Spatiotemporal control of Atg9 vesicle fusion in autophagosome formation.

Atg9 vesicles serve as membrane seeds for autophagosome formation. These vesicles are derived from the Golgi/endosomes and localized to the pre-autophagosomal structure or phagophore assembly site (PAS) upon autophagy induction. How these vesicles are maintained as discrete membrane carriers while diffusing through the cytoplasm and subsequently become competent for downstream events at the PAS has remained unknown. Here, we show that the Atg9-interacting protein Atg23 remains associated with Atg9 vesicles following their formation and protects them from inappropriate fusion with endomembranes during their movement through the cytoplasm. Upon arrival at the PAS, Atg1-mediated phosphorylation of Atg9 triggers the dissociation of Atg23, thereby enabling efficient recruitment of the lipid-transfer protein Atg2. Collectively, these findings define a spatiotemporally regulated mechanism in which Atg23 preserves Atg9 vesicles during cytoplasmic transport, whereas its dissociation enables their productive utilization in autophagosome formation.