Functions of the small proteins in the TOM complex of Neurospora crasssa

The TOM (translocase of the outer mitochondrial membrane) complex of the outer mitochondrial membrane is required for the import of proteins into the organelle. The core TOM complex contains five proteins, including three small components Tom7, Tomb, and Toms. We have created single and double mutants of all combinations of the three small Tom proteins of Neurospora crassa. Analysis of the mutants revealed that Tomb plays a major role in TOM complex stability, whereas Tom? has a lesser role. Mutants lacking both Tomb and Tom? have an extremely labile TOM complex and are the only class of mutant to exhibit an altered growth phenotype. Although single mutants lacking N. crassa Toms have no apparent TOM complex abnormalities, studies of double mutants lacking Toms suggest that it also has a minor role in maintaining TOM complex stability. Our inability to isolate triple mutants supports the idea that the three proteins have overlapping functions. Mitochondria lacking either Tomb or Tom? are differentially affected in their ability to import different precursor proteins into the organelle, suggesting that they may play roles in the sorting of proteins to different mitochondrial subcompartments. Newly imported Tom40 was readily assembled into the TOM complex in mitochondria lacking any of the small Tom proteins.