Modified mitochondrial dynamics in neurodegeneration
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As defined in yeast, in the first step of Atg8 conjugation, the cysteine protease Atg4 proteolytically removes a C-terminal arginine of Atg8, exposing a glycine that is then accessible to the E1-like Atg7, the same enzyme used in the Atg12-Atg5 conjugation system. Atg7 activates Atg8, which is then transferred to another E2-like enzyme, Atg3, and eventually conjugated to PE through an amide bond. Atg8 conjugated to PE behaves like a membrane protein. Unlike the Atg12-Atg5 conjugation, modification of PE with Atg8 is a reversible event, in which Atg4 can again cleave Atg8 after the glycine residue to remove it from the lipid [93]. Atg8-PE is detected on both the forming intermediate vesicle and the completed autophagosome [94]. It is transported to the lysosome/vacuole and is degraded along with the cargo. Accordingly, Atg8-PE (LC3-II or LC3-PE) is the best candidate for a structural component of the autophagosome. However, both the Atg12-Atg5Atg16 complex and the Atg8-PE conjugate localize at the PAS, the initial sequestering membrane structure (Figure 2) [95]. Atg5, which cooperates with LC3-II [96] is an essential autophagy gene involved in the early stages of autophagosome formation [97], and seems to be extremely important to the membrane targeting of LC3 [96]. Indeed, downregulation of Atg5 or expression of Atg5 (K130R) mutant, suppresses vacuole formation and cell death [98]. Atg5 was also found to interact with FADD, Fas-Associated protein with Death Domain, an adaptor molecule that bridges the Fas-receptor, and other death receptors, to caspase-8 to form the death-inducing signaling complex (DISC) during extrinsic apoptosis [98]. Moreover, Atg5 can be cleaved by calpains. Thus, Simon and coworkers [99] reported that in cells overexpressing Atg5, a calpain cleavage product of Atg5 translocates to mitochondria, triggering cell death involving cytochrome c release and partial antagonism of Bcl-2 and Bcl-XL, as well as caspase activation [100]. Thus, Atg5 may turn into a pro-apoptotic signal at two levels, first via death receptor adaptor molecule FADD, activating the extrinsic pathway, and second via translocation of a calpain-cleavage product of Atg5 to mitochondria, activating the intrinsic pathway. As mentioned before, most of the ATG genes are required for both autophagy and selective autophagy, but other genes have a role only in certain types of autophagy. Atg11, an adaptor protein for selective autophagy, is needed along with Atg19 (a receptor protein involved in the formation of the Cvt complex) to recruit the Cvt complex to the PAS [101]. Similarly, during pexophagy Atg30 localizes to peroxisomes. Atg11 binds Atg30 and recruits the peroxisomes to the PAS [80]. In addition, Atg11 is essential for mitophagy, a selective type of autophagy that engulfs mitochondria, suggesting that this organelle is selectively imported into the vacuole. The role of other Atg molecules in mitophagy is described in section 4.1.1.