title: Bax targets mitochondria by distinct mechanisms before or during apoptotic cell death: a requirement for VDAC2 or Bak for efficient Bax apoptotic function.
abstract: In non-apoptotic cells, Bak constitutively resides in the mitochondrial outer membrane. In contrast, Bax is in a dynamic equilibrium between the cytosol and mitochondria, and is commonly predominant in the cytosol. In response to an apoptotic stimulus, Bax and Bak change conformation, leading to Bax accumulation at mitochondria and Bak/Bax oligomerization to form a pore in the mitochondrial outer membrane that is responsible for cell death. Using blue native-PAGE to investigate how Bax oligomerizes in the mitochondrial outer membrane, we observed that, like Bak, a proportion of Bax that constitutively resides at mitochondria associates with voltage-dependent anion channel (VDAC)2 prior to an apoptotic stimulus. During apoptosis, Bax dissociates from VDAC2 and homo-oligomerizes to form high molecular weight oligomers. In cells that lack VDAC2, constitutive mitochondrial localization of Bax and Bak was impaired, suggesting that VDAC2 has a role in Bax and Bak import to, or stability at, the mitochondrial outer membrane. However, following an apoptotic stimulus, Bak and Bax retained the ability to accumulate at VDAC2-deficient mitochondria and to mediate cell death. Silencing of Bak in VDAC2-deficient cells indicated that Bax required either VDAC2 or Bak in order to translocate to and oligomerize at the mitochondrial outer membrane to efficiently mediate apoptosis. In contrast, efficient Bak homo-oligomerization at the mitochondrial outer membrane and its pro-apoptotic function required neither VDAC2 nor Bax. Even a C-terminal mutant of Bax (S184L) that localizes to mitochondria did not constitutively target mitochondria deficient in VDAC2, but was recruited to mitochondria following an apoptotic stimulus dependent on Bak or upon over-expression of Bcl-xL. Together, our data suggest that Bax localizes to the mitochondrial outer membrane via alternate mechanisms, either constitutively via an interaction with VDAC2 or after activation via interaction with Bcl-2 family proteins.
authors.affiliation: Walter and Eliza Hall Institute of Medical Research, 1 G Royal Parade, Parkville, Melbourne, Victoria, Australia.
authors.forename: S B
authors.initials: SB
authors.lastname: Ma
authors.affiliation: Department of Biochemistry, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Australia.
authors.forename: T N
authors.initials: TN
authors.lastname: Nguyen
authors.affiliation: Walter and Eliza Hall Institute of Medical Research, 1 G Royal Parade, Parkville, Melbourne, Victoria, Australia.
authors.forename: I
authors.initials: I
authors.lastname: Tan
authors.affiliation: 1] Walter and Eliza Hall Institute of Medical Research, 1 G Royal Parade, Parkville, Melbourne, Victoria, Australia [2] Department of Medical Biology, University of Melbourne, Parkville, Victoria.
authors.forename: R
authors.initials: R
authors.lastname: Ninnis
authors.affiliation: 1] Walter and Eliza Hall Institute of Medical Research, 1 G Royal Parade, Parkville, Melbourne, Victoria, Australia [2] Department of Medical Biology, University of Melbourne, Parkville, Victoria.
authors.forename: S
authors.initials: S
authors.lastname: Iyer
authors.affiliation: Department of Biochemistry, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Australia.
authors.forename: D A
authors.initials: DA
authors.lastname: Stroud
authors.affiliation: 1] Walter and Eliza Hall Institute of Medical Research, 1 G Royal Parade, Parkville, Melbourne, Victoria, Australia [2] Centre de Cancérologie de Lyon, Université de Lyon, Lyon, France.
authors.forename: M
authors.initials: M
authors.lastname: Menard
authors.affiliation: 1] Walter and Eliza Hall Institute of Medical Research, 1 G Royal Parade, Parkville, Melbourne, Victoria, Australia [2] Department of Medical Biology, University of Melbourne, Parkville, Victoria.
authors.forename: R M
authors.initials: RM
authors.lastname: Kluck
authors.affiliation: Department of Biochemistry, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Australia.
authors.forename: M T
authors.initials: MT
authors.lastname: Ryan
authors.affiliation: 1] Walter and Eliza Hall Institute of Medical Research, 1 G Royal Parade, Parkville, Melbourne, Victoria, Australia [2] Department of Medical Biology, University of Melbourne, Parkville, Victoria.
authors.forename: G
authors.initials: G
authors.lastname: Dewson
mesh_terms: D000818:Animals; D017209:Apoptosis; D002478:Cells, Cultured; D057507:Mice, 129 Strain; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008928:Mitochondria / Q000378:metabolism*; D055503:Protein Multimerization; D021381:Protein Transport; D050996:Voltage-Dependent Anion Channel 2 / Q000378:metabolism*; D050998:bcl-2 Homologous Antagonist-Killer Protein / Q000378:metabolism*; D051028:bcl-2-Associated X Protein / Q000502:physiology*
chemical_list: C494773:Bak1 protein, mouse; C494827:Bax protein, mouse; C494766:Vdac2 protein, mouse; D050996:Voltage-Dependent Anion Channel 2; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein