title: The apoptotic protein tBid promotes leakage by altering membrane curvature.
abstract: The apoptotic protein tBid is effective in promoting both leakage and lipid mixing in liposomes composed of cardiolipin and phosphatidylcholine at a molar ratio of 1:2 in the presence of calcium. When half of the phosphatidylcholine component of these liposomes is replaced with phosphatidylethanolamine, a lipid that promotes negative membrane curvature, the rates of both leakage and lipid mixing caused by tBid are substantially increased. Replacement of cardiolipin with phosphatidylglycerol, a lipid that is structurally similar to cardiolipin but does not promote negative membrane curvature in the presence of calcium, prevents the tBid from promoting leakage. The promotion of leakage by tBid is also inhibited by several substances that promote positive membrane curvature, including lysophosphatidylcholine, tritrpticin, a potent antimicrobial peptide, and cyclosporin A, a known inhibitor of cytochrome c release from mitochondria. We directly measured the effect of tBid on membrane curvature by (31)P NMR. We found that tBid promotes the formation of highly curved non-lamellar phases. All of these data are consistent with the hypothesis that tBid promotes negative curvature, and as a result it destabilizes bilayer membranes. Bcl-X(L) inhibits leakage and lipid mixing induced by tBid. Bcl-X(L) is anti-apoptotic. It reduces the promotion of non-bilayer phases by tBid, although by itself Bcl-X(L) is capable of promoting their formation. Bcl-X(L) has little effect on liposomal integrity. Our results suggest that the anti-apoptotic activity of Bcl-X(L) is not a consequence of its interaction with membranes, but rather with other proteins, such as tBid.
authors.affiliation: Department of Biochemistry, McMaster University Health Sciences Centre, Hamilton, Ontario L8N 3Z5, Canada. epand@mcmaster.ca
authors.forename: Raquel F
authors.initials: RF
authors.lastname: Epand
authors.forename: Jean-Claude
authors.initials: JC
authors.lastname: Martinou
authors.forename: Monique
authors.initials: M
authors.lastname: Fornallaz-Mulhauser
authors.forename: Donald W
authors.initials: DW
authors.lastname: Hughes
authors.forename: Richard M
authors.initials: RM
authors.lastname: Epand
mesh_terms: D017209:Apoptosis; D051023:BH3 Interacting Domain Death Agonist Protein; D002122:Calcium Chloride / Q000494:pharmacology; D002308:Cardiolipins / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D016572:Cyclosporine / Q000494:pharmacology; D003574:Cytochrome c Group / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006367:HeLa Cells; D006801:Humans; D008051:Lipid Bilayers / Q000378:metabolism; D050356:Lipid Metabolism; D008081:Liposomes / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D008928:Mitochondria / Q000378:metabolism; D009842:Oligopeptides / Q000494:pharmacology; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology*; D010714:Phosphatidylethanolamines / Q000494:pharmacology; D010743:Phospholipids / Q000378:metabolism; D011485:Protein Binding; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D013696:Temperature; D013997:Time Factors; D051020:bcl-X Protein
chemical_list: C494811:BCL2L1 protein, human; D051023:BH3 Interacting Domain Death Agonist Protein; C494814:BID protein, human; D002308:Cardiolipins; D002352:Carrier Proteins; D003574:Cytochrome c Group; D004791:Enzyme Inhibitors; D008051:Lipid Bilayers; D008081:Liposomes; D009842:Oligopeptides; D010446:Peptide Fragments; D010714:Phosphatidylethanolamines; D010743:Phospholipids; D019253:Proto-Oncogene Proteins c-bcl-2; D051020:bcl-X Protein; C402895:tritrpticin; D016572:Cyclosporine; D002122:Calcium Chloride