Rab9 effector required for endosome to trans-Golgi network (TGN) transport. Interacts with PIKFYVE; the interaction recruits RABEPK to the endosomal membrane. Interacts with RAB9 in its GTP-bound conformation. Interaction with PIKFYVE and subsequent phosphorylation recruits it to the endosomal membrane. Phosphorylated on Ser residues by PIKFYVE.