Autophagosome formation from membrane compartments enriched in phosphatidylinositol 3-phosphate and dynamically connected to the endoplasmic reticulum

TLDR

Autophagy involves the formation of double‑membrane autophagosomes that require phosphatidylinositol 3‑phosphate and arise near the endoplasmic reticulum, though the precise mechanisms remain unclear. Upon amino‑acid starvation, the PI(3)P‑binding protein DFCP1 and other PI(3)P probes relocate to a PI(3)P‑enriched punctate compartment that forms adjacent to Vps34 vesicles, remains in dynamic equilibrium with the ER, and serves as a membrane platform for autophagosome protein recruitment, membrane expansion, and full autophagosome emergence, indicating a key role for this compartment in autophagosome biogenesis.

Abstract

Autophagy is the engulfment of cytosol and organelles by double-membrane vesicles termed autophagosomes. Autophagosome formation is known to require phosphatidylinositol 3-phosphate (PI(3)P) and occurs near the endoplasmic reticulum (ER), but the exact mechanisms are unknown. We show that double FYVE domain–containing protein 1, a PI(3)P-binding protein with unusual localization on ER and Golgi membranes, translocates in response to amino acid starvation to a punctate compartment partially colocalized with autophagosomal proteins. Translocation is dependent on Vps34 and beclin function. Other PI(3)P-binding probes targeted to the ER show the same starvation-induced translocation that is dependent on PI(3)P formation and recognition. Live imaging experiments show that this punctate compartment forms near Vps34-containing vesicles, is in dynamic equilibrium with the ER, and provides a membrane platform for accumulation of autophagosomal proteins, expansion of autophagosomal membranes, and emergence of fully formed autophagosomes. This PI(3)P-enriched compartment may be involved in autophagosome biogenesis. Its dynamic relationship with the ER is consistent with the idea that the ER may provide important components for autophagosome formation.