Plays a role in regulation of vesicular cargo transport from the trans-Golgi network (TGN) to the plasma membrane (PubMed:15107860). Regulates Golgi phosphatidylinositol 4-phosphate (PtdIns(4)P) levels and activates the PtdIns(4)P phosphatase activity of SACM1L when it binds PtdIns(4)P in 'trans' configuration (PubMed:30659099). Binds preferentially to PtdIns(4)P (PubMed:11001876, PubMed:15107860). Negatively regulates APOB secretion from hepatocytes (PubMed:30659099). Interacts with GTP-bound ARF1 (PubMed:15107860, PubMed:21454700). Interacts with SACM1L and VAPA and/or VAPB to form a ternary complex (PubMed:30659099). Localizes to ER-trans Golgi network (TGN) contacts sites. Widely expressed. The PH domain binds the small GTPase ARF1 and phosphatidylinositol-4-phosphate (PtdIns4P) with high selectivity, and is required for its recruitment to the trans-Golgi network (TGN).