It is not known where the initiation of autophagy takes place, but in the yeast, autophagosome formation is suggested to start at the pre-autophagosomal structure (PAS).
Atg9 is thought to be involved in the early phase of PAS formation, since many Atg proteins are not localized in the PAS in the budding yeast lacking ATG9. Atg2-Atg18 complex binds to Atg9 on the surface of the PAS. Atg18 is known to interact with PI3P, however, the function of Atg2-Atg18 complex remains to be elucidated. In the yeast with mutated ATG18 and ATG2, Atg9 is highly accumulated at the PAS. Meanwhile, Atg9 is not localized at the surface of autophagosome. These findings suggest there must be mechanisms for Atg9 to detach from the PAS surface. Atg2-Atg18 complex may be involved in this process.