Fig. 1.

Overview of autophagy. Cellular stresses, including reactive oxygen or reactive nitrogen species (ROS, RNS), or nutritional/energetic stress activate AMP-activated protein kinase (AMPK) and inhibit mammalian target of rapamycin (mTOR) to signal autophagy through Atg1. Once initiated, Beclin-1 and VPS34, a class I phosphatidylinositol 3-kinase, trigger downstream events leading to activation of the first of two ubiquitin-like pathways. First, Atg12 is activated similarly to ubiquitin by Atg7, an E1-like enzyme, which then transfers it to Atg10, an E2-like enzyme, which then conjugates it to lysine 130 of Atg5. The Atg5/Atg12 conjugate then complexes with a homodimer of Atg16, and the complex assembles on a membrane structure termed the phagophore. This event is a prerequisite for the second ubiquitin-like pathway, which involves the cleavage of the terminal Cys residue of Atg8 (LC3) by Atg4, a cysteine protease, which exposes a terminal glycine residue. Atg4 is redox regulated and appears to upregulate autophagy in the face of oxidative stress. Atg4 in conjunction with Atg7 facilitates conjugation of LC3 onto phosphatidylethanolamine in the lipid bilayer of the membrane. The cup-shaped phagophore is recruited to engulf targets via adaptor proteins such as p62, which binds ubiquitinated protein aggregates and LC3. The growing ends of the phagophore eventually meet and fuse to enclose the target within the double-membrane structure of the autophagosome. At this point Atg12/5/16L are released, and the autophagosome fuses with a lysosome; fusion and degradation of contents requires acidification, which is mediated by the vacuolar proton ATPase (VPATPase); lysosomal enzymes such as cathepsins, and lipases have pH optima well below 6.0. The entire process of formation and destruction occurs on a time-scale of minutes. PIP₃, phosphatidylinositol 3,4,5-trisphosphate.