title: Apoptosis is triggered when prosurvival Bcl-2 proteins cannot restrain Bax.
abstract: 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-x(L) remained able to block apoptosis induced by Bax D68R. Whereas cells with sufficient Bcl-x(L) tolerated expression of Bax D68R, it provoked apoptosis when Bcl-x(L) was absent, downregulated, or inactivated. Moreover, Bax D68R rendered membrane bound by a C-terminal anchor mutation overwhelmed endogenous Bcl-x(L) 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-x(L) also controls Bax by other mechanisms.
authors.affiliation: The Walter and Eliza Hall Institute of Medical Research, Parkville, Victoria 3050, Australia.
authors.forename: Jamie I
authors.initials: JI
authors.lastname: Fletcher
authors.forename: Sarina
authors.initials: S
authors.lastname: Meusburger
authors.forename: Christine J
authors.initials: CJ
authors.lastname: Hawkins
authors.forename: David T
authors.initials: DT
authors.lastname: Riglar
authors.forename: Erinna F
authors.initials: EF
authors.lastname: Lee
authors.forename: W Douglas
authors.initials: WD
authors.lastname: Fairlie
authors.forename: David C S
authors.initials: DC
authors.lastname: Huang
authors.forename: Jerry M
authors.initials: JM
authors.lastname: Adams
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D002470:Cell Survival / Q000502:physiology*; D051379:Mice; D008969:Molecular Sequence Data; D011485:Protein Binding; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism / Q000502:physiology*; D017386:Sequence Homology, Amino Acid; D051028:bcl-2-Associated X Protein / Q000235:genetics / Q000378:metabolism / Q000502:physiology*
chemical_list: D019253:Proto-Oncogene Proteins c-bcl-2; D051028:bcl-2-Associated X Protein