Signaling Pathway

Autophagy-related proteins (ATG) regulate this process in yeast and many ATG proteins are conserved in mammals. Individual ATG proteins and ATG-complexes support specific steps in the dynamic process of autophagy. The formation of a double membrane vesicle for the entrapment and delivery of cytosolic content to lysosomes is the hallmark of autophagy. Formation of the mature organelle or autolysosome involves a sequence of coordinated events. In each step, ATG proteins catalyze specific reactions critical for the maintenance of autophagic flux.

Learn about autophagosome biogenesis: Autophagy Webinar

Explore the Autophagy Interactive Pathway

Autophagic Flux

The process of autophagy involves a series of events including:

Induction:

• May be triggered by starvation or reduced nutrient availability.
• Inhibition of mTOR kinase (mammalian target of rapamycin), a key regulator of nutrient signaling.
• Upon mTOR inhibition, decrease phosphorylation status of a protein complex including ULK1 (unc-51-like kinase 1), ATG13/ATG101 and FIP200/RB1CC1 (RB1-inducible coiled-coil 1), increases the activity of ULK1 and induces autophagy.
  
  Laverne >>> Regulation of Autophagy

Nucleation, Phagophore Formation:

• Release of Beclin1 and AMBRA1 from their inhibitor Bcl-2.
• Formation of the Ptdlns3K (Class lll phosphatidylinositol 3-kinase) complex including ATG14, VPS15 and VPS34 (vacuolar protein sorting).
• PtdIns3K complex phosphorylates lipids and gives rise to phosphatidylinositol-3-phosphate and recruits ATGs to the new membrane.
• The phagophore transmembrane protein ATG9 participates in the addition of membrane during elongation.

Elongation, Autophagosome Formation:

• Mediated by two ubiquitin-like conjugation complexes: ATG7 and ATG10 act as E1 and E2 enzymes to conjugate ATG12 to ATG5.
• Atg12-Atgt5 conjugate with ATG16L1 (Atg16-like1) and promote the lipidation of LC3-II.
• ATG4, ATG7 (E1) and ATG3 (E2) process pro-LC3 to the final lipid conjugated form LC3-II.
• Phagophore’s membrane expands and closes onto itself sequestering the cytosolic content.
• All peripheral ATG proteins dissociate from the autophagosome and return to the cytosol.
• Lipidated LC3 remains associated with the autophagosome’s internal membrane.
• LC3-II interacts with p62/SQSTM1 inside the autophagosome.

Fusion and Degradation:

• SNARE protein STX17 (syntaxin 17 - blue) localizes to completed autophagosomes.
• STX17 interacts with SNAP29 - green and lysosomal VAMP8 - red and leads to autophagosome-lysosome fusion.
• Proton pumps and hydrolases are added to the autophagosome for formation of the autolysosome, cargo acidification and hydrolysis.
  
  Laverne >>> Phagosome-Lysosome Fusion