Probing the Activation Mechanism of BAK in Mitochondrial Apoptosis

Abstract

Proteins of the B‐cell Lymphoma 2 (BCL‐2) family are the main regulators of mitochondrial outer membrane permeabilization (MOMP), the initiating event of the caspase cascade during mitochondrial apoptosis. Upon activation by BID binding to its trigger site, the BCL‐2 effector, Bcl‐2 Antagonist Killer (BAK) mediates MOMP. The mechanism of BAK oligomerization during MOMP remains poorly understood and controversial. It is postulated to involve sequential dimerization at an extensive interface overlapping the trigger site followed by weak affinity association of dimers at a poorly defined interface mediated by helix α6. Here we probed the oligomerization mechanism using disulfide‐bonded dimers of BAK engineered to trap the α6‐α6 interface. Using functional assays based on permeabilization of large unilamellar vesicles that mimic the outer mitochondrial membrane, and size exclusion chromatography multi‐angle light scattering, we probed and refined the current model for BAK‐mediated MOMP.