Molecular details of Bax activation, oligomerization, and membrane insertion.
Bax and Bid are pro-apoptotic members of the Bcl-2 protein family. Upon cleavage by caspase-8, Bid activates Bax. Activated Bax inserts into the mitochondrial outer membrane forming oligomers which lead to membrane poration, release of cytochrome c, and apoptosis. The detailed mechanism of Bax activation and the topology and composition of the oligomers are still under debate. Here molecular details of Bax activation and oligomerization were obtained by application of several biophysical techniques, including atomic force microscopy, cryoelectron microscopy, and particularly electron paramagnetic resonance (EPR) spectroscopy performed on spin-labeled Bax. Incubation with detergents, reconstitution, and Bid-triggered insertion into liposomes were found to be effective in inducing Bax oligomerization. Bid was shown to activate Bax independently of the stoichiometric ratio, suggesting that Bid has a catalytic function and that the interaction with Bax is transient. The formation of a stable dimerization interface involving two Bcl-2 homology 3 (BH3) domains was found to be the nucleation event for Bax homo-oligomerization. Based on intermolecular distance determined by EPR, a model of six adjacent Bax molecules in the oligomer is presented where the hydrophobic hairpins (helices alpha5 and alpha6) are equally spaced in the membrane and the two BH3 domains are in close vicinity in the dimer interface, separated by >5 nm from the next BH3 pairs.
Bleicken
Stephanie
S
Department of Membrane Biochemistry, Max Planck Institute for Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany.
Classen
Mirjam
M
Padmavathi
Pulagam V L
PV
Ishikawa
Takashi
T
Zeth
Kornelius
K
Steinhoff
Heinz-Jürgen
HJ
Bordignon
Enrica
E
D000818:Animals; D051023:BH3 Interacting Domain Death Agonist Protein / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D051379:Mice; D051336:Mitochondrial Membranes / Q000378:metabolism*; D008956:Models, Chemical; D011487:Protein Conformation; D055503:Protein Multimerization; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000737:chemistry / Q000378:metabolism*
D051023:BH3 Interacting Domain Death Agonist Protein; C494815:Bid protein, mouse; D019253:Proto-Oncogene Proteins c-bcl-2