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Apoptotic pore formation is associated with in-plane insertion of Bak or Bax central helices into the mitochondrial

Dana Westphal1, Grant Dewson2, Marie Menard3

  • 1Molecular Genetics of Cancer Division, Department of Medical Biology, University of Melbourne, Parkville, VIC 3010, Australia; and.

Proceedings of the National Academy of Sciences of the United States of America
|September 18, 2014
PubMed

Insights

The B-cell lymphoma-2 (Bcl-2) family proteins Bak and Bax perforate the mitochondrial outer membrane during apoptosis. Our findings reveal these proteins insert shallowly into the membrane, forming pores rather than a hairpin structure.

Area of Science:

  • Cell Biology
  • Biochemistry
  • Molecular Biology

Background:

  • Apoptosis, or programmed cell death, is crucial for development and tissue homeostasis.
  • The mitochondrial pathway of apoptosis is initiated by the permeabilization of the mitochondrial outer membrane (MOM).
  • Oligomers of B-cell lymphoma-2 (Bcl-2) family proteins, Bak and Bax, are key mediators of MOM permeabilization.

Purpose of the Study:

  • To elucidate the mechanism by which Bak and Bax oligomers disrupt MOM integrity.
  • To investigate the role of α5 and α6 helices of Bak and Bax in MOM perforation.
  • To challenge and refine existing models of MOM permeabilization.

Main Methods:

  • Utilized cysteine accessibility assays with membrane-impermeable reagents.
  • Investigated Bak and Bax oligomer structures within the context of MOM permeabilization.
  • Compared experimental findings with the established hairpin insertion model.

Main Results:

  • Bak α5 helix, similar to Bax α5, remains integrated within the protein core of oligomers.
  • Cysteines introduced at multiple positions within Bak and Bax α5 and α6 helices were accessible to membrane-impermeable reagents.
  • These results contradict the hairpin insertion model of membrane disruption.

Conclusions:

  • The α5 and α6 helices of Bak and Bax do not insert as a hairpin across the MOM.
  • An "in-plane" model is supported, where α5 and α6 helices lie parallel to the membrane.
  • These helices shallowly insert into the membrane, driving the formation of proteolipidic pores and MOM permeabilization.
Keywords:
cell deathcysteine-scanning mutagenesismembrane poresmitochondrial permeabilizationprotein-membrane topology

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