Autophagy is a bulk protein and organelle degradation system and is an important homeostatic cellular recycling mechanism. The following kinds are the three types of autophagy: macroautophagy, microautophagy, and chaperone-mediated autophagy. In general, the term “autophagy” indicates macroautophagy. Autophagy is mediated by double-membrane-bound structures called autophagosomes. During the autophagic process, cytoplasmic components are sequestered and engulfed by autophagosomes. Autophagosomes then fuse with lysosomes to form autolysosomes where the sequestered components are digested by lysosomal hydrolases. Microtubule-associated protein 1 light chain 3 (LC3) is an autophagosomal ortholog of the yeast protein ATG8. Autophagy stimulates the upregulation of LC3 expression, and a cytosolic form of LC3 (LC3-I) is conjugated to phosphatidylethanolamine to form LC3-II which is recruited to autophagosomal membranes. Subsequently, LC3-II is degraded by lysosomal hydrolases after the fusion of autophagosomes with lysosomes. Therefore, LC3 is a specific marker of autophagosome formation. Additionally, beclin 1, the mammalian ortholog of the yeast protein ATG6, has been known to play a crucial role in autophagy. Beclin 1 acts in conjunction with the phosphoinositide-3 kinase pathway to enhance the formation of the autophagic vacuole.
Part of the book: Cell Death