期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：11
页码：3350-3355
DOI：10.1073/pnas.1421092112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceAutophagy is a highly conserved degradative process in eukaryotes. In response to starvation, a number of autophagosome-related (Atg) proteins are recruited, and these proteins govern the process of autophagosome formation. Atg9 vesicles are thought to play an essential role in the nucleation step, but it remains unclear how Atg9 vesicles are localized to the site of autophagosome formation. In this study, we found that Atg9 interacts with the HORMA (from Hop1, Rev7, and Mad2) domain of Atg13. Atg13 mutants lacking the Atg9-binding region fail to recruit Atg9 vesicles to the site of autophagosome formation and exhibit severe defects in autophagy. Thus, the HORMA domain of Atg13 facilitates recruitment of Atg9 vesicles during autophagosome formation. Our studies provide a molecular insight into how Atg9 vesicles become part of the autophagosomal membrane. During autophagosome formation, autophagosome-related (Atg) proteins are recruited hierarchically to organize the preautophagosomal structure (PAS). Atg13, which plays a central role in the initial step of PAS formation, consists of two structural regions, the N-terminal HORMA (from Hop1, Rev7, and Mad2) domain and the C-terminal disordered region. The C-terminal disordered region of Atg13, which contains the binding sites for Atg1 and Atg17, is essential for the initiation step in which the Atg1 complex is formed to serve as a scaffold for the PAS. The N-terminal HORMA domain of Atg13 is also essential for autophagy, but its molecular function has not been established. In this study, we searched for interaction partners of the Atg13 HORMA domain and found that it binds Atg9, a multispanning membrane protein that exists on specific cytoplasmic vesicles (Atg9 vesicles). After the Atg1 complex is formed, Atg9 vesicles are recruited to the PAS and become part of the autophagosomal membrane. HORMA domain mutants, which are unable to interact with Atg9, impaired the PAS localization of Atg9 vesicles and exhibited severe defects in starvation-induced autophagy. Thus, Atg9 vesicles are recruited to the PAS via the interaction with the Atg13 HORMA domain. Based on these findings, we propose that the two distinct regions of Atg13 play crucial roles in distinct steps of autophagosome formation: In the first step, Atg13 forms a scaffold for the PAS via its C-terminal disordered region, and subsequently it recruits Atg9 vesicles via its N-terminal HORMA domain.
