The RalA binding protein 1 (RalBP1 or RLIP76) was originally identified as a GTP-RalA associated protein that acted as a downstream RalA effector in regulating Ral-Ras signaling (1). RalBP1 interacts with RalA and the endocytosis protein REPS2 (POB1) through its carboxy-terminal Ral binding domain. RalBP1 has an intrinsic GTPase activating function and interacts with Cdc42 through its centrally located Rho-GAP domain (1-3). A protein complex containing RalBP1/POB1/RalA regulates endocytosis of membrane receptors (4). RalBP1 also functions as a non-ABC transporter that catalyzes the ATP-dependent transport of numerous xenobiotics, including glutathione conjugates and some chemotherapeutic agents. RalBP1 transporter activity may play an important role in detoxification, drug resistance and the stress response (5-7). Increased expression of RalBP1 protein is associated with some forms of cancer and regression of cancer xenografts results from RalBP1 inhibition (8,9). Evidence to date suggests that RalBP1 may be a promising therapeutic target for cancer therapy.
