Apoptosis is triggered when prosurvival Bcl-2 proteins cannot restrain Bax

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 105; no. 47; pp. 18081 - 18087
• Main Authors: Fletcher, Jamie I, Meusburger, Sarina, Hawkins, Christine J, Riglar, David T, Lee, Erinna F, Fairlie, W. Douglas, Huang, David C.S, Adams, Jerry M
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 25-11-2008; National Acad Sciences
• Series: Inaugural Article
• Subjects: Amino Acid Sequence; Animals; Apoptosis; Apoptosis - physiology; bcl-2-Associated X Protein - genetics; bcl-2-Associated X Protein - metabolism; bcl-2-Associated X Protein - physiology; Biological Sciences; Cell death; Cell membranes; Cell Survival - physiology; Family members; Mice; Molecular Sequence Data; Molecules; Physiological regulation; Protein Binding; Proteins; Proto-Oncogene Proteins c-bcl-2 - metabolism; Proto-Oncogene Proteins c-bcl-2 - physiology; Sequence Homology, Amino Acid; T lymphocytes; Viability; Yeasts
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0808691105

A central issue in the control of apoptosis is whether its essential mediators Bax and Bak must be restrained by Bcl-2-like prosurvival relatives to prevent their damaging mitochondria and unleashing apoptosis. The issue is particularly vexed for Bax, which is largely a cytosolic monomer in unstressed cells. To determine whether Bax regulation requires its binding by prosurvival relatives, we replaced a conserved aspartate in its BH3 interaction domain with arginine. Bax D68R functioned and behaved like wild-type Bax in localization and activation but had greatly impaired binding to the prosurvival family members. Nevertheless, Bcl-xL remained able to block apoptosis induced by Bax D68R. Whereas cells with sufficient Bcl-xL tolerated expression of Bax D68R, it provoked apoptosis when Bcl-xL was absent, downregulated, or inactivated. Moreover, Bax D68R rendered membrane bound by a C-terminal anchor mutation overwhelmed endogenous Bcl-xL and killed cells. These unexpected results suggest that engagement of Bax by its prosurvival relatives is a major barrier to its full activation. We propose that the Bcl-2-like proteins must capture the small proportion of Bax molecules with an exposed BH3 domain, probably on the mitochondrial membrane, to prevent Bax-imposed cell death, but that Bcl-xL also controls Bax by other mechanisms.