Fig. 1.

The autophagy pathway and its roles in cell survival and cell death. In the presence of an autophagy inducer, cytoplasmic materials, such as protein aggregates and organelles, are sequestered by a pre-autophagosomal membrane structure, the phagophore. The phagophore membrane then expands and encloses its cargo to form a double-membrane vesicle, the autophagosome. The autophagosome fuses with a lysosome (or a vacuole in yeast) to form an autolysosome, in which the enclosed cargo is degraded by acid hydrolases. After the resulting macromolecules are transported back into the cytosol through membrane permeases, they can either be used to synthesize proteins or can be oxidized by the mitochondria to generate ATP for cell survival. However, when autophagy occurs at excessive levels or under certain physiological conditions it can lead to type II programmed cell death (type II PCD). See the text for additional details.