Fig. 2.

Overview of the general autophagy pathway in mammalian cells. (Bottom) Autophagy can be divided into five major steps: (1) Initiation and nucleation of the double-membrane phagophore, (2) elongation and (3) closure of the phagophore to form the autophagosome, (4) autophagosome-lysosome fusion, and (5) lysosomal degradation and nutrient recycling. (Top left) Autophagy induction is primarily mediated by the ULK1 complex, which is regulated by AMPK and mTORC1. Upon activation, the ULK1 complex activates the class III phosphatidylinositol 3-kinase (PI3K) complex through phosphorylation of beclin 1 (BECN1) and VPS34. The Class III PI3K complex generates PI3P at the site of nucleation of phagophore from ER. (Top middle) ProLC3B is converted to LC3B-I via the cleavage by ATG4B. LC3B-I is conjugated with PE through ubiquitin-like conjugation systems that include ATG7 (E1 ligase), ATG3 (E2 ligase), and ATG12, ATG5, and ATG16L (the E3 ligase complex). The resulting PE-conjugated LC3, which is called LC3B-II (shown as small green circles), is inserted on the phagophore membranes, where it facilitates phagophore elongation and closure. (Pale blue frames) Inhibitors of ULK1/2 (SBI-0206965 [139], MRT68921, MRT 67,307 [140], ULK101 [141]), VPS34 (VPS34-In1 [142], PIK-III [143], SAR405 [144], Compound 31 [145], Spautin1 [146]), ATG4B (S130 [147], FMK-9a [148], NSC185058 [149]), and the lysosome [CQ/HCQ, BafA1, Lys05 [150]]) are shown. ULK Unc-51-like kinase, PI3K phosphatidylinositol 3-kinase, PI3P phosphatidylinositol 3-phosphate, ER endoplasmic reticulum, WIPI WD-repeat protein interacting with phosphoinositide, mTORC1 mammalian target of rapamycin complex 1, AMPK 5' AMP-activated protein kinase, PE phosphatidylethanolamine, p62/SQSTM1 sequestosome 1, NBR1 neighbor of BRCA1