Electron Transfer Activity of Mitochondrial Outer Membrane Protein MitoNEET

Mitochondrial outer membrane protein MitoNEET, a primary target of type II diabetes drug pioglitazone, is a key regulator of energy metabolism in mitochondria. The protein exists as a homodimer with each monomer containing a redox active [2Fe-2S] cluster. Here we report that mitoNEET has a specific interaction with flavin mononucleotide (FMN). In the presence of flavin reductase, FMNH 2 reduced by NADH can rapidly reduce the mitoNEET [2Fe-2S] clusters under anaerobic conditions. Under aerobic conditions, however, the reduced mitoNEET [2Fe-2S] clusters are constantly re-oxidized by oxygen with concomitant oxidation of NADH in the presence of FMN and flavin reductase. The reduced mitoNEET [2Fe-2S] clusters can also reduce ubiquinone-2, a ubiquinone-10 analog, under aerobic or anaerobic conditions, indicating that mitoNEET has the ability to transfer electron from NADH to the membrane-bound ubiquinone in mitochondria. The [2Fe-2S] cluster is required for the electron transfer activity of mitoNEET, as apo-form mitoNEET has no activity to mediate oxidation of NADH or reduction of ubiquinone. The pioglitazone analog NL-1, which has a specific binding affinity for mitoNEET, effectively inhibits the electron transfer activity of the mitoNEET [2Fe-2S] clusters. The results suggest that mitoNEET is a novel electron transfer protein that may directly regulate energy metabolism in mitochondria.