Autophagosome formation machinery.

(1) The initiation of autophagosome formation requires the activation of the ULK complex (composed of ULK1/2 (unc-51-like kinases 1 and 2), ATG13, ATG101, and FIP200 (also known as RB1CC1)), which further activates ATG13 inducing its translocation to the ER where autophagosome formation occurs. (2) The ULK complex also activates and recruits (3) the class III PI3K lipid kinase complex (composed of VPS34, VPS15, Beclin 1, and ATG14) to the ER. (4) There, the PI3K III complex phosphorylates surrounding PI2P from the ER membrane, generating a PI3P-rich membrane. (5) The local PI3P enrichment of the ER membrane allows the recruitment of PI3P-binding proteins such as DFCP1 promoting the formation of a particular compartment termed “omegasome” from which autophagosomes are generated. (6) Other PI3P-binding proteins such as WIPI proteins are required for the formation of the isolation membrane of the future autophagosome, also called phagophore. WIPI proteins bind to and bring the ATG5-ATG12-ATG16L1 complex to the isolation membrane where it acts as a conjugating system, with ATG3, ATG4, and ATG7. (7) This machinery conjugates a PE to the LC3 type I protein converting it into LC3 type II (or LC3-PE), which is therefore incorporated in the isolation membrane, allowing its elongation and closure into an autophagosome. (8) LC3-PE then interacts with the targeted cargo to be degraded through specific adapters that harbour an LIR motif, allowing its sequestration inside the autophagosome, and further degradation through the lysosomal pathway. Created with BioRender.com. DFCP1, double FYVE-containing protein 1; ER, endoplasmic reticulum; LIR, LC3-interacting region; PE, phosphatidylethanolamine; PI2P, phosphatidylinositol-2-phosphate; PI3K, phosphatidylinositol-3-phosphate kinase; PI3P, phosphatidylinositol-3-phosphate; ULK, unc-51-like kinase; VPS34, vacuolar sorting protein 34; WIPI, WD repeat domain phosphoinositide-interacting.