Mechanistic studies of autophagic cargo recruitment and membrane expansion through in vitro reconstitution

Autophagy is a highly conserved catabolic pathway to remove deleterious cytosolic material to maintain cellular homeostasis and cell survival. Upon autophagy induction, a unique double-membraned structure, called a phagophore, forms and expands into a cup shape to engulf these cytosolic substrates. ATG8 proteins are covalently conjugated to autophagic membranes by lipidation of phosphatidylethanolamine (PE) and are thought to localise on both sides of the phagophore membrane. ATG8 conjugated on the inner membrane of the phagophore recruits autophagy cargo receptors, such as p62. To recapitulate events on the inner membrane, we used giant unilamellar vesicles (GUVs) as a model membrane and encapsulated proteins of interest inside GUVs, thus generating a membrane platform to which ATG8 proteins could be localised on the inner leaflet of the vesicles. We reconstituted WIPI2b-directed and cargo-directed ATG8 lipidation inside the GUVs and revealed distinct roles of WIPI2b and p62 in initiating the ATG conjugation cascade. Furthermore, we showed that p62 or p62 droplets were recruited to the inner membrane of the GUVs though interaction with membrane-bound ATG8s. Using a bead-based membrane expansion assay, we demonstrated a redistribution and local enrichment of membrane-bound ATG8s across the membrane upon interaction with p62 and p62 droplets. Our study provides novel model systems to investigate the interactions on the inner membrane of the phagophore and reveals fundamental molecular insights into phagophore membrane bending. This process is directed by ATG8-cargo interaction, during which cargo receptors concentrate ATG8 proteins on the inner surface of the phagophore membrane.