ATG9 promotes autophagosome formation through interaction with LC3.

The autophagosome is a double-membrane organelle that executes macroautophagy. Previous studies have shown that the autophagosome formation is driven by autophagy-related genes, among which ATG9 is the only conserved transmembrane protein and has been shown to play a critical role in the autophagosome formation. However, how ATG9 binds to the growing autophagosome membrane has remained uncertain. Herein, we report that ATG9 binds to LC3, an essential membrane component of the autophagosome, thereby allowing ATG9 to incorporate into the autophagosome membrane. Mechanistically, we show that ATG9 interacts with LC3 through its UIM motives, which bind to the UDS site of LC3. Interrupting such UIM-UDS interaction abolishes the autophagosome association of ATG9 and suppresses the autophagosome formation. Collectively, our findings reveal a novel mechanism regulating autophagosome biogenesis and suggest that the interaction of ATG9 with LC3 is critical for ATG9 binding to the growing autophagosome membrane.