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Summary: In mammals, fusion of the mitochondrial outer membrane is controlled by two DRPs, MFN1 and MFN2, that function in place of a single outer membrane DRP, Fzo1 in yeast. We addressed the significance of two mammalian outer membrane fusion DRPs using an in vitro mammalian mitochondrial fusion assay. We demonstrate that heterotypic MFN1-MFN2 trans complexes possess greater efficacy in fusion as compared to homotypic MFN1 or MFN2 complexes. In addition, we show that the soluble form of the proapoptotic Bcl2 protein, Bax, positively regulates mitochondrial fusion exclusively through homotypic MFN2 trans complexes. Together, these data demonstrate functional and regulatory distinctions between MFN1 and MFN2 and provide insight into their unique physiological roles. [ABSTRACT FROM AUTHOR]

Summary: The Bcl-2 family member Bax translocates from the cytosol to mitochondria, where it oligomerizes and permeabilizes the mitochondrial outer membrane to promote apoptosis. Bax activity is counteracted by prosurvival Bcl-2 proteins, but how they inhibit Bax remains controversial because they neither colocalize nor form stable complexes with Bax. We constrained Bax in its native cytosolic conformation within cells using intramolecular disulfide tethers. Bax tethers disrupt interaction with Bcl-xL in detergents and cell-free MOMP activity but unexpectedly induce Bax accumulation on mitochondria. Fluorescence loss in photobleaching (FLIP) reveals constant retrotranslocation of WT Bax, but not tethered Bax, from the mitochondria into the cytoplasm of healthy cells. Bax retrotranslocation depends on prosurvival Bcl-2 family proteins, and inhibition of retrotranslocation correlates with Bax accumulation on the mitochondria. We propose that Bcl-xL inhibits and maintains Bax in the cytosol by constant retrotranslocation of mitochondrial Bax. [ABSTRACT FROM AUTHOR]

The Bcl-2 family member Bax translocates from the cytosol to mitochondria, where it oligomerizes and permeabilizes the mitochondrial outer membrane to promote apoptosis. Bax activity is counteracted by prosurvival Bcl-2 proteins, but how they inhibit Bax remains controversial because they neither colocalize nor form stable complexes with Bax. We constrained Bax in its native cytosolic conformation within cells using intramolecular disulfide tethers. Bax tethers disrupt interaction with Bcl-x(L) in detergents and cell-free MOMP activity but unexpectedly induce Bax accumulation on mitochondria. Fluorescence loss in photobleaching (FLIP) reveals constant retrotranslocation of WT Bax, but not tethered Bax, from the mitochondria into the cytoplasm of healthy cells. Bax retrotranslocation depends on prosurvival Bcl-2 family proteins, and inhibition of retrotranslocation correlates with Bax accumulation on the mitochondria. We propose that Bcl-x(L) inhibits and maintains Bax in the cytosol by constant retrotranslocation of mitochondrial Bax., (Copyright © 2011 Elsevier Inc. All rights reserved.)