title: Distinct lipid effects on tBid and Bim activation of membrane permeabilization by pro-apoptotic Bax.
abstract: After exposure to stressful stimuli, apoptotic signals can be relayed to mitochondria by pro-apoptotic activator proteins, tBid (truncated Bid/p15) and Bim (Bcl-2 interacting mediator), which activate Bax (Bcl-2 associated X protein) and or Bak (Bcl-2 antagonist/killer) to induce mitochondrial outer membrane (MOM) permeabilization (MOMP). These protein-protein and protein-membrane interactions are critical for apoptosis regulation, since MOMP irreversibly leads to cell death. Whereas the distinct roles of tBid and Bim as sensors of different types of stress are well recognized, it is not known whether the molecular mechanisms whereby they initiate MOMP are the same. In the present study, we compare membrane permeabilization by Bax activated by either cBid [cleaved Bid (p7 and p15)] or Bim and we examine the role of membrane lipids in the recruitment and activation of these three Bcl-2 (B-cell lymphoma 2) pro-apoptotic proteins. We employ fluorescently-labelled proteins and liposomes to quantify the effects of specific lipids on each of the well-characterized steps in Bax-mediated membrane permeabilization. We show that high levels of cholesterol in the membrane inhibit permeabilization by categorically identifying the recruitment of Bax by the activators and Bax insertion in the membrane as the steps being hindered by cholesterol. Furthermore, we show that binding of both cBid and Bim to membranes is facilitated by electrostatic interactions with anionic phospholipids. However, whereas Bim does not require any particular anionic lipids, the conformational change in tBid depends on cardiolipin (CL). This suggests that CL can activate tBid in a similar manner to Mtch2 (mitochondrial carrier homologue 2). Thus, lipids modify multiple aspects of Bax-mediated membrane permeabilization.
authors.affiliation: *Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Aisha
authors.initials: A
authors.lastname: Shamas-Din
authors.affiliation: *Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Scott
authors.initials: S
authors.lastname: Bindner
authors.affiliation: †Department of Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
authors.forename: Xiaoke
authors.initials: X
authors.lastname: Chi
authors.affiliation: *Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Brian
authors.initials: B
authors.lastname: Leber
authors.affiliation: *Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8S 4K1, Canada.
authors.forename: David W
authors.initials: DW
authors.lastname: Andrews
authors.affiliation: *Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8S 4K1, Canada.
authors.forename: Cécile
authors.initials: C
authors.lastname: Fradin
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D051017:Apoptosis Regulatory Proteins / Q000235:genetics / Q000378:metabolism*; D051023:BH3 Interacting Domain Death Agonist Protein / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000072224:Bcl-2-Like Protein 11; D002308:Cardiolipins / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism; D006801:Humans; D008081:Liposomes / Q000378:metabolism; D008563:Membrane Lipids / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D008930:Mitochondria, Liver / Q000378:metabolism; D051336:Mitochondrial Membranes / Q000378:metabolism; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D010539:Permeability; D011487:Protein Conformation; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D050998:bcl-2 Homologous Antagonist-Killer Protein / Q000172:deficiency / Q000235:genetics; D051028:bcl-2-Associated X Protein / Q000235:genetics / Q000378:metabolism*
chemical_list: D051017:Apoptosis Regulatory Proteins; C494826:BAX protein, human; C000606775:BCL2L11 protein, human; D051023:BH3 Interacting Domain Death Agonist Protein; C494773:Bak1 protein, mouse; D000072224:Bcl-2-Like Protein 11; C000606776:Bcl2l11 protein, mouse; C494815:Bid protein, mouse; D002308:Cardiolipins; D008081:Liposomes; D008563:Membrane Lipids; D008565:Membrane Proteins; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein; D002784:Cholesterol