Autophagy is a dynamic membrane phenomenon for bulk protein degradation in the lysosome/vacuole.When cells face starvation conditions, the cytoplasmic components are nonselectively enclosed by a double-membrane structure, autophagosome and delivered to the lysosome/vacuole to be degraded (Fig.1).Autophagy is ubiquitous in eukaryotes and is essential for survival during starvation and cell differentiation.However,the molecular basis of each process in autophagy is still poorly characterized.Taking advantage of yeast genetics, autophagy defective mutants(apg) have been isolated(Tsukada and Ohsumi,1993),and so far 15 APG genes have been cloned.Apg8 is the first molecule found to be localized to the intermediate structures of the autophagosome,and is necessary for autophagosome formation(Kirisako et al., 1999).Our recent studies revealed that the carboxy-terminal arginine of newly synthesized Apg8 is removed by Apg4 protease,leaving a glycine residue at the C terminus (Kirisako et al.,2000).This cleavage reaction is essential for production of tightly bound form of Apg8 to unidentified membrane. Interestingly,the membrane bound form of Apg8 is covalently conjugated to phosphatidylethanolamine (PE) through an amide bond between the C-terminal glycine and the amino group of phosphatidylethanolamine.The biochemical analysis of the mechanism for Apg8-PE conjugation revealed that the Apg8-PE was formed by sequential enzyme reactions,similar to ubiquitination- like system.The C-terminal glycine of processed Apg8 (called as Apg8FG in this thesis)was activated in ATP dependent manner by Apg7 (E1 enzyme),and linked to a cysteine residue of Apg7 via a high-energy thioester bond.Through a transthioester reaction, Apg8FG conjugated with a novel identified E2 enzyme, Apg3.Finally, Apg8FG covalently conjugated with the PE.Notably, Apg7 is a unique E1-like enzyme, activates two different proteins, Apg12 and Apg8, and assigns them to specific E2 enzymes,Apg10 and Apg3, respectively.The reactions of Apg8-PE conjugation system were reconstituted in vitro by purified Apg7,Apg3,Apg8FG,and PE only,under the presence of ATP.These in vitro reconstitution studies demonstrated that the PE content of membrane affects the production of Apg8-PE.Apg8-PE conjugation of the in vitro reconstitution was enhanced by high amounts of Apg7 and Apg3.On the other hand,excessive Apg8FG inhibited their PE conjugation in a cell-free system.Apg8-PE conjugation directly occurs from Apg8-Apg3.However,some specific mechanisms seem to be required for the catalytic reactions in the system of Apg8-PE conjugation.The detailed mechanism of Apg8-PE conjugation step and its intracellular localization in yeast might be elucidated by the in vitro reconstitution study of Apg8-PE.Ubiquitination and Ubiquitination-like system generally mediates a covalent conjugation of modifier to its target protein.However,the target of Apg8 turned out to be a PE.This thesis is the first report that ubiquitination-like system mediates protein lipidation