Assessment of Dynamic BCL-2 Protein Shuttling Between Outer Mitochondrial Membrane and Cytosol.

BCL-2 proteins control stress-dependent commitment to the programmed cell death apoptosis. In nonapoptotic cells the proapoptotic BCL-2 proteins BAX and BAK but also prosurvival family members, like BCL-x or MCL-1, translocate to the outer mitochondrial membrane (OMM) and retrotranslocate from the mitochondria back into the cytosol. The resulting equilibrium produces a broad range of localization pattern observed for BAX and BAK in human cells and shows correlation between relative BAX and BAK localizations and cellular predisposition to apoptosis. The retrotranslocation of BCL-2 proteins from the OMM can be measured using fluorescence-labeled protein in intact cells or endogenous protein from isolated heavy membrane fractions.