Rab9 effector required for endosome to trans-Golgi network (TGN) transport. Interacts with PIKFYVE; the interaction recruits RABEPK to the endosomal membrane (PubMed:14530284). Interacts with RAB9 in its GTP-bound conformation (By similarity). Interaction with PIKFYVE and subsequent phosphorylation recruits it to the endosomal membrane. Phosphorylated on Ser residues by PIKFYVE. It is uncertain whether Met-1 or Met-9 is the initiator.