RLIP76: A Structural and Functional Triumvirate

SIMPLE SUMMARY: The RLIP76 protein is present at high levels in multiple cancers, compared with levels in normal cells. In cancer cells it is thought to be important for removal of chemotherapeutics, while in normal cells it has been implicated in endocytosis, stress response and mitochondrial fission. Although RLIP76 is a potential target for ovary, breast, lung, colon, prostate and kidney cancers, only the middle third of the protein has been structurally characterized. Understanding the full structure of RLIP76 will help us to better understand the signalling pathways in which it is involved and by extension its role in cancer. ABSTRACT: RLIP76/RalBP1 is an ATP-dependent transporter of glutathione conjugates, which is overexpressed in various human cancers, but its diverse functions in normal cells, which include endocytosis, stress response and mitochondrial dynamics, are still not fully understood. The protein can be divided into three distinct regions, each with its own structural properties. At the centre of the protein are two well-defined domains, a GTPase activating protein domain targeting Rho family small G proteins and a small coiled-coil that binds to the Ras family small GTPases RalA and RalB. In engaging with Rho and Ral proteins, RLIP76 bridges these two distinct G protein families. The N-terminal region is predicted to be disordered and is rich in basic amino acids, which may mediate membrane association, consistent with its role in transport. RLIP76 is an ATP-dependent transporter with ATP-binding sites within the N-terminus and the Ral binding domain. Furthermore, RLIP76 is subject to extensive phosphorylation, particularly in the N-terminal region. In contrast, the C-terminal region is thought to form an extensive coiled-coil that could mediate dimerization. Here, we review the structural features of RLIP76, including experimental data and computational predictions, and discuss the implications of its various post-translational modifications.