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(3) BAX IS PRESENT AS A HIGH MOLECULAR WEIGHT OLIGOMER/COMPLEX IN THE MITOCHONDRIAL MEMBRANE OF APOPTOTIC CELLS B. Antonsson, S. Montessuit,
B. Sanchez and J-C. Martinou
Bax is a Bcl-2 family protein with pro-apoptotic activity that can form ion channels in artificial lipid membranes. The localization of Bax has been shown to change from the cytosol to the mitochondria during apoptosis. At the mitochondria the protein triggers cytochrome c release both in vitro and in vivo. In the cytosol of untreated HeLa cells and in cytosolic extract from mouse liver Bax is present as a monomer with an apparent molecular weight of 24,000 Da. In untreated HeLa cells Bax monomers are attached to the mitochondrial membrane. After treatment of HeLa cells with the apoptosis inducer staurosporine the cytosolic Bax concentration decreases. In the staurosporine treated cells Bax associated with the mitochondria is present as both a monomer and as two large molecular weight oligomers/complexs of 96,000 and 260,000 Da. The monomer is only attached to the mitochondria whereas the oligomers/complexs are integrated into the mitochondrial membrane. Similar to staurosporine treatment UV irradiation induces Bax oligomerization and insertion into the mitochondrial membrane. Further, treatment of isolated HeLa mitochondria with recombinant caspase 8 cleaved Bid induces Bax oligomerization and membrane insertion. The outer mitochondrial membrane protein VDAC and the inner mitochondrial membrane protein ANT, both parts of the permeability transition pore, are not co-migrating with the large molecular weight Bax multimers. These results suggest that monomeric Bax is inactive at the mitochondria and Bax oligomerization and possibly complex formation with an unknown protein is required for Bax channel activity and to trigger cytochrome c release.
For further information contact...Carmen Mannella: carmen@wadsworth.org |
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