title: tBid undergoes multiple conformational changes at the membrane required for Bax activation.
abstract: Bid is a Bcl-2 family protein that promotes apoptosis by activating Bax and eliciting mitochondrial outer membrane permeabilization (MOMP). Full-length Bid is cleaved in response to apoptotic stimuli into two fragments, p7 and tBid (p15), that are held together by strong hydrophobic interactions until the complex binds to membranes. The detailed mechanism(s) of fragment separation including tBid binding to membranes and release of the p7 fragment to the cytoplasm remain unclear. Using liposomes or isolated mitochondria with fluorescently labeled proteins at physiological concentrations as in vitro models, we report that the two components of the complex quickly separate upon interaction with a membrane. Once tBid binds to the membrane, it undergoes slow structural rearrangements that result in an equilibrium between two major tBid conformations on the membrane. The conformational change of tBid is a prerequisite for interaction with Bax and is, therefore, a novel step that can be modulated to promote or inhibit MOMP. Using automated high-throughput image analysis in cells, we show that down-regulation of Mtch2 causes a significant delay between tBid and Bax relocalization in cells. We propose that by promoting insertion of tBid via a conformational change at the mitochondrial outer membrane, Mtch2 accelerates tBid-mediated Bax activation and MOMP. Thus the interaction of Mtch2 and tBid is a potential target for therapeutic control of Bid initiated cell death.
authors.affiliation: Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, Ontario L8N 3Z5, Canada.
authors.forename: Aisha
authors.initials: A
authors.lastname: Shamas-Din
authors.forename: Scott
authors.initials: S
authors.lastname: Bindner
authors.forename: Weijia
authors.initials: W
authors.lastname: Zhu
authors.forename: Yehudit
authors.initials: Y
authors.lastname: Zaltsman
authors.forename: Clinton
authors.initials: C
authors.lastname: Campbell
authors.forename: Atan
authors.initials: A
authors.lastname: Gross
authors.forename: Brian
authors.initials: B
authors.lastname: Leber
authors.forename: David W
authors.initials: DW
authors.lastname: Andrews
authors.forename: Cécile
authors.initials: C
authors.lastname: Fradin
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000502:physiology; D051023:BH3 Interacting Domain Death Agonist Protein / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D053181:Caspase 8 / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism*; D031541:Fluorescence Resonance Energy Transfer; D006367:HeLa Cells; D006801:Humans; D007700:Kinetics; D008081:Liposomes / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D008928:Mitochondria / Q000378:metabolism / Q000502:physiology; D033681:Mitochondrial Membrane Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D051336:Mitochondrial Membranes / Q000502:physiology; D008954:Models, Biological; D008958:Models, Molecular; D009154:Mutation; D010539:Permeability; D011485:Protein Binding; D011487:Protein Conformation; D013997:Time Factors; D051028:bcl-2-Associated X Protein / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D051023:BH3 Interacting Domain Death Agonist Protein; D008081:Liposomes; C502207:MTCH2 protein, human; D033681:Mitochondrial Membrane Transport Proteins; C550380:Mtch2 protein, mouse; D051028:bcl-2-Associated X Protein; D053181:Caspase 8
keywords: Apoptosis; Bax; Bcl-2 Proteins; Bid; Fluorescence Correlation Spectroscopy; Fluorescence Resonance Energy Transfer (FRET); Fluorescence Spectroscopy; Mtch2; Protein Conformation; tBid