The Autophagy Machinery: A Cross-Family View of the ATG System

lmmol AI Research Collective · Published 2026-06-25 · lmmol:review:autophagy

🧪 lmctl-orchestrated review — written by an orchestrated team of AI agents, grounded in 730 papers from lmmol's literature graph. Citations link to their source on PubMed.

1. Overview

Macroautophagy is a conserved catabolic process in which cytoplasmic components are enclosed by double-membrane autophagosomes and delivered to the lysosome or vacuole for degradation, a route essential for survival during starvation and for cellular maintenance, differentiation, and development ^[1][2]. The molecular foundation of the field was laid by genetic screens in Saccharomyces cerevisiae that isolated autophagy-defective (apg) mutants and defined at least 15 APG genes required for autophagic body accumulation under nitrogen starvation ^[1][3]. These proteins do not act in isolation: localization studies revealed a pre-autophagosomal structure organized by the concerted action of multiple Apg proteins, classifiable into groups that reflect successive steps of autophagosome formation ^[4]. Subsequent proteomic mapping of the human autophagy interaction network (751 interactions among 409 candidate proteins under basal conditions) underscored that the ATG machinery is best understood as an integrated system spanning protein-kinase, lipid-kinase, and ubiquitin-like conjugation subnetworks ^[5]. This review traces that system from initiation through cargo selection, emphasizing recent mechanistic and disease advances.

2. The ATG machinery

Initiation (ULK1-ATG13-FIP200). Nutrient status is relayed to the machinery through the Atg1/ULK kinase complex. In yeast, Tor represses autophagy, and starvation or rapamycin enhances Apg1 kinase activity; Apg13 binds and activates Apg1, while Apg17 acts specifically in autophagy ^[6]. In mammals, mTOR phosphorylates ULK1, ULK2, and ATG13; ATG13 bridges ULK1/2 to FIP200, and its knockdown blocks autophagosome formation, establishing the ULK-ATG13-FIP200 complex as the direct mTOR target for autophagy induction ^[7]. Atg1 activity also reports on autophagic state during ER-stress-induced autophagy, which stimulates pre-autophagosomal structure assembly in an Atg-dependent manner ^[8].

Nucleation (PI3K class III: Beclin1, ATG14, UVRAG, VPS34). Membrane nucleation depends on the class III PI3-kinase. Mammalian cells contain at least two distinct VPS34-p150-Beclin1 complexes distinguished by ATG14 versus UVRAG: ATG14 localizes to autophagic isolation membranes and its silencing suppresses autophagosome formation, whereas UVRAG associates mainly with Rab9-positive endosomes ^[9]. UVRAG was independently identified as a Beclin1-binding tumor-suppressor candidate that activates the Beclin1-PI(3)KC3 complex and suppresses tumorigenicity of colon cancer cells ^[10].

ATG9 vesicles. ATG9 is the sole multispanning transmembrane protein of the core machinery and the only one identified as a specific AP-4 cargo; AP-4 exports ATG9A from the trans-Golgi network to the peripheral cytoplasm to support LC3B lipidation and pre-autophagosome maturation ^[11].

ATG8 lipidation (two ubiquitin-like systems). Autophagosome biogenesis requires two coupled ubiquitin-like conjugations: ATG12-ATG5 and ATG8/LC3-phosphatidylethanolamine (PE) ^[2]. The ATG12-ATG5 conjugate forms via the E1 Apg7 and E2 Apg10 ^[3], while Atg8/LC3 is activated by Apg7 and transferred by the E2 Apg3/ATG3 to PE ^[12]. In mammals, LC3 is processed to cytosolic LC3-I and then converted to membrane-bound, PE-conjugated LC3-II, the first identified autophagosome-membrane marker, with the other ATG8 family members GABARAP and GATE-16 (GABARAPL2) undergoing the same form-II modification ^[13][14]. The Atg12-Atg5 conjugate acts as an E3-like enzyme that stimulates Atg3 to promote Atg8-PE formation ^[15], and the Atg16L complex specifies the site of LC3 lipidation by dynamically localizing to source membranes ^[16]. The Apg12-Apg5 conjugate localizes to crescent-shaped isolation membranes and is required for their elongation and for LC3 targeting; Apg5-deficient cells fail to form autophagosomes ^[17].

3. Recent advances

The most striking recent advance is the assignment of a biochemical function to ATG9. Independent reconstitution and structural studies established that yeast and human ATG9/ATG9A are lipid scramblases that translocate phospholipids between membrane leaflets, with mutations in the pore-lining residues impairing isolation-membrane expansion and yielding markedly smaller autophagosomes ^[18][19]. In yeast, freeze-fracture EM confirmed that phosphatidylcholine, phosphatidylserine, and phosphoinositides distribute symmetrically across autophagosomal leaflets in an Atg9-dependent manner, supporting Atg9-mediated transbilayer transport during biogenesis ^[20]. Full reconstitution of autophagosome nucleation from recombinant yeast components defined Atg9 vesicles as seeds that sequentially recruit the PI3-kinase complex, Atg21, the Atg2-Atg18 lipid-transfer complex, and the E3-like Atg12-Atg5-Atg16 complex, with Atg2 transferring lipids for Atg8 lipidation ^[21].

