The biochemical properties of a pathogenic variant of human apoptosis inducing factor (AIF) provide insight into its unusual reactivity toward NADH

Apoptosis inducing factor (AIF) is a Janus flavoprotein that plays opposite vital and lethal roles in eukaryotic cells (1). When confined into the intermembrane space of the mitochondrion, it contributes to maintain the integrity of complexes I and II whereas, when is released from the organelle, it triggers caspase-independent apoptosis. The role of AIF in mitochondrion maintenance has been proposed to be mediated by its action as a sensor of the redox state of the organelle and/or of NAD+/NADH concentration (2). Indeed, upon reaction with NADH, AIF yields an oxygen-stable NAD+−FADH- charge-transfer complex (CT), whose formation is strictly coupled to protein dimerization. This transition has been suggested to be at the basis of an as yet uncharacterized signal transduction pathway (3). The mechanism of NADH binding, hydride transfer, CT formation and monomer-dimer transition is not fully understood to date. Recently, a Gly-to-Glu mutation in human AIF was shown to cause major prenatal neurological defects (4). We have introduced the corresponding replacement in the mouse homolog and we have purified the pathogenic AIF-G307E variant. Although G307 is located in the binding site of the adenyl moiety of NAD, the G307E replacement neither prevented the formation of CT nor significantly altered its stability. Rather, it specifically slowed down the kinetics of FAD reduction and CT formation, providing important clues in the dissection of the individual steps of the process. This work has been supported by a PRIN2008 grant awarded by the Italian Ministry of Education and University (MIUR) to M.A. Vanoni. 1. Sevrioukova IF. Antioxid Redox Signal. 2011; 14: 2545-79. 2. Churbanova IY, Sevrioukova IF. J Biol Chem. 2008; 283: 5622-31. 3. Sevrioukova IF. J Mol Biol. 2009; 390: 924-38. 4. Berger I, et al. Mol Genet Metab. 2011; 104: 517-20.