophagy activity in cell biology and aging research.

Identity

• UniProt: Q9GZQ8 (MAP1LC3B_HUMAN; Swiss-Prot reviewed)
• Gene: MAP1LC3B (aliases: MAP1ALC3)
• NCBI Gene ID: 81631
• HGNC: 13352
• Ensembl: ENSG00000197548
• Mouse ortholog: Map1lc3b (one-to-one; function and lipidation machinery fully conserved)
• Length: 125 amino acids (pro-LC3 form; mature form after ATG4 cleavage = 120 aa exposing C-terminal Gly120)
• Molecular weight: ~14.7 kDa (apparent MW on SDS-PAGE: LC3-I ~16–18 kDa, LC3-II ~14–16 kDa despite heavier mass due to PE adduct altering SDS binding; Kabeya 2000 observed 18 kDa / 16 kDa for rat LC3; Klionsky 2021 gives 16–18 kDa / 14–16 kDa as the standard range)

Paralog family — the mammalian ATG8 superfamily

Mammals encode six ATG8 family members divided into two subfamilies (note: Klionsky 2021 lists LC3B2 as an additional LC3 subfamily member, which would bring the canonical count to seven; LC3B2 is omitted from the table below as it is less studied in aging contexts):

Subfamily	Members	UniProt (human)	Notes
LC3	LC3A (MAP1LC3A)	Q9H492	Two splice variants (LC3A-I and LC3A-II); -I/-II/-III notation refers to processing state, not splice variant count
LC3	LC3B (MAP1LC3B)	Q9GZQ8	Canonical aging/autophagy marker
LC3	LC3C (MAP1LC3C)	Q9BXW4	Restricted expression; roles in STING-autophagy
GABARAP	GABARAP	O95166	Autophagosome maturation/fusion
GABARAP	GABARAPL1	Q9H0R8	Neuronal; enriched in brain
GABARAP	GABARAPL2 (GATE-16)	P60520	Golgi-associated; ER-Golgi transport

In yeast, all six functions are compressed into a single gene: ATG8 ¹. The diversification in mammals may reflect expanded cargo-selectivity requirements. LC3B is the most abundant and ubiquitously expressed family member; it is the default choice for Western blot and immunofluorescence readouts of autophagy in most mammalian cell types.

Structure and maturation

LC3B adopts a ubiquitin-like fold (Pfam: PF02991) with an N-terminal alpha-helical extension absent in canonical ubiquitin. The protein is synthesized as pro-LC3 and requires obligate processing:

Maturation cascade

1. Pro-LC3 synthesis — ribosome produces full-length precursor (human LC3B: 125 aa).
2. ATG4 protease cleavage — ATG4B (the primary mammalian isoform; four ATG4 paralogues exist: ATG4A/B/C/D) cleaves C-terminal residues after Gly120, exposing the glycine’s carboxyl group. This produces LC3-I (cytosolic, Gly120 C-terminal) ².
3. ATG7 activation (E1-like) — ATG7 adenylates the Gly120 carboxyl in an ATP-dependent reaction, forming a high-energy LC3-AMP intermediate, then transfers LC3 to an ATG7 active-site Cys via thioester bond.
4. ATG3 conjugation (E2-like) — ATG7 transfers LC3 to ATG3 (E2-like enzyme) via transthioesterification.
5. ATG12–ATG5–ATG16L1 complex (E3-like) — the ATG12~ATG5 conjugate (itself formed by a parallel ubiquitin-like cascade) recruits ATG16L1 and functions as the E3, facilitating transfer of LC3 to phosphatidylethanolamine (PE) on the autophagosomal membrane. The product is LC3-II (PE-conjugated). Note: Ichimura 2000 ¹ established the E1 (Apg7) and E2 (Apg3) steps in yeast Apg8 conjugation; the E3-like role of ATG12–ATG5–ATG16L1 for mammalian LC3-PE was demonstrated in subsequent work (Hanada et al. 2007 Nat Cell Biol; Fujita et al. 2008 Nat Cell Biol). unsourced — a study page for the E3-specific citation is needed.

