Reconstitution of cargo-induced LC3 lipidation in mammalian selective autophagy

Abstract

Selective autophagy of damaged mitochondria, intracellular pathogens, protein aggregates, endoplasmic reticulum, and other large cargoes is essential for health. The presence of cargo initiates phagophore biogenesis, which entails the conjugation of ATG8/LC3 family proteins to membrane phosphatidylethanolamine. Current models suggest that the presence of clustered ubiquitin chains on a cargo triggers a cascade of interactions from autophagic cargo receptors through the autophagy core complexes ULK1 and class III PI 3-kinase complex I (PI3KC3-C1), WIPI2, and the ATG7, ATG3, and ATG12-ATG5-ATG16L1 machinery of LC3 lipidation. This model was tested using giant unilamellar vesicles (GUVs), GST-Ub4 as a model cargo, the cargo receptors NDP52, TAX1BP1, and OPTN, and the autophagy core complexes. All three cargo receptors potently stimulated LC3 lipidation on GUVs. NDP52- and TAX1BP1-induced LC3 lipidation required the ULK1 complex together with all other components, however, ULK1 kinase activity was dispensable. In contrast, OPTN bypassed the ULK1 requirement completely. These data show that the cargo-dependent stimulation of LC3 lipidation is a common property of multiple autophagic cargo receptors, yet the details of core complex engagement vary considerably and unexpectedly between the different receptors.