PI3P binding by Atg21 organises Atg8 lipidation.

Autophagosome biogenesis requires two ubiquitin-like conjugation systems. One couples ubiquitin-like Atg8 to phosphatidylethanolamine, and the other couples ubiquitin-like Atg12 to Atg5. Atg12~Atg5 then forms a heterodimer with Atg16. Membrane recruitment of the Atg12~Atg5/Atg16 complex defines the Atg8 lipidation site. Lipidation requires a PI3P-containing precursor. How PI3P is sensed and used to coordinate the conjugation systems remained unclear. Here, we show that Atg21, a WD40 β-propeller, binds via PI3P to the preautophagosomal structure (PAS). Atg21 directly interacts with the coiled-coil domain of Atg16 and with Atg8. This latter interaction requires the conserved F5K6-motif in the N-terminal helical domain of Atg8, but not its AIM-binding site. Accordingly, the Atg8 AIM-binding site remains free to mediate interaction with its E2 enzyme Atg3. Atg21 thus defines PI3P-dependently the lipidation site by linking and organising the E3 ligase complex and Atg8 at the PAS.

Juris L ，Montino M ，Rube P ，Schlotterhose P ，Thumm M ，Krick R ... -  《-》

Atg8 lipidation is coordinated in a PtdIns3P-dependent manner by the PROPPIN Atg21.

Krick R ，Thumm M 《-》

Atg21 organizes Atg8 lipidation at the contact of the vacuole with the phagophore.

Munzel L ，Neumann P ，Otto FB ，Krick R ，Metje-Sprink J ，Kroppen B ，Karedla N ，Enderlein J ，Meinecke M ，Ficner R ，Thumm M ... -  《-》

Two distinct mechanisms target the autophagy-related E3 complex to the pre-autophagosomal structure.

In autophagy, Atg proteins organize the pre-autophagosomal structure (PAS) to initiate autophagosome formation. Previous studies in yeast revealed that the autophagy-related E3 complex Atg12-Atg5-Atg16 is recruited to the PAS via Atg16 interaction with Atg21, which binds phosphatidylinositol 3-phosphate (PI3P) produced at the PAS, to stimulate conjugation of the ubiquitin-like protein Atg8 to phosphatidylethanolamine. Here, we discover a novel mechanism for the PAS targeting of Atg12-Atg5-Atg16, which is mediated by the interaction of Atg12 with the Atg1 kinase complex that serves as a scaffold for PAS organization. While autophagy is partially defective without one of these mechanisms, cells lacking both completely lose the PAS localization of Atg12-Atg5-Atg16 and show no autophagic activity. As with the PI3P-dependent mechanism, Atg12-Atg5-Atg16 recruited via the Atg12-dependent mechanism stimulates Atg8 lipidation, but also has the specific function of facilitating PAS scaffold assembly. Thus, this study significantly advances our understanding of the nucleation step in autophagosome formation.

Harada K ，Kotani T ，Kirisako H ，Sakoh-Nakatogawa M ，Oikawa Y ，Kimura Y ，Hirano H ，Yamamoto H ，Ohsumi Y ，Nakatogawa H ... -  《eLife》

The amino-terminal region of Atg3 is essential for association with phosphatidylethanolamine in Atg8 lipidation.

Hanada T ，Satomi Y ，Takao T ，Ohsumi Y ... -  《-》