A uvs-5 Strain Is Deficient for a Mitofusin Gene Homologue, fzo1 , Involved in Maintenance of Long Life Span in Neurospora crassa

Mitochondria are highly dynamic organelles that continuously fuse and divide. To maintain mitochondria, cells establish an equilibrium of fusion and fission events, which are mediated by dynamin-like GTPases. We previously showed that anmus-10 strain, a mutagen-sensitive strain of the filamentous fungusNeurospora crassa , is defective in an F-box protein that is essential for the maintenance of mitochondrial DNA (mtDNA), long life span, and mitochondrial morphology. Similarly, auvs-5 mutant accumulates deletions within its mtDNA, has a shortened life span, and harbors fragmented mitochondria, the latter of which is indicative of an imbalance between mitochondrial fission and fusion. Since theuvs-5 mutation maps very close to the locus offzo1 , encoding a mitofusin homologue thought to mediate mitochondrial outer membrane fusion, we determined the sequence of thefzo1 gene in theuvs-5 mutant. A single amino acid substitution (Q368R) was found in the GTPase domain of the FZO1 protein. Expression of wild-type FZO1 in theuvs-5 strain rescued the mutant phenotypes, while expression of a mutant FZO1 protein did not. Moreover, when knock-in of the Q368R mutation was performed on a wild-type strain, the resulting mutant displayed phenotypes identical to those of theuvs-5 mutant. Therefore, we concluded that the previously unidentifieduvs-5 gene isfzo1 . Furthermore, we used immunoprecipitation analysis to show that the FZO1 protein interacts with MUS-10, which suggests that these two proteins may function together to maintain mitochondrial morphology.