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 (PubMed:16769818, PubMed:9230071). Interaction with PIKFYVE and subsequent phosphorylation recruits it to the endosomal membrane. Phosphorylated on Ser residues by PIKFYVE.