The G308E variant of the apoptosis inducing factor, responsible of a rare encephalopathy, is hampered in NAD+/H binding

The apoptosis inducing factor (AIF), a highly conserved mitochondrial flavoprotein, plays two opposite roles in eukaryotic cells: while in mitochondria, it is required for efficient oxidative phosphorylation (OXPHOS), but it triggers caspase-independent apoptosis when released into the cytosol (1). AIF undergoes dimerization upon reaction with NAD+/H, via formation of an unusually air-stable charge-transfer (CT) complex. This peculiar property of AIF is likely to be linked to its biological function. Recently, the G308E mutation of human AIF was identified as the cause of severe neurodegeneration associated with OXPHOS defect (2). We introduced the equivalent amino acyl substitution in murine AIF (G307E) and characterized the resulting protein variant in vitro. This replacement dramatically decreases the rate of reaction between AIF and NAD+/H, but neither the rate of dissociation nor the O2 reactivity of the resulting dimeric CT complex were affected. A detailed rapid-mixing and steady-state kinetic study of the reaction between AIF and NAD+/H allowed us to develop a two-step mechanism for CT complex formation. In addition, we found that FAD reduction induced a partial conformational reorganization of AIF, triggering dimerization which is therefore independent from ligand binding and CT complex formation. Our results shed new light on the mechanism of the possible redox-sensing role of AIF and show that the pathogenic G308E replacement may disrupt its functions, while in mitochondria, by specifically slowing down the formation of its dimeric CT complex. 1. Sevrioukova (2011) Antioxid Redox Signal, 14: 2545-2579 2. Berger et al (2011) Mol Genet Metab, 104: 517-520