ATG9 vesicles comprise the seed membrane of mammalian autophagosomes

As the autophagosome forms, its membrane surface area expands rapidly, while its volume is kept low. Protein-mediated transfer of lipids from another organelle to the autophagosome likely drives this expansion, but as these lipids are only introduced into the cytoplasmic-facing leaflet of the organelle, full membrane growth also requires lipid scramblase activity. ATG9 harbors scramblase activity and is essential to autophagosome formation; however, whether ATG9 is integrated into mammalian autophagosomes remains unclear. Here we show that in the absence of lipid transport, ATG9 vesicles are already competent to collect proteins found on mature autophagosomes, including LC3-II. Further, we use styrene-maleic acid lipid particles to reveal the nanoscale organization of protein on LC3-II membranes; ATG9 and LC3-II are each fully integrated into expanding autophagosomes. The ratios of these two proteins at different stages of maturation demonstrate that ATG9 proteins are not continuously integrated, but rather are present on the seed vesicles only and become diluted in the expanding autophagosome membrane. Olivas et al. demonstrate that ATG9 vesicles are the membrane seed for mammalian autophagosomes using nanodisc technology to determine co-residency of ATG9 and LC3 on autophagosomes and their precursor membranes. This work establishes a key piece of the model for autophagosome expansion via direct lipid transfer.

Automated summary

ATG9 vesicles serve as the membrane seed for mammalian autophagosomes, as shown by Olivas et al. using nanodisc technology. The integration of ATG9 with expanding autophagosome membranes demonstrates the importance of lipid transfer in autophagosome expansion. This work provides significant insights into the model of autophagosome formation and expands our understanding of the cellular process of autophagy.