Study of an FBXO7 patient mutation reveals Fbxo7 and PI31 co-regulate proteasomes and mitochondria

Mutations in FBXO7 have been discovered associated with an atypical parkinsonism. We report here a new homozygous missense mutation in a paediatric patient that causes an L250P substitution in the dimerization domain of Fbxo7. This alteration selectively ablates the Fbxo7-PI31 interaction and causes a significant reduction in Fbxo7 and PI31 levels in patient cells. Consistent with their association with proteasomes, L250P patient fibroblasts have reduced proteasome activity and proteasome subunits. We also show PI31 interacts directly with the MiD49/51 fission adaptor proteins, and unexpectedly, PI31 acts as an adaptor enabling SCFFbxo7 ligase to ubiquitinate MiD49. Thus, the L250P mutation changes the function of Fbxo7 by altering its substrate repertoire. Although MiD49/51 expression was reduced in L250P patient cells, there was no effect on the mitochondrial network. However, patient cells had higher levels of ROS and reduced viability under stress. Our study shows that Fbxo7 and PI31 affect each other's functions in regulating both proteasomal and mitochondrial function and demonstrate a new function for PI31, as an adaptor for the SCFFbxo7 E3 ubiquitin ligase.