E HDX-MS reveals structural constraints of BAK on a membrane. BAK-6H (150 nM) on liposomes was deuterated for 0, 10s, 60s or 600s and the incorporation of deuterium was assessed by mass spectrometry. Deuteration is expressed relative to a theoretical maximum. BH3 domain (orange), BH4 domain (blue) and hydrophobic surface groove comprising α helices 3-5 (green) are shown on the structure of BAK (PDB:2IMS). 