title: Mitochondrial outer membrane proteins assist Bid in Bax-mediated lipidic pore formation.
abstract: Mitochondrial outer membrane permeabilization (MOMP) is a critical step in apoptosis and is regulated by Bcl-2 family proteins. In vitro systems using cardiolipin-containing liposomes have demonstrated the key features of MOMP induced by Bax and cleaved Bid; however, the nature of the "pores" and how they are formed remain obscure. We found that mitochondrial outer membranes contained very little cardiolipin, far less than that required for liposome permeabilization, despite their responsiveness to Bcl-2 family proteins. Strikingly, the incorporation of isolated mitochondrial outer membrane (MOM) proteins into liposomes lacking cardiolipin conferred responsiveness to cleaved Bid and Bax. Cardiolipin dependence was observed only when permeabilization was induced with cleaved Bid but not with Bid or Bim BH3 peptide or oligomerized Bax. Therefore, we conclude that MOM proteins specifically assist cleaved Bid in Bax-mediated permeabilization. Cryoelectron microscopy of cardiolipin-liposomes revealed that cleaved Bid and Bax produced large round holes with diameters of 25-100 nm, suggestive of lipidic pores. In sum, we propose that activated Bax induces lipidic pore formation and that MOM proteins assist cleaved Bid in this process in the absence of cardiolipin.
authors.affiliation: Department of Pathology, University of Iowa Carver College of Medicine, Iowa City, 52242, USA.
authors.forename: Blanca
authors.initials: B
authors.lastname: Schafer
authors.forename: Joel
authors.initials: J
authors.lastname: Quispe
authors.forename: Vineet
authors.initials: V
authors.lastname: Choudhary
authors.forename: Jerry E
authors.initials: JE
authors.lastname: Chipuk
authors.forename: Teddy G
authors.initials: TG
authors.lastname: Ajero
authors.forename: Han
authors.initials: H
authors.lastname: Du
authors.forename: Roger
authors.initials: R
authors.lastname: Schneiter
authors.forename: Tomomi
authors.initials: T
authors.lastname: Kuwana
mesh_terms: D000818:Animals; D051023:BH3 Interacting Domain Death Agonist Protein / Q000494:pharmacology*; D002308:Cardiolipins / Q000494:pharmacology; D020285:Cryoelectron Microscopy; D008055:Lipids / Q000096:biosynthesis* / Q000302:isolation & purification; D051336:Mitochondrial Membranes / Q000187:drug effects* / Q000378:metabolism* / Q000648:ultrastructure; D024101:Mitochondrial Proteins / Q000378:metabolism*; D010539:Permeability / Q000187:drug effects; D011510:Proteolipids / Q000378:metabolism; D053835:Unilamellar Liposomes; D014981:Xenopus; D051028:bcl-2-Associated X Protein / Q000494:pharmacology*
chemical_list: D051023:BH3 Interacting Domain Death Agonist Protein; D002308:Cardiolipins; D008055:Lipids; D024101:Mitochondrial Proteins; D011510:Proteolipids; D053835:Unilamellar Liposomes; D051028:bcl-2-Associated X Protein; C020485:proteoliposomes