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      LC3 conjugation system in mammalian autophagy

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          Abstract

          Autophagy is the bulk degradation of proteins and organelles, a process essential for cellular maintenance, cell viability, differentiation and development in mammals. Autophagy has significant associations with neurodegenerative diseases, cardiomyopathies, cancer, programmed cell death, and bacterial and viral infections. During autophagy, a cup-shaped structure, the preautophagosome, engulfs cytosolic components, including organelles, and closes, forming an autophagosome, which subsequently fuses with a lysosome, leading to the proteolytic degradation of internal components of the autophagosome by lysosomal lytic enzymes. During the formation of mammalian autophagosomes, two ubiquitylation-like modifications are required, Atg12-conjugation and LC3-modification. LC3 is an autophagosomal ortholog of yeast Atg8. A lipidated form of LC3, LC3-II, has been shown to be an autophagosomal marker in mammals, and has been used to study autophagy in neurodegenerative and neuromuscular diseases, tumorigenesis, and bacterial and viral infections. The other Atg8 homologues, GABARAP and GATE-16, are also modified by the same mechanism. In non-starved rats, the tissue distribution of LC3-II differs from those of the lipidated forms of GABARAP and GATE-16, GABARAP-II and GATE-16-II, suggesting that there is a functional divergence among these three modified proteins. Delipidation of LC3-II and GABARAP-II is mediated by hAtg4B. We review the molecular mechanism of LC3-modification, the crosstalk between LC3-modification and mammalian Atg12-conjugation, and the cycle of LC3-lipidation and delipidation mediated by hAtg4B, as well as recent findings concerning the other two Atg8 homologues, GABARAP and GATE-16. We also highlight recent findings regarding the pathobiology of LC3-modification, including its role in microbial infection, cancer and neuromuscular diseases.

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          Most cited references 79

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          Beclin 1, an autophagy gene essential for early embryonic development, is a haploinsufficient tumor suppressor.

          The biochemical properties of beclin 1 suggest a role in two fundamentally important cell biological pathways: autophagy and apoptosis. We show here that beclin 1-/- mutant mice die early in embryogenesis and beclin 1+/- mutant mice suffer from a high incidence of spontaneous tumors. These tumors continue to express wild-type beclin 1 mRNA and protein, establishing that beclin 1 is a haploinsufficient tumor suppressor gene. Beclin 1-/- embryonic stem cells have a severely altered autophagic response, whereas their apoptotic response to serum withdrawal or UV light is normal. These results demonstrate that beclin 1 is a critical component of mammalian autophagy and establish a role for autophagy in tumor suppression. They both provide a biological explanation for recent evidence implicating beclin 1 in human cancer and suggest that mutations in other genes operating in this pathway may contribute to tumor formation through deregulation of autophagy.
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            A ubiquitin-like system mediates protein lipidation.

            Autophagy is a dynamic membrane phenomenon for bulk protein degradation in the lysosome/vacuole. Apg8/Aut7 is an essential factor for autophagy in yeast. We previously found that the carboxy-terminal arginine of nascent Apg8 is removed by Apg4/Aut2 protease, leaving a glycine residue at the C terminus. Apg8 is then converted to a form (Apg8-X) that is tightly bound to the membrane. Here we report a new mode of protein lipidation. Apg8 is covalently conjugated to phosphatidylethanolamine through an amide bond between the C-terminal glycine and the amino group of phosphatidylethanolamine. This lipidation is mediated by a ubiquitination-like system. Apg8 is a ubiquitin-like protein that is activated by an E1 protein, Apg7 (refs 7, 8), and is transferred subsequently to the E2 enzymes Apg3/Aut1 (ref. 9). Apg7 activates two different ubiquitin-like proteins, Apg12 (ref. 10) and Apg8, and assigns them to specific E2 enzymes, Apg10 (ref. 11) and Apg3, respectively. These reactions are necessary for the formation of Apg8-phosphatidylethanolamine. This lipidation has an essential role in membrane dynamics during autophagy.
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              Dissection of Autophagosome Formation Using Apg5-Deficient Mouse Embryonic Stem Cells

              In macroautophagy, cytoplasmic components are delivered to lysosomes for degradation via autophagosomes that are formed by closure of cup-shaped isolation membranes. However, how the isolation membranes are formed is poorly understood. We recently found in yeast that a novel ubiquitin-like system, the Apg12-Apg5 conjugation system, is essential for autophagy. Here we show that mouse Apg12-Apg5 conjugate localizes to the isolation membranes in mouse embryonic stem cells. Using green fluorescent protein–tagged Apg5, we revealed that the cup-shaped isolation membrane is developed from a small crescent-shaped compartment. Apg5 localizes on the isolation membrane throughout its elongation process. To examine the role of Apg5, we generated Apg5-deficient embryonic stem cells, which showed defects in autophagosome formation. The covalent modification of Apg5 with Apg12 is not required for its membrane targeting, but is essential for involvement of Apg5 in elongation of the isolation membranes. We also show that Apg12-Apg5 is required for targeting of a mammalian Aut7/Apg8 homologue, LC3, to the isolation membranes. These results suggest that the Apg12-Apg5 conjugate plays essential roles in isolation membrane development.
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                Author and article information

                Contributors
                Journal
                Int J Biochem Cell Biol
                Int. J. Biochem. Cell Biol
                The International Journal of Biochemistry & Cell Biology
                Elsevier Ltd.
                1357-2725
                1878-5875
                13 August 2004
                December 2004
                13 August 2004
                : 36
                : 12
                : 2503-2518
                Affiliations
                Department of Biochemistry, School of Medicine, Juntendo University, 2-1-1 Hongo, Bunkyo-ku, Tokyo 113-8421, Japan
                Author notes
                [* ]Corresponding author. Tel.: +81-3-5802-1031; fax: +81-3-5802-5889. kominami@ 123456med.juntendo.ac.jp
                Article
                S1357-2725(04)00211-0
                10.1016/j.biocel.2004.05.009
                7129593
                15325588
                Copyright © 2004 Elsevier Ltd. All rights reserved.

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                Categories
                Article

                Biochemistry

                atg8, autophagy, autophagosomes, gabarap, gate-16

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