FIGURE 6.

Dissection of Atg18-related processes during autophagy. A summary of Atg18-related processes during autophagy is shown. The process that requires PtdIns(3)P binding of Atg18 is boxed. Atg18 can form a protein complex with Atg2 independent of binding to PtdIns(3)P. The Atg18-Atg2 complex associates with autophagic membranes; this depends on a cooperation of Atg18-PtdIns(3)P binding and Atg2-X (existing at autophagic membrane) interaction. On the autophagic membranes, the Atg18-Atg2 complex exerts an unknown function(s), which does not require direct interaction between PtdIns(3)P and Atg18. These processes are involved in starvation-induced autophagy and the Cvt pathway. The exact components and stoichiometry of the Atg18-Atg2 complex are to be clarified in the future. Direction of the complex toward the membrane is also unknown at present. Binding to PtdIns(3,5)P₂ (and to some vacuolar protein component) is required for Atg18 to localize to the vacuolar membrane, where it functions to maintain vacuolar homeostasis (29).