Acts as an effector of RABF2A and RABF2B. Involved in vacuolar transport of storage proteins. Regulates membrane trafficking to protein storage vacuoles (PSVs). Binds specifically to phosphatidylinositol 3-monophosphate (PtdIns3P). Interacts (via PX domain) with RABF2A and RABF2B. The endosomal localization depends on the active state of RABF2B. Binding of phosphatidylinositol 3-monophosphate via the PX domain is required for the endosomal localization. No visible phenotype under normal growth conditions, but the double mutant plants erex and erel1 exhibit severe growth retardation at a juvenile stage.