(E) Schematic showing Parkin ubiquitination of BAK monomer in non-apoptotic cells, but not the BAK homo-dimer in cells undergoing apoptosis. In response to mitophagy stimuli, PINK1 phosphorylates ubiquitin to recruit Parkin, which in turn is phosphorylated by PINK1 to become activated to ubiquitinate monomeric BAK on K113. Mitochondrial outer membrane (MOM) permeabilisation driven by BAK oligomers during apoptosis provokes Parkin activity in a PINK1-dependent manner. Although Parkin can ubiquitinate mitochondrial outer membrane substrates (e.g. Mfn2) to promote mitochondrial clearance, it cannot ubiquitinate dimerised BAK on K113. 