ATG9A supplies PtdIns4P to the autophagosome initiation site.

The identity of the platform supporting the initiation and formation of the nascent autophagosome, the phagophore, is not fully understood. Nucleation and expansion of the phagophore membrane requires a coordinated flux or activation of specific proteins and membrane lipids at the initiation site. The transmembrane protein ATG9A is essential for macroautophagy/autophagy and proposed to be an initiator of the phagophore by directing or facilitating the delivery of proteins and lipids to the initiation site. Upon amino acid starvation, ATG9A-containing vesicles are formed from the Golgi complex and endosomal compartments and translocate to the initiation site. Unravelling the complement of proteins and lipids brought by ATG9A vesicles to the forming autophagosome is essential to further understand the initiation of autophagy.