BIM-Mediated Membrane Insertion of the BAK Pore Domain Is an Essential Requirement for Apoptosis

Summary BAK activation represents a key step during apoptosis, but how it converts into a mitochondria-permeabilizing pore remains unclear. By further delineating the structural rearrangements involved, we reveal that BAK activation progresses through a series of independent steps: BH3-domain exposure, N-terminal change, oligomerization, and membrane insertion. Employing a “BCL-XL-addiction” model, we show that neutralization of BCL-XL by the BH3 mimetic ABT-737 resulted in death only when cells were reconstituted with BCL-XL:BAK, but not BCL-2/ BCL-XL:BIM complexes. Although this resembles the indirect model, release of BAK from BCL-XL did not result in spontaneous adoption of the pore conformation. Commitment to apoptosis required association of the direct activator BIM with oligomeric BAK promoting its conversion to a membrane-inserted pore. The sequential nature of this cascade provides multiple opportunities for other BCL-2 proteins to interfere with or promote BAK activation and unites aspects of the indirect and direct activation models.

Graphical Abstract

Highlights • A BH3 mimetic displaces BAK from BCL-XL consistent with the indirect activation model • Released BAK undergoes a series of distinct independent conformational changes • Oligomerization is not the final step in the activation cascade • BIM promotes pre-pore-to-pore conversion supporting the “direct” activation model

BAK activation is a key step in apoptosis, but how it converts into a mitochondrial permeabilizing pore remains unclear. By analyzing the structural rearrangements involved, Cohen and colleagues now define how BAK activation occurs in a stepwise manner. Whereas the authors find some aspects consistent with the indirect activation model (where antiapoptotic BCL2 proteins sequester effectors), they provide data arguing that association of the direct activator BIM with oligomeric BAK is required for commitment to apoptosis.