LC3-II is membrane-anchored and appears on both the inner and outer autophagosomal membrane leaflets. On the outer leaflet, it can be deconjugated back to LC3-I by ATG4B after autophagosome closure — enabling recycling. LC3-II on the inner leaflet is degraded upon lysosomal fusion.

Slower SDS-PAGE migration of LC3-I vs LC3-II

A common source of confusion: despite the added PE adduct making LC3-II heavier, it runs faster (apparent lower MW) on SDS-PAGE than LC3-I. This is because the hydrophobic PE moiety alters SDS binding and the lipid anchor reduces aqueous migration rate differently from the expected mass shift. Apparent MW values vary by system: Kabeya 2000 observed LC3-I at ~18 kDa and LC3-II at ~16 kDa for rat LC3; Klionsky 2021 gives 16–18 kDa (LC3-I) and 14–16 kDa (LC3-II) as the standard mammalian range. The two forms are distinguishable on standard PAGE ³.

LC3-II as an autophagy flux readout

What LC3-II level measures

LC3-II accumulation = autophagosome formation rate minus degradation rate. A static increase in LC3-II can reflect either:

• Increased autophagy induction (more autophagosomes forming), OR
• Impaired autophagy completion (block in fusion or lysosomal degradation)

These are biologically opposite scenarios. They cannot be distinguished from a single Western blot time-point ³.

Flux assay — lysosomal inhibitor blockade

Standard practice (per Klionsky 2021 guidelines): compare LC3-II levels with and without lysosomal inhibitors (bafilomycin A1, chloroquine, or ammonium chloride) ⁴:

• If LC3-II increases further upon inhibitor treatment → autophagic flux is active (degradation was occurring)
• If LC3-II does not increase upon inhibitor treatment → flux is blocked upstream (autophagosomes not forming or not delivering cargo)

This “flux assay” is the minimal requirement for concluding that a treatment increases autophagy rather than merely blocking degradation.

LC3-II in aging

In post-mitotic neurons, elevated basal LC3-II in aged animals may reflect impaired lysosomal clearance rather than enhanced autophagy initiation. Conditional knockout of Atg5 in neural precursor cells (Hara 2006, nestin-Cre) and conditional knockout of Atg7 in CNS cells (Komatsu 2006, nestin-Cre) each independently cause progressive neurodegeneration in mice, with ubiquitin-positive inclusion body accumulation — establishing that basal autophagy is essential for neuronal proteostasis ^{5 6}. needs-human-replication — these KO studies are mouse only; human neurodegeneration causally linked to autophagy impairment is indirect.

Dimension	Status
Pathway conserved in humans?	yes — ATG8 lipidation cascade identical in human cells
Phenotype conserved in humans?	partial — neurodegeneration from autophagy failure is conserved; aging-specific LC3 accumulation profiling is incomplete
Replicated in humans?	no — Atg5/Atg7 neural KO neurodegeneration is mouse-only; human genetic equivalents not identified

LIR motif — cargo receptor binding

LC3 family members interact with LIR (LC3-interacting region) motifs on cargo receptors. The consensus LIR is:

W/F/Y - x - x - L/I/V

Also called the AIM (Atg8-interacting motif) in yeast. LIR-containing cargo receptors include:

Receptor	Gene	Cargo targeted	LIR core
p62 / Sequestosome-1	SQSTM1	Ubiquitinated protein aggregates	WSTL
NBR1	NBR1	Ubiquitinated cargo, complement	YIII
NDP52	CALCOCO2	Ubiquitinated bacteria, mitochondria	ILVV
Optineurin	OPTN	Ubiquitinated bacteria, mitochondria	FAVI
BNIP3	BNIP3	Damaged mitochondria (hypoxia)	YEVL
FUNDC1	FUNDC1	Mitochondria (hypoxia)	YEVL
TEX264	TEX264	ER (ER-phagy)	—

p62/SQSTM1 is the best-characterized cargo receptor — it binds polyubiquitinated substrates via its UBA domain and LC3 via its LIR motif, bridging cargo to the autophagosomal membrane ⁷. p62 level itself is often used as a secondary autophagy readout: p62 accumulates when autophagy is impaired (because it is itself a cargo), so elevated p62 combined with elevated LC3-II suggests a block in flux rather than enhanced initiation.

