title: Bcl-2 proteins bid and bax form a network to permeabilize the mitochondria at the onset of apoptosis.
abstract: The most critical step in the initiation of apoptosis is the activation of the Bcl-2 family of proteins to oligomerize and permeabilize the outer-mitochondrial membrane (OMM). As this step results in the irreversible release of factors that enhance cellular degradation, it is the point of no return in programmed cell death and would be an ideal therapeutic target. However, the arrangement of the Bcl-2 proteins in the OMM during permeabilization still remains unknown. It is also unclear whether the Bcl-2 protein, Bid, directly participates in the formation of the oligomers in live cells, even though it is cleaved and translocates to the OMM at the initiation of apoptosis. Therefore, we utilized confocal microscopy to measure Förster resonance energy transfer (FRET) efficiencies in live cells to determine the conformation(s) and intermolecular contacts of Bid within these Bcl-2 oligomers. We found that Bid adopts an extended conformation, which appears to be critical for its association with the mitochondrial membrane. This conformation is also important for intermolecular contacts within the Bid oligomer. More importantly for the first time, direct intermolecular contacts between Bid and Bax were observed, thereby, confirming Bid as a key component of these oligomers. Furthermore, the observed FRET efficiencies allowed us to propose an oligomeric arrangement of Bid, Bax, and possibly other members of the Bcl-2 family of proteins that form a self-propagating network that permeabilizes the OMM.
authors.affiliation: Laboratory of Molecular Biophysics, Biochemistry and Biophysics Center, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda 20892, MD, USA.
authors.forename: Robert F
authors.initials: RF
authors.lastname: Gahl
authors.affiliation: Laboratory of Molecular Biophysics, Biochemistry and Biophysics Center, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda 20892, MD, USA.
authors.forename: Pallavi
authors.initials: P
authors.lastname: Dwivedi
authors.affiliation: Laboratory of Molecular Biophysics, Biochemistry and Biophysics Center, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda 20892, MD, USA.
authors.forename: Nico
authors.initials: N
authors.lastname: Tjandra
mesh_terms: D000818:Animals; D017209:Apoptosis; D051023:BH3 Interacting Domain Death Agonist Protein / Q000737:chemistry / Q000378:metabolism*; D003432:Cross-Linking Reagents / Q000378:metabolism; D031541:Fluorescence Resonance Energy Transfer; D051379:Mice; D008928:Mitochondria / Q000378:metabolism*; D051336:Mitochondrial Membranes / Q000378:metabolism; D008954:Models, Biological; D010539:Permeability; D011487:Protein Conformation; D021381:Protein Transport; D051028:bcl-2-Associated X Protein / Q000737:chemistry / Q000378:metabolism*
chemical_list: D051023:BH3 Interacting Domain Death Agonist Protein; D003432:Cross-Linking Reagents; D051028:bcl-2-Associated X Protein