Structural bases of the altered functional properties of a pathological variant of the apoptosis inducing factor (AIF)

The apoptosis inducing factor (AIF) is a mitochondrial NAD-dependent flavoprotein, which, in addition to its long-known function in apoptosis, plays an essential role in respiratory chain biogenesis and maintenance, possibly via its interaction with CHCHD4/MIA40.1 Twenty human allelic variants of AIF are currently known as the cause of different neurodegenerative diseases. A strong correlation has been observed between the loss of ability of the pathological AIF forms to stabilize the charge-transfer (CT) complex between FADH- and NAD+ and the severity of the resulting illnesses, pointing to a pivotal role played by its redox activity and the related monomerdimer transition of AIF in its physiological function.2 Recently, we demonstrated that the G307E replacement in murine AIF (equivalent to the pathogenic G308E in the human protein) dramatically decreases the rate of CT complex formation by hampering AIF interaction with NAD+/H.3 Here we report the crystal structures of murine AIF-G307E in both its oxidized and CT complex states. From their comparison with the corresponding structures of wild type AIF, we ascertained how the increased steric hindrance of the replacing side chain leads to changes in the network of protein-ligand contacts. In particular, the G307E mutation causes the disruption of the interactions that, in the wild-type protein, Glu335 and Lys341 establish with the adenosine ribose and the pyrophosphate of NAD+, respectively. This in turn results in a 2 Å shift and reorientation of the adenylate moiety of bound NAD+, which appears to be the main cause of the observed decrease in the rate of CT complex formation. Moreover, we found that the altered balance between the binding of the adenine/nicotinamide moieties of the coenzyme determines a large drop in AIFG307E ability to discriminate between NADH and NADPH. References 1 Hangen E et al. (2015), Mol Cell, 58: 1001–1014. 2 Brosey CA et al. (2016), Structure, 24: 1-13. 3 Sorrentino L et al. (2015), Biochemistry, 54: 6996−7009.