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

Significance To trigger cell death (apoptosis), two members of the B-cell lymphoma-2 protein family, Bak and Bax, change shape and convert from inert monomers into the oligomers that disrupt the outer mitochondrial membrane, but how they perturb the membrane is poorly understood. A longstanding model is that they rearrange and insert two central helices, α5 and α6, as a hairpin through the membrane. We show, however, that the hairpin insertion model does not hold. Instead, these helices in the oligomers insert only shallowly in the membrane, in its plane. The results favor a model in which these and probably other helices of Bak and Bax crowd the outer leaflet of the membrane, producing membrane curvature that leads to its disruption.