Purified hydrophobic proteins, chargerins, are essential for energy transduction in oxidative phosphorylation.

Studies on anisotropic inhibitors, a unique type of inhibitor of energy transduction in oxidative phosphorylation, suggested that redox reactions generate two kinds of negative charges on the outer surface of mitochondrial inner membranes, on redox complexes and on F0, and that the inhibitors inhibit energy transduction by binding to these negative charges. Recent experiments on photoaffinity labeling of mitochondria with monoazide ethidium, which is an anisotropic inhibitor, showed that the inhibitor specifically binds to a hydrophobic protein of the membranes. In the present work the mitochondrial components labeled with monoazide ethidium were further purified and two kinds of hydrophobic proteins (apparent molecular masses, 8 and 13 kDa) were found to be specifically labeled with the inhibitor. These proteins were named chargerin I and II, respectively. Redox reactions greatly increased the molar ratio of ethidium bound to chargerin I and II in mitochondria, reflecting a conformational change of the chargerins coupled with the redox reactions. It was also shown that antibody against chargerin II specifically inhibited ATP synthesis in mitoplasts (inner membranes plus matrix) prepared from rat liver mitochondria. Thus, the present findings show that chargerins have an essential role in energy transduction in oxidative phosphorylation in rat liver mitochondria, in good accord with the conformational coupling model of the H+ pumps and ATP synthesis.