title: Mechanistic issues of the interaction of the hairpin-forming domain of tBid with mitochondrial cardiolipin.
abstract: BACKGROUND
  The pro-apoptotic effector Bid induces mitochondrial apoptosis in synergy with Bax and Bak. In response to death receptors activation, Bid is cleaved by caspase-8 into its active form, tBid (truncated Bid), which then translocates to the mitochondria to trigger cytochrome c release and subsequent apoptosis. Accumulating evidence now indicate that the binding of tBid initiates an ordered sequences of events that prime mitochondria from the action of Bax and Bak: (1) tBid interacts with mitochondria via a specific binding to cardiolipin (CL) and immediately disturbs mitochondrial structure and function idependently of its BH3 domain; (2) Then, tBid activates through its BH3 domain Bax and/or Bak and induces their subsequent oligomerization in mitochondrial membranes. To date, the underlying mechanism responsible for targeting tBid to mitochondria and disrupting mitochondrial bioenergetics has yet be elucidated.
  
  
  RESULTS
  The present study investigates the mechanism by which tBid interacts with mitochondria issued from mouse hepatocytes and perturbs mitochondrial function. We show here that the helix alphaH6 is responsible for targeting tBid to mitochondrial CL and disrupting mitochondrial bioenergetics. In particular, alphaH6 interacts with mitochondria through electrostatic interactions involving the lysines 157 and 158 and induces an inhibition of state-3 respiration and an uncoupling of state-4 respiration. These changes may represent a key event that primes mitochondria for the action of Bax and Bak. In addition, we also demonstrate that tBid required its helix alphaH6 to efficiently induce cytochrome c release and apoptosis.
  
  
  CONCLUSIONS
  Our findings provide new insights into the mechanism of action of tBid, and particularly emphasize the importance of the interaction of the helix alphaH6 with CL for both mitochondrial targeting and pro-apoptotic activity of tBid. These support the notion that tBid acts as a bifunctional molecule: first, it binds to mitochondrial CL via its helix alphaH6 and destabilizes mitochondrial structure and function, and then it promotes through its BH3 domain the activation and oligomerization of Bax and/or Bak, leading to cytochrome c release and execution of apoptosis. Our findings also imply an active role of the membrane in modulating the interactions between Bcl-2 proteins that has so far been underestimated.
authors.affiliation: Institut Cochin, CNRS UMR8104 (Centre National de la Recherche Scientifique), INSERM U 567, Université Paris-Descartes, Paris, France.
authors.forename: François
authors.initials: F
authors.lastname: Gonzalvez
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Pariselli
authors.forename: Olivier
authors.initials: O
authors.lastname: Jalmar
authors.forename: Pauline
authors.initials: P
authors.lastname: Dupaigne
authors.forename: Franck
authors.initials: F
authors.lastname: Sureau
authors.forename: Marc
authors.initials: M
authors.lastname: Dellinger
authors.forename: Eric A
authors.initials: EA
authors.lastname: Hendrickson
authors.forename: Sophie
authors.initials: S
authors.lastname: Bernard
authors.forename: Patrice X
authors.initials: PX
authors.lastname: Petit
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D017209:Apoptosis; D051023:BH3 Interacting Domain Death Agonist Protein / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002308:Cardiolipins / Q000235:genetics / Q000378:metabolism*; D002478:Cells, Cultured; D045304:Cytochromes c / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D005260:Female; D022781:Hepatocytes / Q000166:cytology / Q000378:metabolism; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D008297:Male; D051379:Mice; D020297:Mice, Congenic; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008856:Microscopy, Fluorescence; D008928:Mitochondria / Q000378:metabolism; D024101:Mitochondrial Proteins / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D011485:Protein Binding; D017382:Reactive Oxygen Species / Q000378:metabolism; D014162:Transfection; D018909:src Homology Domains
chemical_list: D051023:BH3 Interacting Domain Death Agonist Protein; C494815:Bid protein, mouse; D002308:Cardiolipins; D008164:Luminescent Proteins; D024101:Mitochondrial Proteins; D017382:Reactive Oxygen Species; D045304:Cytochromes c