Dynamic re-configuration of pro-apoptotic BAK on membranes

BAK and BAX, the effectors of intrinsic apoptosis, undergo major reconfiguration to an activated conformer that self-associates to damage mitochondria and cause cell death. However, the dynamic structural mechanisms that describe this reconfiguration in the presence of a membrane have yet to be fully elucidated. To explore the metamorphosis of membrane-bound BAK, we employed hydrogen-deuterium exchange mass spectrometry (HDX-MS) on liposomes comprising mitochondrial lipids. The HDX-MS profile of BAK on a membrane was broadly consistent with the known solution structures of inactive BAK. Following activation, HDX-MS resolved major reconfigurations in BAK. Mutagenesis led by our HDX-MS profiling revealed that the BCL-2 homology (BH) 4 domain maintains BAK in its inactive conformation and disrupting this was sufficient for constitutive BAK activation. Moreover, the entire BAK N-terminus that precedes the BAK oligomerisation domains became disordered post-activation and remained disordered in the activated oligomer. Cleavage of the N-terminus potentiated BAK-mediated membrane permeabilisation on liposomes and mitochondria. Together, HDX-MS reveals new insights into the dynamic nature of BAK activating conformation change in a membrane that will reveal new opportunities for therapeutic targeting.