Macroautophagy (henceforth autophagy) is an evolutionary conserved mechanism that culminates with the lysosomal degradation of superfluous or potentially dangerous cytosolic proteins and organelles. Autophagosomes evolve from open double-membrane structures called phagophores, and their formation involves multiple sequential steps that need to be coordinated and linked. In this new paper from the Rubinsztein lab, Claudia Puri and colleagues describe in mammalian cells that the transferrin receptor (TfR) links LC3 family conjugation to phagophore membranes, an early step in autophagosome biogenesis, with subsequent autophagosome closure.
TfR depletion impairs autophagic flux and its overexpression stimulates this catabolic process in an iron-independent manner. TfR is ubiquitinated by the ubiquitin ligase MARCH8 in the RAB11A—LC3B-positive membranes that are conjugated by LC3 family members, from which phagophores emanate. Ubiquitinated-TfR recruits the VPS34 component VPS15 enabling PI(3)P (Phosphatidylinositol 3-phosphate) synthesis on nascent autophagosome membranes. This PI(3)P is not only important for LC3-lipid conjugation, but also for subsequent phagophore closure, where TfR-dependent PI(3)P recruits the ESCRT complex. This TfR activity occurs after endocytosis of iron-containing transferrin, its canonical function, as TfR only binds VPS15 after iron detachment from transferrin that is enabled by pH lowering in the endocytic compartment.