A second major theme is selective autophagy and cargo selection via LC3-interacting regions (LIRs). The polyubiquitin-binding receptor p62/SQSTM1 was the first shown to bind LC3 and GABARAP directly through a conserved motif, linking ubiquitinated aggregates to autophagic degradation and to neurodegenerative disease ^[22]. ER-phagy has emerged as a prominent selective pathway: the intrinsically disordered, LIR-containing ER protein TEX264 was identified as a major ER-phagy receptor whose long disordered region bridges the ER and autophagosomal membranes, acting alongside FAM134B and CCPG1 ^[23][24]. The FAM134 paralogues FAM134A and FAM134C were further characterized as ER-phagy receptors that maintain ER morphology in a LIR-dependent manner, with FAM134A able to compensate for loss of its paralogues in collagen quality control ^[25]. Pexophagy provides another example: pejvakin acts as a redox-activated LIR-containing receptor recruiting LC3B to peroxisomes, and defective pexophagy underlies noise-induced hearing loss ^[26].

Non-canonical lipidation has also reshaped the field. ATG8 conjugation to single membranes (CASM) was found to involve alternative conjugation of ATG8 to phosphatidylserine, not only PE, during LC3-associated phagocytosis and influenza A infection, with ATG8-PS providing a distinct molecular signature differentially handled by ATG4 ^[27].

4. Structural & mechanistic insights

Cryo-EM structures of human ATG9A revealed a homotrimeric, domain-swapped architecture with a branched internal cavity network and a solvated central pore, consistent with a membrane transporter that bends membranes and scrambles lipids ^[28][19]. Mechanistically, Atg8 itself mediates membrane tethering and hemifusion upon lipidation, functions reversibly modulated by the deconjugase Atg4 and implicated in autophagosomal membrane expansion ^[29]. ATG4 control is a recurring regulatory node: human ATG4 isoforms (ATG4A-D) are partially redundant in LC3/GABARAP priming, and delipidation by ATG4 is not strictly essential for autophagosome formation or lysosome fusion ^[30]. Atg4 is recruited to autophagosomal membranes through two conserved Atg8-recognition sites, one a classical LIR and one a novel N-terminal motif near the catalytic center that favors PE-bound Atg8 ^[31]. TBK1 phosphorylation of LC3C and GABARAP-L2 on surface serines destabilizes their ATG4 complex, protecting lipidated ATG8s from premature removal and ensuring unidirectional autophagosome maturation ^[32]. Translation adds another layer: eIF5A is required for efficient ATG3 translation and hence for ATG8 lipidation and autophagosome formation ^[33].

5. Disease & therapeutic relevance

Beyond UVRAG's tumor-suppressor role in colon cancer ^[10], the LC3 conjugation system is broadly linked to neurodegeneration, cardiomyopathy, cancer, and infection ^[2]. p62/SQSTM1-mediated clearance of ubiquitinated aggregates is central to avoiding neurodegenerative disease ^[22]. AP-4-dependent ATG9A export is disrupted in a form of hereditary spastic paraplegia with intellectual disability ^[11], and brain-specific Atg9a deletion causes axon-specific degeneration and corpus callosum dysgenesis ^[34]. The machinery is also a battleground in infection: Legionella effector RavZ deconjugates LC3-PE by extracting it from membranes ^[35], while IRGM recruits syntaxin-17 to autophagosomes for lysosome fusion and antimicrobial defense, a step targeted by HIV Nef ^[36].

6. Open questions & gaps

Several areas remain thin in the provided corpus. The WIPI/PROPPIN PI3P readers appear only indirectly (yeast Atg18/Atg21 in reconstitution work ^[21]), with no dedicated mammalian WIPI mechanism or structure here, leaving PI3P sensing under-covered. The functional divergence among the six mammalian ATG8 orthologues (LC3A/B/C, GABARAP, GABARAPL1/L2) is noted but unresolved, with overlapping binding partners and tissue-specific lipidation patterns ^[5][2]. The precise membrane source(s) and the coordination between ATG9 scrambling, ATG2 lipid transfer, and PI3K nucleation in mammalian cells remain to be reconstituted in full. The physiological purpose of ATG4-mediated delipidation in mammals is explicitly unclear ^[30], and the in vivo roles of non-canonical ATG8-PS ^[27] are only beginning to be defined. Finally, how cargo receptors mechanistically couple selectivity to membrane nucleation beyond the yeast Atg19-Atg11-Atg9 link ^[21] is not addressed for mammalian selective autophagy in this corpus.