Selective autophagy variants

LC3 participates in all major selective autophagy subtypes via cargo-specific receptors:

• mitophagy — BNIP3/NIX (hypoxia-induced), FUNDC1, or the PINK1-Parkin-driven ubiquitin pathway (p62/NDP52/OPTN as receptors)
• Aggrephagy — p62/NBR1 targeting ubiquitin-decorated protein aggregates
• Xenophagy — NDP52/OPTN/p62 targeting ubiquitin-coated intracellular bacteria
• ER-phagy — TEX264, FAM134B, RTN3L, SEC62
• Pexophagy — NBR1 in mammals
• Lysophagy — galectin-3 / p62 after lysosomal membrane permeabilization

Role in aging

Autophagy flux declines with age

Multiple lines of evidence indicate that autophagy flux decreases in aged tissues:

• LC3-II accumulation in aged mouse brain without compensatory increase in degradation markers (impaired flux interpretation) needs-human-replication
• ATG5, ATG7, and Beclin-1 protein levels decrease in aged rodent tissues in multiple studies unsourced — a systematic multi-tissue census citation is needed
• Neural-specific Atg5 deletion (Hara 2006, nestin-Cre) and neural-specific Atg7 deletion (Komatsu 2006, nestin-Cre) each independently produce a neurodegeneration phenotype with ubiquitin-positive inclusions, progressive behavioral deficits, and early death in mice ^{5 6}
• Atg5 transgenic overexpression in mice extends median lifespan ~17.2% (n=65 WT / 70 Tg; χ²=17.32 p<0.001) — see ulk1 for details on this finding via the autophagy initiation axis

GABARAP subfamily in aging

The GABARAP subfamily may play distinct roles from LC3 subfamily in autophagosome maturation and lysosomal fusion — age-related changes in GABARAP subfamily members are less characterized than LC3B. unsourced — a comparative aging-specific profiling citation is needed.

Pathway membership

• autophagy — core effector; ATG4-ATG7-ATG3-ATG5/12/16L1 cascade; marks phagophore and autophagosome
• mitophagy — required for receptor-mediated mitophagic targeting; LIR-docking of BNIP3/FUNDC1/NDP52/OPTN
• mtor — mTORC1 suppresses autophagy initiation via ulk1 phosphorylation; LC3-II accumulation is a downstream readout of mTOR inhibition
• ampk — activates ulk1, relieving mTOR brake; AMPK-dependent autophagy flux read out by LC3-II increase

Key interactors

• atg4b — required for pro-LC3 cleavage; also deconjugates LC3-II from outer membrane post-fusion (recycling arm)
• atg7 — E1-like activating enzyme in lipidation cascade
• atg3 — E2-like conjugating enzyme
• beclin-1 — part of PI3K complex (VPS34-Beclin-1-ATG14L) that generates PI3P to recruit phagophore nucleation machinery; upstream of LC3 lipidation
• ulk1 — initiating kinase; ULK1 complex activation leads to downstream ATG cascade activation and LC3-II accumulation
• sqstm1 — primary cargo receptor for ubiquitinated substrates; LIR-docking partner

Biochemical assay notes

• Western blot: Urea-containing sample buffer or methanol fixation improves LC3-II band intensity (lipid anchor requires careful lysis). Standard RIPA lysis may underestimate LC3-II.
• Immunofluorescence: LC3 puncta (dots) = autophagosomes; diffuse cytoplasmic signal = LC3-I. Puncta count per cell is an autophagosome number readout, not a flux readout — combine with lysosomal markers (LAMP1/2) or inhibitor treatment.
• GFP-LC3 constructs: Tandem mCherry-GFP-LC3 reporters allow flux monitoring — GFP is quenched in the acidic lysosome while mCherry is not; ratio reports autophagosome-lysosome fusion status.
• LC3-II/I ratio vs absolute LC3-II: Klionsky 2021 guidelines explicitly discourage using LC3-II/LC3-I ratio as the primary metric because LC3-I levels vary independently; LC3-II should be normalized to a housekeeping protein (e.g., actin or GAPDH) rather than to LC3-I, with the caveat that even housekeeping proteins may vary under autophagy-inducing conditions ⁴.

