Structural insights into the coenzyme mediated monomer-dimer transition of the pro-apoptotic Apoptosis Inducing Factor
The apoptosis-inducing factor (AIF) is a mitochondrial-flavoprotein that, after cell death induction, is distributed to the nucleus to mediate chromatinolysis. In mitochondria AIF is present in a monomer-dimer equilibrium that after reduction by NADH gets displaced towards the dimer. The crystal structure of the human AIF (hAIF):NAD+-bound dimer revealed one FAD and, unexpectedly, two NAD+/H molecules per protomer, being this stoichiometry confirmed in solution. The here newly discovered NAD+-binding site includes residues mutated in human disorders, and accommodation of the coenzyme in it requires restructuration of an hAIF portion within the 509-560 apoptogenic segment. Disruption of interactions at the dimerization surface by production of the hAIF E413A/R422A/R430A mutant resulted in a non-dimerizable variant considerably less efficient stabilizing charge-transfer complexes upon coenzyme reduction than WT hAIF. These data reveal that the coenzyme-mediated monomer-dimer transition of hAIF modulates the conformation of its C-terminal proapoptotic domain, as well as its mechanism as reductase. These observations indicate that both the mitochondrial and apoptotic functions of hAIF are interconnected and coenzyme controlled: a key information in the understanding of the physiological role of AIF in the cellular life and death cycle.