Molecular Physiology of Uncoupling Proteins in the Central Nervous System: Self-Association and Proton Transport

Abstract

Uncoupling proteins (UCPs), located in the mitochondrial inner membrane, facilitate the transmembrane proton flux and, consequently, reduce the membrane potential and ATP production. Found in the central nervous system (CNS), based on several studies on animal models and clinical investigations, three human UCP homologs (UCP2, UCP4, and UCP5) could have crucial physiological functions. Despite these studies, the detailed structural features and molecular physiology landscape of these proteins remain relatively unexplored. Recently, we reported a novel expression system for obtaining functionally folded UCP1 in bacterial membranes (Hoang et al., 2013). In the current study, employing similar expression and reconstitution methods, we will report our new findings for the three human neuronal UCP homologs. The reconstituted CNS proteins display high helical contents and transport protons in the presence of several physiologically-relevant fatty acid activators. In addition, experimental results from CD, fluorescence spectroscopy, mass spectrometry and semi-native electrophoresis suggest self-association of these proteins in the membranes. While sharing comparable secondary structures in the liposomes, neuronal UCPs differ in their proton transport rates (and possibly mechanism) in the presence of different fatty acid activators. The protein-fatty acids interaction is further investigated using near-UV CD spectroscopy. The differences in fatty-acid activated UCP-mediated proton transport could serve as an essential clue in understanding and differentiating the physiological roles of UCP homologs in the CNS.Hoang T, Smith MD, Jelokhani-Niaraki M (2013) Expression, folding, and proton transport activity of human uncoupling protein-1 (UCP1) in lipid membranes: evidence for associated functional forms. J. Biol. Chem. 288, 36244-36258