Limitations and gaps

• Paralog compensation: LC3A, LC3B, LC3C, and GABARAPs are partially redundant. LC3B knockout alone may be compensated. Studies using only LC3B antibodies may miss redistribution among family members. needs-replication
• Aging-tissue LC3-II profiling: Systematic multi-organ aging atlas of LC3-II flux (formation rate vs degradation rate separately) in humans is lacking. unsourced
• GABARAP subfamily aging biology: LC3 subfamily is far better characterized than GABARAP subfamily in aging contexts. unsourced
• LC3B as a proxy for all autophagy: Selective autophagy subtypes may be regulated differently from bulk macroautophagy; LC3B alone cannot distinguish these. no-mechanism for subtype specificity in aged cells.
• Human genetic evidence: No human loss-of-function variant in MAP1LC3B with a confirmed aging phenotype is known. The aging-relevance of LC3 in humans is inferred from mouse KO and flux studies. needs-human-replication

Footnotes

Footnotes

1. ichimura-2000-atg8-ubiquitin-lipidation · doi:10.1038/35044114 · in-vitro + in-vivo · model: yeast + mammalian cell-free · 2,016 citations · Ichimura Y et al., Nature 2000. Established ubiquitin-like conjugation cascades for ATG8 and ATG12; defined E1/E2/E3 analogy for autophagy conjugation systems. Archive: local PDF available. ↩ ↩²

2. kabeya-2000-lc3-autophagosome · doi:10.1093/emboj/19.21.5720 · in-vitro + in-vivo · model: rat brain + COS-7 cells · 6,474 citations (OpenAlex) · Kabeya Y et al., EMBO J 2000. First characterization of LC3 as mammalian Atg8 homologue; demonstrated LC3-I→LC3-II conversion on autophagosome membranes. Archive: pending download. ↩

3. mizushima-yoshimori-2007-lc3-immunoblotting · doi:10.4161/auto.4600 · review · model: N/A · 2,575 citations · Mizushima N & Yoshimori T, Autophagy 2007. Definitive guide to interpreting LC3 Western blots; clarified that LC3-II runs faster than LC3-I despite heavier mass; established flux-measurement requirements. Archive: pending download. ↩ ↩²

4. klionsky-2021-autophagy-guidelines-4th · doi:10.1080/15548627.2020.1797280 · review · model: N/A · 2,434 citations · Klionsky DJ et al., Autophagy 2021. Fourth edition consensus guidelines for autophagy assays; advises comparing LC3-II to housekeeping proteins rather than to LC3-I (LC3-II/LC3-I ratio is discouraged as a primary metric); mandates flux controls (lysosomal inhibitors or equivalent); stresses LC3/GABARAP levels alone do not address autophagic flux. Archive: local PDF available. ↩ ↩²

5. hara-2006-atg5-neural-neurodegeneration · doi:10.1038/nature04724 · in-vivo · model: Atg5 neural conditional KO mice (nestin-Cre) · 3,796 citations · Hara T et al., Nature 2006. Showed that loss of basal autophagy (via Atg5 deletion) in neural cells causes progressive neurodegeneration with ubiquitin-positive cytoplasmic inclusion bodies; established requirement for autophagy in neuronal protein homeostasis. Archive: local PDF available. ↩ ↩²

6. komatsu-2006-atg7-cns-neurodegeneration · doi:10.1038/nature04723 · in-vivo · model: Atg7 CNS-specific KO mice (Nestin-Cre) · 3,439 citations · Komatsu M et al., Nature 2006. Companion paper to Hara 2006; independently showed CNS-specific Atg7 loss causes inclusion body neurodegeneration; replicated the essential role of autophagy in neuronal proteostasis. Archive: local PDF available. ↩ ↩²

7. pankiv-2007-p62-lir-lc3 · doi:10.1074/jbc.m702824200 · in-vitro · model: HEK293 cells · 4,465 citations · Pankiv S et al., J Biol Chem 2007. Showed p62/SQSTM1 binds LC3 directly via LIR motif (WSTL); point mutations in LIR abolished co-IP and autophagic degradation of ubiquitinated cargo. Archive: pending download. ↩