The WIPI homolog Atg18 binds to and tethers membranes containing phosphatidylinositol-3,4,5-triphosphate.
Atg18 --for Autophagy-related gene 18-- is a member of the PROPPIN ({beta}-propellers that bind polyphosphoinositides) family, which is known for its binding to phosphorylated phosphoinositides through two conserved binding sites. Although Atg18 binding to polyphosphoinositides is crucial for its roles in both autophagic and non-autophagic functions within cells, the precise molecular mechanism by which Atg18 selectively binds to specific phosphatidylinositols remains unresolved. Here, we combined molecular dynamic simulations and biophysical methods --including isothermal titration calorimetry (ITC), cosedimentation, fluorescence resonance energy transfer (FRET), and dynamic light scattering (DLS)-- to characterize the interaction between Atg18 and polyphosphoinositides. In contrast to previous findings, we demonstrate that Atg18 binds to and clusters liposomes containing phosphatidylinositol-3,4,5-triphosphate (PtdIns(3,4,5)P3), suggesting that Atg18 oligomerizes and tethers opposing membranes containing this physiological phosphatidylinositol. Hence, our results provide new insights into how Atg18 and its mammalian homologs, WIPI --for WD-repeat domain phosphoinositide-interacting-- proteins, may regulate organelle membrane organization and vesicle trafficking required for both autophagic and non-autophagic functions.