Regulates autophagy by controlling the spatial distribution and levels of the intracellular phosphatidylinositol 4-phosphate (PtdIns(4)P) pools (By similarity). Modulates (PtdIns(4)P) levels by regulating the ER-to-Golgi trafficking of the phosphatidylinositide phosphatase SACM1L (By similarity). Belongs to the TMEM39 family.